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6 Biosynthesis By RUSSELL J. COX School of Chemistry University of Bristol Cantock’s Close Bristol UK BS8 1TS 1 Introduction The study of the origin of natural products continues to drive the development of organic chemistry. This review is intended to highlight some of the major contributions made to the field during 1996 and 1997 continuing the coverage of previous Reports.1,2 The study of biosynthetic pathways has long held the promise of the development of controlled in vivo organic synthesis; these promises are beginning to be repaid and this Report will outline some of the significant progress made. 2 Polyketide metabolites The biosynthesis of numerous individual polyketides has been extensively reviewed.3 The growth in the use of molecular biological methods in this field has advanced at such a pace that a short review such as this cannot hope to be comprehensive.Indeed a recent issue of Chemical Reviews devotes over 240 pages to discussion of the biosynthesis of a few representative polyketides and polypeptides.4 In almost all cases the conclusions drawn from the new molecular methods have backed-up and reinforced the models for polyketide and fatty acid synthesis arrived at through classical labelled feeding experiments.5 Importantly the new methods allow for the redirection of secondary metabolite production through genetic engineering. A crucial sphere of understanding now revolves around central questions of ‘programming’ i.e. the ability of polyketide synthase complexes to control the structures they produce. The ability to reprogram a synthase complex allows the in vivo or in vitro production of metabolites at will.This section will focus on progress made in e§orts to achieve novel metabolite production. Modular bacterial PKS complexes The genetic sequences for a number of PKS complexes responsible for the production of macrolides such as erythromycin 1 and tylosin aglycones were among the first PKS gene clusters to be examined. Sequence comparison with FAS clusters immediately identified putative functional domains in the deduced peptide sequence (Scheme 1). These deduced domains consist of Acyl Transferases (AT) for the loading of starter extender and intermediate acyl units; Acyl Carrier Proteins (ACP) which hold the 187 S S S S S S S O O O O O O O HO HO HO HO HO HO HO HO HO HO HO O O O O HO HO HO O O R OH OH OH O DEBS1 Module 1 Module 2 KS AT ACP KS AT DH ER KR ACP DEBS2 Module 3 Module 4 KS AT KR ACP KS AT KR ACP TE AT ACP KS AT KR ACP KS AT KR ACP DEBS3 Module 5 Module 6 R = Me 6-dEB 1 R = H 1a Scheme 1 The biosynthesis of 6-deoxyerythronolide-B (6-dEB).Intermediates are presumed to be bound as thiol esters to the acylcarrier proteins (ACP). Other protein units AT Acyl Transferase; KS Keto-Synthase; KR Keto-Reductase; DH Dehydratase; ER Enoyl Reductase; TE Thiol Esterase. growing macrolide as a thiol ester; b-Keto-Acyl Synthases (KS) which catalyse chain extension; b-Keto Reductases (KR) responsible for the first reduction to an alcohol functionality; Dehydratases (DH) which eliminate water to give an unsaturated thiol ester; Enoyl Reductases (ER) which catalyse the final reduction to full saturation; and finally a Thiol Esterase (TE) to catalyse macrolide release and cyclisation.The observation that there was a separate functional domain for each and every transformation required to produce the macrolide suggests that for the macrolides the programming of the PKS is achieved through a production-line approach. Early experiments to modify biosynthesis focused upon deletion of functionality these experiments have been reviewed elsewhere.6 For the construction of the erythromycin aglycone 6-deoxyerythronolide B (6-dEB 1) three large proteins are required (DEBS1 DEBS2 and DEBS3 Scheme 1). Each contains two modules responsible for chain elongation and modification. A similar gene cluster codes for the RAPS proteins responsible for rapamycin aglycone production. Early experiments (Scheme 2) showed that DEBS1 (both with and without a fused TE domain) was su¶cient to produce triketides (starter plus two extensions) such as the expected 2 and the acetate starter derived 2a.More recent experiments have shown that expression of DEBS1 with the first module of DEBS2 fused to a TE [DEBS2 (module 3)]TE] will produce tetraketides in vivo (Scheme 3).7 Two compounds 3 and 4 were recovered from strains of Streptomyces coelicolor containing the relevant 188 R. J. Cox O O R OH DEBS1 + TE Module 1 Module 2 AT ACP KS AT KR ACP KS AT KR ACP TE R = Me 2 R = H 2a Scheme 2 Biosynthesis of a triketide by DEBS1 DEBS1 O OH O O Module 1 Module 2 KS AT ACP TE O OH OH DEBS2 Module 3 AT ACP KS AT KR ACP KS AT KR ACP 3 4 Scheme 3 Biosynthesis of tetraketides by DEBS1 and fused DEBS module 3 genetic constructs.Classical feeding experiments with labelled propionates proved their polypropionate origin. Interestingly the compounds di§er in the mode of cyclisation –the mixture of d-lactone and pyrones indicating a more complex role for the TE than previously expected.A more thorough investigation of the TE specificity has been carried out.8 The above in vivo results have been duplicated in vitro.9,10 Purification of DEBS1 DEBS1]TE and DEBS2 (module 3)]TE followed by in vitro reconstitution gives rise to the expected compounds. The k#!5 for DEBS1]TE has been measured at 4.8 min~1 comparable with values measured in crude cell-free preparations and the rate of secondary metabolite production in vivo. Interestingly the rate of production of 3 and 4 from the reconstituted DEBS1 with DEBS2 (module 3)]TE was significantly slower (k#!5 0.23 min~1) even when an excess of module 3 was present.This was taken as evidence for ine¶cient or unproductive binding of the DEBS1 and DEBS2 proteins to form the macromolecular complex. Many PKS systems have been demonstrated to have lax specificity for the acyl starter unit. One way of generating new polyketides which has been exploited in the past has been to supplement growing cultures of the producing organism with a high concentration of a suitable carboxylic acid which in some circumstances can prime polyketide synthesis. With the controlled expression of the DEBS proteins in S. coelicolor however a more sophisticated approach has been demonstrated. Sitedirected mutation of the initial KS domain of DEBS1 led to the production of an in vivo PKS complex unable to process the usual propionate starter unit and unable to produce 6-dEB under normal circumstances (Scheme 4).11 Synthesis of 6-dEB 1 was restored when the cultures were supplemented with the natural diketide 5 as an N-acetyl cysteamyl thiol ester.Novel macrolides were produced when growing cultures of the S. coelicolor expression host were supplemented with synthetic di- 5a and 5b and tri-ketides 9. These experiments proved the ability of the modified PKS to process unnatural substrates. Further experiments were performed in which the modi- 189 Biosynthesis R SNAC O OH O O R OH OH OH O O O R O O OH O OH O O N OH OH OH SNAC OH O O O O OH OH OH DEBS1 KS1° Module 1 Module 2 KS AT ACP KS AT DH ER KR ACP DEBS2 Module 3 Module 4 KS AT KR ACP KS AT KR ACP TE AT ACP KS AT KR ACP KS AT KR ACP DEBS3 Module 5 Module 6 R = Me 1 R = Pr n 6 R = Ph 6a R = Me 7 R = Pr n 8 R = Ph 8a 9 S.erythrae R = Me 5 R = Pr n 5a R = Ph 5b Scheme 4 Inactivation of DEBS module 1 and feeding studies with PKS precursors O O R O DEBS1 + TE Module 1 Module 2 AT ACP KS at KR ACP KS AT KR ACP TE from RAPS module 2 R = Me 10 R = Et 10a Scheme 5 Modification of module 1 AT substrate specificity fied aglycones 1 6 and 6a were fed to cultures of an erythromycin producing strain of Saccharopolyspora erythrae deficient in 6-dEB synthesis. 6-dEB 1 was transformed to 7 as expected post-PKS modifications of the unnatural aglycones were successfully carried out. The production of the novel antimicrobial compounds 8 and 8a was confirmed by structural elucidation and antimicrobial assays.The combination of modules from PKS clusters responsible for the synthesis of erythromycin and rapamycin aglycones has been achieved (Scheme 5).12 The first reported experiment involved the insertion of the RAPS module 2 AT domain into DEBS1 module 1 of a DEBS1]TE construct in which KR of module 2 had been inactivated. The RAPS module 2 AT is specific for the transfer of a malonyl unit rather than the DEBS specified methylmalonyl unit. This replacement resulted in the produc- 190 R. J. Cox DEBS1 Module 1 KS AT ACP TE DEBS2 Module 3 AT ACP KS AT KR ACP KS AT dh kr ACP from RAPS module 4 R OH O R = H 11 R = Me 11a Scheme 6 Addition of functionality to module 2 O O R OH DEBS1 + TE Module 1 Module 2 at acp KS AT KR ACP KS AT KR ACP TE R = Me 12 R = Et 12a R = Pri 12b R = Bus 12c from AVR module 1 Scheme 7 Variation of starter unit flexibility tion of the nor-triketide ketones 10 and 10a as expected.The insertion of KR and DH domains from module 4 of the rapamycin PKS into the DEBS1–DEBS2 (module 3)]TE construct in place of the natural KR domain of DEBS module 2 was also achieved (Scheme 6).13 Expression of the modified construct led to the synthesis of the unsaturated ketones 11 and 11a resulting from the addition of functionality of the PKS. More ambitious replacements have also been made. Replacement of the DEBS1 module 1 loading domains (AT]ACP) with the loading domains from the avermectin PKS (AVRS) resulted in the production of four triketide lactones 12 12a 12b and 12c (Scheme 7).14 The AVR synthase specifically loads branched starter units derived from amino acid catabolism and these branched units were e¶ciently loaded into the new compounds by the hybrid synthase.Additional rational replacement experiments have also been carried out with the full set of DEBS proteins (Scheme 8). Replacement of module 6 methylmalonyl-specificAT from DEBS3 with a malonyl-specific AT domain from the rapamycin PKS module 2 resulted in a genetic construct which enabled the biosynthesis of the nor-6-dEBs 13 and 14.15 These compounds were also transformed to their corresponding biologically active glycosidated homologues 15 and 16 when fed to S. erythrae deficient in 6-dEB synthesis. Similar gains of function experiments have been used to produce the eight-membered lactone 17.16 Insertion of the RAPS module 1 DH ER and KR domains in place of the DEBS module 2 KR in the DEBS1–DEBS2 (module 3)]TE construct led to production of a synthase in which full reduction to a methylene was carried out after the second condensation (Scheme 9).Removal of oxygen functionality then prevented formation of the d-lactone by TE forcing production of 17. Reduction of the b-keto group presumably occurred via adventitious enzyme activity within the S. coelicolor expression strain. A similar experiment in which the corresponding DEBS domains from module 4 were inserted at the same position resulted in production of 18 (Scheme 10). These domains presumably were not able to fully process the intermediates 191 Biosynthesis O O R1 OH OH OH O R2 O O R1 O O OH O R2 OH O O N OH OH OH DEBS1 KS1 Module 1 Module 2 KS AT ACP KS AT DH ER KR ACP DEBS2 Module 3 Module 4 KS AT KR ACP KS at KR ACP TE AT ACP KS AT KR ACP KS AT KR ACP Module 5 Module 6 from RAPS module 2 S.erythrae DEBS3 R1 = H R2 = H 13 R1 = Me R2 = H 14 R1 = H R2 = H 15 R1 = Me R2 = H 16 Scheme 8 Variation in late-stage programming and onward conversion to erythromycin analogues O O OH DEBS1 Module 1 from RAPS module 1 DEBS2 Module 3 AT ACP KS AT KR ACP KS AT dh er kr ACP KS AT ACP TE 17 Scheme 9 Production of new lactones leaving the unsaturated ketone which spontaneously decarboxylates as previously observed. Fungal PKS complexes Very few fungal PKS systems have been investigated at the genetic biochemical or biosynthetic levels and recent advances are limited to the biosynthetic machinery involved in the production of the potent mycotoxin aflatoxin B1 19 and the nonproteinogenic amino acid methyl Bmt 20.Methyl-Bmt 20 forms part of the cyclic peptide immunosuppressant cyclosporin produced by the fungus Tolypocladium niveum. The carbon framework of 20 is formed via a classical fungal polyketide pathway from acetate and methionine leading to the enzyme-bound tetraketide 21.17 Post-PKS processing then provides the a-amino moiety of 20. Recent work has concentrated on the extraction and purification of a functional Me-Bmt synthase.18 The synthase would appear to be a very large single protein similar to the DEBS and RAPS proteins perhaps not surprisingly considering the highly reduced nature of the product. Through a series of careful biochemical experiments involving acetyl and malonyl-CoAs S-adenosyl methionine and other potential intermediates assayed by 192 R.J. Cox DEBS1 Module 1 from DEBS module 4 KS AT ACP TE R OH O DEBS2 Module 3 AT ACP KS AT KR ACP KS AT dh er kr ACP 18 Scheme 10 DEBS module 4 cannot fully process ‘unexpected’ precursors HPLC a biosynthetic route has been established (Scheme 11). The early steps of the synthesis up to the production of the C 7 triketide 22 have been shown conclusively to be processive. Later steps may or may not be processive although a processive model for assembly would be preferred in analogy to the DEBS proteins. The substrate specificity of the complex has been assessed and importantly alternative starter units may be used producing the fully saturated 23 from butyryl CoA used as a starter unit in preference to a crotonyl-CoA intermediate.Hexanoyl-CoA is processed as a starter unit as far as the first round of condensation and reduced to an octanoyl species but can proceed no further towards methylated intermediates. Interestingly hexanoyl- and octanoyl-CoAs are implicated in the biosynthesis of at least two other fungal polyketides. Recent work on the biosynthesis of piliformic acid 24 in species of Xylaria and Poronia piliformis has shown that an octanoyl unit (likely derived from fatty acid biosynthesis because of the deduced stereochemistry of the ER step) is incorporated intact into 24 the rest of the molecule being derived from oxaloacetate (Scheme 12).19 Likewise the synthase responsible for synthesis of norsolorinic acid 25 the first free intermediate during aflatoxin biosynthesis hijacks a hexanoyl starter unit produced by a dedicated FAS (Scheme 13).Again specificity is a little lax and recent work has shown that a number of alternative starter units may be incorporated.20 Examples include compounds derived from pentanoate 26 and various fluorohexanoates but butyrate heptanoate and octanoate were not incorporated. Much recent genetic and biochemical evidence for the operation of the separate norsolorinic FAS and PKS clusters is reviewed elsewhere.21 Type II bacterial PKS complexes The PKS enzymes which are responsible for the biosynthesis of the polyaromatic bacterial polyketides such as actinorhodin (act) oxytetracycline (otc) griseusin (gris) tetracenomycin (tcm) and others are fundamentally di§erent from the modular multifunctional enzymes which catalyse the production of the macrolides.A number of discreet monofunctional enzymes act iteratively to produce a putative poly-b-keto intermediate of defined chain length. The cyclisation aromatisation and later oxidation and reduction steps are then catalysed by a host of additional discreet enzymes. The programming of the synthase chain length is cryptic dependent upon the interaction of the PKS subunits as much as the subunits themselves. Despite the di¶culty in understanding programming significant progress has been made in the directed synthesis of novel compounds. The development of a Streptomyces coelicolor non-polyketide producing strain coupled with the use of a flexible plasmid system in which individual genes and gene clusters may be expressed has enabled the early biosynthetic steps toward a number of polyaromatic compounds to be investigated.The production of new compounds has 193 Biosynthesis SCoA O S O O E S O E O O S E O O S E O S E O S E O S O E OH S O E OH X O OH X O O OH X O NH2 KS malonyl CoA KR DH NADPH –H2O KS malonyl CoA MT SAM KR DH NADPH –H2O ER NADPH KS malonyl CoA KR NADPH Release [O] [NH3] OH Me-Bmt 20 OH O Post PKS Modification Enzyme Bound Assembly 23 21 22 Scheme 11 Deduced biosynthesis of Me-Bmt been used to infer the biosynthetic steps controlled by the expressed genes. For example the use of limited cloning methods has led to the conclusion that the mithramycinone aglycone 27 moieties of the aureolic acid group of anti-tumor agents are derived from a single polyketide chain a conclusion not obviously drawn from examination of carbon labelling patterns (Scheme 14).22 Similar experiments with the genes responsible for mithramycin and anthracycline biosynthesis showed that the 194 R.J. Cox HO2C HO2C HO2C HO2C HO2C HO2C HO CO2H HO2C Acetyl CoA Malonyl CoA FAS Octanoate 24 Oxaloacetate –CO2 –H2O Scheme 12 Biosynthesis of piliformic acid HO2C X X O OH O OH HO O O O Me O O Acetyl CoA Malonyl CoA X = CH3 Norsolorinic Acid 25 X = CH3 Hexanoate X = CH2F Fluorohexanoate X = H Pentanoate 26 PKS Aflatoxin B1 19 FAS Scheme 13 Starter unit flexibility during norsolorinic acid biosynthesis minimal PKS was responsible for chain synthesis but that downstream enzymes were involved in dictating the initial regioselective cyclisation.23 3 Terpenoids The biosynthesis synthesis structural elucidation and chemistry of numerous individual terpenoids has been extensively reviewed.24 The major development in this area has been the discovery of a new biosynthetic route to the C 5 precursor isopentenyl pyrophosphate (IPP) of all terpenoids.The route has been termed the ‘Mevalonate- Independent Pathway’ and has been found to operate in plants (specifically the 195 Biosynthesis O O O O O O O O O O O OH O HO OH O O OH OH OH OH HO OH CO2H OH O OH OH Me HO OMe O OH OH mtm KS CLF act ACP act KR full mtm PKS mithramycinone 27 1 1 20 O SEnz post PKS operations 9 14 9 14 20 OH 20 1 9 20 14 9 14 Scheme 14 Mithramycinone biosynthesis plastids) green algae and diverse bacteria from both the Gram positive (e.g. Corynebacterium and Bacillus species) and Gram negatives (e.g. Escherichia coli).The first hints of an alternative pathway came from observations of isotope incorporation into terpenoids from labelled glucose 28 which did not fit the accepted biosynthetic pathway. The conversion of glucose to pyruvate and then to acetyl-CoA via glycolysis followed by synthesis of acetoacetyl-CoA and then 3-hydroxy-3-methylglutaryl- CoA (HMG-CoA) and finally mevalonic acid has formed the cornerstone of accepted wisdom in this area (Scheme 15). Additional observations that the very potent HMG-CoA reductase inhibitor mevinolin was not able to abolish terpenoid production in plant cells were initially rationalised in terms of in vivo partitioning of biosynthetic machinery. However persistent experimentation by classical isotope feeding experiments has elucidated the major carbon–carbon bond forming steps as well as some of the functional group interconversions in the new pathway.An initial key experiment in the elucidation of the new pathway involved the feeding of isotopically labelled trioses to triose phosphate metabolism mutants of E. coli.25 A number of mutants were used each blocked in a di§erent enzyme on the pathway between glycerol and pyruvate. Incorporation of label into ubiquinone Q8 31 was observed for each mutant strain for each of two feeding experiments. In the first experiment 13C labelled pyruvate was fed with unlabelled glycerol and in the second experiment unlabelled pyruvate was fed with 13C labelled glycerol. The results of these 196 R. J. Cox O OH HO HO HO OH PO OH O PO O OH O O OH O SCoA O SCoA HO OH O OH HO OH O OP O OH OH TPP Pyruvate Glyceraldehyde 3-phosphate Dihydroxyacetone phosphate OP OH 1-13C-D-Glucose 28 OH OH Acetyl CoA TPP OP OH O O H OP O HO OH D-1-Deoxyxylulose-5-phosphate 29 SCoA O O Acetoacetyl CoA HMG CoA Mevalonate OP OH HO OH 2- C-Methyl-D-erythritol 4-phosphate 30 Isoprene Labelling Pattern • • • • • • • • • • • • • • • • • • • • • • • • • • Scheme 15 Classical mevalonate and the ‘non-mevalonate’ pathways to isoprenoids 197 Biosynthesis O O MeO MeO OR OH OH OH O OH OH NH2 OH NH2 OH OH OH HO 7 R = H Ubiquinone Q8 31 Hopanoids 32 experiments showed that pyruvate specifically labelled two positions of each isoprene unit of ubiquinone for all mutants and that these positions were unlabelled by glycerol in agreement with the proposed biosynthetic route (Scheme 15).Pyruvate labelled the C 3 subunit only in mutants blocked after glyceraldehyde-3-phosphate.Glycerol labelled the C 3 unit only in mutants blocked before glyceraldehyde 3-phosphate. These experiments showed that glyceraldehyde 3-phosphate was the source of the C 3 unit. Further evidence for the incorporation of intact C 2 and C 3 units came from the feeding of UL-13C glucose to the bacterium Zymomonas mobilis and the isolation of labelled hopanoids 32. Examination of 13C/13C1J 2J and 3J coupling constants in the isolated hopanoids clearly indicated the incorporation of intact C 2 and C 3 units. A biosynthetic rationalisation for these results was formed (Scheme 16) in which thiamine pyrophosphate (TPP) catalysed condensation of pyruvate with glyceraldehyde- 3-phosphate. This reaction has been proposed previously for the synthesis of the product 1-deoxyxylulose-5-phosphate 29.In Streptomyces species at least this compound also serves as a precursor to thiamin26,27 and pyridoxal.28,29 The proposed pathway requires that the first free C 5 intermediate 1-deoxyxylulose- 5-phosphate 29 should undergo a rearrangement and then presumably reductive modification to give 2-C-methyl-D-erythritol-4-phosphate 30. This compound has been observed as a product of Corynebacterium ammoniagenes when grown under oxidative stress. Feeding of 1-13C and 6-13C glucose to this organism followed by extraction of methylerythritol 30 as the tetraacetate and examination of the 13C NMR spectrum clearly showed incorporation of label in the expected positions.30 After supplementation with UL-13C glucose 13C/13C 1J coupling was observed between C5 and C2 as well as between C3 and C4.Additional long range coupling was observed between C4 and C1 in methylerythritol in agreement with the rearrangement hypoth- 198 R. J. Cox O O OH N S O O S N H OH S N HO OP O OH OP OH O OH H+ +H D-1-Deoxyxylulose-5-phosphate 29 CO2 • • • • Scheme 16 Proposed mechanism for the biosynthesis of C 5 isoprene precursors Sitosterol 37 O O H HO 6-7 Menaquinones 33 X X trans-Phytol 34 X = H b-Carotene 35 X = OH Lutein 36 199 Biosynthesis esis. Examination of the labelling pattern in dihydromenaquinones 33 produced by C. ammoniagenes simultaneously with methylerythritol also showed the expected incorporation pattern. Furthermore the level of incorporation of label (4–5%) was equivalent in methylerythritol 30 and dihydromenaquinone 33 indicating the likeliness of methylerythritol as a precursor for IPP.Strong evidence for the intermediacy of 2-C-methyl-D-erythritol-4-phosphate 30 came with the synthesis of 2H labelled material. Initial experiments supplementing 2H labelled racemic methylerythritol as the free tetrol to growing cultures of C. ammoniagenes were unsuccessful with no incorporation of 2H label into menaquinones 33 observed. However supplementation of this material to E. coli was more successful and 2Hlabel was observed incorporated into ubiquinone-8 at C4 of each isoprene unit. The presumed reason for positive incorporation in E. coli is the presence of an e¶cient kinase of methylerythritol the 4-phosphate being the true intermediate. This study was extended by the synthesis of both D- and L-isomers of 2-C-methylerythritol in 2H labelled form.FAB mass spectrometry was used to determine around ca. 10% incorporation for the D-isomer with no observed incorporation of the L-isomer. Large scale incubations using [1,1-2H 2 ]-2-C-methyl-D-erythritol followed by extraction of ubiquinone 31 and menaquinone 33 allowed 2HNMRinvestigations which confirmed 2H incorporation at positions derived from C4 of IPP. The intermediacy of 1-deoxyxylulose 29 in Catharanthus roseus has been proven by independent synthesis of singly and multiply 13C-labelled material.31 C. roseus is photosynthetically active and the trans-phytol 34 moiety of chlorophyll as well as b-carotene 35 lutein 36 and sitosterol 37 were examined for label incorporation. In experiments with [1-13C]-deoxyxylulose carbons arising from C5 of IPP in phytol 34 b-carotene 35 and lutein 36 were specifically labelled.Incorporation was high between 16% and 18% specific incorporation at each labelled position. In contrast incorporation of the singly labelled deoxyxylulose in sitosterol 37 was observed at specific incorporation rates of 1.5% although at the expected C5 IPP derived positions. Results from experiments with [2,3,4,5-13C 4 ]-deoxyxylulose supplemented to C. roseus mirrored the above results with overall specific incorporations of label at 7–8% into the non-steroidal terpenoids. Incorporation into sitosterol 37 was again around 1.5%. Results from the multiply labelled incorporation experiments back up conclusions from the feeding of UL-13C glucose to C. ammoniagenes which showed the rearrangement of a straight-chain C5 unit to the branched IPP skeleton.Di§erences in incorporation rates between sterols and non-steroidal metabolites was explained by the operation of dual pathways to IPP in C. roseus. From the results obtained in this feeding study it would appear that IPP derived from trioses via the non-mevalonate pathway are channelled towards carotenes and quinones while mevalonate derived IPP is channelled toward sterols. This observation is consistent with operation of the non-mevalonate pathway within plastids and operation of the classical pathway in the cytosol. The partitioning of IPP synthesis has been investigated more fully in a range of higher plant species.32 By supplementing growing cultures of Lemna gibba (duckweed) Hordeum vulgare (barley) and Daucus carota (carrot) with [1-13C]-glucose the operation of both the mevalonate and non-mevalonate pathways could be observed (Scheme 15).Four plastidic terpenoids were examined for label incorporation; phytol 34 b-carotene 35 lutein 36 and plastoquinone-9 39. Two cytosolic sterols were also 200 R. J. Cox O O O O H O OH O O H Premarrubiin 40 Marrubiin 41 O O 8 Plastoquinone-9 39 Stigmasterol 38 extracted and examined; sitosterol 37 and stigmasterol 39. In all cases sterols contained the highest specific labelling at carbons derived from C2 C4 and C5 of IPP consistent with operation of the mevalonate dependent pathway in the cytosol. All the plastidic terpenoids were labelled most highly at positions derived from C1 and C5 of IPP consistent with the non-mevalonate pathway.Close examination of the labelling patterns however indicated that there must be some level of IPP exchange between the cytosol and the plastids as C5 derived positions generally received the highest labelling (consistent with label coming from both pathways) while C1 derived positions of sterols received significant enrichment. In all compounds examined the C3 derived positions received the lowest incorporations consistent with both pathways. The biosynthesis of marrubiins 40 and 41 isolated from Marrubium vulgare (Horehound or Stinking Roger) has been studied in detail as early reports of mevalonate incorporation into marrubiin 41 are not reproducible.33 Mevalonate labelled with 14C was incorporated to an extent of only 0.005% by these compounds. Geranylgeranyl pyrophosphate labelled with 14C was incorporated at a rate of 0.15% however indicating the terpenoid origin of the marrubiins and mevalonate was shown to be incorporated into sterols.[1-13C]-Glucose was used to show the operation of the non-mevalonate pathway in the biosynthesis of the marrubiins. Carbons deriving from C1 and C5 of IPP were enriched with 13C and carbons C2 C3 and C4 were not significantly labelled indicating biosynthesis via the non-mevalonate pathway. 201 Biosynthesis P O OH O O OH OH P O OH P O OH OH NH2 O O HO OH NH2 O O OH OH NH2 O O OH NH2 HO O OH OH P O OH OH O O PEP 42 aminoDHS AHBA 44 Erythrose-4 -phosphate 43 aminoDAHP 45 aminoDHQ 46 [N] DAHP 47 Scheme 17 Parallel pathway to amino-shikimate 4 Shikimate metabolites The biosynthesis of shikimate derived compounds continues to fascinate both chemists and biochemists.34 The biosynthesis of shikimate derived precursor units for commercial microbially derived pharmaceuticals has attracted interest due its importance for strain improvement or genetic engineering.In particular the biosynthesis of the m-C 7 N units of the ansamycin antibiotics has been examined.35 A new parallel variant of the classical shikimate pathway was shown to be in operation in cell free extracts of ansamycin producing organisms (Scheme 17). In these cell free extracts it was shown that labelled phosphoenolpyruvate 42 and erythrose-4-phosphate 43 gave 3-amino-5-hydroxybenzoic acid (AHBA 44) and aminoDAHP 45. The complex precursors aminoDAHP 45 as well as aminoDHQ 46 were e¶ciently converted to AHBA 44. DAHP 47 the usual shikimate precursor was not incorporated into AHBA 44.Interestingly no obvious nitrogen donor could be identified despite a number of labelled feeding studies. The final unsolved step in the biosynthesis of the C 7 shikimate derived starter unit 48 of the polyketide ascomycin has been determined in the bacterium Streptomyces hygroscopicus (Scheme 18). The steric course of the 1,4-conjugate elimination has been demonstrated. Stereospecific synthesis of ([)-(6R)-[6-2H 1 ] and ([)-(6S)-[6-2H 1 ]- 202 R. J. Cox HO HproR HproS OH OH O OH OH OH O OH HproS O OH OH OH O OH HproS ascomycin w-cyclohexyl fatty acids 49 48 50 Scheme 18 Stereochemistry of 1,4-conjugate elimination during cyclohexane carboxylate biosynthesis O HO HproS HproS HO O OH O OH HproS HO O OH HproS w-cycloheptyl Fatty Acids 52 51 Scheme 19 Biosynthesis of cycloheptyl carboxylate shikimic acid 49 from mannose allowed the stereochemistry of this step to be determined unambiguously as anti with the loss of the 6-pro-R hydrogen.36 Similar work by the same authors showed the same process to be operative in the parallel biosynthesis of the cyclohexane-carboxylic acid 50 starter unit of x-cyclohexyl fatty acids in Streptomyces collinus and Alicyclobacillus acidocaldarius.37 In work related to the above investigations the origin of the cycloheptyl carboxylic acid 51 starter unit of the x-cycloheptyl fatty acids produced by Alicyclobacillus cycloheptanicus has been investigated (Scheme 19).Initial thoughts that this compound could be derived from a ‘homo-shikimate’ pathway were disproved by showing 203 Biosynthesis NH2 OH O D3C NH2 OH O D D O HO O O O N OH H D3C O CD3 HO O O O N OH H D D Senecio pleistocephalus Rosmarinine 53 54 • • •• • • Scheme 20 Incorporation of C 4 units into rosmarinine NH HO HO HO NH MeO HO HO NH MeO HO MeO HO N O O MeO 59 58 55 56 NH HO MeO MeO HO 57 Scheme 21 Biosynthesis of erythrina alkaloids phenylacetic acid 52 to be the precursor.38 Phenylacetic acid 52 evidently undergoes an oxidative ring expansion followed by a series of reductions to give the observed metabolite prior to incorporation into the fatty acids.5 Alkaloids The chemistry biochemistry synthesis and biosynthesis of alkaloids has been very extensively reviewed.39 Interesting work investigating the biosynthesis of the diacid 204 R. J. Cox moiety of pyrrolizidines has been carried out using labelled a-aminobutanoic acids (Scheme 20).40 The complete labelling patterns of senecic acid moieties of rosmarinine 53 have been determined.Using [3,4-13C 2 ]- and [3,4-2H 5 ]-a-aminobutanoic acids 54 in feeding experiments with Senecio pleistocephalus plants and S. vulgaris root cultures it has been shown that the senecic acids are derived from two C 4 moieties and that retention of 2H at C13 and C20 indicates that biosynthesis does not involve keto intermediates at these carbons. (S)-Norreticuline 55 has been shown to be the precursor to the erythrina alkaloid 56 by feeding labelled 55 to Erythrina crystagalli plants and plant cell cultures (Scheme 21). Previous reports that (S)-norprotosinomenine 57 could act as a precursor were disproved by synthesising and feeding a number of isomeric nortetrahydrobenzylisoquinolines.41 None of the (R) isomers were incorporated but (S)-coclaurine 58 was incorporated and is suggested to be a precursor of (S)-norreticuline 55. The precursor of (S)-coclaurine 58 is known to be norcoclaurine 59. References 1 R.A. Hill Ann. Rep. Prog. Chem. Sect. B 1993 90 311. 2 R.A. Hill Ann. Rep. Prog. Chem. Sect. B 1995 92 283. 3 L. Zeng Q. Ye N. H. Oberlies G. Shi Z.-M. Gu K. He and J. McLaughlin Nat. Prod. Rep. 1996 13 275; M.S. F. Lie Ken Jie M. K. Pasha and M. S. K. Syed-Rahmatullah Nat. Prod. Rep. 1997 14 163; B. J. Rawlings Nat. Prod. Rep. 1997 14 335; B. J. Rawlings Nat. Prod. Rep. 1997 14 523. 4 See the full issue of Chem. Rev. 1997 97 2463. 5 D. O’Hagan in The Polyketide Metabolites Ellis Horwood Chichester UK 1991.6 R. Pieper C. Kao C. Khosla G. Luo and D. E. Cane Chem. Soc. Rev. 1996 297. 7 C.M. Kao G. Luo L. Katz D. E. Cane and C. Khosla J. Am. Chem. Soc. 1996 118 9184. 8 R. Aggarwal P. Ca§rey P. F. Leadlay C. J. Smith and J. Staunton J. Chem. Soc. Chem. Commun. 1995 1519. 9 K.E.H. Wiesmann J. Cortes M.J. B. Brown A. L. Cutter J. Staunton and P. F. Leadlay Chem. Biol. 1995 2 583. 10 R. Pieper R. S. Gokhale G. Luo D. E. Cane and C. Khosla Biochemistry 1997 36 1846. 11 J. R. Jacobson C. R. Hutchinson D. E. Cane and C. Khosla Science 1997 277 367. 12 M. Oliynyk M. J. B. Brown J. Cortes J. Staunton and P. F. Leadlay Chem. Biol. 1996 3 833. 13 R. McDaniel C. M. Kao H. Fu P. Hevezi C. Gustafsson M. Betlach G. Ashley D. E. Cane and C. Khosla J. Am. Chem. Soc. 1997 119 4309. 14 P.F. Leadlay J. Staunton A. F. A. Marsden B. Wilkinson N. J. Dunster J. Cortes M. Oliynyk U. Hanefeld and M. J. B. Brown in Industrial Micro-Organisms Basic and Applied Molecular Genetics eds R. H. Batltz G. D. Hegeman and P. L. Skatrud American Society for Microbiology Washington DC 1997. 15 L. Liu A. Thamchaipenet H. Fu M. Betlach and G. Ashley J. Am. Chem. Soc. 1997 119 10 553. 16 C.M. Kao M. McPherson R. N. McDaniel H. Fu D. E. Cane and C. Khosla J. Am. Chem. Soc. 1997 119 11 339. 17 H. Kobel H. R. Loosli and R. Voges Experimentia 1983 39 873. 18 M. O§enzeller G. Santer K. Totschnig Z. Su H. Moser R. Traber and E. Schneider-Scherzer Biochemistry 1996 35 8401. 19 N. C. J. E. Chesters and D. O’Hagan J. Chem. Soc. Perkin Trans 1 1997 827. 20 D. S. J. McKeown C. McNicholas T. J. Simpson and N.J. Willett Chem. Commun. 1996 301. 21 R. E. Minto and C. A. Townsend Chem. Rev. 1997 97 2537. 22 G. Blanco H. Fu C. Mendez C. Khosla and J. A. Salas Chem. Biol. 1996 3 193. 23 J. Kantola G. Blanco A. Hautala T. Kunnari J. Hakala C. Mendez K. Ylihonko P. Mantsala and J. Salas Chem. Biol. 1997 4 751. 24 J. R. Hanson Nat. Prod. Rep. 1996 13 59; J. D. Connolly and R. A. Hill Nat. Prod. Rep. 1996 13 151; D. H. Grayson Nat. Prod. Rep. 1996 13 195; B. M. Fraga Nat. Prod. Rep. 1996 13 307; J. R. Hanson Nat. Prod. Rep. 1996 13 529; S. Fujioka and A. Sakurai Nat. Prod. Rep. 1997 14 1; P.M. Dewick Nat. Prod. Rep. 1997 14 111; B. M. Fraga Nat. Prod. Rep. 1997 14 145 A. Rahman and M. I. Choudhary Nat. Prod. Rep. 1997 14 191; J. R. Hanson Nat. Prod. Rep. 1997 14 245; J. R. Hanson Nat.Prod. Rep. 1997 14 373; D. H. Grayson Nat. Prod. Rep. 1997 14 477. 25 M. Rohmer M. Seeman S. Horbach S. Bringer-Meyer and H. Sahm J. Am. Chem. Soc. 1996 118 2564. 205 Biosynthesis 26 R. H. White Biochemistry 1978 17 3833. 27 S. David B. Estramarieix J.-C. Fischer and M. Therisod J. Am. Chem. Soc. 1981 103 7341. 28 R. E. Hill B. G. Sayer and I. D. Spenser J. Am. Chem. Soc. 1989 111 1916. 29 K. Himmeldirk I. A. Kennedy R. E. Hill B. G. Sayer and I. D. Spenser Chem. Commun. 1996 1187. 30 T. Duvold J.-M. Bravo C. Pale-Grosdemange and M. Rohmer Tetrahedron Lett. 1997 38 4769. 31 D. Arigoni S. Sanger C. Latzel W. Eisenreich A. Bacher and M.H. Zenk Proc. Natl. Acad. Sci. U.S.A. 1997 94 10 600. 32 H. K. Lichtenthaler J. Schwender A. Disch and M. Rohmer FEBS Lett. 1997 400 271. 33 W.Knoss B. Reuter and J. Zapp Biochem. J. 1997 326 449. 34 H. G. Floss Nat. Prod. Rep. 1997 14 433. 35 C.-G. Kim A. Kirshning P. Bergon P. Zhou E. B. Sauerbrei S. Ning Y. Ahn M. Breuer E. Leistner and H. G. Floss J. Am. Chem. Soc. 1996 118 7486. 36 K. A. Reynolds K. K. Wallace S. Handa M.S. Brown H. A. I. McArthur and H. G. Floss J. Antibiot. 1997 50 701. 37 S. Handa and H. G. Floss Chem. Commun. 1997 153. 38 B. S. Moore K. Walker I. Tornus S. Handa K. Poralla and H. G. Floss J. Org. Chem. 1997 62 2173. 39 R. B. Herbert Nat. Prod. Rep. 1996 13 45; K. W. Bentley Nat. Prod. Rep. 1996. 13 127; J. R. Liddell Nat. Prod. Rep. 1996 13 187; M. Ihara and K. Fukumoto Nat. Prod. Rep. 1996 13 241; J. R. Lewis Nat. Prod. Rep. 1996 13 171; J. E. Saxton Nat. Prod. Rep. 1996 13 327; J. R. Lewis Nat. Prod. Rep. 13 1996 435; J. P. Michael Nat. Prod. Rep. 1997 14 11; J. P. Michael Nat. Prod. Rep. 1997 14 21; D. C. Gournelis G. G. Laskaris and R. Verpoorte Nat. Prod. Rep. 1997 14 75; J. R. Lewis Nat. Prod. Rep. 1997 14 303; R. B. Herbert Nat. Prod. Rep. 1997 14 359; K. Bentley Nat. Prod. Rep. 1997 14 387; M. Ihara and K. Fukumoto Nat. Prod. Rep. 1997 14 413. 40 I. R. Stirling I. K. A. Freer and D. J. Robins J. Chem. Soc. Perkin Trans. 1 1997 677. 41 U. Maier and M. H. Zenk Chem. Commun. 1997 2313. 206 R. J. Cox

ISSN:0069-3030    

DOI:10.1039/oc094187

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

7 Synthesis of man-made polymers By S. RIMMER The Polymer Centre Lancaster University Bailrigg Lancaster Lancs UK LA1 4YA 1 Introduction and scope The following article reviews the literature on the synthesis of man-made polymers that has appeared during 1997. The scope of the review covers mainly synthetic procedures that either alter the molecular structure or topology of the main chain or o§er new improved procedures for known polymers and oligomers. The review will not cover the following areas of polymer synthesis synthesis of functional vinyl monomers; synthesis of polymers for solid phase synthesis; immobilized catalysts and reagents; hybrid synthetic/biopolymers; chemical modification of polymers or noncovalently linked polymers. 2 Chain growth polymerizations Radical polymerization Controlled radical polymerization Control over radical polymerization can be exerted by reversible end capping of growing chains either by stable radicals or by catalytic transfer of groups such as halide.The first of these methods with substituted tetraphenylethane as the precursor to the stable radical was used to prepare polyurethane block copolymers1 and telechelic oligomers.2 Vinyl ethers prepared by cationic polymerization with trityl end groups are claimed to be useful as precursors to block copolymers of vinyl ethers and methacrylates.3 Similar work with trityl ended polyurethanes yielded poly(urethaneblock- vinylbenzyl chloride)s.4 Further examples of styryl monomers that can be polymerized in a controlled way using the aminoxyl method have appeared. For example chlorostyrene and chloromethylstyrene have recently been polymerized.5,6 Controlled photopolymerization of styrene using diphenylselenede as initiator followed by reductive elimination of the terminal seleno groups yielded chain-end unsaturated polystyrene.7 A multi-functional iniferter capable of initiating controlled radical polymerization of styrenes anionic ring opening and anionic vinyl polymerization has been presented.8 Atom transfer radical polymerization (ATRP) continues to attract considerable attention. Barvah and Goswami have used an iron(III) complex to control the polymerization of methyl methacrylate (MMA). They also report that the polymerization is 207 SiO)m (SiO)n SiMe3 Me3SiO( O Et Et Ph Ph Et Et Ph Ph SiO)m (SiO)n–p (SiO)p Me3SiO( SiMe3 + Scheme 1 • • n AIBN Scheme 2 accelerated by the presence of Lewis acids.9 The ATRP method has now been applied to acrylonitrile polymerization.10 2-Pyridinecarbaldehydeimine11 and 4,4@-di(5- nonyl)-2,2@-bipyridine ligands12 have been introduced for ATRP of MMA while 1,10-phenanthroline–copper(I) complexes have been used for styrene (S) polymerization.13 NiII(PPh) 3 Cl NiII(PPh) 3 Br and FeIICl 2 plus organic halides were used as the initiating system for the ATRP of MMA.14,15 Of a variety of halides studied as initiators in the ATRP of MMA trichloroacetates and methyl 2-bromoacetate were found to give the highest degree of control.16 Monohydroxypolymethyl methacrylate (PMMA) has been prepared by ATRP with initiation from hydroxy functionalized alkyl bromides.17,18 Palladium–triphenylphosphine complexes were active in the controlled radical polymerization of MMA.19 Catalytic chain transfer with cobaloxime complexes has been used as a method of molecular weight control in emulsion polymerization.20 The partition of the catalyst had a bearing on chain transfer activity in that the more water-soluble catalyst appeared to be less active.Branching has now been reported as inevitable in catalytic chain-transfer polymerizations carried out to high conversion.21 Other radical polymerizations The Diels–Alder adduct formed by reaction of maleimide and furan has been shown to copolymerize by UV initiation with vinyl monomers such as acrylamide and acrylic acid (Scheme 1).22 Silicon-containing fluoroacrylate monomers have been copolymerized.23 Copolymerization of maleimide with 3,4-epoxycyclohexylmethyl alkyl ether has been carried out.24 New hydrogels can be synthesized from lactose functional styryl monomers.25 Butatrienes have been polymerized by radical means to give the polymers 208 S. Rimmer shown in Scheme 2.26 Maleimide with L-menthyl functionality was polymerized and yielded optically active polymers.27 Novel phosphatidyl choline (PC) monomers have been prepared and polymerized radically.28,29 An intermediate based on a Diels–Alder mechanism was postulated as being responsible for the regular copolymer microstructure found in copolymerizations of chloroprene and ethyl cyanoacrylate.30 Multi-functional 2-vinylcyclopropanes were prepared by esterification of carboxyl functionalised vinylcyclopropanes with various polyols.31 Ionic and co-ordination polymerizations Metathesis Ring-opening metathesis polymerization (ROMP) of ester and cyano functionalized alkenes has been carried out.32 A new carbazole functionalized norbornene has also been polymerized.33 New ruthenium catalysts with phosphine and aryl ligands were shown to tolerate a wide variety of functional groups and were useful in the polymerization of low strain olefins such as cyclooctene by ROMP.34 Living polymerization of cyclobutenes with a variety of functional groups has been achieved.35 7,8-Bis(tri- fluoromethyl)tricyclo[4.2.2.02,5]deca-3,7,9-triene was polymerized.This polyene precursor can then be converted by elimination of hexafluoroxylene.36 Triphenylenylnorbornene and triphenylenylbutadiene have been polymerized with a ruthenium catalyst to yield discotic side-chain liquid crystalline polymers.37 ROMP of 5- (trimethylgermylmethyl)norborn-2-enewas achieved using ruthenium catalysts.38 The catalyst Rh(nbd)Cl 2 has been shown to be e§ective for the polymerization of acetylene monomers containing cyano functionalized mesogens.39 Other classical metathesis catalysts such as WCl 6 were not e§ective.A new iodopentylacetylene monomer has been polymerized.40 Lightly cross-linked networks were prepared by ROMP of 5- (cyclohex-3-en-1-yl)norborn-2-ene.41 Ruthenium catalysts were found to be more e¶cient than similar rhodium species in acyclic diene metathesis of diallyldiorganosilanes. 42 Penicillin and thymine functionalized polymers have been synthesized by ROMPof functionalized monomers.43,44 Strained olefins such as norbornene have been found to be susceptible to polymerization by MgCl 2 .The structures of the resulting polymers were found to be identical to those prepared by ROMP.45 Procatalysts that are active inROMPonly after activation with alkylating agents have been reported.46 Polymer-supported tungsten catalysts have been introduced that o§er the usual environmental advantages of immobilized reagents.47 Ring-opening polymerizations A useful modification of the anionic insertion polymerization of e-caprolactone (CL) using either hydroxyethyl methacrylate or 4-(hydroxymethyl)vinylbenzene as the initiating alcohol and aluminium alkoxide as the catalyst yielded an a-hydroxy x-vinyl macromonomer (see Scheme 3).48 The preparation worked due to the low tendency of aluminium alkoxides to promote transesterification reactions.Poly(b- butyrolactone-block-e-caprolactone[P-(BL-b-CL)] was prepared by anionic polymerization of b-butyrolactone (BL) followed by coordinative insertion polymerization of CL.49 Trimethylene carbonate was copolymerized with CL using a variety of rare earth compounds.50 It was found that if the chlorides were used as catalysts it was 209 Synthesis of man-made polymers O O O O O OH O 5 5 n HO AI(OR)3 Scheme 3 necessary to add an epoxide as an accelerator. CL has also been copolymerized with adipic anhydride.51 Nitrophenylethyl alcohol was used as coinitiator in the polymerization of trimethylene carbonate giving nitro chain-end functional polymers that after palladium catalysed reduction gave amine chain-ended polymers.52 Schi§’s base with aluminium methoxide has been used as an initiator for lactide polymerization.53 Other ring-opening polymerizations Polymer supported ion exchangers have found a use as acid scavengers in a copolymerization of octamethylcyclotetrasiloxane and dimethyldiphenylsilane.54 Polymerization of spirocyclic orthocarbonates is useful for low shrinkage applications and recently sulfur analogues have been polymerized.55 Similar low shrinkage but higher T' polymers are prepared from rigid cyclic ketene acetals.56 Chiral polyamides were prepared from ring-opening polymerization of b-lactam-11 derived from Dglutaraldehyde.57 Anhydro-sugar monomers were polymerized using Lewis acid catalysis. 58–60 Anionic polymerization Polymers with alicyclic structures in the main chain have been produced from copolymerization of cyclohexa-1,3-diene.61 Polyphosphazines with trifluoroethoxy and alkoxy groups were prepared by anionic polymerization of phosphoramines.62 A novel lanthanoid alkoxide initiator was found to be activated by reaction with a ketene.63 Cationic polymerization The cationic copolymerization of endo-dicyclopentadiene and penta-1,3-diene gave polymers that arose because of propagation by 1,3-addition rather than the usual 1,2-addition.This feature was a result of cationic rearrangement as shown in Scheme 4.64 Living cationic polymerization of a-pinene with the HCl adduct of chloroethyl vinyl ether and TiCl 3 initiator has been reported.65 Several liquid crystalline monomers have been polymerized cationically.66,67 Multifunctional epoxy monomers based on a silsesquioxane core have been polymerized using photoinitiation.68 D-Glucose functional polyvinyl ethers were synthesized by living cationic polymerization.69 Living cationic polymerization of isobutyl vinyl ether has been carried out using water and amine tolerant Lewis acid Yb(OTf) 3 .70 Coordination polymerization Monovalent rhenium complexes were used to polymerize styrene.However the resultant polymers were not stereoregular and displayed bimodel molecular weight 210 S. Rimmer R + R H H + R + Scheme 4 distributions.71 A range of catalysts based on molybdenum and tungsten were used to polymerize propargyltriphenylphosphonium tetraphenylborate.72 Polyolefins Reduction of polydienes A method of producing polyolefins that often receives little attention is the reduction of polydienes. The advantage of this route is the wide range of methods available for diene polymerization in comparison to the polymerization of monoalkenes.Ruthenium catalysts are useful in the reduction step.73–75 Similarly styrene–butadiene rubber lattices have been reduced using hydrazine as the source of hydrogen with copper(II) as the catalyst.68 A related synthesis is the hydroformylation of styrene–butadiene copolymers.76 Metallocenes The use of metallocene catalysts in the polymerization of olefins is currently attracting a great deal of interest in both industry and academia. Much of the work concentrates on preparing and applying new catalysts which hopefully will give either improvements in process or can deliver new materials with improved properties. Malmberg and Lofgren77 reported the use of Et(Ind) 2 ZrCl 2 –MAO and Et(Ind) 2 HfCl 2 –MAO in the copolymerization of ethene–propene and the terpolymerization of ethene–propene–5-ethylidenenorborn-2-ene mixtures.Unusual di§erences between hafnium and zirconium metallocene catalysis have been observed.78 Copolymers of styrene and 4-methylstyrene were prepared.79,80 Chloro functionalized polyolefins can be synthesized by metallocene catalyzed polymerization of a-chloro x-alkenes.81 Step growth polymerizations Polyarylenes Novel high performance polymers with phthalazinone repeat units which give lower melt viscosities have been prepared (Scheme 5).82 Polyethylene glycol modified polysulfones were made by carrying out a polyamide condensation using bis(aminophenyl) sulfone and carboxylic acid-ended polyethylene glycol. The resultant amine-ended polymers were then used to modify epoxy resin systems.83 Nanofoams can be prepared by thermolysis of polypropylene oxide blocks contained within poly(imide-block-propylene oxide) copolymers.The polyimide 211 Synthesis of man-made polymers HO N N O H O N N O F O F + + S O O Cl Cl K2CO3 140–165 °C O n O N N O n S O O co Scheme 5 blocks contained ethynyl blocks which provide cross-linking at the thermolysis temperature. 84 Fluorinated aromatic polymers can be synthesized by condensation of various fluorine-containing monomers.85–88 Poly(aryl ether ketone)s synthesized by the nucleophilic substitution route with excess diol gave phenol-ended oligomers that after reaction with chloromethylstyrene yielded styryl-ended polyarylates.89 Aromatic nucleophilic substitution on 2,5-bis(4-fluorophenyl)-1,3,4-oxadiazole gave rigid polymers some of which were crystalline.90,91 Liquid crystal poly(aryl ether ketones) functionalized with chloro groups have been synthesized.92 Cyano functionalized poly(aryl ethers) can be prepared from cyano functionalized diphenyl ethers.High T' and heat resistant polymers based on aryl phosphine oxides were synthesized using the nickelcatalyzed coupling reaction shown in Scheme 6.93 Polyxanthanes have been prepared from benzoyl functionalized polyaryl ethers.94,95 The polymerization was also carried out with 1,4-iodobenzene but the resultant material had much lower fluorescence emission intensity. Other new alkyl functional polythiophenes have also been reported.96,97 Dicoumarin monomers that can be polymerized by light-induced poly 2p]2p cycloaddition have been prepared.98 A polymer capable of chelating metal ions was prepared by Schi§’s base condensation.99 Poly(thioarylene)s have been prepared by the radical-mediated condensation of aryl thiols and aryl bromides.100 Bisphenol A based poly(S-thioesters) have been prepared using the reaction of bisthiiranes and diacyl chlorides.101 Polyarylates have been synthesized from the condensation of bis(aminonitrile) benzenes with difluoro aromatics102 and with piperazines.103 New polyamides containing the fluoromethyl group have been prepared.104 Firstly a fluoromethylaryldiamine was synthesized as shown in Scheme 7. This monomer was then condensed with pyromellitic anhydride to give the polyimide via the polyamic acid. Cross-linkable phosphorous-containing polyimides were synthesized.105 A new 212 S.Rimmer Cl P O Cl P O n Ni Scheme 6 CF3 CF3 HO OH 2 Cl NO2 CF3 + K2CO3/DMAc/Cu tetrabutylammonium chloride O NO2 CF3 CF3 CF3 O CF3 O2N H2NNH2H2O Pd/C O NH2 CF3 CF3 CF3 O CF3 H2N Scheme 7 route to polyimides has been reported.106 The approach involves the trapping of photochemically generated dienes with bismaleimides. New polyimides have been prepared by condensation of various novel monomers.107–126 Soluble polimides have been prepared by incorporating cyclohexane derivatives into the main chain127 and by using a dinaphthyl dianhydride monomer.128 Polyimides with in-chain pyridinium salts were synthesized from pyridinium-containing aryl amines.129 A route to poly(amide-imide)s uses a new amide-containing diamine in the polyamic acid synthesis.130 Many high performance polymers contain main-chain aromatic units and are di¶cult to process. One method of improving the processability of these materials is to use polymers that are processable precursors to the desired final material. Thus the Diels–Alder adducts of anthracene-containing polymers are processable but on heating the retro Diels–Alder reaction yields an intractable material.131,132 In a similar approach the presence of bicyclo[2.2.2]octyl in chain units gave processable polybisbenzoxazole precursors which could be later aromatized.133 The same concept was 213 Synthesis of man-made polymers used to prepare processable precursors to helicene polyimides. Treatment with bromine aromatized the alicyclic units within the precursor.134 Fluorine-containing arylpolyesters were synthesized by interfacial polymerization.135 A new polyesteramide which contains norbornadiene units has been synthesized by reaction of the acid-ended norbornadiene compound 1 with diepoxides. 136 Carbonyl-terminated nylon 12 was chain extended with a dioxazoline.137 Hexamethylene diamine units were evenly spaced along the main-chain of aryl polyamides. The authors used individually prepared building blocks.138 Arylpolyamides have been prepared from the condensation of acid chloride functionalized arylsilanes with diamines.139 Other polyamides were synthesized by interfacial polymerization of aromatic diamines and diacyl functionalized triazines140 and by using the new monomer 4,4@-(naphthalene-2,7-dioxy)dibenzoic acid.141 The diamine monomer 2 was used to synthesise fully aromatic cyano and amino functionalized polyamides.In this work the pyrazole amino group did not participate in the condensation with terephthaloyl chloride.142 ortho-Linked aromatic polyamides have been prepared.143 Aliphatic aromatic polyamides were prepared by both interfacial and solution polymerizations. 144 OH O N O H R1 N H O OH O NH2 N N NH2 NC H2N 2 1 Synthesis of polyarylenes can be e§ected by the use of nickel(0) on arylene mesylates145 and aryl halides.146 Another recently introduced method involves the use of FeCl 3 as both oxidant and Lewis acid.147 Pyridine functionalized poly(enaryloxynitrile)s have been prepared from the polynucleophilic substitution of pyridine-containing bisphenols.148 A polysulfonate polymer has been prepared by interfacial polymerization but no molecular weight data were recorded.149 Cyclopentadiene–iron complexes are known to form aryl complexes in which the aryl ligand becomes more susceptible to nucleophilic attack.This has now been applied to the synthesis of polyaryl ethers.150 Reactive polyethers have been synthesized by the reaction of diepoxides with bis(haloacetoxy) esters.151 New reactive polymers have also been prepared by the polyhaloboration of bisallenes.152 Other step growth polymers Biomimetic polymers are attracting continued attention of these phosphatidyl choline (PC)-containing polymers show useful antifouling properties. With this in mind new polyurethanes functionalized with alkyl groups and PC moieties have been prepared. 153 Cyclotriphosphazene polyols have been incorporated into polyurethane syntheses.154 214 S.Rimmer O N O O O NR R1 NH2 H2N + n CO CONH R NHCO CONH R1 NH n Scheme 8 N N O O O Si O O O n CsF Scheme 9 Other groups that have been incorporated into polyurethanes include sulfate (within the soft-segment)155 and side-chain liquid crystals.156,157 A synthesis of a phosphorous-containing polymer involves the ring opening of epoxides with aryl phosphorodichloridates.158 Photosensitive polyesters incorporating triazine units which were derived from triazine-containing diols have been prepared.159 New bisphenols containing phenolphthalein functionality have been polyesterified using interfacial polymerization.160 Diester siloxane telechelic oligomers were incorporated in a melt transesterification to yield poly(ester-block-siloxane) materials.161 Polyesters with sulfo-para-phenylene nitroterephthalate show liquid crystal properties.162 Solid phase techniques have been used to prepare liquid peptides.163 Further results on the use of phase transfer catalysts in poly(amide carbonate) and poly(amide thiocarbonate) syntheses involved the successful use of benzyltriethylammonium chloride.164 Microwave radiation has also been shown to be beneficial in phase-transfer catalysed condensations.165 The addition of dithiols to bisalkynes is catalysed by phosphines and yields polymers which contain dithioacetal linkages.166 However similar reactions with electron-deficient alkynes give polymers containing alkoxyenoate moieties in the main chain.167 The method of ring-opening polyaddition has been applied to isomaleimides (see Scheme 8).168,169 Halogen functionalized isophthalamides have been condensed with aromatic diamines.170 The natural diamine L-lysine has been used to prepare new polyamides.171 Spirochroman dicarboxylic acids have been condensed with various diamines.172 An important step forward has been the introduction of the Tishchenko reaction in polymer synthesis of polyesters.173,174 The polyaddition of bisoxazolines to dithiols has been shown to be possible in water.175 Aliphatic polycarbonates have been synthesized at moderate temperatures by condensation of an imidazoloylsiloxane monomer (Scheme 9).176 Polynucleophilic substitution using sodium sulfides on alkyl halide substrates yields liquid polysulfide polymers.177,178 Hydrosilylation of vinylchlorosilane with a fluorinated dihydrosilane yielded material that after hydrolysis polycondensed to a fluoropolysiloxane.179 215 Synthesis of man-made polymers H2N (CH2)3 Si O Si (CH2)3 NH2 6 + ClC O O O CCl O n O NHC O O C O n HN (CH2)3 Si O Si (CH2)3 6 p TEA Scheme 10 3 Designed molecular architectures Block copolymers New poly(carbonate-block-siloxane) polymers have been prepared by a coupling of oligomers method (see Scheme 10).180 The macromonomer technique has been extended to the preparation of poly(siloxane-block-N-vinylpyrrolidinone).181 The two macroinitiators (3 and 4) shown below were found to act synergistically.182,183 Polyethylene glycols (PEG) with azo main-chain functionality have been used to polymerize dicyclohexyl itaconate.184 Azo functionalized macroinitiators were used to synthesize Bisphenol Z block copolymers with either styrene or methyl methacrylate as the second monomer.185 Polyethylene oxide (PEO) block copolymers have been prepared via initiation of the PEO block from alkoxy-ended polyisoprene or poly(ethylene-stat-propylene).186 The synthesis is shown in Scheme 11.O O N R Ph N O ( ) 3 O O O O ( ) 2 S O 3 4 Poly(styrene-block-semifluorinated side-chain) block copolymers have been prepared by fluorination of poly(styrene-block-isoprene)s prepared by anionic polymerization. 187 Poly(vinyl methyl ether-block-styrene)s have been prepared by coupling between living polystyrene anions and chlorine-ended poly(methyl vinyl ether).188 Coupling of oligomers of 2,6-dimethyl-1,4-phenylene oxide and isoprene oligomers with dicyclohexyl carbodiimide-mediated amidation has been used to prepare new 216 S.Rimmer OH 1) ButLi 2) CH2CH2O n OH n reduction O n O H Cumyl Potassium /CH2CH2O Scheme 11 athermoplastic elastomers.189 Poly(caprolactam-block-ether sulfone-block-caprolactam) was prepared by initiation of a CL polymerization by chlorine-terminated polyethersulfone.190 The concept of changing the mode of polymerization to yield block copolymers has been further extended.191 Alkylene polysiloxanes were synthesized by the hydrosilylation of a,x-dienes followed by condensation of the resultant chlorosilane-ended oligomers.192 A new route to polyphenylene sulfide-block-bismaleimides involved the addition of mercapto functionalized oligophenylene sulfides to bismaleimides.193 Poly(imide-block-ethylene oxide)s were prepared by the catalytic condensation of polyamic acids with PEGs.194 Nylon 6 block copolymers have been prepared by the ring-opening polymerization initiated by amino functionalized polybutadiene poly(propylene oxide) or PEO.195 Poly(MMA-block-perfluroalkyl methacrylate) s formed by living screened anionic polymerization have found use as stabilizers in dispersion polymerization in supercritical CO 2 .196 Many AB ABA and ABC block copolymers have been prepared by living polymerization techniques.197–212 Also living anionic techniques have been used to synthesize an ABCBA pentablock213 –215 and A(BA) 2 A(BA) 3 (AB) 3 A(BA) 3 non-linear block copolymers.216 Lithium chloride has been shown to a§ect the molecular weight distribution in the living anionic block copolymerization and homopolymerization.217,218 Living polystyryl anions have been coupled to a,x-dichloropoly(phenylmethylsilane).219Group transfer polymerization has been used to prepare AB block copolymers from tertiary amine methacrylates.220 Similar triblock versions of these polymers have been betainized by reaction of 2-(dimethylamino)ethyl methacrylate residues.221 Bromo-terminated polymers from ROMP222 or cationic polymerization223 were used as initiators in ATRP.Poly(tetrahydrofuran) was copolymerized with deca-1,9-diene by metathesis methodology. 224 Graft copolymers Grafting of block or random copolymers in which one repeat unit is poly(4-methylstyrene) is achieved by metallation of the latter block with a superbase followed by 217 Synthesis of man-made polymers anionic polymerization from the block copolymer backbone.225,226 Similar ionic techniques were used for the grafting of copolymers of isobutylene and chloromethylstyrene with oxazolines227 and Kevlar with CL.228 Hydroxy functionalized oligostyrene was converted to a macromonomer suitable for synthesis of graft copolymers by ROMP via reaction with bicyclo[2.2.1]hept-5-ene-2,3-trans-dicarbonyl chloride.229 Polystyrene was grafted onto poly[bromomethylphenyl)methylsilane] by using the bromomethyl groups as initiators in an ATRP procedure.230 Telechelic oligomers Lubczak has shown that tetrafunctional polyetherols can be prepared without solvents by using the amino groups of the melamine derivatives 3 which is soluble in the monomers to initiate ring-opening polymerization of oxiranes.231 N N N NH2 HN NH O O 5 Dichlorofunctionalized oligoisoprene and oligostyrene were produced by reaction of the living anions with dichlorides.the synthesis was less successful when dibromide or diiodides were used.232 The use of an acetal functionalized anionic initiator in the anionic polymerization of ethylene glycol followed by quenching with methacrylic anhydride has yielded PEG with methacrylate and after deprotection aldehyde end-groups.233 Telechelic polyisobutylene was prepared by termination of a living polymerization with isobutenyltrimethylsilane in the presence of TiCl 4 .234 Tosylated polybutadienes have been used to generate amine- and phosphate-ended oligomers.235 A cleaner route to sulfonate ester-ended oligomers uses a supported base.236 Oligopolystyrenes have also been tosylated and these tosylated oligomers were quaternized with 1-methylpyrolidine.237 Oligoimides with benzhydrol endgroups were acrylated to yield photopolymerizable materials.238 In the ring-opening polymerization of ethylene oxide it was found that it was possible to use aminoalcoholates with negligible initiation from the free amine group.Thus it was possible to prepare amino-ended oligomers.239 Nucleophilic substitution on PEGs a§orded azide functionalized telechelics that could undergo 1,3-dipolar cycloaddition.240 ROMP procedures have been used to prepare polybutadiene with ester end groups.241 Macromonomers Oligolactone macromonomers with methacrylate end-groups have been prepared by initiation of insertion polymerizations by bisphenol-A-bis(2-hydroxypropyl methacrylate). 242 a-x Ended macromonomers with a-methacrylol and x-formyl end-groups have been prepared by using 3,3-diethoxypropan-1-ol as a protected initiator followed by reaction of the hydroxy end of the resultant oligomer with methacrylic anhydride.243 New block oligomers have been prepared by chain extension from aldehyde functional oligomers.244 a-Hydroxy a-chloroformyl and a-thiol oligobutylene terephthalate have been synthesized by using benzoyl chloride or 4-hydroxybutyl benzoate as chain limiters.245 Perfluoropolyether macromonomers have been prepared from 218 S. Rimmer SiO)m (SiO)n SiMe3 Me3SiO( O Et Et Ph Ph Et Et Ph Ph SiO)m (SiO)n–p (SiO)p Me3SiO( SiMe3 + Scheme 12 hydroxy-ended perfluoro polyethers.246 Tetrahydrofuran was polymerized with functionalized initiators containing acrylate methacrylate and allyl functionalized triflate esters giving poly(tetrahydrofuran) macromonomers.247 New macromonomers with norbornene end-groups can be polymerized using living ROMP techniques.Sequential polymerization of di§ering macromonomers provides routes to block-graft copolymers.248 Networks The polysiloxanes known as C-gums have been modified by carrying out the Diels–Alder reaction shown in Scheme 12.249 Toughened thermoset polymers have been reported. The precursors of these materials were composed of aryl dimaleimides and dimaleimides with butadiene and acrylonitrile repeat units. Thermal polymerization gave the networks.250 b-Ketoesterended polypropylene glycols have been cross-linked in the presence of base by Michael addition to multifunctional acrylates.251 These authors also found it beneficial to add a final UV curing stage.PEO with in-chain acetylene dicarboxylate units has been cross-linked with an oligobutyl methacrylate with furan end-groups.252 PEO has also been cross-linked by simple irradiation of a sample with UV light in the presence of benzophenone.253 New biodegradable networks have been prepared from the radical polymerization of triacrylated lactic acid oligomers with monoacrylated polyethylene glycol.254 Cross-linking of siloxanes was achieved by hydrosilylation of allyl end groups.255 Carborane polymers that also contain diacetylenic groups in the main chain were prepared by polynucleophilic substitution of the butadiyne dilithium nucleophile on bis(chloro dimethyl silyl)benzene or bis(tetramethylchlorodisiloxane)- m-carborane.256 Other carborane polymers were formed from the reaction of aminocarboranes on dihaloalkanes.257 The diacetylenic groups could be polymerized to form cross-links.258 The cyanate ester synthesis from trimerization of diisocyanates was used to prepare rigid aryl networks.259 A polymer containing phosphoramide main-chain units is shown in Scheme 13.This material can also be photo-crosslinked by a 2p]2p cycloaddition.260 A new approach to the cross-linking of polyaniline uses the acid catalysed formaldehyde –aniline reaction.261 Photocross-linking is often achieved by cyclobutane formation of cinnamates. This method has been extended to the cross-linking of poly(vinyl amine)s.262 Maleimido resins have been cross-linked by adduction of allyl amine and further reactions.263 New polyurethane networks have been prepared from both dihydroxy telechelic poly(2-chloroethyl vinyl ether)s and poly(2-chloroethyl vinyl ether)s with hydroxy groups situated along the main-chain.264 219 Synthesis of man-made polymers CH HO CH O OH OMe MeO + NH P Cl O Cl CH O MeO O CH O OMe P O NH n Scheme 13 O O O N O O O O O O O N O O [Rh(bhd)Cl]2 Scheme 14 Cyclics and catanenes Cyclic oligomers derived from naphthalene dicarboxylic acid were prepared under high dilution conditions.265 Cyclic arylene ether ether sulfide and ether ether ketone oligomers have been similarly prepared by nucleophilic substitution of fluoro bisaryls.266,267 Macrocyclic arylates have been prepared using nucleophilic substitution with bisphenols.268,269 Surprisingly high concentrations of reagents can be used in the cyclization reaction of polyethylene glycol alkoxide with 1,4-dibromobutane.270 Cyclic aryl ether ketone polymers with main-chain phenylacetylene groups can be synthesized and can be cross-linked at high temperatures ([340 °C).271 The method of cyclopolymerization at high dilution was used to prepare the crown ether functional polyphenylacetylenes shown in Scheme 14.272 During the synthesis of hyperbranched polymers from the monomers dimethyl-5- 220 S.Rimmer (hydroxyalkoxy) isophthalates substantial amounts of cyclic structures were found.273 A recent advance in the area uses bulky dimethyl tert-butyl siloxane groups to ensure that the two monomer moieties (in this case alkyne groups) remain in the cisoid conformations favourable to cyclopolymerization.274 The Knoevenagel condensation has been used to prepare cyclic tetramer carbazoles.275 The synthesis of a poly(aryl thioether ketone) generated significant quantities of macrocycle even at moderate dilution.While at high dilution the cyclo dimer became the major product.276 High yields of cyclic polyesters and poly(aryl ether)s were obtained by ring–chain equilibration at high dilution.277,278 A cyclic aromatic ether sulfone was prepared by condensing a bisphenol-ended trimer with carboxylic acid functionality.279 Rotaxanes New radical initiators containing bulky groups such as 6 have been used to synthesize polystyrene with dumbell-like architecture.280 Rotaxanes have been formed from polymers with hydroquinol ether units linked by oligoethylene oxide units after threading of the tetracationic cyclophane cyclo[bis(paraquat-p-phenylene)].281 6 R Stars Eight-armed stars of polyisobutylene (PIB) were prepared from octafunctional calixarene–BCl 3 and calixarene–TiCl 4 initiators282 or by hydrosilation of allylicended PIB to octa(dimethylsiloxy)octasilsesquioxane.283 Work in the same laboratories has led to the synthesis of stars with many arms and cores of condensed methylcyclosiloxanes (see Scheme 15).284 Other workers used the sequential addition of divinyl benzene to produce multi-arm stars of PIB.285 A novel method of making stars involves the self assembly of oligomers with pyrrolidinium salts at one chain-end with carboxylic acids followed by ring opening of the pyrrolidinium ring.286 Manyarmed star-like polymers were synthesized by cross-linking the inner 2-vinylpyridine core of poly(styrene-block-2-vinylpyridine-block-styrene) micelles with 1,4- diiodobutane.287 Amphiphilic star-graft polymers were prepared by grafting polyethylene glycols to polymers containing pendant triol units.288Poly(methyl methacrylate) stars were prepared by adding living PMMA polymers to difunctional crosslinkers.289 Heteroarm stars have been prepared in a similar way.290 221 Synthesis of man-made polymers R O Si O Si O Si O OH H PIB R O Si O Si O Si O OH H HO R O Si O Si O Si O OH H H R O Si O Si O Si O O O O Si Coupling between H-Si and HO-Si O Si O Si O R O O O O R O Si O Si O Si O PIB R O Si O Si O Si O O O R O Si O Si O Si O OH O O R O Si O Si O Si O O O R O Si O Si O Si O O Si O R O Si O Si O PIB O Scheme 15 Hyperbranched polymers and dendrimers Hyperbranched polycarbonates were recently reported.291 Poly(ester-amide) hyperbranched polymers from the condensation of 3,5-dihydroxybenzoic acid and 3,5- diaminobenzoic acid derivatives.292 New dendrimers with alkyl chain surfaces have been prepared.293 Water soluble dendrimers have been prepared that are made hydrophilic by adding a final layer of carboxylic acid moieties.294 Polyaryl ether dendrimers were prepared using an activation–condensation approach with caesium phenolate as the nucleophile of choice.295,296 Polyaryl ether dendrimers with ferrocene functions at the peripheries were synthesized using the convergent approach.297 Melt polymerization ofN-acryloyl-1,2-diaminoethane hydrochloride gave hyperbranched polymers.298 ATRP of acrylates can be used to prepare hyperbranched polymers.299–301 Amphiphilic dendrimers were synthesized by divergent growth of x- 222 S.Rimmer NO2 NH2 N2 + NaNO2 Na2S2O4/ OH– H+ Scheme 16 O R P N O R O R P N O R O O O n n O R P N O R O O O n NH N H ( ) 6 NH NH H2N ( ) 6 O O O R¢• Scheme 17 ethylene diamine-terminated poly(2-methyl-2-oxazoline).302 Group-transfer polymerization with 2-(2–methyl-1-triethylsiloxyprop-1-enyloxy)ethyl methacrylate a selfcondensing monomer gave hyperbranched polymers.303 Carbosilane dendrimers with fluoroalkyl surfaces were prepared by radical addition of fluoroalkyl mercaptans.304 The divergent synthesis of a dendrimer containing tris(a,a-bispentachlorophenyl- 2,4,5,6-tetrachlorotolyl)methane was not possible due to steric crowding. So it was necessary instead to chlorinate the parent hydrocarbon.305 Palladium-catalysed amination has been used to prepare dendrimers.306 New routes to functionalized polymers (excludes main-chain modification and copolymerization) Functionalized polymer lattices are an important class of material for the paints adhesives and pharmaceutical sectors.Since the latex particles sit in an aqueous medium their preparation requires water-tolerant methods. With this in mind some progress on the synthesis of isocyanate functionalized lattices has been reported.307 Lattices with carboxylic and aldehyde functionality have been prepared.308,309 Another route to lattices of functionalized oligomers is telomerization.310 Non-aqueous emulsion particles with oxazolone functionality have been prepared from oxazoline functionalized methacrylate.311 Lattices with monosaccharide functionality have been prepared by polymerization of sugar-containing vinyl ketones.312 A new method for preparing diazonium functional polymers has been reported.313 Polyvinyl alcohol was first cross-linked with glutaraldehyde in the presence of 4- nitrobenzaldehyde the resultant gel was then ground.This nitrophenyl functional polymer was then reduced and diazotized as shown in Scheme 16. Polyphosphazines are susceptable to UV degradation but they can be protected by grafting with hindered amines as shown in Scheme 17.314 223 Synthesis of man-made polymers Co PPh3 PPh3 Co PPh3 + R1 C C R2 C C R1 R2 R1 R1 Scheme 18 Fluorinated polymers with liquid crystal properties may be synthesized by quaternization of polyamines with semifluorinated 1-bromoalkanes.315 Liquid side-chain polymers with polynorbornene and polybutadiene main-chains were prepared with ruthenium catalysts.316 Functionalized polycaprolactone can be prepared from functional e-caprolactone monomers.317 Light-stable poly(ethylene-propylene)s have been prepared by adding a hindered amine to the chain end.This the authors achieved by terminating a living polymerization of isoprene followed by reduction of the polyisoprene. 318 Organometallic polymers are of growing interest so that Endo and co-workers319 have reported the synthesis of a cobalt cyclopentadiene polymer. The synthesis is shown in Scheme 18. The complexes can be converted to polymers with main-chain pyridine units by reaction with nitriles.320 Europium functional polymers with interesting luminescence properties have been prepared by polymerizing the europium complex 7.321 O O Eu O O O O N N 7 ROMP of vinylene ferrocene has been used to prepare ferrocene functionalized polymers.322 Polymers containing cyclodextran side-units have been prepared.323 New reactive polymers containing silyl enol ethers have been synthesized.324 Degradable polymers Environmental issues and medical applications are currently driving workers to prepare degradable polymers with more controllable degradation profiles and which produce less toxic products.Shuai and Tan have produced new types of polyether-copolyesters using mixed anhydride chemistry.325 The new monomer (3S)-[(benzyloxycarbonyl) methyl]morpholine-2,5-dione has been polymerized. Removal of the carboxy side group then led to poly(glycolic-s-L-aspartic acid).326 A new method for the synthesis of poly(L-aspartic acid) via poly(succinimide)327,328 has appeared.The introduction of a-amino sequences continues to be a popular route for imparting biodegradability.329,330 Cross-linked polycaprolactone can be prepared by using polyfunctional lactones as comonomers.331 New silica–poly(e-caprolactone) hybrid 224 S. Rimmer S S Cl Cl Cl Cl n ButSH CF3COOH Scheme 19 S I I+ S n PdCl2(PR3) Scheme 20 biodegradable networks have been prepared by inclusion of a star shaped poly(e- caprolactone) with triethoxysilane functionality in a sol–gel silica synthesis.332 New biodegradable polymers from renewable resources were prepared from furan derivatives. 333 New drug delivery systems have been devised that consist of polyethylene glycols connected via biodegradable peptides.334 Biodegradable polyurethanes have been prepared by polycondensation of ditoluene-p-sulfonic acid salts of bis(phenylalanine) alkylene diesters with di-p-nitrophenyl trimethylenedicarbonate.335 Polymers for electrical applications Poly(isothianaphthene) is a new low band gap polymer that can be prepared from the reaction of 1,1,3,3-tetrachlorothiophthalan with tert-butylmercaptan (Scheme 19). Poly[bicarbazolylene-alt-phenylene–bis(cyanovinylene)] is a new type of conjugated polymer.336 Scheme 20 shows the synthesis of a new polythiophene.337 Various substituted polythiophenes have been prepared.338–342 Polypyrrole has been synthesized from pyrrole-2-carboxylic acid in both supercritical CO 2 and fluoroform.343 Aromatic azomethine conjugated polymers can be prepared by condensation of aryl diamines with aryl dialdehydes.344 Phthalocyanine functionalized aryl–silicon polymers have been synthesized.345 Heck chemistry has been applied to the synthesis of conjugated poly(phenylenevinylene) copolymers with regular non-conjugated mainchain sequences.346 Chiral poly(p-phenylene)-containing cyclophane derivatives have been prepared.347 The precursor route to poly(aryl vinylene) has been applied to poly(pyridine vinylene).348 Miscellaneous New aliphatic poly sulfoxides were prepared by selective oxidation with H 2 O 2 of sulfides.349 Fullerene functionalization has been achieved by the 1,3-dipolar addition of an azide functional polymer;350 reaction of living polyacrylonitrile;351 reaction of living poly(a-methylstyrene);352 addition of polybromostyrene;353 addition of chloromethyl styrene via the benzyl anion;354 Bingel reaction with bismalonates;355 addition of living vinyl ether oligomers356 and addition of the TEMPO ends of living polystyryl radical.357 Similar carborane-containing polyether ketones were prepared by condensation of bis(4-phenoxyphenyl)-1,12-dicarbadodecaborane.358 Functionalization of a 225 Synthesis of man-made polymers OH HO O O NH O O Ar NH H2N Ar NH2 + n P(OPh)3 NMP–pyr 110 °C [Rh(bhd)Cl]2 NH O O Ar NH Scheme 21 cyclophane was carried out by reacting a poly(tert-butyl methacrylate) with carboxylic end-groups to a cyclophane.The poly(tert-butyl methacrylate) was then hydrolysed to the acrylic acid.359 Cubane polyamides have been prepared and used as precursors to cyclooctatetraene polymers (Scheme 21).360 Conjugated polyradicals for use as polymer magnets have been synthetic targets for many years.Progress towards these species involved the synthesis via irradiation of precursors.361 Coumarin and its derivatives dimerize on irradiation at [300nm but resplit into the starting coumarins at wavelength\300 nm. 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Winningham and D. Y. Sogah Macromolecules 1997 30 862. 233 Synthesis of man-made polymers mmmm

ISSN:0069-3030    

DOI:10.1039/oc094207

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

8 Synthesis highlights a review of the literature for 1997 By PETER QUAYLE Department of Chemistry University of Manchester Manchester UK M13 9PL 1 Introduction This year almost more than any other in the recent past has demonstrated the seemingly limitless ability (of certain research groups at least) to approach and complete the synthesis of complex structures. The synthesis of natural products once an esoteric pastime whose main purpose was to act as a vehicle for the exemplification of synthetic methodology has taken a di§erent direction this year due mainly to the isolation in 1996 of the antitumour agents epothilone A 1 and B 2. these macrocyclic lactones have elicited much attention from molecular biologists and synthetic chemists alike as they are able to stabilise tubulin formation in a manner similar to TaxolTM and have similar antitumour activities.Importantly however the structurally less complex epothilones are potentially available by total synthesis unlike the more structurally elaborate and synthetically more challenging taxane system. A number of reports this year have been concerned with the total synthesis of these natural products and combined with the renewed interest in the use of solid supports in organic synthesis combinatorial techniques have been adopted in the synthesis of ‘libraries’ of analogues which have generated compounds which are more potent than the natural products themselves.1b,c 2 Total synthesis of epothilones A and B Preamble Any approach to the synthesis1a of epothilone A 1 has to address a number of key issues including the introduction of seven chiral centres a cis-epoxide moiety two hydroxy groups two secondary methyl groups one gem-dimethyl group a trisubstituted double bond and of course the construction of the 16-membered ring itself.An obvious disconnection for the lactone ring would be to synthesise the corresponding hydroxy acid 3 which hopefully could be cyclised using standard methodology (Scheme 1). Common to all the approaches disclosed to date is the introduction of the epoxide moiety at a late stage in the synthesis the realisation of such a strategy relies upon the chemoselective epoxidation of the more exposed face of the less sterically congested double C 12 –C 13 bond rather than epoxidation at C 16 –C 17 or indeed oxidation of the thiazole ring. A more subtle question is concerned with the 235 O OH R O HO O O N S 12 R 11 10 9 8 S 7 S 6 R 5 4 3 S 2 1 15 S 14 13 S 16 17 18 19 20 21 23 22 1 R = H epothilone A 2 R = Me epothilone B O OH O HO O O N S 12 13 15 17 19 20 21 esterification 1 6 7 olefin metathesis; epoxidation aldol reaction 1 epothilone A O OR RO O O N S OH OR RO O O N S OH 4 3 Scheme 1 stereochemical outcome of this epoxidation sequence from the outset it was envisaged on the basis of an examination of the X-ray crystal structure of 1 that approach of the epoxidising agent would proceed from the more exposed b-face of the macrocycle leading to the introduction of the epoxide with the correct relative stereochemistry.As all the syntheses to date employ this end-game strategy there remain the final choices with regard to the strategic disconnections concerning the construction of the macrocycle itself.Two basic approaches have been reported this year for the construction of the lactone via standard macrolactonisation of the corresponding hydroxy acid or utilising metathesis chemistry for the direct installation of the C–C double bond using the now fashionable metathesis chemistry recently popularised by Grubbs. 236 P. Quayle Nicolaou’s macrolactonisation approach to epothilone A In this convergent synthesis the key intermediate 20 was prepared from the three fragments 8 13 and 17 each of which possessed a single stereogenic centre. The phosphonium salt 8 was readily available (61% overall yield from 7) in high levels of optical purity ([98% ee) starting from the hydrazone 5 using the SAMP methodology developed by Enders (Scheme 2a).A di§erent asymmetric alkylation strategy that developed by Brown was also put to good use in the preparation of the thiazole aldehyde 13 in which the chiral centre at C15 was introduced with high levels of enantioselectivity ([95% ee). The remaining stereogenic centres those at C6 and C7 could in principle be installed by aldol methodology. Hence conversion [i OsO 4 NMO; ii Pb(OAc) 4 ] of the olefin 12 to the aldehyde 13 followed by Wittig homologation with the non-stabilised ylid 9 derived from 8 a§orded the (Z)-olefin 14 in 69% yield (Z:E\9 1). Unmasking the hydroxy group of 14 and oxidation (SO 3 ·py–DMSO) a§orded the aldehyde 16. Unfortunately coupling of the dianion 18 derived from 17 with the aldehyde 16 a§orded a 1 1 mixture of aldol products 19a and 19b in ‘high yield’ (Scheme 2b).In retrospect this initial approach did supply su¶cient quantities of the diastereoisomeric seco-acids to enable the total synthesis of the target molecule to be completed. Cyclisation of the ‘natural’ diastereoisomer 20 using Yamaguchi’s conditions a§orded the desired 16-membered ring lactone 21 in 90% isolated yield. Selective oxidation of the C12–C13 double bond was e§ected with MCPBA at 0 °C to a§ord epothilone A 1 together with the diastereoisomeric a- epoxide 22 (b a\2.7 1) Scheme 2b. An obvious limitation of this approach is the poor stereoselection observed in the crucial aldol reaction used to install the stereocentres at C6 and C7. A number of reports have appeared which addressed this problem. For example Shinzer2 has developed an aldol strategy which overrides the natural Cram selectivity observed in these aldol reactions treatment of the lithium enolate 24 derived from 23 with the aldehyde 25 a§orded the aldol product 26 with the desired 6,7-syn-7,8-anti relative stereochemistry as the sole diastereoisomer in 70% yield presumably the product of a matched transition state (Scheme 3).Similarly Kalesse3 has shown that treatment of the enolate derived from the ketone 27 with the b-alkoxy aldehyde 28 a§orded the aldol product 29 as a single diastereoisomer with the correct epothilone relative stereochemistry in 64% isolated yield Scheme 4. Metathesis approach to epothilone A With the advent of well defined metathesis catalysts use of olefin metathesis reactions in target-orientated synthesis has as anticipated,4,5 increased dramatically.6 Indeed ring closure metathesis reactions (RCM) feature in Nicolaou’s7a,d and Schinzer’s2 synthesis of epothilone A and Danishefsky’s synthesis of epitholone B.7e In Nicolaou’s approach Scheme 5 metathesis of the bis-olefin 30 with the Grubs catalyst 31 in dichloromethane at 25 °C a§orded the macrocycles 32 and 33 in good overall yield (85%) as a mixture of separable C12–C13 isomers with the desired (Z)-isomer (32) predominating (32 33\1.4 1).These results were mirrored in Schinzer’s synthesis in that metathesis of the same substrate 30 a§orded a 1 1 mixture of olefins 32 and 33 in a slightly improved yield of 94%. Of note is the observation that the stereochemical outcome of these metathesis reactions is to some extent dependent upon the conformational properties of the diene undergoing macrocyclisation as poignantly observed 237 Synthesis highlights a review of the literature for 1997 N N OMe N N OMe OBn HO OBn i ii 5 6 7 PPh3 I– OTBS 8 PPh3 OTBS 9 steps N S O H N S OH N S OTBS 10 11 12 N S OTBS HCO 13 15 (>95% ee) iii iv v N S OTBS OTBS N S OTBS OH N S OTBS O 9 69% 14 ( Z E = 9:1) 15 16 Scheme 2a Reagents and conditions i a 1.1 equiv.LDA THF 0 °C; b I(CH 2 ) 3 OBn THF,[100 °C; ii a O 3 CH 2 Cl 2 ,[78 °C; b NaBH 4 ; MeOH; iii (])-Ipc 2 B(allyl) 1.5 equiv. Et 2 O [100 °C; iv TBSCl imidazole THF 0 °C to 25 °C; v OsO 4 (1 mol%) NMO (1 equiv.); b Pb(OAc) 4 EtOAc 238 P. Quayle CO2H O OTBS 17 CO2 – –O OTBS N S CO2H OTBS O HO OTBS H N S CO2H OTBS O TBSO OTBS 18 16 vi 19a 20 HO O 19b N S O O TBSO OTBS N S O O HO OTBS vii O O viii ix 21 22 2Li+ O Scheme 2b vi LDA 3 equiv.THF 0 °C; vii 2,4,6-Cl 3 C 6 H 2 COCl 5 equiv. Et 2 N 6 equiv. THF 25 °C; viii 20% TFA–CH 2 Cl 2 0 °C 1 h; ix MCPBA CH 2 Cl 2 by both Nicolaou7d and Danishefsky.7e Also of interest in this report6d was the observation that stereoselective and regioselective epoxidation of the (Z)-C12–C13 double bond of the diol 34 was best e§ected using DMDO (35) in dichloromethane at [35 °C the desired epoxide 37 was isolated in 48% yield (Scheme 6). In Nicolaou’s full paper,7c oxidation of 32 with methyl(trifluoromethyl)dioxirane in acetonitrile a§orded a mixture of epoxides 37 and 38 (37 38\5 1) with a combined yield of 75% Scheme 6. Suzuki-macrolactonisation approach to epothilone B The synthesis of epothilone B introduces a further complication in that the C12–C13 double bond is now trisubstituted.The key step in Danishefsky8 synthesis was to be a Suzuki coupling reaction of the vinyl iodide 39 with the borane 40 (Scheme 7). The 239 Synthesis highlights a review of the literature for 1997 O O O O O OLi O O O OH O 23 24 25 70% 26 Scheme 3 TBSO O O OBn TBSO O OH OTBS OTBS OH i 64% 27 28 29 Scheme 4 Reagents and conditions i LDA THF 15 min,[78 °C N S O O O HO OTBS N S O O O HO OTBS N S O O O HO OTBS PCy3 Ru PCy3 Ph Cl Cl 30 32 33 33 i Scheme 5 Reagents and conditions i 33 cat. CH 2 Cl 2 25 °C 8 h 240 P. Quayle N S O O O HO OH O O O O CH3 CF3 35 36 34 37 (b-Epoxide) 48% 37 + 38 (a-Epoxide) Scheme 6 stereocentre at C15 was installed using Keck’s asymmetric allylation reaction a§ording the acetate 41 in acceptable yield (60%) and with good levels of asymmetric induction ([95% ee).The olefin 41 was then converted to the Z-vinyl iodide 39 using standard methodology ready to be coupled with the borane 40 which itself was readily available from the alkene 429 upon hydroboration with 9-BBN. As anticipated treatment of the iodide 39 with the borane 40 in the presence of catalytic palladium proceeded in a highly stereoselective manner a§ording only the desired Z-olefin in 77% isolated yield. Completion of the synthesis proceeded along conventional lines involving macrolactonisation followed by epoxidation of the C12–C13 double bond Scheme 7. The intrinsic methodological interest in this approach lies in the fact that prior to this study the synthesis of trisubstituted double bonds had not been reported using this operationally robust chemistry.3 Extensions and diversifications The current interest in solid phase synthesis,10 combinatorial chemistry11 (especially as applied to problems derived from medicinal chemistry),12 interesting biological profiles and structural simplicity have enabled a number of groups to develop generic synthetic strategies for the synthesis of analogues of the epothilones and in itself presents a landmark in total synthesis. The solid phase syntheses of epothilones A and B has been disclosed as have the syntheses of a variety of analogues using solid phase and combinatorial techniques.1b,7b 4 Related topics The pace of synthetic achievement is nicely exemplified again by Nicolaou by the synthesis of sarcodictyin A 4313 and eleutherobin 44.14 Both of these natural products exhibit potent antitumour activity and were synthesised within a year of their structure determination being reported in the primary literature.The key feature of the sarcodictyin synthesis13 was the realisation that the dihydrofuran moiety of the tricyclic core could be prepared by way of the intramolecular ketalisation of the hydroxy ketone 48 which was to be prepared via an intramolecular 241 Synthesis highlights a review of the literature for 1997 O S N O O O OH OH S N I OAc H 1 S R2B CH(OMe)2 OTBS OTPS 39 40 S N CHO S N OAc 41 (> 95% ee) 39 i 60% CH(OMe)2 OTBS OTBS 42 O BnO OH 40 CH(OMe)2 S N OTPS OTBS OAc H 39 + 40 ii 77% Scheme 7 Reagents and conditions i a CH2 ––CHCH 2 SnBu 3 (S)-([)-2,2@-dihydroxy- 1,1@-biphenyl Ti(OPr*) 4 CH2 Cl 2 [20 °C b Ac 2 O Et 3 N DMAP; ii [Pd(dppf) 2 ] Cs 2 CO 3 Ph 3 As H 2 O DMF rt 242 P.Quayle H H O OH CO2Et O N N O H H O OMe O N N O O O OH OH OAc Sarcodictyin A 43 Eleutherobin 44 OR O OR OH H H CHO OR OR OTMS H H O 47 46 (+)-Carvone OTBS OEt O OTBS H H 10 i steps 46 OR OH OR OTMS H H ii iii OR O OR OTMS H H OR O OR OH H H H H O OMe OR OR iv 47 48 49 45 Scheme 8 Reagents and conditions i (EtO) 3 CCH 3 EtCO 2 H 170 °C; ii LHMDS (2 equiv.) THF 25 °C; iii Dess–Martin; iv a PPTS; MeOH b H 2 /Pd-C 243 Synthesis highlights a review of the literature for 1997 CHO O OTES OTES H H O OTBS OTBS OPMP Eleutherobin 44 50 Scheme 9 alkylation reaction of the acetylenic aldehyde 46 (Scheme 8). In practice an enantiospecific synthesis was accomplished starting from (])-carvone from which the known alcohol 45 was readily obtained.A Claisen rearrangement was used to good e§ect for the introduction of the side-chain at C10 with the correct relative stereochemistry whilst elaboration of the side-chains at C1 and C10 to a§ord the acetylenic aldehyde 46 was relatively straightforward. The pivotal step treatment of 46 with LiHMDS followed by oxidation (Dess–Martin) a§orded the acetylenic ketone 47 in 85% overall yield. Vindication of the overall synthetic strategy was realised as semireduction of the C5–C6 acetylenic bond in 47 and deprotection of the TMS ether at C7 presumably generated the ketone 48 which spontaneously a§orded the tricyclic intermediate 49 in good yield (75%). The synthesis of eleutherobin14 followed a similar synthetic strategy except that the initial cyclisation–oxidation sequence was carried out on the fully elaborated aldehyde 50 (80% overall yield) Scheme 9.5 Zaragozic acids The zaragozic acids (squalestatins) a novel class of fungal metabolites which are inhibitors of squalene synthase have been the focus of much synthetic attention in recent years. The synthesis of this class of compounds poses a major synthetic challenge given the densely functionalised dioxabicyclo[3.2.1]octane core. Several new approaches and a total synthesis of zaragozic acid C have appeared this year all of which are worthy of discussion. The key feature of Hashimoto’s synthesis15 is the realisation that the basic skeleton 51 containing C1–C3 could be constructed from an aldol reaction between an enolate 52a derived from D-tartaric acid and an a-keto ester O O OH HO2C O OH HO2C CO2H Ph O OAc Ph Zaragozic acid C 8 5 3 4 244 P.Quayle O HO OH RO2C O OH RO2C OAc Ph O O RO2C OR O O O Y OM RX OR 51 53 52a L-Tartaric acid D-Tartaric acid Scheme 10 O O EtO2C OMEM O O O OTMS MeS OBn O O HO OMEM EtO2C OBn O O MeSCO i 83% 52b 54 + Diastereoisomers 5 4 Scheme 11 Reagents and conditions i Sn(OTf) 2 (2 equiv.) EtCN,[70 °C component 53 derived from L-tartaric acid (Scheme 10). In the critical aldol step treatment of the ketene acetal 52b with ketone 53 in the presence of tin(II) triflate at [70 °C a§orded the adduct 54 in 83% yield as a 1.6 1 mixture of diastereoisomers at C4 C5. Although the level of stereoselection for the desired isomer was not great the simplicity of the reaction leading to the creation of two quaternary stereogenic centres is a useful development and enabled a rapid synthesis of the target compound Scheme 11.Patterson16 has reported a conceptually di§erent and very elegant approach to the oxygenated core of zaragozic acid C which utilised a sequential Sharpless dihydroxylation –epoxidation sequence to set up the functionality for a domino-cyclisation strategy. Regiospecific dihydroxylation of the diene 55 (54% yield;[95% de) followed by a regio- and stereoselective epoxidation of the resulting allylic alcohol 56 a§orded the triol 57 in 78% yield as a single isomer (Scheme 12). Exposure of 57 to CSA initiated a cascade of reactions resulting in the isolation of the acetal 58 in 95% yield. In an alternative strategy Johnson17 utilised the optically pure ketone 59 prepared from cycloheptatriene via a chemoenzymatic route as starting material.Installation of the quaternary centre at C5 was readily accomplished by reaction of the ketone 59 with ‘BnOCH 2 Li’. Ozonolysis of 60 and acidification led to the fully functionalised zaragozic acid core 61 Scheme 13. 245 Synthesis highlights a review of the literature for 1997 Ph OBn Ph OH OTBS OBn OTBS OBn O OBn O OBn OH OH OH OH Ph OBn OTBS OBn O OBn OH OH OH i 54% ii 78% O 55 56 57 iii HO OBn BnO OBn O R O H OH OH O HO OBn O OH OTBS Ph HO BnO OBn 95% 58 Scheme 12 Reagents and conditions i AD-mix b MeSO 2 NH 2 ButOH; ii VO(acac) 2 ButOOH 20 °C; iii PPTS CDCl 3 60°C 6 Asymmetric reactions A number of interesting asymmetric processes have been applied to the synthesis of heterocyclic systems this year. A rather elegant and direct approach to the synthesis of ([)-epibatidine18 relied upon a relatively uncommon asymmetric enolate protonation.Hence treatment of the enol acetate 62 with methyllithium followed by the alcohol 63 at [90 °C a§orded the ketone 64 in 63% yield (82% ee) (Scheme 14). Kozikowski19 has utilised an asymmetric 1,3-dipolar cycloaddition strategy in the synthesis of derivatives whilst Gallagher has reported further on a cycloaddition strategy for carbapenem synthesis,20 Scheme 15. Desymmetrization strategies continue to gain popularity as exemplified by Oppolzer’s four step synthesis21 of the Prelog–Djerassi lactone from the meso-dialdehyde 65. In this sequence aldol reaction of the boron enolate of the sultam 66 with aldehyde 65 246 P. Quayle O O OMOM O OMOM steps O O OMOM OMOM OH OBn 88% O OAc OAc O BnO OAc AcO 59 60 61 i Scheme 13 Reagents and conditions i O3 MeOH–CH 2 Cl 2 then NaBH 4 ; ii TFA–Ac 2 O DMAP O O OAc N Cl O O O N Cl NH N Cl i ii 63% (82% ee) steps 62 64 (–)-Epibatidine S O F3C OH 63 Scheme 14 Reagents and conditions i MeLi 2 equiv.Et 2 O 0 °C; ii 63 [90 °C to [60 °C a§orded the lactols 67a and 67b in 86% yield with the anti-Felkin product 67a predominating (67a 67b\7 1). Conversion of 67a to the Prelog–Djerassi lactone is outlined in Scheme 16. 7 Functional group preparations The synthesis of medium ring ethers continues to be an area of interest as exemplified by the two approaches to laurencin which have appeared this year. In Ho§mann’s approach,22 the key step involved a diastereoselective intramolecular alkylation reaction of the allyl borane 69 (Scheme 16).The borane 69 was prepared from malic acid via the Weinreb amide 68. Notably reduction of the amide carbonyl of 68 was conveniently accomplished by reaction with Dibal-H forming the stable tetrahedral intermediate prior to metallation. Cyclisation of 69 took place between [90 °C and 247 Synthesis highlights a review of the literature for 1997 N Ph O– Me S p-Tolyl O Ph O S N CH3 O p-Tolyl major product N O H CO2PNB O O N H CO2PNB R O (ref. 19) (ref. 20) Scheme 15 N O SO2 O O O X O OH O X O OH i 75–88% 66 65 67a 67b O X O O O HO O O Prelog–Djerassi lactone ii 86% iii 94% Scheme 16 Reagents and conditions i Et 2 BOTf Pr* 2 NEt CH 2 Cl 2 0 °C; ii TPAP NMO; iii LiOH H 2 O H 2 O 2 THF–H 2 O room temperature a§ording 70 as a single diastereoisomer in 38% overall yield from 68.Holmes’23 synthesis of (])-laurencin also starts from malic acid and provides a nice example of the use of the Yamaguchi lactonisation procedure for the preparation of eight-membered lactones an approach which has been under utilised thus far in the synthesis of such compounds (Scheme 18). The seco-acid 71 was conveniently prepared using standard Wittig chemistry and cyclisation to the advanced intermediate 72 proceeded in excellent yield (84%). Methylenation of 72 using the Petasis reagent rather than the more usual Tebbe methylenating agent a§orded the enol ether 73 in high yield. Intramolecular hydrosilation of the enol ether 73 and subsequent oxidative unmasking of the silane (‘Tamao oxidation’) to the key diol 74 proved to be capricious but provided direct entry to the hydroxymethyl group at C1.Conversion of 74 into (])-laurencin was accomplished using standard methodology. In a related context the 248 P. Quayle MeO N O OTBDMS CH3 O MeO N O OTBDMS CH3 OLi+ Malic acid i 68 O OTBDMS ii O B O iii; MeO O O O B O OTBDMS HO 38% overall 69 70 steps O Br (+)-Laurencin H OAc Scheme 17 Reagents and conditions i Dibal-H,[78 °C; ii BusLi THF,[78 °C synthesis of polyether natural products such as ciguatoxin24 and brevetoxin25,26 presents a formidable synthetic challenge. A Yamagouchi lactonisation–Stille sequence developed by Nicolaou25 (Scheme 19) and Clark’s26 RCM approach (Scheme 20) to annelated medium ring ethers is most timely. With the isolation of the annonaceous acetogenins many of which display antitumour immunosuppressive and cytotoxic properties general approaches to the synthesis of oligo(tetrahydrofurans) have been sought.Two rather elegant approaches to the syntheses of such compounds have appeared this year. That of McDonald27 utilises an acylrhenium mediated tandem oxidation of hydroxypolyenes. For example treatment of 75 with the complex 76 a§orded the trans,trans,trans-tris(THF) 77 in 39% yield in a single step (Scheme 21). An alternative strategy leading to similar intermediates has been developed by Koert28 (Scheme 22). This bi-directional strategy employs readily available scalemic starting materials such as 78 and utilises two reactions which proceed with a predictable stereochemical outcome the Williamson ether synthesis (S N 2) and the Sharpless asymmetric dihydroxylation reaction. Alkylation of 78 with lithium acetylide and subsequent realkylation of this homologated acetylene 79 a§ords the bis-acetylene 80 which on controlled dissolving metal reduction a§orded the cis-bis-alkene 81.Sharpless asymmetric dihydroxylation of 81 a§orded the tetraol 249 Synthesis highlights a review of the literature for 1997 HO OTPS O OH OBOM ( R)-Malic acid O OTPS BOMO O i 84% O OTPS RO 71 72 73 O OTPS HO O OTPS HO O OTPS HO HO OH ii iii iv v 1.5 1 74 Scheme 18 Reagents and conditions i 2,4,6-Cl 3 C 6 H 2 COCl Et 2 N THF DMAP; ii Cp 2 TiMe 2 PhCH 3 *; iii (Me 2 SiH) 2 NH; iv Pt(DVS) 2 ; v KOH H 2 O 2 MeOH THF O O OH CO2H Ph H H H O O O R i ii 90% 70% H O O O O H H H H H H steps O O O O O O Ph Ph Ph Scheme 19 Reagents and conditions i 2,4,6-Cl 3 C 6 H 2 COCl Et 3 N THF 0 °C; ii a KHMDS THF,[78 °C b (PhO) 2 POCl HMPA THF,[78 °C 250 P.Quayle O O O R1 Ar R2 ( ) n O O Ar O H R2 R1 H RCM ( ) n Scheme 20 O Et O O n-C12H25 Et n-C12H25 OH H H H H H H H i 39% 75 77 Scheme 21 Reagents and conditions i (Cl 3 CCO 2 )ReO 3 76 THF 20 °C O O Br O O i 81% O O O O O O 67% 78 79 80 O O O O O O 81 O O 82 OH OH OHO OH O O O O O O O OH HO H H H H H H H H 83 ii iii iv v 56% Scheme 22 Reagents and conditions i LiC–– – C–H H 2 NCH 2 CH 2 NH 2 ; ii a LiNH 2 NH 3 b 78; iii Na NH 3 THF; iv AD-mix-b; v a TsCl; Py b H` C NaH 251 Synthesis highlights a review of the literature for 1997 S S S S OTBS BnO TBS TBSO OH i ii iii 84 56% Scheme 23 Reagents and conditions i But Li Et 2 O [78 °C; ii (S)-benzyloxymethyloxirane Et 2 O [78 °C; iii (R)-tetrabutylsilyloxirane Et 2 O HMPA [78 °C]20 °C O O Br OH O O i ii O O Br OH O O Br OH O O HO OMe OH OH HO OH O O 85 8% 6.7% 2.5% (±)-Pinitol steps Scheme 24 Reagents and conditions i OsO 4 hl NaBrO 3 ; ii CH 3 COCH 3 H` 82 in a highly diastereoselective manner (90% de).Per-tosylation of the tetraol 82 acetonide deprotection and multiple Williamson etherification (NaH THF 40 °C) a§orded the tetrakis(THF) 83 in 56% isolated yield from 82. The four rings which were generated in this reaction sequence were formed in a stereospecific fashion with tetrahydrofuran rather than tetrahydropyran ring formation taking place. A two directional synthesis of 1,3-polyols has been developed by Smith.29 This procedure utilises the silylated dithiane 84 in a repetitive alkylation–Brook rearrangement which is apparently only favourable in HMPA as solvent (Scheme 23).This simple procedure should find application in the synthesis of natural products possessing the ubiquitous 1,3-diol moiety. Motherwell30 has reported a remarkably rapid synthesis of (^)-pinitol starting from benzene (Scheme 24). The key feature of this synthesis is the catalytic photoinduced charge-transfer osmylation of benzene using barium chlorate as re-oxidant leading directly to the bromohydrin 85 after protection. Conversion of 85 into (^)- pinitol was readily accomplished using well established chemistry (a total of five steps from benzene!). This methodology presents a useful alternative albeit in racemic form to other methods of cyclitol synthesis (e.g. from cyclohexadiene diols available from enzyme catalysed oxidation of benzene).252 P. Quayle PMBO O H OTBS PMBO OTES H OTBS OTES H OTBS O SPh H O O Br O O O OPMB i >95% steps ii 61% 86 87 Scheme 25 Reagents and conditions i a,KHMDS(3 equiv.) TESCl THF,[78 °C b [78 °C to 45 °C; ii Me 3 SnSnMe 3 (5 equiv.) C 6 H 6 hl 8 C–C Bond forming reactions Radical reactions are now used extensively for the construction of C–C bonds. A number of reports this year have underscored their synthetic utility as described below. Nicolaou31 has described an intramolecular 5-exo-S H 2@ radical cyclisation in a model study concerned with the synthesis of CP-225,917 (Scheme 25). Treatment of the bromide 86 with hexamethyldistannane under photolytic conditions a§orded the lactone 87 as a single diastereoisomer in 61% isolated yield. Of mechanistic interest here is the apparent stereoelectronic requirement (syn-disposition of incoming radical and leaving group) for a successful outcome of this displacement reaction and the use of a divinylcyclopropyl rearrangement for the construction of the bicyclo[4.3.1] decane core itself.Malacria32 has developed a one-pot diastereoselective cascade reaction in the synthesis of epi-illudol in which the cycloundecyne 88 underwent two consecutive transannular reactions (Scheme 25). The eleven-membered ring itself was prepared (88% yield) using a Nozaki–Kishi coupling reaction between an iodoalkyne and an aldehyde whilst the silicon tether used in the radical reaction was subsequently cleaved using the Tamao oxidation protocol. In another cascade reaction that reported by Lee,33 a 5-exo-7-endo cyclisation pathway was utilised in the construction of the guaianolide skeleton.Although there is precedent for 7-endo over 6-exo radical cyclisations this example is notable in that the yield of the endo addition mode is so high (99% yield for both cyclisation reactions) Scheme 27. Finally Hart34 has published details of his synthesis of (^)-gelsemine which utilises a 5-exo-trig cyclisation in the key oxindole forming reaction Scheme 28. The use of palladium-catalysed carbon–carbon coupling reactions has revolution- 253 Synthesis highlights a review of the literature for 1997 I OTBDMS CHO HO OTBDMS O Si Br OTBDMS i ii 100% 88% OH H H H H OH OTBDMS OH OH OH H 88 (4:1 mixture of diastereoisomers) iii iv 47% v epi-Illudol Scheme 26 Reagents and conditions i CrCl 2 (THF) 2 ; ii BMDMSCl DMAP NEt 3 CH 2 Cl 2 ; iii Bun 3 SnH AIBN *; iv H 2 O 2 KHCO 3 KF THF–MeOH; v Bun 4 NF THF O H O H H THPO OH H H THPO O H H THPO H H THPO H H O Br OEt O O OEt O L-Carvone steps steps ii 99% i 90% Scheme 27 Reagents and conditions i,CH 2 –– CHMgBr THF 0 °C; ii Bu 3 SnH AIBN PhH * ised synthetic endeavours over the last decade.Illustrative of this point is a synthesis of cephalotaxine by Tietze,35 in which two consecutive palladium reactions are utilised firstly in the construction of the spirocycle (Godleski reaction) and then in an intramolecular Heck reaction to give the seven-membered ring. Of note is that the very robust catalyst 89 recently reported by Herrmann is essential in promoting the Heck 254 P. Quayle Br N R O MOMO N Ph Ph Me O OR¢ N Ph Ph Me O OR¢ OMOM N O R N Me O N O H O i (±)-21-Oxogelsemine Scheme 28 Reagents and conditions i Bu 3 SnH AIBN PhH hl O O N Br H OAc O O Br N O O N H O O N H HO OMe P Pd—OAc o-Tolyl o-Tolyl 2 89 ii 80% i 85% 4 steps (±)-Cephalotaxine Scheme 29 Reagents and conditions i cat.Pd(Ph 3 P) 4 NEt 3 CH 3 CN *; ii 6mol% 89 Bun 4 NOAc CH 3 CN DMF H 2 O reaction of the electron rich aryl bromide Scheme 29. A number of workers have described the beneficial e§ect of adding copper salts to palladium coupling reactions. Indeed Liebskind reported that copper(I) thiophene-2-carboxylate promotes Stille cross coupling reactions without the requirement for palladium to be present. This coupling procedure has been utilised most successfully by Paterson36 in an approach to elaiophylin in which the 16-membered ring macrolide core was assembled in a cyclodimerisation reaction of 90 in 70% isolated yield Scheme 30.The use of organotitanium chemistry in organic synthesis is a rapidly developing area at present the Kulinkovich reaction in particular is proving to be quite versatile for the synthesis of a variety of fused carbocyclic and heterocyclic systems. For example Cha37 has developed a facile synthesis of the mitomycin skeleton starting from readily available imides. In the case of 91 treatment with c-C 5 H 9 MgCl–TiCl(OPr*) 3 followed by oxidative work-up (O 2 ) led directly to the tricyclic lactam 92 in 60% overall yield 255 Synthesis highlights a review of the literature for 1997 SnMe3 O O O OH HO OH O O O I i 70% 90 Scheme 30 Reagents and conditions i NMP CuTC (10 equiv.) 20 °C OMe N O O MeO N OH O OH i 91 92 Scheme 31 Reagents and conditions i c-C 5 H 9 MgCl ClTi(OPr*) 3 0 °C; ii O 2 N CO2Me H OH N O H OH N OH H OH L-Proline i 67% steps 93 Allopumiliotoxin Scheme 32 Reagents and conditions i Ti(OPr*) 4 Pr*MgCl Et 2 O [50 °C to[5 °C (Scheme 31).Sato,38 one of the major players in this field employed this reaction in an asymmetric synthesis of allopumiliotoxin 267A 93 using L-proline as the starting material (Scheme 32). A key feature of this synthesis is the overall stereospecific carbotitanation of an acetylenic bond with a pendent ester moiety which installs the endocyclic C9–C10 double bond with exclusiveZ-stereochemistry. This synthesis of 93 is the shortest yet (seven steps from L-proline) proceeding in 27% overall yield. Issues of chemoselectivity are often a problem when considering the application of group I and II organometallics to the selective functionalisation of polyfunctionalised intermediates.However the fact that halogen–lithium exchange may be very rapid and hence a potentially selective process has been highlighted by Kim39 in a synthesis of (^)-oppositol (Scheme 33). Exposure of the bromide 94 (itself prepared in an interesting manner via a double diastereodi§erentiating alkylation reaction) to ButLi in diethyl ether at [90 °C cleanly e§ected bromine–lithium exchange the resultant alkyl lithium then su§ered intramolecular acylation to a§ord the ketone 95 in 72% yield. 256 P. Quayle O O R EtO2C Br Br O O EtO2C PMBO O O EtO2C Br i 94 O O O ii 72% Br HO steps 95 (±)-Oppositol Scheme 33 Reagents and conditions i a KHMDS THF [78 °C to [25 °C b ButOK THF 0 °C; ii ButLi Et 2 O,[100 °C to [75 °C The Diels–Alder reaction is extensively used to prepare functionalised cyclohexane rings.In an interesting application Shiori40 has utilised an asymmetric Diels–Alder reaction to control the axial chirality at C7 of the unusual dipeptide radiosumin. Olefination of the ketone 98 prepared from the diene 97 and the a-chloro nitroso compound 96 with Elder’s phosphonate 99 a§orded the olefin 100 as a 94 6 mixture of diastereoisomers (E:Z\94 6) (Scheme 34). Deprotection of the acetonide enabled easy purification of the E-diasteroisomer 100E and at this point the diol functionality which served as a stereocontrol element in the olefination reaction was then removed using the Corey–Winter procedure. The axially chiral olefin 101 which was prepared in this sequence served as a common intermediate for both amino acid residues of the natural product.The synthesis of ‘simple’ hydrocarbons can pose many problems primarily due to the lack of functionality in the final target from which the molecule can be pieced together. A conceptually simple three step synthesis of corannulene,41 illustrates how such systems may be constructed in a very rapid manner Scheme 35. 9 Synthesis of complex natural products The synthesis of a number of highly complex natural products including erythromicin B,42 spongiastatin 2,43 glycopeptides,44 (])-duocarmycin,45 (])-rampamycin46 and (])-dynemicin47 should provide inspiration to all those engaged in organic synthesis. 257 Synthesis highlights a review of the literature for 1997 NH CH3 NH O O NH CH3 O CO2H NH2 7 NH2 CO2R¢ NHR NH2 CO2R¢ NHR radiosumin O O O N Cl O O O O O O O O NHBoc steps 96 97 98 (MeO)2P CO2Me N OMe O 99 O O NHBoc i N CO2 Me OMe NHBoc N CO2 Me OMe steps 100E (Major isomer) 101 Scheme 34 Reagents and conditions i BunLi DME 258 P.Quayle C O CH3 C H3C O O O O O O C Cl CH2 C H2C Cl i 72% ii 85% iii 35–40% Scheme 35 Reagents and conditions i glycine toluene reflux; ii PCl 5 toluene reflux; iii flash pyrolysis 1100 °C References 1 (a) K. C. Nicolaou F. Sarabia S. Ninkovic and Z. Yang Angew. Chem. Int. Ed. Engl. 1997 36 525; (b) D.-S. Su A. Balog D. Meng P. Bertinato S. J. Danishefsky Y.-H. Zheng T.-C. Chou L. He and S. B. Horwitz Angew. Chem. Int. Ed. Engl. 1997 36 2093; (c) K.C. Nicolaou D. Vourloumis T. Li J. Pastor N. Winssinger Y.He S. Ninkovic F. Sarabia H. Vallberg F. Roschanger N. P. King M. R. V. Finlay P. Giannakakou P. Verdier-Pinard and E. Hamel Angew. Chem. Int. Ed. Engl. 1997 36 2097. 2 D. Schinzer A. Limberg A. Bauer O. M. Bo� hm and M. Cordes Angew. Chem. Int. Ed. Engl. 1997 36 523. 3 E. Claus A. Pahl P. G. Jones H. M. Meyer and M. Kalesse Tetrahedron Lett. 1997 38 1359. 4 K.C. Nicolaou N. Winssinger J. Pastor S. Ninkovic F. Sarabia Y. He D. Vourloumis Z. Yang T. Li P. Giannakakou and E. Hamel Nature 1997 387 268. 5 P. Quayle Annu. Rep. Prog. Chem. Sect. B Org. Chem. 1996 93 69. 6 A.Fu� rstner Top. Catal. 1997 4 285; J. Tsuji J. Synth. Org. Chem. Jpn 1997 55 1101. 7 (a) Z. Yang Y. He D. Vourloumis H. Vallberg and K. C. Nicolaou Angew. Chem. Int. Ed. Engl. 1997 36 166; (b) K.C. Nicolaou S. Ninkovic M.R. V. Finlay F. Sarabia and T. Li Chem. Commun. 1997 2343; (c) K. C. Nicolaou S. Ninkovic F. Sarabia D. Vourlumis Y. He H. Vallberg M. R. V. Finlay and Z. Yang J. Am. Chem. Soc. 1997 119 7974; (d) K.C. Nicolaou Y. He D. Vourloumis H. Vallberg F. Roschanger F. Sarabia S. Ninkovic Z. Yang and J. I. Trujillo J. Am. Chem. Soc. 1997 119 7960; (e) D. Meng D.-S. Su A. Balog P. Bertinato E. J. Sorensen S. J. Danishefsky Y.-H. Zheng T.-C. Chou L. He and S. B. Horowitz J. Am. Chem. Soc. 1997 119 2733. 8 D.-S. Su D. Meng P. Bertinato A. Balog E. J. Sorensen S. J. Danishefsky Y.-H. Zheng T.-C. Chou L. He and S. B. Horowitz Angew. Chem. Int. Ed. Engl. 1997 36 757. 9 Unfortunately prepared in an over lengthy sequence A. Balog D. Meng T. Kamenacka P. Bertinato D.-S. Su E. J. Sorensen and S.J. Danishefsky Angew. Chem. Int. Ed. Engl. 1996 35 2801. 10 P. H. H. Hermkens H. C. J. Ottenheim and D. C. Rees Tetrahedron 1997 53 5643. 11 R. Giger Chimia 1997 51 819; J. Trias and E. M. Gordon Curr. Opin. Biotechnol. 1997 8 757. 12 P.M. Cowley and D. C. Rees Curr. Med. Chem. 1997 4 211. 13 K. C. Nicolaou J.-Y. Xu S. Kim T. Ohima S. Hosokawa and J. Pfe§erkorn J. Am. Chem. Soc. 1997 119 11 353. 14 K. C. Nicolaou F. van Delft T. Ohshima D. Vourloumis J. Xu S. Hosokawa J. Pfe§erkorn S. Kim and T. Li Angew. Chem. Int. Ed. Engl. 1997 36 2520. 15 H. Sato S. Nakamura N. Watanabe and S. Hashimoto Synlett 1997 451. 16 I. Patterson K. Fesner and M. R. V. Finlay Tetrahedron Lett. 1997 38 4301. 259 Synthesis highlights a review of the literature for 1997 17 Y. Xu and C. R. Johnson Tetrahedron Lett.1997 38 1117. 18 H. Kosugi M. Abe R. Hatsuda H. Uda and M. Kato Chem. Commun. 1997 1857. 19 G. L. Araldi K. R. C. Prakash C. George and A. P. Kozikowski Chem. Commun. 1997 1875. 20 S. Martel D. Planchenault R. Wisedale T. Gallagher and N. J. Hales Chem. Commun. 1997 1897. 21 W. Opolzer E. Walther C. P. Balado and J. De Brabander Tetrahedron Lett. 1997 38 809. 22 J. Kru� ger and R. W. Ho§mann J. Am. Chem. Soc. 1997 119 7499. 23 J. W. Burton J. S. Clark S. Derrer T. C. Stork J. G. Bendall and A. B. Holmes J. Am. Chem. Soc. 1997 119 7483. 24 T. Oishi K. Maeda and M. Hirama Chem. Commun. 1997 1289. 25 K. C. Nicolaou Z. Yang M. Oulette G.-Q. Shi P. Ga� rtner J. L. Gunzer K. A. Agrios R. Huber R. Chadha and D. H. Huang J. Am. Chem. Soc. 1997 119 8105. 26 J. S. Clark and J.G. Kettle Tetrahedron Lett. 1997 38 127. 27 T. B. Towne and F. E. McDonald J. Am. Chem. Soc. 1997 119 6022. 28 U. Koert M. Stein and H. Wagner Chem. Eur. J. 1997 3 1170. 29 A. B. Smith III and A. M. Boldi J. Am. Chem. Soc. 1997 119 6925. 30 P.M. J. Jung W. B. Motherwell and A. S. Williams Chem. Commun. 1997 1283. 31 K. C. Nicolaou M. H. D. Postema N. D. Miller and G. Yuang Angew. Chem. Int. Ed. Engl. 1997 36 2821. 32 M.R. Elliot A.-L. Dhimane and M. Malacria J. Am. Chem. Soc. 1997 119 3427. 33 E. Lee J. W. Lim C. H. Yoon Y. Sung and Y. M. Kim J. Am. Chem. Soc. 1997 119 8391. 34 S. Atarashi J.-K. Choi D.-C. Ha D. J. Hart D. Kuzmich C.-S. Lee S. Ramesh and S. C. Wu J. Am. Chem. Soc. 1997 119 6226. 35 L. Tietze and H. Schirok Angew. Chem. Int. Ed. Engl. 1997 36 1124. 36 I. Paterson and J.Man Tetrahedron Lett. 1997 38 695. 37 J. Lee J. D. Ha and J. K. Cha J. Am. Chem. Soc. 1997 119 8127. 38 S. Okamoto M. Iwakubo K. Kobayashi and F. Sato J. Am. Chem. Soc. 1997 119 6984. 39 D. Kim and I. H. Kim Tetrahedron Lett. 1997 38 415. 40 H. Noguchi T. Aoyama and T. Shioiri Tetrahedron Lett. 1997 38 2883. 41 L. T. Scott P.-C. Cheng M. M. Hashemi M. S. Bratcher D. T. Meyer and H. B. Warren J. Am. Chem. Soc. 1997 119 10 963. 42 S. F. Martin T. Hida P. R. Kym M. Loft and A. Hodgson J. Am. Chem. Soc. 1997 119 3193. 43 D. A. Evans P. J. Coleman and L. C. Dias Angew. Chem. Int. Ed. Engl. 1997 36 2738. 44 S. J. Danishefsky S. Hu P. F. Cirillo M. Eckhardt and P. H. Seeberger Chem. Eur. J. 1997 36 1617. 45 D. L. Boger J. A. McTie T. Nishi and T. Ogiku J. Am. Chem. Soc. 1997 119 311. 46 A. B. Smith III S. M. Condon J. A. McCauley J. L. Leazer Jr. J. W. Leahy and R. E. Maleczka Jr. J. Am. Chem. Soc. 1997 119 962. 47 A. G. Myers N. J. Tom M. E. Fraley S. B. Cohen and D. J. Madar J. Am. Chem. Soc. 1997 119 6072. 260 P

ISSN:0069-3030    

DOI:10.1039/oc094235

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

9 Reaction mechanisms Part (i) Polar reactions By IAN W. ASHWORTH Zeneca Specialties Process Studies Group Huddersfield Works Leeds Road Huddersfield UK HD2 1FF 1 Introduction Single electron transfer (SET) in nucleophilic substitution reactions continues to be an area of active research which has been reviewed.1 The use of bulky substituents to stabilise enols2 and the initial addition complexes in electrophilic bromination reactions3 have been reviewed. The generation of reactive intermediates by laser flash photolytic techniques has continued to be an area of active research. 2 Solvolysis and carbocations The chemistry of stable carbocations has been thoroughly discussed in a recent book,4 which includes chapters on vinyl cations elucidation of carbocation structure and the role of cationic species in electrophilic aromatic substitution reactions of benzene.The X-ray crystal structure of the cumyl cation 1 has been determined as the hexafluoroantimonate salt.5 The short C`–Cipso bond and bond lengths within the ring are consistent with strong benzylic delocalisation. Stable ion conditions have been used to prepare and characterise the 2,6-dimet7hylmesitylene-2,6-diyl dication at [75 °C in SO 2 ClF.6 The unusual stability of this dication is rationalised in terms of the charges separating to give a dienyl–allyl dication structure 2 on the basis of DFT/IGLO calculations of theoreticalNMRchemical shifts. Adication has also been implicated in the Nazarov-type cyclization of aryl vinyl ketones under superacidic conditions.7 A linear relationship between the acidity of the reaction medium (in terms of the Hammett acidity function H 0 ) and the rate of reaction was taken as evidence of a reaction proceeding through the O,O-diprotonated dication 3.O H H + + + + + 1 2 3 261 The 6,6-dimethyl-3-phenylbenzenium ion 4 has been generated solvolytically in aqueous solution containing 20% acetonitrile from the acetate 5 and the dienol 6.8 Solvolytic generation of the cation 4 in the presence of sodium azide provided rate constants for the rate of capture by water using the azide clock methodology. The regiochemistry of attack by water was discussed and compared to the biphenyl-4- ylnitrenium ion 7. A similar methodology has been used by Dalby and Jencks9 to obtain lifetimes for imidinium ions 8 in aqueous solution. The ions were generated Ph H OX Ph NH Ph N N Ar R1 R2 + + + 4 5 X = Ac 6 X = H 7 8 solvolytically from the corresponding fluoro or chloro formamidine precursor in a process which exhibits common ion inhibition in the presence of added fluoride ion.Trapping of the imidinium ions with thiolacetate anion was concluded to occur in a di§usion limited process in contrast to the behaviour observed with azide. Tidwell and co-workers have generated the antiaromatic 3-(trifluoromethyl)indenyl cation 9 solvolytically in TFA.10 The solvolysis of the tosylate parent of 9 was proposed to occur by the formation of an intermediate allyl cation–tosylate ion pair which undergoes an allylic rearrangement to yield the tosylate 10 in competition with capture by solvent. CF3 CF3 OTs Ph S Cl O + 9 10 11 The e§ect of solvent on the rate of solvolysis of 1-alkyl-1-chloro-1-(4-methyl)phenylmethanes has been studied and found to correlate with the Grunwald–Winstein equation using the recently proposed Y B/C- scale.11 It was found that increasing the size of the alkyl substituent from methyl to tert-butyl led to a change to an S N 1 mechanism with no nucleophilic participation by solvent.The tert-butyl derivative was therefore proposed as a suitable reference standard with which to extend the Y B/Cscale. In contrast the application of a range of di§erent methods of treating dispersion on Grunwald–Winstein plots for the solvolysis of benzylic substrates has been discussed by Kevill and D’Souza.12 Based on data for the solvolyses of a range of b-arylalkyl toluene-p-sulfonates the authors concluded that an approach involving the aromatic ring parameter I in an extended Grunwald–Winstein treatment was more generally applicable.Evidence for a change in mechanism of solvolysis of phenyl chlorothioformate 11 was found when an extended Grunwald–Winstein equation using the solvent nucleophilicity term N T was applied.13 Two distinct correlations were observed. In highly nucleophilic weakly ionizing solvents the reaction was concluded to occur by the addition–elimination mechanism observed in phenyl 262 I.W. Ashworth chloroformate solvolysis. In fluoro alcohol rich mixtures and in water the reaction occurs by a rate determining ionization process. 3 Nucleophilic substitution UV–VIS spectroscopic evidence for the operation of a SET pathway in the nucleophilic substitution reaction between 9-mesitylfluorene anion 12 and methyl iodide has been found.14 The methylation of the carbanion was concluded to occur through an inner sphere electron transfer to methyl iodide generating a radical pair which gave methylation by recombination within the solvent cage.Di§usional separation of the radical pair at higher temperatures led to the build up and detection of the persistent 9-mesitylfluorenyl radical. The reactions of substituted N-methylbenzohydroxamate anions 13 with substituted phenyldimethylsulfonium fluoroborates were studied in [2H 4 ]methanol by 1H NMR techniques.15 The a-e§ect observed for this S N 2 reaction was found to increase with increasing electron donation in the aryl substituent of the nucleophile. Correlations between the rates of reaction and the oxidation potential of the nucleophiles are discussed in terms of SET character in the substitution reaction.N O O– X OSO2Ar S OMe – n + Cl– 13 14 n = 1-4 15 12 The cross-interaction constants oXZ for the nucleophilic substitution reaction between cycloalkylmethyl arenesulfonates 14 and substituted anilines have been determined and are consistent with a tight transition state and independent of the size of the cycloalkyl ring.16 Application of the Taft equation enabled the p* values for n\1 and 4 to be estimated based on the o* (17.4) and S (2.3) values obtained for n\2 and 3. The large magnitude of o* relative to S shows the sensitivity of the reaction to polar substituent e§ects to be greater than the sensitivity to steric e§ects. Studies of the e§ects of salts on the nucleophilic displacement reactions of (4-methoxybenzyl)dimethylsulfonium chloride 15 with a range of nucleophiles have been carried out.17 A range of mechanistic behaviours were observed ranging from a mixed S N 1/S N 2 mechanism in the case of pyridine to a combination of concerted S N 2 and preassociation-concerted mechanisms in the cases of azide and sulfite at low nucleophile concentrations.4 Elimination reactions An E1 elimination reaction proceeding via a primary carbocation has been proposed for the solvolytic elimination of 9-[(4@-bromophenyl)sulfonyloxymethyl]fluorene 16 in 263 Reaction mechanisms Part (i) Polar reactions 25% acetonitrile in water.18 Evidence for this was provided by the small kinetic deuterium isotope e§ect measured for the elimination. Subsitution products consistent with the carbocation reacting with the solvent were also observed.Increasing the acidity of the b-hydrogen by substituting the ring to give 17 decreases the rate of substitution by solvent in agreement with the proposed mechanism. However none of the experimental results presented in this paper requires formation of a primary carbocation intermediate and the authors acknowledge that these elimination reactions may proceed by an E2 mechanism through a carbocation-like transition state. This alternative mechanism must be given equal consideration in view of the large instability of primary carbocations and the paucity of experimental evidence for their formation as intermediates of substitution and elimination reactions. X X OBs N X (D)H (D)H MeHN Ar 16 X = H 17 X = Br 18 X = Cl 19 X = SO2Ar 20 Studies of the imine forming elimination reaction of the N-chloramine 18 in 25% acetonitrile in water have demonstrated the operation of solvent and base promoted E2 pathways based on the primary deuterium kinetic isotope e§ects.19 In contrast the N-benzenesulfonylamine derivatives 19 with poor leaving groups undergo elimination via a reversible E1cB reaction with hydrogen incorporation at C9 in the deuterated derivative and large b1'.The 4@-nitrophenylsulfonyl derivative (19 X\ SO 2 C 6 H 4 -4-NO 2 ) fails to undergo elimination generating the rearrangement product 20 through a nucleophilic aromatic substitution reaction at the nitrophenyl ring. The alkaline hydrolysis of aryl carbazates 21 has been studied and shown to involve an E1cB pathway.20 Evidence for this pathway is provided by the high dependence of the reaction rate upon the pK! of the leaving group the observed levelling of the pH rate profile at high pH and by the low value of DSt.The methyl substituted derivative 22 O N NH2 R O X X H(D) N O But O But But But But 21 R = H 22 R = Me 23 24 which has no acidic proton on nitrogen displays di§erent behaviour and is postulated to hydrolyse by a B!#2 mechanism. Cho and co-workers have studied the elimination reactions of a range of (E)- and (Z)-benzaldehyde O-pivaloyloximes 23 and found that the oximes undergo an E2 elimination.21 Comparison of the Hammett q and k H /k D values for the two isomers suggest that the transition state for the anti elimination from the (Z)-oxime is more symmetrical with less proton transfer and negative charge build up on the b-carbon than for the (E)-oxime.264 I.W. Ashworth 5 Addition reactions The electrophilic bromination of ring-substituted E- and Z-stilbenes has been investigated in a range of solvents.22 The observed kinetic solvent e§ects chemoselectivity and dependence of product stereochemistry on the solvent and substituents were discussed and interpreted in terms of a mechanistic scheme in which the products form by competing preassociation free-ion and ion-pair pathways. Spectroscopic evidence has been reported for the 2 1 bromine–alkene complex which had been inferred to be an intermediate in the electrophilic bromination of alkenes based on thermodynamic data.23 UV spectroscopic studies of solutions of the highly hindered olefin tetraneopentylethene 24 in 1,2-dichloroethane (DCE) containing bromine demonstrated the existence of a new absorption band when high concentrations of bromine were used.The bromination of a range of 1,1-diarylethenes and their 2,2-dideuterio derivatives has been investigated in DCE.24 It was found that 1,1-diphenylethene underwent reaction to yield the dibromide 25 and the vinyl bromide 26 which were concluded to be formed via a rate limiting ionisation step to give an open b-bromocarbonium ion. Increasing electron withdrawal by the aryl groups led to the reversible formation of the ionic intermediate. Ph Ph CL2Br Br CLBr Ph Ph Br Ar N O R 26 27 25 28 The substitution reactions of 9-(a-bromo-a-arylmethylene)fluorenes 27 with methanethiolate and p-toluenethiolate have been studied by Rappoport and Shainyan.25 The reactions exhibit clean second order kinetics large negative values of DSt and a Hammett o of 1.07 which were taken as evidence of a nucleophilic vinyllic substitution mechanism via a carbanion intermediate.The hydrolysis of the C––Ngroup of the benzoxazine 28 has been studied and shown to proceed by an A2 mechanism in which nitrogen protonation is followed by the nucleophilic addition of water.26 The rectilinear dependence of the rate on [bu§er] was rationalised in terms of a change from rate limiting tetrahedral intermediate formation at high [bu§er] to rate limiting deprotonation of the tetrahedral intermediate at low [bu§er]. 6 Carbonyl derivatives The water catalysed hydrolysis of trifluoroacetanilide has been studied. A rate limiting addition of water via a transition state involving at least two protons in flight has been proposed supported by proton inventory experiments in water–D 2 O mixtures.27 Experiments using 18O labelled water found no incorporation of the label into the starting material suggesting that C–N cleavage occurs in preference to OH expulsion.Studies of the hydroxide ion mediated ring-opening and reclosure (Scheme 1) of a 265 Reaction mechanisms Part (i) Polar reactions S N R1 R2 S N R1 R2 OH H N R1 R2 S CHO + - OH– 29 OH– 31 30 Scheme 1 number of thiazolium ions 29 have been reported.28 The reclosure reaction of the enethiol 30 is pH independent at low pH and exhibits general acid catalysis at higher pH. Two kinetic processes were observed which both generate the thiazolium ion via the tetrahedral intermediate 31. These were concluded to be the reactions of the two amide rotamers which are kinetically distinct due to their interconversion occuring more slowly than the reclosure reaction.The development of 4-substituted norsnoutanones 32 as a probe for the investigation of long range electronic e§ects on the p-facial selectivity of nucleophile addition to C–– O has been reported.29 The intramolecular participation of carbonyl hydrates in a range of reactions has been studied by Bowden and co-workers.30 These reactions included the hydrolysis of the acrylate ester 33,30a 2-formylbenzonitrile andN-(2-formylphenyl)-3-X-benzamides 34.30b The base catalysed ring fission of 2,2-dihydroxyindane-1,3-diones 35 was concluded to occur by rate limiting 1,2-nucleophilic attack by the anion of the dione to yield the epoxide 36 which then opens with fission of the C–C bond.30c 3,4-Diarylcyclobut- 3-ene-1,2-diones 37 were found to undergo a base catalysed ring opening reaction to give ‘aryl–acyl’ cleavage,30d in contrast to substituted benzocyclobutene- 1,2-diones 38 which give ‘acyl–acyl’ cleavage.30e This observation was rationalised in terms of ring strain in the benzofused system rendering the benzylic acid-type rearrangement energetically unfavourable (path a Scheme 2).In the absence of this pathway 1,2-nucleophilic attack by the hydrated carbonyl group occurs which generates an epoxide (path b). O R Ph Ph O OMe Ar O O H N O Ar O O OH OH X O OH O– O X O O Ar Ar O O X Y O H OR 39 32 33 H 34 35 36 38 37 The use of a rigid structural framework has facilitated the comparison of the e§ects 266 I.W.Ashworth R R O O R R R R O O– OH O O– OH O R R O OH R CO2 – R H O R R O– CO2H H R R O CO2 – OH– (a) (b) – Scheme 2 of syn- and anti-periplanar lone pairs on the rates of hydrolysis of tetrahydropyranyl acetals.31 A range of acetals containing synperiplanar lone pairs 39 have been synthesised their hydrolysis rates measured and crystal structures determined. The b1' for this system was found to be very large ([1.4) which was taken as evidence of considerable ([0.4 units) positive charge build up in the ground state which is lost on proceeding to a transition state with about a full negative charge on the oxygen leaving group. Bond lengths for the C–Obonds in the tricyclic system correlate with the pK! of the leaving group although the sensitivity is lower than for the more flexible tetrahydropyranyl acetal system.These observations led to the conclusion that the synperiplanar lone pair in 39 is less e§ective at facilitating the hydrolysis of the acetal than the antiperiplanar lone pair in the simple tetrahydropyranyl system. The reactions of aryl dithioacetates with substituted anilines N,N-dimethylanilines and benzylamines have been studied in acetonitrile.32 Substituted anilines and dimethylanilines were shown to react through a mechanism in which the breakdown of the zwitterionic tetrahedral intermediate is rate limiting. This conclusion was based on the observation of large values of Hammett o and Brønsted b for both the nucleophile and leaving group and of large cross correlation coe¶cients. The more basic benzylamines react in a process with lower sensitivity to changes in ring substituents which is consistent with a change to rate limiting attack by the nucleophile.7 Reactive intermediates Two groups have reported the generation and spectroscopic study of singlet phenylnitrene by flash photolysis of phenylazide.33 The kinetics of the decay of the transient species generated initially have been studied and a lifetime of 0.1–1.0 ns predicted for singlet phenylnitrene in pentane at 298 K.33a Attempts to trap the nitrene were unsuccessful and decay occurred by intersystem crossing to the triplet nitrene coupled with ring expansion to the cyclic ketenimine 40.33b The reaction of singlet penta- fluorophenylnitrene with alkenes to yield the azirine 41 has been studied in competitive trapping experiments with pyridine.34 The e¶ciency of the alkene at trapping the nitrene was found to increase as the nucleophilicity of the alkene increased.Diphenylnitrenium ion 42 has been generated flash photolytically and the regiochemistry and 267 Reaction mechanisms Part (i) Polar reactions N Ar R H Ar C N C H R Ar H N H R Ar H N OR R H – + + hn H + ROH 50 51 49 Scheme 3 rates of its reactions with electron rich alkenes studied.35 Addition of the alkene was found to occur at the ortho and para positions of the phenyl rings but not on nitrogen. Highly nucleophilic silyl ketene acetals 43 gave poor yields of nitrogen addition products as well as products arising from reaction on the phenyl groups of the nitrenium ion. N N R1 R3 R3 R4 Ph N Ph Me3SiO OR F + 40 41 42 43 Acetylketene 44 and N-propylacetacetimidoylketene 45 have been generated in solution and their chemoselectivities towards a range of functional groups assessed in competitive trapping experiments.36 The trends in reactivity observed are in agreement with a reaction proceeding through a pseudopericyclic transition structure which had previously been proposed for the addition of water to formylketene based on ab initio calculations.Tidwell and co-workers have prepared a number of trimethylsilyl stabilized allenylketenes 46 by flash photolysis of substituted methylenecyclobutenone precursors and studied their rates of hydration.37 X-Ray crystallographic study of the bisphenyl substituted derivative confirmed the anti-planar conformation predicted by computational studies. C O X Me3Si C O SiMe3 C R1 R2 C N R PMP PMP PMP NHR HO PMP 44 X = O 45 X = NPr 46 47 48 Highly hindered N-substituted keteneimines (PMP\pentamethylphenyl) 47 have been isolated and their hydration studied.38 Rather than undergoing protonation on the b-carbon followed by attack of water the highly hindered keteneimines underwent a pre-equilibrium protonation on nitrogen.This was followed by rate determining nucleophilic attack by water to form the amide enols 48 which then ketonised. Azaallenium cations 49 have been identified spectroscopically as intermediates in the photochemical ring opening of 2H-azirines 50 in alcohols.39 The azaallenium 268 I.W. Ashworth cations arise from the protonation of the nitrile ylide 51 (Scheme 3) and are trapped by the alcohol solvent or added nucleophiles. Based on the large kinetic isotope e§ect for the protonation of the ylide by alcohol a linear transition state was proposed.The investigation of the keto–enol tautomerism of carboxylic acids has been facilitated by the preparation of significant quantities of 1,1-enediols which were generated by hydration of the relevant ketene.40 Kresge and co-workers have synthesised a O OH O Ph OH O OH CO2H 52 53 54 number of ketenes flash photolytically and studied the pH rate dependence of their hydration reactions and subsequent ketonisation. These studies have been performed on the 2-oxocyclopentanecarboxylic acid 52,40a mandelic acid 53,40b and fluorene-9- carboxylic acid 5440c keto–enol systems generating pK! values for the ketones acting as carbon acids. Sterically hindered enols of carboxylic acids have also been produced by hydrating bis(PMP)ketene and bis(2,4,6-trimethylphenyl)ketene.40d In these compounds hydration occurs in preference to protonation on the b-carbon due to steric hindrance facilitating the study of the ketonisation reaction.A study of the kinetics of formation of the amino acid ester enolate of glycine methyl ester by base-catalyzed deprotonation of the parent carbon acid at room temperature and neutral pD in D 2 O has been reported. It was shown that the addition of an NH 3 ` substituent to ethyl acetate results in a 3500-fold increase in the second-order rate constant k DO for lyoxide-ion catalyzed exchange of the acidic a-carbonyl protons and an estimated decrease in the pK! of the carbon acid from 25.6 to 21.40e 8 Aromatic substitution The reactivity of S,S-diphenylsulfilimine (Ph 2 S––NH) with a range of electron deficient haloaromatic species has been investigated.41 The reaction was concluded to be a nucleophilic aromatic substitution reaction in which the sulfilimine displays anomalously high reactivity for an sp2 hybridised nucleophile.This high nucleophilicity was postulated as arising from reaction proceeding through the sp3 hybridised ylide of the sulfilimine. S R NO2 L O2N L NO2 R NO2 NMe2 O2N NO2 NO2 Ar H O2N NMe2 NO2 NO2 H H Ar – 55 R = H or Me 56 R = H or Me 57 58 A combination of kinetic and X-ray crystallographic techniques have been used to investigate the di§erence in secondary steric e§ects in the S N Ar reactions of thiophenes 269 Reaction mechanisms Part (i) Polar reactions 55 and benzenes 56.42 Methyl substitution causes a large fall in the rate of reaction in the case of the substituted benzene whereas the thiophene derivative shows only a small e§ect.This correlates well with the crystallographic data which show the nitro groups of 56 R\Me to be rotated further out of the plane of the molecule than in the corresponding thiophene derivative. An unusual ring protonation reaction was observed when the Meisenheimer complexes 57 formed by the reaction of N,N-dimethylpicraminide with phenoxide or 2,6-di-tert-butylphenoxide inDMSOwere treated with acid.43 Protonation was observed to occur at C-2 by 1H and 13C NMR spectroscopy to give the nitro tautomer 58. This behaviour was rationalised in terms of a steric interaction between the 2-NO 2 group and the NMe 2 group which favours protonation at the 2-position due to strain relief.Studies of the S E Ar reaction between 3-methoxythiophene 59 and the super-electrophile 4,6-dinitrobenzofuroxan 60 (DBNF) have been carried out in DMSO–water S OMe N O N O2N NO2 O S OMe DNBF - 59 60 61 mixtures.44 The rate of formation of the anionic adduct 61 was found to be virtually una§ected by hydrogen or deuterium labelling at the a-carbon of the thiophene. This led to the conclusion that the addition of DBNF to the thiophene is rate limiting. The rate constants were found to have a strong dependence upon the water content of the reaction solvent indicating a highly polar rate limiting transition state in agreement with the postulated mechanism.A pK! of[6.5 was estimated for the protonation of 59 at the a-carbon in water based on a Brønsted-type relationship and discussed in relation to other thiophene derivatives.References 1 M. Patz and S. Fukuzumi J. Phys. Org. Chem. 1997 10 129. 2 Z. Rappoport J. Frey M. Sigalov and E. Rochlin Pure Appl. Chem. 1997 69 1933. 3 R. S. Brown Acc. Chem. Res. 1997 30 131. 4 G.K. S. Prakash and P. v. R. Schleyer (Eds.) Stable Carbocation Chemistry Wiley New York 1996. 5 T. Laube G. A. Olah and R. Bau J. Am. Chem. Soc. 1997 119 3087. 6 G.A. Olah T. Shamma A. Burrichter G. Rasul and G. K. S. Prakash J. Am. Chem. Soc. 1997 119 3407. 7 T. Suzuki T. Ohwada and K. Shudo J. Am. Chem. Soc. 1997 119 6774. 8 R.A. McClelland D. Ren D. Ghobrial and T. A. Gadosy J. Chem. Soc. Perkin Trans. 2 1997 451. 9 K.N. Dalby and W.P. Jencks J. Am. Chem. Soc. 1997 119 7271. 10 A. D. Allen M. Fujio N. Mohammed T. T. Tidwell and Y.Tsuji J. Org. Chem. 1997 62 246. 11 K.-T. Liu L.-W. Chang D.-G. Yu P.-S. Chen and J.-U. Fan. J. Phys. Org. Chem. 1997 10 879. 12 D. N. Kevill and M. J. D’Souza J. Chem. Soc. Perkin Trans. 2 1997 257. 13 D. N. Kevill M.W. Bond and M.J. D’Souza J. Org. Chem. 1997 62 7869. 14 L.M. Tolbert J. Bedlek M. Terapane and J. Kowalik J. Am. Chem. Soc. 1997 119 2291. 15 K. R. Fountain T. W. Dunkin and K. D. Patel J. Org. Chem. 1997 62 2738. 16 H. K. Oh S. J. Song D.-S. Jo and I. Lee J. Phys. Org. Chem. 1997 10 91. 17 N. Buckley and N. J. Oppenheimer J. Org. Chem. 1997 62 540. 18 Q. Meng and A. Thibblin J. Am. Chem. Soc. 1997 119 4834. 19 Q. Meng and A. Thibblin J. Am. Chem. Soc. 1997 119 1224. 270 I.W. Ashworth 20 P. Vlasa� k and J. Mindl J. Chem. Soc. Perkin Trans. 2 1997 1401. 21 B. R.Cho N. S. Cho and S. K. Lee J. Org. Chem. 1997 62 2230. 22 M.-F. Ruasse G. L. Moro B. Galland R. Bianchini C. Chiappe and G. Bellucci J. Am. Chem. Soc. 1997 119 12 492. 23 R. Bianchini C. Chiappe D. Lenoir P. Lemmen R. Herges and J. Grunenburg Angew. Chem. Int. Ed. Engl. 1997 36 1284. 24 G. Bellucci and C. Chiappe J. Chem. Soc. Perkin Trans. 2 1997 581. 25 Z. Rappoport and B. A. Shainyan J. Phys. Org. Chem. 1997 10 871. 26 W.J. Dixon F. Hibbert and J. F. Mills J. Chem. Soc. Perkin Trans. 2 1997 1503. 27 H. Slebocka-Tilk C. J. Rescorla S. Shirin A. J. Bannet and R. S. Brown J. Am. Chem. Soc. 1997 119 10 969. 28 E. C. Carmichael V. D. Geldart R. S. McDonald D. B. Moore S. Rose L. D. Colebrook G. D. Spiropoulos and O. S. Tee J. Chem. Soc. Perkin Trans. 2 1997 2609. 29 G. Mehta C. Ravikrishna B.Ganguly and J. Chandrasekhar Chem. Commun. 1997 75. 30 (a) K. Bowden and J. M. Byrne J. Chem. Soc. Perkin Trans. 2 1997 123; (b) K. Bowden S. P. Hiscocks and M.K. Reddy J. Chem. Soc. Perkin Trans. 2 1997 1133; (c) K. Bowden and S. Rumpal J. Chem. Soc. Perkin Trans. 2 1997 983; (d) A. Al-Najjar K. Bowden and M. V. Horri J. Chem. Soc. Perkin Trans. 2 1997 993; (e) K. Bowden and M.V. Horri J. Chem. Soc. Perkin Trans. 2 1997 989. 31 P. Deslongchamps P. G. Jones S. Li A. J. Kirby S. Kuusela and Y. Ma J. Chem. Soc. Perkin Trans. 2 1997 2621. 32 H. K. Oh S. Y. Woo C. H. Shin Y. S. Park and I. Lee J. Org. Chem. 1997 62 5780. 33 (a) N. P. Gritsan T. Yuzawa and M.S. Platz J. Am. Chem. Soc. 1997 119 5059; (b) R. Born C. Burda P. Senn and J. Wirz J. Am. Chem. Soc. 1997 119 5061. 34 H.Zhai and M. S. Platz J. Phys. Org. Chem. 1997 10 22. 35 R. J. Moran C. Cramer and D. E. Falvey J. Org. Chem. 1997 62 2742. 36 D.M. Birney X. Xu S. Ham and X. Huang J. Org. Chem. 1997 62 7114. 37 W. Huang D. Fang K. Temple and T. T. Tidwell J. Am. Chem. Soc. 1997 119 2832. 38 A. F. Hegarty J. G. Kelly and C. M. Relihan J. Chem. Soc. Perkin Trans. 2 1997 1175. 39 E. Albrecht J. Mattay and S. Steenken J. Am. Chem. Soc. 1997 119 11 605. 40 (a) Y. Chiang A. J. Kresge V. A. Nikolaev and V. V. Popik J. Am. Chem. Soc. 1997 119 11 183; (b) Y. Chiang A. J. Kresge V. V. Popik and N. P. Schepp J. Am. Chem. Soc. 1997 119 10 203; (c) J. Andraos Y. Chiang A. J. Kresge and V. V. Popik J. Am. Chem. Soc. 1997 119 8417; (d) B.M. Allen A. F. Hegarty and P. O’Neill J. Chem. Soc. Perkin Trans. 2 1997 2733; (e) A. Rios and J. P. Richard J. Am. Chem. Soc. 1997 119 8375. 41 J. P. B. Sandall C. Thompson and N. J. D. Steel J. Chem. Soc. Perkin Trans. 2 1997 513. 42 G. Consiglio V. Frenna A. Mugnoli R. Noto M. Pani and D. Spinelli J. Chem. Soc. Perkin Trans. 2 1997 309. 43 R. A. Manderville and E. Buncel J. Org. Chem. 1997 62 7614. 44 E. Kizilian F. Terrier A.-P. Chatrousse K. Gzouli and J.-C. Halle� J. Chem. Soc. Perkin Trans. 2 1997 2667. 271 Reaction mechanisms Part (i) Polar reactions

ISSN:0069-3030    

DOI:10.1039/oc094261

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

9 Reaction mechanisms Part (ii) Pericyclic reactions By IAN D. CUNNINGHAM Department of Chemistry University of Surrey Guildford UK GU2 5XH 1 Introduction Recent advances in our understanding of the mechanisms for pericyclic reactions have been summarised in reviews which cover quantum mechanical methods for modelling pericyclic reactions,1 metal-assisted cycloaddition reactions,2 radical cation cycloaddition reactions,3 frontier molecular orbital (FMO) theory4 and solvent e§ects on Claisen rearrangement reactions.5 2 Cycloaddition reactions Theoretical studies Pericyclic cycloaddition reactions with well established mechanisms are often used as benchmarks to test proposed advances in computational methodology.6 These studies provide justification for the routine use of computational modelling to rationalise the rates and stereochemical course of pericyclic reactions.For example a frontier molecular orbital analysis nicely rationalizes the syn regioselectivity observed for the reaction of penta-1,3-diene with juglone 1 (Scheme 1) by a consideration of secondary orbital interactions (SOI). The di§erences in the FMO coe¶cients for the diene determined by semi-empirical and ab initio calculations are large for non-bonding secondary sites (C2\C3) but minimal for the primary bonding sites. BF 3 coordinates to the Cl carbonyl of 1 and the observation that addition of this complex to penta-1,3- diene results in a reversal to anti regioselectivity can also be rationalised by frontier molecular orbital theory where calculation shows that there is a reversal in the magnitude of the LUMO coe¶cients at the secondary sites (C1[C4).Al(OR) 3 with bulky R groups (e.g. 2,6-diphenylphenyl) coordinates to the C4 carbonyl group but does not lead to the predicted syn regioselectivity. Steric e§ects are invoked to explain the failure to observe the predicted reaction products. Finally the results of an AM1/ab initio study suggest an extremely asynchronous approach to the transition state for the reaction catalysed by BH 3 so that the structure of this transition state approaches that expected for a zwitterionic reaction mechanism.7 The still controversial question of the explanation for the observation of endo selectivity in Diels–Alder reactions has been considered for the butadiene–cyclo- 273 O O OH O O OH O O OH syn 3 2 1 4 1 anti Scheme 1 Si Si Si Si Scheme 2 propene cycloaddition reaction using ab initio computational methods [HF CAS QCISD(T)].While it is clear that the endo selectivity is the result of a C–H · · · p interaction the authors argue whether this should be defined as a true secondary orbital e§ect (SOE).8 A related DFT study also identifies the C–H· · · p interaction as dominant in determining the endo selectivity for this reaction.9 The interplay between calculations and experiments is often fruitful. A prediction from density function theory of a novel and facile [4]2] cycloaddition of cyclohexa- 1,3-diene onto a Si(100)-2]1 surface (Scheme 2)10 has been supported by vibrational spectroscopic evidence for formation of a buta-1,3-diene adduct obtained in a separate study.11 Novel dienes and dienophiles There were many reports in the 1997 literature of cycloaddition reactions of novel dienes and dienophiles but relatively few of these provide significant new insights into the mechanisms of these reactions.However the reaction of alkynyldihaloboranes 2 (R\Bun Ph But Si(Pr*) 3 ; X\Cl Br) with 2-substituted dienes is of interest. Although some of the products of these reactions are consistent with a pericyclic [p44 ]p24] reaction mechanism,* the observation of anomalous regioselectivity solvent e§ects and reaction by-products is cited as evidence to support alternative reaction mechanisms including one that proceeds via the [4]3] adduct 3 (R@\Me But Ph).12 B R' X X R R BX2 2 – + 3 *The distinction of H.-W. Fru� hauf (ref. 2) between the mechanistic notation e.g. [p44 ]p24] and the topological or product-based notation e.g.[4]2] is used in this report. 274 I.D. Cunningham EtO N O N NO2 N –O O- O N O N NO2 O– O N –O OEt 5 + + 4 + + Scheme 3 NPh R O S R' R R' N O S Ph S N O R R' 7 6 Ph Scheme 4 The [4]2] cycloaddition of ethyl vinyl ether to the C––C–N––O part of 4,6-dinitrobenzofuran (DNBF) 4 has been studied (Scheme 3). The authors propose that the endo monoadduct 5 and the diadduct (not shown) form by consecutive Diels–Alder reactions in which there is inverse electron demand.13 The results of a theoretical density functional theory study of the related cycloaddition reaction between nitroethene and ethenol support a concerted pericyclic reaction mechanism but it was not possible to clearly identify the transition state of the reaction by a one-step mechanism in an AM1 study of the cycloaddition reaction between 4 and methyl vinyl ether.14 A detailed experimental study of the Diels–Alder reactions of the ambident dienophile 6 has shown that dienes with C- and Z-type substituents R and/or R@ show high C––S vs.C–– C chemoselectivity while those with X-type substituents (alkyl) show little chemoselectivity (Scheme 4). High regioselectivity as shown by 7 in Scheme 4 is observed for the cycloaddition reaction of the C–– S functional group and the stereoselectivity of the reaction generally favours formation of the endo product. The regioselectivity for the reaction of the thiocarbonyl group cannot be adequately modelled by FMO/AM1 theory because the coe¶cients for theLUMO C�S are similar at carbon and sulfur. However an AM1 analysis of transition state structures provides a better prediction for the regioselectivity and overall stereoselectivity for this reaction.These calculations model the experimental trend in chemoselectivities but they overestimate the di§erence between the activation barriers for the cycloaddition reactions of the C–– C and C–– S centres.15 The P–– S groups in Lawesson’s reagent 8 (Ar\4-MeOPh) show high 1,3- dipolarophilicity towards 9 and a value of k 2 \2.21dm3 mol~1 s~1 has been determined for the reaction in acetone at 37 °C. By comparison the P–– S group of 8 is 1500- and 1.4-fold more reactive as a dipolarophile respectively than representative C––C and C––S groups.16 275 Reaction mechanisms Part (ii) Pericyclic reactions Ar P S S N N N Ph N Ph Ph Ph 9 + – 8 2 Kinetics and mechanism of [4]2] and [2]2] cycloaddition reactions The qualitative categorization of pericyclic reactions as proceeding with normal or inverse electron demand is now well established.Sauer has published the results of a systematic study of the kinetics of the cycloaddition of polyhalogenated cyclopentadienes to triazolinedione maleimide styrene acrylate and other dienophiles. The dependence of the rate constant for these reactions on the Hammett substituent constant r is used to define the electron demand for the reaction. A surprising number of reactions give V-shaped Hammett plots in which the Hammett reaction constant q changes from positive for electron-withdrawing substituents to negative for electrondonating substituents. This result is indicative of neutral electron demand.17 N N N N R R N N N N R 12 11 10 The di¶culty in establishing a scale to define electron demand is illustrated by the results of a study of the cycloaddition of tetradehydrodianthracene 10 with its relatively high C–– C HOMO to the electron-deficient diene 11 (R\H Me CO 2 Me CF 3 ).The rate of this reaction decreases as R becomes more electron-withdrawing which is the opposite of the change expected for a reaction with inverse electron demand. It has been proposed instead that the rates for these reactions are sensitive to the steric bulk of R. This conclusion is supported by the results of PM3 calculations and by the observation of the expected inverse electron demand trend when the substituentsR separated from the reaction center in 12 (R\MeO H Cl).18 The question of how to distinguish ‘true’ concerted pericyclic and stepwise reaction mechanisms continues to tax the ingenuity of chemists particularly when the reactions involve novel reactants.Arenediazonium ions 13 with electron-withdrawing substituents at the phenyl ring undergo a [4]2] cycloaddition reaction with certain dienes such as (E)-penta-1,3-diene to form 14 and 15 and the aromatised analogs of these compounds (Scheme 5). A stepwise mechanism has been considered for this reaction involving initial electrophilic addition of the terminal nitrogen to the diene followed by cyclisation because of the observation that dienes such as 4-methylpenta-1,3-diene yield azo coupling products such as 16 derived from the intermediate allylic cation 17. Aconcerted mechanism is favoured for the formal [4]2] addition reaction because of 276 I.D.Cunningham N N Ar N N Ar N N Ar N N Ar OMe Ar N N MeOH 13 + 16 17 + 15 14 Scheme 5 O O Me Me R R Me R R Me CO2 18 + D Scheme 6 the failure to observe the products of trapping of an allylic cation intermediate by nucleophiles and because of the low reactivity of the Z isomer of penta-1,3-diene. The preference for formation of the regioisomer 14 as the product of reaction of highly electron-deficient 13 is attributed to the stabilization of a transition state with some cationic character at C4 (C6 of product).19 The concept of asynchronicity in pericyclic reactions is well established and there have been several studies of the borderline between concerted ansynchronous and stepwise reaction mechanisms. The thermal decarboxylation of b-lactone (oxetan-2- one) 18 is formally a [2]2] cycloreversion reaction (Scheme 6).The results of ab initio (RHF and MP2) calculations predict a concerted reaction mechanism through a highly asynchronous planar transition state with considerable biradical character. There is good agreement between the values of E!#5 determined by experiment and calculation for unsubstituted 18 (R\H) and fluoro-substituted 18 (R\F) which is 725-fold less reactive than the unsubstituted compound. The large 18 kJ mol~1 change in DG8 observed for a change in solvent from cyclohexane to acetonitrile is consistent with a change to a strongly zwitterionic transition state for a stepwise reaction mechanism in solution. The authors conclude that the high asynchronicity for the gas phase reaction provides a mechanism to circumvent the Woodward–Ho§man rules which forbid the ‘planar’ [p24 ]p24]-like transition state and they invoke a spectrum of transition states with structures that are strongly influenced by solvent.20 The authors of a second theoretical study prefer a concerted reaction mechanism through a polar asynchronous transition state for the gas phase reaction whose geometry corresponds to that expected for an asynchronous [p24 ]p24] reaction mechanism.Although this reaction is formally forbidden for 4-p electron systems the authors suggest that it can be explained using frontier molecular orbital theory.21 277 Reaction mechanisms Part (ii) Pericyclic reactions O N (CH2) n Ph Ph NO (CH2) n 20 + 19 k2 k–1 Scheme 7 Solvent e§ects The relative rates of cycloaddition of polyhalogenated cyclopentadienes to 4-phenyl- 1,2,4-triazoline-3,5-dione in solution were found to show a good correlation with the solvent parameters AN (acceptor number) DN (donor number) and SB (solvent basicity) but not with E5.The small e§ect of changing solvent on the reaction rate and the tendency of reactivity to increase with increasing AN are characteristic of a pericyclic cycloaddition reaction mechanism.22 Pericyclic reactions have been described as the ‘showpiece’ in the study of organic transformations in aqueous media.23 In an attempt to separate the contributions of hydrophobic and hydrogen bonding interactions to the large rate accelerations observed for pericyclic reactions in water Engberts has studied the cycloreversion of the naphthoquinone–cyclopentadiene adduct and has argued that the small value of DV8 observed for this reaction requires a reactant-like transition state and a minimal hydrophobic e§ect on the rate of the reaction.The observed 13 kJ mol~1 decrease in DG8 on going from hexane to water was therefore taken as a measure of the stabilization of this transition state by hydrogen bonding and it was concluded that the same stabilization will be observed in the reverse cycloaddition reaction. Application of this type of analysis to a related Diels–Alder reaction where both hydrophobic and hydrogen bonding interactions contribute to the rate acceleration in water allows calculation of a value of ca. 8 kJ mol~1 for transition state stabilization by a hydrophobic e§ect.24 A rather unusual rate-retarding e§ect of water has been observed for the heteroretro- Diels–Alder reaction of 19 (n\1).This e§ect has been quantified for the reaction of 20 (n\2 Scheme 7) by estimating the value of k ~1 for its cycloreversion reaction from the experimental values for k 2 and the overall equilibrium constant K\k ~1 /k 2 . The decrease in k ~1 observed for the reaction in water compared to organic solvents is attributed to the decrease in the hydrogen bonding interactions to 19 proposed for the change from reactant to transition state. This transition state is reactant-like but shows some resemblance to nitrosobenzene which an infrared spectroscopy study shows to form weaker hydrogen bonds than 19.23 The possibility that Diels–Alder reactions in supercritical CO 2 proceed with unusual regio- or stereoselectivity has attracted attention in recent years.However a report published in 1997 casts doubt on earlier findings of a reversal in the selectivity for the cycloaddition reaction of isoprene with methyl acrylate near the critical point of CO 2 . The authors suggest that incomplete sampling or an unknown complex phase behavior may have resulted in erroneous experimental results.25 Catalysis There were many reports in 1997 of Lewis acid-catalysed Diels–Alder reactions which 278 I.D. Cunningham Al O O Cl O MeO O MeO OH HO Ph Ph 21 22 Mg2+ O O O O MeO MeO OMe OMe H Ar H Ar OH OH HO HO 23 24 Ar = 4-chlorophenyl involve the lowering of theLUMOof the dieneophile. An interesting example involves autoinduction in the cycloaddition of methyl acrylate to cyclopentadiene in the presence of (S)-VAPOL 21 and Et 2 AlCl. It is proposed that the enantioselectivity for formation of the product complex (e.g.22) is higher than when the reaction proceeds in the presence of (S)-VAPOL alone. The further increase in enantioselectivity to[99% observed upon addition of bulky achiral ligands such as di-tert-butyl 2,2-dimethylmalonate suggests that the enhancement of enantioselectivity (e.g. in 22) is due mainly to an increase in the steric bulk around the chiral catalyst.26 Catalysis of Diels–Alder reactions by solid materials is of current interest. Convincing evidence has been obtained that catalysis and stereoselectivity in the cycloaddition reaction of acrolein and cyclohexa-1,3-diene occurs within the cavities of solid 23. Non-catalytic cycloaddition of methyl acrylate to cyclohexa-1,3-diene is also facilitated by solid 23 and an X-ray crystallographic analysis has identified the presence of reactants within the cavities of this solid.The enhanced reactivity of these bound substrates was attributed to a proximity e§ect even though it was noted that the reagents embedded in the solid are not aligned towards the classical Diels–Alder transition state. The lack of catalysis of the reaction of methyl acrylate compared to acrolein was attributed to the stronger binding of the cycloadduct of the former within the cavities of 23.27 Studies of the rate enhancement of cycloaddition reactions caused by alkali and alkali earth metal ions continue. Evidence has been presented in a kinetic study of the MgClO 4 -catalysed Diels–Alder reaction of cyclopentadiene and 2-(4-chlorobenzylidene) malonic acid dimethyl ester that the rate-limiting step for the reaction involves addition of the diene to a dienophile–Mg2` complex 24.The value of DS8[[167 J mol~1K~1 is typical of an uncatalysed Diels–Alder reaction and it was therefore suggested that the enhanced reactivity of 24 is due mainly to a tendency of the metal ion to lower DH8.28 Cumulenes The question of whether cycloadditions of allenes to alkenes proceed by biradical or concerted pericyclic [2]2] reaction mechanisms continues to be debated. The ob- 279 Reaction mechanisms Part (ii) Pericyclic reactions O N O H H R NR O Ph D H H NR O • Ph D 26 + 25 LUMOC1'-C2' 2' 1' 4 LUMO HOMOC4-C1' O O Scheme 8 served stereoselectivity for the cycloaddition reaction of 25 with a range of alkenes and alkynes provides evidence for the concerted reaction mechanism (Scheme 8).Two frontier molecular orbital-based schemes of which 26 appears to be the more credible are discussed and the observed high reactivity of 25 for cycloaddition is attributed to a lowering of the energy of the p* C1{–C2{ LUMOby an interaction with the rather distant p* N–R (R\-SO 2 Tol or -COPh) orbital. The e§ect of the increase in the energy of the pC4–C1{ HOMO by the adjacent nitrogen does not seem to have been considered.29 1,3-Dipolar cycloaddition reactions Reactive ylides 27 (RA\H Ph) have been generated by laser flash photolysis reactions of 28 in acetonitrile and values of 106–1010dm3 mol~1 s~1 have been determined for the second-order rate constants for the reaction of 27 with dipolarophiles 29 (R R@\H CO 2 Me CN Scheme 9). The values of log k for the reaction of 27 (RA\H) with acrylonitrile show a linear dependence on both the values of p0 for the aryl ring substituents and of *E(LUMO$*10-!301)*-% [HOMO:-*$%) determined by AM1 calculations.30 Asymmetric cycloaddition reactions Asymmetric cycloaddition reactions have a profound significance in synthetic organic chemistry which has led inevitably to an interest in their mechanism. The mechanism for the Sharpless alkene asymmetric dihydroxylation reaction has long been the subject of contentious debate with the controversy centering around whether the key-step for the reaction is a [3]2] or a [2]2] cycloaddition reaction (Scheme 10). A comparison of the 13C and the 2H kinetic isotope e§ects determined by experiment with the calculated isotope e§ects for [3]2] and [2]2] reactions favours the [3]2] mechanism via a symmetrical transition state.This is o§ered to counter the recent evidence supporting the [2]2] reaction mechanism which is also discussed in this paper.31 280 I.D. Cunningham N Ar R¢¢ H N R R R' R' R'' H Ar R2C CR'2 Ar C N C H R'' 27 + – 29 hn 28 Scheme 9 Os O O O O L Os O O O L O Os O O O L L = amine ligand + [2 + 2] [3 + 2] O Scheme 10 The results of mixed density function–molecular mechanics calculations also favour a [3]2] cycloaddition reaction. However it is proposed that the dihydroxylation reaction of styrene proceeds with the initial formation of a complex that is stabilised by a p–p interaction between the ligand L and substrate.32 The reactive species in the Ti–TADDOLate-catalysed (TADDOL\a,a,a@,a@-tetraaryl- 1,3-dioxolane-4,5-dimethanol) asymmetric cycloaddition reaction has been the subject of speculation.Jorgenson has modelled the cycloaddition reaction of 3-[(E)- but-2@-enoyl]-1,3-oxazolidin-2-one to benzylidinephenylamine N-oxide (R\Pr*O) and proposed the structure 30 for the transition state for an exo-approach of reactants. The decrease in exo selectivity with increasing bulk of the ligand X (Cl Br OTf) is cited as evidence that these cycloaddition reactions are subject to axial ligand control.33 Cl Cl Cl Cl L S M Ti O O N O R R R X C H N Ph Ph O steric interaction 30 31 Cl Cl The ‘Inside-Alkoxy’ model for stereoselectivity has been proposed to hold for the inverse electron demand cycloaddition of alkenes to hexachlorocyclopentadiene. The diastereoselectivities observed by experiment and from the results of ab initio calculations are modelled by the transition state 31 where the largest group L is anti to the incoming diene and the medium-sized alkoxy groupMis positioned ‘inside’ in order to minimise the electrostatic and steric repulsion with the chlorine atoms of hexachlorocyclopentadiene.34 281 Reaction mechanisms Part (ii) Pericyclic reactions N O R R' N O R R' N C O R' R 2 32 R = H Me CF3 R' = H Me CF3 F H Scheme 11 3 Electrocyclic reactions Spin-coupled theory has been shown to be a useful computational approach to model electrocyclic reactions. This form of modern valence bond theory yields orbitals which are highly localised and therefore more compatible with the traditional view of organic reactivity. Application of spin-coupled theory to model the ring-opening reaction of cyclobutene to form butadiene results in the prediction that most of the changes in the geometry observed along the intrinsic reaction coordinate occur after the reaction transition state and that this trend is most pronounced for the high-energy disrotatory process.35 The mechanism for ring-opening of the cyclobutene radical cation has received relatively little experimental attention.Wiest has reported a theoretical study using UHF MP2 QCISD(T)/QCISD and DFT methods. The results of these calculations mostly favour a concerted reaction mechanism with the lone electron localised on one methylene group and the positive charge distributed across the other three carbon atoms. Steric factors are invoked to rationalise the calculated preference for conrotatory ring opening.36 The above report highlights concerns that the observation of a stereoselective reaction may not be a suitable criterion for a concerted pericyclic reaction mechanism in cases where steric e§ects favour a stereoselective stepwise reaction.Dolbier et al. have examined the cyclisation reaction of isocyanate 32 (Scheme 11) in an attempt to determine the significance of torquoselectivity in a 6p system where the steric e§ects on product formation are minimal (Scheme 11). Values of E!#5 and DS8 are typically 105 kJ mol~1 and [54 J mol~1K~1 respectively and are consistent with a 6p pericyclic electrocyclisation reaction mechanism. However the results of calculations at the MP2/6-31G*//RHF/6-31G* level predict a transition state geometry in which the terminal p-orbital on C2 is twisted and oriented for overlap with a C–– Orather than an N––C p-orbital and the overall movement of electrons corresponds to a pseudopericyclic process.However the variation in the relative rate constants for the cyclisation of the Z and E isomers of 32 is still interpreted within the framework of the torquoselectivity for formation of a product-like transition state.37 The cyclisation of 33 has been studied in superacidic media (Scheme 12). The linear dependence of the rate constant on the acidity function H 0 and the lack of a plateau observed at values ofH 0 above the pK! for monoprotonated 33 (ca.[6) indicates that the reaction proceeds via the O,O-diprotonated intermediate 34. The results of 282 I.D. Cunningham O R¢ R O R¢ R R R¢ O H H + + 34 33 R = H CF3 Me R' = H Ph 4-CF3Ph 4-MePh Me Et Scheme 12 HB HA HAHB 35 repulsion rotation 1 2 Scheme 13 deuterium exchange studies exclude a Friedel–Crafts-type reaction of the protonated alkene and the relatively small e§ect of substituents R@ on the rate of the reaction is consistent with a 4p electrocyclisation reaction.The phenylallyl carbocation that is generated by the initial protonation of the carbonyl group is apparently unreactive towards electrocyclization and additional activation by a second protonation of the substrate to give a strongly activating –H 2 O` substituent is required for observation of a cyclisation reaction.38 An ab initio study of substituent e§ects on a prototype 4p pentadienyl cation electrocyclisation reaction has appeared.39 4 Sigmatropic rearrangements [1,n] Shifts Several computational studies have been reported which attempt to provide a rationalisation for novel experimental findings.The rearrangement of vinylpropene to cyclopentene (Scheme 13) shows some characteristics of both stepwise and concerted reaction mechanisms and the formation of products of all possible reaction modes si (suprafacial–inversion) ar (antarafacial–retention) ai and sr is observed with the si product predominating as expected for a Woodward–Ho§man allowed [p24 ]r2!] process. There is an additional ring opening–closing reaction which results in the geometric isomerisation of the ring. DFT calculations for reaction by the preferred si mode show a concerted mechanism for the rearrangement reaction but with an extensive plateau region running through the diradical-like transition state 35 and with the C2 methylene group twisted towards the final inverted configuration.The energies of the transition states leading to the other reaction products are within 3–7 kJ mol~1 of the transition state for the si reaction. The twisting of the C2 methyl- 283 Reaction mechanisms Part (ii) Pericyclic reactions H Ar N O O R N R O O Ar 36 E + Scheme 14 CO2Me CO2Me CO2Me AlCl3 38 37 + + Scheme 15 ene group is attributed to a repulsive interaction between the C––C p-orbital and the r-orbital of the cleaving C–C bond.40 Similar results are obtained for a CASSCF calculation and a transition state for the ring isomerisation reaction is also reported.41 Not all experimental findings are so easily rationalised by theory. Several computational studies on [1,3] migrations of Si in allylsilanes and siloxyacetylenes favour a sr reaction mode in apparent contradiction to experimental results.42 An experimental study of a [1,5]-intermolecular migration between maleimides and arylpropenes (an ene reaction) has appeared (Scheme 14).The concerted pericyclic reaction mechanism is supported by the observation of a very small aryl substituent e§ect on reactivity. The exclusive formation of the E product 36 is proposed to reflect a reaction through an endo transition state which is stabilised by a secondary orbital interaction between the ene HOMO and the enophile LUMO.43 Jenner has reported that the AlCl 3 - or ZrCl 4 -catalysed reaction of cyclohexene with methyl propiolate yields 37 from a [2]2] cycloaddition reaction and the product 38 of an ene reaction (Scheme 15).The chemoselectivity is constant with changing pressure and this is consistent with similar values of DS8 for both reaction pathways.44 Houk et al. have published the results of a computational study of the ene reaction of cyclopropene with ethene propene and cyclopropene using ab initio (RHF and MP) and DFT methods including CASSCF. The endo transition state for the dimerisation of cyclopropene by an ene reaction is predicted to be stabilised by the secondary orbital interaction between C–H and C––C. In some cases a degree of diradical character is suggested for this transition state and the reaction pathway has been illustrated by use of a More O’Ferrall–Jencks diagram.45 The temperature dependent broadening of the NMR peaks from the spectrum of 7-phenylsulfanylcyclohepta-1,3,5-triene has been attributed to migration of the PhS group by a series of [1,7] shifts.46 [n,n] Shifts The allyl thiocyanate to allyl isothiocyanate rearrangement follows clean first-order kinetics and there is no rate acceleration observed upon addition of thiocyanate ion.These results are consistent with a pericyclic [3,3] reaction mechanism. The results of 284 I.D. Cunningham N NH+ + 39 N N N N 40 –H+ [3,3] Scheme 16 ab initio calculations are in agreement with this conclusion and predict that there is some separation of charge in the reaction transition state. This conclusion cannot simply account for the small rate retardation observed for reactions in solvents of increasing polarity and these solvent e§ects are rationalised in terms of opposing e§ects of solvent polarity and hydrogen bonding on the reaction rate.47 The interconversion of the products 39 and 40 of the cycloaddition reaction shown in Scheme 16 has been shown to occur by a [3,3] rearrangement rather than by cycloreversion.It is not clear whether this rearrangement proceeds in one or two steps.48 The benzidine rearrangement reaction of 42 is proposed to proceed by a [9,9] sigmatropic shift through the transition state 41. The reaction is second-order in the concentration of acid with an observed third-order rate constant of 0.13dm6 mol~2 s~1. Only indirect evidence is cited for the proposed concerted pericyclic reaction mechanism and the authors admit that they cannot rule out the possibility that the reaction occurs by two consecutive [5,5] shifts. However the observation that the well documented [5,5] shift observed for the reaction of the less hindered hydrazobenzene is slower (0.024dm6 mol~2 s~1) than the reaction of 42 and the failure to observe any other products of rearrangement reactions is consistent with a reaction by a [9,9] sigmatropic shift.49 A study of the kinetics of the O toNallyl migration of 1-aryl-5-allyloxytetrazoles 43 inDMSOand in 1,1,2,2-tetrachloroethane has been reported (Scheme 17).A concerted mechanism for this reaction has been proposed on the basis of the lack of evidence (CINDP EPR) for a biradical intermediate. While the values of E!#5 are roughly independent of the aryl ring substituent and the alkene substituent R the values of *H8 and *S8 show significant compensating variations. These variations are consistent with a dipolar transition state in which there is a partial positive charge on the allyl fragment of substrate and a partial negative charge on the tetrazole fragment.However there is no evidence for the formation of ions in a polar LiClO 4 –ether medium.50 The variability of substituent e§ects on Claisen and related [3,3] rearrangements is made apparent in the above paper. Houk has formulated a theory for the e§ect of substitution of O-donor groups based upon RHF DFT and CASSCF calculations. A 285 Reaction mechanisms Part (ii) Pericyclic reactions O N N O H H N O N O HH HH N N N N Ar O R N N N N Ar R [3,3] 43 41 2+ 42 O Scheme 17 Ar Ar Ar Ar Ar Ar 45 46 44 + • Scheme 18 modification of Marcus theory has been used to model the relative contribution of the intrinsic reaction barrier and the thermodynamic driving force to the observed activation barrier.The results are interpreted using frontier molecular orbital theory and the reactant transition state and product orbitals obtained from RHF/3-61G* calculations. 51 The di¶culties in distinguishing stepwise and concerted reaction mechanisms are readily apparent for the rearrangement reactions of radical cations. However this question appears to have been resolved for the photo-induced electron-transfer (PET) Cope rearrangement of 44 to form 45 (Ar\4-MeOC 6 H 4 -) where the radical cations 44·` and 45·` and a distonic reaction intermediate 46·` have been detected following pulse radiolysis of the substrate (Scheme 18). It is not yet clear whether this stepwise mechanism is strongly favoured by the methoxyphenyl group.52 [n,m] Shifts where nDm Most of the literature reports of these rearrangement reactions concern their use in chemical syntheses.However a novel [3,4] shift has been described for the conversion of allene oxides such as 47 to the ketone 48 (Scheme 19). The proposed shift in the 6p 286 I.D. Cunningham CHO O O– CHO O CHO 47 + [3,4] 49 48 Scheme 19 O O C O O O O C O O CO 51 50 + D Scheme 20 electron system 49 is allowed by the Woodward–Ho§man rules but it has not yet been demonstrated whether this reaction proceeds by a ‘true’ pericyclic reaction mechanism. 53 5 Miscellaneous Birney et al. have published the results of an ab initio computational study of thermal chelotropic decarbonylation reaction mechanisms which have been described as pseudopericyclic.These calculations predict a transition state 51 for decarbonylation of 50 which is consistent with the ‘in-plane’ departure of CO and the orbital topology shown in Scheme 20 in which the lack of formal cyclic orbital overlap characterises the reaction as pseudopericyclic.54 References 1 O. Wiest D. C. Montiel and K. N. Houk J. Phys. Chem. 1997 101 8378. 2 H.-W. Fru� hauf Chem. Rev. 1997 97 523. 3 M. Schmittel C. Wo� hrle and I. Bohn Acta Chem. Scand. 1997 51 151. 4 N.T. Anh and F. Maurel New J. Chem. 1997 21 861. 5 J. J. Gajewski Acc. Chem. Res. 1997 30 219. 6 Advances in theoretical methods are covered elsewhere in Annual Reports but a good example is by M. K. Diedrich F.-G. Kla� rner B. R. Beno K. N. Houk H. Senderowitz and W. C. Still J. Am. Chem. Soc. 1997 119 10 255.7 J. Motoyoshiya T. Kameda M. Asari M. Miyamoto S. Narita H. Aoyama and S. Hayashi J. Chem. Soc. Perkin Trans. 2 1997 1845. 8 M. Sodupe R. Rios V. Branchadell T. Nicholas A. Oliva and J. J. Dannenberg J. Am. Chem. Soc. 1997 119 4232. 287 Reaction mechanisms Part (ii) Pericyclic reactions 9 B. S. Jursic J. Org. Chem. 1997 62 3046. 10 R. Konecny and D. J. Doren J. Am. Chem. Soc. 1997 119 11 098. 11 A. V. Teplyakov M.J. Kong and S. F. Bent J. Am. Chem. Soc. 1997 119 11 100. 12 S.-W. Leung and D. A. Singleton J. Org. Chem. 1997 62 1955. 13 J.-C. Halle� D. Vichard M.-J. Pouet and F. Terrier J. Org. Chem. 1997 62 7178. 14 S. Pugnaud D. Masure J.-C. Halle� and P. Chaquin J. Org. Chem. 1997 62 8687. 15 Y. Tamaru H. Harayama H. Sakata H. Konishi K. Fugami M. Kimura S. Tanaka T. Okajima and Y.Fukazawa Liebigs Ann./Recuil 1997 907. 16 R. N. Butler E. C. McKenna and D. C. Grogan Chem. Commun. 1997 2149. 17 E. Eibler P. Ho� cht B. Prantl H. Roßmaier H. M. Schuhbauer H. Wiest and J. Sauer Liebigs Ann./Recuil 1997 2471. 18 J. Sauer J. Breu U. Holland R. Herges H. Neumann and S. Kammermeier,Liebigs Ann./Recuil 1997 1473. 19 M. Hartnagel K. Grimm and H. Mayr Liebigs Ann./Recuil 1997 71. 20 R. Ocampo W.R. Dolbier Jr. M. D. Bartberger and R. Parades J. Org. Chem. 1997 62 109. 21 I. Morao B. Lecea A. Arrieta and F. P. Cossý� o J. Am. Chem. Soc. 1997 119 816. 22 J. Sauer and H. M. Schuhbauer Liebigs Ann./Recuil 1997 1739. 23 J. W. Wijnen and J. B. F. N. Engberts Liebigs Ann./Recuil 1997 1085. 24 J. W. Wijnen and J. B. F. N. Engberts J. Org. Chem. 1997 62 2039. 25 A. R.Renslo R. D. Weinstein J. W. Tester and R. L. Danheiser J. Org. Chem. 1997 62 4530. 26 D. P. Heller D. R. Goldberg and W. D. Wul§ J. Am. Chem. Soc. 1997 119 10 551. 27 K. Endo T. Koike T. Sawaki O. Hayashida H. Masuda and Y. Aoyama J. Am. Chem. Soc. 1997 119 4117. 28 G. Desimoni G. Faita M. Mella M. Ricci and P.-P. Righetti Tetrahedron 1997 53 13 495. 29 M. Kimura Y. Horino Y. Wakamiya T. Okajima and Y. Tamaru J. Am. Chem. Soc. 1997 119 10 869. 30 E. Albrecht J. Mattay and S. Steenken J. Am. Chem. Soc. 1997 119 11 605. 31 A. J. DelMonte J. Haller K. N. Houk K. B. Sharpless D. A. Singleton T. Strassner and A. A. Thomas J. Am. Chem. Soc. 1997 119 9907. 32 G. Ujaque F. Maseras and A. Lledo� s J. Org. Chem. 1997 62 7892. 33 K. V. Gothelf and K. A. Jørgensen J. Chem. Soc. Perkin Trans.2 1997 111. 34 J. Haller S. Niwayama H.-Y. Duh and K. N. Houk J. Org. Chem. 1997 62 5728. 35 J. M. Olivia J. M. Gerratt P. B. Karadakov and D. L. Cooper J. Chem. Phys. 1997 107 8917. 36 O. Wiest J. Am. Chem. Soc. 1997 119 5713. 37 L. Luo M.D. Bartberger and W. R. Dolbier Jr. J. Am. Chem. Soc. 1997 119 12 366. 38 T. Suzuki T. Ohwada and K. Shudo J. Am. Chem. Soc. 1997 119 6774. 39 D. A. Smith and C. W. Ulmer II J. Org. Chem. 1997 62 5110. 40 K. N. Houk M. Nendal O. Wiest and J. W. Storer J. Am. Chem. Soc. 1997 119 10 545. 41 E. R. Davidson and J. J. Gajewski J. Am. Chem. Soc. 1997 119 10 543. 42 T. Yamabe K. Nakamura Y. Shiota K. Yoshizawa S. Kawauchi and M. Ishikawa J. Am. Chem. Soc. 1997 119 807; M. Takahashi and M. Kira J. Am. Chem. Soc. 1997 119 1948; M. Oblin F. Fotiadu M. Rajzmann and J.-M.Pons J. Chem. Soc. Perkin Trans. 2 1997 1621. 43 I. D. Cunningham A. Brownhill I. Hamerton and B. J. Howlin Tetrahedron 1997 53 13 473. 44 G. Jenner New J. Chem. 1997 21 1085. 45 Q. Deng B. E. Thomas IV K. N. Houk and P. Dowd J. Am. Chem. Soc. 1997 119 6902. 46 G. A. Dushenko I. E. Mikhailove A. Zschunke N. Hakam C. Mu� gge and V. I. Minkin Mendeleev. Commun. 1997 50. 47 M. Kotani Y. Shigetomi M. Imada M. O3 ki and M. Nagaoka Heteroat. Chem. 1997 8 35. 48 K. Beck P. Ho§man and S. Hu� nig Chem. Eur. J. 1997 3 1588. 49 K. H. Park and J. S. Kang J. Org. Chem. 1997 61 3794. 50 M.L. S. Cristiano and R. A. W. Johnstone J. Chem. Soc. Perkin Trans. 2 1997 489. 51 H. Y. Yoo and K. N. Houk J. Am. Chem. Soc. 1977 119 2877. 52 H. Ikeda A. Ishida T. Takasaki S. Tojo S. Takamuka and T. Miyashi J. Chem. Soc. Perkin Trans. 2 1997 849. 53 I. Erden F.-P. Xu and W.-G. Cao Angew. Chem. Int. Ed. Engl. 1997 36 1516. 54 D.M. Birney S. Ham and G. R. Unruh J. Am. Chem. Soc. 1997 119 4509; see also J. A. Ross R. P. Seiders and D. M. Lemal J. Am. Chem. Soc. 1976 98 4325 for a definition of ‘pseudopericyclic’.

ISSN:0069-3030    

DOI:10.1039/oc094273

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

9 Reaction mechanisms Part (iii) Bioorganic enzyme-catalysed reactions By NIGEL G. J. RICHARDS Department of Chemistry University of Florida Gainesville FL 32611 USA 1 Introduction The advent of powerful biophysical methods for macromolecular structure determination 1,2 access to large amounts of pure enzymes using the techniques of molecular biology and significant advances in computational approaches for modeling reactions 3,4 preclude a comprehensive description of recent developments in enzymology and biochemical catalysis.5 This report seeks to discuss enzymes involved in both primary and secondary metabolism that (i) have not been the subject of recent detailed reviews and (ii) employ catalytic mechanisms which are not well precedented by the chemical behavior of model compounds.As a result advances in our understanding of proteases,6 glycosidases,7 nucleoside hydrolases8 and enzymes involved in the manipulation of nucleic acids,9,10 and in intracellular signalling11–13 are not addressed. Recent work on the structural and mechanistic basis of abzyme14 and ribozyme15 catalysis is also omitted fromthis review althoughstudies on both of these classes of catalyst will be discussed in a subsequent report. 2 Fundamental ideas in enzyme catalysis Despite the rapid pace of enzyme structure determination using X-ray crystallography and NMR-based methods the fundamental molecular basis of enzyme catalysis continues to be debated. The success of rational methods for discovering potent enzyme inhibitors,16 coupled with evidence from site-directed mutagenesis has supported the hypothesis that transition state stabilization rather than substrate binding is critical to the rate enhancements observed for many enzyme-catalyzed reactions.17,18Arecent discussion focussing upon detailed analysis of the reaction coordinate however points out that the ground-state of the enzyme–substrate (ES) complex is related to the transition state through the mean force acting along the reaction path.19 The consequence of such an idea is that catalytic strategies cannot be cleanly analyzed in terms of transition state stabilization meaning that attention must focus on the dynamical molecular processes that concentrate energy into the reaction coordinate. In this model the ability of an enzyme to accelerate a chemical reaction is related solely to its ability to lower the work required for the reacting systemto pass 289 over the energy barrier separating reacting species from products.Hence the barrier to reaction is removed in the ES complex by eliminating slow dynamic events such as the solvent reorganization that must occur in the non-catalyzed transformation. In e§ect substrate binding energy is employed to preorganize the reactant(s) so that the mean restoring force along the reaction coordinate is decreased lowering the free energy of activation. Such a model raises questions concerning the magnitude of contributions to catalysis arising from reactant orientation in the ES complex. Such ideas have been formulated in the ‘orbital steering’ hypothesis in which the energy of the transition state is a function of the orbital overlap between substrates.20 An implicit assumption in this model is that the entropic component of the activation energy can be approximated by [(1/kT)ln(v0/v8) where v0 and v8 are the configurational volumes of the bound substrate and transition state respectively and k is the Boltzmann constant.21 As the enzyme channels energy along the reaction coordinate into the transition state v8 is small thereby giving rise in this model to large estimates of the entropy of activation.On the other hand there are several assumptions in estimating the configurational volumes v0 (especially if the protein is very flexible) and it can been argued that even if v8 is small the entropy of activation need not necessarily be large.21 While there are significant technical problems in using theoretical methods such as molecular dynamics (MD) simulations,22,23 in carefully investigating the energetic contributions of orbital steering to enzyme catalysis high-resolution protein crystal structures may allow insight into the importance of orbital steering in enzyme catalysis.An interesting example of this experimental approach has been provided in recent studies employing isocitrate dehydrogenase (IDH).24 Using cryocrystallographic methods the structures N N N N R O O OH O O– P O O– P O O O O +N HO OH NH2 O O P O– O– 1 R = NH2 2 R = OH of the Michaelis complexes of IDH bound to (i) isocitrate,NADP` 1 and Ca2` and (ii) isocitrate Mg2` and the modified substrate 2 were determined and compared to that of the Y160F IDH mutant complexed with isocitrate 1 and Mg2`.25 The latter reference structure was obtained previously using time-resolved X-ray crystallography.26 Although the observed changes in the orientation of the substrate analogs in the IDH active site were small the rates of reaction were decreased by 3–5 orders of magnitude. These results suggest that deviations from the optimal overlap of molecular orbitals in enzyme-bound reactants may play a significant role in the energetics of catalysis. While the experimental error in the structure determinations complicates interpretation of the significance of the small changes in substrate orientation these results provide a useful starting point for more detailed theoretical studies upon this interesting hypothesis. 290 N.G. J. Richards Fig. 1 Moving to a more traditional (albeit controversial) vision of enzyme catalysis experimental and theoretical investigations into the importance of very strong (‘lowbarrier’) 27 hydrogen bonds (LBHBs) in transition state stabilization continue to be reported (Fig.1).28 These studies are motivated by the hypothesis that the hydrogen bond formed between two groups with similar pK! values acquires partial covalent character giving rise to (i) unusual spectroscopic properties and (ii) an additional energetic contribution that may be employed in lowering the barrier to reaction. At issue given compelling evidence for the existence of LBHBs in the gas phase,29 is whether such bonds can be formed in active site environments. Although experimental evidence for the existence of LBHBs in enzymes has been reported,30,31 theoretical objections have been raised which suggest LBHBs should be ‘anticatalytic’.32 Studies to resolve this issue have been reported in which the energetics of intramolecular hydrogen bonds have been measured as a function of the pK! di§erence (*pK!) between the donor and acceptor groups.33,34 The e§ects of environment were modeled by comparing the behavior of these model systems in water and DMSO.The conclusions of this work were that there is no additional stabilization of the hydrogen bond when *pK! \0 over that expected from electrostatic interactions arising from the asymmetric distribution of charge. Clearly the magnitude of this energetic contribution is higher in a medium of lower relative permittivity and so it can be argued that stabilization of the transition state by hydrogen bonding will be larger within the enzyme active site than in water.The contribution of the hydrogen bond to catalysis will therefore appear to be greater than for the non-catalyzed reference reaction. LBHB e§ects which imply a high degree of covalency may not be strictly necessary to rationalize enzyme catalysis for reactions in which there is significant rearrangement of charge. Evidence against the existence of LBHBs in active sites at least as defined in current models has been claimed in recent NMR experiments involving a-lytic protease which possesses a Ser-His-Asp catalytic triad,35 and the model compound cis-urocanic acid 3c.36 It seems generally agreed that the least ambiguous evidence for a LBHB is the observation of the proton chemical shift at extremely low field (d[16). Such a low-field resonance is observed for the strongly hydrogen bonded proton that is present in the His-Asp diad of serine proteases.37 Predictions of the LBHB model are (i) that this proton is shared equally between the histidine and aspartic acid residues and (ii) that this interaction is shielded from water a hydrogen bonding solvent.The absence of a similar low-field NMR resonance associated with the intramolecularly hydrogen bonded proton when 3c is placed in an aqueous environment has been cited as evidence for the latter restriction. Using 15N-labeled histidine it was shown that the proton in the His-Asp hydrogen bond present in a-lytic protease is associated primarily with the nitrogen atom and exchanges with deuterium when the enzyme is dissolved in D 2 O even in the presence of a transition-state analog inhibitor!36 The 291 Reaction mechanisms Part (iii) Bioorganic enzyme-catalysed reactions O H -O O Tyr14 H O O H Asp99 CO2H Asp38 O H –O O O Asp99 CO2H Asp38 H O H Tyr14 (A) (B) 4 4 Fig.2 N N H HO2C N N H HO2C 3 c 3 t absence of the expected low-field resonance in the 1H NMR spectrum of 3c dissolved in water appears to be due to rapid exchange with solvent protons at 25 °C. Cooling a solution of 3c in 85 15 [2H 6 ]acetone–H 2 O gave the anticipated signal at 18.5 ppm.36 On the basis of these observations it was suggested that the fundamental properties of LBHBs should be re-assessed. On the other hand a detailed investigation of the hydrogen bonding at the active site of *5-3-keto steroid isomerase can be interpreted in favor of a catalytic role for a LBHB.31 The principal mechanistic issue is the molecular basis for stabilization of the dienolate intermediate 4 (Fig.2). In one model 4 is stabilized by two hydrogen bonds of normal strength in a similar manner to that proposed for residues in the oxyanion hole of serine proteases (Fig. 2A).37 The alternative proposal derived fromUVresonance Raman measurements that indicated that the Tyr-14 both accepted and donated hydrogen bonds,38 suggests that Asp-99 hydrogen bonds to Tyr-14 thereby polarizing the O–H bond and enhancing the strength of its interaction with the dienolate intermediate 4 (Fig. 2B). Evidence consistent with the hypothesis that the Asp-99–Tyr-14 interaction is a LBHB is 292 N.G. J. Richards H H O N R1 R H O Ser N N His H H O –O Asp N N Oxyanion Hole Fig. 3 provided by the appearance of two deshielded proton resonances at 18.15 and 11.60ppm on mixing the enzyme with analogs of 4 such as dihydroquilenin 5 (DHE).39 Detailed NMR studies showed that while mutation of Asp-99 to alanine (D99A) removes the signal at 18.15ppm in the DHE–D99A complex both of the low-field resonances disappear in isomerase mutants lacking Tyr-14.31NOEmeasurements and fractionation factors were also cited as evidence for the existence of the LBHB in the steroid isomerase–DHE complex.HO H OH 5 A key assumption of discussions concerning the origin of the catalytic properties of enzymes is that the protein framework must somehow recognize and bind to the transition state. As pointed out in an excellent review,40 however this type of molecular recognition must be ‘dynamic’ given that substantial changes in bonding are underway in the transition state giving rise to associated rapid alterations in charge distribution.For example the initial step in amide bond hydrolysis involves the creation of up to five partial bonds in the transition state being accompanied by a substantial relocation of charge (Fig. 3).41 An important feature of dynamic recognition as compared to the intermolecular interactions between molecules in their ground states (the more traditional view of molecular recognition) is that partial covalent bonding may be present. The magnitude of this energetic contribution to catalysis remains to be established for many enzyme-catalyzed reactions. One approach to studying this question is to employ chemical models in which functional groups are forced to undergo intramolecular reaction.42,43 For reactions involving acid/base catalysis (the majority of enzyme-catalyzed reactions!) the situation is less clear giving rise in part to the debate over LBHBs and catalysis.Model studies have 293 Reaction mechanisms Part (iii) Bioorganic enzyme-catalysed reactions Lys166 NH2 Lys166 +NH3 N N His297 H +N N His297 H H CO2 – +N N His297 H H H OH Lys166 +NH3 CO2 – HO H O– O– HO 6 Scheme 1 established that a high e¶ciency of catalysis can be achieved in a situation in which hydrogen-bonding can stabilize the ‘in-flight’ proton in the transition state but is absent (or very weak) in the ground state of the reactant. An ideal example of such a reaction is the enolization of carbon acids,44 such as in the transformation catalyzed by mandelate racemase for which k#!5 is 700 s~1 (Scheme 1).45 The overall rate acceleration in the enzyme-catalyzed reaction may in part be due to stabilization of the dianionic intermediate 6.On the other hand it is likely that stabilization of the ‘in-flight’ proton by hydrogen bonding to either the lysine or histidine base which cannot occur prior to cleavage of the C–H bond in mandelate represents a significant contribution to catalysis. 3 New perspectives on old co-enzymes Bioorganic chemical studies employing model compounds have been instrumental in elucidating the mechanistic roles of co-enzymes such as thiamin diphosphate (TDP),46 riboflavins,47 S-adenosylmethionine pyridoxal phosphate and tetrahydrofolate. It is therefore surprising to realize that many details of the enzyme-catalyzed reactions involved in their biosynthesis remain unknown as outlined in an excellent recent overview of current knowledge concerning these transformations.48 For example the mechanisms underlying the synthesis of the pyridoxal ring from 3-hydroxy-Lthreonine 4-phosphate 7 (Scheme 2),49 or riboflavin by the disproportionation of lumazine 8 (Scheme 3),50 a reaction that can occur non-enzymatically (!),51 appear to 294 N.G.J. Richards O OH OH OH CO2 – H3N+ O OH H P O– O O– HO OH N CO2 H O P O O– O– OH HO OH N CO2 H O P O O– O– O O OH N CO2 H O P O O– O– OH N O P O O– [O] O– – H2O – CO2 HO 7 – H2O Scheme 2 be completely speculative. The other product of lumazine disproportionation 9 is recycled to 8 by the enzyme 6,7-dimethyl-8-ribityl lumazine synthase.52 Significant progress continues to be made however in understanding the mechanistic details of several reactions leading to TDP due to the cloning and expression of many of the genes encoding the relevant enzymes.53–55 For example the enzyme encoded by the ThiE gene in Escherichia coli which synthesizes thiamin phosphate from pyrimidine and thiazole precursors has been overexpressed and used to demonstrate that the p-hydroxybenzyl group initially present in the thiazole precursor 10 is removed prior to the coupling reaction (Scheme 4).55 On the other hand the mechanism of the complex rearrangement of 5-aminoimidazoleribose 11 to give the pyrimidine moiety of TDP remains unknown,56,57 in part because over-expression of the ThiC gene product in Escherichia coli has not yet given active enzyme.A mechanism that is consistent with labeling studies has been proposed (Scheme 5),58 but exploration of this complex transformation using model compounds has not yet been reported.The mechanism by which TDP is activated for reaction by the protein also remains controversial.59,60 Deprotonation of the thiazolium ring at C-2 to form the anion 12 is a key catalytic step because the rate constant for this reaction in the free coenzyme is much smaller than that observed for transformations catalyzed by TDP-dependent enzymes.61 Several proposals have been made for the role of the protein environment 295 Reaction mechanisms Part (iii) Bioorganic enzyme-catalysed reactions N N N N R O O H N N N N HO R O O H H N N N N HO R O O H H N N N N R O O H H N HO R N N N N R O O H N N O NH2 O H H N N N N R O O H OHN R N N O NH2 O H H H N N N N R O O H OH N N N N R O O H * – H2O * * * * * * * * * * 9 6,7-Dimethyl-8-ribityl lumazine synthase 8 Scheme 3 N S O OH O– P O– O N N O NH2 O– P O O– P O– O O N S O O– P O– O N N O NH2 OP O OP OO O N+ S O OH O– P O– O N N H2N N+ S O O– P O– O ThiE low specific activity N N H2N ThiE 600 nmol min–1 mg–1 10 Scheme 4 296 N.G.J. Richards O HO HO OH N N NH2 HO OH N OH N H2N O N N +NH2 OH OH OH HO N N +NH2 OH N N OH NH2 HO H HO2C +N N H O NH2 OH HO H O HO aldose–ketose isomerization (several steps) cyclization N N OH NH2 – CO2 oxidation ring-opening ring-closure – CH2O HO– 11 Scheme 5 N N +N S O O– P O O O– P O O– NH2 12 – in activation and crystallographic studies of several TDP-dependent enzymes,62–65 in combination with studies on site-specific mutant proteins have identified an invariant glutamate residue as having a key role in catalysis.66 NMR studies on the thermodynamics and kinetics of TDP-deprotonation when the co-enzyme is bound within transketolase and pyruvate decarboxylase have been reported that address the role of this residue in thiamin activation.59 A significant conclusion of these experiments was that the pyrimidine ring of the co-enzyme participates in the deprotonation of C-2 possibly by tautomerization to the imino-substituted heterocycle 13 (Fig.4). Incubation of transketolase with the TDP-analogs 1467 and 1568 gave inactive enzyme even 297 Reaction mechanisms Part (iii) Bioorganic enzyme-catalysed reactions O– O Glu N N H N +N H S O O– P O O P O O– O– H 13 Fig. 4 O H H3C H3C HO S +N H N N N N Bu O H O H3C O S +N N N N N Bu O H O H H S +N N N N N Bu O H O H H S +N N N N N Bu O H O 16 CH3OH O2 Pt + CH3CO2CH3 – – Scheme 6 N +N S O O– P O O O– P O O– NH2 N N +N S O O– P O O O– P O O– 15 14 though X-ray crystallographic analysis of these complexes showed that no significant structural changes had taken place relative to the transketolase–TDP complex.69 This observation implies the participation of 13 in TDP-activation and supports a catalytic role for the conserved glutamic acid residue in C-2 deprotonation.A particularly interesting organic model system for TDP-dependent enzymes has been developed for the oxidation of aldehydes to acids or methyl esters (Scheme 6).70 The flavothiazolo cyclophane derivative 16 was synthesized and shown to bind aromatic substrates.The ability of 16 to act as an enzyme mimic was then investigated 298 N.G. J. Richards N N N N Bu H O O O +N O S +N O +N O (CH2)4 (CH2)4 16 CHO 17 in combination with an electrochemical system for regeneration of the oxidized form of the flavin. When 2-naphthaldehyde 17 was used as a substrate k#!5 was observed to be 0.24 s~1 which is 200–700 times faster than the cognate oxidation reaction in control experiments employing compounds lacking the binding cavity. Destruction of 16 during re-oxidation of the reduced flavin however severely limited the catalytic turnover for the transformation carried out by this enzyme mimetic. The di¶culties inherent in the design and characterization of model compounds for even simple co-factor mediated transformations are illustrated by recent studies on the conversion of urocanic acid 3t to imidazolone propionic acid 18 by the enzyme urocanase.71 The urocanase-catalyzed reaction requires NAD` and has been shown to proceed via formation of a covalent adduct between urocanic acid and the co-factor (Scheme 7).72 The complex and unstable model compound 19 was synthesized so as to investigate the mechanistic proposal for urocanase and gave complex mixtures of products in the presence of small amounts of water.Treatment of 19 with heat-treated alumina in the presence of dry acetonitile–methanol as eluent gave the isoquinolinium salt 20 rather than the desired pyridinium derivative 21 (Scheme 8). Introduction of the methylene bridge in 19 therefore disfavors formation of the imidazole tautomer that must participate in the enzyme-catalyzed reaction.This interesting example nicely illustrates how the small molecular modifications that must be introduced into model compounds can lead to dramatic changes in chemical mechanism. 299 Reaction mechanisms Part (iii) Bioorganic enzyme-catalysed reactions N N H CO2H +N R H2NOC N R H2NOC N N H CO2H +N R H2NOC N N H CO2H 3 t OH H3O+ 18 steps N R H2NOC +N N H CO2H HO Scheme 7 +N CH2Ph H2NOC CO2Et H N N H 19 alumina CH3OH–CH3CN 20 21 not observed Br – +N CH2Ph H2NOC CO2Et N N H +N CH2Ph H2NOC CO2Et N N H OCH3 Scheme 8 300 N.G. J. Richards 4 Glutamine-dependent amidotransferases and ammonia-mediated nitrogen transfer L-Glutamine plays a central role in many aspects of primary metabolism especially as a nitrogen donor in the biosynthesis of (i) purines and pyrimidines (ii) the amino acids histidine73 and asparagine,74 (iii) the NAD co-factor75 and (iv) glucosamine-6-phosphate 22 an amino sugar that is not only involved in glucose regulation in mammals,76 but is also a key precursor in bacterial and fungal cell-wall biosynthesis.77 Despite the central importance of glutamine in cellular metabolism detailed structural and mechanistic studies of glutamine-dependent amidotransferases,78 the enzymes catalyzing nitrogen transfer from glutamine to suitable acceptors have only been reported O NH2 OH HO OH O P O– –O O O NH2 O O P O– –O CO2 – H3N+ N2 O H 22 23 24 recently.Glutamine-dependent amidotransferases possess two domains or subunits each of which contain a separate active site and can be divided into two classes on the basis of their amino acid sequence.79 The overall glutamine-dependent transformation then involves hydrolysis of glutamine to glutamate in the glutamine-activating (GAT) active site and subsequent transfer of the nitrogen atom as ‘nascent’ ammonia to an electrophilic center present in the second synthetase site.The molecular basis for coordination of the chemical and binding events in the two active sites of these amidotransferases as well as the involvement of ammonia in mediating nitrogen transfer are significant mechanistic issues. Class I amidotransferases an enzyme family that includes carbamoyl phosphate synthetase (CPS)80 and GMP synthetase (GMPS),81 possess a common glutamineutilizing site in which there is a conserved triad of cysteine histidine and aspartate residues.82 The crystal structure of CPS which catalyzes the synthesis of carbamoyl phosphate 23 from glutamine bicarbonate and ATP,80 revealed the existence of a ‘channel’ linking the active site of the small subunit (which binds and activates glutamine) with two further active sites in the large subunit (which binds ATP and bicarbonate).83 Passage of reaction intermediates including ammonia released by amide hydrolysis from glutamine in the small subunit of the enzyme containing the conserved Cys-His-Asp triad through this channel explains the lack of uncoupling of the various partial reactions at saturating levels of the three substrates.The futile hydrolysis of glutamine is also avoided by sequestering ammonia within the protein structure. On the basis of the crystal structure it appears that the nitrogen atom originally present in glutamine moves up to 96Å from the small subunit to the point at which carbamoyl phosphate exits the third active site of the enzyme! Another intriguing and unexpected aspect is that the channel is not defined by hydrophobic amino acids but by the side chains of residues capable of forming hydrogen bonds with ammonia.This presumably prevents protonation of neutral ammonia as it moves through the protein due to the formation of intermolecular hydrogen bonds within the 301 Reaction mechanisms Part (iii) Bioorganic enzyme-catalysed reactions channel. The existence of similar channels through which ammonia can di§use cannot be assumed as yet for other Class I amidotransferases. For example no similar structure for channeling ammonia was observed in the ternary complex between GMPS AMP and inorganic pyrophosphate.81 Enzymes in the Class II amidotransferase family which include glutamine PRPP amidotransferase (GPA),84 glutamine fructose-6-phosphate amidotransferase (GFAT),85 glutamate synthase86 and asparagine synthetase (AS),74 have a homologous N-terminal domain that mediates glutamine-binding and utilization.78 This family of glutamine-dependent amidotransferases are usually single-chain polypeptides and are characterized by an N-terminal cysteine residue (Cys-1) in the mature form of the enzyme.Covalent modification of Cys-1 by the glutamine analog 5- dioxonorleucine 24 (DON) therefore irreversibly inhibits their glutamine-dependent activities. In a similar manner to Class I amidotransferases there is substantial evidence that suggests these enzymes have two active sites located in two separate domains connected by a ‘hinge’ region.An important series of X-ray crystal structures have been described for bacterial GPAs both for the free enzymes,87,88 and their complexes with various inhibitors allosteric regulators and reaction products.89,90 The mechanism of nitrogen transfer from glutamine has been an important issue for Class II amidotransferases given that inhibitors for GFAT and AS have potential clinical application as antibacterial91 and anti-leukemia agents respectively. Kinetic isotope e§ect (KIE) measurements have been obtained that appear inconsistent with ammonia-mediated nitrogen transfer in Escherichia coli AS-B,92 the glutamine-dependent asparagine synthetase encoded by the asnB gene.93 On the other hand a recent structure of Escherichia coli GPA in which Cys-1 is covalently modified by DON and the non-hydrolyzable PRPP analog 25 is bound in the synthetase active O O P O O P O O– O– O– O P –O O– HO OH 25 site revealed the existence of a channel linking the two active sites in the enzyme.94 Comparison of the structure of this ‘doubly-inhibited’ form of GPA with that of the DON-modified form of the enzyme showed that a significant conformational change had taken place in several segments of the protein leading to channel formation.This exciting result is the first direct evidence that similar structural mechanisms might be employed by both families of glutamine-dependent amidotransferases to sequester and transfer the ammonia molecule liberated from glutamine by the GAT-domain active site.In contrast to the structure of CPS the channel in GPA is defined principally by hydrophobic amino acid side chains. Furthermore the lack of structural and sequence homology in these two enzyme families implies that these channel structures for sequestering ammonia probably evolved independently.95 Kinetic measurements–similar to those obtained in studies on tryptophan synthase96 where indole moves through a channel connecting the active sites of di§erent subunits97–to 302 N.G. J. Richards CO2 – H3N+ H N CO2H O S H CHF2 CO2 – H3N+ O H S Enzyme H3N+ O H CO2H H2N CHF2 H S F O O– O H GFAT 26 27 28 – NH3 29 Scheme 9 show the functional importance of channeling remain to be reported for any Class II amidotransferase and are anxiously awaited. These recent reports suggest that channels which control the transport of small neutral molecules between multiple active sites may prove to be a general structural feature of complex enzymes.Despite all of the recent progress in understanding the structure and mechanism of Class II amidotransferases potent inhibitors exhibiting selectivity for either GFAT or AS have still to be obtained. An interesting approach to developing such compounds however is represented by the glutamine derivative 26 which is the first mechanismbased GFAT inhibitor (Scheme 9).98 Reaction of 26 with the Cys-1 thiolate in GFAT yields a thioester 27 and liberates 28 an intermediate that can spontaneously break down to give the electrophilic quinonoid 29 ammonia and glyoxylate. Incubation of GFAT with 26 resulted in a time-dependent inactivation that could be prevented by the addition of nucleophiles such as dithiothreitol.The nature of the covalent adduct that presumably forms by reaction of GFAT with 29 however remains to be established especially as the thioester intermediate is presumably present when 29 is formed in the GAT-domain active site. 5 N-terminal nucleophile (Ntn) hydrolases Interest in the active site features that allow serine proteases and cysteine proteases to catalyze amide bond hydrolysis remains high given the important physiological roles of many members of these enzyme families. Indeed the discovery that caspases cysteine proteases that specifically cleave peptide bonds after aspartic acid residues,10 are important mediators of apoptosis has stimulated significant e§orts to discover potent inhibitors.99 The mechanism by which an active site histidine is employed in general acid/base catalysis to promote attack at the amide bond by serine or cysteine residues has been the subject of numerous structural100 and theoretical studies.101,102 303 Reaction mechanisms Part (iii) Bioorganic enzyme-catalysed reactions The limitations on active site structure imposed by the chemical requirements for amide bond hydrolysis have also been studied.Ntn hydrolases catalyze amide bond hydrolysis using an N-terminal nucleophilic residue such as serine threonine or cysteine but appear to lack a histidine residue that plays an important role in catalysis of related enzymatic reactions.103 Enzymes in this family which include penicillin acylase,104 the 20S proteasome,105 aspartylglucosaminidase 106 GPA and other Class II amidotransferases are all of medical interest.For example proteasome inhibitors such as lactacystin 30,107 are of signifi- cant interest as potential treatments for many diseases including cancer.108 In current mechanistic models for Ntn hydrolase catalysis the N-terminal amino-group of the protein functions as an acid/base catalyst in place of histidine.103,106 Recent computational studies on aspartyl-glucosaminidase provide support for this proposal.109 The mechanistic relationship of the GAT-domain of Escherichia coli AS-B a putative Ntn hydrolase and the amidohydrolase activity of papain a thiol protease have been demonstrated through re-engineering the catalytic properties of AS-B.110 AS-B glutaminase activity was eliminated by replacement of Asn-74 (thought to define the oxyanion hole in the AS-B GAT-domain) with aspartic acid (Scheme 10).The resulting NH O O O OH 30 N74D AS-B mutant catalyzed the synthesis of glutamine from the nitrile 31 a compound that is a reversible inhibitor of the wild-type enzyme. The pH-dependence of the nitrile hydratase activity suggested that the carboxylate of Asp-74 did not function as a general acid in the AS-B mutant. Similar results are obtained when a cognate mutation is made to Gln-19 of papain,111 the residue defining the oxyanion hole in this cysteine protease.112 It is interesting to note however that replacement of Cys-1 by serine in Class II amidotransferases yields enzymes that no longer possess the ability to hydrolyze glutamine to glutamate,74 even though serine and threonine are the catalytic nucleophiles in penicillin acylase and the 20S proteasome respectively.Further serine can be substituted for threonine to give a 20S proteasome mutant that retains catalytic activity.113 Therefore the presence of the N-terminal amine and a nucleophilic side chain may not be su¶cient for catalysis of amide hydrolysis. Although other features of the active site must also be important in mediating amide bond cleavage the catalytically relevant residues of all Ntn hydrolases for which crystal structures are available can be superimposed almost exactly.103 6 Oligosaccharyltransferase and N-glycosylation of the Asn-Xaa-Ser/Thr motif N-Glycosylation of asparagine residues an essential post-translational modification of many eukaryotic proteins is catalyzed by oligosaccharyl transferase (OT) (Scheme 304 N.G.J. Richards CO2 – H3N+ C N H O– O Asp74 O– O Asp98 +NH2 N H2N H Arg49 SCys1 H3N+ H CO2 – H3N+ C +N H O– O Asp74 OO Asp98 +NH2 N H2N H Arg49 SCys1 H3N+ H H CO2 – H3N+ H O– O Asp74 O– O Asp98 +NH2 N H2N H Arg49 S N H H3N+ Cys1 H CO2 – H3N+ H O– O Asp74 O– O Asp98 +NH2 N H2N H Arg49 S H3N+ Cys1 H OH H2N CO2 – H3N+ H O– O Asp74 O– O Asp98 +NH2 N H2N H Arg49 NH2 O S– Cys1 H3N+ H CO2 – H3N+ H O– O Asp74 O– O Asp98 +NH2 N H2N H Arg49 S H3N+ Cys1 H O CO2 – H3N+ CO2H H 31 – H+ H+ H2O several steps – NH3 H2O H+ X Scheme 10 305 Reaction mechanisms Part (iii) Bioorganic enzyme-catalysed reactions N H O N R N H O NH2 O H O HO R1 O NHAc O P O OO P O OO HO HO O O R2O HO HO NHAc N H O N R N H O N O H O HO R1 O NHAc HO HO O O R2O HO HO NHAc Nascent Peptide R1 = H CH3 N = 13–17 N – Dolichol-PP H R2 = (Man)6(Glc)3 Scheme 11 11).114 The reaction is of chemical interest in that the primary amide of asparagine acts as a nucleophile attacking the anomeric center of an activated saccharide.The molecular basis for this unusual reactivity is likely to be correlated with the observation that the asparagine residue which undergoes N-glycosylation is located within a minimum Asn-Xaa-Ser/Thr motif where Xaa represents any amino acid other than proline.115 Experiments employing conformationally defined model peptides such as 32 and 33 are consistent with a mechanism in which intramolecular hydrogen bonding possibly coupled with enzyme-catalyzed proton transfer results in tautomerization of the primary amide.116 The requirement for the Asn-Xaa-Ser/Thr motif is also consistent with a model in which intramolecular deprotonation of the asparagine side chain by the proximal serine hydroxy group with suitable activation by the enzyme yields the reactive anion 34.117 A third mechanistic proposal in which nitrogen transfer proceeds via the transient formation of ‘nascent’ ammonia from the asparagine 306 N.G.J. Richards N O O H N H H2NOC O N H O N HO H N S S N O O H N H H2NOC O N H O N HO H O H H3N+ N H OH CO2 – O N O H N O N H R O N O H H N -O H H H Base-Enz 32 O N Ph H N H O O N HO 33 H N H O H2NSC 35 34 H N O O OH N H H N O Gly-Ser-Ile 38 O Ile-Thr-Pro O HO AcHN O H AcHN AcHN OH OH Ile-Thr-Pro-Asn-Gly-Ser-Ile 37 36 N O O H N H O N H O N HO H S H3N+ side chains in a similar manner to that proposed for glutamine-dependent amidotransferases has been investigated using a series of 13C/15N-labeled tripeptides as OT substrates.118 Mass spectroscopic andNMRmeasurements show that free ammonia is not lost from the asparagine side chain and that exogenous ammonia does not intercept any enzyme-bound oligosaccharyl intermediates.Resolution of this intriguing mechanistic problem using high-resolution structural data is complicated by the fact that OT is membrane-associated and the enzyme isolated from Saccharomyces cerevisiae is comprised of six polypeptide chains.119 Inhibition of the enzyme by substrate analogs has been reported.120 In these studies the most interesting observation was that the thioamide 35 is a substrate for the enzyme but the relative maximal 307 Reaction mechanisms Part (iii) Bioorganic enzyme-catalysed reactions O O– O O– P O –O H Enz+ O O– O O– P O –O CO2 – O OH O O CO2 – P O O– O– O P –O O– CO2 – O OH O CO2 – O P –O O– CO2 – O OH OH O P –O O– 42 40 41 + Scheme 12 velocity for the glycosylation is only 8% of that observed for the true substrate.Whatever the details of the glycosylation mechanism the conformationally constrained compound 36 is a potent OT inhibitor that demonstrates slow tight binding. 121 The structural e§ects of glycosylation have also been explored using a series of model peptides that have sequences cognate to those of b-turn surface loops present in hemagglutinin.122 In the case of the model peptide 37 NMR methods indicated that the free peptide adopted an extended conformation in aqueous solution.OT-catalyzed glycosylation of the asparagine residue with chitobiose to give 38 resulted in a conformational preference for a compact type I b-turn. 7 EPSP synthase The commercial importance of the herbicide glyphosate 39,123 which blocks the synthesis of aromatic amino acids derived from the shikimate pathway,124 has stimulated extensive mechanistic studies on its target enzyme EPSP synthase. The generally O AcNH O– O OH HO OH HO H N P O HO OH CO2H H 43 39 + accepted reaction mechanism for EPSP synthase (Scheme 12) based on rapid kinetic measurements,125 involves the formation of a tetrahedral intermediate 42 produced from reaction of shikimate-3-phosphate 40 with carbocation 41. Elimination of phosphate from 42 then yields the coupled product. In this model glyphosate therefore acts 308 N.G.J. Richards O O– O O– P O –O H Enz+ CO2 – O OH O X CO2 – Enz O P –O O– CO2 – O OH O CO2 – O P –O O– CO2 – O OH OH O P –O O– HX Enz O X CO2 – Enz P O– –O O H CO2 – Enz X H Enz+ Enz – PO4 3- 46 45 44 Scheme 13 as a transition state analog of 41.126 The active site of EPSP synthase lies in the cleft between two globular domains,127 and solid-state magic angle spinning (MAS)NMR experiments have been used to study the enzyme–glyphosate complex.128 REDOR measurements of internuclear distances,129 using compounds that are either isotopically enriched (13C and/or 15N) or which contain nuclei not normally found in proteins (19F and/or 31P) indicate that EPSP synthase undergoes a conformational change on substrate binding.128,130,131 An intriguing feature of the accepted mechanism is the need for formation of the carbocation intermediate 41 which has little chemical precedent.In principle electrostatic stabilization of 41 is feasible. Recent theoretical studies on the putative carbocationic intermediate 43132 in the reaction mechanism of cytidine monophosphate N-acetylneuraminidate (CMP-NeuAc) do provide support for such a hypothesis.133 On the other hand evidence for an alternate mechanism for EPSP formation by EPSP synthase has been reported (Scheme 13)126 in which an enzyme side chain reacts to form a covalent bond with PEP 44. The resulting adduct 45 then undergoes loss of phosphate followed by addition of shikimate- 3-phosphate to form the final product. A general solid-state NMR approach to observing long-lived species formed during enzyme-catalyzed reactions has recently been developed.134 The method employs substrates containing a combination of nuclear labels (13C 15N 19F or 31P) which are incubated with the enzyme at sub-zero temperatures.The resulting mixture is then carefully lyophilized to give the solid sample used in the NMRmeasurements. By observing the evolution of chemical shifts in the enzyme-catalyzed reaction in the solid state and using rotational echo double resonance (REDOR) experiments to measure internuclear distances,129 structural information on various reaction intermediates can be obtained.135 In recent work this NMR approach has been applied to study the mechanism of EPSP synthase.134 In these experiments EPSP synthase was incubated with PEP specifically labelled at C-2 with 13C. Using a combination of 31P chemical shift and 13C–31P distance determinations it was found that the bound phosphate of PEP was 6.1Å distant from the 309 Reaction mechanisms Part (iii) Bioorganic enzyme-catalysed reactions 13C-label in the early stages of the enzyme-catalyzed reaction.These NMR observations can be interpreted as evidence for the presence of an enolpyruvylenzyme covalent intermediate 45 raising questions about the role of the tetrahedral intermediate 42 in the catalytic mechanism of EPSP synthase. The authors proposed that the formation of 42 takes place in an enzyme-catalyzed side-reaction of EPSP with enzyme bound phosphate. Although this conclusion is controversial the application of solid-state NMR methods to the elucidation of enzyme mechanisms represents an important addition to current methodologies.A significant advantage is that there are no size restrictions on the system for which data can be obtained unlike the situation in solution-phase NMR where the di§usional properties of the protein are an important experimental constraint.135 The use of the novel lyophilization procedure reported in this study of EPSP synthase should also simplify sample preparation in time-resolved CP-MAS NMR compared with previous rapid quench techniques.136 8 Sesquiterpene cyclases Studies of biosynthetic pathways leading to a myriad of biologically active natural products have been energized in the past few years by the cloning expression and biochemical characterization of enzymes that mediate many of the transform- O O OH O P O– O O P O– O –O 46 47 ations involved in polyketide,137 terpene138 and penicillin biosynthesis.139 Several recent reports have outlined elegant experiments in which modular polyketide synthase modules have been combined to yield transformed organisms with the ability to synthesize new ‘natural products’ such as the eight-membered lactone 46.140 These studies open the way for the combinatorial biosynthesis of complex ‘designer’ polyketides that can be used in high-throughput screens for the identification of new antibiotics.141 Similar developments are underway in the area of biologically active peptides derived by non-ribosomal synthesis.142 The structural diversity of cyclic sesquiterpene metabolites biosynthesized from the linear precursor farnesylpyrophosphate 47 has long fascinated organic chemists143 and prompted the characterization of enzymes responsible for mediating the synthesis of these natural products.144 It is now clear that sesquiterpenes are formed by cyclization reactions which are initiated by heterolytic bond cleavage to form pyrophosphate and an allylic carbocation.The protein plays many di§erent roles in catalysis of these reactions. These include (a) activating the pyrophosphate for bond cleavage (b) maintaining a relatively electron-rich environment required for stabilization of the allylic carbocation and subsequent intermediates and (c) controlling the conformation and reactivity of the allylic carbocation. An important constraint is that capture of the reactive cationic intermediates by nucleophilic amino acid side chains 310 N.G. J. Richards R R2 R1 Phe O N H H Asn/Gln d- d- + Fig.5 must be avoided. In principle the creation of an active site containing alkyl side chains could avoid potential covalent modification of the enzyme but would have the disadvantage of lacking groups capable of stabilizing cationic structures. Further product release might be problematic given the lack of hydrogen bonding groups in many sesquiterpenes. Insight into the structural solutions to these chemical problems has been obtained with the recent reports of crystal structures for the two unrelated sesquiterpene cyclases 5-epiaristolochene synthase145 and pentalenene synthase.146 Both enzymes employ a hydrophobic active site that is primarily composed of aromatic amino acid side chains. It has been proposed that the p-electron clouds of these side chains provide stabilization of cationic reaction intermediates.147 In addition there is a flexible segment containing hydrophilic residues (predominantly aspartates) that can bind Mg2` ions.In both structures the non-hydrolyzable substrate analogs that were co-crystallized with these enzymes corresponding to the pyrophosphate moiety lie close to this flexible segment. It has therefore been proposed that this segment initiates cleavage of the bond between the leaving group and the rest of the substrate. It is likely that pyrophosphate remains bound to the enzyme during the cyclization reaction possibly facilitating the stabilization of cationic intermediates. As might have been anticipated water is excluded from the active site during cyclization by a flexible loop that changes position on substrate binding.In pentalenene synthase residues such as Phe-77 and Asn-219 are optimally placed to stabilize reactive carbocations (Fig. 5) and a number of proton transfer reactions appear to be mediated by a histidine residue acting as a general acid/base (Scheme 14).146 In contrast in 5-epiaristolochene synthase (EAS) the p-electron system in the indole ring of a tryptophan side chain is proposed to participate in electronic stabilization of cationic intermediates.145 This residue is also hypothesized to be involved in proton transfer steps in vetispiradiene synthase (VS),148 an enzyme that is closely related in sequence to EAS. The active sites in both enyzmes must act as templates to ‘channel’ the conformations of the cationic intermediates along the appropriate reaction coordinate.Since a large number of carbon atoms in the farnesylpyrophosphate precursor must undergo significant changes in hybridization and therefore shape during the cyclization reaction a ‘loose’ active site structure is observed in both enzymes raising questions of 311 Reaction mechanisms Part (iii) Bioorganic enzyme-catalysed reactions His309 N N H His309 N N+ H H H H H His309 N N H His309 N N+ H H H Pentalenene + – P2O7 4– + O P O– O P O– O –O O 47 Mg2+ Scheme 14 H 48 49 whether sesquiterpene cyclases might catalyze formation of a range of products. It is interesting to note an experiment that might bear on this question where the domain structure of these enzymes has been exploited to produce new catalytic activities. Chimeric cyclases composed of domains from EAS and VS give mixtures of 5- 312 N.G.J. Richards H2O FeII N O S N N N Asp216 O H H His270 His214 N R H H N H O H H –O2C H2O FeIII N O S N N N Asp216 O H H His270 His214 N R H H N H O H H –O2C O •O H2O FeIV N O S N N N Asp216 O H H His270 His214 N R H H Hydrogen abstraction Cyclization O O Hydrogen abstraction Cyclization Isopropyl rotation N H –O2C H H2O FeII N O S N N N Asp216 O H H His270 His214 N R H H O HO N H –O2C H Scheme 15 epiaristolochene 48 and vetispiradiene 49 when incubated with farnesylpyrophosphate 47 even though the parent enzymes do not appear to synthesize such product mixtures.149 These observations and the observation that many plant sesquiterpenes probably possess similar three-dimensional structures,150,151 o§er the possibility that sesquiterpene cyclases can be reengineered to yield new structures with interesting biological activity.9 Isopenicillin N synthase The biosynthetic pathway to the penicillin antibiotics continues to provide significant insights into the molecular mechanisms by which complex transformations can be performed with complete control of reactivity and stereochemistry. Briefly penicillin biosynthesis entails (i) the assembly of the tripeptide L-(a-aminoadipoyl)-L-cysteinyl-Dvaline (ACV) and (ii) its oxidative modification to give the bicyclic ring system of isopenicillin N by the enzyme isopenicillin N synthase (IPNS).139 ACV is synthesized by a peptide synthetase,152 which is a multifunctional enzyme capable of condensing 313 Reaction mechanisms Part (iii) Bioorganic enzyme-catalysed reactions the amino acid precursors and epimerizing L-valine.153 The IPNS-catalyzed cyclization reaction however has been the focus of considerable study given the complete lack of precedent for this transformation in organic chemistry.This has recently culminated in the first structural characterization of IPNS bound to its substrate and NO an unreactive oxygen analog.154 A critical element in obtaining this important structure was the use of anaerobic crystallization conditions.155 Using the structure and the results of extensive studies employing isotopically labelled IPNS substrates the mechanism for the oxidation reaction has been elucidated (Scheme 15). The essential non-heme FeII is ligated by two histidine side chains and a glutamine (Gln-330) that is displaced by the thiolate of the substrate.Oxygen coordination to the metal then initiates the cyclization reaction. Unexpectedly the b-CH bond of the D-valine residue which must be broken during the reaction is oriented away from the FeII center. This implies that there must be a rotation about the Ca–Cb bond before the second hydrogen abstraction. In addition given that Gln-330 is not essential for the catalytic activity of IPNS,156,157 the chemical significance of amide coordination to the metal remains unclear. 10 Nitrile hydratase The synthesis of primary amides (RCONH 2 ) by enzyme-catalyzed hydration of nitriles (RCN) is an important transformation for the removal of potentially toxic metabolites from a variety of plants and microorganisms. This reaction is catalyzed by metaldependent nitrile hydratases,158 that contain either FeIII159 or CoIII centers.160 Organisms expressing such enzymes have biotechnological and commercial importance,161 and can convert a wide range of reactive nitriles such as acrylonitrile into their corresponding amides in very high yield.162 Indeed this is almost the only example of the synthesis of a bulk chemical by an enzyme-based procedure that is economically favorable when compared with chemical processes using metal-based catalysts.Uniquely among known metalloproteins the FeIII present in the putative active site of nitrile hydratase has a low-spin electronic configuration but is not bound within a heme co-factor.159,163 Despite excellent spectroscopic investigations of nitrile hydratase 164–166 questions concerning the catalytic mechanism of the enzyme and the importance of the unusual spin properties of the FeIII center remain to be resolved.For example three mechanisms by which the metal might participate in transforming a nitrile to a primary amide have been proposed (Scheme 16). An important step in understanding the catalytic properties of nitrile hydratases was the structural characterization of the enzyme isolated from Rhodococcus sp. R312 by X-ray cyrstallography. 167 This study revealed that (i) the enzyme possessed a novel fold and (ii) the FeIII was coordinated by three cysteine thiolates and two backbone amide bonds. The latter result was unexpected on the basis of previous spectroscopic studies. The sixth ligand in the octahedral FeIII center however was not observed in the crystal structure although EPR evidence suggests that it is probably a water molecule.159 Interestingly none of the three proposed mechanisms for the enzyme-catalyzed hydration of the nitrile moiety can be ruled out on the basis of the three-dimensional structure.314 N.G. J. Richards Fe3+ N C R H O H N Fe3+ H O R H Fe3+ -O H O H H R C N H B+ Enzyme H H O Fe3+ C R N H O H B Enzyme C R N H O Fe3+ –O H H B+ Enzyme H H B+ Enzyme B Enzyme H B+ Enzyme N R H H O Fe3+ –O H R C N H B+ Enzyme C R N H O Fe3+ H H H O Fe3+ C R N H O (A) (C) H Fe3+ H+ transfer B Enzyme (B) H+ transfer B Enzyme H+ transfer Scheme 16 11 Future trends Significant advances in structural and mechanistic enzymology have been made during the past year. 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ISSN:0069-3030    

DOI:10.1039/oc094289

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

9 Reaction mechanisms Part (iv) Free-radical reactions By DANIEL E. FALVEY Department of Chemistry and Biochemistry University of Maryland College Park MD 20742 USA 1 Theoretical and thermochemical studies A summary of all theoretical and computational work on odd-electron species would easily fill an entire volume. Therefore this report will focus on studies of large organic free radicals in solution and properties that are likely to be of general interest to organic chemists. Applications of free radicals in synthesis and other areas have benefited from the wealth of readily available thermochemical data. Through properties such as bond dissociation energies (BDEs) it is possible to predict stabilities of free radicals and forecast their reactive behavior. For this reason interest in BDEs for particular radicals continues both from experimentalists and theorists.One area of recent interest is the e§ect of a-heteroatoms on the stability of carboncentered radicals. Wayner et al.1 have examined the e§ect of alkyl substitution on the stabilities of a-amino radicals 1 using both calculations and photoacoustic calorimetry (PAC). Both methods showed that there is little e§ect of increasing alkylation on the nitrogen as judged by the C–H BDE. The possibility of stabilization of a-aminocarbonylmethyl radicals 2 by captodative e§ects was considered by Welle et al.2 These workers estimated the stabilization energies of these radicals by first determining the equilibrium constant for their dimerization (using EPR spectroscopy) and then correcting for steric e§ects. There was a good correlation between the radical stabilization energy (RSE) and the EPR a-H hyperfine coupling constant.The values of the RSE of the radicals in this study as well as those estimated from hyperfine coupling constants were taken to support the existence of a captodative e§ect in such systems. In a related study Brocks et al.3 examined the equilibrium constant for homolytic dissociation of sarcosine anhydride 3 to form 4 (Scheme 2). The results of this study were also interpreted to favor a captodative e§ect. The stability of nitrogen-centered radicals also attracted attention in 1997. A PAC study4 of the N–H bond dissociation energies for a series of arylamines showed that there was little change in the N–H bond strength with changes in substituents at the nitrogen atom. Notably theN–H BDE of aniline and diphenylamine di§er by no more than 3 kcal mol~1.Using a combination of electrochemical oxidation potentials and 321 N R2 R1 Y R3 R1 O N R2 R3 R4 1 2 Scheme 1 N N O H O H N N O H O H N N O H O H 3 4 2 Scheme 2 N O O N O O N O O N O O• 5a 5b 5c Scheme 3 acidity measurements Zhao et al.5 determined N–H bond dissociation energies of a number of hydrazides (RCONHNH 2 ). It was shown that the NH 2 substituent results in a substantial weakening of the N–H bond relative to the corresponding bond in the amide RCONH 2 . The succimindyl radical 5 (Scheme 3) has historically been a controversial species. Recently Gainsforth et al.6 carried out a series of density functional theory (DFT) calculations (B3LYP) along with CAS correlated Hartree–Fock (HF) calculations.The results of DFT calculations which were apparently regarded by the authors as being the more definitive predict that the lowest energy configuration is an oxygen sigma radical (5c) rather than the nitrogen-p (5a) or the nitrogen-r (5b) radicals whose relative significance were the subject of debate in earlier work. The stabilities of aryloxyl (phenolic) radicals (6) have been the subject of continued O• R1 R2 R3 R5 R4 6 322 Daniel E. Falvey H3C Cl R H O– O CH3 R H R O H Cl– CH3 • Cl– 7 Scheme 4 interest because of the role of tocopherols in the inhibition of biological free radicals. Wright et al.,7 for example applied DFT to the prediction of O–H bond dissociation energies in various ring-substituted phenols. The results of DFT calculations gave excellent agreement with the experimental data.Application of this method to some 35 phenols with various ring substituents validated simple substituent additivity rules. Virtually all p-donor substituents were found to weaken the O–H bond with amino groups in the ortho-positions being predicted to cause the most substantial change in BDE. In a parallel study Brinck et al.8 examined a series of phenol derivatives also using DFT methods. In this study the computations were also extended to phenols having electron-withdrawing substituents on the aromatic ring. In agreement with the other DFT study electron-donating substituents were found to weaken the O–H bond towards homolytic cleavage and electron-withdrawing groups were found to tend to strengthen the O–H bond. In contrast to the situation with phenols a combined gas phase and solution PAC study9 revealed no discernable e§ect of aromatic ring substituents on the carbon–bromine bond dissociation energies of substituted benzyl bromides nor on the C–CH 3 bond dissociation energies of ring-substituted tertbutylbenzenes.This finding is at odds with the results of earlier work although no explanation for the discrepancy in these results was o§ered. Laarhoven and Mulder10 determined the C–H BDE for the benzylic position in tetralin and found it to be 82.9 kcal mol~1. Bartberger et al.11 examined the e§ect of fluorine substituents on both the rate constants for bond cleavage determined by laser flash photolysis and bond dissociation energies from DFT calculations. The results from both experiment and calculation suggest a destabilizing e§ect of b,b-difluoro substitution which results in a more rapid rate of hydrogen atom abstractions and higher bond dissociation energies.On the other hand a,a-difluorination results in a modest decrease in the bond dissociation energy but accelerates the rate of H-atom abstraction. Dakternieks et al.12 have carried out QCISD ab initio calculations on the reaction of various alkyl radicals with tin hydride reagents. The results of these calculations successfully model the observed e§ect of alkyl substitution on the relative rates of these reactions. Chandra and Nguyen13 have applied DFT to the studies of regioselectivity of radical additions to alkenes. They found that condensed Fukui functions and local softness could successfully predict the regiochemistry of radical addition to monosubstituted ethylenes.Nau14 has argued that bond dissociation energies can be divided into three parts a steric part a radical stabilization part and a polar part. The radical stabilization 323 Reaction mechanisms Part (iv) Free-radical reactions P O HO HO O P O OH HO O P O HO HO O P O OH HO [2,3] [1,2] O H synelimination 8 Scheme 5 9 10 Scheme 6 energy can be derived from the BDE of symmetric compounds where polar e§ects should cancel. The polar component can be determined from the deviation of an observed BDE from that predicted on the basis of the radical stabilization energy alone. In addition to the prediction of structural and thermochemical properties of radicals theorists have recently begun to model reaction mechanisms by computing transition state energies and mapping out the potential energy surfaces for these reactions.A recent investigation by Shaik et al.15 examined the reactions of aldehyde radical anions (7 Scheme 4) with alkyl halides. The results from their calculations (ROHF 6-31G*) predict both a dissociative electron transfer pathway and an S N 2-like pathway. Benassi et al.16 have examined the radical anions of various substituted benzyl chlorides as well as 4-picolyl halides. In all cases these radical anions are predicted to undergo barrierless cleavage of the benzylic carbon–chlorine bond. Extensive DFT (B3LYP) calculations carried out by Zipse17 on the 2-(phosphatoxy) propyl radical (8 Scheme 5) have helped to define the potential energy surface for this species. Saddle points corresponding to a [2,3] and a [1,2] shift were detected as well as a previously undetected syn [1,3] elimination reaction pathway.Wiest18 has carried out calculations on the mechanism for the ring opening reaction of cyclobutene radical cation (9 Scheme 6) using QCISD. Contrary to earlier reports it was found that the reaction proceeds via a concerted C1 pathway leading to cis-butadiene radical cation (10). The results of DFT methods were in agreement with this proposal. 2 Generation of free radicals Several novel pathways for the generation of unusual free radicals have been reported. For example Montevecchi et al.19 have discovered a novel method for the generation 324 Daniel E. Falvey N3 R1 R2 R3 PhS N• R1 R2 R3 PhS• 11 12 + N2 Scheme 7 N2 + S O R S O R S O R I– 13 14 15 Scheme 8 Ar S R O Ar SO• + R• hn 16 Scheme 9 N O• N OH R—H hn R• + 17 Scheme 10 of iminyl radicals (12 Scheme 7).Addition of the phenylthiyl radical (11) to vinyl azides produces an aminyl radical accompanied by the loss of N 2 . The resulting iminyl radicals may then undergo either hydrogen atom transfer or b-scission. Crich and Hao20 have developed a clean method for the formation of acyl radicals. Their approach relies on the formation of an aryl radical from reduction of a diazonium ion salt by iodide anion (13 Scheme 8). This aryl radical (14) then adds to the sulfur atom of a tethered thioester group. This results in the expulsion of the corresponding acyl radical (15) and formation of 2,3-dihydrobenzothiophene. Photochemical methods for the generation of free radicals continue to be of interest. Guo and Jenks21 have examined the initial events following photolysis of alkyl aryl sulfoxides.It was shown that photolysis results in C–S bond scission (i.e. b-bond scission) to form a sulfinyl radical–alkyl radical pair (16 Scheme 9). No evidence for excited-state hydrogen atom abstraction was found. Bottle et al.22 have shown that photolysis of the aminoxyl radical 1,1,3,3-tetramethylisoindolin- 2-oxyl (17 Scheme 10) causes this radical to abstract a hydrogen atom from a number of simple hydrocarbons including cyclohexane isobutane and n-butane. Products of the coupling of an alkyl radical and unreacted aminoxyl radical were isolated. 325 Reaction mechanisms Part (iv) Free-radical reactions N O• H N N R N O H N N R + 18 Scheme 11 O O O H3C O CH3 2 Bu t• CO2 19 Scheme 12 SPri CO2Et EtO2C RS CO2Et CO2Et SR CO2Et CO2Et + n 20 Scheme 13 Hydrogen atoms can be readily abstracted from monoalkyl diazenes (18 Scheme 11).Myers et al.23 have proposed that this reflects a late transition state for the reaction. In fact even the stable aminoxyl radical TEMPO is proposed to be capable of abstracting a hydrogen atom from these substrates. Nakamura et al.24 examined the thermal decomposition of a peroxy ester (19 Scheme 12) to form a pair of tert-butyl radicals. In this case the alcohol fragment from the peroxy ester was designed to undergo a secondary fragmentation generating the tert-butyl radical and acetone through a b-scission reaction. Engel et al.25 prepared a tertiary alkoxyamine Et 2 NOCH(CH 3 )Ph and examined the thermolysis reaction of this compound. At 150 °C dissociation to form diethylaminoxyl and 1-phenylethyl radicals is observed and this radical pair is found to undergo subsequent coupling and dimerization reactions.The spontaneous polymerization of mixtures of electron rich (isopropyl vinyl sul- fide) and electron-poor (diethyl fumarate) olefins was examined by Mash et al.26 On the basis of spin-trapping experiments and EPR measurements it was concluded that the propagation step involves primarily alkyl diradical species (e.g. 20 Scheme 13) rather than vinyl radicals. Grossi27 examined the formation of cycloalkoxy radicals from photolysis of nitrite esters. Two processes predominate b-scission and 1,5-hydrogen atom shifts. The alkyl radicals formed from these processes were observed to couple with NO and the resulting adducts were trapped by an additional alkyl radical to form a persistent aminoxyl species which was characterized by EPR spectroscopy.326 Daniel E. Falvey OCH3 CH3OH 21 Scheme 14 3 Free radical rearrangements One of the more attractive features of free radical chemistry is the ready availability of kinetic data for various free radical reactions. This makes it possible to predict the outcomes of radical reactions where several competing pathways may exist. It also aids in the optimization of reaction conditions. For this reason there is continuing interest in evaluating rate constants for free radical reactions and in developing correlations between these rate constants and structural properties of the reactants. The cyclopropylcarbinyl rearrangement has a long tradition in radical chemistry as a mechanistic probe for the halftimes of radical reactions (‘radical clock’).This has resulted in continued interest in the development and characterization of additional probes that might be applicable in situations where the cyclopropylcarbinyl is ine§ective. For example there is interest in characterizing rates of rearrangement of probes for radical ions. Wang and Tanko28 undertook careful electrochemical characterization of the radical cation of a number of arylcyclopropanes (e.g. 21 Scheme 14). One electron oxidation of these substrates results in the opening of the cyclopropyl ring. However the ring opening reaction was slow and required nucleophilic addition to the cyclopropyl ring carbons. This is proposed to proceed through a product-like transition state where a significant amount of positive charge accumulates at the incipient primary carbon atom.Chen et al.29 also argue against the use of arylcyclopropyl probes for cation radical formation. The results of ab initio calculations on p-cyclopropylaniline demonstrate that in the radical cation state the spin and charge are localized largely on the nitrogen rather than on the aromatic ring or cyclopropyl group. While rate constants for the parent cyclopropylcarbinyl rearrangement are well known there has been confusion about the values of the rate constants for opening of cyclopropyl rings with tertiary radical substituents (e.g. the dimethylcyclopropylcarbinyl radical 22 Scheme 15). Engel et al.30 attempted to clarify this problem by determination of this rate constant through competitive trapping using PhSeH and some stable aminoxyl radicals.These experiments provide a rate constant of 1.8]107 s~1 for ring-opening. Oxiranylcarbinyl radicals provide even faster probes for free radical formation according to Krishnamurthy and Rawal.31 Using thiophenol (PhSH) as a radical trap these workers estimate a rate constant of 3.2]1010 s~1 for the ring-opening reaction of 23 (Scheme 16). Le Tadic-Biadatti et al.32 have used laser flash photolysis as well as competitive trapping experiments to examine the kinetics of 5-exo cyclization of iminyl radicals such as 24 (Scheme 17). This species cyclizes with a rate constant of 1.6]106 s~1. The 327 Reaction mechanisms Part (iv) Free-radical reactions CH3 CH3 CH3 CH3 22 Scheme 15 O O • 23 Scheme 16 R1 N• R3 R2 N R2 R3 R1 24 Scheme 17 O OCH3 H3CO O OCH3 H3CO Scheme 18 rates of the cyclization and the hydrogen atom abstraction reactions of 24 were found to be slower than for similar reactions of the corresponding alkyl radicals.Schepp et al.33 have designed several cyclizable probes for cation radical formation (e.g. Scheme 18). Each of these probes relies on the oxidation of a p-methoxystyrene fragment followed by cycloaddition of the resulting radical cation to a tethered alkene. The kinetics of the reaction of radicals generated by laser flash photolysis indicated that cyclization occurs on the subnanosecond timescale. Kim et al.34 have used competition experiments to determine the rate constant for 5-exo-trig cyclization of a primary alkyl radical onto the C––N bond of an oxime (25 Scheme 19). The alkoxy group apparently accelerates the rate of this process relative to that observed for the corresponding imines.328 Daniel E. Falvey R2 R1 N OCH3 R2 R1 N OCH3 25 Scheme 19 Ph O Ph O O H Ph O Ph O O H Ph O Ph O O H Ph OH C O O Ph H 26 hn Scheme 20 O O CH2 • 27 Scheme 21 Cyclopropylcarbinyl probes were successfully employed in the characterization of the lifetimes of diradicals derived from the photolysis of phenylglyoxalate esters (Scheme 20). Hu and Neckers35 have determined the distribution of products from the photolysis of the phenylglyoxylate ester from Scheme 20 and concluded that the lifetime for the postulated diradical intermediate 26 is ca. 5 ns. Chatgilialoglu et al.36 studied the cyclization of hex-5-enoyl (27 Scheme 21) radicals. As with the well studied hex-5-enyl radical cyclization of the acyl radical 27 by the 5-exo pathway is faster than cyclization by the 6-endo process.Rate constants for the exo-cyclization were determined through competitive trapping experiments with Bun 3 SnH. Horner et al.37 employed radical clock methodology and laser flash photolysis in order to examine rate constants for hydrogen atom transfer from tris[(per- fluorohexyl)ethyl]tin hydride. This reagent has been advanced as a replacement for Bun 3 SnH as it is more easily removed from reaction mixtures. These experiments showed that the new reagent is slightly (ca. two times) more reactive than Bun 3 SnH. Lowe and Porter38 examined the rearrangement of allylic peroxyradicals (28 Scheme 22). These experiments required the synthesis of an unsymmetrically 18O labeled peroxide which was converted to the peroxyradical 28.This radical was observed to undergo competitive rearrangement reactions one involving dissociation and recombination of the O 2 within a radical pair and the other involving intramolecular migration of the O 2 which results in transposition of the 18O-label. 329 Reaction mechanisms Part (iv) Free-radical reactions O *O• O* •O O •O* O O* 28 Scheme 22 X C C• C C X– 29 Scheme 23 Ar X Y O RO Ar Y O RO H Ar Y O RO Y = H OCH3 X = OPO3-R OTs Cl 32 X– 30 31 –H+ 3' 2' 4' Scheme 24 4 Free-radical elimination reactions Among the more interesting discoveries in recent years is that certain neutral free radicals can undergo elimination of anionic groups from a b-carbon center (29) to form an alkene cation radical (Scheme 23). Earlier Giese and others had postulated that such a process might account for the strand scission step that results from the attack of free radicals (e.g.·OH) on DNA. The postulated reaction mechanism involves abstraction of the C4@ hydrogen from deoxyribose to form the radical 30 followed by expulsion of the phosphate group from the C3@ position. The result is scission of the DNA strand and formation of a radical cation 31 which undergoes loss of a proton to give 32 (Scheme 24). Recent experiments39 have presented spectroscopic evidence in favor of the pathway in the form of CIDNP polarizations from the stable products formed when model radicals are generated. Likewise an allylic radical (32) from the same model system was 330 Daniel E. Falvey Ar X R1 Ar R2 R1 R2 R1 Ar Ar R1 X R2 X R2 dimerization X– 33 Scheme 25 detected by EPR spectroscopy.40 It was proposed that this species forms from deprotonation of the radical cation that was generated by the phosphate elimination illustrated in Scheme 24.Crich and Mo41 have examined the e§ect of alkoxy groups in the 2@-position on this reaction (30 Y\OCH 3 ). The purpose of these experiments was to determine how the rate and mechanism of phosphate elimination may di§er in DNA (Y\H) and RNA (Y\OH). It was found that the 2@-alkoxy substituent has a retarding e§ect on the rate of elimination presumably due to an inductive destabilization of the incipient radical cation. In a related study Crich and Mo42 have determined the e§ect of changing the nucleic acid base at the C1@ position (Ar Scheme 24) on the rates of strand cleavage.Robins et al.43 have shown that the partitioning of electrons in the elimination reaction of 30 is dependent on the nature of the leaving group X. Homolytic fragmentation is observed with a chloride ion leaving group at C3@ (X\Cl Scheme 24) but a change to heterolytic cleavage is observed for reaction of the more weakly basic tosylate leaving group (X\OTs). Cozens et al.44 studied a more general example of the mechanism the b-heterolysis reactions of radicals embodied in decay pathways of the alkyl aryl radicals 33 shown in Scheme 25. Using laser flash photolysis experiments these workers were able to distinguish between the elimination pathway which is favored in highly polar solvents and a competing radical dimerization process which is favored in non-polar solvents and by the addition of weakly electron-donating substituents to the aromatic ring.The mechanism for the enzymatic reaction catalyzed by ribonucleotide reductases has been considered in related work. Here it is thought that generation of a C3@ radical (34) results in elimination of a leaving group in the 2@-position (Scheme 26). This process is coupled to deprotonation of the C3@ OH group and the overall mechanism results in formation of an a-keto radical. Lenz and Giese45 have modeled these reactions using a C3@ seleno ester as the radical initiator. Kinetic studies indicated that the rate of this elimination reaction is rapid and depends upon the concentration of bu§er. Thus a pathway involving concerted elimination and proton transfer was inferred. Mu� ller et al.46 have extended these mechanistic concepts to the free-radical degradation of phospholipids.On the basis of model studies these workers argue that phospholipid degradation can occur via free-radical formation at C2 of the glycerol residue (35 in Scheme 27) followed by b-elimination of the phosphate group. Although not strictly an elimination reaction it is appropriate to mention here the results of studies of b-acyloxy and b-phosphatoxy radical migrations of 36. Crich et al.47 showed through isotope labeling experiments that in solvents as polar as tertbutyl alcohol the migration proceeds through a dissociative mechanism involving elimination and recombination reactions (Scheme 28). The same substrates were 331 Reaction mechanisms Part (iv) Free-radical reactions Ar O Y O RO H Ar O RO O Y– 34 Scheme 26 RO3P OR' O H O O OR' –H+ + –OPO3R 35 Scheme 27 O O CH3 O –O O CH3 O R2 R1 R1 R2 R1 R2 36 Scheme 28 O– X Y O Y X– 37 Scheme 29 demonstrated to undergo concerted migration reactions in non-polar solvents such as benzene.In addition to studies of reactions of the neutral radicals noted above there has also been intense interest in elimination reactions involving ion radicals. For example Ghamini and Simonet48 have studied the electrochemical reduction of diarylalkylsulfonium salts. When the aryl rings were substituted with strongly electron-withdrawing groups it was found the reduction was reversible. However without such substituents the rapid elimination of an alkyl radical was observed. Mehta et al.49 have examined the behavior of the indol-1-ylacetic acid radical cation along with the behavior of derivatives of this radical cation.These species were found to undergo elimination of CO 2 with rate constants ranging from 102 to 104 s~1 depending on the redox potential of the substrate. Heterolytic scission reactions of a-substituted acetophenone anion radicals (37 Scheme 29) were examined by Andrieux et al.50 These workers have examined the e§ect of variations in the aryl ring-substituents and the leaving group X. In certain cases cleavage of the C–X bond was determined to be concerted with transfer of an electron from the electrode. However the authors stress that this does not mean that 332 Daniel E. Falvey H3C N H3C N CH2 • N H 38 Scheme 30 N R1 R2 O O– Y N R1 R2 Y CO2 39 Scheme 31 Ph CH2 Si(CH3)3 Ph CH2 • (H3C)3Si Nuc Nuc 40 Scheme 32 the corresponding radical anions do not have a finite lifetime but merely that the electrochemical process does not proceed through this particular intermediate.The authors also establish correlations between their experimental data for bond scission and various molecular parameters for the substrate with the most notable being a correlation between the reduction potential and the–X bond dissociation energies. In a similar study Andersen et al.51 have examined the cathodic reduction of a-aryloxyacetophenone derivatives. As in the aforementioned study the rate of radical anion bond scission was compared to the change in free energy for the reaction with the latter being estimated by construction of thermochemical cycles. Parker et al.52 also used cyclic voltammetry along with kinetic isotope e§ects to argue that the deprotonation of 9-methylanthracene radical cation (38 Scheme 30) proceeds through a mechanism involving addition of the base (lutidine) to the cation radical followed by elimination.Su et al.53 examined the heterolytic cleavage of a-anilinoacetate radical cations (39 Scheme 31) using laser flash photolysis. It was shown that the rate constant for decarboxylation is accelerated by electron-withdrawing groups on the aromatic ring and retarded by the tight coordination of a counterion to the carboxylate group. The fragmentation of C–Si bonds in benzyltrialkylsilane radical cations (40 Scheme 32) was studied by Dockery et al.54 Results from laser flash photolysis studies indicated that desilylation reaction proceeds through an S N 2-like pathway involving nucleophilic attack and concerted bond scission.Unimolecular bond scission was 333 Reaction mechanisms Part (iv) Free-radical reactions CN CN R2 R1 R4 R3 R4 R3 Nuc R1 R2 CN CN CN R2 R1 R4 R3 Nuc CN– Nuc 41 Scheme 33 judged to be much slower if it occurred at all. Perrott et al.55 examined C–H and C–C bond scission of b-phenethyl ether radical cations. It was found that the bond scission reactions are controlled by stereoelectronic as well as thermochemical factors. The stable products of the reduction of cumyl chlorides by sodium naphthalide have been identified by Denney et al.56 They conclude that the cumyl chloride radical anion itself does not dissociate to form a cumyl radical. Instead it is argued that the elimination of chloride occurs upon a second reduction.5 Free-radical addition reactions The recently discovered addition pathway shown in Scheme 33 is becoming known as the nucleophile olefin combination aromatic substitution or NOCAS reaction. This process is initiated by electron transfer (usually promoted by light) from the olefin to the aromatic compound that has the appropriate electron-withdrawing substituents. The resulting olefin radical cation is trapped by a nucleophile to form a radical which then couples with the aromatic radical anion. The overall process is a termolecular addition reaction. A novel photo-NOCAS reaction involvingCH 3 CNas the nucleophile was recently characterized by deLijser et al.57 Although CH 3 CN is generally regarded as a weak nucleophile the high e¶ciency of this process is attributed to the high instability of the isobutylene radical cation (41 R1 R2\H; R3 R4\CH 3 ) which makes it more inclined to react with the nitrile.In contrast the isobutylene radical cation does not give photo-NOCAS products when CN~ is the nucleophile.58 This nucleophile appears to undergo oxidation by alkenes with high redox potentials. However with more easily oxidized alkenes (e.g. 2,3-dimethylbut-2-ene 41 R1 R2 R3 R4\CH 3 ) high yields of the corresponding photo-NOCAS product are obtained. Torriani et al.59 have studied a similar photo-NOCAS reaction involving 1,4- dicyanobenzene 2,3-dimethylbut-2-ene and methanol. The results of studies of the quantum yields showed that the overall e¶ciency of the reaction increases with increasing concentrations of nucleophile. Several side reactions were also characterized.Lew et al.60 used laser flash photolysis to examine the rate constants for reactions of the radical cations of 1,3-dienes (e.g. 42 Scheme 34). These species react with anionic nucleophiles (e.g. CN~ and NO 3 ~) with rate constants approaching the di§usioncontrolled limit but the reactions with alcohols are slower than di§usion. Rate 334 Daniel E. Falvey H3CO CH3OH (–H+) 42 Scheme 34 43 OCH3 CH3OH Scheme 35 constants for reaction with various alkenes were also characterized. Experiments by deLijser and Arnold have characterized the mechanism for reaction of 1,2-bis(methylene)cyclohexane radical cation with nucleophiles such as CH 3 OH.61 In the absence of added nucleophiles the addition of the solvent acetonitrile and deprotonation/dimerization reaction pathways are observed.No evidence for an intramolecular cyclization reaction was found in this study. Nakamura et al.62 have examined the addition of alkoxy and alkyl radicals to styrene using the aminoxyl trapping technique. They find that the rate of the radical addition reaction increases with the nucleophilic character of the radical (e.g. But · reacts faster than Me·) and that this e§ect is greater than o§setting unfavorable steric factors. Roth et al.63 have explored the behavior of the radical cations of several ring constrained vinylcyclopropanes (e.g. 43 Scheme 35) using CIDNP. Among the many interesting reactions observed was nucleophilic substitution at the quaternary center of the cyclopropyl ring. References 1 D.D.M. Wayner K. B. Clark A. Rauk D. Yu and D. A. Armstrong J.Am. Chem. Soc. 1997 119 8925. 2 F.M. Welle H. D. Beckhaus and C. Ru� chardt J. Org. Chem. 1997 62 552. 3 J. J. Brocks F. M. Welle H. D. Beckhaus and C. Ru� chardt Tetrahedron Lett. 1997 38 7721. 4 P.A. MacFaul D. D.M. Wayner and K. U. Ingold J. Org. Chem. 1997 63 3413. 5 Y.Y. Zhao F. G. Bordwell J. P. Cheng and D. F. Wang J. Am. Chem. Soc. 1997 119 9125. 6 J.L. Gainsforth M. Klobokowski and D. D. Tanner J. Am. Chem. Soc. 1997 119 3339. 7 J. S. Wright D. J. Carpenter D. J. McKay and K. U. Ingold J. Am. Chem. Soc. 1997 119 4245. 8 T. Brinck M. Haeberlein and M. Jonsson J. Am. Chem. Soc. 1997 119 4239. 9 L. J. J. Laarhoven J. G. P. Born I. W. C. E. Arends and P. Mulder J. Chem. Soc. Perkin Trans. 2 1997 2307. 10 L. U. Laarhoven and P. Mulder J. Phys. Chem. B 1997 101 73. 11 M.D. Bartberger W.R. Dolbier J. Lusztyk and K. U. Ingold Tetrahedron 1997 53 9857. 12 D. Dakternieks D. J. Henry and C. H. Schiesser J. Chem. Soc. Perkin Trans. 2 1997 1665. 13 A. K. Chandra and M. T. Nguyen J. Chem. Soc. Perkin Trans. 2 1997 1415. 14 W.M. Nau J. Phys. Org. Chem. 1997 10 445. 15 S. Shaik G. N. Sastry P. Y. Ayala and H. B. Schlegel J. Am. Chem. Soc. 1997 119 9237. 335 Reaction mechanisms Part (iv) Free-radical reactions 16 R. Benassi C. Bertarini and F. Taddei J. Chem. Soc. Perkin Trans. 2 1997 2263. 17 H. Zipse J. Am. Chem. Soc. 1997 119 2889. 18 O. Wiest J. Am. Chem. Soc. 1997 119 5713. 19 P. C. Montevecchi M. L. Navacchia and P. Spagnolo J. Org. Chem. 1997 62 5846. 20 D. Crich and X. L. Hao J. Org. Chem. 1997 62 5982. 21 Y. S. Guo and W. S. Jenks J. Org. Chem. 1997 62 857.22 S. E. Bottle U. Chand and A. S. Micallef Chem. Lett. 1997 857. 23 A. G. Myers M. Movassaghi and B. Zheng Tetrahedron Lett. 1997 38 6569. 24 T. Nakamura Y. Watanabe and H. Tezuka Chem. Lett. 1997 11 1093. 25 P. S. Engel S. M. Duan and G. B. Arhancet J. Org. Chem. 1997 62 3537. 26 E. A. Mash H. G. Korth and S. M. DeMoss Tetrahedron 1997 53 15 297. 27 L. Grossi Tetrahedron 1997 53 6401. 28 Y. H. Wang and J. M. Tanko J. Am. Chem. Soc. 1997 119 8201. 29 H. Chen M.J. deGroot N. P. E. Vermeulen and R. P. Hanzlik J. Org. Chem. 1997 62 8227. 30 P. S. Engel S. L. He J. T. Banks K. U. Ingold and J. Lusztyk J. Org. Chem. 1997 62 1210. 31 V. Krishnamurthy and V. H. Rawal J. Org. Chem. 1997 62 1572. 32 M.H. LeTadic-Biadatti A. C. CallierDublanchet J. H. Horner B. Quiclet Sire S. Z. Zard and M.Newcomb J. Org. Chem. 1997 62 559. 33 N. Schepp D. Shuckla H. Sarker and K. U. Ingold J. Am. Chem. Soc. 1997 119 10 325. 34 S. Kim Y. Kim and K. S. Yoon Tetrahedron Lett. 1997 38 2487. 35 S. K. Hu and D. C. Neckers J. Org. Chem. 1997 62 755. 36 C. Chatgilialoglu C. Ferreri M. Lucarni A. Venturini and A. A. Zavitsas Chem. Eur. J. 1997 376. 37 J. H. Horner F. N. Martinez M. Newcomb S. Hadida and D. P. Curran Tetrahedron Lett. 1997 38 2783. 38 J. R. Lowe and N. A. Porter J. Am. Chem. Soc. 1997 119 11 534. 39 A. Gugger R. Batra P. Rzadek G. Rist and B. Giese J. Amem. Soc. 1997 119 8740. 40 S. Peukert R. Batra and B. Giese Tetrahedron Lett. 1997 38 3507. 41 D. Crich and X. S. Mo J. Am. Chem. Soc. 1997 119 249. 42 D. Crich and X. S. Mo Tetrahedron Lett. 1997 38 8169. 43 M.J.Robins Z. Q. Guo and S. F. Wnuk J. Am. Chem. Soc. 1997 119 3637. 44 F. L. Cozens M. O’Neill R. Bogdanova and N. Schepp J. Am. Chem. Soc. 1997 119 10 652. 45 R. Lenz and B. Giese J. Am. Chem. Soc. 1997 119 2784. 46 S. N. Mu� ller R. Batra M. Senn B. Giese M. Kisel and O. Shadyro J. Am. Chem. Soc. 1997 119 2795. 47 D. Crich J. Escalante and X. J. Jiao J. Chem. Soc. Perkin Trans. 2 1997 627. 48 A. Ghamini and J. Simonet New J. Chem. 1997 21 257. 49 L. K. Mehta M. Porssa J. Parrick L. P. Candeias and P. Wardman J. Chem. Soc. Perkin Trans. 2 1997 1487. 50 C. P. Andrieux J. M. Save� ant A. Tallec R. Tardivel and C. Tardy J. Am. Chem. Soc. 1997 119 2420. 51 M.L. Andersen W. Long and D. D. M. Wayner J. Am. Chem. Soc. 1997 119 6590. 52 V. D. Parker E. T. Chao and G. Zheng J. Am. Chem. Soc.1997 119 11 390. 53 Z. Y. Su D. E. Falvey U. C. Yoon and P. S. Mariano J. Am. Chem. Soc. 1997 119 5261. 54 K. P. Dockery J. P. Dinnocennzo S. Farid J. L. Goodman I. R. Gould and W.P. Todd J. Am. Chem. Soc. 1997 119 1876. 55 A. L. Perrott H. J. P. deLijser and D. R. Arnold Can. J. Chem. 1997 75 384. 56 D. B. Denney D. Z. Denney and S. P. Fenelli Tetrahedron 1997 53 5397. 57 H. J. P. deLijser and D. R. Arnold J. Org. Chem. 1997 62 8432. 58 D. R. Arnold K. A. McManus and M.S. W. Chan Can. J. Chem. 1997 75 1055. 59 R. Torriani M. Mella E. Fasani and A. Albini Tetrahedron 1997 53 2573. 60 C. S. Q. Lew J. R. Brisson and L. J. Johnston J. Am. Chem. Soc. 1997 119 4047. 61 H. J. P. deLijser and D. R. Arnold J. Chem. Soc. Perkin Trans. 2 1997 1369. 62 T. Nakamura W. K. Busfield I. D. Jenkins E. Rizzardo S. H. Thang and S. Suyama J. Org. Chem. 1997 62 5578. 63 H. D. Roth H. X. Wang and T. Herbertz Tetrahedron 1997 53 10 051. 336 Daniel E. Fa

ISSN:0069-3030    

DOI:10.1039/oc094321

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

10 Theoretical organic chemistry By IAN ALBERTS EMBL Outstation European Bioinformatics Institute Wellcome Trust Genome Campus Hinxton Cambridge UK CB10 1SD and Department of Biological Sciences University of Stirling Stirling UK FK9 4LA 1 Introduction Adiverse range of publications appeared in the theoretical organic chemistry literature in 1997 from highly accurate electronic structure calculations of small molecules to computer simulations of large bio-organic systems and reactions in solution. In a selective review of the literature this report is divided into two main sections; advances in theoretical methodology and their applications. 2 Theoretical advances Traditionally methods used in theoretical organic chemistry could be classified as molecular orbital (MO) and empirical force-field based methods.MO methods are used for predicting the energetics structures properties and reactivity of relatively small to medium-sized molecular systems. Empirical force-field based methods in conjunction with molecular dynamics (MD) and Monte-Carlo (MC) simulations are used for determining thermodynamic data including hydration free energies and structural information of much larger biomolecular systems. However with the advent of quantum mechanical (QM) methods that scale more favourably with system size and the use of quantum mechanical potentials in dynamic simulations the division is no longer clear cut. In this section recent advances in quantum mechanical methodology are reviewed particularly with regard to the study of ‘large’ molecular systems and the inclusion of solvation e§ects.This section also includes a summary of studies that assess the performance of highly correlated wavefunctions and the latest density functional methods and model chemistries. Large molecular systems The problem of scalability is one of the most serious bottlenecks in quantum chemistry calculations. Until recently the main limiting factor in the application of Hartree –Fock (HF) and Density Functional Theory (DFT) methods to large molecular systems has been the computation of the two-electron integrals which scales as N4 where N is the number of basis functions. However as reviewed by Gao in last year’s 337 report,1 methods have recently been introduced to overcome this problem and nearlinear scaling has been achieved in integral evaluation in the large system limit.In 1997 Schwegler and co-workers applied a linear scaling approach to benchmark HF calculations on water clusters polyglycine a-helices and the P53 tetramerization monomer and reported microhartree accuracy in total energies.2,3 The latter calculation involved 698 atoms and 3836 basis functions. As a result of the above advances the most time intensive step in HF and DFT calculations for large systems is the diagonalisation of the Hamiltonian. This is an N4 step with anN2 memory requirement. In the last year several alternatives to diagonalisation have been developed most of which use a basis of localised functions. Rubio et al. propose solving the Self-Consistent Field (SCF) equations using localised molecular orbitals so avoiding the use of canonical MOs which require diagonalisation of the Hamiltonian.4 The method involves an iterative scheme to satisfy Brillouin’s theorem and involves diagonalisation of single Configuration Interaction (CI) matrices.Millam and Scuseria report a conjugate-gradient density-matrix search (CG–DMS) method,5 that is based on the approach of Li et al.,6 and involves searching for a density matrix that minimises an energy functional subject to density matrix idempotency. The reduction of the computational cost and memory requirements to linear scaling for large systems is achieved by setting a neglect threshold for the elimination of elements in the sparse Fock and density matrices. In HF/3-21G and local density functional (LDA) calculations on a polyglycine chain containing 20 glycine units (20-glycine) a neglect threshold of 10~6 au gave microhartree accuracy in the total energy.The cross-over in cpu time between diagonalisation and CG–DMS for a sequence of polyglycine chains with LDA/3-21G was found to occur at 50-glycine. For 70-glycine CG–DMS was about 25% faster than diagonalisation. For water clusters which are more compact with less sparse Fock and density matrices the cross-over took place at larger systems; with neglect thresholds of 10~4 5]10~5 10~5 and 10~6 au the cross-over occurred at 160 200 380 and 400 water molecules respectively. The cross-over points for the latter two cluster sizes were estimated by extrapolation (the largest calculation performed was for 300 water molecules). The CG–DMS scheme has also been implemented into semi-empirical procedures and benchmark AM1 calculations were performed using this method on polyglycine chains (20 000 atoms) water clusters (up to 1800 atoms) and nucleic acids (up to 6304 atoms).7 Baer and Head-Gordon propose an alternative scheme involving a Chebyshev polynomial expansion of the density matrix as a power series in the Hamiltonian matrix.8 The method was applied to large saturated hydrocarbon systems containing over 200 carbon atoms using a tight binding Hamiltonian and again the cross-over point relative to direct diagonalisation for various density matrix accuracy levels was assessed.In this procedure the expansion length of the Chebyshev series determines the numerical accuracy. Dixon and Merz describe a divide and conquer scheme for avoiding the diagonalisation of the full Fock matrix in semi-empirical calculations.9 In this approach a large system is divided into a series of smaller overlapping subsystems each of which requires diagonalisation of a comparatively small matrix from which the global description can be constructed.Again above a certain cross-over point the divide and conquer scheme becomes cheaper than the full diagonalisation approach. Using this scheme semi-empirical calculations were carried out on proteins with up to 1960 atoms on a 32Mb memory workstation. 338 Ian Alberts Finally Lee and Head-Gordon describe the use of polarised atomic orbitals (PAOs) in HF and DFT calculations of large systems.10 PAOs are a small set of atomic orbital basis functions typically of minimal basis set size which are defined as a linear combination of a much larger set of basis functions.A PAO–SCF calculation involves the simultaneous optimization of the PAOs and SCF density matrix (which is defined in the PAO basis set). This calculation is applicable to large systems which may be inaccessible with a basis set larger than minimal. SCF calculations of the energies dipole moments and geometries of several molecules show that well-chosen PAOs are superior to the STO3G minimal basis set and may even approach 6-31G* quality. Assessment of methods Traditional quantum mechanical methods The assessment of the accuracy of electronic structure methods has been a topic of much activity during the last year. Traditional quantum mechanical methods tend to give results that vary in a systematic manner as the level of electron correlation and basis set is improved. The determination of this systematic variation is of interest since calculations could be extended to more complex systems and an empirical correction applied to account for the systematic errors.Such an assessment is in fact at the core of Pople’s composite GAUSSIAN-2 (G2) procedure which assumes certain additivity assumptions particularly relating to the e§ects of basis set extensions to yield final energies at e§ectively the QCISD(T) level of theory with a 6-311]G(3df,2p) basis set.11 Pople and co-workers assessed G2 in 1991 by calculating the enthalpy of formation of 55 molecules for which the experimental results were known to have high accuracy. In 1997 this data set of molecules was extended to 148 including 29 radicals 35 non-hydrogen systems 22 hydrocarbons 47 substituted hydrocarbons and 15 inorganic anhydrides.12 The G2 theory proves to be reliable for the systems studied giving an average absolute deviation of 1.58 kcal mol~1 although the maximum deviaton was 8.2 kcal mol~1.When the G2 method is applied to larger molecules errors tend to accumulate. More accurate heats of formation can be obtained via isodesmic or homodesmic reactions rather than from atomisation energies in combination with experimentally known heats of formation of the constituent atoms as in standard G2. Isodesmic reactions are defined by ‘bond separation reactions’ where bonds between nonhydrogen atoms are separated into the simplest parent molecule containing these same linkages. Raghavachari et al. applied these ideas to 40 molecules; 37 from the G2 test set plus cyclopentane methyl acetate and tert-butyl alcohol and using G2 energies for the isodesmic reactions obtained an absolute mean deviation of only 0.5 kcal mol~1 in the heats of formation.13 Petrie reported calculations applying G2 and its variants G1 G2(MP2) G2(Q) and G2Q(QCI) to the energetics of 39 molecular dications and found that the various G2 methods perform relatively poorly for dications compared to neutral or singly charged species.14 The warning is given that ‘such theoretical methods should therefore be used with caution in determining the properties of dications’.The convergence characteristics of correlation consistent basis sets with high level correlated wavefunctions for the calculation of bond energies and equilibrium geometries have been investigated.15,16 Significant basis set dependencies were found.For example for the CHn and C 2 Hn (n\1–4) series of molecules average errors using the 339 Theoretical organic chemistry coupled cluster CCSD(T) wavefunction were 5.6^3.0 1.4^0.8 and 0.5^0.4 kcal mol~1 for bond energies and 0.020 0.003 and 0.002Å for equilibrium bond lengths with the cc-pVDZ cc-pVTZ and cc-pVQZ basis sets respectively. Martin and Taylor investigated the basis set convergence of total atomization energies of small polyatomic molecules.17 The CCSD(T) wavefunction was used with a systematic series of correlation consistent basis sets combined with basis set extrapolations of the SCF and correlation energies. Molecular total atomisation energies were determined with a mean absolute error as low as 0.12 kcal mol~1. In other areas Murphy et al.describe a generalised valence bond (GVB) method augmented by local second order Møller–Plesset perturbation theory (LMP2) in a pseudospectral scheme (PSGVB-LMP2).18 Pseudospectral methods use grid-based techniques to construct the coulomb and exchange matrix elements and so do not require the computation of the 2-electron integrals thus making them more e¶cient and applicable to larger systems than traditional MO methods. The GVB-LMP2 formulation constitutes a well-defined multi-reference method which is applicable automatically to molecular systems. This is unlike other multi-reference techniques which require significant e§ort to determine a suitable set of reference wavefunctions. In applying the method to the calculation of conformational energy di§erences in 36 molecules Murphy and co-workers found an error bar of 0.4 kcal mol~1 relative to experimental energy di§erences except in one of the cases formic acid where it was postulated that the experimental value may not be as reliable as the theoretical result.A parallel version of the PSGVB-LMP2 method has been developed which can be applied to larger systems.19 Density functional theory In recent years density functional theory methods have become very popular. It is clear that DFT methods have many advantages; they scale well with system size implicitly include electron correlation e§ects and the accuracy of DFT methods is comparable to correlated ab initio methods such as MP2 which do not scale as well. DFT methods can therefore be applied to larger molecular systems than traditional ab initio methods.However despite much work to understand the form of the exchange-correlation energy functional20,21 there is as yet no clear approach to improve the quality of the density functionals in a systematic fashion and so it is important to test the performance of existing functionals. The 1997 literature showed a lot of activity in the prediction of DFT molecular energies and properties in comparison to accurate experimental results if available or high level calculations using traditional correlated methods. Curtiss et al. used seven DFT methods including pure and hybrid functionals to predict the heats of formation of the 148 molecules in the G2 test set12 (see ref. 12 for a description of these functionals). It was found that the hybrid B3LYP method gave the smallest mean absolute deviation from experiment of 3.1 kcal mol~1 about double the G2 error.Raghavachari et al. used these seven functionals to calculate the heats of formation of the set of 40 molecules mentioned earlier.22 Using atomization energies the mean absolute errors in the calculated heats of formation for the various DFT methods varies greatly from 153.0 kcal mol~1 for local DFT to 33.3 kcal mol~1 for B3P86 and 2.7 kcal mol~1 for B3LYP. Using the isodesmic scheme there is an improvement in values for all the functionals. This is particularly significant for the local 340 Ian Alberts DF where the error drops from 153.3 to 4.0 kcal mol~1. Again B3LYP gives the smallest error of 1.5 kcal mol~1 about triple the G2 error. Interestingly the best results are obtained with a large 6-311]G(3df,2p) basis set however using a smaller basis set such as 6-311G(d,p) does not significantly reduce the accuracy; the mean deviation with B3LYP/6-311G(d,p) is only 1.9 kcal mol~1.The suitability of DFT methods for determining the electronic structure of atomic and molecular anions and hence electron a¶nities has recently been assessed.23,24 This is potentially a problem in DFT due to the use of non-exact functionals. The electron–electron Coulomb repulsion in the Kohn–Sham equations contains electron self-repulsion which must be cancelled precisely by the exact exchange-correlation term. However the functionals currently in use are approximations which do not cancel the self-interaction terms exactly leading to incorrect asymptotic behaviour of the potential. The outermost electron in anions is loosely bound and samples the region far from the nucleus and so the incomplete cancellation of self-repulsion may be the most severe in anions.Bearing this in mind Tschumper and Schaefer report the electron a¶nities of the 8 first row atoms and 27 first row diatomic and triatomic molecules using six di§erent DFT or hybrid HF/DFT methods.24 These species were chosen as they have relatively well established experimental electron a¶nities. In general it was found that the hybrid DFT methods give the shortest and most accurate bond lengths and that electron a¶nities increase in the following order BHLYP\expt.\BLYP B3LYP\BP86 B3P86 (@LSDA). The BLYP functional provides the best agreement with experiment for the anions studied with an average absolute error of 0.21 eV.The agreement is also reasonable for the B3LYP BHLYP and BP86 functionals which give errors of about 0.3 eV. B3P86 and LSDA give larger absolute errors of 0.71 and 0.75 eV respectively. The DZP]] basis set was employed in this study since Galbraith and Schaefer previously showed that electron a¶nities are relatively insensitive to the basis set size.25 In general for the systems studied most of the DFT methods compute electron a¶nities that are too high whereas traditional QM methods generally give electron a¶nities that are too low and very sensitive to the basis set employed. The DFT results suggest that introduction of a scaling factor to calibrate the theoretical methods against experimental electron a¶nities may be possible. It was also assumed that some kind of fortuitous cancellation of errors must be taking place such thatDFT predictions of electron a¶nities lie reasonably close to the experimental values and the functionals perform almost as well as highly correlated ab initio methods.Furthermore Tschumper and Schaefer failed to find that the ill e§ects arising from self-repulsion were most severe for atomic anions as had been suggested earlier26 since for the systems studied the best agreement between theory and experiment was for the values of the atomic electron a¶nities. In a series of papers Jursic examined the performance of DFT methods in determining activation barriers. It was shown that hybrid DFT methods can correctly model the activation barriers for cycloaddition,27 rearrangement,28 ring opening29 and isomerisation30,31 reactions. However Jursic found that DFT methods were not able to reproduce the experimental activation barriers for the hydrogen radical reactions32 –34 [reactions (1) and (2)].C 2 H 4 ]H]C 2 H 5 (1) 341 Theoretical organic chemistry H 2 ]OH]H 2 O]H (2) For the former reaction DFT methods had great di¶culty in locating the transition state structure and the activation barrier was poor. Only B3LYP/6-311G(2d,2p) gave a computed activation barrier in reasonable agreement with experiment. Interestingly hybrid DFT methods were able to reproduce the experimental value for the reverse reaction. MP2 and QCISD also failed to reproduce the experimental barriers for the forward and reverse reactions however G2 gave accurate values in good agreement with experiment. For the latter reaction only the hybrid DFT methods were able to locate a transition state on the potential energy surface however the activation barriers predicted for the forward and reverse reactions were up to 5 kcal mol~1 below the experimental values.Jursic postulated that DFT methods are likely to fail to produce correct activation barriers for reactions involving a hydrogen radical due to a systematic problem in evaluating the total energy of this species. In a systematic analysis Soliva et al. examined the quality of the molecular electrostatic potential (MEP) and MEP-derived properties computed using DFT methods.35 For a set of 35 neutral and charged molecules electrostatic properties such as electrostatic potential (ESP) derived charges and dipole moments were examined. The hybrid non-localDFTfunctionals B3LYP and B3P86 gave the best results in comparison with those derived from MP4 computations.It is clear that quantum mechanical methods are an important tool in chemical investigations. In particular despite some problems noted above non-local hybrid DFT methods generally give reliable results for a remarkable range of chemical problems. These functionals have been used extensively in the applications discussed below. Solvation Solvent e§ects play an important role in the structure and reactivity of organic molecules and so any appropriate theoretical model must treat solvation. Methods for including solvent are typically divided into two types continuum and explicit solvent models. In continuum models the solute is inside a cavity which is immersed in a polarizable dielectric medium.This model is typically used to assess the e§ect of bulk solvent inQMcalculations of relatively small species or in simulations of biomolecular structures. In the alternative approach solvent molecules are explicitly considered using classical force-field based methods. This approach is computationally more demanding yet includes details of specific solute–solvent interactions. In molecular dynamics and Monte Carlo simulations explicit solvent treatments are preferred due to the current quality of empirical potential functions and the development of combined quantum mechanical-molecular mechanical potentials. In this review developments in the two areas will be discussed separately. Continuum models These methods are significantly cheaper computationally than explicit solvent treatments and can readily incorporate polarization of solute and solvent.The self-consistent reaction field (SCRF) is a typical method for incorporating electrostatic solute –solvent interactions. In this scheme the solute is treated quantum mechanically and a reaction field potential V R is introduced into the Hamiltonian as a perturbation 342 Ian Alberts to account for solute–solvent interactions such that eqns. (3) and (4) apply H™ 0W0\E0W0 in vacuo (3) [H™ 0]V R ]W\EW in solution (4) whereH™ 0 is the Hamiltonian of the solute in vacuo W0 and W are the solute wavefunctions in vacuo and in solution respectively and V R is the reaction field operator. Various SCRF schemes have been developed that di§er principally in the description of the cavity shape and the treatment of the reaction field.The most commonly used schemes are the apparent surface charge (ASC) model in which the reaction field is represented by apparent charges spread on the cavity surface and the multi-polar expansion (MPE) model in which the solute charge distribution is expanded in terms of spherical or elliptical harmonic functions. Initial implementations were limited by the use of spherical (or ellipsoidal) cavities however current implementations use molecular shaped cavities which are defined by the union of van der Waals spheres or isodensity surfaces determined self-consistently. The solvation free energy can be calculated as a sum of the contributions from the electrostatic energy and estimates for the dispersion36 and cavitation37 energies. The polarizable continuum model (PCM) is one of the most commonly used ASC methods.Pomelli and Tomasi describe their implementation of a new formulation of the PCM solvation model for which analytical gradients can be evaluated.38 Most reaction field methods treat only isotropic homogeneous dielectrics. In this respect Cance` s et al. present a new integral equation formalism of the PCM method which allows di§erent types of dielectrics to be treated including anisotropic solvents such as liquid crystals solid matrices or ionic solutions.39 Dielectric continuum models often assume that the solute charge is completely enclosed in the cavity. However a quantum mechanical description of the solute charge distribution which decays exponentially leads to a tail in the wavefunction or solute charge density that penetrates outside the cavity.In recent developments improved approaches to treat solute charge tails have been implemented in ASC40,41 and multipolar expansion42 methods. Richardson et al. describe an alternative scheme involving solution of the Poisson –Boltzmann (PB) equation for the electrostatic potential that has been implemented in the Amsterdam Density Functional (ADF) program.43 The PB approach is similar in many ways to the polarization continuum model and can incorporate ionic strength e§ects an ionic exclusion radius or Stern layer and multi-dielectric media. The method was used to predict the pK! values of several organic acids and overall the results were in good agreement with experiment. Interestingly solution of the non-linear Poisson–Boltzmann equation has been used successfully in classical models of electrostatic interactions in macromolecular systems for a number of years.During 1997 this approach was extended to treat monovalent and divalent salt e§ects44 and electrostatic binding of proteins to membranes.45 Lee and Tidor report a novel method to determine the ligand charge distribution that results in optimal binding to a given receptor using analytical solutions to the Poisson equation.46 The Poisson–Boltzmann equation is usually solved by finite-di§erence grid-based methods. As an alternative David and Field47 report a basis set approach to the PB equation. The electrostatic potential /(r) is expanded as a linear combination of N 343 Theoretical organic chemistry basis functions fi to give eqn. (5) /(r)\ N; i/1 ci fi(r) (5) where c* the expansion coe¶cients are obtained by minimization of the electrostatic energy.Gaussian basis functions were chosen to represent the charge distribution of the molecule. In studies of the solvation energies of methanol formate ion acetic acid and methazolamide in water it was found that basis sets with only a small number of functions of the order of N where N is the number of atoms gave results that were in good agreement with those obtained from the finite-di§erence solution of the PB equation. Honjou et al. describe a new method to determine bulk solvent e§ects on the absorption maxima of molecules.48 The method employs the PCM model coupled with a semi-empirical CI procedure. The key to the derivation of the CI matrix elements is that the solvent polarization is correctly partitioned into orientational and electronic contributions.For the vertical excitations considered in this study only the change in the electronic polarization of the solvent is included in the calculations as the solvent orientation is frozen. Test calculations reproduce quite well the experimental solvatochromic shifts for a merocyanine dye in a variety of solvents. Explicit solvent models When there are strong interactions between solute and solvent methods that include the treatment of explicit solvent molecules are appropriate such as the supermolecule approach integral-equation methods and models in which solvent molecules are represented by empirical force fields. The ab initio supermolecule approach involves an extension of the quantum mechanical treatment to include explicit solvent molecules that directly interact with the solute.Continuum models can be used to solvate the supermolecule. This scheme has been applied to the study of tautomeric equilibria and reaction pathways in aqueous solution (see Section 3). A related approach is the e§ective fragment potential (EFP) model in which the solute is treated quantum mechanically while the solvent molecules are modelled by a parametrized potential that includes terms for electrostatics polarizability exchangerepulsion and charge-transfer. The EFPs are one-electron potentials and so only modestly increase the cost of the soluteQMcalculation. Such a scheme is somewhat in the spirit of the ab initio supermolecule approach but while the latter can in practise only accommodate a small number of solvent molecules EFP is computationally much cheaper and can accommodate full solvent shells.This can have important consequences in studying solvent e§ects. For example Day and Pachter have used the EFP model to study the equilibrium involving aqueous glutamic acid.49 It is known that the neutral isomer is favoured in the gas phase and the zwitterion is preferred in solution. EFP calculations with one and two solvent molecules and an HF description of the solute were shown to be in good agreement with full ab initio supermolecule calculations thus giving confidence for use of the EFP model with more water molecules. The neutral isomer was found to be more stable with zero one and two water molecules however EFP calculations with 10 water molecules where lowenergy conformations were determined by Monte Carlo simulations found the zwitterion to be more stable as expected.The EFP scheme has also been incorporated into 344 Ian Alberts the MCSCF method and used to study the excited states of formamide.50 As a test of the EFP method it was found that the structures and energetics of formamide with three water molecules modelled by EFP were in good agreement with the all electron case. Moriarty and Karlstro� m used a similar method in a Monte Carlo scheme to model the structure of a water molecule in liquid water.51 Hirata and co-workers report further developments and applications of the reference interaction site model (RISM). This is an integral-equation method based on statistical mechanics that lies between continuum and explicit solvent descriptions. The algorithm for solving the RISM equations for pure water was extended to solutions with finite salt concentrations by employing the theory of dielectric consistency.Using an extended simple point charge model (SCP/E) for water the method was used to analyse the structure of salt solutions in the bulk medium and near a noble gas solute atom.52 The RISM method was also used to study the structure and stability of peptide conformations in aqueous solutions using the SCP/E water model. It was found that the peptide conformation greatly influences the hydration free energy and water structure near the peptide. For met-enkephalin which has a rather compact form in the gas phase due to intramolecular hydrogen bonding the calculations showed that a fully extended conformation has the lowest energy in water in qualitative agreement with experiment.53 An attractive method to incorporate explicit solvent molecules is the hybrid quantum mechanical molecular mechanical (QM–MM) scheme.In this procedure the system is partitioned into a smallQM fragment that describes the solute and a larger empirically treated fragment that represents the solvent. Alternatively in the study of enzyme catalysis the QM fragment could describe an enzyme active site with the surrounding protein represented by the MM fragment. A classical Hamiltonian consisting of Coulombic and van der Waals terms is used to describe the interactions between the QM andMMatoms. Derivatives of the QM–MM energy with respect to the atomic coordinates can be computed in a standard way and used in energy minimizations and MD or MC simulations.Of course simulations involving QM or QM–MM potentials are far more computationally expensive than purely classical simulations and so are applied to smaller systems. An important advantage of using a QM potential is that bond breaking/making phenomena on the potential energy surface can be followed. In most fluid simulations using the QM–MM potential the QM fragment is treated by semi-empirical methods however recent rapid advances in computing performance mean that it is now possible to treat theQMregion using more accurate non-empirical SCF post-SCF and DFT methods. For example Tun8 o� n and co-workers describe the application of hybrid DFT–MM molecular dynamics to the simulation of chemical processes in solution,54,55 specifically the bromination of ethylene in water and proton transfer between a water molecule and the hydroxide ion in water.Alternatively it would be possible to implement the QM–MMpotential in a Car–Parinello (CP) scheme. In general a DFT approach is used in the CP scheme often the local density approximation with pseudopotentials to describe core electrons and plane wave basis sets rather than Gaussian atomic-orbital like basis sets. In principle CP–MD is computationally more e¶cient than traditional QM–MD however in practice very short time steps may be required which may reduce the e¶ciency.56 In 1997 Curioni et al.57 reported a DFT CP–MDsimulation of the induction phase of the acid catalysed 345 Theoretical organic chemistry polymerisation of 1,3,5-trioxane the cyclic trimer of formaldehyde. The authors claimed that this was the first attempt to determine the free-energy profile of a complex system in the condensed phase using non-empirical methods.The lack of solvent polarization e§ects has been one of the major deficiencies of the QM–MM approach. Based on earlier work of Thompson and Schenter,58 Gao describes the implementation of a solvent polarizability model that uses a polarizable intermolecular potential function to represent the interactions between theQMsolute and the solvent-induced dipoles.59 The induced dipole ki at the ith MM atom depends upon the total electric field E(ri) at position ri [eqn. (6)] ki \aiE(ri)\ai[E QM (ri)]E MM (ri)] (6) where ai is the polarizability of atom i and E QM (ri) and E MM (ri) are the electric fields due to theMMandQMatoms respectively at position ri.The soluteQMwavefunction is also a function of the solvent-induced dipoles and thus a coupled iterative scheme must be applied until self-consistency is reached. An energy decomposition scheme was also proposed in this work and the method was used in the Monte Carlo simulations of a series of organic solutes in aqueous solution. Polarization e§ects were found to contribute 10–23% of the total solute–solvent interaction energies and so their neglect can be a serious problem. The polarizable QM–MM scheme was also used to study the e§ects of solvation on the structure and electronic absorption energy of conted cyclic and heterocyclic compounds.60,61A semi-empirical configuration interaction singles (CIS) wavefunction was used to describe the electronic structure of the solute excited states.The computed structural changes in passing from the gas phase to solution and the solvatochromic shifts were found to be in agreement with the experimental results. In a similar approach Sakuma et al. used an atom polarizable model to study the chlorophyll dimer in the photosynthetic reaction centre.62 In this study the system was divided into a QM region that describes the chlorophyll dimer and a classical region in which the atoms were considered to be classical point charges with induced electric dipoles that represents the rest of the protein. The Poisson–Boltzmann equation was used to model bulk solvation of the system. The induced dipole at the ith classical atom therefore depends upon the electric fields due to the continuum classical and quantum regions at the atom position.The primary charge-separation step in the photosynthetic reaction centre involves donation of an electron from the chlorophyll dimer. The calculations show that the inclusion of polarization e§ects is important for this process and that there is an unequal distribution of electron densities for the unpaired electron between the two chlorophyll molecules in agreement with the most recent experimental results. Alternative approaches to incorporate solvent polarization based on the fluctuating charge (FC) model have been described by Field63 and Gao.64 These schemes employ the principle of electronegativity equalisation and are related to methods developed by Rappe� and Goddard65 and Rick et al.66 The first approach is a QM FC model in which each solvent molecule is represented by a QM wavefunction and interactions with all other solvent molecules are modelled by the hybrid QM–MM potential.Solvent polarization is accounted for in the QM wavefunction. In the second approach the FC model is applied to the molecular mechanics atoms in the QM–MM procedure and involves iteratively solving a set of linear equations in a straightforward 346 Ian Alberts implementation. In both approaches derivatives of the energy with respect to the atomic coordinates can be computed and used in energy minimizations or computer simulations. Field used the QM FC approach to simulate liquid water and the QM–MM FC scheme to model methane and formaldehyde in water.63 The main limitation in these studies was the use of an AM1 or STO3GQMwavefunction which was not able to reproduce the gas phase properties of the solute or the water monomer and dimer.Bersuker et al. report a QM–MMprocedure that includes charge transfer between theQMandMMfragments.67 In addition to the usual partitioning of the system into a smallerQMfragment and a largerMMfragment this scheme involves the identification of an intermediate fragment (which could be the whole of theMMregion if it is not too big). The intermediate fragment is treated by both QM and MM methods allowing double self-consistency to be reached and a smooth transition from QM to MMatoms including charge transfer between them. The method was applied to iron picket-fence porphyrin since charge transfer is known to be particularly significant in molecular systems that contain transition metals. INDO/1 semi-empirical QM–MM calculations gave the correct out-of-plane position of the iron with respect to the porphyrin ring and an estimated charge transfer 3.6 electrons from theMMto theQM region.Sanchez et al. describe a mean field theory in which the solvent distribution is given by a set of point charges that reproduce the average solvent electrostatic potential determined from molecular dynamics.68 The method was implemented in aQM–MM scheme and using HF and MP2 solute wavefunctions was able to adequately reproduce the energetics and properties of formamide solutions. Computer simulations can su§er from the e§ects of truncating the long-range electrostatic interactions with standard spherical cut-o§s. Several methods have been developed to include these long-range interactions including reaction field approaches the Ewald lattice sum technique and fast multi-pole methods.Gao and Alhambra introduced the Ewald lattice-sum method into the QM–MM procedure and conducted dynamics simulations to compare the free energy of hydration of chloride ions determined with the Ewald method and with standard spherical cuto §s.69 A di§erence of[13.2 kcal mol~1 was obtained between the two schemes due to the long-range electrostatic contributions. 3 Applications Tautomerism and conformational analysis Continuing with their earlier work on the tautomers of heterocyclic aromatic compounds Herna� ndez et al. investigated the tautomerism of 2-azaadenine and 2-azahypoxanthine using quantum mechanical methods including DFT and SCRF continuum models.70 This is a challenging problem as many tautomers are possible for each molecule.The tautomerisation free energy in solution was determined according to eqn. (7) DG!2 A/B \DG'!4 A/B ]DG):$ B [DG):$ A (7) where DG'!4 A/B is the tautomerisation free energy in the gas phase and DG):$ A and DG):$ B 347 Theoretical organic chemistry are the hydration free energies of each tautomer. 2-Azaadenine has a clear preference for amino tautomers and in the gas phase 1 is the most stable structure with an energy separation of more than 2 kcal mol~1 to the next stable tautomer. In solution tautomer 2 is preferentially stabilised however it is still about 2 kcal mol~1 higher in energy than 1. The other tautomers are more than 5 kcal mol~1 higher in energy therefore 2-azaadenine is expected to exist predominantly (97%) as tautomer 1 in solution.Keto tautomer 3 is the most stable species for 2-azahypoxanthine in the gas phase but it is only about 1 kcal mol~1 lower in energy than another keto tautomer 4. Solvation does not significantly a§ect the tautomeric energy di§erences therefore 3 is the predominant species in solution but a significant proportion of tautomer 4 is expected. The tautomeric stability of the related compounds hypoxanthine and xanthine has also been studied.71 The results of these studies have been used to qualitatively explain the susceptibility di§erences exhibited by these compounds to xanthine oxidase enzymatic action. N N N N N H NH2 N N N N N NH2 N N N N N H O N N N N N O H H H H 1 2 3 4 Luque et al. report studies on the tautomeric equilibria of several heterocyclic molecules.72 High level ab initio calculations up to MP4 were performed for the gas phase molecules and the PCM model was used to describe solvation.An incorrect ordering of tautomer stability was found for 3-hydroxypyrazole. Three tautomers were studied in detail for this molecule and in the gas phase the stability ordering was found to be 5[6[7. UsingPCMhydration energies tautomer 7 was predicted to be the least stable species in aqueous solution. However experimental results indicate that 7 is the most stable tautomer. Monte Carlo free energy perturbation (FEP) simulations performed previously showed the importance of first-solvation shell e§ects which are not considered in the PCM model and combining MC–FEP hydration energies with the ab initio gas phase results leads to a stability ordering in agreement with experiment.Similarly Adamo et al. find that both specific and bulk solvent e§ects play a role in the tautomerisation of formamide via proton transfer.73 N N O H H N N O H N N O H H H 5 6 7 Craw et al. studied the tautomeric amine–imine equilibrium in creatinine and the barrier to rotation about the amine C–N bond in aqueous environments.74,75 Two tautomeric forms of creatinine are known to exist. In the gas phase high level QM calculations show that the imine form 8 is preferred whereas in the condensed phase the amino species 9 is predominant according to experiment and theory.75 The amino 348 Ian Alberts N C N C H2C O C N H H H H H N C N C H2C O C N H H H H N C N C H2C O C N H H H H H H O H N C N C H2C O C N H H H H H H O H O H H 9 10 11 form involves a degree of conjugation along the H 2 NC–N–C–O moiety and the gas phase C–NH 2 barrier is predicted to be 7.3 kcal mol~1.Employing the continuum solvation model leads to a predicted barrier of 9.0 kcal mol~1 due to greater solute polarisation in the planar amine form however this value is still below the experimental barrier of 13.0^1.0 kcal mol~1. The inclusion of an explicit water molecule preferentially stabilises the ground state structure 10 due to the formation of a six-membered ring that involves strong hydrogen-bonding interactions and increased delocalisation. Solvation of this structure using the PCM model increases the barrier to 11.2 kcal mol~1. Incorporation of a second explicit water molecule giving structure 11 further increases the barrier to 12.2 kcal mol~1 in excellent agreement with the experimental result.The interactions between creatinine and an artificial receptor in aqueous solution have also been examined.76 The major interactions in the guest–host complex involve H-bonding contacts that stabilise a high energy zwitterionic form of the receptor by 25.2 kcal mol~1 relative to the lowest energy neutral form. This stabilisation is reduced to 12.2 kcal mol~1 in solution due to the lower solvation energy of the complex compared to that of the guest]host. Basis set superposition errors reduce this value by a further 6.3 kcal mol~1 however the guest–host complex is still bound by about 6 kcal mol~1. Urban et al. used semi-empirical and ab initio methods to study the conformational preferences of 2-phenethylamines in the gas phase and in solution.77 Neutral and charged phenethylamines are predicted to prefer a folded gauche conformation in the gas phase due to a stabilising interaction between the amino group and the aromatic ring.Empirical force fields only gave reasonable results for this system when AM1- CM1A78 electrostatic charges were used. As supported by experiment solvent e§ects modelled by GB/SA79 and SM280 lead to a more equally populated mixture of the anti and gauche conformations in aqueous solution. This results from preferential solvation of the anti form since hydrogen-bonding with solvent would be expected to become competitive with the intramolecular p–amino interaction. The best agreement with experiment for solvation free energies was obtained when the MP2/6- 311]G(d,p)//MP2/6-31G(d,p) method was used in concert with the SM2 continuum model.Conformational preferences were also a§ected by the electron donating or withdrawing power of substituents introduced to the ring. Smallwood and McAllister used high level ab initio methods to investigate the e§ect of geometrical distortions on the strength of the short-strong low-barrier hydrogen bond (LBHB) between formic acid and formate ion in the gas phase.81 Lengthening the hydrogen bond by 0.5 and 1.0Å severely weakens the bond strength by 6 and 12 kcal mol~1 respectively and a 30° angle distortion also leads to a 5 kcal mol~1 weakening. The intramolecular LBHB in the cis isomer of the hydrogen maleate anion 349 Theoretical organic chemistry C N H H OH C N H H OH C N H H OH2 + C NH2 H O H + HC N H OH2 + . .. H + 1,2-H-shift BR Scheme 1 is calculated to about 20 kcal mol~1 stronger than the normal intramolecular hydrogen bond in maleic diacid.82 The authors discuss these results in terms of the implications of short-strong or low-barrier hydrogen bonds in enzyme catalysis. Reaction pathways and transition states This section focuses on the location and characterisation of transition states on potential energy surfaces and the calculation of associated activation barriers of organic reactions. The determination of definitive activation barriers is a di¶cult problem and the calculated values are very dependent on the level of theory. The HF method generally does not yield accurate barriers however high level post-HF and in particular DFT methods have been found to yield reliable values for many reactions.The calculation of transition states allows theory to provide an important and in many ways unique contribution to the study of organic reactivity. Rearrangement reactions The Beckmann rearrangement has proved to be one of the most versatile and widely used approaches to convert oximes into amides. Nguyen et al. used correlated ab initio methods to explore substituent and solvent e§ects on the Beckmann rearrangement pathway of formaldehyde oxime.83 The reaction proceeds by protonation of the oxime to give the more stable and thus more populated N-protonated species followed by a 1,2-H shift to the O-protonated species (Scheme 1). The 1,2-shift connecting the two protonated isomers is rate-determining and the gas phase barrier is predicted to be about 54 kcal mol~1 at the MP4/6-311]]G(d,p) level well above the experimental value of 24 kcal mol~1.The bulk solvent model has a small e§ect on the activation barrier however ab initio calculations involving a supermolecule with one explicit solvent molecule lead to a 50% reduction in the barrier height thus approaching the experimental result. The water molecule acts like a catalyst by interacting strongly with the migrating hydrogen giving an oxime–H 3 O` complex in the transition state 12 which leads to the oxygen-protonated oxime. C N H H O H H O H H 12 Singlet phenylnitrenes and phenylcarbenes undergo a series of fascinating ring expansion reactions that were compared theoretically by Karney and Borden using (8/8)CASSCF/6-31G* and CASPT2N/6-311G(2d,p) methods.84 The reactions are expected to follow the steps as depicted in Scheme 2.Although both phenylnitrene and phenylcarbene have triplet ground states the ring expansions are thought to be singlet processes. Ring expansion of the open-shell 1A 2 state of phenylnitrene 13a takes place 350 Ian Alberts XN2 X X X D or hn PhN3/PhCHN2 13a,b 14a,b 15a,b a X = N b X = CH Scheme 2 via the intermediate azirine structure 14a. The cyclization of 13a to 14a is believed to be the rate determining step with an activation barrier of 6 kcal mol~1. Azirine then rearranges quickly to 15a with a small barrier of about 3 kcal mol~1. The 13a–15a rearrangement on the singlet surface is exothermic by 1.6 kcal mol~1. The correspond- N F F N F N F 17 18 16 ing ring expansion reaction of the 1A@ state of phenylcarbene has a barrier that is about 9 kcal mol~1 higher however the overall reaction is 19 kcal mol~1 more exothermic.This can be explained on the basis of the di§erent electronic structures of the lowest singlet-states of 13a and 13b. The e§ects of fluorine substituents on the ring expansion reactions of phenylnitrene were also studied.85 The barrier of the rate determining cyclization step for 2,6-difluorophenylnitrene 16 is 4.1–4.3 kcal mol~1 higher in energy than that for 4-fluorophenylnitrene 17 and the parent system 13a. Similarly for 2-fluorophenylnitrene 18 the barrier for cyclization towards the fluorine is 3.1 kcal mol~1 higher than that for cyclization away from the fluorine. It is clear that o-fluoro substituents increase the barrier heights for ring expansion. Analysis of the corresponding transition states shows that substantial steric repulsions as well as electrostatic repulsions are in operation when the nitrogen cyclises towards fluorine and are responsible for the increased barriers.This is supported by further calculations on the corresponding cyclization reactions of 2-chlorophenylnitrene and 2-methylphenylnitrene which also show higher barriers when the nitrogen cyclises towards the ortho-substituent. In a related study Xie et al. examined the complex potential energy hypersurface of naphthylcarbene and located transition states for various rearrangement processes.86 In complementary work Cramer and Falvey usedDFTto examine the S–T splitting in a series of isovalent compounds including arylnitrenium ions.87,88 These ions typically have singlet ground states for example the singlet state of phenylnitrenium is over 20 kcal mol~1 lower in energy than the triplet.In contrast arylcarbenes have triplet ground states. The di§erence is due to the much greater p acceptor ability of the positively charged nitrogen atom. The electronic structure of nitrenium ions has important biological consequences. Arylnitrenium ions which can be formed in vivo 351 Theoretical organic chemistry CO2 – OH O CO2 – O OH – O2C CO2 – O Scheme 3 from aromatic amines are carcinogens and can lead to modifications in the genetic material. Only singlet arylnitrenium ions can behave in this way. For such divalent compounds it is well known that when the valence angle at the divalent centre is increased the singlet-state is destabilised relative to the triplet. Cramer and Falvey observed this trend as the substituents at the nitrene were made bulkier.For the bulkiest substituent used bis(2,6-di-tert-butyl)phenyl steric interactions between the two aromatic rings were so severe that the triplet structure was predicted to be linear about nitrogen and the sign of the S–T gap was reversed with the triplet lower in energy. The Claisen rearrangement of chorismate to prephenate (Scheme 3) has been the subject of much investigation recently due to its synthetic and biochemical importance. The reaction occurs at the active site of the enzyme chorismate mutase which is involved in the shikimic acid pathway for the formation of aromatic amino acids. The enzyme is able to stabilise a relatively high energy diaxial conformation of chorismate. Davidson et al.continued their study of this reaction by examining solvent e§ects using both a continuum model and the Monte Carlo FEP method.89 In agreement with experimental observations solvation reduces the activation barrier for the reaction with respect to the lowest energy diequatorial form by 17.3 kcal mol~1 (PCM) and 23.7 kcal mol~1 (MC). This is due to the enhanced polarity of the transition state and the greater number of hydrogen bonds formed by the transition state with the solvent. Despite the significant inherent di§erences between the two solvation models they give qualitatively similar results for this reaction. Interestingly in this paper the authors report that the preferential solvation of the transition state relative to the diaxial conformer is about 16 kcal mol~1 using the results of the MC simulation.This can be compared to the corresponding value of 27 kcal mol~1 in the enzyme active site computed using a hybrid QM–MM approach,90 showing that the enzyme exerts a strong catalytic influence. Guest et al. analysed the e§ect of solvation on the Claisen rearrangement of allyl vinyl ether to pent-4-enal (Scheme 4).91 Calculations were performed at the B3LYP/6- 31G* level and the system was solvated with the PCM model. With no explicit water molecules solvation reduces the rearrangement barrier by 0.6 kcal mol~1 relative to the gas phase. Including two explicit waters in a supermolecule calculation leads to a reduction in the barrier of 6.3 kcal mol~1 compared to the experimental barrier lowering of 3.5–4.7 kcal mol~1. This indicates that the continuum model may underestimate the e§ect of solvation whilst the explicit water model may overestimate specific solute–solvent interactions for this reaction.Interestingly MC simulations with geometrical structures and formal charges derived from ab initio gas phase wavefunctions give a barrier lowering in good agreement with the experimental value. Yoo and Houk have examined substituent e§ects on the Claisen rearrangement of allyl 352 Ian Alberts O O 5 6 1 2 3 4 Scheme 4 vinyl ether.92 It was found that 1- 2- 4- and 6-hydroxy substituents lower the activation barrier whilst the 5-hydroxy substituent raises the barrier. This is in agreement with experimentally measured activation energies for alkoxy-substituted compounds. Frontier orbital theory was used to rationalise these substituent e§ects.Zipse studied the [3,2] and [1,2] phosphatoxy rearrangements in the 2-(phosphatoxy) ethyl and 2-(dimethylphosphatoxy)ethyl radicals using high level correlated wavefunctions.93 In contrast to the analogous acyloxy rearrangements,94 the [1,2]- shift is the preferred rearrangement pathway. A novel syn-1,3-phosphate elimination reaction is also described and the barrier for this process is predicted to be lower than the barrier for syn-1,2-elimination but above the barriers for the 1,2-phosphatoxy rearrangements. For the acyloxy rearrangement solvation was found to have little e§ect on the activation barrier however protonation significantly reduced the barrier suggesting the importance of acid catalysis in this reaction.94 Cycloaddition reactions The competition between single-step and two-step mechanisms in cyclopropanation reactions has been studied by Bernardi et al.They used (4/4)CASSCF and multireference (MR)-MP2 wavefunctions with 4-31G and 6-31G(d) basis sets to model the addition of singlet and triplet CF 2 and C(OH) 2 to the ethylene double bond.95 Both CF 2 and C(OH) 2 have singlet ground states in contrast to CH 2 which has a triplet ground state. At the CASSCF level which is known to overestimate the stability of biradical species stable CH 2 CH 2 CX 2 diradical intermediates are identified for the addition of both singlet carbenes with small barriers to the subsequent ring closure processes. However on inclusion of the dynamical correlation via MR-MP2 calculations at the CASSCF stationary points the small ring closure barriers disappear and the diradical cannot be located.Only when CF 2 is added to a bulky substituted alkene isobutene in this case can the diradical intermediate be located and a ring closure barrier assigned. This suggests that addition to an unsubstituted double bond is likely to be a concerted asynchronous process whereas a two-step process is likely to be observed in addition to bulky substituted alkenes. The corresponding triplet reactions are two-step processes through a diradical intermediate. Furthermore activation barriers for rotation of the terminal methylene groups in the triplet diradicals are lower than that for the singlet diradicals (at the CASSCF level) suggesting that the triplet reactions can lose stereospecificity more easily. In comparison the hypothetical addition of the parent CH 2 radical to ethylene is a barrierless process which has been rationalised on the basis of the di§erent (triplet) ground state electronic structure for this radical.Nguyen et al. systematically examined model [2]1] cycloaddition reactions of hydrogen isocyanide HNC to a series of dipolarophiles of the form CH 2 –– X and 353 Theoretical organic chemistry PH––X (X\CH 2 NH O SiH 2 Ph S) and showed them to be highly regioselective and stereospecific.96 The silicon-containing dipolarophiles were found to have relatively small barriers for cycloaddition to isocyanides. The [4]2] and [2]2] cycloaddition reactions of ketenimine and acrolein have been studied by ab initio methods up to MP4(SDTQ)/6-31/]/G*//MP2/6-31G*97 According to thermodynamic and kinetic arguments the ‘allowed’ [4]2] pathway is favoured over the ‘forbidden’ [2]2] route and may be followed by rearrangement processes.The Diels–Alder (DA) reaction is probably the most well-studied of all pericyclic processes and it appears to be accepted that the reaction proceeds by a synchronous concerted pathway rather than a step-wise scheme involving a diradical intermediate. 98 In 1997 theoretical investigations focussed on the e§ect of hetero-atoms and the stereoselectivity of DA reactions. The hetero-DA reactions of butadiene with formaldehyde and thioformaldehyde have been studied at a variety of theoretical levels.99 Concerted non-synchronous mechanisms were predicted for these reactions and with traditional QM methods converged energetics required the inclusion of most of the dynamical correlation via QCISD(T) with extensive basis sets of at least valence TZ2P quality.However for these reactions the cheaper hybrid density functional method B3LYP with a smaller 6-31G(d) basis set gave excellent geometries and energetics. The endo/exo selectivity of the Diels–Alder reaction between cyclopropene and butadiene has been studied using HF and high level post-HF methods; (6/6)CASSCF QCISD(T) MP2 and DFT with a range of basis sets as well as the semi-empirical AM1 method.100,101 In agreement with the observed cycloadduct stereochemistry the endo transition state structure was predicted to be lower in energy than the corresponding exo transition state. This is due to the stabilising secondary orbital overlap involving the methylene hydrogen of cyclopropene and the butadiene p bond between the central carbons which is only possible in the endo transition state.Interestingly AM1 is the only method that fails to predict the preferred endo pathway. The e§ects of polar solvents on the Diels–Alder reaction of cyclopentadiene with acrylates have been studied using hybrid semi-empirical AM1 QM–MM and SCRF approaches.102 Despite the failure of AM1 to predict the correct gas phase stereoselectivity theAM1QM–MMpotential gives the correct endo/exo selectivity in solution as important hydrogen bonding interactions are explicitly included. Ring opening reactions The central question in the ring opening reactions of cycloalkanes is whether they occur via a concerted mechanism or a step-wise process in which one ring bond is cleaved to form an intermediate diradical followed by cleavage of the CC central bond.Calculations using extensive correlated wavefunctions find that the thermal ring-opening reactions of cyclobutane and cyclopropane take place via the tetramethylene and trimethylene diradical intermediates respectively.103,104 For comparison high-level multi-reference ab initio calculations have been performed to determine the ring-opening reaction mechanisms of the analogous silicon compounds silacyclobutane105 and silacyclopropane.106 In agreement with experimental results it was predicted using CASSCF that the most likely reaction pathway for the thermal decomposition of silacyclobutane to ethylene]silene involves initial cleavage of the ring CC bond giving the CH 2 SiH 2 CH 2 CH 2 trans diradical followed by cleavage of the central SiC bond to give the products.The diradical is a minimum on the potential 354 Ian Alberts energy surface however the transition state that leads to the products disappears at the MR-MP2/6-311G(d,p) level when the dynamical correlation is included. Breaking the initial C–C bond is the rate determining step and at the highest level of theory employed in this study MR-MP2 the reaction appears to follow a highly asynchronous concerted route with considerable diradical character. The barriers for initial cleavage of the SiC bond and the synchronous concerted decomposition are about 6 and 10 kcal mol~1 respectively higher in energy than that for initial CC rupture. Interestingly using CCSD(T) an alternative process leading to propylsilylene via a concerted SiC ring opening and 1,2-H shift is found to be energetically competitive with the route towards ethylene and silene.The latter mechanism involving hydrogen migration in concert with ring opening is predicted to be the preferred mode for the ring opening of silacyclopropane and leads to the formation of ethylsilylene.106 This is completely di§erent to the ring opening of cyclopropane. The calculated barrier heights for the silacyclopropane to ethylsilylene reaction and its reverse are 24.7 and 13.2 kcal mol~1 at the (12/12)CASPT2N/6-31G* level of theory in reasonable agreement with the experimental values for the reactions of alkylsilacyclopropanes. The diradicals SiH 2 CH 2 CH 2 and CH 2 SiH 2 CH 2 formed by C–Cor Si–C cleavage are computed to be transition states about 20 kcal mol~1 higher in energy than the transition state that leads to ethylsilylene.Ethylsilylene can subsequently convert to 1-silapropene with a barrier of 30.7 kcal mol~1. The di§erence between the reactions for the cycloalkanes and silacycloalkanes has been explained in terms of the relative strengths of Si–H and C–H bonds in primary silyl and alkyl radicals. SN2 Reaction Botschwina et al.107 conducted very accurate calculations on the F~]CH 3 Cl]FCH 3 ]Cl~ reaction using CCSD(T) with a large correlation consistent basis set to yield energetics of ‘chemical accuracy’. Ion–dipole complexes were located in the reactant and product channels separated by a small activation barrier of 3.3 kcal mol~1 and the enthalpy of reaction was predicted to be[31.5 kcal mol~1. In a high-level theoretical study HF MP2 and B3LYP methods with the 6-31]G* basis set were used to compare the S N 2 reactions of RCH 2 F and RCH 2 Cl with F~ and Cl~ and the ion pair displacement reactions of the alkyl halides with LiF LiCl and NaF.108 It was shown as is well known that the barriers for the traditional S N 2 reactions with anionic nucleophiles get higher as the bulkiness of the alkyl group in the alkyl halide increases.The situation for the ion-pair reactions is di§erent. The ordering of the barrier heights is reversed; the bulkier alkyl groups give lower barriers. The ion-pair reactions proceed via a dipole–dipole complex and a cyclic transition state involving two cations and two anions with highly bent X–C–X (X\F Cl) bonds. This leads to a reduction in steric e§ects and together with the electrostatic e§ect of the cation allows greater charge polarization and delocalization within the alkyl group.Therefore the greater stabilising e§ect exerted by bulkier carbocations can appear. Truong et al. describe a method for determining the free energy profile of reactions in solution using the generalised conductor-like screening model (GCOSMO).109 This method was applied to the type II S N 2 Menshutkin reaction NH 3 ]CH 3 Cl]NH 3 CH 3 `]Cl~ with a DFT description of the reacting species. In the gas phase the reaction is very endothermic however solvation stabilises the 355 Theoretical organic chemistry R R R R CH2 Cl Zn R R + Cl Zn Cl 'ClCH2ZnCl' Three-centred transition state Cl Scheme 5 charged products and the reaction is exothermic in aqueous solution with a free energy of activation of 24.8 kcal mol~1.This type II S N 2 reaction is very di§erent to type I S N 2 reactions such as Cl~]CH 3 Cl]ClCH 3 ]Cl~. Solvation reduces the reaction rate of type I reactions however the rate of the Menshutkin reaction significantly increases in aqueous solution since the polar solvent stabilises the charge separation in the transition state and the ionic products. Catalysis One of the powerful features of DFT is that it is a universal theory applicable to all atoms including transition metals which are notoriously di¶cult to treat with traditional ab initio methods. In this respect Morokuma and co-workers continued their studies of the reactions of organometallic systems with unsaturated molecules.110,111 The transfer of a methyl group from lithium organocuprate reagents to acetylene and acrolein was examined using the B3LYP method.The C–C bond formation mechanisms in the two systems were found to be very similar with co-operative action of the copper and lithium and involvement of a CuIII species. Bernardi et al. used the B3LYP method to study the Simmons–Smith cyclopropanation reaction of ethylene with ClCH 2 ZnCl.112 The favoured reaction pathway (Scheme 5) was found to be a concerted addition leading to cyclopropane with an activation barrier of 24.7 kcal mol~1. The transition state corresponds to a threecentred structure that allows the olefinic stereochemistry to be retained. The alternative insertion pathway leading to propene has a higher barrier of 36.0 kcal mol~1. In comparison the addition of singlet methylene to ethylene is barrierless and the insertion reaction which is competitive has a very small barrier.Theoretical studies of ethylene polymerization reactions catalysed by transition metal complexes such as Zr and Ti chelating alkoxide complexes and NiII diimines have been performed by several groups.113,114 For example the combined DFT–MM model was applied by Deng et al. to study the role of bulky substituents in the NiII diimine catalysed polymerisation of ethylene (Brookhart-type reaction). The system studied was the [ArN––C(R)–C(R)––NAr]NiII based catalyst where R\CH 3 and Ar\2,6-C 6 H 3 (Pr*) 2 in which Ar and R are treated byMMand the remainder of the system by DFT. The chain propagation branching and termination steps were found to have barriers in agreement with experiment in contrast to the barriers given by pure QM calculations which did not include the substituent groups R and Ar.115,116 The substituent groups exert both steric and electronic e§ects which lower the propagation barrier and raise the termination barrier relative to the values given by the pure QM model.The Lewis acid catalysed [2]2] cycloaddition reaction of ketene and formalde- 356 Ian Alberts Fig. 1 Reproduced with permission from J. Am. Chem. Soc. 1997 119 10 815. © 1997 American Chemical Society. CI denotes the conical intersection. hyde leading to the formation of oxetanone has been studied using semi-empirical AM1 and ab initio HF/6-31G* and MP2/6-31G* methods.117 It was found that BF 3 catalysis leads to a substantial reduction in the activation for initial C–C bond formation from 40.9 to 11.9 kcal mol~1 at the HF/6-31G* level.Inclusion of electron correlation using MP2/6-31G* reduced the barrier further to 3.6 kcal mol~1. Solvent a§ects estimated using AM1/COSMO lead to the formation of an earlier transition state. The BH 3 catalysed reaction was found to be more complex since hydride transfer can compete with the C–C bond forming process. Photochemistry Robb and co-workers have recently conducted detailed theoretical studies of photochemical processes. The photochemical ring opening reaction on the S 1 potential energy surface of benzopyran was explored by CASSCF calculations (Fig. 1).118 Starting from the Franck–Condon region (FC) on S 1 which is about 107 kcal mol~1 above the equilibrium S 0 structure evolution of the excited state leads from a shallow minimum (M*) through a C–O ring-opening transition state (TS*) to an acyclic structure that is much lower in energy than M*.This point corresponds to a conical intersection (CI) where S 0 and S 1 are degenerate. At the CI S 1 decays very e¶ciently back to S 0 . The model system pyran was then used to study the relaxation on S 0 . Two relaxation coordinates were identified; regeneration of the reactant pyran or formation of the open structure cZc-penta-2,4-dienal. The latter process may be more e¶cient for inertial reasons. The justification for the use of pyran to model the S 1 ]S 0 decay and relaxation on S 0 is that the conical intersection structures for benzopyran and pyran are found to be very similar. 357 Theoretical organic chemistry The photochemical interconversion of cyclohexa-1,3-diene (CHD) and cZc-hexa- 1,3,5-triene (cZc-HT) has been revisited in order to examine the product formation mechanism.119 Initial irradiation of either CHD or cZc-HT leads to the 2A 1 excited state which decays towards the 2A 1 /1A 1 conical intersection.Three relaxation directions were identified from the CI. Two of these pathways are directly connected to the CI and lead to CHD and cZc-HT. The third pathway which leads to methylenecyclopentene (MCPD) is predicted to be less accessible as it is not directly connected to the CI and has a higher energy profile. The decay dynamics obtained using hybrid MMVB potentials and the trajectory surface hopping method suggest that the yields of CHD and cZc-HT will be very similar and that of MCPD will be very low in agreement with experimental observations.The [4]4] photochemical cycloaddition of butadiene]butadiene has been studied by CASSCF/4-31G calculations.120 A pericyclic minimum was located on the S 1 surface which connects to two S 1 /S 0 conical intersections by low barriers. Relaxation from the conical intersections leads to a mixture of products. Garavelli et al.121 studied the photoisomerisation pathway of the protonated Shi§ base cis-C 5 H 6 NH 2 `. Evolution of the excited state leads to an S 1 /S 0 conical intersection. The CI provides a barrierless fully e¶cient decay channel that is in accordance with the fast cis–trans isomerisation process. References 1 J. Gao Ann. Rep. Prog. Chem. Sect. B Org. Chem. 1997 93 3. 2 M. Challacombe and E. Schwegler J. Chem. Phys. 1997 106 5526. 3 E. 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Soc. 1997 119 6891. 3

ISSN:0069-3030    

DOI:10.1039/oc094337

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

11 NMR Spectroscopy By B. A. SALVATORE Department of Chemistry and Biochemistry University of South Carolina Columbia SC 29208 USA 1 Introduction It appears that most recent breakthroughs in NMR are driven by research involving biological macromolecules. The fact that many of these developments are occurring in neighboring fields does not preclude their application within the realm of organic chemistry.A survey of the recent literature reveals that many newNMRtechniques are indeed important to chemists who study relatively small organic molecules. Thus it is instructive to distill the recent literature from a cross-section of disciplines and pinpoint those NMR techniques which are of interest to the organic community. It is in that spirit that this review focuses on recent advances in NMR from a variety of areas.The material pertains to both isotropic and oriented NMRsample systems and the author has adopted a selective (rather than comprehensive) approach in presenting some of the most significant developments. 2 Monitoring chemical reactions by NMR The utility of NMR in monitoring the progress of chemical reactions is continually being enhanced with recent advances demonstrating its analytical power in both solution and solid-phase chemistry. This includes the development and refinement of new techniques and of existing hardware. Solution-phase chemistry NMR is particularly well-suited for detecting and identifying intermediates in chemical reactions. Organocuprate chemistry is one area in which mechanistic details remain obscure. A recent study of several cuprates helped clarify the dichotomous reactivity of these agents which often participate in both electron-transfer and conjugate addition processes.1 In the reaction between Me 3 CuLi 2 and trimethyl ethylenetricarboxylate the cuprate was found to be a particularly strong reducing agent with the reduction product predominating over the conjugate addition adduct by more than 4 1.Standard 1H-decoupled 13C spectra of Me 3 CuLi 2 and trimethyl ethylene- 361 tricarboxylate [13C-enriched at C(2)] in ether at 0 °C revealed a single carbon resonance representing the intermediate which follows single electron-transfer from the cuprate. However no resonances from an intermediate along the conjugate addition pathway were detectable by NMR for reasons that remain unclear. This is puzzling in light of the fact that the conjugate addition adduct still accounts for nearly 20% of the total product yield and thus further investigations are warranted.Chemically Induced Dynamic Nuclear Polarization (CIDNP) is a powerful NMRbased method for probing the molecular structure of species in which a photochemically generated free-radical electron spin polarizes a nuclear spin on the same atom. Using this technique Giese et al. provided the first spectroscopic proof for the existence of a radical cation intermediate in a chemical reaction that models the C,O-bond scission process of 4@-DNA radicals.2 Such radical species are believed to be important intermediates in the cleavage of DNA by bleomycin and enediyne-based natural products. NMR is also a powerful tool in assessing the formation of organometallic complexes.Particularly important are new metallo-NMR methods for characterizing the structure of ionic species in solution. Koga et al. described the 6Li and 15Nanalysis of a labeled chiral bidentate lithium amide. 6Li–15N couplings established the basis for studying these complexes in solution.3 Note that 6Li is quadrupolar (nuclear spin\1). Such experiments should help in explaining the dependence of enantioselectivity on solvent composition in proton transfer of asymmetric carbon. These investigators concluded that an observed drop in enantioselectivity in certain solvents was caused by formation of a dimer of the chiral lithium amide species. Other atom-pairs have also recently been investigated by NMR. Gunther and Bohler reported the first 2D heteronuclear shift correlation experiment for the spin pair 6Li–29Si,4 which they used to study the structure of a silyl-substituted organolithium anion.The experiments were performed unlocked on a doubly-tuned probe with proton decoupling. The retuned deuterium lock coil served as the 6Li channel and the X-coil was used to pulse 29Si nuclei (natural abundance\5%). Oneand three-bond correlations were visible in the 2D HSQC spectrum which displayed two sets of 6Li–29Si scalar couplings. Such experiments are very powerful because they can establish the site of chelation within an ionic complex. New hardware developments have also broadened the applications of NMR in monitoring chemical reactions. Baumann et al. described a simple apparatus which is particularly applicable (but not limited) to kinetic and mechanistic studies of reactions between dissolved gases and organometallic complexes.5 Such studies make it possible to monitor the appearance and disappearance of intermediates that ordinarily cannot be isolated.Several reactions have been studied with this apparatus including an unusual one in which two molecules of ethylene add to a zirconocene–alkyne complex displacing the alkyne.6 Other researchers have sought to ally NMR with liquid chromatographic instrumentation. 7 LC-NMR consists of linking an HPLC in series with a specially-developed NMR probe capable of detecting flow-through samples. A temporary pause in the flow as the compound moves through the probe allows the sample to remain within the NMR coil long enough to obtain adequate signal averaging. Other implementations of this technique have also been introduced.For example elimination of the HPLC column and introduction of an autoinjector establishes the basis for another 362 B.A. Salvatore new analytical system known as Versatile Automated Sample Transport (VAST) NMR which promises to be a very powerful tool in the analysis of combinatorial libraries.7 Applications in solid-phase synthesis One long-standing problem in solid-phase synthesis (SPS) involves monitoring the progress of reactions during synthesis. FT-IR is often employed to accomplish this but it lacks the analytical power of 1H and 13C NMR. The primary obstacle to NMR analysis of SPS is local discontinuities in magnetic susceptibility that result from the heterogeneous nature of resin-bound samples. This heterogeneity causes variations in the e§ective magnetic field around the attached molecules resulting in chemical shift dispersions (i.e.broad lines!). As a result the solid-phase synthetic chemist often must cleave some product from the resin after each step which is used for solution NMRor mass spectrometric analyses. This is time-consuming and wastes synthetic intermediates. Fortunately this is changing with the development of fast non-destructive NMR methods for characterization of the products of SPS. 13C NMR of solvent-swelled resin samples (‘gel-phase NMR’) is suitable in certain situations.8 Solvating the resin sample as much as possible before acquiring spectra increases motional freedom of the resin-bound molecules thereby reducing linewidths. This is a convenient adaptation of conventional solution NMR methods requiring no specialized equipment.Yet even though this method often provides carbon NMR spectra suitable for analysis of the products of chemical syntheses the problem of broad lines remains and renders the method unsuitable for high-resolution 1H NMR studies. Magic-angle-spinning (MAS) of solvent-swelled resin beads drastically reduces line widths by averaging out magnetic susceptibility di§erences within the sample thus improving resolution and making it suitable for high-resolution 1H NMR applications. 7 This technique is useful in monitoring the progress of SPS and e§orts are being made to optimize its utility. It was found that poly(ethylene glycol) (PEG)-tethered resins commonly called TentaGel give the narrowest NMR line-widths.9 Spin echo experiments and spin-locking are commonly employed to attenuate unwanted polymer peaks and enhance resolution obtained with other resins.However one disadvantage of spin echoes is that J-couplings are lost as the echo refocuses. If it is important to retain the J-coupling information it can be recovered through 2D J-resolved spectroscopy. Shapiro et al. reported a useful experiment in which an ‘untilted’ 2D J-resolved spectrum is projected along a single axis.10 This technique capitalizes on the benefits of the spin echo experiment by attenuating unwanted polymer resin peaks while retaining proton scalar coupling information along the chemical shift axis of a standard one-dimensional 1H-spectrum. MASNMRis an excellent tool for analyzing the products from peptide SPS. Opella et al.have shown that it is now possible to determine the three-dimensional structural characteristics of resin-bound molecules.11 They acquired 2D NOESY spectra and established the conformation of a resin-bound hexapeptide. Such information is relevant to drug design because binding assays are sometimes performed on resinbound libraries of ligands. Lippens et al. employed similar techniques to investigate the structural basis for the di¶culties encountered during the SPS of certain peptide sequences.12 Specifically 363 NMR Spectroscopy Fig. 1 A schematic representation of the 1Hnano-probe. The plug for the nanoprobe cell and the RF leads have been omitted for clarity. (Reproduced with permission from J. Magn. Reson. Series A 1996 119 65 they used NOE and chemical shift data to establish a correlation between coupling di¶culties and the degree of interchain aggregation as the synthesis progressed.Advances have been made in hardware development as well. Keifer et al. investigated magic-angle-spinning with new high-resolution probes (Fig. 1) that were optimized for very small sample volumes (ca. 40ll).13 The excellent resolution obtained with these ‘nano-NMR probes’ demonstrates the important benefits of minimizing magnetic susceptibility discontinuities in probe design as well as in the sample. Shapiro et al. have adapted MAS NMR methods to analyze products on multipin crowns thus extending its utility in parallel combinatorial synthesis.14 Application of MAS NMR in the solid-phase synthesis of oligosaccharides15 and other non-peptidebased combinatorial libraries10 has also been documented.Supramolecular chemistry NMR is a powerful tool for probing non-covalent molecular assemblies along with the dynamic exchange processes that occur in those assemblies. Lehn et al. have used 109Ag-NMR spectroscopy to monitor the formation of a rectangular supramolecular grid assembled from the combination of tritopic ligand 1 ditopic ligand 2 and silver triflate in a 2 3 6 stoichiometric ratio.16 The 109Ag NMR spectrum displayed two signals in a 2 1 ratio corresponding to four peripheral and two central silver ions respectively. This provides evidence for formation of the complex 3 in which the signal from the peripheral silver ions is shifted downfield by 59ppm from the resonance generated by the central silver ions. Thus it was consistent with the formation of a 2]3 rectangular grid via mixed-ligand recognition.364 B.A. Salvatore N N N N N N Me Me N N N N Me Me 1 2 N N N N Me Me N N N N N N Me Me N N N N N N Me Me N N N N Me Me N N N N Me Me 3 +6 Another elegant example of NMR’s utility in supramolecular chemistry was reported in an investigation of the reversible dimerization and guest exchange in C 2 -symmetric calixarenes.17 O§-resonance rotating frame NMR experiments provide a useful way of discriminating between exchange and the direct cross-relaxation transfers that are commonly witnessed in standard NOE experiments. O§-resonance spectroscopy also eliminates complications from TOCSY transfers that sometimes arise during on-resonance rotating frame experiments. Spin locking was performed in these experiments such that the angle between the e§ective field and the external magnetic field (H\35.6°) was in between those used in standard NOE experiments (H\0°) and on-resonance rotating frame experiments (H\90°).That value was 365 NMR Spectroscopy chosen because it resulted in a cancellation of the cross-relaxation contributions from the laboratory frame and the on-resonance rotating frame for the observed protons. Thus only exchange cross-peaks were detected. Determination of acid dissociation constants by NMR It is often a challenge to accurately measure dissociation constants of organic acids in non-aqueous media. Pehk et al. have devised a general approach for measuring relative acid dissociation constraints by 13C NMR.18 This technique is based on the measurement of frequency di§erences at varying degrees of protonation between a known reference compound and a compound whose dissociation constant is unknown.The degree of protonation of the reference compound (e.g. acetic acid) is known exactly at each stage of the process. The ratio of dissociation constants for the acid under study and the reference compound can be determined from the following relationship in eqn. (1) K/K!\(d[d1)(d! $ [d!)/(d$ [d)(d![d! 1) (1) where d represents the chemical shift for the partly protonated reference acid while d1 and d$ represent the chemical shifts of its fully protonated and fully deprotonated species respectively. The corresponding chemical shift values for the acid under investigation are represented by d! d! 1 and d! $ accordingly. In the most convenient implementation of this technique one measures the chemical shift d for the reference acid and d! for the compound under investigation and then plots the di§erence d[d! against the degree of protonation (n) according to eqn.(2) d[d!\d$ [d! $ [n(d$ [d1)]nK/K!(d! $ [d! 1)/[1]n[(K/K!)[1]] (2) where K/K! represents the ratio of dissociation constants between the reference acid and the acid under study. The derivation of this equation (based on the law of massaction and two expressions relatingNMRchemical shift to the distribution of molecular populations during the exchange process) was presented by the authors. Plotting the experimental data for d[d! vs. n one obtains bell-shaped plots which are fit to obtain the sought quantity K/K! according to eqn. (2). In principle the dissociation constant of a particular acid can be determined by this technique when it di§ers from that of the reference acid by as little as 4 J mol~1.This technique has minimal requirements for sample purity and it can be carried out without any determination of pH. Additional methods are available for treating the experimental data which simplify the data interpretation when the chemical shift di§erences between the fully protonated and deprotonated forms of the compounds are not equal.18 The authors illustrate several practical examples involving branched carboxylic acids. 3 Defining structure and conformation through NMR Correlation spectroscopy Rychnovsky et al. reported a strategy for assigning the configuration of 1,3-skipped polyols.19 This method was demonstrated by analysis of a pair of polyacetonide acetate derivatives of the natural product dermostatin A which are ‘frame-shifted’ with respect to each other (4 and 5 Fig.2). Thus this strategy is particularly well-suited for 366 B.A. Salvatore O O O O O O O O O O AcO O O OAc O O O O O O O O 4 5 15 15 16 16 17 17 19 19 21 21 23 23 25 25 27 27 29 29 31 31 Fig. 2 Two isomeric (‘frame shifted’) polyacetonide acetate derivatives (4 and 5) of dermostatin A polyols which contain an odd number of hydroxy groups. The technique relies on a combination of 1H–1H COSY 1H–13C DQF-HSQC and 1H–1H ROESY experiments. After assigning all the protons from COSY spectra,HSQC and ROESYspectra are acquired to determine which of the acetonides in 4 are syn and which are anti (Fig. 3). The relative stereochemistry between each pair of acetonides in 4 is then established by analyzing the HSQC and ROESY spectra from an isomeric ‘frame-shifted’ acetonide (5 Fig.2). A small level of 13C-enrichment within the acetonides (ca. 10%) is optimal. This is a very powerful NMR-based method for assigning the relative stereochemistry within polyol chains. Mosher ester analysis can then be used to subsequently establish the absolute stereochemistry. Gervay et al. have explored long-range 13C–1H NMR connectivity in carbohydrates. 20 They applied a 1DInverse Detected Single Quantum Long Range (INSQLR) experiment to establish the existence of a highly-labile sialic acid lactone moiety. Selective excitation at the 13C-labeled carbonyl in one of the possible lactones resulted in magnetization transfer through the lactone oxygen which was detected at a proton three bonds away.This signified the presence of the lactone. Had that lactone been absent this correlation would not have been observed since it would have required 13C–1H magnetization transfer through seven r-bonds. One of the major problems in three-dimensional structure determination of oligosaccharides by solution NMR results from the limited number of distance and angular restraints which are generally defined from 1H–1H NOEs and three-bond 1H–13C coupling constant measurements. To alleviate this problem Homans et al. have enhanced existing NMR methods for deriving information from exchangeable protons (i.e. -OH -NH protons).21 This technique was demonstrated on N-acetyllactose with three experiments (TOCSY-HSQC ROESY-HSQC and NOESY-HSQC). 367 NMR Spectroscopy Fig.3 ROESY (left) and HMQC (right) of the acetonide methyl group region for compound 4. There are three syn- and two anti-acetonides [two of the axial methyl groups in distinct syn acetonides coincidentally have the same HSQC chemical shifts (1H 1.44 ppm and 13C 20 ppm)]. No ROE correlations appear with the equatorial methyl groups in the syn acetonides. (Reproduced with permission from J. Org. Chem. 1997 62 2925) The samples were prepared in an H 2 O–[2H 6 ]acetone mixture which was cooled ([17 °C) to minimize proton exchange. The investigators employed 3D 13C-editing techniques to overcome chemical shift overlap of non-exchangeable protons a common problem in most proton-detected 2D NMR experiments involving carbohydrates. Water suppression in these experiments was performed without pulsed-field gradients (vide infra).These experiments produced a vast number of additional distance restraints which were useful in conformational analysis of the disaccharide. Symmetry often poses an obstacle in conformational studies with NMR since NOEs and ROEs normally cannot be detected between chemically-equivalent protons. Thus fewer distance constraints are available for analysis. Wagner and Berger reported an e§ective solution to this problem,22 building upon prior work in which HMQC-ROEtransfers were performed between two chemically equivalent protons on a 13C/12C atom-pair. Unwanted signals resulting from ROEs between protons on 12C-atoms were successfully suppressed with pulsed field gradients. These authors also reported an improved 1D version of this experiment based on the selective excitation of a single 13C resonance bearing one member of a chemically-equivalent pair of protons (the other proton being on a 12C-atom within the same molecule).Enclosing the selective 180 ° pulses within a gradient sandwich facilitates calibration of the pulses. A negatively-phased signal appears in the center of each C–H doublet (representing two equivalent protons) where an NOE exists (Fig. 4). Jimenez-Barbero et al. have reported a similar 1D experiment for extracting NOE (ROE) information from chemically equivalent anomeric protons in the C 2 symmetric disaccharide trehalose.23 Their technique is based on the selective inversion of one anomeric 13C resonance with a DANTE-Z pulse train during which time the proton magnetization is spin-locked.368 B.A. Salvatore Fig. 4 1D HSQC-NOESY spectrum of phenanthrene obtained through a selective pulse on C(4). A negative peak in the centre of the 1H–13Csplitting for H(4) is apparent above. This represents an NOE between the two chemically-equivalent H(4) protons. (Spectrum reproduced with permission from Magn. Reson. Chem. 1997 35 199) Deriving information from scalar coupling constants It is possible to garner a tremendous amount of structural and conformational information through the accurate measurement of scalar (J) coupling constants. In a recent review Thomas reminds the chemical community not to lose sight of the importance of coupling constants in conformational analysis.24 He attributes recent neglect in part to the explosive development of new multidimensional NMR experiments which have relegated J-couplings to an uninteresting role.Investigators who simply view J-couplings as the basis for multidimensional correlations rather than a direct source of information as well may be ignoring potentially valuable data. Conformational analysis based on scalar couplings has in fact been undergoing a renaissance recently with many investigators gathering information about 1H–13C and 13C–13C J-values. Serianni et al. have developed synthetic and spectroscopic methods for measuring the complete set of one- two- and three-bond 1H–13C and 13C–13C scalar couplings in b-D-ribofuranose and 2-deoxy-b-ribofuranose rings.25,26 Since all the furanosyl rings within DNA (or RNA) have the same chemical structure comparisons between related J-couplings from the sugars within discrete segments can yield important information about the topological structure of nucleic acids.This same group has devised an empirical method for predicting the magnitude and sign of two-bond 13C–13C scalar coupling constants (2J CC ) in aldopyranosyl rings.27 369 NMR Spectroscopy Fig. 5 Serianni’s projection rule for determining 2J CC . To determine J C1–C3 for this carbohydrate one must first inspect the angles made by each oxygen substituent on C1 and C2 with the projection anti to the C(2)–C(3) bond [viewed along C(1)–C(2)] and then inspect the angles made by each oxygen substituent on C2 and C3 with the projection anti to the C(1)–C(2) bond [viewed along C(2)–C(3)]. Summing the cosines of all of these angles provides a resultant ([0.5) which predicts a small negative coupling constant (ca.[2 Hz).The measured value is[2.4Hz The values of such couplings have been shown to depend on the orientation of electronegative substituents relative to the C–C bond. Serianni’s projection rule for estimating 2J CC is based on an inspection of the angle that each electronegative substituent on each of the two C–C bonds makes with a projection anti to the other C–C bond (Fig. 5). Then the cosines of all these angles are simply added together. A small positive sum (\1.0) or a negative sum is indicative of a negative 2J CC while a larger positive value ([1.0) predicts a positive 2J CC . This report includes data for several di§erent sugars and demonstrates that ab initio calculations of 2J CC in model compounds agreed with the authors’ predictions.This empirical rule also applies to 2J CC couplings through oxygen (i.e. C–O–C) and this should prove particularly useful in the conformational analysis of O-glycosidic linkages in oligosaccharides. In structural studies of complex natural products it is often desirable to detect long-range 1H–13Cscalar couplings. The standard 2DHMBCexperiment is one of the most powerful methods for accomplishing this. Yet it is often hampered by low sensitivity for some long-range correlations due to the di¶culty in setting a universally- optimal delay time after the first 90 ° 13C-pulse. As a compromise a 50–60 ms delay is generally prescribed but this is often not optimal particularly for couplings involving fast-decaying signals (e.g. methylene signals). Seto et al. have proposed a way to overcome this commonly encountered problem which entails displaying a 2D projection from a three-dimensional HMBC experiment.28 The delay time following the first carbon pulse in the conventional 2D experiment becomes variable and is increased uniformly in 4ms increments from 20 ms up to 80 ms thus establishing the ‘third dimension’.The results of this 3D experiment are viewed as a projection of the 370 B.A. Salvatore sum of sixteen separate 2D spectra onto the f2,f3 plane. By employing gradient-based coherence selection requiring only 4 scans per t 2 point this experiment takes no longer than a standard 2D HMBC experiment performed with 64 scans per t 1 point. In a comparison run with the natural product monazomycin the results of a ‘3D HMBC’ were far better than those derived from a standard 2D HMBC.This experiment is particularly useful for detecting long-range J-couplings involving protons that are broadened in complex spin systems. Fructose exhibits a complex mutarotational equilibrium between five isomeric forms including the pyranose furanose and open (straight chain) forms. In an e§ort to develop receptors that will selectively bind to one isomeric form in solution Eggert and Norrild characterized the various boronic acid complexes of fructose on the basis of one-bond 13C–13C coupling constants (1J CC ).29 Their work is based on the important observation that exceptionally low 1J CC (35–40 Hz) result when the O–C–C–O fragment in a vicinal diol is incorporated into a five-membered ring (as in vicinal cyclic boronic esters). It is believed that this e§ect results from a change in the orientation of the oxygen lone pairs with respect to the C–C bond.Since this species is selectively bound (albeit in DMSO) as its 2,3 4,5-bis(p-tosylboronate) ester the authors believe that they may be able to design a bis-boronic acid-based receptor that selectively binds to the b-D-fructopyranose anomer in water. Structure–activity relationships by NMR (‘SAR by NMR’) Individuals engaged in drug discovery today have the choice of pursuing rational design methods or concentrating on the many recently-developed combinatorial approaches. The latter techniques have rapidly gained popularity because the rational approach to drug design continues to be hampered with di¶culties such as those involved in predicting the enthalpic and entropic e§ects of ligand binding to drug targets.These factors are key in determining the stability of most drug-protein complexes. For example water molecules are often released upon ligand binding or alternatively they move to fill gaps at the binding site in unpredictable ways. Problems of this sort make life di¶cult for those pursuing a strictly rational approach to drug discovery. However combinatorial chemists also encounter di¶culties due to the limited sensitivity of most biological assays which generally facilitate the identification of only the most active compounds in a given library. Weaker binding is often obscured by background signals that result from high ligand concentrations. Thus it is likely that many key lead compounds are missed in high throughput combinatorial assays.While the debate continues between those espousing combinatorial methods for drug discovery and those who remain firmly entrenched with rational drug design Fesik et al. devised a strategy which blends these two strategies into a very powerful new approach. This is the so-called ‘Structure–Activity Relationship by NMR’ (SAR by NMR) approach to drug discovery. This technique which has been previously reviewed,30 is very powerful yet elegant in its simplicity. It blends the advantages of rational drug design and combinatorial chemistry with NMR studies of 15N-enriched proteins. The 1H–15N pairs in folded proteins generally yield well-resolved HSQC spectra. Addition of a substrate with a moderate a¶nity for the protein results in a shift of the HSQC NMR signals for all the 1H–15N atom pairs within the binding site.The chief advantage of SAR byNMRis that it allows one to obtain reliable SAR for compounds 371 NMR Spectroscopy N O O OCH3 OCH3 H3CO OCH3 O N O H OH HO N O O OCH3 OCH3 H3CO O O N O H OH O Kd = 2 mM Kd = 100 mM Kd = 19 nM Fig. 6 Two fairly weak-binding FKBP ligands discovered through ‘SAR by NMR’ screening (top). These two ligands bind to two distinct sites on FKBP. Linkage of the two ligands produced a much stronger-binding ligand (in box) which bind to the target with low a¶nity (millimolar range). In their seminal paper on this technique Fesik et al. designed a ligand which binds to FKBP in the nanomolar range by first identifying two ligands that bind to distinct sites on FKBP with moderately weak binding constants (Fig. 6).31 A key feature of this technique is the use of 1H–15N HSQC spectra to detect the binding of small ligands and to di§erentiate between multiple binding sites on the protein surface.Due to 15N-spectral editing no signal from the ligands is observed in the spectra just changes in the proton and nitrogen resonances for protein 1H–15N atom pairs within the binding site(s). NOEs between the ligands and specific 1H–15Npairs on the protein can also be used to assess the manner in which the ligands bind to the protein. Fesik et al. acquired HSQC data with help from an automated sample-changer. Selection of the strongest binding ligands was then accomplished by considering a weighted average of the chemical shift changes for 1H and 15N upon addition of each ligand. The overall strategy involves 372 B.A.Salvatore five steps (1) screening of binding of first ligand by NMR (2) optimization of binding of the first ligand (3) screening of binding of second ligand by NMR (4) optimization of binding of the second ligand (5) linking the first and second ligands. When two or more ligands which bind to a protein at distinct sites are linked together one obtains a total free energy of binding based on the sum of the two individual binding energies plus an additional negative free-energy term associated with the entropy decrease that results from linking the two ligands together [eqn. (3)]. DG\DG 1 ]DG 2 ]DG%/5301*# (3) A similar approach was adopted to identify powerful inhibitors of stromelysin a zinc-dependent matrix metalloproteinase.32 Fesik et al. also applied SAR by NMR to find initial leads for inhibitors of the E2 protein from the human papilloma virus which binds DNA at a single site.33 In this case rather than physically linking the two independently-binding ligands together SAR from two distinct series of weaklybinding lead compounds were gathered and merged in the design of a single lead that displayed an IC 50 in the micromolar range.The two main disadvantages of SAR by NMR are that it requires a minimum of 200mg of 15N-labeled protein overall and the size of the protein should not exceed the 30 kDa limit imposed by solution NMR. However those problems are largely overcome with relaxation- and di§usion-edited NMR screening techniques.34 These new methods are complementary to the HSQC-based technique since spectra of the ligands (instead of the protein) are observed.The spectra acquired by these methods do not allow characterization of the protein binding site. However they minimize the amount of protein required eliminate the need for isotopically labelled protein and they are amenable to binding studies with very large proteins. Advances in water suppression For NMR structural studies involving water-soluble compounds (e.g. carbohydrates peptides) from which information about exchangeable protons is gathered H 2 O is often a necessary component of the sample as well as a significant source of problems during spectral acquisition. For studies in which the sample concentration is in themM range the water proton concentration may be up to five orders of magnitude higher. This large di§erence in concentrations complicates the acquisition process.Most problematic is the fact that the huge water resonance makes it impossible to set the receiver gain at a level suitable for analyzing the sample. Secondly the water resonance often overlaps with some of the sample resonances. For these reasons the development of e¶cient water suppression methods continues to be an active research area. Today the most popular solvent suppression techniques involve selective excitation of the water resonance. This is particularly useful in (but not limited to) protein NMR applications since protein resonances have shorter relaxation times than water and thus return to equilibrium much faster. ‘Radiation damping’ can be a serious problem. This phenomenon results when transverse water magnetization induces a current in the receiver coil.The induced current results in a magnetic field that causes the water proton magnetization to return to equilibrium at a rate much faster than that prescribed by its true T 1 thus ruining the solvent suppression. For this reason a lot of research has been done to develop gradient-enhanced frequency-selective water suppression techniques. The general utility of pulsed-field gradients in NMR has been 373 NMR Spectroscopy recently reviewed by Canet.35 Gradient-based solvent suppression methods are generally classified into two categories (1) frequency-selective excitation followed by dephasing (i.e. spoiling) with gradient pulses and (2) frequency-selective refocusing flanked by gradient pulses which dephase unwanted transverse magnetization. A recent straightforward application of the first category is the WANTED (waterselective DANTE using gradients) sequence.36 Here radiation damping is suppressed during a water-selective pulse train by keeping the transverse water magnetization defocused during the period in between each DANTE pulse.In homonuclear 2D NMR however frequency selective refocusing of the solvent resonance is more common. This is usually achieved by inserting a WATERGATEsequence near the end of a pulse sequence prior to acquisition. However splicing this segment within the middle of a pulse sequence often complicates the phase cycling and timing within the entire sequence. Thus each experiment must be developed and optimized independently. For example Ni et al. recently reported a new gradient-enhanced WATERGATE- TOCSY experiment in which pulsed-field gradients were used to maintain precise control of the water magnetization vector.37 This experiment demonstrated marked improvements over a standard z-filtered TOCSY which used water presaturation but one should not underestimate the e§ort needed to develop and optimize similar experiments of this nature.Several more sophisticated methods for solvent suppression have been developed which like WATERGATE are based on the frequency-selective refocusing of the water resonance where the refocusing RF pulses are flanked by gradients that dephase unwanted transverse magnetization. These are the so-called ‘excitation sculpting’ and MEGA techniques38 and they are less sensitive to flip-angle errors than WATERGATE. This alleviates the phase problems commonly encountered when water magnetization is spin-locked.In these experiments the water magnetization is fully returned to equilibrium prior to each acquisition. This improves water suppression alleviates attenuation of the sample signal from saturation and eliminates radiation damping. As an aside it is instructive to point out that ‘excitation sculpting’ has other useful applications besides solvent suppression. Frenkiel et al. have extended it to selective excitation of other (i.e. non-solvent) resonances and it is particularly applicable in identifying correlations between specific protons in small molecules.39 Such techniques often allow one to get the same amount of information from a 1D experiment that would otherwise require 2D NMR. Depending on the sample however simpler water-suppression techniques are sometimes adequate or even more desirable.As previously mentioned Homans et al.21 opted not to use a gradient-based water suppression technique. This was due to the small di§erence in chemical shift between water and the anomeric protons in the selected carbohydrates. This made it virtually impossible to selectively suppress the water resonance without also suppressing the anomeric protons in the disaccharide. Thus these investigators opted for a technique which used long water-selective ‘purge pulses’ after all the disaccharide proton magnetization had been temporarily transferred to 13C. Other solvent suppression techniques sometimes have distinct advantages over pulsed-field gradient-based methods. The water-PRESS sequence is deployed just before the main part of a pulse sequence.40 In this technique a p-RF pulse inverts all of 374 B.A.Salvatore the magnetization (i.e. sample and water resonances) and then any transverse magnetization is removed with a homospoil. During a delay both the water and protein resonances relax along the z-axis. However since the T 1 for water is inherently much longer than the T 1 for the sample the delay is chosen such that the water magnetization is approximately zero when the protein resonances are almost all fully relaxed. The homospoil succeeds in dephasing spurious amounts of water magnetization in the transverse plane thus preventing the onset of damping during the delay. A subsequent ‘read pulse’ is then applied to observe the sample magnetization while flipping the water magnetization back to the[z-axis.Since the water-PRESS suppression module is appended in front of the pulse sequence and not spliced within it the main part of the experiment begins with the sample magnetization at thermal equilibrium. This eliminates the phase errors often seen with gradient-based techniques particularly when some of the pulse-widths or gradients are not calibrated properly. This technique is useful because it facilitates the observation of sample resonances that lie ‘underneath’ the water resonance something which cannot be achieved with selectiveinversion methods like WATERGATE. In contrast to most other methods the Water-PRESS technique is extremely simple to implement and optimize. It does not require accurately calibrated RF pulses nor excellent lineshape. Moreover there is no loss of sample intensity from di§usional e§ects which are especially problematic for small molecules.One disadvantage of the Water-PRESS method is the length of time it adds to each acquisition but this may be outweighed by some of the above advantages. Using computers in structure determination Traditional automated-NMR-resonance-assignment strategies are based on grouping resonances into spin systems that represent distinct components of a molecule (e.g. amino acid residues within a protein). This is followed by the identification of these segments and the sequential connection of the spin systems ultimately allowing assignment of all the NMR spectral resonances. Advances in double isotopic labeling (13C 15N) techniques have greatly facilitated this process for proteins. Now the e¶ciency is being further increased by computers.Montelione et al. have developed a computer program for assigning NMR resonances in proteins called AUTOASSIGN. 41 It requires the amino acid sequence and input data generated from 1H–15N HSQC as well as data from the eight most common 3D triple-resonance NMR experiments [H(CA)(CO)NH CA(CO)NH CBCA(CO)NH HNCO H(CA)NH CANH CBCANH and HN(CA)CO]. The program employs five sequential stages of analysis. Depending on the stage di§erent methods and criteria are used to designate chemical shifts and establish sequential links between individual spin systems. Each stage uses ‘constraint-based matching’ which progressively relaxes the criteria used in designating chemical shifts and sequential links along the protein carbon backbone. Prestegard et al.42 have pursued an alternative approach which employs a neural network to make connections between input data and output structural assignments from 15N-edited TOCSY-HSQC spectral data.This approach is more flexible in that probabilities are evaluated at each stage resulting in several choices instead of just one definitive choice. One disadvantage of a neural network approach however is the need for a large number of correctly assigned examples to ‘train’ the network. This can ultimately be o§set by the small amount of data required to make the actual assignments. 375 NMR Spectroscopy Emerenciano et al. have applied a similar heuristic approach to the structure determination of natural products by 13C NMR. This has resulted in a collection of modular programs based on the main program called SISTEMAT.This program operates on the premise that virtually all natural products are divisible into three components the skeletal atoms the heteroatoms directly bound to these atoms and the carbon side-chains. These researchers recently reported a subroutine which is suited for the identification of common side-chains (e.g. angelate tiglate etc.) attached to any of the atoms in a natural product.43 This module identifies subspectra of the carbon atoms representing specific substituent groups amid the raw 13C NMR spectral data. Thus side-chain peaks are identified and distinguished from the skeletal carbons whose values in turn can be fed to SISTEMAT to identify the carbon skeleton. SISTEMAT itself was recently upgraded to identify aromatic molecules and it was trained with a library of over 700 flavinoids which represent 72 distinct skeletal types.44 Given a set of 13C chemical shifts for an unknown flavinoid the program was able to suggest a list of probable carbon skeletons eliminating 70 of the 72 possible carbon skeletons in one demonstration.Another useful application of computers in the structure identification of natural products (albeit one that does not utilize AI methods) involves the simulation of complex NMR spectra based on molecular mechanics-derived structural data. Laatikainen et al. have developed one such spectral-simulation program called PERCH.45 They demonstrated this program’s usefulness by performing a complete 1H NMR spectral analysis of b-pinene which possesses a highly complex spin system containing 16 coupled protons.46 Oriented-sample NMR techniques It is ultimately desirable to study the properties of a natural molecule in an environment that resembles its native environment.Unfortunately this is more easily said than done. Natural oligosaccharides for example are often fixed on the outer phospholipid bilayer of cells and yet conformational information is still generally derived from these molecules through solution NMR experiments in which they tumble isotropically. However newNMRmethods for studying such molecules in membranelike environments have been developed over recent years. Isotropic micelles o§er one option for studying these molecules by NMR,47 and magic angle spinning of multilamellar liposomes has facilitated the acquisition of some high-resolution NMR spectra.48 Most appealing though are techniques which not only allow one to study such molecules at membrane surfaces but to also extract information about structure and conformation which is not available from solution or MAS NMR experiments.One such technique is liquid crystal NMR where molecules are studied in a fieldorientable liquid crystalline matrix that facilitates the measurement of dipolar couplings. These ‘through-space’ couplings do not exist in isotropic or MAS samples because they average to zero as the molecules tumble randomly or spin at the magic angle. Liquid crystal samples are often prepared by adding the compound of interest to a concentrated aqueous lipid-micelle solution. Dimyristoyl phosphatidylcholine (DMPC) and dihexanoyl phosphatidylcholine (DHPC) (or alternatively DMPC and the bile salt CHAPSO) are commonly used to form orientable micelle solutions.The resulting bilayer micelles (i.e. bicelles) are planar and thus possess an anisotropic magnetic susceptibility which causes them to orient in a magnetic field such that the 376 B.A. Salvatore HO O NH O OH O OH NH O HO HO HO O O HO O NH OH O OH NH O HO HO HO O O O bilayer normal Bo Fig. 7 Incorporation of GM4-lactam glycolipid molecules into a DMPC–CHAPSO bicelle. Note that the bicelle’s bilayer normal aligns perpendicular to the static magnetic field of the spectrometer (B 0 ) normal to the bicelle surface on average lies perpendicular to the external magnetic field (Fig. 7). In liquid crystal bicelle systems unlike solid crystals dipolar couplings are partially averaged (scaled down) by the motion of the bicelles and are hence termed ‘residual dipolar couplings’.The size of a particular residual dipolar coupling D*+ is defined by eqn. (4) D*+ \ [c*c+h 2p2r3 S.*#%-%S!9*!-T3 cos2 h*+ [1 2 U (4) where c* and c+ are the gyromagnetic ratios of the two nuclei r is the distance between the nuclei h*+ is the angle that the internuclear vector makes with the external magnetic field and the two S terms are order parameters whose product is sample-dependent and can be measured experimentally. The angular quantity (h*+) is the key term on the right side of the equation where the brackets denote the average value of the enclosed quantity on the NMR timescale. The presence of this angular term indicates that a relationship exists between D*+ and h*+. Thus one can infer that a correlation also exists between D*+ and a molecule’s orientation/conformation in an oriented system.This is the basis for liquid crystal NMR. The data are generally analyzed in terms of an order matrix using NMR dipolar (or quadrupolar) coupling data along with structural models from which distance information (r) is derived. Prestegard et al. employed oriented planar bilayer micelles (i.e. bicelles) to determine oligosaccharide headgroup orientation at membrane surfaces. Recent studies explored the conformations of GM4-lactam glycolipid49 (Fig. 7) a ganglioside analog with potential applications in the development of cancer vaccines as well as sulfoquinovosyldiacylglycerol a glycolipid with strong inhibitory activity against HIV- 1.50 These investigations were based on measurements of 1H–13C 13C–13C and 377 NMR Spectroscopy 1H–15N residual dipolar coupling measurements as well as site-specific 13C- and 15N-chemical shift anisotropy measurements.The dipolar coupling values were derived from labeled samples mainly via direct detection of 13C spectra (proton-decoupled 1D 13C–13C INADEQUATE and 2D 13C–13C DQF-COSY) and molecular mechanics-minimized structures were used to obtain distance (r) values. Vold and Prosser reported a modified bicelle system for which the diamagnetic anisotropy and hence the sense of bicelle orientation is flipped. Thus the bilayer normal axis aligns parallel to the magnetic field (S.*#%-%[0).51 This was achieved by doping a DMPC–DHPC solution with lanthanide salts (e.g. EuCl 3 ) and it resulted in a two-fold increase in the order parameter (S.*#%-%) as determined by quadrupolar splitting measurements in oriented-chain perdeuterated DMPC.This modified bicelle system enhances the resolution of dipolar couplings inNMRspectra and makes bicelle systems more applicable to the study of large slow tumbling molecules like proteins. Bax and Tjandra found that even greater resolution spectra are attainable even without metals by simply diluting the bicelle concentration (down to 3%w/v).52 This substantial decrease in bicelle concentration does not disrupt orientation but rather results in much higher resolution spectra which facilitate the measurement of 1H–15N 1H–15N and 1H–13C dipolar couplings. Moreover this bicelle system is amenable to indirect detection which provides far greater sensitivity than the direct 13C- and 15N-detection methods employed previously.In such dilute bicelle systems it is unlikely that the sample molecules are incorporated into the individual bicelles but they are still influenced and thus oriented by the cooperative alignment e§ects within the bicelle matrix. These results expand the power of oriented-bicelles making them more applicable to small molecules which are not necessarily membrane-anchored. The same investigators also reported a new pulse sequence for determining the residual dipolar contributions to C–H splittings within methinyl groups based on the quantitative measurement of peak intensities in 1H–13C HSQC spectra.53 They accomplished this with a modified constant time HSQC experiment in which the proton pulse is fixed in at the center of the constant time evolution period and only the carbon pulse is stepped through this period.4 Miscellaneous On a final note one paper which did not conveniently fall into any one of the above sections and yet is potentially applicable to all of them is a comprehensive list of the 1H- and 13C-chemical shift data of virtually all common laboratory solvents as trace in impurities in a variety of deuterated NMR solvents.54 Such a useful compendium will find application in the interpretation of a myriad of NMR spectra of organic compounds. References 1 T. Chounan H. Horino T. Ibuka and Y. Yamamoto Bull. Chem. Soc. Jpn. 1997 70 1953. 2 A. 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ISSN:0069-3030    

DOI:10.1039/oc094361

出版商:RSC    

年代:1998

数据来源: RSC