ISSN:0069-3030    

DOI:10.1039/OC98380FX001

出版商:RSC    

年代:1983

数据来源: RSC

ISSN:0069-3030    

DOI:10.1039/OC98380BX003

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

2 Physical Methods and Techniques Part (ii) Mass Spectrometry By M. JARMAN Drug Metabolism Team Section of Drug Development Cancer Research Campaign Laboratory Institute of Cancer Research Clifton Avenue Sutton Surrey SM2 5PX 1 Introduction The period since the previous review' has been noteworthy for the explosive growth in the number of papers dealing with the new soft ionization technique of fast atom bombardment. This has not however been at the expense of other areas of mass spectrometry with the possible exception of field desorption. The Proceedings of the 9th International Mass Spectrometry Conference held in Vienna in 1982,2 continue to reflect the diversity of the subject as do the Proceedings of another regular series of meetings the 4th International Symposium on Quantitative Mass Spe~trometry.~ Subjects of substantial reviews not cited elsewhere have included fundamental aspects of electron impact i~nization,~ gas-phase chemistry of collision- ally activated ions,5 and analytical capabilities and 2 Ion Structure and Fragmentation Molecular orbital calculations predict and experiment confirms that stable ions exist which are unusual in being low-energy species without stable neutral counter- parts.' Thus the methyleneoxonium radical cation CH2-OH2t was found to be 45 kJ mol-' lower in energy than CH3-OHt and was subsequently generated from ethylene glycoL8 Other ions of the type CH,XH+ (X = halogen NH2 or SH) have been generated.' These ions have been referred to as 'radical ion dipole complexes' since CH2XH* can formally be represented as a tightly bound complex between ionized methylene and HX.Thus and typically the structure HF-+ CH2t was assigned" to the ion CH2FHt generated'.10 by loss of COz from CH2FC02H since collision activation afforded the ions HFt and abundantly CH2t but unlike the isomeric species CH,F$ no CH,'. Analogous more complex structures have been ' M. Jarman Annu. Rep. Prog. Chem. Secr. B 1982 78 3. * In?. J. Mass Spectrom. Ion Phys. 1982 Vol. 45; 1983 Vols. 4648. Biomed. Muss Specrrom. 1983 10 113-235. T. D. Maerk In?. J. Muss Spectrom. Ion Phys. 1982 45 125. K. Levsen and H. Schwarz Mass Spectrom. Rev. 1983 2 77. ' R. G. Cooks K. L. Busch and G. L. Glish Science 1983 282 273. ' W. J. Bouma J. K. Macleod R.H. Nobes and L. Radom Int. J. Mass Spectrom. Ion phys. 1983,46,235. ' W. J. Bouma J. K. Macleod and L. Radom J. Am. Chem. SOC. 1982 104 2930. J. L. Holmes F. P. Lossing J. K. Terlouw and P. C. Burgers Can. J. Chem. 1983 61 2305. lo H. Halim B. Ciommer and H. Schwarz Angew. Chem. Int. Ed. EngL 1982 21 528. 19 M. Jarman described such as ionized oxycarbenes‘ I (e.g. CH,COHt and CH,OCOH’) and halonium radical ions’’ (e.g. CH,XCH,t). The latter are noteworthy as containing a divalent halogen atom bonded to two carbons. Ion-molecule complexes as intermediates in gas-phase reactions are exten- sively discussed as part of a review’ of gas-phase analogues of solvolysis reactions which emphasizes the connection between chemistry in the mass spectrometer and reactions in solution.For example in solution alkyl 4-pyridyl ethers readily elimi- nated olefins following N-methy1ati0n.l~ In the gas-phase analogy the olefin was produced as an ion-molecule complex with C4H5NOt following proton transfer from the alkyl residue to the pyridyloxy moiety to form e.g. ion (1). If the olefin contained allylic hydrogen as in the isopropyl13 but not in the ethylI5 ether a second hydrogen-transfer yielded the ion (2) and m/z 96 as well as m/z95 was abundant in the mass spectrum. The propensity for pyridine nitrogen to abstract hydrogen intramolecularly has a practical application in determining the positions of branching or unsaturation in long-chain fatty acids by mass spectrometry of their picolinyl esters.16 Interest in C6H6 radical-cations continues and an interesting example is benzvalene’’ which appeared to retain the structure (3) when ionized as evidenced for example by the decompositions of its gas-phase adduct with buta-173-diene which are most readily interpreted in terms of the structure (4).Another recurrent theme is the comparative behaviour of odd- and even-electron ions and the McLafferty rearrangement has been studied from this viewpoint.’8 Using even-electron ions generated from aromatic diesters and diketones McLaff erty rearrangement of ketones [(5) -* (7) X = CH,] but not of analogous esters (X = 0) was established on the criteria of specific y-deuterium transfer (Y = ’H) and by the presence of an appropriate metastable peak but was judged less common than for radical cations.” J. K. Terlouw J. Wezenberg P. C. Burgers and J. L. Holmes J. Chem. SOC.,Chem. Commun. 1983 1121. ’’ Y. Apeloig B. Ciommer G. Frenking M. Karni A. Mandelbaum H. Schwarz and A. Weisz J. Am. Chem. SOC.,1983 105 2186. l3 T. H. Morton Tetrahedron 1982 38 3195. l4 G. Schmid and A. W. Wolkoff Can. J. Chem. 1972 50 1181. Is A. Maquestiau Y. van Haverbeke C. de Meyer A. R. Katritzky M. J. Cook and A. D. Page Can. J. Chem. 1975 53 490. l6 D. J. Harvey Biomed. Mass Spectrom. 1982 9 33. l7 D. L. Miller and M. L. Gross J. Am. Chem. SOC.,1983 105 4239. M. Zollinger and J. Seibl Inf. J. Mass Specrrom. Ion Phys. 1983. 47 363. Physical Methods and Techniques -Part (ii) Mass Spectrometry 3 Chemical Ionization The alkanols were among the earliest compounds studied by CI using methane” and isobutane,20 and their isobutane spectra were recently studied in detail.2’ Formation of the Lewis adduct [M + C4H9]+ requires more energy than protonation but becomes relatively more abundant for the higher members within a homologous series since these larger molecules are more efficient at distributing this energy.The ratio [M + H -H,O]+/[ M + H]+ was 3-4 fold greater in the spectra of the axial 3-and 4-methylcyclohexanols than in their equatorial counterparts since more steric strain is released in eliminating water from the axial position. For dihydroxy derivatives [A4 + HI’ was more abundant for isomers (8) and (9) where it is stabilized by hydrogen bonding than for Owing to the low internal energy content of [M -HI- anions negative ion CI is particularly effective for revealing such sterochemical differences and a recent example of its use is the assignment of ring D stereochemistry to 17[-alkyl-5a 14P-androstane- 14,17(-diol~.~~ (8) (9) (10) New re%gent gases have been introduced.Dimethyl ether produces reactive CH,CH=OH ions and among other discriminating gas-phase reactions was able to distinguish polycyclic hydrocarbons which would form Diels-Alder adducts giving intense [M + C,H,O]’ ions from isomers which would not.24 Reagents for locating olefinic bonds continue to be sought and dimethyl disulphide is a recent example.25 However the use of a selective reagent gas can be misleading should it react at other positions of unsaturation.In a different approach designed to circum- vent this problem epoxidation of the olefinic linkage and ring opening with ammonia or an amine to form the corresponding amino-alcohol introduced a heteroatom having a higher proton affinity than any already present and hence directing sub- sequent fragment at ion.26 Dimeric ions have received little attention as gas-phase products of chemical ionization. Although their formation obviously requires sufficient sample concentra- tion for intermolecular reactions to occur other factors influence their formation. ”) F. H. Field Acc. Chem. Rex 1968 1 42. 20 F. H. FieM J. Am. Chem. Soc. 1970 92 2672. ” F. J. Winkler F. 0. Gulacar F. Mermoud and A. Buchs Helu. Chim. Acta 1983 66 929.22 F. J. Winkler F. 0. Gulacar F. Mermoud D. Stahl T. Gauman and A. Buchs Inr. J. Mass Specrrom. Ion Phys. 1983 46 321. 23 J. C. Beloeil M. Bertranne D. Stahl and J. C. Tabet J. Am. Chem. Soc. 1983 105 1355. 24 T. Keough Anal. Chem. 1982 54 2540. 25 H. R. Buser H. Arn P. Guerin and S. Rauscher Anal. Chem. 1983 55 818. 26 M. Cervilla and G. Puzo,Anal. Chem.. 1983 55 2100. M. Jarman Thus at a given total ion current methyl a-D-glucopyranoside gave [M2 + H]+ more abundantly than did the p-anomer." Evidence points to hydrogen bonding as a factor stabilizing dimers as in the hydroxyazadamantanes (1 I)** and it is therefore likely that dimer formation will depend upon stereochemistry and prove a useful additional means of distinguishing stereoisomers.Gas-phase analogies with solution chemistry have been numerous in CI and the reaction between benzene and alkyl cations is a particularly well studied example. The large entropy loss for the reaction t-C,H,' + C6H6-t-C,H,.C,H,+ attributed to the large mass and moments of inertia of the reactants and to the restricted motions of the t-butyl group in the product ion contributed significantly to its thermal in~tability.'~ However the stability of benzenium ions increases with the number of alkyl substituents on the benzene ring owing to greater stabilization of the charge by the multiple alkyl substituents and o-xylene for example forms a stable adduct with t-C4H9+.30 Attention has been given to the experimental conditions needed to detect and analyse adduct ions in gas-phase aromatic sub~titution.~' They were collisionally stabilized in the ion source ion-selected using a double-focusing mass spectrometer and their spectra after collision-induced decomposition (CID) taken by scanning a final analyser.Wheland-type adducts from allyl (12; R = CH2CH=CH2) and isopropyl [R = CH(CH,),] cations were detected and their structures validated by comparison with the spectra of protonated allyl and isopropyl benzenes. Conversely the structures of the C3H5+ ions produced from the corres- ponding isomeric halides were determined by reacting them with benzene and examining the CID spectra of the ad duct^.^^ Although the generally accepted transition state for electrophilic aromatic substitu- tion in the liquid phase is a cT-bonded complex (12) n-complexes have been favoured in certain cases.In a study of gas-phase analogues of these processes,33 proton transfer from CH5+ to toluene was markedly disfavoured by an o-or p-fluorine substituent but only slightly in rn-fluorotoluene where alone the mesomeric effect of fluorine as in (13) and (14) opposed its electron-withdrawing inductive effect. In the T-complex with NO+ only the inductive effect operated and the reduction in affinity for NO+ on fluorine substitution was independent of the orientation of the substituent. 27 P. Finch R. A. Hancock M. C. Matulewicz H. Weigel and M. Jarman Curbohydr. Res. 1982 104 C9. 28 M. D. Bezoari P. Kovacic and A. R. Gagneux Org. Muss Spectrom. 1982 17 493.29 D. K. Sen Sharma S. Ikuta and P. Kebarle Can. J. Chem. 1982 60,2325. 30 F. Cacace G. Ciranni and P. Giacomello J. Chem. Soc. Perkin Trans. 2 1982 1373. 3' D. L. Miller J. 0. Lay Jr. and M. L. Gross J. Chem. SOC. Chem. Commun. 1982 970. 32 J. 0. Lay Jr. and M. L. Gross J. Am. Chem. SOC.,1983 105 3445. 33 J. A. Stone D. E. Splinter and S. Y. Kong Can. J. Chrrn. 1982. 60. 910. Physical Methods and Techniques -Part (ii) Mass Spectrometry (12) (13) (14) (15) Reactions other than aromatic substitution have been studied as gas-phase analogues. The carbanion produced from dimethyl adipate in the gas phase yielded the anion (15),34 a proposed intermediate in the analogous Dieckmann ester con- densation in solution. In contrast the acid-catalysed acyl-transfer reaction between methanol and vinyl acetate proceeded as with other esters in the gas phase via acyI -cation trans fe r3 -+ HY + AcXH' [HY--Ac+--XH]-+ AcYH+ + HX and not as previously thought possible via tetrahedral addition-complexes invoked for the corresponding reaction in solution.4 Fast Atom Bombardment A review36 of this technique provides a useful background against which recent progress can be assessed. Among the issues raised are the choice of bombarding gas matrix effects comparisons with other soft ionization methods particularly field desorption and the key question of how large a molecule can be lifted from the matrix. Regarding yields of ions in FAB spectra xenon is generally reckoned superior to the more common inert gases argon and neon on account of the higher momentum of the heavier gas at constant accelerating voltage.Thus in a study of peptides ion yields from the three gases were extrapolated to equivalent momentum and found to be identical.37 Other factors appear to operate for less polar molecules. Thus the yield of ions from vitamin B,z using Xe Ar and Ne closely reflected the ratio between their atomic weights (1 0.3 0.15) but for protoporphyrin IX dimethyl ester a chloroform-soluble material the ratio was 1 0.8 0.6 a much less steep decline than predicted.38 On the basis of volatility and atomic weight other less expensive elements should be suitable for FAB. Thus mercury and caesium have given ion yields similar to or greater than ~enon.~~,~' Glycerol is the most widely used matrix being relatively involatile but having good solvent properties.Not all compounds dissolve in glycerol and other media such as triethyl citrate diethanolamine and polyethylene glycol have advantages in certain cases.4' The characteristic [ M + HIf ion seen in positive-ion FAB spectra using glycerol or other hydroxylated solvents is a function of the matrix since the use of a non-hydroxylated medium such as Fomblin oil3' results in Mt instead. .3 4 D. J. Burinsky and R. G. Cooks J. Org. Chern. 1982 47 4864. 3 -5 J. K. Kim and M. C. Caserio J. Am. Chern. Soc. 1982 104 4624. 36 K. L. Rinehart Jr. Science 1982 218 254. 37 S.A. Martin C. E. Costello and K. Biemann Anal. Chern. 1982 54 2362.1x H. R. Morris M. Panico and N. J. Haskins Inr. J. Mass Spectrom. Ion Phys. 1983 46 363. 39 R. Stoll U. Schade F. W. Roellgen U. Giessman and D. F. Barofsky Inr. J. Mass Specfrorn. Ion Pbys. 1982 43 227. s. S. Wong R. Stoll and F. W. Roellgen Z. Naturforsch. Ted A 1982 37 718. " J. Meili and J. Seibl. In!. J. Mass Spectrom. [on Phys. 1983 46 367. 24 M. Jarman Also the nitrophenols studied by gas-phase FAB gave Mt and fragmentation patterns qualitatively like their EI though the negative-ion spestra differed in that EI gave predominantly [M -HI- whereas FAB afforded MY base peak. In glycerol solution FAB afforded the conventional [M + H]+ and [M -HI- ions. Interference from glycerol clusters can be a nuisance where these coincide with peaks in the spectrum of the sample.This can be overcome by adding acids when [M + HI+ is enhanced.37 Conversely the molecular radical ion Mt can be enhanced at the expense of [M + HIt by charge transfer complexation particularly if glycerol is replaced by the aprotic solvent dimethyl s~lphoxide.~~ Turning to specific applications FAB has been most extensively used in the analysis of peptides. Both molecular weights in excess of 5000 and amino-acid sequences have been determined. Studies of bovine insulin4u6 exemplify the high molecular weights (5729) accessible using FAB and high-field magnets. Careful examination of the molecular ion region has revealed three sets of overlapping isotope clusters corresponding to reduction of one two or all three of the disulphide bonds showing that the two component peptide-chains remain together even after reduction of all S-S linkages4' The spectra afford sequence information.Thus the positive-ion spectra of angiotensins exhibited systematic fragmentation between CH-CO bonds and between CON-C bonds in the peptide chain the N-terminal fragment retaining the charge.47 Negative-ion spectra afforded less fragmentation but were complementary in giving a series of fragments by NH-CH cleavage the C-terminal fragment retaining the charge. Complex proteins can be sequenced if the base sequence of the gene coding for that protein is Conventional GC-EI-MS sequencing of small peptide fragments and molecular weight determina- tion of larger fragments from trypsin digests using FAB can afford information as to how the base sequence of the gene should be read and help correct any errors in this sequence.Another exciting innovation is the study of enzymatic reactions by FAB. Hydrolysis of arginine methyl ester by trypsin and of angiotensin by carboxypeptidase Y proved compatible with the glycerol matrix. Moreover intermit- tent exposure to the fast-atom beam did not reduce the activity of the enzymes although continuous exposure for 20 minutes virtually abolished it.49 Other types of macromolecule give sequence information. Oligodeoxyribonucleo- tides were most successfully examined in the negative-ion Sequence-defining fragments in the negative-ion FAB spectrum derived from P-0 cleavage both at the 3' and the 5' end of the internucleotide linkage and were exemplified for d(A-C-T-C-G-A-T-G) ([M -HI- 2407 daltons) enabling controlled synthesis of such compounds to be monitored using FAB.52 42 B.Kralj V. Kramer and V. Vrscaj lnr. 3. Mass Spectrom. Ion fhys. 1983 46 399. 43 E. De Pauw Anal. Chem. 1983 55 2195. 44 M. Barber R. S. Bordoli G. J. Elliott R. D. Sedgwick A. N. Tyler and B. N. Green J. Chem. SOC. Chem. Commun. 1982 936. 45 A. Dell and H. R. Morris Blochem. Brophys. Res. Commun. 1982 106 1452. 46 A. M. Buko L. R. Phillips and B. A. Blair Biomed. Mass Spectrom. 1983 10 408. 47 M. Barber R. S. Bordoli D. R. Sedgwick and A. N. Tyler Biomed. Mass Specfrom. 1982 9 208. 48 K. Biemann Inr. J. Mass Specfrom. Ion fhys. 1982 45 183. 49 L. A. Smith and R.M. Caprioli Biomed. Mass Specfrom.,1983 10 98. 50 G. Sindona N. Uccella and K. Weclawek J. Chem. Res. (S) 1982 184. M. Panico G. Sindona and N. Uccella J. Am. Chem. Soc. 1983 105 5607. 52 G. Lutz. R. Frank and H. Bloecker. Nuckic Acids Res.. 1982. 10 4671. Physical Methods and Techniques -Part (ii) Mass Spectrometry h Carbohydrates have been studied using FAB. The negative ion spectra of simple monosaccharides contained prominent [M -HI- ions and the major fragmentations appeared to be unimolecular decompositions of this ion and were diagnostic of structure.53 For example the prominent ion at m/z 89 from [M -13-in the FAB spectrum of ribose could be accounted for by the sequence (16) --+ (18). Oligosac-charides gave sequence-defining fragments by cleavage at glycosidic linkage^.'^ Another application to carbohydrate derivatives was the analysis of intact in gluc~ronides,~~ particular a new class of drug conjugates quaternary glucuronides in which tertiary nitrogen is quaternized by the glucuronyl residue and which are therefore intractable to analysis by EI and CI.56 Sulphates are a class of conjugate which usually decompose on attempted derivatization but can be analysed intact using FAB as exemplified by dehydroepian- drosterone sulphate" and other steroid sulphate~.~~"~ Other classes of natural product which have been examined using FAB have included corrins,6' prostaglan- dins,62 and penicillin^.^^ Finally an application having relevance to solution chemistry is the use of FAB to study the complexes formed between crown ethers and metal ions and their salts.64 Metallic salts yielded complexes of the type [crown + M"++ For example the complex ion from PbCI2 contained the anion and had the structure [crown + PbCI]' whereas univalent cations e.g.the alkali metals gave complexes containing the cation only. The well known high affinity of 18-crown-6 for K+ is reflected in the increase in ion yielded with increasing [K'] which becomes asymptotic as the ratio [18-crown-6]:[K+] approaches unity. 5 Other Soft Ionization Procedures Desorption-ionization mass spectrometry has been used as a general term to describe those methods in which ions are sampled from the condensed phase.6s The term '' M. Barber R. S. Bordoli R. D.Sedgwick and J. C. Vickerman J. Chem. SOC.,Faraday Trans. I 1982 78 1291. 54 J. P. Johannis W. Heerma J. F. G. Vliegenthart B. N. Green A. S. Ivor G. Strecker and G. Spik Biomed. Mass Spectrom. 1983 10 420. 55 C. Fenselau L. Yelle M. Stogniew D. Liberato J. Lehman and P. Feng Int. J. Mass Spectrom. Ion Phys. 1983 46 41 I. 56 J. P. Lehman and C. Fenselau Drug Merab. Dispos. 1982 10 446. 57 S. J. Gaskell B. G. Brownsey P. W. Brooks and B. N. Green Biomed. Mass Specrrom. 1983 10 215. 58 J. G. Lier C. F. Beckner A. M. Ballatore and R. M. Caprioli Steroids 1982 39,599. 59 C. H. L. Shackleton V. R. Mattox and J. W. Honour J. Steroid Biochem. 1983 19 209. 60 M. C. Dumasia E. Houghton C. V. Bradley and D. H. Williams Biomed. MassSpectrorn.1983 10,434. " H. Schwarz K. Eckart and L. C. E. Taylor Org. Mass Specrrom. 1982 17 458. 62 R. C. Murphy W. R. Mathews J. Rokach and C. Fenselau Prostaglandins 1982 23 201. 63 M. Barber R. S. Bordoli R. D. Sedgwick A. N. Tyler B. N. Green V. C. Parr and J. L. Cower Biomed. Mass Spectrom. 1982 9 I I. 64 R. A. W. Johnstone and I. A. S. Lewis Int. J. Mass Spectrom. Ion Phvs..1983 46 451. 65 K. L. Busch S. E. Unger A. Vincze R. G. Cooks and T. Keough J. Am. Chem. SOC.,1982 104 1507. 26 M. Jarman encompasses FAB and other methods described in this section. Secondary-ion mass spectrometry (SIMS) is closely allied mechanistically to FAB. A major difference is that cationized ions are formed in SIMS by interaction between the sample and the metal surface on which it is conventionally coated.Thus a silver support gave [M + Ag]’ ions for example from peptides66 and from oligo~accharides,~~ which were useful in defining molecular weight. Molecular ions were predictably more abundant6’ from Xe’ bombardment than from Ar’. An interesting deveiopment of this principle is the use of heavy organic ions exemplified by [A4 -CHJ’ from trimethylpentaphenyltrisiloxane(m/z 53 l).40 Despite the growth of FAB as a relatively routine method field desorption (FD) continues to be a useful complementary technique particularly in the hands of investigators skilled in its use and has been recently reviewed.69 Alone among desorption methods FD commonly affords odd-electron molecular ions (Mt) although molecules lacking functional groups of low ionization potential such as carbohydrates or aliphatic amino-acids tend to yield protonated or cationized molecular ions.However the low tendency of cobyrinic esters to form this type of ion may be the reason why FAB afforded no molecular ion species from these corrins whereas FD of cobyrinic acid heptamethyl ester could afford Mt exclus-ive]~.~’ Another advantage over FAB exemplified in a comparative study of bis- pyridinium salts7’ is the often abundant formation of doubly charged ions in FD. It was proposed that the ions induced an ‘image’ charge of equal intensity and opposite sign which resists desorption of the ions but which is easily counteracted by the field in FD allowing desorption even of doubly charged ions.Plasma desorption (PD) continues to lead other desorption methods in its capabil- ity for analysing compounds of very high molecular weight. With oligonucleotides positive-ion PD afforded molecular weights and negative-ion spectra gave sequence determining fragments formed by C(3’)-0 and C(5’)-0 fission.72 The method has now been applied to peptides and sequence determining fragments have been observed.73 Insulin was studied using ‘271 ions which are more energetic than the conventional 252~f fission fragment^.'^ 6 Other Studies In parallel with the foregoing developments in desorption methods investigations continue into techniques for obtaining mass spectra on refractory compounds using conventional EI and CI sources and specially designed probes.Formerly the emphasis was on the use of CI and the term desorption chemical ionization (DCI) has been used to describe the technique. Recently reports of similar investigations using the EI mode have been numerous. It is likely however that CI mechanisms bb H. Kambara S. Hishida and H. Naganawa Org. Ma55 Spectrom. 1982 17 67. 67 K. Harada M. Suzuki and H. Kambara Tetrahedron Lett. 1982 23 2481. 68 H. Kambara Org. Mass Spectrom. 1982 17 29. 69 G. W. Wood Tetrahedron 1982 38 1125. 70 H. M. Schiebel and H. K.Schulten Biomed. Mass Spectrom. 1982 9 354. ” D. N. Heller J. Yergey and R. J. Cotter Anal. Chem. 1983 55 1310. 72 C. J. McNeal K. K. Ogilvie N. Y. Theriault and M. J. Nemer J. Am. Chem. Soc. 1982 104 972. 71 B.T. Chait B. F. Gisin and F. H. Field J. Am. C‘hem. SOC.,1982 104 5157. 74 P. Haakansson I. Kamensky B. Sundqvist J. Fohlman P. Peterson and C. J. McNeal J. Am. Chem. SOC.,1982. 104 2948. Physical Methods and Techniques -Part (ii) Mass Spectrometry often contribute to the formation of the ions observed in 'direct electron impact' as it has been termed.75 Thus amino-acids were reported as affording M' in one such but [M + HI' ions in another.76 Penicillins mainly gave Mt but did give [M + HI' in one example.77 The addition of ammonium chloride to salts of penicil- lins and bile acids afforded molecular ions of the free acids but above 300 "C sodium salts of bile acids also gave RCO,Na+ ions.7' Alkali attachment ions [M + Na]+ could also be produced at similar temperatures from sucrose and glucose in an EI source.79 Protonated molecular ions were also produced under DCI conditions in a CI source when only N2 was present implying that adjacent molecules on the probe served as the proton source.'o In this study FAB SIMS and DCI gave similar spectra for ecdysterone suggesting common features in the desorption ionization processes.However the 'in beam' techniques were thought unlikely to be competitive in studying really large polar molecules and an upper mass limit of ca. 1500 daltons was suggested. Finally the potential of Fourier transform mass spectrometry (FTMS) has been further explored.81 Collision-induced decompositions can be studied without the need for a tandem instrument and proton-bound dimers and metal-bound ions have been investigated,'* as has high-resolution mass measurement in the separation of isobaric product ions.'3 The range of ion lifetimes (lop3+-> Is) in FTMS was exploited in studying methyl nitrite as a CI reagent gas whose ion products change with time.'4 Short ion trapping times generated NO' which reacted mainly by hydride and hydroxide abstraction.Longer times afforded CH,O(NO)NO' which transferred NO' to yield [M + NO]+. 75 P. Traldi Org. Mass Spectrorn. 1982 17 245. 76 D. Dessort A. Van Dorsselaer S. J. Tian and G. Vincendon Tetrahedron Lett. 1982 23 1395. 77 M. Ohashi R. P. Barron and W. R. Benson J. Pharrn. Sci. 1983 72 508. 7X A. K. Bose B. N. Pramanik and P. L. Bartner J. Org. Chern. 1982,47 4008.79 E. Constantin Org. Mass Specrrorn. 1982 17 346. 80 R. N. Stillwell D. I. Carroll J. G. Nowlin and E. C. Homing Anal. Chern. 1983 55 1313. '' C. L. Wilkins and M. L. Gross Anal. Chern. 1981 53 1661A. X? R. C. Burnier K.B. Cody and B. S. Freiser J. Am. Chern. SOC.,1982 104 7436. H3 R. B. Cody and B. S. Freiser Anal. Chern. 1982 54 1431. x4 W. D. Reents Jr. R. C. Burnier R. B. Cody and B. S. Freiser Anal. Chern. 1982 54 1245.

ISSN:0069-3030    

DOI:10.1039/OC9838000019

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

3 Theoretical Chemistry By C. THOMSON Department of Chemistry University of St. Andrews St. Andrews KY16 9ST Scotland 1 Introduction As in last year’s report coverage of the literature is restricted to ab initio calculations and owing to the ever increasing number of Ipplications must be very subjective a large number of references being omitted. Two useful new text books have appeared,’32 and a recent issue of THEOCHEM3 was devoted to papers in honour of K. Fukui Nobel Laureate (with R. Hoffman) in 1981. The papers presented at the 4th International Congress in Quantum Chemistry in Uppsala have been p~blished.~ A number of useful reviews have appeared including one on substituent effect^,^ a review of the rotational barrier in C2H6,6 the interpretation of electron distributions in molec~les,~ calculation of excited state potential energy (PE) surfaces,8 and reviews on Valence Bond (VB) theory’ and its use in organic reactivity problems.” The increasing application of quantum chemistry to biochemical problems is reflected in the appearance of a new edition’ ’ of Richards’ ‘Quantum Pharmacology’.2 Advances in Theoretical Techniques Basis Sets and Integral Evaluation.-Further additions to the standard basis sets developed by Pople and co-workers have been reported. The widely used STO-3G minimal basis sets have been extended to include the first and second row transition metals,12 and appear to be useful in the description of the bonding in various organometallic compounds. Improvements to the standard first row atom STO-3G basis sets have been tested by Yurtsever et a113 These yield energies -500-1400 kJmol-’ lower than the original STO-3G basis sets.’ R. L. Flurry ‘Quantum Chemistry’ Prentice Hall New York 1983. ’ D. A. McQuarrie ‘Quantum Chemistry’ Oxford University Press Oxford 1983. ’ THEOCHEM 1983.93 1-250. Int. J. Quantum Chem. 1983 23 1-1688. R. D. Topsom Acc. Chem. Res. 1983 16 292. R. M. Pitzer Acc. Chem. Res. 1983 16 207. ’ M. B. Hall in ‘Electron Distributions and the Chemical Bond’ ed P. Coopens and M. B. Hall Plenum Press New York 1982. E. R. Davidson and L. E. McMurchie Excited States 1982 5 I. Z. S. Herman Znt. J. Quantum Chem. 1983 23 921. lo A. D. Ross and S. S. Shaik Acc. Chem. Rex 1983 16 363. I’ W. G. Richards ‘Quantum Pharmacology’ 2nd Ed.Butterworths London 1983. ’’W. J. Pietro and W. J. Hehre J. Comput. Chem. 1983 4 241. E. Yurtsever W. Schoeller and D. D. Shillady Chim. Acta Turc. 1982 10 165. 29 30 C. Thornson The standard 3-21G basis set can be augmented with a set of diffuse SP functions to give the 3-21 + G basis set which is suitable for calculating the geometries and proton affinities of molecular anions.14 A medium size energy optimized basis set (15s 1 Ip 6d) has been developed for the elements In-Xe.” Burton and co-workers16 have studied procedures for sys- tematically analysing and improving on basis set deficiencies especially for configur- ation interaction (CI) calculations Different algorithms for the calculation of the electron-electron repulsion integrals have been studied by Hegarty and Van der Velde,” particularly with respect to their use on vector computers.A recent book has appeared devoted to the problem of evaluating integrals over Slater-type orbitals.” However it seems likely that Gaussian-type orbitals (GTO) will continue to dominate polyatomic molecule calcu- lations. Rys” et al. and Harris2’ have discussed improved algorithms for GTO integrals and Habitz and Clementi” have also developed faster programmes for these integrals which should be useful for large molecules. Self-consistent Field Theory.-Further work has appeared on the convergence prob- lem in Self-consistent Field (SCF) calculations. The criteria proposed by Stanton” have been generalized for an Unrestricted (UHF) wave function,23 and related to the stability conditions.The latter have been further in~estigated.~~ A further method for accelerating SCF convergence has been propo~ed.’~ A new procedure for SCF energy minimization has been proposed and tested in RHF calculations on both closed- and open-shell systems.26 Substantial time savings were achieved. The calculation of localized orbitals has been disc~ssed,~’ as have population and the calculation of one-electron proper tie^.^' A new SCF interaction energy scheme has been proposed by Kaufman and co-workers3’ which includes the counterpoise correction. Leroy3* has examined the concept of chemical stability from the point of view of SCF results. A new and very general form of valence bond theory has been described (VB-SCF theory),33 which contains MC-SCF theory as a subclass and gives results for P.E.l4 T. Clark J. Chandrasekhar G. W. Spitznagel and P. von R. Schleyer J. Comput. Chem. 1983 4 294. Is A. Stromberg 0. Gropen and U. Wahlgren J. Compu!. Chem. 1983 4 181. I‘ P. G. Burton P. D. Gray and U. E. Senff Mol. fhys. 1982 47 785. ” D. Hegarty and G. Van der Velde Int. J. Quantum Chem. 1983 23 1135. ETO Multicentre Molecular Integrals ed. C. A. Weatherford and H. W. Jones Reidel Dordrecht 1982. l9 J. Rys M. Dupuis and H. F. King J. Comput. Chem. 1983 4 154. 20 F. Harris In!. J. Quantum Chem. 1983 23 1469. 21 P. Habitz and E. Clementi Comput. Phys. Commun. 1983 29 301. 22 R. E. Stanton J. Chem. fhys. 1981 75 5416. 23 J.C. Facelli and R. H. Contreras J. Chem. fhys. 1983 79 3421. 24 P. Karadakov and 0.Castano In!. J. Quantum Chem.. 1983 24 453. 25 I. Balint and M. 1. Ban Chem. Phys. Lett. 1983 101 153. 26 J. Fernandez Rico J. M. Garcia de la Vega J. I. FernLndez Alonso and P. Fantucci J. Comput. Chem. 1983 4 33 41. 27 J. M. Leonard and W. L. Luken 7heor. (-‘him. Actu 1982 62 107. 28 B. T. Thole and P. Th. van Duijnen Theor. Chim. Acta 1983 63 209. 29 1. Mayer Chem. fhvs. Lett. 1983 97 270. “I G. De Rrouckkre Int. J. Quantum Chern. 1983 23 1677. 31 W. A. Sokalski S. Roszak P. C. Hariharan and J. J. Kaufman In!. J. Quantum Chem. 1983 23 847. 32 G. Leroy Int. J. Quantum Chem. 1983 23 271. 33 J. H. van Lenthe and G. Balint-Kurti J. Chem. Phys.1983 78 5699. The0retical Chemistry 31 curves which are very accurate. Extensions to polyatomic molecules will be awaited with interest. There have been several papers dealing with localized bond and their use in the calculation of rotation barriers.36 Electron Correlation.-A large number of papers have appeared concerning the various methods of including electron correlation in quantum chemistry calculations. Space permits only a brief mention of some of these. The configuration interaction method (CI) is in principle capable of yielding the exact non-relativistic wave function and much recent work has appeared. One important aspect is the convergence characteristics of the e~pansion,~’ which has been studied by Cooper and Pounder.Payne has investigated the use of a basis of biorthogonal states.38 Multi-reference CI techniques have been implemented by several groups and references are to be found in a recent article by Ahlri~hs~~ in the Proceedings of a meeting devoted to computational methods in quantum chemistry. An important implementation of the shape-driven graphical unitary group approach has been reported by Schaefer’s A variational calculation on C2H4 included more than lo6 configurations and examined the contributions of triple and quadruple excitations. Several other a~thors~~,~’,~~ have discussed the unitary group method and its relationship to the symmetric group approach. The paper by Schaefer marks the beginning of a series of full CI calculations (where all possible N-tuple excitations are included for N-electrons) on molecules with small or medium-size basis sets.A full CI double zeta (DZ) basis calculation on H2043 was followed by DZ + P calculations on BH,44 HF NH3 and more recently Be and ( H2)2 with large basis sets.45 Bartlett and co-~orkers~~ have compared the results with calculations including correlation by different methods in particular the coupled-cluster doubles (CCD) and many body perturbation theory (MBPT) methods at the full fourth-order level. The agreement with the full CI is to within <8 kJ mol-’ for CCD and to -40 kJ mol-’ for the MBPT(4) method. A similar comparison of full CI to a CI extrapolation method has been reported for H20by Burton and Gray.47 Schweig and co-~orkers~~ have looked at the effect of triple and quadruple excitations on one-electron densities in CI calculations.34 V. Barone. G. del Re A. Lami and G. Abbate THEOCHEM 1983 105 198. ’’ V. Barone G. del Re A. Lami and G. Abbate THEOC‘HEM 1983 105 IY I. 36 G. F. Musso G. Figari and V. Magnasco J. Chem. Soc. Faraduy Trans. 2 1983 79 93 1283. ” 1. L. Cooper and C. N. M. Pounder J. Chem. Phys. 1982 77 5045. 3x P. W. Payne J. Chem. Phys. 1982 77 5630. 39 R. Ahlrichs 5th Seminar on Computational Methods in Quantum Chemistry Max Planck Institute Groningen 198 1. 40 P. Saxe D. J. Fox H. F. Schaefer 111 and N. C. Handy J. Chem. Phys. 1982 77 5584. 4’ P. W. Payne lnr. J. Quantum Chem. 1982 22 1085. 42 W. Duch and J. Karwowski Inr. J. Quantum Chem. 1982 22 783.43 P. Saxe H. F. Schaefer 111 and N. C. Hardy Chem. Phys. Lett. 1981 79 202. 44 R. J. Harrison and N. C. Handy Chem. Phys. Lerr. 1983 95 386. 45 R. J. Harrison and N. C. Handy Chem. Phys. Lett. 1983 98 97. 46 R. J. Bartlett H. Sekino and G. D. Purvis Ill Chem. Phys. Lett. 1983 98 66. 47 P. G. Burton and P. D. Gray Chem. Phys. Lett. 1983 96,453. 48 H Meyer A. Schweig and W. Zittlau. Chem. Phvs. Lett. 1982 92 637. 32 C. Thornson Interest continues in developing the Multi-Configuration SCF (MC-SCF) method particularly since the earlier problems with convergence seem to have largely been solved. The recent developments have been reviewed,49 and several later papers by the same group have a~peared.~'-~~ A different convergence method has been proposed by Yurtsever and shill ad^^^ and tested.MC-SCF equations for excited states have been in~estigated,~~ and also time-dependent aspects of MC-SCF problem^.^^,^^ R~eggen~~ has further developed his extended geminal method6' and in an application to F2 obtained excellent agreement with experiment for the dissociation energy and bond length. There have been various developments in coupled cluster theory.61 Potential Energy (PE) Surfaces.-The recent efforts in this area are expected ulti- mately to be of enormous benefit to the chemist in the understanding of reaction mechanisms and much current research is concerned with the problem of the analysis of the molecular potential energy hypersurfaces. Mezey in particular has made important contributions in the description of chemical structure based on the topology of the nuclear configuration spa~e,6~~' have also and other a~thors~~-~' studied this aspect of PE surfaces.The problems of locating saddle points on such surfaces have been the subject of several paper^.^"^^ S~hafer~~ has reviewed the use of gradient methods in structural chemistry and Miller74 has similarly reviewed the dynamic aspects of reaction path studies. Sim~ns~~ and co-workers have described an interesting new automated method for walking on PE surfaces. This should enable one to locate stationary points at lower cost and has been successfully implemented. Central to this problem is that of determining the first and second derivatives of the potential energy surface.A very clear review of the analytic calculation of 49 D. L. Yeager D. Lynch J. Nichols P. Jbrgensen and J. Olsen J. Phys. Chem. 1982 86 2140. 50 A. Igawa D. L. Yeager and H. Fukutome J. Chem. Phys. 1982 76 5388. 5' P. J~rgensen,P. Swanstrom and D. L. Yeager J. Chem. Phys. 1983 78 347. 52 J. Olsen and P. Jbrgensen J. Chem. Phys. 1982 77 6109. 53 P. Jbrgensen P. Swanstrom D. L. Yeager and J. Olsen Int. J. Quantum Chem. 1983 23 959. 54 J. Olsen P. Jbrgensen and D. L. Yeager Int. J. Quant. Chem. 1983 23 25. 55 E. Yurtsever and D. Shillady Chem. Phys. Lett. 1983 94 316. 56 R. Colle and 0.Salvetti Mol. Phys. 1982 47 959. 57 R. McWeeny Int. J. Quantum Chem. 1983 23 405. 58 R. McWeeny THEOCHEM 1983,93 1. 59 I.Roeggen J. Chem. Phys. 1983 78 2496. 60 I. Roeggen Int. J. Quantum Chem. 1982 22 149. 6' W. H.Fink A Banerjee and J. Simons J. Chem. Phys. 1983 79 3104. 62 P. G. Mezey Int. J. Quanrum Chem. 1982 22 101. 63 P. G. Mezey THEOCHEM 1983 103 81. h4 P. G. Mezey Theor. Chim. Acra 1982 62 131. 65 P. G. Mezey J. C'hem. Phys. 1983 78 6182. 66 M. V. Basilevsky THEOCHEM 1983 103 139. 67 M. V. Basilevsky Chem. Phys. 1982 67 337. 68 R. B. King Theor. Chim. Acta 1983 63 103. '" P. G. Mezey Can. J. Chem. 1983 61 956. 70 M. R. Peterson I. G. Csizmadia and R. W. Sharpe THEOCHEM 1983 94 127. 7' I. Balint and M. I. Ban Theor. Chim. Acta 1983 63 255. 12 I. Balint and M. I. Ban Int. J. Quantum Chem. 1983 24 161. 73 L. Schafer J.Mol. Struct. 1983 100 51. 74 W. H. Miller J. Phys. Chern. 1983 87 3811. 75 J. Simons P. Jbrgensen H. Taylor and J. Ozment J. Phys. Chem. 1983 87 2745. Theoretical Chemistry 33 molecular gradients and Hessians has been given by Jargensen and Simon~,~~ who derived the relevant equations for several different classes of wave functions using exponential unitary operator methods. A more recent paper77 dealt with third and fourth derivatives which relate to anharmonicities on PE surfaces. P~lay~~ has also discussed gradients and coupled electron pair theories. Schaefer and co-~orkers~~ have also obtained expressions for analytic second derivatives for high-spin open-shell RHF wave functions and also for the simplest" two-configuration post-SCF wave function.A new approach" to the solution of the coupled perturbed HF equations has been described which avoids the four index transformation. The rotational invariance properties of the analytic first second and third energy derivative integrals have been exploiteds2 to save time in the integral derivative computations. King and co-workersS3 have also investigated the use of symmetry in the coupled- HF formalism and an alternative expression for MCSCF force constants was de~cribed.'~ An alternative to gradient methods has been proposed by Nakatsuji et aLS5in a series of papers. An extension of the usual basis set by inclusion of derivative basis functions gives a wave function which satisfies the Hellman-Feynman theorem and this yields additional insight into the driving force in chemical reactions.However it is substantially more expensive than gradient methods,8688 but there will probably be increased interest in such approaches. 3 Applications Applications of the methods referred to in earlier sections are increasing enormously every year and this section will be of necessity very subjective. Small Molecule PE Surfaces and Reactions.-A large basis set CI calculation includ- ing all single and double excitations has been reporteds9 for the reaction (1) C(3P)+ H -+ CH2(3B,) (1) The reaction proceeds through a weakly avoided crossing of the 3A2and 3B,potential energy surfaces the lowest energy pathway giving a barrier height of -4 kJ mol-' 76 P. J~rgensen and J. Simons J.Chem. Phys. 1983 79 334. 77 J. Simons and P. J~rgensen,J. Chem. Phys. 1983 79 3599. 7x P. Pulay THEOCHEM 1983 103 57. 79 P. Saxe Y. Yamaguchi and H. F. Schaefer 111 J. Chem. Phys. 1982 77 5647. no Y. Yamaguchi Y. Osamura G. Fitzgerald and H. F. Schaefer Ill J. Chem. Phys. 1983 78 1607. " Y. Osamura Y. Yamaguchi P. Saxe D. J. Fox M. A. Vincent and H. F. Schaefer Ill THEOCHEM 1983 103 183. '* M. A. Vincent P. Saxe and H. F. Schaefer 111 Chem. Phys. Lett. 1983 94 351. 133 T. Takada M. Dupuis and H. F. King J. Comput. Chem. 1983 4 234. 84 R. N. Camp H. F. King J. W. Mclver jun. and D. Mullally J. Chem. Phys. 1983 79 1088. 85 H. Nakatsuji M. Hada K. Kanda and T. Yonezawa Int. J. Quantum Chem. 1983,23,357 and references therein.86 P. Pulay J. Chem. Phys. 1983 79 2491. 87 H. Nakatsuji K. Kanda M. Hada and T. Yonezawa J. Chem. Phys. 1983 79 2493. xn H. Nakatsuji M. Hada and T. Yonezawa Chem. Phys. Lett. 1983 95 573. x9 L. B. Harding J. Phys. Chem. 1983 87 441. 34 C. Thomson but the value is very sensitive to the basis set. A similar study of the reaction B+(]S) + H + BH'(21;+) + H (2) using an MRD-CI wave functiong0 was reported by Hirst. The best available calculations on the electron affinity of CH29' yield a value corrected for zero-point vibrational energy of 0.42 eV and it is estimated that this value is low by -0.2 eV. The source of this remaining error has yet to be determined. The singlet-triplet gap of 37 kJ mol-' is in good agreement with other estimates.The insertion of Be into H2 has been studiedg2 using the coupled cluster singles and doubles (CCSD) method and compared to full CI results. The method provides an excellent description of this process in good agreement with full CI results. The structure and stability of a large number of dicationsg3 XHF(X = N 0 P or S) fourteen in all have been studied using large basis sets in RHF calculations and the calculated activation barriers and deprotonation energies were in reasonable agreement with charge stripping experiments. Botschwina has used the Self Con- sistent Electron Pairs (SCEP) method to calculate the PE surface and vibrational frequencies of FH2+ C1H2+,94 and H30+ 95 The i.r. intensities were also predicted for H30+. The minimum energy pathway for the reaction (3)96 H,O + OH + H (3) has been calculated using a variety of methods and a DZ + P basis.It was concluded that the MC-SCF procedure without CI is the simplest way to obtain reliable dissociation curves. A detailed study of the reaction surface for the addition of C(3Por ID) to H20 has a~peared.~' Geometries were optimized at the 3-21G level and single point UMP3/6-3 1G** calculations carried out at the stationary points. The calculations agree with experiment in predicting that only the singlet state of C is reactive towards H2O. Carbenoids involving Li have been studied by several groups starting with CLi and Li2C.98 The former has a 'Z ground state in contrast to the 2rI state of CH. Various isomeric structures of CH2LiF have been studied with a better than DZ + P basis set and their vibrational frequencies calculated at the SCF level.The PE surface is extremely flat and the lowest energy isomer is the ion pair H,CLi+-..F- at both SCF levels and also with CI calculation^.^^ Related organometallic molecules are H2CBe and HCBeH where the low lying singlet and triplet states have been studied by Pople's group."' Calculated energy 90 D. M. Hirst Chem. Phys. Lett. 1983 95 591. 9' D. Feller L. E. McMurchie W. T. Borden and E. R. Davidson J. Chem. Phys. 1982 77 6134. 92 G. D. Purvis 111 R. Shepard F. B. Brown and R. J. Bartlett Int. J. Quantum Chem. 1983 23 835. 93 S. A. Pope I. H. Hillier M. F. Guest and J. Kendric Chem. Phys. Lett. 1983 95 247. 94 P.Botschwina 'Molecular Ions' ed. J. Berkowitz and K. 0. Groeneveld Plenum Press 1983 p. 41 I. 95 P. Botschwina P. Rosmus and E. A. Reinsch Chem. Phys. Lett. 1983 102 299. 96 S. A. Alexander and F. A. Matsen Int. J. Quantum Chem. 1982 516 445. 97 S. N. Ahmed M. L. McKee and P. B. Shevlin J. Am. Chem. SOC.,1983 105 3942. 98 A. Mavridis and J. F. Harrison J. Am Chem. SOC.,1982 104 3827. 99 M. A. Vincent and H. F. Schaefer 111 J. Chem. Phys. 1982 77 6103. I00 B. T. Luke J. A. Pople and P. von R. Schleyer Chem. Phys. Lett. 1983 97 265. Theoretical Chemistry differences were obtained using the 6-31G** basis set and the MP4 procedure following 6-3 1G* geometry optimizations at the SCF level. The first computation of the structure of a transition state in the reactions of carbenoids has been reported."l The reaction of LiCH2F with C2H4 has been studied using a 3-21G basis set.The computed transition state structure (I) is related to the butterfly structure (2) proposed for the Simmons-Smith reaction where the car- benoid is IZnCH,I and L,L' are ligands. The structure (1) also resembles the transition-state structure for the isomerization of LiCH2F to H2CLiF which is nearly linear.99 L' I . ,CH2 a. R,C-CR A very detailed study has appeared of CH;+ which has shown that this is a viable species of hexaco-ordinated carb~n.'~~,~~~ The geometry (3) was obtained in RHF/6- 3 1G* gradient calculations and the relative energies on the surface by single point MP3/6-31G** calculations.Results were also reported for CH;+ which has D4h symmetry and C,Hi+. The structure of the latter has each carbon pentaco-ordinate (4). There is a large barrier -119 kJ mol-' to deprotonation of CHi+. It was suggested that both CHi+ C,Hi+ and C,Hi+ might be observable in the gas phase. A related paperIo4 dealt with the PE surface for C,H:+ which has the perpendicular structure (5). The rotational barrier to the D2h transition state (6) is high (115 kJ mol-I). Other workerslo5 have studied CH4+. Transition-state structures for the neutral cationic and anionic vinylidene-acetyleneIo6 rearrangement have been found using the 4-3 1G basis and relative energies computed at the MP3 level with a 6-31 +G** basis set with added diffuse functions for the anions.Rearrangement of the anion proceeds with a high activation energy (193 kJ mol-I) uia a perpendicular transition state (7). J. Mareda N. G. Rondan K. N. Houk T. Clarke and P. von R. Schleyer J. Am. Chem. SOC.,1983 105 6997. ItIZ K. Lamrnemma G. A. Olah M. Rar~aphi and M. Simonetta J. Am. Chem. Soc. 1982 104 6851. I03 K. Lammertsma M. Barzaghi G. A. Olah J. A. Pople P. von R. Schleyer and M. Simonetta J. Am. Chem. SOC.,1983 105 5258. I04 K. Lammertsma M. Barzaghi G. A. Olah J. A. Pople A. J. Kos and P. von R. Schleyer J. Am. Chem. SOC.,1983 105 5252. I05 J. M. Garcia de la Vega J. Fernandez Rico M. Paniagua and J. J. Fernandez-Alonso THEOCHEM 1983 105 31. lo' G. Frenking Chem. Phys. Lerr. 1983 100 484. 36 C.Thornson A comprehensive study of the PE surfaces for the set of compounds H,ABH (A B = C N 0 and F) has been p~b1ished.l~' A wealth of important information is contained in this paper. Cations containing oxygen have been much studied recently. Isotope effects have been investigatedlo8 in the protonation and deprotonation of CH30H a problem studied earlier in detail,'" together with the transition-state structures. Various dications such as CH30H2+ CH,OH;+ CH202+ HCOHZf and other related species have been studied by Bouma and Radom."' CH,OH;+ is much more stable than CH30H2+ in agreement with experiment. The structures and stabilities of the species C2H30t,'l' C2H50+,Ii2 have been studied by the same and C2H60+113 group. The lowest energy isomer of C2H,0f is the acetyl cation CH3CO+ but CH2COH is also predicted to be stable.In the case of C2H60+ the two lowest energy isomers are the oxonium ions (8) and (9). The acidity of the acetylenic proton in 21 mono-substituted acetylenes has been investigated the STO-3G geometries being fully optimized and the acidities calcu- lated at the 4-31G//3G Potential energy surface calculations become more expensive as the number of atoms increases and a thorough search of the surface is not feasible except for quite small basis sets. Nevertheless several larger systems have been studied and we refer to a few of these results. An investigation of the reactions of 'Aa O2 with C2H4 with the CASSCF method shows that the pathway via a peroxirane intermediate has a lower activation energy than the biradical pathway.' The mechanism of hydroboration in ether has been studied using as a model H3B-OH2.The reaction with C2H4 was studied and shown to resemble an SN2 displacement of the solvent by the olefin."6 A large scale calculation in the dissociation of H,C0"7 gives an H,CO -CO + H (4) I07 J. A. Pople K. Raghavachari M. J. Frisch J. S. Brinkley and P. von R. Schleyer J. Am. Chem. Soc. 1983 105 6389. Io8 I. H. Williams THEOCHEM 1983 105 105. I IIY R. H. Nobes and L. Radom Org Mass Specfrom. 1982 17 340. I10 W. J. Bouma and L. Radom J. Am. Chem. Soc. 1983 105 5484. 'I1 R. H. Nobes W. J. Bouma and L. Radom J. Am. Chem. Soc. 1983 105 309. I I2 R. H. Nobes and L. Radom Chem. Phys. Leu. 1983 99 107.I13 W. J. Bouma R. H. Nobes and L. Radom J. Am. Chem. Soc. 1983 105 1743. I14 M. F. Powell M. R. Peterson and I. G. Csizmadia THEOCHEM 1983 9 323. 1 I5 M. Hotokka B. Roos and P. Siegbahn J. Am. Chem. Soc. 1983 105 5263. 1 I6 T. Clark D. Wilhelm and P. von R. Schleyer J. Chem. Soc. Chem. Commun. 1983 606. I I7 M. Dupuis W. A. Lester jun. B. H. Lengsfield 111 and B. Liu J. Chem. Phys. 1983 79 6167. Theoretical Chemistry 37 activation energy of 322 * 13 kJ mol-' and accurate structures and harmonic frequencies were also calculated. A similar MC-SCF study was carried out by Feller and Davidson"' of the D2hdissociation of the 'A excited state of C2H4. C2H -+ 2CH,('A,) (5) Several authors have calculated fundamental vibrational frequencies for optimized structures.Among these we mention calculations on the formate anion,'I9 and on cyclobutadiene.12' A more recent development is the ability to calculate infrared intensities and the application to H30+ H2DO' HD20+ and D20+.I2l Schaefer and co-workers'22 have examined the (C,H,) surface. The absolute minimum corresponds to vinylacetylene but singlet cyclobutyne is a relative minimum lying some 320 kJ mol-' above the absolute minimum. The [1,3] dipolar cycloaddition of fulminic acid to C2H2 has been investigated at the 4-3 IG basis level including ~orrelation'~~ with -2 x lo6 configurations! Exten- sive CI is necessary for biradical problems. The transition-state structure for the [ 1 51 sigmatropic hydrogen transfer in cis-1 3-pentadiene has been determined,'24 and in the case of the 1,2-hydride shift in C6H5+ a high quality calculation predicts a very large barrier of -170 kJ m01-I.'~~ l~~ Bicerano et ~ 1 .in an interesting paper on malonaldehyde have determined the geometry and vibrational frequencies of the asymmetric equilibrium structure and also that of the transition state. The geometry and frequencies were evaluated at the SCF level and the barrier height using C1 calculations the latter quantity being 33 kJ mol-l. Finally the dynamical tunnelling has been calculated using a simple one-dimensional model with results in good agreement with experiment. (10) (1 1) (12) The question of the structure of the 2-norbornyl cation has been addressed once more with high quality CI calculations.127 Optimization of the various structures was performed and the classical structure (10) is the minimum energy structure at the 4-3 1G level with C- and H-bridged structures corresponding to saddle points.Correlation calculations with up to 1.5 million configurations stabilize the bridged structure relative to the classical as was also the case with 6-3 1G" SCF calculations. It was concluded that the C-bridged structure should be a true minimum and that there is no true minimum corresponding to the classical structure (10). Similar results were reported by other workers.12* I IX D. Feller and E. R. Davidson J. Phys. Chem. 1983 87 2721. 1 I9 A. R. Gregory K. G Kidd and G. W. Burton THEOCHEM 1983 104 9. I20 B. A. Hess jun.P. Chrsky and L. J. Schaad J. Am. Chem. SOC.,1983 105 695. 12' M. E. Colvin G. P. Raine H. F. Schaefer Ill and M. Dupuis. J. Chem. Phys. 1983 79 1551. 122 G. Fitgerald P. Saxe and H. F. Schaefer 111 J. Am. Chem. SOC.,1983 105 690. I23 P. C. Hiberty G. Ohanessian and H. B. Schlegel J. Am. Chem. SOC.,1983 105 719. I24 B. A. Hess jun.and L. J. Schaad J. Am. Chem. SOC.,1983 105 7185. I25 P. von R. Schleyer A. J. Kos and K. Raghavachari J. Chem. SOC.,Chem. Commun. 1983 1296. I26 J. Bicerano H. F. Schaefer 111 and W. H. Miller J. Am. Chem. SOC.,1983 105 2550. I27 M. Yoshimine A. D. McLean B. Liu D. J. DeFrees and J. S. Binkley J. Am. Chem. SOC.,1983,105,6185. 128 K. Raghavachari R. C. Haddon P. von R. Schleyer and H. F. Schaefer 111 J. Am.Chem. SOC.,1983 105 5915. 38 C. Thomson The structures of various C9H9+ cations and their interconversions have been studied both with MNDO and STO-3G ab initio ~tudies.'~' The cation of C symmetry is the most stable in agreement with experiment. Calculations on the molecular ions of allene and propyne have been rep~rted.'~~~'~' Transition structures for reactions representative of the additions of model nucleophiles electrophiles and radicals to propene have been reported by Houk and ~o-workers.'~~ Finally the question of cyclic N6of Dshsymmetry'33 (hexa-azabenzene) has been explored at the SCF level. The cyclic structure is predicted to be a relative minimum and the energy surface is very flat. Radical and Other Open-shell Species and Reactions.-We restrict the coverage in this section to radicals of interest primarily in organic chemistry.has reviewed his various papers on the structure of free radicals and the use of the Unrestricted Hartree Fock (UHF) method. An extensive comparison of structural predictions for small radicals using a wide variety of basis sets and correlation up to the MP3 level has been reported by P0p1e.l~~ Provided spin contamination is small the UHF and ROHF predictions are in good agreement and spin contamina- tion is least for the largest basis sets. The radicals considered in this study were mainly diatomic or triatomic molecules apart from CH3. The ammonium radical' 36 undergoes dissociation by tunnelling through a low barrier the ground state being the 3s 'A Rydberg state.Havriliak and King'37 have also studied this species. The reactions of singlet and triplet NH radicals'38 have been studied using a MRD-CI wave function and 4-31G or 4-31G** basis set. The calculation^'^^ on HCO show it to have a a-ground-state and a low lying 7r-excited-state. Transition states for the fragmentation to H + COz were determined. Two papers deal with different aspects of the structure of methoxy CH,O. MBPT calculations'40 of the geometry Jahn-Teller energy surfaces spin orbit splitting and Zeeman effects have been reported. A very detailed study of the surface for CH30 and its rearrangement to CH,OH together with vibrational frequencies and transition-state structures has appeared.141 CH20H lies some 21 kJ mol-' lower than CH30 and the favoured isomerization mode is uia an intramolecular rearrangement.Theoretical studies'42 on [CH,COH]+ agree with experiment in predicting that it does exist as a stable C2H40+ isomer and as a key intermediate in the isomerization- dissociation processes of the cation radical of gaseous vinyl alcohol. 129 M. B. Huang 0.Goskinski G. Jonsall and P. Ahlberg J. Chem. Soc. Perkin Trans. 2 1983 305. 130 G. Frenking and H. Schwartz 2. Naturforsch. Teil B 1982 37 1602. 131 G. Frenking and H. Schwartz Znt. J. Mass Spectrom. Ion Phys. 1983 52 131. 132 M. N. Paddon-Row N. G. Rondan and K. N. Houk J. Am. Chem. Soc. 1982 104 7162. 133 P. Saxe and H. F. Schaefer 111 J. Am Chem. SOC. 1983 105 1760. 134 G. Leroy J. Mol. Struct.1983 93 175. 135 L. Farnell J. A. Pople and L. Radom J. Phys. Chem. 1983 87 79. 136 B. N. McMaster J. Mrozek and V. H. Smith jun. Chem. Phys. 1982 73 131. 137 S. Havnliak and H. F. King J. Am. Chem. SOC.,1983 105 4. 138 T. Fueno V. BonaEiE-Kouteckf and J. KouteckL J. Am. Chem. SOC. 1983 105 5547. 139 D. Feller E. S. Huyser W. T. Borden and E. R. Davidson J. Am. Chem. SOC.,1983 105 1459. 140 G. D. Bent G. F. Adams R. H. Bartram G. D. Purvis and R. J. Bartlett J. Chem. Phys. 1982,76,4144. 141 S. Saebo L. Radom and H. F. Schaefer 111 J. Chem. Phys. 1983 78 845. 142 Y. Apeloig M. Karni B. Gommer G. Depke G. Frenking S. Meyn J. Schmidt and H. Schwarz J. Chem. SOC. Chem. Commun. 1983 1497. Theoretical Chemistry (13) (14) (15) Calculations on five different states'43 of CH2N have been reported and a study on CH2CN.144 The frequencies and i.r.spectrum intensities have been calculated for C2H5,145 and a vibrational analysis performed from an MC SCF wave function for all~l.'~~ Correlation effects in several small radicals including C2H5 have been studied by Nakatsuji et ~21'~' Two important papers by Schlegel and co-workers have dealt with the reactions H + C2H4 S C2H5 (5) F +CZH4 + C2H4F + H + C2H3F (6) using fully optimized structures at HF/3-21G HF/6-31G* and MP2/3-21G levels. The geometries are relatively insensitive to basis set variations unlike the energies and the transition-state structures have geometries which are reactant-like (13). Energy differences computed at the MP4 and including zero-point energy corrections were compared with experimental data where available for instance AH for reaction (6) was computed to be -61 *9 kJ mol-'.The structures of a variety of radicals derived from addition of F to substituted ethylenes have been studied at the SCF level with a 3-21G basis set,'50 and the radicals formed in the reaction of OH with pyridine and pyridinium ion with a minimal STO-3G basis.'" Several P-substituted cyclopropyl radicals have been studied by Clark el A variety of radical ions have been the subject of calculations. Hyperfine coupling constants inversion barriers and geometries have been computed'53 for CF3 NFf and BFJ. There is a continuing controversy over the structure of the radical cation derived from C2H,.Calculations by Merry and Thomson predict C2HZ to be planar at the 6-31G*'54 SCF level of theory a conclusion confirmed by Belville and Ba~ld,'~~ but an extensive CI cal~ulation'~~ predicted a twisted structure (by -23") in agreement with MNDO results155 and the analysis of the photoelectron ~pectra.'~' High quality calculations are needed on this species. I43 G. F. Adarns D. R. Yarkony R. J. Bartlett and G. D. Purvis Inr. J. Quantum Chem. 1983 22 437. I44 F. Delbecq Chem. Phys. Letr. 1983 99 21. '45 J. Pacansky and B. Schrader J. Chem. Phys. 1983 78 1033. I46 T. Takada and M. Dupnis J. Am. Chem. Soc. 1983 105 1713. I47 H. Nakatsuji K. Ohta and T. Yonezawa J. Phys. Chem. 1983 87 3008. I48 H. B. Schlegel J.Phys. Chem. 1982 86 4878. 149 H. B. Schlegel K. C. Bhalla and W. L. Hase J. Phys. Chem. 1982 86 4883. Is" F. Delbecq and J. M. Lefour Terruhedron Lett. 1983 24 3613. 151 M. C. Anthony W. L. Waltz and P. G. Mezey Can. J. Chem. 1982 60,813. 152 T. Clark A. J. Kos P. von R. Schleyer W. P. Cofino W. H. de Wolf and F. Bickelhaupt J. Chem. SOC.,Chem. Commun. 1983 685. IS3 M. A. Benzel A. M. Maurice R. L. Belford and C. E. Dykstra J. Am. Chem. SOC.,1983 105 3802. I54 S. Merry and C. Thomson Chem. Phys. Left. 1981 82 373. 155 D. J. Belville and N. L. Bauld J. Am. Chem. SOC.,1982 104 294. 15h R. J. Bueuker S. D. Peyerirnhoff and H. L. Hsu Chem. Phys. Lett. 1971 11 65. I57 H. Koppel W. Domke L. S. Cederbaurn and W. von Niessen J. Chem. Phys.1978 69 4252. 40 C. Thornson Calculations have been reported on several substituted ethylene cation radi~a1s.l~~ The cation radical of ethane C2H6+ contains an elongated one-electron bond ( 1.98 8 at 6-31G** Level) in this a-radi~a1.I~~ The structures of the cyclic cation radicals derived from cyclopropane,16' cyclopen- tane,I6l and cyclobutane'62 have been reported. In the case of the cyclopentane cation a non-planar C symmetry is found whereas for C4Hi the lowest energy structure is rhomboidal with a rectangular structure slightly higher in energy. Ab initio spin densities'63 have been calculated for a large number of mono- and bi-cyclic azine radical anions using both MBS and DZ basis sets. Reasonable agreement was found with experimental ratios of hyperfine coupling constants.Finally there have been two studies of Jahn-Teller distortions in C6Hi,164'165 and in C6Fl,164 in one of these papers including 7r-electron correlation. Molecules Containing Other Than First-row Atoms.-Applications to second-row molecules with ever increasing accuracy continue as a result of improvements in methodology and new programmes. Only a few of the many papers are cited in this review. There is much interest in silicon-containing compounds. Spectroscopic observa- tions on various fluorosilylenes have prompted a careful study of the three lowest lying states of SiH, SiHF and SiF2.166 Earlier theoretical work has appeared on SiH2 and SiF2I6' but it is now known that a two configuration treatment must be used for the lowest singlet state of carbenes.This DZ + P study included the three low lying states and agreement with experiment was good. Vibrational frequencies were also calculated many of these not having been observed to date. A model reaction for the insertion of singlet carbenes into alkanes is the reaction SiH + H + SiH4 (7) Following earlier work by Gordon,'68 the reaction has been re-investigated with a two configuration description of SiH2 and a DZ + P basis The barrier was 20 kJ mol-' lower than the single configuration result. The barrier was further lowered to -28 kJ mol-' with CISD which is in good agreement with experiment. The transition state is depicted in (14). has studied a variety of abstraction reactions XH, +H,+ XH,,+ +H (X =C,N,Si,orP) "'J.Kalcher and G. Olbrich THEOCHEM 1983 104 489. D. J. Bellville and N. L. Bauld J. Am. Chem. Soc. 1982 104 5700. D. D. M. Wayner R. J. Boyd and D. R. Arnold Can. J. Chem. 1983 61 2310. I6l H. B. Huang S. Lunnell and A. Lund Chem. fhys. Lett. 1983 99 201. 16* J. W. Bouma D. Pottinger and L. Radom THEOCHEM 1983 103 209. M. H. Palmer and I. Simpson Z. Nuturforsch. Teil A 1983 38 415. 164 K. Raghavachari R. C. Haddon T. A. Miller and V. E. Bondybey J. Chem. Phys. 1983 79,1387. I65 H. Kato K. Hirao and M. Sano THEOCHEM 1983 104 489. I hh M. E. Colvin R. S. Grev H. F. Schaefer 111 and J. Bicerano Chem flip Leff 1983. 99. 390 I67 C. Thomson Theor. Chim. Actu 1973 32 93. I68 M. S. Gordon J. Chem. SOC.,Chem. Commun. 1981 890.I69 R. S. Grev and H. F. Schaefer 111 J. Chem. SOC.,Chem. Commun. 1983 785. I70 M. S. Gordon D. R. Gano and J. A. Boatz J. Am. Chem. Soc. 1983 105 5771. Theoretical Chemistry 41 using a variety of methods including electron correlation. Abstraction by CH is found to proceed more easily than by silyl. The POL-CI'71 method seems to be particularly good for this type of reaction. Further calculations on the Si2H2 system have appeared. Lishka and KOh1e1-l~~ have shown that large scale calculations including correlation and d-functions are essential to get the geometry correct. In particular the twisted structure does not correspond to a local minimum as reported earlier the lowest energy singlet being a non-planar bridged structure (15).For the triplet planar H2SiSi is a global minimum. The addition of HCl to ~ilaethene'~~ differs significantly from the analogous reaction involving C2H4. Gradient calculations with a 3-2 1G basis found the transi- tion state (16) and an intermediate complex which forms early in the reaction. The transition state is two-centre-like whereas the analogous state for the C2H4 reaction is four-centre-like. A careful study (with geometry optimization at the 3-21G level) of the nine possible isomers of CH2N2' and SiH2N2 has been reported by Thomson and G1ide~ell.I~~ All the silicon-containing molecules are unstable with respect to decomposition to SiH2 + N2 and the ab initio results are in disagreement with MNDO predictions for the SiH2N2 isomers.The most stable isomer of CH2N2 is cyanamide but it seems for the Si-containing compounds that the relative energies are incorrect for the 3-2 1G basis sets and one should use the 6-3 1G* basis for these molecules. Correlation corrections do not alter the order of the stabilities but in view of the results on Si2H2 the geometric parameters probably need further investigation with larger basis sets. Hopkinson et al. have studied the singlet and triplet energy surfaces for CSiH2 and the transition states.'75 A DZ basis was used and single point DZ + P or DZ + CI calculations. A similar study of the singlet surface but with more extensive CI was carried out by Hoffmann et ~l.,'~~ who also calculated the harmonic frequencies. Finally Gordon and co-~orkers'~~ have examined the question of the aromaticity of silacyclopentadienyl and silacyclopropenyl cations using an STO-2G basis set.These molecules are not significantly stabilized by delocalization unlike ~i1abenzene.l~~ Ill I72 I73 I75 I76 I77 I78 P. J. Hay and T. H. Dunning jun. J. Chem. Phvs. 1976 64 5077. H. Lishka and H.-J. Kohler J. Am. Chem. Soc. 1983 105 6646. S. Nagase and T. Kudo J. Chem. SOC. Chem. Commun. 1983 363. C. Thomson and C. Glidewell J. Comput. Chem. 1983 4 1. A. C. Hopkinson M. H. Lieu and I. G. Csizmadia Chem. Phys. Left. 1983 95 232. M. R. Hoffmann Y. Yoshioka and H. F. Schaefer 111 J. Am. Chem. Soc. 1983 105 1084. M. S. Gordon P. Boudjouk and F. Anwari J. Am. Chem. Soc. 1983 105 4972. H. B. Schlegel B.Coleman and M. Jones J. Am. Chem. Soc. 1978 100 649. 42 C. Thornson Calculations on similar germanium-containing compounds are now appearing more frequently such as H2Ge=GeH2,'793180 H,Ge=NH H,Ge=O and related molecules.'79 Dixon et ~1.'~' have determined the structure of the ylides and the energetics of the reaction (8) CH,XH + CHYXH;, for X = P N S or 0,using the GVB-POL-CI method. Another paper by the same author'82 has dealt with the proton affinities of RNH2 ROH ROH, and RSH for R = H CH3 SiH3 using a DZ + P basis for the heavy atoms and including zero-point vibrational effects. A very detailed study has appeared of the phosphonium cyclopropylide model H3P=C(CH2)2.'83 The pyramidal carbanion centre of the ground state geometry is shown ( 17).Geometries and energy barriers were given in detail and were consistent with experimental work on more complex systems. Koopman's theorem ionization potentials for a series of hydrides methyls and silyls H,X (CH3),X (SiH3),X; X = F C1 n = 1; X = 0 S n = 2; X = N P n = 3 have been reported.IS4 Excellent agreement with experimental values is found without including d-functions and this conclusion has been verified using several different basis sets. Clark'" has continued his investigations of three-electron bonds with studies of the radical cations H3PPH; H,PSH,' H3PClH+ and HCICIHf. The optimized geometry with S and P ligands tends to be increasingly trigonal bipyramidal. Turning now to sulphur containing molecules it has been observed by ScheinerIS6 that most basis sets correctly predict that NH; has a higher proton affinity than OH2 but it is necessary to go to a 4-31G" basis before the correct ordering of OH and SH2 is obtained.In contrast to the results on the phosphines noted above complete geometry optimization results show that 3d orbitals on S are essential to describe properly the bonding in d-thiocarbanions -CH2SH and -CH2SCH3.'87 Previous work used values which were too long for the C-S bond which are obtained if 3d functions are not included in the basis set. Minimal basis sets (STO-3G) and the larger STO-3G* have been used to optimize the geometries of several sulphur di-imides,Is8 following single point MP3//6-3 1G* calculations. Several isomers and isomerization pathways were studied.A similar study has appeared of the singlet states of six isomeric forms of C2H2S by two different group^,'^^,'^^ the paper by Siegbahn et ~1.'~' using the largest basis I79 G. Trinquier J.-C. Barthelat and J. Satge J. Am. Chem. SOC.,1982 104 5931. I80 S. Nagase and T. Kudo THEOCHEM 1983 103 35. 181 D. A. Dixon T. H. Dunning jun. R. E. Eades and P. G. Gassman J. Am. Chem. Soc. 1983 105 701 1. I82 M. L. Hendewerk R. Frey and D. A. Dixon J. Phys. Chem. 1983.87 2026. I83 M. A. Vincent H. F. Schaefer 111 A. Schier and H. Shmidbaur J. Am. Chem. SOC.,1983 105 3806. 184 C. Glidewell and C. Thornson J. Compur. Chem. 1983 4 9. I85 T. Clark J. Comput. Chem. 1983 4 404. 186 S. Scheiner Chem. Phys. Lerr. 1982 93 540.187 S. Wolfe L. A. LaJohn F. Bernardi A. Mangini and G. Tonachini Tetrahedron Lett. 1983 24 3789 407 1. 188 K. Raghavachari and R. C. Haddon J. Phys. Chem. 1983 87 1308. I89 R. K. Gosavi and 0. P. Strausz Can. J. Chem. 1983 61 2596. I90 P. Siegbahn M. Yoshimine and J. Pacansky J. Chem. Phys. 1983 78 1384. 7%eoretical Chemistry 43 sets. Both singlet and triplet states were studied and the two sets of calculations agree in their main conclusions. Contrary to earlier work the lowest singlet isomer is thioketene CH2=C=S with HCrCSH 30 +70 kJ mol-' higher in energy. Thiirene exists as a minimum on the singlet surface but lies some 160 kJ mol-' above the minimum. Mezey et ~1.'~' have reported the results of a study of the minimum energy reaction paths for two thioketone-enethiol equilibria using an STO-3G basis set.Finally we mention a study of the isomers of C2H4Br+,192 a series of calculations on conformers of 2-fluoro- and 2-~hloro-ethanols,'~~ and three papers which attempt a clarification of some important organic reactions. In the first the energetics of the reactions of chromyl chloride and molybdyl chloride with alkanes alcohols and alkenes has been studied with GVB wave function^,'^^ and in the second a model for the insertion of CO into the Pt"-CH3 bond has been ~tudied.''~ The increasing power of ab initio methods is well illustrated by these two papers. The last paper'96 deals with the first stages of the Stephen Gattermann and Houben- Hoesch reactions and involved determination of the reaction pathway and a large portion of the energy hypersurface for HCl + RCN + RCIC=NH R = N,CH3 (9) Structural Investigations.-The use of ab initio methods for the investigation of the equilibrium configuration of polyatomic molecules continues to grow and only a few selected studies are mentioned.Pulay and co-workers have systematically 19' investigated the effects of basis set and electron correlation on the computation of force constants and separated out the most important effects in studies on HF HCN and NH3. A subsequent careful study of the force fields of glyoxal acrolein butadiene formaldehyde and ethylene was reported19* to reproduce very well the frequencies using a 4-21G basis set. There have been several calculations on various cyclopropanes.Dupuis et have computed the vibrational spectra of cyclopropane itself and the cyclopropyl radical. There is no significant hyperconjugative interaction between the radical centre and the P-C-H bonds in the latter. A similar study of cyclopropane ethylene oxide and ethylene imine has appeared.*" Skancke and co-workers2" have investigated the strain energies in gem-difluorocyclopropanes and rotational isomers of cyclopropanecarboxaldehyde202 and vinylcyclopropane203 have been investigated. 191 A. E. Bruno R. P. Steer and P. G. Mezey J. Comput. Chem. 1983 4 104. 192 R. A. Poirier G. Demare K. Yates and 1. G. Csizmadia THEOCHEM 1983 94 137. 193 J. Murto M. Rasanen A. Aspiala and L. Homanen THEOCHEM 1983 92 45.194 A. K. Rappe and W. A. Goddard 111 J. Am. Chem. SOC.,1982 104 3287. 195 S. Sasaki K. Kitaura K. Morukuma and K. Ohkubo J. Am. Chem. Soc. 1983 105 2280. 196 G. Alagona and J. Tomasi THEOCHEM 1983,91 263. I97 P. Pulay J.-G. Lee and J. E. Boggs J. Chem. Phys. 1983 79 3382. 198 P. Pulay G. Fogarasi G. Pongor J. E. Boggs and A. Vargha J. Am. Chem. SOC.,1983 105 7037. I99 M. Dupuis and J. Pacansky J. Chem. Phys. 1982 76 251 1. 200 A. Komornicki F. Pauzat and Y. Ellinger J. Phys. Chem. 1983 87 3847. 201 A. Greenberg J. F. Liebrnan W. R. Dolbier jun. K. S. Medinger and A. Skancke Tetrahedron 1983 39 1533. 202 G. R. de Mare and M. R. Peterson THEOCHEM 1983 104 115. 203 G. R. de Mare and M. R. Peterson THEOCHEM 1982 89 213.44 C. Thornson Cyclobutane derivatives204 have been studied as have cyclopentane cyclopentene and cy~lopentadiene.~~~ Calculations on homocyclopropenylium cation206 show that highly sophisticated calculations (MP4/6-3 1G") are required to provide accurate information on this type of homoaromatic species. A similar example is provided by the 2-norbornyl cation (see earlier refs. 127 and 128). An important paper by Schleyer and K0s207has addressed the question of negative (anionic) hyperconjugation uia calculations on several fluorosubstituted molecules such as FCH,CH, F2BCH, and F2AlCH,. The stabilization energies are found to be large and it is concluded that the effect is definitely established. Hexahalogenoben~enes~~~*~~~ have been studied at DZ level and a variety of properties evaluated with different basis sets for C6H6 and C6F6.All the compounds are found to be planar at the SCF level. The C6H;' isomers2" have been studied with geometry optimization and their fragmentation to C5H3f and CH,' investigated. A full optimization study of the planar hydrogen2" maleate anion at the SCF level predicts a slightly asymmetric structure. Finally borepine (18)2'2 and its valence isomers have been studied in more accurate calculations. Molecular Interactions.-We mention a few of the many papers on this topic in this section. The important role of the solvent and H20 in particular on reactions continues to be an active area of research. For instance the influence of the solvent on the reaction ( 10) H2C0 + LiBH4 -* products (10) has been investigated using a modified SCF formalism with quite large effects on the reaction energy profile although the mechanistic conclusions are not altered appre~iably.~'~.~'~ The hydration of formaldehyde2I5 has been studied utilizing Ko10s'216suggestion to eliminate the basis set superposition error.The enthalpies of a large number of gas-phase hydration reactions X + H,O -* H,OX (X = H+ Li+ H20) (1 1) have been computed using MP3 level of theory with excellent agreement with e~periment.~" 204 T. Jonvik and J. E. Boggs THEOCHEM 1983 105 201. '05 S. Saeb~i F. R. Cordell and J. E. Boggs THEOCHEM 1983 104 221. 206 R. C. Haddon and K. Raghavachari J. Am. Chem. Soc. 1983 105 118. '07 P.von R. Schleyer and A. J. Kos Tetrahedron 1983 39 1141. 208 J. Almlof and K. Faegri jun. J. Chem. Phys. 1983 79,2284. 209 J. Almlof and K. Faegri jun. J. Am. Chem. Soc. 1983 105 2965. 210 K. Lammertsma and P. von R. Schleyer J. Am. Chem. Soc. 1983 105 1049. ZI I P. George C. W. Bock and M. Trachtman 1.Phys. Chem. 1983 87,1839. 212 R. L.Disch M. L. Sabio and J. M. Schulman Tetrahedron Lett. 1983 24 1863. 213 R. Bonnacorsi P. Pala and J. Tomasi THEOCHEM 1982 87,181. 214 R. Bonnacorsi P. Cimiraglia J. Tomasi and S. Mierths THEOCHEM 1983 94 1 I. 215 G. M. Maggiora and I. H. Williams THEOCHEM 1982 88 23. 216 W. Kolos Theor. Chim. Acta 1979 51 219. 217 J. del Bene H. D. Mettee M. J. Frisch B. T. Luke and J. A. Pople J. Phys. Chem.1983 87,3279. Theoretical Chemistry Scheiner has continued to investigate proton-transfer including effects of electron correlation and the structure of H302 has been examined221 in detail. The hydration energies for NH; and H30+ have also been studied at the SCF as well as the hydration of ketenimine with H20 and (H20)2.224 Sapse has reported studies of various complexes with HF,225and Scheiner226 has investigated the role of d-functions when the other molecule is H2S. Various authors have proposed other techniques for calculating intermolecular interaction^.^^'-^^^ Numerous papers on hydrogen bonding have appeared for example a series by Hin~hliffe,~~' and an application of a new AESCFdecomposition scheme23' to 12 hydrogen-bonded dimers has given very good results but has shown that STO-3G basis sets are very poor for this problem.There have been several calculations on dimers such as (NO)2,232(H20)2,233,234 (CO)2,235 and (CS)2.235In the latter study bound triplet ground states were predicted. Finally it is clear that with the advent of more powerful computers and improved programs for ab initio calculations these methods will have an even greater impact on chemistry in the future. Acknowledgement. The author wishes once again to thank Mrs. Maureen Thomson for her invaluable help with the literature search. 218 S. Scheiner Int. J. Quantum Chem. 1983 23 739. 219 S. Scheiner Int. J. Quantum Chem. 1983 23 739. 220 S. Scheiner Int. J. Quantum Chem. 1983 23 753. 221 C.M. Rohling L. C. Allen C. M. Cook and H. B. Schlegel J. Chem. Phys. 1983 78 2498. 222 S. Ikuta Cfiem.Phys. Lett. 1983 95 604. 223 S. Ikuta Mass Spectroscopy 1982 30,297. 224 M. T. Nguyen and A. F. Hegarty J. Am. Chem. Soc. 1983 105 381 1. 225 A. M. Sapse J. Chem. Phys. 1983 78 5733 5738. 226 S. Steiner J. Chem. Phys. 1983 78 599. 227 J. Hoinkis A. Ahlrichs and H.-J. Bohm Int. J. Quuntum Chem. 1983 23 821. 228 J. B. Peel Int. J. Quantum Chem. 1983 23 653. 229 0. Navaro Int. J. Quantum Chem. 1983 23 1611. 230 A. Hinchliffe THEOCHEM 1983 105 335 and references therein. 23 I W. A. Sokalski P. C. Hariharan and J. J. Kaufman J. Phys. Chem. 1983 87 2803. 232 R. D. Bardo J. Phys. Chem. 1982 86 4658. 233 M. D. Newton and N. R. Kestner Chem.Phys. Lett. 1983,94 198. 234 L. A. Curtiss Chem. Phys. Lett. 1983 96 442.

ISSN:0069-3030    

DOI:10.1039/OC9838000029

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (i) Pericyclic Reactions By R. S. ATKINSON Department of Chemistry University of Leicester Leicester LEI 7RH 1 Electrocyclic Reactions A helical transition state (I) is suggested for the 877 -+ 677 conversion of diazo- alkadiene (2) into the diazepine (3).' This geometry (reminiscent of the [1,7]sig-matropic rearrangement of hydrogen) it is claimed requires less distortion of the diazoalkane and accounts for the alternative pathway followed (67r -B 47r cycliza-tion) when R2 = Me. (65-9 3 % ) More compelling evidence for helicity involvement in an electrocyclic reaction comes from thermolysis of (E)-and (2)-2-butenedioates (4)and (5) respectively.* In each case two substituted pyrrolizine stereoisomers are formed by [1,6] sigmatropic rearrangement of deuterium followed by 677 -B 477 ring-closure of the 1,5-dipole.There is stereospecificity in the formation of (6)-(9) which is revealed by the different chemical shifts of the CHDC02Me protons as indicated and the logical conclusion is that there is no equilibration of the clockwise and anti-clockwise helices from which the [1,6] sigmatropic rearrangement has taken place. The authors assume that electrocyclic ring-closure from these helices is invariably disrotatory and to account for the formation of (6) and (8) postulate that stereomutation within the 1,5-dipole [ fi in (lo)] takes place without interconversion of the helices. This seems remarkable and these results could perhaps be better accommodated by assuming competitive disrotatory ring-closure and direct carbanion-iminium cation combination within the I $dipole (but still without interconversion of the two helices involved).' I. R. Robertson and J. T. Sharp J. Chem. SOC.,Chem. Commun. 1983 1003. ' D. N. Reinhoudt G. W. Visser W. Verboom P. H. Benders and M. L. M. Pennings J. Am. Chem. Soc. 1983. 105. 4775. 47 R. S. Atkinson E D D (4) (5) E = C0,Me 1) b 6 CHDE (4) R'= E R2 = H + (6) 6 2.77 + (7) 6 3.12 (5) R'= H R2= E (8) 6 3.99 + (9) 6 2.97 Comparison of electrocyclization rates for the donor-acceptor substituted trienes (1 1) with that of 1,2-divinylcyclohexene shows that the donor-acceptor properties of the substituents have very little influence on the rate of cyclization.' R COzEt (1 1 ) R = H Ph or p-MeOC,H4 2 Cycloaddition Reactions Without doubt this is the age of the intramolecular cycloaddition and the intramolecular [47r + 27~1 in particular.Ingenious and delightful syntheses of com- plex natural products or models for natural product synthesis using intramolecular Diels-Alder and intramolecular 1,3-dipolar cycloaddition reactions continue to appear in profusion. There has been little evidence of a sktckening of interest in the inter-molecular Diels-Alder reaction in the past year either. The ideal diene (or set of dienes) for the Diels-Alder should of course possess the following attributes (a) be regio- specific (in either sense as required) (b)be endo (or exo?) stereospecific (c) be perispecifically reactive as the 47r component under the mildest stimulation with least reactive dienophiles (d)offer versatility for functional group interconversion in the cycloadduct (e)yield only one diastereoisomer when reacted (in chiral form) with a prochiral dienophile.The ideal matching set of dienophiles would have E. N. Marvell C. Hilton and M. Cleary J. Org. Chem. 1983 48 4272. Reaction Mechanisms -Part (i) Pericyclic Reactions identical attributes mutatis mutandis. A set of dienes/dienophiles is required not least to satisfy the inherently different requirements of the normal and inverse Diels-Alder reactions. Much of present work in this area is in practice an examination of regioselectivity effects [(a) above] resulting from changes in diene/dienophile catalysis or reaction conditions.Although prediction of regioselectivity has been available for some years using frontier molecular orbital (FMO) theory it has also been recognized that neglect of secondary orbital interactions could lead to erroneous conclusions. For certain Diels-Alder reactions notably (12) with methyl vinyl ketone it has been shown that regioselectivity is dominated by secondary orbital interaction^.^ The corollary which follows from the above that there should be a direct link between regioselectivity and endo-stereoselectivity [(a)and (b)above] has also been recognized for some time but not explored. Dienes (13) are ambident reacting both Me0 8-20 kbar PhSfi CO,R1 - OR2 rl-" '"* with dienophiles substituted with electron-accepting (A) and electron-withdrawing (D) groups.It has been found that application of high pressure brings about reaction at room temperature and that the (thermally unstable) adducts are formed with high regioselectivity and endo-~electivity.~ NHC0,R (14) X = SPh (15) X = SOPh (16) X = S0,Ph x The acylamino group in the new (stable and crystalline) dienes (14)-( 16) has been found to exercise powerful regio-control in reactions with a#-unsaturated carbonyl compounds dominating the effects of a sulphur substituent at position 4 whether this be sulphenyl sulphinyl or sulphonyl endo-stereospecificity is also high.6 It is claimed that this dominance by the acylamino group is not rationalized by conven- tional FMO considerations.The cycloadducts from (14)-( 16) and e.g. acrolein score well on (d) above -versatility in functional-group interconversion in the cyclohexene adduct. P. V. Alston M. D. Gordon R. M. Ottenbrite and T. Cohen J. Org. Chem. 1983 48 5051. G. Revial M. Blanchard and J. d'Angelo Tetrahedron Lett. 1983 24 899. ' L. E. Overman C. B. Petty. T. Ban. and G. T. Huang J. Am. Chem. Soc. 1983. 105. 6335 R. S. Atkinson In contrast the regioselectivity of addition of (17) to glyoxylate esters can be reversed by the apparently slight modification as in (1 8). This reversal of regioselec-tivity has been exploited in a synthesis of uranicine (19).7 o-Quinodimethanes will always be popular because of their reactivity [(c) above] and the route for their generation as in (20) has been extended to obtain (21)8 and (22).' F -3 SiMe R' I SiMe CsF -SiMe Additional dienes which have been prepared with a view to their use in Diels-Alder reactions include (23)-(26).10-13 (PhSe) (23) Hal = C1 or Br R = H or C1 ' R.R. Schmidt and A. Wagner Tetrahedron Lett. 1983 24 4661. ' Y. Ito Y. Amino M. Nakatsuka and T. Saegusa J. Am. Chem. Soc. 1983 105 1586. ' Y. Ito E. Nakajo M. Nakatsuka and T. Saegusa Tetrahedron Left. 1983 24 2881. lo A. J. Bridges and J. W. Fischer Tetrahedron Lett. 1983 24 445. " T. Minani H. Sako T. Ikehira T. Hanamoto and I. Hirao J. Org. Chem. 1983 48 2569. '' A. V. Rama Rao G. Venkatswamy S. M. Javeed V. H. Deshpande and B. Ramamohan Rao J. Org. Chem. 1983 48 1552.E. Negishi and F.-T. Luo J. Org. Chem.. 1983 48 1560. Reaction Mechanisms -Part ( i) Pericyclic Reactions R' Z ,c=c, \/ + C5H11y Pd(PPh,) H M X Z Z = OEt SEt or SiMe R' M = ZnC1 AIR, or BR,Li (26) X = Br or I Regiospecific preparation of 3-or 4-substituted 2,6-dimethylbenzoates is accom- plished by reaction of the pyrone (27) with morpholinoenamines. By using the appropriate enamine either of the isomeric benzoates may be obtained as a single pr~duct.'~ 0 CO,R (27) (50%) "'0"' Ph One of the sternest challenges in (C=C) dienophile design is to find surrogates for alkyl-substituted alkenes (which are notoriously unreactive in the intermolecular Diels-Alder). Synthons for ethylene or 1-alkenes and acetylene or monosubstituted alkynes are (28) and (29) respecti~ely.'~ Phenyl vinyl sulphone is a moderately reactive dienophile is particularly effective in regio-control and after [4~ + 2~1 cycloaddition is easily reductively removed (29) is an alkyne synthon since treatment of its adducts with 4~ systems with fluoride ion restores a double bond with elimination of trimethylsilicon and sulphonyl groups.Me,Si -SO,Ph -SO,Ph (28) (29) Introduction of the sulphonyl group likewise increases the dienophilicity of other alkenes and this is accomplished using phenylseknenyl benzene sulphonate (Scheme l)." MeoJ+ SO,R Reagents i PhSeSO,Ph hv; ii H,OI; iii OSiMe iv H,Of; v Zn-HOAc Scheme 1 j4 H. L. Gingrich D. M. Roush and W. A. Van Saun J. Org.Chem. 1983 48 4869. l5 R. V. C. Cam R. V. Williams and L. A. Paquette J. Org. Chem. 1983 48 4976. '' W. A. Kinney G. D. Crouse and L. A. Paquette J. Org. Chem. 1983 48 4986. R. S. Atkinson The compound trans-1,2-disulphonyletheneis also reactive in normal Diels-Alder reactions and an ethylene synthon." Surprisingly high dienophile reactivity is exhibited by Fischer carbene complexes (30) which react at rates 104-timesgreater than methyl acrylate. The potential of the metal complex residue iii the cycloadducts for conversion into other functional groups is considerable.'8 (70%) A number of new chiral dienophiles (31)-(33) have been introduced which with cyclopentadiene yield adducts with high enantiospecificity [(e)ab~ve].'~-~' 0-\ Bu' \ Ro (31) (32) K XJ,IY rrz or S0,Ph R = SO,NPr;orSO,Ph (33) Recent recognition of the carbonyl group as a useful dienophile under the influence of catalysis by Lewis acids has been exploited to good effect as the first step in a synthesis of amino-octose derivatives (iincosamine) (Scheme 2).23 I OMe Scheme 2 A remarkable observation in this heter~-Diels-Alder*~ reaction is that the role of the Lewis acid catalyst can be assumed by 'oxaphilic' shift reagents e.g.Eu(fod), " 0. De Lucci and G. Modena Tetrahedron Lett. 1983 24 1653. l8 W. D. Wulff and D. C. Yang J. Am. Chem. SOC.,1983 105 6726. l9 W. Oppolzer and C. Chapuis Tetrahedron Lett. 1983 24 4665. 20 W. Choy L. A. Reed and S. Masamune J. Org. Chem. 1983 48 1137; S.Masamune L. A. Reed J. T. Davis and W. Choy ibid. 1983 48 4441. 'I W. Oppolzer C. Chapuis and M. J. Kelly Helv. Chim. Acta 1983 66,2358. 22 See also D. Horton T. Machinami Y. Takagi C. W. Bergmann and G. C. Cristoph J. Chem. SOC. Chem. Commun. 1983 1164; P. A. T. W. Porskamp R. C. Haltiwanger and B. Zwanenburg Tetrahedron Lett. 1983 24 2035. 23 E. R. Larson and S. Danishefsky J. Am. Chem. SOC. 1983 105 6715. 24 For a review of the synthetic aspects of Diels-Alder additions with heterodienophiles see S. Weinreb and R. R. Staib. Tetrahedron. 1982 38 3087. Reaction Mechanisms -Part (i) Pericyclic Reactions 53 and asymmetric induction in the product is brought about when the ligands on the shift reagent are chiral e.g. E~(hfc)~.~~ Thioaldehydes are unstable species which are nevertheless well-behaved dienophiles in normal Diels-Alder reactions.Their regioselectivity of reaction with electron-rich dienes for Z = alkyl (Scheme 3 path a) is reversed with Z = acyl or cyano (path b).26 R= SiMe,Bu' Ib Z Scheme 3 A number of new routes to ethoxycarbonyl-substituted thioaldehydes (thio-oxa- acetates) have been devised.27 The importance of pressure effects on the Diels-Alder reaction is increasingly receiving recognition. Furan and benzoquinone react to give endo-and em-products under high pressure,28 but both of the latter are unstable at room temperature and pressure which explains why these adducts have not previously been obtained (Scheme 4). Increased pressure affects not only rates of reaction but also endo exo ratios in addition of benzoquinones to f~rans.~~ ST,12h.I bar + -8 "C I bar ov Scheme 4 25 M. Bednarski and S. Danishefsky J. Am. Chem. SOC.,1983 105,6968; M. Bednarski and S. Danishefsky ibid. 1983 105 3716; M. Bednarski C. Maring and S. Danishefsky Tetrahedron Lett. 1983 24 3451. 26 E. Vedejs D. A. Perry K. N. Houk and N. G. Rondan J. Am. Chem. SOC.,1983 105 6999. 27 G. M. Bladon I. E. G. Ferguson G. W. Kirby A. W. Lochead and D. C. McDougall J. Chem. Soc. Chem. Commun. 1983 423; G. W. Kirby and A. W. Lochead ibid. 1983 1325. 28 J. Jurczak T. Kozluk S. Filipek and C. H. Eugster Helv. Chim. Acta 1983 66 222. 29 J. Jurczak T. Kozluk M. Tkacz and C. H. Eugster Helu. Chim. Acta 1983 66 218.54 R. S. Atkinson In the Lewis-acid catalysis of the reaction of quinones with alkyl-substituted butadienes TiCI (at -78 "C)brings about the same regioselectivity as in the thermal reaction (at 200 "C) but BF3 catalysis gives the opposite regioselectivity. It is suggested that a different site in the quinone may be complexed in each case.3o Further striking examples of the accelerated rates of some Diels-Alder reactions in water have a~peared.~' Considerable efforts have been made to try to understand facial stereoselectivity of dienes (34)-(37) which are grafted on to a [2.2.1] skeleton. The face which is preferentially attacked by dienophiles is not necessarily that predicted on the basis of steric effects and the selectivity has been ascribed to r-lobe tilting of the sub-HOMOS (subjacent) resulting from u/T coupling.32 Other factors are held to be responsible in the case of (38).33 ,Me pJ (34) (35) (37) A number of cation radicals of some dienes show exceptional reactivity as dienophile components in the Diels-Alder reaction (Scheme 5).Though still in a primitive state of development this concept is intere~ting.~~ OMe A I 71% endo:exo = 2:l OMe Scheme 5 Heating the ring-fused oxazines (39) in xylene generates the corresponding nitroso- olefins whose Diels-Alder diene and/or 1,3-dipolar character has also been 30 J. B. Hendrickson and V. Singh J. Chem. SOC.,Chem. Commun. 1983 837. 3' P. Grieco P. Garner and Z. He Tetrahedron Lett. 1983,24 1897 R. Breslow U. Maitra and D.Rideout ibid. 1983 1901; P. A. Grieco K. Yoshida and P. Garner J. Org. Chem. 1983 48 3137. 32 L. A. Paquette P. Charumilind M. C. Bohm R. Gleiter L. S. Bass and J. Clardy J. Am. Chem. Soc. 1983 105 3136; L. A. Paquette P. C. Hayes P. Charumilind M. C. Bohm R. Gleiter and J. F. Blount ibid. 1983 105,3148; L. A. Paquette A.-G. Shaefer and J. F. Blount ibid. 1983 105,3642; L. A. Paquette T. M. Kravetz M. C. Bohm and R. Gleiter J. Org. Chem. 1983 48 1250. 33 M. Avenati and P. Vogel Helv. Chim. Acta 1983 66 1279. 34 R.A. Pabon D. J. Bellville and N. L. Bauld J. Am. Chem. SOC.,1983 105 5158. 15 D. E. Davis and T. L. Gilchrist J. Chem. SOC. ferkin Trans. I 1983 1479 T. L. Gilchrist and T. G. Roberts ibid. 1983 1283 D. E. Davies T. L. Gilchrist and T.G. Roberts. ihid. 1983 1275. 55 Reaction Mechanisms -Part (i) Pericyclic Reactions (39) R = COMe CO,Et or Ph An elegant synthesis of 3,4,5,6-substituted cyclohexene derivatives (40) used the proximity of the anthracene nucleus to direct stereospecificity/ regiospecificity of attack on the benzoquinone-derived moiety. Accelerated retro-Diels-Alder reaction takes advantage of the 'oxido effect' which results in a sloughing off of the anthracene at room temperat~re.~~ ,? OR ' Turning to 1,3-dipolar cycloadditions ;the use of N-trimethylsilylmethyl iminium salts as precursors for azomethine ylides has been improved by using thiolactams (Scheme 6).37Variations on this original method include those in Schemes 7 and 8.38 S STf C0,Me ___ (66'/o ) Reagents i MeOTf; ii CsF; iii CH,=CHCO,Me Scheme 6 XR p"y i,ii Ph& + phbE Me X = SMe or OEt Me Me (E = C0,Me) Reagents i Me3SiCH20S02CF3;ii CsF ECHLCHE Scheme 7 A non-stabilized azomethine ylide is believed to be intermediate in the formation of pyrrolidine (41) from trimethylamine oxide,39 This reaction has the hallmarks of a concerted reaction and even normally sluggish 1,3-dipolarophiles are reactive.36 S. Knapp R. M. Ornaf and K. E. Rodriques J. Am. Chem. SOC.,1983 105 5494. 37 E. Vedejs and F. G. West J. Org. Chem. 1983 48 4773. 38 A. Padwa and Y. Y. Chen Tetrahedron Lett. 1983 24 3447; A. Padwa G. Haffmanns and A. Tomas; T. Livinghouse and R.Smith J. Chem. SOC. Chem. Commun. 1983 210; R. Smith and T.Livinghouse J. Org. Chem. 1983 48 1554; see also S. Chen J. W. Ullrich and P. S. Manano J. Am. Chem. SOC. 1983 105 6160. 39 R. Beugelrnans G. Negron and G. Roussi J. Chem. Soc. Chem. Commun. 1983 31. R. S. Atkinson Scheme 8 Me\+/ Me + 7 LiNfii, N OR /\ R R = n-C,H, 63% -0 Me N Me (41) cis-Cyanohydroxylation or cis-carboxyalkylation of alkenes is accomplished by cleavage of their isoxazoline-nitrile oxide adducts (42) and (43) re~pectively.~' 0-)+lilN+ -___* NaOH I -R R 5"yo" C I CN CN CN CO H (42)R = Ph 77% dipole dipolarophile 1 :3 0-I 0+ Ill -O~OTHP bHooco2H N+ C I CH20THP (43) Replacement of the sulphonyl group in isoxazolines (44) by organolithiums makes these more versatile 1,3-hydroxycarbonyI ~ynthons.~' 0-R' R' R3 R' (44) R3 = alkyl aryl alkenyl 0-alkyl or CN Nitrilimine (45) is the first monosubstituted member of this class to be intercepted by dipolarophile~.~~ 40 A.P. Kozikowski and M. Adarnnyk J. Org. Chem. 1983 48 366. 41 P. A. Wade H.-K. Yen S. A. Hardinger M. K. Pillay N. V. Amin P. D. Vail and S. D. Morrow J. Org. Chem. 1983 48 1796; see also A. P. Kozikowski B. B. Mugrage B. C. Wang and Z. Xu Tetrahedron Lett. 1983 24 3705; D. P. Curran J. Am. Chem. SOC.,1983 105 5826. 42 R. Huisgen W. Fliege and W. Kolbeck Chem. Ber. 1983 116 3027. Reaction Mechanisms -Part ( i) Pericyclic Reactions 57 H 0,NCH =N-NPh A [HC-A-NPh] / Na' Normal nitrone regioselectivity is believed to be LUMO (nitrone) controlled until the dipolarophile becomes very electron-deficient or the nitrone becomes very electron-rich.Dicyclopropyl- N-methylnitrone (46) has a lower ionization potential (higher HOMO) due to the electron-donating ability of the cyclopropyl rings and reaction with electron deficient dipolarophiles results in inversion of normal regioselectivity since the addition is now HOMO (nitrone) controlled.43 Cycloaddition of the nitrone (47) to ethyl vinyl ether gives the isoxazolidine (48) as a single diastereoisomer which has been converted in one step into the methyl glycoside of daunosamine (49).44 Me HO NH2 MFoH (47) 93% (48) The facile generation of thiobenzophenone S-methylide (50) by extrusion of N2 from the thiadiazoline (51) even at -45 "C has allowed a study of the character of this 1,3-dipole in its reactions with various C=S C=C C=C and N=N bonds.The relative rate constants for cycloaddition (obtained by using competition experi- ments) have been shown to vary over a range of 10' and the high selectivity of this dipole has been rationalized using Sustmann's cla~sification.~~ 43 A. Z. Bimanand and K. N. Houk Tetrahedron Lett. 1983 24 435. 44 P. Deshong and J. M. Leginus J. Am. Chem. Soc. 1983 105 1686. 45 L. Xingya and R. Huisgen Tetrahedron Lett. 1983. 24 4181. 4185. R. s. Atkinson Ph +N, p2 N (51) (50) The hitherto unknown 1,2,3-triazolidine ring-system has been prepared by CYC~O-addition of azimine (52) to enamines. X-Ray data on (53) show that the three nitrogen atoms are Y .\yo P 0 Oy00 0 >-OAN ?'-\ N /=( d e+o y-7-?' (52) 0 (53) The reactivity of tetrafl~oroalkene,~' and ~inylsilanes~~ cy~loalkenynes,~~ towards various I ,3-dipoles has been probed.3 Other Cycloadditions Attempted intramolecular [67~+ 477-1cycloaddition of the dienyl-fulvene (54) at 210 "C for 48 h yielded a gross mixture from competing [47~+ 277-1 and [677 + 4r] cycloadditions. In accordance with theoretical predictions the greater reactivity of the diethylaminodienyl-fulvene(55) resulted in [6~ + 4r]periselectivity (at 40 "C for 12 h) and the overall transformation after dehydrogenation is an azulene fused to a third ring." K -NEt S b __* 40 "(' R (54) R = H (55) R = NEt Intramolecular [8 T + 2 7~1 cycloaddition of the heptafulvene with the 'tetraenophilic' double bond in (56) results in (57)as the major product calculations support (56) as the preferred transition-state ge~metry.~' Cyclopentyne generated by base treatment of dibromomethylenecyclobutane (58) has been found to react stereospecifically with cis-and trans-but-2-ene and on this 46 N.Egger R. Prewo J. H. Bieri L. Hoesch and A. S. Dreiding Helu. Chim. Acta 1983 66 1608. 47 G. B. Blackwell R. N. Haszeldine and D. R. Taylor J. Chem. Soc. Perkin I 1983 1. 48 P. Konig J. Zountsas K. Bleekmann and H. Meier Chem. Ber. 1983 116 3580. 4Y A. Padwa and J. G. MacDonald J. Org. Chem. 1983 48 3189. ''I J. C. Wu J. Mareda Y. N. Gupta and K.N. Houk J. Am. Chem. SOC.,1983 105 6996. C.-Y. Lin. J. Mareda. K. N. Houk and F. R. Frouczek. J. Am. Chem. Soc.. 1983. 105. 6714. Reaction Mechanisms -Part (i) Pericyclic Reactions 0 &o @ /\ / -\ ,H 0 H H C0,Et Et 0,c H (57) (56) w;;=[a] (58) basis has been assumed to have an antisymmetric singlet ground-state. Concerted [T~S+ T~S]cycloaddition is thus allowed as in the case of 1,8-dehydronaphthalene. This highly strained cycloalkyne preferentially adds to one double bond of b~tadiene.~~ 4 Sigmatropic Rearrangements Claisen rearrangement of (59) shows a preference for axial attachment to the ring in the product but relatively minor substitution in the vinyl ether portion of the molecule can alter this preference to eq~atorial.~~ CH,COR R = OEt 87 13 ax :eq TCH2 -QHz Bu' Bu' (59) R = Et OEt OSiBu'Me Control of enolate geometry has been achieved using the allylic glycolate (60) where internal chelation of lithium fixes the double bond configuration Claisen rearrangement occurs with high stereoselectivity (-10 1) to give (61) and (62) from (E) and (2)-substituted (allyl) double bonds re~pectively.~~ 4' C0,Li R~ = H R3 = Me R2 = Me R3 = H (61) OR * R3 (60) '' L.Fitjer and S. Modaressi Tetrahedron Lerr. 1983 24 5495. R. E. Ireland and M. D. Varney J. Org. Chem. 1983 48 1829. s4 J. Kallrnerten and T. J. Could Tetrahedron Lett. 1983 24 5177. R. S. Atkinson 6-Allyloxyindoles appear to undergo highly regioselective Claisen rearrangement (Scheme 9).55 60-70% Scheme 9 It has been suggested that regioselectivity in the trifluoroacetic acid (TFA)-catalysed Claisen rearrangement of ester-substituted allyl phenyl ethers e.g.(63) may be correlated with the response of the chemical shift of the I3Cn.m.r. resonances of the respective ortho-carbons to protonation by TFA the major products are those derived by rearrangement to the (sometimes more sterically hindered) ortho-position whose carbon resonance is shifted downfield the less.56 New [3,3] sigmatropic rearrangements are involved in the formation of reaction products from allyl sulphides and dichlor~ketene~’ (Scheme 10) and in the benzoyla-tion of nitrones (Scheme 1 The overall conversion in the latter case corresponds to a a-benzoyloxylation (hydroxylation) of the aldehyde from which the nitrone was derived.0 I1 Is’ CJ0-Q’ + /\ --.+ 0 C II CI c1 CI cj CI CI 2045% Scheme 10 The impressive potential of the anion-acceleratedClaisen rearrangement is shown by the formation of the highly crowded product (64) on heating the sodium salt of N-methoxycarbonylhydrazone (65). Heating the corresponding cyclohexane-I ,2-dione (66)brings about a rare reverse Claisen rearrangement the driving force for which must be the steric crowding present.59 55 C. J. Moody J. Chem. SOC.,Chem. Commun. 1983 1129. 56 L. M. Harwood J. Chem. SOC.,Commun. 1983 530. 57 R. Malherbe G. Rist and D. Bellus J. Org. Chem. 1983 48 860. 58 C. H. Cummins and R.M. Coates J. Org. Chem. 1983 48 2070. 5Y A. A. Ponaras J. Org. Chem.. 1983. 48 3866. Reaction Mechanisms -Part (i) Pericyclic Reactions J H' R 0 ? '0 e-H Scheme 11 Me Me Me Me 115°C' --+ x Q: N- N'I / (65) (64) (85%) Me Me Me Me 0' Me Among new [2,3]sigmatropic rearrangements6' is the oxidation of allylic iodides to allylic alcohols (with rearrangement) which has been shown to follow the pathway indicated in Scheme 12.6' It has been claimed that the method is comparable in yield to that obtained in allylic sulphoxide -sulphenate ester (followed by phos- phine scavenging of the latter) for effecting allylic alcohol rearrangement but has the advantage that no heating is required. (It is also subject to the availability of the required iodide.) Scheme 12 hO Review K.Hiroi and S. Sato Annu. Rep. Tohuku Coll. Pharm. 1982 (29) 1-37. '' S. Yamamoto. H. ltani T. Tsuji and W. Nagata J. Am. Chem. Soc. 1983 105 2908. R. S. Atkinson The ally1 selenoxide selenenate equilibrium (Scheme 13) lies more on the side of the ester than in the corresponding sulphoxide S sulphenate ester because of (a) the weaker Se-C bond strength compared to S-C and (b) the smaller degree of multiple bonding in Se=O by comparison with S=O." Y = SorSe Scheme 13 The [2,3] sigmatropic rearrangement has in general exhibited only moderate levels of diastereoselection by comparison with the [3,3]sigmatropic rearrangement. Attempts have been made using the [2,3] Wittig rearrangement (Scheme 14) to H Me -\ Jh 1 I R "? -H 0-Scheme 14 examine the changes in stereoselectivity with changes in substitution.To accommo-date the effects observed the authors propose a transition-state geometry (67) [rather than the more usually assumed envelope (68)] in which repulsion between R and the olefin H is the major factor determining stereoselectivity. A gauche relationship between R and R' or R3 explains why the stereoselectivity is little affected by the nature of R2(R3) (whether an alkyl group or hydr~gen).~~ (67) (68) Two examples of [2,3]sigmatropic rearrangement which do show high stereoselec- tivity are the conversion of the individual ally1 sulphoxides (69) and (70) into (71) and (72) respectively [for which a transition state corresponding to (73) is assumed with a methyl group preferring the 'equatorial' position]64 and rearrangement of the 62 H.J. Reich K. E. Yelm and S. Wollowitz J. Am. Chem. SOC.,1983 105 2503. 63 K. Mikami Y. Kimura N. Kishi and T. Nakai J. Org. Chem. 1983 48 279. 64 R. S. Garigipati and S. M. Weinreb J. Am. Chem. SOC..1983 105. 4499. Reaction Mechanisms -Part (i) Pericyclic Reactions OH J P(OMe) + HCO CH Ph csc-vr NHC02CH2Ph R' R2' Rl (69) R' = Me R' = H (71) R' = Me,R' = H (70) R' = H,R2 = Me (72) R1 = H,R2 = Me N HC0,CH ,Ph allylic sulphinate (74) to the sulphone (75) in which chirality at sulphur is efficiently transferred to carbon.65 A dramatic solvent-effect in the latter reaction using dimethylformamide remains to be explained.8- Ar-S-0 R' ,\ I CH2 -ArS02CCH=CH2 I .. R12 R2 R2 (75) (74) R' R' = H Me or (CH,),Me Other sigmatropic rearrangements include the circumambulation of the (sub- stituted) bridging carbon in e.g. (76) which proceeds at 0°C with inversion at the migrating group in agreement with orbital symmetry control. The low activation- barrier is attributed to the high ground-state enthalpy of the bicyclopentene.66 Heating (78) results in formation of (77). This is viewed by the authors as a concerted [1,5] hydrogen-shift within the dipolar resonance structure followed by ring-closure. However 1,Shydride shift in this system also explains the effects of changes in R and also that the reaction fails with CH3 instead of CF3.Distinction between these two routes is a fine but real one particularly in terms of the geometries involved." A [1,5] sigmatropic rearrangement of the nitro group in the ipso-nitration product (79) occurs and there is good evidence that this is concerted.68 6S K. Hiroi R. Kitayama and S. Sato J. Chem. SOC.,Chem. Commun. 1983 1470. 66 F.-G. Klarner and F. Adamsky Chem. Ber. 1983 116 299. 67 W. Verboom B. G. VanDijk and D. N. Reinhoudt Tetrahedron Lett. 1983 24 3923. 6X G. S. Bapat. A. Fischer. G. N. Hendersen and S. Raymahasay. J. Chem. Soc.. Chem. Commun. 1983 119. R. S. Atkinson MeFf MeFf B;02 __* O;BMe R (79) R = H (95%) The cyclononatetraene ring in (80) has a pronounced helicity (X-ray) which results in diastereotopic hydrogens within the methylene group.Heating at 100 "C results in an unusual [1,5] vinyl shift to give (81).69 A -w I00 "C \/ Ph Ph An ab initio calculation of the preferred transition-state for [1,5] sigmatropic rearrangement of hydrogen in cis-1,3-pentadiene does not support the conclusion (drawn recently from the temperature dependence of the kinetic isotope effect) that transfer of hydrogen takes place via a linear (CIv) (82) rather than an envelope (C,) (83) geometry." n (82) (83) A detailed study of the Stevens rearrangement of acyl-stabilized ammonium ylides e.g. (84) to (85) has revealed some surprising results including the finding that in water at O'C rearrangement of the salt (84) is essentially intramolecular with 69 A.G. Anastassiou H. S. Kasrnai and M. Sabahi Helu. Chim. Acta 1983 66,718. 70 B. A. Hess and L.J. Shaad. J. Am. Chern. SOC.,1983 105 7185. Reaction Mechanisms -Part ( i) Pericyclic Reactions 65 almost complete retention of configuration in the migrating group. Nevertheless the rearrangement is believed to be proceeding uia a non-concerted (biradical) route based on the decrease in intramolecularity and stereospecificity of the reaction with decrease in solvent viscosity." 5 Other Pericyclic Reactions Introduction of an ester group on the carbon of the N-acylimine precursor (86) facilitates the (retro-ene) removal of acetic acid as shown by a lowering of the reaction temperature required by 200 "C the superior yield of product is attributed to enhanced enophilic character (lower energy LUMO) in the C=N of (87).72 __* 0-I NOCOMe -+ [?HI CH,R CH,R R = H (40%) (86) (87) R = CO,Et (100%) High asymmetric induction has been obtained in the stannic chloride catalysed ene reaction of chiral a-ketoester (88) with hex- 1-ene.73 RO%:r __* HO+Zr Me O<--78 -L-111 diastereoisomer excess > 90% I Me (88) Cleavage of the individual cyclic sulpliinamides (89) and (90) with base is accom- panied by stereospecific retro-ene elimination of sulphur dioxide the chirality at C" in (90) and (92) is presumed to be determined by the preference of the methyl group for the equatorial position in a chair-type transition state (93).74 Me Me R' R 'R (89) R = H R1= Me b (91) R = H R1= Me (85%) (90) R = Me R' = H + (92) R = Me R' = H(83%) w.D. Ollis M. Rey and I. 0. Sutherland J. Chem. SOC. Perkin Trans. I 1983 1009. 71 K. Koch J.-M. Lin and F. W. Fowler Tefruhedron Leff. 1983 24 1581. 73 J. K. Whitesell D. Deyo and A. Bhattacharya J. Chem. SOC.,Chem. Commun. 1983 802. 71 R. S. Garigipati J. A. Morton and S. M. Weinreb. Tetrahedron Lett.. 1983. 24 987. R. S. Atkinson Me Elimination of tropolone from the ether (94) proceeds with high stereoselectivity with elimination of the hydrogen trans to the ether oxygen as shown by deuterium labelling and mass spectral data. It has been suggested that this is a concerted [7r2s + a2s + a2a]reaction but it is difficult to visualize (by drawing or models) the transition state which leads preferentially to trans elimination.The authors do not appear to have completely eliminated the possibility that the reaction is a trans elimination with the solvent (DMSO) acting as the base." D (94) H. Takeshita and Mametsuka J. Chern. Soc. Chern. Cornrnun. 1983 483.

ISSN:0069-3030    

DOI:10.1039/OC9838000047

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (ii) Polar Reactions By D. G. MORRIS Department of Chemistry University of Glasgow Glasgow G12 8QQ 1 Introduction Monographs in this area have dealt with the anomeric effect' and electrophilic additions to unsaturated systems.2 Pertinent reviews have been concerned with the reactivity of tetrahedral intermediate^,^ a qualitative valence-bond approach to organic rea~tivity,~ the thermodynamics of metastable intermediates in s~lution,~ electron-deficient carbocations,6 homoenolate anions,' aromatic substitution reac- tions,* and chemical models of enzymic transimination.' 2 Substitution Reactions Attempted aldol condensation of (1) with paraformaldehyde and ButOK gave the intermediate (2) from which a high yield of (3) was formed almost certainly uia SiMe2Bu' 0 OR (1) R = SiMe,Bu' (2) R = SiMe2Bu' Bu'Me,Si I 0 (3) R = SiMezBu' ' A.J. Kirby 'The Anomeric Effect and Related Stereoelectronic Effects at Oxygen' Springer-Verlag Heidelberg 1983. P. B. D. de la Mare and R. Bolton 'Electrophilic Additions to Unsaturated Systems' Elsevier Amsterdam 1982. R. A. McClelland and L. J. Santry Acc. Chem. Res. 1983 16 394. A. Pross and S. S. Shaik Acc. Chem. Rex 1983 16,363. J. P. Guthrie Acc. Chem. Res. 1983 16 122. 'P. G. Gassman and T. T. Tidwell Acc. Chem. Res. 1983 16 279. ' N. H. Werstiuk Tetrahedron 1983 39 205. J. G. Traynham 1.Chem. Educ. 1983,60 937. M. J. Makela and T. K. Korpela Chem. SOC. Rev. 1983 12 309. 0' 67 D. G. Morris an endocyclic nucleophilic substitution reaction whose transition state contained eight atoms one of which was silicon." Me-S 8 MeO-+ +MeOMe + Sd Scheme 1 The 13C isotope effect of 7-8% observed in the reaction outlined in Scheme 1 is close to the maximum expected for complete loss of the stretching vibration of the C-S bond in the transition state.The miniscule a-deuterium secondary isotope effect indicates that softening of the vibrational potential brought about by departure of sulphur and flattening of the methyl group is compensated by approach of nucleophilic oxygen. The solvent isotope effect kCD3,,/ kCH30Hca. 2 is large enough to suggest that desolvation of MeO- is largely complete in the transition state though this is short of the value 2.47 estimated for complete desolvation." Aspects of the sN2 reaction have been consideredI2 in terms of the correlation diagram developed by the author and co-workers which uses the energy gap (I -ARX) (IN is the ionization potential of the nucleophile N A, is the electron affinity of the substrate RX) as a parameter.A fraction of (I -ARX)enters the activation barrier and is a function of the degree of delocalization of the three- electron bonds (Re-.X)-. The fraction is larger the more delocalized the three-electron bond; in turn greater localization is shown by a-halogeno substitution of R associated with which is a small improvement in the acceptor stability of the modified substrate. The largest delocalization and the largest rate reduction is shown when the halogen introduced into R corresponds to the leaving group; and it has long been known that CH,CI is much less reactive toward nucleophiles than is CH3Cl.a-Substituents which are .rr-acceptors e.g. CN significantly increase the acceptor ability exemplified by CH3CI -30.0 but NCCH2Cl -5.5 kcal mol-' respectively. This has the effect of enhancing reactivity the more so when the nucleophile is powerful. A frequently used method for deprotection of synthetic peptides employs HF with the inevitable consequence that carbocations can intrude. A modified protocol has been developed and involves the use of a weak base typically Me,S (pK = -6.8) in conjuction with HF in order to lower its acidity f~ncti0n.l~ The nucleophilic sulphide which gives inert products is a weaker base than the protecting groups and these alone become protonated (Scheme 2).At an operational level a mixture ") V. H. Rawal and M. P. Cava Tetrahedron Lett. 1983 50 5581. 0.S. L. Wong and R. L. Schowen J. Am. Chem. Soc. 1983 105 1951. l2 S. S. Shaik J. Am. Chem. SOC.,1983 105 4359. l3 J. P. Tam W. F. Heath and R. B. Merrifield J. Am. Chem. Soc. 1983 105 6442. Reaction Mechanisms -Part (ii) Polar Reactions H Ph + PhCH,SMez Scheme 2 HF-Me,S-p-cresol (25 :65 10 v/v) is employed. The procedure has also found application in solid-phase peptide synthesis. Many reactions are considered to obey rate-equilibrium relationships of the type A(AG') = aA(AG") with a in the range 0-1 ; implicit in this relationship is the belief that influences on equilibria are only partially reflected in reaction rates.No such correlation is shown for the reaction family in Scheme 3 in which K = 1 whereas a change in CH,Cl ClCH Q-0 c1-+ + cf-Y Y Scheme 3 Y does influence the reaction rate. The authorI4 points out that a plot of log kus. log K is linear with a = 00 'the highest value yet observed and substantially larger than previously reported values of ca. 1.5-1.8'. It is proposed that reaction families in which the substituent is adjacent to a site at which charge is neither generated nor destroyed will not exhibit a rate-equilibrium relationship though one is likely for example in the methylation of pyridines. A combination of water and the polar aprotic solvents hexamethylphosphoramide or N-methyl-2-pyrrolidone alone provides a potent and selective source of nucleophilic oxygen; buffering can be provided by NaHCO3.I5 Thus alkyl halides and sulphonates may be converted into alcohols; increased nucleophilicity of alcohols leads to ethers.Terminal epoxides can also be converted into 1,2-diols. Vinyl halides react with sodium alkanethiolates e.g. NaSMe in hexamethylphos- phoramide to give substitution products in high yield and with retention of configur- ation (Scheme 4);I6 little further mechanistic information is available for what appears to be a general reaction. NaSMe Ph Scheme 4 l4 A. Pross Tetrahedron Lett. 1983 24 835. l5 R. 0. Hutchins and I. M. Taffer J. Org. Chem. 1983 48 1360. l6 M. Tiecco L.Testaferri. M. Tingoli D. Chianelli and M. Montanucci J. Org. Chew. 1983 48 4795. D. G. Morris Stohrer17 has commented that the syn-preference generally exhibited by SN2' reactions is favoured on account of stereoelectronic stabilization which may be offset by steric and electrostatic repulsions. An additional factor has been considered ; thus in the anti transition-state (4) the central carbon is planar whereas this is not the case for the syn counterpart where the larger lobe remains on the same side of the molecule along the reaction co-ordinate (5). It is also proposed that a difficult to cleave nucleophile should favour the syn reaction. Nu *-(41 Clean cyclization of the (R)-trans-ester (6) in refluxing trifluoroethanol in the presence of 2.2 equiv.KOBu' gave (7) which was almost entirely racemized though a small amount of chirality was transferred with an anti relationship between entering and leaving groups.'* However in the presence of a catalytic quantity of tetrakis(tripheny1phosphine)palladiumand Et,N (6) reacted to give (7) with essen- tially complete transfer of chirality in the syn sense. The acrylate (8) was reduced by LiBEt,H (Super Hydride) to (9) in a reaction that is both regio- and trans-selective only the E-isomer being detected ('H n.m.r.). This reaction is noteworthy in that it is an SN2' reaction which occurs readily at low temperature (-50 "C) notwithstanding the fact that acetate is generally regarded as a poor leaving group." The reaction of the dieneyl ether with the dianion of methanol as in (10) shows a pronounced endo stereoselectivity which may be rationalized in terms of a " W.-D.Stohrer Angew. Chem. Znf. Ed Engl 1983 22 613. G. Stork and J. M. Poirer J. Am. Chem. Soc. 1983 105 1073. l9 J. Rabe and H. M. R. Hoffmann Angew. Chem. lnt. Ed. Engl. 1983 22 796. Reaction Mechanisms -Part ( ii) Polar Reactions ,--OH six-membered transition-state leading via an SN2'reaction to (1 1).20 The key feature is the chelation of the organolithium reagent to the ether oxygen. Addition of the dichloride (12) to two moles of lithium diphenylphosphide at -70 "C gave (13) via two successive Sy2' reactions.21 3 Carbocations The first carbocation with an a-carbonyl group (14) has been prepared; it has a finite though limited life under normal laboratory conditions.22 Arnett's has so chosen a carbocation and a carbanion that they can co-exist in the same solvent (Scheme 5).The crystal structure of (15) reveals an unusually long C -C bond 1.588(4) A. The orange colour of the carbanion is apparent in acetone and more polar solvents. In MeCN the value AH:, = +5.5 f0.5 kcal mol-' was determined. (B No Scheme 5 20 D. G. Farnum and T. Monego Tetrahedron Lett. 1983 24 1361. '' D. G. Gillespie and B. J. Walker J. Chem. SOC.,Perkin Trans. 1 1983 1689. 22 K. Takeuchi T. Kitagawa and K. Okamoto J. Chem. SOC.,Chem. Commun. 1983 7. 23 E. M. Amett E. B. Troughton A.T. McPhail and K. E. Molter J. Am. Chem. SOC.,1983 105 6172. 72 D. G. Morris The "0-labelled exo-2-norbornyl brosylate ( 16) (and the sulphonyl oxygen- labelled counterpart) have been prepared and the ethanolysis monitored by "0 n.m.r.spectro~copy.~~ In the presence of praseodymium( 111) nitrate as a shift reagent it is possible to monitor the rates of scrambling of the label and the ethanolysis separately. The authors state that return occurs without the oxygen atoms having fully equilibrated. Further 170-enriched acid and natural-abundance ester gave no hint of ( 16) (or the sulphonyl oxygen-labelled counterpart) thereby excluding external return. A proposition that the volume of activation of a solvolysis reaction to produce Coates' non-classical ion from (17) should show an activation volume significantly different from one in which a classical ion is unequivocally involved was tested using t-butyl chloride as a ~ubstrate.,~ In 80% aqueous ethanol at 30 "C the respective values were -19 cm3 mol-' and -40 cm3 mol-I.Charge dispersal within an ion leads to a smaller interaction with the medium; however the difference found here is larger than expected. pg In HOS0,F-S0,ClF at -120 "C diazomethane is protonated to give the methyl diazonium ion which absorbed at 4.75 6 (singlet).26 In the more strongly acidic solvent HOS02F-SbFS the methylenediazenium ion (18) is also formed in a kineti- cally controlled process; (18) can however be converted into the more thermody- namically stable C-protonated isomer. Asymmetric introduction of deuterium into a molecule and subsequent analysis of the n.m.r.spectrum can serve to distinguish between carbocations which equili- brate rapidly over a low barrier and one which possesses a single minimum energy surface. The 'H n.m.r. spectrum of the monodeuteriated norbornyl cation (prepared from 2-[2H,]norbornan-2-01) showed a peak to highfield of the averaged H- I H-2 24 S. Chang and W. J. le Noble J. Am. Chem. SOC.,1983 105 3708. 25 G. Jenner S. Srivastava and W. J. le Noble TetrJhedron Lett. 1983 24 2429. 26 J. F. McCarrity and D. P. Cox J. Am. Chem. Snc. 1983 105 3961. Reaction Mechanisms -Part (ii) Polar Reactions 73 H-6 signal between -105 and -43 "C. This arises from perturbation of the average shift of the three remaining protons by an equilibrium isotope effect in that fraction of molecules which possess deuterium on C-1 C-2 or C-6.27 The observed isotope induced shift is 0.146 p.p.m.upfield whereas a classical ion is predicted to show a downfield shift of 0.059 p.p.m. at -43 "C; a non-classical ion structure is thus preferred. Exposure of the acid (19) to trifluoroacetic acid gives rise to the virtually racemic lactone (20) (one enantiomer shown) and it is proposed that a novel and racemizing 6,2-endo methyl migration in the ion (21) accounts for the loss of optical activity.28 (19) (20) (21) A residual possibility that (20) simply has a low specific rotation slightly mars this elegant experiment. The novel bishomoaromatic allylic dication (23) has been generated from (22) at -1 20 "C in FS03H-SbF5 (Scheme 6).29 The twelve absorptions in the I3C nmr.Scheme 6 spectrum of (23) relate closely to those of the corresponding monoanions. At -40 "C a new dication is formed from (23) and is assigned structure (24) on the basis of only six absorptions in the I3C n.m.r. spectrum (Scheme 7); here the rearranging part of (23) is shown as a classical ion for clarity. Me Me Me Me (24) (23) Scheme 7 77 -' M. Saunders and M. R. Kates J. Am. Chem. SOC.,1983 105 3571. 20 W. R. Vaughan B. A. Gross S.E. Butkle M. A. Langell R.Caple and D. B. Oakes J. Org. Chem. 1983 48 4792. 29 G. A. Olah M.Arvanaghi and G. K.Surya Prakash Angew. Chem. Int. Ed. Engl. 1983 22 712. D. G. Morris The triflates (25) and (26) undergo trifluoroacetolysis via the corresponding vinyl cations without participation ; some internal return is found however.30 The much more rapid trifluoroacetolysis of (27) occurs with w-participation involving ion (28) ; the major product from this compound is the triflate (26) and the corresponding trifluoroethyl ether.OTf (27) The same group has shown that trifluoroethanolysis of (29) gives rise to the cyclobutanones (30) and (31) (Scheme 8).31 The distribution of label in the product is nicely rationalized by postulating the intermediacy of (32) and subsequent conver- sion of this ion into product via the non-classical ions shown. +Cc=C-Me I '** j + *\y-(: C-%D2 D Me I c CH2 + -I Ill C=C-Me C CD,' (30) Scheme 8 A phenyl carbocation whose collisional activation (CA) mass spectrum was identical to that of C6H5+ obtained from the molecular ion of bromobenzene has been obtained from (33) which does not cyclize concomitantly with formation of molecular ions but rather after rupture of the C-Br bond.32 This is supported by 30 M.Hanack K.-A. Fuchs and C. J. Collins J. Am. Chem. SOC.,1983 105 4008. 3' C. J. Collins M. Hanack H. Stutz G. Auchter and W. Schoberth. J. Org. Chem. 1983 48 5260. 32 G. Depke M. Hanack W. Hummer and H. Schwarz. Angew. Chem. Int. Ed. Engb 1983 22 786. Reaction Mechanisms -Part (ii) Polar Reactions 75 the finding that the CA mass spectra of the molecular ions of (33) and bromobenzene are different and also since the kinetic energy released on decomposition of (33)'.to C6HSt differs from that of the corresponding reaction of C6H,+'. 4 Elimination Reactions Interest in elimination reactions has been bullish. Tritylpotassium brings about rapid dehydrohalogenation of secondary alkyl halides at 0 "C within 5 min to produce olefin presumably via an E2 mechanism; the reaction does not work well with primary alkyl halides.33 Large I4C kinetic isotope effects k/k = 1.061 (1.044) and k/ kp = 1.036 (1.040) are shown for the syn elimination of the parent (X = H) members of a series of para-substituted (2-phenylethy1)dimethylamine oxides (Scheme 9).34 Values in + 65"c CpH2-C,H2-NMe 10% aq. Me,SO XOCH=CH2 + Me,NOH I 0-Scheme 9 parentheses refer to the values for base-promoted anti elimination of the correspond- ing trimethylammonium bromides.These data are construed to indicate a greater bonding change at C in the amine oxide pyrolysis for which a Hammett p value of 2.1 1 was found. Accordingly the reaction is characterized by extensive rupture of both the C,-N and C,-H bonds with relatively little C,-C double-bond character. In the presence of the complex base NaNH,-NaOR in tetrahydrofuran trans- 1 -bromo-2-chlorocyclohexane underwent syn elimination with dehydrochlorination preferred (54-65°/i).35 Preferred loss of the normally poorer leaving group is accounted for by an interaction between the metal (e.g. Na) and the leaving group (34). In the presence of the strong complexing agent for Na+ 15-crown-5 loss of Br takes precedence and dehydrochlorination is reduced to 3% for the NaNH,- NaOBu' system.Koch and McLennan and their co-workers have measured the chlorine isotope effects in a series of alkoxide-ion promoted dehydro- and dedeuterio- chlorination^.^^ For eliminations by ethanolic sodium ethoxide the chlorine isotope effect k3J k3 values were 1.00590 * 0.00013 and 1.00507 * 0.00036 for (35) and (36) respectively. R / Ph-C-CH,Cl / Me B--M (35) R = H (34) (36) R = D 33 D. R. Anton and R. H. Crabtree Tetrahedron Lett. 1983 24 2449. 34 D. R. Wright L. B. Sims and A. Fry J. Am. Chem. SOC.,1983 105 3714. 35 A. P. Croft and R. A. Bartsch J. Org. Chem 1983,48 876. 3h H. F. Koch D. J. McLennan J. G. Koch W. Tumas B. Dobson and N. H. Koch J. Am. Cbem.Soc. 1983 105 1930. D. G. Morris These values were taken as indicative of an E2 reaction for dehydrochlorination. Calculations indicated that the latter figure would be 1.00090 in the case of an E lcb reaction when the substrate was (36). The rates of eliminations of 1-arylethyl chlorides at 45 "C to give the corresponding styrene are significantly increased by both electron-releasing and electron-with- drawing substituents upon reaction with the conjugate base from the solvent system bis(2-hydroxyethyl) ether-lO% v/v DMS0.37 The change in substituent character in the above sequences causes a change from an El-like E2 mechanism to one which is E lcb-like. This is supported by the P-14Ckinetic isotope effects such that kl+/k14~ratios are 1.038 1.058 and 1.068 for (37) (38) and (39) respectively.(37) X = Me x7 (38) X = H \-CI (39) x = c1 In acyclic systems the propensity for syn elimination increases with the steric requirements of the p-substituents in protic solvents.38 The anti transition-state (40) contains non-bonded interactions between the P-substituents and the bulky leaving group; relief of any interaction by torsion around the C,-C bond only induces other interactions. Thus whereas EtOH-EtO- at 60 "C reacts via anti elimination (40) to the extent of > 95% the syn pathway (41) is followed to the extent of 62% H when R' = C6H5,R2 = p-MeOC& in 50% Me2SO-H20 in the presence of -OH. A corresponding figure of 68S0/0 for syn elimination via (41) when R' = C6H5 R2 = Me2CH suggests that electronic effects are not important.After analysis of deuterium isotope effect data the authors propose that the principal difference is that the syn transition state has less carbon-nitrogen cleavage than the anti counterpart. Full details have appeared39 of the mechanistic study of the eliminations of (42) (NR3 = trimethylamine or quinuclidine) with -OH and buffer bases. These indicate a change in mechanism from (E Icb) to (E Icb) ;indeed the E Icb mechanism was proposed for (42; R = Me) fifty years ago. (42) 37 T. Hasan L. B. Sims and A. Fry J. Am. Chem. SOC.,1983 105 3967. 38 Y.-T. Tao and W. H. Saunders jun. J. Am. Chem. SOC.,1983 105 3183. 39 J. R. Keeffe and W. P. Jencks J. Am. Chem. SOC. 1983 105 265. Reaction Mechanisms -Part (ii) Polar Reactions Menger's group has used elimination and kindred reactions to address the question of the directionality of proton transfer which has relevance to both chemical and biochemical processes.a Thus in the reaction in Scheme 10 (a) the separation 0-H in (43) is 2.9 A and (b) the angle 0-HC is 82" and little torsional freedom is possible.In the event the reaction is completely intermolecular either because the separation in (a) is too great or the angle in (b) is too small. OH (43) Scheme 10 In (44) with the corresponding dimensions as shown the exchange of endo-3-H is 27 times faster at pD = 13.9 than with the endo-5-OMe derivative and a true intramolecular catalysis by endo-5-OH is indicated. These data correspond to an intramolecular catalysis of 1 O5 and one mechanism is initiated by the electron sequence shown in (49 although a molecule of water may intervene as in (46).A rapid intramolecular elimination ( < 1 h) at 53 "C is indicated in Scheme 11 ;40 direct involvement of solvent in the reaction mechanism is considered unlikely. Scheme 11 The authors suspect that the separation -0-a-H is the critical factor in bringing about an intramolecular reaction with the crossover point between 2.2 and 2.9 A. This finding has ramifications in that 'long distance catalysis' appears to be unlikely both in organic molecules and at the active site of enzymes. Further it appears39 that assertions that the three atoms involved in an intermolecular proton transfer must necessarily be collinear are probably suspect.F. M. Menger J. F. Chow H. Kaiserman and P. C. Vasquez J. Am. Chem. Soc. 1983 105 4996. 78 D. G. Morris The variable transition-state theory of bimolecular eliminations proposed several years ago by Bunnett has been subject to intense and sophisticated scrutiny; but it has not yet been tested to destruction and serves as an admirable model. Probably the most contentious aspect of alkene-forming reactions is the E2C mechanism with which two papers have been concerned. The temperature depen- dence of the kinetic deuterium isotope effect has been used as an approach for elucidating transition-state geometry in reactions having a single rate-determining step.41 -+ PhCHDCH,X PhCH=CH + PhCD=CH (47) X = OTs (48) X = Br (49) X = 'SMe (50) X = +NMe Scheme 12 Reactions of F- in MeCN with substrates (47)-(50) Scheme 12 were studied.For (47)-(49) the ratios of frequency factors AH/AD were respectively 3.6 4.6 and 6.6 and in particular were >> 21/2,by which criterion a bent transition state with non-linear H-transfer is indicated ; support was provided by the essentially temperature-invariant deuterium kinetic isotope effects. By way of contrast A,/A is 0.212 for (50) and kH/kDvaries from 6.177 (293 K) to 3.514 (353 K); the authors associate these data with a linear transition-state and hydrogen tunnelling. The authors state that the angle of H transfer is a direct function of A,/AD although values of 110" (47) 125" (48) 160" (49) and 180" (50) are quoted.For the first three substrates an E2C mechanism is proposed. Between 78.5 "C and 128.6 "C the bromide ion promoted elimination of (51) in MeCN gave a mean kinetic isotope effect of 2.606 and an unusually large a-deuterium effect of 1.214 was also measured for (52).42 An angle of ca. 100" was PhCH R1 CR2 BrCO Et (51) R1 = D,R2 = H (52) R' = H,R2 = D calculated for the angle of H transfer rather smaller than that for P-phenylethyl tosylates. From these results the authors conclude that an E2C mechanism is operative. At room temperature in DMSO the sulphonium salts (53) and (54) gave almost exclusively the respective terminal olefins (55) and (56) after reaction with Bu'OK. However under the same conditions (57) gave (58) and the non-terminal olefin (59) in essentially equal amounts whereas (60) again gave predominantly terminal 01efin.4~ 4' H.Kwart K. A. Wilk and D. Chatellier J. Org. Chem. 1983 48 756. 42 H. Kwart and A. Gaffney J Org. Chem. 1983,48 4502. 43 B. Badet M. Julia J. M.Mallett and C. Schmitz Tetrahedron Lett. 1983 24 4331 Reaction Mechanisms -Part (ii) Polar Reactions (53) Z = H (55) Z = H (54) Z = OH (56) Z = OH (57) Z = OPh (58) Z = OPh The mechanism of dehydrohalogenation of the dihalides derived from 1,4-benzoquinone has been studieda in the solvent mixture EtOH :H20:Me2C095 :5 1. Acid catalysis is unimportant and loss of HBr from ClC-CBr is a little faster than loss of HC1 from C1C-CC1 after statistical correction.The geometric requirements for rapid concerted E2 reactions are not present in either of the conformationally isomeric transition-states of which one (61) is shown; accordingly the mechanism is well towards the Elcb end of the spectrum of transition states available for elimination with the dominant feature being proton loss which was essentially ( 99%) irreversible. Cyclization of (62) to (63) occurs readily in ButO-Bu‘OH in the temperature range 20-80°C and has t,,2 ca. 5 min at room temperat~re.~’ The mechanism involves nucleophilic attack of alkoxide ion on an isolated double bond and is formally the reverse of an Elcb reaction. In accord with this proposal a small solvent isotope effect kRo,/kRo = 1.7 together with ASS + 67.4 J K-’ mol-l was observed.0’ % 0 %OH R. C. Atkinson P. B. D. de la Mare and D. S. Larsen J. Chem. SOC.,Perkin Trans. 2 1983 271. A. A. Smeaton W. V. Steele G. M. R. Tombo and C. Ganter Helv. Chim. Acta 1983 66 2449. D. G. Morris 5 Ester Hydrolysis and Reactivity of Carbonyl Derivatives Since the ester (64) is hydrolysed ca. lo7 times faster than (65) in basic solution a dissociative path via (66) is proposed for the phenolic ester; the increased reactivity of the conjugate base of (64) over (67) ca. lo3 is attributed to enhanced nucleophilic- ity of the phenoxy anion in the former case resulting in the more rapid expulsion of the 2,4-dinitrophenoxy R' (64) R' = Me,R2 = H (65) R' = Me,R2 = Me (67) R' = H,R2 = H A comparison of the kinetics of hydrolysis shows that at alkaline pH the cyahester (69) is more reactive than (68) by both E lcb and BAc2 mechanism^.^' In the former reaction the greater reactivity of (69) by a factor of 70 stems from relief of strain in proceeding from the ground state to the transition state.The rate constant kpl for the BAC2 hydrolysis of (69) is ca. 20 times that of (68) when the pH >> pK of the ionizable proton; under the opposite conditions however the more bulky substrate (69) is ca.8-times slower. The increase of k, on t-butylation is ascribed to the increase in the pK of the acid proton by ca. 2 units. CN / (68) R = H R-CH \ (69) R = Me,C C0,Et In 60/40 v/v dioxan-water the initial specific rate of the reaction between hydroxide ion and the cyclic diester (70) to give the rjcyclic half-ester was greatly increased upon addition of low concentrations of NaCl KCl RbCl or C~c1.4~ The complex formed between (70) and the metal ion decreases in reactivity in the 46 S.Thea G. Guanti N. Kasheh-Naini and A. Williams 1.Chem. Soc. Chem. Carnmun. 1983 529. 47 M.lnoue and T. C. Buice J. Org. Chem. 1983,48 3559. 48 D. S. Baker and V. Gold 1.Chem. Soc. Perkin Trans. 2 1983 1129. Reaction Mechanisms -Part (ii) Polar Reactions sequence K+ > Rb+ > Na+ > Cs'; Lif was without effect. It is inferred that the Bjerrum model which relates to the effect of remote charges on ionization equilibria may be relevant to the effects of guest metal-ions on ionic reactions of crown ether molecules with possible ramifications for the chemistry and physiological activity of ionophoric antibiotics and enzymes.The authors propose that the bulk relative permittivity attenuates electrostatic interaction of the charges. With respect to the uncatalysed hydrolysis of the phospholene (7 I) a rate enhance- ment of more than fifty is brought about by a 2.4-fold excess of imidazole in 50% aq. methanol in a reaction which is first order in each component; methanol is also formed.49Two independent and competitive routes were proposed one involving nucleophilic attack on the methoxy carbon by water which is involved in general-base catalysis by imidazole as in (72) leading to (73). A ring-opened intermediate H detectable by n.m.r.spectroscopy which probably results from analogous general- base catalysed attack at phosphorus followed by reclosure is thought to be involved. The effective molarity of -COT which is a powerful nucleophilic catalyst for the hydrolysis of phosphate esters is 6 x 108M; the rate-determining step is loss of the apical phenolate anion (Scheme 13). The pH-rate profile for the hydrolysis of OPh OPh Scheme 13 (74) shows a pronounced maximum at pH ca. 4.5 in which region the concentration of the dianion is expected to be a maximum.50 The rate enhancement brought about through the introduction of one carboxy and one carboxylate group is of the order of lo'' which is taken as evidence for nucleophilic catalysis. The observed rate of hydrolysis of the dianion (75) is ca.four times the maximum value calculated; this small but significant value firmly suggests intramolecular general-acid catalysis by the carboxy group (Scheme 14). Even so this catalysis is relatively inefficient probably on account of an early transition-state for the cleavage of (76) ;accordingly there is little negative charge on the departing oxygen to induce the proton-transfer step readily. Hydrolysis of (77) in which the trans-ring junction locks the leaving group in an equatorial position is appreciably (x105) slower than expected for a 49 R. S. Macomber J. Am. Chem. SOC.,1983 105 4386. 50 K. W. Y. Abell and A. J. Kirby J. Chem. SOC.,Perkin Trans. 2 1983 1171. D. G. Morris 0 (74) (75) II O,p/O--+ salicylate 0 (76) Scheme 14 conformationally flexible acetal on account of the stereoselective barrier to cleavage of the acetal C-0 bond denied assistance from .rr-donation from the donor oxygen atom.5' Hydrolysis of (77) is not reversible thereby excluding equilibration with (78).Under most conditions the products consist of a mixture of (78) and (79) itself a mixture of isomers. The spontaneous cleavage of (78) yields a zwitterion and recyclization is antici- pated to be more rapid than addition of water to give (79); support for this contention is provided by the solvent isotope effect kHzo/ kDzo = 1.74 at 100 "C.This reaction is now characterized by a different rate-determining step viz. hydration to form (79) and this accounts for the slow reaction.The cis axial bis-acetal (80) which contains the remote oxygen atom as a .rr-donor in a favourable stereoelectronic environment reacts ca. 200 times faster than the trans isomer (81) in reactions in which the leaving group departs irreversibly and in which there is a relatively early transition-state for spontaneous cleavage.52 In '' A. J. Kirby and R. J. Martin J. Chem. SOC.,Perkin Trans. 2 1983 1627. A. J. Kirby and R. J. Martin J. Chem. Soc. Perkin Trans. 2 1983. 1633. Reaction Mechanisms -Part (ii) Polar Reactions I H the trans isomer the remote oxygen is thought to be functioning solely as a u-acceptor thereby destabilizing the developing oxocarbocation. The depiction of the fragmenta- tion shown in (80) is incomplete in respect of the curly arrow formalism.Lysozyme catalyses the hydrolysis of a polysaccharide constituent of bacterial cell walls and cleaves a P-glycosidic linkage with retention of configuration. As with the conformationally fixed acetal (82) which is hydrolysed only with extreme reluctance the lone pairs on the ring oxygen of P-glycoside (83) cannot overlap significantly with because two ring bonds are anti-periplanar to the C-OR bond.53 The stereoelectronic barrier to C-OR cleavage is ca. 19 kcal mol-’ when the OR group is held equatorial; as an alternative an energetically less costly reaction was proposed via a relatively facile conformational change around the (ring)O-C( OR) bond and lone-pair-&, overlap becomes progressively more efficient as the ring flattens.The energy required to form the reactive conformation of higher energy can be readily recouped providing that the conformational barrrier is appreciably less than the energy of activation for the cleavage reaction. It is conceivable that the reaction of (83) proceeds via (84) (part structure) in which the conformation is akin to that of a half chair.53 The normally very rapid hydrolysis of ketene diethyl acetals is diminished by introduction of chloro or cyano substituents. Reactions catalysed by perchloric acid (in H20 and D20)indicate rate-determining proton transfer to the P-carb01-1.~~ The observed isotope effects were 2.46-2.96 ; the smallest difference between observed and calculated values was observed with the least bulky substrate cyanoketene dimethyl acetal.Bulky substituents at the P-carbon reduce the reactivity more so than for the hydrolysis of vinyl ethers. A solvent isotope effect of 2.19 was found for the water-catalysed hydrolysis of the immonium ion (85).55From proton inventory techniques in H20-D20 a transi- tion state exemplified by (86) and containing an immature hydronium ion is impli- cated. 53 A. J. Briggs C. M. Evans R. Glenn and A. J. Kirby J. Chem. SOC.,Perkin Trans. 2 1983 1637. 54 A. J. Kresge and T. S. Straub J. Am. Chem. SOC.,1983 105,3957. 55 R. L. Erhardt G. Gopalakrishnan and J. L. Hogg J. Org. Chem. 1983 48,1586. D. G. Morris Ph Me \ C=N+/ 6 Aromatic Reactivity The rate of hydrolysis of 3-methyl- 1-picryl-imidazolinium ion (87) is strongly cata- lysed by oxygen bases between pH 1.7 and 9.3 and gives picric acid q~antitatively.~~ A reversibly formed (Ll ca.10's-10'6 s-') and short lived intermediate (88) resulting from concerted addition of water to the aromatic ring is proposed and support is provided by the value AS' -28.6 cal mol-' deg-' at 25 "C when water functions as the base. cj'"' N No (87) In benzene the observed second-order rate constant for the reaction of 2,4-dinitrofluorobenzene with o-anisidine (89) shows a quadratic dependence on ( 89).57 H A dimer nucleophile ArN.-.H-NAr is implicated and reaction to form (91) is H H considered to occur via the cyclic intermediate (90). NHAr 56 R. H. de Rossi and A.Veglia J. Org. Chem. 1983 48 1879. 57 N. S. Nudelman and D. Palleros J. Org. Chem. 1983 38 1613. Reaction Mechanisms -Part (ii) Polar Reactions 7 Micelle-aneous and Other Reactions The anilinium ion precursor of (92) gave vesicles of diameter ca. 9008 after sonication in aq. HBr; these led to (92) diameter ca. 660 A at pH 5.58Azo coupling of vesicular (92) with &naphthol was biphasic consisting of a rapid pseudo first-order reaction (85%) attributed to an exo-vesicular diazonium group and a slower endo counterpart (1 5%). However the endo-vesicular reaction of (92) with (93) is completely suppressed. Thus whereas P-naphthol permeates vesicular (92) at a rate competitive with that of azo coupling such permeation does not occur with (93).(92) (93) The micro-environment of the micelles (94) and (95) has been examined by monitoring the solvent-sensitive chemical shifts of the acetylenic For aqueous solutions in the micellar state these occur at 2.16 and 2.10 S respectively and being downfield from the anticipated region of 1.&I .8 S indicate that the chain termini are wet on a time-averaged basis.59 Water-hydrocarbon contact is considered to occur throughout the micellar region external to the relatively small apolar core. Equimolar amounts of solubilized alcohol shifts the absorption of the terminal carbon to upfield as water is displaced from surface irregularities. H-C rC -C D (C H2),,NMe (94) H-CrC-CD,(CH,),,OSO; (95) Energies for N-to-N proton transfer in H2NCH2hH3 were calculated with the migrating proton confined to the bisecting xz plane (Figure 1).60 The transition state so obtained places the mobile proton in the NCN plane 21 kcal mol-' above the ground state with a partial N-H bond distance of 1.27 8 and an LNHN of 103".Movement of the proton from this transition state can be carried out at comparatively little energetic cost exemplified by a destabilization of 1.1 kcal mol-' for a proton movement of 0.2 8,out of the xy plane. The proton thus appears to have considerable freedom of motion in the transition state. Although the value of the activation energy is high the values achieved in practice may be so modified by solvent as to make rates in the region 106-108s-' possible. Figure 1 58 R. A. Moss and J.-S.Shin J. Chem. SOC.,Chem. Commun. 1983 1027. 59 F. M. Menger and J. F. Chow J. Am. Chem. SOC.,1983 105 5501. 60 F. M. Menger J. Grossman and D.C. Liotta J. Org. Chem. 1983 48,905. D. G. Morris Gas-phase basicity of a number of conformationally stable P-amino-alcohols has been examined by ion cyclotron resonance spectrometry. The dihedral angles (8) between the functional groups of the compounds ranged from 0" for (96) to 180" for (97).61 A continuous increase in basicity is observed as 8 decreases and in the conjugate acid of (96) stabilization brought about by hydrogen bonding is ca. 5 kcal mol-'. In the case of (97) a localized N-protonated species is obtained. Calculations suggest that the most stable form of 2-aminoethanol occurs when 8 = 19.2" and the stabilization amounts to ca.15.5 kcal mol-' with respect to the anti-periplanar conformation. Conversion of (98) into (99) was readily achieved with the aid of electrophilic catalysis of mercuric ions which have the effect of blocking attack at C-6 and also weakening the S-C-6 bond (Scheme 15).62 This mechanism may be compared with one of S~hubert,~~ who uncovered an example of an SN1reaction of an a-amino sulphide without electrophilic catalysis. Scheme 15 A useful paper describes the relative efficiency of the methods for drying alcohols and is addressed principally to methanol ethanol butanols and 1,2-ethanedi01.~ " R. Houriet H. Riifenacht P.-A. Carrupt P. Vogel and M. Tichy J. Am. Chem. Soc, 1983 105 3417. 62 C.Tea-Gokou J. P. Pradere J. Villieras and H. Quiniou Tetrahedron Lett. 1983 24 3713. 63 W. M. Schubert and Y. Motogama J. Am. Chem. SOC.,1965 87 5123 64 D. R. Burfield and R. H. Smithers J. Org. Chem. 48 2420.

ISSN:0069-3030    

DOI:10.1039/OC9838000067

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (iii) Free-radical Reactions By D. GRILLER Nationai Research Council of Canada Ottawa Ontario Canada KIA OR6 1 Synthesis A new method has been discovered for the reductive dehalogenation of aryl vinyl cyclopropyl and bridgehead iodides.' The approach involved photolysis of mixtures containing the halide di-t-butyl peroxide and lithium aluminium hydride in THF; yields (g.c. analysis) were normally ca. 90%. The chain propagation steps are shown in Scheme 1. When Ar was neophyl substantial amounts of product were derived from the rearranged form of the radical suggesting that the second step of the reaction was relatively slow (rate constant <lo3dm3 mol-' s-'). ArX + AlH, -+ Ar-+ AlH3X-Ar. + AIH; -ArH + AIH35 Scheme 1 Barton et al.devised a high-yield 'one pot' method for the decarboxylation of acids.2 The 0-ester of N-hydroxypyridine-2-thionewas prepared and was then reduced by tri-n-butyltin hydride or t-butyl mercaptan giving >70% yield of isolated product (Scheme 2). However when R was tertiary the mechanism involved forma- tion of the corresponding pyridyl sulphide which was then reduced by the tin hydride to the desired product (Scheme 2). Several reports have dealt with the use of radical cyclization reactions in chemical synthesi~.~~ In the most intriguing of these the cyclized radical was scavenged by t-butyl isocyanide. Subsequent elimination of t-butyl left the chemically versatile cyano-group neatly incorporated in the product (Scheme 3). Photolysis of hexaphenylditin was used to generate the stannyl radicals needed for the initial bromine abstraction.Tin hydrides could not be used since they proved to be far more effective as radical scavengers than the isocyanide. ' A. L. J. Beckwith and S. H. Goh 1.Chem. SOC.,Chem. Commun. 1983 907. ' D. H. R. Barton D. Crich and W. B. Motherwell J. Chem. SOC.,Chem. Commun. 1983 939. E. J. Corey G. Schmidt and K. Shimoji Tetrahedron Lett. 1983 24 3169. G. Stork and R. Mook jun. J. Am. Chem. SOC.,1983 105 3720. G. Stork R. Mook jun. S. A. Biller and S. D. Rychnovsky J. Am. Chem. SOC.,1983 105 3741. G. Stork and P. M. Sher J. Am. Chem. SOC.,1983 105 6765. 87 D. Griller S (1) + Bu",Sn. -+ R. + C02 + SSnBu R-+ Bu",SnH 3 RH + Bu",Sn* For R = tertiary alkyl SR Scheme 2 OEt c!AJBr Ph,Sn- -• OEt OEt Scheme 3 The synthesis of eight trans-alkyl hyponitrites RO-N=N-OR has been repor- ted.' They were generally prepared by the addition of silver hyponitrite to the corresponding alkyl iodide or bromide at 0°C in pentane as solvent.Most were white crystalline solids which could be stored for long periods without change below ' C. A. Ogle S. W. Martin M. P. Dziobak M. W. Urban and G. D. Mendenhall J. Org. Chem. 1983 48 3728. Reaction Mechanisms -Part (iii) Free-radical Reactions 0 "C. The hyponitrites decompdsed at 66 "C in iso-octane solvent with first-order kinetics and with half-lives (min) ranging from 32.3 (R = Me) to 5.5 (R = 1-phenylethyl). They should prove to be useful low-temperature thermal initiators.2 Mechanism 1983's most fascinating and controversial papers on mechanism dealt with the possibility that radicals can be generated thermally in two distinct electronic states. Skell and May investigated the reactions of acyloxyl radicals at -78 "C in mixtures containing methylene chloride and neopentane.' They found that in addition to the normal Hunsdiecker reaction a small amount (ca. 10%) of hydrogen abstraction was also taking place. The abstracting agent was taken to be the acyloxyl radical on the basis of the relative reactivities displayed by the two substrates which were reported to be quite different to those expected if alkyl radicals or halogen atoms had been the abstracting agents. However product yields for the carboxylic acids were thought to be unreliable and were not investigated in detail (Scheme 4).Hunsdiecker RCO,. -+ R-+ C02 Re + RC0,Br -+ RBr + RCO,. Abstraction RC02. + R'H + RCO2H + R'. R'. + RC0,Br --* R'Br + RC02* Scheme 4 The experiments were carried out first in the presence of bromine and then in the presence of vinylidene chloride which was used as a bromine atom scavenger. The relative reactivities of the substrates differed by a factor of ca. 2 under these conditions and it was therefore proposed that the acyloxyl radical could be generated in two electronic states (Scheme 5). With bromine present R'. + Br -+ R'Br + Br-Br. + RC0,Br -+ Br + RCO,. 'lT' With a bromine scavenger R'. + RCOzBr -+ R'Br + RC024 'IT' Scheme 5 Numerous control experiments were carried out in an attempt to show that the presence of other chain carriers was not responsible for the effect.Moreover when 1-bromobutane isobutane and n-butane were used as substrates the selectivities P. S. Skell and D. D. May J. Am. Chem. SOC.,1983 105 3999. 90 D. Griller for the proposed u and T acyloxyls continued to show small but significant differen- ces which overall were quite different to those for bromine atoms and alkyl radicals. Almost identical experiments and arguments were advanced to support the inter- mediacy of (T and T succinimidyl radicals.' These have now been challenged and two groups have argued against the concept.lO," One group accounts for the changing patterns of selectivity in terms of a bromine atom chain a succinimidyl chain or a mixture of the two depending upon the experimental conditions." The second favours as possible chain carriers the bromine atom the succinimidyl radical and a bromine atom-N-bromosuccinimide complex.' I However it is already clear that some of the experimental evidence involved in one of the challenges'' is suspect.' However the literature contains an additional counter argumentI2 to the u,T concept which has yet to be addressed by its proponents.Baban Brand and Roberts have shown that borane radical anions can be gener- ated by photo-induced electron transfer from hydroborate anions.I3 The reaction was only successful when the light was absorbed by the charge-transfer-to-solvent band of the anion in solution.Thus photolysis of Bu",NBH in liquid ammonia-1,2- dimethoxyethane (A < 290 nm) gave the e.s.r. spectra of H3BT and the solvated electron which had lifetimes of ca. 108 and 134 ms at -23 "C.When t-butyl chloride was added to the reaction mixture these spectra were replaced by that of t-butyl (Scheme 6). A typographically corrected paper on borane radical anions was also p~b1ished.l~ H,B.&, + Bu'Cl -+ Bu'. + H3BCl-e& + Bu'Cl -+ But. + C1-Scheme 6 Product studies on the fate of conformationally biased cyclohexyl radicals such as 4-t-butyl- 4-t-butyl-cis,czs-2,6-dimethyl- and 4-t-butyl-cis,trans-2,6-dimethyl-cyclohexyl showed that loss of hydrogen to form olefin was favoured when the C-H bond involved lay close to the plane of the adjacent semi-occupied 0rbita1.l~ Quinga and MendenhallI6 investigated chemiluminescence intensities during the thermolysis of alkyl hyponitrites containing a-hydrogens.The data were consistent with the mechanism of Scheme 7 where substantial recombination between triplet RON=NOR 4 [RO. t + N + RO. 41 -+ ROH + R'R~CO (Os) 11 fast [RO. t + N2 + RO. t ] -+ ROH + R'R2C0 (3T) Scheme 7 ' P. S. Skell R. L. Tlumak and S. Seshadri J. Am Chem. Soc. 1983 105 5125 and references cited therein. 10 D. D. Tanner T. C.3. Ruo H. Takiguchi A. Guillaume D. W. Reed B. P. Setiloane S. L. Tan and C. P. Meintzer J. Org. Chem. 1983 48 2743. I' C. Walling G. M. El-Taliawi and C. Zhao J. Am. Chem. Soc. 1983 105 51 19. '' A. G. Davies B.P. Roberts and J. M. Smith J. Chem. Soc. Perkin Trans. 2 1973 2221. I3 J. A. Baban J. C. Brand and B. P. Roberts J. Chem. Soc. Chem. Commun. 1983 315. l4 J. R. M. Giles and B. P. Roberts J. Chem. Soc. Perkin Trans. 2 1983 743. l5 A. L. J. Beckwith and C. J. Easton J. Chem. Soc. Perkin Trans. 2 1983 661. '' E. M. Y. Quinga and G. D. Mendenhall J. Am. Chem. Soc. 1983 105 6520. Reaction Mechanisms -Part ( iii) Free-radical Reactions radical pairs took place within the solvent cage. The yield of the triplet pathway was proportional to the exothermicities of the reactions calculated for the ground-state products and not to those of the triplet products. It was estimated that the quantum yield would be as high as 0.5 if the exothermicity of the reaction were ca.385 kJ mol-’. This is probably the first unequivocal demonstration that an appropri- ately chosen triplet radical pair can undergo a disproportionation reaction. The selectivity of chlorine atoms in their reaction with 2,3-dimethylbutane was investigated in benzene-trichlorofluoromethane mixture^.'^ Previous investigations showed that benzene moderated the reactivity of the chlorine atom and a .n-complex had been proposed to rationalize the results. In the more recent work,” it was shown that the selectivity depended upon the concentration of 2,3-dimethylbutane. This should not have been the case if the wcomplex were in rapid equilibrium with free chlorine atoms and benzene and the presence of another intermediate namely a cyclohexadienyl radical was invoked (Scheme 8).However it would seem that the data could be consistent with the irreversible formation of a single intermediate be it cyclohexadienyl or .n-complex. c1 CIS + R* + HCI +C,H Scheme 8 Details of a I3C CIDNP study of arylethyl radicals have been reported.” In the case of the 2-phenylethyl radical no polarization transfer could be found that was consistent with its rearrangement to the corresponding spirocycloalkylcyc- lohexadienyl radical (Scheme 9). However if evidence for the rearrangement is to be obtained from polarization transfer then ring closure would have had to take place before relaxation of the nuclear spin states of the radical (ca. s). A kinetic e.s.r. study” indicates that the rate constant for the rearrangement to the spirocyc- loalkylcyclohexadienyl radical must be several orders of magnitude lower than this and hence the absence of polarization was entirely to be expected.Scheme 9 ” P. S. Skell H. N. Baxter 111 and C. K. Taylor J. Am. Chem. SOC.,1983 105 120. Is G. A. Olah V. V. Krishnamurthy B. P. Singh and P. S. Iyer J. Org. Chem. 1983 48 955 5414. A. Effio D. Griller K. U. Ingold J. C. Scaiano and S. J. Sheng J. Am. Chem. SOC.,1980 102 6063. D. Griller Whereas vicinal 1,2-shifts are actually quite common for carbon-centred radicals they are scarcely known for silicon-centred radicals. Wilt and Keller found that the vicinal migration of an acetoxy-group can take place for a silicon-centred radical.20 Thus photolysis of a mixture containing di-t-butyl peroxide and (acetoxy-methy1)dimethylsilane in hexadecane solvent gave radicals (2) and (3) which were trapped by carbon tetrachloride (Scheme 10).It is clear from the experimental conditions and product yields that the rearrangement in this instance must be far more rapid than that for the carbon analogue which presumably reflects the thermochemistry involved. (2)/(3) + CCl -+ (2)C1/(3)C1 + CCI Scheme 10 Sonication of argon-saturated aqueous solutions was found to give .OH and H. which appeared to have been formed in the cavitation bubbles2’ Evidence for the formation of these species was obtained by using a variety of spin traps. The evidence was reinforced with careful control experiments. For example it was shown that the concentrations of the spin-adducts grew from the start of sonication to steady- state levels and that either or .OH could be diverted by reaction with specific Ha scavengers.Obviously the technique could develop into a very useful method for initiating radical processes in water. 1-Adamantyl and 1-bicyclo[2.2.2]octyl radicals were generated by photolysis of the corresponding azo-compounds in various hydrocarbon solvents.22 Remarkably high isotope effects (kH/kD = 25; kH/k = 104) were found for hydrogen abstrac- tion at cyclohexane by 1-adamantyl. The result was rationalized in terms of a contribution from quantum mechanical tunnelling brought about because of a ‘thin’ activation barrier for the reaction. This was thought to have arisen because very little molecular motion was required to convert the I-adamantyl radical into its parent hydrocarbon.Cyclohexyl and ally1 radicals were detected by e.s.r. spectroscopy when cyclo- hexane and 2,5-dimethylhex-3-ene were deposited respectively on a clean surface of sodium chloride at -196 0C.23 The precise details of the reaction mechanism were unknown but the very high electrostatic fields on the crystal surfaces were thought to play a critical role. In the experiment layers derived from hot sodium chloride vapour and from the organic substrate were deposited alternately on a cold rotating 20 J. W. Wilt and S. M. Keller J. Am. Chem. Soc. 1983 105 1395. 21 K. Makino M. M. Mossoba and P. Riesz J. fhys. Chem. 1983 87 1369. 22 P. S. Engel W.-K. Chae S.A. Baughman G. E. Marschke E. S. Lewis J. W. Timberlake. and A. E. Luedtke J. Am. Chem. Soc. 1983 105 5030. 23 H. Dahrnane B. Mile H. Morris J. A. Howard and R. Sutcliffe J. Chem. Soc. Chem. Cornmun. 1983 1068. Reaction Mechanisms -Part ( iii) Free-radical Reactions drum and attempts were made to eliminate the obvious possibility that simple pyrolysis was responsible for radical formation. Bilayers of phosphatidylcholine induced differences in the rates of thermal decomposition of the diastereomers of (4).24 The meso-form decomposed six times faster than the (*)-diastereomer in aqueous emulsions of the multilamellar vesicles. Thus the bilayer host had a significant effect on the orientations of the diastereomeric compounds and/or their transition states for decomposition.It was concluded that such bilayers might be used to induce diastereomeric selectivity in synthesis. N It N Niki et al. showed that vitamins C and E could each inhibit the oxidation of methyl linoleate in t-butyl alcohol-methanol solvent.25 The reduction in the oxidation rate during the inhibition period was greater for vitamin E than C presumably because the former acted as a more effective peroxy radical scavenger. However when the two were used in combination the reduction in the rate was the same as that observed for E alone but the length of the induction period reflected the total vitamin concentration. These observations led to the conclusion that vitamin C was regenerating E during the induction period until the two were completely consumed.Similar conclusions were reached by Barclay et a1.,26who carried out a detailed kinetic study of the initiated oxidation of methyl linoleate localized in sodium dodecylsulphate micelles a system which represents a primitive model for biological membrane oxidation. Vitamin E located within the micelles functioned as an effective inhibitor. Again there was a synergistic effect when vitamin C was added even though the latter was localized in the aqueous phase. It was argued that the phenolic function of vitamin E could easily approach the water-impregnated periphery of the micelle where reduction by vitamin C could take place. Reaction of methylthiyl radicals with Z-methyl-4,5-dihydrofurantook place by abstraction of the allylic hydrogen or by addition to the double bond abstraction being favoured at higher temperature^.^' Whereas the radical derived by abstraction could be detected by e.s.r.that derived by addition could not. Instead a rearranged form of the radical was observed which was thought to have resulted from a 1,4 hydrogen migration (Scheme 11). 24 R. C. Petter J. C. Mitchell W. J. Brittain T. J. Mclntosh and N. A. Porter J. Am. Chem. SOC.,1983 105 5700. 25 E. Niki T. Saito and Y. Kamiya Chem. Lett. 1983 631. 26 L. R. C. Barclay S. J. Locke and J. M. MacNeil Can. J. Chem. 1983 61 1288. 27 L. Lunazzi G. Placucci and L. Grossi Tetrahedron 1983 39 159. D. Griller +Q Me Scheme 11 3 Structure There has been a good deal of interest in the structure of boron-containing radicals.The triborane(7) radical anion was detected by e.s.r. spectroscopy when mixtures of di-t-butyl peroxide and Bun4NB3H in a variety of solvents were photolysed in the cavity of the spectrometer.28 The hyperfine splittings were a(l ''B) 7.89 a(2l'B) 2.91 a( 1H) 10.79 a(2H) 11.60 and a(4H) 2.61 G at -45 "C and were consistent with structure (5). This anion was found to be very much less reactive than H3BT as was expected for a delocalized species of higher ionizatibn potential.28 .~~ Symons et ~1 obtained the powder spectrum due to .BH4 on y-irradiation of sodium borohydride or b~rodeuteride.~~ The hydrogen atoms were found to be equivalent in pairs. MNDO-UHF calculations were consistent with these findings and gave the optimized geometry (6).H H \B<,' 58.3"\-/ /-%. B%)I 14.50 H 122.0'H Reaction of silver and gold atoms with acetylene and phenylacetylene at 77 K gave e.s.r. spectra due to metal substituted vinyl and styryl radicals re~pectively.~' The hyperfine splittings were not dissimilar to those observed for the hydrocarbon analogues and the atoms were therefore thought to be a-bonded to carbon. However copper formed mono- and bis-.rr-complexes with acetylene but was a-bonded to phenylacetylene. The e.s.r. spectrum of the tetrafluoroethylene radical cation was obtained by y-irradiation of a solid solution of FCCI containing tetrafluoroethylene and was found to have four equivalent fluorine at~rns.~' The result suggested that the cation had a planar structure which was consistent with INDO calculations.An additional small fluorine hyperfine splitting was thought to be due to a matrix fluorine atom. 28 J. R. M. Giles V. P. J. Marti and B. P. Roberts J. Cbem. SOC.,Cbem. Commun. 1983 696. 29 M. C. R. Symons T. Chen and C. Glidewell J. Cbem. SOC.,Chem. Commun. 1983 326. 30 J. H. B. Chenier J. A. Howard B. Mile and R. Sutcliffe J. Am. Chem. SOC.,1983 105 788. 3' A. Hasegawa and M. C. R.Symorts J. Cbem. SOC.,Faraday Trans. I 1983 79 93. Reaction Mechanisms -Part (iii) Free-radical Reactions 95 By contrast calculation showed that the tetrafluoroethylene radical anion had a chair structure.32 Careful analysis of the e.s.r. spectra of this radical which again showed four equivalent fluorine atoms at -170 "C,was now found to be consistent with the chair structure on the assumption that the radical can rotate about an axis perpendicular to the plane containing the four fluorine atoms.Readers are advised to study these papers in detail since the arguments are more compelling than this summary indicates. N,N'-Bis(ary1thio)benzamidinyl radicals (7) were found to be a new class of persistent nitrogen-centred radicals.33 They were generated by oxidation of the corresponding benzamidines using lead dioxide and had half-lives of hours even in the presence of oxygen. Their persistence was thought to be associated with their delocalized electronic structures rather than with steric protection of the radical centre. N-Alkyl-N-(a1kylthio)aminylradicals were not persistent except for the case when the alkyl was t-b~tyl.~~ However even in this instance reaction with oxygen was rapid suggesting that the longevity of the radical under oxygen-free conditions was due to steric protection of the radical centre.A number of copper( 11) dithiolate complexes were characterized by e.s.r. spec- troscopy and may be of particular biochemical imp~rtance.~' For example d-penicillamine is effective in treating physiological disorders associated with excessive copper levels and was indeed found to produce a planar complex (8) with a half-life of several minutes. The e.s.r. spectra of the trans and cis radical ions (9) and (10) were obtained by photolysis of a-complexes formed between dimerized t-butylmethylacetylene and aluminium chloride.36 The radicals had slightly different hyperfine splittings a(2Me) 9.00G a(2Bu') 0.20G (9) and a(2Me) 8.00G a(2Bu') 0.24G (lo) at -80°C in methylene chloride.It was possible to interpret these differences in terms of breaking the degeneracy of molecular orbitals in the trans-isomer by the more powerful electron release of the t-butyl groups. However it was concluded that such arguments were unreliable because of the likelihood that steric perturbations could have been the origin of these effects. HU' Bu' Bu' (9) 32 A. Hasegawa and M. C. R. Symons J. Chem. SOC.,Faraday Trans. I 1983 79 1565. 33 Y. Miura T. Kunishi and M. Kinoshita Chem. Lett. 1983 885. 34 Y. Miura H. Asada M. Kinoshita and K.Ohta J. Phys. Chem. 1983 87 3450. 35 F. J. Davis B. C. Gilbert R. 0.C. Norman and M. C. R. Symons J. Chem. SOC.,Perkin Trans. 2 1983 1763. 36 J. L. Courtneidge A. G. Davies and J. Lusztyk,J. Chem. Soc. Chem. Commun. 1983 893. 96 D. Griller Photolysis of pentamethylcyclopentadienes with 13C in the methyl groups or in the ring gave the corresponding pentamethylcyclopentadienyl radical^.^' The 13C hyperfine splittings in hexane solvent at -25 "C were a(I3C,j 2.68 G and a(13Cp) 3.35 G demonstrating that they are .rr-delocalized radicals which can be treated by rr-electron theory. 4 Kinetics Rate constants for the reactions of radicals with oxygen were measured in solution using a flash photolysis technique in which the lifetimes of the radicals were monitored as a function of the oxygen c~ncentration.~~ For hydrocarbon radicals the reactions were approximately diffusion-limited bearing in mind that only one- third of the encounters between the doublet radicals and triplet oxygen should lead to doublet products.The rate constants k at 25 "C were therefore relatively insensi- tive to structure e.g. k(t-butyl) = 4.9 x lo9 (in cyclohexanej and k(cyclohexadieny1) = 1.64 x lo9 dm3 mol-' s-' (in benzene). The rate constant for tri-n-butylstannyl radicals was unusually high (7.5 x lo9 dm3 mol-' s-lj a fact that was attributed to relaxation of the spin selection criteria as a result of a heavy-atom effect. Interestingly diphenylaminyl did not react with oxygen in the time-scale of these experiments and the rate constant for its reaction was therefore less than lo7dm3 mol-' s-I.Phenyl radical kinetics were also investigated by flash photolysis technique^.^^ Representative rate constants for its reactions are chlorobenzene I .2 x lo6 carbon tetrachloride 7.8 x lo6,and methyl methacrylate 1.8 x lo8dm3 mol-' s-I. Since the reaction products were not investigated these rate constants represent global rate constants for all possible modes and sites of attack at the substrates. No evidence could be found for the participation of benzoyloxyl radicals when phenyl was generated by photolysis of benzoyl peroxide and the lifetime of the former was concluded to be less than lop8s. This contrasts with the trapping of benzoyloxyl in thermally initiated reactions of benzoyl peroxide and could be taken as support for the existence of two electronic states of the radical (see above).Two groups found similar results for the decarbonylation of the phenylacetyl radical and obtained A factors of ca. 10' 13 s-' and activation energies of 26 kJ mol-' which were little affected by ~olvent.~~~~~ Similarly the @-scission of cumyloxy was described by an A factor of 10'2.4 s-l and an activation energy of 36 kJ mol-' by combination of data obtained in chlorobenzene and cumene as solvents.42 Tri-n-bytylgermanium hydride was found to react with primary alkyl radicals about 20 times more slowly than the corresponding tin hydride at 25"C.43 This property may make it a useful alternative to tin hydride for competitive scavenging of alkyl radicals formed in slow rearrangement reactions.37 A. G. Davies E. Lusztyk J. Lusztyk V. P. J. Marti R. J. H. Clark and M. J. Stead J. Chem. SOC.,Perkin Trans. 2 1983 669. 38 B. Maillard K. U. Ingold and J. C. Scaiano J. Am. Chem. SOC.,1983 105 5095. 39 J. C. Scaiano and L. C. Stewart J. Am. Chem. SOC.,1983 105 3609. 40 L. Lunazzi K. U. Ingold and J. C. Scaiano J. Phys. Chem. 1983 87 529. 41 N. J. Turro 1. R.Gould and B. H. Baretz J. Phys. Chem. 1983 87 531. 42 A. BaignCe J. A. Howard J. C. Scaiano and L. C. Stewart J. Am. Chem. SOC.,1983 105 6120. 43 J. Lusztyk B. Maillard D. A. Lindsay and K. U. Ingold J. Am. Chem. SOC.,1983 105 3578. Reaction Mechanisms -Part ( iii) Free-radical Reactions 97 Livingston and Zeldes investigated the pyrolysis of benzyl ether using a high-pressure-high-temperature e.s.r.cell.44They found the reaction described in Scheme 12 to be an essential feature of the mechanism and determined its A factor ( s-') and activation energy (65.1 kJ mol-'). The technique itself is a considerable innova-tion for e.s.r. spectroscopy and will hopefully be adopted by other investigators. PhCHOCH,Ph -+ PhCHO + CH2Ph Scheme 12 Rate constants were measured for the self-reactions of substituted ben~yl,~~ iso-propylol,46 and a series of 'capto-dative' stabilized radicals.47 Those for the benzyl radicals45were not perfectly described by the von Schmolochowski equation an effect which was ascribed to the unreliability of estimated reaction diameters for bulky radicals.As expected rate constants for isopropyl01~~ were much lower than predicted by theory in solvents capable of hydrogen bonding to the radical. Not surprisingly the capto-dative stabilized radicals ( 1 1)-( 13) all underwent self-reaction at the diffusion-controlled rate.47Obviously the stabilization is insufficient to weight the radical-dimer equilibrium in favour of the radical at ambient tem-peratures. Similarly the stabilized radicals cyanomethyl 2-cyano-2-propy1 syn-and anti-1-cyanoallyl radicals were not persistent and all underwent diff usion-controlled self-reaction?' BU~-O-CH-CN BU~-S-CH-CN MeO-CH-C0,Me (1 1) (12) (13) Burton et al. investigated the properties of a number of phenols e.g. (14)-( 17) which were structurally related to a-tocopherol (14) to see whether the latter had a fully optimized structure as a chain-breaking anti~xidant.~~ Of these (17) was substantially more reactive towards peroxyls than a-tocopherol itself.The results were found to be related to polar and conformational effects manifest in the e.s.r. spectra of the phenoxyl radicals derived from these inhibit01-s.~' Me Relative n R' R2 X reactivity 0 1.o Ho@yf~ (15) C,,H,, (14) 2 Me C(0)OMe 0 0.56 Me X (16) 2 H H NC(0)Me 0.04 I Me (17) 1 H Me 0 1.66 5 Thermochemistry Heats of formation and ionization potentials for a series of a-aminoalkyl radicals R' NCR2 were determined from measurements of appearance energies for the 44 R. Livingston and H.Zeldes J. Phys. Chem. 1983 87 1086. 45 R. F. C. Claridge and H. Fischer J. Phys. Chem. 1983 87 1960. 46 M. Lehni and H. Fischer Int. J. Chem. Kiner. 1983 15 733. 47 H.-G. Korth R. Sustmann R. Merinyi and H. G. Viehe J. Chem. SOC.,Perkin Trans. 2 1983 67. 48 H.-G. Korth P. Lommes W. Sicking and R. Sustmann Int. J. Chem. Kinet. 1983 15 267. 49 G. W. Burton L. Hughes and K. U. Ingold J. Am. Chem. SOC.,1983 105 5950. 50 T. Doba G. W. Burton and K. U. Ingold J. Am. Chem. SOC.,1983 105 6505. 98 D. Griller fragmentations of a series of ethylenediamines.' ' Stabilization energies increased and ionization potentials decreased with increasing C-or N-alkylation. For example when R' = Me R2 = H the methane-based stabilization energy was 84 kJ mol-' and the ionization potential was 5.7 eV as compared with 7-10 eV for most alkyl radicals.E.s.r. spectroscopy was used in several instances to determine thermochemical properties. Jenkins and Perkins used such measurements to determine the 0-H bond strengths in a series of N-t-butylhydroxamic acids with respect to that of (1 8) (Scheme 13).52For example those for (19) and (20) were found to be 334 and 3 15 kJ mol-' respectively. In general these bond strengths were greater than those for dialkyl- nitroxides and increased with electron demand in the acyl group. 0 0 Bu' ROX-N'''' \0. + (18)-H \OH + I tk (19) R = NO 0. (20) R = OMe (18) Scheme 13 Schlosser and Steenken found that (CF3S) N-N(SCF,) underwent reversible homolytic cleavage in perhalogeno-alkane solvents in the temperature range 250-315 K to give the corresponding aminyl radicals which were easily detected by e.~.r.~~ The bond dissociation energy was found to be only 32 f2 kJ mol-'.This facile homolysis was explained in terms of both steric and electronic factors. The rotational barriers for aminopropynyl and aminocyanomethyl were deter- mined from the temperature dependencies of their e.s.r. spectra which were in turn used to calculate their methane-based stabilization energies (107 and 95 kJ mol-' re~pectively).~~ These were equal within experimental error to the sum of the stabilization energies associated with each part of the radical e.g. E (aminocyanomethyl) = E (aminomethyl) + E (cyanomethyl). If the popular 'capto-dative effect' has any substance if should be revealed by tests of this kind.That is the stabilizing effect of the combination of substituents should be greater than the sum of the parts. However such experimental verifications have not yet been adequately demonstrated. A recent claim55 for an additional capto-dative stabilization of 16 kJ mol-' for CH(0Me)CN is almost certainly insignificant when compared with the experimental errors and assumptions built into the experiment although the approach represented a serious test of the capto-dative idea. While theory supports the capto-dative concept it is arguable that the present widespread use of the term lacks definitive experimental support. Two st~dies~~,~' dealt with bond dissociation energies in fluoroalkanes.A gas-phase bromine buffer system was used to determine D(i-C3F7-H) as 433.3 f 5' T. J. Burkey A. L. Castelhano D. Griller and F. P. Lossing J. Am. Chem. Soc. 1983 105 4701. 52 T. C. Jenkins and M. J. Perkins J. Chem. SOC.,Perkin Trans. 2 1983 717. 53 K. Schlosser and S. Steenken J. Am. Chem. Soc. 1983 105 1504. 54 D. Griller D. C. Nonhebel and J. C. Walton J. Chem. Soc. Perkin Trans. 2 1983 1373. 55 R. Louw and J. J. Bunk Recl. J. R. Nerh. Chem. Soc. 1983 102 119. 56 J. P. Martin and G. Paraskevopoulos Can. J. Chem. 1983 61 861. 57 B. S. Evans 1. Weeks and E. Whittle J. Chem. Soc. Faraday Trans. 1 1983 79 1471. Reaction Mechanisms -Part (iii) Free-radical Reactions 2.4 kJ mol-I. However (CF&C-H was found to be particularly unreactive towards bromine atom attack so that the technique could not be applied.Finally an elegant of the thermolysis of tetrasubstituted succinonitriles led to a ‘resonance energy’ of 21 kJ mol-’ for tertiary a-cyanoalkyl radicals when careful account was taken of the steric and electronic factors involved in the dissociation process. W. Barbe H.-D. Beckhaus and C. Ruchardt Chem. Ber. 1983 116 1042.

ISSN:0069-3030    

DOI:10.1039/OC9838000087

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

5 Arynes Carbenes Nitrenes and Related Species By M. S. BAlRD School of Chemistry University of Newcastle upon Tyne Newcastle upon Tyne NEl7RU 1 Arynes Calculations on 0-,rn- and p-benzynes using MNDO UMNDO and MNDO/CI methods agree well with earlier results. The 0-and rn-species are predicted to be similar in energy whereas the p-form should be considerably less stable. Both rn-and p-species are predicted to occur in two isomeric forms a bicyclohexatriene and a phenylene biradical with the latter of lower energy in each case. The bicyclic and biradical species should be separated by appreciable barriers (9 and 3 kcal mol-' respectively). The degenerate rearrangement of hex-3-ene- 1,5-diyne is found to involve the intermediacy of p-benzyne.' Benzynes are obtained on treatment of the readily available 6-chloropentafulvenes with strong base.Thus reaction of the di-t-butyl derivative (1) with lithium piperidide leads to I -piperidyl-3,5-di-t-butylbenzene, and the intermediate benzyne (2) may be trapped by [4 + 21 cycloaddition to benzofuran.2 I I (1) (2) (3) (4) The oxazoline (3; X = C1 Y = H) reacts with organolithium reagents to give selectively the benzyne (4). This was reported to react further by addition of the organolithium to produce (3;X = alkyl Y = Li). However it is now found that with n-butyl-lithium most products are derived from the alternative addition to produce (3; X = Li Y = Bu"). This is thought to be due to the kinetic effect of complexation of the organolithium to the oxazoline group.3 Reaction of the bromides (5; X = Br Y = H) and (5; X = H Y = Br) with the complex base t-butoxide ion-amide ion leads to the benzynes (6) and (7) respectively which are identified as their furan adducts.The regioselectivity of the former reaction is explained in terms of the lower bond length and angle deformations in the benzyne pr~duced.~ ' M. J. S. Dewar G. P. Ford and C. H. Reynolds J. Am. Chem. Soc. 1983 105 3162. ' K. Hafner H.-P. Krimmer and B. Stowasser Angew Chem. Int. Ed. EngL 1983 22 490. ' A. I. Meyers and P. D. Pansegrau Tetrahedron Letr. 1983 24 4935. B. Halton and C. J. Randall J. Am. Chem. Soc. 1983 105. 6310. 101 102 M. S. Baird Treatment of 1,2,4,5-tetrabromobenzenewith two equivalents of butyl-lithium in the presence of furan leads to the formation of (8),5 and 1,4-dilithio-tetrahalogenoarenes may be prepared from the corresponding hexahalides at low temperature and undergo elimination to give arynes at higher temperatures6 2 Carbenes STO-3G calculations show that triplet di-t-butylcarbene is ca.25 kcal mol- ' more stable than singlet. Because of steric repulsions between the alkyl groups the central CCC angles are 132.5' (singlet) and 142" (triplet). The normal alkyl-group stabiliz- ation of the singlet is overridden by destabilization due to sterically forced increase in this central angle. The value for the triplet is very close to the experimental figure.' Ab initio calculations on the lowest closed-shell singlet states of six isomers of C2H2S show that (9) (So),(10) (So),and (I 1) (S,) are the least stable species with stability decreasing in the order given.* Theoretical studies have also been reported CO,H (12) (13) on the ground triplet electronic state of cyanocarbene; these suggest a near degeneracy between the bent carbene and a linear allene-related geometry.' Calcula- tions relating to the reactions of atomic carbon with water and ammonia include an examination of hydroxycarbene and aminocarbene ;the barrier for rearrangement of the former species to methanal is found to be 38.9 and 41.3 kcal mol-' respectively for the singlet and triplet forms."." In practice the reaction of arc-generated carbon atoms with ammonia leads to NH-insertion and H-abstraction.The former leads to formaldimine and hydrogen cyanide and the latter to methylene which reacts with ammonia to produce methylamine.Hydrolysis of non-volatile residues leads to amino-acids." The reaction of atomic carbon with methanal has also provided evidence for the involvement of the first excited singlet state (I B,) of methylene. As predicted this appears to add to alkenes in a non-stereospecific manner.I3 H. Hart N. Raju M. A. Meador and D. L. Ward J. Org. Chem. 1983.48 4357. H. Hart and G. C. Nwokogu Tetrahedron Lett. 1983 24 5721. ' P. H. Mueller N. G. Rondan K. N. Houk J. E. Gano and M. S. Platz Terrahedron Lett. 1983,24,485. R. K. Gosavi and 0. P. Strausz Can. J. Chem. 1983 61 2596. K. S. Kim H. F. Schaefer L. Radom J. A. Pople and J. S. Binkley J. Am. Chem. SOC.,1983 105 4148.lo S. N. Ahmed M. L. McKee and P. B. Shevlin J. Am. Chem. SOC.,1983 105 3942. I' D. W. McPherson M. L. McKee and P. B. Shevlin J. Am. Chem. SOC.,1983 105 6493. l2 P. B. Shevlin D. W. McPherson and P. Melius J. Am. Chem. Soc, 1983 105 488. l3 S. N. Ahmed and P. B. Shevlin J. Am. Chem. Soc.. 1983 105 6488. Arynes Carbenes Nitrenes and Related Species Diazirines have been shown in recent years to be a very effective source of carbenes. It is now reported that exchange reactions of halogenodiazirines (12; X = C1 or Br) allow ready access to a variety of substituted derivatives e.g. (12; X = CN) with tetra-n-butylammonium cyanide and (1 2; X = N,) with tetra-n-butylammonium azide. The former is a source of cyanophenylcarbene which may be trapped by alkenes while thermolysis of the latter leads to benzonitrile either through azido- phenylcarbene or (12; X = N:).I4 The diazirine (13) is a highly photolabile carbene- generating label which is readily fixable to biochemical agent~.'~ Normally stable lithio-derivatives of diphenyl thioacetals decompose cleanly to carbenes when another negative charge is present nearby in the same molecule.The derived carbenes can be highly selective and their reactions depend on the nature of the second anionic site and its relationship relative to the carbene centre. Thus (14) is converted into (15) at 0°C via a 1,2-hydrogen shift and (16) leads to (17).16 Crown ether- catalysed dehydrobromination of o-(bromomethy1ene)adamantane with potassium t-butoxide provides a convenient and efficient source of adamantylidenecarbene which can be trapped by alkenes.In competition experiments the carbene is very similar to isopropylidenecarbene and it appears to be an unencumbered electrophilic singlet. Plots of their relative reactivities versus the HOMO levels of the alkenes both show non-linearity in keeping with considerable steric retardation." I (14) (15) (17) Interest in absolute rate measurements for carbene reactions has continued. Photolysis of a series of 3-aryl-3-chlorodiazirinesin 3-methylpentane matrices at 77 K led to arylchlorocarbenes which showed U.V. spectra similar to those observed when the carbenes were generated by laser flash photolysis in iso-octane at 300 K. When the laser excitation was carried out in the presence of a series of alkenes the decay of the transient absorption followed pseudo-first-order kinetics.Analysis of the kinetic data gave good Hammett correlations p = +I.4-1.6 with o+-constants of the aryl substituent ; electron-donating substituents in the carbene retard the additions. Moreover although each carbene shows the expected electrophilic selec- tivity the carbenes all show a comparable range of selectivities towards the series of alkenes.18 Absolute rates of reaction of fluoro- bromo- and chloro-phenyl carbenes again generated by laser flash photolysis indicate that with each alkene the reactivity order is Br > C1 > F; this is the reverse of the carbene stability order based on expected halogen lone-pair resonance stabilization of an adjacent singlet carbene p-orbital.In addition the selectivities of the carbenes towards a series of alkenes as measured by the rate spread in each case follows the inverse order i.e. F > CI > Br. This provides direct evidence of the inverse relationship between l4 D. P.Cox R. A. Moss and J. Terpinski J. Am. Chem. SOC.,1983 105 6513. Is M. Nassal Liebigs Ann. Chem. 1983 1510. l6 T. Cohen and L.-C. Yu J. Am. Chem. Suc. 1983 105 281 I. I' T. Sasaki S. Eguchi M. Tanida F. Nakata and T. Esaki 1. Org. Chem. 1983 48 1579. IR R. A. Moss L. A. Perez N. J. Turro 1. R. Could and N. P. Hacker Tetrahedron Lett. 1983. 24 685. 104 M. S. Baird absolute reactivity and selectivity such a 'normal' relationship has been assumed for some time.'' It is interesting to note that the reactions of halogenocarbynes with alkenes produce a very similar pattern of absolute rate versus selectivity.20 An analysis of the effect of a two-step mechanism for carbene addition involving a carbene-alkene complex on the relationship between absolute rates as determined by laser flash photolysis and relative rates as determined by product ratios has been provided.It is concluded that the widespread occurrence of valid correlations means that complex formation may not be a general phenomenon or that when it does occur complexation is either much faster or much slower than the product- forming step.2' The absolute rates of reaction of phenylchlorocarbene and p-anisylchlorocarbene with carboxylic acids have been found to be very high compared with rates of reaction with alkenes; rates are relatively insensitive to the acid strength suggesting an early transition state.The high reactivity must be associated with the HO-bond since ethyl acetate is not an effective quencher.22 In cycloaddition reactions of methyl-substituted alkenes with halogenocarbenes the steric repulsion between the methyl groups and the carbene substituents appears to be negligible. However when a series of alkenes (18)-(20) is examined each set of alkenes gives a different correlation for the relative reactivities of.dibromo- and dichloro-carbenes i.e. no common Skell-Moss linear correlation exists between the reactivities of the two carbenes towards alkenes if the number and bulkiness of the substituents is changed.23 R R R )=: L P= '* Me (18) (19) (20) (21) Phenoxychlorocarbene derived by thermolysis of phenoxychlorodiazirine acts as an ambiphile in addition reactions with styrene derivatives.This is consistent with frontier orbital predictions and with its ambiphilic behaviour towards sub- stituted ethenes; however it is in contrast to the reported nucleophilic behaviour of the carbene towards styrenes when it is generated from a,a-dichloroanisole and base under phase-transfer conditions. It is suggested that this latter process actually involves attack of the dichlorophenoxymethyl anion on the styrene rather than the ~arbene.~~ Addition of carbenes (2 1) to the unsymmetrical alkenes 2-methylpropene and 3,3-dimethylbut-1-ene leads in all cases to predominantly E -rather than 2-products with increased stereoselectivity for the second alkene and when R is changed from ethyl to t-butyl.The highest stereoselectivity observed was about 10 1 corresponding to an energy difference of ca. 1 kcal mol-' at -20 "C.Model studies of the vinylidene- ethylene reaction by MNDO and ab initio methods and of more complex examples using MNDO indicate that attack of the carbenes on 2-methylpropene leads to the '' D. P. Cox I. R. Could N. P. Hacker R. A. Moss and N.J. Turro Tetrahedron Lett. 1983 24 5313. 20 B. P. Ruzsicska A. Jodhan H. K. J. Choi 0. P. Strausz and T. N.Bell J. Am. Chem. Soc. 1983 105 2489. " M. S. Platz Tetrahedron Lett. 1983 24 4763. 22 D. Griller M. T. H. Liu C.R. Montgomery J. C. Scaiano and P. C. Wong J. Org. Chem. 1983 48 1359. 23 B. Giese W. Bung Lee and C. Stiehl Tetrahedron Lett. 1983 24 881. 24 R. A. Moss and L. A. Perez Tetrahedron Lett. 1983,24,2719;see W. Bruck and H. Durr Angew. Chem. Int. Ed. Engl. 1982 21 916. Arynes Carbenes Nitrenes and Related Species favoured transition state (22) whereas with 3,3-dimethylbut- 1-ene the transition state resembles the perpendicular form (23).25 An ab initio study of the reaction of the carbenoid (24) with ethylene leads to a transition state (25) in which the methylene group is in a plane nearly parallel to that of the alkene. The alignment allows the LUMO to interact in an electrophilic sense with the double-bond HOMO on one side and with the fluorine lone pair on the other.This transition state is quite similar to that for free halogenocarbene cycloadditions except that the carbene fragment is less strongly bound.26 Phenyl(tri- bromomethy1)mercury reacts with either fumaronitrile or styrene to give high yields of cyclopropanes but in both cases the reaction is not stereospecific even though a triplet reaction does not appear to be involved. Kinetic analysis indicates that two distinct intermediates are involved and that the loss of stereospecificity is directly proportional to the concentration of the mercury deri~ative.~’ A detailed analysis of ionization potentials electron affinities and .rr-orbital shapes of 2-substituted bicyclo[2.2. llheptadienes has allowed an interpretation of the reac- tivities and selectivities of these species in carbene cycloadditions.A 2-substituent not only affects the 2,3-.rr-bond but also influences the 5,6-bond by through-space interaction. Orbital energy changes and polarization induced by the substituents provide a rationale of variations in 1,2-and homo- 1,4-addition processes and confirm the electrophilic nature of both reactions.28 The main product from the reaction of dilithiopentalene with dichloromethane and methyl-lithium is (26). This is thought to arise by addition of chlorocarbene followed by loss of chloride ion to produce (27) which undergoes 1,4-addition to give (28). Reaction of (29) with methyl-lithium leads to (30) in moderate yield.” (29) (30) ’’ Y Apeloig M. Karni P. J. Stang and D.P. Fox J. Am. Chem. SOC.,1983 105 4781. 26 J. Mareda N. G. Rondan K. N. Houk T. Clark and P. von R. Schleyer J. Am. Chem. SOC.,1983 105 6997. 27 J. B. Lambert E. G. Larson and R. J. Bosch Tetrahedron Lett. 1983 24 3799. 28 K. N. Houk N. G. Rondan M. N. Paddon-Row C. W. Jefford P. T. Huy P. D. Barrow and K. D. Jordan J. Am. Chem. SOC.,1983 105 5563. 29 U. Burger and B. Bianco Helc. Chim. Acta. 1983 66 60. 106 M. S. Baird Intramolecular addition of the keto-carbenes derived from (3 1) and (32) occurs at the bonds indi~ated.~'?~' Related reactions have been used in syntheses of sarkomy- cin and of cyclopentanoid terpenic acids,32 and copper-catalysed addition of the carbene derived from (33) gave a bicyclo[4.1 .O]heptane which is a key intermediate in a synthesis of sirenin;'3 a rather more exotic 6,7-addition is reported in the case of the carbene (:4) when the product is (35).34 A range of esters (36) undergo rhodium-catalysed cyclization to cyclopentenes (37) with relatively high (ca.85 15) diastereoselection in favour of the isomer shown.Furthermore the major and minor diastereoisomers are separable chromatographi- cally providing a simple route to cyclopentane derivatives of high optical purity. The reaction is explained in terms of a transition state such as (38) with the transferred hydrogen occupying one position in a chair six-membered ring and assuming an insertion which proceeds with retention of c~nfiguration.~~ R (36) (37) (38) Singlet excitation of the oxirane (39) leads to the carbonyl ylide (40) together with two carbenes (41) and (42).The former rearranges to a cyclopropenc but the latter gives (43),the product of apparent carbene insertion into the C-C bond of (41).36 Photolysis of (44) in acetonitrile leads to cleavage of the oxirane to (45) and (46) as the main singlet processes. The former carbene rearranges to a cyclopropene but 30 T. Hudlicky D. B. Reddy S. V. Govindan T. Kulp B. Still and J. P. Sheth J. Org. Chem. 1983,48,3422. 3' R. J. Sundberg and T. Nishiguchi Tetrahedron Left. 1983 24 4773. 32 S. V. Govindan T. Hudlicky and F. J. Koszyk J. Org. Chem. 1983 48 3581; R. P. Short J.-M. Revol B. C. Ranu and T. Hudlicky ibid. p. 4453. 33 T. Mandai K. Hara M. Kawada and J. Nokarni Tetrahedron Lett.1983 24 1517. 34 J. A. Marshall J. C. Peterson and L. Lebioda J. Am. Chem. SOC.,1983 105 6515. 35 D. F. Taber and K. Rarnan J. Am. Chem. Soc. 1983 105 5935. 36 N. Bischofberger B. Frei and 0. Jeger Helu. Chim. Acta 1983 66 1638. 107 Arynes Carbenes Nitrenes and Related Species )= (43) OH (44) ) OH OH (45) the latter again inserts this time into the adjacent carbinol C-H bond to give the en01.~’ Treatment of dibromides (47; X = Br) with methyl-lithium leads to cyclopropyl- idenes which undergo efficient insertion into the 5,6-related C-H bonds of the R group; no insertion into 3,4-related bonds is observed possibly owing to co-ordina- tion of the intermediate lithio-bromide (47; X = Li) to the acetal oxygen^.^^ The carbene (48) generated by thermolysis of the !ithium salt of the corresponding tosyl hydrazone inserts exclusively into the C -H bond indicated.39 The photolysis of several a-diazo-amides in mixed solvent systems gave unusually large OH CH insertion selectivity ratios of 103-104 1 far higher than those observed for photolysis of diazoacetate esters.Photolysis of the amides in t-butyl alcohol or 2,3-dimethylbutane led to OH-and tertiary-CH insertion products respec- tively and no primary CH-insertion product was isolated in either case indicating a very discriminating intermediate. The effect of the carboxamide group is thought to be electronic in origin and is consistent with an increased electrophilic character of the intermediates in respect of OH-insertion and more discriminative behaviour in respect of CH-in~ertion.~’ 37 G.de Weck N. Nakamura K. Tsutsumi H. R. Wolf B. Frei and 0.Jeger Helu. Chim. Acta 1983 66 2236. 3n J. Arct L. Skattebbl and Y. Stenstrom Acta Chem. Scand. (B),1983 37 681. 39 C. A. Andruskiewicz and R. K. Murray J. Org. Chem. 1983 48 1926. 40 J. Wydila and E. R. Thornton Tetrahedron Lett. 1983 24 233. 108 M. S. Baird Lithium alkoxides of alkyl allyl and benzyl alcohols react with chloroform in the presence of lithium t-butoxide to give dichloromethylcarbinols by insertion of dichlorocarbene into the a-C-H bond of the alkoxide (Scheme I). X / R’ :C \ R‘ X Scheme 1 Potassium alkoxides react in an analogous manner with benzal chloride and potassium t-butoxide by insertion of phenylchlorocarbene into the a-C-H bond followed by cyclization of the resulting 2-chloro-2-phenethyl alkoxide to the stereoisomeric oxiranes (Scheme 2).41 Dimethylvinylidenecarbene also inserts regioselectively into the a -C-H bond of alkoxides giving allenylcarbinols e.g.(49) in moderate yield.42 Scheme 2 Rhodium-catalysed addition of diazo-compounds to (50) leads to ring expansion to (51) in high yield apparently through the ylide (52).43 Similarly (53) reacts with diphenyldiazomethane to produce a 1 :2 adduct (54) apparently via another S-vlide.44 Treatment of (55) with chloroform and base under phase-transfer conditions gave (56)in addition to the product of apparent 1,2-dichlorocarbene addition to the double bond.The first product is analogous to one earlier reported between (55) and diphenylpropenylidene and various routes to it are possible including 1,Ccarbene addition or intermediate formation of the carbonyl ylide (57).45Thermal decomposi- tion of several phenyltrihalogenomethylmercury compounds in the presence of 41 T. Harada E. Akiba and A. Oku J. Am. Chem. Soc. 1983 105 2771. 42 T. Harada Y. Nozaki and A. Oku Tetrahedron Lett. 1983 24 5665. 43 W. D. Crow I. Gosney and R. A. Ormiston J. Chem. SOC.,Chem. Commun. 1983 643. 44 D. M. McKinnon Can. J. Chem. 1983 61 1161. 45 H. Hart and J. W. Raggon Tetrahedron Lerr. 1983 24 4891. Arynes Carbenes Nitrenes and Related Species substituted benzaldehydes and dimethyl acetylenedicarboxylate gave 2-halogeno-5- arylfuran-3,4-dicarboxylatesby selective attack of the resulting carbene on the aldehyde followed by trapping of the carbonyl ylide by the acetylene and elimination of HCl.46 Thermolysis of phenyl(bromodichloromethy1)mercury in the presence of benzophenone leads to Ph,CClCOCl as the only major product.An intermediate carbonyl ylide could not be trapped by addition of dimethyl acetylenedicarboxylate. It is suggested that the difference in behaviour between benzophenone and benzal- dehyde is due to the adoption of a different geometry in the ylide derived from benzophenone because of endqendo-interaction of chlorine and the aromatic rings (58) leading to rapid closure to (59) which rearranges to the observed produ~t.~’ PhAvA;h phAQA;h 0 c1 c1 (55) (56) I Ph 0 c1 %% Ph CI (57) (58) (59) The formation of carbon monoxide from the reaction of dibromocarbene with various aldehydes is extremely general contrary to earlier reports ;when carried out on benzaldehyde labelled at the aldehyde carbon the label is retained indicating that the reaction involves deoxygenation rather than decarbonylation.Variations in the yield of carbon monoxide with the structure of aliphatic aldehydes coupled with earlier results lead to the mechanism shown in Scheme 3 in which steric and conformational effects on the rate of step b play an important part in determining the yield. For aromatic compounds there is a linear correlation between carbon monoxide production and increasing electron release by the substituent,.indicating a dominance of electronic factors.48 alkenes lb \ Br-0-C-Br C+ -co Br t \c’ ‘ ‘Br \C/ ‘Br /\Br Scheme 3 46 H. S. Gill and J. A. Landgrebe J. Org. Chem. 1983 48 1051. 47 C. W. Martin H. S. Gill and J. A. Landgrebe J. Org. Chem. 1983 48 1898. 48 Z. Huan J. A. Landgrebe and K. Peterson Tetrahedron Lett. 1983 24 2829; J. Org. Chem. 1983 48 4519. 110 M. S. Baird Thermolysis of 2-methoxy-A3- 1,3,4-0xadiazoline involves loss of nitrogen to form a carbonyl ylide which mainly fragments to a carbonyl compound and a carbene the latter of which can be trapped by alkene~.~~ Thermolysis of (60) also leads to a carbonyl ylidt which can be trapped by cycloaddition to acetone but also undergoes fragmentation either to methyl acetate and 1 -methylethylidene or to acetone and 1-methoxyethylidene.Both carbenes react with acetone to generate carbonyl ylides and also insert into C-H(D) bonds of the ketone. The carbonyl ylides themselves react with acetone by 1,3-~ycloaddition.~~ Reaction of cyclopentadienylidene with oxygen in a low-temperature matrix leads to an intermediate which has been formulated as the carbonyl oxide (61) on the basis of i.r. data.5' Laser photolysis of 1-naphthyldiazomethane in acetonitrile has been postulated to produce the nitrile ylide (62). Photolysis in acetonitrile gives the same absorption spectrum and the transient species in each case reacts with acrylonitrile at the same rate; the products are (63; X = CN Y = H) and (63; X = H Y = CN).Photolysis of the diazirine (64) in acetonitrile-acrylonitrile leads to both carbene- and nitrile- ylide-derived products.52 eo/o ... MkoMe NO h+Me Me N=N NP H \Me Y // (62) Np = I-naphthyl (63) (64) It is widely believed that singlet arylcarbenes insert into 0-H bonds and add stereospecifically to alkenes but that the triplet ground states are efficient hydrogen- abstracting agents and add non-stereospecifically to alkenes. Rapid quenching of optical absorption spectra due to triplet carbenes by methanol has been explained in terms of a thermal equilibrium between triplet and singlet states; however it is also possible that the triplet reacts directly with methanol. It is now reported that the triplet state of dimesitylcarbene does not convert into the singlet state and that the two carbenes do indeed show quite different chemistries.The triplet species generated from the corresponding diazo-compound does not react with its precursor to give an azine but instead undergoes dimerization; the lifetime of the carbene is essentially unaffected by the addition of methanol although the quantum yield of the triplet is reduced owing to scavenging of the singlet carbene precursor.53 Photoly- 49 M. Bekhazi and J. Warkentin Can. J. Chern. 1983 61 619. 50 M. Bekhazi and J. Warkentin J. Am. Chem. SOC.,1983 105 1289. '' G. A. Bell and I. R. Dunkin J. Chem. SOC.,Chem. Commun. 1983 1213. 52 R. L. Barcus B. B. Wright M. S. Platz and J.C. Scaiano Tetrahedron Left. 1983 24 3955. 53 A. S. Nazran and D. Griller J. Chem. Soc. Chem. Cornrnun. 1983 850. Arynes Carbenes Nitrenes and Related Species sis of dimesityldiazomethane in a glass leads to a strong e.p.r. signal due to dimesitylcarbene. The signal changes on annealing the glass and it is suggested that the rigid environment prevents the carbene from adopting its preferred near-linear minimum-energy ge~metry.'~ Photolysis of diphenyldiazomethane in (RS)-butan-2-01 between ambient tem- perature and -1 15 "C gives (65) as the only volatile product. Photolysis in polycrystal- line (S)-butan-2-01 at -196 "C also gives the ether together with products of carbene insertion into each non-equivalent C-H bond of the alcohol. Insertion into the tertiary C-H bond gives a single enantiomer.At -196 "C an intense and relatively long lived e.s.r. signal is seen for triplet diphenylcarbene. It is felt to be unlikely that all the tertiary alcohol is derived from the singlet carbene and a substantial amount must be formed from the triplet uia the radical pair (66). Although each component is achiral the radical pair is chiral and the solid environment allows coupling in only one sense.55 Photoacoustic calorimetry has been used to determine the heat of reaction for formation of triplet diphenylmethylene from diphenyldiazomethane; the value obtained -12 f2 kcal mol-' is in contrast to the endothermic heat of reaction of ca. 33 kcal mol-' for triplet methylene from diazomethane. The heat of reaction for photolysis of diphenyldiazomethane in ethanol which leads to (67) is -54 f 2 kcal mol-I leading to a heat of reaction of -47 * 2 kcal mol-' for formation of the ether from singlet diphenyl~nethylene.~~ Photolysis of 2-biphenyldiazomethane at 77 K leads to a single set of triplet carbene resonances in the e.s.r.spectrum assigned to an unresolved superposition of signals due to syn- and anti-rotamers of 2-biphenylmethylene. Kinetic studies indicate that the triplet carbene decays faster in ether than in inert matrices and an abstraction-recombination mechanism is predicted to be the major decay route in an ether glass.57 Photolysis of diazoindene generates indenylidene (68) which reacts with simple alkenes by addition or by insertion into an allylic C-H bond.The stereochemistry of the addition has been investigated as a function of dilution of the alkenes with octafluorocyclobutane or 2,3-dimethylbuta- 1,3-diene. In the presence of 97 mol% of the fluoro-compound ca. 47% of the adduct obtained with cis-butene was derived from triplet indenylidene but in the presence of 25 mol% of the diene the triplet reactions were largely eliminated.58 Irradiation of 9-diazofluorene with a pulsed laser on a picosecond or nanosecond time-scale either at ambient temperature or at 10 K in a glassy matrix leads to fluorenylidene which exists as a rapidly equilibrating singlet-triplet mixture. Analysis of kinetic data indicates that the singlet 54 A. S. Nazran E. J. Gabe Y. Le Page D. J. Northcott J. M.Park and D. Griller J. Am. Chem. Soc. 1983 105 2912. 55 J. Zayas and M. S. Platz Tetrahedron Lett. 1983 24 3689. Sh J. D. Simon and K. S. Peters J. Am. Chem. Soc. 1983 105 5156. 5' E. C. Palik and M. S. Platz J. Org. Chem. 1983 48 963. 58 R. A. Moss and C. M. Young J. Am. Chem. SOC.,1983 105 5859. 112 M. S. Baird is only 1.1 kcal mol-' above the triplet ground state and rates of reaction with penta- 1,3-diene suggest that this small energy gap may be due to increased energy of the triplet caused by angle strain.59 N2 PhKCH-Ph I Me Photolysis of (69) at low temperature leads to an e.s.r. signal due to the triplet ground state of the corresponding carbene. The only kinetically important process at low temperature in a glass is hydrogen abstraction from the matrix.The triplet carbene is long lived in polycrystals but at 103-1 19 K 1,2-hydrogen shifts do occur in polyfluorinated matrices albeit only slowly; the mechanism of this process is not known. The singlet carbene undergoes 1,2-shifts without appreciable product isotope effects when in a matrix but more appreciable effects are seen in fluid solution.60 Thermolysis of the diazirine (70; X = MeO) in the presence of tetramethylethylene leads to 83% intramolecular trapping of the carbene by way of a 1,2-hydrogen shift. When X = Me the intramolecular contribution is reduced to 64'70,and with X = Ph the carbene is exclusively trapped by intermolecular addition to the alkene. Apparently the methoxy-group is better able to stabilize the positive charge developed on carbon in the transition state of the 1,2-hydrogen shift.61 Reaction of the trihalides (71 X = Br C1) with n-butyl-lithium leads to good yields of the alkenes (72) with the 2-configuration.The reasons for the high stereoselectivity are not immediately apparent; however examination of the alternative transition states in a 1,2-hydrogen shift to an intermediate carbene centre (73) and (74) respectively indicate the possibility of dominant lone pair to silicon repulsion or of hyperconjuga- tion of the lone pair to low-lying C-0 a*-orbital favouring the former confor- mation.62 X The carbene (75) generated from the corresponding diazo-compound ring- expands to cyclopentyne which is trapped in a [2 + 21 cycloaddition with 2,3- dihydrofuran ;labelling studies confirm the intervention of a symmetrical intermedi- SQ P.B. Grasse B.-E. Brauer J. J. Zupancic K. J. Kaufmann and G. B. Schuster J. Am. Chem. SOC.,1983 105 6833. 00 H. Tomioka N. Hayashi Y. Izawa V. Senthilnathan and M. S. Platz J. Am. Chem. SOC.,1983 105,5053. " M. T. H. Liu and M. Tencer Tetrahedron Lett. 1983 24 5713. 62 M. C. Pirrung and J. R. Hwu Tetrahedron Leu. 1983 24 565. Arynes Carbenes Nitrenes and Related Species ate.63 However cyclopentyne generated from dibromomethylenecyclobutane and alkyl-lithium undergoes stereospecific 1,2-addition to alkenes and preferred 1,2- addition to buta- lY3-diene; this has been interpreted in terms of an antisymmetrica! singlet ground state (76).64 Flash vacuum pyrolysis of 5-adamantylidene-2,2-dimethyl- 1,3-dioxane-4,6-dione leads to (77) by an unusual rearrangement of an intermediate carbene which can be formulated as in (78); presumably the rearrange- ment occurs because lY2-alkyl shifts to an anti-Bredt alkene or homoadamantene are di~favoured.~~ Labelling studies have shown that the rearrangement of (79) to (80) occurs by a hydrogen rather than an acyl-group migration and no evidence has been obtained for the rapid reversibility of the process.66 (78) Dicyclopropylcarbene can be generated by thermolysis of 5,5-dicyclopropyl-2- methoxy-2-methyl-A3- 1,3,4-0xadiazoline at 80 "C.The carbene undergoes ring expansion to 1-cyclopropylcyclobutene but will also abstract a chlorine from carbon tetrachloride and insert efficiently into the C-H bond of chloroform; it also adds to tetrachloroethylene albeit in low yield.67 Treatment of (8 1) with alkyl-lithium reagents gave a complex product mixture which included (82) and (83).These are thought to arise through rearrangement of the carbene (84) to (85) followed by trapping with ether or intramolecular insertion to produce a bicyclo[ 1.1 .O]butane followed by trapping with water on work-up.68 In the presence of tetramethylethylene the cyclopropylidene (84) is trapped by cycloaddition. However with 1,l -diethoxyethylene or di-t-butyl maleate the ap- parently ambiphilic cyclobutylidene (85) is trapped.69 Treatment of the dibromo- cyclobutane (86) with an alkyl-lithium in the presence of diphenylisobenzofuran 63 J.C. Gilbert and M. E. Baze J. Am. Chem. SOC.,1983 105 664. 64 L. Fitjer and S. Modaressi Tetrahedron Lett. 1983 24 5495. 65 J. Scharp and U. E. Wiersum J. Chem. Soc. Chem. Commun. 1983 629. 66 M. Koller M. Karpf and A. S. Dreiding Helu. Chim. Acra 1983 66 2760. 67 M. Bekhazi P. A. Risbood and J. Warkentin J. Am. Chem. Soc. 1983 105 5675. 68 M. Bertrand A. Tubul and C. Ghiglione J. Chem. Res. (S) 1983 250. 69 A. Tubul A. Meou and M. Bertrand Tefrahedron Lerr. 1983 24 4199. 114 M. S. Baird Br produces (87); this apparently originates by a 1,2-alkyl shift in an intermediate carbene to produce the strained alkene (88) followed by [4 + 23 cy~loaddition.~’ Cleavage of (89; R = H) with sodium methoxide produces a vinylcyclopropylidene by deprotonation of an intermediate diazonium salt.The carbene leads to penta- 1,2,4- triene cyclopentadiene and 4-methoxycyclopentene the latter two products by trapping of the rearranged ‘foiled’ carbene (90). In the presence of methyl vinyl ether and methanol (90) may be trapped as (91) and (92). Protonation of the foiled carbene leads to (93) as a major species which is then trapped by methanol to give 4-methoxycyclopentene; this reaction is dependent on the stereochemistry of the initial vinylcyclopropane (89; R = D) leading to (94) and this rules out the inter- mediacy of a planar intermediate.7’ Vinylcyclopropylidene rearrangements in a number of species derived by treatment of mono- and bis-dibromocarbene adducts of hexatrienes with methyl-lithium have been the subject of a careful labelling study.72 R-Tq-CoNH,R NO @ b moMe (89) (90) (91) H D Pyrolysis of the sodium salt of 2-vinylcyclobutanone tosylhydrazone gives a complex of products including 2-vinylmethylenecyclopropane,1-vinylcyclobutene and methylenecyclopentene ; however no products appear to be derived from cyclohex-3-en-1-ylidene the product expected from the cyclobutane analogue of a vinylcyclopropylidene rearrar~gement.~~ However flash vaccum pyrolysis of the sodium salt of the tosylhydrazone corresponding to (95) leads to (96) (97) and (98).The first two products may be explained by a cyclobutylidene-methylenecyc-70 C. W. Jefford J. C. Rosier J. A. Zuber 0. Kennard and W. B.T. Cruse Tetrahedron Leff.,1983 24 181. ” W. Kirmse P. V. Chiem and P. G. Henning J. Am. Chem. SOC. 1983 105 1695. 72 I. Fleischhauer and U. H. Brinker Tetrahedron Lett. 1983 24 3205. 73 U. H. Brinker and L. Konig Chem. Ber. 1983 116 882. Arynes Carbenes Nitrenes and Related Species lopropane rearrangement and a vinyl migration respectively; the final product is thought to result from a vinylcyclobutylidene-cyclohexenylidenerearrangement to (99) followed by a 1,2-alkyl shift.74 (95) Although the reaction of 1,1 -dihalogenocyclopropanes with methyl-lithium is one of the simplest routes to cyclopropylidenes the corresponding reaction of several I 1,2-trihalogenocyclopropanesleads instead to a 1,2-dehaIogenation to produce cyclopropenes; in the case of (100) the derived cyclopropene undergoes ring expansion under the reaction conditions to generate the vinylcarbene (101).75 Vinyl- carbenes are also generated from the thermolysis or photolysis of allenes.Photolysis of cyclonona-1,2-diene in pentane leads to (102). Two carbenes (103) and (104) appear to be likely intermediates in this rearrangement and indeed when generated independently both lead to some cyclopropane. However other products are also formed and it seems unlikely that either is a true intermediate in the photochemical process. A concerted mechanism involving a 1,2-hydrogen shift and 1,3-bonding (a formal a2a + 1r2a process) may occur.76 Irradiation of tetraphenylallene leads to 1,2,3-triphenyIindene and products derived from it in a process initiated by a singlet reaction ; irradiation of triphenylallene leads to 1,3,3-triphenylcyclopropene diphenylindenes and 1,3,3-triphenyIpropyne.Vinylcarbenes are implicated in these reactions.77 Flow pyrolysis of the allene (I 05) gave 2-methylfuran ; this rearrangement may involve initial 1,2-hydrogen shifts to produce the vinylcarbenes (106) or ( 107).78 Further examples of cyclopropenes as vinylcarbene sources are also reported. Photolysis of the spiro-alkene (108) using far-u.v. radiation leads to (109) as the major product together with ethynylcyclopentane and 1-vinylcyclopentene. Ther- molysis leads largely to the acetylene and gives no allene. The formation of (109) 74 U. H. Brinker and L. Konig Chem. Ber. 1983 116 894.75 M. S. Baird and W. Nethercott Tetrahedron Lett. 1983 24 605. 76 T. J. Stierman and R. P. Johnson J. Am. Chem. Soc. 1983 105 2492. 77 M. W. Klett and R. P. Johnson Tetrahedron Lett 1983 24 2523. 78 W. D. Huntsman and T.-K. Yin 1. Org. Chem.. 1983 48,3813. 116 M. S. Baird in the photochemical process is explained by ring-opening of the excited spiran to the S1 excited vinylcarbene ( I 10) followed by radiationless decay to proximate maxima on the ground-state energy surface corresponding to transition states in the thermal isomerization. The least favoured thermal transition state (that to allene) offers the closest approach to the S excited state and is therefore photochemically preferred. A considerable proportion of the photoproduct appears however to derive from the Sovinylcarbene and the thermal reaction shows that the conversion of So into S vinylcarbenes does not occur.79 Photolysis of (1 11 ; R = H) and (1 12) gave unsaturated carbenes which do not cyclize to cyclopropenes.When the former compound is photolysed in furan the carbene (1 13; R = H) is trapped as a 34 1 mixture of endo-and exo-1,2-adducts; photolysis of (1 12) leads to the acetylene (1 14).80Photolysis of (I 1 I) in vinyl ether or cyclopentadiene leads to the trapping of two formal carbene species (1 13) and (1 IS> depending on the nature of R. Thus with vinyl ether and R = Me exclusive trapping of (I 15) occurs whereas with R = C0,Me it is (1 13) which is exclusively trapped.” R Irradiation of (116) (R’ R2 = H; R’ R2 = CH2-CH2; or R1,R2 = CH=CH) does not lead to ketenes unless carried out in an inert matrix.Under these conditions (1 16; R’ R’ = Hj gives the corresponding triplet carbene as the sole primary product and this can be characterized spectroscopically. The triplet is the ground state and can be trapped as (1 17) and (I 18) respectively by oxygen and carbon monoxide. Longer irradiation leads to the ketene (I 19) and the yield of this product is inversely proportional to the concentration of oxygen or carbon monoxide in the matrix. The triplet has a high barrier to Wolff rearrangement and it is suggested that activation 79 M. G. Steinmetz Y.-P. Yen and G. K. Poch J. Chem. SOC.,Chem. Commun. 1983 1504. 80 M. Franck-Neumann P. Geoffroy and J.J. Lohmann Tetrahedron Lett. 1983 24 1775. M. Franck-Neumann and P. Geoffroy Tetrahedron Lett. 1983 24 1779. Arynes Carbenes Nitrenes and Related Species R1 RZ // of the ground state To to T and intersystem crossing of the latter to S”’ carbene occur followed by rapid rearrangement.82 A comparative study of the conformational control in the Wolff rearrangement has been carried out using low-temperature photolysis of either matrix-isolated Q -diazoketones or vinylene thioxocarbonate (1 22). Under these conditions equili- brium between the s-E (123) and s-Z (124) conformers is hindered. Results indicate that the conformational control in the rearrangement is not per se a proof of a concerted mechanism and that a ketocarbene intermediate is most rea~onable.’~ 3 Nitrenes An ab initio study of the chemical reactions of singlet and triplet :NH is reported.84 Photolysis of 1-azidobicyclo[2.2.llheptane in an argon matrix at 10 K leads pre- dominantly to (1 25) which reacts with methanol to give (126) even at 100 K.85Similar strained imines are obtained on photolysis of 1-azidobicyclo[2.2.2.]octanes and 1-azidoadeamantanes.86 ( 125) (126) ( 127) Thermolysis of phenyl azide with sulphides (R1SCH2R2)leads to 2-substituted anilines (127) by Sommelet-Hauser rearrangement of intermediate N-phenylsul- phimides arising from phenylnitrene attack on sulphur. However with ethyl phenyl sulphide the products are PhSNHPh and ethylene which are formed by cycloelimina- tion in an intermediate N-phenylsulphimide.With acyclic benzyl sulphides the intermediate sulphimides undergo a Stevens rearrangement whereas cyclic benzyl sulphides give largely Sommelet-Hauser-derived product^.^' R. A. Hayes T. C. Hess R. J. McMahon and 0. L. Chapman J. Am. Chem. SOC.,1983 105 7786. M. Torres J. Ribo A. Clement and 0. P. Strausz Can. J. Chem. 1983 61 996. x4 T. Fueno V. Bonacic-Koutecky and J. Koutecky J. Am. Chem. SOC.,1983 105 5547. XS R. S. Sheridan and G. A. Ganzer J. Am. Chem. SOC.,1983 105 6158. nt I. R. Dunkin C. J. Shields H. Quast and B. Seiferling Tefrahedron Lett. 1983 24 3887. H’ L. Renati. P. C. Montevecchi and P. Spagnolo J. Chem. Soc. Perkin Trans. I 1983 771. 118 M. S. Baird Photorearrangement of I -azatriptycene (1 28) at 77 K leads to an absorption spectrum attributed to the nitrene (129) which is also obtained from the correspond- ing azide and one attributed to the azanorcaradiene (130); the latter could be a key intermediate in a number of reactions.Analysis of the kinetics of the process and of chemical and spectroscopic data from the two precursors suggested the involve- ment of two conformers of the nitrene (I 3 1) and (1 32).88In order to prove this the conformationally fixed azides (133) and (1 34) were synthesized and indeed were found to lead to different nitrenes and to quite different reaction products.89 (131) X = H Y = N (132) X = N Y = H Flash vacuum pyrolysis of o-azidodiphenylmethane leads to acridan and to acridine whereas at lower temperature in solution the main product is 10H-azepinoindole.Since the singlet nitrene is thought to be favoured by high tem- perature it does not appear that the acridan can be derived from triplet nitrene. It is suggested that the reactions occur via the singlet which leads to (135). Cleavage of bond a to form a biradical is a relatively easy process and occurs at low temperature producing the azepine. Cleavage of bond b is relatively more difficult and only occurs at higher temperatures; this leads to acridan and acridine.” Ther-molysis of (136) leads to (137) through a nitrene intermediate; the insertion process provides a key step in the synthesis of the bacterial coenzyme methoxatin.” Me0,C Me0,C N N RO / NHAc RO / NHAc (135) ( 136) (137) 88 T.Sugawara N. Nakashima K. Yoshihara and H. Iwamura J Am. Chem. SOC.,1983 105 858. 89 S. Murata T. Sugawara and H. Iwamura J. Am. Chem. SOC.,1983 105 3723. 90 M. G. Hicks and G. Jones J. Chem. SOC.,Chem. Commun. 1983 1277. 9‘ A. R. MacKenzie C. J. Moody and C. W. Rees J. Chem. SOC.,Chem. Cornmun. 1983 1372. Arynes Carbenes Nitrenes and Related Species On heating in benzene (1 38) leads to (1 39). This is explained in terms of parallel decomposition pathways involving loss of benzonitrile to produce (140) and loss of sulphur dioxide to give the vinylnitrene (141); cycloaddition of (140) and (141) would then lead to the product. Some support for this mechanism is provided by the trapping of both species. However an alternative reaction pathway involving the sulphene (140) and (142) could not be ruled out.92 PhXi "k= =& R-R The reaction of ethoxycarbonylnitrene with 1,4-di-t-butyl- or 1,rl-di-isopropyl-benzene gave the heterocycles (143) and (144) together with dialkyla~epines.~~ C0,Me K I ,Me C0,Me (143) R = Pr'or But (144) 4 Silylenes Thermal decomposition of (145) has been shown to lead not only to silene (146) but also to methylsilylene and to silylene itself.All three species are trapped by cycloaddition to buta- 1,3-diene. The formation of methylsilylene is intriguing and from an examination of the temperature dependence of the reaction products it appears that this species is derived by rearrangement of the ~ilene.~~ Heats of formation of 1-methylsilaethylene and dimethylsilylene have been found to be 18 and 46 kcal mol-' respectively using ion cyclotron double-resonance spectroscopy.The difference in favour of the sila-alkene contradicts earlier experimental and theoretical A matrix study of the dimethylsilylene to sila-alkene rearrange- ment suggests that the previously observed formation of products characteristic of Y2 B. F. Bonini G. Maccagnani G. Mazzanti P. Pedrini B. H. M. Lammerink and B. Zwanenburg J. Chem. SOC.,Perkin Trans. 1 1983 2097. 93 T. Kurnagai K. Satake K. Kidoura and T. Mukai Tetrahedron Lett. 1983 24 2275. 94 R.T. Conlin and R.S. Gill J. Am. Chem. SOC. 1983 105 618. 95 C. F. Pau W. J. Pietro and W. J. Hehre J. Am. Chem. SOC.,1983 105 16. 120 M. S. Baird the silylene when the sila-alkene is warmed to 100-120 K may be due to rearrange- ment in the act of trapping the intermediate^.^^ Flash vacuum pyrolysis of (1 47) led to trimethylmethoxysilane trimethylvinyl- silane and (148).The latter two products are consistent with isomerization of trimethylsilylvinylsilylene to 1-trimethylsilyl-I -silacyclopropene and a subsequent 172-trimethylsilyl shift to produce a silacyclopropanylidene (l49) which can lose silicon to give the vinylsilane or insert into a C-H bond of a methyl group and give a product which can then rearrange to (148). In the presence of 2,3-dimethyl- butadiene the first formed silylene is readily trapped ; however a minor product from this reaction is (150). The latter may be rationalized in terms of trapping of the silacyclopropanylidene (149) by the diene to produce (151) which loses vinyltrimethylsilane to generate a third silylene (152) which again is trapped by the added diene.97 SiMe, I Me,Si-Si I-Me,SiASiH, ElH2 CH,=Si H OMe L/ (145) ( 146) (147) ( 148) pgMe MeDizMe SiMe Me Me SiMe Me Me Flash vacuum pyrolysis of methoxydisilanes has also been used to generate 1- 2- and 3-propenylsilylenes each of which gives siletene products although by different pathways.Thus (1 53) leads to (I 54; R = H) apparently through the bicyclic species (1 59 and (156) leads to the same product by a C-H bond insertion. The facility with which this occurs has led the authors to question the report that formation of (157) from (158) represents a 1,4-hydrogen migration and to suggest instead an insertion to form a cyclobutene followed by ring-opening to the diene.98 R 96 C.A. Arrington R. West and J. Michl J. Am. Chem. Soc. 1983 105 6176. 97 T. J. Barton and G. T. Burns Tetrahedron Lett. 1983 24 159. 98 G. T. Burns and T. J. Barton J. Am. Chem. Snc. 1983 105 2006. Arynes Carbenes Nitrenes and Related Species Thermolysis of (1 59) in benzene in a flow system at 450 "C leads to the disilacyc- lobutane (160) and methoxytrimethylsilane. The cyclobutane is readily explained in terms of an insertion of an intermediate silylene (161) into a y-C-H bond of a methyl group. However when (159) was copyrolysed with alcohols (160) was obtained together with (162); the latter is formally derived by addition of the alcohol to an intermediate silaethylene (163) itself formed by a silylene to silaethylene rearrangement.The intervention of the silaethylene was confirmed by trapping with benzophenone to give (164). It is interesting to note the preference for trimethylsilyl rather than hydrogen migration to the silylene. Thermolysis of the disilacyclobutane (160) in the presence of trapping agents is consistent with the formation of the disilabutanylidene (I 65).99 (Me,Si),Si -CH,Si Me Me,Si-Si-CH(SiMe,) ORI (Me,Si),Si=CHSiMe (161) (162) (163) A :Si\/SiMe2 I Ph2C=C HSi Me SiMe (164) (165) Thermolysis of (166) leads to dimesitylsilylene in a reaction similar to the thermoly- sis or photolysis of epoxides. The silylene may be trapped in various reactions for example as (167) on reaction with 1,1,3,3-tetramethylindane-2-thione, and undergoes intramolecular reaction to produce (168).'0° Mesity1 I Me3Si- Si- SiMe, I Mesityl Mesityl YY A.Sekiguchi and W. Ando Tetrahedron Lett. 1983 24 2791. I00 W. Ando Y. Hamada and A. Sekiguchi J. Chem. SOC. Chem. Commun. 1983 952; W. Ando Y. Hamada A. Sekiguchi and K. Ueno Tetrahedron Lett. 1983 24 4033.

ISSN:0069-3030    

DOI:10.1039/OC9838000101

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

6 Electro-organic Chemistry* By M. SAINSBURY School of Chemistry University of Bath Claverton Down Bath BA2 7AY 1 Introduction The subject of electro-organic chemistry continues to expand and it is an increasingly difficult task to summarize all those papers which best reflect trends and innovation within this general subject area. Thus it is appropriate to note that several reviews have appeared which deal with specialized topics; for example a number of surveys of the use of electrochemical reactions in synthesis both on .laboratory’.’ and on industrial scale^,^>^ have been published. Other summaries cover the synthesis of heterocycle^,^ some interconversions of p-lactam antibiotics and related structures,6 and the application of electrochemical methods to the reactions of polyenes and carotenoid~.~ For those readers unfamiliar with cyclic voltammetric techniques an introductory article by Mabbott is recommended,* and for a detailed study of reaction mechanisms by electrochemical methods the review by Parker’ is noteworthy.2 General and Mechanistic Aspects Savkant and Tessier have determined” the electrochemical electron-transfer rate constant as a function of the electrode potential for a series of simple electron- transfer processes occurring at a mercury electrode in media consisting of acetonitrile or N,N-dimethylformamide and a quaternary ammonium salt as the supporting electrolyte. In this work the reactions and the experimental conditions were selected in order to emphasize ‘outer-sphere’ processes and to minimize the effects of double-layer corrections.Convolution potential-sweep voltammetry and impedance methods were employed in order to obtain the kinetic data and it was found that the electrochemical transfer coefficient varied beyond experimental * Based on Chemical Abstracts during the period November 1982-November 1983. J. Simonet Actual. Chim. 1982 19. J. Grimshaw and D. Pletcher Electrochemistry 1983 8 171. M. M. Baizer Electrochem. Znd. [Proc. Znt. Symp. J.] 1980 (Pub. 1982) 101. J. H. Wagenknecht J. Chem. Educ. 1983 60 271. Y. Ban Yuki Gosei Kagaku Kyokai Shi 1982 40 866; Chem. Abstr. 1983 98 54254~. S. Torii H. Tanaka M. Sasoaka N. Saitoh T. Siroi and J. Nokami Bull. SOC.Chim. Belg. 1982,91,951. ’J. H. P. Utley Carotenoid Chem.Biochem. Proc. Znt. Symp. Carotenoids 6th 1981 (Pub. 1982) pp. 97-105 ed. by G. Britton and T. W. Goodwin Pergamon Oxford. G. A. Mabbott J. Chem. Educ. 1983,60 697. V. D. Parker Adv. Phys. Org. Chem. 1983 19 13. J. M. Saviant and D. Tessier Faraday Discuss. Chem. SOC. 1982 No. 74 p. 57. I23 124 M. Suinsbury error with the electrode potential. The magnitude of the variation is of the same order as that predicted by the Marcus theory of outer-sphere electron-transfer. This behaviour appears to be general in reductions of organic substrates for which charge transfer is fast and is governed mainly by solvent reorganization. Sirnonet's group have provided an interesting example of an electron-transfer reaction which occurs during the reduction of the indanone (1) in the presence of primary alkyl bromides (RBr)." The products -methyleneindanones (2) -are formed with a low overall consumption of electricity implying a chain reaction possibly following the path outlined in Scheme 1.Me Me m\ Me Me Scheme 1 Potential-pH diagrams have been constructed12 for a number of phenols and from them the relationships between chemical and electrochemical phenomena associated with the oxidation-reduction of the substrates have been analysed in an effort to provide a better understanding of these and similar reactions which involve proton transfer. In the case of arylsulphonyl halides the half-wave potentials of the corresponding chlorides and fluorides differ by >1 V the fluorides being the more negative.I3 The effect of para-substituents is small for the chlorides but large for the fluorides.In agreement with the values of the half-wave potentials arylsulphonyl chlorides are considerably more chemically active than the corresponding fluorides. A number of cyclic voltammetric studies have been conducted this year. Thus the behaviour of 2-alkylthiocycloalkanoneson anodic oxidation has been examined J. Delaunauy M.-A. Orliac-Le Moing and J. Sirnonet J. Chem. SOC.,Chem. Commun. 1983 820. 12 S. I. Bailey I. M. Ritchie and F. R. Hewgill J. Chern. SOC.Perkin Trans. 2 1983 645. 13 L. Horner and R. E. Schmitt Phosphorus Sulphur 1982 13 189. Electro-organic Chemistry 125 in order to assess the possible uses of these compounds in ~ynthesis,'~ and the cathodic reactions of a series of azo-compounds have been similarly ana1y~ed.I~ Substituent effects in the electrochemical oxidation of arylmethyl anions (lithium salts) have also been considered16 and a body of data has been accumulated which provides information about electronic distributions and geometrical preferences in such molecules.The direct electrochemical oxidation of aromatic hydrocarbons at a lead oxide anion has been investigated using the method of electrode kinetic^.'^ The results obtained were then analysed to predict conditions under which the reactions could be employed in large-scale syntheses of aromatic alcohols and aldehydes from arenes. The Kolbe reaction still attracts interest and the kinetics of the dimerization process and its side reactions have been measured for a series of alkali-metal alkanoates.18 Dimer formation is a second-order reaction whereas by-products are formed by first-order processes.Aprotic conditions for the Kolbe reaction have also been studied and the basis for preparative-scale reactions of this type has been established." Trifluoromethyl radicals are generated by the electrolysis of sodium trifluoroace- tate in the presence of activated dienes (3). These react to yield cyclic structures (4)and (5) through addition and the coupling of the biradicals so formed.20 In F3c\ C0,Et / CH=CHCO,Et CH2-CHC0,Et CH-CH (~72~ (A2)"/ / 1 (C\"/ 2)n \ CH=CHCO,Et CH-CHC0,Et /CH-CH\ I F3c C02Et CF3 (3) (4) (5) the case of simple alkenes trifluoromethyl radicals normally give the corresponding 1,2-bis(trifluoromethyl)alkanes but it has been shown that when such alkenes bear an isopropenyl group only one trifluoromethyl group is added.21 For example isopropenyl acetate (6) gives only the ketone (7) when electrolysed in an aqueous medium containing sodium trifluoroacetate (see Scheme 2).H3C CF3CH2 CF3CH (6) (7) Scheme 2 14 H. D. Cajon and H. Viertler An. Simp. Bras. Electroquim. Electroanaf. 3rd 1982,1 197;-Chem. Abstr. 1983,98 80419e. 15 A. J. Bellamy 1. S. MacKirdy and C. E. Niven J. Chem. SOC.Perkin Trans. 2 1983 183. 16 S. Bank A. Schepartz P. Giammatteo and J. Zubieta J. Org. Chem. 1983 48 3458. 17 J. A. Harrison and J. M. Mayne Electrochim.Acfa 1983 28 1223. 18 K. Kase N. Sato and T. Sekine Denki Kagaku Oyobi Kogyo Butsuri Kagaku 1982 50 914; Chem. Abstr. 1983 98 125182r. 19 R. Engels C. J. Smit and W. J. M. Van Tilborg Angew. Chern. Znt. Ed. Engl 1983 22 494. 20 R. N. Renaud C. J. Stephens and D. Berube Can. J. Chem. 1982,60 1687. 21 N. Muller J. Org. Chem. 1983 48 1370. 126 M. Sainsbury Nelsen and his associates in a series of paper^^^-^^ have shown that the radical cations derived from various bridged heterocycles such as azaoxabicyclo-octenes (8) and azabicyclo-octenes (9) are long lived -a fact associated with the implementa- tion of Bredt's rule. Oxidation of 2-phenylnorbornene (10) leads to the dehy- drodimer (ll),and an analysis of the anodic behaviour of the substrate by cyclic voltammetry shows a novel curve-crossing which is claimed to be the first experi- mental observation of this kind.2s An ECE mechanism with a significant redox cross-reaction is proposed to explain this undsual result.Positional selectivity and isotope effects have been studied for the chemical and electrochemical oxidations of polyalkylbenzenes.26 Si,milar results were obtained for those reactions occurring at the anode and for those using cerium(1rr) ammonium nitrate as oxidant. Both techniques generate radical cations which then deprotonate to give benzylic radicals in the selectivity-determining step. However in oxidations promoted by cobalt(II1) acetate no correlations with the electrochemical method were found and here it is proposed thaf the mechanism involves hydrogen atom transfer as the key step.A number of diverse unsaturated compounds have been subjected to electroreduc- tion at a mercury pool cathode in acetonitrile solution containing methyl chlorofor- mate.27 These structures include activated alkenes ketones aromatic imines nitro- compounds and nitrogen heterocycles. From the products isolated and from the voltammetric data obtained possible reaction mechanisms are advanced for the reduction of each type of compound. Most follow predictable paths as for example in the formation of 9-methoxycarbonylfluoren-9-yl methyl carbonate ( 13) from the reduction of fluorenone (12) (see Scheme 3). R' RZ ClCOMe R' R' R2 CIC0,Me R' RZ RTOR~ 5 'f + -0 OC0,Me OC0,Me OC0,Me 22 S.F. Nelsen D. J. Steffek G. T. Cunkle and P. M. Gannett J. Am. Chem. SOC.,1982 104 6641. 23 S. F. Nelsen and J. A. Thompson-Colon J. Org. Chem. 1983 48 3364. 24 S. F. Nelsen G. T. Cunkle D. H. Evans and T. Clark J. Am. Chem. Soc. 1983 105 5928. 25 M. A. Fox and R. Akaba J. Am. Chem. SOC.,1983,105 3460. 26 E. Baciocchi L. Eberson and C. Rol J. Org. Chem. 1982 47 5106. 27 J. Armand C. Bellec L. Boulares and J. Pinson J. Org. Chem. 1983 48 2847. Electro-organic Chemistry 127 On reduction l-alkyl-2,4,6-trisubstituted pyridinium salts (14) afford v-radicals which are stable on the time-scale of cyclic voltammetry but the radicals from the corresponding 1-benzyl- and 1-allyl-compounds are not and undergo cleavage of the C-N bond at rates which are dependent on the size of the 2,6-~ubstituents.~~ A cyclic voltammetric study of the pyrazolidinones (15) shows that here the chemical reaction step after formation of the radical cation is deprotonation at position-2." In the absence of added base and at substrate concentrations >2 mM it is the parent compound which scavenges the proton.The lifetimes of the radical cations are relatively insensitive to the nature of the substituents at C-4 and C-5 but are increased markedly by substitution at N-2. R3 Ph\ N-N /R' R2A N+ R4-R4qo R3 R2 R' = alkyl; R2 R3,R4= alkyl or phenyl R' R2,R3,R4= H alkyl or phenyl (14) (15) It has been argued that the N-radical cation (16) of laudanosine through a form of homocon jugation promotes aryl-aryl coupling which eventually leads to 0-methylflavinantine (19).This radical cation has now been synthesized by non- electrochemical means but in dilute solution instead of undergoing the coupling reaction (16) -+ (19) it decomposes by eliminating the benzyl group at C-1 thus producing the 3,4-dihydroisoquinolinium salt ( 17).30An alternative mechanism for the formation of 0-methylflavinantine is now proposed namely one involving homogeneous electron-transfer from the aromatic m-system of a neutral laudanosine molecule to the radical cation (16). The newly formed rr-radical cation (18) then forms 0-methylflavinantine by a conventional aryl-aryl coupling process. Clearly such a reaction will be concentration dependent and may well be favoured near to the electrode surface (see Scheme 4).A study has been made of the anodic oxidation of a-methyldopamine (20a) a-methylnoradrenaline (20b) and dopamine (20c) at a carbon-paste electrode in 1M perchloric acid and McIlvaine buffers of varying pH and at temperatures ranging from 15 to 30 "C in attempts to simulate the formation of natural melanins.31 Cyclic voltammetric analyses show that each catecholamine undergoes an ECC sequence reaction which begins with a two-electron oxidation to a quinone (21) and then involves deprotonation to the free amines (22). These products cyclize rapidly to 5,6-dihydroxyindolines (23). Further oxidation then gives the aminochromes (24) which in the case of the dopamines (20a) and (20c) rearrange to electrochemically detectable 5,6-dihydroxyindoles (25).No peak for 5,6-dihydroxy-2-methylindole can be discerned probably because loss of water from the precursor (24b) yields 28 J. Grimshaw S. Moore N. Thompson and J. T. Grimshaw J. Chem SOC.,Chem. Commun. 1983,783. 29 A. J. Bellamy D. I. Innes and P. J. Hillson J. Chem. SOC.,Perkin Trans. 2 1983 179. 30 M. Hutchins M. Sainsbury and D. I. C. Scopes J. Chem. SOC.,Perkin Trans. 1 1983 2059. 31 T. E. Young and B. W. Babbitt J. Org. Chern 1983 48 562. 128 M. Sainsbury Me0 Me0 -+ =?Me M e O T M e Me0 (17) Me0 M eO OMe OMe A OMe Me0 Me0 _.._ NMe Me0 Scheme 4 (25 a and c) (24 oxidation 11 [H(aT$R] +melanoid pigments (26 a+> a series R' = R3= H; R2 = Me b series R' = H; R2 = Me; R3= OH c series R' = R2 = R3= H Scheme 5 Electro-organic Chemistry 129 the transient iminoquinone (26b).In all cases on further oxidation melanin-like pigments are ultimately produced. Anodic oxidation of benzophenone hydrazones (27) gives several products dcpending upon the electrolysis conditions used.32 At a platinum anode in acetonitrile containing lithium perchlorate azines (28) are the major products but in sodium methoxide however methanol dimethylacetals (29) are formed. If the electrode is changed to one made of graphite arylmethyl methyl ethers (30) and diarylmethanes (3 1) result. It seems likely that diaryldiazomethanes or their equivalents are intermediates in these last reactions and that in the undivided cell diarylmethanes form through cathodic reduction (see Scheme 6).Ar Ar Ar OMe Ar Ar >N-N< X >OMe ) Ar Ar Ar OMe Ar Ar (28) (29) (30) (31) Scheme 6 3 Cathodic Processes The electrochemical behaviour of a series of a,w-dibromo-esters (32) has been studied in an attempt to optimize the intramolecular cyclization reaction (32) -* (33).33 Although a yield as high as 90% is claimed for the conversion of ethyl a,y-dibromobutyrate (32; n = 1) into ethyl cyclopropanecarboxylate (33; n = l) reactions with other dibromo-esters are much less productive and often mono-debromination is found to be a competitive process to cyclization. Two-electron reductions of dibromocyclopropanes (34)afford mixtures of cis and trans mono-brominated products (35) and (36).34When the substituent groups are R' = H or alkyl and R2 = phenyl only a low stereoselectivity is noted but when R2is a carboxyl or alkoxycarbonyl function the cis form of the products predominates.Possibly in such cases the carbonyl group of the substrate is best orientated away from the negatively charged electrode surface thus inducing the observed stereochemical preference. Conformation plays an important role in determining the product ratios in electrochemical reductions of vicinal dibromides and it has been dem~nstrated~~ that there is a strong preference for reductions to occur in substrates which allow 32 T. Chiba M. Okimoto H. Nagai and Y. Takata J. Org. Chem. 1983,48 2968. 33 C. Giornini A. Inesi and E.Zeuli J. Chem. Res. 1983 280. 34 R. Hazard S. Jaouannet and A. Tallec Electrochim. Actu 1983 28 1095. '' K. M. O'Connell and D. H. Evans J. Am. Chem. SOC.,1983,105,1473. 130 M. Sainsbury conformations with an antiperiplanar disposition of bromine atoms. Thus conforma- tional interconversions prior to electron transfer are observed for trans-l ,2-dibromo-(37) and 1,l -dimethyl-trans-3,4-dibromo-cyclohexanes (38) and also in the cases of other similar substrates. BrCH2(CH2),CH( Br)CO,Et &H2- (CH,) -kHCO,Et (32) (33) R:-Br (34) (35,cis) (36 trans) (j:,,=Br (37) Further examples of the electrochemical synthesis of catalytic species capable of initiating coupling reactions have appeared this year. As an illustration the reduction of nickel(I1) bromide yields a form of nickel which causes ethene to couple with aryl halides giving 1,l-diarylethanes as the principal products.36 Electrochemical generation of organonickel-phosphine complexes similarly allows a new catalytic route to substituted styrenes via the coupling of aromatic halides and alkenes (see Scheme 7).37 ArX + RCH=CH 2 ArCH=CHR + HX Scheme 7 Primary and secondary alcohols and a,w-diols may be converted into carboxylic acids ketones and dicarboxylic acids respectively by heterogeneous oxidation with nickel(o) hydroxide electrochemically generated at a nickel(I1) ele~trode.~' In this way butan-1-01 in aqueous 1 M sodium hydroxide at 70 "C gives butanoic acid in 85% yield and 1,6-dihydroxyhexane at 25 "C affords adipic acid in 84% yield.Electrochemically generated superoxide ion has been used as a base to deprotonate secondary nitro alkane^.^^ The anions thus formed are then oxidized by molecular oxygen bubbled through the cathode compartment of the electrolysis cell thereby giving rise to the corresponding ketones. Productivity is good and for example the 36 Y. Rollin G. Meyer M. Troupel and J.-F. Fauvarque Tetrahedron Lett. 1982 23 3573. 37 Y. Rollin G. Meyer M. Troupel J.-F. Fauvarque and J. Perichon f. Chem. SOC.,Chem. Commun. 1983,793. 38 J. Kaulen and H.-J. Schafer Tetrahedron 1982 38 3299. 39 W. T. Monte M. M. Baizer and R. D. Little J. Org. Chem. 1983 48 803. Electro-organic Chemistry nitroacetal (39) is converted into the ketone (40) in 71% yield.Using the same reagent ethyl cyanoacetate (41) is transformed directly (a) into ethyl glyoxylate (42) and (b) by a 'one-pot' two-step sequence into ethyl oxomalonate (43)."' (39) NCCH,CO,Et OHCC0,Et O=C(CO,Et) (41) (42) (43) The anion (45) generated by electrochemical reduction of 2-pyrrolidone (44) in dimethylformamide solution containing tetraethylammonium toluene-4-sulphonate is sufficiently basic to promote the formation of trichloromethyl alcohols (47) from ketones (46) and chloroform4' (see Scheme.8). H (44) (45) (45)+ CHCl -+ CCI3 + (44) R'COR + Cc13 (46) (47) Scheme 8 Reduction of the bicyclic enone (48)affords the tetracyclodecanone (52).42 Cyclic voltammetric analysis indicates that the reduction requires the formation of the radical anion (49)? which then undergoes intramolecular hydrodimerization-aldol condensation leading via the isomeric radical anion (50) to the dianion (51) and thence to the final product (see Scheme 9).This compound is also produced when the starting material is reduced with magnesium in the presence of trimethylsilyl chloride and a catalytic amount of titanium(1v) chloride in HMPT solution. Cathodic reduction of fluorenes (53; R' = H or Br R2 = CN) or (53; R1= H R2 = C02Et) in dimethylformamide solution gives dianions which are efficient in converting phosphonium salts into the corresponding ylide~.~~ The electrode poten- tials required are low enough to allow the generation of the bases in the presence of several phosphonium salts and of aldehydes so that the method provides a 40 M.Sugawara and M. M. Baizer TetrahedronLett. 1983,.24 2223. 41 T. Shono S. Kashimura K. Ishizaki and 0.Ishige Chem. Lett. 1983 1311. 42 P. Margaretha and P. Tissot Helv. Chim. Acta 1982 65 1949. 43 R. Mehta V. L. Pardini and J. H. P. Utley J. Chem. SOC.,Perkin Trans. 1 1982 2921. 132 M. Sainsbury Scheme 9 convenient way of carrying out the Wittig reaction under very mild conditions. It is interesting that the stereoselectivity of the process depends upon the nature of the counter cation. Lithium cation for instance gives predominantly cis-isomers. Cathodic reduction of nitroalkenes (54) formed from aldehydes and nitroalkanes in a divided cell containing methanol and 20%aqueous sulphuric acid affords mainly oximes (55),together with small amounts of acetals (56)and ketones (57).44 Since the subsequent transformations of the oximes and the acetals into the ketones are readily achieved by hydrolysis this constitutes a very convenient three-step synthesis of longer-chain ketones from aldehydes.R1 R' R' R2CH=&-N02 A. R2 (54) NC 1,I,3,3-Tetraphenylallene (58) undergoes reduction at a mercury pool cathode to give the alkenes (60) although if carbon dioxide is introduced into the catholyte mixture the acid (61)is formed.45 In the presence of dimethylformamide the aldehyde (64) is produced and it is proposed that the first two products arise from the unrearranged anion (59),whereas in the case of the aldehyde it seems likely that 44 T.Shono H. Hamaguchi H. Mikami H. Nogusa and S. Kashimura J. Org. Chern. 1983 48 2103. 45 G. Schlegel and H.-J. Schafer Chern. Ber. 1983 116 960. Electro-organic Chemistry isomerization of this intermediate initially to the anion (62) and thence to the species (63) is necessary before reaction with the electrophile occurs (see Scheme 10). Ph )=C<PhPh ph4PhPh Ph (58) (60) Ph Scheme 10 4 Anodic Processes Electrochemical methoxylation is a very convenient procedure which has application in the synthesis of heterocycles and acetals as well as in the a-functionalization of amides and carbamates. Thus the derivatives of L-ornithine and L-lysine (65; n = 0) and (65; n = 1) may be cyclized to the corresponding optically pure a’,P’-unsatur-ated a-amino-acids (67;n = 0) and (67; n = 1) through electrolysis in the presence of methanol followed by acid treatment and elimination of methanol from the intermediate products (66)46(see Scheme 11).( * )-Lupidine (72) and its epimer (73) are prepared similarly by a sequence of reactions wherein the lactam (68) is methoxylated anodically to give the ether (69). This is converted into the iminium salt (70) or its equivalent by treatment with a Lewis this species then undergoes intramolecular cyclization to the diester 46 T. Shono Y. Matsumura and K. Inoue J. Chem. SOC.,Chem. Commun. 1983 1169. 47 M. Okita T. Wakamatsu and Y. Ban Hererocycles 1983 20 401. 134 M. Sainsbury ,,OH-Y (CH2)" Me0/r>&+(J C0,Me C02Me I 1 C0,Me C0,Me (66) (67) Scheme 11 (71) which through a reductive sequence eventually gives an epimeric mixture of the alkaloids (see Scheme 12).Me0,C C0,Me Me0,C C0,Me *qfJ QNJ TiCI b 0 0 1 (73) (72) Scheme 12 The products of anodic methoxylation of ergolines (74; R' = R2 = H) previously thought to be the 2-methoxylated derivatives (74; R1 = H R2 = OMe) are in fact l-hydromethylergolines (74; R' = CH20H R2 = H).48 However when the 1-position is blocked anodic cyanalation does yield 2-cyanoergolines (74; R' = Me R2 = CN).49 In the case of the p-lactam (75; R = H) anodic methoxylation gives the ether (75; R = OMe). The yield 90% is remarkably high bearing in mind that there are two potential sites for meth~xylation.~' Swenton" and PedlerS2 have 48 B.Danieli G. Fiori G. Lesma and G. Palmisano Tetrahedron Lett. 1983 24 819. 49 K. Seifert S. Hartling and S. Johne Tetrahedron Lett. 1983 24 2841 T. Shono Y. Matsumura and K. Inoue J. Org. Chem. 1983 48 1389. 51 J. S. Swenton Synthesis 1983 74. 52 G. M. Elgy W. B. Jennings and A. E. Pedler J. Chem. Soc. Perkin Trans. 1 1983 1255. Electro-organic Chemistry H2NOC8 H Me02CN R R2 0 ’?Me ‘N CO,Me R’ (74) (75) published further details concerning the electrochemical synthesis of quinone bis- and mono-acetals from aromatic precursors and it has been shown that annulenes behave similarly. Thus oxidation of 2-methoxy- 1,&methano[ 1Olannulene (76) in methanolic 1% potassium hydroxide solution gives the 1,4-addition product (77) which on deprotection and base treatment affords the hydroxy compound (78)53 (see Scheme 13).-2e (i)HCl ,&Me @ / KOH-MeOH \ (ii) Et,N \ H OMe OMe (76) (77) (78) Scheme 13 Electrochemical methoxylation of 3-furoic acid yields a mixture of isomeric 2,4,5-trimethoxylated tetrahydrofurans (79).54 Bond breaking reactions are also possible and during the oxidation of the cyclohexanone (80) in the presence of methanol cleavage of the C-1 and C-2 bonds occurs to give the methyl ester (81) later used in a synthesis of methyl trans-~hrysanthemate.~~ A new method of 1,4-transdisposition of a carbonyl group has been developed in which the key step is the anodic methoxylation of a dienol acetate (82).56 The product (83) is then reduced to the corresponding allylic alcohol (84) and this is esterified with toluene-4-sulphonyl chloride.Solvolysis of the ester (85) in acetone ’’ R. Neidlein and G. Hartz Synthesis 1983 463. 54 1. Stibor J. Srogl M. Janda N. Piricova and K. Vlazny Collect. Czech. Chem. Commun. 1982,47,3261. 55 S. Torii T. Inokueki and R. Oi J. Org. Chem. 1983 48 1944. 56 T. Shono and S. Kashimura J. Org. Chem. 1983 48 1939. 136 M. Sainsbury containing 10% water gives the 1,4-transposed carbonyl product (86) in good yield (see Scheme 14). NaBH 1 (86) (85) Scheme 14 Anodic methoxylation of carbamates (88) prepared from primary amines (87) gives the a-methoxycarbamates (89) as transient intermediates which are converted into the acetals (90) by the acid generated in situ at the anode during the electrolysis step.57 Yields are good for aliphatic substrates and the route is probably the simplest yet devised for the interconversion of amines to carbonyl compounds (see Scheme 15).R' CIC0,Me ' R' FNHCOzMe -2e R2 R2 (87) (88) (89) MeoH I"' R1 H+ R' OMe R2>o H,O R ~ ~ O M ~ (90) Scheme 15 Secondary aliphatic nitro-compounds (91) may be converted into the correspond- ing ketones (92) by oxidation in methanol or ethanol solution containing sodium formate or sodium acetate respectively. Yields range from 40 to 90% depending upon the nature of the substituents.s8 No mechanism for the process is suggested but it is likely that nitro-esters are intermediates which hydrolyse and lose nitrous acid on aqueous work-up (see Scheme 16).R2 NaOCOR3 ' R' R' OCOR~ R' (91) (92) Scheme 16 57 T. Shono Y. Matsumura and S. Kashimura J. Org. Chem. 1983 48 3338. 58 J. Nokarni T. Sonoda and S. Wakabayashi Synthesis 1983 763. Electro-organic Chemistry A useful application of the Kolbe reaction is to be found in the synthesis of 1,4-diketones (94) through the anodic oxidation of the P-acetal acids (93)59 (see Scheme 17). r (93) R2 R' 0 R 3 w R 3 0 R' R2 Scheme 17 Last year Simonet and his associates described an electron-transfer chain reaction for the conversion of epoxides (95) into ketones (96) (Annu. Rep. Progr.Chern. Sect. B. 1982 79 109). In a complementary study Torii now reports a practical route to ketones which is promoted by the electrochemical generation of catalytic amounts of acid.60 Aprotic solvents such as dichloromethane are normally employed so that the acid is produced from small amounts of water present in the medium. Should acetone be used as solvent acetonides (97) are formed directly. R2 R1 (95) (96) (97) a-Fluoroketones (99) have been prepared by the anodic oxidation of enol-acetates (98) in the presence of triethylammonium fluoride and hydrofluoric acid.61 The mechanism of the reaction probably follows the course shown in Scheme 18. R3 R2 R3 R2 )yt AcO R' AcO R' AcO R'. (98) R3 RZ -e R3 R2 )+R' -Ac' -)-tR1 OF AcO F (99) Scheme 18 59 J.Einhorn J. L. Soulier C. Bacquet and D. Lelandais Can. J. Chern. 1983 61,584. 60 K. Uneyama A. Isimura K. Fujii and S. Torii Tetrahedron Lett. 1983 24 2857. 61 E. Laurent R. Tardivel and H. Thiebaut Tetrahedron Left. 1983 24 903. 138 M. Sainsbury A mild method for the deprotection of thioacetals is provided by the potential- controlled oxidation of such compounds at platinum or vitreous-carbon electrodes in aqueous acetonitrile solution. Applied to the phosphonium nitrates (100) this allows the synthesis of p-branched &-unsaturated ketones and aldehydes ( 101)62 (see Scheme 19). Platinum electrodes coated first with a silylated polypyrrole film and then with poly-( L-valine) have been used in the asymmetric oxidation of phenyl cyclohexyl sulphide (102) to phenyl cyclohexyl sulphoxide ( 103).63The optical yield is 54%.Electrochemical oxidation of the commercially available nitroxyl from 2,2,6,6-tetramethylpiperidine gives the oxoammonium ion (104) which effects the oxidation of primary alcohols to aldehydes at a potential ( -+ 0.4V) well below that normally required thus avoiding over-oxidation to the corresponding carboxylic acids.64 Catechol on electrochemical- oxidation in the presence of nucleophiles such as 4-hydroxycoumarin ( 105) and indane- 1,3-dione ( 106) affords the coumestan deriva- tives ( 107) and ( 108) re~pectively.~~ OH Me y+ Me 0 fyJoa *: ' 00 0 i107) (108) 62 H. J. Cristau B. Chabaud and C. Niangoran J. Org.Chem. 1983 48 1527. 63 T. Komori and T. Nonaka J. Am. Chem. SOC.,1983,105 5690. 64 M. F. Semmelhack C. S. Chou and D. S. Cortes J. Am. Chem. SOC.,1983 105 4492. 65 I. Tabakovik Z. Grujik and Z. BejtoviC J. Heferocycl. Chem. 1983 20 635. Electro-organic Chemistry The anodic oxidation of a series of benzyl-and phenethyl-tetrahydroisoquinolines has been studied ; these afford tetracyclic products through six-membered ring forming reactions. Thus the substrates (109) and (1 10) yield compounds (1 11) and OMe OMe Me0 (111) Me0 (112) (1 12) respectively.66 In the case of the isochromanone (113) the anodic product is the y-lactone ( 1 15) not the 8-lactone (1 14) as previously supposed (see Scheme (20). OMe OMe I II Me0 Me0 OMe (115) P.Bird M.Powell M. Sainsbury and b. I. C. Scopes J. Chem. SOC.,Perkin Trans. 1 1983 2053.

ISSN:0069-3030    

DOI:10.1039/OC9838000123

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

7 Photochemistry By J. D.COYLE Chemistry Department The Open University Milton Keynes MK7 6AA 1 General The term 'photocatalysis' and related phrases have been used with different mean- ings and sometimes they have clearly been misused. In a review' of catalysis of photochemical reactions a definition has been given of a catalyst for a photochemical reaction together with kinetic and other criteria for establishing that catalysis is operating. Photoreactions in which species other than the substrate play an essential mechanistic role are of widespread importance and both organic heterogeneous photocatalysis* (i. e. chemical reactions sensitized by irradiated semiconductors) and homogeneous metal catalysis in organic photochemistry3 have been reviewed. A more general article4 on photoelectron-transfer catalysis covers both organic and inorganic photochemistry.Interest in electron-transfer mechanisms for organic photoreactions continues unabated and a comprehensive account' of the chemistry of excited complexes provides a good grounding in the photophysical aspects of systems that might undergo reaction by such mechanisms. A new tool in the investigation of photochemical mechanisms is holography,6 which can be used to give information about the photophysical or photochemical processes occurring in certain solid-state reactions. 2 Alkenes The trans isomers of cycloheptenes and cyclohexenes have often been proposed as reaction intermediates but now a species stable at -78 "C has been prepared7 by the sensitized irradiation of cis-cycloheptene and assigned the trans-cycloheptene structure.The use of copper(1) chloride to promote the photochemical reactions of alkenes has been extended to the photoaddition of halogenoalkanes to electron-deficient alkenes* to give for example a-halonitriles (Scheme 1). If dichloromethane is employed as the addend,' 1,3-dichloro-compounds are formed that can be converted ' G. G. Wuebbels Acc. Chem. Res. 1983 16 285. M. A. Fox Acc. Chem. Res. 1983 16 314. R. G. Salomon Tetrahedron 1983 39 485. 'M. Juillard and M. Chanon Chem. Rev. 1983 83 425. ' R. S. Davidson,.Adv. Phys. Org. Chem. 1983 19 1. 'D. M. Burland Ace. Chem. Res. 1983,16 218; Angew. Chem. Znt. Ed. Engl. 1983 22 582. 'Y. Inoue T. Ueoka T. Kuroda and T. Hakushi J.Chem. Soc. Perkin Trans. 2 1983 983. ' M. Mitani I. Kata and K. Koyarna J. Am. Chem. SOC. 1983 105 6719. M. Mitani Y. Yamamoto and K. Koyama J. Chem. SOC.,Chem. Commun. 1983 1446. 141 142 J. D. Coyle hu,BuBr dcN &CN cuci 62'/O Scheme 1 c1 &CN hv,cUci CH,CI 'CI &CN FCN 90% 83% Scheme 2 electrochemically into cyclopropanes (Scheme 2); this provides a useful alternative to the Simmons-Smith reaction which is not successful for electron-poor alkenes. A different metal-promoted photoreaction of alkenes involves oxidation in the presence of iron( 111) chloride. lo Cyclohexene leads to 2-chlorocyclohexanone but 1,2- and 1,6-dimethylcyclohexene give ring-opened dichloroketones (Scheme 3)that can be usefully modified for synthetic purposes.'" The reaction probably involves electron transfer to Fe3+ from the alkene and a similar electron-transfer mechanism is proposed' to account for the photochemical cyclization of a 5-hydroxypentene in the presence of 9,lO-dicyanoanthracene. The product (Scheme 4)is a tetrahy- drofuran compared with the 2,2-diphenyltetrahydropyran obtained by acid-promoted cyclization. c1 c1 0 Scheme 3 Ph Ph-OH up to 60% &Ph Ph Scheme 4 Direct irradiation of simple alkenes or dienes often leads to complex mixtures of dimers and the problem is exacerbated when two alkenes are irradiated together. However when a mixture of cyclohexene and cycloheptene reacts in the presence of copper(1) triflate,12 one stereoisomer of the cross-adduct (1) can be isolated in yields up to 37%.The photodimerization of cyclohexa- 1,3-diene normally gives 10 A. Kohda K. Nagayoshi K. Maemoto and T. Sato J. Org. Chern. 1983 48 425. 'I Z. Q. Jiang and C. S. Foote Tetrahedron Lett. 1983 24 461. 12 P. J. J. A. Tinnemans G. M. T. de Ruiter A. H. A. Tinnemans and A. Mackor Tetrahedron Lett. 1983,24 1419. Photochemistry [2 + 21 cycloadducts as major products but when 9,lO-dicyanoanthracene is present as an electron-acceptor sensitizer [4 + 21 dimers can be predominant (>60%).13 It is likely that the sensitizer generates the radical cation of the diene as the reactive species. In a related reaction radical cations from 1-arylpropenes can be produced non-photochemically using (Ar3N.)+(SbCl6)- and are found14 to lead to [2 + 21 cycloadducts.The reverse of alkene [2 + 21 cycloaddition namely the cleavage of cyclobutanes can also be achieved using electron-transfer photosensitization and irradiation of a mixture of 1,2-diarylcyclobutane and dicyanoanthracene in the presence of oxygen givesI5 a 1,2-dioxan (2) in 60-70% yield by trapping of a ring-opened radical cation. Under similar conditions 1-butyl-2,3-diphenylaziridine yields (83%) a 1,2,4-dioxazolidine (3).16 The di- v-methane reaction of 3-arylpropenes gives arylcyclopropanes and this has been used" in a patent description of the production of arylcyclopropanes with oxygen substituents in the aryl group for use in fragrances. 3-Vinylcyclopropenes are converted photochemically into cyclopentadienes and the most likely mechanism for the rearrangement has been elucidated" by careful study of suitably substituted systems.3 Aromatics Substituted thiophenes furans and pyrroles undergo photochemical ring-transposi- tion or ring-contraction reactions often in low yield. Furans substituted with trimethylsilyl groups are now shown" to give allenic aldehydes or ketones (Scheme 5) in good yield in a clean reaction at -78 "C. Scheme 5 The mechanisms of aromatic photosubstitution reactions have been reviewed.*' An unusual nucleophilic photosubstitution by cyanide ion in a tricyclic pyrrole gives rise (Scheme@ to a product in which the leaving group (hydride ion) has been 13 C. R. Jones B. J. Allman A. Mooring and B.Spahic J. Am. Chem. SOC.,1983,105 652. 14 N. L. Bauld and R. Pabon J. Am. Chem. SOC.,1983,105 633. 1s K. Mizuno K. Murakami N. Kamiyama and Y. Otsuji J. Chem. Soc. Chem. Comrnun. 1983 462. I6 A. P. Schaap G. Prasad and S. D. Gagnon Tetrahedron Lett. 1983 24 3047. Jpn. Kokai Tokkyo Koho JP 58 21,634 (Chem. Abstr. 1983 98 178935). 18 H. E. Zimmerman and S. A. Fleming J. Am. Chem. SOC.,1983,105 622. 19 T. J. Barton and G. P. Hussmann J. Am. Chem. SOC.,1983 105 6316. 20 C. Parkanyi Pure Appl. Chem. 1983 55 331. 144 J. D. Coyle transferred within the molecule to effect the reduction of a ketone.2' A p-chloro-phenylurea has been employed2* as a radical source to effect substitution at the 3-position in pyridine (Scheme 7); the resultant 3-arylpyridine can be converted in two stages into 3-phenylpyridine which is not readily accessible by more conven- tional routes.Scheme 6 Scheme 7 Aryl or vinyl halides irradiated with visible light in the presence of carbon monoxide and dicobalt octacarbonyl are converted into arylcarboxylate salts;23 if there is an orfho substituent with a suitably placed amino or hydroxyl (Scheme 8) group lactams or lactones are readily formed. *mo T O H 95% 0 Scheme 8 Although the mefu photocycloaddition of alkenes to benzenes can give complex mixtures of isomeric adducts each in fairly low yield there have been a number of useful applications of the reaction in natural product synthesis. Another example is the synthesis of c~riolin~~ in eleven stages from the intramolecular photoadduct obtained from a 5-phenylpentene (Scheme 9).Photocycloaddition of conjugated dienes to anthracenes no longer offers the original fairly straightforward picture with major products derived by addition across the 9,lO-positions. If the initial adducts are protected from shorter-wavelength radiation it is now apparent that for 9,lO-dichloroanthracene and cyclohexadiene the major product (57%) involves [2 + 21 addition across the 1,2-positions of the anthra~ene.~~ 21 Y. Girard J. G. Atkinson P. C. BClanger J. J. Fuentes J. Rokach C. S. Rooney D. C. Remy and C. A. Hunt J. Org. Chem. 1983 48 3220. 22 F. S. Tanaka R. G. Wien and B. L. Hoffer Synth. Commun. 1983 951. 23 J.-J. Brunet C. Sidot and P.Caubere J. Org. Chem. 1983 48 1166. 24 P. A. Wender and J. J. Howbert Tetrahedron Lett. 1983 24 5325. 25 W. K. Smothers M. C. Meyer and J. Saltiel J. Am. Chem. Soc. 1983 105 545. Photochemistry Ac$ +-I= H Scheme 9 A number of interesting photocyclizations to aromatic rings have been reported. 2,6-Dichlorocinnamate esters or the corresponding amides undergo photochemical ring-closure’“ to give 5-chlorocoumarin (Scheme lo) in near quantitative yield if the solution is dilute enough to prevent significant photodimerization of the product. Such aryl-oxygen bond formation is unusual in a photochemical reaction and the cyclization step is formally an oxa-analogue of the photocyclization of 1-phenyl-butadienes. Also rather unexpected is the ring-clo~ure~’ of 2-alkoxy-3-arylcyclohex-2-enones (Scheme 11); this process cannot be considered as a 67-electron cycliz- ation and the reaction is initiated by intramolecular hydrogen abstraction involving the oxygen of the enone group.C1 CI Scheme 10 Scheme 11 When benzophenone is irradiated with acetone oxime in methanol fluoren- 1-01 (4)is formed in 60% yield.28 Irradiation of the oxime of benzophenone alone produces the same compound in 34% yield,’” increasing to 60% if benzophenone 26 R.Arad-Yellin B. S. Green and K. A. Muszkat J. Org. Chem. 1983 48 2578. 27 J.-P. Pete and D. Scholler Tetrahedron Lett. 1983 24 5887. 28 B. Kumar N. Kaur and G. Kaur Synthesis 1983 115. 2Y B. Kumar and N. Kaur J. Org. Chem. 1983,48 2281.146 J. D. Coyle is added. The mechanism proposed to account for this process involves attack by the oxygen of excited benzophenone on the nitrogen of ground-state oxime to give an intermediate 1,4-biradical with a new N-0 bond which undergoes cleavage and then cyclization to fluorenol. Finally in this section the application of a photo- chemical 1,3-shift related to the photo-Fries reaction provides an efficient route to fused medium-ring lactams [e.g. the model compound (5)],which are starting points for the synthesis of a variety of alkaloids of the strychnos aspidosperma schizozy- gane or eburnamine fa mi lie^.^' Off (4) H 90% H 0x-/ (5) 4 Carbonyl Compounds Two of the best known photochemical reactions of ketones and other organic carbonyl compounds are the Norrish type 1 and type 2 reactions and these can often be useful in syntheses of systems that are not readily obtainable by other routes.Photochemical extrusion of carbon dioxide from lactones can yield small-ring compounds and from fused oxazolidinones 1-azabicyclo[ 1.1 .O]butanes [e.g. (6)]can be ~btained.~’ The epoxides of cyclohex-3-enols are not easily oxidized to the corresponding cyclohexanones because of their sensitivity to acid reagents but photolysis of the pyruvate esters (7) achieves this oxidation by a Norrish type 2 ph~toelimination.~’ The pyruvic part of the substrate is converted into a hydroxy-ketene and the analogous species from a cyclohexyl benzoylformate can be trapped by an imine (Scheme 12) to give a P-la~tam.~~ 0 -Ph I I0 U rii 0 30 Y.Ban K. Yoshida J. Goto T. Oishi and E. Takeda Tetrahedron 1983 39 3657. 31 R. Bartnik Z. Cebulska and A. Laurent Tetrahedron Lett. 1983 24 4197. 32 H. A. J. Carless and G. K. Fekarurhobo Tetrahedron Lett. 1983 24 107. 33 H. Aoyama M. Sakamoto K. Yoshida and Y. Omote J. Heterocycl. Chem. 1983 20 1099. Photochemistry OH Ph P h A o o *Ho>=o PhCH=NCH,Ph Ph-cf:, Ph 0 76% Scheme 12 Irradiation of nonan-5-one in fluid solution gives cyclobutanols and elimination products (hexan-2-one and propene) in a ratio of 0.32 1;when the urea inclusion complex of the ketone is irradiated this ratio increases to 0.67 :1 and mainly one stereoisomer of the cyclobutanol is formed.34 Similarly N,N-dialkylpyruvamides give P-lactams (42-74%) when irradiated as their inclusion complex with desoxycholic acid,35 whereas in solution these are at best minor products.Some a#-unsaturated ketones undergo photochemical isomerization to P,y-unsaturated isomers by way of intramolecular hydrogen abstraction. However many enones are inert under normal conditions but when irradiated in the presence of a mild base (e.g.pyridine) the isomerization can be achieved su~cessfully.~~ 2-Vinyl-lactones (8) can be obtained in an analogous process by photoisomerization of the more readily available 2-methylenela~tones.~~ Photoreduction of benzophenone in acidified methanol gives Ph,CHCH( OMe), the acetal of diphenyla~etaldehyde,~' by way of the initially formed mixed pinacol 1,l-diphenylethane-172-diol; the formation of the acetal from the diol is a thermal process.Some cyclic ketones have long been known to give their ketals on irradiation in methanol and it is now suggested39 that this is a genuine photoreaction involving attack by solvent on the ketone excited state. do%& (8) The oxetanes formed photochemically from furans and aldehydes undergo facile ring-opening and the overall process (Scheme 13)is equivalent to an aldol condensa- ti01-1.~' The stereochemical course of the reaction is well defined and the highly functionalized products are useful for subsequent transformations. Aromatic imides -d: hw + R1dR2 RIGR2 R3CHo R1 R2 HO R3 Scheme 13 34 H.L. Casal P. de Mayo J. F. Miranda and J. C. Scaiano J. Am. Chem. Soc. 1983 105 5155. 35 H. Aoyarna K. Miyazaki M. Sakarnoto and Y. Ornote J. Chem. SOC.,Chem. Commun. 1983 333. 36 S. L. Eng R. Ricard C. S. K. Wan and A. C. Weedon J. Chem. Soc. Chem. Commun. 1983 236. 37 F. HCnin R. Mortezaei and J.-P. Pete Synthesis 1983 1019. 38 N. J. Bunce and E. J. Toone J. Chem. Res. (S) 1983 115. 39 V. Malatesta M. Jennings and P. Hackett Can. J. Chem. 1983 61 366. 40 S. L. Schreiber A. H. Hoveyda and H.-J. Wu J. Am. Chem. Soc. 1983 105 660. 148 J. D. Coyle do not normally give oxetanes on irradiation with alkenes and the course of the reaction depends on the solvent. A 5-phthalimidopent-1-ene (Scheme 14) gives a fused 2-benzazepinedione in acetonitrile but the reaction takes a different course in methan01.~' ,OMe -k 0 Scheme 14 [2 + 21 Photocycloadditions involving a,P-unsaturated ketones are some of the best ways of making fused cyclobutane systems.Intramolecular reaction in 1-or 2-acylhexa-l,5-dienes or in hexa-l,5-dien-3-ones can give products derived from either initial 1,5-or initial 1,6-bond formation and an extensive study4* has high- lighted the substituent and ring effects that can promote the 1,&mode of ring-closure. 3-Alkenylcyclohex-2-enones have a 1 -acylhexa- 1,Sdiene unit and an intramolecular photocycloaddition in such an enone has been employed43 in a synthesis of acoradiene (Scheme 15); the overall synthesis requires eight stages from 3-methoxycyclohex-2-enone.Coumarin dimerizes on irradiation to give a mixture of cyclobutane products; addition of BF increases the quantum yield by a factor of a hundred and changes the ratio of isomers formed.44 Scheme 15 5 Nitrogen Compounds Many aliphatic azo-compounds undergo &-trans isomerization and/or eliminate nitrogen on irradiation but some of the products formed with loss of nitrogen seem 41 P. H. Mazzocchi P. Wilson. F. Khachik. L. Klingler and S. Minamikawa J. Org. Chem. 1983.48. 2981. 42 S. Wolff and W. C. Agosta. J. Am. Chem. SOC.,1983 105. 1292 1299. 43 W. Oppolzer F. Zutterman and K. Baettig Helv. Chim. Am 1983 66,522. 44 F. D. Lewis. D. K. Howard and J. D. Oxman. J. Am. Chem. Soc. 1983 105 3344. Photochemistry to require a carbene rather than a radical intermediate.This suggests that initial azo-diazo-compound isomerization may occur and by using a laser source for the irradiation it has now been demonstrated4’ that a diazo-compound is generated in solution at room temperature from the tricyclic azo-compound (9). & hunm) +6’ (333.6 (9) Useful cyclization reactions involving the formation of heterocycles from iminium salts have been reported in recent years and these have now been reviewed.46 Photocycloadditions of alkenes to C=N compounds have become familiar and 2-phenylbenzoxazole gives a high yield of a 1,3-diazetidine dimer (Scheme 16) on irradiati~n.~’ Scheme 16 45 W. Adam N. Carballeira and W. D. Gillaspey Tetrahedron Lett. 1983 24 5473. 4h P. S. Mariano Ace. Chem. Rex 1983 16 130; Tetrahedron 1983 39 3845. 47 J. Roussilhe E. Fargin A. Lopez B. Despax and N. Paillons J. Org. Chem. 1983 48 3736.

ISSN:0069-3030    

DOI:10.1039/OC9838000141

出版商:RSC    

年代:1983

数据来源: RSC