ISSN:0069-3030    

DOI:10.1039/OC97471FX001

出版商:RSC    

年代:1974

数据来源: RSC

ISSN:0069-3030    

DOI:10.1039/OC97471BX003

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

2 Physical Methods and Techniques Part (i) Mass Spectrometry By T. P. TOUBE Chemistry Department. Queen Maty College London. El 4NS The literature on organic mass spectrometry continues to proliferate. Several books in the field have appeared during the period reviewed in this Report notably ones on metastable ions,’ gas chromatography-mass spectroscopy,2 pesticides and pollutant^,^ and solvents and imp~rities.~ Tables of metastable ions’ of m/e 1-500 and a four-volume registry of mass spectral data6 have also been published. Selected areas have been the subject of reviews among them structure determinati~n,~ steroids,* indoles,’ and Hammett-type correlations. lo The relationship between mass spectrometric thermolytic and pyrolytic re- activity has been reviewed.’ ’ An extremely compact and comprehensive survey of recent literature (1647 references in 25 pages!)12 has appeared.In this Report an attempt has been made to select those topics which the Re- porter believes will be of greatest interest to the majority of organic chemists. R. G. Cooks J. H. Beynon R. M. Caprioli and G. R. Lester ‘Metastable Ions’ Elsevier Amsterdam 1973. * W. McFadden ‘Techniques of Combined Gas Chromatography-Mass Spectroscopy’ Wiley-Interscience New York 1973. S. Safe and 0.Hutzinger ‘Mass Spectrometry of Pesticides and Pollutants’ CRC Press Cleveland 1973. M. Spiteller and G. Spiteller ‘Massenspektrumsammlung von Losungsmitteln Verun- reinigungen Saulen belegmaterialen und einfache aliphatische Verbindungen’ Springer- Verlag Vienna 1973.’ R. Neeter and C. W. F. Kort ‘Metastable Precursor Ions’ Elsevier New York 1973. E. Stenhagen S. Abrahamsson and F. W. McLafferty ‘Registry of Mass Spectral Data’ Wiley-Interscience New York 1974. J. M. Wilson in ‘Structure Determination in Organic Chemistry’ ed. W. D. Ollis MTP International Review of Science Organic Chemistry Series One Vol. 1 Butter-worth London 1973. * ‘Modern Methods of Steroid Analysis’ ed. E. Heftmann Academic Press New York 1973. R. Kmel’nitskii Khim. geterotsikl. Soedinenii 1974 291. lo M. M. Bursey in ‘Advances in Linear Free Energy Relationships’ ed. N. B. Chapman and J. Shorter Plenum London 1973. I ’ R. C. Dougherty Fortschr. chem. Forsch. 1974 45 93. A. L. Burlingame R.E. Cox and P.J. Derrick Analyr. Chem. 1974 46 248R. 7 T. P.Toube 1 Metastable Ions Interest in the properties and behaviour of metastable ions' has been stimulated by the introduction of MIKES (mass analysed ion kinetic energy spectroscopy)' or DAD1 (direct analysis of daughter ions).I4 These techniques provide methods for the study of ions decomposing between the source and collector regions of a mass spectrometer which are more selective than those previously available. Studies on metastable ions for the process (2)-(3) in the mass spectrum of (1)" showed that the formation of (3)involved the transfer of hydrogen only from PhNHCO(CH,),CHJ -+ ,OH1:PhNHC -+ C,H,Nf \CH (1) (2) (3) the CH group of (2). Deuterium labelling suggested that the bulk of the trans- ferred hydrogen came from the a-carbon atom of (l) but the absence of a meta- stable peak for deuterium transfer in the second stage of the reaction implied that only ions of comparatively high internal energy transfer the relevant hydrogen atom.Allylic alcohols are known16 to give spectra similar to those of the correspond- ing ketones after electron impact (EI) at 70eV. Investigation by metastable techniques of those C4H80t ions from hept-1-en-3-01 which underwent de- composition after EI indicatedI7 that they probably had structure (4) or (5). These results confirmed conclusions' from ion cyclotron resonance (ICR)which showed that the C4H,0t ion in the mass spectrum of this compound was not derived oia ketonization of the molecular ion.Using MIKES Beynon" has been able to measure kinetic isotope effects and the effect of isotopic substitution on the release of kinetic energy for the loss of methane from the molecular ion of butane using the I3C1and 2H,isotope peaks at natural abundance. Such data can normally be derived only by examination of labelled compounds. l3 J. H. Beynon R. G. Cooks J. W. Amy W. E. Baitinger and T. Y. Ridley Analyt. Chem. 1973,45 1023A. I4 K. H. Mauer C. Brunee G. Kappus K. Habfest U. Schroder and P. Schulze 19th Conference of Mass Spectroscopy Atlanta Georgia 1971. l5 K. B. Tomer and C. Djerassi Tetrahedron 1974 30 17. M. Kraft and G. Spiteller Org. Mass spectrometry 1968 1 617. l7 B. Grant and C. Djerassi J. Amer. Chem. SOC.,1974 96,3477. J.H. Beynon D. F. Brothers and R. G. Cooks Analyt. Chem. 1974. 46 1299. Physical Methods and Techniques-Part (i) Mass Spectrometry 9 The 1,2-elimination of H from ethane CH2=OH+ CH,=NH,+ and similar species if it occurs uia a planar transition state constitutes a 'symmetry- forbidden' process. Measurements from metastable ion characteristics of the kinetic energy released in the known 1,Zelimination of H from these molecules s~ggested'~ that the energy penalty for violation of the Woodward-Hoffmann rules appears as excess kinetic energy (ca. 1 eV in the cases studied). This con- clusion was then extended2' to the examination of the loss of H2 in species in which scrambling precluded investigation by 2H labelling. The absence of kinetic energy release for the loss of H2 from C6H7+ C2H5+ and C2H4 ions was interpreted as indicating a possible 'symmetry-permitted' 1,l-elimination of H, while for C7H9+ kinetic energy release supported a mechanism involving 1,2-elimination.A 1,l-elimination in this case of HNO ,has also been reported" from the molecular ion of Me02C(CH2),N02. Collisionally activated (CA) metastable ions have been studied using spectro- meters in which as for MIKES the magnetic analyser precedes the electrostatic analyser. Collision with an inert gas induces decomposition in previously stable ions. Field-ionized (FI) molecules have been studied by this technique using helium at ca. lop4Torr as the activating species. As would be expected the ions behaved differently from species produced in EI sources as FI yields ions whose internal energy is low.It was observed that collisional activation itself appeared to promote a certain amount of isomerization in these cases. A CA of some C2H6N + and C,H,N + ions established that only ions possessing apparently immonium structures had lifetimes greater than 10-s. Among the isomeric C2H6N+ ions it was observed that on actiuation NH2CH2CH2+ and CH3CH2NH+ isomerized to a common CH,CH=NH,+ structure which + + differed from the CH,NH=CH species formed by rearrangement of CH,NCH . The subsequent decomposition of these immonium ions proceeded without significant hydrogen scrambling. Negative-ion CA studies have been ~ndertaken.~~ Collision of the stable negative molecular ions of carboxylic acids2 with toluene induced decomposition pathways based on the acid function which were lacking in the conventional spectra.The ejection of ethane from the [M -C1]+ ion from exo-norbornyl chloride2" was shown by isotopic labelling to proceed with statistical loss of hydrogen27 and carbon.26 The 'metastable peak'26 for the process could be resolved into a l9 D. H. Williams and G. Hvistendahl J. Amer. Chem. Soc. 1974 96,6753. 'O D. H. Williams and G. Hvistendahl J. Amer. Chem. Soc. 1974 96,6755. *l T. A. Molenaar-Langeveld and N. M. M. Nibbering Org. Mass Spectrometry 19 4. 9 257. 22 K. Levsen and H. D. Beckey Org. Mass Spectrometry 1974 9 570. 23 K. Levsen and F. W. McLafferty J. Amer. Chem. Soc. 1974 96,139. 24 J. H. Bowie and S.G. Hart Internat. J. Mass Spectrometry ion Phys. 1974 13 319 25 J. H. Bowie Org. Mass Spectrometry 1974 9 304. 2b J. L. Holmes D. McGillivray and N. S Isaacs Org. Mass Spectrometry 1974 9 5 0. " J. L. Holmes D. McGillivray and N. S. Isaacs Canad. J. Chem. 1970 48 2791. 10 T. P.Toube superimposition of 'dished' and 'Gaussian' shapes suggesting that two isomeric ions differing in heat of formation by ca 0.4 eV decomposed in this fashion. It is reasonable to suppose that the kinetic energy released in the unimolecular decomposition of an ion should not exceed the sum of the reverse activation energy for the reaction and the excess energy of the ion above the threshold for the process. The reverse activation energy for the process C3H,+ -+ C3H5++ H, as estimated from the appearance potentials of C3H ions produced from + thermal C3H radicals has been compared** with the kinetic energy release for the reaction measured from the 'dished metastable peak'.Thermochemical calculations showed that nearly all the reverse activation energy appeared as a loss of kinetic energy. This result implies that maximum kinetic energy release cannot alwayhbe equated with the above sum. h 2 The C7H7+Ion Since the original proposal2' that the C,H,+ ion from toluene was the ring- expanded tropylium ion (6) rather than a benzyl cation (7) the structures of the C,H ions from a large variety of sources have been extensively studied. The + problem is of particular importance in view of the gay abandon with which ring-expanded structures for all sorts of bicyclic and heterocyclic species have appeared in the literature since Meyerson's first suggestion sometimes on no firmer basis than tenuous analogy or aesthetic preference! An extensive study by McLafferty3' of C7H7+ ions from a number of ortho- meta- para- and side- chain-substituted toluenes substituted cycloheptatrienes and substituted nor- bornadienes has used CA methods to attempt to distinguish between the structures (6)-(11).The technique has particular merit as it permits the study of the (6) (7) (8) (9) (10) (1 1) fragmentation of C,H,+ ions which would not have had enough energy to decompose were they not activated by collision and the manner in which the induced fragmentations occur is not dependent on the internal energies of the ions at formation (as collisions lead to a Boltzmann distribution of these energies) but only on their structures.The relative abundances of fragment ions indicated for example that a significant proportion of C,H,+ ions from cycloheptatriene derivatives had the benzyl structure (7). In fact the ratio (6) :(7) did not vary very greatly irrespective of whether the starting material was a cycloheptatriene or a toluene. Calculated energy barriers suggested that the equilibration between these skeleta was easier at the molecular ion stage than in the C7H,+ fragment 28 J. L. Holmes and G. M. Weese Org. Mass Spectrometry 1974 9 618. 29 P. N. Rylander S. Meyerson and H. M. Grubb J. Amer. Chem. SOC. 1957 79 842. 30 F.W. McLafferty and J. Winkler J. Amer. Chem. SOC.,1974 96,5182. Physical Methods and Techniques-Part (i) Mass Spectrometry 11 itself. The tolyl ions (8)-( 10) appeared to be distinguishable from each other and the relative ease with which they interconverted with one another and with (6) and (7) was examined. The C7H7+ ions from norbornadiene derivatives remained largely norbornadienyl (11) and showed less tendency to isomerize to (6)or (7) than did the aromatic species. Differences in the kinetic energy released in the process C7H7+ --* C5HS+ previously reported31 for the metastable decomposition of C7H ions from different sources were reinterpreted3' on the + basis of the CA data as reflecting differences in internal energy distributions in the ions rather than variations in structure.E~amination~~ of the C,H,+ problem on the different time and energy scales of ICR produced the interesting result that whereas the C7H7+ ions from benzyl methyl ether and 7-methylcycloheptatriene reacted with dimethylamine no such reaction was observed with the corresponding ion from 7-methoxycyclohepta- triene. These data would normally be regarded as evidence for a structural difference between the reacting and non-reacting species. Isotopic scrambling results33 on a 13C,-labelled C7H,+ fragment from 6-hepta-2,4-dien-6-ynyl chloride have been interpreted as indicating that a proportion of such ions from an open-chain precursor isomerize to a tropylium structure (6). In view of McLafferty's observations3' such conclusions should not be drawn without great circumspection.Similar caution is necessary in dealing with data reported34 on isotope scrambling in the C,H,+ ion from [ 3C,Jtropylium iodide. More cautious conclusions were drawn from ion kinetic energy spectroscopy (IKES) on the [M-11' ions from benzyl fluoride and the isomeric monofluorotoluenes :35 the differences in the behaviour of the C7H6F+ ions were ascribed to variations in their internal energies and/or structures. The assumption that C,H,+ ions have a tropylium structure is implicit in a of stable C13H1 ions from a variety of aromatic compounds. Heats of + formation of these ions were calculated (AH for each starting material being derived from 'group equivalent' values). On the basis of their magnitudes it was concluded that diphenylmethane and p-phenyltoluene gave rise to phenyltropy- lium ions but that the other compounds examined produced C 3H + species which had partially acyclic structures.A benzyl cation figured in a different role in a study3 of the loss of a benzyl radical from the molecular ion of a,o-dibenzyloxyalkanes. The magnitude of a deuterium isotope effect and results from l80labelling were consistent with a 31 R. G.Cooks,J. H. Beynon M. Bertrand and M. K. Hoffmann Ore. Mass Spectrometry 1973. 7. 1303. 32 A. Venema and N. M. M. Nibbering Tetrahedron Letrers 1974 3013. 33 C. Koppel. H. Schwarz and F. Bohlmann Org. Mass Spectrometry 1974,9 332. 34 A. Siegel J. Amer. Chem. Soc. 1974 96 1251. 35 S. Safe W.D. Jamieson and D. J. Embree Canad. J. Chem. 1974 52 867. 36 F. Bohlmann C. Koppel B. Miiller H. Schwarz and P. Weyerstahl Tetrahedron 1974 30,1011. " A. P. Bruins and N. M. M. Nibbering Tetrahedron 1974 30,499. 12 T. P.Toube mechanism which involved an S,i-type intramolecular migration of a benzyl cation. 3 Fragmentation Processes The effects of interaction between functional groups in polyfunctional alkanes on their mass spectra have been reviewed.38 Several examples of such intra- molecular effects have recently appeared in the literature. The base peak [M-PhC,H,] ,in the spectrum of N-acetyl-N-(4-phenyl)butylamine + has been shown3' to arise with the transfer of a benzylic hydrogen atom to nitrogen. Analogous processes are absent in the spectra of N-acetyl-N-butylamine and butylbenzme.It has been suggested that interactions between functional groups may depend less on their nature than on their relative positions. This proposal was made on the basis of the observation that the fragrnentation40 of dimethyl glutamate MeO,C(CH,),CH(NH,)CO,Me and of some of its derivatives includes some processes found in compounds containing a -N(CH2)3N- unit. On the other hand measurement4' of the appearance potentials of [M -Me]+ ions from bistrimethylsilyl ethers separated by 2-7 methylene groups gave values independent of the separation of the functional groups indicating that in this series no interaction between the oxygen functions was apparent. In the chemical ionization (CI) spectra (isobutane reagent gas) of cis-and trans-cyclohexanediols steric effects have been investigated42 The trans- 1,3- and 1,Cisomers behaved like monohydric alcohols preferring to form [M -HI+ ions while the cis-compound yielded [M + HI+ species presumably by forma- tion of a hydrogen bridge.The cis-and trans-1,2-diols formed such bridged ions equally well but possibly because of the greater strain involved this process was less favoured in the cyclopentane-1,2-dioIs. Relationships derived from condensed-phase reactions involving hydrogen stereochemistry have been extended to the study43 of the EI-induced 1,Zloss of acetic acid froucetates and the 1,4-loss of water from alcohols. By deuterium labelling it was shown that dehydration was stereoselective.The expulsion of acetic acid had the same trans,antipreference and deuterium isotope effect as the corresponding pyrolysis reaction suggesting a common structure for the transi- tion states for the two processes. The effect of steric hindrance on fragmentations of the type (12)-+ (13) has been in~estigated.4~ With an electron-donating group in the para-position of the 38 H. Bosshardt and M. Hesse Angew. Chem. 1974 86 256. 39 R. Wild and M. Hesse Helv. Chim. Acta 1974 57 452. 40 E. Lerch and M. Hesse Helv. Chim. Acta 1974 57 1584. 4' C. Koppel H. Schwarz and F. Bohlmann Org. Mass Spectrometry 1974 9 567. 42 J. Winkler and F. W. McLafferty Tetrahedron 1974 30 2971. 43 M. M. Green J. M. Moldowan and J. G. McGrew J. Org. Chern. 1974 39 2166. 44 H.Kuschel and H.-F. Grutzmacher Org. Mass Spectrometry 1974 9 403. Physical Methods and Techniques-Part (i) Mass Spectrometry 13 benzene ring this process was suppressed. However if the electron-donating substituent was an aryl group chosen to produce sufficient steric congestion for conjugation between the two aromatic rings to be prevented the process involving loss of the ortho hydrogen atom was once more observed. Steric effects have also been adduced45 to support the mechanism usually proposed for the losses of toluene and xylene from the molecular ions of carote- noids uia intermediates such as (14).46 The bulky end-groups of the unusual R2 C, carotenoids would crowd the cyclobutane ring in (14). The [M-92]/ [M -1061ratios in such Cs0 compounds were measured and found to have been altered in the sense which would have been predicted.The contribution of the stability of the radical lost in the formation of species which can be written as five- and six-membered cyclic bromonium ions from a variety of bromides has been assessed.47 For the loss of PhCH ,thermochemical criteria prevail. With most other radicals the reaction proceeds under kinetic control. An attempt to correlate the relative abundances of fragment ions from aryl benzoates with Hammett Q values failed.48 The lack of linearity in the plot was ascribed to some form of oxygen bridging in the excited state. The expulsion of a carbene unit from an ion in the mass spectrometer has commonly been regarded on energetic grounds as an improbable process.However several examples of this fragmentation have recently been reported. The molecular ion of perfluoropropene decomposes49 to give a C,F4t ion a formal loss of :CF . A variety of substituted trifluoromethylbenzenes have been + shown to lose :CF .” The iminosulphurane Me,SNHCO,Et C1-gives an [A4-C1]+ ion which then expels :CH .51 In the fragmentation of the corre- sponding diethyl compound it was proposed that the ejection of :CHCH occurred. Such loss of a carbene from a sulphonium ylide has previously been 45 W. Francis S. NorgBrd and S. Liaaen-Jensen Acta Chem. Scand. 1974 B28,244. “ E.g. U. Schweiter G. Englert N. Rigassi and W. Vetter Pure Appl. Chem. 1969 20 365. 47 H. Schwarz F. Bohlmann G. Hillebrand and G.Altnau Org. Mass Spectrometry 1974 9 707. 48 S. A. Shamshurina 0. S. Chizhov and B. M. Zolotarev Zhur. org. Khim. 1974 10 913. 49 B. S. Freiser and J. L. Beauchamp J. Amer. Chem. SOC.,1974 96 6260. 50 G. P. Vdovin Y. P. Egorov A. P. Krasnoshchek and L. M. Yagupol’skii Zhur. org. Khim. 1974 10 1355. 51 G. F. Whitfield H. S. Beilan D. Saika and D. Swern J. Org. Chem. 1974 39 2148. T. P.Toube po~tulated.~’Examination of the CA spectra of PhCH,CH2Br at low energies (11-15 eV) suggested53 that the C8H9+ ion varied in structure with energy. Carbene loss from this ion was observed. Energetic considerations would also be expected to discriminate against processes in which an even-electron species loses a radical to yield an odd- electron ion.Such fragmentations are not unknown in mass spectrometry. A particularly amusing example of this reaction however is provided by octaethyl- porphorin (15),54 which in the mass spectrometer exhibited the successive expulsion of all eight methyl radicals. Labelling with 13C revealed that the methyl radical lost from the molecular ion ofp-methyl~tyrene~’ arose from the positions shown in (16).No loss ofcarbon from the ring was observed. 20.1 % f/32.2 % 47.7 % (16) (17) Labelling results and metastable data for isobutenes6 led to a postulated mechanism for scrambling involving the interconversion of methylpropene but-1-ene and but-Zene structures via opening and closing of a methylcyclo- propane intermediate. Selective deuteriation of indolo[2,3-~]quinolizidine (17) revealed” that the [M -13 ion arose by expulsion of H-12b (37 %) H-7 (32 ”/,) H-6 (19”/,),and H-4 (13%).The ejection of ethane from the molecular ions of such compounds is well known. From the spectra of the labelled derivatives it was concluded that loss of 28a.m.u. occurred by a retro-Diels-Alder type of 52 P. Robson P. R. H. Speakman and D. G. Stewart J. Chem. SOC.(C) 1968 2180. 53 N. M. M. Nibbering T. Nishishita C. C. Van de Sande and F. W. McLafferty J. Amer. Chem. SOC.,1974 % 5668. 54 P. S. Clezy C. L. Lim and J. S. Shannon Austral. J. Chem. 1974 27 1103. 55 C. Koppel H. Schwarz and F. Bohlmann Org. Mass Spectrometry 1974 9 343. 56 M. S.-H. Liu and A. G. Harrison Canad. J. Chem. 1974,52 1813.57 G. W. Gribble and R. B. Nelson J. Org. Chem. 1974 39 1845. Physical Methods and Techniques-Part (i) Mass Spectrometry 15 pathway (loss of C-2 + C-3) to the extent of 207$ while loss of C-1 + C-2 yielded 80 % of the [M-281 ions. 4 Ion Cyclotron Resonance Vapour-phase reactions have been studied by ICR yielding results related to reactions in solution. On electron impact followed by collision of the ions with their parent molecules esters produced [M + H]+ ions.58 Reaction of these species with molecules of added alcohols led to an ion formed with the ejection of water by a process analogous to esterification. The trifluoroacetoxyl anion produced by EI was allowed to react with trifluoroacetic anhydride molecule^.^^ On the time-scale 1O-j s this yielded a 1 1 adduct whose structure was deduced to resemble (18) a species typical of solution chemistry.On the other 0-I FC,-C-OCOCF, I OCOCF (18) hand in solution esters tend to protonate at the carbonyl group; deuterium labelling in conjunction with ICR has shown6' that in the CI spectrum of esters CH5+ protonates the ether oxygen not the carbonyl. The basicities of mono- di- and tri-methylphosphines have been measured' by ICR by observation of their proton affinities. From these data P-H bond strengths have been estimated. From kinetic studies62 rates for some vapour-phase ion-molecule reactions of t-butyl alcohol have been determined. 5 Miscellaneous Applications An unusual correlation of properties was reported in a study of some aromatic dyes,63 PhN=NAr in which it was shown that the ratio [Ph+]/[PhN,+] in their EI mass spectra increased in the same way as the light-fastness of their 1% dyeings on cellulose acetate or polyester fabric.The volatile products of pyrolysis of some 70 compounds have been examined by mass spe~trometry.~~ The pyrolysis patterns could be used to distinguish the parent compounds. 58 J. K. Pau J. K. Kim and M. J. Caserio J.C.S. Chem. Comm. 1974 120. 59 J. H. Bowie and B. D. Williams Austral. J. Chem. 1974 27 1923. 6o C. V. Pesheck and S. E. Buttrill J. Amer. Chem. SOC. 1974 96,6027. 61 R. H. Staley and J. L. Beauchamp J. Amer. Chem. SOC. 1974 96,6252. 62 J. L. Beauchamp M. J. Caserio and T. B. McMahon J. Amer. Chem. SOC.1974 % 6243. 63 H. P. Mehta and A. T. Peters Appl. Spectroscopy 1974 23 241. 64 R. Belcher G. Ingram and J. R. Majer Microchem. J. 1974 19 191. T. P.Toube Substrates with two or more oxygen atoms or with nitrogen functions re- acted6' with added ethanolamine or ethylenediamine introduced via the liquid inlet system of a CI mass spectrometer. This procedure would provide a means of imparting additional reagent gas selectivity in CI mass spectrometry without the need to change gas cylinders! The kinetics of some unimolecular reactions of field-ionized species have been Field desorption (FD)techniques have been used to obtain molecular weights for compounds which fail to give molecular ions under other conditions. A representative recent example of this class is the production of a significant molecular ion of an unstable ~yloside~~ by FD.Benoit6* has re-examined some ionization potentials by the authors of last year's Report7' and compared the results with values from his own earlier Not surprisingly he comes to the conclusion that his own figures were the more reliable! The area of computer-aided interpretation remains a popular one. A Japanese computer program has been devised73 which is claimed to generate the structure most likely to give rise to a given mass spectrum even for compounds not stored as references. 65 D. V. Bowen and F. H. Field Org. Mass Spectrometry 1974 9 195. 66 P. J. Derrick and A. L. Burlingarne Accounts Cliem. Res. 1974 7 328. 67 J. J. Karchesy M.L. Lauer D. F. Barofsky and E. Barofsky J.C.S. Chem. Comm. 1974 649. 68 F. Benoit Org. Mass Spectrometry 1974 9 626. " T. W. Bentley and R. A. W. Johnstone J. Chem. SOC. (B),1971 263. '' R. A. W. Johnstone and F. A. Mellon J.C.S. Faraday II 1973,69 36. 71 R. A. W. Johnstone and F. A. Mellon Ann. Reports (B),1973 70 7. l2 F. Benoit Org. Mass Spectrometry 1972 6 1289. 73 F. Erni J. T. Clerc and S. Hishida Kuguku No Ryoiki 1974 71 101 1.

ISSN:0069-3030    

DOI:10.1039/OC9747100007

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

2 Physical Methods and Techniques Part (ii) Nuclear Magnetic Resonance By R. B. JONES and L. PHILLIPS Department of Chemistry Imperial College of Science and Technology South Kensington London S W7 2A Y 1 Introduction In this Report we have concentrated attention upon two major areas of continuing importance namely I3C n.m.r. and conformational analysis. Two obvious 'growth points' selective TI measurements and studies of the less-common isotopes of hydrogen are also considered. Needless to say the coverage of each of these areas is highly selective and reflects the authors' impression of what is important; apologies are therefore in order to the originators of all the work which lack of space has forced us to ignore! 2 Conformational Analysis This is one of the largest areas of application of n.m.r.techniques and we cannot cover the many papers of a 'routine' nature which have appeared in the past year. We concentrate upon two relatively new techniques of importance 3C n.m.r. and lanthanide-induced shifts (LIS); in addition we cover just one area to which the techniques of conformational analysis have been applied biological systems. Applications of l3CN.M.R.-HinderedRotation about C-C Bonds. This has been examined in some n-conjugated systems. In 2,5-dimethyh3,4-di-isopropyl-2,4-hexadiene the barrier to internal rotation about the (formally single) C-3--C-4 bond is unusually high with AG* > 1lOkJmol-'. The methyl groups of the isopropyl functions are diastereotopic because of the chirality induced by the hindered rotation and their non-equivalence persists at temperatures up to 195"C.' An analogous study of the s-cisSs-trans equilibrium in some enamino-ketones and -thiones of the form X=C(R)CH=CHNR (X = OS R = alkyl) shows that only in the case ofthe thiones is there significant restriction to rotation resulting in non-planarity.2 The power of the I3C technique in this area is D.S. Bomse and T. H. Morton Tetrahedron Letters 1974 3491. J. Dabrowski K. Kamienska-Frela and L. Kozerski Org. Magn. Resonance 1974 6 499. 17 18 R. B. Jones and L.Phillips illustrated by a study of the diastereomeric modifications of some 2,3-disubsti- tuted succinic acids and a 3,4-disubstituted adipic acid. The diastereomers show significant differences in chemical shift for all carbons and this enables conforma- tional populations to be ded~ced.~ Conformational Properties of Glycosidic Linkages.These have been studied by the 3C n.m.r. method. The '3C-'H coupling between anomeric hydrogens and carbon in the aglycone moiety enables torsion angles to be deduced and the '3C chemical shifts give a measure of relative steric compression at the anomeric ~entre.~ The barrier to internal rotation around this bond in some nucleosides has been measured by using aglycone bases which have C, symmetry to ensure equal popu!ations of the two rotamers. The changes in lineshape of the I3C n.m.r. spectrum of the aglycone enable the barriers to be mea~ured.~ Alicyclic Compounds. Alicyclic compounds have been examined using the concept of relative high-field shifts for sterically compressed carbons.6-* Of particular interest is a study of cyclohexylmercuric derivatives which utilizes the fact that 3J199Hg-L3C is much larger (275 Hz) for equatorial mercury than for axial ( -70 Hz).For bromomercuri- and acetoxymercuri-substituents there is greater preference for axial orientation than equatorial and there is no steric compression shift of C-3,5 in the axial isomer.' The Conformations of Heterocyclic Compounds. ' 3C Studies resolve an ambiguity evident in previously published work on 1,Zdimethylhexahydropyridazines which suggested a significant population of the diaxial conformer ;the '3Cstudy shows that this is not so.'' In a similar study of double inversion at two N atoms this time in a bicyclic hydrazine the process was shown to be trans trans (1) rather than cisccis (2)." The variation of 'J13C-lH with the nature and 3 L.Ernst and W. Trowitsch Chem. Ber. 1974 107 3771. 4 R. U. Lemieux and S. Koto Tetrahedron 1974 30,1933. 5 H. von Voithenberg A. Skrzelewski J. C. Jochims and W. Pfleiderer Tetrahedron Letters 1974 4063. 6 R. J. Abraham C. M. Holden P. Loftus and D. Whittaker Org. Magn. Resonance 1974 6 184. 7 Y. Senda S.Imaizumi S. Ochiai and K. Fujita Tetrahedron 1974 30 539. 8 Y. Senda and S. Imaizumi Tetrahedron 1974 30 3813. 9 F. A. L. Anet J. Kraus W. Kitching D. Doddrell and D. Praeger Tetrahedron Letters 1974 3255. 10 S. F. Nelson and G. R. Weisinan J. Amer. Chem. Soc. 1974 % 711 1.11 Y. Nomura N. Masai and Y. Takeuchi J.C.S. Chem. Comm. 1974 288. Physical Methods and Techniques-Part (ii) Nuclear Magnetic Resonance geometrical arrangement of substituents in the vicinity of the C-H bond has been used to examine some hexahydro-3H-oxazolo[3,4-c]pyridines. l2 Long-range coupling between tin and carbon 5J13C-119Sn has been used as a probe for hyperconjugation from a carbon-metal bond into aromatic n-systems. It is shown to have marked stereochemical dependence and hence is a suitable conformational probe for such systems.' Application of Lanthanide Shift Reagents (LSR's).-An excellent review-type article has appeared which specifically examines the usefulness of LSR's as tools for the study of conformational equilibria in ~olution.'~ The difficulties encoun- tered in the differentiation between contact and pseudo-contact contributions to the observed shift and the location of the chelate with respect to the substrate by means of computerized search-and-fit programmes are discussed in detail.An equally thorough examination of the use of LSR's to determine conformational properties of acyclic alcohols is given by Williamson et al.' This points out the usefulness of Yb(d~m)~* for '3C studies since all contact shifts are eliminated and specifically examines the questions of distortions of the substrate by the complexation and of its influence upon the conformational equilibrium. Other work also affirms the usefulness of Yb(dpm) for 13Cstudies and points out the necessity of correcting for non-paramagnetic 'complexation shifts' using La(dpm) as a probe for this.16 The danger of distortion of structure or of favouring one partner of a conforma- tional equilibrium by shift reagents is obvious.Several pieces of work show that this is sometimes not a problem as in studies on dialkylnitro~amines'~ and NN-dimethylamides. Other work clearly demonstrates that the converse may apply however. Thus complexation of 1-methylcyclohexanol with a LSR causes the equatorial conformer of the alcohol to be more favoured.Ig N-Substituted-4- phenylpiperidines show differential complexation of equatorial and axial nitrogen lone pairs with the shift reagent Co" acetylacetonate; this work also presents evidence that the shift mechanism is pseudo-contact for this reagent.20 Studies in aqueous solution are of potential importance in systems of biological interest.Thus the perchlorates of Pr3 + and Ho3+have been used to examine the l2 Y. Takeuchi P. J. Chivers and T. A. Crabb J.C.S. Chem. Comm. 1974 210. l3 W. Adcock B. D. Gupta W. Kitching D. Dodrell and M. Geckle J. Amer. Chem. SOC. 1974 % 7360. l4 J. D. Roberts G. E. Hawkes J. Husar A. W. Roberts and D. W. Roberts Tetrahedron 1974 30 1833. l5 K. L. Williamson D. R. Clutter R.Emch M. Alexander A. E. Burroughs C. Chua and M. E. Bogel J. Amer. Chem. Sac. 1974 % 1471. l6 D. J. Chadwick and D. H. Williams J.C.S. Perkin 11 1974 1202. R. A. Perry and Y. L. Chow Cunud.J. Chem. 1974 52 315. G. Montaudo P. Maravigna S.Caccamese and V.Librando J. Org. Chem. 1974 39 2806. l9 J. Bouquant M. Wuilmet A. Maujean and J. Chuihe J.C.S. Chem. Comm. 1974 778. I. D. Blackburne A. R. Katritzky and Y. Takeuchi J. Amer. Chem. SOC.,1974 % 682. * dpm = dipivalomethanato. 20 R. B. Jones and L. Phill@s structure of cyclic j?-adenosine 3',5'-phosphate in aqueous solution ;the conform- ation is the same as in the crystalline state.2' Use has been made of edta com- plexes of La"' Pr'll Eu'" and Gd"' to examine the conformations in aqueous solution of adenosine 5'-monophosphate and cytidine 5'-rnonopho~phate.'~ A major advantage of these reagents is that they may be used at much higher pH values than the lanthanide aquo-ion probes which are insoluble above pH 6.23 Conformational Studies of Biological Interest.-The con formational properties of polypeptides are of continuing interest.Two series of macromolecular sequen- tial polypeptides containing up to four y-benzyl-~-glutamyl and one y-benzyl-L- glutamyl or L-leucyl residue in the repeating unit have been examined by 'H n.~.r.'~ The ordered conformations seem to be a distorted form of the a-helix with special types of cross lj-structure. A similar study of random copolymers of benzyl L-aspartate with 0-and p-nitrobenzyl aspartate shows that the presence of nitro-groups induces a reversal of the helix sense from left-hand to right-hand Complete assignment of the peptide NH resonances of angiotensin I1 (Asn' Vals) in aqueous solution has enabled 3JHNCH to be used for conformational analysis; no definite conclusions could be reached but the data exclude many possible arrangemenkZ6 A similar study has been made of the repeating penta- peptide unit of elastin." Following the identification by PerutzZ8 of two different quaternary structures for high-ligand-affinity deoxyhaemoglobins (T and R,conformational isomers) 'H n.m.r.has been used to observe the switch from T to R forms in solution at pH 9-7. The addition of inositol hexaphosphate at pH 7 reverses the change.29 Restricted rotation of a methyl group in a haem side-chain in some ferric myo- globin complexes has been examined. The barrier to this rotation may serve as a sensitive probe for the study of interactions between the haem side-chain and the ap~protein.~' Many molecules of biological importance form associations with metal ions in solution.The conformational implications of such associations have been re-examined for the case of the valinomycin-K+ complex. A 'H n.m.r. study at 220 MHz shows that the 3J, (aCH-CHD) of the valyl residue is 11.0 0.3 Hz rather than the previously suggested 3.54.0 Hz. This necessitates a revision of the hitherto accepted conf~rmation.~ The conformational change which occurs 21 D. K. Lavallee and A. H. Zeltmann J. Amer. Chem. SOC. 1974 96,5552. 22 C. M. Dobson R. J. P. Williams and A. V. Xavier J.C.S. Dalton 1974 1762. 23 C. D. Barry C. M. Dobson R. J. P. Williams and A. V. Xavier J.C.S. Dalton 1974 1765. 24 H. T. Storey R. C. Thompson P. M. Hardy and H. N. Rydon Polymer 1974 15 690.25 M.-H. Loucheux-Lefebvre A. Forchioni and C. Duflot Polymer 1974 15 474. 26 J. D. Gluckson J. Dadok and G. R. Marshall Biochemistry 1974 13 11. 27 D. W. Urry W. D. Cunningham and T. Ohnishi Biochemistry 1974 13 609. 28 M. F. Perutz Nature 1970 228 726. 29 S. Ogawa D. J. Patel and S. R. Simon Biochemistry 1974 13 2001. 30 I. Morishima and T. Iizuka J. Amer. Chem. Soc. 1974 96,7365. 31 D. W. Urry and N. G. Kumar Biochemistry 1974 13 1829. Physical Methods and Techniques-Part (ii) Nuclear Magnetic Resonance 2 1 when carp muscle calcium-binding parvalbumin releases its Ca2 + ion has been studied by a combination of 3C n.m.r. chemical shifts Nuclear Overhauser Effects and TI measurement^.^' There have been studies of small molecules of biological interest.For example in a study of some C-%substituted adenine nucleotides by ‘H and 1H[31P] Fourier transform n.m.r. it has been shown that the bulky C-8 substituent favours the syn orientation about the glycosidic linkage (3) rather than the more usual anti ~rientation.~~ This change is accompanied by distortion of the sugar phosphate backbone conformation about the C-4’-C-5’ bond. In a study of HO OH (3) X = Br or MeS 8-phenylpurines (3 ;sugar replaced by H X = Ph) conformational information concerning the imidazole-phenyl bond is obtained by studying the separation between the ortho proton resonance and the meta/pura resonances as a function of the nature of substituents in the base. The method calculates this separation as a function of the purine ring current the effect of which varies with the torsion angle.34 2 The Use of Spin-Lattice (Longitudinal) Relaxation Times in Structure Determination An excellent introduction to this potentially important area is given by Wehrli,35 and the reader is referred to this for an account of theory and practice up to the beginning of 1974.The most useful parameter is TyD,the contribution to TIobs (the observed longitudinal relaxation time) from dipole-dipole interactions with other dipolar nuclei in the molecule; this may be calculated if TIobs and the Nuclear Overhauser Enhancement* are known. A straightforward application of the method is the investigation of the variation of the 3C TI TyD,and NOE in cycloalkanes of different ring size.As the ring J2 S. J. Opella D. J. Nelson and 0.Jardetzky J. Amer. Chem. SOC.,1974 96,7157. 33 R. H. Sarma C. H. Lee F. E. Evans N. Yathindro and M. Sundaralingham J. Amer. Chem. Sor. 1974 96 7337. 34 F. Bergmann I. Tamir Z. Neiman and D. Lichtenberg Telrahedron 1974 30 3045. 35 F. W. Wehrli ‘Advances in Magnetic Relaxation’ ed. W. J. Orville-Thomas Academic Press London 1974. * NOE the increase in intensity of an observed resonance obtained upon saturation of the resonances in the system which are responsible for its relaxation. 22 R.B. Jones and L. Phillips size increases TlObs and T?D decrease markedly.36 'H Tl measurements may also show important variation with structure as has been demonstrated for the alkaloid vindolene in which each of the individual Tl values has been measured ; they vary from -0.45 to 2.7s and reflect the degree of crowding of each 'H by the others which are responsible for its rela~ation.~~ A rather more sophisticated study of 'H and 13C T,'s in small molecules (cis-and trans-crotonaldehyde) enables internuclear distances involving the methyl-group protons to be calcu- lated to a degree of accuracy comparable to that obtained by microwave methods.An important p~blication~~ studies dipole-dipole relaxation between protons in 3,4,6-tri-O-acetyl-1-O-benzoyl-2-chloro-2-deoxy-~-~-glucopyranose, using TI and NOE measurements based upon a new pulse technique. This enables the dipolar contribution to T of a given line in the 'Hspectrum arising from all other protons to be compared with the contributions from just its nearest neigh- bours and the ratio of the distances of these from the observed proton to be calculated.The specific replacement of hydrogens in a molecule by deuterium provides a useful structural probe for it is much less efficient in relaxing carbon to which it is bonded than is the lighter isotope. This has been exploited in the assignment of the I3C spectrum of thiamine hydr~chloride,~' where use is made of the fact that if all attached protons are replaced by *H the value of TYDfor 3C increases by a factor of ten; the effects are also seen by nearby non-hydrogen-bearing carbons which depend upon neighbouring hydrogens for relaxation. A I3C TIstudy of the back-bone and side-chains of oxytocin and lysin vaso- pressin gives a complete picture of their conformational fle~ibility.~' The suggestion is made that similar measurements on '3C-enriched neurohypophy- seal peptide hormones may allow a study of the conformational changes and dynamic processes which occur on binding to the neurophysins (intracellular protein carriers) A Tl study of the eleven resolvable "F resonances in fluoro- tyrosine alkaline phosphatase (corresponding to the eleven known tyrosines per subunit) has similarly enabled a detailed study of the general tertiary and quater- nary structure of the protein to be made.42 The differences in TI caused by different local environments of particular residues are a very useful probe and it is suggested that the fluorination approach may be of wider applicability.Several papers report the use of paramagnetic relaxation agents as alternatives or complements to lanthanide shift reagents as structural probes in solution. 36 S. Berger F. R. Kreissl and J. D. Roberts J. Amer. Chem. SOC.,1974 % 4348. 37 L. D. Hall and C. M. Preston Cunud.J. Chem. 1974.52 829. 38 R. Rowan tert. J. A. McCammon and B. D. Sykes J. Amer. Chem. SOC.,1974 % 4773. 39 R. Freeman H. D. W. Hill B. L. Tomlinson and L. D. Hall J. Chem. Phys. 1974,61 4466. 40 R. E. Echols and G. C. Levy J. Org. Chem. 1974 39 1321. 41 R. Deslauriers I. C. P. Smith and R. Walter J. Amer. Chem. SOC.,1974 % 2289. *' W. E. Hull and B. D. Sykes Biochemistry 1974 13 3431. 43 G. C. Levy and R.A. Komoroski J. Amer. Chem. SOC.,1974 % 678. Physical Methods and Techniques-Part (ii) Nuclear Magnetic Resonance 23 Their advantage is that the effects upon line-broadening (T,) or Tl show a simple r-6 dependence from the site of the metal. A definitive work by Levy and Komoro~ki~~ considers the effects of Cr and Fe trisacetylacetonates upon 13C T l’s in various organic substrates. The use of Gd(dpm) has also been e~amined.~~.~’ The selective relaxation effects upon 3Cresonance in adenosine 5’-triphosphate (ATP) which occur on binding Mn2 have been used to establish that the binding + site is the N-7 atom of the adenine base.46 Smith et aL4’ make use of the fact that TI relaxation of ,H resonance is entirely by an intramolecular quadrupole mechanism and consequently the interpretation of Tl’s is much easier than for ‘Hor I3C.They illustrate this by considering its implications with regard to stereochemistry intramolecular rotations and anisotropic motions in a variety of deuteriated alkane and aromatic deriva- tive~.~’ A note of caution is sounded in a paper which examines some of the experi- mental problems encountered in making Tl measurements?* From a study of I3C Tl in dioxan-D,O solutions by the progressive-saturation method35 it is noted that the dependence of this parameter on concentration roughly parallels the inverse of solution viscosity. A 2 1 (v/v) dioxan-D,O mixture has Tl of 6.2 0.6 s at 30 “C and is recommended as a standard for checking instrument performance.Using this the authors discuss the effects of dissolved oxygen intermolecular interactions and radiofrequency (H field inhomogeneity upon the experimental value of T ; in addition they discuss the experimental advan- tages of the progressive-saturation technique. Finally two papers deal with the determination of T in spin systems which are not in a Boltzmann equilibrium state. Ernst et al.49 point out that these may arise as a result of CIDNP effects,” Overhauser saturation experiments or radiofrequency pulses during a Tl measurement. They also occur in any repetitive Fourier transform experiment with a close pulse spacing (i.e.normal experimental conditions for I3C n.m.r.). Relative signal intensities vary as a result particularly in systems with homonuclear spin-spin coupling which is second order.No distortion of intensities will arise however in a weakly coupled system without degeneracy if a 90” pulse is used. The second paper5 deals specifically with the determination of TIin species with CIDNP using the polarized products of benzoyl peroxide decomposition for an example. 44 J. W. Faller and M. A. Adams Tetrahedron Letters 1974 699. 45 G. N. La Mar and E. C. Metz J. Amer. Chem. SOC.,1974 96 561 1. 46 Y. F. Lam G. P. P. Kuntz and G. Kotowycz J. Amer. Chem. SOC. 1974 % 1834. ” H. H. Mantsch H. SaitB L. C. Leitch and I. C. P. Smith J. Amer. Chem. SOC.,1974 % 256. 48 I. M. Armitage H. Huber D. H. Live H. Pearson and J. D. Roberts J. Magn. Resonance 1974 15 142. 49 S. Schaublin A. Hohener and R.R. Ernst J. Magn. Resonance 1974 13 196. I. Sadler Ann. Reports (B) 1973 70 22. ” K. A. Christensen D. M. Grant E. M. Schulman and C. Walling J. Phys. Chem. 1974 78 1971. 24 R.B. Jones und L. Phillips 4 The Study of ’H and 3H N.M.R. A survey52 of recent advances in deuterium and tritium n.m.r. covers work up to 1972. The field has since expanded and is obviously a growth point so a Report of progress in 1974 is in order. There have been studies of liquid-crystal structure by 2H In one of these,54 double-quantum transitions are observed in the case of a non-macro- scopically aligned lyotropic mesophase (a situation which is particularly favour- able for such observations). The observed intensities are so high that double- quantum transitions must be taken into account in the analysis and interpretation of 2H n.m.r.spectra of such systems. A complementary study is that of 2H n.m.r. in molecules which are aligned by liquid crystals. One report examines CD,SiH CH,SiD, and germanium analQgues in a nematic-phase liquid crystal. The calculated ratios of interatomic distances and bond angles which result are in good agreement with microwave data.55,56 2H N.m.r. has been used to study lipid bilayer~.~’-’~ In one of these ~tudies,~’ selectively deuteriated lipids are used to build up a liquid-crystalline bilayer and the anisotropic motion of the lipid molecules is studied. The results are compared with those obtained using a nitroxide spin label which the authors assert causes a significant distortion of the structure; the ’Hn.m.r.results represent a real improvement on the spin-label method. The point is reiterated in a second paper,58 and the same workers also examine the dynamic structure of the fatty acyl chains in a phospholipid bila~er.~~ Much the same comments emerge from work by Reeves et who examine the possibility of using selective or massive deuteriation together with 2Hn.m.r. to examine the local degree of micro-orientation in chain segments in membrane systems. The first application of 3H n.m.r. to a biosynthetic investigation has now appeared. [3H]Acetate is incorporated into penicillic acid by Penicillium cyclo-pium and the stereospecificity of labelling of the vinyl methylene group was directly demonstrated.6’ The authors point out that adequate spectra may be obtained from samples containing as little as 1 mCi of 3H,and there is no signifi- cant radiological hazard.The same group has used 3H n.m.r. to study the steric course of the A’-dehydrogenation of testosterone by a highly stereospecific 52 P. Diehl ‘Nuclear Magnetic Resonance Spectroscopy of Nuclei ather than Protons’ ed. T. Axenrod and G. A. Webb Wiley New York 1974 p. 275. 53 Z. Luz. R. C. Hewitt and S. Meiboom J. Chem. Phys. 1974 61 1758 ” H. Weunerstrom N. 0.Persson and B. Lindrnan J. Mugn. Resonance 1974 13 348. 55 R. Ader and A. Loewenstein Mol. Phys. 1974 27 11 13. 56 R. Ader and A. Loewenstein J. Amer. Chem. SOC.,1974 96,5336. ’’ J. Seelig and W. Niederberger J. Amer. Chem. SOC.,1974 96,2069.58 J. Seelig and W. Niederberger Biochemistry 1974 13 1585. ’9 A. Seelig and J. Seelig Biochemistry 1974 13 4839. 6o F. Fujiwara L. W. Reeves A. S. Tracey and L. A. Wilson J. Amer. Chem. SOC.,1974 % 5249. 61 J. M. A. Al-Rawi J. A. Elvidge D. K. Jaisival J. R. Jones and R. Thomas J.C.S. Chem. Comm. 1974 220. Ph-ysical Methods und Techniques-Part (ii) Nuclear Magnetic Resonunce 25 microbial lor,2P-truns elimination.62 In a full paper,63 they demonstrate the equality between 3H and ‘H chemical shifts and from an accurate measurement of the ratio between the gyromagnetic ratios of the two nuclei propose that ‘ghost references’ derived from observed ‘H n.m.r. frequencies may be used for 3H n.m.r. In the same work they give a simple example of a mechanistic application of 3H n.m.r.by a study of the Reimer-Tiemann reaction. 5 Carbon-13 N.M.R. Theory.-Factors influencing I ‘C chemical shifts remain poorly understood and the calculation of ‘useful‘ values is a difficult task. Extensive studies of polar substituent effects in alk~l~~ derivatives have been published but no and meaningful correlation of shieldings with 6-or total electron densities (calculated by extended HMO techniques) was obtained particularly for /3 or y effects where qualitative trends could not be reproduced. The use of the Average Excitation Energy (AEE) approximation AE in calculating paramagnetic* contributions (cP)to carbon shieldings in alkanes has been criticized and other approaches have been proposed allowing (a) variation in AE between different molecules obtained by calculation (modified MIND0/2) of all available excitation energies66 and (b)dependence of a ‘localized AE’ of a given carbon nucleus on substitution elsewhere in the molec~le.~ Both papers describe encouraging correlations with experiment.Good correlation has been described between alkane shieldings calculated within the framework of the AEE approximation by a population analysis modified to allow for polarity of the C-H bond.68 Variation in AE has been invoked to explain the carbon shifts of thiocarbonyl resonances com- pared with their carbonyl analogues69 and the protonation shifts of pterine and its derivative^.^' Deshielding S effects observed in various systems for carbon atoms in sterically crowded situations must lead to re-evaluation of the much-used steric explanation of shielding effects.’ In methyl decalin and androstane derivatives a 1,3-diaxial interaction between Me and OH deshields the former by 24 ~.p.m.’~ 62 J.M. A. Al-Rawi J. A. Elvidge R. Thomas and B. J. Wright J.C.S. Chem. Comm. 1974 1031. h3 J. M. A. Al-Rawi J. P. Bioxsidge C. O’Brien D. E. Caddy J. A. Elvidge J. R. Jones and E. A. Evans J.C.S. Perkin 11 1974 1635. 64 G. Miyajima and K. Nishimoto Org. Magn. Resonance 1974 6 313. 65 G. Miyajima K. Takahashi and K. Nishimoto Org. Magn. Resonance 1974 6 413. 66 I. Ando A. Nishioka and M. Kondo Bull. Chem. SOC.Japan 1974 47 1097. h7 K. Takaishi I. Ando M. Kondo R. Chujo and A. Nishioka Bull. Chem. SOC.Japan 1974 47 1559.6* S. Fliszar A. Goursot and H. Dugas J. Amer. Chem. SOC.,1974 96 4358. h9 H. 0. Kalinowski and H. Kessler Org. Nagn. Resonance 1974 6 305. 70 U. Ewers H. Gunther and L. Jaenicke Chem. Ber. 1974 107 3275. ’I J. B. Stothers ‘Carbon-13 N.M.R. Spectroscopy’ Academic Press New York 1972 p. 112. 72 S. H. Grover and J. B. Stothers Canad. J. Chem. 1974 52 870. * The paramagnetic term in the Ramsey expression for shielding. We consider the frequent use of ‘paramagnetic’ and ‘diamagnetic’ as synonyms respectively for ‘downfieid’ and ‘upfield’ inadvisable. 26 R. B. Jones and L. Phillips Similarly the peri interaction between methyl groups in 1,8-dimethylnaphthalene deshields both by 6.6 p.p.m. relative to 1-meth~lnaphthalene.~ Other examples of downfield 6 and E effects have been described for the methoxy carbon atom in 2,6-dialkylated an is ole^^^ and the methyl carbon in related 2,6-dialkylated methyl phenyl ~ulphides,~ which increase with increasing size of the alkyl substituents for the peri interaction between methyl groups on C-3 and C-4 of ben~ofuran,~ and for interactions between methyl groups in cyclohexanone derivative^.^^ Of possibly related origin are the shielding effects seen on reson- ances from sterically compressed aromatic carbons in 2,2-metacy~lophanes,~~ although the authors suggest that n-electron redistribution due to direct trans- annular repulsion is responsible.Shielding E effects have been seen in 43-dimethylphenanthrene resonances from sterically interacting methyls being to high field (4.6p.p.m.) of the methyl resonance of 4-meth~lphenanthrene,~~ and on the -S(0)CH3 in 2,4,6-tri-isopropylphenylmethyl sulphoxide (relative to phenyl methyl sulphoxide) although deshielding E effects were seen in both the related sulphides and sulphones.75 Further understanding of the origin of the y effect must derive from data obtained from aliphatic molecules of rigid conformation where both Ygauche and yfranseffects may be simultaneously studied. The study of a number of 'frozen-out' cyclohexane derivatives" illustrates not only the application of 3Cn.m.r. to this type of conformational problem but also provides direct comparison of Ygaucfte (i.e.substituent axial) and ytrans (equatorial) effects. The significant size of the latter and its correlation with electronegativity (increasing electronegativity f increasing shielding) suggests that electronic (through bond?) contributions cannot be ignored in explaining the y effect.Further ygouche correlates with ytrans (ygauche = -2.5ylrans) for small electronegative substituents behaviour seen in other systems where steric interactions are thought to be unimportant.81 The relationship persists even where similarly sized substituents (i.e.CN and NC) produce greatly differing Ygauehe effects and may therefore be evidence for pre- dominance of (stereospecific?) electronic ygouche effects in these systems. yrrans Effects have also been measured in 1-substituted adamantanes,82 where electro- negativity dependence in the series Me NH, OH and F was also observed.However the trend for the halogens differs in sign from that of the equatorial cyclohexyl halides. Attempts to separate steric from electronic shielding contri- 73 N. K. Wilson and J. B. Stothers J. Magn. Resonance 1974 15 31. 74 G. W. Buchanan G. Monlaudo and P. Finocchiaro Canad. J. Chem. 1974,52 767. 75 G. W. Buchanan C. Reyes-Zamora and D. E. Clarke Canad. J. Chem. 1974,52,3895. 76 N. Platzer J. J. Basselier and P. Demerseman Bull. SOC.chim. France 1974 905. 77 J. B Stothers and C. T. Tan Canad. J. Chem. 1974,52 308. 78 T. Sato T. Takemura and M. Kainosho J.C.S. Chem. Comm. 1974 97. 79 J. B. Stothers personal communication quoted in ref. 75. H. J. Schneider and V. Hoppen Tetrahedron Letters 1974 579.' R. B. Jones and L. Phillips unpublished results. 82 G. E. Maciel H. C. Dorn R. L. Greene W. A. Kleschick M. R. Peterson and G. H. Wahl Org. Magn. Resonance 1974 6 179. Physical Methods and Techniques-Part (ii) Nuclear Magnetic Resonance 27 butions in 9cc-substituted cortisolss3 illustrate further the inadequacy of present shielding theories. The failure of CND0/2 to estimate electronic shielding effects and the inverse correlation of ygauchewith substituent size were noted although for a given substituent correlation of ygauckeshielding with internuclear distances (X-ray data) was seen. The downfield Y~,~,,~ effect noted for the C-19 methyl group on the introduction of the 9a-F has also been observed in other related systems" and contrasts with the upfield effects seen at methylene and methine centres described in the above This illustrates the structural dependence of the y effect and emphasizes the caution with which 13C substituent additivity rules must be treated.Studies on unsaturated fatty acids have shown the importance of n-bond polarization by intramolecular dipolar electric fields on olefinic chemical shifts ;84 substituent effects were interpreted in terms of steric and linear electric field effects. Contributions to coupling constants between directly bonded '3C nuclei arising from various mechanisms have been calculated and good agreement is claimed with e~periment.~' Vicinal 3J13c-,3c couplings were found to follow closely a calculated 'Karplus-type' curve.86 Applications.-Applications of 3Cn.m.r.are growing rapidly with a consequent enormous increase in the volume of data published although it is disturbing to note cases in which assignment of 13C spectra of available compounds has become an end in itself. The use for full unknown structure determination remains impossible in the absence of closely related models because of the general lack of understanding of l3C shielding but information available from noise- decoupled and 'off-resonance' spectra (carbon types and numbers of bonded protons) has frequently proved useful in structural problems. Physical Organic Chemistry. Emphasis here is given to selected applications of 13Cn.m.r. to physical organic problems and much work has been directed to protonation-deprotonation effects on carbon resonances.Predominant 0-protonation of HCONMe in aqueous acidic media has been confirmeds7 by the observation of discrete I3C methyl resonances from the protonated species. Slow isomerization (uia N-protonation) sufficient to cause coalescence of the 'H methyl resonances because of the small chemical shift differences limited the application of 'H n.m.r. to this problem. Rates of hindered rotation and acid- base exchange of benzaldehydes in superacid media have been determined by analysis of I3C spectra with a four-site exchange programme the proton 83 D. D. Giannini P. A. Kollman N. S. Bhacca and M. E. Wolff J. Amer. Chem. SOC. 1974 96 5462. J. G. Batchelor R. J. Cushley and J. H. Prestegard J. Org. Chem. 1974 39 1698.'* J. M. Schulman and M. D. Newton J. Amer. Chem. SOC.,1974 96,6295. 86 D. Doddrell I. Burfitt J. B. Grutzner and M. Barfield J. Amer. Chem. SOC.,1974 96. 1241. " R. A. McQelland and W. F. Reynolds J.C.S. Chem. Comm. 1974 824. 88 T. Drakenberg S. Forsen and J. M. Sommer J.C.S. Perkin ZZ 1974 520. 28 R. B. Jones and L. Phillips spectra were far too complex to be handled in this way. I3C Chemical shifts have been used to estimate n-electron densities in several mesoionic sydnones and related compoundsg9 and confirm calculated predictions. Protonation of 3-aryl-sydnones in strong acid media has been studied by * 3C and 15N n.m.~.~O Changes in 13C shifts as a function of pH obtained by titration have much application for analysis of spectra of peptides and proteins.” Titration curves for amino-acids have been de~cribed.~~,~~ The ionization shifts on deprotonation at adjacent sites have been attributed to competition between deshielding re- sulting from a decrease in AE and shielding by increased electron density whereas at remote sites only the latter effect is important.92 Some additivity of protonation shifts has been ~laimed.’~ Both 3513c-lH coupling9’ and the pH dependence of ’ 3C resonances93 have been used to determine side-chain configurations in amino-acids.The co-ordination of organic molecules to metal ions can be examined where discrete carbon resonances are visible from both bound and free molecules. The solvation of A13+ and Mg2+ by alcohols96 and the complexation of A13+ with amides” have been thus studied.I3C and I5N spectra of chlorophyll u and its magnesium-free derivative pheophytin have been used to study the binding of the central magnesium atom ; quaternary carbons were assigned by their 2J, and 3JcHcouplings to visible protons by INDOR techniques. The effect of the magnesium atom was analysed in terms of perturbation of the interaction of the nitrogen lone pairs with the unoccupied n*-orbitals leading to variation in AE(n,n*)and thereby alteration of op,and a qualitative picture was obtained of the nature of the Mg-N bond-ing.’” ’ 3C Studies on coproporphyrin-type molecules and their zinc@) and thallium(w) derivatives have been used to support the existence of an 18-atom 18-n-electron delocalization system in these molecules over the alternative proposal of a 16-atom 18-n-electron system.’” Significant broadening of the signal (15-20 Hz) of the a-pyrrole carbon atoms was attributed to tautomerism of the NH protons between two available sites at a rate corresponding to coales- cence of the carbon signals from each distinct environment.The unpaired- 89 M. T. W. Hearn and K. T. Yotts J.C.S. Perkin II 1974 875. 90 A. J. Buglass J.C.S. Chem. Comm. 1974 313. 91 J. Feeney P. Partington and G. C. K. Roberts J. Mugn. Resonance 1974 13 268. ” A. R. Quirt J. R. Lyerla I. R. Peat J. S. Cohen W. F. Reynolds and M. H. Freedman J. Amer. Chem. Soc. 1974 % 570. 93 S. Tran-Dinh S. Ferrnandjian E. Sala R. Mermet-Bouvier M. Cohen and P. Fromageot J. Amer.Chem. Soc. 1974 % 1484. 94 K. F. Koch J. A. Rhaades E. W. Hagaman and E. Wenkert J. Amer. Chem. Sot. 1974 % 3300. 95 J. Feeney P. E. Hanson and G. C. K. Roberts J.C.S. Chem. Comm. 1974 465. 96 G. W. Stockton and J. S. Martin Canad. J. Chem. 1974,52 744. 97 D. Canet J.-J. Delpuech M. R. Khaddar and P. Rubini J. Magn. Resonance 1974 15 325. 9* T. T. Nakashima D. D. Traficante and G. E. Maciel J. Phys. Chem. 1974 78 124. 99 I. D. Gay J. Phys. Chem. 1974 78 38. S. G. Boxer G. L. Gloss and J. J. Katz J. Amer. Chem. Soc. 1974 % 7058. lo’ R. J. Abraham G. E. Hawkes and K. M. Smith J.C.S. Perkin II 1974 627. Physical Methods and Techniques- Purr (ii) Nuclear Magnetic Resonance electron spin density in low-spin iron(m) complexes with protoporphyrin has been determined by measurement of the 13C hyperfine shifts.lo2 The data have been used to prove the interaction of peripheral substituents on the molecule with the delocalized n-system to assist understanding of the relationship between electronic structure and the biological role of these molecules.The association of acetone and phenol in CCl solution has been studied by observation of the carbonyl 3C re~onance.~~ Thermodynamic data for self- association of these species were also obtained. 13CShifts and 1J13c-lH couplings have been measured for simple organic molecules adsorbed on to silica ;99 carbon atoms adjacent to probable binding sites generally showed downfield shifts relative to the neat liquid (0-10 p.p.m.) whereas other carbons showed upfield shifts (0-2 p.p.m.).Extensive work on the 13C n.m.r. of onium ionslo3 and carbocation~'~~ in superacid media has been completed by Olah. Organometallic Chemistry. Extensive use has been made of 3C n.m.r. in organo- metallic chemistry to aid structure determination and deduce information about C-metal bonding. As frequently admitted the latter deductions are handicapped by ignorance of the factors influencing the observed shieldings. Studies on car- bony1 complexes are widespread and early work has been reviewed;'05 reson- ances usually lie in the region 150-250 p.p.m. below TMS with a general trend to lower field with increased bridging. The application of the relaxation agent Cr(acac) is widespread as is the use of 13C-enriched CO to overcome sensitivity problems caused by low solubility at the low temperatures frequently required.Application to structure determination is illustrated by the study on [Rh 2(CO),o(p2-CO)2(p3-CO)8]2 -in the presence of Cr(acac) ,where resonances due to terminal and doubly and triply bridged carbonyls could be distinguished and integrated. lo6 'JRhXcouplings are inversely related to Rh-C bond lengths. O6 The qualitative solution dynamics of M3(CO),2 systems (M = Fe Ru or 0s) have been studied by variable-temperature '3C n.m.r.lo7 The activation energy for the isomerization of the [(Cp)(OC)Fe(p-CO),Fe(CO)(Cp)]molecule has been estimated by simulation of the temperature dependence of the carbonyl signals. lo* Severe quadrupolar broadening of carbonyl resonances has been demonstrated for carbonyl derivatives of 59C0(I = 7/2) and 55Mn (I = 5/2) which is reduced considerably by recording spectra at lower temperatures.'02 K. Wuthrich and R. Baumann Helv. Chim. Acra 1974 57 336. Io3 G. A. Olah G. Liang and J. Staral J. Amer. Chem. Soc. 1974 % 81 12 and references therein. lo' G. A. Olah G. Liang A. Babiak and R. K. Murray J. Amer. Chem. SOC. 1974 96 6794 and references therein. '05 L. J. Todd and J. R. Wilkinson J. Organometallic Chem. 1974 77 1. lob P. Chini S. Martinengo D. J. A. McCaffey and B. T. Heaton J.C.S. Chem. Comm. 1974 310. lo' A. Forster B. F. G. Johnson J. Lewis T. W. Matheson B. H. Robinson and W. G. Jackson J.C.S. Chem. Comm. 1974 1042. lo*D. C. Harris E. Rosenberg and J.D. Roberts J.C.S. Dalton 1974 2398. Io9 L. J. Todd and J. R. Wilkinson J. Organometallic Chem. 1974 80 C31. 30 R.B. Jones and L. Phillips Several attempts have been made to examine the nature of the metal-C=O bonds from I3C measurements. It is argued"' that correlation of I3C shifts with carbonyl i.r. stretching frequencies and metal-C bond lengths leads to the hypothesis that increasing metal-warbonyl n-back donation causes progressive deshielding of the carbonyl resonance. The shielding of the carbon nucleus is inversely related to the carbon electron density and the authors therefore attribute the observed trends to variation in cp,but were unable to justify this assumption qualitatively. Studies' ' in some thiocarbonyl derivatives demonstrated the con- siderable deshielding of thiocarbonyl resonances over carbonyl resonances [by 218 p.p.m.for (Z-C,H,)M~(CO)~(CS) and 105 p.p.m. for (7t-C5H5)Fe(CO),(CS)PF6]. This was attributed to changes in both AE in cpand increased metal n-back donation to the thiocarbonyl ligand. The interpretation of chemical shifts of metal-bound carbonyls purely in terms of changes in oPalone has been strongly and it has been pointed out that variation in the calculated diamag- netic shielding terms (ad)of carbonyl groups bound to different heavy metals can amount to several hundred p.p.m. Further evidence for the importance of c,,has come from studies of the carbonyl shifts of Ni(CO) and Fe(CO) relative to those of free C0.113Independent attempts to calculate both cdand cpterms have led to the conclusion that the carbonyl shifts on binding to Ni (12.5 p.p.m.) could be explained by changes in either cdor CT, (or both) whereas the shift on binding to Fe (31 p.p.m.) could only be explained by changes in cP.I3C Studies on both carbonyl and aryl nuclei on complexation of Cr(CO) with monosubstituted benzenes were interpreted as indicating a net withdrawal of electron density by the Cr(CO) fragment from the framework of the arene ring.114 Several studies have appeared on 13C shifts and 19sPt-'3C couplings in organo-plati-num(Ir),' 15-i17 and organo-platinum(1v)' ' derivatives and the 'cis and trnns n.m.r.influences' of other co-ordinated ligands have been discussed. The former is tentatively attributed to steric interactions"7 and the latter which follows the same qualitative relationship in both Pt" and Pt'" derivatives,"' to a-orbital rehybridization.Studies on the complexation of Ag' with ketones have suggested that the carbonyl group acts as an n-rather than a n-donor."' Simple olefins bound to Ag' have been studied both in solution120 and absorbed on to an AgNaX-type G. M. Bodner and L. J. Todd Inorg. Chem. 1974 13 1335. ]I1 G. M. Bodner Inorg. Chem. 1974 13 2563. I I J. Evans and J. R. Norton Inorg. Chem. 1974 13 3042. *I3 H. Mahnke R. K,Sheline and H. W. Spiess J. Chem. Phys. 1974 61 55. 'I4 G. M. Bodner and L. J. Todd Inorg. Chem. 1974 13 360. 'I5 H. C. Clark and J. E. H. Ward J. Amer. Chem. SOC.,1974 96 1741. H. C. Clark L. E. Manzer and J.E. H. Ward Canad. J. Chem. 1974,52 1165. H. C. Clark and J. E. H. Ward Canad. J. Chem. 1974,52 570. H. C. Clark t.E. Manzer and J. E. H. Ward Canad. J. Chem. 1974 52 1973. D. R. Crist Z.-H. Hsieh G. J. Jordan F. P. Schinco and C. A. Maciorowski J. Arner. Chem. Soc. 1974 96,4932. 120 D. M' ichel W. Meiler and E. Angele 2.phys. Chem. (Leipzig) 1974 255 389. Physical Methods and Techniques-Part (ii) Nuclear Magnetic Resonance 3 1 zeolite.12' The olefinic carbon resonances showed high-field shifts on binding of similar magnitude in each case. Biochemistry.* Uses of 13C as biosynthetic label are growing the feeding of doubly labelled acetate ['3CH3'3C02Na] becoming increasingly popular for obtaining information about bond fission during biosynthesis.Incorporations required for visible peak enhancements (generally >0.2%)have largely limited the application of '3Cn.m.r. to micro-organisms and fungal metabolites although incorporation of a I3C-labelled precursor into a higher plant has been detected.' 22 The incorporation of singly labelled acetate@) alone has been used to study the biosynthesis of hirsutic acid C,123 taji~anthone,'~~ cytochalasinsB and D,125and metacyclo- and undecyl-prodigiosin.'26 Feeding of doubly labelled acetate (with the subsequent observation of 'Jcc satellite lines arising from acetate subunits which remain intact during biosynthesis) has been used to prove methyl migration during the biosynthesis of the triprenylphenol asc~chlorin,'~' to suggest oxidative fission of a preformed aromatic precursor during the biosyn- thesis of multicolic and multicolosic acids,*28 to prove C-C bond fission during the biosynthesis of the fungal sesquiterpenoid o~alicin,'~~ to distinguish between alternative folding patterns of the polyketide chains prior to cyclization in the biosynthesis of the metabolite ~terigmatocystin,'~~ to study the polyketide origin of the metabolite ~chrephilone,'~' and to determine the partial acetate origin of the metabolite tenellin.'32 The latter authors argue that the 'doubly labelled acetate' method is superior to the singly labelled technique where only low 3C incorporations are achieved if it may be assumed that contributions to spin-lattice relaxation from bonded '3C are negligible.This has been disproved for non-protonated carbon atoms,133 where I3C-' 3Cdipolar relaxation provides a very important contribution to the overall relaxation rate (1/Tl).Further where coupled carbon resonances are close together they constitute an AB system and signal intensity errors (a function of relaxation rates and flip angles) are seen in tightly coupled homonuclear systems in non-Boltnnann equilibrium states as is the case for rapidly pulsed FT experiment^.'^^ l2 I D. Michel W. Meiler and D. Hoppach Z. phys. Chem. (Leipzig) 1974 255 509. A. R. Battersby P. W. Sheldrake and J. A. Milner Tetrahedron Letters 1974 3315. lZ3 T. C. Feline G. Mellows R. B. Jones and L. Phillips J.C.S. Chem. Comm. 1974 63. IZ4 J. S. E. Holker R. D. Lapper and T. J. Simpson J.C.S.Perkin I 1974 2135. W. Graf J. L. Robert J. C. Vederas C. Tarnm P. H. Solomon I. Miura and K. Nakanishi Hefu. Chim. Acta 1974 57 1801. '26 H. H. Wasserrnan C. K. Shaw R. J. Sykes and R.J. Cushley Tetrahedron Letters 1974 2787. M. Tanabe and K. T. Suzuki J.C.S. Chem. Comm. 1974,445. J. A. Gudgeon J. S. E. Holker and T. J. Simpson J.C.S. Chem. Comm. 1974 636. M. Tanabe and K. T. Suzuki Tetrahedron Letters 1974 4417. I30 H. Seto L. W. Cary and M. Tanabe Tetrahedron Letters 1974 4491. 13' H. Set0 and M. Tanabe Tetrahedron Letters 1974 651. 13' A. G. McInnes D. G. Smith J. A. Walter L. C. Vining and J. L. C. Wright J.C.S. Chem. Comm. 1974,282. C. G. Moreland and F. I. Carroll J. Magn. Resonance 1974 15 596. S. Schaublin A. Hohener and R.R. Ernst J. Magn. Resonance 1974 13 196. * See also Ch. 15. 32 R. B. Jones and L. Phillips Examples of the use of other 13C precursors in biosynthetic studies have been described. [2-13C]Mevalonic acid has been synthesized and incorporated into the sequiterpenoid trichothecolone. (2RS,3S)-[4-'3C]Valine has been syn- thesized and incorporated into penicillin V.'36 [1-' 3C]Glucosamine and [6-'3C]Glucose have been incorporated into the antibiotic neomycin. 37 I3C-Labelled methionine baminolevulinic acid porphobilinogen and uroporphy- rinogens I-IV have been incorporated into vitamin B ,.'38 Selective bio- synthetic enrichment studies with labelled amino-acids have been used to assign all carbon resonances seen from the decapeptide gramicidin S-A.139 13Chas been utilized in many other diverse areas of biochemistry. An n.m.r. probe designed to take sample tubes of 20 mm 0.d. has been developed and has enabled observation of 13Cresonances from some minor bases in tRNA which were assigned by comparison with models.'40 '3C7'''s have been used to confirm the random coil conformation of the linear pentadecapeptide gramicidin A in E2H6]DMS0 and the existence of its double-helical dimer in CD30D.14' The binding of '3C0 to various haemoglobins has been monitored by means of the carbonyl resonances. The two discrete resonances seen were specifically assigned to 13C0bound to either Q or /Isubunits and both the thermodynamic stability and the ease of replacement of CO by O2were found to differ for each site.'42 An unusually high pK value of '3C-enriched carboxymethyl-cobalamin (HO '3C'3CH,-cobalamin) was determined by the titration shifts of both methylene and carboxyl ~arb0ns.l~~ The interaction of the Cu2 + ion with pyrimi- dine nucleosides and nucleotides has been studied by observation of the changes in relaxation rates (estimated by conventional inversion-recovery techniques and from signal linewidths) of 13C nuclei due to the paramagnetic ion.144 This technique has been criticized.14' The specific co-ordination of Ca2+ and Na' (but not K+ or Zn2 +)with the macrocyclic antibiotic rifamycin S has been proven by the magnitudes of the shifts on the 13C resonances observed on addition of the metal ion to the antibi~tic.'~~ Assignment ofCarbon Resonances.Several novel assignment techniques have been described in the period of this Report. 'Partially Relaxed Fourier Transform' 135 J. R. Hanson T. Marten and M. Siverns J.C.S. Perkin I 1974 1033. 13' D. J. Aberhart and L. J. Lin J.C.S. Perkin I 1974 2320. 13' K. L. Rinehart J. M. Malik R. S.Nystrom R. M. Stroshane S. T. Truitt M. Tani- guchi J. P. Rolls W. J. Haak and B. A. Ruff J. Amer. Chem. Soc. 1974 96,2263. 13' A. I. Scott C. A. Townsend K. Okada M. Kajiwara R. J. Cushley and P. J. Whitman 1. Amer. Chem. SOC.,1974 96,8069. 139 J. A. Sogn L. C. Craig and W. A.Gibbons J. Amer. Chem. SOC., 1974 96,3306. laa R. A. Komoroski and A. Allerhand Biochemistry 1974 13 369. la' E. T. Fossel W. R. Veatch Yu. A. Ovchinnikov and E. R. Blout Biochemistry 1974 13 5264.R. B. Moon and J. H. Richards Biochemistry 1974 13 3437. 143 T. E. Walker H. P. C. Hogenkamp T. E. Needham and N. A. Matwiyoff J.C.S. Chem. Comm. 1974 85. ja4 G. Kotowycz Canad. J. Chem. 1974,52 924. Ia5 W. G. Esperin W. C. Hutton S. T. Chow and R. B. Martin J. Amer. Chem. SOC. 1974 96,8111. D. Leibfritz Tetrahedron Letters 1974 4125. Physical Methods and Techniques-Part (ii) Nuclear Magnetic Resonance 33 spectra (PRFT) have been used as an alternative to off-resonance double ir- radiation for determination of numbers of directly bonded protons during the assignment of the sesquiterpenoid ~uanhternone'~~ and the polyketides cyto- chalasins €3 and D.125 The use of paramagnetic relaxation agents to complement lanthanide shift reagents has been mentioned above.The interaction of Cr(acac) and Fe(acac) with a number of various functionalized substrates has been studied and the advantages and disadvantages of their use have been discussed.43 The lanthanide relaxation agent Gd(fod) has been used to assign olefinic carbons in the conjugated side-chain of all-tr~ns-retinal.'~~ The Gd(fod) binds to the terminal aldehyde function and addition of successive aliquots causes the se- quential disappearance (through line-broadening) of resonances from carbon atoms at increasing distances from the binding site. The use of TiCl as an alternative shift reagent to Eu(fod) in 13Cn.m.r. has been ~uggested.'~~ Observed shifts are generally similar although TiCI appears to produce greater shifts of carbonyl resonances; its smaller size may also be of advantage in sterically hindered systems.An attempted use of long-range 13C-'H couplings to assign quaternary atoms in polynuclear aromatic hydrocarbons failed because of the difficulties of corre- lating the observed couplings with structure.' 50 The assignment of carbonyl resonances in large molecules (especially in peptides and proteins) is particularly difficult and several approaches have been made to this problem. The pH dependence of carboxylate functions is useful where pK,'s are known or accurately estimable." In cases where carboxylate resonances are hidden they may be detected and assigned by a subtraction of a spectrum obtained at one pH from another obtained at a different pH.pH- insensitive resonances cancel and therefore disappear revealing the hidden carboxylate resonance. This technique was used during a study on the basic pancreatic trypsin inhibitor.' 5' Two further general techniques are the solvent dependence of carbonyl resonances and their assignment by solvent-mixture titration' 52 and the detection and specific collapse by secondary irradiation of long-range C-H coupling^.^^ Amide resonances may be assigned by recording spectra in a 50 50mixture of H20and D,O. The NH/ND exchange is sufficiently slow to show two lines in the I3C spectrum (corresponding to -CONH- and -COND-species) separated by a typical deuterium isotope effect ofO.1 p.p.m9' The well known 'second-order 'H off-resonance fingerprint' effects,' 53,1 54 of considerable value in interpreting off-resonance spectra have been quantitatively explained for a AA'X system.' '4i K.Nakanishi R. Crouch I. Miura X. Dominguez A. Zamudio and R. Villareal J. Amer. Chem. SOC.,1974 % 609. 148 R. Rowan and B. D. Sykes J. Amer. Chem. SOC.,1974 96,7000 149 A. K. Bose and P. R. Srinvasan J. Magn. Resonance 1974 15 592. R. S. Ozubka G. W. Buchanan and I. C. P. Smith Cunad. J. Chem. 1974,52 2493. 151 W. Maurer W. Haar and H. Ruterjans Z. phys. Chem. (Frankfurr) 1974 93 119. '" D. W. Urry L. W. Mitchell and T. Ohnishi Biochemistry 1974 13 4083. Is' G. Jikeli W. Herrig and H. Gunther J. Amer. Chem. SOC. 1974 96,323. J. B. Grutzner J.C.S.Chem. Comm. 1974 64. ' 55 H. Fritz and H. Sauter J. Magn.Resonance 1974 15 177. 34 R .B. Jones and L. Phillips Several very closely related techniques have been described for the assignment and sign determination of '3C-'H coupling constants involving pulsed hetero- nuclear decoupling to produce total' 56-' 58 or partial population in~ersion'~~-'~~ of selected proton transitions seen as intensity variations in the carbon spectrum. Total population inversion produces FT spectra analogous to CW INDOR spectra. 156 K. G. R. Pachler and P. L. Wessels J. Magn. Resonance 1973 12 337. 57 A. A. Chalmers K. G. R. Pachler and P. L. Wessels Org. Magn. Resonance 1974 6 445. ls8 A. A. Chalmers K. G. R. Pachler and P. L. Wessels J. Magn. Resonance 1974 15 415. i59 J. Runsink J. de Wit and W. D. Weringa Tetrahedron Letters 1974 55.160 A. Kiewiet J. de Wit and W. D. Weringa Org. Magn. Resonance 1974 6 461. 16' S. Sorensen R. S. Hansen and H. J. Jakobsen J. Magn. Resonance 1974 14 243. '62 H. J. Jakobsen S. A. A. Linde and S. Sorensen J. Magn. Resonance. 1974.15. 385.

ISSN:0069-3030    

DOI:10.1039/OC9747100017

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

2 Physical Methods and Techniques Part (iii) Circular Dichroism By P. M. SCOPES Department of Chemistry Westfietd Coltege London NW3 7ST 1 Introduction In the two years since this subject was last discussed in Annual Reports circular dichroism (c.d.) has essentially replaced optical rotatory dispersion (0.r.d.) as the main chiroptical technique for the study of dissymmetric molecules in the visible and U.V. regions of the spectrum. During the same period several related but specialized topics have been developed which augment ‘normal’ c.d. These topics include important work on circularly polarized Raman spectra by Barron and Buckingham’ and on i.r. c.d. by Holzwarth and his colleagues.2 Studies of induced c.d. in achiral aromatic compounds have been made by two independent groups of worker^,^*^ and several more papers on magnetic c.d.have also ap- peared.’ The proceedings of the NATO advanced study institute on 0.r.d. and c.d. held at Pisa in September 1971 were published during 19736 and a special collection of papers has appeared’” which commemorates the centenary of van’t Hoff and Le Bel’s stereochemical work of 1874 An article surveying developments in stereochemistry since 1874and giving many key references has also appeared.7b This particular Report is concerned chiefly with the c.d. of discrete organic molecules and is arranged according to the dominant chromophore. It does not include a discussion of work on macromolecules which has been reviewed separately ;see ref. 8 for a monograph on the optical activity of proteins.’ L. D. Barron and A. D. Buckingham J.C.S. Chem. Comm. 1973 152; L. D. Barron M. P. Bogaard and A. D. Buckingham J. Amer. Chem. SOC.,1973 95 603; L. D. Barron and A. D. Buckingham J.C.S. Chem. Comm. 1974 1028. G. Holzwarth E. C. Hsu H. S.Mosher T. R. Faulkner and A. Moscowitz J. Amer. Chem. SOC. 1974 96 25 1 ;T. R. Faulkner A. Moscowitz G. Hoizwarth E. C. Hsu and H. S. Mosher ibid. p. 252. ’ S. Takenaka N. Matsuura and N. Tokura Terruhedron Letters 1974,2325; N. Tokura T. Nagai S. Takenaka and T. Oshima J.C.S. Perkin II 1974 337; S.Takenaka K. Kondo and N. Tokura ibid. p. 1749. F. D. Saeva P. E. Sharpe and G. R. Olin J. Amer. Chem. SOC. 1973,95,7656 7660; F. D. Saeva and G. R. Olin ibid. p. 7882. A. J.McCafTery and M. D. Rowe J.C.S. Faruday II 1973 1779 and preceding papers; R. E. Linder G. Barth E. Bunnenberg C. Djerassi L. Seamans and A. Moscowitz J.C.S. Perkin II 1974 1712 and preceding papers. ‘‘Fundamental Aspects and Recent Developments in Optical Rotatory Dispersion and Circular Dichroism’ ed. F. Ciardelli and P.Salvadori Heyden London 1973. ’ (a) Tetrahedron 1974. 30,1477-2007; (b)E. L. Eliel Chemrech. 1974 758. * B. Jirgensons ‘Optical Activity of Proteins and Other Macromolecules’ Springer New York 1973 Second Edn. 35 P.M. Scopes 2 The Carbonyl Chromophore Ketones For many years the octant rule' has provided the framework for the semi- quantitative interpretation of saturated carbonyl Cotton effects. The rule relates the sign and to some extent the magnitude of the Cotton effect to the geometry of the molecule around the carbonyl group but precise quantitative interpretation has never been possible and several anomalies have been noted.A new survey of carbonyl n -+ n* Cotton effects in decalones and related compounds has now been published. lo This paper based on c.d. measurements describes an empirical analysis for ketones of the 'extended decalone' type which leads to numerical contributions for ring systems and alkyl substituents (~AE values). One strength of the analysis is the very wide range of ketones examined more than 120 indi-vidual compounds including trans-fused cis-fused and 'middle-ring' types. A number of new key compounds were synthesized for the work and although most of the data refer to methanol and hexane solutions many representative compounds were also examined in dioxan and acetonitrile.From this extensive data collection the empirical analysis gives 6Ae values which may be summed to provide calculated A& values usually within kO.2 units of the experimental value i.e. close to the limits of experimental error common for c.d. measurements. Two main hypotheses arise from the analysis. The first follows recent work by Hudec' which showed that heteroatoms can exert significant long-range effects on the carbonyl n-+ 1~* Cotton effect if the conformation is such that a planar 'zig-zag' of bonds connects the heteroatom and the carbonyl oxygen. The present analysis" develops the idea for extended decalones and leads to the suggestion that the length of a 'primary zig-zag' of C-C bonds (see octant projection Figure 1) is the major factor in determining the magnitude of the Cotton effect ! 1 pic.cfc. I Figure 1 W. Moffitt R.B. Woodward A. Moscowitz W. Klyne and C. Djerassi J. Amer Chem. Soc. 1961 83 4013. lo D. N. Kirk and W. Klyne J.C.S. Perkin I 1974 1076. " J. Hudec Chem. Comm. 1970 829; M. T. Hughes and J. Hudec ibid. 197 805; G. P. Powell and J. Hudec ibid.,1971 806. Physical Methods and Techniques-Part (iii) Circular Dichroism in any given compound. The second hypothesis concerns the effect of a p-axial methyl group which appears to be consignate12 when the group is a substituent on a primary zig-zag of more than two C-C bonds but dissignate if the primary zig-zag has only one or two bonds.Although the latter finding is entirely empirical the two ideas provide a rationalization of all available data and in particular provide for the first time an interpretation of data for the cis-and the truns-decalones within one scheme. Anomalies concerning the octant rule have been investigated by Lightner and his colleague^'^ who have synthesized specific compounds to test potentially anomalous situations. The norbornane derivative (1)’3a and the adamantane derivative (2)13‘ each have perturbing substituents in front octants only and show Cotton effects of the sign predicted by the octant rule (positive and negative respectively). The same authors have studied the anti-octant behaviour of a methyl group in certain geometrical situations in the rigid bicyclo[2,2,l]heptan- 7-ones (3a and b).’ 3b (3) a; R1= Me RZ= H b; R’= €3 R2= Me The effect of ring heteroatoms (N 0,and S) on the Cotton effect of cyclo-hexanones has been described by Djerassi;14 0 and N are weakly ‘anti-octant’ and S ‘octant’ in behaviour as compared with the methylene group.The rigid cyclopentanone (4) which contains two fused norbornyl systems has an excep- tionally high rotatory power for a saturated ketone (A&z 20). This has been attributed15 to the non-polarity of the five-membered ring enhanced by a large number of perturbing atoms all falling within octants of the same sign. W. Klyne and D. N. Kirk Tetrahedron Letters 1973 1483.l3 (u)D. A. Lightner and D. E. Jackman J.C.S. Chem. Comm. 1974,344; (6)D. Lightner and D. E. Jackman J. Amer. Chem. SOC. 1974 96 1938; (c) D. A. Lightner and T. C. Chang ibid. p. 3015. l4 M. M. Cook and C. Djerassi J. Amer. Chem. Soc. 1973,95 3678. E. Weissberger J. Amer. Chem. SOC., 1974,96 7219. 38 P.M. Scopes 3 ’Ihe Aromatic Chromophore Biaryls and Alkaloids Recent papers on aromatic compounds fall into three main groups. One group of papers is concerned with the c.d. spectra of more complex aromatic compounds including twisted biaryls and cyclophanes another is concerned with the absolute configuration of natural products chiefly alkaloids and the third is based on compounds in which the aromatic group is introduced as a substituent to produce a chromophoric derivative suitable for chiroptical investigation.A wide range of chiral biaryls including biphenyls binaphthyls and certain 9,9’-bianthryls has been studied by Mason and his colleagues.16 They have investigated the relationship between the c.d. spectra and the stereochemical configuration of the compounds as a function of the dihedral angle between the planes of the aromatic rings. The c.d. of 2,2’-dimethyl-l,l’-bianthryland its anion radical and dianion have also been re~0rted.I~ C.d. data have been recorded for a series of 9,10-ethano-9,1O-dihydroanthracene derivatives.” The results confirm previous observation^'^ that for 1,5-disubstituted compounds it is the chirality of the dihydroethanoanthracene skeleton that determines the sign of the Cotton effect and not the nature of the substituents at C-11 which have negligible in- fluence.For derivatives lacking substituents at C-5 only relatively weak Cotton effects are observed which are.less easy to interpret. Several types of chiral cyclophane have also been studied,” and their c.d. rationalized in terms of a sector rule. Apparent discrepancies” between the results of the Bijvoet X-ray method and the exciton chirality method which greatly perturbed stereochemical thinking in early 1973 were soon resolved ;22923 c$ also the discussion in the X-ray section of last year’s Annual Reports (B) pp. 54-55. The absolute configurations of several groups of Iboga and Voacanga alkaloids have been allotted by X-ray crystallographic studies on (+)-coronaridineZ4 as well as by chemical studies relating (+ hdihydrocatharanthine to ~leavamine.’~ Preliminary c.d.work on (-)-coronaridineZ6 has now been extended and detailed studies of the chiroptical properties of this group of alkaloidsz7” and of the 5,16- cyclo-3,4-secocorynane have appeared. In addition the chiroptical properties of the pentacyclic 5,16-cyclocorynane group have been correlated l6 S.F. Mason R. H. Seal and D. R. Roberts Tetrahedron 1974,30 1671. l7 0.Ito and M. Hatano J. Amer. Chem. SOC. 1974,96,4375. M. J. Brienne and J. Jacques Bull. SOC. chim. France 1974 2647. l9 S. Hagishita and K. Kuriyama Tetrahedron 1972 28 1435. *O E. Langer H. Lehner and K. Schlogl Tetrahedron 1973 29 2473. 21 J. Tanaka C.Katayama F. Ogura H. Tatemitsu and M. Nakagawa J.C.S. Chem. Comm. 1973,21. 22 S. F. Mason J.C.S. Chem. Comm. 1973 239. 23 A. M. F. Hezemans and M. P. Groenewege Tetrahedron 1973,29 1223. 24 J. P. Kutney K. Fuji A. M. Treasurywala J. Fayos J. Clardy A. I. Scott and C. C. Wei J. Amer. Chem. SOC. 1973,95 5407. 25 N. Camerman and J. Trotter Actu Cryst. 1964 17 384. 26 K. Biiiha Z.Koblicova and J. Trojiinek Tetrahedron Letters 1972 2763. 27 (a)K. Blaha Z. Koblicova and J. Trojanek CON. Czech. Chem. Comm. 1974,39,2258; (6) K. B1aha and J. Trojiinek ibid. 1973 38 929; (c) K. Bliiha Z. Koblicova and J. Trojanek ibid. 1974 39 3168. Physical Methods and Techniques-Part (iii) Circular Dichroism 39 with those of related indole alkaloids particularly the yohimbane and eburnane types.27c In other series the absolute configurations of cryptostyline-I -11 and -111 (1-phenyltetrahydroisoquinolinetype) have.been allotted by X-ray analysis and by c.d. measurements;28 details of earlier work on rhoeadine and iso- rhoeadine derivatives have been published29 and the c.d. of a series of sparteine alkaloids has been de~cribed.~' Seven theaflavins pigments isolated from black tea have been studied by a wide range of physical techniques including c.d. ;31 among oxygenated heterocyclic products the chiroptical properties of 4-substituted flavan~~~ and of the related compound siccanin have been discussed.33 For compounds in which the aromatic ring has greater conformational mobility solvent studies and low-temperature measurements have been used to provide additional information about the conformation of the molecule or about the electronic transitions involved.Conformational transmission has been studied in nitrobenzoic esters of chiral alcohols such as menthol,34 and c.d. data for dansyl amino-acids have been rationalized in terms of the twisting of the sulphonamide group in these derivative^.^^ Some a-halogenopropion- anilides have been studied in a range of solvents and the data explained in terms of the preferred conformation^.^^ The c.d. of isomeric 1-pyridylethanols at low temperature provides some additional evidence3' for the existence of a second n+ R.* electronic transition of the pyridine chromophore at ca. 240 nm. 4 Sulphur-cmtainingCompounds The optical activity of chiral disulphides has been studied both the~retically~~.~~ and empirically40341 in order to try and correlate the observed chiroptical behaviour with the CSSC dihedral angle.Previous work on model dithiadecalins has been extended4* and the effects of methyl and phenyl substituents on 2,3-dithiadecalins and 2-thiahexahydroindanes have been disc~ssed.~' New c.d. data have also been reported for gliotoxin and related compounds.42 In further work on steroidal ally1 sulphoxides the relative orientation of the olefinic double bond and the sulphoxide group has been shown to have significant influence on J. F. Blount V. Toome S. Teitel and A. Brossi Tetrahedron 1973 29 31. 29 J. Hrbek L. Hruban V. Simhnek F. Santavy and G.Snatzke CON. Czech. Chem. Comm. 1973,30,2799. 30 W. Klyne P. M. Scopes R. N. Thomas J. Skolik J. Gawronski and M. Wiewiorowski J.C.S. Perkin I 1974 2565. 31 P. D. Collier T. Bryce R. Mallows P. E. Thomas D. J. Frost 0.Korver and C. K. Wilkins Tetrahedron 1973 29 125. 32 G. Snatzke F. Snatzke A. L. Tokes M. Rakosi and R. Bognar Terrahedron 1973 29 909. 33 S.Nozoe K. Hirai F. Snatzke and G. Snatzke Tetrahedron 1974 30,2773. 34 U. Nagai E Abe. and R. Sano Tetrahedron 1974,30,25. 35 T. Polonski A. Chimiak and M. Kochman Tetrahedron 1974 30,641. 36 G. Snatzke M. M. El-Abadelah and M. 2. Nazer Tetrahedron 1973 29 487. 37 G. Gottarelli and B. Samori J.C.S. Perkin ZZ 1974 1462. 38 J. Webb R. W. Strickland and F. S. Richardson J. Amer. Chem. SOC.,1973,95,4775.39 R. W. Woody Tetrahedron 1973,29 1273. 40 L. A. Neubert and M. Carmack J. Amer. Chem. SOC. 1974,96,943. *' S. Hagishita and K. Kuriyama J.C.S. Perkin II 1974 686. 42 R. Nagarajan and R. W. Woody J. Amer. Chem. SOC. 1973,95 7212. 40 P.M.Scopes the c.d. whereas the chirality at sulphur does not necessarily play a dominant roie.43 5 Olefins Mason and his colleagues have reported the existence of a Cotton effect associated with the olefin Rydberg transition and have measured it for a-pinene in the gas phase.44 Solvent effects associated with this and other olefin transitions have been discussed by Andersen and his who have also published an extensive discussion of allylic bond polarization in olefins and related homo- conjugated systems.46 The c.d.of a series of olefins derived from cedrane has also been rep~rted.~” 6 The Carboxyl Chromophore Acids Esters Lactones and Related Compounds The exciton chirality method was originally applied to steroid dibenzoates and related compounds48 but has been extended over the years.49 A recent paper5’ describes the existence of long-range effects as detected in the c.d. spectra of bis- (p-dimethylaminobenzoate) esters of steroid glycols. In these compounds the two ester groups are widely separated e.g. diesters of 3p,1 la- 38,l lp- and 3j,15p-dihydroxy-5a,lk-steroids. Despite the distance separating the two aryl ester chromophores the A&values are still much greater (up to 10 times) than those for the isolated groups ;calculations yield predicted A&values in reasonable agree- ment with observed values.Two groups of workers have studied the absolute configuration of abscisic acid and related compounds using c.d. measurement^.^'^^^ The c.d. spectra show strong exciton coupling arising from interactions between the chromophores in the ring and in the side-chain and the absolute configuration deduced is in agree- ment with that determined by chemical methodss3 The extension of the exciton chirality method to other chromophores has been discussed for conjugated 43 D. N. Jones J. Blenkinsopp A. C. F. Edmonds E. Helmy and R. J. K. Taylor J.C.S. Perkin I 1974 937. 44 A. F. Drake and S. F. Mason J.C.S. Chem. Comm. 1973 253. ” N. H. Andersen and Y. Ohta J.C.S. Chem. Comm. 1974 730.46 N. H. Andersen C. R. Costin D. D. Syrdal and D. P. Svedberg J. Amer. Chem. Soc. 1973,95 2049; N. H. Andersen C. R. Costin and J. R. Shaw ibid. 1974,96 3692. 47 M. Fetizon Y. Le Bigot and J. Rens Tetrahedron 1973 29 2815. 48 N. Harada M. Ohashi and K. Nakanishi J. Amer. Chem. SOC. 1968 90 7349; N. Harada and K. Nakanishi ibid. p. 735; 1969 91 3989; N. Harada K. Nakanishi and S. Tatsuoka ibid. p. 5896. 49 N. Harada and K. Nakanishi Accounts Chem. Res. 1972,5 257. 50 Sow-mei Lai Chen N. Harada and K. Nakanishi J. Amer. Chem. SOC.,1974,96,7352. ’’ G. OhlofF E. Otto V. Rautenstrauch and G. Snatzke Helv. Chim. Acta 1973 56 1874. 52 M. Koreeda G. Weiss and K. Nakanishi J. Amer. Chem. Soc. 1973,95,239; N. Harada ibid. p. 240. 53 G. Ryback J.C.S.Chem. Comm. 1972 1190. Physical Methods and Techniques-Part (iii) Circular Dichroism 41 enones and related corn pound^'^ and for N-salicylidenimino derivatives.’ ’ An apparent exception to the dibenzoate rule has been discovered in connection with the absolute configuration of ~aryoptin.’~ Among simple acids and esters there Is continuing emphasis on the c.d. of amino-acids because of their relevance to work with proteins. In a detailed study of L-alanine and its conformational isomer^,^' the rotatory strengths calculated from LCAO-MO-SCF-Cl wavefunctions have been compared with experi- mental c.d. spectra and with predictions based on various sector rules for a-amino-acids. The far-u.v. c.d. of L-tryptophan has also been discussed in detail.58 The chiroptical properties of 2-methyl-substituted acids and esters have been studied together with n.m.r.data obtained with optically active shift reagent^.'^ With certain assumptions about the rotamer population the results are inter- preted in terms-of an empirical rule relating the sign of the n +n* Cotton effect with the geometry of the molecule round the chromophore. The c.d. of cx-tri-methylammonium carboxylic acids has been studied and compared with that of a-amino-acids.60 An empirical analysis for the c.d. data has been made for about 70 secondary acetates and acetamides in which the acetoxy- or acetamido- group is a substituent on a cyclohexane ring.61 Two assumptions are made firstly that acetates and acetamides adopt a strongly preferred conformation in solution (as in the crystal) and secondly that the contributions 6Ae of individual C-C bonds in the molecule to the total AEvalue are additive.The results appear to confirm these assumptions and to suggest the position of one of the nodal surfaces of the carboxy chromophores. The chiroptical properties of some simple five- and six-membered lactones62 and of some adamantane lac tone^^^ have been studied and interpreted in terms of their second- and third-sphere contributions to the n-+ T* transition. C.d. data have also been reported for a series of five-membered lactones derived from the aldopentonic acids.64 The absoIute configuration of (+)-canthark acid (5) (5) 54 M. Koreeda N. Harada and K. Nakanishi J. Amer. Chem. SOC.,1974,96 266.55 H. E. Smith J. R. Neergaard E. P. Burrows and Fu-Ming Chen J. Amer. Chem. Sac. 1974,96 2908. 5h S. Hosozawa N. Kato and K. Munakata Tetrahedron Letters 1974 3753. 57 J. Webb R. W. Strickland and F. S. Richardson Tetrahedron 1973 29 2499. 58 H. E. Auer J. Amer. Chem. Soc. 1973,95 3003. 59 0. Korver and M. van Gorkom Tetrahedron 1974,30,4041. 6o M. Gacek and K. Undheim Tetrahedron 1973 29 863. ” L. Bartlett D. N. Kirk and P. M. Scopes J.C.S. Perkin I 1974 2219. 62 0. Cervinka L. Hub F. Snatzke and G. Snatzke CON. Czech. Chem. Comm. 1973 38 897. 63 M. Keller and G. Snatzke Tetrahedron 1973 29 4013. 64 J. J. K. Novak Coll. Czech. Chem. Comm. 1974 39 869. P.M.Scopes has been determined on the basis of c.d. measurement^^^ and by application of the Horeau method.7 Miscellaneous Chromophores The c.d. curves of many natural products are so complex that assignment of individual bands to particular electronic transitions is difficult and correlations between the stereochemistry of a molecule and the sign of the observed dichroism can only be made on an empirical basis. This is true for example of caro- tenoids ;66-68 a recent paper66 uses c.d. curves to show that a-zeacarotene (6) (6R)-7’,8’-dihydro-~rl/-carotene, has the same absolute configuration at C-6 as natural ( +)or-carotene (6‘R)-P&-carotene. Cotton effects have also been reported for the visible region of the spectrum for six compounds and on this basis it is suggested that all carotenoids with C-6I(R)-chirality show a positive Cotton effect at their longest wavelength absorption band (450-500 nm).The first assignment of absolute stereochemistry to a C,,-carotenoid follows from c.d. correlation with natural (2R,2’R)-PP-~arotene2,2’-diol.~~ The c,d. spectra of corrinoids are also very complicated but the solvent- and temperature-dependence of the spectra have been studied for vitamin B12 and many related corn pound^.^^ The c.d. of the polycyclic polyether antibiotic X-537Ahas been studied in a variety of solvents and in the presence of amines and of univalent and bivalent metals for which it acts as a cation carrier.7o Other groups of compounds investigated include steroidal nitroxides7 and carbohydrate azide~.~~ 8 Hydrocarbons In the past two years several groups of workers have reported c.d.curves at short wavelengths for saturated hydrocarbons and closely related compounds. These include steroids (studied in ~olution),~ (3S,5S)-2,2,3,5tetramethylheptane 65 M. G. Peter G. Snatzke F. Snatzke K. N. Nagarajan and H. Schmid Helv. Chim. Acta 1974,57 32. 66 R. Buchecker and C. H. Eugster Helv. Chim. Acta 1973 56 1124. 67 R. Buchecker P. Hamm and C. H. Eugster Helv. Chim. Acra 1974,57 631. A. G. Andrewes S. Liaaen-Jensen and G. Borch Acta Chem. Scand. 1974 28,737. 69 R. Bonnett J. M. Godfrey V. B. Math P. M. Scopes and R. N. Thomas J.C.S. Perkin I 1973 252. ’O S. R. Alpha and A. H. Brady J. Amer. Chem. SOC.,1973,95 7043. ’* R. Ramasseul A. Rassat and P. Rey Tetrahedron 1974 30,265. ’’ D.R. Dunstan W. P. Mose and P. M. Scopes J.C.S. Perkin I 1973 2749. 73 D. N. Kirk P. M. Scopes and B. M. Tracey Tetrahedron Letters 1973 1355. Physical Methods and Techniques-Part (iii)Circular Dichroisrn 43 studied in the vapour phase,74 and the (1S,2R)-dimethyl-(2-methylcyclopropyl)-carbenium ion the first experimental observation of a Cotton effect in a carbenium Full details have also been published of the chiroptical behaviour of twistane and the related unsaturated hydrocarbon twi~tene.~~ Compounds in which the chirality is due solely to the replacement of one hydrogen atom by deuterium include [l-2H]neopentyftosylate,77(-)(S)-[4-’H]-2,2-para~yclophane,~* and 1R-[ l-2H]-ol-fenchocamphorquinone.79This latter paper also contains the first report of the c.d.of a compound in which chirality arises from the replacement of one oxygen atom by the isotope ‘*O. 74 S. D. Allen and 0.Schnepp J.C.S. Chem. Comm. 1974,904. ’’ R. G. Ghirardelli,J. Amer. Chem. SOC.,1973,95 4987. I’ M. Tichg CON. Czech. Chem. Comm.. 1974,39,2673. l7 P. H. Anderson B. Stephenson and H. S. Mosher J. Amer. Chem. Sac. 1974,96 3171. ’* P. H. Hoffman E. C. Ong 0. E. Weigang and M. J. Nugent J. Amer. Chem. Soc. 1974,% 2620. 79 W. C. M. C. Kokke and L. J. Oosterhoff J. Amer. Chem. SOC.,1973,95,7159.

ISSN:0069-3030    

DOI:10.1039/OC9747100035

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

2 Physical Methods and Techniques Part (iv) Chromatography (a) High-pressure Liquid Chromatography By J. P. LEPPARD and E. REID Wolfson Rioanalytical Centre University of Surrey Guildford GU2 5XH 1 Introduction Terminology is in a state of turmoil. ‘HSLC’ (high-speed liquid chromatography) seems a misnomer where as practised by the pioneer group at Oak Ridge National Laboratory runs may occupy nearly 24 hours in the interests ofgood ion-exchange separation (e.g. refs 1 and 2). Whilst the term ‘HPLC’ is widely accepted some practitioners regard the ‘P’ as an abbreviation not for ‘pressure’ -which can indeed be low with some systems even 40p.s.i. -but for ‘performance’. This as a purist might object pre-supposes optimization of the conditions for a particular separation (e.g.refs 1 and 2). Whilst the term ‘HPLC’ is widely accepted some manufacturers’ literature and even in research papers but is ambiguous :it could wrongly imply demise of the old-established practice of running columns under gravity with a coarse packing where this gives adequate speed and resolution. Indeed with packings such as dextran gels HPLC is precluded because of blockage under pressure. During 1973 and 1974 this being the period covered by the present survey general introductions to HPLC have appeared in several books3 and including reviews that cater for particular interests such as petroleum,’ pharma- ceuticals,8 and nucleic acids.g Reviews of ion-exchange methods allude to HPLC,’O and recent issues of two journals” are rich in HPLC reviews and ’ C.D. Scott D. D. Chilcote S. Katz and W. W. Pitt jun. J. Chromatog. Sci.,1973 11 96. J. E. Mrochek S. Katz W. H. Christie andS. R. Dinsmore Clin. Chem. 1974,20,1086. P. R. Brown ‘High Pressure Liquid Chromatography Biochemical and Biomedical Applications’ Academic Press New York 1973; L. R. Snyder and J. J. Kirkland ‘Introduction to Modern Liquid chromatography’ Wiley New York 1974. J. H. Knox Ann. Rea. Phys. Chem. 1973 24 29. G. E. Zweig and J. Sherma Analyt. Chem. 1974 46 73R (see 78R). J. H. Knox Chem. and Ind. 1975,29. ’ D. L. Camin and A. J. Raymond J. Chromatog. Sci.,1973 11 625. A. F. Michaelis D. W. Cornish and R. Vivilecchia J. Pharm. Sci. 1973 62 1399. C. Horvath Methods of Biochemical Analysis 1973 21 79.lo P. Jandera and J. ChurhEek J. Chromatog. 1974 98 1 55; H. F. Walton Analyt. Chem. 1974 46 398R. 44 Physical Methods-Part (iva) High-pressure Liquid Chromatography 45 original papers. Reviews of apparatus12 may be helpful to uninitiated readers as may a surveyi3 of theory and capabilities in relation to improved packing materials. Developments in packing^:.'^ including chemically bonded stationary phases,I5 have in fact been a notable feature of recent HPLC technology. 2 Column Packings The following categories as used below (with abbreyiations as naw given parenthetically) in a tabulation of illustrative separations are somewhat arbitrary but will serve as a reminder of alternative separatory principles. (a) Steri’c exclusion (‘Steric’) also termed ‘Molecular size exclusion’ or inappropriately in the case of porous glass beads,16 ‘Gel permeation’.(b) Adsorption (‘Adsorb’) also termed ‘Liquid-solid,. (c) Partition (‘Partit’) also termed ‘Liquid-liquid’. The further term ‘Reversed phase‘ (‘RP’)is applied to this and the following category if necessarily or option- ally it is the mobile phase rather than the stationary phase that is aqueous (polar). (d) Bonded phase (‘Bonded’) where a compound such as an alkyl silicone is chemically bonded on to the surface groups of the particles ;this category is really a sub-category of (c) or possibly (b),unless in (e). (e) Anion-exchange (‘AX’) or Cation-exchange (‘CX’) usually made by bond- ing. As is considered below packings within most of these categories are now available in a ‘pellicular’ (also termed ‘superficially porous’ or ‘porous layer’) form as an alternative to the (totally) porous form.A range of particle sizes down to ‘micro-particulate’ may also be available as may a choice between spherical and irregular particles the irregularity being without adverse effect on separ- ations.13 Unfortunately for readers whose memory for catalogue pages is poor journal editors often allow authors to use trade names for packings with no reminder of their nature. A useful list is to be found in an informative survey of column packings. l4 Pellicular packings have a thin ‘skin’ of the chromatographically active material coated on the outside of hard inert spheres of diameter only ca.40 pm. Comparable efficiencies are obtainable with totally porous packings (usually non-spherical)-only if their apparent diameter is much smaller commonly < 10pm. For both types of packing a narrow particle size distribution is essential for the highest efficiency Pellicular packings allow of faster speeds for given pressures.* Hansen’s prediction’ that the optimal pellicle thickness is 5-10 ”/ I J. Chromatog. Sci. 1974 12,425 (to end of Volume); J. Chromatog. 1973,83 I et seq.; ibid. 1974 99 13 et seq. l2 C. D. Chandler and H. M. McNair J. Chromatog. Sci. 1973 11,468; H. M. McNair ibid. 1974 12 425; J. H. Knox Lab. Practice. 1974 23 505. l3 G. R. Laird J. Jurand and J. H. Knox Proc. SOC.Analyt. Chem. 1974 11 310. l4 R. E. Leitch and J.J. DeStefano J. Chromnrog. Sci. 1973 11 105; cf. R. E. Majors Proc. Analyt. Diu. Chem. Soc. 1975 12 25. A. Pryde J. Chromatog. Sci. 1974 12 486. l6 ’’ M. J. Telepchak J. Chromatog. 1973 83 125. L. C. Hansen and T. W. Gilbert J. Chromarog. Sci. 1974. 12 464. 46 J. P. Leppard and E. Reid of the particle diameter has been borne out by tests on polystyrene-divinylbenzene beads having varying thicknesses of ion-exchange coating. Whilst on theoretical grounds the column efficiency should increase with diminution of particle diameter investigation has suggested that factors such as detector dead volume and recorder response time will limit the usefulness of diameters below 5pm.I’ This study was performed with specially prepared totally porous silica spheres of closely controlled diameter a form of packing known to give high column efficiencies.20 Much work with porous silica gel makes use of irregularly shaped micro-particles and the effects of various activation and deactivation steps upon these have been examined.21 Silica gel packings from different commercial sources have been compared.2 Impregnation of silica gel with cadmium ions has given improved selectivity in the separation of various chloroaniline and toluidine isomer^.^ Moreover pellicular silica gel has beqn impregnated with dimeric rhodium(i1) acetate for the separation of alkenes through complex formation.24 Whilst adsorption chromatography is conventionally performed with materials such as silica gel alumina,13 or polyamide various other materials have been investigated.The porous acrylic resin XAD-7 has been used to separate phenols.25 Compounds such as phenols and organic bases have likewise been separated with essentially aqueous eluents by use of the porous polystyrene-divinylbenzene resin XAD-2 crushed and sieved to give particle sizes down to 45 pm ;the HETP values were 3 mm or poorer and the separation mode was apparently adsorp- tive.26 Such material may however behave like a reversed-phase partition packing. A packing coated with Bentone-34 an organo-clay derivative has enabled various phenols and polycyclic aromatic hydrocarbons to be separated.28 Engelhardt29 has described how to apply a heavy coat of liquid phase to small- particle adsorbents in situ as may also occur inadvertently through adsorption of water from chromatographic solvents used for separation of non-polar compounds on silica gel.Rigid and flexible foams based on polymers such as phenol-formaldehyde polyurethane and polyethylene have been described for use in ion-exchange ’’ L. C. Hansen and T. W. Gilbert J. Chromatog. Sci. 1974 12,458. R. Endele I. HalBsz and K. Unger J. Chromatog. 1974 99 377. *O J. J. Kirkland J. Chromatog. 1973,83 149; K. Unger J. Schick-Kalb and K.-F. Krebs ibid. p. 5. I. I. Frolov R. G. Vorobyoeva I. V. Mironova A. Z. Chernov and Ya. I. Yashin J. Chromatog. 1973 80 167. 22 R. P. W. Scott and P. Kucera J. Chromatog. Sci. 1974 12 473. 23 D. Kunzru and R. W. Frei J. Chromatog. Sci. 1974 12 191. 24 F. Mikes V.Schurig and E Gil-Av J. Chromatog. 1973 83 91. 25 J. S. Fritz and R. B. Willis J. Chromatog. 1973 79 107. 26 C. H. Chu and D. J. Pietrzyk Analyt. Chem. 1974 46 330. 27 H. Takahagi and S. Seno J. Chromatog. Sci. 1974 12 507. 28 D. W. Grant R. B. Meiris and M. G. Hollis J. Chromatog. 1974 99 721. 29 H. Engelhardt J. Asshauer U. Neue and N. Weigand Analyt. Chem. 1974 46 336. Physical Methods-Par t (iva) High-pressure Liquid Chroma tograp hy 47 ~hromatography.~’ Polyurethane has also been used in the form of agglomerated particles in column form the polymerization being done in sit^.^' Although the agglomerates consist of spherical particles of only 1-10 pm diameter rapid flow rates are obtained readily the resistance being slight and sample capacity is high.Separations by steric exclusion have been successfully performed with silica gel of diameter as little as 3 pm with high controlled porosity.32 A re-evaluation of a porous silica for steric exclusion has shown that contrary to the findings in previous work pore structure was not homogeneous but that the material was nevertheless suitable for the separation of macr~molecules.~ Experiments with columns made of glass rods sintered together have shown that it is possible to separate molecules of different molecular weights by flow alone i.e. without the operation of steric exclusion principles.34 An evaluation of various steric exclusion packings has been reported.35 Unfortunately as in the case of a com-pendium of separations with porous glass beads,16 the enthusiasm of manu- facturers concerned with HPLC apparatus or materials in encouraging their staff to publish is not always matched by the depth and objectivity of the study.Besides ion-exchange packings mentioned above and below triethylaminoethyl cellulose (TEAE-cellulose) warrants mention because of its successful use for HPLC notwithstanding the susceptibility of such materials to compre~sion.~~ When packed by dry tamping it enabled prostaglandins and organic acids to be separated without blockage. Chemically Bonded Packins.-The development and application of ‘bonded phase’ packings has been and will continue to be a notable growth area in HPLC as well discussed by Locke3 and by Pryde.’ Such packings are almost invariably for use in an ion-exchange or predominantly partition mode.In the latter mode they may obviate a need that can arise with non-bonded packings i.e.to circum- vent stationary-phase ‘bleeding’. Yet as mentioned below they are not neces- sarily free from deterioration problems such as may be encountered (e.g. ref. 38) with non-bonded packings. Gratifyingly however the phenomenon of bleeding of traces of U.V.-absorbing material (with detriment to U.V. detection) was not encountered with a novel type of cation-exchanger having sulphobenzyl groups bonded to ~ilica.~’.~’ ” T. Braun 0. Bekeffy I. Haklits K. Kadar and G. Majoros Analyt. Chim. Acta 1973 64 45. 3’ L. C. Hansen and R. E. Sievers J. Chromatog. 1974 99 123. 3z K. K. Unger R. Kern M. C. Ninou and K.-F.Krebs J. Chromatog. 1974 99 435. 33 A. R. Cooper and E. M. Barrall J. Appl. Polymer Sci. 1973 17 1253. 34 S. Mori R. S. Porter and J. F. Johnson Analyt. Chem. 1974 46 1599. 3J W. A. Dark and R. J. Limpert J. Chromatog. Sci. 1973 11 114. 36 W. Morozowich J. Chromatog. Sci. 1974 12,453; J. Pharm. Sci. 1974,63 800. 37 D. C. Locke J. Chromatog. Sci. 1974 12,433. 38 G. G. Vaughan B. B. Wheals and M. J. Whitehouse J. Chromatog. 1973 78 203. ’9 R. A. Barford L. T. Olszewski D. H. Saunders P. Magidman and H. L. Rothbart J. Chromatog. Sci. 1974 12 555. 40 D. H. Saunders R. A. Barford P. Magidman L. T. Olszewski and H. L. Rothbart Analyt. Chem. 1974 46,834. 48 J. P. Leppard and E. Reid The preparation and characterization of a number of bonded phase packings with hydroxy cyano and ester functional groups have been described:’ and some commercially available packings evaluated.42 A novel polar bonded phase l-trimethoxysilyl-2-chloromethylphenylethane, has been described and tested.43 Gilpin et have described a technique for forming the bonded phase on the support after it has been packed into the column.Many of the earlier bonded phases employed Si-0-C or Si-0-Si bonds to link the functional group to the support ;but these are comparatively easily broken by hydrolysis. The C-Si bond should be much more stable and is a feature of some recently developed packings such as the above-mentioned sulphobenzylsilica cation exchangers4’ and those described by Sebestian and Halasz4’ The latter group have also described stationary phases involving the Si-N linkage.46 Packing Techniques.-Columns can be dry-packed or slurry-packed.Almost every author has his own variation on these two basic approaches as compared by Laird et a1.I3who feel that slurry packing is the only successful method for particles below 10 pm. In most hands dry packing gives efficient columns only with particles of 20 pm diameter or more. The simplest technique is to tamp small aliquots of packing with a rod of appropriate diameter (cj ref. 36) but a generally more satisfactory method is to use a variant of the ‘rotate-bounce-tap’ method,13 in which adsorbent is slowly added to the column while it is being rotated and bounced and tapped at the level attained by the packing.Hazelton4’ has designed a machine to do this automatically. A widely used method of slurry packing is that of Majors4* in which the adsorbent is slurried in a mixture of solvents whose density is adjusted to that of the adsorbent; the slurry is then pumped at high speed into the column. Cassidy et aL4’ have studied a number of packing techniques for small-particle silica gels and recommend an incremental slurry-packing method. Asshauer and HalaszS0 favour a high-viscosity slurry-packing method. A centrifugal system has been described” for packing and running microparticulate silica gel columns. 3 Applications and Approaches Choice of Packing and Separation Conditions.-Model separations that may give guidance on if not a blueprint for a particular separation problem have been collated into a useful compendi~m,~~ which however pre-supposes that the 41 M.Novotny S. L. Bektesh and K. Grohmann J. Chromatog. 1973,83 25. 42 J. H. Knox and G. Vasvari J. Chromatog. 1973,83 181. 43 E. Grushka and E. J. Kikta jun. Analyt. Chem. 1974 46 1370. 44 R. K. Gilpin J. A. Korpi and C. A. Janicki Analyr. Chem. 1974 46 1314. 4s I. Sebestian and I. Halasz Chromatographia 1974 7 371. 46 0. E. Brust I. Sebestian and I. Halasz J. Chromatog. 1973 83 15. 4i H. R. Hazelton Lab. Practice. 1974 23 178. 48 R. E. Majors J. Chromatog. Sci. 1973 11 88. 49 R. M. Cassidy D. S. LeCay and R. W. Frei Analyt. Chem. 1974 46,340. 50 J. Asshauer and I. Halasz J. Chromatog. Sci. 1974 12 139. 51 C. R.Wynne-Roberts and D. L. Davis J. Chromarog. Sci. 1973 11 406. 52 J. N. Done J. H. Knox and J. Loheac ‘Applications of High Speed Liquid Chroma- tography’ Wiley London 1974. Physica1 Methods-Part (iua) High-pressure Liquid Chromatography 49 sample is already in a fit state for loading. The latter aspect as further considered later is touched on in Table 1 which collates some recent literature with a classification based primarily on the type of packing employed (see beginning of Section 2 above). With the steric exclusion approach the molecular weight distribution of polymers can readily be determined particularly with on-line data handling;53 but the tendency of some polymers to adhere to surfaces can be detrimental.54 Whilst Table 1 gives only one example of the separation of small molecules by steric exclusion another is to be found in a report73 of the influence of hydrogen bonding the behaviour of chloroform methanol and benzoic acid on a polystyrene packing was studied with carbon tetrachloride as the eluent.Steric exclusion would be the approach of choice for small molecules only if other approaches were ineffective. Whilst porous adsorptive or partitioning packings are commonly used (Table l) for some purposes it is advantageous to use pellicular packings particularly those of bonded reversed-phase or ion- exchange type. The choice of packing and of running conditions hinges on empiricism and experience although a few of the papers cited do report compari- sons. Where conventional approaches are ineffective an ion-pair technique66 may warrant trial with attempted optimization of hydrophobic interactions of the solutes with the stationary (aqueous) phase.Initial derivatization is sometimes practised either to facilitate chromato- graphic separation as with the triacetyl derivatives of adrenaline and noradren- ali~~e,~~ or more commonly to aid detection through the formation of e.g. a coloured metal derivative7' or a fluorescent product (Table l).69The fluorogenic approach may not only improve sensitivity but also make u.v.-absorbing 53 W. W. MacLean J. Chromatog. 1974 99 425. 54 P. M. James A. C. Ouano E. Gipstein and A. R. Gregges J. Appl. Polymer Sci. 1972 16 2425. 55 G. P. Belue J. Chromatog. 1974 100 233. 56 A. F.Machin Proc. Soc. Analyt. Chem. 1973 10 92; J. Pesticide Sci. 1973 4 425. 57 C. D. Chandler G. R. Gibson and W. T. Bolleter J. Chromatog. 1974 100 185. A. J. Falk J. Pharm. Sci. 1974 63 274. 59 M. Koreeda G. Weiss and K. Nakanishi J. Amer. Chem. SOC.,1973 95 239. 6o E. H. Pfadenhauer J. Chromatog. 1973 81 85. 61 J. N. Seiber and D. P. H. Hsieh J. Assoc. Ofic. Analyt. Chemists 1973 56 827. 62 D. W. Smith T. H. Beasley R. L. Charles and H. W. Ziegler J. Pharm. Sci. 1973,62 1691. 63 W. C. Landgraf and E. C. Jennings J. Pharm. Sci.,1973,62,278. 64 A. Stolyhwo and 0. S. Privett J. Chromatog. Sci. 1973 11 20; c/ J G. Lawrence J. Chromatog. 1973 84 299. 65 W. Mechlinski and C. P. SchaRner J. Chromatog. 1974 99 619. 66 B.-A. Persson and €3. L. Karger J.Chromatog.Sci. 1974 12 521; B. L. Karger S. C. Su S. Marchese and B.-A. Persson ibid. p. 678. 67 C. V. Manion D. W. Shoeman and D. L. Azamoff J. Chromatog. 1974 101 169. 68 M. C. Olson J. Pharm. Sci.,1973,62 2001. 69 R. W. Frei J. F. Lawrence J. Hope,and R. M. Cassidy J. Chromatog. Sci.,1974 12 40. 'O W. Dunges G. Naundorf and N. Seiler J. Chromatog. Sci.,1974 12 655. 71 T. F. Gabriel and J. E. Michalewsky J. Chromatog. 1973 80 263. 72 J. H. Knox and J. Jurand J. Chromatog. 1973,87,95. 73 D. H. Freeman and R. M. Angeles J. Chromatog. Sci. 1974 12 730. 74 J. Merzhauser E. Roder and Ch. Hesse Klin. Wschr. 1973 51 883. 75 J. S. Fritz and J. N. Story Analyt. Chem. 1974 46 825. VI 0 Table 1 Illustrative separations of organic compounds in recent literature Starting materials used Chromatographic Notes on conditions Remarks especially on results besides pure solutions; category -usually e.g.mobile phase and conclusions (sensitivities soh = solvent pellicular packing if (grad = gradient) refer to amount chromatographed; Compound(s) separated extract (ion) not stated ‘porous’ detector ifnot U.V. typically x 10‘ to get amount [e.g. Juor(imetric) per ml of sample) refiactometric)] Sulphur-containing polymers Steric (gel) Aqueous; refr Troubles due to surface adherence Polymers ;blood proteins ;drugs ; Blood plasma etc. Steric (glass beads Various arbitrarily stated Theme of paper (from a flavours; etc. porous) conditions ;refr and U.V. cowpany laboratory) is merit of the packing Polyhydric alcohols e.g.Adsorb porous Partly aqueous ;refr Relevant to degradation studies isoerythritol ethylene glycol on polysaccharides Hydroxylated metabolites of Urine solv Adsorb porous Iso-octane (mainly) Hard to get good resolution on diazinon (a phosphorothionate) a preparative scale Nitrate esters e.g. Waste water (effluent Adsorb porous Dichloromethane/n-hexane; nitroglycerin monitoring) grad (optional) o-Iodohippurate Na salt Pharmaceutical solution Adsorb porous G.1.c. (derivatization difficulty) solv two-stage or t.1.c. poorer than HPLC ( +)-Abscisic acid precursor Adsorb porous n-Hexane ;9-ft column Diastereoisomers separated on ‘MTP esters’-cz and cis-diols several recycles preparative scale Xanthine and hypoxanthine Blood plasma Adsorb porous Solvent mixture Sensitivity 7 ng (endogenous) ultrafiltrate Aflatoxins Mould cultures solv Adsorb Chloroform-iso-octane T.1.c.gave better resolution Thebaine (an alkaloid) Poppies -roots etc. Adsorb Solvent mixture ;pressure XAD-2 (resin) extract only 40 p.s.i. Steroids e.g. fluocinonide Lipids (polar and non-polar) Steroids; dansyl amino-acids Polyenes (anti-fungal) Biogenic amines sulpha drugs thyroid hormones etc. Theoph ylline Polycyclic hydrocarbons Cortisone and other steroids N-Methyl carbamates (insecticides) Barbiturates Acetaminophen (4hydroxyacet- anilide) and metabolites Oligodeoxyribonucleotides Morphine alkaloids etc. Creams ointments soh (elaborate) Red blood cells soh Blood plasma solv ; urine XAD-2 extract Used engine oil Creams ointments solv Water and soil solv then dansylate Blood plasma soh then dansylate Blood serum ultrafiltrate; urine Adsorb and Partit (modified surface) Partit (pre-treat with NH solution) Partit porous Partit RP Partit porous ion-pair Bonded (nitrile) porous Bonded RP Bonded R.P.Bonded R.P. Bonded RP AX porous AX (weak type) CX (AX for one) Ternary mixture novel Ammoniacal solvent mixture grad; FID Coating applied in situ Methanol-THF-water (30,- or N(butyl),+ in stationary phase. Solvent mixtures. Propan-2-ol/n-hexane ; grad (optional) Aqueous methanol; fluor and U.V. Water with 1% methanol n-Hexane-ethanol ;fluor Aqueous methanol ;grad fluor 70 "C,grad 21 h ;fluor after Ce"' and U.V.Sulphate grad at 50°C Cations + acetonitrile b Good discussion of the methods 63 $ & Good discussion of the methods 64 ts 2 High load can be tolerated 29 8-65 I Ion-pair separation mechanism 66 discussed s e. E Caffeine etc. in plasma 67 -jeopardize g.1.c. but not HPLC 3 5 Forensic application envisaged 38 6 5 Degradation products examined 68 2 ;E Sensitivity 1 ng;initial clean-up 69 Q obviated by the dansylation 9 3 Sensitivity 1 ng 70 2 Glucuronisidates sulphates 2 $ etc.,detected incidentally 71 39 'rJ Good discussion of the methods 72 ,$ 52 J. P.Leppard and E.Reid contaminants less trouble~ome.~~ Yet good sensitivity may be attainable with U.V. detection if a suitable concentrate can be prepared for loading and if the compound (without derivatization) has a sufficiently high U.V. absorbance at the detection wavelength (commonly 254 nm). Differences in absorbance preclude generalization about sensitivity from published values such as are cited in Table 1. It has to be taken into account that with a crude sample as distinct from the authentic compound in pure solution sensitivity may be impaired due to inter- fering substances and that the loading volume to which sensitivity values com- monly refer is typically only 101.11. Thus a ‘sensitivity’ of 1ng referring to the amount loaded may imply that a concentration below (say) 1OOpgl-l in the starting sample would hardly be detectable.Many trace constituents of practical interest occur at lower concentrations than this or absorb so poorly that this sensitivity is not attainable with a U.V. detector. In Table 1 and in additional examples of separations now to be mentioned the emphasis is on ‘realistic’ applications often with a crude starting material which can hardly be successfully chromatographed without initial semi-purifi- cation by a procedure such as solvent extraction. Pilot HPLC runs with authentic material in pure solution are of course an essential step in method development ; but literature on so-called applications often from manufacturers’ laboratories is rather dominated by model separations which have not been tested on the ‘real’ starting material even to ensure that there will not be detriment to the column.A disposable ‘pre-column’ may give some protection besides being of use (e.g.refs. 6,66) to ensure that the mobile phase is saturated with the stationary phase in partition systems. Analysis of Fluids especially for Trace Constituents.-Table 1 cites a study5’ related to water pollution (cf.refs. 28 and 90) and several studies on blood or urine in relation to an ingested foreign substance such as a pesticidd6 or drug whose metabolism or pharmacokinetics can be elucidated with the aid of HPLC methodology. Other examples (see also ref. 8) are the assay of a diphenylhydan- toin metabolite in hydrolysed urine by a partition method after solvent extrac- ti~n,~~ and assays (on raw samples) for p-aminobenzoic acid and metabolites in serum or urine7? and for cefoxitin cephalothin and metabolites in urine,78 by ion-exchange methods.Beverages and soluble foods can be examined for various additives including saccharin by an anian-exchange method without pre- treatment of the sample.79 Other fluids that have been analysed by HPLC include culture media6’ and for forensic reasons engine {Table 1). Appli-cations in the petroleum industry have been re~iewed.~ Analysis of Pharmaceuticals and Materials.-The HPLC approach is being widely adopted in the assay of pharmaceutical preparations for active ingredients as evidenced by a review8 and diverse papers in the Journal of Pharmceuticai 76 T.Inaba and J. F. Brien J. Chromatog. 1973 80 161. 77 N. D. Brown R. T. Lofberg and T. P. Gibson J. Chromarog. 1974 99 635. 78 R.P.Buhs T. E.Maxim N. Allen T. A. Jacob and F. J. Wolf J. Chromatog. 1974 99 609. 79 J. J. Nelson J. Chromatog. Sci. 1973 11 28. Physical Methods-Part (iva) High-pressure Liquid Chromatography 53 Sciences during 1973 and 1974. Illustrative examples are given in Table 1 including a steroid notable for its exposition of the methodology. High sensitivity is needed only exceptionally as may be the case when stored pre- parations have to be examined for degradation products.6g Such products in multi-vitamin preparations the HPLC assay of which has been can vitiate the official fluorimetric assay for riboflaving A further instance of a field of applied chemistry where HPLC has proved useful is the analysis of food wrappings :the examination of polyolefins and PVC for phenolic antioxidants and organophosphite or organotin stabilizers has been reported.g2 HPLC as an Aid in Pure Chemistry and Biochemistry.-HPLC proved useful in an investigation of the mechanism of oxidation of xylenol by oxygen in the presence of copper and ~yridine.~~ In the reaction kinetics field HPLC has also been used to facilitate study of the rate and the mechanism of azo-group migration from a cyclic position.84 HPLC has enabled diastereoisomers to be separated sometimess9 (Table 1) but not necessarilyg5 with painstaking methodology.HPLC in the partition mode was helpful in distinguishing different 1,2-dicarba- closo-dodecaborane derivatives and discerning impurities.g6 It can also be a means of determining activity coefficients." Anion-exchangers can handle oligodeoxyribonucleotides7 (Table 1) and other nucleic-acid component^.^ 4 Apparatus Developments Detectors.-For organic compounds which absorb light poorly no highly sensitive detector is on the market.Different photometric detectors have been compared.gg One advance is the design of a reagent-addition facility :this enables Ce"' formed by Ce" oxidation of the separated solutes to be measured fluori- metri~ally~~~~~ (cf. ref. 69). Electrometric detectors offer promise but are not without limitation^.^^ The approach may be electrochemi~al,~' coul~metric,~~ polarographic,6 or condu~timetric.~~ Descriptions have appeared94 of novel systems which make use of flame-ionization or electron-capture detectors as in g.1.c.A novel detection principle is involved in the 'spray-impact detector' in R. C. Williams D. R. Baker and J. A. Schmidt J. Chromatog. Sci. 1973 11 618. " D. Wittmer and W. G. Haney,jun. J. Pharm. Sci. 1974 63 588. J. Protivovh J. PospiSil and L. Zikmund J. Polymer Sci. Polymer Symposia 1973 No. 40,233 (Chem. Ah. 1974,80 15 575). 83 A. Revillon Analyt. Chem. 1974 46 1589. 84 J. N. Done J. H. Knox R. McEwan and J. T. Sharp J.C.S. Chem. Comm. 1974 532. 85 G. Helmchen and W. Strubert Chromatographia 1974 7 713. J. H. Kindsvater P. H. Weiner and T. J. Klingen Analyt. Chem. 1974 46 982. 87 P. Alessi and I.Kikic Chromatographia 1974 7 299. 88 D. R. Baker R. C. Williams and J. C. Steichen J. Chromarog. Sci. 1974 12 499. R9 S. Katz W. W. Pitt jun. and G. Jones Clin. Chem. 1973 19 817; S. Katz W. W. Pitt jun. J. E. Mrochek and S. Dinsmore J. Chromatog. 1974 101 193. 90 A. W. Wolkoff and R. H. Larose. J. Chromatog. 1974 99 731. 9' P. T. Kissinger L. J. Felice R. M. Riggin L. A. Pachla and D. C. Wenke Clin. Chem. 1974 20 992; F. A. Schultz and D. E. Mathis Analyt. Chem. 1974 46,2253. 92 L. R,Taylor and D. C. Johnson Analyt. Chem. 1974 46 262. 9J K. Tesarik and P. Kalab J. Chromatog. 1973 78 357. 54 J. P.Leppard and E. Reid which the effluent is sprayed past a glassy carbon or gold electrode generating a high potential on the latter which varies with the type and amount of the com-ponent.Mass Spectrometry (MS) in Conjunction with HPLC.-In efforts to couple an MS detector to HPLC a major difficulty is the rather long time needed to volatilize each portion of the effluent for MS examination of components. Each such cycle took 5-7 minutes in a system designed around electron-impact MS.96 Accordingly the effluent flow sometimes had to be interrupted,as was also the case with a chemical-ionization MS systemg7 (effective if the HPLC peak contained a few micrograms of solute) in which a small proportion of the effluent was continuously fed into the MS apparatus via a capillary. Whilst atmospheric pressure ionization like chemical ionization is unsuitable for poorly volatile solutes it has given promising results with test compounds such as progesterone in an ethanolic medium the whole effluent is vaporized through a corona discharge and the resulting ions enter the MS chamber via a narrow apert~re.~' High-resolution MS seems rather slow for HPLC,98 and field desorption MS seems unsuitable for on-line ~peration.'~ A particularly interesting approachg9 (tried with electron-impact MS)is the use of a moving wire to convey a proportion of the effluent into an evaporation chamber and thence carrying the dried solute into the MS chamber.This approach is not handicapped by a long cycle time and allows of latitude in the nature of the solvent since it is pre-evaporated. Other Apparatus Developments.-Carr loo has described a sequential separator which with a moving-bed arrangement enables samples to be purified at a throughput rate of up to 100 g h- A more conventional approach to preparative HPLC is the use of wide-bore columns up to 8 cm in diameter."' Pressure can be dispensed with in a liquid chromatography approach where electro-osmotic solvent flow is created by applying a high potential across the ends of the Another way to achieve solvent flow is to use a ~entrifuge.~'The centrifugal multi-column system reported from Oak Ridge'03 is an adaptation 94 F.W. Willmott and R.J. Dolphin J. Chromatog. Sci. 1974 12 695; J. J. Szakasits and R. E. Robinson Anafyt. Chem. 1974 46 1648. 95 R. A. Mowery and R. S. Juvet jun. ibid. p. 687. 96 R. E. Lovins S. R. Ellis G. D. Tolbert and C. R. McKinney Adu.Mass Spectrometry 1974 5 457. 97 M. A. Baldwin and F. W. McLafferty Org. Muss Spectrometry 1973 7 1111; P. Arpino M. A. Baldwin and F. W. McLafferty Biomed. Mass spectrometry 1974 1 80; P. J. Arpino B. G. Dawkins and F. W. McLafferty J. Chromatog. Sci. 1974 12 574. 98 E. C. Horning D. 1. Carroll I. Dzidic K. D. Haegele M.G. Horning and R. N. Stilwell J. Chromatog. 1974 99 13. 99 R. P. W. Scott C. G. Scott M. Munroe and J. Hess jun. J. Chromatog. 1974,99 395. 100 P. J. Carr Proc. Analyt. Div. Chem. SOC. 1975 12 28. 101 D. R. Baker R. A. Henry R. C. Williams D. R. Hudson and N. A. Parris J. Chromatug 1973 83 233; E. Godbille and P. Devaux J. Chromatog. Sci. 1974 12 564. 102 V. Pretorius B. J. Hopkins and J. D. Schieke J. Chromarog. 1974 99 23. 103 C.D. Scott W. W. Pitt jun. and W. F. Johnson J. Chromatog. 1974 99 35. Phy sica1 Methods-Part (iua)High-pressure Liquid Chroma tograp hy of the centrifugal fast analyser' with in situ monitoring and with computerized data handling Chil~ote''~ has advocated a scheme to facilitate computer identi- fication of chromatographic peaks. As an aid to the automation of HPLC a high-pressure sample injection valve has been designed. 'O5 5 Theoretical Aspects Basic theory which was cardinal to the development of HPLC has been well recapitulated by Knox's gro~p.~,~,' Differences between theory and practice have been discussed for both g.1.c. and HPLC.'06 A unifying concept has been proposed for the mechanism of action of chromatographic gels."' Snyder'** has critically reviewed three theories of adsorption chromatography and con- cluded that his own is the most generally useful.Whether or not the theory of Scott and Kucera'" is valid it does enable a 'rational series of solvents' to be developed for gradient elution. Retention volume bandwidth and resolution have been theoretically related in an exposition of gradient-elution performance.' The causes and importance of solute band spreading in HPLC have been discussed."' For partition chromatography retention volumes' ' and partition coefficients' can be predicted as can suitable conditions for 'ion pair' operation.66 For resolving overlapping HPLC peaks a mathematical approach has been proposed in conjunction with an array of different detectors.114 With a given detector and a given weight load of solute great improvement in sensitivity should be obtain- able by increasing the sample injection volume say from 5 to 150pl ;there may be little impairment of chromatographic performance.' 15,' Other ways to optimize experimental conditions on the basis of theory have been well surveyed by Kirkland.' In connection with the advent of packings of ultra-small diameter,I3 Martin et a[.' have argued that there need be no great increase in pressure to achieve a given separation since the column length can be reduced as the particle size decreases. The benefit to laboratory budgets is obvious. D. D. Chilcote J. Chromatog. 1974 99 243. '05 J. Fleischer Chromatographia 1974 7 80. M. J. E. Golay Chromatographia 1973,6 242.lo' D. H. Freeman J. Chromatog. Sci. 1973 11 175. lo* L. R. Snyder Analyt. Chem. 1974,46 1384. log R. P. W. Scott and P. Kucera Analyt. Chem. 1973,45 749. P. Jandera and J. ChurBCek J. Chromatog. 1974, 91 207 223. 'I' E. Grushka Analyr. Chem. 1974,46 510A. 'l2 C. Eon B. Novosel and G. Guiochon J. Chromatog. 1973 83 77. 'I3 P. Menheere C. Devillez C.Eon,and G. Guiochon Analyr. Chem. 1974 46 1375. 'I4 N. Ostojic Analyr. Chem. 1974 46 1653. '" B. L. Karger M. Martin and G. Guiochon Analyt. Chem. 1974 46 1640. '" J. J. Kirkland Analyst 1974 99 859. M. Martin C. Eon and G. Guiochon J. Chromatog. 1974 99 357; M. Martin G. Blu C. Eon and G. Guiochon J. Chromatog. Sci. 1974 12 438.

ISSN:0069-3030    

DOI:10.1039/OC9747100044

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

2 Physical Methods Part (iv) Chromatography (b) Affinity Chromatography By H. GUILFORD The Radiochemical Centre Amersham Bucks 1 Introduction Affinity -or more properly bioaffinity -chromatography is a separation technique depending on biospecific interaction between an immobilized ligand and a species for which this ligand is in the broadest sense a substrate. Introduced into general use cu. 1968 its wide acceptance as a useful biochemical tool may be adjudged from the appearance in 1974 of nearly two hundred pertinent papers a chemical review,‘ and an excellent book covering advances to mid-1973,2 and the commercial availability of many reagents. Biospecific adsorbents are now being used not only for chromatography but as mechanistic probes and matrix- bound reactants.Recently after the initial euphoria there has been time for reflection leading to some rigorous re-investigations which show that the term ‘bioaffinity’ is not strictly applicable to some well-known oft-quoted cases. Many biochemists have one reservation about affinity chromatography. They doubt their ability to undertake the synthesis of some of the more sophisticated affinity systems. Indeed at a recent international conference biochemists were described as scientists who cheerfully isolate minute amounts of fragile proteins but baulk at manipulating ten grams of simple chemicals. Whatever the truth of this cynical view (voiced by a biochemist it should be added) clearly there is a need for chemists and engineers to contribute to this field of biological study.Hopefully this chapter in Annual Reports will stimulate more interest among this section of the scientific fraternity. 2 Matrix Chemistry Cyanogen bromide (CNBr)-activated agarose continues to be the most widely reported support matrix. Two new simplifying modifications of the activation procedure eliminate the need for careful pH control. The cyanogen halide can be added either in aqueous solution to a suspension of gel in concentrated phosphate buffer at pH 11-12 or in acetonitrile solution to a suspension in ~arbonate.~ The influence of concentrations temperature and duration of H. GuiIford Chem. SOC.Rev. 1973 2 249. C. Lowe and P. D. G. Dean ‘Affinity Chromatography’ Wiley New York,1974. J. Porath K. Aspberg H.Drevin and R. Axen J. Chromatog. 1973 86 53. S. C. March I. Parikh and P. Cuatrecasas Analyr. Biochem. 1974,60 149. 56 Physical Methods-Par t (ivb) Afinity Chromatography 57 reaction on coupling is discussed as it is also in a study of immobilizing glycine and IgG.’ Prehydrolysis of the activated gel before coupling as a means of controlling yields has been investigated.6 The coupling of amino-acids to activated polysaccharides has been shown to involve isourea rather than carbon- ate or cyclic imidocarbonate linkage^.^ Such ammonia as is liberated is indepen- dent of the coupling reaction. The monomeric model compound methyl-46- 0-benzylidene-a-D-glucopyranoside, when activated reacts with glycine to form the isourea (1) only. A linear relationship exists between the optimum pH of /o-CH2 0-C-NHR I + NH (1) R = CH,CO,-or CH,CONHCH,CO,-coupling and the pK of the conjugate acid of the coupled amine.8 The slight pH-dependent leakage of ligands from CNBr-activated polysaccharides and from polyacrylamide supports is now well established.’ The enhancement of stability by multivalently coupled ligands demonstrated by Wilchek with poly- (L-lysyl)agarose has been quantified.lo It appears that increasing the number of linkages from 1 to 2 gives a much more worthwhile increase in stability than an increase from say 5 to 10. However this leakage problem is significant only when extremely small amounts of material are involved (e.g.in receptor localiza- tion). More details are available of the preparation and properties of gels derivatized by the relatively hydrophilic bifunctional oxirans suitable for attaching ligands uia hydroxy-groups.Affinity chromatography of a p-amylase on cyclohexa- amylose linked to agarose through 1,4-bis-(2,3-epoxypropoxy)butanehas been demonstrated.” Ovine hormones have been coupled to a new matrix divinyl- sulphonylagarose for isolation of antibodies. 3 Spacer-arms It is a widely accepted tenet of affinity chromatography that for a tailored adsorbent to function as intended its biospecific ligand must be separated from 5 D. F. Stage and M. Mannik Biochim. Biophys. Acta 1974 343 282. 6 Separation News 1974 March. 7 K. Brostrom S. Ekman L. KAgedal and S. Akerstrom Acta Chem. Scand. 1974 B28 102.8 L. Kagerdal Biochem. SOC.Trans. 1974 2 1328. 9 G. I. Tesser H.-U. Fisch and R. Schwyzer Helv. Chim. Acta 1974 57 1718. 10 T. C. J. Gribnau and G. I. Tesser Experientia 1974 in press. 11 L. Sundberg and J. Porath J. Chromatog. 1974 90 87. 12 P. Vretblad F.E.B.S. Letters 1974 47. 86. 13 M. R. Sairam W. C. Clarke D. Chung J. Porath and C. H Li Biuchem. Biophys. Res. Comm. 1974 61 355. H. Guilford the carrier matrix so as to render it sterically available to the (often much larger) moiety for which it has affinity. This effect is achieved by inserting a spacer-arm between the ligand and its support. Matrices which are recognized as capable of introducing non-specific effects by being either ionic e.g. ion-exchangers or hydrophobic e.g.polystyrenes have been avoided in affinity chromatography. Agarose glass and polyacrylamide have been carefully chosen as supports which minimize both types of interaction.' The choice of spacers however has usually been much less rigorous being dictated mainly by convenience of chemical modification commercial accessibility and the procedure for immobilization on a matrix. Typical examples are &-amino-caproic acid am-diamines and small peptides (e.g.glycylglycyltyrosine). It has now become apparent that the nature of the spacer can be highly significant since some so-called 'sticky' enzymes have such an affinity for the spacer moiety that biospecific interaction with the ligand is almost eclipsed. The involvement of spacers has been particularly intensively investigated by O'Carra and his colleagues at Galway some of their earlier results being sum- marized in an excellent review.14 A number of enzymes have been chromato- graphed not only on a supposed biospecific assembly but also on the matrix- spacer system either alone (i.e.lacking a ligand) or coupled to a residue having structural analogy with the ligand but lacking affinity for the enzyme.A per- suasive example of the involvement of a spacer is illustrated in Figure 1. Cuatre-casas had earlier shown that when p-aminophenyl-p-D-thiogalactopyranoside (2),a weak inhibitor of P-galactosidase is attached directly to agarose it shows no affinity for the enzyme. If a 10A spacer N-(2-aminoethyl)acetamide,is inserted there is retardation of fl-galactosidase and if system A (ca.21 A) is used the enzyme is strongly adsorbed. These observations have been widely quoted as proof of the importance of spacer length and ligand accessibility in bioaffinity chromato- graphy. However not only can the enzyme not be displaced from the long-arm gel by j3-thiogalactose but replacing the inhibitor ligand by an a-glycoside having no affinity for the enzyme does not reduce the binding of P-galactosidase to the gel. Rood and Wilkinson have attempted to purify influenza virus sialidase (muco- polysaccharide N-acetyl neuraminylhydrolase) by the method of Cuatrecasas and Ilhairo on columns prepared by coupling the diazonium derivative of the inhibitor N-(p-aminopheny1)-oxamicacid (3) to agarose-glycyl-tyrosine.Several * P.O'Carra F.E.B.S. Symposium 'Industrial Aspects of Biochemistry' ed. B. Spencer 1974 p. 107. Physical Methods-Part (ivb)Afinity Chromatography 0 I M Borate h pn 10 APPLICATION 1 2 3 4 I L I -EFFLUENT VOLUME (in COLUMN-VOLUME UNITS) E NH-CHI-CH,-CHI-NH-CHr~HI-CHI-NH-M-CH,-CH,-C~NH -Q-lde 41 i--06 0 1 M Borate -04 APPLICATION -OF SAMPLE (Reproduced by permission from F.E.B.S. Symposium ‘Industrial Aspects of Bio-chemistry’ 1974 p. 124) other protein components of the crude mixture were adsorbed on this column and omitting the inhibitor ligand had little effect on the chromatography which was concluded to be predominantly ion-exchange.” Clearly in both cases a non-biological interaction with the spacer was operating.It is usual to distinguish only between biospecificity and ‘non-specific’ effects due to gross electrostatic hydrophobic or other interactions. O’Carra points out l4 that this is inaccurate since the latter may be highly specific as in the former case discussed above which indeed resulted in a useful purification of the enzyme but in a way that has little to do with bioaffinity. J. I. Rood and R.G. Wilkinson Biochim. Biophys. Acta 1974 334 168. H. Guilford $-NH-CH2-CH,-CH,-CH,-CH,-CH2-NH-CO-CH,-NH 0 Sepharose 4B Figure 2 Some typical matrix-spacer arm assemblies bearing no biospecific ligand It is hydrophobic interactions between enzymes and spacers that have been particularly ignored in many studies of affinity chromatography.Systems such as those illustrated in Figure 2 should have minimal biological or ionic affinity for proteins yet a number of so-called ‘sticky’ enzymes e.g P-galactosidase yeast alcohol dehydrogenase xanthine oxidase xanthine dehydrogenase and AMP aminohydrolase are strongly adsorbed on a variety of such gels. The thesis that the hydrophobicity of the spacers is the dominant effect has recently been examined.I6 A number of hydrophilic spacers have been attached toagarose a typical synthesis being summarized in Figure 3. Each was constructed so that I o Agarose 1.3-DAP $-NHCH,CHCH,NH OH 0 I 1.3-DAP -NHCH,CHCH,NHCOCH,Br N~HCOI OH OH I I 3-NHCH2CHCH,NHCOCH,NHCH,CWCHCHzNHz Figure 3 Synthesis of a typical hydrophilic spacer-arm based on 1,3-diaminopropan-2-oI (I,3-DAP) at least every alternate atom forms part of relatively hydrophilic groups such as amido secondary amino or carbinol.The behaviour of several NAD +-dependent dehydrogenases was studied using the systems illustrated in Figure 4. Lactate dehydrogenase recognizes the immobilized oxamate derivative as a substrate analogue. Owing to its ordered kinetic mechanism it can only bind biospecifically in the presence of NADH and is released when the coenzyme is omitted from the eluant. With the hydrophobic system A it is necessary to use buffers of high ionic strength to overcome non-biospecific adsorption. With gels B and C this pre- caution is unnecessary. Lactate dehydrogenase binds to either B or C in the presence of NADH and elutes in its absence at low ionic strength.Similar effects P. O’Carra S.Barry and T. Griffin,F.E.B.S. Letters 1974 43 169. Physical Met hods-Pa rt (iub) Afinity Chromatography +9 z I z I1 0-u I I u Iz3 0-uI u I3:z I I O=U I 3:z I V I xz 0-uI u I z7 z7 $ z7 z? " z UI xx 0-0I u I zz 77777 3? I 3:z I V I zx 0-0 3 u I zz I uIO=U I 3:z I u I X3 0-u 3:u I 3? IN z? z7 1- I xz 'SIuI xz 0-uI u I zz I rn " 3? I 3 z Is 1-z? 3? IN IN 0 3:u I I V u x L) 3:u I Xz I m I 3 z I I3:z 0-u I u I3 z I 8 s IT7n zz I mmr zz 'I T7ml 4c m u W LLO H. Guilford are observed when comparing adsorbents D E and F. The 'sticky' enzyme alcohol dehydrogenase is strongly adsorbed on D from which it cannot be eluted with NAD' or NADH yet it is not significantly retarded by gels E or F implying that this enzyme has little bioaffinity for the C-8-linked azo-substituted NAD' residue.In contrast the chromatographic behaviour of this enzyme on gels G and H is similar suggesting that biospecific adsorption predominates on the 6-thiol-linked NAD'. These and other results show that much interference can be eliminated by using hydrophilic spacer-arms. It is hoped that commercial suppliers of functional matrices will take note. The effect of increasing spacer-arm length on the efficacy of a homologous series of N6-(o-aminoalkyl)-AMP's (4) as bioaffinity ligands for dehydrogenases HO OH (4) has been reported.' The binding of the lactate dehydrogenase isoenzymes H and M increases with values of n from 2 to 5 and increasing n above 7gives no better binding of any enzyme studied.It appears that above a certain spacer- length no greater steric availability can be attained as predicted.' 4 Bioafflnity Chromatographic Purifications The examples given in this section are intended to illustrate the range of appli- cations of affinity chromatography having been selected with a bias towards chemical novelty. The selection is far from exhaustive. 4-Phospho[ 14C]pantetheine-labelled pigeon liver fatty acid synthetase has been separated into two half-molecular weight subunits by affinity chroma- tography on &-aminocaproylpantetheinecoupled to CNBr-activated agarose. Acetyl CoA transacylase is retained by this adsorbent whereas the p-keto-acyl thioester reductase subunit containing 4-phospho[ 14C]pantetheine is not." The human serum cortisol-binding protein transcortin has been purified on an immobilized steroid analogue synthesized from corticosterone (5) as shown in Figure 5 from which it is eluted by a pulse of corti~ol.'~ M.C. Hipwell M. J. Harvey and P. D. G. Dean F.E.B.S. Letters 1974 42 355. F. A. Lornitzo A. A. Qureshi and J. W. Porter J. Biol. Chem. 1974 249 1654. l9 F. Le Gaillard A. Racadot N. Racadot-Leray and M. Dautrevaux Biochimie 1974 56 99. Physical Methods-Pa rt (ivb) Afinit y Chroma tograp h y COCH,OH CO,H HCIO, + H,N(CH,),NH(CH,),NH-E & { fi (5) &NH(CH,I,UH(CH&NH Figure 5 Synthesis of an immobilized cortisol analogue The troponin complex of muscles involved in regulating ATP-ase activity has at least three components troponins I C and T.Troponin I is particularly difficult to purify by conventional techniques because of its susceptibility to breakdown by endogenous cathepsins (proteolytic enzymes). This component forms a urea-stable complex with troponin C in the presence of Ca2+. This observation forms the basis-of a biospecific purification of troponin I from a crude rabbit muscle homogenate in Ca2+-containing buffered urea. A column of troponin C immobilized a CNBr-activated agarose adsorbs only troponin I from the mixture. Elution is effected with ethanedeoxybis(ethy1amine)tetra-acetate known to dissociate the complex.20 Bacterial luciferase has been chromatographed using the immobilized sub- strate flavin mononucleotide as bioadsorbent.2 ' Thymidylate synthetase cata- lyses an interesting reaction in DNA metabolism in which a single carbon unit is transferred from a folate coenzyme with concomitant reduction according to 2'-deoxyuridylate + 5,10-methylene-5,6,7,8-tetrahydrofolate*5-methyl-2'-de-oxyuridylate(thymidy1ate)+ 7,8-dihydrofolate.0 OH (6) '* H. Syska S. V. Perry and I. P. Trayer F.E.B.S. Leiiers 1974 40,253. '' C. A. Waters J. R. Murphy and J. W. Hastings Biochem. Biophys. Res. Comm. 1974 57 1152. 64 H. Guilford The inhibitor 5-fluoro-2’-deoxyuridine 5’-(p-aminophenylphosphate) (6) attached to CNBr-activated agarose through a spacer-arm provided a bioaffinity ligand on which thymidylate synthetase could be isolated from a cell-free extract of amethopterin-resistant Lactobacillus ~asei.~’ An adsorbent designed for fractionation of nucleic acids has been prepared by synthesizing an immobilized actinomycin analogue.3-Hydroxy-4-methyl-2-nitrobenzoic acid is coupled to aminohexylagarose. Reduction of the nitro-group followed by oxidative coupling with 2-amino-3-hydroxy-4-methylbenzoyl-/l-(N~-diethylamino)ethylamid~ affords the adsorbent (7) which shows a prefer- ential affinity for DNA over RNA.23 6-Phosphogluconic acid (8)linked through gNH(CH,),NHCO CONHCH ,CH ,NEt I I Me Me OH an amide bond to aminohexylagarose provides an adsorbent for glycerol-3- phosphate dehydrogenase which can be retrieved by elution with the natural substrate @-glycerolphosphate.The novelty of this system is that in solution 6-phosphogIuconic acid exhibits no affinity for this enzyme.24 Bilirubin has been purified on an albumin-agarose gel ;conversely albumin can be removed from serum with a bilirubin-agarose ad~orbent.’~ Cyclic AMPdependent Enzymes.-.The activity of a number of protein kinases which catalyse the phosphorylation of proteins according to Protein + ATP S phosphorylated protein + ADP is stimulated by adenosine 3’,5’-cyclic phosphate cyclic AMP. In several cases these enzymes have been shown to dissociate reversibly into catalytic and inactive cyclic AMP-binding components in the presence of cyclic AMP. In the one previous report on chromatography of a protein kinase on an immobilized cyclic AMP analogue [N6-(.+aminocaproyl)-cyclicAMP-agarose] the enzyme did dissociate the catalytic fraction now fully active i.e.no longer stimulated by cyclic AMP eluting unpurified in the void volume while the receptor unit defied all attempts to liberate it from the adsorbent. More recently three of these enzymes have been subjected to affinity chroma- tography on immobilized cyclic AMP analogues with spacer-arms attached at 22 J. M. Whiteley I. Jerkunica and T. Deits Biochemistry 1974 13 2044. ’’ F. Seela 2.Nururforsch. 1974 29 521. 24 J. F. McGinnis and J. de Vellis Biochem. Biophys. Res. Comm. 1974 60,186. 25 P. H. Plotz P. D. Berk B. F. Scharschmidt J. K. Gordon and J. Vergalla J. Clin. Invest. 1974 53 778; M. Hierowski and R. Brodersen Biochim.Biophys. Acta 1974 354 121. Physical Methods-Part (iub) Afinity Chromatography I NH I C-8 of the adenine ring instead of to the 6-amino-group. 8-(y-Carboxypropytthio)-cyclic AMP coupled to poly(1ysyl)agarose using a di-imide yields the adsorbent (9) on which pig brain histone kinase is adsorbed. In contrast to the above example an increase in ionic strength is required to release a fully active catalytic component along with much non-specifically adsorbed protein. A further increase released an eiectrophoretically homogeneous cyclic AMP-binding component thus achieving an apparently biospecific purification of the receptor component of a histone kinase." A partial purification of the receptor unit of pig muscle protein kinase has also been reported27 using 8-(6-aminohexyl)- amino-cyclic AMP (lo),' synthesized from 8-bromo-cyclic AMP and 1,6-diamino- hexane coupled to CNBr-activated agarose.The free ligand was shown to function as a competitive inhibitor of cyclic AMP binding to the receptor site but with much lower affinity. After incubating the adsorbent with the kinase a denatured protein was eluted by urea removal by dialysis of which regenerated cyclic AMP binding capacity. The catalytic part of this enzyme was not adsorbed or purified on this adsorbent. 8-(6-Aminohexyl)amino-cycIicAMP is not only an activator of protamine kinase from trout testes but at higher concentrations it inhibits competitively with respect to ATP as do both cyclic AMP and AMP. The catalytic part not 26 E.S. Severin S. N. Kochetkov M. V. Nesterova and N. N. Gulyaey F.E.B.S. Letters 1974 49 61. 27 J. Ramseyer H.R. Kaslow and G. N. Gill Biochem. Biophys. Res. Comm. 1974,59,813. 66 H. Guilford further stimulated by cyclic AMP is again eluted at increased ionic strength but also by a pulse of AMP with up to 100-fold purification. It seems possible that the enzyme dissociates on the immobilized ligand (which acts as an effector causing a conformational change and binding the receptor unit) and that the catalytic component is subsequently adsorbed on to other vacant ligands acting at the ATP binding site from which it can be eluted by the competitive inhibitor AMP. If this is the case then an interesting dual biospecificity is operative.8-(6-Aminohexyl)amino-AMPwas also synthesized in an attempt to exploit the inhibition by AMP in affinity chromatography. This failed because neither this analogue nor N6-(6-aminohexyl)-AMP inhibits protamine kinase and this enzyme has no affinity for these immobilized ligands2' These studies illustrate an advantage of using complete ligand-spacer molecules in affinity chroma- tography :the behaviour of the whole unit can be tested in solution often demon- strating differences in affinity between an inhibitor and the analogue for immobil- ization. 5 General Ligands The concept of general ligands for group-specific bioaffinity chromatography continues to be the focus of much attention. Purification of lectins by this method is particularly attractive since each of these carbohydrate-binding proteins can be eluted specifically with an appropriate saccharide.Barondes and co-workers have exploited the definitive ability of lectins to agglutinate erythrocytes by using formalinized erythrocytes as the affinity adsorbent on which lectins from jack bean meal wheat germ lipase Ulex europeus seeds and lima beans have been fra~tionated.~' Immobilized lectins have been used to fractionate b-galactosi- da~es.~' Immobilized streptomycin and gentamicin provide useful adsorbents for ribosome^.^' Adenyl Ligands.-Many enzymes function only in conjunction with a cofactor which is generally a chemically well-defined compound of low molecular weight. Some part of its structure can be identified with its catalytic involvement e.g.the dihydropyridino-moiety of NAD(P)H the cobalt-alkyl of 5-deoxyadenosyl-cobalamin the y-phosphate of ATP etc. Immobilization of these compounds has been the focus of much effort because of their potential as bioaffinity ligands and in the search for matrix-bound coenzymes retaining biological activity and readily retrievable for re-use. With the latter consideration in mind synthetic operations have been concentrated at sites remote for that of catalytic function. A component of many coenzymes [e.g.ATP NAD(P)' FAD and CoA] is the adenyl moiety itself rarely involved in chemical reaction but often having a separate binding site on the enzyme sometimes acting as inhibitor or effector. 28 B. Jergil H. Guilford and K. Mosbach Biochem.J. 1974 139 441. 29 R. W. Reitherman S. D. Rosen and S. H. Barondes Nature 1974 2411 599. 30 A. G. W. Norden and J. S. O'Brien Biochem. Biophys. Res. Comm. 1974 56 193; Separation News 1974 September. 31 F. Le Goffic,B. Baca and N. Moreau F.E.B.S. Letters 1974 41 69. Physical Methods-Part (ivb)Afinity Chromatography NH -(CH,),-NH2 NH-(CHz),-NH 0 OH 0 II IOH CH,-0-P-OR OH OH OH OH A B 00 II II 00 OH OH OH OH C D Figure 6 Adenosine phosphate and pyrophospha~e Iigands (R= H monophosphate diphosphate or imidodiphosphate) Hence much elegant chemistry has been evolved in the search for AMP- analogues coupled to supports. Several points of attachment have been con- sidered allowing comparison of affinities of one enzyme for the same basic ligand presented with different topographies (cJ Figures 4 and 6) sometimes providing insight into the necessary features for interaction in a particular case.Several significant advances have been made during 1974 including an excellent summary of the status quo of matrix-bound adenine n~cleotides.’~ Trayer’s group at Birmingham has compared and contrasted three series of adenosine 5’-mOnO- 5’-di- and 5’-imidodi-phosphates (and in addition simple pyrophosphate) (Figure 6). In each case the ligand-spacer has a terminal amino- group for attachment to CNBr-activated agarose. Affinity chromatography of several ATP-utilizing enzymes has been studied using the ADP-agarose gels. The muscle protein myosin and its biologically active subfragments are adsorbed and can be eluted with ATP.Glucokinase is specifically adsorbed on to the C8-ADP-agarose [cf. Figure 6 A R = -PO(OH),]. Hexokinase however 32 P.-0. Larsson ‘The Synthesis of Polymer-Bound Adenine Nucleotides and Their Use as Affinity Adsorbents and Active Cofactors’ University of Lund 1974. H. Guilford is not retained on this system but is adsorbed on to N6-(ti-aminohexyl)-ADP- agarose illustrating how apparently minor differences in ligand construction can drastically affect bi~specificity.~’ Several groups have synthesized adenyl analogues in which the 6-amino-group is replaced by a thiol. 6-Mercaptopurine riboside riboside-5’-phosphate riboside-5’-triphosphate and nicotinamide-6-mercaptopurinedinucleotide have been coupled to the matrix-spacer N ‘-agarosyl-N6-bromoacetyl-1,Bdiamino-hexane.Anderton and co-workers prepared the ATP analogue (11) by treating 2’,3’- 0-isopropylidene-6-mercaptopurineriboside with 2,4-dinitrofluorobenzene,then sequential phosphorylation. Coupling to ‘thiolated agarsse’ was accomplished under mild conditions affording adsorbent (12) used in chromatography of HNAc I 02NDNo2 S S(CH,),CHCONH(CH,)3NH(CHz)3NH I R R (1 1) (12) R = ribosyl 5‘-triphosphate Na* K+ATpa~e.~~ An alternative synthesis of N6-(o-aminoalky1)-AMP’s (4) differs from the original method only in that the am-diaminoalkane is treated with 6-mercaptopurineriboside 5’-phosphate instead of the 6-chloro-derivative.’ Mosbach’s group in Lund has further extended its studies of general ligands tailored to dehydrogenases.N6-(6-Aminohexy1)adenosine2,Sdiphosphate and N6-(6-aminohexy1)adenosine 3’,5’-diphosphate have been synthesized in a manner analogous to the corresponding 5‘-monophosphate (4;n = 6) except in that phosphorus trichloride is substituted for phosphoryl chloride to provide 2(3’),5’-diphosphorylation.Several NADP+-dependent enzymes which show little or no affinity for the 5’-monophosphate immobilized or otherwise are retained on the matrix-bound 2’,5’-diphosphate (cf. Figure 7 for structural relationship to the coenzyme). The 3’,5’-diphosphate (more formally related to CoA)is a relatively ineffective affinity ligand as expected from its lack of inhibition of most dehydrogenases in s~lution.’~ I.P. Trayer H. R. Trayer D. A. P. Small and R. C. Bottomley Biochem. J. 1974 139 609; I. P. Trayer and H. R. Trayer Biochem. J. 1974 141 775. B. H. Anderton F. W. Hulla H. Fasold and H. A. White F.E.E.S. Letters 1973 37 338. D. B. Craven M. J. Harvey C. R. Lowe and P. D. G. Dean European J. Biochem. 1974 41 329. P. Brodelius P.-0. Larsson and K. Mosbach Europcwn J. Biochem. 1974 47 8 1 Physical Methods-Part (ivb) Afinity Chromatography 69 Figure 7 summarizes the first synthesis of a chemically defined immobilized NADP+ analogue. The key step is the quaternization of the N' position of the adenine ring by iodoacetic acid which provides a mild method for introducing a functionality into NADP' (and NAD')32 suitable for attaching a spacer and subsequent immobilization.This NADP+-analogue retains a considerable measure of its coenzyme function when coupled to soluble dextran and appears to be useful as a bioaffinity ligand for NADP+-dependent dehydrogenases when insolubilized on agar~se.~~ Applications of General Ligands.-The above studies have come from laboratories cancerned with developing affinity chromatography per se. The acceptability of the concept of general ligands is apparent from its application to various purifi- cations. Myo-inositol-1-phosphate synthetase from rat testes and anaemic chicken blood3' has been purified using NAD+-agarose which has also been effective in purifying diphtheria toxin39 and 8-hydroxybutyrate dehydrogena~e.~' Human erythrocyte glucose-6-phosphate dehydrogenase has been partially purified on NADP+-agaro~e.~' The isoenzymes of horse alcohol dehydrogenase have been separated on AMP-agaro~e.~~ 6 NewTechniques Assay of Coupling Yields.-Proteins coupled to polysaccharides or collagen have been determined by measuring the tryptophan content by the acid-ninhydrin reaction.43 A high blank was reported with CNBr-activated agarose.A fluoro-metric assay has been developed in which fluorescamine is attached to amino- ligands liberated from the agarose matrix by basic hydroly~is.~~ Affinity Electrophoresis.-The technique of crossed immuno-affinity electro- phoresis has been designed to predict the efficacy of biospecific affinity chroma- tography systems. In the pilot study a concanavalin A-agarose gel component was incorporated on to the electrophoretic plate and the behaviour of serum proteins on this modified system was compared with that on a standard immuno- electrophoretic plate.45 The migration of glycoprotein components with o[-D-glucosidic or a-D-mannosidic termini with which concanavalin A binds was clearly affected in a way which paralleled the behaviour of serum protein on a conventional biospecific column.Concanavalin A is an example of a phyto- haemagglutinin or lectin. These proteins reminiscent of antibodies interact with 37 C. R. Lowe and K. Mosbach European J. Biochem. 1974 49 51 1. 38 F. Pittner W. Fried and 0. Hoffman-Ostenhof 2.physiof. Chem. 1974 355 222; R. Schwarcz W. Fried F. Pittner and 0.Hoffmann-Ostenhof Monatsh. 1974,105,445.39 C. Cukor J. D. Readio and R. J. Kuchler Biotechnof. and Bioeng. 1974 16 925. 40 A. K. Grover and G. G. Hammes Biochim. Biophys. Acta 1974 356 309. 4' A. De Flora F. Giuliano and A. Morelli Ital. J. Biochem. 1973 22 258. O2 L. Anderson H. Jornvall A. Akeson and K. Mosbach Eiochim. Biophys. Acta 1974 364 1. 43 A. Eshkami T. Chase jun. J. Freudenberger and S. G. Gilbert Anafyt. Biochem. 1974 57 421. 44 M. Naoi and Y.C. Lee Analyt. Biochem.. 1974.57 640. 45 T. C. Beg-Hansen Analy!. Biochem. 1973,56,480. H. Guilford X 2 2" V O=Y (=y I a I ci P d r" 5 z 8 G2-f X I 2 a v I o=v X -2 2" Y +$ Physical Methods-Part (iub)Afinity Chromatography 71 cell membrane components agglutinating embryonic and tumour cells and erythrocytes.Many have a sugar-binding specificity an observation exploited in affinity chromatographic purification A range of sugar-containing neutral hydrophilic gels has recently been prepared by copolymerization of alkenyl-0- glycosides with acrylamide and NN'-methylene-bisacrylamide. An example immobilized 0-a-L-fucopyranoside is tentatively assigned structure (13).46 \ / NH,-CO-CH / CH \2 /CH-CO-NH CH-CO-NH CH, \ CH\,CH /CH NH NH CH / \ /\ /\co/\FH2 CH co cHz CH-CO-NH CH-CO-NH, / \ /CH2 CH\, oO-CH,-(CH ) -CH NH,-CO-CH "/ \ 10 CH /CH, (q / CH-CO-NH CH-CO-NH OH \ CH /CH, \,CH NH NH CH / \ /\ /\ /\ CH\ CO CH CO /CH2 NH,-CO-CH NH,-CO-CH / \ CH /CH OH /\,CH-CO-NH CH-(CH,),-CH, \ p&& CH /CH, \2 NH,-CO-CH NH,-CO-CH / \ CH (1 3) Partial tentative structure of an 0-a-L-fucopyranosyl derivative; when ally1 CL-L-fucopyranoside is used for copolymerization n = 0 Subsequently these preparations have been used in conjunction with normal acrylamide gel electrophoresis.For instance the crude haemagglutinating protein fraction from pea seeds was applied to polyacrylamide and to gels consisting of normal polyacrylamide covered by a layer of 0-a-D-galactosyl- and 0-a-D-mannosyl-polyacrylamide, respectively. The resolutions were identical on the first two gels but on the third gel one band had lost its mobility owing to its specific affinity for mar~nose.~' 46 V.Horejsi and J. Kocourek Biochim. Biophys. Acta 1973 297 346. " V. Horejsi and J. Koeourek Biochim. Biophys. Acta 1974 336 338. 72 H. Guilford Resolution of Optical Isomers.-The differential binding of optical isomers by certain proteins has been exploited in a resolution of DL-tryptophan on immobilized bovine serum albumin which has affinity for the L-isomer only. D-Tryptophan eluted in the void volume whereas L-tryptophan was adsorbed but could be eluted with acetic acid.48 Adsorbent with Digestible Spacer.-Most immunoadsorbents used for fractiona- tion of lymphocyte populations according to cell-surface antigens suffer from the limitation that the antigen-specific cells cannot be recovered. This problem has been overcome by using an enzymically digestible bridge between matrix and ligand.Cells are applied to antiglobulin coupled with glutaraldehyde to gelatin insolubilized on agarose. The B lymphocytes [i.e.those concerned with antibody production) adsorbed on the column were released by collagenase digestion of the gelatin spacer. More than 90 % of the eluted lymphocyte population carried surface immunoglobulins with full cell viability.49 Magnetic Matrices.-An aspect of affinity chromatography on which little has been published is that of its application to completely crude homogenates. A special problem is that even should the component of interest be taken up specifically on a tailor-made affinity gel its physical separation from cell debris and viscous cellular components can be prohibitively difficult.An ingenious technique has now been introduced which may facilitate such separations in batch adsorption-desorption processes. Magnetic polyacrylamide gels can be prepared by polymerizing the usual monomeric ingredients in the presence of iron oxide. The biospecific ligand is then attached using conventional techniques e.g. the L-asparaginase inhibitor D-asparagine was coupled to magnetic amino- ethylpolyacrylamide using butane- 1,4-diol diglycidyl ether. This adsorbent was incubated with a crude cell homogenate of Escherichia coli from which the gel particles could subsequently be separated by magnetic sedimentation. That the preparation is an effective biospecific adsorbent was shown by releasing L-asparaginase with a pulse of ~-asparagine.” This technique may well simplify in particular large-scale biospecific purifications.7 Molecular Probes Use of immobilized biospecific ligands as ‘molecular probes’ provides one of the most fascinating aspects of this field of investigation. Ligands are usually spaced from the matrix with arms long enough to ensure optimum steric availability. In a study of the binding of cobalamin to hog intrinsic factor (the protein con- cerned with B, adsorption and transport across the ileum wall) a short spacer was chosen deliberately. The reaction of cobfr)alamin with bromoethylamino- agarose affords a BI2analogue immobilized so that the cobalt atom is no more than 5 %t from the matrik Intrinsic factor is strongly bound tothe cobalamin gel 48 K.K. Stewart and R. F. Doherty 9th F.E.B.S. Internat. Biochem. Symposium Stock- holm 1973 Abs. la 7. 49 D. B. Thomas and B. Phillips European J. Immunol. 1973 3 740. P. Dunnill and M. D. Lilly Biotechnol. and Bioeng. 1974 16 987. Physical Methods-Part (ivb)A&%ity Chromatography 73 indicating that since the Stokes radius of the intrinsic factor-B, complex is 32.8 A,the binding site is at ordose to the protein’s surface.” Much interest has been focused on the glycoprotein interferon because of its involvement in inducing antiviral activity against a broad spectrum of viruses. Studies on human interferon have been hindered by the crudeness of available material-a litre of Ieucocyte preparation contains about ten micrograms of interferon.Anfinsen and co-workers have developed a biospecific purification leading to much purer enriched material.52 Some insight has been gained into the mode of induction of resistance to viral infection by examining the effect of mouse interferon coupled to agarose on mouse L-cells which are several times smaller than the gel beads. Retention of biological activity was observed suggesting that interferon operates without entering the cell probably by binding to a specific receptor on the cell ~urface.’~ The rabbit muscle enzyme phosphorylase b which catalyses the reaction glycogen + glucose-1 -phosphate * glycogen-glucose + Pi is totally inactive in the absence of the effector AMP (or structural analogues). Mosbach and Gestrelius have capitalized on this fact by succeeding in ‘freezing’ the enzyme in its active c~nformation.’~ It was first shown that when phosphory- lase b is coupled to CNBr-activated agarose or to glass ca.30 % of its activity is retained on adding AMFto a suspension of the immobilized enzyme. Also N6-(6-aminohexyl)-AMP (4 ;n = 6)can replace AMP as activator either free in solution (ca.80% as effective as AMP) or when coupled to soluble dextran (ca. 30 % as effective). This analogue is also ca. 20 % as effective as AMP when immobilized on agarose. Continuous cycling of a mixture of enzyme and sub- strates led to a linear release of phosphate over thirty minutes. Finally the enzyme was applied to a column of N6-(6-aminohexyl)-AMP-agarose,unbound enzyme was washed away and the substrates alone were cycled through the column.Continuous liberation of phosphate proved that phosphorylase b was ‘frozen’ in its active conformation on its immobilized activator AMP. A similar effect was observed when the enzyme and N6-(6-aminohexyl)-AMP were simul- taneously coupled to agarose such that enzyme and activator were fixed in sufficiently close proximity to one another to provide a permanently active phosphorylase b. The activity is not optimal since it can be increased by addition of AMP showing that only a proportion of the enzyme activator sites are orien- tated towards the immobilized AMP. Unfortunately when phosphorylase b was coupled in the presence of AMP which was then washed away no activity was observed unless more AMP was added.5‘ E. L. Lien L. Ellenbogen P. Y. Law and J. M. Wood J. Biol. Chem. 1974 249 890. 52 C. B. Anfinsen S. Bose L. Corley and D. Gurari-Rotman Proc. Nut. Acud. Sci. U.S.A. 1974 71 3139. 53 E. Knight jun. Biochem. Biophys. Res. Comm. 1974,56 860. 54 K. Mosbach and S. Gestrelius F.E.B.S. Lerters 1974 42 200. H. Guivord DNA coexists as chromatin in the cellular nucleoplasm with histones and other proteins. Although nucleotide-protein interactions have evoked much interest protein-protein interactions are much less investigated. Histone-histone binding has now been studied by fractionating calf thymus histones on a column of the arginine-rich histone H,-agarose. The most lysine-rich histone H,is not retarded histones H, and H, are eluted at increased ionic strength but histone H4 could only be retrieved with HCl confirming earlier speculation that self-interaction of histones is the strongest.” 8 Physical Parameters in Afinity Chromatography Wankat has produceds6 a theoretical analysis of affinity chromatography (c$ ref.14). Studies of the effects of other physical and kinetic parameters in affinity chromatography have shown that column geometry ligand concentration and total ligand must be balanced carefully to achieve optimum res~lts.’~ The affinity of glycerokinase and yeast alcohol dehydrogenase for immobilized N6-(6-aminohexyl)-AMP decreases with increasing temperature. ’* Glutamate synthetase however has affinity for AMP only at 50°C or above at which temperature it sticks to immobilized (4;n = 6).Elution is achieved by cooling the column. J. Mizon M. Mayne-D’Haultfoeuille C. Mizon-Capron P. Sautiere and G. Biserte F.E.B.S. Letters 1974 47 125. 56 P. C. Wankat Analyr. Chem. 1974 46 1400. 57 C. R. Lowe M. J. Harvey and P. D. G. Dean European J. Biochem. 1974 41 341. 58 C. R. Lowe M.J. Harvey and P. D. G. Dean European J. Biochem. 1974 41 353. 59 R. P. Bywater Biochem. Soc. Symposium on Affinity Chromatography Galway 1974.

ISSN:0069-3030    

DOI:10.1039/OC9747100056

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

3 Photoelectron Spectroscopy and Related Phenomena of Gaseous Species By J. P. MAlER Ph ysikalisch -chemiscbes lnstitut University of Basel Klingelbergstrasse 80 4056 Basel Switzerland Previously there has been a tendency to separate the ultraviolet- (u.v.)and X-ray- excited processes; however in this Report it seems opportune to encompass the whole energy region and to make the division by reference to progress and study of the species in the gas phase. The field of X-ray p.e. spectroscopy and the implications in organic chemistry wzre covered in Annual Reports by Clark up to 1972 and for an up-to-date panorama the reader is referred to a conference publication reviewing the state of the art up to 1974.’ This Report highlights what in the author’s opinion have been the fruitful advances in p.e.spectroscopic and related techniques in the past two years with emphasis placed on the applications and information available predomi- nantly for the valence region of organic species. An extensive compilation’ exists for the U.V. p.e. spectroscopic studies of heterocyclic molecules up to 1973 and aspects of inorganic and organometallic molecules are thoroughly reviewed and discussed annually in the Specialist Periodical Re~ort,~ and are not reported here. Application of the technique to non-metallic species has also been sur~eyed.~ Two more books devoted to p.e. spectroscopy have appeared’ and these with those of Turner et al. and Siegbahn et al. provide the necessary introductions whilst the Faraday Discussionsare representative of the develop- ments up to 1972.6 J.Electron Spectroscopy 1974 5. E. Heilbronner J. P. Maier and E. Haselbach in ‘Physical Methods in Heterocyclic Chemistry’ ed. A. R. Katritzky Academic Press New York 1974 Vol. 5. A. Hamnett and A. F. Orchard in ‘Electronic Structure and Magnetism of Inorganic Compounds’ ed. P. Day (Specialist Periodical Reports) The Chemical Society London 1974 Vol. 3 p. 218. H. Bock and B. G. Ramsey Angew. Chem. Internat. Edn. 1973 12 734. ’ (a)A. D. Baker and D. Betteridge ‘Photoelectron Spectroscopy’ International Series of Monographs in Analytical Chemistry 1972 Vol. 53; (b)J. H. D. Eland ‘Photoelectron Spectroscopy’ Butterworths London 1974. Faraday Discuss. Chem. SOC.,1972 No. 54. 75 J.P.Maier 1 Photoelectron Spectroscopic Studies of Organic Molecules The technique has continued to be used extensively to obtain ionization energies (IE) and to use these as a basis for the description of the electronic structure of the molecule in terms of MO models.By this means the bands in the p.e. spectra are related to electron ejection from orbitals. The additional detail discernible Franck-Condon profiles excitation of vibrational modes in the ion spin-orbit coupling vibronic features and the band intensity dependence on the angular asymmetry parameter and photoionization cross-section provide substantial indications of the respectability of the model. The past two years have witnessed advances in sophistication in interpretive methods and more appreciation of their limitations.These aspects are exemplified in an article by Heilbr~nner.~ There has been considerable success in the use of the technique to investigate specific problems and to relate the data with other experiments. In this area the He-Ict (21.22eV) photon source has by and large been utilized in order to benefit from the resolution attainable. Photoelectron-Electronic Spectroscopy Correlations.-The IEs and the assign- ment of the bands have been used to draw inferences regarding the interpretation of the electronic absorption spectra of molecules or their cations. In the latter the correspondence between the separation of the bands in the p.e. spectra and the electronic bands of the cation is illustrated by the absorption and emission spectra of the cations in the vapour phase of diatomic and triatomic molecules.* For organic molecules the correspondence is illustrated by the absorption spectra of their radical cations trapped in matrices.The benefits of the comparison (and complemented by open-shell MO studies) are thoroughly discussed in a recent review.’ The gas-phase absorption spectra of the cations are ‘simulated’ to an extent by the technique introduced by Dunbar of photodissociation spectroscopy. lo The ion currents of fragments obtained by photon irradiation of the parent ions are monitored in an ion cyclotron spectrometer. In Figure 1 is shown such a spectrum where the loss of hydrogen yields the fragment ion part of the work involving substituted benzene cations.lo Differences from the p.e. spectra are ex- pected as the method relies on dissociation products the thermodynamic dissociation pathway has to be accessible and the excited states of the parent cation generated are dependent on optical selection rules. The important indica- tion of these experiments is that the ions examined by the two techniques are structurally similar. Electronic transitions of ions are also observed in the X-ray emission spectra. Some of sufficient resolution to identify the valence states E. Heilbronner in ‘The World of Quantum Chemistry’ ed. R. Daudel and B. Puliman D. Reidel Publishing Co. Dordrecht 1974. G. Herzberg Quart. Rev. 1971 25 201. P. Carsky and R. Zahradnik Topics Current Chem. 1973 43 1. lo P. P. Dymerski E.Fu and R. C. Dunbar J. Amer. Chem. Soc. 1974 96,4109 and references therein. Photoelectron Spectroscopy and Related Phenomena of Gaseous Species Energy feV1 Figure 1 Photoelectron and photodissociation spectra of p-Juorotoluene (Reproduced by permission from J. Amer. Chem. Soc. 1974,96,4109) have been reported by Siegbahn and co-workers,'' for diatomic and triatomic ions and for ions of C,H, C2H40 CHF, and CF in the gas phase. Following the generation of the core vacancy by electron impact relaxation transitions corresponding to core from valence states are observed. This approach can help in the assignment of the valence bands due to the selection rules governing the transitions. For example in CF4+ only transitions from the t,-orbitals are strongly allowed when the Cls electron is ionized and the Cls X-ray spectrum consists of one line.The similarities in vibrational structures observed on p.e. and Rydberg bands converging to the same ionic state have been discussed for fluorobenzenes.12 The consideration of the quantum defects and selection rules for the Rydberg transitions with the type of MO with which the p.e. band is associated allows the interpretation of both sets of data to be advanced. Similarly the assigned p.e. spectra of fluoromethanes and cyclopropenone allow the prominent features of electron-impact spectra of these molecules to be identified.I3 An approach has been made to correlate the separation of the cationic states (z,x) from the p.e. spectra with the electronic absorption spectrum of the molecular states where the two excited states of the molecule correspond in the orbital picture to excitation of the electrons to an unoccupied orbital and which L.0. Werme B. Grennberg J. Nordgren C. Nordling and K. Siegbahn UUIP-815 Uppsala University Institute of Physics Report March 1973. R. Gilbert P. Sauvageau and C. Sandorfy Chem. Phys. Letters 1972 17 465; R. Gilbert and C. Sandorfy ibid. 1974 27 457. l3 W. R. Harshbarger N. A. Kuebler and M. B. Robin J. Chem. Phys. 1974 60 345; W. R. Harshbarger M. B. Robin and E. N. Lassettre J. Electron Spectroscopy 1973 1 319. 78 Maier 8e-la2 8e-lb I I p-AE(1,2) _cI I A-i 0.75 h .z 0.50 5 a .C. -3 Ii f-+ 23 "C Y a 0.25 0.0 20OOO 30 0oO 4oOOo 50OOO (b) an-' Figure 2 (a) Photoelectron spectrum of spiro[4,4]nonatetraene;(b) electronic absorption spectrum of spiro[4,4]nonatetraene (Reproduced by permission from J.Amer. Chem. Soc. 1974,96,7662) Photoelectron Spectroscopy and Related Phenomena of Gaseous Species 79 when photoionized leave the ion in the respective (8or 2)state.14 In an in-dependent-electron model such as Huckel theory the energy separation of the cationic states would equal that of the excited states when Koopman's theorem is used for the IEs. When electron interactions are considered provided that configuration interaction is not important then by evaluating the coulomb and exchange integrals from SCF MO calculations the excitation energies may be estimated This has been done recently for pyrazine and its 2,6-dimethyI deriva- tive.' A striking example is that of spiro[4,4]nonatetraene1 where the separa- tion between the first two bands in the p.e.spectrum (Figure 2a) corresponding to electron ejection from the la2 (7t)and lbl (z)MOs respectively is equal to the separation of the first two bands in the U.V. spectrum (Figure 2b). The latter are associated with transitions of E symmetry represented by 8e (z*) +la2 (7t) and 8e (z*)t-lbl (7t). In this instance the symmetry and spiroconjugation result in approximate cancellation of the two-electron integrals. A study involving the p.e.-electronic spectra correlation for benzene with the aid of PPP-SCF-CI calculations resulted in assignment of the 12.3eV IE band to the sca,,,M0.16 In a series of related compounds it has been assumed that electron interactions are comparable and the variations of the orbital energies derived from the p.e.spectra have been used to suggest the location of the olefinic z-+ cr* band of Me2C=CMe2.l7 Conformational and Hydrogen-bonding Phenomena.-P.e. studies of conforma-tional phenomena have illuminated both the potential and the limitations of the method. Requisite are well defined changes of the IEs of the pertinent bands which are dependent on the conformation adopted. Through the study of structurally related series the variations may be used to establish empirical correlations. For example in the case of conjugated molecules twisting around the essentially single bonds is readily perceptible from the IE of the z-bands whereas around double bonds the resolution limits preclude definite deductions.The former situation is encountered in cyclo-octatetraene and its hydrogenated derivatives,' * biphenyls and phenylethylenes,' alkyl aryl ethers sulphides and amines,20 and hydrazine-type systems.' This enables an estimation of the E. Haselbach and A. Schmelzer Helv. Chim. Acta 1972 55 1745; E. Haselbach Z. Lanyiova and M. Rossi ibid. 1973 56 2889. Is C. Batich E. Heilbronner E. Rommel M. F. Semmelhack and J. S. Foos J. Amer. Chem. SOC. 1974,96 7662. '' F. Marschner and H. Goetz Tetrahedron 1973 29 3105. W. Fuss and H. Bock J Chem. Phys. 1974,61 1613. C. Batich P. Bischof and E. Heilbronner J. Electron Spectroscopy 1973 1 333.l9 J. P. Maier and D. W. Turner J.C.S. Faraday ZZ,1973,69 196; Faraday Discuss. Chem. SOC.,1972 No. 54 p. 149; T. Kobayashi T. Yokota and S. Nagakura J. Electron Spectroscopy 1973 3 449. 2o J. P. Maier and D. W. Turner J.C.S. Faraday II 1973 69 521; S. A. Cowling and R. A. W. Johnstone J. Electron Spectroscopy 1973 2 161;J. P. Maier Helv. Chim. Acta 1974,57,994; P. S. Dewar E. Ernstbrunner J. R. Gilmore M. Godfrey and J. M. Mellor Tetrahedron 1974 30,2455; A. D. Baker M. Brisk and M. Gellender J. Electron Spectroscopy 1974 3 227. 21 S. F. Nelsen and J. M. Bushek J. Amer. Chem. SOC. 1973 95,2011 2013; ibid. 1974 96 2392 7930; P. Rademacher Angew. Chem. Znternat. Edn. 1973 12,408. 3.P.Maim I "'""'" Figure 3 Photoelectron spectrum of 1,2-dimethylhexahydropyridazine (Redrawn by permission from J.Amer. Chem. Soc.,1973,95,2013) dihedral angle between the coupling ?t-systems to be derived. The study of deformed double bonds illustrates the latter case.22 When there is a significant proportion of more than one conformation in flexible systems these may be detected provided that the criteria specified above are satisfied. The work on hydrazines is a nice instance where the nitrogen lone pairs provide such a probe.,' In Figure 3 is shown such a p.e. spectrum the analysis of which reveals that at least two conformers are present. By contrast in some substituted hydrazines where several conformers dominate the differences are not clear in the p.e. spectra. In a study of geometrical isomers of cis-and trans-Zsubstituted cyclopentyl and cyclohexyl bromides small differences in the first IEs were found and some change in the shape of bands.23 Open-chain conformations for (a-naphthyl)(CH,),(a-naphthyl) (n = 4 or 6) at -500 K were deduced by comparison with the p.e.spectrum of a-n-butyl-na~hthalene.,~ This supports the mass spectral studies for n = 4-16 which indicate that these species exist in the open-chain form with rapid intramolecular 22 C. Batich 0. Ermer E. Heilbronner and J. R. Wiseman Angew. Chem. Internat. Edn. 1973 12 312. 23 R. Botter F. Menes Y. Gounelle J. M. Pechine and D. Solgadi Internut. J. Mass Spectrometry Ion Phys. 1973 12 188. 24 J. Berkowitz J. L. Dehmer K. Shimada and M. Szwarc J. Electron Spectroscopy 1974 2 21 1 ; ibid.3 164. Photoelectron Spectroscopy and Related Phenomena of Gaseous Species 8 1 AA 114°C MeAA 075°C Me2AA ~ 8 10 8 10 I.P./eV -Figure 4 Photoelectron spectra of acetylacetone (AA) 3-methylacetylacetone (MeAA) and 3,3-dimethylacetylacetone(Me,AA) (Reproduced by permission from Chem. Phys. Letters 1974,26,229) collisions to form a ring form after ionization. The temperature dependence of the p.e. spectra has also been examined for inter- and intra-molecularly hydrogen- bonded systems. In the first approach the dimer formation of carboxylic acids was encouraged by lowering the temperature to 200-245K and comparing these p.e. spectra with those at -300 K.*' However the complexity of overlap of the bands allows only a few comparisons.The p.e. spectra of intramolecularly hydrogen-bonded amino-alcohols at -300 and 500 K suggest that the decrease in IE of the nitrogen lone pair at the higher temperature may be taken as an indication of the hydrogen-bonding stabilization.26 In Figure 4 is shown the variation of relative intensity of the lowest IE bands of acetylacetone and its '' R. K. Thomas Proc. Roy SOC.,1972 A331 249. 26 S. Leavell J. Steichen and J. L. Franklin J. Chem. Phys. 1973 59 4343. 82 J. P.Maier 3-methyl and 3,3-dimethyl derivatives as a function of temperat~re.~~ This is interpreted as a consequence of the keto-enol tautomerism and from the band intensities equilibrium constants and reaction enthalpies were inferred.Series Correlations and Substituent Effects.-A successful way to interpret the p.e. spectra of organic molecules has been through the study of series of molecules where the trends in the IEs establish a recognizable at tern.^ When the IEs of the n-bands correlated with the el,(s)and a2”nbands of benzene of the hetero- cyclic derivatives of benzene C5H5X (X = N P As or Sb) are plotted against the first IEofthefree heteroatoms,linear regressionsareobtained.28Thegradients of0.36 and 0.16 are in accord with the values 4 and 4 predicted from perturbation consideration of the benzene orbitals. The changes in the IEs have then been regarded in terms of simple models such as the LCBO (linear combination of bond orbitals) type. If the molecules contain a heavy atom particularly iodine or bromine the spin-orbit coupling discernible yields a sensitive probe of the electronic environment and hence of the adequacy of the model.In the p.e. spectroscopic study of hal~genodiacetylenes,~~ X-C-C-C=C-H X-CSC-C-C-X and X-C-C-C-C-Me (X = Cl Br or I) the spin-orbit splittings reveal that even though parametrization may be success-ful for IEs of the bands the predicted spin-orbit splittings are erroneous. The p.e. spectra are of considerable simplicity. Figure 5a shows a spectrum and Figure 5b the model used. The consistency of the interpretation is witnessed by the spin-orbit coupling vibrational excitation Franck-Condon profiles and correla- tion with data on other halogenoacetylenes. The p.e. spectra of iodoethylenes are rather more complex and were interpreted with the aid of modified extended Huckel calculations with spin orbitals as basis.30 Semi-empirical MO calculations have continued to be used in obtaining cor- roboration of the assignments.However such assignments may be unreliable. For instance the study of the IEs of cycloalkenes by ab initio SCF MO calculation3’ demonstrated that for small rings especially the results obtained from the SE MO theories are rather poor. In other work the IEs of the n-bands of the a~enes~~ were considered in relation to models of differing approximations from Huckel to SCF calculations. In an impressive compilation of PIE data of 65 eata-and peri-condensed aromatic hydrocarbon^^^ the n-IEs below 11 eV are identified with the aid of SE MO calculations.For molecules larger than naphthalene the a-IE onset is placed at 10.6 f0.2 eV. In the sterically overcrowded systems the authors argue that the Franck-Condon profiles of the n-bands suffice to 27 A. Schweig H. Vermeer and U. Weidner Chem. Phys. Letters 1974 26 229. ’* C. Batich E. Heilbronner V. Hornung A. J. Ashe D. T. Clark U. T. Cobley D. Kilcast and I. Scanian J. Amer. Chem. Sac. 1973 95 928. 29 E. Heilbronner V. Hornung J. P. Maier and E. Kloster-Jensen J. Amer. Chem. Sac. 1974 96,4252. ’O K. Wittel H. Bock and R. Manne Tetrahedron 1974 30,651. ’l D. C. Clary A. A. Lewis D. Morland J. N. Murrell and E. Heilbronner J.C.S. Faraday II 1974 70 1889. 32 P. A. Clark F. Brogli and E. Heilbronner Helv. Chim. Acta 1972 55 1415.33 R. Boschi E. Clar and W. Schmidt J. Chem. Phys. 1974,60,4406. Photoelectron Spectroscopy and Related Phenomena of Gaseous Species 1 r. Iv) 9 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 I.PJeV t %-If -Be# Figure 5 (a)Photoelectron spectrum of di-iododiacetylene;(b)orbital correlation diagram for di-iododiacetylene (Reproduced by permission from J. Amer. Chem. SOC.,1974,96,4252) distinguish deviations from coplanarity. The good correlation generally attained in such series with the calculated orbital energies suggests that the 'defects' of the Koopmans' theorem are similarly related. In one approach the results of ab initio calculations for azabenzenes were corrected empirically for reorganiza- tion and correlation by dividing the NOS into related categories.34 A para-metrization of the iterative Extended Huckel method also gave good agreement 34 J.Almlof B. Roos U. Wahlgren and H. Johansen J. Electron Specfroscopy 1973 2 51. 84 J. P.Maier with the p.e. spectra of azabenzenes and azanaphthalenes by considering groups of similar orbitals in separate regressions and thus ‘compensating’ for the frozen orbital notion.35 The n-IEs of alkyl-f~lvenes~~ provide an example where the relaxation process on ionization is strongly dependent on the nature of the state of the cation generated. Substituent E’ects. The effect of alkyl groups on the n-IE in 63 alkenes as well as in alkynes and carbonyl compounds has been disc~ssed.~’ The influence of alkyl substituents on the Jahn-Teller splitting of the first band has also been considered in allene derivative^.^^ In the study of 4-substituted quinuclidines the dependence of the nitrogen lone pair IEs was viewed in terms of local potential at the nitrogen due to the dipole of the sub~tituent.~~ These IEs were also found to be linearly related to o* substituent parameters.Also in substituted pyridines the first n-IEs give linear correlation with o+ constants and the nitrogen lone pair IEs with the pK values of the m01ecules.~~ In para-substituted pyridine N-oxides the effect of substituents has been investigated for the ground state and two excited states corresponding to ionization of electrons from correlated MOs discernible throughout the series.41 The IEs of these three processes gave good linear correlations with dipole moments pK values and cr and o+ substituent constants.In the above-mentioned empirical correlations the sig- nificance of the changes of IEs in regard to the electronic mechanism changes are not clear. The effects of halogen (F C1 or Br) substitution on benzene and pyridine x-IEs have also been discussed in terms of a model comprising short- and long-range inductive terms as well as the rnesomeric term.42 Through-bond and Through-space Notions. Questions of through-bond/space interactions have continued to attract the interest of p.e. spectroscopy and here the necessity of data on related molecules is especially evident. The analysis of the n-IE p.e. data of butadiene hexatriene and their methylated derivatives shows the influence of the methyl groups and affords the postulation of through-space interaction between the n-MO and the non-bonded pseudo n-MOs of the methyl group in the 2-or 3-po~itions.~~ Interactions between nitrogen lone pairs have been investigated in 2,3-diazabicyclo[2,2,n]alk-2-enes(n =14) 35 J.Spanget-Larsen J. Electron Spectroscopy 1973 2 33; ibid. 1974 3 369. 36 F. Brogli P. A. Clark E. Heilbronner and M. Neuenschwarider Angew. Chem. Internat. Edn. 1973 12 422. 37 P. Masclet 0. Grosjean G. Mouvier and J. Dubois J. Electron Spectroscopy 1973 2 225; P. Carlier R. Hernandez P. Masclet and G. Mouvier ibid. 1974 5 1103; G Hentrich E. Gunkel and M. Klessinger J. Mol. Struct. 1974 31 231.38 F. Brogli J. K. Crandall E. Heilbronner E. Kloster-Jensen and S. A. Sojka J. Electron Spectroscopy 1973 2 455. 39 G. Bieri and E. Heilbronner Helv. Chim. Acra 1974 57 546. 40 B. G. Ramsey and F. A. Walker J. Amer. Chem. SOC. 1974 96 3315. 41 J. P. Maier and J.-F. Muller J.C.S. Faraday IZ 1974 70 1991. 42 D. G. Streets and G. P. Ceasar Mol. Phys. 1973 26; 1037; J. N. Murrell and R. J. Suffolk J. Electron Spectroscopy 1973 1 471. 43 M. Beez G. Bieri H. Bock and E. Heilbronner Helc. Chim. Acta 1973 56 1028. Photoelectron Spectroscopy and Related Phenomena of Gaseous Species (1) and 3,4-diaza-analogues of tricyclo[4,2,1 ,02*5]nonadiene (2) and its 7,8-dihydro-deri~atives.~~ n-Orbital interactions were also discussed from the p.e.spectra of anti-and syn-tricyclo[4,2,0,02*5]octadienes(3) homoconjugative interactions of polyunsaturated [4,4,2]propellanes and bridged bicyclic dienes (4; n = l--4).4’ In bridged cyclohexanes (5; X = 0,S or C=CH2) the interac- tions of the substituent X with the cz ‘ribbon orbitals’ of the framework were con- sidered from the p.e. spectral IEs.~~ In 1,5-cyclo-octadiyne (6)the through-space/ bond interactions have been shown to be of comparable magnitude as far as the n-IEs are concerned and the effects of cis-bending of the acetylene moiety on the n-IEs have also been disc~ssed.~’ Homoconjugative interactions between exo- cycIic and endocyclic double bonds have also been considered by p.e. spectro- scopy as has spiroconjugation.’’,48 The assignments of the p.e.spectra of quadricyclanes have been used to discuss their cycloaddition reactions.49 Fluorosubstitution has been used to change the balance of through-bond/space interactions by preferentially stabilizing the o-MOs relative to the n-MOs. This has been demonstrated in [2,2]paracyclophanes diazabenzenes and diazanaphthalenes.’ 2 Transient Species and Free Radicals The stable nitroxide radicals di-t-butyl nitroxide and 2,2,6,6-tetramethylpiperi-dine N-oxyl have been studied in the gas phase by p.e. spectroscopy with U.V. 44 J. C. Bunzli D. C. Frost and L. Weiler J. Amer. Chem. SOC.,1973 45 7881 ;F. Brogli W. Eberbach E. Haselbach E. Heilbronner V. Hornung and D. M. Lemal Helv. Chim. Acta 1973 56 1933. 45 R. Gleiter E. Heilbronner and H.D. Martin Chem. Ber. 1973 106 28; R. Gleiter E. Heilbronner L. A. Paquette G. L. Thompson and R. E. Wingard Tetrahedron 1973 29 565; M. J. Goldstein S.Natowsky E. Heilbronner and V. Hornung Helv. Chim. Acta 1973 56 294. 46 R. Hoffmann P. D. Mollere and E. Heilbronner J. Amer. Chem. SOC.,1973 95 4860; R. J. Boyd J. C. Biinzli J. P. Snyder and M. L. Heyman ibid. p. 6478. 47 G. Bieri E. Heilbronner E. Kloster-Jensen A. Schmelzer and J. Wirz Helv. Chim. Acta 1974 S7 1265; H. Schmidt A. Schweig and A. Krebs Tetrahedron Letters 1974 1471. 48 P. Asmus and M. Klessinger Tetrahedron 1974 30 2477; A. Schweig U. Weidner R. K. Hill and D. A. Cullison J. Amer. Chem. Soc. 1973 95 5426 and references therein. 49 H. D. Martin C. Heller E. Haselbach and Z. Lanyiova Helv.Chim. Acta 1974 57 465; E. Haselbach and H. D. Martin ibid. p. 472. 50 E. Heilbronner and J. P. Maier Helv. Chim. Acta 1974 57 151; D. M. W. van den Ham D. van der Meer and D. Feil J. Electron Spectroscopy 1974,3,479; R. J. SuRolk ibid. p. 53; D. M. W. van den Ham and D. van der Meer ibid. 1973 2 247. 86 J. P.Maier excitation and the former species also with X-ray excitation in the 1s binding energy regi~n.~ From the multiplet splitting of the core levels due to exchange interactions between the unpaired core and valence electrons the spin density distribution is inferred. The unpaired electron is mainly in the n* MO of the NO group and is delocalized and the Nls and 01s binding energies reflect the net flow of charge from alkyl to NO.The valence p.e. spectrum indicates an IE of 7.31eV for the photoejection of the odd electron from the '7t* MO resulting in a singlet ground ionic state. The subsequent bands IE = 8.94 and 9.27 eV correspond to ionization of the nonbonding electrons of oxygen and the exchange splitting between the unpaired electrons in the ion results in triplet and singlet states whose intensity ratio in the p.e. spectrum 3 :1 is as expected on the basis of spin multiplicities. These two studies illustrate the complementary nature of the experiments to provide the complete data. Transient species so far studied have been reasonably long-lived. Most of the species have been generated in sufficient amounts from the precursor(s) to domi- nate the spectrum or else the bands have been separated or well defined from those of the precursor(s) and/or side-products.Pyrolysis or microwave discharge methods can also be used and fast-flow systems enable steady-state concentrations to be attained. The p.e. spectra of CS,O2('Ag) SO (%-) P2 HBS NF2,HCP S20 H2CS F2CS,and others have been obtained.6*s2 The first band of the methyl radical (from Me2Hg as precursor; lifetime <2 ms) has been observed.53 The IE of 9.837 eV is in good agreement with the known spectroscopic value and the dominance of the 0-0 transition is indicative of similar geometry for the ground ionic state to that of CH ,which is expected to be planar. The latter is in accord with the planarity of NH,' which is iso-electronic with CH,.. A single vibrational excitation of 2720 & 30cm-' is ascribed to the vl stretching mode.The identification of the species involved is one problem encountered. One means of characterizing the reactive species is by mass analysis. This has been demonstrated by Turner and co-workers where in situ analysis is achieved by utilizing the same analyser for electron detection and for mass identification from the time-of-flight of the ions.54 Even in routine measurements this gives reassurance that the sample is not only pure but of the correct structure. Recently phase-sensitive detection with a modulated micro- wave discharge has been used successfully to separate the p.e. spectra of the Oz (lAg) (10% in concentration) and 0 (3P)transients from that of O2(,&-) and SO (,I=-) from SOz." These authors also report the p.e.spectrum of the 51 I. Morishima K. Yoshikawa T. Yonezawa and H. Matsumoto Chem. Phys. Letters 1972 16 336; D. W. Davis and D. A. Shirley J. Chem. Phys. 1972,56,669. 52 References given by D. C. Frost in ref. I. L. Golob N.Jonathan A. Morris M. Okuda and K. J. Ross J. Electron Spectroscopy 1972 1 506. 54 D. L. Ames J. P. Maier F. Watt and D. W. Turner Faraduy Discuss. Chem. SOC. 1972 No.54 p. 277. '' N. Jonathan A. Morris M. Okuda K. J. Ross and D. J. Smith J.C.S. Faraduy II 1974,70 1810; J. M. Dyke L. Golob N. Jonathan A. Morris M. Okuda and D. J. Smith ibid. p. 1818; J. M. Dyke L. Golob N. Jonathan A. Morris and M. Okuda ibid. p. 1828. Photoelectron Spectroscopy and Related Phenomena of Gaseous Species 87 CF2 radical produced by microwave discharge through C2F4-He.The extensive vibrational excitation on the first band suggests considerable bond-angle change on ionization. 3 Coincidence Experiments Although coincidence measurements have been widely used by physicists the application of these powerful techniques to probe the electronic structure and nature of the ionic states of organic molecules is as yet limited. Photoelectron- photoion coincidence measurements aim at mass and kinetic energy analysis of the ions obtained corresponding to selected delayed photoelectrons and uia the latter at determining the initial internal energy of the ion. Alternatively the p.e. spectrum for a selected mass ion is determined. Thus when dissociation products are detected the ionic state which was at first produced is identified.The ionic states are correlated with ejection of electrons from MOs and thus one may try to link the reaction of ions of known initial energy to the description of the electronic configuration. For the triatomic molecules studied COS N20,C02 CS2 SO2,and HzO it was found that all ionic states that lie above the thermodynamic dissociation limits fragn~ent.~ Quasi-equilibrium theory or direct dissociation models did not suffice for rationalization of the branching ratios and the kinetic energy of the ions. The fragmentation pathways can be postulated from the energetics data and Wigner-Witner correlation rules. In Figure 6 are shown the mass ion spectra of benzene taken in coincidence with the electronic states of the benzene which are represented in the Figure via the MOs from which the electron is removed.The ground (1elg-')and first excited (1e2g-') ionic states which lie at IEs of 9.25 and 11.49eV respectively are not shown. The Franck-Condon region of the hypersurfaces reached in the ground and first two excited states is below dissociative pathways while for higher states the fragments shown are detected on removal of electrons as shown. The studies on CF4+confirm that the ground ionic state is fully dissociated with the CF3 + fragment possessing large kinetic energy releases ( -1eV),5* whereas for C2F6+ the results suggest that C2F6+ ions in their first excited state fragment directly to C2F5' rather than via the ground ionic state which yields only CF3+.59 The authors relate the difference to the ionization of electrons from the strongly C-C a-bonding MO and n-MOs of the fluorines for the first two ionization processes respectively.The fragmentations of CH4+ CD4+ C2H6+ and C2D,+ have also been studied but instead of the fixed photon source (He 584 A) a continuous light source with a monochromator was used and only electrons of sf J. H. D. Eland Ado. in Mass Spectrometry 1974 6 917; references given in ref 5(6). ST J. H. D. Eland Internal. J. Mass Spectrometry Zon Phys. 1974 13 457. B. Brehm R. Frey A. Kiistler and J. H. D. Eland Internat. J. Muss Spectrometry Ion Phys. 1974 13 251. '' I. G. Simm C. J. Danby and J. H. D. Eland Internat. J. Mass Spectrometry Zon Phys.1974 14 285. J. P. Maier m/e 39 52 63 78 f 5 2.5 0 5 0 A m/e 39 52 63 78 Figure 6 Coincidence mass spectra of benzene (Reproduced by permission from Internat. J. Mass Spectrometry ion Phys. 1974,13,457) zero kinetic energy were detected.60 This obviates compfications arising in cross-section and autoionization. Another approach has been to use fast electrons (3.5 keV) to simulate photon impact and to detect the scattered and ejected electrons in coincidence. This provides a means of varying the impact wavelength and in this way partial ionization cross-sections have been determined for NH ,CO and H20which are in agreement with the available photon impact data.61 Application of this approach to larger molecules can yield the cross-sections as well as the asymmetry parameter fl.6o R. Stockbauer J. Chem. Phys. 1973,58 3800. M. J. van der Wiel and C. E. Brion J. EIectron Spectroscopy 1973 1 309 439 443; G. R. Branton and C. E. Brion ibid. 1974 3 123 129. Photoelectron Spectroscopy and Related Phenomena of Gaseous Species 89 4 Angular Distribution of Photoelectrons from Organic Molecules Measurements of the angular distribution of ejected photoelectrons from some organic molecules have been carried out in order to determine how this can be related to their electronic structure. The asymmetry parameter /I (-1 to + 2) describes the results. For the molecules of interest here the He-Icc (21.22eV) resonance line has been used throughout with the exception of benzene where the Ne-I line was also used as well as the synchrotron radiation at these wave- lengths (as a source of polarized radiation).These data illustrate the intensity differences as a consequence of polarization of the radiation.62 The energy dependence of /I was found to follow the pattern suggested by calculations for CT-and n-orbitals of ethylene. Carlson and co-workers have determined /I for the p.e. bands of halogen-substituted methanes CH,,X4-, (n = 0-3 X = F or C1) and MeX (X = Br or I) and tetramethyl compounds of Group IV Me,X (X = C Si Ge Sn or Pd) and attempts were made to correlate the data with 1.6 1.2 0.8 0.4 0.0 .0.4 I I I I I I I I I 1 1 19 18 17 16 15 14 13 12 11 10 9 8 Binding energy (eV) Figure 7 He-I photoelectron spectrum of butadiene and the angular parameter /3 (Reproduced by permission from J.Electron Spectroscopy 1974,3,59) 62 J. A. Kinsinger and J. W. Taylor Internat. J. Mass Spectrometry Ion Phys. 197213 10. 445. J. P.Maier the associated MO parameter^.^^ Such studies on pyridine and the three diaza- benzenes also revealed trends.54 In Figure 7 is shown the B variation in the p.e. spectrum of butadiene. In the homologous series of unsaturated linear hydro- carbons it is noted that IT-and a-orbitals may be so di~tinguished.~~ The difficulty arises in interpreting the data via theoretical models and recently semi-empirical SCF calculations have been used to provide the link.65 5 X-Ray-excited P.E. Spectra and Complementary Uses of He-I and He-I1 Excitation The development in performance of X-ray-excited p.e.spectrometers by Siegbahn and co-workers66 has allowed the study of the valence bands of gaseous species with sufficient resolution to permit band-intensity corn par is on^.^^ In Figure 8 -... eV 293 290 35 3b 2k 20 1'5 10 Binding energy Figure 8 Photoelectron spectrum of benzene excited by monochromatized A1 Ka radiation (Reproduced by permission from J. Electron Spectroscopy 1974,5985) 63 T. A. Carlson and R. M. White Faraday Discuss. Chem. SOC. 1972 No. 54 p. 1972; A. E. Jonas G. K. Schweitzer F.A. Grimm and T. A. Carlson J.Electron Spectroscopy 1972 1 29. 64 R. M. White T. A. Carlson and D. P. Spears J. Electron Spectroscopy 1974 3 59. 6s B.Ritchie J. Chem. Phys. 1974 60 898; ibid. 1974 61 3279 3291; S. Iwata and S. Nagakura Mol. Phys. 1974 27 425; J. N. Rabalais and T. P. Debies ref. 1 p. 847. 66 B. Wannberg U. Gelius and K. Siegbahn J. Phys. (Ej 1974 7 159. 67 K. Siegbahn ref. 1 p. 3; U. Gelius ibid. p. 895. Photoelectron Spectroscopy and Related Phenomena of Gaseous Species t I TRITHIAPENTALENE s GASEOUS PHASE s2.3 ‘2 2Pq2 .2V\- AE, =I .4eV -eV 166 164 162 160 0 I I I 75 100 125 Channel number Figure 9 The S 2p photoelectron spectrum of irithiapentalene excited by monochromatized X-ray irradiation (Reproduced by permission from J. Electron Spectroscopy 1974,5,985) are shown the monochromatized A1 Ka-excited valence bands of benzene and here the intensities of the bands show clearly that the 16.9 eV band is that asso- ciated with the 3al MO and the preceding band with the 2bl MO owing to its increased intensity.In contrast to u.v.-excited p.e. spectra in the X-ray spectra s-type bands are stronger than the p-type. All the deeper valence bands can be excited and the intensity reversal is an asset in the location of bands that are difficult to detect with He-I1 radiation especially for IE >25 eV (e.g. the 2a, benzene band in Figure 8). Intensity models have been proposed and related to gross atomic populations from MO calculation^.^^ The higher resolution has revealed interesting data on the core level bands. The spectrum of trithiapentalene (Figure 9) shows clearly how the S 2p bands overlap and the assignment is shown.The old spectrum is shown in the inset and the authors note that this was ‘deconvoluted’ by three groups differently and then interpreted. These latest data suggest that there is a flat minimum around the S-S equilibrium position. The Clsband of CH4+is shown in Figure 10,and the contour shows vibrational excitation associated with the relaxation in the hole state leading to change of geometry; the equilibrium at 0.05 A shorter C-H distance is calculated Other molecules whose valence structures have been studied with these improvements include C,H, C2C14 and SF, as C302 well as some diatomic and triatomic molecules.67 The gas-phase X-ray-excited J. P.Maier EXP. c1s -LEAST SQUARES FIT 0.43 eV 291.5 291.0 290.5 Binding energy/eV Figure 10 The C 1s photoelectron spectrum of methane (Reproducedby permission from Chern.Phys.Letters 1974,28 1) p.e- spectrum of the pyridine-iodine monochloride charge-transfer complex has been reported68 and the technique has also been applied to He-IIa (40.80eV) radiation has been used as a source of excitation with two objectives firstly energy extension of the accessible ionic states and secondly for changes in the photoionization cross-section. The p.e. spectra of inner valence shells of saturated and unsaturated hydrocarbons have been pre~ented.~’ In Figure 11 the division of the inner valence region is seen and the s-type MO pattern is evident. The band positions which are less complex than of the outer valence shell have been found to fit a simple Huckel model.Shake-up bands often seen in X-ray-excited spectra have also been observed with He-I1 radiation. These forbidden transitions involving simultaneous electron excitation accom- panying the ionization gain their intensity through configuration interaction and have been discussed for diatomic and triatomic molecules in relation to their detection in the He-11-excited p.e. spectra.70 In the He-I-excited p.e. spectrum of butatriene an additional band of comparable intensity to the other bands is 68 A. Mostad S. Svensson R. Nilsson E. Basilier U. Gelius C. Nordling and K. Siegbahn Chem. Phys. Letters 1973 23 157. 69 A. W. Potts and D. G. Streets J.C.S. Faraday II 1974 70 875; ibid. 70 1505. ’O A. W.Potts and T. A. Williams f.Electron Spectroscopy 1974 3 3. Photoelectron Spectroscopy and Related Phenomena of Gaseous Species p-type bands s-type bands 15 20 25 30 ionization energy/eV Figure 11 The He-IIa photoelectron spectra of cycloalkanes (Reproduced from J.C.S. Faraday II 1974,70,875) also attributed to such a two-electron process and the increased transition probability has been di~cussed.~ The change of photoionization cross-section has been used as a means of identification of MOs in which a high contribution of an A0 is indicated. This has been used predominantly in instances where the changes in relative intensity of the bands are drastic on passing from He-I to He-I1 radiation. This is the case with the 3p A0 in comparison with the 2p.The former represented by chlorine- sulphur- and phosphorus-containing molecules decrease appreciably in cross- section towards the shorter wavelengths. The differential cross-sections have been used in connection with the assignment of the p.e. spectra of halogeno- ethylene~~~ and of oxygen- ~hlorine-,~’and fluorine-containing corn pound^.^^ There has also been some work in this direction by correlation with calculation^.^^*^^ 71 F. Brogli E. Heilbronner E. Kloster-Jensen A. Schmelzer A. S. Manocha J. A. Pople and L. Radom Chem. Phys. 1974,4 107. 72 A. Katrib T. P. Debies R. J. Colton T. H. Lee and J. W. Rabalais Chem. Phys. Letters 1973 22 196. ” A. Schweig and W. Thiel J. Electron Spectroscopy 1974 3 27 and references therein.74 F. 0.Ellison J. Chem. Phys. 1974,61 507; J. W. Rabalais T. P. Debies J. L. Berkosky J.-T. J. Huang and F. 0.Ellison ibid. p. 516.

ISSN:0069-3030    

DOI:10.1039/OC9747100075

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (i) Orbital Symmetry Correlations and Pericyclic Reactions By D.W. JONES Department of Organic Chemistry The University Leeds LS2 WT 1 General and Theoretical Aspects New books,’ general review articles,2 useful mnemonic^,^ and numerous papers on both the fundamental and synthetic aspects of pericyclic reactions testify to a continued high level of interest in this area. Stimulus for further activity has undoubtedly been provided by Dewar’s MINDOj3 calculations which e.g. predict that the prototype Diels-Alder addition between butadiene and ethylene proceeds via a very unsymmetrical biradical transition state (TS)(1) in which one new o-bond is almost completely formed while the other has hardly formed at aIL4 Both the boat-like Cope rearrangement of hexa-l,5-diene and the conversion of (2) into (3) could be regarded as involving TS’s related to the biradical (4).A. J. Bellamy ‘An Introduction to Conservation of Orbital Symmetry A Programmed Text’ Longman London 1974; G. B. Gill and M. R. Willis ‘Pericyclic Reactions’ Chapman and Hall London 1974; W. B. Smith ‘Molecular Orbital Methods in Organic Chemistry HMO and PMO’ Marcel Dekker New York 1974 p. 127. (a) N. D. Epiotis ‘The Theory of Pericyclic Reactions’ Angew. Chem. Internat. Edn. 1974 13 751; J. B. Hendrickson ‘The Variety of Thermal Pericyclic Reactions’ ibid. p. 47; R. Gleiter ‘Effects of Through-bond Coupling’ ibid. p. 696; (6)T. Kauff-mann ‘1.3-Anionic Cycloadditions of Organolithiums’ ibid. p. 627. R. H. Wollenberg and R.Belloli Chem. in Britain 1974 10 95; F. G. Holliman ibid. p. 361; J. Mathieu Bull SOC.chim. France 1973 807; A. Rassat Compt. rend. 1972 274 730. M. J. S. Dewar A. C. Griffin and S. Kirschner J. Amer. Chem. SOC. 1974 96 6225. 95 D. W.Jones Yet heating (2)gives none of the isomer (5) expected from a boat-like species (4). The TS's for the boat-like Cope rearrangement and the isomerization (2)-+ (3) are clearly distinct. The concept of orbital isomerism5acan be used to rationalize this ob~ervation.~'The TS for the (2)into (3) conversion is akin to the through-space coupled biradical (6) in which the bonding combination + G2) is the HOMO. On the other hand the TS for the boat-like Cope reaction is related to the biradical (7)in which through-bond coupling places -t,b2) at lower energy; (6)and (7)are therefore lumomers [HOMO (6)converts into LUMO (7)] and their interconversion is forbidden and must proceed via the higher-energy biradical (4) as TS.The biradicaloids (8) and (9) formed respectively from two ethylenes and cyclobutane are also lumomers whose interconversion is for-bidden ;there is thereforeopportunity for loss of configuration in the dimerization of substituted ethylenes by rotation about the C,-Cb bond.5c Forbidden reac-tions will try to cross the biradical barrier at a point corresponding to the most stable form of the biradical. Symmetrical disrotatory opening of cyclobutene would lead to the antiaromatic (0 0)-biradical (10). MIND0/3 suggests that this is avoided; the TS corresponds to rotation of one methylene group by 45" while the other has moved little.Thus the TS is more closely allied to the orthogonal methylene-ally1 biradical (11) where destabilizing interaction of the allyl and carbon 2p orbitals is avoided. As the twist angle increases beyond 45" bond formation in the allyl radical moiety leads to a decrease in energy and the HOMO-LUMO crossing occurs after the TSSd H .. ,H 9 HX $ H-D 'D C H -.. H -H D H H (5) (6) (71 (8) (9) (10) (1 1) (12) The TS for the .2 + ,2 cycloreversion of dioxetan (12) to formaldehyde is calculated to lie ca. 65 kcal mol-' above (12) and the TS for cleavage via a biradical some 45 kcal mol-' above (12). These compare poorly with observed activation energies (21-29 kcal mol-in substituted cases).Calculation of the (a) M. J. S. Dewar S. Kirschner and H. W. Kollmar J. Amer. Chem. Soc. 1974 96 5240; (b) M. J. S. Dewar S. Kirschner H. W. Kollmar and L. E. Wade ibid. p. 5241; (c) M. J. S. Dewar and S. Kirschner ibid. p. 5246; (d) M.J. S. Dewar and S. Kirschner ibid. p. 5244 6809; (e) M. J. S. Dewar and S. Kirschner ibid. p. 7578; v) M. J. S. Dewar S. Kirschner and H. W. Kollmar ibid. p. 7579; (g) R. S. Case M. J. S. Dewar S. Kirschner R. Pettit and W. Slegeir ibid. p. 7581 ;(h)W. H. Richard-son F. C. Montgomery M. B. Yelvington and H. E. O'Neal ibid. p. 7525. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations 97 triplet energy surface shows it to intersect the singlet surface at 38.3 kcal mol-’.It is t herefore suggested that the cleavage of dioxetans involves intersystem crossing to the triplet surface which is pursued with formation of triplet-excited carbonyl compound and consequent light emi~sion.’~ Because the triplet surface for Dewar benzene intersects the singlet surface after the TS for opening to benzene only a small amount of triplet benzene is formed.’/ Circumstances appear to be more auspicious for conversion of (13) into (1 4) which gives rise to light emission in the presence of 9,10-dibromoanthracene,5” An extended biradical mechanism for dioxetan cleavage is favoured by others.5h (13) (14) An ab initio study of the methylenecyclopropane rearrangement6 implicates the orthogonal methylene-allylic biradical (15) which lies 8.8 kcal mo1-l below the TS and is suggested asa target for experimental detection.This result contrasts with the conclusion of an experimental study that the biradical is a TS in the racemization and geometrical isomerization of cyclopropanes. While thermo- chemical arguments consistently predict biradical intermediates quantum mechanics yield a biradical TS when no secondary stabilization of the radical is possible and a biradical intermediate when one odd electron is stabili~ed.~ The singlet orthogonal trimethylenemethane (16)reacts with olefins by addition 3 (15) (16) at C-1 and C-4; addition at C-3 and C-4 would leave an orthogonal n-system between C-1 and C-2. In contrast the additions of the related triplet are orienta- tionally and stereochemically non-specific.’ Doubts that the Stevens rearrangement proceeds exclusively via biradicals have been raised.The process detected by CIDNP may not be the only biradical process involved,’ nor indeed the only process involved for the concerted- forbidden pathway is predicted to have a very low activation energy (ca. 4 kcal mol -’).’ The CIDNP enhancements accompanying rearrangement are much less than the theoretically estimated values. l1 W. J. Hehre L. Salem and M. R. Willcott J. Amer. Chem. Soc. 1974 96,4328. ’ W. von E. Doering and K. Sachdev J. Amer. Chem. SOC.,1974 96 I 168. a J. A. Berson C. D. Duncan and L. R. Corwin J. Amer. Chem. SOC.,1974 96 6175; J. A. Berson L. R. Corwin and J. H. Davis ibid. p. 6177. F. Gerhart and L.Wilde Tetrahedron Letters 1974,475. lo M. J. S. Dewar and C. A. Ramsden J.C.S. Perkin I 1974 1839. ’ * G. L. Closs Sheffield Stereochemistry Symposium 1974. D. W.Jones For reactions observed in the mass spectrometer the extra activation energy requirement of a concerted-forbidden reaction results in kinetic energy release and the accompanying metastable peaks are broad (flat-topped or dished). Thus forbidden 1 ,Zhydrogen elimination from protonated formaldehyde to give the formyl cation and the decomposition of C3H7+to the ally1 cation give broad metastables. But for the allowed 1,l-hydrogen eliminations in the frag- mentation of CzH5+to the vinylium cation and the conversion of protonated benzene into the phenyl cation the metastables are normal.I2 2 Electrocyck Reactions Ab initio SCF and CI calculations support the qualitative predictions of orbital symmetry theory for the cyclopropyl-ally1 interconversions (17) C(18).In the forbidden processes as well as in the allowed interconversion of the anions ring-opening fails to coincide with methylene rotation.' In agreement with preferred conrotatory opening of cyclopropyl anions (19) opens to an ally1 CN anion some lo4 times more rapidly than (2O).I4 Attention has been drawnI5 to the first example'$ ofan electrocyclic reaction of an an-ally1 anion (21) -+(22) and a possible example of the disrotatory opening of a cyclopropyl radical is provided17 by pyrolysis of (23) in ethylbenzene to give (24). The bicyclobutane FN '' D.H. Williams and G Hvistendahl J. Amer. Chem. SOC.,1974 96 6753 6755. l3 P. Merlet S. D. Peyerimhoff R. J. Buenker and S. Shih J. Amer. Chem. SOC.,1974 96 959 and cited references. M. Newcomb and W. T. Ford J. Amer. Chem. SOC.,1974 96 2968. H. Quast and C. A. Weise-Velez Angew. Chem. Internat. Edn. 1974 13 342. *' A. T. Bottini and R. E. Olsen J. Amer. Chem. SOC.,1962 84 195. I' A. Barmetler C. Ruchardt R. Sustmann S. Sustmann and R. Verhulsdonk Tetra-hedron Letters 1974 4389. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations (25) opens thermally to (26) and photochemically to (27); it is now shown'* that the related radical anion resembles excited (25) in opening to the radical anion of pleiadiene (27). It is pointed out that both excited (25) and the radical anion have one electron in the same frontier orbital ;the ring-opening is regarded as a rare ,,2* +u2aprocess (28).The cation (29) gives (30) by conrotatory closure1g and the anion (31) closes to (32) in a disrotatory way.** (25) (26) (27) (33) (34) (35) The six-election photocyclization of (33) to (34) is followed by 1,4-sigmatropic hydrogen shift to (35).21 Related photocyclization of the amides (36) affords intermediates(37) which go to products (38) by a 1,hhift of the methoxy or acyl group R.22 J. R. Dodd R. F. Winton R. M. Pagni C. R. Watson andJ. Bloor J. Amer. Chem. Sac. 1974 96 7846. l9 C. W. Shoppee and B. J. A. Cooke J.C.S. Perkin I 1974 189. 2o C. W. Shoppee and G. N. Henderson J.C.S. Chem. Cumm.1974 561. *'A. G. Schultz and M. B. De Tar J. Amer. Chem. SOC.,1974 96 296. 22 I. Ninomiya T. Kiguchi and T. Naito J.C.S. Chem. Cumm. 1974 81. 100 D. W. Jones Full papers have appearedz3 dealing with the generation of the enol(39) either by irradiation of o-methylbenzaldehyde or by thermal opening of benzocyclo-butanol. The preference of the hydroxy-group for an outside position shown in these reactions extends to the opening of (40) to the enol (41).24 The cyclo- butene (42) opens by the conrotatory path that places both acetylene moieties in internal positions and the product (43) is the first stable benzo~yclobutene.~~ The effect of substituents on the cycloheptatriene-norcaradiene equilibrium has been studied.26 Homoporphyrins undergo 18~-electron electrocyclic reac- tions and 1,17-sigmatropy analogous to the Berson-Willcott rearrangement of cycloheptatrienes.3 Cycloaddition and Cheletropic Reactions The usually observed reaction of carbenes with dienes is 1,Zaddition via a non-linear approach (44).The alternative 1P-addition involving a linear approach (45)has now been realized with nucleophilic carbenes. Difluorocarbene is thought to be more nucleophilic than dichlorocarbene owing to better 2p2p than 3p2p orbital overlap and consequently more important donation of the halogen lone pairs into the carbon p-orbital; addition of the former to norbornadiene gives both the 1,2-and 1,4-adducts (46) and (47).'* Similarly the nucleophilic carbene 23 B. J. Arnold S. M. Mellows P. G. Sammes and T.W. Wallace J.C.S. Perkin I 1974 401; B. J. Arnold P.G. Sammes and T. W. Wallace ibid. p. 409 415. 24 C. W. Jefford A. F. Boschung and C. G. Rimbault Tetrahedron Letters 1974 3387. 25 H. Straub Angew. Chem. Internat. Edn. 1974 13 405. 26 F. G. Klarner Tetrahedron Letters 1974 19; W. Betz and J. Daub Chem. Ber. 1974 107 2095. 27 H.J. Callot and T. Tschamber Tetrahedron Letters 1974 3155. 28 C. W. Jefford n'Tanda Kabengele J. Koyacs and U. Burger Tetrahedron Letters 1974 257; Helv. Chim. Acta 1974,57 104. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations (48) is believed to add 1,4 to the electron-deficient diene (49) to give after de- carbonylation the product (50). The reaction of (51) with (49) is more complex but may also involve initial 1,4-additi0n.~~ Reaction of (51) with the triene (52) may proceed via the 1,4-adduct (53) which rearranges to (54) by a u2 + u2 + u2s R2 0-/A &yo (55) R' (56) R' = Me R2= H (57) R' = Br RZ= C0,Me Me The INDO-MO technique suggests that 1,3-interaction in the 2-oxidoallyl cation (55)is so strong that it is best described as a monohomocyclopropenone.3 Species related to (55) add 1,4 to pyrroles to give e.g.(56) and (57) which offer access to tropane alkaloids ;32 the 2-methoxyallyl cation will even add to benzene giving after acid work-up the ketone (58).33 The betaine (59) incorporates an oxidoallyl cation moiety and undergoes [4 + 21-addition to 2,3-dimethyl-butadiene to give (60).3" Betaines of the type (61) likewise add as oxidoallyls 29 T.Mitsuhashi and W. M. Jones J.C.S. Chem. Comm. 1974 103. 30 K. Saito Y. Yamqshita andT. Mukai J.C.S. Chem. Comm. 1974 58. 31 B. K. Carpenter J.C.S. Perkin ZZ 1974 1. 32 G. Fierz R. Chidgey and H. M. R. Hoffmann Angew. Chem. Znternat. Edn. 1974 13 410; R. Noyori Y. Baba and Y. Hayakawa J. Amer. Chem. SOC.,1974 96 3336. 33 H. M. R. Hoffmann and A. E. Hill Angew. Chem. Znternat. Edn. 1974 13 136. 34 (a)K. L. Mok and M. J. Nye J.C.S. Chem. Comm. 1974,608; (b)N. Dennis B. Ibrahim and A. R. Katritzky ibid. p. 500;N. Dennis A. R. Katritzky T. Matsuo S. K. Parton and Y. Takeuchi J.C.S. Perkin I 1974 746. 102 I). w.Jones to dienes but with olefins they behave as 4n-electron oxidopentadienyl cations giving adducts of the type (62); the reversible dimerization of (61) is therefore a .4 + .2 addition.34b ((i@,? pqo no N N R xx A number of studies have been concerned with retro-cy~loaddition.~~ In agreement with a concerted ,,2s + ,2 + (r2,process (63) loses nitrogen stereo- specifically to give cis,cis-octa-2,6-diene ; nitrogen loss is however much slower than for (64)in which the 'bent-bond' of the bridge ensures better overlap with the breaking C-N bonds.35" Such overlap would be expected to increase with the angle 8 in (65); the rates of deamination and photoelectron spectra of (65 ; n = 0 and n = 1) support this Full papers have appeared dealing with the synthesis of semibullvalenes from diazasnoutenes by reaction of the type (66)-+ (67).35c The activation energy for the related loss of nitrous oxide from azoxy-compounds is much greater.However the process is thought to be concerted and shows the same response to the presence of a 'bent-bond'; (68) loses nitrous oxide lo6 times more rapidly than (69).35dA similar large rate difference is observed for the decomposition of (70) and (71) to butadiene and sulphur dioxide; (70)decomposes at ca. 0 "C and (71) requires heating to 120°C. The effect of pressure on fhe relative amounts of [2 + 2} and [2 + 43 adducts formed in the dimerization of chloroprene and in the addition of tetrachloro-benzyne to norbornadiene has been studied.36 Only for the first process is the [2 + 41 reaction strongly favoured by pressure increase. In the benzyne addition 3s (a)J.A. Berson E. W. Petrillo and P. Bickhart J. Amer. Chem. SOC.,1974 96 636; J. A. Berson S. S. Oh E. W. Petrillo and P. Bickhart Tetrahedron 1974 30 1639; (6) H. Schmidt A. Schweig B. M. Trost H. B. Neubold and 9. H. Scudder J. Amer. p. 7454 7465; (4H. Olsen and J. P. Snyder ibid. p. 7839; (e) F. Jung M Molin R. Chem. SOC.,1974 96 622; (c) D. R. James G. H. Birnberg and L. A. Paquette ibid. Van Den Elzen and T. Durst ibid. p. 935. 36 (a) W. J. le Noble and R. Mukhtar J. Amer. Chem. Suc. 1974 96 6191; (b) W. G. Dauben and A. P. Kosikowski ibid. p. 3664; J. Rimmelin and G. Jenner Tetrahedron 1974,30 3081; K. Seguchi A. Sera and K. Maruyama Bull. Chem. SOC.Japan 1974 47 2242. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations the [2 + 21 adduct is thought to arise via the zwitterion (72),solvation of which is associated with a large volume decrease.Thus only those two-step additions proceeding via non-polar intermediates can be distinguished from concerted processes by the effect of The effect of pressure on several Diels- Alder additions supports a concerted mechanism.36b C1 H a++JSAr (77) (75) R' = H R2= R,X = CH (76) R' = R,R2 = H,X = 0,NRZ The conformation and the ethanol trapping of zwitterion intermediates in the reaction of tetracyanoethylene with en01 ethers has been discussed. Most zwitterions arise in U-conformations like (73) in which a donor-acceptor inter- action exists.37 Whilst quadricyclane (74; X = CHI)undergoes .2 + ,2 + ,2 addition to acetylenes (RC-CR) to give adducts of type (73 3-heteroquadri-cyclanes (74 ;X = 0or NR)give adducts of type (76)by initial.opening to dipolar intermediates( The 13-anionic cycloaddition of organolithiums to olefins hasbeen reviewed2b and used in a neat construction of a five-ring compound (78)-+ (79),by addition to ethyl a~rylate.~' The addition of the lithium derivative (80)to trans-azobenzene is a clear-cut two-step process; the intermediate (81) can be trapped by quenching and gives (82) on heating.40 Ph Ph 37 R. Huisgen R. Schug and G. Steiner Angew. Chem. Internat. Edn. 1974 13 80 81. 38 E. Haselbach and H. D. Martin Heiu. Chim. Acta 1974 57 472; H. D. Martin C. Heller E. Haselbach and 2.Lanyjova ibid. p. 465. 39 J. P. Marino and W. B. Mesbergen J.Amer. Chem. SOC.,1974 96,4050. 40 W. Bannwarth R. Eidenschink and T. Kauffmann Angew. Chem. Internut. Edn. 1974 13,468. 104 D. W.Jones Several studies emphasize the role of steric factors in determining endo/exo ratios in the Diels-Alder rea~tion.~' In additions to cyclopentadiene the domi- nant steric factor appears to involve a methylene hydrogen of the diene and an exo-substituent on the dien~phile.~'" Destabilization of the endo-TS in additions to 1,2-diphenylbutadiene may be associated with non-coplanarity of the 2-phenyl group with the diene In additions to hexachlorocyclopenta- diene the stereochemistry of the olefin may be lost and with unsymmetrical olefins the bulkier substituent takes the endo-p~sition.~" More subtle factors influencing stereochemical course41d and reaction rate4'" have also been dis- cerned.Domino-Diels-Alder reactions have been explored with a view to the synthesis of dodecahedrane ;e.g. (83) gives (84) and (85) with dimethyl acetylene- dicarb~xylate.~'A neat synthesis of ( & )-patchouli alcohol involves an internal addition (86) -P (87) which requires base catalysis ; this could involve activation of the diene system in (86) by the nearby ionized hydro~y-group.~~ Me ,H & (84) X = C0,Me;Y = H (87) (85) X = H;Y = C0,Me The isomerization of simple cyclo-octatetraenes e.g. (88) -+ (89) involves [4+ 21 addition in the bicyclic valence tautomer (90). This gives (91) which by reverse [4 + 2) addition in the manner shown (91; arrows) gives (89) after valence tautomerism ;an alternative mechanism involving 1,5-shift of the bond a in (90)would give 1,3-dirnethylcyclo-octatetraene and so cannot operate here.44 41 (a)J.M. Mellor and C. F. Webb J.C.S. Perkin 11 1974 17,26; B. C. C. Cantello J. M. Mellor and C. F. Webb ibid. p. 22; D. Bellus K. von Bredow H. Sauter and C. D. Weis Helv. Chim. Actu 1973 56 3004; S. Tsuboi Y. Ishiguro and A. Takeda Bull. Chem. SOC.Japan 1974 47 1673 ; D. Bellus H. C. Mez and G. Rihs J.C.S. Perkin 11 1974 884; (b) P. C. Jain Y. N. Mukerjee and N. Anand J. Amer. Chem. SOC.,1974 96,2996; (c) V. Mark J. Org. Chem. 1974,39,3 179,3 181 ; (6)W. Oppolzer Tetrahedron Letters 1974 1001 ; K. Mackenzie ibid. p. 1203; (e) C. K. Bradsher W. A. Porter and T.G. Wallis J. Org. Chem. 1974 39 1 172. 42 L. A. Paquette and M. J. Wyvratt J. Amer. Chem. SOC.,1974 96 4671 ; D. McNeil B. R. Vogt J. J. Sudol S. Theodoropulos and E. Hedaya ibid. p. 4673. '' F. Naf and G. Ohloff Helv. Chim. Actu 1974 57 1868. 44 L. A. Paquette and M. Oku J. Amer. Chem. SOC.,1974 96 1219. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations 105 Examination of the acrolein dimerization using the frontier-orbital method with orbital coefficients calculated by an ab initio SCF procedure allows pre- diction of the observed regio-isomer (92); use of HMO coefficients is unsatis- fa~tory.~'8,8-Dirnethylisobenzofulvene (93) and tropone give the [6 + 41 adduct (94) rather than the isomer (95) theoretically ~redicted.~~" It has now been shown that (95) is indeed the kinetically controlled product which is readily converted into (94) by Cope rearrangement.46b Since 6,6-dimethylfulvene gives an adduct like (94) with tropone the possibility exists that it too is formed via a kinetically controlled isomer of type (95).OCHO 0 H (94) (95) 0 0 R2 HH' Me Pr" 0 0 6 (96) (99) (97) R = Me RZ = Pr" (98) R = Pr" R2 = Me Although originating from the triplet state the photo-rearrangement of (96) to (97)and (98) is regarded as a ,2 + ,2 process cf. (99);a biradical intermediate is not consistent with the retention of optical activity observed.47n Optical 45 P. V. Alston and D. D. Shillady J. Org. Chern. 1974 39 3402. 46 (a) M. N. Paddon-Row Austral.J. Chern. 1974 27 299; (b) M. N. Paddon-Row and R. N. Warrener Tetrahedron Letters 1974 3797. 47 (a) D. I. Schuster and B. M. Resnick J. Amer. Chern. Soc. 1974 96 6223; (6) R. L. Coffin R. S. Givens and R. C. Carlson ibid. p. 7554; (c) H. E. Zimrnerman J. D. Robins R. D. McKelvey C. J. Samuel and L. R. Sousa ibid. p. 1974 4630. 106 D.W.Jones activity is also retained in the sensitized oxa-di-z-methane rearrangement (100)-(101) which can likewise be regarded as a =2 + u2 though the .2 + u2a+ .2 description favoured for the di-n-methane rearrangement47C is also possible. A possible =4 + =2 photoaddition has been and it has been pointed out that the 4 + 2 photo-reaction is feasible for a donor-acceptor partner~hip.~~ ( 102) The secondary isotope effect for the addition of [a-'H]styrene to dimethylketen (kdkD = 0.8) indicates important bonding to Ca in the TS ;"' the result contrasts with that for the addition of the same olefin to diphenylketen (kdkD = 1.12).Sob The addition of t-butylcyanoketen to norbornadiene gives the .2 + .2 + .2 adduct (102) as well as the expected cyclobutanone.sl" Ketenimmonium cations + (Me,C=C==NMe,) add in a similar way to cisoid dienes e.g.(103) from cyclo- ~entadiene,~~~ adducts of the more but with transoid dienes and 01efins~'~ familiar type (104) are formed. 4 Sigmatropic Reactions Unlike the normal 1,2-shift in a carbonium ion degenerate rearrangement of the bicyclobutyl carbonium ion (105) should proceed with inversion at the migrating group.The inversion TS (106) permits better overlap of the C-4 orbital with a filled Walsh orbital of the cyclopropane ring than is present in the retention TS (107) between the same C-4 orbital and the double bond. The process is suggested 48 H. Hart T. Miyashi D. N.Buchanan and S. Sasson J. Amer. Chem. Soc. 1974 96 4857. 49 N. D. Epiotis and R.L. Yates J. Urg. Chem. 1974 39 3150. 50 (a) N. S. isaacs and B. G. Hatcher J.C.S. Chem. Comm. 1974 593; (b) J. E. Baldwin and S. Kapecki J. Amer. Chem. SOC.,1970 92 4874. 51 (a)P. R. Brook and K. Hunt J.C.S. Chem. Comm. 1974,989; (b)J. Marchand-Brynaert and L. Ghosez Tetrahedron Letters 1974 377; (c) A. Sidani J. Marchand-Brynaert, and L. Ghosez Angew. Chem. Internat. Edn. 1974 13 267. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations 107 as an extreme example of subjacent orbital contr01.'~ The photoconversion of cyclobutanones to or-oxocarbenes has been investigated in detail ;53 (108) gives (109) stereospecifically indicating a direct 1,2-~hift.'~' aH H Ally1 vinyl ether anions are thought to rearrange to oxyanions of the type (1 10) by cleavage to vinyl anion and enone and recombination.Since the process is stereospecific cleavage to configurationally unstable vinyl radicals is ~nlikely.'~ The anions (111;X = CN or COMe) undergo the indicated 2,3-shift,"" and a related rearrangement (112) is thought to intervene in the base-catalysed re- arrangement of catechol monoallyl ethers.' 5b Allylic alcohols react with di- methylformamide diethyl acetal via carbenes (1 13) which undergo 2,3-shift to #+unsaturated amide~.'~' The use of allylic sulphoxides as synthetic inter- mediates via 2,3-shift and phosphite desulphurization has been reviewed,56a and several related processes are described.56* 52 A.J. P. Devaquet and W. J. Hehre J. Amer. Chem. SOC.,1974,96 3644. 53 (a)G.Quinkert K. H. Kaiser and W.-D. Stohrer Angew. Chem. Internat. Edn. 1974 198;W.-D. Stohrer G.Wiech and G. Quinkert ibid. p. 199 200; (6) G. Quinkert P. Jacobs and W. D. Stohrer ibid. p. 197. 54 V. Rautenstrauch G.Buchi and H. Wuest J. Amer. Chem. SOC.,1974 96 2576. 55 (a)B. Cazes and S. Julia Tetrahedron Lerrers 1974,2077;A. F. Thomas and R. Dubini Helv. Chim. Acra 1974,57,2084;(b)W. D.Ollis R. Somanathan and I.0.Sutherland J.C.S. Chem. Comm. f974 494; (c) G.Buchi M. Cushman and H. Wuest J. Amer. Chem. SOC.,1974 96 5563. 56 (a) D.A. Evans and G. C. Andrews Accounts Chem. Res. 1974,7 147;(6) G.Buchi and R. M. Friedinger J. Amer. Chem. Soc. 1974,96 3332;P. T. Lansbury and J. E. Rhodes J.C.S. Chem. Comm. 1974 21; J. G.Miller W. Kurz K. G. Untch and G. Stork J. Amer. Chem. SOC.,1974,96 6774. 108 D. W.Jones The intramolecular 1’3-benzyl shift in the conversion of (114) into (1 15) involves 295% inversion at the benzyl carbon.57 There is retention of optical activity in the rearrangement of (116) to (117) consistent with the indicated antarafacial shift.58u When the migrating bond does not initially lie in the nodal plane of the z-bond an antarafacial shift is not ~bserved.’~’ The acid-catalysed rearrangement of (1 18) gives inter alia the 1,3-shift product (119).’“ CH,Ph 0 Ph [l]Ph H4H Me+H7-Me Me I D-C-H Me Ph (1 14) Full papers have appeared dealing with the circumambulation of polymethyl derivatives of the cation (120).59uSince electron-releasing groups at C-6 speed 1A-shift build-up of positive charge at C-6 in the TSis indicated cf.(121). This Mobius aromatic TS becomes the ground state for the aluminium chloride complexes ( 122) of various acylpentamethylcyclopentadienes. These undergo rapid degenerate rearrangement by 1,5-shifts involving bridged TS’s related to ’’ J. W. Lown M. H. Akhtar and R. S. McDaniel J. Org. Chem. 1974 39 1998; J. W. Lown and M.H. Akhtar Tetrahedron Letters 1974 179. 58 (a)H. Bertrand J. L. Gras and G. Gil Tetrahedron Letters 1974 37. (6) J. A. Berson T. Miyashi and G. Jones J. Amer. Chem. SOC.,1974 96 3468; cf. J. E. Baldwin and R. H. Fleming ibid. 1973 95 5256 5261. (c) B. Miller ibid. 1974 96 7155. 59 (a) R. F. Childs M. Sakai B. D. Parrington and S. Winstein J. Amer. Chem. Soc. 1974 96,6403; R. F. Childs and S. Winstein ibid. p. 6409; (b) R. F. Childs and M. Zeya ibid. p. 6418; (c) W. 3. Hehre ibid. p. 5207. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations I09 (120).59b The rapid rearrangement of the cations (120) may be due to their antiaromatic character ;related rearrangement is not observed for the aromatic homo tropylium cation. 9c The 1,fi-shift involved in the Berson-Willcott rearrangement of (123) to give e.g.(124) proceeds with inuersion suggesting the dominance of least-motion over orbital-symmetry control in this instance ;60 this result has implications for related rearrangements shown to proceed with inversion in accord with orbital-symmetry control. Me C02Me \ ,JI Deuterium labelling studies show that hexa- 1,s-diene undergoes two stereo- chemically distinct Cope rearrangements in preference to either 1,3-sigmatropy or homolytic dissociation. The easier process involves either a chair or a 'twist' (125) TS and the less easy either the boat or a 'plane' (126) TS6'" In contrast the ''0-scrambling ofacetyl peroxide involves competing 1'3- and 3,3-sigmatropy and at higher temperatures dissociation overtakes both these processes.61 b.The rearrangement of (127) to (128) viu a necessarily boat-like TS requires a temperature some 50-75 "C higher than required for the chair (or twist) re- arrangement of hexa- 1,5-diene.6'' Semibullvalene shows the lowest known barrier to Cope rearrangement (AG* = 5.5 & 0.1 kcal mol-' at -140 oC).61d The fragmentation of (129; R = Ph) proceeds in a concerted manner. The estimated activation energy for the reaction of (129; R = H) is only 2-3 kcal mol-' greater than for the related six-electron process in ethyl vinyl ether.61e The process (129; arrows) can be viewed as 1,7-sigmatropy in which o-bonds replace two of the three participating double bonds; a similar reaction 6o F.-G. Klarner Angew.Chem. Internat. Edn. 1974 13 268; CJ R. B. Woodward and R. Hoffmann 'The Conservation of Orbital Symmetry' Academic Press 1970 p. 124. 61 (a) M. J. Goldstein and M. R. De Camp J. Amer. Chem. SOC.,1974 96 7356; (b) M. J. Goldstein and W. A. Haiby ibid. p. 7358; (c) J. J. Gajewski L. K. Hoffman and C. N. Shih ibid. p. 3705; (d)A. K. Cheng F. A. L. Anet J. Mioduski and J. Meinwald ibid. p. 2887-;(e) A. Viola S. Madhavan R. J. Proverb B. L. Yates and J. Larrahondo J.C.S. Chem. Comm. 1974 842. 110 1).W.Jones H in which only one double bond is replaced by a a-bond has been suggested (130 ; arrows).62 5 Miscellany D-labelling and asymmetric induction in the ene-reaction of 8-pinene with maleic anhydride (131; arrows) establish attack on the side of the molecule remote from the gem-dimethyl group as well as the endo nature of the TS.63" The attack of benzyne on 8-pinene occurs from the same direction.63b The kinetics of the reverse ene-reactions (132; arrows) agree with a concerted mech- anism,64" but for X = NPh a radical mechanism The reaction (133 ; arrows) has been used to prepare thiobenzaldehyde and thi~acrolein.~~~ R SiMe (132) X = CH, NH or 0;R = CH=CH (133) X = S;R = Ph or CH=CH Further examples of dyotropic reactions [(134) and (135); arrows] have been discussed.6s The first process may be made allowed by use of low-energy vacant orbitals of silicon.The second is an allowed @2,+ ,2 + ,2 process in which at the TS,the migration of the trimethylsilyl group is more advanced than that of the ally1 group.62 J. S. Hastings H. G. Heller H. Tucker and K. Smith J.C.S. Chem. Comm. 1974 348. 63 (a)R. K. Hill J. W. Morgan R. V. Shetty and M. E. Synerholm J. Amer. Chem. SOC. 1974 96,4201; (6)V. Garsky D. F. Koster and R. T. Arnold ibid. p. 4207. 64 (a) K. W. Egger and P. Vitins J. Amer. Chem. SOC.,t974 % 2714; J.C.S. Perkin II 1974 1289 1292; (b)K. W. Egger and P. Vitins Helv. Chim. Acta 1974 57 214; (c) H. G. Giles R. A. Marty and P. de Mayo J.C.S. Chem. Comm. 1974 409. 65 M.T. Reetz M. Kliment and M. Plachky Angew. Chem. Internat. Edn. 1974 13,813; M. T. Reetz ibid. p. 402.

ISSN:0069-3030    

DOI:10.1039/OC9747100095

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (ii) Polar Reactions By T. W. BENTLEY Department of Chemistry University Colfege of Swansea Swansea SA2 8PP 1 Introduction Electronic theories have dominated the interpretations of organic reactions during the past forty years and within the past ten years the concept of orbital symmetry has received much attention. Whilst orbitals and curly arrows the pictorial accompaniments of these qualitative theories are both useful and appealing other important factors including the effects of solvent and ion pairing are often neglected.* As shown by recent work on addition and elimination (see later) the stereochemistry of products may be controlled by formation and solvation of ion pairs rather than by stereoelectronic shifts of electron pairs or orbitals.There are other indications that these electronic theories are not panaceas. Both curly arrows and orbital-symmetry arguments suggest that the Diels-Alder reaction between butadiene and ethene would proceed via the symmetrical transition state shown in Scheme 1. However it is a fundamental postulate of r 1 i+ Scheme 1 theoretical chemistry that electrons ‘move’ faster than nuclei (e.g. Born-Oppen-heirner approximation Franck-Condon principle). Thus the representation in Scheme 1 requires that the nuclei move in a remarkably concerted manner especially remarkable considering that the collision complex of butadiene and ethene must be highly vibrationally excited. Recently using a quantitative MO treatment (MIND0/3) without symmetry constraints it has been calculated that one of the new bonds (e.g.C-1 to C-6)is almost completely formed while the other has hardly begun to form at all.’ Similar arguments as well as additional * It is particularly unfortunate that effects of solvent and ion pairing are often disregarded whereas it appears to be ‘well known’ that the iobes of a p-orbital are blue and green! ‘ M.J. S. Dewar A. C. Griffin and S. Kirschner J. Amer. Chem. Soc. 1974 96 6225; see also Chapter 4 part (i). 111 T. W. BmtieJ experimental evidence have led Bordwell to question the prevalence of concerted reactions involving formation and cleavage of four or more bonds (e.g.E2 S,2’),2 It is often difficult to rule out the formation of reactive ion-pair intermediates.Consequently there is little room to sit back complacently thinking that a particular mechanism ‘is’ as shown by Sneen and co-workers’ serious challenge of the classical S,2 mechanism -however some major deficiencies of Sneen’s interpretation are presented in the following section. Studies of reaction mechanisms are important aids to calculations of reaction rates and help to ensure that reactions the rates of which are to be calculated or correlated are proceeding by the same mechanism. This chapter includes a section on empirical force-field calculations of bridgehead reactivity which now spans a range of relative rates of over twenty powers of ten. Also a timely review has been published encouraging the inductive approach to chemistry and pointing out some of the approximations in transition-state the~ry.~ Isotopes are being used increasingly for studies of reaction mechanisms.Many of these investigations provide indirect mechanistic evidence because the structure of the product (i.e.the arrangement of atoms) is characterized more completely. Further mechanistic information is provided by kinetic isotope effects which are subtle probes of mechanism and cause minimum disturbance to the original system unlike for example substitution of a methyl group for a hydrogen. Primary deuterium isotope effects are particularly relevant to the sections on elimination and oxidation which include a discussion of a remarkable deuterium isotope effect of 36.5 for CrV* oxidation of glycolic acid. The inter- pretation of secondary deuterium isotope effects provides the basis for Shiner and co-workers’ mechanisms of aliphatic nucleophilic substitution but these are not generally accepted.A more comprehensive annual coverage of the literature of organic reaction mechanisms is presented el~ewhere.~ 2 Aliphatic Nucleophilic Substitution An advanced undergraduate text updating Bunton’s monograph (1 963) has been published,’ and there is also an account of solvolytic reactions of simple alkyl systems including detailed discussion of ion pairs.6 Since substitution and elimination usually occur concurrently research discussed in this section is relevant to both types of reaction.* Study of concurrent F. G. Bordwell Accounts Chem. Res. 1972,5 374; 1970 3 281.D. L. Bunker Accounts Chem. Res. 1974 7 195; see also L. P. Hammett ‘Physical Organic Chemistry’ McGraw-Hill New York 1970 pp. 1-3. ‘Organic Reaction Mechanisms 1974’ ed. A. R. Butler and M.J. Perkins Wiley London 1975. .S. R. Hartshorn ‘Aliphatic Nucleophilic Substitution’ Cambridge University Press London 1973. J. M. Harris Pragr. Phys. Org. Chem. 1974 11 pp. 89-173;(a) p. 140; (b) p. 155; see also D. J. Raber J. M. Harris and P. von R. Schleyer in ‘Ions and Ion Pairs in Organic Reactions’ ed. M. Szwarc Vol. 2 Wiley New York 1974 pp. 247-374. * If one accepts the principle of microscopic reversibility mechanistic conclusions from elimination reactions are also relevant to addition reactions. However some critical comments on applications of this principle to solution chemistry are presented later.Reaction Mechanisms-Part (ii) Polar Reactions elimination and substitution during solvolyses of t-butyl halides showed that the products depended on the leaving group (see Ann. Reports 1963,60 262). For solvolyses of 2-adamantyl arenesulphonates in 70 % v/v ethanol-water (Scheme 2) only substitution products were formed but the proportions of alcohol and 70"LEtOH-H, D O H + GOEt aoso20x ROH ROEt Scheme 2 ethyl ether depended markedly on the substituent X in the arylsulphonate; see Table l.7 Similar results have been obtained for 1-adamantyl solvolyses.8 Table 1 Dependence ojsubstitution product on leaving group (Scheme 2) X OMe Me H Br NO [ROH]/[ROEtJ 1.94 2.56 3.25 3.05 5.20 Therefore it appears that all the above reactions proceed through ion-pair intermediates rather than through free (i.e.symmetrically solvated) cations. The Mechanism of Sneen and Co-workers.-Many different types of ion pairs may be present in solution but recent attention has been focused on the simple ion-pair mechanism shown in Scheme 3. In the Straightforward? S,1 reaction ki h [Nuc] RX * R'X-LR-Nuc+X-k-i contact ion-pair when ~~[NUC] >> k-1 kobs = k straightforward S,1 but when k2[Nuc] -K k- kobs= k,k,"ucl . ion-pair SN2 or SN ip ~ k-1 and when k2[Nuc] x k-borderline mechanism. Scheme 3 ' J. M. Harris A. Becker J. F. Fagan and F. A. Walden J. Amer. Chem. SOC.,1974 96 4484. D. J. Watson (University College Swansea) unpublished results; P.Luton and M. C. Whiting to be published; P. Luton Ph.D. Thesis Bristol 1972. t Use of 'straightforward' is particularly appropriate since there is much current interest in internal return from contact ion-pairs (k- > k, Scheme 3) in some S,1 reactions -see also discussion of ref. 15. 114 7‘.W. Bentley k is rate-limiting and products are produced by rapid destruction of the contact ion-pair -for simplicity elimination is excluded from Scheme 3 but the mech- anism can readily be extended to include it [see Ann. Reports (B) 1973’70 1201. Although in the classical interpretation of Ingold and co-workers it was assumed that a free cation was formed in sN1 reactions there is nothing in Ingold’s defini- tion of the term SN1 to rule out rate-limiting formation of the contact ion-pair shown in Scheme 3.As there is a long history to the debate about whether a free cation shielded on one side by the leaving group is chemically or just semantically different froma contact ion-pair suffice it to state here that some form of sequential ionization via ion pairs is now widely accepted ;6 see also Scheme 6. However Sneen’s reinterpretation of the classical sN2 mechanism has not been received favourably. His ion-pair SN2 mechanism (Scheme 3) requires rapid formation of an ion-pair followed by its rate-limiting destruction. Possibly this may occur for systems capable of stabilizing the developing positive charge (e.g. allyl) but the mechanism was also intended to account for the behaviour of simple primary and secondary substrates.Perhaps the best single piece of evidence against this spectacular generalization is the calculation of Abraham who showed that heterolysis of an alkyl chloride without assistance from rearside nucleophilic attack would require about 40kcal mol- more than the experi- mentally observed activation energy [Ann. Reports (A) 1973’70 1501. As might be expected the energetics for heterolysis of secondary substrates were calculated to be borderline but there are other good reasons to doubt the interpretation of borderline behaviour shown in Scheme 3. The experimental basis of Sneen’s generalization is a study of the effect of azide ion on the solvolysis of 2-octyl methanesulphonate in aqueous dioxan.Unfortunately it is difficult to separate the effect of azide on the ionic strength of the medium from the effect of azide as nucleophile. One approach to ‘buffer’ the solution with a large excess of the non-nucleophilic salt NaClO, led to the conclusion that the correlation of rates and products (with varying concentrations of azide) was consistent with the classical sN2 mechani~rn.~ It has also been pointed out that azide concentrations may be poor approximations for activities and that salt effects may be specific in nature so that corrections for ‘normal’ salt effects on the medium are highly questionable.” OBs kHlkD klZclk14c c6H13teH3 ROH 1.097 & 0.007 1.063 & 0.007 N -+D OBs aq. acetone I IH RN,C~H~*,CCHJ 1.106 f0.007 1.105 f0.007 ROBS Scheme 4 D.J. McLennan J.C.S. Perkin II 1974 481. lo G. A. Gregoriou Tetrahedron Letters 1974 233; P. J. Dais and G. A. Gregoriou,ibid. p. 3827. Reaction Mechanisms-Part (ill Polar Reactions There are also completely independent criticisms based on secondary kinetic isotope effects. The results given in Scheme 4 show the I4C-and a-deuterium- isotope effects on the formation of both azide and carbinol from 2-octyl bromo- benzene-p-sulphonate in aqueous acetone-NaN, using carbon- 14 as a tracer for deuterium.'' It was argued that the relatively large carbon isotope effects indi- cated that the transition state must be nearly symmetrical which is consistent with the classical S,2 mechanism not the ion-pair SN2 mechanism. Also as the a-deuterium isotope effects of 1.10 for formation of both azide and carbinol are the same within experimental error similar mechanisms must be operating but detailed interpretation is a matter ofdisagreement and speculation.The authors'" surprising suggestion that 1.10 represented competing classical S,2 (kH/kD = 1.0) and SNl (k,/k x 1.2) mechanisms is inconceivable because as weH as ignoring all the evidence for sequential ionization it requires that reactions with both azide and water as nucleophiles proceed by the same proportions of S,1 and SN2 mechanisms (ca. 50 50). An alternative explanation of the value of 1.10 is that varying amounts of nucleophilic assistance to ionization are possible in a spectrum of sN2 mechanisms,'* [with SN1as the limit where nucleophilic assis- tance is zero (cf.Ann. Reports 1951,4'8 121)]. Since a-deuterium isotope effects appear to reflect hybridization changes e.g. addition of MnO -to cinnamic acids involves a hybridization change from sp2 to sp3 and gives inverse a-PhCH=CHCO,H & PhHF-qHCO,H 5 PhCHO + HCOCO,H(?) + + Mn0,-Mn02-for k fork [a-2H]cinnamic acid k JkD = 0.77 kdkD = 1.09 [B-'H]cinnamic acid kJkD = 0.75 k JkD = 1.09 Scbeme 5 deuterium isotope effects (Scheme 5)," it might be expected that the ion-pair SN2mechanism should also give inverse isotope effects. Thus depending on the kinetic-isotope effects on k and k- (Scheme 3) the value of 1.10 may not be consistent with an ion-pair SN2 mechanism. The Mechanism of shiner and Co-workers.-Shiner and co-workers have pro- posed a sequential mechanism for ionization in which under different conditions as many as four steps may be rate-limiting.I4 A simplified form of their mechan- ism is shown in Scheme 6 which has been drawn to show the similarity with Scheme 3.Additional steps are the classical SN2 process and k the dissociation V. F. Raaen T. Juhlke F. J. Brown and C. J. Collins J Amer. Chem. Soc. 1974 % 5928. '* J. M. Harris R. E. Hall and P. von R. Schleyer J. Amer. Chem. Soc. 1971 93 2551. l3 D. G. Lee and J. R. Brownridge J. Amer. Chem. Soc. 1974 96 5517; see also E. A. Halevi Progr. Phys. Org. Chem. 1963 1 168. l4 V. J. Shiner jun. in 'Isotope Effects in Chemical Reactions' ed. C. J. Collins and N. S. Bowman Van Nostrand Reinhold New York 1970.116 T. W.Bentley R-Nuc R + /IX-solvent-separated ion-pair Scheme 6 of a contact ion-pair to a solvent-separated ion-pair. It is claimed that the four possible rate-limiting steps can be identified by their characteristic a-deuterium isotope effects. As these are somewhat dependent on the leaving group typical values for solvolyses of secondary sulphonates are given below (i) k,/k z 1.0-classical S,2 mechanism ; (ii) k,/k = 1.15-1.16 -k is rate-limiting e.g. solvolysis of 3,3-dimethyl-2- butyl sulphonates (1) in protic solvents (Scheme 7); (iii) k,/k = 1.22-1.25-k is rate-limiting e.g. trifluoroacetolysis of 2-propyl toluene-p-sulphonate solvolyses of 2-adamantyl sulphonates ;l2 (iv) k,/k zz 1.05-1.15 (depending on the solvent) -k is rate-limiting; this interpretation corresponds to Sneen’s.The above interpretation is supplemented by studies of additions to double bonds thought to give rise to the contact ion-pair (R’X-). Addition of bromo- benzene-p-sulphonic acid to propene in trifluoroacetic acid leads to 2-propyl- bromobenzene-p-sulphonate whereas a similar reaction of t-butylethylene leads to the rearranged trifluoroacetate (2). Consequently it appeared that for propene collapse of the contact ion-pair was more rapid than attack by nucleophile (k,) or further dissociation (k3,Scheme 6) but that for t-butyl- ethylene collapse of the contact ion-pair was prevented by rapid rearrangement CH H I ICH3C-CCH CH H k I IS CH,C-$CH CH3 H HOBs 1 1 -CH,C-C=CH &H3 OBs k-’ AH3 OBs bH3 (1) Ik3 I I OBs CH CF3COO CH3 (2) Scheme 7 (k3,Scheme 7).” Thus it was argued that the relative rates of secondary solvo- lyses were influenced by ‘hidden return’ i.e.k2 < k- (Scheme 3) or k and k3 < k-(Scheme 6). As an independent test of these far-reaching proposals the analogous 1-adamantylmethyl carbinyl system (3) was st~died.’~ Because of the extra ring (a) V. J. Shiner jun. and W. Dowd J. Amer. Chem. Soc. 1969 91,6528; (b) V. J. Shiner jun. R. D. Fisher and W. Dowd ibid. p. 7748. I6 T. W. Bentley S. H. Liggero M. A. Imhoff and P. von R. Schleyer J. Arner. Chem. Sot. 1974 96 1970. Reaction Mechanisms-Part (ii) Polar Reactions Figure 1 Logarithms of rate constants at 25 "C for trifluoroacetolysis of secondary tosylutes (X = OTs) plotted against Co*(data from refs.16 and 17) strain introduced by rearrangement acetolysis of (3) leads mainly to the un- rearranged acetate (4) and the alkene (6),along with small amounts of the re- arranged acetate (5). Accordingto Shiner's proposals this lack of rearrangement Scheme 8 118 T. W.Bentley indicates that hidden return in (3) should increase and therefore the rate of solvolysis of (3) should be slower than that of (1). The experimental results showed that after allowing for the well-established OBs/OTs rate factor of three (3) solvolysed slightly faster (not slower) than (l) in agreement with classical carbo-cation theory. Also the linear correlation (Figure 1) of the logarithms of trifluoroacetolysis rate constants of simple secondary toluene-p-sulphonates (tosylates) including 2-propyl and (l) with CF* suggests that all the compounds have the same rate-limiting step presumably formation of contact ion-pair (k,).16 The point for 3-methyl-2-butyl” is slightly off the correlation line (Figure l) possibly owing to anchimeric assistance (rate x ca.3) by the neighbouring hydrogen. These criticisms cast considerable doubt on the basis of Shiner’s proposals i.e. both the interpretation of a-deuterium isotope effects and certain applications of the principle of microscopic reversibility are brought into question. It appears that ion pairs produced from additions to double bonds behave differently from those produced by heterolysis of a covalent bond in that the former appear to be more likely to collapse to covalent substrate.This may be due to different solvation of short-lived intermediates and suggests that the principle of micro- scopic reversibility should be used cautiously to interpret reaction mechanisms in solution. ?Ihe Mechanism of Schleyer and Co-workers.-Recent studies of 2-adamantyl tosylate (7) a hindered secondary substrate which solvolyses without detectable nucleophilic assistance (i.e. &l) have caused reconsideration of the role of solvent as nucleophile in the solvolyses of other secondary substrates [cJ Ann. Reports (B) 1972 69 1621. Until recently it could have been argued that the relative rates of secondary solvolyses depended so markedly on solvent that it was not possible to make a meaningful comparison of solvolysis rates.However because the relative rates of secondary solvolyses in hexafluoroisopropyl alcohol and trifluoroacetic acid are very similar (Table 2) it may be argued that these solvolyses are close to limiting (i.e. S,1) and therefore reflect true carbenium ion reactivity.Is This accounts for the linear correlation of the trifluoroacetolysis data in Figure 1,16 which is not observed in more nucleophilic solvents.6b The relative rate data (Table 2) suggests that solvolysis of secondary substrates (except 2-adamantyl) in more nucleophilic solvents is assisted by the solvent behaving as nucleophile. Both acetic and formic acids appear to be relatively A. Pross and R. Koren Tetrahedron Letters 1974 1949.Is F. L. Schadt P. von R. Schleyer and T. W. Bentley Tetrahedron Letters 1974 2335. Reaction Mechanisms-Part (ii) Polar Reactions Table 2 Relative rates of secondary solvolyses" Solt'ent EtOH CH3C0,H HC0,H Tosyiate 2-adamantyl{7) 1.P exo-2-norbornyl(8) 10 400 endo-Znorbornyl(9) 34 cyclopentyl(l0) 6 250 3-pentyl 1 560 I.@ 4 140 14 280 40 1 .ob 1940 1.14 27 5.3 cyclohexyl 108 8.3 1.5 CF,CO,H 1.P 52W 0.46' 3.w 0.85 0.30 97 wt. % (CF,),CHOH 1.P ----0.19d 2-propyl 910 13 0.9 6' 0.01 0.024' a Titrimetric or conductimetric rate constants at 25 "C. Defined as 1 .O for each solvent. 'Ref. 21. Ref. 18. nucleophilic compared with trifluoroacetic acid. Also less-hindered substrates appear to be more susceptible to nucleophilic attack and there appears to be a regular gradation of reactivity within the series.A quantitative estimate of the magnitude of nucleophilic solvent assistance (kJk ratio) can be obtained from Table 2 by dividing the relative rates in any solvent by the relative rates in tri- fluoroacetic acid e.g. in acetic acid 2-propyl tosylate appears to be nucleo- philically assisted by a factor of 13/0.024 = 540 whereas cyclopentyl is assisted by 280/3.0 = 93. Consequently it is clear that the common tendency to compare directly the acetolysis rates of secondary substrates is meaningless. Correction should be made for nucleophilic solvent assistance or comparisons should be made between data for solvolyses in hexafluoroisopropyl alcohol or in trifluoro- acetic acid.This leads to a previously unpublished analysis of the problem of the rates of norbornyl solvolyses discussed below. Solvolyses of Norbornyl Systems a Compromise Solution.t-Although inter-pretation of the relative rate ratio of about lo00 for solvolyses of exo-and endo- 2-norbornyl systems (Table 2) is a controversial topic it is generally agreed that the problem is a subtle one. Attempts to 'probe' the system by introducing extra substituents (e.g. by substitution at the l-position,lg or by comparison of the endo-2-norbornyl system with the corresponding tertiary compound20) into an environment which is already sterically crowded must be suspect. Ideally exo-(8) should be compared with endo-(9) but this does not help to determine l9 D.Lenoir Tetrahedron Letters 1974 1563. 2o J. M. Harris and S. P. McManus J. Amer. Chem. SOC.,1974 96 4693. 7 Helpful discussions with H. C. Brown B. Capon and P. von R. Schleyer are gratefully acknowledged. 120 T. W.Bentrey whether solvolysis of exo-(8) is unusually or solvolysis of endo-(9)is unusually slow.22 In the past there has been considerable disagreement about how to interpret the kinetic data and what constituted normal solvolytic be- haviour. If the conclusions discussed in the previous section are accepted a revised analysis of the kinetic data is necessary. Also ‘normal’ behaviour can be defined by the good linear correlation of trifluoroacetolysis rate constants with G* for acyclic tosylates (Figure l),which suggests that inductive/hyperconjugative effects dominate the effect of structure on reactivity.I6 Even in trifluoroacetolysis of cyclopentyl tosylate (lo) where relief of torsional strain may occur the rate constant is less than four times greater than that of the acyclic C5system 3-pentyl tosylate; however (10)reacts over 100 times faster than 2-propyl tosylate (Table 2).An estimate of the ‘normal’ trifluoroacetolysis rates of exo-(8)and endo-(9) can be made by correction of cyclopentyl tosylate (10)for the normal inductive/ hyperconjugative effects of the two extra carbon atoms and a rate correction factor of 3-8 is indicated by the following evidence based on a wide variety of model systems (i) Solvolysis of the tertiary norbornyl derivative (11) is about 5 timesfaster than that of the corresponding cyclopentyl derivative (12),e.g.when X = C1 k,,/k12 = 5.4,23and when X = OPNB k,,/k12 = 4.4.24 This is of the expected magnitude and direction for inductiuelhyperconjugativeeflects.2 I (10) CH -3 (1 1) (12) P Following Sargent’s argument,26‘ if it is assumed that cyclopentyl is accelerated over norbornyl by conformational effects such as relief of extra torsional strain the observed rate ratios would have to be explained by another rate-accelerating effect in the tertiary norbornyl system (I 1).Anchimeric assistance is ruled out since it has now generally been agreed that tertiary 2-norbornyt cations are essentially clas~icai.~~~~~’.~~ Also as similar results are obtained for chlorides and p-nitrobenzoates * .24 steric acceleration by relief of leaving-group strain is not appreciably different in (I 1) and (12).29Therefore it appears that only inductive/hyperconjugativeeffects lead to a consistent explanation.21 J. E. Nordlander R. R. Gruetzmacher W. J. Kelly and S. P. Jindal J.Amer. Chem. Soc. 1974 96 181. 22 (a) H. C. Brown M. Ravindranathan and E. N. Peters J. Amer. Chem. SOC. 1974 96 7351 ; (b)H. C. Brown Accounfs Chem. Res. 1973,6 377. 23 H. C. Brown and F. J. Chloupek J. Amer. Chem. Sor. 1963,85 2322. 24 H. C. Brown and M.-H. Rei J. Amer. Chem. SOC.,1964,86 5004. 2s A. Streitwieser jun. J. Amer. Chem. soc. 1956 78 4935. 26 (a)C. D. Sargent Quart. Rev. 1966 20 301 ; (b) G. D. Sargent in ‘Carbonium Ions’ Vol.111 ed. G. A. Olah and P. von R. Schleyer Wiley New York N.Y. 1972 pp. 1099-1 200. Zf H. L. Goering and J. V.Clevenger J. Amer. Chem. Soc. 1972 94 1010; H. L. Goering and K. Humski ibid. 1969 91 4594. 28 G. A. Oiah G. Liang G. D. Mateescu and J. L. Riemenschneider J. Amer. Chem. Soc. 1973 95 8698. 29 J. Slutsky R. C. Bingham P. von R. Schleyer W. C. Dickason and H. C. Brown J. Amer. Ghem. SOC.,1974 96 1969. Reaction Mechanisms-Part (ii)Polar Reactions 121 (ii) In trifluoroacetic acid trans-2-methylcyclopentyl tosylate solvolyses 2.6 times faster than cyclopentyl tosylate,2 and allowing for the extra y-carbon this would lead to a rate factor of 3.4. (iii) A maximum factor of 7-8 would be predicted from the r~* correlation for the more flexible acyclic secondary tosylates ;Figure 1.Comparison of the trifluoroacetolysis rates relative to cyclopentyl with the predicted relative rate of 3-8 leads to the residual rate factors (Table 3). It Table 3 Relative rates oj'secondary solzlolyses Tosylare CF3C0,H (Predicted)" Residuul rate factors cyclopentyl (10) 1 .ob (1 .oy __ endo-(9) 0.15 (3-8)c 20-60 too Slow em-(8) 172 (3-8)' 20-60 too fast See text. Defined as 1.0. 'Based on an estimate of normal inductive/hyperconjugative effects of two extra carbon atoms -see text. A higher rate factor would probably be obtained using polarimetric rate constants. appears that by comparison with models (lo)-( 12) exo-(8) solvolyses more rapidly than expected whereas endo-(9) solvolyses more slowly than expected.Thus at least two additional kinetic effects must be postulated. Published interpretations for the behaviour of exo-(8) include anchimeric assistance with formation of a bridged non-classical ion during heterolysis of the sulphonate bond,26a or stabilization of the incipient cation by C-C hyper-c~njugation,~' ' The behaviour of endo-(9)may or by a vertical ~tabilization.~ be explained by 'steric hindrance to ionization',22b possibly caused by decreased ability of the solvent to form hydrogen bonds to the three oxygen atoms of the sulphonate anion. Since the rate-limiting step is probably formation of a contact ion-pair 'steric hindrance to solvation of the leaving group' may be a preferable modification of the interpretation.The residual rate factors (Table 3) are similar and correspond to energies of only 2 kcal mol-'. Thus if the above analysis is correct the remaining problems of the interpretation are extremely subtle. Recently calculations purporting to show that the rate constant for 1,2-shift in a classical 2-norbornyl cation would be as high as 1OI2 s-' did not adequately account for the con- current 1,Zshift of the counterion and its hydrogen-bonded solvent molecules ;32 the same paper contains an interesting but biased analysis of the norbornyl problem including criticisms of the ESCA data28 of stable ions [cf.Ann. Reports (B) 1972 69 801 and of transition-state theory. '"F. R. Jensen and B. E. Smart J. Amer. Chem. SOC.,1969 91 5688. '*T. G. Traylor W. Hsnstein H.J. Berwin N. A. Clinton and R. S. Brown J. Amer. Chem. SOC.,1971 93 5715. 32 F. K. Fong. J. Amer. Chem. SOC.,1974 96 7638. 122 T. W. Bentley 3 Elimination Since there is not general agreement on the mechanistic interpretations of nucleophilic substitution formally involving cleavage of only one bond and formation of only one bond it is not surprising that the mechanisms of elimination are a matter of great debate. As with substitution reactions it is possible to recog- nize relatively clear mechanistic categories e.g. Elcb El E2H (classical E2) but consideration of ion pairs leads to a variety of possible mechanistic interpretations. General agreement on the mechanism of substitution would appear to be a prerequisite for agreement on mechanisms of elimination particularly for solvo- lytic reactions and for eliminations by those weak bases which are good nucleo- Scheme 9 philes.This can be illustrated by the results of a detailed study of the products from cyclopentyl bromobenzene-p-sulphonate (Scheme 9 Table 4).33 Whilst Table 4 Elimination from cyclopentyl bromobenzene-p-sulphonate" Reaction conditions Alkene k,lkDb yield (%) antilsyn" 1SM-NaOEt-EtOH ca. 20 5.77 1.15-1.18 trifluoroethanol-water aqueous ethanol 50-76 12-27 0.27 1.37 1.23-1.25 1.15-1.18 See Scheme 9. a-Deuterium isotope effect including competitive substitution. reaction with sodium ethoxide gave predominantly anti-elimination (E2H-like) in trifluoroethanol-water syn-elimination was favoured by about 4 1.A possible rationalization of the solvolytic results for which no base is available to remove the P-hydrogen involves the five-co-ordinate intermediate first suggested nearly twenty years ago (Scheme 10);6a*34 this proposal is similar to the E2C mechanism which usually leads to anti-elimination except that bonding between the nucleo- phile or leaving group and the B-hydrogen has not been specified in Scheme 10. The authors'33 proposal that the reaction in aqueous ethanol was an El elimina- tion does not appear to be consistent with the a-deuterium isotope effect (Table 4); 33 K. Humski V. SendijareviC and V. J. Shiner jun, J. Amer. Chem. SOC.,1974 96 6187. 34 S. Winstein D. Darwish and N. J. Holness J. Amer. Chem. Soc. 1956 78 2915. Reaction Mechanisms-Part (ii) Polar Reactions substitution (inversion) syn-elimination OBs 6-anti-elimination Scheme 10 their proposed mechanism for trifluoroethanol involved attack on a preformed ion-pair (see also refs.2 and 39 and closely parallels the mechanism of substitu- tion (Scheme 3) criticized above. Recent studies of the E2C mechanism [Ann. Reports (B) 1968,65 138 ; 1972 69 1801 have concentrated on the isotope-effect criterion. Primary hydrogen isotope effects on the rates of bimolecular elimination of substituted cyclohexyl tosylates and bromides are small (kH/kD -2-3) for very E2C-like reactions pass through a maximum of about 6 for E2H-like reactions and are small again (2-3) for very E2H-like reactions. Movement within the E2C-E2H spectrum was achieved by increasing the acidity of the substrate or the basicity of the base or by changing the leaving group from tosylate to bromide.36 Secondary isotope effects indicated significant changes in hybridization at both C and C, con-sistent with an E2C me~hanism.~’ As there is now considerable evidence for the E2C mechanism one wonders to what extent an inability to represent the mech- anism in an aesthetically pleasing way (e.g.using curly arrows) is proving to be an ‘activation’ barrier to its general acceptance. It is known that crown ethers can affect the amount of product formed by syn-elimination. When the ether complexes with the cation more of the base is dissociated. In some cases it appears that anti-elimination is favoured by disso- ciated base whereas syn-elimination is favoured when the base is associated which can be explained by simultaneous co-ordination of the metal counterion (M) with the base and the leaving group (13).A striking example of this effect is the influence of 18-Crown-6 (1 moll-’) on elimination from exo-2-norbornyl [exo-3-’H]tosylate (14) by the sodium salt of 2-cyclohexylcyclohexanol in triglyme (Table 5).38 35 W. T. Ford Accounts Chem. Res. 1973 6,410. 36 D. Cook R. E. J. Hutchinson J. K. McLeod and A. J. Parker J. Org. Chem. 1974 39,534. ” I>. Cook R. E. J. Hutchinson and A. J. Parker J. Org. Chem. 1974 39,3029. 38 R.A. Bartsch and R. H. Kayser J. Amer. Chem. SOC.,1974 96,4346; R. A.Bartsch E. A. Mintz and R. M. Parlman ibid.p. 4249. 124 T. W. Bentley Table 5 Eliminationfrom exo-2-norborny~[exo-3-2H]tos~~lute (14) 18-Crown-6 % of total hydrocarbon present (15) (16) Nortricyclene No 98.0 0" 2.0 Yes 70.0 27.2 2.8 a Lower limit of detection <2 % (n.m.r.). Similar research has been carried out on acyclic systems. In a detailed analysis of the stereochemistry of elimination reactions of 3-hexyltrimethylammonium iodide (17) promoted by phenoxide bases in mixtures of t-butyl alcohol and dimethyl sulphoxide it has been shown that trans-3-hexene is formed in both syn-and anti-pathways whereas at least 94% of cis-3-hexene is formed by the anti-pathway. The cation of the metal phenoxide influences the proportion of syn-elimination and in 20 % dimethyl sulphoxide the syn -+ trans pathway ranges from 34% for potassium to 74% for lithium phenoxide.Because the leaving group is positively charged attack by associated base leading to a transition state resembling (13) is unfavourable. Thus in contrast to the previous example (14) attack by dissociated base favours syn-elimination possibly oia the cyclic transition state (18).39 CH3CH2CHCH2CH2CH3 I I-+N(CH,) (17) (18) As it requires only 1.4 kcal mol-I to change a product distribution from 50 50 to 90 10 generalizations about the factors influencing products should be applied cautiously e.g.base association plays an important role in determining the trans-cis-alkene ratios in elimination reactions of 1-phenylprop-2-yl chlorides but in contrast to the behaviour of non-activated halides an associated base leads to preferential formation of trans-alkene~.~' From a study of a simple system 2-iodobutane for which there is considerable evidence for a classical E2 mechanism it appears that the positional orientation of the elimination products is mainly controlled by the base strength.Orientation was most sensitive to base strength when the ,&hydrogen was removed by an oxygen base the order oxygen > nitrogen > carbon favouring 'hard' bases. Only for very hindered bases such as 2,6-di-t-butylphenoxide did steric effects become important .41 39 J. K. Borchardt and W. H. Saunders jun. J. Amer. Chem. SOC.,1974 96 3912. 'O S. Alunni E. Baciocchi R. Ruzziconi and M. Tingoli J. Org.Chem. 1974 39 3299. 41 R. A. Bartsch K. E. Wiegers and D. M. Guritz J. Amer. Chem. SOC.,1974 96 430. Reaction Mechanisms-Part (ii) Polar Reactions 4 Electrophilic Addition In accordance with Ingold’s systematic notation the term Ad will be used for electrophilic addition reactions. Akenes.-Because the kinetic expressions for addition of hydrogen halides to alkenes are often complex mechanistic interpretations benefit from detailed studies of selected systems. Furthermore rigorous experimental techniques may be required to prevent isomerization of the starting alkenes. For addition of hydrogen bromide in acetic acid to cis- or trans-but-2-enes7it appears that the presence of hydrogen ion bromide ion and oxygen or light is necessary for alkene isomerization.Although addition of a bromine atom to the alkene to give a #I-bromoalkyl radical is implicated even a large excess of a typical radical inhibitor did not quench the reaction and reactions were carried out on degassed solutions in the dark under a helium atm0sphe1-e.~~ The stereochemistry of addition of both deuterium bromide and {O-2H]acetic acid to both cis- and trans-but-2-ene is 84 f2‘%; anti (16 & 2 ”/ syn) and is invari- ant over a 100-fold concentration range of deuterium bromide and unaffected by the presence of added lithium bromide or lithium perchlorate. Three mechanistic possibilities were considered. The first an Ad,2 (formally the reverse El) process involving formation of a carbenium ion-bromide ion contact ion-pair which collapses to syn-addition product or rearranges to produce the anti-addition product was ruled out because added bromide ion did not affect the stereo- chemistry.The second competing syn A42 and anti Ad,3 (formally the reverse E2H) processes was ruled out because when taken with the kinetic expression +d[RBr],dt = k,[alkene] [HBr] + k,[alkene] [HBrI2 (1) [equation (l)],it predicts that product stereochemistry should be dependent on the concentration of hydrogen bromide. The results appear to be consistent with competing syn and anti AdE3 mechanisms (19) and (20) in which alkyl bromide is derived only from the third-order term and alkyl acetate is derived only from the second-order term in equation (1). Kinetic isotope effects support the inter- pretation that the hydrogen in transition states (19) and (20) is supplied by co- valent hydrogen bromide rather than protonated acetic Nuc = HBr Br- or CH,C02H Allowing for pre-equilibria (e.g..n-complexes or ion pairs) there is not neces- sarily a termolecular elementary step. The products may be determined by ‘* D. J. Pasto G. R. Meyer and B. Lepeska J. Amer. Chem. SOC.,1974 96 1858; see also R. C. Fahey C. A. McPherson and R. A. Smith ibid.,p. 4534. 126 T. W.Bentley addition of either acetic acid not explicitly included in equation (l),or hydrogen bromide to a complex of alkene and another molecule of hydrogen bromide. Since this complex could be attacked by hydrogen bromide acetic acid or bromide ion the identical stereochemistry of addition of both acetate and bromide is explained as well as the dependence ofthe ratio of acetate to bromide products on the initial concentrations of hydrogen bromide.However the reaction of both cis-and trans-but-2-enes cannot proceed through a common intermediate (at least not all the reaction). Consequently although it may be fortuitous that both cis- and trans-alkenes undergo the same amount of unti- addition this aspect of the interpretation needs further attention. Using gas-phase 'H n.m.r. spectroscopy it has been found that the uncatalysed reaction between alkenes and hydrogen chloride at room temperature and ten atmospheres pressure has a half-life of about four days (k = 4.2 f1 x 1 mol-' s-at 25 "C). The reaction rate is not affected by an increase in surface area (e.g.added glass capillary tubing) but appears to be retarded by increased temperature. Similar results were obtained for 2-methylpropene and the reaction at room temperature appears to be ca. lo9 times faster than predicted from the equilibrium established at high temperatures. The low-energy path (not available at higher temperatures?) may involve reversible formation of a complex which is unstable at high temperatures. It was suggested that this pathway could not be the reverse of the decomposition pathway although some charge separation appears to be present because no 1-chloropropane was obtained from pr~pene.~~ No doubt the issues raised between the above interpretation and the principle of microscopic reversibility will require further arbitration.Whilst the broad outline of the mechanism of addition of bromine to alkenes is generally agreed detailed interpretations are still being debated.44 Relative rate data for addition of bromine to a series of alkenes in 1,1,2-trichlorotri- fluoroethane solution at -35 "C show a considerably smaller range than rates previously obtained for methanol at 25°C. This was interpreted as resulting from a change in the position of the transition state along the reaction co- ordinate so that it was more like the reversibly formed n-complex than the ethylene bromonium ion a-comple~.~~ Alkynes.-If protonation of an alkyne by hydrogen chloride yielded a 'free' vinyl cation which would be linear subsequent attack by nucleophile should lead to both anti- and syn-addition products.However recent data for addition of hydrogen chloride to a variety of alkynes are consistent with competing Ad,2 and A43 mechanisms with the predominant mode of addition being syn and anti respectively. Contact ion-pairs are clearly implicated in the AdE2 mechanism because unlike the ,443 (see above) addition of chloride ion does not increase the percentage of chloride formed. For example phenylethyne 43 F. Amar D. R. Dalton G. Eisman and M. J. Haugh Tetrahedron Letters 1974 3033 3037. 44 S. P. McManus and D. W. Ware Tetrahedron Letters 1974 4271 and references there cited. 45 G. A. Olah and T. R. Hockswender jun. J. Amer. Chem. SOC.,1974 96 3574. Reaction Mechanisms-Part (ii) Polar Reactions 127 undergoes addition of hydrogen chloride (0.8moll-') in acetic acid to give the expected chloride (90%) along with acetophenone (10 %).Added tetramethyl- ammonium chlaride (0.2 moll- ') reduces the yield of chloride slightly whilst the reaction rate increases by a factor of two probably due to a medium effect. In contrast [1-'HI hex-1-yne undergoes addition of hydrogen chloride (0.75 mol 1-I) in acetic acid to give anti-(21) and syn-(22) products in the ratio of 40 :60; addition of tetramethylammonium chloride (0.2 moll- ') changes the ratio to 90 10,and considerably decreases the yield of ketone (23).46 V ci H CI D \ D Ph H \ /D Ph \/ Ph-CEC-D-/c=c\ /c=c\ + Although the syn contact ion-pair is almost certainly formed first in the A42 mechanism both syn-and anti-addition products may result if 'leakage' occurs i.e.the syn ion-pair may isomerize to the anti ion-pair at a rate competitive with the rate of collapse of the ion-pair to covalent product. Consequently both the solvent and the presence of catalyst may influence the stereochemistry (Table 6),47 Table 6 Stereochemistry of addition of hydrogen chloride to phenyl[1-'H]ethyne (24)" Ratio Solvent Catalyst syn(25):anti(26)* Acetic Acid -60 :40' Dichloromethane -65:35 Sulpholane -75 :25 Sulpholane ZnC1 50 :50 Results from ref. 47. Estimated error 5% See also ref. 46. but the effects are not large in energy terms as both products are probably formed from the syn ion-pair by very rapid reactions. Whilst Table 6 does show one 50 :50 syn :anti ratio theauth01-s'~~ alternative interpretation that this implicated a symmetrically substituted free vinyl cation seems unlikely as other workers observed that added chloride ion reduced the amount of alkyl chloride formed in competition with ketone.46 46 R.C.Fahey M. T. Payne and D.-J. Lee J. Org. Chem.. 1974 39 1124. 47 F. Marcuzzi G. Melloni and G. Modena Tetrahedron Letters 1974 413. 128 T. W.Bentley Similarly addition of trifluoroacetic acid to both the allene (27) and the alkyne (29) (Scheme 11) gave the same ratio of 2 :E trifluoroacetates (30) and (32) which was interpreted as indicating that the vinyl cation (28) was a common intermediate.48 However an alternative explanation is that in the weakly nucleophilic solvent trifluoroacetic acid the two ion pairs (31) and (32) may interconvert more rapidly than they collapse to products (30)and (33).-H H 7 \+ H+ H,C=C=C /H++ +-H,C-C_C-CH \ /C=C-cH3 CH3 H3C (28) (27) or (29) H CH3 \ +/ / \ H3C/c=c OCOCF H H ,CH, \ I \ /OCOCF3 (31) /c=c\ H3C OCOCF 11 H3C CH E -(30) H -OCOCF3 Z-(33) H3C';'\CH3 Scheme 11 5 Bridgehead Reactivity Currently semi-empirical quantum mechanical' and empirical valence force- field (molecular mechanics)49 calculations are being applied to problems of structure conformation mechanism and reactivity. In sharp contrast to the above discussion emphasizing the roles of ion pairs and solvation effects attempts to interpret mechanism and reactivity in polar reactions by calculation usually require the approximation that the reactions are carried out in the gas phase.As might be expected if these calculations are compared directly with solution chemistry some alarming discrepancies arise. Bridgehead reactivity is one of the more successful areas of polar reactions in solution where calculations have been attempted. The change in strain (AHcalc)during the reaction is evaluated from empirical independent terms for bond stretching angle strain torsional strain and non- bonded strain. To reduce the number of empirical parameters required the hydrocarbon is used as a model for the bridgehead halide or sulphonate. Also R. H. Summerville and P. von R. Schleyer J. Amer. Chem. SOC.,1974 96 1 I 10.49 W. Parker R. L. Tranter C. 1. F. Watt L. W. K. Chang and P. von R. Schleyer J. Amer. Chem. SOC.,1974 96 7121. Reaction Mechanisms-Part (ii) Polar Reactions I 1 1 I I 1 I 1 I 1 I 1 18 ---16 -+Fey ii -14 -El-a- 12 - -10 8--&--@ I -@a-@ -+-p @-a 6-2-C1 Y,-4-$-4 2-CI 0-H&H -2 -H -4 --& -I I I I I I I I I 6 I I 0 12 3 4 5 6 7 8 9 10 -log k(70 "C) Figure 2 Calculated diflerences in strain energy between hydrocarbon and bridgehead cation (AHcalc)plotted against -log of the experimental rate constants in 80% ethanol ar 70 "C cf. R.C. Bingham and P. von R. Schleyer 3. Amer. Chem. SOC.,1971,93 3 189 with additional data for l-chZorobicyclo[2,2,2]octanecalculated from results for the corresponding bromide assuming k,,/k, = 40,and for l-chlorobicycio[3,3,3]undecane (35)jronare$ 49 the free cation is assumed to be a good approximation to the transition state.Then (AHcalc) is plotted against the experimentally observed rate constant. The results for chlorides at present the most extensive series available having a common leaving group are shown in Figure 2. Considering the wide range of relative rates (> 10") correlated the errors in extrapolations of experimental data at various temperatures to 70 "C,and the 130 T. W.Bentley approximations made in the calculations the results in Figure 2 are quite en- couraging. Predictions of reactivity within a rate factor of 10 should be possible using values of (AHcalc)for new systems by extrapolation or interpolation of a correlation line for data in Figure 2 e.g.for l-chlorobicyclo[4,4,4]tetradecane (34) and l-chlorobicyclo[3,3,3]undecane (39 Scheme 12."' Although the re1 rates -ca. lo4 1 (predicted) ca. lo6 ca. lo3 1 Scheme 12 predicted very high reactivity of (34) has yet to be tested experimentally the high reactivity of (35) indicates how much progress has been made from the time when all bridgehead positions were thought to be very unreactive in S,1 reactions. 6 hide Hydrolysis The site of protonation of amides in dilute acids and hence the mechanism of acid-catalysed hydrolysis of amides has received further attention [cf Ann. Reports (B),1972,69,390] with experimental evidence favouring hydrolysis by a displacement reaction of the N-conjugate acid.Spectroscopic evidence suggests that addition of water to benzamide in 100% sulphuric acid causes a tautomeric change from 0-protonated to N-protonated amide.50 For acid-catalysed (pH 5) exchange of the primary amide (36) it appears that exchange of H is slower than that of He because restricted rotation about the C-N bond of RCONH3+ preserves the inequivalence of the hydrogens. The interpretation is con-sistent with hydrogen exchange in amidinium ions (37).51 In the alternative 0\ /*, C-N /\ R He interpretation involving protonation of oxygen (38) He and H are non-equiva- lent but it is not clear why H exchanges more slowly than He 50 M. Liler J.C.S. Perkin II 1974 71.51 C. L. Perrin J. Amer. Chem. SOC.,1974 96 5628 5631. Reaction Mechanisms-Part (ii) Polar Reactions The extent to which these conclusions can be generalized is not yet clear e.g. deamination of the N-nitroso-2-pyrrolidone appears to proceed by hydrolysis I NO H/\NO H’ \“-OH Scheme 13 of the N-protonated species (Scheme 13),’ but deamination of N-n-butyl-N-nitrosoacetamide may occur by rate-limiting attack by water on the 0-protonated amide.’ 7Ester Aminolysis Many reactions involving the carbonyl group proceed via tetrahedral inter- mediates (T),but the rate-limiting step may be markedly dependent on the pH of the solution [Ann. Reports (B) 1970 67 751. Previous mechanistic inter- pretations of ester aminolysis have recently been modified and extended to the (general) Scheme 14.54 For most esters direct attack of the amme on the ester 0 H 0-OH 0 k1 k2 II I I I I1 RNH + C-OR s RN+C-OR e RN-C-OR -+ RN-c + -OR / k- I \ k-2 I I \ H H Tk (41) To (42) (product) HO 0-I + II I RN-C -OR RN-C-OR I\ II H H (product) T-(43) Scheme 14 s2 B.C. Challis and S. P. Jones J.C.S. Chem. Comm. 1974 748. 53 C. N. Berry and B. C. Challis J.C.S. Perkin ZI 1974 1638. ’4 A. C. Satterthwait and W. P. Jencks J. Amer. Chem. SOC.,1974,% 7018,7031 ;see also P. Y. Bruice and T. C. Bruice ibid. p. 5533. 132 T. W.Bentley (k,) appears to be rapid and reversible and for uncatalysed reactions of alkyl acetates it is suggested that the rate-limiting step is proton transfer through water (k2).General-base-catalysed reactions may involve rate-limiting removal of a proton from T' (41) by a second molecule of amine so that T' is trapped before it reverts to starting material.Similarly the general-acid-catalysed reac- tion involves trapping of T' by rate-limiting proton donation to give T+ (39) which proceeds to products via To(42). Because the proton exchanges are not sufficiently rapid to equilibrate the tetrahedral intermediates the same intermediate(s) are not necessarily formed by hydrolysis of imidates (40). Consequently the ratio of products formed by imidate hydrolysis does not necessarily indicate which step is rate-limiting in the corresponding ester aminolysis. This evidence modifies previous interpreta- tions which assumed that direct attack of amine on ester (k,) was rate-limiting [Ann.Reports (B),1968,65 781.Apparently the direct attack of amine on ester (k,) is only rate-limiting for reactive phenyl esters which accounts for a small sensitivity to substituents observed for such reactions. Less reactive phenyl acetates may react by rate- limiting expulsion of phenolate ion (k3)from T* (41),which explains why this step is independent of pH and of buffer. 8 Three-electron Oxidation One of the most extensively studied mechanisms of oxidation is the CrV' oxidation of alcohols which is usually drawn as a two-electron process involving rate-limit- ing hydrogen transfer in a chromate ester to yield ketone and Cr" (Scheme 15). Although the detailed mechanisms of the process are still under in~estigation,~~ HH HH H & \/ o rate \ Cr"' + \/ / c \ Ii __+ /C=O + Cr"' C -limiting /\ CO2H OH C0,H 0-Cr-OH CO,H II 0 Scheme 15 Cr" appears to be responsible for one-electron oxidations [Cr" -+ Cr"'] giving rise to side-reactions involving carbon-carbon bond cleavage reactions.56 Good evidence for the thermodynamically favourable three-electron oxidation [CrV1-+Cr'"] has recently been obtained from studies of primary deuterium isotope effects in the CrV1 oxidation of glycolic acid (Table 7).57 The primary isotope effect of 6.15 at low concentrations is consistent with a two-electron oxidation (Scheme 15).As both the rate constants and the isotope effects increase at higher concentrations it appears that there is a competing " H.Kwart and J. H. Nickle J. Amer. Chem. SOC.,1974 96 7572. ''" K. B. Wiberg and S. K. Mukherjee J. Amer. Chem. SOC.,1974 96,6647. F. Hasan and J. RoEek J. Amer. Chem. SOC.,1974,% 6802. Reaction Mechanisms-Part (ii) Polar Reactions Table 7 Dependence of deuterium-isotope effect on concentration for CrV' oxidation of glycolic acid (44). [Glycolic acidl/mol I-' 103kJs-' 103kD/s-' k JkD 0.0145 0.0826 0.0134 6.15 3.20 4.2 0.334 12.6 6.01 13.3 0.365 36.5 reaction in which two C-H or C-D bonds are broken in a single rate-limiting step. A possible mechanism involving three-electron oxidation of the complex (45) is outlined in Scheme 16. Among the other evidence for three-electron H HOC'HCO~H + /\ + C02-+ Cr"' I 0 G' CHO 'H Scheme 16 oxidation is the effect of radical scavengers on stoicheiometry and several co-oxidations (e.g.2-propanol with glycolic acid or with oxalic acid) appear to involve three-electron oxidations.

ISSN:0069-3030    

DOI:10.1039/OC9747100111

出版商:RSC    

年代:1974

数据来源: RSC