ISSN:0069-3030    

DOI:10.1039/OC97673FX001

出版商:RSC    

年代:1976

数据来源: RSC

ISSN:0069-3030    

DOI:10.1039/OC97673BX003

出版商:RSC    

年代:1976

数据来源: RSC

摘要:

2 Physical Methods and Techniques Part (ii) Gas Chromatography ~~~~ ~~~~~ ~ ~~~ By D. A. COWAN Pharmacy Department Chelsea College Manresa Road London S W3 6LX 1 Introduction The intention of this Report is to highlight some of the newest techniques and developments in gas chromatography. A comprehensive review is published in the annual applied review issue of Analytical Chemistry,’ and a bibliography appears four times a ear^-^ with an annual subject index6 in the Journal of Chromatography. Reviews of publications in specific areas of chromatography are published as a part of the Journal of Chromatography and a list covering the period 1959 to 1976 has recently a~peared.~ Among books and reviews of a general nature published recently are a text by Miller8 on ‘Separation Methods in Chemical Analysis’ a compilation of 11 papers’ from a meeting of the American Chemical Society in 1974 entitled ‘New Developments in Separation Methods’ a review by Ettre,” on the development of gas chromatography and a dictionaryof chromatography by Denney,” almost half of which is devoted to gas chromatography.A book covering organic functional group analysis by gas chromatography by Ma and Ladas12 is referenced here but other specific texts will be referred to in later sections. 2 Columns Various methods may be used for assessing the efficiency of a chromatographic column and Ettre13 has discussed the advantages of each. The ideal column will have a sufficiently high separation efficiency and be as short as possible to ensure rapid analysis with low column temperatures thus minimizing the risk of decomposition.The physicochemical processes which occur between the sample and the column will determine whether a sufficient differential rate of migration will occur with a S. P. Cram and R.S. Juvet jun. Analyt. Chem. 1976,48 411R. 2 J. Chromatog. 1976,118 B1. 3 J. Chromatog. 1976 121 B71. J. Chromatog. 1976,124 B145. J. Chromatog. 1976 128 B219. 6 J. Chromatog. 1976 128 B341. J. Chromatog. 1976 127 259. * J. M. Miller ‘Separation Methods in Chemical Analysis’ Wiley New York 1975. E. Grushka ‘New Developments in Separation Methods’ Marcel Dekker. New York 1976. lo L. S. Ettre J. Chromafog.,1975 112 1. l1 R.C. Denney ‘A Dictionary of Chromatography’ Macmillan London 1976.l2 T. S.Ma and A. S. Ladas ‘Organic Functional Group Analysis by Gas Chromatography’ Academic Press London 1976. 13 L. S. Ettre Chromatogruphiu 1975.8. 291. 11 12 D. A. Cowan minimum of solute band br~adening.’~ The factors which affect these processes are therefore continually under investigation and include column diameter and length quantity and type of stationary phase and its particle size column temperature and carrier gas and its pressure and flow rate. Many papers on retention indices continue to appear largely as a result of the research programme initiated by Janik and co-workers in 1964. Examples of subjects discussed are the precision of the determination of the retention indices of alkylbenzenes,” considering the method of time measurement the non-ideality of the carrier gas tile column temperature and the ageing of the column; the use of retention illdiceh in the identification of some organochlorine pesticides;16 the pre-calculanon of retention indices of alkanes on the basis of their molecular structures;l7 the application of Rohrschneider’s and McReynold’s concepts in a discussion of the contribution to the polarity of stationary phases expressed by retention indices;18 the use of topology-information correlation^;'^ and the predic- tion of resolutions from Kovats retention indices as an aid to column selection.” Instrumental contributions to band broadening have been evaluated experimentally by Cram and Glenn,21 and a new equation has been derived from the kinetic viewpoint to describe the effect of the column length on the height equivalent to a theoretical plate (HETP).22 A systematic approach for the optimization of gas chromatographic separations using the simplex method has been described by Morgan and Demi~~g,~~ and more recently by Holderith T6th and Varadi.24 Solid Supports.-Diatomaceous earth or kieselguhr is still the most popular basis for solid supports.Vermiculite from a deposit in the Soviet Union (Kovdor) is to make a support which will improve the selectivity towards aromatic hydrocarbons compared with commonly used bentones. The use of carbon in various forms has attracted quite a lot of attention during the past few A method for coating glass capillary columns with graphite using and its use in packed columns3’ have been described.Carbon supports have also been modified by treatment with various quantities of organic vapour or liquid Di Corcia and co-workers have investigated the analysis of various 14 S. P. Cram and T. H. Glenn jun. J. Chromatog. 1975 112 329. 15 L. Sojlk and J. A. Rijks J. Chromatog. 1976,119 505. 16 F. I. Onuska and M. E. Comba J. Chromatog. 1976,119 385. 17 T. S. Lombosi E. R. Lombosi I. Bernat Zs. Sz. Bernat E. C. Taklcs and J. M. Takacs J. Chromatog. 1976,119,307. 18 G. Tarjln A. Kiss G.,Kocsis S. Meszaros and J. M. Taklcs J. Chromatog. 1976 119 327. 19 J. R. ChrCtien and J.-E. Dubois J. Chromatog. 1976 126 171. 20 S. D. West and R. C. Hall J. Chromatog. Sci. 1976 14 339.21 S. P. Cram and T. H. Glenn jun. J. Chromatog. 1976 119,55. 22 K. Ohzeki T. Karnbara and K. Kodama J. Chromatog. 1976 121 199. 23 S. L. Morgan and S. N. Deming J. Chromatog. 1975,112 267. 24 J. Holderith T. T6th and A. Varadi J. Chromatog. 1976,119 215. 25 V. G. Berezkin and V. S. Gavrichev J. Chromatog. 1976 116 9. 26 V. Patzelovl 0.Kadlec and P. Seidl J. Chromatog. 1974,91 313. 27 T. V. Barmakova A. V. Kiselev and N. V. Kovaleva KofloidZhur. 1974 36 133. 28 T. V. Barmakova A V. Kiselev and N. V. Kovaleva Kolloid Zhur 1974 36 934. 29 F. Bruner G. Bertoni and P. Cicciolo J. Chromatog. 1976 120 307. 30 G. Nota G. C. Goretti M. Armenante and G. Marino J. Chromatog. 1974,95 229. 31 F. Bruner P. Ciccioli and G. Bertoni J. Chromatog. 1974,90 239. 32 A.Di Corcia A Liberti and R. Samperi Analyt. Chem. 1973 45 1228. 33 A. Di Corcia and R. Sarnperi J. Chromatog. 1973 77,277. 34 A. V. Kiselev and K. D. Shcherbakova J. Chromatog. Sci. 1974 12 788. Physical Methods -Part (ii) Gas Chromatography 13 aliphatic and aromatic hydrocarbons using a 2,4,5,7-tetranitrofluorenone-modified graphitized carbon black following their earlier work which used a 2,4,7-trinitrofluorenone-modified stationary phase.36 In addition they have used a graphitized carbon black column partially coated with poly(ethy1ene glycol) 1500for the separation of C and C5 hydrocarbons at 50 0C.37 Other supports include aluminium glass beads,39 and organic materials such as Rohm and Haas XAD-type resins.,' Some applications of molecular sieves in the gas chromatographic analysis of hydrocarbons and alcohols have been described.41 Liquid Phases.-There are many hundreds of liquid phases but the search for a few standard phases continues.Hawkes and co-workers4* have selected five preferred phases 24 phases of secondary preference and 13 phases for special applications. Moff at43 has reviewed the use of SE-30 for the gas chromatographic analysis of some 480 drugs. A recent book4 summarizes the basic terms used in gas chromatography in connection with retention data; it deals with the selectivity of gas chromato- graphic packings and considers the chemical characterization of the main types of phase. A nematic liquid crystal NN'-bis(p-methoxybenzy1idene)-act'-bis-p-toluidine has been used as the stationary phase €or the gas chromatographic separation of naphthalene homologue~,~~ aza-heterocyclic and steroid epimers.,' Di-n-butyltetrachlorophthalate has been used by R~ba,~ for the identification of 78 hydrocarbons.He points out that this stationary phase displays certain features not found in other stationary phases especially in that alkylbenzenes can be disting- uished from alkylcyclohexanes. Polyols on Chromosorb P have been used to separate volatile compounds from excess amounts of ethanol in the analysis of The properties of column packings containing small amounts of stationary phase have been in~estigated,~' and it was shown that uni- and bi-molecular layers of the stationary phase were formed on the support surface.A new set of test compounds has been recommended5' for the determination of the selectivity of liquid phases at temperatures between 250 and 400 "C. The upper temperature limit of various poly(ethy1ene glycols) has been raised to 230°C by 35 A. Di Corcia A. Liberti and R. Samperi J. Chromatog. 1976 122 459. 36 A. Di Corcia R. Samperi and G. Capponi .I. Chromatog. 1976 121 370. 37 A.Di Corcia and R. Samperi J. Chromatog.. 1976 117 199. 38 E. E. Kugucheva Khim. Teckhnol. Topl. Masel 1976 57. 39 L.Zoccolillo and F. Salomoni J. Chromatog. 1975 106 103. 4O D. F.Fritz and R. C. Chang Analyt. Chem. 1974,46 938. 41 M. Long V. Raverdino G. Di Tuilio and L. Tomarchio J. Chromatog. 1976,117 305. 42 S.Hawkes D. Grossman A. Hartkopf T. Isenhour J. Leary and J. Parcher J.Chromatog Sci. 1975 13,115. 43 A. C. Moffat J. Chromatog. 1975 113 69. 44 G.E.Baiulescu and V. A. Hie 'Stationary Phases in Gas Chromatography' Pergamon Press Oxford 1975. 45 S. Wasik and S. Chesler J. Chromatog. 1976 122 451. a 46 M. Pailer and V. Hloiek J. Chromatog. 1976. 128 163. 47 W.L. Zielinski jun. K. Johnston and G. M. Muschik Analyt. Chem. 1976,48,907. 48 M. Ryba J. Chromatog. 1976 123 327. 49 H.Verachtert D. van Oeveen and J. Bevers J. Chromatog. 1976 117 295. 50 A Waksmundzki and J. Rayss J. Chromatog. 1976,119 557. 51 R. D.Schwartz and R. G. Mathews J. Chromatog. 1976,126,113. 14 D.A.Cowan bonding them to silaceous supports by the formation of oxirons in the preparation A hydrocarbon of empirical formula C87H176 has been proposed as a possible standard non-polar stationary phase.53 This paper describes the disadvantages of using either squalane or methylsilicanes as standard phases and explains the design of a hydrocarbon to fulfil as far as possible the following requirements (i) the hydrocarbon should be a pure substance with no chiral centres; (ii) its melting point should be as low as possible (iii) it should be of low viscocity; (iv) the upper temperature limit of its use should be determined by its pyrolytic stability and not by volatility; and (v) the price of the substance should not be prohibitive for its use as a stationary phase.Consideration of these criteria and preliminary investigations led to the synthesis of the hydrocarbon 24,24-diethyl-19,29-dioctadecylheptatetra-contane (C87HI87) which may be used between 30 "Cand at least 250 "C.Grant54 has reviewed the highlights of gas chromatographic column development from a kinetic rather than from a thermodynamic standpoint. He compares the theoretical and practical performances of columns in relation to the distribution and load of the liquid phase and considers the feasibility of using packed and open tubular columns in the optimization of column parameters to achieve the minimum analysis time. Open Tubular Columns.-Discussion still continues on the use of capillary columns for routine gas chromatographic analysis despite the fact that high-efficiency columns were first used successfully more than 15 years ago. In a critical but short review55 the progress in the production and connection of glass capillary columns is surveyed the authors pointing out that the commercial gas chromatographs now available have been developed mainly for packed columns and hence the various components of the instruments must be re-optimized if high-performance analyses are to be undertaken.The typical size of sample per component for a capillary column is about 200 times less than that for a packed column and is in the order of nanograms. Flow splitters are normally used with capillary columns to allow between one-thousandth and one- twentieth part of the sample onto the column. Schomburg and co-worker~~~ have discussed the implications of the restricted sample load and present in their review article the design of an improved splitter.An injector using a pre-column and a pneumatically controlled split point is described in a paper by Hartigan and Ettre56 with the interesting title of 'Questions related to gas chromatographic systems with glass open-tubular columns'. It is generally accepted that capillary columns are invaluable for the analysis of complex mixtures which cannot be separated by the use of packed columns. Capillary columns however are more easily damaged than packed columns and some skill is needed in their preparation. The novice to capillary columns is advised to purchase a commercially prepared column of guaranteed quality before he attempts to prepare his own. Nevertheless many new and simpler techniques have been developed to 52 S.Ho Chang K. M. Gooding and F. E. Regnier J. Chromatog. 1976 120 321. 53 F. Riedo D. Fritz G.Tarjhn and E. Sz. KovLs J. Chromatog. 1976 126 63. s4 D. W. Grant I. Chromatog. 1976 122 107. 55 G. Schomburg R. Dielmann H. Husmann and F. Weeke J. Chromatog. 1976,122 55. 56 M. J. Hartigan and L. S. Ettre J. Chromatog. 1976 119 187. Chysical Methods-Part (ii) Gas Chromatography 15 enable the preparation of laboratory-made columns. Grob and GrobS7 have discus- sed some of the factors which influence the successful preparation of glass capillary columns. Indeed they recommend that individuals prepare their own columns on psychological grounds claiming that the analyst who can rely on his own ability to replace a ruined column has far better pre-conditions for acquiring a correct technique in their use.The liquid phase must be spread homogeneously and permanently over the walls of the capillary column and this spreading will depend on two independent factors. First the liquid film may be stabilized by intermolecular forces that attract the liquid to the solid as occurs when untreated glass is coated with apolar polysiloxane phases. The second stabilizing factor is known as micro-roughness in which the surface of the column is roughened to a degree which is of similar magnitude to the thickness of the liquid film. Usually both factors will operate to some extent at the same time although the predominance of the former is likely to produce higher separation efhcienciesss because of the more homogeneous liquid film produced.Unfortu- nately more polar phases require a higher degree of roughening since the surface tension of these phases tends to decrease their ability to wet the glass surface. The glass surface may be modified by etching or by the growth of crystals or by both. Gaseous hydrogen chloride may be used to etch soft glass when a layer of sodium chloride crystals is simultaneously pr~duced.~~,~* Repeating the etching process has enabled even the most polar phases to be successfully coated.61 However columns made from soda glass tend to be alkaline and the crystal growth depends on the glass structure which makes the production of a standard degree of micro-roughness ~ncertain.~’ In addition the solubility of the crystals in polar liquids may be a limitation.Gaseous hydrogen fluoride has also been used6* for the modification of the surface of both soft soda-lime glass and sodium borosilicate (Pyrex)glass. Sodium chloride crystals may be deposited on the glass surface by coating with a solution of sodium chloride and the size and number of crystals deposited when solutions of different strengths are used has been in~estigated.~~ Barium carbonate is a useful interlayer as it is easily used and is insoluble in water. Although the micro-crystals of barium carbonate produced on borosilicate glass differ from those on soda glass the variations usually observed within a selected type of glass can be neglected. Dehydration of the glass during the drawing of the tubes is claimed to produce an oxide-type glass surface which is completely wettable by non-polar and by most polar liquid phases without further treatment.This procedure results in high efficiency columns of 3000 to 10 000 theoretical plates per metre. The stationary phase may be bonded to the glass as has been demon~trated~~ using octadecyltrich- lorosilane to form an octadecylsilane bonded to the capillary wall or by using a base-catalysed reaction of a siloxane polymeric mixture with the previously etched 57 K. Grob and G. Grob J. Chromatog. 1976 125 471. 58 L.Blomberg J. Chromatog. 1975 115 365. s9 J. J. Franken G. A. F. M. Rutten and J. A. Riiks J. Chromatog. 1976 126 117. 6o J. KrupEik M. Kristin M. ValachoviCova and S. Janiga J. Chromatog. 1976,126 147. 61 J.L.Marshal and D. A. Parker J. Chromatog. 1976,122,425. F.I. Onuska and M. E. Comba J. Chromatog. 1976,126 133. 63 C.Watanabe and H. Tomita J. Chromatog. 1976 121 1. 64 J. Simon and L. Szepesy J. Chromatog. 1976,119,495. 6s R. G.Einig and J. L. MacDonald Analyt. Chem. 1976,48 2281. 16 D.A. Cowan glass surface.66 Various other means of stabilization of the stationary phase include the addition of a polymer which has an improved ability to wet glass,67 the addition of a surfactant,68 first reported in 1962 or the coating of the stationary phase together with a finely powdered Metal capillaries may also be used and the importance of the initial cleaning process of stainless-steeI capillaries on their adsorption properties has been st~died.~’ Tapeworm columns formed by flattening a 1mm internal diameter tube in a ‘micro-mangle’ are claimed71 to combine high separating power with high capacity.3 Detectors A book by SevEik7’ covers all the main types of detector in use today. His systematic treatment of the subject describes not only the function but also the optimal working conditions of the devices including gas flow effect of additives in the gases and the temperature. The use of selective detectors in gas chromatography has been reviewed with a bias towards their application to forensic science,73 and ~~~ A u has reviewed detectors for use in the gas chromatographic analysis of pesticides. Flame Ionization.-The fundamental treatment by Blades75 on the mechanism of ion formation has been added to by Sevc‘ik and co-worker~.~~*~’ The normal mixing of the eluate and hydrogen after the gas chromatograph column gives a constant but temperature-dependent response related to the methylene groups of organic com- pounds.The ionization efficiency of flame ionization detection has been shown to increase rapidly with the number of CH functional groups in the organic sample and the proportion of methyl radicals is correlated with the resulting detector current. A semi-specific flame ionization detector (FID) called an ‘energized flame ionization detector’ (EFID) has been designed77 and is based on a pre-reaction zone containing a heated hydrogen path leading to a FID. The output of the EFID and the signal from a normal-mode FID are measured simultaneously both detectors being connected in parallel to the outlet of the gas chromatograph column.The EFID signal depends on sample cracking leading to radical formation and in the presence of electronegative groups or atoms to the higher probability of recombination. Branched alcohols were found to produce a greater signal than straight-chain alcohols owing to the greater number of CH groups per molecule. The technique shows promise for making qualitative identifications. 66 C. Madani E. M. Charnbaz M. Rigaud J. Durand. and P. Chebroux J. Chromatog. 1976,126 161. 67 A. L. Gordon P. J. Taylor and F. W. Harris J. Chromafog.Sci. 1976 14 428. 68 F. Farre-Rius J. Henniker and G. Guichon Nature 1962 196 63. 69 R. S. Deelder J. J. M. Ramaekers J. H.M. Van Den Berf and M. L. Wetzels J. Chromatog. 1976,119 99. 70 M. Ryba J. Chromatog. 1976 123 317. 71 H._D. Papendick and J. Baudisch J. Chromatog. 1976,122 443. 72 J. SevEik ‘Detectors in Gas Chromatography’ Journal of Chromatography Library Vol. 4 Elsevier New York 1976. 73 J. F. Taylor Roc. Analyt. Div. Chem. SOC.,1976 13 168. 74 W. A. Aue J. Chromatog. Sci. 1975,13 329. 75 A.,T. Blades J. Chromafog.Sci. 1973 11 251. 76 J. SevEik and M. Klima Chromatogruphia 1976 9 69. 77 J. SevEik R. E. Kaiser and R. Rieder J. Chromatog. 1976,126 263. S Physical Methods-Part (ii) Gas Chromatography 17 A flame ionization detector using a negatively polarized collector electrode mounted high above the flame burning in a mixture of hydrogen and air to which a small amount of silane is added has been to respond well to organometallic compounds of aluminium chromium iron lead and tin.As little as 6 X g of one test compound aluminium hexafluoroacetylacetonate could be detected and the minimum detectable amount of a typical hydrocarbon standard tetradecane was only 4 X g. It appears that this detector will be suitable for determining certain trace metal organics contained in complex mixtures. The use of ammonia instead of hydrogen for flame formation in a FID increases the sensitivity of detecting chlorinated hydrocarbon^.^^ In addition the low viscosity of ammonia as carrier gas especially in long packed columns makes it possible to obtain high velocities using a decreased pressure gradient. A new thermionic detector has been described" by Verga and Poy which uses a potassium chloride pellet as the alkali source and which has high sensitivity and variable selectivity towards nitrogen and phosphorus.The alkali source may be positioned over the flame to produce the maximum ionization current or lowered beneath the flame to allow normal flame ionization detection. The polarization electrode may also be moved to alter the electric field; movement towards the flame increases the standing current and gives an increased phosphorus/nitrogen signal ratio. Conversely movement of the electrode away from the flame reduces the standing current and improves the selectivity towards nitrogen. The detector is claimed to be 50-100 times more sensitive to nitrogen compounds and 500-1000 times more sensitive to phosphorus compounds than the conventional FID.The Optimization of a three-electrode thermionic detector by adjustment of the hydrogen flow rate may allow increased selectivity between nitrogen and Bleeding 2,6-dinitrotoluene onto the column from a 7 cm 100 pI syringe needle has been used8* to simplify the optimization of a thermionic detector towards nitrogen. Brazhnikov and Shmide183 have proposed a mechanism for the processes that occur in the thermionic ionization detector. Alkali-metal atoms are formed in the volume of the flame not on the salt surface by reaction of the vapours of alkali-metal salts with hydrogen. The metal atoms are excited in the flame and collide with other atoms to produce metal ions.Aerosols of alkali-metal particles are formed near the collector electrode because of its lower temperature and the background current is probably due to thermoemission of the aerosols since they absorb radiant energy from the flame. When organic compounds containing phosphorus or nitrogen enter the flame a useful thermionic detector signal appears which may be explained by consideration of the following facts (i) alkali-metal salts are active inhibitors of combustion; (ii) heavy ions of low mobility are formed by combination of alkali-metal ions with those produced by the combustion of phosphorus- or nitrogren- containing organic compounds; (iii) a reducing pyrolysis of phosphorus-and nitrogen-containing organic compounds resulting in the formation of hydrocarbon radicals may occur.The flame temperature is substantially reduced by the presence 78 H. H. Hill jun. and W. A. Aue J. Chromatog. 1976 122,515. 79 V. G. Berezkin and L. A. Shkolina J. Chromatog. 1976 119,33. G. R. Verga and F. Poy J. Chromatog. 1976 116,17. 81 N. Mellor J. Chromatog. 1976 123,396. ** A. T. Chamberlain J. Chromatog. 1976 116,180. 83 V.V. Brazhnikov and E. B. Shmidel J. Chromafog.,1976 122,527. 18 D.A. Cowan of alkali-metal ions and the efficiency of ionization described above depends on the temperature that is established. When phosphorus- or nitrogen-containing organic compounds enter the flame the temperature of the flame increases because the concentration of alkali-metal salt in the flame is reduced as a result of heavy-ion formation.In addition combustion of hydrocarbon radicals formed will raise the flame temperature and this will lead to an increase in the efficiency of the ionization of the alkali-metal atoms and to an increase in the thermoemission of aerosol particles and hence to the appearance of the useful detector signal. The design of a new dete~tor,'~ the sensitivity of which is not so dependent on the flow-rates of the gases as other designs was also discussed by Brazhnikov and Smidele3 and a comparison made with other thermionic detectors. Flame Photometry.-The flame photometric detector has been extensively used for the selective detection of volatile sulphur- and phosphorus-containing compounds in the atmosphere pesticides urinary volatile sulphur-containing compounds and sulphur gas in soil atmospheres (see paper by Bl~mberg'~ for list of references).The problems encountered in the use of the flame photometric detector for gas chromatographic analysis include (i) extinguishing of the flame when the solvent peak elutes; (ii) recorder baseline drift due to thermal instability of the photomultip- liers; (iii) damping of luminescence by the presence of other compounds which may lead to a loss of specificity; (iv) small dynamic range for sulphur-containing com- pounds; (v) the optimum flow-rates of feed gases are different for detecting phosphorus- and sulphur-containing compounds; (vi) sulphur produces interference signals on the phosphorus channel. Several designs of detector have been publi~hed~~-*~ which do not suffer the problem of the flame extinguishing when using liquid samples of as much as 50 p1by passing the column effluent to the detector by means of the hydrogen line instead of the oxidant line.The design of Hasinski86 uses a thermal insulation plate between the luminescence and burner parts enabling changes to be made to the geometric arrangement of the system to optimize the signal-to-noise ratio the background noise with the flame on the detector sensitivity to carbon-containing compounds and the independent thermal control of the detector system to maintain the dark current of the photomultipliers at a constant level. The detector of Joonson and LOO^^^ overcomes the problem of the dependence of the detector sensitivity on changes in the gas flow.In addition the optimum flow-rates for phosphorus- and sulphur-containing compounds are equal and this enables the reduction of the interference to the phosphorus signals from the sulphur signals by electrical compen- sation. Blomberg" uses ice-cooled acetone to keep the photomultiplier at 4"Cto reduce the detector noise and he has successfully used his apparatus with capillary columns to analyse the gas phase of fresh tobacco smoke. The response of the Pye flame photometric detector has been optimized88 and the detection limits for some phosphorus- and sulphur-containing insecticides have been determined. 84 E. B. Shmidel L. I. Kalabina E. N. Vorona and K. I. Sakodynskii Bull. Isobret. 1975,No.30 123. 85 L. Blomberg J.Chromatog. 1976 125 389. 86 S.Hasinski J. Chromatog. 1976 119 207. 87 V. A. Joonson and E. P. Loog J. Chromatog. 1976,120 285. 88 R.Greenhalgh and M. A. Wilson J. Chromatog. 1976 128 157. Physical Methods-Part (ii) Gas Chromatography 19 Electron Capture.-The effect of the nitrogen carrier gas pressure on the response of a commercial direct current nickel-63 electron capture detector has been measuredSg using 2,4,5,6-tetrachloronitrobenzenein the pressure range 1-5 atm by making the detector leak-tight and by the connection of a pressure gauge and an outlet valve. The response increased linearly with the cell pressure in the range 2-5 atm and as little as 20 fg of the test compound was easily detected at the higher pressures. The response under the normal operating conditions of 1atm was found to be less dependent on pressure.The advantages of iron-55 as an Auger electron emitter over conventional sources have been rep~rted.~' Signal fluctuations which determine the smallest useable signal in electron capture detectors are assumed to be as a consequence of the statistical nature of radioactive decay. Thus the p -emitting nuclides tritium and nickel-63 are generally used as the radiation source as a-emitters are considered too 'noisy' and y -emitting sources would cause radiological exposure problems at the strength required to give suitable ion currents. The current flowing at the detector anode is made up of a series of random pulses which varies as a function of the square root of the number of ionizing particles (N)emitted in a chosen time interval and the number of ion pairs (A) generated by each ionizing particle.Low noise for a given current is favoured by making N as large as possible and A as small as possible. A source giving a low ion current is required for the pulse feedback method of electron capture detection to achieve a linear response characteristic. The orbital electron which may be emitted in extra-nuclear re-adjustments which follow radioactivity decay by orbital electron capture is known as an Auger electron. The Auger yield of iron-55 the number of ions generated by each particle the half-life and the specific activity are all satisfactory for its use as the source in an electron capture detector. A source using iron-55 has been devised which is stable to moisture and to heat up to 4OO0C which is above the temperature to which these detectors are normally subjected.A 5mCi iron-55 source gave a better signal-to-noise ratio than that obtained for other more conventional sources. An atmospheric pressure ionizer using a nickel-63 source for mass spectrometry has been usedg1 to study the formation of ions as well as electrons produced in what is essentially an electron capture detector. This technique has demonstrated that in the electron capture detector the positive and the negative charge densities are numerically equal in the active volume and that the charged particle density is relatively insensitive to whether the negative charge consists of electrons or negative ions.It has also been shown that the differential and ultimate sensitivity and the dynamic range of the electron capture detector is strongly affected by the nature of the positive ions present. It is suggested that atmospheric pressure ionization coupled to the mass spectrometer allows the systematic determination of the operating conditions for specific sample types. MassSpectrometer.-The mass spectrometer is perhaps the best gas chromatograph detector at present available for selectivity and sensitivity. The mass spectrometer analyser and generally its ion source must operate at very low pressures often less 89 S. Kapila and W. A. Aue J. Chromatog. 1976 118 233. 90 D. J. Dwight E. A. Lorch and J. E. Lovelock,J. Chromatog.. 1976 116 257.91 M. W.Siege1 and M. C. McKeown J. Chromatog. 1976,122 397. 20 D. A. Cowan than 1x Torr and since the gas chromatograph column outlet is usually at atmospheric pressure a coupling system which will produce the necessary pressure reduction without too much loss of eluted compound must be used. Various interfacing devices are available but with the use of faster pumping systems on modern mass spectrometers and the small volume flow-rate of capillary columns direct coupling to the ion sources becomes a possibility. A recent designg2 uses such a direct coupling together with a device to enable the removal of the large volume of the eluted solvent peak to prevent pressure build-up in the ion source. Ryhageg3 has described the design of a mass spectrometer ion source which may be switched rapidly between the electron impact and chemical ionization modes enabling a mass spectrum to be recorded in each mode for an eluted gas chromatog- raph peak over the mass range of m/e 5 to m/e 500 in less than nine seconds.A simple modification is described94 to allow the dual operation of a packed and a capillary column with switching of either column outlet to the mass spectrometer. This modification is intended to reduce the need to change columns when packed columns are required and removes the limitations of sample injection volume associated with capillary columns and stream splitting. Palmer and Holmstedtgs have described briefly the principle of selective ion monitoring (mass fragmentography) and provided a list of 319 references on the technique and its use as a selective and sensitive detection system for quantitative gas chromatographic analysis.The use of glass capillary columns with the mass spec- trometer as the selective ion monitor for the quantitative analysis of trace compo- nents in extracts of biological fluids has been de~cribed.’~ The review by Burlingame Kimble and Derrick” covers all aspects of mass spectrometry as well as aspects relating to gas chromatograph linked systems. A minicomputer-based method has been described98 which will produce mass spectra free from neighbouring compo- nent contributions. The systematic errors which may occur when isotope ratios are measured using gas chromatography coupled mass spectrometry have been discus- ~ed.~~ It was concluded that unidirectional scanning is superior to bidirectional scanning and that a bias of less than one per cent may be obtained by its use for ten or more cycles on any given gas chromatographic peak.Radio-isotope.-A very sensitive flow-through proportional counting tube is now available for gas chromatography. Some of the theoretical and practical aspects of the use of this equipment have been discussed,’oo in particular the calculation of specific activities of gas chromatograph peaks the optimization of the tube voltage using external radiation sources and the effect of the composition of the counting gas. Under otherwise identical conditions the width of the gas chromatographic peak has no effect on the count number and thus the activities of narrow and broad peaks can be evaluated equally well.92 F. A. Thome and G. W. Young Analyt. Chem. 1976,48,1423. 93 R. Ryhage Analyt. Chem. 1976,48 1829. 94 L.Kazyak Analyt. Chem. 1976,48 1826. 95 L.Palmtr and B. Holmstedt Sci. Tools 1975 22 25. 96 J. Eyem Sci Tools 1976,23,43. 97 A.L.Burlingame B. J. Kimble and P. J. Derrick Analyt. Chem. 1976,48,368R. 98 R. G. Dromey M. J. Stefik T. C. Rindfleisch and A. M. Duffield Anafyr. Chem. 1976,48 1368. 99 D.E.Matthews and J. M. Hayes Analyt. Chem. 1976,48 1375. 100 I. Kiricsi K. Varga and P. Fejes J. Chromatog. 1976 123 279. Physical Methods-Part (ii ) Gas Chromatography 21 Electrochemical.-The Hall"' microelectrolytic conductivity detector surpasses the earlier Coulson'02 detector in sensitivity.A valve has been to vent the solvents away from a Hall conductivity detector thus significantly reducing solvent tailing peak masking and detector contamination. The influence of furnace temper- ature flow-rate of water through the conductivity cell and mode of operation (pyrolytic or reductive) has been investigated."* The use of the detector in the pyrolysis mode for N-nitrosamines gave a specificity lo7times greater than that for hexane and a detection limit of 50 pg with a linear response up to at least 200 ng. Miscellaneous.-A piezo-electric with an operation range of 25-100 "C will it is suggested increase the scope of analytical capabilities over the normal device despite a reduction in response with an increase in the temperature.Some further comments have been made by Novhk Guha and JanBklo6 following their earlier work'" on the effect of columnn temperatures on the sensitivity of the katharometer detector. Although the effect of katharometer temperature on peak area was found to be insignificant the column temperature does have an effect on the response which has not been explained. A gas chromatograph using glass capillary columns has been coupled'o8 to a multiple detector arrangement consisting of a flame ionization detector a flame photometric detector in the sulphur mode a thermionic ionization detector for nitrogen and a sniffing port for the sensoric evaluation of column effluent. This exotic arrangement has been used for the analysis of flavours including that obtained from roasted meat.4 Pyrolysis Gas Chromatography The pyrolysis of samples and subsequent gas chromatographic separation of the products continue to find new applications. A regulated temperature pyrolyser has been describedlog which uses a filament whose resistance is a linear function of the temperature and which reaches the required temperature in less than 5 ms with very good reproducibility. This pyrolyser has been used to study epoxy polymers. Other applications include the determination of alkylene oxides in their co-polymers,"o the characterization of bitumens,"' and the investigation of some of van Meegeren's faked Vermeers and Pieter de Hooghs."' 5 Derivatization Without the preparation of suitable derivatives the gas chromatographic analysis of many compounds would not be possible because of their low volatility or thermola- 101 R.C. Hall J. Chromatog. Sci. 1974 12 152. lo2 D. M. Coulson J. Gas Chromatog. 1965 3 124. 103 J. MacDonald and J. W. King J. Chromatog. 1976,124 364. '04 E. von Rappard G. Eisenbrand and R. Preussrnann,J. Chromatog. 1976,124 247. lo5 F. W. Karasek P. Guy H. H. Hill jun. and J. M. Tiernay J. Chromatog. 1976,124 179. 106 J. Novak 0.K. Guha and J. Janik J. Chromatog. 1976 123 497. 10' J. Novik 0.K. Guha and J. Janlk J; Chromatog. 1975 112 365. 108 M. HiivniE W. Frischknecht and L. Cechovi Analyt. Chem. 1976,48 937. Io9 C. Waysrnan D. Matelin and C. L. Duc J. Chromatog. 1976 118 115. ll0 I. Zernan L. Novik L. Mitter J. Stckla and 0.Holendova J.Chromatogr. 1976 119 581. 111 Z. Ramljak D. Deur-Siftar and A. Solc J. Chromatog. 1976,119 445. R. Breek and W. Froentjes Sad. Consem. 1975 20 183. D.A. Cowan bility. The general principles involved in the preparation of derivatives and the application of derivatives in individual groups of compounds have been reviewed by Dr0~d.l'~ The consultation of this excellent paper with its 604 references is highly recommended before any derivatization work is undertaken. The preparation of derivatives should also be considered for improving the separation of closely related compounds or to increase the detector response; for example chlorinated and fluorinated derivatives usually yield very large responses from the electron capture dete~tor."~ However despite its usefulness derivatization is not a panacea.For instance low volatility caused as the result of intermolecular cohesion from disper- sion forces because the substance has a large molecule obviously cannot be increased by derivatization. The use of derivatization in the gas chromatographic analysis of pharmaceutical compounds has been reviewed.' '5*116 Recent examples of derivatization methods and their applications include the formation of N-isobutyloxycarbonylaminoacid methyl esters of protein amino-acids (except arginine) by simple treatment with isobutylchloroformatein aqueous medium117 and the separation in 35 min of the 20 protein amino-acids as their N-trifluoroacetyl n-butyl derivatives.' l8 J. Drozd J. Chromatog.1975,113 303. 114 J. E. Lovelock Nature 1961,189 729. llS S. Ahuja J. Phamt. Sci. 1976,65 163. J. Nicholson Proc. Analyr. Diu. Chem. Soc. 1976 13 16. 117 M. Makita S. Yamamoto and M. K6no J. Chromatog. 1976 120 129. L.-A. Appelqvist and B. M. Nair J. Chromatog. 1976 124 239.

ISSN:0069-3030    

DOI:10.1039/OC9767300011

出版商:RSC    

年代:1976

数据来源: RSC

摘要:

3 Theoretical Chemistry By G. KLOPMAN and P. ANDREOZZI Chemistry Department Case Western Reserve University Cleveland Ohio 44106 U.S.A. 1 Introduction In this chapter we review three major areas in which quantum mechanical methods have been extensively applied in the past year. The topics include the electronic structure and conformation of molecules where the contributions of quantum mechanical methods have now found widespread acceptance; the theoretical study of dynamic processes which is becoming a popular area thanks to improved computational techniques that allow the determination of fairly accurate reaction hypersurfaces; and finally the study of molecular associations which is one of the most promising new areas of quantum mechanical research.Indeed with the development of more sophisticated theoretical techniques methods are now availa- ble for investigating ‘supermolecules’ consisting of two or more aggregated molecules as in a solute-solvent cluster. 2 Electronic Structure and Conformation of Molecules Structural Properties.-Ab initio methods with extended basis sets and varying level of configuration interaction continue to be applied to the calculation of the proper- ties of the ground and excited states of small molecules. The structure of xenon difluoride has been investigated by Bagus et al.’ They found the bond distances and dissociation energies to be in reasonable agreement with the experimentally determined values. The molecular structures of ClF and ClF radicals and their ions have been investigated by Ungemach and Schaefer.2 The geometries of the ions were compared with those of the isoelectronic sulphur fluorides and interesting disparities were found.Nitrogen derivatives seem to have been particularly popular and several calculations have been published notably on the electronic structure and geometry of N2H,3 N2H2, HN02,6and NOCl and NCC1.’ The bulk of the calculations however remain associated with organic molecules. In the small carbon-containing systems these studies often aim at determining the ’ P. S. Bagus B. Liu D. H. Liskow and H. F. Schaefer tert. J. Amer. Chem. SOC. 1975,97 7216. * S. R. Ungemach and H. F. Schaefer tert. J. Amer. Chem. SOC.,1976,98 1658. 3 K. Vasudevan S. D. Peyerimhoff and R. J. Buenker J.Mol. Structure 1975,29 285. R. Ahlrichs and V. Staemmler Chem. Phys. Letters 1976 37 77. C. F. Jackels and E. R. Davidson J. Chem. Phys. 1975,63 4672. 6 S. Skaarup and J. E. Boggs J. Mol. Structure 1976,30 389. 7 A Stagard Chem. Phys. Letters 1976,40 429. 23 24 G.Klopman and P.Andreozzi geometries and wavefunctions of the ground and low-lying excited states. Examples of such calculations are those performed on CH2,8 CH0,9 HCHO," and NH2CH0.l1 The interaction between two substituents on the same carbon atom has recently attracted the attention of several research groups. Nielssen and Skancke12 found that the planar form of CH2Li2 is only 8.7 kcal mol-' less stable than the tetrahedral one. A systematic study of a large series of CH2XY molecules where X and Y can be any of the Li BeH BH2 CH3 NH2 OH or F groups was presented by Dill Schleyer and P0p1e.l~ These authors found large bond-separation energies (for the isodesmic reaction XCH2Y + CH -+CH3X+ CH3Y) only if both substituents were strongly electronegative or both strongly electropositive.Rotational barriers have also been discussed in the cases where Y is either BH or NH,. Barriers to internal rotation have also been determined in the groundl4 and excited states of glyoxal (OHC-CHO)." In the ground state the trans-form was found to be 5.9kcalmol-' more stable than the cis-form. The structure of ethylidene (CH3-CH) and its transformation into ethylene was studied by Gordon et a1.,16using MIND0/2 and INDO methods and by Altman et al.," from an ab initio viewpoint.MIND0/2 finds no energy minimum for singlet ethylidene while INDO predicts a non-classical bridged structure (l),and the ab initio calculations point towards a classical staggered geometry (2). Other small organic systems of interest have been investigated such as methyleneimide and its fluoro-derivatives,18 keten,19*20 and acrolein21 (both in the ground and excited states) md trimethylenemethane.22 Single-determinant ab initio MO theory has also bgen applied to all possible singlet and triplet states of the C3H2 isomers23 and to C4 hydrocarbon^.^^ The structure of cyclobutadiene continues to arouse controver~y.~~*~~ The latest entry is a Multi Configuration LCAO-MO calculation by Nakayama et ~l.,~' H HH H -H-'H H (1) (2) * J.H. Meadows and H. F. Schaefer tert. J. Amer. Chem. SOC.,1976,98,4383. 9 P. I. Bruna R. J. Buenker and S. D. Peyerimhoff J. Mol. Structure 1976,32 217. 10 L. B. Harding and W. A. Goddard tert. J. Amer. Chem. SOC.,1975,97,6293. 11 L. B. Harding and W. A. Goddard tert. J. Amer. Chem. Soc. 1975 97 6300. " E. W. Nielssen and A. Skancke J. Organometallic Chem. 1976 116 251. 13 J. D. Dill P. von R. Schleyer and J. A. Pople J. Amer. Chem. SOC.,1976,98 1663. 14 C. E. Dykstra and H. F. Schaefer tert. J. Amer. Chem. SOC.,1975,!?7 7210. 15 C. E. Dykstra and H. F. Schaefer tert. J. Amer. Chem. SOC., 1976 98 401. 16 M. S. Gordon P. M. Saatzer and R. D. Koob Chem. Phys. Letters 1976 37 217. 1' J. A. Altmann 1. G. Csizmadia and K. Yates Chem. Phys.Letters 1976,41 500. 18 J. M. Howell J. Amer. Chem. SOC.,1976 98 886. 19 L. B. Harding and W. A. Goddard tert. J. Amer. Chem. SOC.,1976,98 6093. 20 C. E. Dykstra and H. F. Schaefer tert. J. Amer. Chem. SOC.,1976 98 2689. 21 C. E. Dykstra J. Amer. Chem. SOC., 1976,98 7182. 22 W. T. Borden J. Amer. Chem. Soc. 1976,98 2695. 23 W. J. Hehre J. A. Pople W. A. Lathan L. Radom E. Wasserman and A. R. Wasserman J. Amer. Chem. SOC.,1976,98 4378. 24 W. J. Hehre and J. A. Pople J. Amer. Chem. SOC.,1975 97 6941. 25 W. T. Borden J. Amer. Chem. SOC., 1975,97 5968. 26 R. C. Haddon and G. R. J. Williams J. Amer. Chem. SOC.,1975,97,6582. z7 M. Nakayama M. Nishihira and Y. J. I'Haya Bull. Chem. SOC.Japan 1976,49 1502. Theoretical Chemistry indicating not only that a Jahn-Teller distortion favours a rectangular (&) ground state (by 0.3-0.9 eV) but that even in the square form (D4,,), the singlet would be more stable than the triplet by ca.0.3 eV. The influence of single methyl and fluoro substituents on the conformation of cyclopentane and cyclohexane was studied by Cremer Binkley and Pople.28 They found that methyl groups tend to exist in the equatorial positions while the fluoro-group prefers the equatorial position in cyclohexane but the axial position in cyclopentane. The importance of non-bonded attractions in determini g the prefer- .~~ red conformation of molecules has been stressed by Epiotis et ~ 1 Thee showed that the preferred geometry depends not only on steric effects but also on one-electron non-bonded attractive effects that are determined by the symmetry of the inherent orbitals.The structures of various ketones have been investigated by several groups. Fitzpatrick and Fanning using STO-3G found that the length and dipole of the CO bond in disubstituted cyclopropenones reflect both the n-and cr-effects of the s~bstituents.~' The influence of substituents on the electron density of the oxygen of the CO bond of a series of compounds of the type X-Ar-COR was also investigated but here a CND0/2 approximation was ~sed.~' A large number of ring systems containing one or more heteroatoms have also been investigated recently. The MIND0/3 method was used to study the stability of a series of possible intermediates in the u.v.-initiated decomposition of (3).32 The reaction occurs with loss of C02 and is believed to yield the lactone (4) rather than (5).The stabilities and geometries of a number of meso-ionic species derived from m oxazole and imida~ole~~ were also calculated with MIND0/3. The meso-ionic compounds were found to be highly polar and much less stable than their classical counterparts. The technique of using molecular fragments in the ab initio calculation of large .~~ molecules was used by Christoffersen et ~ 1to calculate with some success various properties of the ground and excited states of carbazole. The electronic structures of phosphole (6)35and thieno[3,4-~]thiophen (7)36were also studied but by semi- empirical methods only. Both systems are postulated to be aromatic despite their unusual geometrical constraints.Phosphole possesses a pyramidal structure and is aromatic only because strong hypercon jugation exists between the PR orbital and 28 D. Cremer J. S. Binkley and J. A. Pople J. Amer. Chem. Soc. 1976,98 6836. 29 N. D. Epiotis R. L. Yates and F. Bernardi J. Amer. Chem. SOC. 1975,97 5961. 30 N. J. Fitzpatrick and M. 0.Fanning J. Mol. Structure 1976 33 257. 31 D. Beauptre J. P. Stguin R. Urzan and J. P. Doucet Canad. J. Chem. 1976 54 297. 32 C. S. C. Chung J. Mol. Structure 1976 30 189. 33 M. J. S. Dewar and I. J. Turchi J.C.S. Perkin 11 1976 548. 34 L. E. Nitzche C. Chabalowski and R. E. Christoffersen J. Amer. Chem. Soc. 1976,98,4794. 35 W. Schafer A. Schweig and F. Mathey J. Amer. Chem.Soc. 1976,98 407. 36 C. Muller A. Schweig M. P. Cava andM. V. Lakshmikantham J. Amer. Chem. SOC.,1976,98,7187. G. Klopman and P.Andreozzi R the 7r-system of the cis-butadiene fragment. In the case of the thienothiophen it is found that although the 7r-system is strongly stabilized (aromatic) the molecule as a whole is not particularly stable because of the unfavourable arrangement of its molecular orbitals. The ability of the sulphur atom to interact with 7r-systems has also been considered by Chen and Hoffman37 in a study of the structure of sulphurane SR,. The molecule which is found to exist in a C, geometry (B) bears many similarities to the corresponding phosphorane molecule. Both seem to generate the same axial preference for electronegative substituents.However the site preference of 7r-substituents is different in the two molecules presumably because of the presence of the lone pair in an equatorial position of the sulphurane. 0 R -Sb-R (axial) 4 '*. R R (equatorial) Carbocations. Both semi-empirical an ab initio methods continue to be used to calculate the structure of carbocations. Thus it was found to no one's surprise that both CH3+ and CH2F+ cations tend to ex% in a planar form.38 On the other hand the structure of the ethyl cation (C2Hs+) continues to be debated. Thus the energies of both the classical and hydrogen-bridged structures were calculated using various quantum methods and evaluated with respect to each other by Heidrich and his collaborators.39 Wilcox et aL40not only calculated the stability of the bridged ethyl cation but also determined the stretching force constants that apply to the migrating hydrogen atom.This calculation allowed an estimate to be made of the deuterium kinetic isotope effect to be observed in the reactions involving the ethyl cation. The stabilities of the classical and hydrogen-bridged vinyl cation (C&+) were also The a6 initio plus CI indicated that both structures are equally stable. Finally it was also found by STO-3G and 4-31G methods that among the various isomers of the C3H3+ cation,,' the most stable is the cyclopropenium ion (9) H H 3-1 M. M. L. Chen and R. Hoffmann J. Amer. Chem. SOC.,1976,98 1647. 38 J. Burdon D. W. Davies and G. D. Conde J.C.S. Perkin ZI 1976 1193.39 D. Heidrich M. Grimmer and H. J. Kohler Tetrahedron 1976 32 1193. 40 C. F. Wilcox I. Szele and D. E. Sunko Tetrahedron Letters 1975 4457. 41 J. Weber M. Yoshimine and A. D. Mclean J. Chem. Phys. 1976,64,4159. 42 L. Radom P. C. Hariharan J. A. Pople and P. von R. Schleyer J. Amer. Chem. Soc. 1976.98 10. 27 Theore tica 1 Chemistry +CH,-C~CH followed at ca. 34 kcal mol-’ by the propargyl cation (10). A similar study involving C2H6N+ isomers resulted in the following order of ~tabilities:~~ Other interesting studies involving carbocations include the STO-3G/4-3 1G calcu- lation of the electronic structure of the phenyl cation44 and that of a series of pyridylmethyl and pyrimidylmethyl cations by an INDO appr~ximation.~~ Carbanions Carbenes and Free Radicals.Owing to the difficulties associated with electron correlation carbanions are among the most difficult carbon derivatives to be studied by ab initio methods. Although some studies have been undertaken most attempts to determine the structural characteristics of carbanions are made by semi-empirical methods. For example the MIND0/2 method was used by Shan~hal~~ to determine the structure energy and barrier to pyramidal inversion of a series of cyclic carbanions ranging from the cyclopropyl to the norbornyl carbanion. A modified Pariser-Parr-Pople program was also used to calculate the electron affinity of a series of conjugated systems including some with heteroatom~.~’ An excellent correlation was found between the energy of the lowest unoccupied MO and some recently obtained values of adiabatic electron affinities.The electronic structure of carbenes presents an equally interesting challenge. The difficulty here comes from the degeneracy of the frontier orbitals resulting some- times in the existence of a triplet ground state. Unrestricted Hartree-Fock calcula- tions are often applied to this problem and this has been done again by Takabe el ~l.,~* using a MIND0/2’ framework to determine the electronic properties of CH2 CHF and CF,. It was shown however that a simple analysis can be sufficient to determine most properties or at least the trends in properties of carbene~.~~ Indeed Liebman Politzer and Sanders showed that their method of structural fragments allows valuable information to be gained about the species merely by counting the number of electrons in the appropriate fragments.Amongst relatively few studies of moderately sized free radicals Rossi and WoodS0 examined the conformational preferences of p -substituted alkyl radicals. They found that hyperconjugative substituents tend to favour eclipsed conforma- tions while bulky substituents prefer quite predictably a staggered arrangement. Barriers to Inversion and Rotation.-The relation between the barrier to inversion of AH3 molecules and the nature of the central atom A has been discussed in terms of 43 F. Jordan J. Phys. Chem. 1976,80 76. 44 J. D. Dill P. von R. Schleyer J. S. Binkley R. Seeger J. A. Pople and E. Haselbach J. Amer. Chem. SOC.,1976,98 5428.” C. U. Pittman M. R. Smith G. D. Nichols K. Wuu Shing and L. D. Kispert J.C.S. Perkin 11 1975 1515. 46 M. Shanshal Z. Naturforsch. 1976 3111,494. 47 J. M. Younkin L. J. Smith and R. N. Compton Theor. Chim. Acta 1976,41 157. 48 T. Takabe M. Takahashi and H. Fukutome Progr. Theor. Phys. 1976,56 349. 49 J. F. Liebman P. Politzer and W. A. Sanders J. Amer. Chem. SOC.,1976,98 5115. A. R. Rossi and D. E. Wood I. Amer. Chem. SOC.,1976,98 3452. 28 G.Klopman and P.Andreozzi Walsh-type diagrams" and one-electron MO An ab initio method has been used to calculate an inversion barrier of 2.6 kcal mol-' for dimethylamine and of 3442 kcal mol-' for diff oro oar nine,^^ the value of 42 kcal mol-' being obtained when the nitrogen d-orbitals were included in the basis set.The inversion barriers of a series of simple amines have also been calculated by several semi-empirical methods,54 but in general they compared poorly with the experimentally deter- mined values. The deformation of methane has been investigated at a STO-3G and 4-31G level and the deformation modes were related to those involved in the formation of a few small-ring The intriguing possibility of finding stable tetraco-ordinated planar carbon systems has been investigated by a STO-3G method.56 It was found that several systems might indeed be more stable in a planar configuration than in the ordinary tetrahedral one. Among these the most favourable structures appear to be the planar 1,l-dilithiocyclopropane(1 l) more stable than the tetra- hedral form by 7 kcal mol-' and the planar 3,3-dilithiocyclopropene (12) stabilized by ca.10 kcal mol-'. The dilithio-derivatives also exhibit extraordinary structural properties in other respects; for example the 1,l-dilithioethylene molecule tends to exist as a triplet in a perpendicular geometry (13).57 H Li Li Li H Li (11) (12) (13) The barrier to internal rotation has been studied in several more derivatives such as in nitrous and in a series of molecules of the type XH3YH3,where X and Y were C Si or Ge.59 The ability of the MIND0/3 method to predict rotational barriers and ring puckering of strained ring compounds was tested by Combs and his collaborators. Barriers on all systems studied were under-estimated by MIND0/3 as well as by its predecessors.60 However a re-parameterization of the INDO technique gave dramatically improved results.61 The relative energies of several conformers of 1,3-diene~~~ have been calculated by ab initio and cy~lohexane~~ methods while the use of the PCILO method demonstrated that dimethylcyclohep- tan one^^^ prefer to exist in twist-chair conformations.The early semi-empirical methods rarely provide a satisfactory quantitative measure of internal rotation energies yet for large systems their results are often 51 C. C. Levin J. Amer. Chem. SOC.,1975,97 5649. 52 W. Cherry and N. Epiotis J. Amer. Chem. SOC.,1976,98 1135. 53 S. Skaarup L. L. Griffin and J. E. Boggs J. Amer. Chem. Soc. 1976,98 3140. 34 K. Ohkubo Y. Azuma and M. Okada Bull. Chem. SOC.Japan 1976,49,1397.55 K. B. Wiberg G.B. Ellison and J. J. Wendoioski J. Amer. Chem. Soc. 1976,98 1212. s6 J. B. Collins J. D. Dill E. D. Jemmis Y. Apeloig P. von R. Schleyer R. Seeger and J. A. Pople J. Amer. Chem. SOC.,1976,98 5419. 57 Y. Apeloig P. von R. Schleyer J. S. Binkley and J. A. Pople J. Amer. Chern.Soc. 1976,98,4332. 58 P. Benioff D. Gurupada and A. Wahl J. Chem. Phys. 1976,64 710. 39 G. Nicolas J. C. Barthelat and Ph. Durand J. Amer. Chem. SOC.,1976,98 1346. 60 L. L. Combs and M. Rossei J. Mol. Structure 1976 32 1. 61 L. L. Combs and M. Holloman J. Mol. Structure 1976 33 289. 62 A. J. P. Devaquet R. E. Townshend and W. J. Hehre J. Amer. Chem. Soc. 1976,98,4068. 63 T. D. Davis and A. A. Frost J. Amer. Chem. SOC.,1975,97 7410. 64 M. St.-Jacques C.Vaziri D. A. Frenette A. Goursot and S. Fliszar J. Amer. Chem. SOC.,1976 98 5759. Theoretical Chemistry qualitatively useful. Thus CND0/2 calculations have been carried out to determine the molecular conformation of phenylcyclopropane and those of various of its hetero-substituted derivatives (14),65of benzoyl(diazo)phenylmethane (15),% of di-2-pyridyl sulphide (16),67and of a series of phosphate esters.68 The effect of optimization of the geometry of the conformers on the rotational barriers was seen to be of fundamental importance in an INDO study of butadiene glyoxal benzal- dehyde and 2,2’-difl~orobiphenyl.~’ (14) X =CH2 0,S NH or CC12 (15) (16) Semi-empirical potential mapping is becoming an important tool in the arsenal of theoreticians interested in the chemical behaviour of biologically important molecules.The influence of the molecular configuration of a molecule on the potential field seen by an approaching reagent is a particularly significant problem that can be studied by such a technique. Jordan7’ applied it to the study of 2-(a!-hydroxyethyl)thiamine (17) in an effort to assess the importance of a sub- stituent at position 2. Me OH (17) In another study related to natural products Popkie Koski and Kaufman’l presented an impressive ab initio calculation of the conformation of morphine and some if its derivatives. The eigenvalues were in satisfactory agreement with rneas- ured photoelectron spectral data and the charge densities were compatible with experimental ESCA results.A report on theoretical conformational analysis would not be complete without mention of some of the excellent work performed with the force-field or molecular- mechanics methods. These methods are extremely powerful in dealing with confor- rnational problems and can easily be used to study very large systems. However these techniques are still in their infancy and considerable work is now aimed at 6s S. Sorriso and F. Stefani J.C.S. Perkin 11 1976 374. S. Sorriso and A. Stggard J.C.S. Perkin 11 1976 538. 67 C. Chachaty G. Pappalardo and G. Scarlata,J.C.S. Perkin 11 1976 1234. 68 D. G. Gorenstein D. Kar B. A. Luxon and R. K. Momii J. Amer. Chem. SOC.,1976,98 1668. 69 J. C. Rayez and J. J. Dannenberg Chem. Phys. Letters 1976,41,492.70 F. Jordan J. Amer. Chem. SOC.,1976,98 808. H. E. Popkie W. S. Koski and J. J. Kaufman J. Amer. Chem. SOC.,1976,98 1342. G.Klopman and P. Andrmzzi developing appropriate potential functions mostly for non-bonding intera~tion,~~ such as the H--H potential.73 The methods however provide rapid and often reliable values for the conformational energies of hydrocarbons and have recently been used to calculate the most favourable conformations of sterically hindered hydrocarbon^,^^ 1,2-disubstituted ethane~,~' and a series of alcohols and ether^,'^ the latter with mitigated success. A unique case has been described by Hagler et aZ.,77 where both ab initio and molecular mechanics methods were used to determine the barriers to rotation about the N-C and C-C bonds (4 and 4)in amides and peptides (18).The values of 4 and 4were determined experimentally to be 180"and O" respectively. Interestingly enough both theoretical methods yielded the correct value for 4 but both also gave an incorrect value for (I. 3 Theoretical Studies of Dynamic Processes Isomerization Reactions.-The possible rearrangement paths of cyclopropane have been It was found that the radical cation79 opens along a disrotatory mode and then fragments into CH,*+ and ethylene. The fragmentation into CH and CH2=CH2*+ is energetically more favourable but could occur only along a forbid- den pathway. The degenerate circumambulatory rearrangement of the homocyclo- propenyl cation (19)has been investigated by Hehre and Devaquet" and explained as being due to subjacent orbital control.Jorgensen'l performed MIND0/3 calculations for various conformations of trishomocyclopropenyl cations (20) and found that the most favourable rearrangement yields (21) rather than (22). The (19) (20) (21) (22) photochemical disrotatory ring closure of butadiene has been investigated by an ab initio method,82 and the possible rearrangements of C6HIo hydrocarbons have been studied by MIND0/2." In the latter case 1,4-cyclohexylene (23) emerged as the 72 S. Fitzwater and L. S. Bartell J. Amer. Chem. SOC.,1976 98 5107. 73 D. N. J. White and M. J. Bovill J. Mol. Structure 1976 33 273. 74 H. Braun and W. Luttke J. Mol. Structure 1976 31 97. 75 A. W. Burgess L. L. Shipman R. A. Nemenoff and H. A. Scheraga J.Amer. Chem. SOC.,1976,98,23. 76 N. L. Allinger and D. Y. Chung J. her. Chem. SOC.,1976,98,6798. 77 A. T. Hagler L. Leiserowitz and M. Tuval J. Amer. Chem. SOC.,1976 98 4600. 78 X. Chapuisat and Y. Jean J. Amer. Chem. Soc. 1975,97,6325. 79 S. Beran and R. Zahradnik Coll. Czech. Chem. Comm. 1976,41 2303. 80 W. J. Hehre and A. J. P. Devaquet J. Amer. Chem. SOC.,1976,98,4370. 81 W. L. Jorgensen Tetrahedron Letters 1976 3029. 82 D. Grimbert G. Segal and A. Devaquet J. Amer. Chem'. Soc. 1975.97,6629. 83 A. Komornicki and J. W. McIver jun. J. Amer. Chem. Soc. 1976,98 4553. Theoretical Chemistry central structure involved both in the Cope rearrangement of hexa- 1,5-diene (24) and in the cleavage and inversion of bicyclo[2,2,0]hexane (25). (23) (24) (25) Ab initio methods have been used in studies of other ring formations.The energy of the oxiren molecule (26) postulated to be an intermediate in the Wolff rearrangement was shown to be 11.8 kcal mol-' greater than that of the isomeric formylmethylene molecule (27) and the interconversion ring-opening reaction was (26) (27) found to carry an activation energy of 7.3 kcal m01-l.'~ A study of the interconver- sion of the azido-tetrazole (29) has also been made.85 From the STO-3G results it was postulated that polar solvents should activate the cyclization reaction in spite of the fact that the open chain possesses more charge separation. (28) (29) The cis-trans isomerization of diazenes (R'-N=N-R2) was investigated by an ab initio method by Howell and Kirschenbaum.86 They found that unless both R' and R2 are fluorine atoms the trans-form is always favoured.cis-trans Isomeriza- tion was seen to occur by inversion rather than rotation. Some tautomerism equilibria have also been investigated recently; the CND0/2 method was used to study the enamine-imine equilibriums7 and the use of the INDO technique allowed Westhof and Flossman8' to postulate that the imidazole radical anion generated within an argon matrix by photoelectron transfer is in reality the radical (30) obtained by addition of H to C-5 of imidazole and not the (Y -pyrrolamine (31). (30) (3 1) Cycloaddition Reactions.-Cycloaddition reactions are among the most amenable to theoretical calculations and continue to arouse the interest of theoreticians.84 0.P. Strausz R. K. Gosavi A. S. Denes and I. G. Csizmadia J. Amer. Chern. Suc. 1976,98 4784. L. A. Burke J. Elguero G. Leroy and M. Sana J. Amer. Chem. Suc. 1976,98 1685. 86 J. M. Howell and L. J. Kirschenbaum J. Amer. Chem. Suc. 1976,98 877. 87 J. Teysseyre J. Arriau A. Dargelos J. Elguero and A. R. Katritzky Bull. SUC.chim. belges 1976,85 39. 88 E. Westhof and W. Flossmann J. Amer. Chem. Suc. 1975 97 6622. G. Klopman and P.Andreozzi Several tudies have been carried out for 1,3-dipolar cycloaddition reactions such as the addision of a carbonyl-ylide to ethylene*’ and the ozonolysis of 01efins.~**~’ Ab initio calculations using STO-3Gand 4-3 1G basis sets have also been carried out for the 1,3-dipolar additions of fulminic acid (HCNO) to ethylene acetylene ethynamine and pr~pynenitrile.~~ These calculations indicated that as predicted by the rules of conservation of symmetry the reactions occur along a synchronous path.It was also found that the geometry of the transition state is insensitive to substitu- tion. The forbidden 2 +2 cycloaddition reaction has also received some attention. Epiotis et ~21.~~ found in contrast to experimental results that the most favourable path for approach by the two cycloaddends of a polar 2+2 reaction is along a transoid configuration. Tatsumi and his ~011aborator~’~ found in their study of the metal-catalysed 2 +2 cyclodimerization of ethylene and acetylene that the rectan- gular approach of the reagents is accompanied by the appearance of significant biradical character.This occurred in spite of the fact that the transition-metal catalyst removed the formal restrictions imposed by orbital symmetry on the geometry of the transition state. Finally an interesting study of the effect of cyano substituents in dienophiles on the rate of 2 +4 cycloadditions has been reported by Houk and M~nchausen.~~ Protonation Reactions.-The protonation of benzene to form a Wheland inter- mediate has been studied by several groups. Ermler Mulliken and Clemer~ti~~ have determined the geometry of the benzenium ion using a large contracted gaussian basis set. They also determined a value of 189 kcal mol-’ for the proton affinity of benzene as compared with an experimental value of 183.1 kcal mol-’.The proton affinity of benzene as well as those of toluene and other benzene derivatives has also been calculated by Gleghorn and McConkey using the MIND0/2’ techniq~e,~’ and by Devlin et al.98 and McKelvey and his collaborator^,'^ using the STO-3G method. In the latter study,99 the authors found that there is a good correlation between the AE value for the isodesmic process [equation (l)] and the value of sigma plus (Brown’s sigma values) used to correlate electrophilic aromatic substitution. X X 89 G. Leroy M.-T. Nguyen and M. Sana Tetrahedron 1976,32 1529. 90 P. C. Hiberty J. Amer. Chem. SOC.,1976,98,6088. 91 G. Leroy and M. Sana Tetrahedron 1976,32 1379. 92 D. Poppinger Austral. J. Chem. 1976,29,465. 93 N. D. Epiotis R.L. Yates D. Carlberg and F. Bernardi J. Amer. Chem. SOC.,1976,98 453. q4 K. Tatsumi K. Yamaguchi and T. Fueno Tetrahedron 1975,31 2899. 9s K. N. Houk and L. L. Munchausen J. Amer. Chem. SOC.,1976,98,937. 96 W. C. Ermler R. S. Mulliken and E. Clementi J. Amer. Chem. SOC. 1976,98,388. 97 J. T. Gleghorn and F. W. McConkey J.C.S. Perkin IZ 1976 1078. 98 J. L. Devlin tert. J. F. Wolf R. W. Raft and W. J. Hehre J. Amer. Chem. SOC.,1976 98 1990. 99 J. M. McKelvey S. Alexandratos A. Streitwieser jun. J. L. M. Abboud and W. J. Hehre J. Amer. Chem. SOC.,1976,98,244. TheoreticaI Chemistry 33 Other protonation studies include that of cyclobutane,'OO where an ub initio calculation found both the edge- and corner-protonated forms to be essentially similar in energy and lying 126 kcalmol-' below that of cyclobutane and a proton.Although this value is quite large it is still significantly lower than that found for cyclopropane and is thus consistent with the fact that protonated cyclobutane is seldom seen experimentally. The electronic structure of protonated methanol has been studied by Flood and Nilssen,'" and that of methyl-anisoles by Greenberg and collaborators,102 who found predominance of ring protonation over oxygen protona- tion for all three anisole isomers. Protonation at oxygen has been found to be energetically more favourable than N-protonation in urea,1o3 and C-protonation is favoured over N-protonation in dia~omethane.''~ The proton affinity of a series of amines has been measured by an equilibrium ion cyclotron resonance technique by Aue and his c~llaborator~''~ and found to correlate reasonably well with values from ab initio and CNDO calcula-tions.The influence of alkyl substituents on the proton affinity of a series of amines as well as alcohols and ethers was also investigated by Umeyama and Morokuma,106 by the energy-decomposition technique using an ab initio method. They found the order of proton affinity to be controlled predominantly by the inductive and polarization stabilization effects of the methyl groups attached to the protonated atom. Finally MO calculationsof the controversial BHS molecule the protonated form of BH4- continue to be performed. BH5 is isoelectronic with CHSC and while in the SCF approximation it is found to be unstable with respect to BH3 and H2,107 the inclusion of correlation energy yields a slightly bonding situation (ca.2 kcal mol-') in the C,configuration."* Nucleophilic Substitution Reactions.-The SN2reaction path has been investigated by Dannenberg"' for the reactions between water and protonated alcohols and between F-and alkyl fluorides. The results obtained from an INDO method seem to over-emphasize hydrogen bonding in the transition state. Keil and Ahlrichs,"' using an ab initio method studied several SN2reactions of the kind ACH,+ B-+ A-+ CH3B where A and B are H F or C1. They found that electron correlation contributes up to 7 kcal mol-' to the activation energy and consequently should not be neglected. The importance of non-bonded attraction in the stereochemistry of the SN2'reaction has been emphasized by Yates et aZ.," while De Tar and Tenpas112 showed using molecular mechanics that steric hindrance must be taken into account in order to correlate the rate of ester hydrolysis.loo T. Pakkanen and J. L. Whitten J. Amer. Chem. SOC.,1975,97,6337. lol E. Flood and E. W. Nilssen J. Mol. Structure 1976 30 129. lo* R. S. Greenberg M. M. Bursey and L. G. Pedersen J. Amer. Chem. SOC.,1976.98 4061. Io3 Y.A. Panteleev and A. A. Liporskii Zhur. strukt. Khim. 1976.17 2. lo4 H. M. Niemeyer Helu. Chim. Acta 1976 59 1133. loS D. H. Aue H. M. Webb and M. T. Bowers J. Amer. Chem. SOC.,1976,98 311. lo6 H. Umeyama and K. Morokuma J. Amer. Chem. SOC.,1976,98,4400. lo7 I. M. Pepperberg T. A. Halgren and W.N. Lipscomb J. Amer. Chem. SOC.,1976,98 3442. Io8 C. Hoheisel and W. Kutzelnigg J. Amer. Chem. SOC.,1975,97 6970. lo9 J. J. Dannenberg J. Amer. Chem. SOC.,1976 98 6261. 110 F. Keil and R. Ahlrichs J. Amer. Chem. SOC.,1976 98 4787. R. L. Yates N. D. Epiotis and F. Bernardi J. Amer. Chem. SOC., 1975 97 6615. 112 D. F. DeTar and C. J. Tenpas J. Amer. Chem. Soc. 1976 98 4567. 34 G.Klopman and P.Andreozzi Nucleophilic attack on carbonyl systems has also been investigated by MO methods. For example Lipscomb and his ~~llaborator~"~ studied the reactions of F- and OH- with FCHO NH,+HCHO and CH,O-+H,NCHO with a minimal basis set using the PRDDO procedure. A complete potential surface for the reaction between OH- and H2NCH0 has been calculated by Tomasi et by a STO-3G procedure.They found that the tetrahedral intermediate is 104 kcal mol-' more stable than the reagents in the gas phase. The nucleophilicity of the hydride ion has been investigated by Allen et al."' in the POH3+H-reactions and by Olah et al.' l6 in the H4Nf +H- reaction. In both cases a trigonal bipyramidal intermediate is postulated where the central atom (P and N respectively is pentaco-ordinated (32) and (33). In the case of the phosphine oxide it was also found that the oxygen prefers to exist in the equatorial position [see (32)] rather than in the axial position [see (34)]. HH HH HO \I \I P-0 N-H 'PLH ,// I // I /' I HH HH HH The aromatic nucleophilic substitution reaction has also received some attention as Epiotis and Cherry"' showed that a perturbation method allowed them to rationalize the substitution pattern encountered in molecules of the type C&Y where X is a halogen and Y any other substituent.Dissociation and Recombination Reactions.-Ab initio calculations have been used to study the dissociation of diazomethane"* into CH,('A,) and N, of keten'" into CH and CO and the insertion reaction of CH into H2 to give methane.',' The interaction of H2 with a large series of Lewis acids (H+ Li' BeH' . . . )has also been investigated,12' and binding energies were found to range from high values of 109.5 kcal mol-' for the system H2+H' and 43.2 kcal mol-' for H,+CH,' to low values of 2-5 kcal mol-' for H2 +BeH' BH2+ LiH and BH,. In general it was found that the ability of electron-deficient species to bind a hydrogen molecule is entirely governed by charge polarizability.The photodissociation of formaldehyde into H2and CO was seen to proceed with an activation energy of 4.5 eV.12* Since this value is considerably larger than the experimentally determined threshold energy (3.66 eV) it was suggested that either energy pooling occurs to produce H2C0 with energy in excess of the exciting radiation or that dissociation occurs uiu a catalysed bimolecular pathway. 113 S. Scheiner W. N. Lipscomb and D. A. Kleier J. Amer. Chem. SOC., 1976 98 4770. 114 G. Alagona E. Scrocco and J. Tomasi J. Amer. Chem. SOC.,1975,97,6976. 115 C. A. Deakyne and L. C. Allen J. Amer. Chem. SOC.,1976,98,4076. 116 G. A.Olah D. J. Donovan J. Chen and G. Klopman J. Amer. Chem. SOC.,1975 97 3559. 117 N. D. Epiotis and W. Cherry J. Amer. Chem. SOC. 1976 98 5432. 118 J. Lievin and G. Verhaegen Theor Ckim. Acta 1976,42 47. 119 P. Pendergast and W. H. Fink J. Amer. Chem. SOC.,1976,98 648. 120 C. W. Bauschlicher jun. H. F. Schaefer tert. andC. F. Bender J. Amer. Chem. SOC.,1976,98,1653. 121 J. B. Collins P. von R. Schleyer J. S. Binkley J. A. Pople and L. Radom J. Amer. Chem. SOC.,1976,98 3436. 122 R. L. Jarre and K. Morokuma J. Chem. Phys. 1976,64 4881. Theoretical Chemistry The deoxygenation of ethylene oxide by atomic carbon [equation (2)] has been investigated by the MIND0/3 method.lZ3 The reaction was found to occur downhill (via path i) all the way by direct oxygen abstraction by the carbon atom to form carbon monoxide.Other abstraction reactions that were investigated by ab initiu methods are the reaction of triplet methylene CH2(3B,)with hydrogen and with methane,124 and the abstraction reaction between chlorine and methane. lZ5 ko'-c-+ 11 + co 4 Solvation and Other Molecular Associations Solvation.-The past couple of years have seen the birth of new kinds of quantum mechanical calculations. Once it had been established that the methods are satisfac- tory for dealing with bonds and that the algorithm had been made sufficiently flexible to deal with relatively large systems people started to look at intermolecular interactions such as chemical reactions. More recently molecular associations such as those encountered in molecules in solution have begun to be dealt with by relatively sophisticated quantum mechanical methods.A review of the various existing molecular models for the solvation of small ions and polar molecules has recently been published by Schuster Jakubetz and Marius.126 Considerable interest also presently exists in determining physical properties of gas-phase molecules such as the basicity of alkylamine~~~' and pyridines128 or the acidity of halogenoacetic acids,lZ9 in order to compare these values with the corresponding solution values in an attempt to establish a data base for the study of hydration enthalpies. McCreery Christoffersen and Hall proposed to separate solvent effects into macroscopic and microscopic contributions.The macroscopic solvation eff ectI3' results essentially from electrostatic contributions related to the dielectric constant of the solvent while the contributions are due to interactions between specific sites of the solute and the solvent molecules e.g. hydrogen bonding. For both of these solvation contributions the authors have presented a possible algorithm and preliminary results. So far however most calculations of solvation energies have been conducted by considering clusters of solvent molecules around an ion and comparing the energy of formation of such supermolecules with that of its components. By properly minimizing the co-ordinates of all atoms of the system information can be gained not only about the stability of the cluster but also about its structural characteristics.'23 J. M. Figuera P. B. Shevlin and S. D. Worley J. Amer. Chem. SOC.,1976 98 3478. 124 C. W. Bauschlicher jun. C.F. Bender and H. F. Schaefer tert. J. Amer. Chem. SOC.,1976,98,3072. 125 E.L. Motel1 and W. H. Fink J. Amer. Chem. SOC.,1976,98 7152. l26 P. Schuster W.Jakubetz and W. Marius Topics Current Chem. 1975,60 1. 127 D.H.Aue H. M. Webb and M. T. Bowers J. Amer. Chem. SOC.,1976,98 318. 128 D.H. Aue H. M. Webb M. T. Bowers C. L. Liotta C. J. Alexander and H. P. Hopkins jun. J. Amer. Chem. SOC.,1976,98 854. 129 P. Haberfield and A. K. Rakshit J. Amer. Chem. SOC.,1976,98 4393. 130 J. H. McCreery R. E. Christoffersen and G.G.Hall J. Amer. Chem. SOC.,1976,98 7191. 131 J. H. McCreery R. E. Christoffersen and G. G. Hall J.Amer. Chem. Soc. 1976,98 7198. 36 G.Klopman and P.Andreozzi The hydration energies of various cations and anions have been determined in this fashion by a variety of methods ranging from empirical potential functions to ab initio quantum mechanical calculations. For example structural information as well as Gibbs free energies enthalpies and entropies were calculated by Mruzik et ~1.l~~ for a number of alkali-metal and halogen ions using semi-empirical intermolecular potential functions to evaluate the interaction of the ions with up to six water molecules. A somewhat simpler technique has been used by Spears and Kim133 to calculate (with apparently great success) the hydration energy of alkali-metal ions. Here a simple electrostatic treatment with a semi-empirical selection of different repulsion potentials for each ion was used to calculate the stability of a variety of structures involving one to six water ligands.CNDO/2 calculations were presented for the solvation of ions by methan01.l~~ For this study the stabilization energies were greatly over-estimated although differences in stabilization energies for different ion-methanol clusters agreed with experiment. The evolution of the energies of stepwise addition of NH3or H20ligands to NH4+ has been computed in a STO-3G basis These calculations reproduced all the qualitative features of the corresponding experimental data (initial preference for NH over HzO,but a crossing over of preferential affinities in the second shell) and the error in the calculated energy values was found to decrease with increasing n.The structure of H30+embedded in a solid (X-= C1- NO3- C104-) has been studied using a modified version of CND0/2. Trends in struc-tural parameters were compared with the chemical properties of hydrogen bonds and paralleled the Hammett acidity scale. The structures of the dimethylformamide fragment and its complexes with alkali metals13’ have been studied using ab initio techniques. The amide form of the ion solvate is highly favoured over the isocyanate form. Ab initio techniques have also been used to study the mechanism for formation of the hydrated electr~n,’~~ and also to eluci- date the structure and properties of hydrated and ammoniated electrons. 139 An interesting attempt to study the influence of the dielectric medium on the conformation of natural products has been presented by Kumbar and Sankar.14’ It was found that the properties of tryptamine and serotonin changed considerably as a result of solvation.In particular the potential barrier between the trans and gauche forms changes as well as the shape of the contour maps around the molecule indicating potential modifications of the reactivity. Hydrogen Bonding.-The theoretical study of hydrogen bonding continues to be of special interest as many new contributions help to enrich our knowledge in this especially important field. A simple model of hydrogen bonding has been proposed by Allen,141 based on the knowledge of three characteristics of the monomer the AH 132 M.Mruzik F. Abraham and D. E. Schreiber J. Chem. Phys. 1976,64 481. 133 K. G. Spears and S. H. Kim J. Phys. Chem. 1976,80,673. I34 M. Salomon Canad. J. Chem. 1975 53 3194. 135 A. Pullman and A. Armbruster Chem. Phys. Letters 1975.36 558. 136 M. Fournier M,Allavena and A. Potier Theor. Chim. Acta 1976.42 145. 137 B. M. Rode and R. Ahlrichs Z. Naturforsch. 1975,30a 1792. $38 B. Webster J. Phys. Chem. 1975 79 2809. 139 M. D. Newton J. Phys. Chem. 1975,79 2795. 140 M. Kumbar and D. V. Siva Sankar J. Amer. Chem. SOC.,1975,97 7411. 14* L. C. Allen J. Amer. Chem. Soc. 1975,97,6921. Theoretical Chemistry 37 dipole moment the difference between the ionization potential of the electron donor and that of the noble-gas atom in its row and the length of the hydrogen-bonding lone pair.Allen has shown that most of the properties of the hydrogen-bonded dimer can be inferred from these three properties. Amongst the most common hydrogen-bonded species are those containing at least one water molecule. For example a large-scale CI study has been made of the system HO-HOH in an effort to assess the effect of correlation energy142 on the energy barrier to proton transfer. The most stable species was found to be the symmetrical one with the hydrogen-bonding H atom midway between the two oxygen centres. This result contrasted with SCF calculations where an asymmetric configuration was found to be more stable. The potential energy for the water dimer in various geometrical configurations has also been studied.Both STO-3G'43 and C1144 methods were used and the linear motion of the water dimer was investigated by Curtiss and P~ple.'~~ A value of the dimer.I4' The nature of the potential surface corresponding to the tunnelling motion of the latter dimer was investigated by Curtiss and P0p1e.l~~ A value of 1.1kcal mol-' for the barrier was obtained indicating that bond breaking occurs quite readily in the system. The stability of chains of HF molecules has also been in~estigated,'~' and it was found that some extra binding energy results from infinite chains as compared to smaller clusters. Other interesting papers report the study of hydrogen-bonded dimers containing mixtures of H20 and HF,14* of H20,H02 and NH3,149 of H20 HF and NH3,lS0 of H2S,15' and of H20,H2S,and a series of other electron donor^.'^^ An unexpected result of the latter work is that in 1:1complexes of amines with H20 neither the hydrogen-bond distance 0--H-N nor the amount of charge transfer seems to depend on the ionization potential of the lone pair of the amine.The interaction of water and carbon dioxide has also been investigated by an ab initio technique with CI and found to be repul~ive.'~~ It was thus suggested that carbonic acid (H2C03) is unlikely to be observed in the gas phase. Ab initio calculations have been used to investigate structural and conformational properties of hydrogen-bonded dimers of organic molecules also. Del Bene has continued to investigate the water complexes of substituted carbonyl derivative^"^ and nitrogen heterocycles.'55 An ab initio study of the cyclic nitrosomethane dimer indicated that the interaction between the two units is better described as a contact of the van der Waals type rather than as hydrogen bonding.ls6 On the other hand the 142 B.0.Roos Theor. Chim. Acta 1976,42 77. 143 G. Leroy G. Louterman-Leloup and P. Ruelle Bull. Soc. chim. belges 1976,85 205. 144 0.Matsuoka E. Clement and M. Yoshimine J. Chem. Phys. 1976 64 1351. 145 G. Leroy,G. Louterman-Leloup and P. Ruelle Bull. SOC. chim. belges 1976,85 229. 146 L. A. Curtiss and J. A. Pople J. Mol. Spectroscopy 1976 61 1. M. Kerttsz J. Koller and A. Azman Chem. Phys. Letters 1975,36 576. 14* G. Leroy G. Louterman-Leloup and P. Ruelle Bull. SOC.chim. belges 1976,85 393. 149 E.J. Hamilton and C. A. Naleway J. Phys. Chem. 1976,80 2037. 150 J. D. Dill L. C. Allen W. C. Topp and J. A. Pople J. Amer. Chem. Soc. 1975 97 7220. ls1 G. Leroy G. Louterman-Leloup and P. Ruelle Bull. Soc. chim. belges 1976,85 219. 152 C. N. R. Rao P. C. Dwivedi A. Gupta H. S. Randhawa H. Ratajczak M. M. Szczesniak K. Romanowska and W. J. Orville-Thomas J. Mol. Structure 1976 30 271. IS3 B. Jonsson G. Karlstrom H. Wennerstrom and B. Roos Chem. Phys. Letters 1976 41 317. Is* J. E. Del Bene J. Chem. Phys. 1975.63 4566. 155 J. E. Del Bene Chem. Phys. 1976,15 463. 156 H. J. Talberg and T. Ottersen J.Mol. Structure 1975 29 225. 14' 38 G.Klopman and P.Andreozzi dimers of both formic acid and hydroxyacrolein are satisfactorily described as being hydrogen-bdnded.15' Intramolecular hydrogen bonding has been studied by ab initio techniques for the enol tautomer of mal~ndialdehyde'~~ the latter with relatively and gly~oaldehyde,'~~ little success. Other Molecular Associations.-The lithium hydride dimer and its negative ion have been studied by Jordan.'60 The square form of (LiH) was found to be the most stable and 1.9 eV below that of two LiH molecules. The square (antiparallel) form (35) of the acetonitrile dimer was also found to be the most stable16' when compared to the head-to-tail structure (36). The stabilities of three possible geometrical vJ_C-CH3 I H3C-CrN CH~-C~N--CH,-CZN (35) (36) isomers of the hydrogen cyanide dimers [(37)-(39)] were considered by Moffat,16* using an ab initio technique.Of these iminoacetonitrile (37) was quite predictably found to be the most stable and the aminocyanocarbene (39) the least stable. H H H \/ /H \ N=C C N-C-CrN / \ 11 CZN N=C/ H H/ \N (37) (38) (39) The failure of CNDO to calculate correctly the stability and geometry of molecular pairs has been demonstrated again,'63 this time on a dimer consisting of formamide and imidazole. Several charge-transfer complexes have been investigated by Morokuma and his collaborators such as the 7.r -complex between benzene and carbonyl cyanide'64 and a series of complexes between various amines and b0~ane.l~~ A similar study involving amines and halogens was conducted by Leuchese and Schaefer. 166 Only a couple of years ago there was absolutely no way in which quantum mechanical calculations could have been carried out on molecules containing heavy elements such as transition metals.New parameterizations however now allow such calculations to be done at least with relatively simple MO methods. For example Summerville and Hoff man'67 calculated the geometries and electronic structures of a series of dimeric tetrahedral and square-planar transition-metal 157 J. E. Del Bene and W. L. Kochenour J. Amer. Chem. Soc. 1976,98,2041. G. Karlstrorn B. Jonsson B. Roos and H. Wennerstrom J. Amer. Chem. SOC.,1976,98 6851. I59 H. H. Jensen H. Mollendal and E. Wisloff-Nilssen J. Mol. Structure 1976 30 145. K. D. Jordan Chem. Phys. Letters 1976 40 441. 161 M. R. Dagnino G. LaMann and L.Paoloni Chem. Phys. Letters 1976 39 552. 162 J. B. Moffat J.C.S. Chem. Comm. 1975 888. 163 S. F. Abdulnur and R. L. Flurry jun. Chem. Phys. Letters 1975 36 586. 164 W. A. Lathan G. R.Pack and K. Morokuma J. Amer. Chem. SOC.,1975,97,6624. 165 H. Urneyama and K. Morokurna J. Amer. Chem. SOC.,1976,98,7208. 166 R. R. Lucchese and J. F. Schaefer tert. J. Amer. Chem. SOC.,1976 97 7205. 16' R. H. Summerville and R. Hoffrnann J. Amer. Chem. SOC.,1976,98,7240. Theoretical Chemistry complexes and determined the factors governing the preferred geometries. Block and his collaborators'68 showed that correlations do exist between the site of nucleophilic attack and the location and configuration of the lowest unoccupied orbital of some chromium and manganese carbonyl complexes of organic molecules.168 T. F. Block R. F. Fenske and C. P. Casey J. Amer. Chern. SOC., 1976,98 441.

ISSN:0069-3030    

DOI:10.1039/OC9767300023

出版商:RSC    

年代:1976

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 9JT 1 General and Theoretical Aspects A valuable new text’ illustrates the utility of the frontier molecular orbital (FMO) approach in treating reactivity in ionic free-radical and pericyclic reactions. Several useful reviews have also appeared.* Application of ab initio MO theory to the Diels-Alder addition of butadiene to ethylene3” provides predictions which contrast with those of earlier MIND0/3 calc~lations.~~ The addition is now calculated to proceed in a concerted manner via a symmetric transition state (TS); the hex-2-ene- 1,6-diyl diradical although of lower energy than this TS is only connected to the reaction by a TS 8.36 kJmol-’ higher in energy than the concerted TS.3“ A distinction has been discerned3‘ between cycloadditions in which the dominant FMO interaction is between antisymmetric orbitals and those cycloadditions in which the principal FMO interaction is between symmetric orbitals.For the first type e.g. normal Diels-Alder reactions concerted formation of two bonds is expected. However for the second type e.g. Diels- Alder addition with inverse electron demand the addition will be less concerted or involve complex formation. For additions of a series of cyano-olefins to e.g. cyclopentadiene a test has been made3d of the relationship between log(addition rate constant) and the interaction energy (AE) of the diene HOMO and cyanoethylene LUMO.The interaction energy was calculated using equation (l),where p is the resonance integral and CAand CDare 1 I. Fleming ‘Frontier Orbitals and Organic Chemical Reactions’ Wiley London 1976. 2 (a ) J. J. Dannenberg ‘Predictive Molecular Orbital Calculations in Organic Chemistry’ Angew. Chem. Intentat. Edn. 1976,15,519; (b)N. Dennis A. R. Katritzky and Y. Takeuchi ‘Synt‘hetic Applications of Heteroaromatic Betaines with Six-Membered Rings’ ibid. p. 1; (c) A. Padwa ‘Intramolecular 1,3-Dipolar Cycioaddition Reactions’ ibid. p. 123; (d)E. A. Halevi ‘Orbital Correspondence Analysis in Maximum Symmetry’ ibid. p. 593; (e) T. Wagner-Jaurregg ‘Reactions of Azines and Imines with Dienophiles’ Synthesis 1976 349; cf) F. McCapra ‘Chemical Mechanisms in Bioluminescence’ Accounts Chem.Res. 1976,9,201;(g)W. J. Hehre ‘Ab Initio Molecular Orbital Theory’ ibid.,p. 399; P. D. Bartlett ‘Four-membered Rings and Reaction Mechanisms’ Chem. SOC.Rev. 1976 5 149; D. Bryce-Smith and A. Gilbert ‘The Organic Photochemistry of Benzene’ Tepahedron 1976,32,1309;c. W. Spangler ‘Thermal [l,j] Sigmatropic Rearrangement’ Chem. Reu. 1976 76 187. (a) R. E. Townshend G. Ramunni G. Segal W. J. Hehre and L. Salem J. Amer. Chem. SOC.,1976,98 2190; (b)M. J. S. Dewar A. C. Griffin and S. Kirschener ibid. 1974 96 6225; (c) H. Fujimoto S. Inagaki and K. Fukui ibid. 1976,98,2670;(d)K. N. Houk and L. L. Munchausen ibid. 1976,98,937. 41 D. W.Jones the coefficients of the electron acceptor and donor respectively at the sites of interaction (CND0/2 calculations).In the denominator of the equation the experi- mentally determined donor ionization potential (IP,) and acceptor electron affinity (EA,) replace the energies of the diene HOMO and cyano-olefin LUMO respec- tively. The quantity C is the amount by which the charge-transfer configuration drops in energy as the molecules approach; it is estimated as 4eV. Plots of log(rate constant) against hE are not linear there being a levelling off of reactivity for more reactive olefins. Earlier TSs for the more reactive olefins would result in different values of /3 and C and so account for this result; the neglect of other MO interactions could also be more serious for one cyano-olefin than another. In nucleophilic attack on cyano-olefins (or two-step Diels-Alder reactions) AE will depend only on the larger cyanoalkene LUMO coefficient.As a result the order of reactivity of a series of cyanoalkenes to nucleophilic attack will differ from the order of reactivity in the Diels-Alder reaction. An orbital mixing rule has been derived4 for the interaction of three MOs; with the aid of the rule orbital bias can be estimated and the results applied to explain preferred ex0 -attack on norbornene syn -selectivity in the Diels-Alder addition of certain cyclopentadienes and regioselectivity in the attack of diazomethane on thiocarbonyl compounds and vinyl and ethynyl thioethers. Structural data from several X-ray analyses of 1,6-methanoannulenes (1) and their cyclized forms (2) have been used in an interesting attempt to map the reaction path for the ring-cl~sure.~" The 1,6-interaction is attractive over the range of distances (1.5 A covalently bonded to 2.48 A non-bonded) considered.The results represent a structural expression of an attractive interaction that follows immediately from the rules of orbital symmetry conser~ation.~" Related X-ray as well as calculations5' suggest that the preferred attack on a carbonyl group by a nucleophile is as in (3) whereas attack on an acetylene should proceed as in (4).These considerations have led Bald~in'~ to suggest rules for ring-closure; favoured ring-closures being those in which the linking chain enables the reacting centres to achieve the trajectories indicated in (3)and (4). Based on a 'banana-bond' model of unsaturation the processes (3) and (4) imply inversions at the reacting carbons.Since geometric processes other than inversions are often observed in concerted electrocyclic reactions the rules may not apply to them. However the preferred approach to an acetylenic bond (4)may serve to explain why the linear 4 S. Inagaki H. Fujimoto and K. Fukui J. Amer Chem. Soc. 1976,98,4054. 5 (u)H. B. Burgi E. Shefter and J. D. Dunitz Tetrahedron 1975,31,3089;(b) H. B. Burgi J. D. Dunitz J. M. Lehn andG. Wipff ibid.,1974,30,1563;G. Wegner PolymerLetters 1971,9,133;R. H. Baughman J. Appl. Phys. 1972,43,4362; (c)H. B. Burgi J. M. Lehn and G. Wipff J. Amer. Chem. Soc. 1974,% 1956; (d)J. E. Baldwin J. C.S. Chem. Comm. 1976,734,738;(e)D. B. Bigley and R.H. Weatherhead J.C.S.Perkin ZZ 1976,592; (f) G.Stork and G. Kraus J. Amer. Gem. SOC.,1976,98 6747. Reaction Mechanisms -Part (i) Orbital Symmetry Correlations acetylene may so readily replace the bent olefin in pericyclic processes e.g. the Cope rearrangement (5) and the decarboxylation (6).'' It seems possible that other reactivity problems in pericyclic reactions could benefit from consideration of preferred angles of approach of reacting centres. An intramolecular ene-reaction employing an acetylenic enophile (7; arrows) is useful in the construction of five-mem bered rings. sf "-pI & 8 CSHII H 0R' (5) (6) (7) In structure (8)orbital interaction between the aromatic n-system and the olefinic bond operating uia the C-1 -C-2 and C-5 -C-6 a-bonds results in a lowering of the LUMO energy so that the olefinic bond in (8) is readily reduced under Birch conditions.6" Orbital interaction through space in the anion (9)imparts 6~-electron aromatic character and accounts for exclusive methylation syn to the methoxy- group.6b This effect is related to the unusual stability of cis-dihalogeno- and dialkoxy-ethylenes which has been treated theoretically.6' Similar through space interaction between entering nucleophile and leaving group will secure 6~-electron aromaticity for the preferred syn-TS in SN2' reactions.6d The endo-preference of alk-2-enyl anions6= may have a similar origin.Quantitative application of the principle of least motion7" to the 1,3-shift implied in (10) favours the inversion pathway shown rather than the intuitively preferred retention pathway; thus the inversion observed to accompany such rearrangemet~t~~ is consistent with either orbital symmetry or least-motion control.2 Cycloaddition and Cheletropic Reactions From the reaction of the very reactive diene (11)with sulphur dioxide the (4+2) w-adduct (12) is formed under conditions of kinetic control and the (n +4)w-adduct 6 (a)M. N. Paddon-Row R. Hartcher and R. N. Warrener J.C.S. Chem Comm.,1976 305; (b) R. R. Fraser and K. L. Dhawan ibid.,p. 674; (c)R. Hoffmann and R. A. OlofsonJ. Amer. Chem.Suc. 1966.88 943; N. D. Epiotis S. Sarkanen D. Bjorkuist L. Bjorkuist and R. Yates ibid. 1974,% 4075; (d)cf. R. L. Yates N. D. Epiotis and F. Bernardi ibid.1975,97 6615; (e)M. Schlosser and J. Hartmann ibid.. 1976,98,4674. 7 (a)J. A. Altmann 0.S. Tee and K. Yates J.Amer. Chem. Soc. 1976,98,7132;(b)J. A. Berson and G. L. Nelson ibid.,1967,89 5503. D. W.Jones / Me Me Me Me (11) (12) (13) (13) is formed under conditions of thermodynamic control.*" Non-linear cheletropic extrusion of sulphur monoxide from (14) occurs in boiling dichloromethane and in the presence of diazo-compounds (R,CN,) sulphines (15; R = Aryl) are obtained.8* Addition of dimethoxycarbene to dimethyl maleate gave an adduct in which the methoxycarbonyl groups are trans; addition of the carbene to cis-and trans+-deuteriostyrene is now shown to be stereospecific in accord with a non-linear cheletropic process.*c Further examples' of carbene-like addition of 1,3-dipoles to double bonds have appeared; (16) affords (17) possibly via a dipole produced as in (16; arrows).(14) (15) (16) (17) In the reaction of singlet oxygen with olefins and dienes the most important FMO interaction is between the HOMO of the olefin and the LUMO of singlet oxygen. Ionization potentials of olefins and dienes accordingly provide a useful guide in predicting the preferred site of singlet oxygen attack;lon in (18) the exocyclic double bond (IP = 8.27 eV) is reactive and the endocyclic double bond (IP = 8.63 eV) is passive. Diels-Alder type addition of singlet oxygen to (19) occurs from the more open anti direction but ene-type attack on (20)occurs from the syn direction. In (20) interaction between the oxygen LUMO and N-1 -N-2 HOMO is preferred (energy anti 8 (a) R.F. Heldeweg and H. Hogeveen J. Amer. Chem. Soc. 1976 98 2341; (b) B. F. Bonini G. Maccagnani and G. Mazzanti J.C.S. Chem. Comm. 1976,431; (c)R. A. Moss and J. K. Huselton ibid. p. 950. 9 A. Padwa A. Ku A. Mazzu and S. I. Wetmore J. Amer. Chem. SOC.,1976 98 1048; L. Garanti A. Vigevani and G. Zecchi Tetrahedron Letters 1976 1527. lo (a)L. A. Paquette D. C. Liotta and A. D. Baker Tetrahedron Letters 1976,2681; (b)L. A. Paquette D. C. Liotta C. C. Liao T. G. Wallis N. Eickman J. Clardy and R. Gleiter J. Amer. Gem. SOC., 1976,98 6413; L. A. Paquette C. C. Liao D. C. Liotta and W. E. Fristad ibid. p. 6412. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations gap 8.2 eV) to interaction between the double-bond HOMO and oxygen LUMO (energy gap 8.7 eV).The singlet oxygen is thereby deactivated and attack on the anti-face of (20) suppressed. In (19) interaction between the oxygen LUMO and diene HOMO is preferred (energy gap 7.4 eV) to deactivation by the hydrazine unit so that anti attack is observed.lob In a theoretical study"" of the addition shown in (21) C-1-C-6 and C-4-C-5 bonding was found to be improved when atoms C-2 and C-3 were brought into proximity with C-7 and 0-8respectively; this supports the novel view that secondary interactions function by improving interaction at the primary bonding sites. The FMO method has provided an explanation for the well-established 'ortho -para ' regioselectivity shown in the majority of Diels-Alder reactions.The hitherto unknown addition of an electron-rich diene to an electron-rich dienophile provides an excellent test as matching the larger coefficients in both HOMO-LUMO pairs as in (22) and (23) predicts predominant formation of the 'meta' isomer. If a diradical X.. .. 3(*...i. :c 2 \..:* xGg '1 08 HOMO LUMO LUMO HOMO (21) (22) (23) were an intermediate in such an addition the para-isomer would be expected to predominate. Trapping the very reactive o -quinodimethanes (24) and (25) with prop-1-yne and ethoxyethylene gave in both cases predominant formation of the unusual 'meta '-adduct e.g. (26).' lb Preparations of several potentially useful Diels- Alder dienes have*appeared.'2 Thus (27) available by electrocyclic ring-opening of the related cyclobutene adds to methyl vinyl ketone to give (28) in which the R2 (24) R' = H R2= Me (25) R' = OMe R2= H phenylthio-group rather than the methoxy-group has controlled the direction of addition.'*" The adduct (29) from 1-trimethylsilylbutadiene undergoes proto- desilylation (29; arrows) offering a potentially useful method of changing the position of the double bond in an adduct.12' The intramolecular Diels-Alder reaction continues to attract attenti~n.'~ An intriguing example'3a is provided by the 11 (a)T. Sugimoto Y. Kobuke J. Furukawa and T. Fueno Tetrahedron Lefrers 1976 1587; (b)I. Fleming F. L. Gianni andT. Mah ibid. p. 881. l* (a)B. M. Trost and A. J. Bridges J. Amer. Chem.SOC.,1976,98 5017 (6) M. J. Carter and I. Fleming J.C.S. Chem. Comm. 1976 679; A. Percival and I. Fleming ibid. p. 681; (c) M. E. Jung and C. A. McCombs Tetrahedron Letters 1976,2935;L. E. Overman G. F. Taylor and P. J. Jessup ibid.,p. 3089; J. F. W. Keana and P. E. Eckler J. Org. Chem. 1976,41,257 1; L. E. Overman and L. A. Clizbe J. Amer. Chem. SOC.,1976 98 2352. l3 (a)R. L. Funk and K. P. C. Vollhart,J. Amer. Chem. SOC.,1976,98,6755; (6) T. Kametani H. Nemoto H Ishikawa K. Shiroyama and K. Fukumoto ibid. p. 3378; P. Yates and H. Auksi,J.C.S.Chem. Comm. 1976 1016; W. Oppolzer R. Achini E. Pfenninger and H. P. Weber Helu. Chim. Acfa 1976,59,1186; L. H. Klemm T. M. McGuire and K. W. Gopinath J. Org. Chem. 1976 41 2571. 46 D. W. Jones addition of bis-trimethylsilylacetylene to compounds of type (30); this proceeds under catalysis by cyclopentadienyldicarbonylcobalt,and probably affords inter- mediate o -quinodimethanes which then undergo intramolecular addition to the C-2 double bond affording products (31).Reaction of the optically active nitroso- (29) (30) (31) compound (32) with E,E-hexa-2,4-diene in methanol gives 3R,6S-(33) of 39% optical p~rity;'~ the configurations of the product accord with preferred exo -addition to the diene which approaches (32) from the front. The reverse Diels-Alder reaction CN Me CI Me (32) (33) (34; arrows) has been used to prepare ~entatetraene,'~~ and fragmentation of (35) involving initial reverse Diels-Alder reaction (35 ; arrows) has provided evidence for the intermediacy of methyleneketen."' The FMO method suggests that electron- rich dienes will add to simple fulvenes in (6 + 4)7r fashion; this prediction has been verified16 by reaction of 1-diethylaminobutadiene with 6-phenylfulvene and dehyd- rogenation of the product to 4-phenylazulene (36).(34) (35) (36) Huisgen has considered in detail17" the case for concerted 1,3-dipolar addition and concludes that the large bulk of the data is in agreement with a ,4 + ,2 process. The observation that diazomethane adds to ethoxyethylene to give (37)17' agrees with a concerted process in which the dipole LUMO-dipolarophile HOMO interac-tion is dominant.' Synthetic applications of six-membered ring heteroaromatic 14 H. Nitsch and G. Kresze Angew Chem.Internat Edn. 1976 15 760. 15 (a) J. L. Ripoll J.C.S. Chem. Comm. 1976 235; (b) R. F. C. Brown F. W. Eastwood and G. L. McMullen J. Amer. Chem. SOC.,1976 98 7421. 16 L. C. Dunn Y. M. Chang and K. N. Houk J. Amer. Chem. SOC.,1976,98,7095. 17 (a)R. Huisgen J. Org. Chem. 1976 41 403; (b)R. A. Firestone ibid. p. 2212; (c)N. Dennis A. R. Katritzky and M. Ramaiah J.C.S. Perkin I 1976,2281 and following papers; (d)P.G. Sammes and R. A. Watt J.C.S. Chem. Comm. 1976.367; (e)W. Oppolzer and M. Petrzilka J. Amer. Chem. SOC.,1976,98 6722; (f) J. Leitich Angew. Chem. Internat. Edn. 1976,15 372; (g) G. Bianchi and D. Maggi J.C.S. Perkin (11),1976 1030. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations (37) betaines have been reviewed,2b and a series of papers describes the substantial contribution of Dennis Katritzky and their collaborators.17' Intramolecular 1,3- dipolar cycloadditions have also been reviewed,*' and a neat example involving addition of a simple olefin to a pyridinium betaine (38) (39) has been pr~vided."~ 0 (39) An elegant synthesis of (f)-l~ciduline'~' has as its key step regioselective intramolecular nitrone addition (40; arrows). The first 1,3-dipolar cycloaddition to a nitro-group in its electronic ground state has been achieved17f in the reaction of 2,E-cyclo-octa- 1,5-diene with p-cyanonitrobenzene giving (41). The high reactivity of E-cyclo-octene towards nitrile oxides may be due to pyramidalization of the olefinic carbon atoms [see (42)].'7g 3 Sigmatropic Reactions The only example18n of a predominantly antara 1,3-shift has been challenged.18' Kinetic data for the thermolysis of (43) were originally taken to indicate 65% rearrangement by 1,3-shift of C-3 to Car with antarafacial use of the ally1 system.The data can be reinterpreted in terms of as much as 100% suprafacial migration; rearrangement could involve 36% suprafacial migration with inversion at C-3 and 64% randomization of C-3 stereochemistry via the biradical (44)produced by ring-opening of (43) in a conrotatory-bevel sense [see arrows in (43)]. It is proposed that cleavage of cyclobutane derivatives generally involves simultaneous 18 (a)J. E. Baldwin and R. H. Fleming J.Amer. Chem. SOC.,1973,95,5261; (b)J. J. Gajewski ibid. 1976 98 5254; (c) H.A. Bampfield P. R. Brook and K. Hunt J.C.S. Chem. Comm. 1976 146; (d)J. A. Berson P. B. Dervan R.Malherbe and J. A. Jenkins J. Amer. Chem. Soc. 1976,98,5937; (e)G. D. Andrews and J. E. Baldwin ibid. p. 6705,6706; (f)J. A. Ross R. P. Seiders and D. M. Lemal ibid. p. 4325; (g) W. Thies and E. P. Seitz,J.C.S. Gem. Comm. 1976,846; (h) B. Franzus M. L. Scheinbaum D. L. Waters and H. B. Bowlin,J. Amer. Chem. Soc.,1976,98,1241; (i) J. F. Garst and C. D. Smith ibid.,p. 1526; (j) M. B. Rubin M. Weiner and H. D. Scharf ibid. p. 5699. D. W.Jones (431 (44) rotation (bevel) about the bond opposite the cleaving bond [C-l-C-4 in (43)]; this serves to minimize steric repulsions and maximize overlap between originally bonded atoms.18b The 1,3-shift with inversion at the migrating centre converting (45) into (46) proceeds with 99% retention of optical purity.18,' The rearrangement is thought to involve the chiral intermediate or TS (47) produced by a conrotatory- bevel ring-opening [see (45)] which gives maximum relief of steric strain.Suprafacial LUMO Bu' CN& ally]cation H Ph Ph H Ph Bu' HOMO Bu' 0 0 enolate (45) (46) (47) pathways are also dominant in the 1,3-rearrangement of 1,2-divinylcyclobutanes to vinylcyclohexenes e.g. (48;arrows),lgd and the vinylcyclopropane rearrangement of (49).Ige In both these cases the allowed suprafacial-inversion pathway is only slightly preferred to the forbidden suprafacial-retention route. It is pointed out18e that the experimental facts regarding vinylcyclopropane rearrangements are directly con- trary to the prediction of MIND0/3 calculations.It has been suggestedlSf that the very rapid 1,3-migration of sulphur in certain episulphoxides is a 'pseudopericyclic' process e.g. (50; arrows) in which there is an interchange of roles between bonding and non-bonding orbitals. Hydroboration (51; arrows) in which the vacant boron orbital switches roles with the C-H bond orbital is a related process. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations Following the discovery that the oxy-Cope rearrangement is enormously acceler- ated in related potassium alkoxides it has now been observedlgg that rearrangement of the potassium salt (52; R = K) is much more rapid than that of the silyl ether (52; R =SiMe,).The formal 1,3-shift (52; arrows) may be less likely than ring-cleavage to an ally1 anion and recombination by Michael addition; this would agree with the failure of 1-vinylcyclononanol to undergo ring-expansion. However a mechanism involving radical-ketyl pairs should be considered for this and a related rearrange- ment.18" Only 16% of the products of Wittig rearrangement of the anion (53)contain cyclopentylmethyl groups formed by cyclization of intermediate hex-5-enyl radicals. The remaining 84% of the reaction is intramolecular giving products containing the hex-5-enyl group. The intramolecular process was thought to differ from secondary recombination of alkyl radical and ketyl and the term 'radical-concerted' was suggested to describe it.18' Photodecarbonylation of (54) is preceded by 1,3-acyl shift (54; arrows) to a cyclobutane- 1,2-dione which on photolysis loses carbon monoxide.18' Ph2C-O-(CH2).+CH=CHZ 0 (52) (53) (54) The ~uggestion'~~ that Cope rearrangement of 2,5-diphenylhexa- 1,5-diene (55) involves an intermediate (56) receives support from MIND0/2 calculations.19' Although 2,5-diphenyl substitution in hexa- 1,5-diene markedly accelerates Cope rearrangement analogous substitution in homotropilidenes (57) and barbaralanes slightly slows rearrangement,'" suggesting that the rearrangement TS (or inter- mediate?) is not of type (58). On the other hand attempts to detect dipolar intermediates of type (59) in the Cope rearrangement of hexa-1,5-dienes with a (55) (56) (57) (58) (59) donor group at C-2 and an acceptor group at C-5 were fruitf~l.''~ Thus (60; R' = H R2=D) rearranges rapidly to (60; R' = D R2= H) and the putative dipolar inter- mediate (61) could be trapped with benzaldehyde or acrylonitrile.Evidence for a dipolar intermediate in the 3-~ulpha[3,3]-sigmatropic shift'" of (62) was also obtained.lgd In an intriguing transformation i.r. laser irradiation of (63) at 10.8 pm results in complete conversion into (64) which is transparent at 10.8pm.'9f 19 (a)M. J. S. Dewar and L. E. Wade J. Amer. Chem. Soc. 1973,95 290; (b)A. Kormornicki and J. W. McIver ibid. 1976,98 4553; (c)H. Kessler and W. Ott ibid. 1976,98 5014; A. Busch and H. M. R. Hoffmann Tetrahedron Letters 1976,2379; (d)R. Gompper and W. R.Ulrich Angew. Chem. Internat. Edn. 1976,15,299,301;(e)Fornomenclature see F. Vogtle and E. Goldschmidt Chem. Ber. 1976,109 1; (f)I. Glatt and A. Yogev J. Amer. Chem. SOC.,1976,98 7087. D. W.Jones 0 N -CO,Me k&' &D2 ~I ',@,Me COPh D D (60) (61) (62) (63) (64) There is increasing evidence that unsaturated groups undergo particularly easy 1,54gmatropy. Thus 3a-H-benzimidazoles e.g. (65) generated as reactive intermediates,"" are believed to undergo a sequence involving [131-butadienyl shift [1,5]-imidoyl shift and [1,5]-hydrogen shift (Scheme). Only under Scheme vigorous thermal conditions are the products of alternative [131- and [1,9]- sigmatropy of the ring-junction methyl of (65)observed.20a In both (65)and (66)"' migration of the unsaturated group to carbon rather than nitrogen is observed.However in the pyrazolenines (67) an acyl group (MeCO or C02Me) migrates to both nitrogen and carbon with the latter process being more important.20C The more rapid acetyl than methoxycarbonyl migration observed in this study accords with the relative migratory aptitudes of these groups in [1,5]-shifts in indenes and cyclohex- adienes;20d this supports the view that more rapid 1,5-migration of acetyl compared to vinyl is in part due to a secondary interaction between the diene HOMO and the T* orbital of the unsaturated group.20d The more rapid 1,Srearrangement of the formyl group compared to hydrogen suggests that formyl may replace hydrogen in other pericyclic processes; the benzoyl group which also migrates more rapidly than hydrogen in the 1,5-shift has already replaced hydrogen in a process (68; arrows) analogous to the ene-reaction.'Oe 20 (a)T.L. Gilchrist C. J. Moody and C. W. Rees J.C.S. Chem. Comm. 1976,414; (b)C. D. Anderson J. T. Sharp E. Stefaniuk and R. S. Strathdee TetruhedronLetters 1976,305; (c)M. Franck-Neumann and C. D. Buchecker ibid.,p. 2069; (d) D. J. Field D. W. Jones and G. Kneen J.C.S. Chem. Comm. 1976 873; (e) A. E. Baydar G.V. Boyd R. L. Monteil P. F. Lindley and M. M. Mahmoud ibid. p 650; (f) M. Kato M. Funakura M. Tsuji and T. Miwa ibid. p. 63; (g) R. F. Childs and C. V. Rogerson J. Amer. Chem. Soc. 1976,98,6391. 51 Reaction Mechanisms-Part (i) Orbital Symmetry Correlations In accord with orbital symmetry considerations there is inversion of the migrating carbon in the photochemical Berson-Willcott rearrangement of (69)which first gives the intermediate (70).20f Photo-rearrangement of the cation (71; R' =OH R2=H) to its isomer (71;R'=H R2=OH) can be viewed as 1,6-sigmatropy in the canonical form (72).20g Although rearrangements of both (69)and (72) proceed with inversion in accord with orbital symmetry control this is also the least-motion pathway.Meoh (LJ 0 d::;H-e Me H Me H ., \/ \\ -_-R2 R' OH (69) (70) (71) (72) 4 ElectrocyclicReactions Marked substituent effects on the rate of opening of cyclobutenones (73; arrows) to vinylketens parallel substituent effects on the opening of cyclobutenes to butadienes;21" introduction of an alkyl group at C-2 slows ring-opening whereas a strong acceleration attends replacement of alkyl groups at C-4 by phenyl groups.It appears that the TSs for ring-opening benefit more from phenyl conjugation than the products. Related vinyl participation may be important in the ring-closure of (74; R =vinyl) which is more rapid than closure of (74; R =Et) (AAH' =13 kJ mo1-').21b It was suggested by Epiotis that the barrier to forbidden pericyclic reactions is lowered by configuration interaction which becomes more important when one component of the pericyclic reaction carries electron-releasing groups and the other component carries electron-withdrawing groups. Support for this idea is provided by the observation2" that (75; R =C02Me) undergoes more rapid disrotatory cyc- lobutene ring-opening than (75; R =Me) (AAH* =35 kJ mol-'); similar substituent Me (a)R.Huisgen and H. Mayr J.C.S. Chem. Comm. 1976,55,57; (6)C. W. Spangler Terrahedron,1976 32,2681; (c)F. van Rantwijk and H. van Bekkum Tetrahedron Letters 1976,3341. D. W.Jones effects were observed for the rearrangement of Dewar-benzene derivatives. The thermal isomerization of 1-and 2-methylbicyclo[2,l,O]pent-2-enehas been the subject of a detailed mechanistic study22 which supports initial ring-opening to a chemically activated cyclopentadiene [Ann.Reports (B) 1975 72 561. Photochemical ring-closure involving an aromatic ring (76; arrows) may be involved in the biogenesis of apolignans. The parent compound (77) is formed by irradiation of (76).230In accord with the principle of least motion cyclohexadienes e.g.(78) in which the pseudo-axial disposition of the methyl group is preferred undergo photochemical opening to E-hexatrienes e.g. (79).236A new index for predicting the preferred mode of photochemical closure of substituted cycloheptat- rienes is derived from the stability gain of the FMOs. In several cases e.g. (80)+(81) the observed pathway is correctly ~redicted.~~' Disrotatory opening of the radical (82) to a cyclohexadienyl radical is slower than sigmatopic rearrangement to (83).24aNeither cyclization of (84)to (85)24bnor of (86)to (87)24cis consistent with orbital symmetry control. Reduction of cup-unsaturated ketones (Ac20 Zn HCI-Et20) to cyclopropyl acetates is believed to involve conrotatory cyclization of intermediate ally1 anions.24d In several cases the product stereochemistry is consis- tent with this mechanism e.g.(88)+(89). 22 W. E. Farneth M. B. D'Amore and J. L. Brauman J. Amer. Chem. SOC.,1976,9% 5546. 23 (a) H G.Heller and P. J. Strydom J.C.S. Chem. Comm. 1976,50;(b)P. Courtot and J. Y. Salaun ibid. p. 124; (c) T. Tezuka and 0.Kikuchi Tetrahedron Letters 1976 1125. Z4 (a)R. Sustmann and F. Lubbe J. Amer. Chem. Soc. 1976,98,6037; (6)G. A. Olah J. S. Staral and L. A. Paquette ibid. p. 1267; (c) R. E. Lehr J. M. Wilson J. W. Harder and P. T. Cohenour ibid. p. 4867; (d) C. W. Jefford and A. F. Boschung Helv. Chim Acfa. 1976,59 962. Reaction Mechanisms -Part (i) Orbital Symmetry Correlations

ISSN:0069-3030    

DOI:10.1039/OC9767300041

出版商:RSC    

年代:1976

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (ii) Polar Reactions By T. W. BENTLEY Department of Chemistry University College of Swansea Swansea SA2 8PP 1 Introduction There is a tendency to become increasingly confident about reaction mechanisms as time elapses after their publication. It is implicitly assumed that any reinterpreta- tions or criticisms would be published without delay. Unfortunately some mechan- isms may become ‘established’ without further experimental verification. In these circumstances critical periodic reassessments of accepted views are necessary. For the classical sN2 and E2 mechanisms ‘revolutionary’ reassessments have already been reported [Ann.Reports (B),1974,71,112], and counter-revolutionary reviews are discussed later (Sections 2 and 5).The ‘revolutionary’ views centre on the role of reactive intermediates in processes previously considered to be concerted. If the reactive intermediate decomposes to the starting material more rapidly than it proceeds to products there are important consequences in correlations of structure and reactivity. Relevant examples are discussed in Sections 2,5 and 7. Two related themes are the effect of increased pressure on reaction rates and products and the nature of mechanistic borderlines. Recent evidence that there is a merging between sN2and SNlmechanisms and between E2 and ElcB mechanisms is presented in Sections 2 and 5. Aspects of carbocations carbanions tetrahedral intermediates and catalysis are also reported but comprehensive annual coverage of the literature of organic reaction mechanisms is presented elsewhere.’ 2 Nucleophilic Substitution at Saturated Carbon The response to Sneen’s criticisms of the classical sN2 mechanism have been overwhelmingly unfavourable [Ann.Reports (B) 1974 71 1141 but preliminary reports of other unfavourable responses continue to be published. A detailed case for the concerted SN2mechanism includes the interesting historical perspective that in the 1930’sthere was considerable opposition to the postulation of ionic inter- mediates whereas now the existence of carbocations and ion pairs is well established. The conclusion was drawn that no currently available evidence demands the formation of intermediates for solvolyses of simple primary and secondary substrates (i.e.these reactions are classical SN2). Also there is no reason why intermediates should intervene in non-solvolytic reactions with stronger nucleophiles.* ‘Organic Reaction Mechanisms 1976’ ed. A. R. Butler and M. J. Perkins Wiley London 1977. D. J. McLennan Accounts Chem. Res. 1976,9 281. 54 Reaction Mechanisms-Part (ii) Polar Reactions The solvolysis of secondary substrates has been a controversial topic for many years partly because it is difficult to fit into the sN2-sN1 framework without ignor- ing evidence that other workers regard as vitally important. The above comments have neglected the evidence (e.g. from ‘80-scrambling in sulphonates and compari- sons of rates of racemization with rates of solvolysis) that ion pairs are formed during solvolytic reactions-but it could be argued that there is no direct evidence that the ion pairs undergoing racemization or 180-scrambling are also undergoing solvolysis.There is independent evidence from comparisons of solvolyses of 2-adamantyl tosylate (1) with other secondary tosylates that secondary solvolyses have varying degrees of sN2 character [Ann. Reports (B) 1974,71 1181. These two interpreta- tions can be reconciled if it is assumed that ion-pair intermediates are nucleophili- cally solvated [(2) where SOH = solvent and X =leaving group]. Furthermore it appears that there is a gradation or merging of mechanism and reactivity from 2-adamantyl to methyl tosylates because the dependence of the reaction rate on solvent ionizing power decreases proportionately as the dependence on solvent nucleophilicity increases.These and other results suggest that there is no clear borderline between sN1 and sN2 reactions and that rear-side nucleophilic assistance by solvent to heterolysis of the R-X bond depends on R X and the solvent. If as in (l) the rear-side is hindered the reaction is sN1 in all solvents examined. Other simple secondary solvolyses (2-propyl cyclohexyl etc.)appear to be SN1 in weakly nucleophilic media (trifluoroacetic acid hexafluoroisopropanol) but sN2 character increases as solvent nucleophilicity increases and/or as the stability of the incipient cation increase^.^ Consistent with this interpretation many secondary solvolyses are now known to proceed with essentially complete inversion of configuration.An alternative SN2 mechanism (Scheme l) in which the solvent attacks the non-nucleophilically solvated ion pair in a slow step requires that k2[SOH]<< k,; i.e. there is ‘hidden’ internal return from an ion-pair intermediate to covalent starting material. Evidence against appreciable hidden return has already been discussed k2~o~’~ ROS +HX SOH = solvent Scheme 1 [Ann.Reports (B),1974,71,116] and additional evidence has now been p~blished.~ The ion-pair SN2 mechanism (Scheme 1) does account for the behaviour of the tertiary allylic systems (3),4 and is not inconsistent with recent studies of other allylic T. W. Bentley and P. von R. Schleyer J. Amer. Chem. SOC.,1976,98,7658; F.L.Schadt T. W. Bentley and P.von R. Schleyer ibid.,p. 7667 and references there cited. * F. G. Bordwell and T. G.Mecca J. Amer. Chem. SOC.,1975,97,123,127;F.G.Bordwell P. F. Wiley and T. G. Mecca ibid.,p. 132. T.W.Bentley ArS02 H Me Me I I /\C=C/ \c/ Me c CH2 C H / \cH/ \ / NC/ H Me CH2 I\ Me Br (31 systems (4).5 Consequently it appears that Scheme 1 should be considered for substrates in which the incipient positive charge is stabilized (e.g. tertiary allylic benzylic) and/or the attack by the nucleophile is hindered sterically. Volumes of activation (A V,') can be determined from the pressure dependence of rate constants [equations (1) and (2)] a positive value of AV,' referring to an In (k,/k,) = a +bp +cp2 AV = -bRT increased 'volume' of the transition state; high-precision data are required to obtain values of A V,' within k0.3 cm3 mol-'.For sN1 reactions A V,' is -20 cm3 mol-' because the positive contributions to A V,' from bond cleavage is outweighed by the negative contribution from solvation of ionic charges. For the fragmentation of y-amino-alcohol derivatives (5) A V' is less negative by 5-10 cm3 mol-' than for the corresponding sN1 reaction of the carbon analogue (5; with CH replacing N). This I/ I/ X- / I I \ C-I\ / \ / \ C-I' (5) suggests that fragmentation may be concerted with heterolysis of the C-X bond which would explain the rate enhancements (up to lo4) observed in some rigid systems (cf the change in the inductive effect when CH is replaced by N should reduce the rate).6 Reaction of triphenylethyl tosylate (6)with sodium ethoxide in ethanol to give (7) is suppressed at high pressures (5500 atm) when the major product is the rearranged ether (8).Interestingly the yield of the rearranged alkene (9) does not increase at OEt EtO-EtOH I Ph3C-CH20Ts Ph3C-CHZOEt + PhZC-CHzPh + PhZC=CHPh (6) (7) (8) (9) high pressure^.^ For the thermal decomposition of optically active N-nitroso- amides the yield of ester of retained configuration increases slightly (59-62%) at high pressures possibly because racemization is hindered by the increased viscosity of the solvent.!-? Since solvation effects appear to dominate A V,'for polar reactions in solution predictions of the effect of pressure are difficult but for some reactions K.B. Astin and M. C. Whiting J.C.S. Perkin ZZ 1976 1160. 6 W. J. le Noble H. Guggisberg T. Asano L. Cho and C. A. Grob J. Amer. Chem. Soc. 1976,98,920. K. K. Lee and Y. Okamoto J. Org. Chem. 1976,41 1552. W. J. le Noble E. H. White and P. M. Dzadzic J. Amer. Chem. Soc. 1976,9%,4020. Reaction Mechanisms-Part (ii) Polar Reactions pressure variation may be a useful empirical way to aid in optimizing yields of desired products. 3 Vinyl Cations Vinyl cations or ion pairs can be produced during substitution reactions of vinyl substrates or by additions to alkynes. Substitution.-The low reactivity of vinylic substrates is often attributed to conjuga- tion between the leaving group X and the double bond (lo),but there is no satisfactory evidence to support this proposal.’ Rather than a stabilized ground state (lo) the low reactivity may be due to a destabilized transition state reflecting the lower stability of vinyl cations and the hindrance to solvation of the developing cationic centre (this also accounts for the tendency towards SN1rather than sN2 reactions).R RR R (10) The electronic effects of p-substituents can be transmitted by the double bond the developing charge may be delocalized and the rates of SNl vinyl solvolyses are less sensitive to solvent ionizing power than expected by comparison with SN1reactions of saturated systems. Also competing reactions such as elimination and substitution by addition-elimination pathways may occur. A sensitive test for competing path- ways is the solvent isotope ef’fect (kRCO2H/kRCo2D); for many vinylic sN1 reactions values are in the range 0.85-1.2 whereas electrophilic addition-elimination path- ways show much higher values; e.g.solvolysis of the tosylate (11) shows kAcOH/kAaD = 0.93 whereas the less reactive substrates (12) show higher values (X = Br 1.45; X = C1 1.94; X = OAc 3.45) due to increasing proportions of the addition-elimination mechanism.’ Ph Ph An H \/ \c=c / TsO/c=c\ph X’ ‘H (11) (12)An = p-MeOC6H4 Much of the classical research on ion pairs investigated acetolysis reactions using the non-nucleophilic perchlorate ion to study the importance of external ion pair return. A rate increase attributed to the ‘special salt effect’ is observed if perchlorate exchanges with the nucleophilic anion [X-; equation (3)] and return to starting RX S R+X-$ R+I(X-$ RfllC104-+ product (3) material is prevented.There is considerable evidence (e.g. from common-ion rate depression) that free vinyl cations are often the product-forming intermediates during acetolysis of a-anisylvinyl halide^'.'^ [see also Ann. Reports (B) 1975,72 Z. Rappoport Accounts Chem. Res. 1976,9 265. lo Z. Rappoport I. Schnabel and P. Greenzaid J. Amer. Chem. SOC.,1976,98,7726. T. W.Bentley 741. By changing to pivalic acid Me3CC02H a solvent of even lower dielectric constant than acetic acid it was possible to study reactions of less dissociated intermediates. Pivalolysis of (13) is accompanied by a much faster isomerization (>lOO-fold) to the trans-isomer (14) and gives a 1:1 mixture of cis-and trans- (13) (14) pivalates.Addition of 0.01M-LiC10 caused only a 1.5-fold increase in the isomeri- zation rate but there was a much higher increase in the solvolysis rate. As rate depression by added bromide ion was not observed the results are consistent with isomerization (13)+ (14) uiu the contact ion pair (R'X-) and solvolysis products formed uia the solvent-separated ion pair (R+l\X-).lo Other mechanistic evidence has been obtained from 1,2-aryl rearrangements. The degenerate anisyl rearrangement of cations (16) and (17) studied by deuteriation of An* An An* An \/ \+ +/ An'c=c -b C=C-An An*-C=C / \Br An 'An (15) (16) (17) one methoxy-group depends on the solvent 100% in CF3CH,0H 35% in AcOH 11.5% in 60% aqueous ethanol and 0% in pivalic acid.As the process appears to be favoured most in the solvent of highest dissociating power and lowest nucleophilicity rearrangement may occur in free dissociated vinyl cations." Similar results were obtained by generating the cations from trianisylvinyl[ 2-14C]phenyltriazene in acetic acid and rearrangement was reduced by the addition of sodium acetate; presumably the lifetime of the cation is reduced when acetate ions are present.12 Addition.-The n.m.r. spectra of some stabilized vinyl cations have been reported.13 but simpler vinyl cations continue to be elusive. Treatment of several alkynes with FS03H-SbF5 at -78 "C led to complex mixtures of unidentified products and at higher temperatures allyl cations are produced in high yields.As reaction of the alkyne (18) with FS03D-SbF5 produced equal amounts of the allyl cations (20)and (21) the intermediate vinyl cation (19) is probably inv01ved.l~ Addition of but-2-H H &.A /" + Me-C C I I I I Me H Me D 11 Y. Houminer E. Noy and Z. Rappoport J. Amer. Chem. SOC. 1976,98,5632; see also Z. Rappoport E. Noy and Y. Houminer ibid. p. 2238. 12 C. C. Lee and E. C. F. KO,Canad. J. Chem. 1976,54 3041; C. C. Lee and M. Oka ibid. p. 604. 13 Ann. Reports (B),1975.12 73; for a review see M. Hanack Accounts Chem. Res. 1976,9 364. 14 G. A. Olah and H. Mayr J. Amer. Chem. SOC. 1976,98,7333. Reaction Mechanisms-Part (ii) Polar Reactions yne to a solution of [2H,]-t-butyl chloride and SbF in SO2at -78 “C produced the ally1 cation (24) presumably via the vinyl cations (22) and (23).” Even in the much Me Me MeCZCMe +(CD&C’ \ + + C=C -Me + (CD&C-C=C/ + (CD3)3C / \Me (22) (23) I CD (24) less polar solvent methylene chloride addition of [2H2]benzyl chloride to 1,3-diphenylpropyne (25) yields equal amounts of the isomers (27) and (28),16 but it was still proposed that the dissociated vinyl cation (26)was the reactive intermediate [cf.Ann. Reports (B),1974 71,1281. PhCD2Cl \+ C=C-Ph +C1- CH2C12 [PhD2C 1 PhCHZCr CPh % PhH2C/ PhD2C C1 PhDzC Ph \c=c’ + \c=c / PhH2C/ ‘Ph PhH2C/ \c1 4 Other Carbocations &I.-From the gas-phase heats of reaction for cation formation (R-Cl+ R’+ Cl-) it appears that formation of the phenyl cation C6H5+is almost as difficult as formaticn of the methyl cation and significantly more difficult than formation of vinyl or ethyl cations.Consistent with these calculations solvolysis of aryl trifluoromethanesulphonates under extremely vigorous conditions leads to nuc- leophilic attack on sulphur rather than heterolysis of the C-0 bond.” Phenyl cations appear to be intermediates in the reactions of benzenediazonium salts. Thermal decomposition of p -‘SN-labelled benzenediazonium tetrafluorobo- 15 G. Capoui V. Lucchini F. Marcuzzi and G. Melloni Tetrahedron Letters 1976 717. 16 F. Marcuzzi and G. Melloni J. Amer. Chem. SOC.,1976,98 3295; see also F. Marcuzzi and G. Melloni J.C.S. Perkin IZ,1976 1517. 17 A.Streitwieser jun. and A. Dafforn Tetrahedron Letters 1976 1435;see also L. R. Subramanian M. Hanack L. W. K. Chang M.A. Imhoff P. von R. Schleyer and F. Effenberger W. Kurz P. J. Stang and T. E. Dueber J. Org. Chem. 1976,41,4099. T. W.Bentley rate in CF3CH20H proceeds with -8% isotopic rearrangement to a-"N-labelled material. When the reaction was carried out under 300 atm of unlabelled nitrogen the 'external' nitrogen was incorporated into the benzenediazonium ion. These results suggest that the phenyl cation can react reversibly with molecular nitrogen [equation (4)];evidence against alternative mechanisms not requiring a phenyl cation intermediate was summarized. l8 C~HSN~+BF~-$ C6H5++N2 +BF4-(4) Aliphatic.-A solution of the s-butyl cation has been prepared quantitatively by the slow addition of a mixture of SbF and S0,ClF to s-butyl chloride in S0,ClF below -100 "C.An exothermic rearrangement to the t-butyl cation occurred above -60 "C and the enthalpy of rearrangement (14.5f0.5 kcal mol-' determined by dynamic calorimetry) is remarkably similar to values obtained in the gas phase (15-17 kcal mol-'). This suggests that the degree of electrostatic solvation varies little between similar carbocations in contrast to the behaviour of ammonium and alkoxide ions which are capable of strong hydrogen-bonding or ion-pairing interac- tions." Other detailed descriptions of procedures for obtaining concentrated (1 moll-') solutions of cations in SbF,-S02CIF should reduce the difficulties of beginning independent research in this area." In many reactions catalysed by aluminium chloride it is assumed that cations are produced.Earlier research indicated that weak donor-acceptor complexes were formed between alkyl chlorides and A1Cl3 but it has now been shown that t-butyl chloride reacts with excess of A1Cl3 in anhydrous liquid hydrogen chloride at 25 "C to produce the t-butyl cation quantitatively [equation (5)]; the anion [A1,C17]- was characterized by its Raman spectrum. Removal of the solvent at -1 12 "C gave a solid which decomposed at ca. -50 "C.'l Me3CCI+A12C16 G= [Me&'] [A12Cl,]-(5 1 The cyclobutenes (29) readily react with SbF in SO2at -78 to -10 "C to produce an equilibrating pair of cyclobutenyl cations (30) and (31); the barrier to rotation of the methoxy-group is 16 kcal mol-' for R = F C1 or MeO.Evaporation of the solvent in an inert atmosphere produces crystalline solid salts stable at room temperature and useful in syntheses as alkylating agents in electrophilic substitution reactions.22 F A (29) (30) (31) 18 R. G. Bergstrom R. G. M. Landells G. H. Wahl jun. and H. Zollinger J. Amer. Chem. SOC. 1976,98 3301; see also C. G. Swain J. E. Sheats and K. G. Harbison ibid. 1975,97 783. 19 E. W. Bittner E. M. Arnett and M. Saunders J. Amer. Chem. SOC.,1976 98 3734; see also T. S. Sorensen Accounts Chem. Res. 1976,9 257. 20 D. P. Kelly and H. C. Brown Austral. J. Chem. 1976,29,957; see also H. Hart and M. Kuzuya J. Amer. Chem. SOC. 1974,% 6436. 21 F. Kalchschmid and E.Mayer Angew. Chem. Internat. Edn. 1976 15 773. 22 B. E. Smart and G. S. Reddy J. Amer. Chem. SOC. 1976,98 5593. Reaction Mechanisms-Part (ii) Polar Reactions Further studies of the remarkable rearrangements of the cation (32) have appeared [see also Ann. Reports (B) 1973,70,219]. Above -60 "C (32)rearranges to (33) possibly via cyclopropylcarbinyl-like intermediates. On standing in deuteriotrifluoroacetic acid all nine methyl groups of the cation (33) become labelled but at different 8.4 & b5 1-2 (33) 5 Elimination Some of the possible mechanisms for E2 eliminations are shown in Scheme 2. In step (a) a carbanion the conjugate base (cB) of the starting material is produced irreversibly (I) in an (ElcB) process which would be first-order in base (B) and first-order in starting material.The classical concerted E2 process is shown in step IS la BH++-c-c-x IS la (c) \/ B+H-C-C-X -B BH++ C=C +X-II /\ B I 01 B +H-C-C+X-II Scheme 2 (c) and the ion-pair E2 mechanism is shown in steps (d) and (e). Included in the classical E2 mechanism are processes in which significant positive charge develops on the a-carbon and/or significant negative charge develops on the P-carbon. Thus a key mechanistic question is whether E2 reactions are concerted and if not whether the reactive intermediate is a carbanion or a carbocation. In the classical E2 mechanism the P-H is removed as a proton and electron release for the C-H bond can provide nucleophilic assistance to ionization of the C-X bond.Also SN2 and E2 processes often occur concurrently. Thus the earlier discussion (Section 2) of the importance of nucleophilic assistance to ion-pair formation is relevant and the ion-pair E2 mechanism seems unlikely except in substrates where the ion-pair sN2 mechanism cannot be excluded e.g. if the positive charge is stabilized (benzylic allylic or tertiary substrates) or if the base/nucleophile 23 H.Hart and M.Kuzuya J. Amer. Chem. Soc. 1976,98 1545. T. W.Bentley B is weak as in the solvolysis of t-butyl halides in 97% trifluoroethanol-water.24 Even then it is not clear whether the base attacks at the a-carbon the 6-hydrogen or both. In a defence of the classical E2 mechanism it was concluded that there was no unequivocal case of the ion-pair E2 mechanism but that the two mechanisms were very difficult to disting~ish.~~ Evidence for the (ElcB) mechanism is becoming more ~ubstantial~~ [see also Ann.Reports (B) 1975,72,78] particularly in activated systems similar to those in which a carbanion intermediate is known from isotopic exchange experiments to be formed reversibly i.e.(ElcB)R Scheme 2 with the reverse of step (a)faster than step (b). The concerted E2 mechanism can be established if it can be shown that the reaction proceeds faster than expected on the basis of a stepwise mechanism (see below). Other criteria such as the extent of bond cleavage to the leaving group and the extent of proton transfer to the base are more arbitrary. Activation volumes have been proposed as a measure of the E2-ElcB character of a reaction.Processes in which there is more bond-making between an anionic base (B-) and the H-C bond than bond-breaking to the leaving group should have negative volume changes (-10 cm3 mol-I) provided that there is no appreciable change in the volume of electrostricted solvent; this limits the method to negatively charged bases reacting with neutral substrates. The (ElcB)R mechanism is charac- terized by bond-breaking and appears to correspond to positive activation volumes (CQ. + 10 cm3 mol-I). As might be expected considerably negative charge is developed during eliminations from the p -sulphonyl compound (34; Z = Cl) and the activation volume is only -1f1cm3 mol-'; various other examples were also reported.26 The results support the interpretation that there is a merging of mechanism between E2 and (E1cB)R mechanisms but this approach does not enable the (ElcB)I mechanism to be distinguished from a classical E2 mechanism with a carbanion-like transition state.PhS02 \ CH-CH2Z -PhS02CH=CH2+ AcOH + Z-/ ACO-+H' (34) (35) The distinction can be made from experimentally determined ionization rates by extrapolations using linear free-energy relationships [see also Ann. Reports (B) 1975,72,78]. For a series of sulphones (34) (Z = Me H Ph NMe2 CH2S02Ph OH OMe OEt SPh or OPh) the observed rate constants for detritiation were corre- lated with the Taft c7"value for the CH2Z group. Correction of these results for the kinetic isotope effect (kH/kT= 7.1) and extrapolation of the correlation line allowed prediction of the ionization rates for other leaving groups.For Z = F C1 OTs or OAc the predicted ionization rates were close (within a factor of 2.1) to the observed elimination rates consistent with an (ElcB)I mechanism. For Z=Br or I the observed elimination rates were over ten times greater than predicted supporting *4 V. J. Shiner jun. W. Dowd R. D. Fisher S. R. Hartshorn M. A. Kessick L. Milakofsky and M. W. Rapp J. Amer. Chem. Soc. 1969,91,4838;D. J. Raber R. C. Bingharn J. M. Harris J. L. Fry and P. von R. Schleyer ibid.,1970,92 5977. 2s W. H. Saunders jun. Accounts Chem. Res. 1976,9 19. *' K. R. Brower M. Mushin and H. E. Brower J. Amer. Chem. SOC.,1976,98,779. Reaction Mechanisms-Part (ii)Polar Reactions 63 the concerted E2 mechanism.Also the reactions classified as concerted showed higher kinetic isotope effects than the reactions classified as (ElcB)I. The authors concluded that these results ‘contra-indicate a mechanistic discontinuity’ at the E2-ElcB b~rderline.~’ Another elimination close to the E2-ElcB borderline is the NaOMe-MeOH- induced dehydrochlorination of substrates in the series (p-YC,H4),CHCHCl2. When Y =C1 H or OMe the small leaving group kinetic isotope effect is consistent with an E2 process. When Y = NO2 no kinetic isotope effect is observed and the rate constant is as predicted from the pK of the substrate consistent with an ElcB mechanism.28 The temperature dependence of kinetic isotope effects is attracting increasing attention.For hydrogen/deuterium primary kinetic isotope effects the ratio of Arrhenius pre-exponential factors AH/AD,is ‘normally’ close to unity. If proton tunnelling occurs A H/A is less than unity and the difference in activation energies ED-EH,exceeds the ‘normal’ values of -1kcal mol-’ e.g. in the Crvr oxidation of di-t-b~tylcarbinol.~’ Anomalous activation parameters with ratios A H/A up to 4.8 and with ED-EH,have been obtained for substrates related to (36). It was proposed that the system may be finely balanced with internal return (Ll)competing favourably with the forward reaction (k2)in Scheme 3.30 Br Br Br I kl -/ k2 I Arc-CF2Br g Arc-CF2Br 3 ArC=CF2 +Br-I k-1 H AOR +HOR +-OR Scheme 3 6 Carbanions Solutions of many carbanions have been examined by spectrophotometry or by conductance measurements and it is known that various ion pairs or free ions can be formed depending on the counterion (Li+ Na’ K+ etc.),the solvent the concentra- tion of carbanion and the temperature.Also different ion pairs from the same carbanion can react at markedly different rates and sometimes can lead to different products. As many polymerization processes and laboratory syntheses utilize car- banion reactions an improved understanding of the behaviour of carbanions in solution would assist in the prediction of optimum conditions needed to carry out particular reactions. This approach is applicable to other areas of reaction mechan- isms e.g. after too many years of discussion about various fanciful gas-phase transition states for LiAlH4 reduction of ketones some progress has recently been made towards understanding the nature of the species present in solutions of complex metal hydrides in ethereal 27 P.J. Thomas and C. J. M. Stirling J.C.S. Chem. Comm. 1976 829. 28 A. Grout D. J. McLennan and I. H. Spackman J.C.S. Chem. Comm. 1976,775. 29 H. Kwart and J. H. Nickle J. Amer. Chem. SOC.,1976 98 2881. 30 H. F. Koch D. B. Dahlberg M. F. McEntee and C. J. Klecha J. Amer. Chem. Soc. 1976 98 1060. 31 E. C. Ashby F. R. Dobbs and H. P. Hopkins jun. J. Amer. Chem. SOC.,1975.97 3158. T. W. Bentley For protonation of the ion pairs of the 1,3-diphenylallyl carbanion (37) by fluorene (38;R = H) with lithium as the counterion the loose ion pair in tetrahydrofuran reacts about 100 times faster than the tight (contact) ion pair in 2,5-dimethyltetrahydrofuran.Also reaction of fluorene with the sodium tight ion pair is about 30 000times faster than with the lithium tight ion pair in the same Equilibrium constants for the addition of the 9-methylfluorenyl carbanion (39; R = Me) to l-phenyl-l-(4-pyridyl)ethene (4PPE) are markedly dependent on the PhPh * + ---* Ph-Ph + M' / R R Mt (37) (38) (39) counterion. In tetrahydropyran KLi+[equation (6)] exceeds Kc,+ by a factor of 1.6x 10'. Generally K decreases with increasing cationic radius and increasing cation solvation. The results can be explained at least partially by the stability of tight FPPE-Li' ion pairs and indicate that ion-pairing effects are potentially very large when the newly generated carbanion differs considerably from the reacting carbanion in charge distrib~tion.~~ 9-MeF-M+ +4PPE FPPE- M+ (6) (39; R = Me) Michael additions and autoxidations of sodium salts of 9-substituted fluorenes (39) (R = CN C02Me or S02Ph) in t-butyl alcohol illustrate the differing reactivities of free and associated carbanions which are evaluated from the kinetic effects of dilution and addition of common-ion salts (e.g.sodium perchlorate). For the addition of the carbanion of 9-cyanofluorene (39; R = CN) to methyl methacrylate no common-ion effect was detected suggesting that free and paired ions were equally reactive. In all other cases examined [additions of (39; R = C02Me) and (39; R = S0,Ph) to methyl methacrylate and additions to methyl acrylate and methyl crotonate] the free ion was 10-100 times more reactive than the ion pair.As the rate-determining step in the autoxidation of carbanions of 9-substituted fluorenes is thought to be electron transfer from the carbanion to a ground-state oxygen molecule differences from Michael additions would be expected. For autoxidations in t-butyl alcohol the kinetic effects of added salts are small and difficult to distinguish from a medium effect. Addition of dimethyl sulphoxide changes the solvating properties ofthe medium and increases the proportion of free carbanions. It appears that (39; R = S02Ph) is autoxidized eight times faster as the free ion than as the ion pair but the ion pair of (39; R =CN)is five times more reactive than the free Autoxidation of the phenol (40) using Bu'O-02 in t-butyl alcohol at 75°C produces the cyclopentadienone (43).At lower temperatures intermediates (4 1)and (42) can be isolated and a partial mechanism is outlined in Scheme 4.35 32 G. C. Greenacre and R. N. Young J.C.S. Perkin IZ 1976 1636. 33 C. J. Chang and T. E. Hogen-Esch Tetrahedron Letters 1976 323. 34 D. Bethell C. S. Fairclough R. J. E. Talbot and R. G. Wilkinson J.C.S. Perkin ZZ 1976 55. 35 A. Nishinaga and A. Rieker J. Amer. Chem. SOC.,1976,98,4667. Reaction Mechanisms-Part (ii) Polar Reactions Ar Al- Ar (40) (41) (42) 75 T RU'O-B~'OH1 0 Scheme4 The pKa for production of the t-butyl carbanion from isobutane has been determined from a thermodynamic cycle [equation (7)].Relative to triphenyl- R-H -+ R'+H' 3 R-+H' -+ R-+H' (7) methane (pK, 3 1.5)and using a fast technique (second-harmonic a.c. voltammetry) to determine the reversible potential for reduction of R-to R- a value of 70.7 for the pKa of isobutane was calc~lated.~~ Previous estimates of the pKa's of simple saturated hydrocarbons have ranged from 42 to 85 but none has involved a thermodynamic method. 7 Tetrahedral Intermediates The rate-determining step in the hydrolysis of a-acetoxy-a-methoxytoluene (44; X = H) in acid solution appears to be the decomposition of the hemiacetal(45); the reaction (Scheme 5)is general acid and general base catalysed and the rate constants /OMe -0COCH2X /OMe PhCH PhCH=6Me -+ PhCH -+ PhCHO 'OCOCH2X \OH (44) (45) Scheme 5 for (44;X = H) and (44; X =Cl) are identical within experimental error.37 At the next higher oxidation level the analogous decomposition of acetoxydimethox-ymethane (46) at -35 "C in a mixture of 12H6]acetone and deuterium oxide to the tetrahedral intermediate (47) (Scheme 6) was observed by n.m.r.The build-up of 36 R. Breslow and R. Goodin J. Amer. Chem. SOC.,1976 98 6076; M. R. Wasielewski and R. Breslow ibid. p. 4222. 37 B. Capon K. Nimmo and G. L. Reid J.C.S. Chem. Comm. 1976,871. T. W.Bentley OMe OMe OMe / / H-C / 4 H-C + H-C 1 1 \OMe \OMe \O OCOMe OD (46) (47) Scheme 6 (47) reached a maximum after 20 min when it accounted for 75% of the starting material and at the same time (47)slowly decomposed to methyl formate.These results show that at least one simple tetrahedral intermediate can be generated by hydrolysis of its 0-acetate and it may be possible to generate related species by similar processes.38 In continuation of a study of the steric and electronic effects in additions to aldehydes and ketones the rates and equilibria for hydration of and bisulphite addition to 1,3-dimethoxyacetone (48) have been investigated. The equilibrium constant (K)for formation of the bisulphite adduct (49) is high (880 1 mol-') and the pK for the hydroxylic hydrogen atom of (49) is 9.89. Decomposition of (49)at 25 "C over the pH range 4.5-8.5 may involve rate-determining decomposition of the corresponding dianion.The rate-constants for additions of hydroxide and bisulphite to (48) were smaller than analogous rate constants for reactions of aldehydes having K values similar to that of (48). This suggests that for these nucleophilic additions to both aldehydes and ketones steric hindrance is greater in the transition states than in the MeOH2C MeOH2C OH K \C=O +NaHS03 $ \C/ MeOH2C/ MeOH2C' \S03-Na' (48) (49) Base-catalysed hydrolysis of esters of aliphatic carboxylic acids is generally accepted to occur via the tetrahedral intermediate (50) and the observed rate constant (/cobs) for this mechanism (Scheme 7) is given by equation (8). The extent of OH -kl k2 R'C02R2 +OH $ R'-d-OR2 + R'CO,H+ OR2 k-1 I 0-(50) Scheme 7 kobs = kl/k2/&1+ k2) (8) reversibility (k-l/kz) depends on R2; when -OR2 is the very stable p-nitrophenoxide l~-~/k, is essentially zero and kobs= kl.The volumes of activation 38 B. Capon J. H. Gall and D. McL. A. Grieve J.C.S. Chern. Comm. 1976 1034. 39 J. Hine L. R. Green P. C. Meng jun. and V. Thiagarajan J. Org. Chem. 1976,41 3343. Reaction Mechanisms-Part (ii) Polar Reactions 67 for p-nitrophenyl acetate propionate dimethylacetate and trimethylacetate are -3 -4,-4,and -10 cm3 mol-' respectively and these values were thought to reflect the volume change on formation of the transition states leading to the tetrahedral intermediate (50). Comparison of these results with those for esters with k-Jk Z0 shows that the ratio k-Jk2 decreases with increasing pressure possibly because R'C02H is more highly solvated than R'C02R2.40The analogous process for hydrolysis of amides is inefficient because -NR2 is a poorer leaving group than -OH and reversion of the tetrahedral intermediate to starting material is preferred.For the hydrolysis of tertiary amides addition of strong base (Bu'O-) produces a dianion which readily ejects -NR2 in a synthetically useful process.41 Further evidence for an ElcB mechanism for ester hydrolysis has been obtained [see also Ann. Reports (B) 1969 66 701. The hydrolysis of monoesters of malonic acid (51) but not of dialkylmalonic acids with good (nitrophenolate) leaving groups is catalysed by low concentrations of general acids and general bases more efficiently than expected.The catalysis disappears at high concentrations of catalyst. Exchange of deuterium from solvent D20 into the methylene group of monoethyl malonate supports the proposal that hydrolysis occurs via the dianion (52) (Scheme 8). This mechanism is products Scheme 8 not observed with p-nitrophenylacetate so there must be a small but decisive stabilizing effect of the C02-on the adjacent carbanion. The dianion (52) may also be an intermediate in certain Knoevenagel reactions.42 Schiff's bases are intermediates in the catalytic mechanism of several enzymes and the kinetics and mechanism of both their formation and their decomposition have been studied. It appears that hydrolysis to the corresponding aldehyde or ketone is subject to general acid-base catalysis but the reaction rates are lower than those observed for enzymic reactions.The rate-determining step for the hydrolysis of (53) is as shown in Scheme 9. As the transition state for attack by water (54)+ (55) NCH2CF HNCH,CF H20 HNCH,CF ZfJ 35fJf-& -I-CF,CH,NH, (53) (54) (55) Scheme 9 R. C. Neuman jun. G. D. Lockyer jun. and J. Marin J. Amer. Chern. SOC.,1976,98,6975. 41 P. G. Gassman P. K. G. Hodgson and R. J. Balchunis J. Amer. Chem. SOC.,1976.98 1275. 42 A. J. Kirby and G. J. Lloyd J.C.S. Perkin If 1976 1762. T. W.Bentley probably involves delocalization of charge it is not surprising that the reaction rate is accelerated in less polar media e.g.reaction in 90% dioxan-water is 18 times faster than in pure water corresponding to a 180-fold change in the presumed second- order rate constant for attack by water.There is an even greater increase in the rate of the general base-catalysed process in less polar media presumably because the charge in the transition state is partially neutralized as well as delocalized. Thus it was proposed that the enzymic processes may utilize a combination of general base catalysis and an apolar active site to facilitate the hydrolysis of Schiff’s bases.43 The question of concertedness in general acid-base catalysis has received more attention [see also Ann. Reports (B) 1975 72 801. Earlier results indicating the possibility of concerted acid- and base-catalysed enolization of oxaloacetic acid (57) with tertiary amine buffers have been checked and extended and an alternative mechanism involving carbinolamines (Scheme 10) has been proposed.Plots of observed rate constants (kobs) versus total amine concentrations (NT including protonated amine) exhibit a break kobs becoming first-order in NT at its higher values. The results suggested that oxaloacetic acid and the tertiary amine form an intermediate (58) which yields the enol(60) on reaction with additional amine and that decomposition of the carbinolamines (58) and (59) is rate-determining at high amine con~entrations.~~ This appears to be the first report of nucleophilic catalysis as a mechanism for enolization. 0 0-OH I1 kl I + H+ I H02CCCHZC02H + N $ -02CCCH2C02-02CCCH2C02-k-1 I -H+ I N+ N+ /I\ /I\ (57) (58) (59) 1 1 8 Neighbouring Group Participation The rates and equilibria of many ring-closure reactions are enhanced by increasing alkyl substitution on the backbone of the ring.Very large rate enhancements for the acid-catalysed lactonization of hydrocoumarinic acids have been reported [e.g. (62) -+ (63) is -lo1’ times faster than (64) -+ (65)] which were attributed to ‘stereopopulation control’ in which restriction of rotational freedom leads to a narrow distribution of conformational populations ideally by removing non-productive conformer^.^^ By comparison with their bimolecular counterparts the 43 R. M. Pollack and M. Brault J.Amer. Chem. SOC.,1976,98,247; see also R.M. Pollack and R. H. Kayser ibid. p. 4174. 44 P. Y. Bruice and T. C. Bruice J.Amer. Chem. SOC.,1976,98 844. 45 S.Milstien and L. A. Cohen J. Amer. Chem. SOC.,1972,94 9158. Reaction Mechanisms-Part (ii) Polar Reactions Me Me (62) (63) (64) (65) rate enhancements correspond to factors of about of similar magnitude to the accelerations observed for some enzyme-catalysed processes. Whereas the possibil- ity that the origin of these rate enhancements is relief of ground-state strain was considered as a minor factor,45 a case has now been made that this is the dominant factor. If the change in ground-state energy in lactonization [(62)+ (63) and (64)+ (65)] can be modelled by [(66)-P (67) and (68)+ (69)] respectively steric Me Me B H&Me M?6Me M P \ \ \ \ H&H accelerations in the range 1012-10’4 can be expected.This ‘rough estimate’ was supported by detailed empirical force-field calculations for (62) (64) and related Comparison with other model compounds led to the suggestion that conformational restrictions may account for a factor of -lo4in rate:’ which is more in line with earlier proposals. There has also been further criticism of the concept of ‘orbital steering’. From molecular orbital calculations of potential energy surfaces for attack of nucleophiles on carbonyl compounds [F- + FCHO HO- + FCHO MeO-+ H,NCHO NH3+ HCHO MeOH + HC02H and MeOH + HC(OH),’] it appears that the potential energy surfaces in the region of the transition states are very flat in those directions corresponding to deformation of the incipient bond. Thus the ‘loose’ transition states are predicted to be relatively insensitive to the orientation of the incoming nucleophile and the carbonyl group.48 Hydrolysis of monoaryl malonate anions is subject to intramolecular general base catalysis by the ionized carboxy-group (71)and appears to be relatively insensitive to structural variations (R).Limiting effective concentrations of 100moll-’ were calculated in contrast to the value of -10’ moll-’ for intramolecular nucleophilic catalysis (70) uncomplicated by strain. It was noted that the efficient intramolecular reactions involve bond formation or cleavage between heavy-atom centres (C 0,N P etc.) and that base catalysis (71) may have a less favourable entr~py.~’ R. E. Winans and C. F. Wilcox jun. J. Amer. Chem. SOC.,1976,98,4281.O7 C. Danforth A. W. Nicholson J. C. James and G. M. Loudon J. Amer. Chem. SOC.,1976,98,4275. O8 S. Scheiner W. N. Lipscomb and D. A. Kleier J. Amer. Chem. SOC.,1976 98 4770. O9 A. J. Kirby and G.J. Lloyd J.C.S. Perkin II,1976,1753;see also T. C. Bruice Ann. Rev. Eiochem. 1976 45. 352. T. W.Bentley The pH-rate profile for cyclization of methyl 2-aminomethylbenzoate (72) to give (74) changes slope at pH 8.6 probably because of protonation of the amine.50 The Brmsted coefficient @ for a series of general base catalysts is 1.O and there is a large solvent isotope effect (kH20/kD20= 2.8) consistent with rate-determining proton transfer [e.g. (72) -+ (73)]. Possible alternative mechanisms were discussed and it was noted that intramolecular aminolysis of (72) is different in several respects from bimolecular aminolysis reactions [see also Ann.Reports (B),1974 71 1311. 50 T. H. Fife and B. R. DeMark J. Amer. Chem. Soc. 1976,98,6978; see also A. J. Kirby and G. J. Lloyd J.C.S. Perkin II 1976 1748.

ISSN:0069-3030    

DOI:10.1039/OC9767300054

出版商:RSC    

年代:1976

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (iii) Electron Spin Resonance Spectroscopy and Free Radical Reactions By A. T. BULLOCK Department of Chemistry University of Aberdeen Old Aberdeen Scotland A89 2UE This yekr’s Report is restricted to two topics namely kinetic studies and chemically induced electron polarization. The year saw the publication of Volume 3 of ‘Electron Spin Resonance’.’ Whilst the standard is excellent throughout of particu- lar relevance to this Report are Chapter 9 by Sealy and that part of Chapter 3 by Atkins which deals with polarization phenomena. 1 Kinetics and Mechanism There have been two independent reports on the observation and kinetic behaviour of several thionitroxide radical^,"^ (l),(2) and (3). Compared with the parent a:/rnT2 * 1.16 0.80 1.09 2.0171 2.017 2.015 g nitroxides it is seen that the thiyl radicals have lower uNvalues and larger g factors indicating more localization of spin on the sulphur atoms.’ The radicals have been prepared by photolysis or thermolysis of the parent disulphides.Using the former technique and following the radical concentration after shuttering the light it was found that (1) and (3) both decayed with second-order kinetics and closely similar rates such that log (k-’/l mol-’ s-’) = 11.8 -16.7/8 (8 = 2.303 RT kJ mol-’). The equilibrium constant k l/k-l [equation (l)]was determined by measuring radical ‘Electron Spin Resonance’ ed. R. 0.C. Norman (Specialist Periodical Reports) The Chemical Society London 1976 Vol. 3. W. C. Danen and D. D.Newkirk J. Amer. Chem. SOC.,1976,98 516. 3 B. Maillard and K.U. Ingold J. Amer. Chem. SOC.,1976 98 520. *lmT=lOG. 71 A. T.Bullock concentrations in solution at different temperatures. From this together with the kinetic data it was found that log (k1/s-') = 16.8-129.7/8 8 being defined as bef~re.~ The activation energy was in good agreement with the result obtained by Danen and Newkirk,' who measured the rate of thermal decomposition of bis(pyrrolidy1-1) disulphide (4) using Banfield's radical as a scavenger. It has been noted3 that the thionitroxides are unique in that they recombine at or near the diffusion-controlled limit but are extremely unreactive to several substrates (e.g. 02 alkenes triethylphosphite and triethylphosphine) which react readily with most free radicals.(4) Following earlier inve~tigations,~*~ an extensive study of the kinetics and products of decay of a series of N-alkoxy-N-alkylamino N-alkoxyamino and N-alkoxy-N- anilino radicals has been described.6 The N-alkoxyamino radicals decayed rapidly with second-order kinetics (k/l mol-' s-l -1-4 x 10' at -60 "C). The rate of the decay was ascribed to lack of steric protection. On the other hand the N-alkoxy-N- anilino radicals decayed very slowly and were found to be in equilibrium with their dimers over the temperature range -63 to +74 "C [equation (2)]. For R = But AH" = -52 f2 kJ mol-' and AS"= -88 * 8 J mol-' K-'. ~RONC~HS $ (RONC~HS)~ (2) The kinetics of addition of a series of radicals R,M- to di-t-butylthioketone have been studied (Scheme l).' From a comparison of aMin radicals (5) and analogous R,M.+ S=CB& -+ R,MS~BU'~ [R,M = CH3 C(CH3)3 CF3 SiH3 Si(CH3)3 S~(BU")~ Sn(CH3)3 S~(BU")~, or Et02P=O] Scheme 1 radicals formed by the addition of R,M* to di-t-butylethylene it was proposed that the adduct group occupied the eclipsed position with respect to the 2p orbital on the a-carbon (6).Support for this structure came from a comparison of the g values W. C. Danen and C. T. West J. Amer. Chem. SOC.,1971,93,5582. W. C. Danen C.T. West andT. T. Kensler J. Amer. Chem. SOC.,1973,95 5716. 6 R. A. Kaba and K. U. Ingold J. Amer. Chem. SOC.,1976,98,7375. J. C. Scaiano and K. U. Ingold J. Amer. Chem. SOC.,1976 98 4727. Reaction Mechanisms-Part (iii) Electron Spin Resonance Spectroscopy 73 (2.0024-2.0033) with those found in unhindered alkylthiyl radicals where delocali- zation onto the sulphur atom c2n take place (g -2.0044-2.0049).The kinetics of the addition of R,M* to the thione were obtained from two competition experiments. Bu'N=NBu' 42Bu'. + N2 But.+ S=CBu'2 -%Bu'SeBu'2 Bu'*+O2 -% Bu'OO. Scheme 2 The first of these involved Scheme 2. At -80°C the adduct was found to be persistent and its concentration increased with time t. Analysis yielded the equation -d[Bu'OO*] kl = k2[Bu'SeBut2],[02]/ dt X t X [Buf2C=S] (3) At -80 "C k = (1.3k0.6)X lo61 mol-' s-l. The second experiment involved competition between the addition of the methyl radical to the thioketone and to Bu'N=O.Analysis of these results gave a value for kl the rate coefficient for addition to the thioketone of 21.1X lo61mol-' s-' at -40°C. Decay kinetics of the adduct radicals (5) were usually complex and of non-integral order. However the methyl adduct was found to be in equilibrium with its dimer. 2CH3SCBut2 dimer (4) Application of the van't Hoff isochore yielded AW=40*6 kJ mol-' and AS"= 134* 13 J mol-' K-' for the dissociation of the dimer. Radical addition to di-t-butylsulphurdi-imide (7) produces the nitrogen-centred radicals (8) according to equation (5).' The adduct radicals were F3C. Me3Si- Bu"Si*,(EtO) P=0,and F3CS*.All the adduct radicals (8)decayed with first-order kinetics and at 20 "C k/s-' = 4 x <lo-' 0.25 and 0.30 respectively.R,M* + Bu'N=S=NBd + Bu'NSN(Bu')MR (5) (7) (8) The persistence of these radicals was attributed to two factors stericprotection of the a-nitrogen and the absence of a-hydrogens. The same paper8 reported addition of CF,O* to di-t-butylcarbodi-imide (9) to give the diaza-ally1 radical (10) [equation (6)]. Radical (10) was found to be appreciably more persistent than previously CF30.+BU'N=C=NBU' -+B~'N=-C:-NB~~ (6) I OCF3 (9) (10) reported diaza-allyls' and decayed with second-order kinetics (presumably to dimer). The rate coefficient was found to fit the expression log (2k,/l mol-' s-I) = (7k l)-(lOk5)/6. The low pre-exponential factor indicates a high entropy of * G. Brunton J. F. Taylor and K. U. Ingold J. Amer. Chem. SOC.,1976,98,4879. 9 W.Ahrens and A. Berndt Tetrahedron Letters 1974 3741. 74 A. T.Bullock activation. Thus although the activation energy is very low the duration of a radical-radical encounter in solution would not normally be long enough to allow the establishment of a configuration favourable for reaction. The dicyanomethyl radical Ht(CN), has been found to decay with second-order kinetics at the diffusion-controlled limit" despite extensive delocalization onto the cyano-groups.'o However in the same work it was found that the rate of decay of the tricyanomethyl radical C(CN) was some two or three orders of magnitude sbwer. No convincing explanation of this was given. Several reports of radical scissions have appeared. 12-14 It is always gratifying when e.s.r.results provide unequivocal evidence in support of a previously proposed reaction mechanism. This was found to be the case in a study of imidoyl radicals and their subsequent fragmentation.' A series of these radicals was produced by radical addition to alkyl isocyanides [equation (7)] and all had g factors in the range 'X 2.0011-2.0016. In particular the truth of an earlier pr~posal'~ was established namely that oxidation of t-butyl isocyanide to the isocyanate by t-butoxyl radicals involves p-scission of an intermediate imidoyl (Scheme 3). Spectra of the imidoyl BU'O.+BU'N=C -+ BU~N=~ /@ 'OBu' Bu'N=C=O +But. Scheme 3 and t-butyl radicals were observed simultaneously. Further when (CD3),COOC(CD3),was used as the source of radicals the imidoyl and perdeuterio- t-butyl spectra were observed thus confirming the p -scission mechanism.The rate coefficient ks was determined relative to the known value for self reaction of the t-butyl radicals and was given by loglo (kp/s-*)= 12.8 -42.2/8. Interest has been maintained in scission reactions of phosphoranyl radicals and configurational effects in the a-scission of such radicals have been reported.I3 The phosphoranyl radicals were produced by the addition of photochemically generated alkoxyl radicals to alkoxyalkylphosphines and their decay with time was measured. It had previously been proposed'6 that in the step-wise reaction given in equation (8) RO.+ROPR -+ (RO)zPRz + (RO),PR+R* (8) the a-scission was a configurationally selective process in which an apical P-C bond 10 R.A. Kaba and K. U. Ingold J. Amer. Chem. SOC.,1976,98,523. 11 A. T. Bullock G. M. Burnett and C. M. L. Kerr European Polymer J. 1971,71 1011. 12 P. M. Blum and B. P. Roberts J.C.S. Chem. Comm. 1976 535. l3 J. W. Cooper and B. P. Roberts J.C.S. Perkin II 1976 808. 14 P. G. Cookson A. G. Davies N. A. Fazal and B. P. Roberts J. Amer. Chem. SOC.,1976,98,616. 15 L. A. Singer and S. S.Kim Tetrahedron Letters 1974 861. 16 A. G. Davies R. W. Dennis and B. P. Roberts J.C.S. Perkin IZ 1974 1101. * A similar conclusion was reached some years ago" for the recombination of 2-cyano-2-propyl radicals (CH&CCN. Reaction Mechanisms-Part (iii Electron Spin Resonance Spectroscopy was cleaved more rapidly than an equatorial P-C bond.The kinetic res~lts'~ for a series of phosphoranyls having different alkyl substituents suppcrted this. It was further suggested that the increase in the rate coefficient for a-scission in the order L2P(OBu') <L P(0Bu')OEt <L2P(OEt) (L = alkyl or R,N) was due to the increase in the proportion of the isomer with an apical P-L bond as the number of ethoxy ligands increased. However if configurational effects were absent then changing the nature of the alkoxy substituents had little effect on the rate of a-scission. This paperI3 also presented the first reported measurement of a bond dissociation enthalpy for a phosphoranyl radical. The temperature variation of the equilibrium constant for reaction (9) gave AW(dissociation) = 29 kJ mol-'.The authors suggested that the low selectivity observed for the overall displacement of alkyl radicals from R'RZP by t-butoxyl radicals was due to the low selectivity of the product-controlling radical formation step. There has been a preliminary report of an e.s.r. study of the reactions of a series of trialkylsiloxyl radical^.'^ These were generated by the photolysis of some t-butyl trialkylsilyl peroxides (Bu'OOSiR3) and bis(trialkylsily1) peroxides (R3SiOOSiR3) in the temperature range -120 to -20 "C. Many of the reactions are summarized in Scheme 4. In addition to these it was noted that for the higher bis(trialkylsily1) XOOSiR3 R Me Et Pr (la) XOOSiR2R(-H) Me Et (lb) 1.5-H CH~ Pr (1c) transfer b R~S~(OH)CH~CH~ 7C2H4 R3SiOCH2CH2 Me Et (Id) (X = Me$ or R3Si) Scheme 4 peroxides (R = Et or Pr) @-scission occurred giving rise to the appropriate alkyl radicals according to reaction (10).This reaction was not found in the t-butyl R3Si0. + R*+ R2Si0 (10) trialkylsilyl peroxides. It is expected to be strongly endothermic which leads the authors to suggest the concerted process shown in Scheme 5. The 1,5-hydrogen atom transfer from carbon to oxygen [Scheme 4 (lc)] is well known in the chemistry A. T.Bullock R3Si-,O R R2Si-0 II 0'-SiR3 O-SiR2.R Scheme 5 of alkoxyl radicals1' but does not seem to have been previously demonstrated by e.s.r. spectroscopy. The trimethyl- and triphenyl-germyl peroxyl radicals (1 1) have been studied." At low temperatures (<-50 "C) they behave in a similar fashion to their carbon analogue^'^ in forming tetroxide dimers [equation (ll)].The variation of K the 2R3Ge02. $ R3Ge04GeR3 (11) (11) R=MeorPh equilibrium constant with temperature gave AW =-48 kJ mol-' and AS"= -1 11J mol-' K-' for both radicals. AH" for dimerization of the germyl peroxides was thus found to be at least 11 kJ mol-' more negative than for alkyl peroxides" whereas the entropy changes were about the same. Above -50°C the radicals decayed according to equations (12) which fits the observed rate equation (13) provided that kl >>kz. The nature of reaction (12b) was not defined. 2R. & R2 (124 k-1 k R. 2,products (12b) -d[R*] k2[R*] -= dt 1+4K[R*] In view of the postulated mediation of electron donor-acceptor complexes in a variety of chemical reactions the first kinetic study of the thermal ionic dissociation of such a complex2o should be noted.It has been shown that tetracyanoethylene in solution in dimethyl sulphoxide and in the absence of light changes quantitatively to its radical anion with first-order kinetics. Over the temperature range 25-45 "C the first-order rate coefficient was given by log (kls-') =6.3 -58.6/8. The authors claim that the slowness of the reaction supports Mulliken's suggestion*' that in polar solvents dissociation of a complex into ions is govered by a slow stabilizing solvation process. The decay kinetics of a series of monosubstituted p-benzosemiquinones (12) have been measured.22 The radical anions were generated by electrolysis in methanol using 0.1M tetraethylammonium perchlorate as the supporting electrolyte.The decay was second-order and the following mechanism was proposed K Et4NQ* +Q'+Et4N+ (144 2QS k2 Q2-+Q (14b) J. K. Kochi in 'Free Radicals' Vol. 11 ed. J. K. Kochi Wiley New York 1973 pp. 686-688. J. A. Howard and J. C. Tait Canad. J. Chem. 1976 54 2669. 19 J. E. Bennett D. M. Brown,and B. Mile Trans.Faraduy Soc. 1970 66 397. 2o N. Kushibiki and H. Yoshida J. Amer. Chem. SOC.,1976,98 268. 21 R. S. Mulliken and W. B. Person in 'Molecular Complexes A Lecture and Reprint Volume' Wiley New York 1969. 22 A. B. Sullivan and G. F. Reynolds J. Phys. Chem. 1976,80 2671. Reaction Mechanisms-Part (iii) Electron Spin Resonance Spectroscopy 0- Q" 0-(12) X =OMe Me Ph COOMe H COMe Br C1 or CN whence -d[Et,NQ.]/dt = (k2K/[Et4N'])[Et4NQ.l2 (15) A linear correlation was found between log kobsd where kobsd= (k2K/[Et4N']) and the normal Hammett substituent coefficients.The observed second-order rate coefficients varied by three orders of magnitude from kobsd/l mol-' s-' = 3.3 (-OMe) to 3300 (-CN). It was suggested that the substituent effect operated on the ion-pair dissociation constant K. Unfortunately the authors did not test the predicted [Et4N+]-l dependence of kobsd. The theories of Marcusz3 have been tested in a study of the rates of electron exchange and electron transfer involving the quinones (13)-(16).24 The rate 6 I It 6 I MI e MeQMe t 1;I ~ I Me Me Me 0 0 0 0 (13) (14) (15) (16) coefficient for the exchange process [Equation (16)] was determined by measuring the e.s.r.linewidth variation with [a]. Values of k, for all four quinones together Q+QS &% Q7+Q (16) with Kii the equilibrium constants for the transfer reactions [equation (17)] allowed k and kb to be calculated from The constants kf and kb were obtained spectrophotometrically by pulse radiolysis of mixtures of the quinones (in pairs) and in general the agreement between theory and experiment was satisfactory. Q~'+Q~& Qi+QjT (17) kb Spin-trapping experiments continue to be used and extended. Phenyl-t-butylnitrone (17)was the trapping agent in a study of the y-radiolysis of nitriles in the liquid phase." The radicals trapped and identified were *CH2CN (acetonitrile) H-and CH3CHCN (propionitrile) *CH(CN) (malononitrile) and Ha -CN and *CH2CH2CN (succinonitrile).The same trap has been used in an investigation of the photoreduction of five quinones by a series of alcohols.26 Alkoxyl radicals were 23 R. A. Marcus Ann. Rev. Phys. Chem. 1964,15,155. 24 D.Meisel and R. W. Fessenden J. Amer. Chem. SOC.,1976,98 7505. 25 S.W.Mao and L. Kevan J. Phys. Chem. 1976,80,2330. 26 K. A. McLauchlan and R. C. Sealy J.C.S. Chem. Comm.,1976,115. A. T.Bullock observed in every case whereas hydroxyl radicals were never trapped. The authors point out the presence of RO. radicals must be taken into account when interpreting electron polarization experiments. Many spin-trapping experiments in the radiolysis of aqueous solutions have involved the use of CH2=N02- the aci-anion of nitromethane.This restricts such studies to alkaline solutions. The restriction has now been removed in a comparative study of the merits of phenyl-t-butylnitrone (17) 5,S-dimethylpyrroline- 1-oxide (18) and 2-methyl-2-nitrosopropane (19).27 The last-mentioned was found to be unsatisfactory largely because of its low solubility. It seems that the best spectra were obtained from (18) and the solvated electron e& H- and *OHwere all successfully trapped during the in situ radiolysis of water with 3 MeV electrons. oCH=y-Bul 1 0 0 BU'NO (17) (18) (19) The investigation of triplet species in solution by e.s.r. is seriously hampered by the fact that for low viscosity solvents at moderate temperatures the short rotational correlation time T, together with the strong dipolar coupling produces very efficient spin relaxation.The lines are thus broadened beyond detection. Two reports of techniques which circumvent this problem have appeared. It has been shown2* that nitric oxide acts as an efficient 'triplet-trap'. Ten substrates known to produce carbenes were photolysed in the presence of NO [equation (18)]. i'.+NO -P T=NO* (18) (20) Characteristic spectra of the resultant iminoxyls (20) were observed. Direct obser- vation of the Ams=*l transition of the triplet species (21) in solution has been reported.29 This was achieved by using a solvent with a viscosity coefficient such that rC 3 8 x s.A mixture of Pr'OH and PrOH was used at temperatures no higher than 143.5 K. The triplet was produced by photolysis of the appropriate diazene as shown in Scheme 6. Cutting off the light resulted in a 'clean' second-order decay with Scheme 6 a rate coefficient (143.5 K) of (2*O0.8)x lo31 mol-'s-' i.e. about 0.13 times the diff usion-controlled encounter frequency. The decay could occur either through a 27 F. P. Sargent and E. M. Gardy Canad. J. Chem. 1976,54 275. 28 A. R. Forrester and J. S. Sadd J.C.S. Chem. Comm. 1976 631. 29 M. S. Platz and J. A. Berson J. Amer. Chem. Soc. 1976 98 6743. Reaction Mechanisms-Part (iii)Electron Spin Resonance Spectroscopy 79 singlet +triplet reaction or uia a triplet +triplet dimerization. Earlier work3' had shown that the singlet state (S) of (21) lay at least 2.5 kJ above the triplet (T).From this it was estimated that [S]/[T] -0.04 and hence if the singlet +triplet mechanism were correct the collisional frequency would have to be at least three times greater than the diffusion-controlled limit to account for the observed value of the second- order rate coefficient. On this basis it was decided that the dimerization took place uia the triplet +triplet reaction. The kinetics of cycloaddition of (2 1) to a number of olefins were followed and shown to be first-order in each reactant. The second-order rate coefficients were claimed to be very similar to those observed for the addition of radicals to 01efins.~~ The field of radical rearrangements remains a fertile one for e.s.r.spectroscopists. The kinetics of isomerization of cyclopropylcarbinyl to allylcarbinyl have been as have those of several neophyl rearrangement^.^^*^^ A valence bond isomerization of two hemi-Dewar naphthalenes to the corresponding naphthalenes is said to take place via the radical anions and the diamagnetic dianions of the naphthalene^.^^ The formation and decay kinetics of the e.s.r. signal of the radical anion of 1,3,7,9-tetra-t-butyl hemi-Dewar naphthalene were measured. Activation energies for formation and decay were 4.2 f2.1 and 17 f5 kJ mol-' respectively. It has been pointed that e.s.r. observations of radical rearrangements are often frustrated by the fact that the rearrangement unless occurring with a very small activation energy cannot compete kinetically with recombination and disproportio- nation reactions.Isolation in an adamantane matrix was therefore used to study the sigmatropic and electrocyclic reactions of the bicyclo[ 3,l ,O]hexenyl radical (22); X2= Y3= H). The radical was produced by X-irradiation of bicyclo[3,1,0]hex-2- ene in adamantane at -196 "C. Below -60 "C (22) was stable but above -60 "C it isomerized to the cyclohexadienyl radical (23) as shown in Scheme 7. Kinetic measurements on the growth of (23) gave a free energy of activation (-50 "C) of AG' = 60.6 kJ mol-'. When X =D and Y = H the cyclohexadienyl spectrum was found to be a composite of monodeuteriated radicals with deuterium in positions 1 2 and 3 in the statistical ratio 2 :2 1. The same result was found for X = H Y = D.The rearrangements shown in Scheme 7 are clearly more rapid than the ring-opening process. The kinetics of isomerization of 2,4,6-tri-t-butylphenyl (24) to 3,5-di-t-butylneophyl (25) have been measured over the temperature range -26 to -160 0C.37 In addition the same radicals deuteriated in the Butgroups were studied between 20 and -150 "C. The authors listed four criteria necessary to establish the presence of quantum mechanical tunneling and showed that their data satisfied all of them. The criteria are (i) a large kinetic isotope effect -k,/k = 80 at -30 "C 1400 at -100 "C and 13000 at -150"C; (ii) non-linear Arrhenius plots -tunneling should become relatively more important with decrease in temperature; (iii) a large 30 M.S. Platz J. M. McBride R. D. Little J. J. Harrison A. Shaw S. E. Potter and J. A. Berson J. Amer. Chem. SOC.,1976,98 5725. 31 J. M. Tedder and J. C. Walton Accounts Chem. Res. 1976,9 183 and references therein. 32 B. Maillard D. Forrest and K. U. Ingold J. Amer. Chem. Soc. 1976,98 7024. 33 B. Maillard and K. U. Ingold J. Amer. Chem. SOC. 1976 98 1224. 34 B. Maillard and K. U. Ingold J. Amer. Chem. Soc. 1976,98 4692. 35 I. B. Goldberg H. R. Crowe and R. W. Franck J. Amer. Chem. SOC. 1976,98 7641. 36 R. Sustmann and F. Liibbe J. Amer. Chem. SOC.,1976,98,6037. 37 G. Brunton D. Griller L. R. C. Barclay and K. U. Ingold J. Amer. Chem. SOC. 1976,98 6803. A. T.Bullock 1 1 1 D Q HH Scheme 7 But Bu' (24) (25) difference in activation energies for H and D transfer -in the absence of tunneling the maximum value of ED-EHshould equal the difference in zero point energies for H-and D-containing reactants (5.7 kJ mol-I) otherwise ED-EH>5.7 kJ mol-' when tunneling is appreciable (at -30 "C ED-EH= 13.4 kJ mol-'); (iv) a large difference in preexponential factors (experimental values were AH= 106.5s-' and AD= lo7.'s-').Detailed calculations were made37 using a one-dimensional Eckart gaussian and truncated parabolic barriers were tried and good agreement with the experimental data was found for the Eckart barrier i.e. Vcxl= Vo/cosh2(xla). Elementary reactions of atoms with molecules in the gas phase continue to be studied using the fast flow microwave discharge technique.As examples the reactions of atomic hydrogen and deuterium with HBr and DBr have been examined,39 and the kinetics and stoicheiometry of the reaction of atomic oxygen with PF3 have been mea~ured.~' Special mention should be made of two papers which significantly extend the scope of the technique. In the first of these4' the rate coefficients for the reactions F+H2+HF +H H +F2+HF+F and F+ CHF3+HF+CF2 were measured at 298 K. Finite difference techniques were used. 38 R. J. LeRoy E. D. Sprague and F. Williams J. Phys. Chem. 1972 76 546. 39 H. Endo and G. P. Glass J. Phys. Chem. 1976,80 1519. 40 I. B.Goldberg and H. R. Crowe J. Phys. Chem. 1976,80 2407. 41 I. B.Goldberg and G. R. Schneider J. Chem. Phys. 1976,65 147. Reaction Mechanisms-Part (iii) Electron Spin Resonance Spectroscopy 81 These permit the use of e.s.r.in complex systems which may not be amenable to study by simple first-order kinetics and where cavity sensitivity as a function of length along the reaction path may be important. Pressure and velocity gradients may also be taken into account. The second paper was concerned with the kinetics of the reaction between hydrogen atoms and molecular chlorine.42 The authors took the obvious but important step of adding an on-line mass spectrometer to the discharge- flow/e.s.r. system. By this means it was shown that C12 consumption and HCl production were much lower than expected; facts which could be accounted for by postulating an enhanced rate for H + HCl -+ H2+ C1 due to vibrational excitation of HCl produced in the initial step (H + C1 +HCl+ CI).In a subsequent study of the reactions of hydrogen and oxygen atoms with the same workers have taken the further step of adding a gas chromatograph to analyse stable condensables (77 K). 2 Chemically Induced Dynamic Electron Polarization An extended kinetic model for CIDEP has been presented which enables kinetic e.s.r. studies to be made at shorter time intervals than hitherto.44 The model incorporates emissive-absorptive (E-A) polarization due to the radical-pair mechanism (RPM) together with initial polarization (IP) arising from the triplet mechanism (TM) and provides a procedure for measuring true radical concentrations when one or both of these mechanisms are operative. For pairs of peaks with the same [xiaimi),the difference in peak heights represents the E-A polarization whilst the sum incorporates the IP.The model was tested using data obtained from a pulse photolysis study of the behaviour of the semidione radicals of pyruvic acid and biacetyl in alcoholic solvents. A spectrometer with a response time of ca. 0.3 ps has been described4’ which allows the observation of e.s.r. signals at time intervals of less than 1ps after a radiolysis pulse. The e.s.r. time profiles were analysed by means of the Bloch equations modified to take account of changes in radical concentration with time any IP on formation and CIDEP effects produced during decay. It was shown that the time dependence of the e&signal clearly indicated a zero initial magnetization and hence equally populated spin states.The relaxation time varied with dose in a manner which suggested a Heisenberg spin exchange mechanism. Secondary radi- cals produced from e all showed zero initial magnetization whilst secondary radicals produced by the reaction of *OH with several substrates gave time profiles showing initial magnetizations corresponding to the Boltzmann distribution. This was a result of spin relaxation of *OH before reaction and it was shown that T,(*OH)< 1ns. Another example of polarization transmission down a reaction pathway was found in a CIDEP study of the photoreductions of some carbonyl compounds in the presence of amine~.~~ The reaction goes via the triplet state of for example benzophenone thus 3Ph2C0+ NEt3 + Ph2COH+ MeCHNEt (19) 42 P.F. Ambidge J. N. Bradley and D. A. Whytock J.C.S. Faraday I 1976 72 1157. 43 P. F. Ambidge J. N. Bradley and D. A. Wbytock J.C.S. Faraday I 1976 72 1870. 44 P. B. Ayscough G. Lambert and A. J. Elliott J.C.S. Faraday I 1976,72 1770. 45 N. C. Verma and R. W. Fessenden J. Chem. Phys. 1976,65,2139. 46 K. A. McLauchlan and R. C. Sealy Chem. Phys. Letters 1976 39 310. 82 A. T. Bullock Only Ph2COH was observed. The failure to detect the counter radical MeCHNEt, the presence and nature of which had previously been established by flash photo- lysisY4’was ascribed to the fast pseudo-first-order reaction (20). The authors found that the polarization of Ph2COH varies not only with [NEt,] but also with [Ph,CO].MeCHNEt +Ph2C0 k CH,=CHNEt +Ph2COH (20) Thus although the absorption signal changed only slightly as [Ph,CO] varied from 0.02M to OSM the emission was much greater for the 0.5M solution. This was attributed to the rate of reaction (20) being sufficiently fast that an appreciable fraction of the primary polarization in the aminPalky1 radical was transferred to Ph,COH. In other words kZO[Ph2CO] -K’(MeCHNEt,). The RPM for two unlike radicals showing CIDEP predicts emission for the species with the higher g value and absorption for the other. However precisely the opposite has been found in the case of eiq (g = 2.0003),48i.e. for all counter radicals having g >2.0003 the e.s.r. signal for eiq was in the emissive mode. The authors suggested that if the RPM is invoked then it must be assumed that reaction between eiq and the counter radical occurs preferentially into the triplet state of the product.Both time-resolved and steady-state photolyses were used in a study of the pho toreduct ions of substituted benzoquinone nap h thoquinone ,and an thraquinone by 2,6-di-t-b~tylphenol.~~ In all cases it was possible to observe both primary radicals in the radical pair. The RPM and TM each contributed to the polarization but the latter was the major contributor. Various aspects of both theories were tested. An interesting feature of this work lies in some preliminary isotope experi- ments which suggested that CIDEP may be a potentially powerful tool for the investigation of kinetic isotope effects involving excited triplet molecules.Polariza- tion enhancements for the species *CH2COO- and CH(COO-)2 produced by electron pulse radiolysis provided another test of the~ry.~’ The E-A enhancements typical of RPM were studied as a function of ionic strength and radical concentra- tion. With regard to the former parameter the theory of Freed and Pedersen” predicts that for non-spherical radicals and small exchange interactions (J) the enhancements should show a stronger dependence on p the ionic strength at low values of p. This was confirmed. Furthermore it was found that *CH20H -CH,COO- and *CH(COO-) all showed polarization enhancements independent of radical concentrations. It was suggested that the initial non-uniform spatial distribution of radicals in spurs may have been responsible i.e.the observations (2-3 ps after the pulse) were made before a uniform distribution was reached. Finally the power of the correlated use of CIDNP and CIDEP in revealing details of major and minor pathways in radical reactions has been demonstrated in a study of the photolysis of tetrafluro-p-benzoquinone in di~xan.~ The photoreactive species was found to be the excited triplet of the quinone and the TM was mainly responsible for CIDEP. The cage recombination of the primary radicals was revealed by the CIDNP measurements and all observations were accounted for by the reactions 47 S. Arimitsu H. Masuhara N. Mataga and H. Tsubomura J. Phys. Chem. 1975.79,1255. *8 R.W. Fessenden and N. C. Verma J. Amer. Chem. SOC.,1976,98,243.49 B.B.Adeleke and J. K. S. Wan J.C.S. Faraday I 1976,72 1799. 50 A. D.Trifunac J. Amer. Chem. SOC.,1976,98,5202. 51 5. H. Freed and J. B. Pedersen Adu. Magn. Resonance 1975,8,2. 52 H. M. Vyas and J. K. S. Wan Canad. J. Chem. 1976,54979. Reaction Mechanisms -Part (iii) Electron Spin Resonance Spectroscopy shown in Scheme 8. The authors make the important point that conventional kinetic e.s.r. only gave information about disproportionation (kd).53 caged tripletpair 0 6 0 OH g1 ’ g2 FFQ diffusion OH 0 OH OH 0 OH Scheme 8 53 H. M. Vyas and J. K. S. Wan Internat. J. Chem. Kinetics 1974,6 125.

ISSN:0069-3030    

DOI:10.1039/OC9767300071

出版商:RSC    

年代:1976

数据来源: RSC

摘要:

5 Arynes Carbenes Nitrenes and Related Species ~~~~~ ~ ~ ~ ~ By S. A. MATLIN Department of Chemistry The City University St. John Street London EClV 4PB 1 Arynes New calculations on the bonding in benzyne based on the previously reported matrix4.r. spectrum indicate a true cycloalkyne structure (1).Based on this model the i.r. spectrum of tetradeuteriobenzyne was predicted and awaits experimental verification. * 138.7 H 141.0 140.5 + H Y -134.4 H (1) distance in pm angles in degrees There is a growing interest in the immobilization of reactants by attaching them to the surface of solid supports,’ amongst the benefits of which is the prevention of intermolecular reactions. One of the most striking publications of the year in the field of aryne chemistry concerns the application of this ‘pseudodilution’ technique to the generation of immobilized ben~yne.~ The polymer-linked benzyne (2; R = carboxylated polystyrene resin) prepared by lead tetra-acetate oxidation of the corresponding aminotriazole did not give dimers but could be trapped up to 70 s after generation by addition of tetracyclone affording (3).The exceptionally long lifetime of the intermediate suggests that this immobilization technique is likely to prove a useful complement to matrix isolation for studying the properties of arynes. Ph ROCH2CH2 ROCH2CH2yy4 h Ph Ph (3) A benzyne-triosmium complex has been isolated as an intermediate in the formation of benzene from benzyl alcohol and triosmium do.decacarbonyl.* J.W. Laing and R. S. Berry J. Amer. Chem. SOC.,1976,98 660. * J. I. Crowley and H. Rapoport Accounts Chem. Res. 1976,9 135. P. Jayalekshmy and S. Mazur J. Amer. Chem. Soc. 1976,98,6710. K. A. &am and A. J. Deeming J.C.S. Gem. Comm. 1976,852. 85 S. A.Matlin Whereas the decomposition of N-nitrosoacetanilide alone in benzene which occurs by competing pathways to benzyne and to phenyl radicals affords up to 80% of biphenyl in the presence of tetracyclone equally high yields of the benzyne adduct 1,2,3,4-tetraphenylnaphthaleneare formed. Cadogan's group5 has demonstrated that the tetracyclone plays a dual role scavenging the phenyl radicals which otherwise initiate a chain radical phenylation of the solvent as well as acting as an arynop hile.Benzyne has been generated by the action of lithium tetramethylpiperidide on phenyl benzenesulphonate. The aryne formed was entirely derived from the phenolic portion of the sulphonate ester but yields of adducts were generally lower than when bromobenzene was used as a precursor.6 No benzyne adduct could be detected when o-dibromobenzene was treated with the strongly basic lithium diphenylphosphide in the presence of furan evidently owing to the instability of the expected product to base.' Cycloaddition of o-benzene to trans -olefins generally proceeds with about 70-80% retention of configuration which can be accounted for either in terms of competing concerted pathways or by a biradical mechanism. If the former explana- tion were correct it would be expected that a monosubstituted deuterium-labelled olefin would give more inversion since the steric effect would favour a ,,2,+,2 process.However it has now been shown that the addition of benzyne to E-deuterio-t-butylethylene gives mainly the product of retention (Scheme l),arguing strongly in favour of the biradical mechanism.8 75% 25% Scheme 1 Steric effects in the addition of 3-methylbenzyne to 2-substituted furans have been examined by Newman and Kannan.9 For a given furan [Scheme 2; R = Me But eHO(CH,),d or CO,Me] the same ratio of products was obtained from each benzyne precursor (X Y =NH2,C02H or F Br) demonstrating that a true benzyne intermediate is involved in each case. The product ratio showed very little sensitivity to polar or steric effects in the furan substituent.Cyclobuta[ 1,2-d]benzyne has been generated from the dihalide (4; n = 1). In the absence of trapping agents the yields of biphenylene-type dimers in the series (4; n = 1-3) increase with decreasing ring size implying that the stability of the benzyne increases with increasing ring strain." Whereas no evidence of ring rotation by any pathway has ever been observed in the [3,3]paracyclophanes the corresponding 5,6-dehydro-derivative (5) is capable J. I. G. Cadogan C. D. Murray and J. T. Sharp J.C.S.PerkinII 1976,583;E. A. Bell J. I. G. Cadogan P. W. Milburn C. D. Murray R. M. Paton and J. T. Sharp ibid. p. 588. I. Fleming and T. Mah J.C.S. Perkin I 1976 1577. D. G. Gillespie B. J. Walker and D.Stevens Tetrahedron Letters 1976 1905. 8 A. T. Browne T. A. Christopher and R. H. Levin Tetrahedron Letters 1976,4111. M. S. Newman and R. Kannan J. Org. Chem. 1976,41,3356. lo R. L. Hillard and K. P. C. Vollhardt J. Amer. Chem. Soc. 1976,98 3579. Arynes Carbenes Nitrenes and Related Species (. Me R y\ H + $. &$ X I R Scheme 2 of undergoing 90"kotation to the perpendicular orientation from which cycloaddi- tion affords the bridged benzobarralene (6) in 66% yield." (5) (6) For the first time a 1,4-dihydronaphthalene-1,4-imine has been isolated from the reaction of a benzyne with a pyrrole from which only 2-naphthylamine derivatives have previously been obtained.12 The adducts (7a; R' = R2= Me) and (7b; R' = But R2= H) of tetrachlorobenzyne and N-alkylpyrroles were of surprising thermal stability eliminating acetylene only on strong heating.In the case of (7b) the isoindole (8; R = But X = C1) could be isolated on thermolysis. A new synthetic route to a variety of polynuclear aromatic hydrocarbons employs the oxidative deamination of dihydroaromatic 1,4-imines generated by aryne addition to py~roles.'~ For example the adduct (9)of 1,2-dehydronaphthalene and (8; R = Me X = F) eliminates nitrosomethane on peracid oxidation to give 8,9,10,11- tetrafluorobenz[a]anthracene. Benzyne reacts with bisethynylsulphone to give exclusively homo-Diels-Alder products probably via zwitterionic intermediates. l4 l1 D. T. Longone and J. A. Gladysz Tetrahedron Letters 1976 4559.l2 M. Ahmed and J. M. Vernon J.C.S. Chem. Comm. 1976 462. l3 G. W. Gribble and R. W. Allen Tetrahedron Letters 1976 3673. l4 E. E. Nunn Tetrahedron Letters 1976,4199. S. A.Matlin Me (7) (8) (9) Studiesof the arynic condensations of ketone enolates contin~e.’~ In the conden- sation of benzyne with cup -unsaturated ketone enolates the indanone products (12) are formed in addition to the previously reported tetralones (13). The ratio of these two products which are formed via opening of the benzocyclobutenol (10) and recyclization of the intermediate (ll) is most sensitive to the nature of the R’ substituen t (Scheme 3). R3 R’CH=d-Y-CHR’ + PhBr Bu‘ONa R2 aR3 NaNH2) R2 &R2 \ 0:-Y O CHR ’ 0 CHR’ R3 R3 mR1 \ + 0 0 (12) (13) Scheme 3 2,3-Thiophyne generated for the first tirnel6 by the flash vacuum thermolysis of the anhydride (14) has been trapped via a variety of dienes to afford benzo[b]thiophens.9,lO-Dehydroanthracene is formed” in the photochemical decarbonylation of the bis-keten (15). 0 c II 0 15 M. Essiz G. Coudert G. Guillaumet and P. Caubere TetrahedronLetters 1976 3185. 16 M. G. Reinecke and J. G. Newsom J. Amer. Chem. SOC.,1976,98,3021. 0.L. Chapman C.-C. Chang and J. L. Kolc J. Amer. Chem. SOC.,1976,98 5703. Arynes Carbenes Nitrenes and Related Species 2 Nitrenes The formation of triplet-state nitrenes from azides has been reviewed." Electrochemical generation of a nitrene from TsNC12 has been achieved by constant-current electroreduction in acetonitrile-dioxan.l9 The thermal unimolecular fragmentation of the NO-bis(trimethy1-sily1)hydroxylamine(16)in cyclohexene afforded mainly (53%)aniline and only 2% of the azirane (17),together with other minor products consistent with the intermedi- acy of phenylnitrene.20 Attempts to trap the nitrene with stilbenes were largely unsuccessful but thermolysis in the presence of amine solvents gave high yields of products from the trapping of the ring-expanded azepines (Scheme 4). The alterna- tive process of ring-contraction of phenyl- and 2-pyridyl-nitrenes has been examined in detail with the aid of I3Clabels.2' Scheme 4 Ethoxycarbonylnitrene evidently in the triplet state can be obtained22 in high yield by the triplet-sensitized photolysis of the ylide (18).I -NCO,Et (18) Solvent effects on the behaviour of nitrenes have been noted by several groups. 1,4-Dioxan stabilizes the singlet state of ethoxycarbonylnitrene presumably by an interaction of the nitrene p-orbital with the lone pairs of electrons of both the ether oxygen^.^^ No comparable effect is seen with tetrahydrofuran or tetrahydropyran and nitrene insertion into the a-C-H bonds of cis- and trans-2,5-dimethylfuran H. Durr and H. Kober Topics Current Chem. 1976,66,89. l9 T. Fuchigarni T. Nonaka and K. Iwata J.C.S. Chem. Comm. 1976 951. 2o F. P. Tsui Y.H. Chang T. M. Vogel and G. Zon J. Org. Chem. 1976,41 3381. 21 C. ThBtaz and C. Wentrup J. Amer. Chem. Soc. 1976 98 1258; R. Harder and C.Wentrup ibid. p. 1259. 22 M. Nastasi H. Strub and J. Streith Tetrahedron Lerters 1976 4719. 23 H. Takeuchi K. Kinoshita S. M. Abdul-Hai M. Mitani T. Tsuchida and K. Koyama J.C.S. Perkin ZZ 1976,1201. 90 S.A.Math proceeds non-stereospecifically consistent with the intermediacy of an 0-N ~lide.~~contrast with the thermally generated ethoxycarbonylnitrene the In photochemically generated species in halogenated solvents such as dichloromethane shows an increased selectivity for insertion into the C-H bonds of deca1i1-1.~~ Similarly dichloromethane solvates and stabilizes pivaloylnitrene without markedly decreasing its reactivity. The singlet nitrene adds stereospecifically to olefins whilst additions of the triplet are stereoselective.26 The effect of temperature on the relative reactivity of the singlet and triplet states has also been empha~ized.~’ Thermally the singlet derived product (20; X =Me) from the azide (19; X=Me) is greatly favoured (98% in boiling PhBr).Direct photolysis of (19; X = Me) at room temperature affords 62% of (20; X =Me) the yield rising to 90% on irradiation in boiling PhCl. However in the sensitized photolysis of (19; X =Me) an increase in temperature favours increased yields of the triplet-derived product (2 1 ; X = CH). Similarly direct room-temperature photo- lysis of (19; X = N3) affords (20; X = N3),whereas triplet sensitization gives mainly azo-dimer and a low yield of (2 1;X = N). Irradiation of (19; X = N3)in a rigid matrix at 77 K affords (21; X= N) quantitatively.X X H In neither of the nitrenes (22) or (23) did reaction lead to any products derived from attack of the sulphur atom on the nitrene.28 In contrast lead tetra-acetate oxidation of the amine (24; R=phthalimido) in the presence of benzothiophen resulted in formation of some (25) and the cis-azo-compound (26) as the major product. Similar results were obtained with other sulphides but in the absence of a sulphide only the trans-isomer of (26) was obtained. The intermediacy of S-N ylides was invoked2’ to explain the formation of both (25) and (26) the cis stereochemistry of the latter being explained by steric effects in the intermediate (27). Reports appearing during the year suggest that investigation of metal-nitrene interactions may prove very fruitful.Thus aryl azides are cleanly reduced to aryl amines by aqueous vanadium dichloride although alkyl azides give complex product mixtures from the vanadium-co-ordinated nitrene interrnediate~.~’ With iron pen- tacarbonyl aryl azides afford ureas in good yield evidently via iron-nitrene com- 24 N. Torimoto T. Shingaki and T. Nagai Bull. Chem. SOC.Japan 1976,49 2572. 25 P. A. Tardella and L. Pellacani J. Org. Chem. 1976 41 2034. 26 G. R. Felt and W. Lwowski J. Org. Chem. 1976 41 96. 27 J. M. Lindley I. M. McRobbie 0.Meth-Cohn and H. Suschitzky TetrahedronLetters 1976 4513; A. Yabe and K. Hondo Bull. Chem. SOC.Japan 1976,49,2495. z8 1. M. McRobbie 0.Meth-Cohn and H. Suschitzky Tetrahedron Letters 1976 929. 29 D.W. Jones J.C.S. Perkin I 1976 1150. 3O T.-L. Ho M. Henninger and G. A. Olah Synthesis 1976,815. Arynes Carbenes Nitrenes and Related Species ~lexes,~’ in the reaction of 2-aryl and similar complexes also seem to be inv01ved~~ azirines with Fe,(CO),. The analogy with carbene-metal complexes is further strengthened by the that the products of reaction of 2-aryl azirenes with dicarbonylchlororhodium dimer are strongly dependent on the azirene :rhodium ratio (cf.the work cited in ref. 75). When this is 10:1 the indole (28) is obtained whereas with a ratio of 2 1 the diary1 pyrrole (29) is the principal product. 3 Carbenes Rearrangements and interconversions of carbenes and nitrenes have been reviewed. 34 The method of structural fragments has been applied to the assessment of singlet-triplet energy diff eren~es.~’ Ab initio MO calculation^^^ on the manifold of 31 A.F. M. Iqbal Helo. Chim. Acta. 1976,59 655. 32 H. Alper and J. E. Prickett J.C.S. Chem. Comm. 1976 191. 33 €3. Alper and J. E. Prickett J.C.S. Chem. Comm.,1976,483. 34 C. Wentrup Topics Current Chem. 1976,62 173; See also ref. 21. 35 J. F. Liebman P. Politzer and W. A. Sanders J. Arner. Gem. SOC 1976,98 51 15. 36 W. J. Hehre J. A. Pople W. A. Lathan L. Radom E. Wasserman and Z. R. Wasserman J. Amer. Chem. Soc. 1976,98,4378. S.A. Matlin C3H2 isomers indicate that cyclopropenylidene is the most stable singlet isomer whereas the most stable triplet species is prop-2-ynylidene for which a 1,3-biradical valence structure is evidently preferred.New ab initio calculations3' suggest vinyl- methylene to have a localized methylene-like triplet ground state but this seems to be inconsistent with e.s.r. evidence,38 which favours a 1,3-biradical structure. Triplet carbenes can be conveniently trapped with nitric oxide to give long-lived iminoxyl radicals which may be detected by e.~.r.~' Previous calculations on the relative stability of oxiren and formylcarbene have produced conflicting data. A new assessment employing a nonempirical SCF-MO study with full optimization of the geometrical parameters has yielded data which are likely to be the most reliable so far obtained and which in broad agreement with the experimentally observed behaviour of ketocarbenes places ground-state oxiren 49.3 kJ mo1-I higher in energy than ground-state formylmethylene.An activation energy of 30.5 kJ mol-1 was computed for the ring-opening of oxiren to ketocar- bene leading to an estimation of a very short lifetime ( < lo-* s)for the unsubstituted oxiren molecule and explaining why attempts to trap oxirens have so far been U~SUCC~SS~U~.~~ The intermediacy of thiiren in the photochemical decomposition of 1,2,3-thiadiazole (30) has been established by 13Clabelling. Interestingly thiiren lies only on the pathway to ethyl mercaptan formation and not thioketen forma- tioa41 Generation.-There has been a continuing development in the use of phase-transfer catalysis for the generation of carbene~.~* Several new methods of forming carbenes have been reported including the Hg(3P1) sensitized photolysis of trifluoroethylene to furnish difl~orovinylenecarbene,~~ and the reaction of sodium iodide in potassium carbonate with ethyl dichloroacetate to give chlorocarbene (Scheme 5)." 0 IIC1&C-aCH2CH3 -+C12CH- Cl-%-H IH I- Scheme 5 37 J.H. Davis W. A. Goddard and R. G. Bergman J. Amer. Chem. SOC.,1976,98,4015. 38 D. R. Arnold,R. W. Humphreys W. J. Leigh and G. E. Palmer,J. Amer. Chem. SOC.,1976,98,6225. 39 A. R. Forrester and J. S. Sadd.,J.C.S. Chem. Comm. 1976 631. 40 0.P. Strausz R. K. Gosavi A. S. Denes and I. G. Csizmadia J. Amer. Chem. SOC.,1976,98,4784. 4' J. Laureni A. Krantz and R. A. Hajdu J. Amer. Chem. Soc. 1976,98,7872. 42 E. V. Dehmlow Tetrahedron Letters 1976,91; E.V.Dehmlow and M. Lissel ibid. p. 1783; S. Kwon Y. Nishimura M. Ikeda and Y.Tamura Synthesis 1976 249; F. De Angelis A. Gombacorta and R. Nicoletti ibid. p. 789; T. Sasaki S. Eguchi M. Ohno and F. Nakata J. Org. Chem. 1976 41 2408. 43 R. J. Norstrom H. E. Gunning and 0.P. Strausz J. Amer. Chem. SOC.,1976 98 1454. 44 J. K. Makrandi and S. K. Grover Tetrahedron Letters 1976 3179. Arynes Carbenes Nitrenes and Related Species 93 The reaction of halide ions with epoxides gives a low concentration of 2-halogenoalkoxide ions sufficient to generate difluorocarbene from chlorodifluoromethane.45Another new method of forming this carbene involves the use of Wittig reagents as bifunctional reactants. The phosphonium ylide (31)serves first as a base to form the carbene from chlorodifluoromethane and then as a trapping agent providing a new synthesis of 1,l-difluoro-olefins (Scheme 6).46 (31) Ph36-CR'R2 +HCF2CI -+ Ph3P+-CHR1R2 +:CF2 -Ph3P+ F2C=CR'C2 (31) Scheme 6 Olofson and co-worker~~~ have described two new approaches to cyclopropanol synthesis.The first involves treatment of 2-chloroethyl chloromethyl ether with lithium tetramethylpiperidide in the presence of olefins furnishing good yields of 2-chloroethyl cyclopropyl ethers from which cyclopropanols can be obtained by the action of n-butyl-lithium. In the second method acyloxycarbenes of which only one example was previously known are generated by the action of hindered base on chloromethyl esters and add to olefins to give cyclopropyl esters in moderate yields.The Meldrum's acid derivatives (32) also decompose on pyrolysis to acyloxycar-benes and in the absence of trapping agents these rearrange to 1,2-diketones in high yield.48 PCO R2C0 0 (32) Carbenes are also formed in the thermal decomposition of arylidene-oxazolones and -isoxaz~lones~~ and in the photochemical fragmentation of cyclic carbonate^.'^ Reactions.-Sensitized photolysis of diazoacetone produces a triplet ketocarbene which reacts by a combination of H-abstration and intersystem crossing to the singlet. In partially chlorinated solvents chlorine abstraction by the singlet ketocar- bene occurs together with H-abstraction. The fact that chlorine abstraction is not seen in the direct photolysis of diazoacetone suggests that the singlet ketocarbene is not an intermediate in the Wolff rearrangement of diaz~acetone.~' Stereochemical aspects of the previously reported vinylogous Wolff rearrange- ment have been examined.52 Interest continues in the stereochemistry of 1,2-shifts 45 M.Kamel W. Kimpenhaus and J. Buddrus Chem. Ber. 1976 109,2351; W. Kimpenhaus and J. Buddrus ibid. p. 2370. 46 G. A. Wheaton and D. J. Burton Tetrahedron Letters 1976,895. 47 R. A. Olofson K. D. Lotts and G. N. Barber TetrahedronLetters,1976,3381,3779; G. N. Barber and R. A. Olofson ibid. p. 3783. 4a R. F. C. Brown F. W. Eastwood S. T. Lim and G. L. McMullen Austral J. Chem. 1976 29 1705. 49 C. Wentrup and W. Reichen Helv. Chim. Acta 1976,59,2615 2618. 50 G.W. Griffin R. L. Smith and A. Manmade J. Org. Chem. 1976,41 338. 5' H. D. Roth and M. L. Manion J. Amer. Chem. SOC.,1976 98 3392. 52 A. B. Smith B. H. Toder and S. J. Branca J. Amer. Chem. SOC.,1976,98,7456. S. A.Matlin to carbenic Examples of 1,2-shifts of groups other than hydrogen are seen in two reports on the thermolysis of tosylhydrazone sodium 6-Norpinanylidene (33) rearranges by an alkyl shift via (34) which furnishes (35) through a methylenecyclopropane rearrangement. It is not clear in this case whether the preference for alkyl over H migration is due to the requirement for the migrating group to be perpendicular to the carbene or simply due to avoidance of the anti-Bredt product of H-migration (36). Phenyl migration is preferred from the axial position in (37a) over the equatorial position in (37b) by a factor of 5 1 H-migration dominating in both cases.However the interpretation of results in this system is complicated by the presence of a certain degree of conformational flexibility. Not only the singlet but also the triplet carbenes generated from the isomers (38a) and (38b) undergo ring-expansion and fragmentation stereospecifically suggesting a singlet-triplet equilibrium with reaction rates favouring rearrangement and frag- mentation via the singlet Ph H N2 (384 (38b) Work on the arylcarbene-aromatic carbene rearrangement Resonance in the dialkynylcarbenes (39) results in formation of a series of isomeric tetra-alkynyl olefins on dimerization. Trapping of (39a) with olefins gives substi- tuted cyclopropanes without stereoselection which implies a triplet-state path~ay.~’ Rearrangements of cyclopropylidenes generated from the corresponding 1,l-dibromocyclopropanes have been used in ~ynthesis.~~ An examination of 53 E.P. Kyba and C. W. Hudson J. Amer. Chem. Soc. 1976,98,5696. 54 U. Langer and H. Musso Annalen 1976 1180; L. Seghers and H. Shechter TetruhedronLetters 1976 1943; for a study of competing phenyl and methyl migrations see J. J. Have] J. Org. Chem. 1976 41 1465. 55 R. R. Gallucci and M. Jones jun. J. Amer Chem. SOC.,1976,98 7704. For other examples of carbene fragmentations see T. L. Gilchrist and D. P. J. Pearson J.C.S. Perkin I 1976 1257; W. R. Dolbier jun. 0.T. Garza and B. H. Al-Sader Tetrahedron Letters.1976 887. 56 N. M. Lan and C. Wentrup Helu. aim. Acta 1976 59 2068; U. H. Brinker and W. M. Jones TetruhedronLetters 1976,577. 57 H. Hauptman Tetruhedron 1976,32 1293. 58 M. S. Baird and C. B. Reese Tetrahedron Letters 1976,2895;J.-C. Damiano J.-L. Luche and P. CrabbC ibid. p. 779. Arynes Carbenes Nitrenes and Related Species C4c/-\c \c e- C-CrC-CrC-R R-CEC-CEC-C R/ 'R R/ \R (39d (39b) (394 intramolecular insertion reactions of cyclopropylidenes into N-H bonds and into C-H bonds adjacent to nitrogen suggests that there is a general requirement for a 1-6 relationship between the carbene and the H atom.59 Pyrolysis of the salt (40 gives tetramethylallene episulphide via the ylide (41) whereas no evidence for ylide formation could be found in the case of the salt (42).f Na+ NNTS I The failure of the oxygen analogue to form an ylide in this case is presumably due to steric factors.60 .Ylide formation with oxygen is readily observed in other cases e.g. in the dichlorocarbene-induced deoxygenation of epoxides to olefins6' and in the carbene-induced cleavage of ethem6* Re~erdy~~ has carried out a detailed examination of the reactions of xanth-enylidenecarbene (43) which shows only weakly electrophilic character as a conse- quence of a resonance contribution by (43b). + (434 (43b) The typically electrophilic behaviour of most carbenes is contrasted by the nucleophilic character of dimethoxycarbene for which a p value of +2.0 has now been determined in its reactions with aryl is~cyanates.~~ The first mechanistically unequivocal demonstration of reaction stereochemistry for this carbene shows that 59 M.S. Baird and A. C. Kaura J.C.S. Chem. Comm. 1976 356. 6o A. G. Hortmann and A. Bhattacharjya J. Amer. Chem. SOC. 1976,98 7081. 61 I. Tabushi Y. Kuroda and Z. Yoshida Tetrahedron 1976 32 997. 62 G. K. Agopian D. W. Brown and M. Jones jun. Tetrahedron Letters 1976 2931; H. Iwamura Y. Imahashi K. Kushida K. Aoki and S. Satoh Bull. Chem. SOC. Japan 1976,49 1690. 63 G. Reverdy BullSoc. chim. France 1976 1131 1136 1141. 64 R. W. Hoffmann and M. Reiffen Chem. Ber. 1976,109,2565. S. A.Matlin its additions to p -deuteriostyrenes are stere~specific.~~ Addition reactions of dimethoxycarbene to heterodienes have also been reported.66 Further work on the 1,4-addition of dihalogenocarbenes to norbornadiene and related dienes has been published,67 and the question of homoallylic delocalization in 7-norbornenylidene has been examined.68 Metal-Carbene Complexes.-Several interesting developments in the field of metal- carbene chemistry have taken place during the year. Reviews have discussed gold-carbene complexes69 and the complexation and activation of diazenes and diazo-compounds by transition metals.70 The first examples of isolable alkyl dialkyl and alkylaryl metal-carbene complexes have been reported.71 Cyclopropanations of alkenes with diazoacetates are efficiently catalysed by rhodium(I1) carboxylate~,~~ as are additions of diazoacetates to carbodi-imides.The latter reaction is also catalysed by copper triflate.73 The importance of the choice of catalyst in determining reaction course is clearly illustrated in a study of the behaviour of the diazoketone (44). This can give either the product of cyclopropana-tion (45) or the furan-3(2H)-one (46) via a carbonyl ~lide,~~ palladium complexes favouring the former rhodium acetate the latter and copper complexes giving either product depending on the ligands present. N2 (44) (45) (46) W~lfman'~ has emphasized that not only the nature of the metal and its ligands but also the catalyst concentration has an important influence on reaction of diazocarbonyl compounds. Detailed studies of the behaviour of diazomalonate with various concentrations of soluble copper catalysts reveal that in the routes to cyclopropanation allylic C-H insertion and dimerization there are competing pathways for the formation of each of the products.It is particularly interesting that 65 R. A. Moss and J. K. Huselton J.C.S. Chem. Comm. 1976,950. e6 R. Hoffmann K. Steinbach and W. Lilienblum Chem. Ber. 1976 109,1759. 67 C. W. Jefford A. Delay T. W. Wallace and U. Burger Helu. Chim. Acta 1976,59,2355;C. W. Jefford J. Mareda J. C. E. Gehret T. Kabengele W. D. Graham and U. Burger J.Amer. Chem. SOC.,1976,98 2585; C. W. Jefford V. de 10s Heros and U. Burger Tetruhedron Letters 1976,703;P. M. Kwantes and G. W. Klumpp ibid. p. 707. R. A. Moss and C.-T. Ho Tetrahedron Letters 1976 1651 3397. 69 H. Schmidbaur Angew.Chem. Internut. Edn. 1976 15 728. 70 H. Albini and H. Kisch Topics Currenf Chem. 1976,65 105. 71 E. 0.Fischer R. G. Clough G. Besl and F. R. Kreissl Angew. Chem. Infernat. Edn. 1976,15,543; R. Aumann H. Wormann and C. Kruger ibid. p. 609. 7* A. J. Hubert A. F. Noels A. J. Aciaux and P. Teyssit Synthesis 1976 600. A. J. Hubert A. Feron R. Warin and P. Teyssit Tetrahedron Letters 1976 1317. 74 S. Bien A. Gillon and S. Kohen J.C.S. Perkin I 1976 489. For another example of carbonyl ylide formation see; T. Ibata T. Motoyama and M. Hamaguchi Bull. Chem. SOC. Japan 1976,49,2298. 75 D. S. Wulfman Tetrahedron 1976,32 123 1 ;D. S. Wulfman R. S. McDaniel jun. and B. W. Peace ibid. pp. 1241 1251; D. S. Wulfman R. G. McGibboney E. K.Steffen N. V. Thinh R. S.McDaniel jun. and B. W. Peace ibid. p. 1257. '3 Arynes Carbenes Nitrenes and Related Species a metallocyclobutane is considered a likely intermediate in cyclopropanation in view of recent results on the olefin metathesis reaction. This year has seen the emergence of convincing proof of the intermediacy of metal-carbene complexes in the olefin metathesis rea~tion.’~ Detailed examinations of the behaviour of deuteriated olefins argue strongly in favour of a mechanism involving exchange of alkylidene moieties between a metal-carbene complex and an olefin via a metallocyclobutane intermediate77 (Scheme 7) and this picture is entirely consistent with the observed stereochemistry of the rea~tion.’~ Scheme 7 The tungsten-complexed ethylidene fragment generated in the metathesis of but- 2-ene has been trapped by addition to ethyl acrylate affording the cyclopropane (47).In a reversal of this addition it was demonstrated that under metathesis conditions alkylcyclopropanes will undergo fragmentation to alk- 1-enes with selec- tive loss of a methylene unit consistent with insertion of tungsten into the three- membered ring to form a transient metallocyclobutane. By combining these two sets of results the first cross-metathesis between an olefin and a cyclopropane was obtained (Scheme 8).79 PhWC13-AIC13 + PCO,Et -b. C0,Et (47) Scheme 8 76 N. Calderon E. A. Ofstead and W. A. Judy Angew. Chem. Internat. Edn. 1976,15,401;M. F. Farona and V. W. Motz J.C.S. Chem. Comm. 1976,930;T. J.Katz S. J. Lee and N. Acton Tetrahednh Letters 1976 4247; T. J. Katz and N. Acton ibid. p. 4251. 77 T. J. Katz and R. Rothchild J. Amer. Chem. Soc. 1976,98,2519. R. H. Grubbs D. D. Carr C. Hoppin and P. L. Burk ibid.,p. 3478. 78 J. L. Bilhou J. M. Basset R. Mutin and W. F. Graydon J.C.S. Chem. Comm. 1976 970. 79 P. G. Gassman and T. H. Johnson J. Amer. Chem. SOC.,1976,98,6055 6057 6058.

ISSN:0069-3030    

DOI:10.1039/OC9767300085

出版商:RSC    

年代:1976

数据来源: RSC

摘要:

6 Organometallic Chemistry Part (i) The Transition Elements By R. PEARCE D. J. THOMPSONf and M. V. TWlGG ICI Corporate Laboratory PO Box 1 I The Heath Runcorn Cheshire WA7 4QE As before we concentrate on applications of organo-transition-metal species in organic synthesis an area for which the recent literature has been reviewed.’ A major section is devoted to olefin metathesis the mechanism of which has been the subject of a remarkable number of publications. This shows strong parallels with the flood of publications in the early 1970’s on metal-catalysed skeletal isomerization which has now subsided and for which we report no major advances in 1976. Reports of reactions involving metal cluster compounds (Section 5) have increased and this appears to be a growth area for the future.1 Metal-catalysed Hydrogenation and Hydrogen-transfer Reactions In metal-catalysed asymmetric synthesis it is now accepted that the matching of ligand to substrate plays a vital part in the optimization of optical yields.’ In this matching additional constraints within the transition state arising from an interac- tion between the ligand and substrate can have a marked and beneficial effect. This is demonstrated in reports on the hydrogenation of ketones and a -acylamido-acrylic acids and the cross-coupling of Grignard reagents with alkenyl halides which involve the ferrocenyl-phosphines (1)and (2) as ligand~.~ Q’”’2 @Z CHMeX R (1) a; X=OH (2) a; R=CHMeNMez b; X = NMe b; R = CH2NMe2 c; X=H c; R=Et Thus in the Rh’-catalysed hydrogenation of pyruvic to lactic acid the hydroxyl- containing compound (la) gave optical yields in the range 59-83% whereas (lc) which contains no polar functional group (other than the tertiary phosphines) that can interact with the carbonyl group in the pyruvic acid gave only 16?40.~* In the A.P. Kozikowski and H. F. Wetter Synthesis 1976,561; ‘Transition Metal Organometallics in Organic Synthesis’ ed. H. Alper Academic Press New York 1976 Vol. 1. * For a review of asymmetric hydrogenation see J. D. Morrison W. F. Masler and M. K. Neuberg Catalysis Rev. 1976 25 81. (a)T. Hayashi T. Mise and M. Kumada TetrahedronLetters 1976,4351; (b)T. Hayashi T. Mise S. Mitachi K. Yamamoto and M. Kumada ibid. p. 1133; (c) T. Hayashi M. Tajika K. Tamao and M.Kumada J. Amer. Chern.Soc. 1976,98,3718. 99 100 R. Pearce D.J. Thompson andM. V.Twigg hydrogenation of a range of ketones optical yields using (la) compare favourably with the alternative procedure of hydr~silylation,~ and are superior to those obtained with DIOP or chiral monophosphines R'R2R3P. In the hydrogenation of the amido-acrylic acids the NMen-substituted (1 b) performed similarly to DIOP.3b In the Nil1-catalysed cross-coupling of a! -phenylethylmagnesium halide with vinyl bromide optical yields in the range 52-63% using the dimethylamino-substituted monophosphines (2a) or (2b) as ligand are markedly better than those with the chelating diphosphine DIOP (7-13%) or with the related (2c) (4%) where interaction with the magnesium of the Grignard reagent is pre~luded.~" The role of the elements of chirality in the ligand in determining asymmetric induction could be examined by comparing (2a) and (2b).The similarities in the optical yields indicated that the planar (ferrocene residue) rather than the central (Rgroup) element was dominant. It may have been expected naively that enantiomerically related ligands would give products differing only in the sign of their optical rotation. Studies on the epimeric neomenthyldiphenylphosphine(NMDPP) and menthyldiphenylphosphine (MDPP) show otherwise. In combination with [Rh(diene)Cl] in the hydrogenation of a-methylcinnamic acid NMDPP gave both a more active catalyst and a higher enantiomeric excess [6O% (R) versus 17% (S)for MDPP].' Conformational factors appear to be responsible.In the search for new chiral ligands reports have included the chelating diamine (3) obtained from a -phenylethylamine," and the sulphoxides R'R2S0.66 Whereas in the Rh'-catalysed hydrogenation of a-acylamido-acrylic acids the ligand (3) behaves similarly to DIOP results with a range of sulphoxides were far from encouraging. PhCHMe PhCHMe (3) The attachment of metal centres to cross-linked resins (heterogenized catalysts) offers many advantages in the ease of catalyst recovery but still poses a number of problems. Conventional resins which swell in non-polar solvents collapse in polar media and are thus unsuitable for use in the hydrogenation of a -acylamido-acrylic acids. To overcome this a new resin has been prepared which is based on a lightly cross-linked hydroxyethyl methacrylate backbone that contains ca.8% of styrene units modified by attachment of a DIOP residue. With Rh'its performance parallels For full details see I. Ojirna,T. Kogure M. Kurnagai S. Horiuchi and T. Sato J. Organometallic Chem. 1976,122,83. 5 A. M. Aguiar C. J. Morrow J. D. Morrison R. E. Burnett W. F. Masler and N. S. Bhacca,J. Org. Chem. 1976,41 1545. (a) M. Fiorini G.M. Giongo F. Marcati and W. Marconi J. Mol. Catalysis 1976,1,45 1; (6) B. R. James R. S. McMillan and K. J. Reimer ibid. p. 439. Organometallic Chemistry -Part (i) The Transition Elements 101 homogeneous DIOP-containing system^.^ Rhodium(1) supported on the novel 2,3- O-bis(diphenylphosphino)-6-O-triphenylmethylcellulose effects stereoselective hydrogenation of 2-phenylbut-1-ene in up to 77% optical yield.This remarkable selectivity with a relatively simple olefin contrasts with the poor optical yield of 17.5% when a!-acetamidocinnamic acid was substrate and emphasizes the need for a careful match of ligand and substrate in the design of asymmetric syntheses.* A surprising improvement in the activity of silica-supported Rh' complexes has been achieved by treatment of silica with a silicone ladder polymer that was subsequently modified via incorporation of organophosphine gro~ps.~ The turnover number with the catalyst was 28 times greater than that obtained with the (Et0)3SiCH2CH2PPh2-modified silica catalyst previously reported by the B.P.Group again highlighting the inadequacy of our knowledge of the role of (modified) inorganic supports in determining catalytic activity. A highly regioselective partial reduction of substituted cyclic anhydrides (e.g. 2,2-dimethylsuccinic anhydride) to y-lactones has been achieved using RuC~~(PP~~)~ as catalyst. lo Hydrogenation occurs at the least hindered carbonyl group contrasting with the reduction with LiA1H4 which affects the more hindered one. Novel catalyst systems have included CoBr(PPh3),/BF3,0Et2,' la which selec- tively hydrogenates conjugated dienes to monoenes via 1,2-hydrogen addition at the more substituted double bond and the Fe(CO)5-photocatalysed system,' lb which complements earlier work on chromium carbonyls.Reactions involving synthesis gas as a chemical feedstock are likely to command increasing attention. In this context it is interesting to report the stoicheiometric production of methoxide ion by bis(pentamethylcyclopentadienyI)zirconium12a(cf. reactions of metal clusters that give methane and ethanediol). Metal formyl inter- mediates are implicated in this reaction (cf. Ref. 126). Transition-metal stearates (Tulupov catalysts) have been the subjects of a number of papers and reviews. In particular homogeneous hydrogenation of aromatics has been claimed. However a recent investigation shows that all is not well they showed neither the reported solubility in ethanol nor were they active in cyclohexene hydr~genation,'~" results confirmed by other workers.13' Monosaccharides are effective donors in Ru"-catalysed [e.g.RuC~~(PP~~)~] trans-fer hydrogenation of a,P -unsaturated ketones. l4 With prochiral substrates asym- metric induction is observed with optical yields up to 34% using 1,2-isopropylidene- a!-D-glucofuranoside as donor. This is the first example of asymmetric transfer hydrogenation. Monosaccharides and related compounds offer a range of readily available optically pure materials that appear to have been largely neglected both as donors and as ligands in asymmetric synthesis (cf.Ref. 8). N. Takaishi H. Imai C. A. Bertelo and J. K. Stille J. Amer. Chem. Soc.,1976 98 5400. Y. Kawabata M. Tanaka and I. Ogata Chem. Letters 1976 1213. J. Conan K. Bartholin and A. Guyot J. Mol.Catalysis 1976 1 375. lo P. Morand and M. Kayser J.C.S. Chem. Comm. 1976 314. l1 (a)K. Kawakami T. Mizoroki and A. Ozaki Chem. Letters 1976,847; (b)M. A. Schroeder and M. S. Wrighton J. Amer. Chem. Soc.,1976,98 551. I* (a)J. M. Manriquez D. R. McAllister R. D. Sanner and J. E. Bercaw J. Amer. Chem. Soc.,1976,98 6733. (b)C. P. Casey and S. M. Neumann ibid. p. 5395. l3 (a)J. W.Larsen and L. W. Chang J. Org. Chem. 1976,41,3332; (b)D. J. Thompson unpublished work. l4 G. Descotes and D. Sinou Tetrahedron Letters 1976 4083. 102 R.Pearce D.J. Thompson,andM. V.Twigg Noble metal salts (e.g.RhCl,) catalyse the homogeneous transfer hydrogenation of aromatic nitro-compounds to aromatic amines. lSQ Indoline was the hydrogen donor. Comparable results have been obtained with the heterogeneous Pd- C/cyclohexene system,lsb which with FeC13 as promoter effects the reduction of aromatic ketones to the corresponding hydrocarbon lbOand gives rapid and selective removal of the benzyloxycarbonyl protecting group from amino-acids.16’ Transition-metal-catalysed activation of saturated hydrocarbons remain as elusive a goal as ever. To this end RuL2H(2-naphthyl) (L =Me2PCH2CH2PMe2)reacts with hydrocarbons RH with loss of CtoHs to give RuL2(H)R.” Hydrocarbons include aromatic compounds activated CH species (e.g. MeCN Me2C0 MeCOzEt or cyclopentadiene) and acetylenes. 2 Dmerization Oligomerization and Polymerization Vollhardt et al. have extended their work which uses benzocyclobutenes as o-xylylene synthons to the generation of naphthalenes and polycyclic systems.*’ The basis of these reactions involves the (q-CsHs)Co(C0)2-catalysed cyclodimerization of Me3SiCrCSiMe3 with 3-substituted hexa- 1,Sdiynes to give benzocyclobutenes (4) as intermediates.With 3-alkoxy-derivatives (R =e.g. OMe or OSiMe,) further reaction with Me3SiCrCSiMe3 yields the naphthalene (5) i.e. the diyne acts as a tetramethynylethylene precursor.18a When R =XCH2YCH,CH=Z (e.g. X =0 Y =CH2 and 2= CH,) intramolecular cycloaddition takes place to give the polycyclic compound (6).186 Me3SimR Me3SicR ~ Me,Si Me,Si \ X-Y Me3Si SiMe Me3Si Me,Si \ / SiMe Me,Si \ Modified organopalladium catalysts (e.g. q 3-allylpalladium acetate in combina- tion with tertiary phosphines) gives good yields (up to 79%) of the head-to-tail dimers of isoprene.’’ These were readily separated from the reaction mixture as 7-chloro-3,7-dimethyloct-1-ene by reaction with HCl.The complex [Pt(MeCN),]- [(BF4)2J catalyses the dimerization of branched olefins. Thus 2-methylbut-2-ene (a)H. Imai T. Nishiguchi and K. Fukuzumi Chem. Letters 1976,655; (b)I. D. Entwhistle R. A. W. Johnstone and T. J. Povall J.C.S. Perkin I 1975 1300. l6 (a)G. Brieger and T.-H. Fu J.C.S. Chem. Comm. 1976,757;(b)A. E. Jackson and R. A. W. Johnstone Synthesis 1976,685. S. D. Ittel C. A. Tolman A. D. English and J. P. Jesson J. Amer. Chem. Soc. 1976,98,6073. (a)R.L. Funk and K. P. C. Vollhardt,J.C.S.Chem.Comm.,1976,833;(b)J.Amer. Chem. Soc. 1976,98 6755. 19 J. P. Neilan R. M. Laine N. Cortese and R.F. Heck J. Org. Chem. 1976,41 3455. Organometallic Chemistry -Part (i) The Transition Elements gave a 3 :1mixture of 2,3,4,4-tetramethylhex- 1-ene and 3,4,4,5-tetramethylhex-2-ene.*O Further insights into the mechanism of diene and acetylene dimerization have come from studies of palladium and platinum complexes.21 Complex (7) has been (7) isolated from the reaction of bis(cyc1o-octa- 1,5-diene)platinum with butadiene and is related to the proposed intermediate in the Nio-catalysed cyclodimerization to 1,2-divinylcycl0butane.~~"The trans-2,5-divinyl structure confirmed by crystal structures of the complexes with L = Bu'NC and L = 1,5-CsHI2 may not be at variance with the cis -2,5-divinyl arrangement in the proposed nickel intermediate but may merely reflect conformational differences imposed by platinum being the larger metal atom.From studies of some hydridoalkynyl-palladiumand -platinum compounds it appears that the pathway for acetylene dimerization involves alkenyl- alkynyl intermediates [M(CH=CHR)CrCR] rather than the alternative hydrido- alkenyl species [M(H)CH=CRCrCR].216 The use of cross-linked resins as carriers in heterogenized catalysts previously largely confined to hydrogenation has been extended to Ziegler-Natta polymeriza- tion catalysts.22 Titanium- or vanadium-based systems have been supported on resins which swell on contact with hydrocarbons e.g. cross-linked ethylene/propylene/diene rubbers grafted with 4-vinylpyridine. In ethylene polymerization polymer was produced in the pores of the resin and was recovered free from catalyst residues either uia subsequent treatment with hot diluent or by conducting the reaction above the melting point of the polymer.Catalysts could be recycled many times without loss of activity or of metal. With mixed metal systems (e.g.with vanadium and nickel species supported on the same resin) both dimeriza- tion and polymerization occurred to give an ethylene/butene copolymer. A particularly active halogen-free homogeneous polymerization catalyst has been described.23 This is based on cyclopentadienyl-titanium(1v)or -zirconium(Iv) complexes in combination with an aluminoxane (the product of reaction of a trialkylaluminium compound with water). Hitherto no related active zirconium catalyst was known and attempts to prepare halogen-free cyclopentadienyltitanium systems had suffered from problems of lop of activity through reduction to species of lower oxidation state.The R2A10AlR2 unit one of the many 'preferred' activators in the prolific patent literature on heterogeneous TiCL catalysts appears to be the essential ingredient for this stabilization and activation. With ethylene conversions 2o A. de Renzi A. Panunzi A. Vitagliano and G. Paiaro J.C.S. Chem. Comm. 1976,47. 21 (a)G. K. Barker M. Green J. A. K. Howard J. L. Spencer and F. G. A. Stone J. Amer. Chem. Soc. 1976 98 3373; (b) Y. Tohda K. Sonogashira and N. Hagihara J. Organometallic Chem. 1976 110 c53. 22 V. A. Kabanov V. I. Srnetanyuk and V. G. Popov Doklady Akad.Nauk S.S.S.R.,1975,225 1377. 23 A. Andresen H.-G. Cordes J. Herwig W. Kaminsky A. Merck R. Mottweiler J. Pein H. Sinn and H.-J. Vollrner Angew. Chem. Internat. Edn. 1976 15,630. 104 R. Pearce,D.J. Thompson andM. V. Twigg were in the region of 2 x lo4g mmol-' h-' but the catalysts appear to be inactive with propylene. Molecular weights were sensitive to and could be controlled by changes in reaction temperature. Much of the confusion surrounding the mechanism of the transition-metal- catalysed polymerization of 1,3-dienes has been cleared up in a recent paper that includes a summary of much of the literat~re.,~ It has been established that (i) q 3-allylic intermediates are involved; (ii) the anti-isomer is formed initially by kinetic control and subsequently isomerizes to the syn-species; and (iii) the type of polymer produced will depend on the relative stabilities of and on the relative rates of monomer insertion into the syn-and anti-isomers.3 Carbonylation Interest in the carbonylation of the olefins continues and a series of ligand-stabilized Pt" or Pd"-Group 4B halide complexes have been shown to be active catalysts.25 Carbonylation of terminal olefins in the presence of alcohol and catalysts of the type PtCl,(AsPh,),/SnCl or PtC1,[P(OPh),],/SnCI2 gives the linear ester with up to 98% selectivity. The related palladium complexes give lower selectivity (85%) but operate at lower CO pressures. Depending on the conditions employed different products can be obtained in the carbonylationof olefins using palladium catalysts.26 Linear a -olefins in the presence of PdC12/CuC12/MeOH give the p-methoxy-esters RCH(OMe)CH,CO,Me in good yield (>50%) whereas addition of an equimolar amount of sodium acetate gives exclusive formation of the succinic ester RCH(C02Me)CH2C02Me.Cyclic olefins give predominantly the diesters even in the absence of base. An interesting use of phase-transfer catalysis has been described in the carbonyla- tion of aryl benzyl and vinyl halides to the corresponding carboxylic acid.,' The halide in an organic solvent is stirred rapidly with a mixture of aqueous NaOH [PdC12(PPhJ2] and Bu4NI under carbon monoxide. The catalyst stays in the organic phase and can be recycled whilst the product is easily isolated in high yield from the aqueous phase as its sodium salt.Moreover the system can be very selective; for example producing p-bromobenzoic acid from p-dibromobenzene in 90% yield. PdC12/sodium acetate has been used to carboxylate aromatic compounds but the yields are poor. It is now reported that sodium palladium malonate in acetic acid/acetic anhydride however gives good yields of the aromatic acid.28 Benzene for example gives benzoic acid in 72% yield but biphenyl (20%) is produced as a side-product. The yields can be improved by the addition of silver acetate. The first transition-metal-catalysed CO insertion into dienes or enones (8) with CO (40 atm; 20°C) in the presence of [RhCl(CO),] (Scheme 1) gives the corres- ponding aromatic compound (10) in 90% yield.The reaction is thought to go via the intermediate (9).29 24 V. A. Kormer M. I. Lobach V. I. Klepikova and B. D. Babitskii,J. Polymer Sci.,Polymer Letters Edn. 1976,14,317. 25 J. F. Knifton J. Org. Chem. 1976,41 793 2885. 26 D.E. James L. F. Hines and J. K. Stille,J. Amer. Chem. SOC.,1976,98 1810. 27 L.Cassar M. Foi and A. Gardano J. Organometallic Chem. 1976,121,C55. 28 T.Sakakibara and Y. Odaira,J. Org. Chem. 1976,41,2049. 29 R.F. Heldeweg and H. Hogeveen J. Amer. Gem. SOC.,1976,98,6040. Organometallic Chemistry -Part (i) The Transition Elements Reagents i [Rh(C0)2Cl], CO (X=CH2 or 0). Scheme 1 Work on asymmetric hydroformylation is continuing to improve optical yields and to increase our understanding of the reaction mechanism.Using the DIOP- related ligand (11).in the presence of rhodium increased optical yields have been obtained for both styrene (44%) and but-1-ene (~OYO).~' A study of the asymmetric hydroformylation of a-methylstyrene using PdC12/( -)-DIOP in the presence of alcohol has revealed that the optical yield varies with the alcohol CO pressure and metal/ligand ratio.31 The optical yield reaches a maximum with t-butyl alcohol increases with CO pressure (going from 3.5to 50% as the pressure is increased from 50 to 700 atm) and is also favoured by low ligand/metal ratios (59% at 0.4 ligand/metal). At the lower ligand ratios however the reaction rate is slower and by-products are formed. In the asymmetric hydroformylation of straight-chain butenes using PtCI2[( -)-DIOP]/SnCl, the chirality of the product is opposite to that when RhH(C0)- (PPh3)J( -)-DIOP] is used suggesting that the asymmetric induction does not just originate by steric interaction between substrate and ligand.32 Interest in organic reactions of CO involving transition-metal complexes has increased but to date very few synthetic advances have been made in this area.The use of copper cyanoacetate acting as a carrier of activated CO for the conversion of propylene oxide into propylene carbonate in 83% yield however has been described.33 The oligomerization of butadiene in the presence of CO using a palladium phosphine complex gives the lactone (14) in low yield via the acids (12) and (13) together with various butadiene 01igomers.~~ 30 M. Tanaka Y.Ikeda and I. Ogata Chem. ktters 1975 11 15. 31 G. Consiglio and P. Pino Chimia (Swirz.) 1976 30 193. 32 G. Consiglio and P. Pino Helv. aim. Acta 1976,59 642. 33 T. Tsuda Y. Chujo and T. Saegusa J.C.S. Gem. Comm. 1976,415. Y. Sasaki Y. Jnoue and H. Hashimoto J.C.S. Gem. Cbmm. 1976,605. R.Pearce D.J. Thompson,andM. V.Twigg 4 Synthesis of N-Heterocyclic Compounds Transition-metal complexes are finding increasing use in the synthesis of organic heterocyclic compounds. They often give better yields and selectivity than the more conventional synthetic methods and it can be anticipated that much progress will be made in this area over the next few years. Alper and Prichett have studied the reaction of 2-aryl-azirines (15)with a variety of transition-metal complexes and have obtained an interesting array of reaction products (see Scheme 2).35 The styrylindoles (16) which could be useful inter- mediates in alkaloid synthesis are obtained in up to 90% yield using either CO~(CO),~~'" or [RhC1(C0)2]2.356 The reaction of (15) with Fe2(CO)9 gives the 2,5-diarylpyrrole (17),35cwhereas reaction with silver perchlorate gives the 2,5- diarylpyrazine (18)in up to 35% yield.35d (15) \ iii Reagents i Fe2(CO),; ii Co2(CO)8 or [Rh(C0)2Cl]2; iii Ag+.Scheme 2 The organocobalt-catalysed synthesis of substituted pyridine from alk- 1-ynes and nitriles has been extended further by the discovery that the reaction is catalysed by the easily obtainable cobaltocene Co(q5-C5H5)2.36 Acetylene reacts with substituted nitriles to give 2-substituted pyridines in up to 60% yield whereas monosubstituted 35 (a)H.Alper and J. E. Prickett Tetrahedron Letters 1976,2589; (b) J.C.S. Chem. Comm. 1976,483; (c) ibid. p. 191; (d)ibid. p. 983. 36 Y. Wakatsuki and H. Yarnazaki Synthesis 1976 26. Organometallic Chemistry -Part (i) The Transition Elements acetylenes react to give mixtures of 2,4,6- and 2,3,6-trisubstituted pyridines [(19) and (20)] in moderate yield. (19) (20) Although $-unsaturated ketoximes (21) are known to cyclize at around 300 "C the reaction can be induced under much milder conditions in the presence of PdC12(PPh3)2/NaOPh.37 The reaction products are substituted pyridines (22) which are obtained in moderate yield. R' R3 Rl,&&,R4 PdC12(PPh3)2-NaOPh ______* :Q:: NOH (21) (22) Substituted indoles (24) can be synthesized by cyclization of compounds of the type (23) in the presence of [Ni(PPh3)4] followed by Cyclization of the related compounds (25) gives the corresponding oxindole (26)in up to 70% yield.386 CH2R oIrR a Ni(PPh3)4 Me Me U (23) ( C R Ni(PPh3)4) (24) CH2Ra* Me Me (25) (26) Another synthesis (Scheme 3) of indoles (29) involves intramolecular cyclization of the o -allylaniline (28) using [PdC12(MeCN)2].39 Yields are good (70-85%) and the starting materials can be readily synthesized by the reaction of ?r-allylnickel bromide with the corresponding o -bromoaniline (27).The reaction of the palladium complex (30) with an isocyanide gives the stable complex (31).40 This complex thermally decomposes at 100"C to give the 3-imino- 2-phenylindazoline (32) whereas the reaction of the complex (31)with COyields the 3-0x0-2-phenylindazoline (33) both reactions proceeding in good yield.37 T. Hosokawa N. Shimo K. Maeda A. Sonoda and S. I. Murahashi Tetruhedron Letters 1976,383. 38 (a)M. Mori and Y. Ban Tetrahedron Letters 1976 1803; (6)ibid. p. 1807. 39 L. S. Hegedus G. F. Allen and E. L. Waterman,J. Amer. Chem. Soc.,1976,98,2674. 40 Y. Yamamoto and H. Yamazaki Synthesis 1976,750. R. Pearce,D.J. Thompson andM. V.Twigg Reagents i (r-allyl)NiBr; ii PdC12(MeCN)2; iii Et,N. Scheme 3 Ph Ph I I N=N C1 AN=N ,Pd C1 L/ \/z u&/ A d ,Pd + 2RNC -+ 100°C X\ NPh \ CNR 0 X X NR x\ NPh e 0 (33) Rearrangement of the aziridine (34) with a catalytic amount of [PdC12(PhCN)2] gives the N-carboxynortropidine (35) in quantitative yield.4' This is a particularly useful reaction since the product (35)is the backbone of the tropane alkaloids.(34) (35) An interesting (4+2) cycloaddition across the diheterodiene system in the copper complex (36) occurs with dimethyl acetylenedicarboxylate to give the substituted 1,4-benzoxazines(37).42 Yields are excellent (ca. 90%) for a variety of substituted complexes and it is noteworthy that no reaction occurs with the uncomplexed nitroso-phenols Ra \; +/ cq + 111 C02Me I I 0-OH (36) (37) 4' G. R. Wiger and M. F. Rettig J. Amer. Chem. Soc. 1976,98 4168. 42 A.McKillop and T. S. B. Sayer J. Org. Chem. 1976,41 1079. Organometallic Chemistry -Part (i) The Transition Elements The reaction of N-sulphinylaniline (38) with diphenylcyclopropenone (39) in the presence of Ni(C0)4 gives the pyrroline-2,5-dione (40) in 78% yield.43 The reaction is thought to go via the complex (41) followed by exchange of the S=O group by CO. In contrast the reaction of N-sulphinylcyclohexylanilinegives the substituted isothiazolone 1-oxide (42) in 36% yield and no product is found in which CO is exchanged for SO. 0 Ni(CO)4 phA,,+ R-N=S=O (R=Ph) 06 0 R (39) (38) (40) Ni(CO)4 (R = cyclohexyl) 1 An extension of the work on co-oligomerization has led to an efficient synthesis of 1,2-diaza- 1,5,9-~yclodecatrienes (43).44 Thus co-oligomerization of butadiene with ketazines or aldazine in the presence of a nickel phosphine or phosphinite catalyst gives the cyclic products (43) in 50-80% yield.R2 R’ R2 I1 I 5 The Use of Metal Cluster Complexes in Catalysis Although a lot of work has been done on the structure and synthesis of metal cluster complexes it is only recently that they have begun to find use as catalysts. A very good review45 was published last year which compared the chemical and catalytic properties of discrete metal clusters with those of metal surfaces. Metal clusters are beginning to emerge which have very interesting catalytic properties. One of the most important industrial applications has been the conver- sion of CO/H mixtures into ethylene glycol.Various rhodium precursors have been employed and appear to work through a common RhI2 intermediate 43 A. Baba Y. Ohshiro and T. Agawa Chem. Letters 1976 11. 44 P. Heimbach B. Hugelin H. Peter A. Roloff and E. Troxler Angew. Chem.Internat. Edn. 1976,15,49. 45 E. L. Muetterties,Bull. SOC.chim.belges 1975 84 959. 110 R. Pearce D.J. Thompson and M. V. Twigg [Rh12(C0)30)2-.46 Typical reaction conditions are 2 10-250 "C and 500-2000 atm; the other major reaction products are methanol glycerine and ethanol. In contrast with the rhodium clusters OS~(CO)~~ and Ir4(C0)12 catalyse the specific reduction of CO to methane.47 The reaction proceeds under fairly mild conditions (140 "C 2 atm) but the reaction rate is slow.Addition of Ph3P to the iridium system increases the rate of production of methane but ethane and propane are also produced. Using trimethyl phosphite the rate is three times faster than the Ph,P/Ir4(C0)12 system and moreover the selectivity for methane production is maintained. A large number of mononuclear complexes were examined but none were active for the reduction of carbon monoxide. The complexes [(Co2B)loRhH6] and [(Ni2B)10Rh15] are prepared from Co'' or Ni" chloride and RhCl in the presence of NaBI-L.48 These cluster complexes are active catalysts for the hydrogenation of olefins and by contrast with Raney nickel give no isomerization products. They are also catalysts €or the reduction of carbon monox- ide to methane. [RU~(CO)~~] is an active catalyst for the isomerization of pent-1-ene giving a mixture of pent-1-ene (3%) cis-pent-2-ene (23'/0) and trans-pent-2-ene (74%).49n In the presence of a small amount of acetic acid there is a ten-fold increase in reaction rate but the initial cis/trans ratio is unchanged.Since no scrambling of hydrogen between the acid and the pentenes is observed on using MeC02D a metal hydride addition-elimination mechanism is excluded and a v-ally1 metal hydride inter- mediate is suggested. os3(co)1249b also show catalytic activity and &RU(CO)~~~" for the isomerization of pent- 1-ene to cis- and trans-pent-2-enes. A series of active supported nickel clusters have been prepared by pyrolysis of [Ni(q '-C5HJ2] [Ni(q 5-C5H5)2(C0)2] and [Ni3(q '-C5H5)3(CO)2] dispersed on silica.50 Although acetylene trimerizes readily at room temperature in 75% yield over dispersed nickel from (q'-C,H,),Ni there is no reaction with the other two nickel clusters.All three clusters however are active for H2/D2 exchange and hydrogenation of ethylene and benzene. Pyrolysis of platinum clusters of the type [Pt,(C0)4][NEt4]2 (x = 3,6,9,12 or 15) dispersed on silica or alumina gives highly dispersed platinum crystallites which are active catalysts for the dehydrocyclization of n-he~ane.~~ The smaller platinum aggregates from Ptd and Pt3 on y-alumina gave methylcyclopentane with up to 94% selectivity and negligible skeletal isomerization. 6 Oleh Metathesis The high level of interest in olefin metathesis has continued with some fifty relevant publications appearing in 1976.Most are concerned with elucidating the mechanism of this reaction. A detailed review was also p~blished.~' 46 e.g. U.S.P. 3974259. 47 M. G. Thomas B. F. Beier and E. L. Muetterties J. Amer. Chem. Soc. 1976 98 1296. 48 R. W. Mitchell L. J. Pandolfi and P. C. Maybury J.C.S. Chem. Gbmm. 1976 172. 49 (a)M. Castiglioni L. Milone D. Osella G. A. Vaglio and M. Valle Zmrg. Chem. 1976,15,394; (6)R. P. Ferrari and G. A. Vaglio Znorg. Chim. Acta 1976,20 141; (c)M. Valle D. Osella and G. A. Vaglio ibid. p. 213. M. Ichikawa J.C.S. Chem. Comm. 1976 26. 51 M. Ichikawa,J.C.S. Chem. Comm. 1976 11. 52 N. Calderon E. A. Ofstead and W. A. Judy Angew. Chem. Internat. Edn. 1976,15,401. Organometallic Chemistry -Part (i) The Transition Elements 111 Mechanistic Aspects.-The current consensus is that the mechanism of olefin metathesis involves a metal-carbene chain process.Failure to detect cyclobutane products was an important factor arguing against earlier proposed mechanisms based on ‘quasicyclobutane’ transition states. It is therefore noteworthy that Gassman and Johnson found that the diolefin (44)is smoothly converted into the thermally stable cyclobutane (45)by a catalyst system typically used for metathesis e.g. [W(Ph)(CO),] activated with AlC13. Some related diolefins do not undergo cyclization but the corresponding cyclobutanes are rapidly converted into diolefins on exposure to cata~yst.~~ (44) (45) Support for the feasibility of metallocyclobutane-carbene interconversions comes from a number of studies (i) Reaction of Metallocyclobutanes.It is suggested that the photolysis of some tungsten complexes causes a q5-C5H5to q3-C5Hs shift in (46);rearrangement to co-ordinated carbene and olefin in the 16-electron species then takes place leading to free olefin that contains one less carbon atom than the initial metallocyclob~tane~~ (Scheme 4). (v-C5H5)*W R2 5 (46) Scheme 4 (ii) Initiation by Metal-Carbene Complexes. Further support has been obtained for the metal-carbene chain mechanism. Several isolable metal-carbene complexes initiate metathesis. In the presence of TiCI4 [W(C0)5C(OEt)R] is a very active catalyst for cyclopentene polymerization but thermal or photo-activation is neces- sar~.’~~ More reactive complexes e.g.[W(C0)5CAr2] are active in the absence of 53 P. G. Gassman and T. H. Johnson J. Amer. Gem. Soc. 1976,98,861. 54 M. Ephritikhine and M. L. H. Green J.C.S. Chem. Comm. 1976,926. 55 (a) E. 0. Fischer and W. R. Wagner J. Organometallic Chem. 1976 116 C21; Y. Chau-vin D. Commereuc and D. Cruypelinck Makromol. Chern. 1976. 177 2637; (6)T. J. Katz and N. Acton TetrahedronLetters 1976,4251;T. J. Katz S. J. Lee and N. Acton ibid. p. 4247; (c)J. McGinnis T. J. Katz and S. Hurwitz,J. Amer. Chem. Soc. 1976,98,605; T. J. Katz J. McGinnis and C. Altus ibid. p. 606; C. P. Casey H. E. Tuinstra and M. C. Saeman ibid. p. 608. 112 R.Pearce,D.J. Thompson andM. V.Twigg co~atalyst.~~~ Results of detailed product analysis of reactions of octa- 1,7-diene and 2,2'-divinylbiphenyl with their specifically deuteriated derivatives are in conflict with mechanistic schemes involving the union of two olefin molecules but in agreement with the carbene (iii) Addition of Carbene Traps.Addition of a typical Michael acceptor (such as ethyl acrylate) to metathesis reactions catalysed by [W(Ph)(C0)5]/AlC13 completely quenched the process and in keeping with the trapping of carbenes low yields of the appropriate cyclopropane derivatives were isolated. These observations suggest that the metal-carbene complex has a nucleophilic carbon. An important feature is that ethyl acrylate does not quench the formation of the cyclobutane (45)from the diolefin (44).56d (iv) In Situ Generationof Curbenes.Solvent dependency in metathesis reactions is well known and it has now been shown that solvent can influence the first formed carbene species. In chlorobenzene propylidene intermediates are initially formed during the metathesis of octa- 1,7-diene catalysed by [Re(CO)5CI]/EtAlCIz but with [Mo(CO)~(~~)]/E~A~C~~/BU~NCI, ethylidene is the initially formed carbene. How- ever propylidene species are involved in the latter system when heptane is the sohen t .56c Although strong evidence is available for the metal-carbene mechanism the mode of carbene generation is not clear. Routes involving alkyl or hydride species have been proposed but in several instances this course is not readily available. In suitable cases 1,2-hydride shift in a co-ordinated olefin via q3-allyl formation could provide a paths7" (Scheme 5).A related co-ordinated olefin-carbene interconver- H 1T Scheme 5 sion takes place in [W(C0)4C(Ph)CHzR] where the co-ordinated carbene rapidly changes to the olefin PhCH=CHR.57b The problem of initiation is particularly pertinent with heterogeneous catalysts. The generation and role of metal hydride 56 (a) R. H. Grubbs D. D. Carr C. Hoppin and P. L. Burk J. Amer. Chem. Soc. 1976,98,3478; (b)R. Rothchild and T. J. Katz ibid. p. 2519; (c) M. F. Farona and V. W. Motz J.C.S. Cltem. Comm. 1976 930; W. S. Greenlee and M. F. Farona Znorg. Gem. 1976 17 2129; (d) P. G. Gassman and T. H. Johnson J. Amer. Chem. SOC.,1976,98,6055. 57 (a)M. Ephritikhine M. L. H. Green and R.E. MacKenzie J.C.S. Chem. Comm. 1976,619; (6) E. 0. Fischer and W. Held J. Orgunometallic Chem. 1976,112 C59; (c) D. T. Laverty J. J. Rooney and A. Stewart J. Cutulysis,1976,45 110;(d)M. T. Mocella R. Rovner and E. L. Muetterties J. Amer. Chem. Soc. 1976,98,4689. Organometallic Chemistry -Part (i) The Transition Elements 113 species has been discussed and carbene formation via a 1,2-hydride shift process is Potential difficulties in obtaining reliable quantitative results (a well- known problem for more conventional radical processes) are emphasized by the fact that traces of oxygen or water are essential for activity of commonly used [WC16]- metal alkyl catalysts. This is not the case for WOCL which can be activated when water and oxygen are rigorously excluded.Similarly new soluble catalysts based on [W(OMe)6] or [WO(Me)J are not impaired by these procedures but for high activity it appears that chlorine must be present in the co~atalyst.~'~ Applications.-'Muscalure' [(Z)-tricos-9-ene] the sex pheromone of the house fly has been obtained from the metathesis of octadec-9-ene and hexadec-2-ene. Typi- cally [WC16]/EtAlC12/EtOH gave the desired cis-product in ca. 10-20% yield. A number of related bioactive compounds were similarly An attempted novel application of olefin metathesis involved the use of 1-methylcyclobutene in the addition of an isoprene equivalent to a terpene but only poor yields were obtained [equation ( 1)].58b Polyisoprene is obtained from the metathesis of 1-methylcyclobutene using a carbene catalyst.It is noteworthy that a consequence of the carbene mechanism is the formation of a (2)('cis ')-polymer.55b e:eAc + dMe + $?"-(1) Me 'Me Me Polymerization double-bond migration alkylation of solvent etc. can be trouble- some. Addition of polar molecules such as esters can suppress these During an investigation of side-reactions it was discovered that WCl6/A1EtCl2 catalyses the conversion of polyalkylbenzenes into monosubstituted species a reaction having possible An unusual use of a metathesis catalyst (WCI6/LiPh) is for the arylation of ethers and aminesSSe [equation (2)] which in some cases gives high yields. Ph / Et2O EtOCH (2) 'Me A major problem associated with olefin metathesis is that with few exceptions functionally substituted olefins normally poison the catalyst.59 The reactivities of a number of substituted acyclic olefins towards metathesis have been reinvestigated.Apart from alkenes with ester groups remote from the double bond only low conversions were observed.60u Other examples of metathesis of unsaturated esters (a)F.-W. Kupper and R. Streck Z. Nuturforsch.,1976,31b 1256; (b)S. R. Wilson and D. E. Schalk,J. Org. Chem. 1976,41,3928; (c)K. Ichikawa and K. Fukuzumi ibid. p. 2633; (d)L. Hocks A. Noels. A. Hubert and P. Teyssie ibid. p. 1631; (e)J. Levisalles H. Rudler and D. Villemin J. Organometuflic Chem. 1976,122 C15. 59 R. Streck Chem. Ztg. 1975,99 397. (a)R. Nakamura S. Matsumoto and E. Echigoya Gem. LRtters 1976,1019; (b)W.Ast G.Rheinwald and R. Kerber Makromol. Chem. 1976,177 1341; (c)ibid. p. 1349. 114 R.Pearce D.J. Thompson andM. V. Twigg include ring opening of lactones with remote double bonds to give unsaturated po1yesters,60bsC and the ring-opening polymerization of (47) to give an unusual polymer (48). p-Alkyl- and p-halogeno-allylbenzenes undergo normal metathesis whereas the presence of a methoxy-group completely inhibits the reaction.61 H C0,Et X CHCO2Et /\ + [=CH( CH2)2 CH-CH(CH& CH=] Alkyl-substituted cyclopropanes are readily fragmented by [W(Ph)CI,]/AICI to give ethylene and substituted olefin. It is thought that the proceeds via metallocyc- lobutanes. In this way bicyclo[2,1,0]pentane is converted into cyclobutene in 70% yield suggesting that this reaction may have use in the preparation of some unusual olefins which reluctantly enter into metathetical reactions.62a The related cross-metathesis of deactivated olefins e.g.(49; R = C02Me) with alkyl-substituted cyclopropanes has some potential but is restricted by low yields [equation (3)].626 H R' H R2 x + x + CH2=CHRZ +CH2=CHR1 (3) (49) 7 Isomerization The smooth isomerization of a variety of cycloalkenones has been shown to be catalysed by rhodium trichloride in ethanol at 100"C. This convenient new enone transposition reaction is also effective when the double bond is not a part of the cyclic [e.g. equation (4)J.Ethylene bis(tri-o-tolyl phosphite)nickel(o) in the 0& -0% presence of HCl is a useful catalyst for the rapid isomerization of alkenes with polar functional groups.For example hex-5-enal is converted into a mixture of cis-and trans-4-isomers and ethyl pent-4-enoate is converted into a mixture of ethyl cis-and trans -pent-3-enoates without the usual formation of crp-unsaturated com- pounds. Alcohols having unsubstituted double bonds are slowly converted into saturated carbonyl compounds in high yield but the catalyst is not long-lived.a 61 P. Chevalier D. Sinou G. Descotes R. Mutin and J. Basset J. Organometalfic Chem. 1976 113 1. 6z (a) P. G. Gassman and T. H. Johnson J. Amer. Chem. Soc.,1976,98,6057; (6)p. 6058. 63 P. A. Grieco M. Nishizawa N. Marinovic and W. J. Ehmann J. Amer. Chem. Soc. 1976,9% 7102. 64 C. F. Lochow and R. G. Miller J.Org. Chem. 1976,41,3020. Organometallic Chemistry -Part (i) The Transition Elements 8 Reactions of Co-ordinated Ligands and Related Topics Reactions of Co-ordmated Ligands.-Chromium tricarbonyl complexes of indanones and tetralones undergo a highly stereoselective Michael addition reaction with methyl vinyl ketone and an unusual annulation reaction involving activation of a benzylic hydr~gen.~' The isomeric 2-methylindanone complexes (50) give (5 1) as the major product (87%) on treatment with methyl vinyl ketone addition occurring selectively at the exo-face. On treatment with base the normal aldol condensation product (52) was obtained as a minor product (5-lo%) the major product being the unexpected (52) (2 isomers) formed through reaction at the activated benzylic position.This unusual annulation takes place only when the exo-face is unhindered with the isomer of (51) of opposite configuration at C-2 where attack is at the relatively hindered endo -face the normal aldol product [isomer of (52)] predomi- nates. 0 Me + Me 0 0 (53) (52) Scheme 6 Another novel annulation (Scheme 7) occurs in the reaction of tricarbonylmyr- ceneiron (54) with oxalyl chloride to give a mixture of (55) and (56) a cyclization involving the unco-ordinated and unprotected double bond."" Recent reports on cyclopentadienyldicarbonyliron complexes confirm their ver- satility in organic synthe~is.~' The reaction of the tropylium complex (57) with [q'-RCH=CHCH2Fe(C0)2(q '-CsHs)] (58) gives (59) a useful intermediate in the 65 G.Jaouen and A. Meyer TefruhedronL.et&rs 1976,3547. A. J. Birch and A. J. Pearson J.C.S. Chem. Comm. 1976,601. 67 (a)N. Genco D. Marten S. Raghu and M. Rosenblum J. Amer. Chem.Soc. 1976,98,848; (b)A. Rosan and M. Rosenblum J. Org. Chem. 1975,40,3621. R. Pearce,D.J. Thompson andM. V.Twigs Reagents i CIC(O)C(O)CI AICI,; ii Ag+. Scheme 7 HR +Fe(CO) H (57) (59) preparation of substituted hydro-a~ulenes~~" which are not readily available. The ion [C5H5Fe(C0)2]' activates the methyl vinyl ketone towards Michael addition under very mild conditions. Reactions of this type which involve regiospecifically generated silylenolates are particularly useful and provide a route to octalones (Scheme Q6'' Reagents i Fe(CO)2(v -C5H5)(CH2=CHCOMe); ii basic Al2O3.Scheme 8 Three papers deal with new routes to .rr-allylpalladium complexes.68 The first which appears to be the most general involves the reaction between vinylmercuric halides (R'CH=CHHgCl) olefins (R2CH=CH2) and [Li2PdC1,] to give (60).68a Benefits of this route are (i) the mild reaction conditions compared with those used in previous methods; (ii) the fact that a range of functional groups can be tolerated (e.g. R2=H Bu CN or COMe); (iii) the formation of complex carbon skeletons from relatively simple starting materials; and (iv) the capability of preparing isomeric .rr-ally1 species that are not available via the allylic substitution route [e.g. (60; R2= C02Et)]. With products from activated olefins treatment with bases provided a (Q) R.C. Larock and M. A. Mitchell J. Amer. Chem. Soc. 1976,98,6718;(b) S. Staicu I. G. Dinulescu F. Chiraleu and M. Avram J. Organometallic Chem. 1976.113 C69; H. Alper H. des Abbayes and H. des Roches ibid. 1976 121 C3 1. 117 Organometallic Chemistry -Part (i) The Transition Elements route to unsymmetrical 1,3-dienes. The second route involves the reaction of diarylacetylenes (PhCrCAr) and olefins (R'R2C=CH2) with [PdC12(PhCN)2] to give (61).'" As with the first route conditions were mild and the .rr-ally1 complexes were prepared uiu carbon-carbon bond formation from simple starting materials. H R1+ cH R2 ci+ cmlR2 H H Ar H PdCl PdCl (60) (61) Examples were restricted to hydrocarbyl substituents.Finally (q3- allyl)C0(C0)~(PR,) complexes have been obtained under relatively mild conditions from CO~(CO)~ and allylic halides by the use of phase-transfer catalysts. This is one of the first examples of the use of phase-transfer catalysis in organometallic synth- esis.68c (q3-Allyl)palladium complexes have been used in the stereospecific alkylation of the side-chain of Depending on the reaction conditions products of opposite configuration were obtained. Path (a) (Scheme 9) is stoicheiometric in palladium whereas path (b) uses catalytic quantities. This catalytic route is effec- tively a stereospecific SN2displacement with net retention of configuration. H \ Reagents i PdC12 base; ii NaCH(C02R),; iii Me4NOAc heat; iv m-ClC6H4C03H; v LiNEt2 MeCOCI; vi Pd(PPh3)4 NaCH(C02R)2.Scheme 9 Reactions of gem -dihalides with nickel(0) species proyide a route to substituted cycl~alkanes.~~ aro -Dihalides [Br(CH2) Br] and gem -dihalides [R1R2CX2] give the cycloalkanes [R'R2mm] in the presence of stoicheiometric quantities of bis(cyclo-octadiene)nickel.70u Nickelacyclic intermediates [L2Ni(CH2),I3 are 69 B.M. Trost and T. R. Verhoeven J. Amer. Chem. SOC.,1976,98,630. 70 (a)S. Takahashi Y. Suzuki K. Sonogashira and N. Hagihara J.C.S. Chem. Comm. 1976 839; (6)J. Furukawa A. Matsumura Y. Matsuoka and J. Kiji Bull. Chem. Soc. Japan,1976,49,829. 118 R.Pearce,D.J. Thompson,andM. V.Twigg involved (cf. Ref. 71) and good yields are therefore restricted to the cases where n =4 (formation of the most stable ring system).Whereas a mixture of PhCOCHBr and Ni[P(OEt)3]4 gave the expected product PhCOCH=CHCOPh reaction in the presence of bis(cyc1o-0ctadiene)nickel gave 1,2,3-tribenzoylcyclopropane,70bthus providing a rare example of dehalogenative cyclopropanation. Extensions to the field of carbon-carbon bond formation via metal-catalysed cross-coupling reactions have been in asymmetric synthesis (see Section 1 for detail^),^' in the use of alkenylaluminium compounds in stereospecific 01efin7," and 1,3-diene7,' synthesis and in the preparation of a versatile organopalladium catal- y~t.~~ trans-Alkenylaluminium complexes (e.g.trans-PhCH=CHAlBui2) are read- ily generated by the stereospecific cis-addition of aluminium hydrides (e.g.HAIBui2) to alkynes which obviates the need for separate preparation of isomerically pure alkenyl halides as is required for the preparation of organo-magnesium and -lithium reagents.Whilst the aluminium complexes are normally unreactive to aryl and alkenyl halides addition of Nio and Pdo triphenylphosphine complexes facilitates coupling between the aluminium compounds and the halides under mild conditions. Yields are high reactions are highly stereospecific (90-99%) and the method appears to be superior to those previously reported for the preparation of (E)-alkenes and (E,E)-and (E,Z)-1,3-diene~.~*"'~ [PdPh(I)(PPh,),] is a convenient and general catalyst for the cross-coupling of Grignard reagents with aryl and alkyl halides and it offers advantages over the existing nickel catalysts.These include (i) an increase in selectivity (e.g.PhMgCl and p-C6&CIBr give PhC6&C1-p); (ii) good yields with ethynyl Grignard reagents (e.g.PhC=CMgBr and PhI give PhCrCPh); and (iii) good yields with hindered arylmagnesium halides (e.g.mesitylmagnesium A newly reported aldehyde-alkene addition reaction appears to share a common hydridoacyl-rhodium(Ir1) intermediate with the well-known Rh'-catalysed decar- bonylation of aldehydes.73 In the presence of chloro- or acetylacetonato-rhodium@) complexes pent-4-enal undergoes cyclization to cyclopentanone while addition of ethylene gives inter alia hex- 1-en-5-one. The reaction sequence presumably involves oxidative addition of the aldehydic C-H insertion into the Rh-H bond (either with ethylene to give an ethylacyl complex or via intramolecular cyclization to give a rhodacyclohexane intermediate) followed by reductive elimination to regenerate the Rh' catalyst.Synthetic Applications.-Rhodium(r1) carboxylates (e.g. the soluble pivalate) are effective catalysts for the cyclopropanation of alkenes by alkyl diaz~acetates.~~ Using this catalyst markedly better yields are obtained from substituted olefins than with the alternative palladium acetate or copper(I1) triflate. A range of di- and tri-oxabicyc10[x72,1]-systemshave been prepared by a novel Pd"/CuC1,-catalysed oxidative intramolecular cyclization reaction.75 Thus the alkenediol (62) obtained via a butadiene telomerization gave the beetle pheromone endo -brevicomin (63).71 M. J. Doyle J. McMeeking and P. Binger J.C.S. Chem. Comm. 1976 376. 72 (a)S. Baba and E.-I. Negishi J.C.S. Chem. Comm. 1976,596; (b)J. Amer. Chem. Soc.,1976,98,6729; (c)A. Sekiya and N. Ishikawa J. OrganometallicChem. 1976 118 349. 73 C. F. Lochow and R. G. Miller J. Amer. Chem. Soc. 1976,98 1281. 74 A. J. Hubert A. F. Noels A. J. Anciaux and P. Teyssit Synthesis 1976,600. 75 N. T. Byrorn R. Grigg and B. Kongkathip J.C.S. Chem. Comm. 1976,216. Organometallic Chemistry -Part (i) The Transition Elements The reaction of acyltetracarbonylferrates [R'COFe(C0)4]- with nitro-compounds (R2N02) provides a new route to amides (R'CONHR2) the ferrate acting as a very mild reducing and acylating agent.76 The hydrido-chromates [K(or Na)HCr2(CO)lo] prepared from potassium/graphite and chromium hexacarbonyl effect the selective reduction of a/3-unsaturated carbonyl compounds and offer an alternative to the well-known hydrido-ferrates [e.g.NaHFe(C0)4].77 A recent patent discloses an unusual homologation reaction of an allylic alcohol. Reactions of allylic alcohols with ketones and carbon monoxide in the presence of K2MCI4/SnCl2(M = Pd or Pt) give the butenols (64).78 R'R2C=CR3CH20H+R4R5C0 3 R'K2C=CR3CH2CR4R50H (64) Secondary kinetic isotope effects in the formation of metal olefin complexes have been used to good effect in the chromatographic separation of the deuteriated ethylenes C2HnD4-n.79 The Rh' complex (65) was used as the stationary phase and was superior to the existing silver nitrate system in this separation.Insertion.-Alkynols in the form of easily prepared titanium complexes [TiC1(OCH2(CH2) C=CH)L2] (L =/3 -diketonate) react regiospecifically under mild conditions with Et2AlCI to give the corresponding trans-ethyl olefins via intramolecular cis-addition to Ti-Et to the triple bond in the ethylated (66) [equation (5) The application of stoicheiometric hydrozirconation reactions to organic synth- esis reported last year has been reviewed and factors such as functional group compatibility have been considered." During 1976 this work was extended to reactions involving 1,3-dienes and halogeno-olefins. The addition of 1,3-dienes to [Zr(q -C5H5)2HC1] in contrast with addition to boron aluminium and many 76 M.Yarnashita Y. Watanabe T. Mitsudo and Y. Takegarni Tetrahedron Letters 1976 1585. 77 G. P. Boldrini A. Umani-Ronchi and M. Panunzio Synthesis 1976 596. 78 U.S.P. 3 9564O8. 79 V. Schurig Angew. Chem. Intemat. Edn. 1976,15 304. R. A. Coleman C. M. O'Doherty H. E. Tweedy T. V. Harris and D. W. Thompson J. Organometallic Chem. 1976,107 C15. J. Schwartz and J. A. Labinger Angew. Chem. Internat. Edn. 1976,15 333. R.Pearce D.J. Thompson andM. V.Twigg Et\ C ,H II H / o'- 0-cH2 / \ Et (66) transition-metal hydrides gives uia simple 1,2-addition the sterically less hindered yS -unsaturated complex (67). Subsequent ready insertion of CO followed by hydrolysis cleanly affords the corresponding yS-unsaturated aldehyde with no products resulting from dimetallation or migration of the double bond.The reaction of (67) with N-bromosuccinimide produces the corresponding unsaturated bromide but with appropriately alkyl-substituted compounds cyclopropane derivatives are also formeds2= (Scheme 10). Cyclopropanes are also obtained in moderate yield uia RR RR P RR Reagents i [Zr(q-C5H5)2HC1]; ii CO H2O;iii NBS. Scheme 10 y-Zr halogen elimination from intermediates obtained from reaction between [Zr(q -CsHs)2HC1] and suitable alicyclic or cyclic halogeno-olefins. Yields are lowered either by p -Zr-halogen elimination or by direct reduction with [Zr(q- CsH5)2HC1](Ref. 826). ZrCL can be used in procedures reminiscent of hydrozirconation. Reaction with LiAlH presumably produces Zr-H species which on addition to an alkene give (after hydrolysis) an alkane or by reaction with bromine an alkyl bromide.82C Mechanistic information is lacking but it will be of interest to see if the more subtle hydrozirconation syntheses can be achieved without employing preformed com- plexes.** (a)C. A. Bertelo and J. Schwartz J. Amer. Chem. Soc. 1976,98,262;(b)W.Tam and M. F. Rettig J. Organometallic Chem. 1976 108 C1;(c) F.Sato S. Sato and M. Sato ibid. 1976,122,C25.

ISSN:0069-3030    

DOI:10.1039/OC9767300099

出版商:RSC    

年代:1976

数据来源: RSC

摘要:

6 Organometallic Chemistry Part (ii) Main-group Elements By K. SMITH Departmentof Chemistry University College of Swansea Swansea SA28PP 1 Introduction As in the previous Annual Report on main-group organometallic compounds,' B Si and As are included but not P Se and Te. In this the last Report from this author it seems appropriate to draw attention to a worrying trend in a nevertheless fascinating area of organometallic chemistry concerning the investigation of reactive intermediates and of novel structural types. A mechanistic rationalization accompanying a publication which is entirely justifi- able for say its synthetic utility causes little concern. On the other hand in main group organometallic chemistry in recent years there have appeared many papers whose very justijkation lies in the involvement of interesting intermediates or novel structural types but which contain little or no evidence for such species.Yet because these proposed species would indeed be highly interesting the publications cannot be ignored and warrant reference here. Fortunately not all of the publications concerning such species are of this type. An example is in the study of multiply-bonded Si compounds an area which has given some cause for concern in the past ' but which is now advancing. Three groups have independently studied the i.r. spectra of sila-alkenes isolated in Ar matrices,* clearly the outstanding method for investigation of such species. The Soviet group investi- gated 1,l-dimethylsila-ethene formed by pyrolysis of 1 1-dimethylsilacyclobutane whilst both American groups investigated 1,1,2-trimethyIsila-etheneproduced by photolysis of trimethylsilyldiazomethane.At the time of writing only the abstract of the Soviet paper has been seen but as far as can be ascertained from the three bands listed therein there appears to be fair agreement between the spectra of the two products. A band at 643 or 645cm-' may be assigned to an olefinic C-H deformation mode but no emphasis is placed on bands near 1410 cm-' the apparent region of the Si=C stretching mode according to the only previous report of an attempt to obtain an i.r. spectrum of a ~ila-alkene.~ It appears that the earlier assignment has now been withdrawn so it is especially interesting how accurately a K.Smith Ann. Reports (B),1975,72 136. (a) A. K. Mal'tsev V. N. Khabashesku and 0.M. Nefedov Izvest. Akad. Nauk S.S.S.R. Ser. khim. 1976,1193 (Chem.Abs. 1976,85,122861); (b) 0.L. Chapman C.-C. Chang J. Kolc M. E. Jung J. A. Lowe T. J. Barton and M. L. Tumey J. Amer. Chem. SOC.,1976 98 7844; (c) M. R. Chedekel M. Skoglund R. L. Kreeger and H. Schechter ibid. p. 7846. T. J. Barton and C. L. Mclntosh J.C.S. Chem. Comm. 1972,861. 121 122 K,Smith computer 'predicted' this earlier freq~ency.~ Also it is claimed5 that a band at 1315 cm-' in the i.r. spectrum of (l) if attributable to the Si=C mode would be inconsistent with a band around 1410 cm-' for free Si=C bonds. New approaches to sila-imines (2) via photolysis or pyrolysis of silyl and to substituted sila-alkenes (3) by photolysis of unsaturated disilanes (4; X =0 or CH2),7 have been reported.The species (3; R' = Me R2= Me3Si R3=Ph) shows unusual behaviour; in the absence of added trapping agents it forms a head-to-head rather than a head-to-tail dimer.7a SiMe,(-Fe(COI3SiMe R2Si=NR XSiR /R$Si=C HXR Si .Si(Rz)C \R3 \R3 (1) (2) (3) (4) Last year we highlighted' an apparently significant report of the production at low temperatures of a relatively stable silicenium ion (5). Fortunately some people working in the organosilicon field were not so easily misled and have shown' that the results obtained on reaction of (6) with Ph3C'CI0,- can be matched using other R3SiH species several of which have previously been widely believed nut to produce stable silicenium ions.After consideration of the lack of conductance by solutions of '(3,and examination of the spectral properties the conclusion is drawn that the supposed silicenium ion is merely a covalent perchlorate (or a tight ion-pair).' This case illustrates the dangers of drawing conclusions from a single piece of datum. Several highly interesting new Group I11 compounds have been claimed this year but again the data are not always conclusive. Amongst them are (7) (8),and (9),all supposedly produced by reactions of halogeno-derivatives with alkali metals. Compounds (7) and (8) were obtained by reaction of a suitably molten form of potassium with MeBCl in the presence of cycl~hexene.~ The authors admit the tentative nature of their structural assignments (based almost entirely upon accurate mass determinations of the supposed molecular ions which in one case did not even H.B. Schlegal S. Wolfe and K. Mislow J.C.S. Chem. Comm. 1975 246. H. Sakurai Y. Kamiyama and Y. Nakadaira J. Amer. Chem. SOC. 1976,98 7453. D. R. Parker and L. H. Sommer J. Amer. Gem. Soc. 1976,98,618; J. Organometallic Chem. 1976 110,c1. 7 (a)A. G. Brook and J. W. Harris J. Amer. Chem. Soc. 1976,98,381; (6)M. Ishikawa T. Fuchikami and M. Kumada J. Organometallic Chem.,1976. 117 C58. * J. B. Lambert and H. Sun J. Amer. Chem. SOC. 1976,98,5611; T.J. Barton A. K. Hovland and C. R. Tully ibid. p. 5695. S. M. van der Kerk J. Boersma and G. J. M. van der Kerk Tetrahedron Letters 1976,4765. Organometallic Chemistry -Part (ii) Main -group Elements agree with a classical molecular weight determination) but give no other justification for the claim to have generated 'methylborylene' (MeB:) the subject of the publication! The evidence'' for (9),produced by the action of K on BukAlCl comes from the production of DZ on treatment with MeOD [equation (l)].It is however possible to think of less obvious ways of obtaining Dz.BU~~AI-AIBU:MeOD) 4Bu'D +2AI(OMe)3+D2 (1) It would be fascinating if the structures (7) (8) and (9) were genuine for they would open up new areas for exploration but given only the evidence presented there must be considerable doubt. Indeed a study'' of the related reaction between BuZBCI and Na-K alloy did not allow a definitive structure to be assigned to the immediate product despite the availability of more data than are given for (7) (8) and (9).The data were at least consistent with an oligomeric borohydride (lo) which has little in common with (7) (8) or (9). With this in mind and in order to encourage definitive work aimed at genuine identification of the reaction products we predict that the compounds (7)-(9) will prove not to be those formed in the reactions described. To the first person to prove this prediction wrong one year's subscription to Annual Reports will gladly be provided.* OBMe BMe BU;AI-AIBU~L n K+-+BHB~~CHP++ (7) (8) (9) (10) Having commented at length on this worrying trend it is proper to proceed to a discussion of other highlights from the 1976 literature.There are several articles of general interest including reviews of the organic chemistry of metal vapours,12 and of metal a-hydrocarbyl~,'~ and a report of a method for estimating the pyrophoricity of metal alkyl~.'~ Volume 43 of Pure and Applied Chemistry contains useful reviews by Makosza (two-phase carbanion reactions) McKillop (TIIII nitrate in organic synthesis) Stork (kinetic enolates) Trost (novel alkylations) Kagan (asymmetric hydrosilylation) and others." 2 Group1 Lithium.-Reviews of the synthetic utility of 2-0xazolines~~ and of nucleophilic acylating agents1' have appeared. Various substituent groups direct lithiation of the aromatic nucleus to the 2-position. Studies of the position of lithiation of a series of 4-substituted methoxyben- lo H.Hoberg and S. Krause Angew. Chem. Internat. Edn. 1976,15,694. K. Smith and K. Swaminathan J.C.S. Dalton 1976 2297. l2 K. J. Klabunde Accounts Chem.Res. 1975,8,393. l3 P. J. Davidson M. F. Lappert and R. Pearce Chem. Rev. 1976,76,219. l4 W. L. Mudry D. C. Burleson D. B. Malpass and S. C. Watson J. fire Flammability 1975,478 (Chem. Abs. 1976,84 105680). Is M. Makosza Pure Appl. Chem. 1975,43,439; A. McKillop ibid. p. 463; G. Stork ibid.,p. 553; B. M. Trost ibid. p. 563; H. B. Kagan ibid. p. 401. l6 A. I. Meyers and E. D. Mihelich Angew. Gem. Internat. Edn. 1976,15 270. l7 0.W. Lever Tetrahedron 1976,32 1943. * Write directly to Dr. Smith -ed. 124 K. Smith zenes indicate that regioselective metallation still occurs and that the groups S02NRMe CONRMe (R= H or Me) and CH2NMe2 take priority over OMe whereas CH2CH2NMe2 NMe2 CF3 and F have less powerful ortho -directing abilities than 0Me.l' At low temperatures it is possible to prepare and use aryl-lithium compounds containing the CN group.'' Lithiated dimethylhydrazones are advocated as useful enolate equivalents (Scheme 1).20 When derived from unsymmetrical ketones they are lithiated highly regioselectively to yield anions which react readily with suitable electrophiles and can easily be converted into a-ketocarbonium ion equivalents.Dimethylhydrazones derived from aldehydes behave like the ketone derivatives unless there is branching at the a-carbon when removal of the aldehydic proton and formation of a nitrile may take over.With an optically active hydrazone the same method gives a-branched optically active ketones with very respectable optical purities.2' /NMe2 NMe2 / N N If II '' R3R'CH-C-CHR4 R3R'CHCN +(R2 1H)R3R'CH-c-R2 (R2= CH2R4f I Li /NMe2 NMe2 / 0 N N II II II R3R'CH-C-CHR4R5 ,V R3R'CH-C-CHR4R5 R3R1CH-C-CHR4 I SMe Reagents i LiNPr'z-HMPA-PhH; ii LiNPr'2-THF; iii (MeQ2; iv R'I; v NaI04-H20. Scheme 1 Dimethylthiopivalamide can be lithiated on one of the NMe groups22 to give a reagent which behaves as a synthetic equivalent to MeNHCH2-. Doubly deproton- ated nitroalkanes are also useful species.23 Purely aliphatic nitroalkanes lose both protons from the a-position and on treatment with electrophiles (E+),then water yield a-substituted products RCH(E)N02 whereas p -arylnitroalkanes lose one proton from each of the a-and @-positions to give species which behave as super-enamine derivatives reacting to give p -substituted products.Since nitro- compounds can readily be converted into amines or ketones the synthetic potential is clear. D. W. Slocum and C. A. Jennings J. Org. Chern. 1976,41,3653. l9 W. E. Parham and L. D. Jones J. Org. Chern. 1976,41 1187. 2o (a) E. J. Corey and D. Enders Tetrahedron Letters 1976 3; (6) T. Cuvigny J. F. Le Borgne M. Larcheveque and H. Normant Synthesis 1976,237; (c) E. J. Corey and S. Knapp TefrahedronLetters 1976,4687. D. Enders and H. Eichenauer,Angew. Chern. Internat. Edn. 1976,15,549. 22 D. Seebach and W. Lubosch Angew. Chem. Internat.Edn. 1976,15,313. 23 D. Seebach and F. Lehr Angew. Chern.Internat. Edn. 1976 15 505; R. Henning F. Lehr and D. Seebach Helv. Chim.Acta 1976,59 2213. Organometallic Chemistry -Part (ii) Main -group Elements Compound (11) prepared by reaction of (12) with Bu"Li is a latent quinone ~arbanion.~~ Useful 1,2-epoxyalkyl-lithium reagents (13) can be obtainedz5 by lithiation of the corresponding epoxides provided that the group X is a second-row substituent group attached via Si P or S. Yet another synthetically useful reagent type is represented by (14; R'= Rz=Me X = 0; R' =Et R2=n-pentyl or Ph X =S) which reacts with ketones to give chain-extended a-substituted a@ -unsaturated aldehydes such as (15) from reaction with cyclohexanone.26 oBr Me0 OMe QLi Me0 OMe R' X Li>Llc R' R2xwy1D Li C H O XR' Me0 OMe Me0 OMe (1 1) (12) (13) (14) (15) Selenium-substituted organolithium reagents have been extensively The reagents are prepared by the action of Bu"Li on gem -diselenyl compounds and can be alkylated hydroxyalkylated acylated or silylated in the same way as the sulphur analogues.The selenium function can be removed reductively by oxidative elimination or in the case of 2-hydroxy-compounds by elimination to form an oxiran so the synthetic potential is great. Scheme 2 illustrates the preparation of an unsaturated ester. PhSe Li PhSe C0,Me 4 90% 60% Reagents i PhSeH; ii BunLi;iii ClCOzMe; iv H202-THF. Scheme 2 Lithiated thioacetal derivatives also continue to provide interesting new applica- tions.For example (16) which does not react with tosylates has been shown to react efficiently with benzenesulphonate esters of primary alcohols thus broadening the scope for alkylation.28 Also whilst compounds of type (17; R = alkyl) cannot be obtained because of elimination of ROLi (17; R = Li) is readily obtained by the (16) (17) (18) 24 M. J. Manning P. W. Raynolds and J. S. Swenton,J. Amer. Chem. Soc. 1976 98 5008. 25 J. J. Eisch and J. E. Galle J. Amer. Chem. SOC. 1976 98 4646; J. Organometallic Chem. 1976,121 ClO. 26 I. Vlattas L. D. Vecchia and A. 0.Lee J. Amer. Chem. Soc. 1976,98 2008; C. N. Skold Synthetic Comm. 1976,6 119. 27 J. N. Denis W. Durnont and A. Krief Tetrahedron Letters 1976,453; D. Van Ende and A. Krief ibid.p. 457; M. Sevrin D. Van Ende and A. Krief ibid. p. 2643; W. Durnont and A. Krief Angew. Chem. Zn&mat. Edn. 1976 15 161. 28 D. Seebach and E.-M. Wilka Synthesis 1976,476. 126 K.Smith action of 2 mol of Bu"Li on (18). The dianion reacts at carbon with electrophiles such as aldehydes.29 1-Lithio- 1 1-bis(pheny1thio)alkanes have previously not been recommended as acyl carbanion equivalents because of their low reactivity towards alkyl halides but they react readily with carbonyl compounds and an interesting new acid-catalysed reductive rearrangement reaction (Scheme 3) gives the same ketone that would be provided by simple deprotection of the alkyl halide Reagents i R1COR2; ii CF3C02H; iii R*CHBrR2; iv H20-HgC12-MeCN. Scheme 3 There have been two significant developments in the use of carbamoyl-lithium reagents (19).Thus when R' and/or R2are methoxymethyl groups the compounds are the synthetic equivalents of (19; R' and/or R2= H) thus providing a direct route to primary and secondary a -hydroxyamides and related Bases such as 0 R' II / Li-C-N LiNPri are the usual deprotonating agents used in the formation of compounds of type (19; R' =R2=alkyl) but provided that the correct solvent is used Bu'Li is also effective and this yields the reagent free from any secondary amine which might subsequently give The necessary solvent is a mixture of THF diethyl ether and pentane (4 :4 :1). A similar solvent mixture though in different propor- tions (4 :1:l) allows the conversion of terminal bromoalkenes into alkenyl-lithium compounds using Bu'Li a procedure which has not previously been developed in any general way.33 Both solvent mixtures are described as 'Trapp's solvent' the confu- sion arising because different original publications by Kobrich and Trapp refer to different proportions of these solvents.Perhaps it would be appropriate to generalize the term 'Trapp's solvent' to include any mixture of the three individual solvents and always to accompany the name by a statement of the proportions involved. The anion from 3-methylpentadienyl-lithium exists substantially in the conforma- tion (20) and is a useful reagent for introducing a diene unit common to several terpenoid species.34 Thus by reaction with R'COR2 it gives (21).29 H. Paulsen K. Roden V. Sinnwell and W. Koebernick Angew. Chem. Internat. Edn. 1976,15,439. 30 P. Blatcher J. I. Grayson and S. Warren J.C.S. Chem. Comm. 1976,547. 3' U. Schollkopf and H. Beckhaus Angew. Chem. Internat. Edn. 1976,15,293. 32 K. Smith and K. Swaminathan J.C.S. Chem. Comm. 1976,387. 33 H. Neumann and D. Seebach Tetrahedron Letters 1976,4839. 34 S. R. Wilson K. M. Jernberg and D. T. Mao J. Org. Chem. 1976,41 3209. Organometallic Chemistry-Part (ii) Main -group Elements (20) (21) Two new methods for estimation of organolithium compounds have been reported.35 Both involve a single titration and one uses readily available diphenylacetic acid as the sub~trate-indicator.~'~ Theoretical-mechanistic studies of organolithium compounds lag far behind their synthetic applications but some are worthy of mention.Ab initio calculations for example reveal that the energy differences between planar and tetrahedral geomet- ries of methane derivatives drop considerably on replacement of hydrogens by highly electropositive substituents such as Li.36 If a narrowing of the energy difference merely accords with naive expectations when the ground-state energies of both geometries are substantially raised it is less obvious that cross-over should occur. Yet 1,l-dilithiocyclopropaneis predicted to be slightly more stable in planar than in tetrahedral Of course the relevance of these gas-phase calculations to the condensed phase where carbon attached to Li seldom has a co-ordination number of four must be questionable.The interesting result reported last year,' that more ketone and less tertiary alcohol are produced the longer a reaction between a lithium carboxylate and an alkyl-lithium reagent is allowed to proceed before quenching is not matched in the reaction of the carboxylic acid itself with the organolithium ~eagent.~' The only species not present in the previous reaction mixture is the acid (apart from alkane which can probably be ignored) so the explanation must involve a further reaction of this acid with one of the reaction intermediates. Equation (2) would seem to accommodate all the data though the authors of the publication propose a much more complicated scheme which is also consistent with the data. Sodium Potassium,Rubidium and Caesium.-The power of Me3SiCH2K as a metallating agent has been dem~nstrated.~~ 3 Group11 Magnesium.-1,3-Benzodithiolium salts formed by reaction of 2-alkoxy- 1,3- benzodithioles with Ph3C'C104- react with Grignard reagents to give 2-substituted 1,3-benzodithioIe~,~~ which were previously available only by much more tedious routes.Furthermore since these thioacetals can be hydrolytically cleaved to give aldehydes this provides a new procedure for formylation of Grignard reagents. 35 J. Kuyper and K. Vrieze J.C.S.Chem. Comm. 1976 64; W. G. Kofron and L. M. Baclawski J. Org. Chem. 1976,41. 1879. 36 J. B. Collins,J. D. Dill E. D. Jemmis Y. Apeloig P. von R.Schleyer,R. Seeger and J. A. Pople,J. Amer. Chem. Soc. 1976,98,5419. 37 R. Levine and M.J. Karten J. Org. Chem. 1976,41 1176. 38 J. Hartmann and M. Schlosser Helv. Chim. Acta 1976 59,453. 39 I. Degani and R. Fochi J.C.S. Perkin I 1976 1886. 128 K.Smith It is interesting that both 1,2- and 1,3-monothiodiketones react with Grignard reagents exclusively at the thiocarbonyl group and by thiophilic The reaction of the latter type gives a new access to cyclopropane derivatives (Scheme 4).40b Reagents i RMgX; ii H30+ Scheme 4 The reaction outlined in Scheme 5 though giving only moderate (ca. 50%) chemical yields of p -substituted aldehydes does so with very high optical induction (ca. 95%).4' R' *CHO '-,A NZ'H(Bu')CO2Bu' ii' iii b R1EH(R2)CH2CH0 * Reagents i BucCH(NH,)C02But; ii R2MgBr; iii H3O+. Scheme 5 Use of a reagent prepared by addition of EtMgI to a highly hindered phenol in a relatively non-polar solvent mixture (CH2C1,-diethyl ether) provides the first gen- eral procedure for incorporation of Mg into metal-free chlorophyll derivative~.~~ The puzzling origin of the secondary alcohol reduction products formed during reactions of ketones with MeMgBr has been traced to magnesium hydride formed during preparation of the Grignard reagent.43 The hydrogen of the hydride origi- nates in the ether solvent and the amount of hydride formed is dependent upon the physical shape and size of the Mg pieces The formation of pinacols in the same reactions is a result of a single-electron transfer (SET)mechanism induced by transition-metal impurities in the Mg used to make the Grignard reagent but for neopentyl Grignards SET mechanisms may play an integral part in uncatalysed reactions.44 Zinc Cadmium and Mercury.-Slurries of finely divided Cd or Zn formed by codeposition of the appropriate metal vapour with a solvent are active enough to form organometallic compounds (presumed to be metal dialkyls) by reaction with simple alkyl halides.45 Although co-ordinating solvents give better results reasona- ble yields can also be obtained in hexane.Respectable yields of a-40 (a) P. Metzner J. Vialle and A. Vibet Tetrahedron Letters 1976,4295;(b)J. L. Burgot J. Masson P. Metzner and J. Vialle ibid. p. 4297. 41 S. Hashimoto S. Yamada and K. Koga J.Amer. Chem. SOC.,1976,98 7450. 42 H.-P. Isenring E. ass K. Smith H.Falk J.-L. Luisier and A. Eschenmoser Helv. Chim. Acra 1975 58 2357. 43 E. C. Ashby T. L. Wiesemann J. S. Bowers and J. T. Laemmle Tetrahedron Letters 1976 21. 44 E. C. Ashby J. D. Buhler I. G. hpp T. L. Wiesemann J. S. Bowers and J. T. Laemmle,J. Amer. Chem. Soc. 1976,98,6561. 45 T. 0.Murdock and K. J. Klabunde J. Org. Chem. 1976,41 1076. Organometallic Chemistry -Part (ii ) Main -group Elements 129 (alky1thio)carbonyl compounds have been on reaction of Reformatsky- type reagents with RSC1. Catalysed reactions of vinylmercuric chlorides continue to provide synthetically useful procedures. Thus treatment of these readily accessible sterically defined compounds with PdClz and LiCl (stoicheiometry 2 :1:4) in HMPA at 0 "C results in essentially quantitative coupling of the vinyl groups without loss of stereochemical definiti~n.~~ Furthermore application of previously established Pd-catalysed car- bonylation of vinylmercuric halides to compounds derived from propargylic alcohols provides a useful new approach to butenolides (Scheme 6).48 Unfortunately the preparation of the organomercurials in these cases proceeds in only moderate yield though the carbonylation is almost quantitative.Finally in the presence of A1Cl3 vinylmercury compounds react readily with acyl chlorides to give crp -unsaturated ketones in high yield.49 ,PR -CO/Li,PdCI ~ Hg CI R2 OH Scheme 6 Peroxymercuration of dienes with hydrogen peroxide results in good yields of Hg-substituted 1,2-dioxacycloalkanes such as (22) from penta- 1,4-diene.These can be converted by standard procedures into Hg-free products thus providing the first high-yielding approach to such compound^.^^ Isopropylidenecarbene (23) is produced by thermal decomposition of (24) at 150"C and reacts with alkenes of various types to give reasonable yields of the corresponding isopropylidenecyclo- propanes.51 The first fully documented example of a 1,4-oxymercuration product of a diene has been reported." It appears that the 1,4-oxymercuration reaction is reversible. 46 I. I. Lapkin G. G. Abashev and F. G. Saitkulova Zhur. org. Khim. 1976,12,976 (Chem.Abs. 1976 85,46 149). 47 R. C. Larock J. Org. Chem. 1976,41,2241. 48 R. C. Larock and B. Riefling Tetrahedron Letters 1976 4661.49 R. C. Larock and J. C. Bernhardt Tetrahedron L.ehi?rs 1976,3097. A. J. Bloodworth and M. E. Loveitt J.C.S. Chem. Comm. 1976,94. 51 D. Seyferth and D. Dagani J. Organometallic Chem. 1976,104 145. 52 A. J. Bloodworth M. G. Hutchings and A. J. Sotowicz J.C.S. Chem. Comm. 1976 578. 130 K. Smith 4 Group111 Boron.-A book on organoborane ~hemistry,~~ and reviews concerned with C-C bond formation using organoborane~,~~ and the chemistry of organoborate~,~~ cate~holborane,~~ have appeared. A comment on some novel types of organoboron compound was made in the Introduction. Other interesting new compounds include (25) (26) and (27). The production of (25a) involves the action of Meerwein’s salt on the known trimethylamine-cyanoborane adduct and it is converted into (25b) by hydrolysis with HCI.Compound (25b) is a boron analogue of an amino-acid and shows potential as an antitumour agent.57 Compounds (26) and (27) are prod~ced~~~,~ by reaction of Mn2(CO)lo with the appropriate boron ligand or an isomer and are of interest as members of a select group of ‘triple-decker-sandwich’ complexes. (25) a; X=NHEt (26) b;X=OH The main interest of organoboranes continues to be in their application as synthetic reagents and studies of alkynylborates in particular are still widespread. Two reports cite the synthesis of unsymmetrical dialkyldialkynylborates and their conversion into unsymmetrical conjugated di-ynes (Scheme 7).59 The essence of YMe I R;BYMe -L Li+[R:BCrCR2]-& R:BCrCR2 Y R2CrC.C=CR3 Li+[R:B(C=CR3)CrCR3]-Reagents i RZC=CLi; ii Y = 0,BF3,OEtz; Y = S no reagent necessary; iii R3C~CLi; iv 12.Scheme 7 both methods is the reaction of R:BYMe (Y=O or S) with an alkynyl-lithium reagent complexation of the initial dialkylalkynylborane by MeY- hindering its further reaction. For Y = 059a this complex must be treated with BF3 to cause decomplexation before the second alkynyl-lithium is added whereas for Y = S because the complex is weaker subsequent reaction with alkynyl-lithium occurs 53 T. Onak ‘Organoborane Chemistry’ Academic London 1975. 54 J. Weill-Raynal Synthesis 1976 633. 55 E. Negishi J. Organometallic Chem. 1976 108 281. 56 C. F. Lane and G. W. Kabalka Tetrahedron 1976,32,981. 57 B. F. Spielvogel L.Wojnowich M. K. Das A. T. McPhail and K. D. Hargrave J. Amer. Chem. Soc. 1976,98,5702. 58 (a)G. E. Herberich J. Hengesbach U. Kolle G. Huttner and A. Frank Angew. Chem. Internat. Edn. 1976 15,433; (b)W. Siebert and K. Kinberger ibid. p. 434. 59 (a)J. A. Sinclair and H. C. Brown J. Org. Chem. 1976,41 1078; (b)A. Pelter R. J. Hughes K. Smith and M. Tabata Tetrahedron Letters 1976,4385. Organometallic Chemistry -Part (ii ) Main -group Elements directly given sufficient time (ca. 1h at 20°C).596 Details6' of the previously reported alkylations of trialkylalkynylborates have appeared and are accompanied by an attempt using MIND0/3 and a6 initio calculations to obtain a mechanistic rationalization of the reaction course. The large calculated energy release accom- panying rearrangement is noteworthy.9,9-Dialkyl-9-boratabicyclo[3,3,l]nonanesare capable of great selectivity in reductions of carbonyl compounds and oxirans,61 and the rearrangement which accompanies reduction gives a useful access to bicyclo-octylboranes (Scheme 8).62 Scheme 8 Reactions of trialkylboranes with 1-methyloxyvinyllithium at -80 "C give pre- sumed intermediate dialkylvinylboranes which have opposite regiochemistry to those obtained by hydroboration of alkyne~.~~ Attempts to oxidize these inter- mediates to give methyl ketones are successful only when the alkyl groups of the original organoborane are unhindered other examples leading to mixtures of the ketones with tertiary alcohols resulting from a second rearrangement.Indeed in aqueous HCl the second rearrangement is quantitative and oxidation then gives the 1,l-dialkylethanol in good yield.63 This gives further evidence that many vinyl- boranes are very susceptible to rearrangement reactions. It has been known for several years that trialkylboranes add 1,4 to @-unsaturated carbonyl compounds by a radical mechanism. This year however several modifica- tions of this general behaviour have been recorded. For example alkenylboron compounds add to cisoid enones in refluxing THF apparently by a non-radical mechanism involving a cyclic transition and the outcome is a useful synthesis of $3 -unsaturated ketones which is even applicable to reactions using methyl vinyl ketone. Another exception is the 1,2-addition under vigorous conditions of triphenylborane to 2-methyla~rolein,~~ and copper organoborates prepared by successive addition of MeLi and CuBr to trialkylboranes allow extension of the conjugate addition process to reaction with acrylonitrile and other compounds.66 The reaction of trialkylboranes with iron(II1) azide is catalysed by HzOz and gives azidoalkanes in respectable yields based on transfer of a single alkyl group.67 The direct replacement of the boron of an organoborane by a carbon atom bearing an extra substituent which can be achieved using several reagents is one of the most 6o A.Pe1ter.T.W. Bentley,C. R. Harrison C. Subrahrnanyam and R. J. hub J.C.S.PerkinZ 1976,2419 Y.Yamarnoto H. Toi A. Sonoda and S.-I. Murahashi J. Amer. Gem. SOC. 1976,98 1965; J.C.S.Chem. Comm. 1976,672. G. W. Kramer and H. C. Brown J. Amer. Chem. Soc. 1976,98 1964. 63 A. B. Levy and S. J. Schwartz Tetrahedron Letters 1976,2201. 64 P. Jacob and H. C. Brown,J. Amer. Chem. Soc.,1976,98,7832. 6s R. Koster H.-J. Zimrnermann and W. Fenzl Annalen. 1976 1116. 66 N. Miyaura M. Itoh and A. Suzuki Tetrahedron Letters 1976 255. 67 A. Suzuki M. Ishidoya and M. Tabata Synthesis 1976 687. 132 K.Smith interesting of all reactions of organoboranes. So far there is no comparable replacement by an element other than carbon but transfer of two alkyl groups to nitrogen using a reagent which is prepared in situ from 2,4-dinitrophenoxyamine and Bu'OCI has now been reported.68 Application of the reaction to a perhyd- roboraphenalene results in a product which can be cyclized to the parent system of the coccinellidae alkaloids (Scheme 9).ClNH I Reagents i react at -78 +25 "C; ii H2@-OH-; iii acid or heat. Scheme 9 Indeed it is pleasing to note that there has been a significant increase in the use of organoboranes in syntheses of natural products or their model systems. For exam- ple a synthesis of epijuvabione and juvabione (28) uses carbonylation of an organoborane as its final whilst formation of the prostaglandin model (29) was accomplished by a modified I2 coupling reaction of the appropriate alkenylalkyl- dimeth~xyborate.~'A number of terpenoid species were obtained from hydrobora- tion products of protected gerani01.~' These included (30),prepared most efficiently using a cyanoborate reaction.C0,Me & GC,H., OSiMe,Bu' G O H Bu' H (28) (29) (30) The same publication7' also reported cyclization caused by treatment of diboryl compounds with AgN03 and other have reported formation of (E)-alkenes by treatment of 172-diboryl compounds (obtained by hydroboration of alkynes) with basic AgN03. Many of the better known reactions of organoboranes involve an intramolecular 1,2-shift which is mechanistically analogous to the Wagner-Meerwein rearrange- ment and as in that reaction the migrating centre retains its stereochemical config- uration during rearrangement. However not all non-radical reactions of 68 R. H. Mueller Tetrahedron Letters 1976 2925. 69 E. Negishi M. Sabanski J.-J. Katz and H.C. Brown Tetrahedron 1976,32,925. 70 D. A. Evans T. C. Crawford R.C. Thomas and J. A. Walker J. Org. am. 1976,41,3947. R. Murphy and R. H. Prager Austral. J. Chem. 1976,29,617. 72 K. Avasthi S. S. Ghosh and D. Devaprabhakara TetrahedronLetters 1976,4871. Organometallic Chemistry -Part (ii) Main -group Elements 133 organoboranes involve this characteristic intramolecular shift and consequently not all need involve retention of configuration. Exceptions have been noted in the base-induced halogenations of organoborane~,~~ and in the formation of cyclo- propanes by hydroboration-eliminationof allylic halides where inversion occurs at both From data obtained using the same techniques that have previously been said to indicate the aromaticity of borazaro- and related compounds Mikhailov has con- cluded that there is no evidence for ar~maticity.~' Careful attention to choice of model systems will obviously be required if a concensus view is to be established.Selective cyclic borylations of polyols and glycosides which were previously achieved with PhB(OH), can now be achieved with triethylborane-pivalic acid systems under mild condition^.^^ This should provide a valuable addition to the repertoire of protection methodology in the carbohydrate field. Aluminium Gallium Indium and Thallium.-The (E)-alkenyldialkylalanes which are obtained by hydroalumination of alkynes have received much attention as potentially important synthetic reagents. The general reaction involves transfer of the alkenyl unit intact to an organic electrophilic group [equation (3)] and several useful procedures have emerged.Reactive electrophiles such as ethyl chloroformate (givingtrans -a@-unsaturated esters) and chloromethyl ethyl ether (ally1 ethers) react readily at moderate temperatures and give good yields of whereas less reactive electrophiles such as alkyl halides and oxirans require prior conversion of the alane into an ate-complex by addition of an alkyl-lithi~m.~~ Reactions with aryl and alkenyl halides can be encouraged by addition of catalytic quantities of Ni and Pd complexes.79 New experimental approaches to oxythallation have been reviewed," and several significant developments have been reported. Undoubtedly the most exciting is the use of a reagent consisting of thallium(II1) nitrate (TIN) deposited on an acidic montmorillonite clay (K-10).81This reagent is apparently capable of all the known oxidative reactions of TTN generally under milder conditions often with better yields and always with an easier work-up procedure.For reactions in solution trimethyl orthoformate as solvent often gives better results than the previously used acidic media.82 73 (a)H. C. Brown N. R. Delue G. W. Kabalka and H. C. Hedgecock J. Amer. Chem. Soc.,1976.98 1290; (6) D. E. Bergbreiter and D. P. Rainville J. Organometallic Chem. 1976 121 19. 74 H. L. Goering and S. L. Trenbeath,J. Amer. Gem. Soc. 1976,98 5016. 75 B. M. Mikhailov and M. E. Kuimova J. Organometallic Chem. 1976 116 123. 76 R. Koster and W. V. Dahlhoff Annalen 1976 1925.77 G. Zweifel and R. A. Lynd Synthesis 1976,625 816. 78 (a)J. J. Eisch and G. A. Damasevitz,J. Org. am. 1976,41,2214;(b)S. Baba D. E. Van Horn and E. Negishi Tetrahedronhtters 1976,1927; (c)E. Negishi S. Baba and A. 0.King J.C.S. Chem. Comm. 1976 17. 79 E. Negishi and S. Baba J.C.S. Chem. Comm. 1976,596; J. Amer. Chem. Soc. 1976,98,6729. A. McKillop and E. C. Taylor Endeaoour 1976,3588. 81 E. C. Taylor C.-S. Chiang A. McKillop and J. F. White J. Amer. Chem. SOC. 1976 98 6750. 82 E. C. Taylor R. L. Robey K.-T. Liu B. Favre H. T. Bozimo R. A. Conley C.-S.Chiang A. McKillop and M. E. Ford J. Amer. Chem. SOC.,1976,98 3037. 134 K. Smith Dithalliation of a single aromatic ring has not previously been recorded but extended reaction of anisole with TI(OCOCF3) at room temperature gives dithal- liated Furthermore the thalliation reaction must be reversible because the monothalliation isomer distribution varies with time (Scheme 10).6 6 ,i OMe &1(0c0cF3)2 ~Tl(ococF3)2 ~ ~ \ \ \ \ n(ocmF3)Z Tl(OCOCF,) Reagents i Tl(OCOCF,), -25 "C; ii TI(0COCF3), 1 mol 20 "C;iii TI(0COCF3),,excess 20 "C 3 days. Scheme 10 5 GroupIV Silicon.-Advances in the field of multiply-bonded Si species and comments con- cerning silicenium ions have been presented in the Introduction. Following on recent success in the synthesis of silacyclopropanes come two reports of syntheses of silacyclopropenes (3 1).84 Both examples are formed by reaction of thermally generated Me2% with the appropriate alkyne and show surprising thermal stability under an inert atmosphere.The 29Si resonance of the ring Si atom of (31b) is over 100p.p.m. upfield from Me4% Amongst other interesting new organosilicon compounds prepared during 1976 are (32) and (33). The former is prepared by successive reaction of Me3SiCECLi with sulphur and Me3SiC1 and is a distillable It reacts exothermally with alcohols and amines losing one Me,Si unit in the process giving a-trimethylsilyl thioesters and thioamides which may be useful for further synthetic transformations. Compound (33) was prepareds6 from 1,8-dilithionaphthalene and Me2SiC12. Fluoride ion is a powerful nucleophile towards Si and can be used to increase the anionic activity of Si-bound groups. Thus when a tetra-alkylammonium fluoride is added to an alkynylsilane it behaves as a solution of R'C_C-R2,N'; addition of an aldehyde or ketone followed by aqueous work-up gives a good yield of the propar- gylic Similarly treatment of (34) with Me4N'F- gives a solution which SiMe SiMe R/-\R CI (31) a;R=Me (32) (33) (34) b; R = SiMe3 83 G.B. Deacon R. N. M. Smith and D. Tunaley J. Organometallic Chem. 1976 114 C1. 84 (a)R. T. Conlin and P. P. Gasper,J. Amer. Chem. SOC.,1976,98,3715;(b)D. Seyferth D. C. Annarelli and S. C. Vick ibid. p. 6382. 85 S. J. Harris and D. R. M. Walton J.C.S. Chem. Comm. 1976 1008. s6 L. S. Yang and H. Schechter J.C.S. Chem. Comm. 1976 775. 87 E. Nakamura and I. Kuwajima Angew. Chem. Zntemat. Edn. 1976,15,498. Organometallic Chemistry -Part (ii) Main -group Elements 135 behaves as a source of isopropylidenecarbene.88This must of course be generated in the presence of the desired trapping agent.Treatment of Si-substituted oxirans with &N'F-in DMSO provides a convenient method for desilylation of such Allylsilanes show potential as synthetic intermediates. Their reactions with electrophiles such as ketones and ketals are promoted by TiCL and the products are 4-hydroxy- or 4-alkoxy-alkenes produced with concomitant rearrangement of the allyl group.go Rearrangement also accompanies reactions of allyl silanes with peracids and phenylsulphenyl tetrafluoroborate which yield allyl alcohols and sulphides respe~tively,~' and with chlorosulphonyl isocyanate followed by pyridine which give py-unsaturated nit rile^.^' Me,SiLi adds in a conjugate manner to cyclohexenone and the product enolate can then be alkylated providing a convenient route to 2-alkyl-3-silylcyclo-hex an one^.^^ Trialkylsilyl alkali-metal derivatives can also be used to effect stereo- specific (trans-) deoxygenation of oxiran~.~~ Lithiated a-(trimethylsilyl)aldimines behave as a synthetic equivalent of the Wittig reagent Ph,P=C(R)CHO providing an effective approach to cup -unsaturated aldehyde^,^^ and reductive alkylation of alkynylsilanes as illustrated in Scheme 11 offers a useful stereoselective approach to vinyl~ilanes,~~ themselves interesting reagents.Me3Sg1 iii iv Me,Si R' R,SiC=CR' A b >=( (R= Me) B&AI H R2 H (R= Et) Et3Si Et3Si H MH iii iv * )=( R' BU~AI R' R2 Reagents i Bu';AIH/heptane-ether; ii BuiAlH/heptane; iii MeLi; iv R'I.Scheme 11 Germanium Tin and Lead.-Specific reductive ring-opening of the cyclopropane ring in vinylcyclopropanes is not normally an easy task but the two-stage process shown in Scheme 12 provides a useful approa~h.'~ Treatment of (35) with two moles of Bu"Li gives (36) a convenient reagent for synthesis of other heterocycle^.^^ The interesting compounds of type R2Sn where R 88 R. F. Cunico and Y.-K. Han J. Organometallic Chem. 1976 105 C29. 89 T. H. Chan P. W. K. Lau and M. P. Li Tetrahedron Letters 1976 2667. 90 A. Hosomi and H. Sakurai Tetrahedron Letters 1976 1295; A. Hosomi M. Endo and H. Sakurai Chem. Letters 1976 941. 91 M.J. Carter and I. Fleming J.C.S. Chem. Comm. 1976 679. 92 G. Deleris J. Dunogues and R. Calas J. Organometallic Chem. 1976 116 C45. 93 W. C. Still J. Org. Gem. 1976,41 3063. 94 P. B. Dervan and M. A. Shippey J. Amer. Chem. SOC.,1976,98 1265; M. T. Reetz and M. Plachky Synthesis 1976 199. 95 E. J. Corey D. Enders and M. G. Bock Tetrahedron Letters 1976,7. 96 K. Uchida K. Utimoto and H. Nozaki J. Org. Gem. 1976,41 2215. 97 M. Ratier and M. Pereyre Tetrahedron Letters 1976 2273. 98 G. Mark1 and P. Hofmeister Tetrahedron Letters 1976,3419. 136 K. Smith R3 R3 A R2 IR' R' R2 R3=H Me Scheme 12 se Sn Li Li Bl5 'Bu" (35) (36) is a bulky group such as bis(trimethylsilyl)methyl show a bent C-Sn-C unit in the crystalline and behave like other Snl' species in their reactivity towards alkyl and aryl halides."' Treatment of Bu2Bu'SnCI with Bu'O- radicals produces the less stable Bun* rather than the But* radical a phenomenon which is tentatively ascribed to steric constraints in a five-co-ordinate intermediate.lo' 6 GroupV The preparation and spectroscopic identification of bismabenzene (37) have been claimed,"* and this completes the series of Group V heterobenzene derivatives. Hetero-Cope rearrangements are involved in the formation of (38),the sole product of treatment of 4-hydroxyarsabenzene with ally1 bromide.lo3 Allylation at As is followed by the hetero-Cope rearrangement and this process is repeated before the third allylation gives the final product.'H N.m.r. spectra suggest that compounds of type (39; E = As or P) are better considered as ylides than as compounds possessing an aromatic sextet.lo4 (37) 9y D. E. Goldberg D. H. Harris M. F. Lappert and K. M. Thomas J.C.S. Gem. Comm. 1976 261. loo M. J. S. Gynane M. F. Lappert S. J. Miles and P. P. Power J.C.S. Chem. Comm. 1976 256. A. G. Davies B. Muggleton B. P. Roberts M.-W. Tse and J. N. Winter J. Organometallic Chem. 1976 118 289. l02 A. J. Ashe Tetrahedron Letters 1976 415. Io3 G. Mark1 and J. B. Rampal Angew. Chem. Internat. Edn. 1976 15 690. Io4 A. J. Ashe and T. W. Smith J. Amer. Chem. SOC.,1976 98 7861.

ISSN:0069-3030    

DOI:10.1039/OC9767300121

出版商:RSC    

年代:1976

数据来源: RSC