ISSN:0306-0012    

DOI:10.1039/CS98514FX001

出版商:RSC    

年代:1985

数据来源: RSC

ISSN:0306-0012    

DOI:10.1039/CS98514BX003

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Polymerization in Organized Systems By Constantinos M. Paleos DEPARTMENT OF CHEMISTRY, NUCLEAR RESEARCH CENTRE ‘DEMOKRITOS’, AGHIA PARASKEVI ATTIKIS, ATHENS, GREECE 1 Introduction The effect of monomer organization on polymerization and polymer properties is illustrated by a critical survey of some typical polymerization experiments con- ducted in certain organized systems. For comparison, in most of the cases, the same polymerizations are performed under isotropic conditions. Monomer organization may prove, at least in principle, of critical importance in polymerization and it can be achieved in various ways including the use of thermotropic liquid crystalline media,1,2 orientation at liquid-liquid interface^,^ and organization in micellar 4-6 or vesicular media.7 It should be noted that these organized media are interrelated and recent findings established the fact that common molecular structural features lead to the formation of organized systems either in the bulk or in the dispersed phase.On the other hand, in discussing organized polymerization, we proceed from thermotropic liquid crystalline media to polymerization at liquid-liquid inter-faces and then to micellar and vesicular phases to coincide with the historical development of the subject, and perhaps with the increasing complexity of the respective media. The significance of polymerization in organized media is primarily attributable to its resemblance to biological reactions, which usually occur in organized system^.^ Furthermore, monomer organization may affect polymerization kinetics, polymer structure, and specifically microstructure, a polymer property that is directly related to the organization of the reacting monomers.Among the polymers of scientific and technological value prepared by polymerization in organized media, are liquid-crystal polymers and polymeric s~rfactants,~ which, depending on their molecular structure, may form intra- and/or inter-molecular micelles or polymerized vesicle^.^ The latter aggregates retain the vesicular structure of their monomeric counterparts and since they exhibit sufficient stability ’ E. M. Barral I1 and J. F. Johnson, J. Macromol. Sci., Rev. Macrom. Chem., 1979, 17, 137. H. Kelker and R. Hatz, Handbook of Liquid Crystals, Verlag Chemie, Weinheim, 1980.’Interfacial Synthesis, ed. F. Millich and C. E. Carracher, jun., Volumes I and JI, Marcel Dekker, Inc., New York and Basel, 1977. C. A. Bunton, in ‘Techniques of Chemistry’, Vol. X Part 11, ed. J. B. Jones, C. J. Sih, and D. Perlman, J. Wiley and Sons, 1976, 731. J. M. Brown in ‘Colloid Science’, A Specialist Periodical Report, Vol. 3, The Chemical Society, London, 1977, p. 253. E. J. R. Sudholter, G. B. van de Langkruis, and J. B. F. N. Engberts, Recueil, 1980, 99, 73.’J. H. Fendler ‘Membrane Mimetic Chemistry’, John Wiley and Sons, New York, 1982. Liquid Crystal Polymers in ‘Advances in Polymer Science’, Vol. 59, 60/’61 ed. M. Gordon, Springer- Verlag, Berlin New York, 1984. C. M. Paleos, C. 1. Stassinopoulou, and A. Malliaris, J.Phys. Chem., 1983, 87, 251. Polymerization in Organized Systems they are considered as the closest analogues of biological membranes. Therefore, a long-term objective, i.e. the formation of stabilized cell-models, has almost been accomplished.'O These models may be used in studying membrane properties and membrane transport phenomena,' in medicine as drug carriers,' 3-1 and in artificial photosynthesis. '6-1 'vl 2 Polymerization in Thermotropic Liquid Crystalline Media The first examples 2o of reactions conducted in nematic liquid-crystalline phases were the thermal decompositions of picric acid, pyrogallol, and trinitroresorcinol. According to these early experiments, performed by Swedberg 2o in 1916, organiz-ation exerted an inhibitory effect on the rate of their thermal decomposition.In 1967, fifty-one years later, Amerik and Krentsel 21 undertook comparative investi- gations of the radiation-induced polymerization of vinyl oleate (a liquid crystal- line monomer) in the solid, mesomorphic, and isotropic phases. They found that the morphology of the polymer depended significantly on the phase in which polymerization had occurred. Crystalline polymer was obtained when the monomer was polymerized in the solid or mesomorphic phases, and the amorphous form resulted when polymerization was conducted isotropically. Amerik et aL2' also studied the polymerization of p-methacryloxybenzoic acid in a liquid crystalline solvent, p-cetyloxybenzoic acid, and they compared it with the polymerization of the same monomer in dimethylformamide.A polymer of higher molecular weight was obtained in liquid crystalline polymerization as compared to the one obtained under isotropic conditions. Following these first liquid crystalline polymerization experiments a series of lo H. F. Mark, Agnew. Chem., Int. Ed. Engl., 1981, 20, 303. J. H. Fendler, Ace. Chem. Res., 1980, 13, 7. R. Mcneil and J. K. Thomas, J. Colloid Interface Sci.,1980, 73, 522. l3 Liposomes in Biological Systems, ed. G. Gregoriadis and A. C. Allison, Wiley-Interscience, Chichester- New York, (1980). l4 B. E. Ryman and D. A. Tyrrell, in 'Essays in Biochemistry', Vol. 16, ed. P. N. Campbell and R. D. Marshall, Academic, New York, 1981, p.49.'' L. Gros, H. Ringsdorf, and H. Schupp, Agnew. Chem., Int. Ed. Engl., 1981, 20, 305. l6 M. Gratzel, Ber. Bunsenges. Phys. Chem., 1980, 84, 981. l7 K. Kurihara, P. Tundo, and J. H. Fendler, J. Phys. Chem., 1983,87, 3777. K. I. Zamaraev and V. N,Parmon, Russ. Chem. Rev., 1983, 52, 1433. l9 M. Kaneko and A. Yamada, in 'Advances in Polymer Science', Vol. 55, Springer-Verlag, Berlin-New York, 1984, p. 2. T. Swedberg, Kolloid. Z., 1916, 18, 54, 101. " Y. B. Amerik and B. A. Krentsel, J. Polyrn. Sci., Part C, 1967, 16, 1383. 22 Y. B. Amerik, I. I. Konstantinov, and B. A. Krentsel, J. Polym. Sci., Part C, 1968, 23, 231. 23 V. E. Perplies, H. Ringsdorf, and J. H. Wendorff, Ber. Bunsenges. Phys. Chem., 1974, 78, 921. 24 C. M. Paleos and S. Voliotis, Isr.J. Chem., 1979, 18. 192. 25 C. M. Paleos and M. M. Labes, Mol. Cryst. Liq. Cryst., 1970, 11, 385. 26 E. C. Hsu and A. Blumstein, J. Polym. Sci., Lett. Ed., 1977, 15, 129. 27 P. L. Magagnini, Makromol. Chem. Suppl., 1981,4, 223. ''B. Bressi, V. Frosini, D. Lupinacci, and P. L. Magagnini, Makromol. Chem., RapidCommun., 1980,1,183. 29 U. P.Shibaev, V. M. Moissenko, Ya. S. Freidon, and N. A. Plate, Eur. Polym. J.,1980, 16, 272. 30 H. Finkelmann, H. Ringsdorf, and J. H. Wendorff, Makromol. Chem., 1978, 179, 273. H. Finkelmann, M. Happ, M. Portugal, and H. Ringsdorf, Makromol. Chem., 1978, 179, 2541 32 V. P. Shibaev and N. A. Plate, Makromol. Chem., 1980, 181, 1393. 33 W. J. Toth and A. V. Tobolsky, J. Pol-vm. Sci..Lett., Ed., 1970, 8, 289.46 Table 1 Some nematic, smectic, and cholosteric liquid crystalline monomers ReJ 23 24 23 25 25 25 26 27 y3 CH2= C-COO 27 CHZ=CH-COO -(CH2 l6-O-28 29 30, 31 30, 32 33 Polymerization in Organized Systems liquid crystalline monomers were prepared, the synthesis of which was based on the functionalization of appropriate mesogenic moieties with polymerizable groups. The monomers that were obtained exhibited nematic, smectic, and cholesteric phases and some of the typical liquid crystalline monomers are shown in Table 1. A major concern during early polymerization experiments in thermotropic liquid crystalline media was the retention of mesomorphic order during poly- merization in order to take complete advantage of the organization of the liquid crystalline phase.The preservation of liquid crystalline order during polymeriz- ation might be achieved by low monomer-to-polymer conversion and also by using suitable liquid crystalline mixtures in which the monomer acts as a low con- centration solute in a liquid crystalline solvent. However, despite such attempts to preserve liquid crystalline order during polymerization a definite trend as far as the effect of liquid crystallinity on polymerization and polymer products is not observed. On the contrary, a critical survey of the literature on this subject reveals that there is little agreement concerning the effect of liquid Crystalline order on polymerization or polymer structure.Thus, in some liquid crystalline polymeriz- ations there is a rate acceleration, in some others retardation, while in certain other cases the polymerization rate remains unaffected. Analogously, polymerization of mesomorphic monomers leads either to the formation of liquid crystalline polymers or to regular polymers. As far as the character of the polymers is concerned (i.e. whether they are mesomorphic or not), it seems that the role of organization is not of crucial importance. The initial thought, that the controversy over the nature of the polymers originates from a possible destruction of the mesophase, did not prove to be an adequate explanation since it has been discovered that polymerization from isotropic melts 33a affords liquid crystalline polymers.In addition, in a specifically designed experiment 24 in which the liquid crystalline order was preserved during polymerization by using a liquid crystalline mixture in which solvent (1) and solute (2) had similar structures, the polymer prepared was of similar structure to the one obtained isotropically, as judged by X-ray analysis. Recently, however, Ringsdorf and his co-workers 34 as well as Shibaev et a1.35,36 have determined the structural criteria required for the formation of liquid crystalline polymers. The first group of workers set forth a model which clearly describes the structural features a polymer should have in order to show liquid crystalline character. According to this model the crucial factor for the exhibition of liquid crystallinity is the existence of mesogenic groups, the motion of which must be decoupled from that of the main chain by appropriate spacer groups.However, the effect of the main chain cannot always be disregarded.28 Depending on the position of the mesogenic group(s), two classes of liquid crystalline polymers are distinguished (Figure 1); main-chain polymers (A) in 33a A. Blumstein, R. B. Blumstein, S. B. Clough, and E. C. Hsu, Macromolecules, 1975, 8, 73. 34 J. H. Wendofi, H. Finkelmann, and H. Ringsdorf. J. Polym. Sci., Polym. Symp., 1978.63, 73. ” V. P. Shibaev, N. A. Plate, and Y. A. Freidzon, J. Polym. Sci., Polym. Chem. Ed., 1979, 17, 1655. 36 V. P. Shibaev, R. V. Tal’rose, F. I. Karakhanova, and N. A. Plate, J. Polym. Sci., Polym. Chem.Ed., 1979, 17, 1671. 37 M. G. Dobb and J. E. McIntyre in ‘Advances in Polymer Science’, Vol. 60/61, Springer-Verlag, Berlin New York, 1984, p. 61 and references cited therein. Paleos which the mesogenic groups are part of the backbone, and side-chain polymers 38,39 (B) where the mesogenic group is located in the side chain. The discussion in this review is limited to side-chain liquid-crystal polymers since only this class of polymers could, in principle at least, be affected by the organizational characteristics of the mesophase. The phenomenon of liquid crystallinity in polymers, therefore, is attributed to specific structural characteristics or otherwise to an intrinsic tendency of the polymers to organize and is irrespective of the mode of polymerization by which they were prepared.Thus, the preservation of liquid crystalline order during polymerization is not a prerequisite for liquid crystalline polymer formation. CH,CH,CH,CH2 eN=CHe-C-CH=CH2\ 1 II 0 Figure 1 The mesogenic groups of main-chain (A)and side-chain polymers (B)are decoupled from the main chain of the polymers by spacer groups of constant or variable length Adirect consequence of these assumptions was the introduction of a novel ''H. Finkelmann and G. Rehage in ref: 37, p. 99 and references cited therein. 39 V. P. Shibaev and N. A. Plate, in ref: 37, p. 173 and references cited therein. 49 Polymerization in Organized Systems method 40*41 for the synthesis under isotropic conditions of liquid crystalline polymers that involved the reaction of a reactive polymer with mesogenic moieties, for example poly(acryloylch1oride) with p-hydroxybiphenyl according to Scheme 1.This polymer as well as similar polymers are liquid crystals since the requirement that the mesogenic groups be decoupled from the polymer backbone is fulfilled. f CHCH2$j -70 fCH -CH2% CO -HOW II I CI 0 I Scheme 1 A modified approach 42 for the synthesis of liquid crystalline polymers involves the interaction of a reactive polymer with a liquid crystalline monomer, as depicted in Scheme 2. POI + H2C=CH -{i?O$ I H (CH210 -2 (CH2)nI I R R Where R is a mesogenic group Scheme 2 Returning to polymerization kinetics in liquid crystalline media, a few typical examples suffice to show clearly the existing controversy.Thus, the rate was enhanced in the liquid crystalline polymerization of vinyl oleate 21 or when p-methacryloyloxybenzoic acid 22 was polymerized in the mesomorphic medium created by the dissolution of this monomer to p-cetyloxybenzoic acid, as compared to the corresponding polymerization in dimethylformamide (Figure 2). On the contrary, the rate was decreased in the polymerization of a Schiff base monomer, e.g. of N-@-acryloyloxybenzylidene)-p-methoxyaniline,43 as shown by the dis- 40 c.M. Paleos, S. E. Filippakis, and G. Margomenou-Leonidopoulou, J. Polym. Sci.,Polym. Chem. Ed., 1981, 19, 1427. 41 C. M. Paleos, G. Margomenou-Leonidopoulou, S. E. Filippakis, A.Malliaris, and Ph. Dais, J. Polym. Sci., Polym. Chem. Ed., 1982, 20, 2267. 42 H. Finkelmann and H. Rehage, Makromol. Chem., Rapid Commun., 1980, 1, 31. 43 E. Perplies, H. Ringsdorf, and J. H. Wendorff, Makromo[. Chem., 1974, 175, 553. 50 Paleos continuity observed at the mesomorphic-isotropic transition of the respective Arrhenius plot (Figure 3). In other experiments, e.g. the polymerization of N-(p-methoxy-o-hydroxybenzy1idene)-p-aminostyrene or N-(p-cyanobenzy1idene)-p-aminostyrene,26 the rate of the polymerization remains unaffected by the organiz-ation of the monomers. This is shown in the corresponding Arrhenius plots (Figures 4and 5) which are practically continuous over the temperature range of mesomorphic and isotropic experiments.60 c E.-0 2~ Q);LO 0 0 20 0 Time / min Figure 2 Conversion-time curves of the thermal polymerization of p-methacryloyloxybenzoicacid in liquid crystalline medium (curve 1) and in DMF (curve 2) c- I I I I -12 - I I -hsot I r ropi c I I 1 1 I 2.4 2.5 lo3. T -' 2.6 2.7 Figure 3 A rrhenius plot of the polymerization of N-(p-acroyloyloxybenzy1idene)-p-methoxy-aniline in the nematic and isotropic phases Polymerization in Organized Systems Figure 4 Arrhenius plot of the polymerization of N-(p-methoxy-o-hydroxybenzy1idene)-p-aminostyrene in mesomorphic and isotropic phases I I 1 I-1 .5 I I1 I I I I1 Paleos From these data and other analogous results one cannot draw general con- clusions about the effect of organization on polymerization in mesomorphic media.These results do, however, provide a hint for its rationalization, i.e. that only monomers of similar structure and polymerized under similar conditions may show the same trend in kinetic behaviour. Thus, the effect of organization on polymerization kinetics cannot easily be predicted and one should usually resort to experiment for determining the acceleration or retardation of the rate of poly- merization. 3 Polymerization at Liquid-Liquid Interfaces Interfaces are regions where two immiscible homogeneous phases come into contact. The properties of the molecules located at interfaces are differentiated from the ones dissolved in the bulk phases.Among these properties orient- ation44-46 is of critical importance and its effect on polymerization will be considered below. Of the various kinds of interfaces, liquid-liquid interfaces created by two liquids of different polarity will be further discussed since this interface can be considered a model of the micro-interface created by the lipophilic core of micelles 4-7 and bulk water phase. This characterization is justified since the factors affecting micellar kinetics are strikingly similar to the ones at liquid- liquid interface^.^^ An example of interfacial organization at a liquid-liquid interface is shown in Figure 6 where a long alkyl-chain quaternary ammonium salt, i.e. a surfactant molecule, is located at a water-oil interface forming an oriented monolayer.A functionalization of this compound or of any similar surfactant by a polymerizable group will result in a monomer which, in principle, can polymerize at liquid-liquid interfaces. Oil CH,-N,-CH,I CH, CH3-N,-CH, CH -N-CH,I I+ Br-CH, Br-CH, Br’ CH3-N,-CH,I CH, Br’ Water Figure 6 Monolayer of a long alkyl-chain quaternary ammonium salt at a water-oil interface 4-Vinyl-N-methylpyridiniummethyl sulphate, whose polymerization was 44 F. M. Menger, Chem. SOC.Rev., 1972, 1, 229. ‘’ F. M. Menger, Pure Appl. Chem., 1979,51,99. 46 M. Spiro, in ‘Essays in Chemistry’, ed. J. N. Bradley, R.D. Gillard, and R. F. Hudson, Vol. 5, Academic Press, 1972, p. 65. Polymerization in Organized Systems studied at a water-toluene interfa~e,4~~~~was probably not the best choice from the point of view of its organizational properties at the interface, since this monomer does not bear balanced lipophilic and hydrophilic moieties.Its inter- facial polymerization was, however, attempted because the mechanism of its spontaneous polymerization in isotropic media was already known 49 and there- fore only the complexities originating from the interfacial mode of polymerization would have to be considered. Actually it is always good practice in novel experiments, such as interfacial polymerization, to try and limit the investigation to the interfacial parameters affecting the polymerization. In this case the quaternary nitrogen of the monomer is solvated into the aqueous phase whereas the heterocyclic ring bearing the vinyl group is located in the organic phase.Interfacial areas were increased by vigorous agitation of a water-toluene mixture leading to the formation of toluene or water droplets in the bulk phases. In fact, since the polar character of the monomer predominates over the non-polar, dispersed particles stabilized by monomer molecules as well as colloidal particles or other conglomerates are almost exclusively formed in the aqueous phase and were made visible by the scattered light in the Tyndal beam. The polymerization model, Figure 7, has been depicted as toluene droplets or cylinders in the aqueous phase stabilized by the monomers, justifying in a way the predominance of the ~yndiotactic~~.~~triad over the isotactic, as shown in Table 2.The small increase of the syndiotactic character of the polymer resulting from the interfacial polymerization as compared to that of isotropic polymerization, is not attributed to the inefficiency of the model (in leading to syndiotactic polymer) but rather to the fact that polymerization primarily occurs isotropically in water. This is due to the high hydrophilicity of the monomer which determines the location of the monomer and consequently its mode of polymerization. The monomer dissolves in the bulk water phase and polymerization is conducted isotropically in water. The experimental methods devised for the enhancement of interfacial modes proved almost inefficient in further controlling the stereospecificity of polymerization.It seems that only a limited number of monomers organize at the water-oil interface and polymerize according to the interfacial mode. It should be noted that in order to avoid simultaneous micellar polymerization of the monomer in the bulk water phase, the concentration of the monomer employed in these interfacial experiments was always below its critical aggregation concentration 50 in water. In contrast to the limited research activity on interfacial addition polymeriz- ation, significant work has been performed on polycondensation interfacial polymerization. Thus, polycondensation of diamines dissolved in water together with acid dichlorides dissolved in chlorinated solvents afforded polyamides ,54 (Nylons) according to Scheme 3.4' C. M. Paleos, J. Polyrn. Sci., Polyrn. Lert. Ed., 1977, 15, 535. C. M. Paleos, G. P. Evangelatos, Ph. Days, and G. Kipouros, J. Polyrn. Sci., Polyrn. Chern. Ed., 1979,17, 1611. 49 J. C. Salamore, B. Snider, and W. L. Fitch, J. Polym. Sci.,Pol.yrn. Chem. Ed., 1971, 9, 1493. V. A. Kabanov, Pure Appl. Chern., 1967, 15, 391. P. W. Morgan, Adv. Chern. Ser., 1962, 34, 191. Paleos WATER WATER Figure 7 Polymerization model of 4-vinyl-N-methylpyridiniummethyl sulphate at a water- toluene interface (toluene is being solubilized by the monomer) Table 2 Relative triad contenfs of poly(4-vinyl-N-rnethylpyridiniummethyl sulphate) obtained under interfacial and isotropic conditions Mode of Isotactic He tero tactic Syndio tact ic P,* polymerization exp.calc. exp. calc. exp. calc. exp. calc. Interfacial 22.13 19.68 44.50 49.46 33.33 30.85 0.465 0.555 Isotropic 25.88 27.24 52.82 50.01 21.34 22.75 0.508 0.522 * P,,,is the probability of isotactic addition according to the Bernoulli mechanism of polymerization 00 1 I1 2 II 1nH2N-R-NH2+ nC1-C-R-C-Cl-fR -NHCO-R2% Scheme 3 Owing to the very nature of polycondensation, properties that are directly associated with organization, such as tacticity, cannot be affected in this case. However, high concentration of the reactants at the interface leads to the formation, at low temperatures, of higher molecular weight polymers compared to the ones formed under isotropic conditions. On the other hand, bulk phases in interfacial polycondensations serve as media for the storage of the reactants and, since polymerization occurs only at the interface, the use of absolutely pure reactants is not necessary as the impurities remain in the bulk phase.Interfacial synthesis in general (and specifically polyamide~,~~ polyester-’’Ref: 3, Vol. 11, p. 157. Polymerization in Organized Systems amide~,~polyurethane^,^^ poly~reas,~ and polyphosphonates s6) have been extensively reviewed recently. 4 Polymerization in Micellar Systems Micelles4-7 are organized systems, usually formed in water by the aggregation of 30-150 surfactant molecules when their concentration exceeds the so-called critical micelle concentration (c.m.c.). The hydrophobic effect 57 is the unique organizing force (repulsion by the solvent instead of attraction) at the site of organization that is responsible for the assembly of surfactants.The shapes taken by micelles depend on the concentration of the surfactant; at relatively low concentration the aggregates may acquire a spherical shape (Figure 8), elongating at higher concentrations and becoming rod-like (Figure 8), and at still higher concentrations becoming transformed to lyotropic liquid crystal~.~~~~~ It has to be noted, however, that these are oversimplified pictures of micellar structure, which is still a matter of constructive controversy.60-62 It is certain, however, that a lipophilic core consisting of the hydrocarbon chains is formed separated by an interface (the so called Stern layer or Stern region) from the bulk aqueous phase in which head groups and (at least) some counter-ions are located.However, most of the counter-ions are found in the Gouy-Chapman electrical double-layer where they are dissociated from the micelle and are free to exchange with ions distributed in the bulk aqueous phase (Figure 9).63As already mentioned, the micro-interfaces that are formed in micelles resemble liquid-liquid interfaces and therefore the chemistry in micellar systems should show similarities to the chemistry at regular liquid-liquid interfaces. The interface of reversed mi~elles,~~,~ formed in an organic solvent, can also be considered of similar nature, having analogous properties. These systems, however, are not further discussed in this review. A characteristic property of molecules assembling to form micelles or of compounds solubilizing into them is organization, and advantage is taken of this property in micellar polymerization.Thus, in micelle-forming molecules poly- merizable groups are introduced and the c.m.c. of the monomers is determined. Polymerization is conducted comparatively under micellar conditions, i.e. at a concentration of the monomer in water exceeding the c.m.c., and in isotropic media in which its concentration is below the c.m.c. Some examples illustrating micellar effects on polymerization and polymer products are described below. 53 Re$ 3, Vol. 11, p. 209. s4 Ref 3, Vol. 11, p. 231. ” Re$ 3, Vol. 11, p.269. ” Ref 3, Vol. 11, p. 309. ” C. Tanford, Science, 1978, 200, 1012. s8 P. A. Winsor, Chem. Rev., 1968, 68, 2. s9 G. W. Gray and P. A. Winsor, in ‘Advances in Chemistry Series’, ed. R. F. Could, American Chemical Society, Washington D.C., 1967, p. 1. 6o F. M. Menger, Acc. Chem. Res., 1979, 12, 11 and references cited therein. 61 F. M.Menger and D. W. Doll, J. Am. Chem. SOC.,1984, 106, 1109. P. Fromherz, Ber. Bunsenges. Phys. Chem., 1981,85, 891. 63 L. R. Fisher and D. G. Oakenfull, Chem. SOC.Reu., 1977.6, 25. 64 H. F. Eicke and H. Christen, Helv. Chim. Acfa, 1978, 61, 2258. ”J. H. Fendler, J. Phys. Chem., 1980, 84, 1485. Paleos Rod-like Micelle A A Spherical Micelle Figure 8 Schematic representation of spherical and rod-like micelles: head group, A counter ion Hydrocarbon Core Stern Layer Gouy-C hapman La yer Figure 9 A cross-section of a micelle showing the lipophilic core as well as Stern and Gouy- Chapman layers The spontaneous polymerization of 4-vinylpyridine protonated salts 66 in aqueous solutions takes two different routes depending upon whether the con- centration of the monomer during polymerization is above or below the critical micelle concentration.Polymer I (1,640nene polymer) was obtained when the concentration of the monomer was below the c.m.c. and polymer I1 (1,2-addition polymer) when it exceeded the c.m.c., the polymerization being conducted in micellar media (Scheme 4). However, although in this case micellar organization modifies drastically the course of polymerization, the microstructure of the 1,2- 66 I.Mielke and H.Ringsdorf, Makromol. Chem., 1972, 153, 307. Polymerization in Organized Systems polymer as shown by I3C n.m.r. studies is not affected by micellar ~rientation.~~ Apparently the organization imposed by the micellar aggregation is not the appropriate one to affect the tacticity of the polymer although the vinyl group is located on the rigid heterocyclic ring. CH2-CH (I1 IX H I concentrat5 solution (111 Scheme 4 Another characteristic example showing the effect of micellar structure is the polymerization of sodium 10-undecenoate (3).68 Polymerization of this monomer was in practice accomplished only in micellar media, leading at high concentration to ‘intermolecular’ micelles composed of ‘intramolecular’ polymeric micelles and establishing the fact that micellar formation is necessary for the polymerization of this salt.The effect observed is attributed to the enhancement of the rate of propagation due to the localization of monomer double bond within the micellar core. In a similar fashion the same monomer has been polymerized69 from its lyotropic liquid crystalline media, showing again the relationship between micellar and lyotropic liquid crystalline media. -COONa Recently a common quaternary ammonium salt, trimethyldodecylammonium bromide, has been functionalized by the introduction of the polymerizable ally1 group in place of a methyl group. Polymerization ’O was achieved by y-irradiation under both micellar and isotropic conditions.The radiolysis products of water, however, exerted a destructive effect on the polymer obtained by the isotropic polymerization. Presumably in those isotropic experiments where micelles are not formed, the long aliphatic chains are totally exposed to the products of water 6’ C. M. Paleos and Ph. Dais, J. Polym. Sci., Polym. Chem. Ed., 1978, 16, 1495. C. E. Larrabee and E. D. Spraque, J. Polym. Sci., Polym Lett. Ed., 1979, 17, 749. 69 R. Thundathil, J. 0.Stoner, and S. E. Friberg, J. Polym. Sci., Polym. Chem. Ed., 1980, 18, 2629. ’O C. M. Paleos, Ph. Dai’s, and A. Malliaris, J. Polym. Sci., Polym. Chem. Ed., 1984, 22, 3383. Paleos radiolysis which are usually responsible for the formation of by-products.The main conclusion of this work is that polymerization occurs in an intramicellar fashion as judged by the fact that the mean micellar aggregation number n of the polymerized micelle was found equal to the aggregation number of the monomeric micelle, i.e. equal to 33 f3 corresponding to a mean molecular weight of the polymer of 11 OOO f 1 OOO daltons. It has, however, to be noticed that micellar structure does not always affect the polymerization or polymer products. Thus the radiation-induced polymerization of 3-n-dodecyl-1-vinylimidazolium iodide and the polymer obtained by this micellar polymerization are not affected by micellar organization. In this case it is reasonable to adopt Menger’s model 60,61 for the interpretation of the experi- mental data.According to this model a spherical micelle is a rather disorganized aggregate in which the Stern region, apart from the polar heads, also includes a significant fraction of the alkyl chains that are free to move about. In this region there is a considerable alkyl-chain-water contact rather than an ion double-layer shielding the non-polar nucleus. Thus, the imidazolium ring bearing the vinyl group is able to move without significant restriction in an environment that is not so much organized as predicted by older models.60 From the above results one cannot, therefore, draw general conclusions concerning the effects of micellar organization on polymerization and polymer products.5 Polymerization in Vesicular Media Liposomes 7*72 and surfactant vesicles 7,72*73 are smectic mesophases of closed phospholipid/synthetic surfactant bilayer structures. They are spherical or ellipsoidal and any that are multicompartmented can be transformed to single compartment vesicles by sonication as shown in Figure 10. The term ‘vesicles’ is used, for simplicity, to describe the above mentioned structures regardless of their chemical composition (see re$ 96). Figure 10 Transformation of a multi-compartment vesicle lo single compartment through sonicalion ” C. M. Paleos, S. Voliotis, G. Margomenou-Leonidopoulou, and Ph. Dais, J. Polym. Sci., Polym. Chem. Ed., 1980, 18, 3463. 72 J. H. Fendler, Arc. Chem. Rex, 1980, 13, 7. Polymerization in Organized Systems Vesicles are closer analogues than micelles of biological membranes and their kinetic ~tability,~’ as compared to the dynamic character of micelles, is responsible for their diverse applications, e.g.in separating charges, in artificial photo- synthesis,’6-19 in drug in reactivity ~ontrol~,~~ and in the duplication of many membrane-mediated processes.’ ’ The molecular structural characteristic 73,7 ’of vesicle-forming amphiphiles is usually the existence of two long alkyl chains, in contrast to the one required for the formation of micelles, coupled with a polar head such as a quaternary ammonium, carboxylate, sulphate, sulphonate, or phosphate group. Formation of vesicles has also been reported from single 76 and triple 77 chain amphiphiles and also from compounds 78-80 bearing a mesogenic moiety, i.e.the typical structure of thermotropic liquid crystals, suitably functionalized by a flexible tail or flexible tails and a polar head. Thus, according to K~nitake,’~ the liquid crystalline nature of bilayers (natural and synthetic) is the cause and not the result of bilayer formation. In these amphiphiles polymerizable groups may be introduced in such a way that the organizational characteristics of the original vesicles are more or less preserved. Some typical vesicle-forming monomers, showing the diversity of molecular structures that exhibit this activity, are given in Table 3. Table 3 Some typical vesicle-forming monomers Monomer Ref: z+/cH3 /N\ CH3-(C H2),dC H Br-CH2= (H3C)C-C00-(CH2),0 -CH2 CH, 82 98 89 ”T.Kunitake, J. Macromol. Sci., Chem., 1979, A13, 587. 74 J. F. Fendler, Pure Appl. Chem., 1982, 54. 1809. 75 T. Kunitake and Y. Okahata, Bull. Chem. SOC.Jpn., 1978, 51, 1872. 76 W.R. Hargreaves and D. W. Deamer, Biochemistry, 1978, 17,3759. ”T.Kunitake, N. Kimizuka, N. Higashi, and N. Nakashima, J. Am. Chern. Soc.. 1978, 106,1978 78 T.Kunitake and Y. Okahata, J. Am. Chem. Soc., 1980,102,549. 79 Y.Okahata and T. Kunitake, J. Am. Chem. SOC.,1980, 102,5231. Y. Okahata and T. Kunitake, Ber. Bunsenges. Phys. Chem., 1980,84, 550. 60 Paleos CHy(CH2I9-CrC-C~C-(CH /\CHT(CH,)~-CSC -CzC-(CH2 19-0 OH CH2=CH(CH2 18-COO-(CH,1,CH, Br-92 /\CH2=CH(CH2 ),-COO-(CH, 1, CH,-CH,OH CH2=CH(CH218 CH2-0 90 CH,=CH(CH,), 91 2 Br-81 + CH20p0 CH2CH2N(CH3), 0-CH ,OC 0-(CH,), COO-C(CH, )=CH,I CHOCO-(CH, I14CH311 + 83 CH20P0 C H,CH2N(CH, 1,I0-CH3-(CH, )12-C C -C C-(CH, -CO0-(CH, )z H \; / Polymerization in Organized Systems HOOC-(CH21a-CZC -C EC-(CH218- COOH 86 CH,-(CH2 )1~0-CH,I 86CH,-(CH2ll70-CH I CH2-OCO-(CH 15-NH-CO UCH,) = C H2 CH,-(CH,),,-COO-CH, I CH,-( CHZ),E-COO -CH I CH2-OCO-C(CH3 )= CH2 86 CH,-( CH, l,2-C C -C C-(CHZ la-COO-CH, I CH,-(CH21,,-C~C-CEC-(CH,)~-COO-CH I 0 107 I II + CH2-0-P-0-N (CH,),I 0-Investigations into the polymerization of ‘monomeric’ to ‘polymerized’ vesicles81-101 were prompted by the need to obtain vesicles of enhanced stability and of controllable permeability and size.In order to take advantage of existing topochemical control, polymerization was conducted in the vesicular phase using conventional polymerization methods. For several vesicle-forming monomers it D. S. Johnston, S. Sanghera, M. Pons, and D. Chapman, Biochem. Biophys. Acta, 1980, 602, 57. 82 S. L. Regen, B. Czech, and A. Singh, J. Am. Chem. Soc., 1980, 102, 6639. ”S. L. Regen, A. Singh, G. Oehme, and M. Singh, J. Am. Chem. Soc., 1982, 104, 791. 84 S. L. Regen, Y. Yamaguchi, N. K. P. Samuel, and M. Singh, J. Am. Chem. Soc., 1983, 105, 6354. 85 H. H. Hub, B. Hupfer, K. Horst, and H. Ringsdorf, Agnew. Chem., In[. Ed. Engl., 1980, 19, 938. 86 A. Akimoto, K. Dorn, L. Gros, H. Ringsdorf, and H.Schupp, Agnew. Chem., In[.Ed. Engl., 1981,20,90.’’ H. Bader and H. Ringsdorf, J. Polym. Sci., Polym. Chem. Ed., 1982, 20, 1623. K. Dorn, R. T. Klingbiel, D. P. Specht, P. N. Tyminski, H. Ringsdorf, and D. F. OBrien, J.Am. Chem. Soc., 1984, 106, 1627. 89 E. Lopez, D. F. O’Brien, and T. H. Whitesides, J. Am. Chem. Soc., 1982, 104, 305. 90 C. M. Paleos, C. Christias, G. P. Evangelatos, and Ph. Dais, J. Polym. Sci., Polym. Chem. Ed., 1982,20, 2565. 91 P.Tundo, D. J. Kippenberger, M. J. Politi, P. Klahn,and J. H. Fendler,J. Am. Chem. Soe.,1982,104,5352. 92 P. Tundo, D. J. Kippenberger, P. L. Klahn, N. E. Priet0.T. C. Jao, and J. H. Fendler, J.Am. Chem. Soc., 1982, 104,456. 93 K. Kurihara and J. Fendler, J. Chem. Soc., Chem. Commun., 1983, 1188.94 W. Reed, L. Guterman, P. Tundo, and J. H. Fendler, J. Am. Chem. Soc., 1984, 106, 1897. 95 J. H. Fendler and P. Tundo, Acc. Chem. Res., 1984, 17, 3. 96 J. Fendler, Science, 1984, 223, 888. 97 T. Kunitake, N. Nakashima, T. Takarabe, M. Nagai, A. Tsuge, and H. Yanagi, J.Am. Chem. Soc., 1981, 103, 5945. 98 M. F. Roks, H. G.J. Visser, J. W. Zwikker, A. J. Verkley, and R. J. M. Nolte, J. Am. Chem. Soc., 1983, 105, 4507. 99 J. H. Fuhrhop, D. Fritsch, B. Tesche, and H. Schmiady, J. Am. Chem. SOL‘.,1984, 106, 1998. loo L. Gros, H. Ringsdorf, and H. Schupp, Agnew. Chem., int. Ed. Engl., 1981, 20, 305. lo’ D. Babilis, Ph. Dais, L. H. Margaritis, and C. M. Paleos, J. Polym. Sci.,Polym. Chcm. Ed., in press. 62 1,-(CH, Paleos has been established that not only do they retain the structure of their monomeric counterparts but that they also exhibit enhanced stability, being less permeable than the respective monomeric ones.Stable vesicles are extremely useful, primarily in the construction of photochemical energy converters and to a lesser extent in their application as drug carriers. The structures and properties of polymerized vesicles are affected by the position, the number, and the nature of the polymerizable group(s) (such as vinyl, acrylate, methylacrylate, diacetylene). Thus, in some polymerized vesicles the head- group mobility is preserved whereas in others the chain mobility is preserved, as shown diagramatically loo in Figure 11. The introduction of two vinyl groups may lead to extra stabilization through crosslinking of the adjacent layers of the vesicles.This has been accomplished by the polymerization of monomeric vesicles of di(undeceny1) phosphate to polymerized vesicles. Their stability (they had still not precipitated three years after their formation) is very probably attributed to the following crosslinked structure (Figure 12). A 50% polymerization of the monomeric vesicles was achieved. Figure 11 Formation of polymerized vesicles with preservation of head groups mobility (a-c) or chain mobility (d)(Taken from L.Gros et a/.'O0) The stabilization of polymerized vesicles is not always so straightforward. For example it has been found that of the ally1 and diallyl vesicle-forming quaternary ammonium salts [(4) and (5)] lo' only the diallyl polymerized vesicles prepared by a mechanism involving alternate intramolecular and intermolecular growth steps O3 exhibit such excellent stability.Polymerized vesicles originating from G. B. Butler and R. J. Angelo, J. Am. Chem. Soc., 1957, 79, 3128. Io3 G. B. Butler in 'Polymeric Amines and Ammonium Salts', ed. E. J. Goethals, Pergamon Press, Oxford- New York,1980, p. 125. lo* P. Tundo, K. Kurihara, D. J. Kipperberger, M. Politi, and J. H. Fendler, Agnew. Chem., Inz. Ed. Engl., 1982, 21, 81. Polymerization in Organized Systems Figure 12 ally1 derivatives are unstable and they dissipate shortly after irradiation. It seems that the insertion of piperidine lo27103 or rather pyrrolidone moieties ‘04* into the backbone of diallyl polymerized vesicles has a profound effect on the stability of the vesicles.On the other hand, monomers bearing the diacetylenic polymerizable gro~p~~,’~’in the middle of their molecules polymerize in the vesicular phase according to Scheme 5 and afford extremely rigid structures which do not, however, show thermotropic phase-transitions. lo5 This indicates that these particular polymerized vesicles have lost the smectic character of biological membranes. A novel approach to the formation of polymerized vesicles is by the oxidation of a thiol group introduced into an appropriate vesicle-forming molecule. Such a ‘monomer’ is 1,2-bis(11-mercaptoundecanoyl)-sn-glycero-3-phosphocholine (6)”“ the bilayer of which is ‘switched on’ (polymerized) by oxidation and ‘switched off’ (depolymerized) by reduction, i.e.the reversibility of the system is governed by the thioldisulphide redox cycle. Using this system biochemical studies could be carried out either in the ‘on’ polymerized or ‘off’non-polymerized mode. A completely different approach to the formation of polymerized vesicles is through the use of ionene polymers” prepared by the interaction of long-chain dibromides with ditertiary amines according to the reaction shown in Scheme 6. A segment of the vesicle structure consisting of ionic and lipophilic functions is also shown. This method of vesicle formation is probably the answer to the question: is the formation of polymerized vesicles exclusively associated with the polymeriz- ation from the vesicular phase, or may the polymer be prepared under conven- tional isotropic conditions and then subjected to one of the usual methods of lo4‘ R.M. Ottenbrite in ‘Cyclopolymerization and Polymers with Chain Ring Structures’, ed. G. B. Butler and J. E. Kreta, ACS Symposium Series, No. 195, American Chemical Society, Washington D.C., 1982. Also, author’s own unpublished data. lo’ H. Koch and H. Ringsdorf, Makromol. Chem., 1981, 182, 255. lo6 D. Day, H. H. Hub, and H. Ringsdorf, isr. J. Chem., 1979, 18, 325. lo’ B. Hupfer, H. Ringsdorf, and H. Schupp, Makromol. Chem., 1981, 182, 247. lo’ K. Dorn, E. V. Patton, R. T. Klingbiel, D. F. OBrien, and H. Ringsdorf, Makromol. Chem., Rapid Commun., 1983, 4, 513.Paleos HC HC'%H I H,C =CH \CH I I H2C\N+/CH2 8r-/\ Scheme 5 CH OC-(CH,)$HIs + CH20P-OCH2CH2N (CH313 vesicle formation? log Judging from this case (ionene polymers) one may assume that polymerized vesicle-formation is not limited to the topochemical mode of synthesis. Actually, there is no way in which dibromides and ditertiary amines could be aggregated into vesicular structures. At the present time, however, there is no experimental evidence, as far as vesicle formation is concerned, from a polymer prepared by addition polymerization in isotropic media. Until recently the emphasis in most of the publications concerning polymeriz- ation in vesicular media was on the synthesis of diverse vesicle-forming monomers, the conditions of their polymerization to the respective polymerized vesicles, and lo9 S.Szoka and D. Papahadjopoulos in 'Liposomes: Physical Structure to Therapeutic Applications', ed. C. G. Knight, Elsevier/North-Holland Biomedical Press, Amsterdam. Polymerization in Organized Systems Scheme 6 on their characterization (primarily stability 7*1 and permeability 7*1lo). How-ever, a systematic work on the kinetics and mechanism of photopolymerization in vesicular media was reported quite recently.94 The study involved a styrene-containing monomer (7) which allowed the polymerization to be conveniently monitored by absorption spectrometry. Thus, continuous irradiation of the monomer or irradiation by laser pulses leads to a decrease in styrene absorbance by a first order process. The calculated rate- constants were independent of the vesicle concentration (vesicles unlike micelles do not break down on dilution) but increased linearly with increasing intensity of laser pulses.Rates of monomer disappearances were considerably slower in ethanol than in vesicles and, in contrast to the vesicular polymerization, depended on monomer concentration. This last observation and the fact that the sizes of vesicles remained practically unchanged on polymerization suggest the occurrence of intravesicular surface polymerization at an apparent reduced dimensionality and analysed on per vesicle rather than on volume basis. As a first approximation a polymerization model is considered, in which the vesicle surface is assumed to be hexagonally packed, each monomer being surrounded by six monomers.The photoinitiated free-radical can react with any one at its neighbours to initiate polymerization, to disproportionate, or to form non-polymeric products or alternatively the free radical may return to the ground state or react with oxygen impurity or the wall of the reacting vessel. Related to this photopolymerization, the same intravesicular model provides an experimentally measurable quantity which relates average polymer chain length (i.e.20 k 30% monomer per chain) to the quantum efficiency of the free-radical formation. In contrast to this short average polymer chain-length it was found lo* that two methacryloyl monomers, a-MA (8) and a-MA (9), which were polymerized catalytically, exhibited chain lengths consisting of about 500 units.For poly(a-MA) the calculated number of 'lo R. Biischl, T. Folda, and H. Ringsdorf, Makromol. Chem., 1984, Suppl. 6, 245. Paleos monomers per vesicle was lo4-3 x lo5 and therefore there must be about 20-600 chains per vesicle, while poly(o-MA) with a number of monomers per vesicle equal to lo4-8 x lo4 contained 20-60 chains per polymerized vesicle. Also, preliminary experiments showed that aof poly(a-MA) varies inversely with the sonication time before polymerization. These results suggest that lower molecular weight polymers are formed with smaller vesicles. We may therefore envisage polymerized vesicles as consisting of several polymeric fragments and not, for example, of only two polymeric chains resulting from the polymerization of each layer, as is the case with one-compartment vesicles.This structure of polymerized vesicles is in some way encountered in intermolecular micelles 70 consisting of polymeric chains (each being an intramolecular micelle) aggregated to above bigger aggregates. 6 Summary and Outlook From the above discussion it is clear that the monomer organization in general affects polymerization and polymer properties. At the present time, however, it is not always possible to predict the nature and extent of the effects. It seems. that this difficulty originates from a limited knowledge of the degree of organization in the various organized systems.Further systematic work on appropriately designed monomers, the organization of which is more definitely known, is required in order to predict and/or to maximize the organizational effects on polymerization. Acknowledgement. The author is indebted to Mr. D. Arapoglou for his assistance in the preparation of the drawings in this work.

ISSN:0306-0012    

DOI:10.1039/CS9851400045

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Reactions Involving the Triple Bond in Dimolybdenum and Ditungsten Hexa-alkoxides and C-C, C-N, and C-0 Triple By Malcolm H. Chisholm, David M. Hoffman, and John C. Huffman DEPARTMENT OF CHEMISTRY AND MOLECULAR STRUCTURE CENTER, INDIANA UNIVERSITY, BLOOMINGTON, IN 47405 1 Introduction The ability of transition metals to form multiple bonds with themselves is now a well recognized and accepted aspect of chemistry.’ Based on bond length, con- formation, electronic and vibronic absorption spectroscopy, theoretical considera- tions employing sophisticated calculations and, above all, an internal consistency, the evidence for a M-M multiple bond in certain compounds is overwhelming. Such is the case for the quadruple bond in the Mo,ClE4- anion and the triple bond in Mo,(OCH,Bu’),, for example.However, not all cases are so clear cut since there may be extensive mixing of metal-metal and metal-ligand bonding such that the significance and validity of a M-M bond order or its assignment is questionable. Whenever there are bridging ligand atoms bonded to both metal atoms, then, in particular, problems arise concerning the detailed nature of the metal-metal interactions. In the first decade following Cotton’s discovery 23 of the quadruple bond in the Re,C1EZ- anion the scope of the field became apparent and our understanding of the electronic structure of M-M multiple bonds matured. It was not, however, until the mid to late 1970s that chemists started to explore the reactivity of M-M multiple bonds in a systematic manner.The initial findings were exciting and exposed a wide variety of reactivity patterns.495 It became apparent that general rules concerning additions and eliminations of the type well established in organic chemistry could not naively be extended to M-M multiple bonds. Triple bonds between molybdenum atoms may have a variety of electronic configurations, dn4, a2n262,x46’, and even 027c4828*2,and the reactivity of the MEMO bond reflects this. By contrast the C=C bond is always of configuration 02n4leading to a more uniform reactivity. Our group has been developing the chemistry of the triple bond in di- molybdenum and ditungsten hexa-alkoxides, compounds of formula M,(OR)6.6 These compounds are members of an extensive group of d3-d3 dimers which ’ F.A. Cotton and R. A. Walton, ‘Multiple Bonds Between Metal Atoms’, Wiley, 1982. F. A. Cotton, N. F. Curtis, C. B. Harris, B. F. G. Johnson, S. J. Lippard, J. T. Mague, R. Robinson, and J. S. Wood, Science, 1964, 145, 1305. F. A. Cotton, Inorg. Chem., 1965, 4, 334. ‘Reactivity of Metal-Metal Bonds’, ed. M. H. Chisholm, ACS Symp. Ser., 1981, vol. 155. M. H. Chisholm and I. P. Rothwel!, frog. Inorg. Chem., 1982, 29, 1. M. H. Chisholm, Polyhedron, 1983, 2, 681. C-C, C-N, and C-0 Triple Bonds OR Ro\ ;/OR M-M \RO’ ORRO adopt a staggered ‘ethane-like’ geometry in the ground state depicted by (1) above. The triple bond (1) has a configuration 02n4 derived from the metal d3-d3 intera~tion.~Taking the M-M axis to be the z axis, the metal atomic d,2 orbitals interact to form oand o*orbitals and the degenerate d,,, dyzatomic orbitals form qYand nx,y*molecular orbitals. The triple bond is cylindrical and these molecules are inorganic analogues of alkynes.He(1) and He(I1) photoelectron spectroscopy reveals that the first ionization (lowest energy) is from the filled M-M n orbitals; the second ionization comes from the M-M oorbital while the third, at still higher ionization energy, is from oxygen lone pairs.*.’ Roughly 0.5 eV separates the filled M-M n and oorbitals. M2(OR), compounds are yellow to reddish-orange and this colour arises from an electronic absorption at the low energy end of the U.V. which tails into the visible region of the spectrum.This absorption can reliably be assigned to the HOMO-LUMO transition. The former is M-M n and the latter is an orbital of e symmetry, being a mix of M-M 6* and n*. These compounds are co-ordinatively unsaturated and can, if steric factors permit, co-ordinate Lewis bases such as pyridine and PMe, to form adducts of formula M2(OR),L2.6 In forming a new metal-ligand o bond there is a loss in oxygen-to-metal 71 bonding. The geometry of these adducts is indicated in (2) below. Each metal atom is in a roughly square planar co-ordination environment and is united to the other metal atom by a triple bond of configuration 02n4.The latter does not impose any conformational restrictions and eclipsed and staggered geometries have been found for M,(OR),L2 compounds depending only upon the ligand-ligand interactions across the M-M bond.The M-M distances in compounds (1) and (2) span a narrow range, 2.2 to 2.4 A, with the W-W distance being roughly 0.1 A longer than the Mo-Mo distance in pairs of otherwise identical molecules. The M-M distances are sensitive to steric ’The X3MrMX3 molecules have been the subject of numerous theoretical studies: K. W. Dobbs, M. H. Francl, and W. J. Hehre, Inorg. Chem., 1984,23,24; T. Ziegler, J. Am. Chem. Soc., 1983, 105,7543; R. A. Kok and M. B. Hall, Inorg. Chem., 1983,22, 728; B. E. Bursten, F. A. Cotton, J. C. Green, E. A. Seddon, and G. Stanley, J. Am. Chem. Soc., 1980,102,4579; M. B. Hall, J. Am. Chem. Soc., 1980,102, 2104; A. Dedjeu, T. A. Albright, and R.Hoffmann, J. Am. Chem. Soc., 1979, 101, 3141; T. A. Albright and R. Hoffmann, J. Am. Chem. Soc., 1977,100, 7736. F. A. Cotton, G. G.Stanley, B. Kalbacher, J. C. Green, E. Seddon, and M. H. Chisholm, Proc. Nurf.Acad. Sci.USA, 1977, 74, 3109. E. M. Kober and D. L. Lichtenberger, results to be published. Chisholm, Hofman, and Hufman RO y;/ ORI 'M RO4i OR factors cJ W-W = 2.296(2) A in W,(OEt),(L-L), where L-L = N,N-dimethyl-ethylenediamine, and W-W = 2.397(1) A in W,(OBu1),(py-4-Me),. These compounds have been found to enter into a wide variety of reactions including oxidative additions lop1 and reductive eliminations 12*13 wherein the M-M bond order is changed in a stepwise manner downward [(MEM)~' --+(M=M)8' (M-M)'O'] and upward [(MEM),' -(MEM)~+],respec-tively. This article deals with a group of reactions involving the M-M triple bond in M,(OR), compounds and CS, C=N, and C=O bonds.These may be viewed as a special class of oxidative-addition reactions in which electron density is removed from the M=M bond to form metal-ligand bonds. The dinuclear centre achieves a reactivity not seen in mononuclear chemistry. 2 Reactions Involving Alkynes Alkynes react with M,(OR), compounds or their adducts M,(OR),L, in hydrocarbon solvents at < 25 "C to give a wide variety of products. Small sterically unencumbered alkynes, such as HCCH, MeCCH, and MeCCMe, are very rapidly polymerized. By careful control of reaction stoicheiometry and conditions, these same alkynes and others also produce new organometallic complexes. The organometallic compounds isolated and characterized include ten simple alkyne adducts stabilized by pyridine ligation, six of which have been characterized by single crystal X-ray crystallography.Alkyne Adducts.-Alkyne adducts of formula M,(oR),(R'CcR')(py),, where n = 1 or 2, adopt one of three structures depicted by (3), (4), and (5) below. Two compounds of each structural type have been characterized by crystallography and these are underlined while the assignment of structure for the other four rests on low temperature limiting n.m.r. spectroscopic data. lo M. H. Chisholm, C. C. Kirkpatrick, and J. C. Huffman, Inorg. Chem., 1981, 20, 871. M. H. Chisholm, J. C. Huffman, and A.L. Ratermann, Inorg. Chem., 1983, 22, 4100.*'M. H. Chisholm, J. C. Huffman, and R. J. Tatz, J. Am. Chem. Soc., 1983, 105, 2075. l3 M. H. Chisholm, K. Folting, J. C. Huffman, and R. J. Tatz, J. Am. Chem. Soc., 1984, 106, 1153. l4 M. H. Chisholm, K. Folting, J. C. Huffman, and I. P. Rothwell, J. Am. Chem. Soc., 1982, 104,4389. l5 M. H. Chisholm, K. Folting, D. M. Hoffman, J. C. Huffman, and J. Leonelli, J. Chem. Soc., Chem. Commun., 1983, 589. l6 M. H. Chisholm, D. M. Hoffman, and J. C. Huffman, J. Am. Chem. Soc., 1984,106,6794. l7 M. H. Chisholm, B. Conroy, D. M. Hoffman, and J. C. Huffman, results to be published. C-C, C-N, and C-0 Triple Bonh R" R' / Ro, RO&\ /M \ PY PYR M = Mo or W R = Pr' R'= R"= H M = MO R= Pr' R'= H R" = Me; R' = R" = Me M= MO R = CH~BU' R' = R"= H R' R' RO'/ wT+OR/RW \ / RO 0 OR R R = CH,Bd R'= H or Mg --R = CH2But R'= Et or Ph Each structural type has a central pseudo-tetrahedral M2C2unit. In (3) there are two alkoxy bridges while in (4) and (5) there is only one. If the bridging alkyne is considered to occupy a single co-ordination site, (3) represents a confacial bioctahedral geometry while in (4) one metal has octahedral co-ordination but the other is only five co-ordinate.The bridging alkyne and the bridging alkoxide occupy, respectively, equatorial and axial positions of a distorted trigonal bipyramid about the five co-ordinate tungsten in (4). In (5), which has only one molecule of ligated pyridine, each tungsten atom is in a distorted trigonal bipyramidal environment with the bridging alkyne and bridging alkoxide in equatorial and axial sites, respectively. What factors favour the adoption of one structure over another? Bulky combinations of R, R', and R" favour (5) while molybdenum has only been found to adopt structural type (3), the confacial bioctahedron.Alkynes bind to the ditungsten centre more tightly, which may account for tungsten's ability to adopt Chisholm, Hoffman, and Huffman structures of type (4) and (5). In solution several of these alkyne adducts are fluxional, showing rapid bridge-terminal OR group site-exchange and reversible dissociation of pyridine. With the exception of compound (3) M = Mo, R' = R" = Me, there is no evidence for reversible dissociation of the p,-alkyne ligand.For compounds of type (5) one must also consider what structures might be present in solution when the pyridine ligand has dissociated. Table 1 Selected bond distances (A)for M~(OR),(py),(lr-c2R'2) compounds of molybdenumand tungsten (n = 1 or 2) Structure M-C Compound Typea M-M C-C (averaged) Re$ Moz(OPri)6(pY)z(VC2H2) (3) 2.544(1) 1.37(1) 2.09 b WZ(~P~')~(PY)~(~-CZHZ)(3) 2.567(1) 1.39(2) 2.09 c W2(0CH2Bu1)6(pY)2(C(-C2H2) (4) 2.610(2) 1.39(3) 2.14 d W~(~CH~B~')~(PY)~(~-CZM~~)(4) 2.602(1) 1.37(2) 2.14 c Wz(OCHzBu1)6(py)(CI-C2Et2) (5) 2.572(1) 1.40(2) 2.14 d Wz(OBu1)6(py)(C(-C2H2) (5) 2.665(1) 1.44(1) 2.10 c (3), (4), and (5) refer to the schematic drawings given in the text.* M. H. Chisholm, K. Folting, J. C. Huffman, and I. P. Rothwell, J. Am. Chem. Soc., 1982, 104,4389. M. H. Chisholm, K. Folting, D. M. Hoffman, and J. C. Huffman, J. Am. Chem. Soc., 1984, 106, 6794. dM. H. Chisholm, B. K. Conroy, K. Folting, D. M. Hoffman, and J. C. Huffman, results to be published. The M-My M-C, and C-C distances of the central M,C2 units in (3), (4), and (5) are given in Table 1. The C-C distances are slightly longer than those typically found in perpendicular alkyne metal complexes which span a small range 1.32-1.38 A.18 For a comparison one might note the C-C double bond distance in ethylene, 1.337(3) A, and the typical C-C single bond distances of 1.54 8,and 1.46 8, for Csp3-Csp3 and Csp2-Cspz, respectively." The C-C distance in Gd,Cl,C,, 1.46 A, formally C,6-, is also worthy of note, since it is a limiting value for a C-C single bond in C, carbides.,' Some '3C n.m.r.data for the bridging C,H, ligand in compounds of type (3), (4), and (9,together with related data for a p2 and p4-C2H2 in cobalt carbonyl complexes are given in Table 2. The values of 'J13C-l~ are reduced from ca. 250 Hz in C,H2 and are approaching that in ethylene, 156 Hz. More remarkable are the extremely small 'J13C-13~values of which in the case of W,(OBut),(py)(p-C2H2) and W,(OPri),(py),(~-C,H,) have not been resolved. This places their magnitude at less than 15 Hz. For comparison the values of in ethyne, ethylene, and 'J13C-13c ethane are 171.5, 67.6, and 34.6 Hz, respectively, while that observed for the q2-C,H, ligand in W2(OPri)6(p-C4H4)(~2-C2H2),a compound discussed later, is For an extensbe listing of C-C distances in perpendicular M2(pC2R,) containing complexes see: D.M. Hoffman, R. Hoffmann, and C. R. Fisel, J. Am. Chem. Soc., 1982, 104, 3858. l9 K. Kuchitsu, MTP Int. Rev. Sci.Phys. Chem. Ser. One, 1972, 11, 221. 2o A. Simon, E. Warkentin, and R. Masse, Angew. Chem., Inr. Ed. Engl., 1981.20, 1013. Table 2 N.m.r. data for the bridging ethyne moiety in selected transition-metal complexes Structure Compound Typea 6b Jw-c' 'Jc-n 'JC-H Jcx 3J~-~Ref: -20 1.4 4.7 26.9 -0 f 44.8 189.7 1.o 19.0 2.9 f 32.8 41.6 -192 -0 <15 -0 g 41.4' -184 -0 <15 -0 g 38 191.9 -1.7 15.8 5.4 g 25 -209.8 15.7 55.9 -0 h -174.7 4.5 21 -0 h (3), (4).and (5) refer to structural types defined in the text. * Chemical shift in p.p.m. relative to Me&. Temperature and solvent in brackets. All coupling constants are in Hz. * JW,< = Jw+. Not reported: data obtained from 'H n.m.r. spectra recorded at -50 "C, CDC13 solvent. f M. H. Chisholm, B. K. Conroy, K. Folting, D. M. Hoffman, and J. C. Huffman, results to be published. M. H. Chisholm, K. Folting, D. M. Hoffman, and J. C. Huffman, J. Am. Chem. SOC.,1984,106,6794. S. Aime, D. Osella, E. Giamello, and G. Granozzi, J. Orgattometal. Chem., 1984, 262, C1 Chisholm, Hoffman, and Huffman -54.5 Hz. Only in small organic rings such as cyclopropane has 1J13c-1Jc10 Hz been previously noted.21 In summary, compounds of the type (3), (4), and (5) may be viewed as dimetallatetrahedranes having M-M and C-C and four M-C bonds, all of roughly order unity.It was noted earlier that bulky combinations of R, R’, and R” favour the adoption of structure (5). However, this is only to a point. Beyond this increasing steric pressure causes C-C bond scission of the co-ordinated alkyne. Alkyne Scission.-Schrock and co-workers 22 first noted the remarkable metathesis reaction shown in equation 1, wherein a W=W bond and a CS bond are cleaved to form two W=C bonds. W,(OBu‘), + R’CXR‘ -2(ButO),WXR’ (1) (R’ = Me, Et, or Pr) The compounds (Bu‘O),WzCR’ are alkyne metathesis catalysts 23,24 which allowed Schrock to extend the synthesis of alkylidyne compounds as shown in equation 2.,* W,(OBu‘), + 2EtCKR’ ----+ 2(Bu‘O),W&R’ + EtC&Et (2)(R‘= Ph, SiMe,, or -CH=CH,) The reactions shown in equations (1) and (2) are rapid at 25 “C in benzene which contrasts with the reported scission of co-ordinated alkynes in cluster carbonyl corn pound^.^^.^^ The latter require much more forcing conditions.The compounds (Bu‘O),W=CR’, where R’ = Me2’ and NMe,,28 have been shown to be weakly associated dimers in the solid state as shown in (6) below. Through the agency of a pair of alkoxy bridges two trigonal bipyramidal tungsten atoms are united along a common equatorial-axial edge. The W-0 axial bonds are much longer, 2.42(1) A, than the equatorial bridging W-0 bonds, 1.95(1) A.The W-C distances are 1.77(2) A and the W-to-W distance is ca. 3.5 A; the latter is a non-bonding distance. In solution I3C n.m.r. signals at ca. 250 p.p.m. with large values of J~SJW-~~C,ca. 300 Hz, are characteristic of the W=C moiety. During our characterization of the ethyne adduct W,(OBu‘),(p-*C,H,)(py).~py, where *C represents 90 atom % I3C, we discovered a 13C signal which could most 21 For listing of C-C and C-H couplings see: J. L. Marshall, ‘C-C and C-H NMR Couplings’ in Methods in Stereochemical Analysis, vol 2, Verlag Chemie Intl., 1983; J. B. Stothers, ‘Carbon-13 NMR Spectroscopy’, Academic Press, New York, 1972; H. Gunther, Angew. Chem., Int. Ed. Engl., 1972.11,861. 21 R.R.Schrock, M. L. Listemann, and L. G. Sturgeoff, J.Am. Chem. Soc., 1982, 104, 4291. 23 J. Sancho and R. R. Schrock, J. Mol. Catal., 1982, 15, 75. 24 R.R. Schrock, ACS Symp. Ser., 1983, 211, 369. 25 J. T.Park, J. R. Shapley, M. R. Churchill, and C. Bueno, J. Am. Chem. SOC.,1983, 105,6182. 26 N. T.Allison, J. R. Fritch, K. C. P. Vollhardt, and E. Walborsky, J. Am. Chem. Soc., 1983,105,1384,4501. 27 M. H. Chisholm, D. M. Hoffman, and J. C. Huffman,Inorg. Chem., 1983,22,2903. Professor F.A. Cotton has informed us that the molecular structure of (Bu‘0)3W%Ph is monomeric in the solid state, being essentially one half of the dimer shown in structure (6). 26 M. H. Chisholm, J. C. Huffman, and N. S. Marchant, J. Am. Chem. SOC., 1983, 105, 6162. 75 C-C, C-N, and C-0 Triple Bonh -OR OR R = But R'= Me or NMez reasonably be assigned to the methylidyne complex (But0)3WSH: 6(W=C) = 252.4 p.p.m.with = 289 Hz and J~~C-HhW-13c = 150 Hz.', The existence of such a compound is not surprising in view of Schrocks findings,22 equation 1, but just how such a species is formed upon dissolution of a crystalline sample of W2(OBu1)6(py)(p-C2H2).:py is of interest. One plausible explanation is that the alkylidyne and the alkyne adduct are in equilibrium, equation 3. Consistent with expectations based on equation 3 , we find that when equi- molar quantities of crystalline samples of Wz(OBut),(py)(p-*C2H2).+py and W2(OBU'),(py)(p-C2D2).~py are dissolved in [2H,]toluene at 25 "C and within 30 min the 3Cspectrum is recorded at -60 "C, there is evidence for the formation of W,(oBu'),(py)(p-H*CcD) in the statistically expected ratio: p-*C2H2 :p-H*CCD = 1:2.Also when hex-1-yne (1 equiv) is added to a C2H8]toluene solution of W,(OBu'), in the presence of pyridine (3 equiv) the 'H n.m.r.spectrum recorded at -60 "C clearly shows the presence Of W,(OBUt),(py)(p-C2H2) along with other signals. These findings are all consistent with equation 3 but do not unequivocally establish this equilibrium. One of the difficulties in working with solutions of W,(OBU1)6(py)(CL-C2H,) is that samples decompose over a 24 h period at room temperature and this decomposition becomes much more rapid at elevated temperatures. [The carbonyl adduct, W2(0BUt),(CI-C2H,)(C0) (discussed later) is more thermally stable but does not show evidence of methylidyne formation.] The decomposition involves Bu'O ligand breakdown to give Me2C=CH2, Bu'OH, and W-0x0 bond formation.Cotton and Schwotzer also have studied the reactions between W,(OBu'), and each of PhCgPh 29 and EtCgEt 30 in hot toluene solutions (5&75 "C)and in 29 F. A. Cotton, W. Schwotzer, and E. S. Shamshoum, OrganometaUics, 1983, 2, 1167. 30 F. A. Cotton, W. Schwotzer, and E. S. Shamshoum, Organometallics, 1983, 2, 1340. Chisholm, Hoffman, and Huffman varying ratios of W,:alkyne from 3: 1 to 1 :1. A variety of crystalline products have been identified including those of alkyne scission, W,(OBu'),(p-CPh),, (Bu'O),W=CPh and [W,(OBU'),(~-O)(~-CE~)O]~.The latter compound again is indicative of Bu'O ligand decomposition.It is important to note that these scissions of the CS bond do not occur in reactions involving Mo,(OBu'),(Mo=Mo), nor for other compounds such as M,(NMe,), and M,(CH,SiMe,),(M=M). The reactions appear to be unique to tungsten but are not exclusively limited to the Bu'O ligand. Other alkoxy groups will give this type of cleavage but the compounds [(RO),W=CR'], are not known. The evidence for CS scission is, however, overwhelming. For example, in the reaction between W,(OPr'),(py), and MeC=CMe (1 equiv), the triangulo-complex W,(p,-CMe)(OPr'), is formed., The structure of the alkylidyne-capped cluster is shown in (7) below. Averaged distances are W-W = 2.74(1) and W-C = 2.06(1) A. Propylidyne and benzylidyne analogues of (7) are formed in reactions involving W,(OPr'),(py), and EtCSEt and PhC=CPh, re~pectively.~~ We believe the W3(p3-CR') formation is understandable in terms of a conproportionation of W,(OPr'), and (Pr'O),W=CR'.This has a parallel in the preparation of the oxo- capped triangulo-compounds M,(p3-O)(OPr'),, by reaction between M,(OPr'), and M0(0Pri),.33,34 Further support for this suggestion comes from the essentially quantitative preparation of (7) by the reaction shown in equation 4 and R = Pri X = CMe (7) the successful synthesis of a mixed-metal cluster, which is isomorphous and isostructural with (7), by the reaction shown in equation 5.35 We have also noted W,(OPr'),(py), + (OBu'),W=CMe p,'~~~~oc~W,(p-CMe)(OPr'), + 3 Bu'OH + 2 py (4) i~~~~Mo,(OPr'), + (Bu'O),W=CMe , , oc* Mo,W(p-CMe)(OPr'), + 3 Bu'OH (5) 31 M.H. Chisholm, D. M. Hoffman, and J. C. Huffman, Inorg. Chem., 1984, 23, 3683. 32 M. H. Chisholm and B. Conroy, results to be published. 33 M. H. Chisholm, K. Folting, J. C. Huffman, and C. C. Kirkpatrick, Inorg. Chem., 1984, 23, 1021. 3* M. H. Chisholm, J. C. Huffman, E. M. Kober, Inorg. Cht-m., 1985, 24, xxx. 35 M. H. Chisholm, K. Folting, J. A. Heppert, D. M. Hoffman, and J. C. Huffman, J. Am. Chem. Soc., 1985,107, xxx. 77 C-C, C-N, and C-0 TripIe Bonds L\ RO' 7M RO M = MO or w R = CH~BJL = PY M=W R = Bu' L = none M=W R = CHzBut L = PMe, M=W R = Pri L = PMe3 R'I RO R' R = Pri R' = H or Me R = CH~BU' R' = H or Me that in the reaction between W2(OPri),(py), and BuC=CH or MeC=CH (1 equiv), the ethyne adduct, W2(OPri),(py),(p-C2H2) is formed." Thus we conclude that alkyne scission is not limited to Bu'O ligands but that with the less bulky alkoxy- groups products other than [(RO),W=CR'), may be formed.Alkyne Coupling Reactions.-Whenever it has been possible to isolate an alkyne adduct it has been possible to show that these are labile towards C-C coupling reactions upon further addition of alkyne. In many instances new compounds are D DCCD D H Scheme The cyclotrimerization of ethyne molecules at the dinuclear centre. The specific combination of labels shown here involved Mo,(OCH,Bu'),(py), in [ZH,]toluene at room temperature formed containing either two or three alkyne fragments bonded to the dimetal centre as shown in (8) and (9) ab~ve.'~,'~-~~ In the case of molybdenum the alkyne adducts, (3), and the p-C4H, compound, (8), were shown to be involved in the catalytic cyclotrimerization of alkynes to give benzene^.'^ Labelling studies have shown that C-C bonds are not broken during the cyclotrimerization (see Scheme).Though not isolated for molybdenum, compounds analogous to those shown for (9) are almost certainly involved in the cyclotrimerization reaction. Interestingly, the ditungsten compounds are reluctant to release the co-ordinated coupled alkynes. If for the sake of electron counting the p-C4R4 and q2-C2R2 ligands are counted as 2-ligands, being a part of metallacyclopentadienyl and metallacyclopropenyl rings, respectively, then the average oxidation state of tungsten in (9) is +5. Of course, oxidation states in compounds of type (9) are rather arbitrary. The main point we wish to emphasize is that in forming new metal-ligand bonds the W-W bond order has been reduced to at most a single bond, consistent with the observed distances W-W = 2.86 A in (9).15e3, C-C coupling is favoured over C-C scission providing steric factors permit.Addition of MeCgMe (>3 equiv) to W,(OPr'),(py), yields 36 W,(OPr'),(p-C4Me4)(C,Me2) in essentially quantitative yield despite the fact that the simple alkyne adduct cannot be isolated and there is strong evidence for the formation of (Pr'O),WSMe. Similarly addition of C2H2 (1 equiv) to a solution of W,(OBut), in [2H,]toluene or ['H,]benzene leads to a 50-50 mixture of W2(OBu'), and W2(OBut)6(p-C,H,).36 W,(OBU')~(~-C,H,) is presumably formed by the reaction between C2H, and W2(OBUt)6(p-C2H2) with the latter being an exceedingly 36 M.H. Chisholm, D. M. Hoffman, and J. C. Huffman, J. Am. Gem. Soc., 1984, 106, 6806. C-C, C-N, and C-0 Triple Bonds W RO OR R = Pr' X = NH Y = OPri reactive molecule in the absence of an additional co-ordinated py or CO ligand. The direct addition of C2H2 to a solution of W2(oBut)6(py)(p-C2H2).:py leads to W2(OBut)6(p-C4H4), quantitatively. 3 Reactions Involving RC=N W2(OBut)6 reacts with nitriles in hydrocarbon solvents at room temperature according to equation 6.22 The nitrido-compound has an interesting linear polymeric structure involving alternating short, 1.74(1) A, and long, 2.66(1) A, W-N distances corresponding to W-N triple bonds and weak dative single bonds re~pectively.~The nitrido-compound is only very sparingly soluble in hexane and Schrock and co-workers 22 have used reaction 6 to prepare alkylidyne derivatives not easily prepared by reactions 1 and 2.W2(oBUt)6 + R'GN -(Bu'O)~W=N+ (But0)3WZR' (6) R' = Ph or Me Reactions involving other ditungsten alkoxides and nitriles lead to [(RO)JW=N], compounds (R = Pr' and CH2Bu') which may be isolated by vacuum sublimation. The fate of the presumably-formed alkylidyne compound [(RO),WKR'], is not certain though in the reaction between W,(OPr'),(py), and MeCEN compound (7) is formed.This once again provides evidence for the formation of the reactive species (Pr'O),W=CMe. If the reaction between W,(OPr'),(py), and MeCsN is carried out in the presence of Pr'OH then an imido- capped compound W,(p3-NH)(OPri),o, (lo), is also formed.37 The imido- compound is isomorphous with the 0x0-capped compound W,(p3-0)(OPr'),,.34 A direct synthesis of (10) is by the reaction outlined in equation 7.37 hexane PriOHbW2(OPri)6(py), + (Bu'O),W=N W3(p-NH)(OPri),, + 3 Bu'OH + 2 py (7) 37 M. H. Chisholm, D. M. Hoffman, and J. C. Huffman, results to be published. Chisholm, Hoffman, and Huffman Figure 1 Comparisonof the Me,NCN-to-metal bonding in W,(OCH,BU'),(NCNM~,)~ and MO,(OCH,BU')~(NCNM~~).The positions of the alkoxy-liganh are denoted by 0 While there is evidence that ditungsten hexa-alkoxides react with alkyl or arylnitriles, R'CzN, to give products derived from scission of the W=W and C=N bonds, no parallel reactions are found for Mo,(OR), compounds.The latter do not even co-ordinate nitriles as N-donor adducts of type (2). This is quite a remarkable difference in the reactivity of triple bonds between molybdenum and tungsten atoms. We believe, however, that it is easily understood in terms of the relative ease of oxidation of the metals (W > Mo) and the relative reducing power of their M=M bonds. The reactions shown in equations 1,2, and 6 can be viewed as oxidative cleavages of the (WEW)~+ moiety to give two W(6+) centres. Though Mo,(OR), compounds show little or no affinity towards simple alkyl or aryl nitriles they do form 1: 1 adducts with dimethyl- and diethylcyanamide, R',N-C=N, and a comparison of the reactivity with W,(OR), compounds is C-C,C-N, and C-0 Triple Bonds particularly pertinent.The spectroscopic characterization of Mo,(OR),(NCNR ,) compounds, where R = But, Pri, or CH,Bu' and R' = Me or Et, leads us to believe they are all similar in structure and bonding.38 Only for R = But and R' = Me or Et do we see reversible binding of the cyanamide in solution. The molecular structure of the compound where R = CH,Bu and R' = Me, deduced from X-ray studies, reveals that the Me,NCN ligand spans the Mo-Mo bond of distance 2.449(1)A.27 The Mo-Mo distance together with Mo-C, Mo-N, and C-N distances allow one to formulate the adduct in terms of a Me,NCN2- ligand bonding to a (Mo=Mo)~+ centre (see Figure 1).The reactions between W,(OR), compounds and dialkylcyanamides are quite different. With the bulky Bu'O ligand the products are those predicted by equation 6, where R' = Me,N or Et2N,27 but with R = CH,Bu' the 1:3 adduct W,(OCH,BU'),(NCNM~,)~ has been isolated at low temperature^.^' Here three different modes of Me,NCN to W bonding are seen in the same molecule (see Figure 1). Each Me,NCN ligand may again be viewed as a dianion and an analogy with the bonding of q2-acyl ligands4' is seen. Of course, assignment of oxidation states in such a molecule is somewhat arbitrary. However, the W-to-W distance of 3.85 A clearly shows that the W=W bond has been cleaved and the ability of W,(OCH,Bu*), to react with three molecules of Me,NCN relative to Mo,(OCH,- But),, which forms only a 1: 1 adduct, testifies to the greater reactivity of the W=W bond and its desire to transfer electron-density to metal-ligand bonding.4 Reactions with Carbon Monoxide Kelly first discovered that hydrocarbon solutions of MO,(OBU'), and C=O react rapidly at room temperature and 1 atm according to the stoicheiometry shown in equation 8.4' 2Mo,(OBu'), + 6CQ-Mo(CO), + -1Mo(OBu'), (8) The first step in equation 8 is the reversible formation of Mo,(OBu'),(p-CO), (1 l).41Carbonylation of Mo,(OPr'), proceeds similarly to give Mo(CO), and in the presence of pyridine an initial carbonyl adduct, Mo,(OPr'),(py),(p-CO) can be i~olated.~~.~~The latter compound adopts the structure shown in (12) which is closely related to (1 1) having a Mo-N bond trans to the Mo-C bond thereby completing the octahedral co-ordination of each metal atom.In the case of the carbonylation of Mo,(OPr'),, the oxidized form of molybdenum is Mo,(OPr'),(CO), which is believed to have the edge-shared bioctahedral structure shown in ( 13).44 Carbonylation of W,(OR), compounds also leads to W(CO), under very mild 38 M. H. Chisholm and R. L. Kelly, Inorg. Chem., 1979, 18, 2266. 39 M. H. Chisholm, J. C. Huffman, and N. S. Marchant, Polyhedron, 1984, 3, 1033. 40 M. D. Curtis, K. B. Shiu, and W. M. Butter, Organometallics, 1983, 2, 1475 and references therein. 41 M.H. Chisholm, F. A. Cotton, M. W. Extine, and R. L. Kelly, J. Am. Chem. SOC.,1979, 101, 7645. 42 M. H. Chisholm, J. C. Huffman, J. Leonelli, and I. P. Rothwell, J. Am. Chem. Soc., 1982, 104, 7030. 43 F. A. Cotton and W. Schwotzer, Inorg. Chem., 1983, 22, 387. 44 M. H. Chisholm, J. C. Huffman, and R. L. Kelly, J. Am. Chem. SOC.,1979, 101, 7615. 82 Chisholm, Hoffman, and Huffman M = MoorW R= But R M = MO or w R = Pri or CH~BU' conditions but the details of the reaction pathways differ from those of molybdenum in two important respects. (a) The oxidized form of tungsten is W(6 + ) and (6)at low CO to W, ratios, where analogues of (1 1) and (12) are formed, the CO ligand is not readily lost from the dinuclear centre.Cotton and Schwotzer 4s isolated a compound of formula W,(OPr'),(CO), '' F. A. Cotton and W.Schwotzer, J. Am. Chem. SOC.,1983, 105, 5639. C-C, C-N, and C-0 Triple Bonds R 0 R = Pri during their studies of the reaction between W2(OPri)6 and CO. The structure of this interesting molecule is indicated by (14) above. The alkoxy-bridges are very asymmetric in (14) implying that a W(OPri)6 molecule is ligated to a W(C0)4 fragment. The stepwise conversion of (1) to (1 1) to (13) to (14)and finally to W(OR), and W(CO), by CO uptake and CO and.OR group migrations is easy to envisage. As indicated earlier the CO ligand binds more tightly to the W2 centre in (1 1) and (12)compounds than to the Mo, centre and some interesting data comparing the molybdenum and tungsten compounds are given in Table 3.46 The following points are worthy of particular note.(a)The M-M and C-0 (carbonyl ligand) distances (2.5 and 1.2A)are comparable to double bond distances in (M=M)'+-containing compounds and ketones, respectively. (b) The values of v(C0) are exceptionally low for bridging carbonyl ligands (p2-CO) in neutral molecules which normally appear in the range 1 70@-1 860~m-'.~~(c) The value of v(C0) is lower, by ca. 70-80 cm-', for compounds where M = W relative to M = Mo. (d)The chemical shifts of the bridging-carbonyl carbon atoms are below 300 p.p.m., which is downfield of the range commonly observed for p-CO Iigand~.~~ All these points are understandable in terms of the mixing of M-M 7c and C-0 7c* orbitals.These molecules are inorganic analogues of cyclopropenones and the simple VB (Valence Bond) structure (15)will have a significant contribution from the ionic resonance forms 15a and 15b. 0 0-0-II I I M=M M-M-M 46 M. H. Chisholm, D. M. Hoffman, and J. C. Huffman, Orgunometullics, 1985, 4, xxx. 4' F. A. Cotton and G. Wilkinson, 'Advanced Inorganic Chemistry', Wiley, 4th Edn., 1980, p. 1072. 48 M. H. Chisholm and S. Godleski, Prog. horg. Chem., 1976, 20, 299. Table 3 Selected i.r. and 13C n.m.r. data for the Mz(p-CO) and [W2(p-C0)]2 containing compounds (M = Mo or W) 3C N.m.r. Temperature/ Compound vcOa(cm-') ~13cO(crn-~) 6(13CO)(p.p.m.)* 1J183W-13c(Hz) Solvent ReJ W2(OBL1t)6(~-CO) 1598 1559 291 .O 192.9 21 "C/C6D6 h (1 605)' 1 587 1 546 1 579 1538 2(OCH2 Bu')6(PY )2 (p-CO) 1 567 321.6 150.3 -60 "C/C7Dg h (1 595)' (1 570)' 1 570 --hW2(0Pri)6(pY)2(cI-CO) 1 579 1533 341' (1 298)' (1 271)' 9E.Mo2(OPri)6(py)2(p-C0) 1655 1618 331.5 -25 "C/CsD6 j a-h -3[w2(oPri)6(p-co)12 1 272 1 243 305.5 188.9f -55 "C/C7Hg 164.0 %$ [W2(oPri)6(py)(cI-Co)12 1 305 1 265 310.4 170.0a 34 OC/C7Dg h (i.r.) ta (1 298)' j,k (n.m.r.) -3 1.r.spectra were recorded for Nujol mulls between CsI plates except when noted. * In p.p.m. relative to TMS. 1.r. spectrum recorded for a toluene solution. 'N.R. = not reported. Magic-angle 'jC n.m.r. value. Solution value from reJj below is incorrect. Consistent with the solid-state structure, there are two le3W-''C coupling % constants.Only one 1s3W-13Ccoupling constant reported. M. H. Chisholm, D. M. Hoffman, and J. C. Huffman, Orgunometullics,submitted. M. H. Chisholm, F. A. 5Cotton, M. W. Extine, and R.L. Kelly, J. Am. Chem. SOC.,1979,101,7645. j M. H. Chisholm, J. C. Huffman, J. Leonelli, and I. P. Rothwell, J. Am. Chem.SOC.,1982,104,oo 7030. 'F. A. Cotton and W. Schwotzer, J. Am. Chem. SOC.,1983,105,4955 3 C-C, C-N, and C-0 Triple Bonds 0i\7iC HC\W /OW-Figure 2, A ball and stick drawing of the central [w,(@o)o& skeleton of the [W,(OPr*),(p -CO)],moleculeshowing the trigonalbipyrumidal co-ordination at each tungsten atom. Pertinent distances are given in the text The lower values of v(C0) for M = W relative to M = Mo reflect the greater reducing power of the W=W bond and the greater W to C-0 n* bonding.The bonding picture presented by (15) and its ionic resonance forms, leads to the prediction that the oxygen atom of the p-CO ligand should be nucleophilic and the metal centre electrophilic. This prediction was realized experimentally by Cotton and Schwotzer 49 who, in attempting to prepare W,(OPr'),(py),(p-CO) isolated instead W4(OPri),,(CO)2(py),. The closely related molecule W,(OPr'), ,(CO),, which has the central W40,,(CO), skeleton shown in Figure 2, is made by the addition of Pr'OH to W,(OBU'),(~-CO).~~The formation of W4(OPri),2(CO), or its pyridine adduct arises from the dimerization of two molecules of (1 1) or (12). The 49 F. A. Cotton and W.Schwotzer, J.Am. Chem. SOC.,1983, 105,4955. Chisholm, Hoflman, and Huffman essential feature of the dimerization is the coupling of two units of (15) to give the resonance isomers shown in (16a) and (166). Of note in the structural characterization of W,(OPr'),,(CO), is that the W-W and C-0 distances increase from those in (11) and (12)to W-W = 2.66(1) A and C-0 = 1.35( 1) A. These are W-W and CsP2-O single-bond distances, respectively. On the other hand the W-C distances, 1.95( 1)A (averaged) are shorter than those in (11) and (12). The stepwise reaction involving W=W + C=O --+ W,(p-CO) followed by 2W2(p-CO) +[W,(p-CO)], converts W-W and C-0 triple bonds first into double bonds and then into single bonds. Formally the ditungsten centre is oxidized (W=W),+ -(W=W)8' -+W-W)''+ and the carbonyl ligand reduced. We believe that the reaction does not stop here but rather by a further reaction between W,(OPr'), and W,(OPr'),,(CO), (note the metal atoms are only five co-ordinate in the latter molecule-see Figure 2) the C-0 bond of the former carbon monoxide molecule is cleaved to give carbido, C4-, and oxide, 0,-tungsten alkoxide clusters.The reduction of C=O to C4-and 0,-,a six-electron process, requires the co-operative effects of two or more (WEW)~+ units which contrasts with the cleavage of C=C and C=N bonds described before. Further work is required to establish these important matters and at present the evidence for Ca cleavage to carbide and oxide is speculative.However, we have observed a facile C==Obond cleavage reaction, equation 9," and also have isolated the carbido butterfly-cluster W4(p4-C)(OPr') ,(p2-NMe) from a thermal decomposition of W,(OPr')6(HNMe,),.6 2W,(OPri),(py), + 2Me,C=O -W,O,(OPr'), ,+ Me,C=CMe, (9) 5 Reactions with N-N Triple Bonds The facile cleavage of C-=C and CzN bonds, the reactions with C=O leading to C-0 single bonds and possibly carbide and oxide lead one to ask: What about N=N? All the chemistry described thus far has been carried out under an atmosphere of N, and we have found no evidence for N, activation. Professor Schrock also informed us that he and his group had looked for a possible reaction between W,(OBu'), and N, at high pressures and moderately elevated temperatures but found none." A plausible explanation for this lack of reactivity may lie in the poor ligating properties of the N, molecule.It is possible that the reaction between W,(OBu'), and N, to give (Bu'O),W=N is thermodynamically favourable but has a high kinetic barrier imposed by the low affinity of the weak Lewis base to the (W=W),' centre. This leads one to speculate whether or not a transition-metal dinitrogen complex 52 might serve as a carrier of a more active N, molecule to the (W=W),' centre. In this regard we note that diaryl-substituted diazomethanes react very T. P. Blatchford, M. H. Chisholm, K. Folting, and J. C. Huffman, J. Chem. Soc., Chem. Commun., 1984, 1295. Personal communication. '* J. Chatt, J. R. Dilworth, and R.L. Richards, Chem. Rev., 1978, 78, 589. 87 C-C, C-N, and C-0 Triple Bonds RO CAr2 N /N= Ar 2C =N /N = OR R = But Ar = p-tolyl rapidly with W2(OR)6 compounds in hydrocarbon solvents even at -78 "C. The molecular structure of the adduct W2(OBut)6(N2CAr2)2 is shown schematically in (17). The geometry depicted by (1 7) is based on two fused trigonal bipyramids sharing a common equatorial-axial edge. The W-W distance is 2.675(1) A and the N-N distance is 1.41(1) A, typical of single-bond distances. In the reaction between W2(OBut)6 and N2CAr2, the (W=W)6+ unit has been oxidized to (W-W)''+ and the diazoalkane reduced to Ar2CN22-, a hydrazido 2-ligand.5"56 6 Reactions of AIkyne Adducts with Nitriles and Carbon Monoxide The observation of facile C-C coupling in the reactions of alkyne adducts, (3), (4), and (5) with additional equivalents of alkyne led us to investigate their reactivity toward CO and RCN molecules.We have found no evidence for C-C bond formation in reactions of alkyne adducts, compounds (3), (4), and (5), or the p-C,R,-containing compounds, (8), with carbon monoxide at room temperature, 1 atm. Simple 1 :1 adducts of the type shown in (18), (19), and (20)below have been isolated." However, nitriles and the alkyne adducts of tungsten react rapidly to give dinuclear compounds containing fused heterocyclic rings. 58 The driving force for these reactions appears to be the formation of both C-C and W-N bonds. W,(OBU'),(py)(p-C2H,).9py reacts at room temperature with R'GN (1 equiv), where R' = Me and Ph to give (21).The VB description is based on the bond distances: W-W = 2.674(1) A, W=C = 1.980(6)A, W=N = 1.903(5) A, W-N = 2.041(5)A and C==C= 1.36(1)81.59 The geometry about each tungsten atom 53 M. H. Chisholm, J. C. Huffman, and A. L. Ratermann, fnorg. Chem. 1984, 23, 2303. 54 J. R. Dilworth, Coord. Chem. Reo., 1976, 21, 29. 55 J. Chatt, R. A. Head, P. B. Hitchcock, W. Hussain, and G. J. Leigh, J. Organomer. Chem., 1977,133, C1. 56 J. Chatt, G. J. Leigh, and R. A. Head, J. Chem. SOC.,Dalton Trans., 1980, 1129. 57 M. H. Chisholm, D. M. Hoffman, and J. C. Huffman, results to be published. '13 M. H. Chisholm, D. M. Hoffman, and J. C. Huffman, J. Am. Chem. SOC.,1984, 106, 6815. 59 For a discussiod of M-N bond distances and bond order assignments see W.A. Nugent and B. L. Haymore, Coord. Chem. Reo., 1980,31, 123. 88 ChishoIm,Hoffman, and Huffman H Ro\ Ro02w\ /W. K'OR RO 0 OR R R = Bu' (18) Me I corresponds to a distorted trigonal-bipyramid in which the W=C and W=N bonds occupy equatorial positions. The compound W,(OCH,Bu'),(py),(~-C,Me,)reacts with MeC=N (>2 equiv) to give W,(OCH,Bu'),(N(CMe),N}(py). In this molecule each tungsten atom is six co-ordinate and the local geometry is a distorted octahedron. The two tungsten atoms share a face formed by two OR ligands and the nitrogen atom of the metallacycle shown in (22). Again the VB depiction is based on observed W-W, W-N, C-N, and C-C distances. It is reasonable to assume that a molecule analogous to (21) is an intermediate in the reaction to form (22).7 Concluding Remarks The reactions described in this account show that the (MEM)~' unit in M,(OR)6 compounds, (l),provides both a source of electrons and a template for CZ-C,C=N, and C-=O groups. These reactions may be viewed as a special class of oxidative additions in which electron density is removed from M-M bonds to form metal- ligand bonds. It is the availability of the empty n* orbitals of the C=C,C=N, and C-C, C-N, and C-0 Triple Bonds C-=Ogroups that leads to the mutal reduction in M-M and C-X bond order (X = C, N, or 0).The W=W bond is a more powerful reducing agent than the MEMO bond leading in certain cases to remarkable oxidative cleavage reactions, equations 1 and 6. The latter are dependent upon steric factors.Whenever possible the formation of more metal-ligand bonds is favoured over cleavage of C-C or C-N bonds. The role of alkyne adducts in the metathesis reactions (equation 1) is also of interest. What reaction pathway leads from the pseudo-tetrahedral M2C2unit in W2(OBut),(py)(p-C2H2) to the alkylidyne (Bu'O),W=CH? Are 1,2 or 1,3-dimetallacyclobutadiene intermediates, (23) or (24), involved? Also what part do bridging alkoxy-ligands play in the pathway leading to C-C cleavage? If we consider the proposed equilibrium shown in equation 3, it is easy to anticipate that it could be close to thermochemical neutrality. The dimetallatetra- hedrane has six a-bonds, four W-C bonds, one C-C, and one W-W bond while.two alkylidyne-tungsten moieties have two W-C o-bonds and four W-C n-bonds.It "'\ i" Chisholm, Hoffman, and Huffman has been estimated 6o that the conversion of two terminal M-OR bonds into four M-OR bridging bonds, 2 M-OR --+ M,(p-OR),, is enthalpically favoured by 10-15 kcal mol- '. Consequently steric factors which permit alkoxide bridge formation might favour the dimetallatetrahedrane while bulky substituents could cause sufficient internal steric pressure to break the p-OR bridges and favour C-C scission. While the present findings raise still many unanswered questions they indicate many avenues for future research and the great potential that compounds with M-M multiple bonds offer toward multi-electron redox reactions.Acknowledgements. We thank the Donors of the Petroleum Research Fund administered by the American Chemical Society, the Department of Energy, Office of Basic Research, Chemical Sciences Division, and the Wrubel Computing Center for financial support and several talented co-workers whose contributions are cited in the references. Note Added in Proof Schrock and co-workers have recently reported6' the cleavage of the MEMO bond in the reaction between Mo,(OBu'), and PhCSH (15 equiv) and the full details6* of their studies of the reactions between alkynes and W,(OBu'),. 6o K. J. Cavell, J. A. Connor, G. Pilcher, M. A. Ribeiro da Silva, M. D. M. C. Ribeiro da Silva, H. A. Skinner, Y. Virmani, and M. T. Zafaram-Moattar, J. Chetn. Soc., Faraday Trans., 1981,77, 1585. 61 H. Strutz and R. R. Schrock, Organometallics, 1984,3, 1600. 62 M. L. Listemann and R. R. Schrock, Organometallics, 1985, 4, 74. 91

ISSN:0306-0012    

DOI:10.1039/CS9851400069

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

x-Allylnickel Halides as Selective Reagents in OrganicSynthesis By David C. Billingtont DEPARTMENT OF PURE AND APPLIED CHEMISTRY, UNIVERSITY OF STRATHCLYDE, GLASGOW, G1 1XL 1 Introduction A central theme in organic synthesis is the formation of new carbon-carbon bonds in a selective manner. In this context, the introduction of an allylic unit into a carbon skeleton is an interesting and useful synthetic operation, which is formalized in Scheme 1. In recent years transition-metal x-ally1 complexes have been recognized as efficient reagents for this type of reaction.’V2 The advantage of these reagents lies primarily in their much higher selectivity, when compared with the more traditional allylmagnesium, lithium, and zinc species. Two classes of complexes have proved particularly useful, the x-allylnickel halides, and the x-ally1 complexes of palladi~m.~*~In synthetic terms these reagents are compli- mentary to a degree, as x-allylnickel halides react with electron-poor centres, and may thus be classed as ‘nucleophilic’ reagents, whereas n-ally1 palladium complexes react with electron-rich centres (normally stabilized anions) and may thus be considered to be ‘electrophilic’ reagents, Scheme 2. The complexes are, however, neither electrophilic or nucleophilic in the normal organic sense of the word.Scheme 1 R-OAc Scheme 2 t Present address: Merck Sharp & Dohme Research Laboratories, Neuroscience Research Centre, Terlings Park, Eastwick Road, Harlow, Essex, CM20 2QR *’ M. F.Semmelhack, Org. React., 1972, 19, 115. L. S. Hegedus, J. Organomer. Chem. Libr., 1976, 1, 329. ’R. Baker, Chem. Rev., 1973,73,487. G. P. Chiusoli and G. Salerno, Adu. Organomet. Chem., 1978, 17 195. B. M. Trost, Acc. Chem. Res., 1980, 13, 385. n-Allylnickel Halides as Selective Reagents in Organic Synthesis The present review is concerned with the selectivity that is available by the use of n-allylnickel halides for the formation of new carbon-carbon bonds. A general outline of the preparation of the complexes, their structure and physical properties is followed by an analysis of their reactivity patterns. Examples of the use of the complexes in natural-product syntheses are then presented, grouped by 'electrophile'. It is hoped that this treatment will demonstrate the utility of these complexes to organic chemists and stimulate their wider use as reagents.2 Preparation, Structure, and Properties The dimeric n-allylnickel halide complexes are most conveniently prepared by the reaction of allylic halides with a zero-valent nickel species, in a non-polar solvent, Scheme 3. The two most commonly used nickel species are nickeltetracarbonyl [Ni(C0),l6 and bis- 1,5-cyclo-octadienylnickel(0)[Ni(COD)2].7 The use of other nickel species e.g. Ni(C0)3P(Ph)3 has been reported.* Ni(C0)4 is commercially available, and although Ni(COD)2 is also an article of commerce, its high price makes its synthesis attractive. Scbeme 3 Allylic systems with other leaving groups such as me~ylates,~ acetates,'O and sulphonium salts l1 also give rise to n-allylnickel halides on treatment with nickel(0) reagents.The use of allylic bromides is, however, by far the most common procedure, as these compounds are easily and reliably prepared from the corresponding alcohols, and give somewhat higher yields of the complexes. A. Preparation from Ni(CO)&--CAUTION The dangerous nature of Ni(CO), CANNOT be over emphasized. The pure liquid is volatile (b.p. 43 "C),flammable, highly toxic, and symptoms of poisoning may not develop until 24 hours after the ingestion of a fatal dose. Before using Ni(CO), t6 M. F. Semmelhack and P. M. Helquist, Org. Synth., 1972, 52, 115.'G. Wilkie, Angew. Chem., Int. Ed. Engl. 1966, 5, 151. E. J. Corey and M.F. Semmelhack, Tetrahedron Lett., 1966, 6237. M. F. Semmelhack and S. J. Brickner, J. Am. Chem. SOC.,1981, 103, 3945. lo N. L. Bauld, Tetrahedron Lett., 1962, 859. l1 M. F. Semmelhack, A. Yamashita, J. C. Tomesch, and K. Hirotsu, J. Am. Chem. SOC.,1978,100,5565. Billing ton the dangers and precautions involved should be fully appreciated, and reference should be made to the technical bulletin, 'Nickel Carbonyl is Dangerous' available from the Matheson Company. Allylic halides react readily with excess Ni(CO), in benzene at 50-70 "C, giving the red dimeric x-allylnickel halide complexes.6 The complexes may be isolated by removal ofsolvent and excess Ni(CO), under reduced pressure, and purified by low- temperature recrystallization under an inert atmosphere.The complexes may also be sublimed, but large losses resulting from thermal decomposition normally preclude this method of isolation/purification. In the solid state the complexes are only moderately air sensitive, and storage for years under an inert atmosphere is possible. Excellent experimental details are available for this method of preparation. 'A B.Preparation from Ni(COD),.-Reduction of nickel bis(acety1acetonate) in the presence of cyclo-octa-1,5-diene yields the yellow crystalline complex Ni(COD),, Scheme 4. 'vl' The reduction is normally performed using aluminiumalkyls as reducing agents. Ni(COD), is an air-sensitive solid which becomes highly air- sensitive in solution/suspension in organic solvents, and requires careful inert- atmosphere techniques for its preparation and use.'v2 Allylic halides react with Ni(COD), at 0 "C or below in non-polar (i.e. non-coordinating) solvents to give almost quantitative yields of the dimeric x-allylnickel halide complexes. '7' The by- product of this reaction, cyclo-octa- 1,5-diene does not normally cause any problems, even when further reactions are performed in situ. The use of Ni(COD), avoids the thermal decomposition observed when Ni(CO), is used at 50-70 "C, and a number of thermally-sensitive complexes may only be successfully prepared using this reagent.13 The preparation and use of Ni(COD), have been well described in the literature.'*7*'2.'3 C. Structureand Properties.-In the solid state, and in solution in non-coordinating solvents, the complexes exist as halogen-bridged dimers, as indicated in Scheme 3.'H N.m.r. spectroscopy confirms the presence of planar delocalized x-ally1 ligands,' and X-ray crystallographic analysis l4 has allowed the assignment of the B. Bogdanovic, M. Kroner, and G. Wilkie, Ann., 1966, 699, 1 l3 L. S. Hegedus and S. Varaprath, Organometallics, 1982, 1, 259. l4 C. H. Dietrich and R. Uttech, 2. Kristallogr., 1965, 122, 60. n-Allylnickel Halides as Selective Reagents in Organic Synthesis structure shown (Figure 1) for n-(2-carbethoxyallyl)nickelbromide. The plane of the ally1 ligand is tilted at ca. 110" to the plane containing the halogen and metal atoms. A discussion of the bonding in the complexes is not germane to the present treatment, and the interested reader is directed to refs.1 and 15-17. COz R I CO,R Figure 1 In polar co-ordinating solvents such as N,N-dimethylformamide, N-methylpyr- rolidone, N,N-dimethylacetamide, or hexamethylphosphoramide, or in the presence of good electron-donor ligands such as triphenylphosphine, the 'H n.m.r. spectra of the complexes change to the characteristic spectra of rapidly equilibrating o-ally1 species.'* This change indicates the splitting of the bis K-allylnickel halide dimers into a highly reactive monomeric species, stabilized by solvent (or ligand) co-ordination, Scheme 5. Although the dimeric complexes are only moderately air-sensitive in the solid state, and may be stored in the absence of air for years, in solution the monomeric species are highly air-sensitive, and atmospheric oxygen must be rigorously excluded.The red complexes form deep- red solutions, and this colour is almost instantly discharged on the admission of air into the reaction system. Solutions of the complexes are also thermally unstable, normally decomposing above 60 "C. q=?---Sol -Sol +Sol 'LNi-X+Sol -Nix), Sol Scheme 5 In practice, reactions with the complexes are normally carried out according to the following protocol. The complex is generated by treatment of an allylic halide with Ni(CO), at 50-70 "C in benzene, or with Ni(COD), at ca. 0 "C in a similar non-polar solvent. The solvent and excess Ni(C0)4 (or COD) are then removed by l5 'Metal Complexes in Organic Chemistry', by R.P. Houghton, Cambridge University Press, 1979. l6 'Advanced Inorganic Chemistry', 3rd Edn., by F. A. Cotton and G. Wilkinson, Interscience, 1972. l7 S. F. A. Kettle and R. Mason, J. Organomet. Chem., 1966,5, 573. D. Walter and G. Wilkie, Angew. Chem., In[. Ed. Engf., 1966, 5, 897. Billingt on NiBr), 46 'lo 0 OH Scheme 6 Reactions which proceed at or below 25 "C application of vacuum, and this leaves the complex as a red solid or gum. The complex may then be isoIated and purified by low-temperature crystallization, (for x-Allylnickel Halides as Selective Reagents in Organic Synthesis simple complexes which are being prepared in bulk) or alternatively a polar co- ordinating solvent is added followed by the substrate.The reaction of the monomeric allyl species with the substrate is then allowed to proceed until the characteristic red colour of the complex is discharged, and the reaction products are then isolated. Reactions are usually conducted between 20 and 60 "C. Excellent practical details for these straightforward reactions are available in the literature, and in the bibliography of this review, asterisks mark those references with particularly useful experimental sections. 3 Reactivity A. Chemose1ectivity.-As previously mentioned, the main advantage of the n-allylnickel halide complexes stems from their high selectivity compared to the traditional allylmetal derivatives. In solution in polar co-ordinating solvents the complexes react at or below room temperature with 1" (primary) and 2" (secondary) alkyl, aryl, vinyl and allyl bromides and iodides, giving allylated product^.'^ Phenyl ketones react to give homoallylic alcohols,20 whereas quinones give allyl-substituted quinones, and/or hydroquinones.2 1,2-Diketones react with the complexes to give a-keto-homoallylic alcohols (even in the presence of excess complex, reaction only occurs at one carbonyl group),20 and 2-pyridyl carboxylates give P,y unsaturated ketones as major products,22 accompanied by the corresponding a,P-unsaturated ketones.These reactions are summarized in Scheme 6. At somewhat higher temperatures (40-53 "C) the complexes have been shown to react with aldehydes, cyclic, and some acyclic ketones, giving homoallylic alcohols,20 allylic chlorides giving diene~,~~ and in one report with certain epoxides.20 a$-Unsaturated ketones and simple ketones react under forcing conditions (55 "C and above) which cause thermal decomposition of the complexes, giving only the products of 1,Zaddition (even in the presence of CUI).~' These reactions are summarized in Scheme 7.In all of the above reactions both allyl ligands are used for carbon-carbon bond formation, so 1 mol of substrate reacts with 3 mol of complex. The complexes do not react with acid chlorides, esters, ethers, nitriles, acetylenic protons, olefins, alcohols, aryl, vinyl, or alkyl chlorides, allylic ethers, or acetals. Allylic acetates are not always compatible with the reagents [Ni(COD), is reported to cleave allylic acetates 24 and Ni(CO), couples allylic acetates lo giving 1,5- dienes].There is also some evidence that easily reduced or oxidized groups, e.g. hydroquinones, are unstable to the nickel reagents. The above reactivity pattern allows both the preparations of complexes with functional groups present in the reagent, and the reaction of the complexes with l9 E. J. Corey and M. F. Semmelhack, J. Am. Chem. SOC.,1967,89, 2755. *'O L. S. Hegedus, S. D. Wagner, E. L. Waterman, and K. Siirala-Hansen, J. Org. Chem., 1975, 40,593.'*L. S. Hegedus, B. R. Evans, D. E. Korte, E. L. Waterman, and K. Sjoberg, J. Am. Chem. Soc., 1976,98, 3901. L2 M. Onaka, T. Goto, and T. Mukiyama, Chem. Lett., 1979, 1483. 23 K.Sato, S. Inoue, and K. Watanabe, J. Chem. Soc., Perkin Trans. 1, 1981, 2411. 24 T. Yamamoto, 1. Ishizu, and A. Yamamoto, J. Am. Chem. Soc., 1981, 103, 6863. Billington yoH + -7,NiBr AI NiBr),0""+ OH Ad+I NiBr), AI 0"O + NiBr), Scheme 7 substrates containing more than one functional group, one or more of which is either less susceptible than the desired reactive centre or completely inert to the nickel reagents. Thus complexes may be prepared bearing functional groups such as ester, acetal, olefin etc. and some examples of this type of complex are given in Scheme 8. These complexes are all prepared from the corresponding allylic halides, and serve to introduce useful functionality into the reaction products.Examples of simple selective reactions with substrates containing more than one functional group are given in Scheme 9. It can be seen that hydroxyl functions are tolerated in the rea~tion,'~,~~ iodides react faster than chlorides in the same molecule," and in the case given, an allylic acetate is not affected.26 co R 1AnOPh-I I I I T / NiBr$ Ni6rI2 NiBrI2 NiSr), NiBr), Scheme 8 The reactivity profile of the n-allylnickel halides thus contrasts markedly with the reactivity displayed by the more conventional allylmetal reagents, and some specific examples of this are identified below. Allyl-lithium, magnesium, and zinc reagents react rapidly with simple ketones, *25 K. Sato, S. Inoue, and R. Yamaguchi, J. Org. Chem., 1972, 37, 1889.*26 K.Sato, S. Inoue, S. Ota, and Y. Fujita, J. Org. Chem., 1972, 37,462. 99 n-Allylnickel Halides as Selective Reagents in Organic Synthesis I Scheme 9 a,P-unsaturated ketones and, in the presence of copper salts, 1,4-addition may be observed. The nickel reagents react only sluggishly with simple ketones and a,p-unsaturated ketones, and do not react in a 1P-fashion. x-Allylnickel halides will tolerate a range of functional groups in the complex and the substrate, including esters, alcohols, and alkynes, whereas these groups interfere with some or all of the more conventional allylmetal reagents. The n-allylnickel halides can also differentiate successfully between closely related groups, such as 1 O (primary) iodide and 1" chloride, and in some cases will react selectively with a single ketone in a dione substrate (e.g.1,2-diketones, see above, and steroidal diones, see Schemes 29 and 34). The ability to perform such selective reactions has led to the use of n-allylnickel halide complexes in the synthesis of complex natural-products, and these syntheses are presented, grouped by the nature of the electrophilic substrate, under the heading synthetic applications. B. Regiose1ectivity.-Substituted n-allylnickel halide complexes which could give products by reaction at either a 1" or 2"/3"terminus almost invariably 13*22*27928 27 K. Sato, S. Inoue, and K. Saito, J. Chem. Soc., Chew. Commun., 1972,953. 28 K. Sato, S. Inoue, Y. Takagi, and S. Morii, Bull. Chem. SOC.Jpn., 1976, 49, 3351.100 Billington react only at the 1" position, and some examples are given in Scheme 10. There are, however, isolated reports 2o of regioisomeric products arising, for example, in the reaction of n-1,l-dimethallyl nickel bromide with benzil, reaction at the primary terminus being the more favoured. OR PRI OR Scheme 10 C.Stereose1ectivity.-In the formation of a carbon-arbon bond between a n-allylnickel halide complex and a substrate, a carbon-carbon double bond is also generated, and if the substituents on this bond are not identical it may be either Eor 2.The products obtained in this type of reaction are normally mixtures of E and 2 isomers, either in a close to 1 :1 ratio or with the E isomer in excess.In certain favourable cases it has proved possible to influence the course of the reaction by varying the reaction solvent, and E:Z ratios of up to 98:2 have been ~btained.'~ Some representative examples of these reactions are given in Scheme 11. In general the use of solvents of low co-ordinating power, e.g. N-methylpyrrolidone or tetrahydrofuran, seems to give high E:Z ratios, but low overall yields, whilst solvents of high co-ordinating power, e.g. hexamethylphosphoramide, give lower E:Z ratios but higher overall yield^,^^,^' see Scheme 11. With regard to the stereochemistry of the substrate, partial epimerization is observed when, for example, trans-4-iodocyclohexanol is treated with n-2-methallyl nickel bromide,' and substrates which are optically active at the electrophilic centre give racemic products. 4 Synthetic Applications The following reactions do not represent all the reported reactions of Ic-allylnickel halides with electrophilic substrates. In particular, many simple examples are not given, rather natural-product syntheses have been selected as they demonstrate both the selectivity and value of the reagents.29 K. Sato, S. Inoue, and K. Saito, J. Chem. Soc., Perkin Trans. I, 1973, 2289. n-Allylnickel Halides as Selective Reagents in Organic Synthesis xBr+VB z -NiBrI2 E:2 = 6:4 Solvent E:Z ratio % Yield HMPA 51:49 a5 DMA 72:2% 79 NMP 80:20 75 Et0+nCl + Q OEt NiBr$ \ Solvent €2ratio % yield DMF 93: 7 50 THF 98: 2 23 HMPA = Hexamethylphosphoramide DMA = N, N -dimethylacetarnide NMP = N-methylpyrrolidone DMF = N,N -dimethylformamide THF = Tetrahydrofuran Scheme I1 A.Reactions with Alkyl and Aryl Halides.-The 1,l-dimethallyl complex has been used to introduce a five-carbon unit into natural-product skeletons. Examples of syntheses with this complex include the preparation of a-santalene (1) and epi-p-santalene (2),' campherenone (3) and epi-campherenone (4) (in optically active forms by using single enantiomers of the alkyl iodides),30 and de~mosterol,~' (5) Scheme 12. It is noteworthy that a more traditional approach to a-santalene oia the Grignard reagent gave only low yields, compared to excellent yields achieved using the nickel complex. 30 G. L. Hodgson, D. F. MacSweeney, R.W. Mills, and T. Money, J. Chem. Sor.,Chem. Commun., 1973,235. 31 S. K. Dasgupta, D. R. Crump, and M. Gut, J. Org. Chem., 1974, 39, 1658. 32 E. J. Corey, S. W. Chow, ad R. A. Schemer, J. Am. Chem. Soc., 1956, 79, 5773. Billington Scheme 12 103 x-Allylnickel Halides as Selective Reagents in Organic Synthesis The preparation of x-methallylnickel bromide and its reaction with bromobenzene in N,N-dimethylforrnamide, giving methallylbenzene in 67-72% yield, has been described in an Organic Syntheses preparation.6 The series of compounds known as coenzyme Q,s (6) where n = 1-10 [ubiquinones (6)] play an important role in electron transport and oxidative phosphorylation processes. In an early study, Sato, et al.25 investigated the synthesis of coenzyme Q1 via the x-1,l-dimethallylnickel bromide complex.In model studies both iodobenzene and iodophenol gave the expected products, but iodotrimethylhydroquinone(7) and iodotrimethylbenzoquinone(8) both failed to give allylated products. These results indicated that although a hydroxyl (phenol) function was tolerated in the substrate, easily oxidized or reduced groups required protection. The synthesis of coenzyme Q1 was then achieved by reaction of the protected aryl bromide (9)with the x-1,l-dimethallyl complex, giving the allylated 0-OM= -Me0 Me0 57 'I. Ql (91 (10) Scheme 13 Br+ OAc~H~oWh~I k0 NiBr), Me0 ' OAc OAc (11) (12 1 (131 Scheme 14 Billington product (10) in 57% yield, followed by hydrolysis and oxidation, Scheme 13.A similar approach was used to prepare a series of coenzyme Q s from the bromo- acetate (11) and substituted complexes (12).33 Thus, reaction of (11) with IT-geranylnickel bromide (12; n = 1) gave (13; n = 1) in 88% yield, which was converted into coenzyme Q,, Scheme 14. Similarly, (1 1) reacted with n-phytyl, IT-sodanesyl, and n-decaprenyl nickelbromide (12; n = 3,8,9) to give (13; n = 3, n = 8, and n = 9) in 45,42, and 40%yields respectively. These (13)s were then converted into coenzymes Q4, Qg, and Qlo by reduction and oxidation. The use of hexamethylphosphoramide as solvent for these reactions gave the highest chemical yields, but poor stereoselectivity for the newly formed C-C double bond (ca.55:45 E to 2 in each case). Less-polar solvents such as N-methylpyrrolidone gave somewhat lower overall yields generally, but higher stereoselectivity, with E: 2 ratios of up to 70:30 being obtained.33 The above strategy has also been applied to the synthesis of vitamins K, and K,,,, (17) by deprotection and oxidation, Scheme 15.29 Thus, vitamins K, (18) and K2(45)(19) were prepared from the requisite n-allylnickel complexes and the acetate (15), with coupling proceeding in 52 and 70% yields, respectively. A solvent effect was again reported in these reactions, with less-polar solvents giving lower yields but higher stereoselectivity than highly polar solvents (the best E:Z ratio achieved was 80:20).29 OR (14) R = CH20Me (15) R = AC (16) (17) 0 Scheme 15 The three isomeric monomethyltocols (22~-22c) have been prepared via the olefinic precursors (21a-21c).These olefins are conveniently obtained by reaction 33 S. Inoue, R. Yamaguchi, K. Saito, and K. Sato, Bull. Chem. SOC.Jpn., 1974, 47, 3098. x-Allylnickel Halides as Selective Reagents in Organic Synthesis of the corresponding diacetoxybromotoluenes (2Oa-204 with the n-phytylnickel bromide complex in hexamethylphosphoramide, giving the allylated products (21a-21c) in yields of between 52 and 93%, Scheme 16.34 OAc OAc + qYW-f2qNiBr )2 OAc R2R'@r ' R3 OAc 3(201 (211 H R' R2 R3 a Me H H b H Me H R' I1cHH Me Scheme 16 A series of isocoumarins and dihydroisocoumarins have been prepared by cyclization of the products obtained from the reaction of x-allylnickel halides with bromo-benzoic acids, present as either their sodium salts or methyl esters.35 Some of these results are summarized in Scheme 17.Yields for the allylation reaction in these cases varied from 59 to 91%.35 B. Reactions with Allylic Halides: Intermolecular Reactions.-A very early report by Webb ef al. established that allylic halides could be coupled to give 1,Sdienes by using Ni(CO), as a reagent.36 This approach to the synthesis of 1,5-dienes is very attractive for the synthesis of terpenoid compounds, by sequential addition of isoprenoid units. For example, the reaction, of x- 1,l -dimethallylnickel bromide with the allylic halide shown in Scheme 18(a), gives the geranyl skeleton.26 Unfortunately this approach is complicated by the fact that the x-allylnickel halide complex can undergo ligand exchange with the substrate allylic halide,37 +34 S.Inoue, K. Saito, K. Kato, S. Nozaki, and K. Sato, J. Chem. SOC.,Perkin Trans. I, 1974, 2097. *" D. E. Korte, L. S. Hegedus, and R. K. Wirth, J. Org. Chem., 1977,42, 1329. 36 I. D. Webb and G. T. Borcherdt, J. Am. Chem. SOC.,1951, 73, 2654. 37 K. Sato, S. Inoue, and S. Morii, Chem. Lett., 1975, 747. Billing t on X=R=H X = H;R =Me X = H ; R = (Me), X = L-CL;R=H X = 4-Cl; R=Me X = S-OMe;R=H but so NiBr), '8 '6 Scheme 18 giving a new x-allylnickel complex, and the allylic halide corresponding to the original x-allylnickel complex.Thus, in solution a mixture of two allylic halides and two x-allylnickel halides will be present, the relative concentrations of each being dependent on the rate of ligand exchange of the complexes, and their rate of reaction with the allylic halides present to give 1,5-dienes. In simple cases, this n-Allylnickel Halides as Selective Reagents in Organic Synthesis situation usually results in production of all possible 1,5-diene products in an approximately statistical distribution, and this is illustrated in Scheme 18(b). Thus in the case illustrated all three coupling products are obtained, the desired crosscoupled product and the two unwanted symmetrical products, in the ratio 25 :52 :23; C, :C, :C8.l' The ratio of these products is not significantly altered by changes in solvent, changes in ligand (iodine, chlorine, triphenylphosphine, or acetylacetonate in place of bromine in the complex), or changes in the leaving group of the allylic substrate (allyl chloride, bromide, iodide, p-toluenesulphonate and N,N-dimethylsulphamate all give the same product ratio).The only case in which a significant effect was observed was the reaction between the allyl pyrroli- dine dithiocarbamate (23) and n-2-methallylnickel bromide which gave the crosscoupled product in 65% yield, accompanied by only 5% of the symmetrical coupling products. S (23 1 Despite the above apparent drawback, this method has found application in ter- pene synthesis due to the fact that certain substituted complexes only undergo slow ligand-exchange and thus give good yields of the desired crosscoupled products.n-1,l-Dimethallylnickel bromide reacts with the allylic bromo-ethers and esters shown to give geranyl derivatives r(24), (25), and (26)] in moderate yields,26 with only small quantities of the symmetrical coupling products, Scheme 19. Some double bond isomerization (E+ 2)was observed in these reactions (between 5 and 30% of the 2compounds were isolated, starting from the E bromo-substrates). Incontrast to these results, the reaction of(27) with thenickel reagent was reported to give (28) (the product of a symmetrical coupling) in 70% yield. Presumably in this yp-*Rn&ar + \ R NiBr), (24)R =COZEt-40 'lo (25) R =OEt--5 'I0 but (26) R =OAc- 60 'Io (27) (28) 70'10 Scheme 19 108 Billington case ligand exchange is fast compared to the rate of coupling of the original complex with the substrate allylic halide, Scheme 19.In a parallel study, the substituted n-allylnickel bromide complexes (29) and (30) reacted with prenyl bromide to give geranyl ethers and with geranyl bromide to give farnesyl ethers, Scheme 20. The products obtained 37 were the expected mixtures of E and 2 isomers (E:Z ratios 84: 16: to 93: 7), but in addition minor products were isolated which arose from reaction of the substrate with the more substituted end of the ally1 complex (3 to 7%). No explanation for this unusual behaviour was advanced. The substituted complex (31) has been used to introduce the isoprenyl group into terpenoid skeleton^.'^ Reaction of (31) with prenyl bromide gave mycene and with geranyl bromide p-farnesene was obtained, both compounds being E/Z mixtures.p-Sinensal has been prepared from the allylic chloride (32) by reaction with the nallylnickel bromide complex (33), Scheme 21. L B r + L I OR-\ OR Ni6rIZ (29) R = Ph, 40'lo ; E:Z= 87: 13 (30) R = CH2Ph,45'/o; E:Z =89:11 Scheme 20 EtO+Cl t OEt (32) Scheme 21 The desired crosscoupled material (50% yield) was accompanied by lesser amounts (20 and 26% yield) of the by-products resulting from symmetrical coup- ling reactions. The E:Z ratio of the desired product was 93:7 in N,N-dimethylformamide and this could be improved to 98:2 by the use of tetrahydrofuran as solvent, although the chemical yield dropped to 23% under these conditions.7c-Allylnickel Halides as Selective Reagents in Organic Synthesis n+4 nt6 Scheme 22 Table 1 Ring closures with Ni(CO)., in DMF; cf. Scheme 22 Possible ring-size Observed ring-size Value of n of product of product % Yield 2 41618 6 42 4 6 6/8/10 81 1 o/1 2 6 12 ? 59 8 101 121 14 14 74 12 1411611 8 18 84 B. Reactions with Allylic Halides; Intramolecular Reactions.-Two allylic halide groups in the same molecule may, under suitable conditions (high dilution to discourage intermolecular reactions), undergo intramolecular coupling, leading to a cyclic product. As each ally1 group may react at either of two positions, three possible cyclic products may be formed, with ring sizes of n + 2, n + 4, or n + 6 in the example shown, Scheme 22.In the simple example shown this corresponds to 2"-2", 1"-2",and 1"-1" reaction respectively. The degree to which each product is obtained for a given value of n depends on normal ring-closure effects, combined with the preference exhibited by n-allylnickel halide complexes to react at their 1" centres. The accepted normal rates of ring-closure reactions are 3 <4 < 5 > 6 >7 > 8 <9 < 10< 11< 12 etc. up to 18. The results of the treatment of a series of bis-allylic bromides with Ni(CO), in N,N-dimethylformamide are presented in Table l.38From the results in Table 1 it can be seen that for n = 63, and 12the ring size which is obtained is dictated by the reaction of the two 1" centres, giving the largest ring in each case.For n = 2 or 4, however the preference for 6-membered ring formation overcomes this 1"-1 ",effect and we see the 6-membered products from 1"-2" and 2"-2" closure only. The high yields for the larger ring sizes (12 and above) coupled with the observation that regardless of the geometry of the substrate double bonds, the products are over 95% E,E isomers, makes this approach attractive for large rings. The stability of esters to the nickel reagents has allowed the extension of this 38 E. J. Corey and E. K. W. Wat, J. Am. Chem. SOC.,1967,89, 2757. Billing ton 75 '1. Scbeme 23 general approach to the synthesis of macrocyclic lactones Scheme 23.39 Again reaction only occurs at the 1" centres giving a 13-membered ring, rather than a 9-or 11-membered ring.Somewhat different results are obtained when the newly formed double bonds are exocyclic rather than endo~yclic.~' Scheme 24 shows the results of studies in this area, and it is clear that the method is of no value for 12-membered rings with exocyclic double bonds. Possible Observed Value of n ringsize ringsire yofield (CH,),,76Ni(Co)L 6 10* 8/10/12 10/12/14 14 Zero 37.10 12/14/16 16 81 'lo 14 16/18/20 20 4O"lo 43 'lo Br (34) Scheme 24 An example with one endocyclic and one exocyclic double bond (34) gives a result between the two extremes, reacting only at the 1" centre of the 1"/2" ally1 unit.Related to these results is the interesting trimerization of (35) to give (36), Scheme 25.8 In an elegant study, Corey ef al. showed that (39, (37), and (38) +(35) all give (36) on treatment with Ni(CO),.' This behaviour is very interesting as (38) preferentially undergoes intermolecular reaction with (35) followed by ring closure to the 9-membered ring, rather than intramolecular reaction to give the 6- 39 E. J. Corey and H. A. Kirst, J. Am. Chem. SOC.,1972, 94, 667. *O E. J. Corey and P. Helquist, Tefrahedron Left., 1975, 4091. n-Allylnickel Halides as Selective Reagents in Organic Synthesis (38) (39 1 Scheme 25 membered product. Indeed treatment of (38) with excess Ni(CO), yields only 10% of the intramolecular product and a series of polymers.Altering the nickel species to Ni(CO),P(Ph), raises the yield of the intramolecular cyclization of (38) to 60%. In the trimerization reaction, the same effect is observed as only minor amounts of (39) can be detected. The above results have led to the application of these methods to natural- product synthesis. Humulene (40) has been synthesized via its geometric isomer (41) obtained from Ni(CO), coupling of the bis-allylic bromide shown Scheme 26.41*42Three other products were reported in the coupling reaction, but not identified. Two 14-membered carbocyclic natural products have been synthesized using this methodology, cembrene (42) 43944 and casbene (43),45,46 Scheme 27.In the cembrene synthesis a low yield of the cyclic coupled product was ascribed to 41 E. J. Corey and E. Hamanaka, J. Am. Chem. Soc., 1967, 89, 2758. 42 E. J. Corey and E. Hamanaka, J. Am. Chem. Soc., 1964,86, 1641. 43 W. G. Dauben, G. H. Beasley, M. D. Broadhurst, B. Muller, D. J. Peppard, P. Pesnelle, and C. Suter, J. Am. Chem. SOC.,1974, %, 4724. *44 W. G. Dauben, G. H. Beasley, M. D. Broadhurst, B. Muller, D. J. Peppard, P. Pesnelle, and C. Suter, J. Am. Chem. Soc., 1975, 97,4973. 45 L. Crombie, G. Kneen, and G. Pattenden, J. Chem. SOC.,Chem. Commun., 1976, 66. ''L. Crombie, G. Kneen, G. Pattenden, and D. Whybrow, J. Chem. Soc., Perkin Trans. I, 1980, 1711. Billing ton (411 X9Scheme 26 OAc Ni KO)& x (421 Scheme 27 interference of the allylic acetate function. In neither case was the coupling reaction as efficient as has been reported for simple bis-allylic substrates giving 14- membered rings.38 Thus, in the synthesis of casbene, three isomeric products were obtained from the coupling reaction, in the ratio of 65: 29: 6, and a total yield of only 10%.Natural casbene was identified as the major component of this mixture. The preference for 6-membered ring formation in the reaction of bis-allylic bromides with Ni(CO), has been exploited in a synthesis of elemol (44),47 Scheme 28. Treatment of the bis-allylic bromide (45) with Ni(CO), in N-methylpyrrolidone gave the 6-membered ring products in 55% yield (a mixture of cis and trans divinylcyclohexanes formed by 2"-2" coupling of the allyls) accompanied by 11% of the 10-membered ring product, formed by 1"-1" coupling.C.Reactions with Carbonyl Groups.-The reaction of a n-allylnickel halide with a carbonyl group produces a homoallylic alkoxide, which may either be stable, and lead to isolation of a homoallylic alcohol as product, or may undergo further reactions. The complexes react with 1,2-diketones, ketones, and aldehydes to give 47 E. J. Corey and E. A. Broger, Tetrahedron Lett., 1969, 1779. 113 n-Allylnickel Halides as Selective Reagents in Organic Synthesis 32 'lo 23 'lo Scheme 28 the expected homoallylic alcohols as products, and some examples are given in Schemes 6 and 7.20This reaction has not been widely used in synthesis, although tagetol (46) has been prepared in this way Scheme 29.13With 1,2-diketones, only one addition occurs, even in the presence of a large excess of nickel reagent, and a$-unsaturated ketones give only 1,2-addition products, even in the presence of added CuI (see Scheme 7).20The complexes also display a very interesting reactivity profile towards carbonyl groups. Thus, 1,Zdiketones are the most reactive substrates, whilst cyclic ketones are less reactive, and both simple ketones and a$-HO9 56Ot0&CHO t T.-) NiBrlz NiBrlZ Scheme 29 Billington unsaturated ketones are less reactive still.Aryl ketones are more reactive than their alkyl counterparts, and both esters and acid chlorides fail to react with the reagents. The above reactivity pattern is in direct contrast to the allyl-lithium, zinc, and magnesium reagents, which normally attack both carbonyls in 1,2-disubstituted ketones, are highly reactive towards aliphatic and a$-unsaturated ketones, and may add in a 1,4 manner to the latter substrates under suitable conditions.Also in contrast to the other reagents, the nickel complexes are able to discriminate between similar ketones in a substrate, see Schemes 29 and 34. 2-Pyridyl carboxylates react with x-allylnickel halides, to give P,y unsaturated ketones in good yields.22 The presence of other functional groups in the substrate including esters, ketones, and even 1" alkyl bromides does not seem to affect the course of the reaction, indicating that 2-pyridyl carboxylates are very reactive electrophiles in this reaction, The products are mixtures of P,y and a,P unsaturated ketones in which the P,y-isomers predominate.Some examples of these reactions are given in Scheme 30.22 Scheme 30 n-Allylnickel halides react with quinones to give substituted quinones or hydroquinones depending on the substrate and reaction conditions.' Electron-transfer processes have been implicated in these reactions:* and product analyses may be shown to correlate well with attachment of the ally1 substituent to the site of the highest spin-density in the quinone radical anions. Recently the involvement of quinols has been demonstrated4' and these quinols were shown to rearrange to the previously isolated substituted quinones/hydroquinones under the reaction conditions employed, Scheme 31.By suitable choice of conditions these 48 L. S. Hegedus and E. L. Waterman, J. Am. Chem. SOC.,1974, 96,6789. 49 L. S. Hegedus and B. R. Evans, J. Am. Chem. SOC.,1978, 100, 3461. 115 n-Allylnickel Halides as Selective Reagents in Organic Synthesis /+MeofJ+ Me0 Me0 Me0 NiBr), 0 P MeO@~O~ Me Me0 \ 0 OH 33 'I. 19 'I0 Ni6rIZ 61 'lo-$& 0 0 (481 30'10 Ni6rIZ"*+Me0 0 Me0 0 (471 Scheme 31 observations have led to one-step syntheses of coenzyme Q1 (47) and plastoquinone-1 (48), Scheme 31.'' A large body of experimental data is available in this area and the interested reader is directed to references 2,21,48,49, and 50 for further examples. D.Reactions Involving Substituted Complexes.-A number of substituted n-allylnickel complexes have been prepared and undergo reaction with substrates in the normal manner. Examples of substituted complexes which exhibit normal stability are given in Scheme 8, and examples of the use of substituted olefinic c~rnplexes,~~.~~and complexes containing protected hydroxyl functions 28*37 50 L. S. Hegedus, E. L. Waterman, and J. Catlin, J. Am. Chem. Soc., 1972, 94, 7155. 116 Billing ton appear in Schemes 14 and 21 and Schemes 11 and 20 respectively. In a study of substituted olefinic complexes,13 the non-conjugated and cross- conjugated complexes (49) and (31) were found to be stable and reacted normally, whereas the conjugated complexes (50) and (51) were very unstable and could not be isolated.Complexes (50)and (51) could be generated and used in situ, but only the most reactive substrates (aryl iodides and vinyl or ally1 bromides) reacted successfully due to ready thermal decomposition of the complexes. In agreement with the above results, the non-conjugated olefinic substituted complexes shown in Schemes 14 and 21 reacted n~rmally.~~*~~ A single example of the formation of a n-allylnickel halide within an a$-unsaturated ketone has appeared in the literat~re,~ ' although other examples are known,52 Scheme 32. b + Ni(COD), -Br Ni Br 1, Scbeme 32 The x-(2-methoxy)allylnickelbromide complex has been shown to have potential as a reagent for the introduction of acetonyl groups,53 by ready hydrolysis of the initially formed enol ethers.This reaction has found application in the synthesis of isoc~umarins,~~and some examples are given in Scheme 33. The n-(2-methoxy)allyl complex exhibits normal stability characteristics, and will react with the usual range of substrate^.^^ Despite early reports of its instability,26 the n-(2-carbethoxy)allylnickelbromide I. T. Harrison, E. Kimura, E. Bohme, and J. H. Fried, Tetrahedron Lett., 1969, 1589. 52 R. Baker and D. C. Billington, unpublished results. *53 L. S. Hegedus and R. K. Stiverson, J. Am. Chem. SOC.,1974,%, 3250. II-Allylnickel Halides as Selective Reagents in Organic Synthesis + I' NiBr), X + scheme 33 complex may be obtained from 2-carbethoxyallyl bromide and Ni(C0)4, and seems reasonably stable.20 Reaction of this complex with aldehydes or ketones gives a-methylene-y-butyrolactones, by addition of the initially formed nickel alkoxide to the ester carbonyl group, followed by elimination of EtO-, Scheme 34.20 This complex has been shown to react with a range of aldehydes and ketones in this way, R' R2 Scheme 34 Billing ton and in favourable cases discrimination between two ketones in the substrate is possible, as is the case for simple complexes, and examples are given in Scheme 34.A logical extension of this reaction is intramolecular reaction between the complex and aldehyde, forming a cyclic system, followed by a second ring-closure giving an a-methylene-y-butyrolactonefused to a second ring.This approach was proved valid by Semmelhack who cyclized the bromo-aldehyde (52), using Ni(COD),, in good yield,54 Scheme 35.Both E and 2allylic bromides gave only the cis-fused lactone (53) as product. This method was then extended to a total synthesis of the sesquiterpene confertin (54), by cyclization of the precursor (55).l1 Here the sulphonium salt acts as a leaving group in place of bromide for the formation of the nallylnickel complex. Treatment of (55)with Ni(COD), gave the desired cis-fused lactone, having the correct P-configuration for elaboration to natural confertin, in 28% yield (plus 14% of the trans-isomer). Cyclization of (55) using a Zn/Cu couple (i.e. Zn'), also gave a cis-fused lactone, in 30% yield, but this material had the a-configuration, and thus could not be elaborated to confertin.(54) Scheme 35 In a related approach, 2-bromo-substituted complexes, derived from allylic bromides such as (56), were shown to undergo ring-closure followed by CO insertion in the presence of Ni(C0)4, giving a-methylene-y-lactones.' This reaction has been used to prepare the sesquiterpene lactone frullanolide (57), by treatment of 54 M.F. Semmelhack and E. C. S. Wu,J. Am. Chem. SOC.,1976,98, 3384. 119 n-Allylnickel Halides as Selective Reagents in Organic Synthesis the bromo-aldehyde (58) with Ni(C0),,9 Scheme 36. Treatment of either E-(58),or Z(58) with Ni(CO), gave a mixture of frullanolide (57), and the intermediate alcohol (59). Attempts to induce this reaction to go to completion failed, and the most efficient procedure is a two-step process, involving treatment of the mixture of(57) and (59)obtained in the coupling reaction with Ni(CO), and triethylamine in benzene, which results in transformation of (59) into (57).This protocol gives frullanolide (57) in 40% yield from E-(58) and 31% yield from 2-(58). Ni(CO)bVoMs (58) (57) 0 NUCOIL tr % Scbeme 36

ISSN:0306-0012    

DOI:10.1039/CS9851400093

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

INDEXES Volume 14, 1985 The indexes in this issue cover Volumes 1-14 (Figures in bold type refer to the volume number) Index INDEX OF AUTHORS Aarons, L. J., 5,359 Ackroyd, J., 11,321 Ager, D. J., 11,493Ahluwalia, J. C., 2,203 Allen, N. S. 4, 533 Angyal, S. J., 9,415 Ambroz, H. B., 8,353 Atkinson, D., 8,475 Attygalle, A. B., 13,245 Baker, A. D., 1,355 Bamfield, P., 13,443 Barker, B. E., 9, 143 Bartle, K. D., 10, 113 Bartlett, P. D., 5, 149 Baxendale, J. H., 7,235 Beattie, I. R., 4, 107 Beddell, C. R., 13,279 Bell, R. P., 3, 5 13 Belson, D. J., 11,4 1 Bentley, P. H., 2,29 Berkoff, C. E., 3,273Billington, D. C., 14,93 Bird, C. L., 10,49 Bird, C. W., 3, 309 Blandamer, M. J., 4,55;14,137 Blundell, T.L., 6, 139 Boelens, H., 7, 167 Bradshaw, T. K., 6,43 Braterman, P. S., 2,271 Breslow, R., 1,553Brown, D. H., 9,217 Brown, I. D., 7,359 Brown, K. S.,Jun., 4, 263 Brundle, C. R., 1,355 Buchanan, G. L., 3,41Burdett, J. K., 3,293; 7, 507 Burgess, J., 4,55; 14, 137 Burnett, M. G., 12,267 Burrows, H. D., 3,139 Burtles, S. M., 7,201 Butterworth, K. R., 7, 185 Cadogan, J. I. G., 3,87 Carabine, M. D., 1,411 Cardin, D. J., 2,99Carless, H. A. J., 1,465 Casellato, U., 8, 199 Cetinkaya, B., 2,99Chamberlain, J., 4,569Chatt, J., 1, 121 458 Chesters, J. K., 10,270 Chisholm, M. H., 14,69 Chivers, T., 2,233 Clark, G. M., 5,269 Clark, R. J. H., 13,219 Collins, C. J., 4,251Colvin, E. W., 7, 15 Connor, J.N. L., 5, 125 Corfield, G. C., 1,523 Cornforth, J. W., 2, 1 Cotton, F. A., 4,27; 12, 35 Coulson, E. H., 1,495 Covington, A. K., 14,265 Cowan, J. M., 8,419 Cox, B. G., 9,381 Coyle, J. D., 1,465; 3, 329; 4,523 Cragg, G. M. L., 6,393 Cramer, R. D., 3,273 Crammer, B., 6,43 1 Cross, R. J., 2,271; 9, 185; 14,197 Curthoys, G., 8,475 Dack, M. R. J., 4,211 Dainton, F. S., 4,323Dalton, H., 8,297 Davies, D. I., 8, 171 de Rijke, D., 7, 167 de Silva, A. P., 10, 181 de Valois, P. J., 7,167 Dickinson, E., 14,421 Dickinson, L. C., 12, 387 Dobson, J. C., 5, 79 Dowle, M. D., 8, 171 Doyle, M. J., 2,99Drummond, I., 2,233 Dunkin, I. R., 9, 1 Durant, G. J., 14,375Duxbury, G., 12,453 Dymond, J.H., 14,3 17 Elliott, M., 7, 473 Emsley, J., 9,9 1 Engberts, J. B. F. N., 14, 237 Eschenmoser, A., 5,377 Evans, D. A., 2,75 Evans, J., 10, 159 Fenby, D. V., 3,193Fensham, P. J., 13,199Fenton, D. E., 6, 325; 8, 199 Ferguson, L. N., 4,289 Fisher, L. R., 6,25 Fleming, I., 10, 83 Flygare, W. H., 6, 109 Forage, A. J., 8,309 Fox, M. F., 9, 143 Frazer, M. J., 11, 171 Fry, A., 1, 163 Funk, R. L., 9,41 Garson, M. J., 8,539 Georghiou, P. E., 6,83 Gheorghiu, M. D., 10, 289 Gibson, K. H., 6,489 Gilbert, J., 10,255 Gilchrist, T. L., 12, 53 Gillespie, R. J., 8,3 15 Goodings, E. P., 5,95Goodrich, J. A., 14,225 Gordon, P. F., 13,443 Gorman, A. A., 10,205 Gray, B. F., 5,359Green, C. L., 2,75 Greenhill, J.V., 6, 277 Greenwood, N. N., 3,231; 13,353 Grey Morgan, C., 8,367 Grice, R., 11, 1 Griffiths, J., 1,481 Grimshaw, J., 10,181 Grossert, J. S., 1, 1 Groves, J. K., 1,73Guilford, H., 2,249 Gutteridge, N. J. A., 1, 38 1 Haines, R. J., 4, 155 Hall, G. G., 2,21 Hall, L. D., 4,401 Hall, T. W., 5,43 1 Halliwell, H. F., 3, 373 Hamdan, I. Y., 8, 143 Hamer, G., 8,143 Harmony, M. D., 1,211 Harris, K. R., 5,215 Harris, R. K., 5, 1 Harrison, L. G., 10,491 Hartley, F. R., 2, 163 Hartshorn, S. R., 3, 167 Hathway, D. E., 9,63, 241 Hayward, R. C., 12,285Heelis, P. F., 11, 15 Henderson, J. W., 2,397 Hepler, L. G.,3, 193 Hilburn, M. E., 8,63 Hinchliffe, A., 5,79 Hoffman, D. M., 14,69 Holbrook, K.A., 12,163 Holland, H. L., 10,435; 11,37Holm, R. H., 10,455 Hore, P. J., 8,29 Horton, E. W., 4,589 Hough, L., 14,357 Hudson, M. F., 4,363 Huffman, J. C., 14,69 Huntress, W. T., jun., 6, 295 Hutchinson, D. W., 6,43 Ibers, J. A., 11,57 Ikan, R., 6,431 Isaacs, N. S., 5, 181 Isbell, H. S., 3, 1 Jaffe, H. H., 5,165 James, A. M., 8,389 Jamieson, A. M., 2,325 Janes, N. F., 7,473 Jencks, W. P., 10,345 Jenkins, J. A., 6, 139 Johnson, A. W., 4, 1; 9, 125 Johnson, S. P., 5,441 Johnstone, A. H., 7,317; 9,365 Jones, J. R., 10,329 Jones, P. G., 13, 157 Josh, C. G., 8,29Jotham, R. W., 2,457 Kalyanasundaram, K., 7, 453 Katritzky, A. R., 13,47 Keenan, A. G., 8,259 Kemball, C., 13,375 Kemp, T. J., 3, 139;8,353 Kennedy, J.F., 2,355; 8, 22 1 Kennewell, P. D., 4, 189; 9,477 Kenny, A. W., 4,90 King, G. A. M., 7,297 Kirby, G. W., 6, 1 Kitaigorodsky, A. I., 7, 133 Koch, K. R., 6,393 Kolar, G. F., 9,241 Korpela, T., 12,309Kresge, A. J., 2,475 Krishnaji, 7, 219 Kroto, H., 11,435 Kruger, H., 11,227 Kuhn, A. T., 10,49 Lappert, M. F., 2,99Lee, M. L., 10,113 Lee-Ruff, E., 6, 195 Leigh, G. J., 1, 121; 4, 155 Lemieux, R. U., 7,423 Leznoff, C. C., 3,65 Lindberg, B., 10,409Lindsay, D. G., 10,233 Lindoy, L. F., 4,421 Linford, R. G., 1,445 Lipscomb, W. N., 1,319 Liu, M. T. H., 11,127 Lynch, J. M., 3,309 Lythgoe, B., 9,449Makela, M. J., 12,309 McCleverty, J. A., 12, 33 1 McKean, D. C., 7,399 McKellar, J.F., 4,533 McKervey, M. A., 3,479 Mackie, R. K., 3,87 McLauchlan, K. A., 8,29McNab, H., 7,345 Maitland, G. C., 2, 181 Maitlis, P. M., 10, 1 Manning, P. G., 5,233 blaret, A. R., 2,325 Markov, P., 13,69 Maslowsky, E., 9,25 Mason, R., 1,431 Mayo, B. C., 2,49 Meadowcroft, A. E., 4, 99 Menger, H. W., 2,415 Midgley, D., 4,549 Millen, D. J., 5,253 Mills, R., 5, 215 Mitchell, J. C., 14, 399 Moore, D. S., 12,415 Moore, H. W., 2,415; 10, 289 Morgan, E. D., 13,245 Morley, R., 5,269 Morris, D. G., 11,397Morris, J. H., 6, 173 Muetterties, E. L., 11, 283 Mulheirn, L. F., 1,259 Munn, A., 4,87Murphy, W. S., 12,213Musumarra, G., 13,47 Newman, J. F.,4, 77 Index Nightingale, W. H., 7, 195 Norman, R. 0.C., 8,l North, A.M., 1,49 Oakenfull, D. G., 6,25 Overton, K. H., 8,447 Page, M. I., 2,295Paleos, C. M., 14,45 Paulsen, H., 13, 15 Pelter, A., 11, 191 Perkins, P. G., 6,173 Pickford, C. J., 10,245 Pletcher, D., 4,471 Poliakoff, M., 3,293; 7, 527 Prakash, V., 7,219 Pratt, A. C., 6,63 Pratt, J. M., 14, 161 Puddephatt, R. J., 12,99 Ramm, P. J., 1,259 Rao, C. N. R., 5,297; 12, 361 Ratledge, C., 8,283 Rattee, I. D., 1, 145 Redl, G., 3,273 Redpath, J., 12,75 Richards, D. H., 6,235Ritch, J. B., jun., 5,452 Roberts, M. W., 6,373 Robinson, F. A., 5,317 Robinson, S. D., 12,415 Roche, M., 5,165 Rodgers, M. A. J., 7,235 Rose, A. E. A., 6,173 Rouvray, D. H., 3,355 Rowlinson, J. S., 7,329;12,251 Sanders, J. K.M., 6,467 Sarma, T. S., 2,203 Satchell, D. P. N., 4,23 1; 6,345 Satchell, R. S., 4,231Scheinmann, F., 11,321 Schlegel,W., 7, 177 Scriven, E. F. V., 12,129 Self, R., 10,255 Senthilnathan, V. P., 5, 297 Sherman, L. R., 14,225 Sherwood, P. M. A., 14,l Shorter, J., 7, 1 Simonetta, M., 13, 1 Simpson, T. J., 4,497 Singh, S., 5,297 Slorach, S. A., 10,280Smith, E. B., 2, 181 Index Smith, I. W. M., 14,141Smith, K.;3,443 Smith, K. M., 4,363Smith, W. E., 6, 173; 9, 217 Snell, K. D., 8,259 Somorjai, G. A., 13, 32 1 Stacey, M., 2,145 Staunton, J., 8,539 Staveley, L. A. K., 13, 173 Stevens, M. F. G., 7,377 Stoddart, J. Fraser, 8,85 Stokes, R. H., 11,257 Strachan, A. N., 11,41 Suckling, C. J., 3, 387; 13,97Suckling, K.E., 3,387 Sutherland, J. K., 9,265 Sutherland, R. G., 1,241 Sutton, D., 4,443Swan, J. S., 7,201 Swindells, R., 7,212 Sykes, A. G., 14,283 Symons, M. C. R., 5, 337; 12, 1,387; 13,393 Takken, H. J., 7,167 Taylor, J. B., 4, 189; 9,477 Taylor, S. E., 10,329 Theobald, D. W., 5, 203 Thomas, T. W., 1,99 Thompson, M., 1,355 Thornber, C. W., 8,563 Tincknell, R. C., 5,463 Toennies, J. P., 3,407 Tolman, C. A., 1,337Trost, B. M., 11,141 Traux, D. R., 5,411 Twitchett, H. J., 3, 209 Tyman, J. H. P., 8,499 Underhill, A. E., 1,99; 9,429 van Dort, J. M., 7, 167 van der Linde, L. M., 7,167 Varvoglis, A., 10,377 Vaughan, K., 7,377 Vidali, M., 8, 199 Vigato, P. A., 8, 199 Vollhardt, K.P. C., 9,41 Wain, R. L., 6,261 Walker, E. R. H., 5,23Walker, I. C., 3,467 Waltz, W. L., 1,241 Ward, I. M., 3,231 Ward, R. S., 11,75Watkins, D. M., 9,429 Wattanasin, S., 12,213 White, A. J., 3, 17 Whitfield, R. C., 1,27 Widom, B., 14, 121 Wieser, H., 5,411 Wiesner, K., 6,413 Williams, D. H., 13, 131 Williams, D. L. H., 14, 171 Williams, G., 7, 89 Williams, R. J. P., 9, 281,325Wilson, A. D., 7,265 Wise, S. A., 10, 113 Yoffe, A. D., 5,51 Zeelen, F. J., 12,75 Index INDEX OF TITLES Abiotic receptors, 12, 285 Absorption bands in the spectra of stars, a crystal field approach,5, 233 Acidity of solid surfaces, 8,475 Across the living barrier, 6, 325 Activation parameters for chemical reactions in solution, 14, 237 Acylation and alkylation catalysts, 4-dialk ylaminopyridines, super,12, 129 -by ketens and isocyanates, a mechanistic comparison, 4, 23 1 Acylation, Friedel-Crafts, of alkenes 1, 73 Adamantane rearrangements, 3, 379 Affinity chromatography, chemical aspects of, 3, 249 Alcohols and amines, conformational analysis of, 5, 411 Aliphatic nucleophilic substitution reactions, new insights into, from the use of pyridines as leaving groups, 13,47Alkali-metal complexs in aqueoussolution, 4, 549 Alkaloids, aconite, synthesis of,6,413 Alkenes, the Friedel-Crafts acylationof, 1, 73 n-Allylnickel halides as selective re-agents in organic synthesis, 14, 93 Aluminium phosphates, the chemistry and binding properties of, 6, 173 Amines and alcohols, conformational analysis of, 5,411 Analysis of trace constituents of the diet, organic and inorganic,10, 245,255 Analytical methods, modern, for en-vironmental polycyclic aromatic compounds, 10, 113 Anionic cyclization of phenols,12, 213 Ants, chemicals from the glands of, 13, 245 Aphids and scale insects, their chem- istry, 4, 263 Application of electrochemical tech-niques to the study of homogeneous chemical reactions, 4,471 Applications of e.s.r.spectroscopy to kinetics and mechanism in organic chemistry, 8, 1 Application of research findings to the development of commercial flavourings, 7, 177 Aqueous carbonate solutions, potentio- metric titrations of, 14, 265 Aqueous mixtures, kinetics of reac-tions in, 4, 55 Aqueous solution, micelles in, 6, 25 Aryl cations-new light on old inter- mediates, 8, 353 -halides, photochemistry and photocyclization of, 10, 181 Aryldiazonium cations, co-ordination chemistry of, 4,443Aryliodine(rrr) dicarboxylates, 10, 377 Atmosphere, interactions in, of drop- lets and gases, 1,411 Autocatalysis, 7, 297 Azidoquinones and related com-pounds, chemistry of, 2,415 Azobenzene and its derivatives, photochemistry of, 1,481 B ,-Dependent isomerase enzymes:how the protein controls the active site, 14, 161 Benzene compounds, substituted,synthesis from acyclic com-pounds, 13,441 Bile pigments, 4, 363 Binding of heavy metals to proteins, 6, 139 Binding properties and chemistryof aluminium phosphates, 6, 173 Bio-active molecules, structural studies on, 13, 131 Biological surfaces, molecular aspects of, 8, 389 Biomimetic chemistry, 1,553Biosynthesis of sterols, 1, 259 Biosynthetic products from arachidonic acid, 6,489 __ studies, carbon- 13 nuclear mag- netic resonance in, 4,497Bis(diphenylphosphino)methane,chemistry of, 12,99Blood groups, human, and carbo-hydrate chemistry, 7, 423 Bond strengths, CH, in simple or-ganic compounds: effects of con-formation and substitution, 7, 399 ___ valences-a simple structural model for inorganic chemistry,7, 359 46 1 Index Boron reagents, carbon-carbon bond formation involving, 11, 191 Bredt’s rule, 3, 41 Br~rnsted relation-recent develop-ments, 2,475 Brownian dynamics with hydro-dynamic interactions: the applica-tion to protein diffusional problems, 14, 421 Butadiene, polymerization and copolymerization of, 6, 235 Calciferols, hormonal: chemistry of ‘Vitamin’ D, 6, 83 Calorimetric investigations of hydro- gen bond and charge transfer com- plexes, 3, 193 Cancer and chemicals, 4,289 Carbohydrate chemistry and human blood groups, 7, 423 Carbohydrate-directed macromole-cules, transition-metal oxide chel-ates of, 8, 221 Carbohydrate-protein complexes, gly- coproteins, and proteoglycans, of human tissues, chemical aspects of, 2,355 Carbohydrates to enzyme analogues, 8, 85 Carbon-carbon bond formation in-volving boron reagents, 11, 191 Carbon-13 nuclear magnetic resonance in biosynthetic studies, 4,497 Carbonium ions, carbanions, and radi- cals, chirality in, 2,397 Carbonyl clusters, metal, relationship with supported metal catalysts,10, 159 compounds, photochemistry of, 1,465 -equivalents, silicon-containing,11,493 -group transpositions, 11, 397 Carcinogens, chemical, mechanisms of reaction with nucleic acid, 9,24 1 Catalysis and surface chemistry, new perspectives, 6, 373 Catalysis, homogeneous, and organo- metallic chemistry, the 16 and 18 electron rule in, 1,337 -of the olefin metathesis reaction, 4, 155 Catalysts, supported metal, rela-tionship with metal carbonyl clus- ters, 10, 159 CENTENARY LECTURE.Biomimetic chemistry, 1,553CENTENARY LECTURE.Cyclo-pentanoids: a challenge for new methodology 11, 141 CENTENARYLECTURE. Hydrocarbon reactions at metal centres, 11, 283 CENTENARYLECTURE. Light scatter-ing in pure liquids and solu-tions, 6, 109 CENTENARYLECTURE.Metal Clusters in biology, 10,455CENTENARYLECTURE. Molecular In-gredients of heterogeneous cataly- sis, 13, 321 CENTENARYLECTURE. Organic re-action paths: a theoretical ap-proach, 13, 1 CENTENARYLECTURE. Phase equili-brium and interfacial structure, 14, 121 CENTENARY LECTURE. Quadruple bonds and other multiple metal to metal bonds, 4, 27 CENTENARYLECTURE. Reactivities of carbon disulphide, carbon dioxide, and carbonyl sulphide towards some transition-metal systems, 11, 57 CENTENARY LECTURE.Rotationally and vibrationally inelastic scattering of molecules, 3,407 CENTENARY LECTURE. Systematic development of strategy in the synthesis of polycyclic poly-substituted natural products: the aconite alkaloids, 6,413CENTENARYLECTURE. Three-dimen- sional structures and chemical mechanisms of enzymes, 1, 3 19 Charge transfer and hydrogen bond complexes, calorimetric investiga-tions of, 3, 193 Chemical applications of advances in Fourier transform spectroscopy,4, 569 aspects of affinity chrom-at ography 2, 249 --of glycoproteins, proteogly- cans, and carbohydrate-proteincomplexes of human tissues, 2, 355 Chemical education, conceptions,misconceptions, and alternative frameworks in, 13, 199 ____ research: facts, findings, and consequences, 9, 365 interpretations of molecular Index wavefunctions, 5, 79 models of enzymic transimin-ation, 12, 309 processes on heterogeneouscatalysis, 13, 375 Chemically-induced dynamic electron polarization (CIDEP), role in chemistry, 8, 29 Chemicals from the glands of ants,13, 245 in rodent control, 1, 381 which control plant growth,6, 261 Chemistry and binding properties of aluminium phosphates, 6, 173 CHEMISTRY AND FLAVOUR I Molecular Structure and Organ- oleptic Quality, 7, 167 I1 Application of Research Findings to the Development of Commercial Flavourings, 7, 177 I11 Safety Evaluation of Natural and Synthetic Flavourings, 7, 185 IV The Influence of Legislation on Research in Flavour Chemistry,7, 195 V The Development of Flavour in Potable Spirits, 7,201 VI The Influence of Flavour Chemistry on Consumer Accept- ance, 7,212 and the new industrial revolu-tion, 5, 317 ,a topological subject, 2, 457 -of aphids and scale insects,4, 263 of azidoquinones and related compounds, 2,415 -, of dental cements, 7, 265 of dyeing, 1, 145 of the gold drugs used in the treatment of rheumatoid arthritis, 9, 217 __ of homonuclear sulphur species, 2, 233 -of long-chain phenols of non-isoprenoid origin, 8,499of the production of organicisocyanates, 3, 209 ___ of peroxonium ions and di-oxygen ylides, 14, 399 of transition-metal carbene complexes and their role as reaction intermediates, 2, 99 of ‘Vitamin’ D: the hormonal calciferols, 6, 83 , some considerations on the philosophy of, 5, 203 Chirality in carbonium ions, car-banions, and radicals, 2, 397 Chlorophyll chemistry, n.m.r.spectral change as a probe, 6,467 Chromatography, affinity, chemical aspects of, 2, 249 cis-and trans-Effects of ligands,2, 163 Clathrates and molecular inclusion phenomena, 7, 65 Collisional transfer of rotational energy and spectral lineshapes,7, 219 Compartmental ligands: routes to homo-and hetero-dinuclear com-plexes, 8, 199 Complex formation between sugarsand metal cations, 9,415 -hydride reducing agents, the functional group selectivity of, 5, 23 Complexes, alkali-metal, in aqueoussolution, 4, 549 -homo-and hetero-dinuclear, routes via compartmental ligands, 8, 199 ,1-D metallic, 9,429 Complexes, square-planar, isomer-ization mechanisms of, 9, 185 Computer resolution of overlappingelectronic absorption bands, 9, 143 Conductivity and superconductivity in polymers, 5, 95 Conformation and substitution, effects of, on individual CH bond strengths in simple organic com-pounds, 7, 399 __ of rings and neighbouring group effects, development of Haworth’s concepts of, 3, 1 Conformational analysis of some alcohols and amines: a comparison of molecular orbital theory, rota-tional and vibrational spectros-COPY, 5,411studies on small molecules, 1, 293 Contribution of ion-pairing to ‘memory effects’, 4, 251 Contributions of pulse radiolysis to chemistry, 7, 235 Conversion of ammonium cyanateinto urea-a saga in reaction mech- anisms, 7, 1 Index Co-ordination chemistry of aryl-diazonium cations: aryldiazenato(arylazo) complexes of transition metals, and the aryldiazenato-nitrosvl analom.4. 443 Corrin synthesis, ‘post-Blz problems in, 5,377 Crystal field approach to absorptionbands in the spectra of stars, 5, 233 Crystal structure determination: a critical view, 13, 157 Crystals and molecules, organic, non- bonded interactions of atoms in, 7, 133 Current aspects of unimolecular reactions, 12, 163 Cyanocobalt(rrr) complexes, the syn-thesis of mononuclear, 12, 267 Cyanoketenes: synthesis and cyclo-additions, 10, 289 Cyclization, initiation of, using 3-methylcyclohex-2-enone derivatives, 9, 265 of phenols, anionic, 12, 213 Cyclopentanoids: a challenge for new methodology, 11, 141 Cyclopol ymerization, 1, 523 Dental cements, chemistry of, 7, 265 Designing drugs to fit a macro-molecular receptor, 13, 279 Development of flavour in potablespirits, 7, 201 4-Dialkylaminopyridines: super acyl-ation and alkylation catalysts, 12, 129 Diazirines, the thermolysis and pho- tolysis of, 11, 127 P-Dicarbonyl compounds, light-induced tautomerism of, 13, 69 Dielectric relaxation in polymersolutions, 1, 49 Diffusion in liquids, the effect of iso- topic substitution on, 5, 215 Diffusional problems, Brownian dynamics with hydrodynamic inter- actions: the application to protein, 14,421 Difluoroamino-radical, gas-phasekinetics of, 3, 17 Droplets and gases, interactions in the atmosphere of, 1, 411 Drug design, isosterism and molecular modification in, 8, 563 --,quantitative 3, 273 Dyeing, chemistry of, 1, 145 Echinoderms, 1, 1 Education, chemical, a reassessment of research in, 1, 27 ~-9 , review of research and development in the U.K., 1972-1976, 7, 317 Effect of isotopic substitution on diffusion in liquids, 5, 215 Electrochemical techniques, applica-tion of to study of homogeneouschemical reactions, 4,471 Electrode and related structures, photoelectron spectroscopic struc-tures of, 14, 1 Electron as a chemical entity, 4, 323 scattering spectroscopy, thresh- old, 3, 467 -spectroscopy, 1, 355 Electronic absorption bands, over-lapping, computer resolution of, 9, 143 Electronic properties of some chain and layer compounds, 5, 51 -transitions, vibrational intensities in, 5, 165 Electrons, solvated, in solutions of metals, 5,337 Electron spin resonance of haemo-globin and myoglobin, 12, 387 Electrophilic aromatic substitutions, non-conventional, and related reac- tions, 3, 167 C-nitroso-compounds, 6, 1 Electrophoresis, historical develop-ment of sodium dodecyl sulphate- polyacrylamide gel, 14, 225 Elimination reactions, isotope effect studies of, 1, 163 Enaminones, 6, 277 Energetics of neighbouring groupparticipation, 2, 295 Enumeration methods for isomers, 3, 355 Environmental lead in perspective,8, 63 -polycyclic aromatic compounds, modern analytical methods for, 10, 113 -protection in the distribution of hazardous chemicals, 4, 99 ___ regulation: an international view, 5, 431 Enzyme analogues from carbo-hydrates, 8, 85 Enzymes, immobilized, 6, 215 in organic synthesis, 3, 387 -, the logic of working with, 2, 1 -, three-dimensional structures and chemical mechanisms of,1, 319 Enzyme-catalysed reactions, reactive intermediates in, 13,97 Enzymic reactions, stereochemical choice in 8,447 E.s.r.spectroscopy, applications to kinetics and mechanism in organic chemistry, 8, 1 Experimental Studies on the structure of aqueous solutions of hydro-phobic solutes, 2, 203 FARADAYLECTURE.The electron as a chemical entity, 4, 323 FARADAYLECTURE.The molecular theory of small systems, 12, 251 Fast reactions, techniques for the kinetic study of, 11, 227 Fats grown from wastes, 8, 283 Fe(C0)4, 7, 527 5-Substituted pyrimidine nucleosides and nucleotides, 6, 43 Fixation, of nitrogen, 1,121 Flavins (isoalloxazines), the photo-physical and photochemicalproperties of, 11, 15 Forces between simple molecules, 2, 181 Foreign compounds in mammals, importance of non-enzymic chem-ical reaction processes to the rate of, 9, 63 Formation of hydrocarbons by micro- organisms, 3, 309 Fourier transform spectroscopy,chemical applications of advances in, 4, 569 Four-membered rings and reaction mechanisms, 5, 149 Friedel-Crafts acylation of alkenes, 1, 73 Functional group selectivity of complex hydride reducing agents,5, 23 Gas-phase kinetics of the difluoro-amino-radical, 3, 17 Gases, and droplets, interactions in the atmosphere of, 1,411 Index Glass transition: salient facts and theoretical models, 12, 361 Glycoproteins, proteoglycans, and car- bohydrate-protein complexes of human tissues, chemical aspectsof, 2, 355 Glycoproteins, synthesis of complexoligosaccharide chains of, 13, 15 Gold drugs used in the treatment of rheumatoid arthritis, chemistry of, 9, 217 Growth of computational quantumchemistry from 1950 to 1971, 2, 21 Guanidine derivatives acting at his-taminergic receptors 14, 375 Haemoglobin and myoglobin, electron spin resonance of, 12, 387 Handling toxic chemicals-environ-mental considerations, 4, 77 Hard-sphere theories of transportproperties, 14, 317 HAWORTH MEMORIAL LECTURE.The consequences of some projects in- itiated by Sir Norman Haworth,2, 145 HAWORTH MEMORIAL LECTURE. The Haworth-Hudson controversy and the development of Haworth’s con- cepts of ring conformation and of neighbouring group effects, 3, 1 HAWORTH MEMORIAL LECTURE. The sweeter side of chemistry, 14, 357 HAWORTH MEMORIAL LECTURE. Human blood groups and carbo-hydrate chemistry, 7, 501 HAWORTH LECTURE.MEMORIAL Struc-tural studies of polysaccharides,10,409 HAWORTH MEMORIAL LECTURE.Synthesis of complex oligosacchar- ide chains of glycoproteins,13, 15 Hazards in the chemical industry-risk management and insurance, 8, 419 Health hazards to workers from in- dustrial chemicals, 4, 82 Heterogeneous catalysis, chemical Drocesses on. 13. 375 H;gh resolution laser spectroscopy,12.453 Histaminergic receptors, guanidinederivatives acting at, 14, 375 Historical development of sodium 465 Index dodecyl sulphate-polyacrylamidegel electrophoresis, 14, 225 Homogeneous catalysis, and organo- metallic chemistry, the 16 and 18 electron rule in, 1,337 Homogeneous chemical reactions, application of electrochemical techniques to the study of, 4,471 Human blood groups and carbo-hydrate chemistry, 7,423 Hydrocarbon formation by micro-organisms, 3, 309 -reactions at metal centres, 11, 283 Hydrogen bond and charge transfer complexes, calorimetric investiga-tions of, 3, 193 -bonded liquids, thermodynamics of, 11, 257 -bonding, very strong, 9, 91 -isotope effects, kinetic, recent advances in the study of, 3, 513 Hydrophobic solutes, experimentalstudies on the structure of aqueous solutions of, 2, 203 Imines, photochemistry of, 6, 63 Immobilized enzymes, 6, 215 Importance of (non-enzymic) chemical reaction processes to the fate of foreign compounds in mammals, 9, 63 Importance of solvent internal pres-sure and cohesion to solution phenomena, 4,211 Inclusion phenomena, molecular, and clathrates, 7, 65 Individual CH bond strengths in simple organic compounds: effects of conformation and substitution, 7,399 Industry, chemical, hazards in: risk management and insurance, 8, 419 Influence of flavour chemistry on consumer acceptance, 7,212 Influence of legislation on research in flavour chemistry, 7, 195 Infrared and Raman vibrational spec- troscopy in inorganic chemistry,4, 107 INGOLD LECTURE.Four-membered rings and reaction mechanisms, 5, 149 INGOLDLECTURE.How does a reaction choose its mechanism? 10,345 Initiation of cyclization using 3-methyl- cyclohex-2-enone derivatives, 9, 265 Inorganic chemistry, bond valences, a simple structural model for, 7, 359 Inorganic pyro-compounds Mo[(xZO’l)b], 5, 269 Insect attractants of natural origin, 2, 75 Insecticides, a new group of: syntheticpyret hroids, 7,473 Interactions in the atmosphere of droplets and gases, 1,411 -, ion-solvent, thermodynamics 0f, 9, 381 -, metal-metal, in transition-metal complexes containing infinite chains of metal atoms, 1, 99 -, non-bonded, of atoms in or-ganic crystals and molecules, 7, 133 Introducing a new agricultural chem- ical, 4, 77 Ion-molecule reactions in the evolution of simple organic molecules in interstellar clouds and planetary atmospheres, 6, 295 Ion-pairing, contribution to ‘memory effects’, 4, 251 Ion-solvent interactions, thermo-dynamics of, 9, 381 Isocyanates and ketens, a mechanistic comparison of acylation by, 4, 231 -, organic, chemistry of the produc- tion of, 3, 209 Isocyanic acid, preparation and prop- erties of, 11, 41 Isomer enumeration methods, 3, 355 Isomerization mechanisms of square- planar complexes, 9, 185 Isosterism and molecular modification in drug design, 8, 563 Isotope effect studies of elimination reactions, 1, 163 Isotopic hydrogen exchange in purines: mechanisms and applications,10, 329 substitution effects on diffusion in liquids, 5, 215 JOHN JEYES LECTURE.Chemicals which control plant growth, 6, 261 KELVIN LECTURE. Across the livingbarrier, 6, 325 Ketens and isocyanates, a mechanistic comparison of acylation by, 4, 231 Index Kinetics and mechanism in organicchemistry, applications of e.s.r.spec- troscopy to, 8, 1 , gas-phase, of the difluoroamino- radical, 3, 17 -of reactions in aqueous mixtures, 4, 55 P-Lactams, synthetic routes to, 5, 181 Lanthanide shift reagents in nuclear magnetic resonance spectroscopy,2, 49 Laser light scattering, quasielastic,2, 325 Laser spectroscopy of ultra-trace quan- tities, 8, 367 Lasers, tunable, 3, 293 Lead, environmental, in perspective,8, 63 LENNARD-JONESLECTURE. Recent ex-perimental and theoretical work on molecularly simple liquid mix-tures, 13, 173 Leukotrienes; a new class of biologicallyactive compounds including SRS-A, the synthesis of, 11, 321 Ligand substitution reactions of square-planar molecules, 14, 197 Ligands, cis-and trans-effects of, 2, 163 -, compartmental: routes to homo- and hetero-dinuclear complexes,8, 199 Light-induced tautomerism of P-dicar-bony1 compounds, 13, 69 Lignans and neolignans, the synthesis of, 11,75 Liquid mixtures, recent experimentaland theoretical work on molecularly simple, 13, 173 Liquid, surface of, 7, 329 LIVERSIDGE On first looking LECTURE.into nature’s chemistry: I The r61e of small molecules and ions: the transport of elements, 9, 281 I1 The r81e of large molecules, especially proteins, 9, 325 LIVERSIDGE Recent advances LECTURE. in the study of kinetic hydrogen isotope effects, 3, 513 LIVERSIDGE The surface of a LECTURE. liquid, 7, 329 Macrocyclic ligands, synthetic, transi- tion-metal complexes of, 4, 421 Macromolecular receptor, designingdrugs to fit a, 13, 279 Main-group elements, ring, cage, and cluster compounds of, 8, 315 Matrix isolation technique and its application to organic chemistry, 9, 1 Mechanisms, chemical, and three-dimensional structures of enzymes,1, 319 , isomerization, of square-planar complexes, 9, 185 of the microbial hydroxylation of steroids, 11, 371 of reaction between ultimate chemical carcinogens and nucleic acid, 9, 241 MELDOLA MEDAL LECTURE.Chem- ical aspects of glycoproteins, pro- teoglycans, and carbohydrate-pro-tein complexes of human tissues, 2, 355 MELDOLAMEDAL LECTURE. Fe(C0)4,7, 527 MELDOLA MolecularMEDAL LECTURE. collisions and the semiclassical ap- proximation, 5, 125 MELDOLA MolecularMEDAL LECTURE.shapes, 7, 507 MELDOLA MEDAL LECTURE. N.m.r. spectral change as a probe of chlorophyll chemistry, 6,467 MELDOLAMEDAL LECTURE. The rela- tionship between metal carbonylclusters and supported metal cata- lysts, 10, 159 Meldrum’s acid, 7, 345 Metal carbonyl clusters, relationship with supported metal catalysts,10, 159 centres, hydrocarbon reactions at, 11,283 clusters in biology, 10,455 Metal-metal bonding and metallobor- anes, 3, 231 bonds of various orders, synergic interplay of experiment and theory in studying, 12, 35 Metal-ion-promoted reactions of organo-sulphur compounds, 6, 345 1-D Metallic complexes, 9,429 Metalloboranes and metal-metal bonding, 3, 231 bonds, multiple (especially quad- ruple), 4, 27 interactions in transition-metal Index complexes containing infinite chains of metal atoms, 1, 99 Metals, binding to proteins, 6, 139 Methyl group removal in steroid bios ynt hesis, 10,435 Micelle-forming surfactant solutions, photophysics of molecules in,7,453 Micelles in aqueous solution, 6, 25 Microbes, use in the petrochemicalindustry, 8, 297 Micro-organisms, protein production by, 8, 143 Mixed-valence complexes, the chem-istry and spectroscopy of, 13, 219 Molecular aspects of biological sur-faces, 8,389 -beam reactive scattering, 11, 1 collisions and the semiclassical approximation, 5, 125 ___ orbital theory, comparison with rotational and vibrational spec-troscopy in conformational ana-lysis of alcohols and amines 5,411 -shapes, 7, 507 -tectonics, the construction of polyhedral clusters, 13, 353 structure and organoleptic qual- ity, 7, 167 -theory of small systems, 12, 251 -wavefunctions, chemical inter-pretations of, 5, 79 Molybdenum and tungsten; alkoxy,amido, hydrazido, and related com- pounds of, 12, 331 Monoalkyltriazenes, 7, 377 Morphogenisis, biological, the physical chemistry of, 10,491 Motion, molecular, and time-cor-relation functions, 7, 89 Multistability in open chemical reaction systems, 5, 359 Myoglobin and haemoglobin, electron spin resonance of, 12, 387 Natural products from echinoderms, 1, 1 --,polycyclic polysubstituted, systematic development of strategyin, 6,413 Neighbouring-group effects and ringconformation, development of Haworth’s concepts of, 3, 1 participation, energetics of,2, 295 New insights into aliphatic nucleo-philic substitution reactions from the use of pyridines as leaving groups, 13,47 New perspectives in surface chemistry and catalysis, 6,373 Nitrogen fixation, 1,121 S-Nitrosation and the reactions of S-nitroso compounds, 14, 171 Nitroso-alkenes and nitroso-alky-nes, 12, 53 C-Nitroso-compounds, electrophilic, 6, 1 N.m.r.and vibrational spectroscopicstudies, structure in solvents and solutions, 12, 1 Non-bonded interactions of atoms in organic crystals and molecules, 7, 133 Non-conventional electrophilic arom-atic substitutions and related reac-tions, 3, 167 Nuclear magnetic resonance and the periodic table, 5, 1 ---,carbon-13, in bio-synthetic studies, 4,497 ---methods (new) for tracing the future of hydrogen in biosynt hesis, 8, 539 spectral change as a probe of chlorophyll chemistry,6, 467 ---spectroscopy, Ian-thanide shift reagents in, 2, 49 ----: spin-latticerelaxation, 4,401 Nucleic acid, mechanisms of reaction with ultimate chemical carcinogens, 9,241 Nucleosides and nucleotides, pyrim- idine, 5-su bsti tuted, 6, 43 Nutritional chemistry of inorganic trace constituents of the diet,10, 270 NYHOLM ChemicalMEMORIAL LECTURE.education research: facts, findings, and consequences, 9,365 NYHOLM COnCep-MEMORIAL LECTURE.tions, misconceptions, and alternative frameworks in chemical educa-tion, 13, 199 NYHOLM ForwardMEMORIAL LECTURE. from Nyholm’s March On Lecture, 3, 373 NYHOLM LECTURE.MEMORIAL Growth, change, challenge, 5, 253 -? NYHOLM MEMORIAL LECTURE.Ring,cage, and cluster compounds of the main group elements, 8, 315 NYHOLMMEMORIALLECTURE.Solvingchemical problems, 11, 171 NYHOLM LECTURE.MEMORIAL Synergicinterplay of experiment and theory in studying metal-metal bonds of vari-ous orders, 12,35 Olefin metathesis and its catalysis,4, 155 Olefinic compounds, photochemistry of, 3, 329 On first looking into nature’s chemistry: I The r81e of small molecules and ions: the transport of the ele-ments, 9, 281 I1 The r81e of large molecules, especially proteins, 9, 325 Organic chemistry of superoxide,6, 195 Organic reaction paths: a theoretical approach, 13, 1 Organoboranes as reagents for organic synthesis, preparation of, 3, 443 Organoborates in organic synthesis: the use of alkenyl-, alkynyl-, and cyano- borates as synthetic intermediates, 6, 393 Organometallic chemistry and hom-ogeneous catalysis, the 16 and 18 electron rule in, 1, 337 Organomethyl compounds, synthesis, structure, and vibrational spectra,9, 25 Organosulphur compounds, metal-ion- promoted reactions of, 6, 345 Organo-transition-metal complexes:stability, reactivity, and orbital cor- relations, 2, 271 Oxygen, singlet molecular, 10, 205 PEDLERLECTURE.Porphyrins and related ring systems, 4, 1 Peroxonium ions and dioxygen ylides, the chemistry of, 14, 399 Phase boundaries, reactivity of organicmolecules at, 1, 229 Phase equilibrium and interfacial structure, 14, 121 Phenols, anionic cyclization of, 12, 213 long-chain, of non-isoprenoid origin, 8,499 Index Philosophy of chemistry, some con-siderations, 5, 203 Phosphates, aluminium, the chem-istry and binding properties of, 6, 173 Phosphorus compounds, tervalent, in organic synthesis, 3, 87 Photochemistry of azobenzene and its derivatives, 1,481 -of carbonyl compounds, 1,465 -of imines, 6, 63 of olefinic compounds, 3, 329 of organic sulphur compounds, 4, 523 of the uranyl ion, 3, 139 of transition-metal co-ordination compounds-a survey, 1, 241 Photocyclization and photochemistry of aryl halides, 10, 181 Photodegradation and stabilization of commercial polyolefins, 4, 533 Photoelectron spectroscopic studies of electrode and related struc-tures, 14, 1 Photophysical and photochemicalproperties of flavins (isoal-loxazines), 11, 15 Photophysics of molecules in micelle- forming surfactant solutions, 7, 453 Plant growth, control by chemicals, 6, 261 Plastocyanin, structure and electron- transfer reactivity of the blue copper protein, 14, 283 Platinum metal complexes, q5-cyclo- pentadienyl and q6-arene as pro-tecting ligands towards, 10, 1 Polyhedral clusters, the construction of, 13, 353 Polymer solutions, dielectric relaxation in, 1, 49 supports, insoluble, use in organic chemical synthesis, 3, 65 Polymerization and copolymerization of butadiene, 6, 235 Polymerization in organized sys-tems, 14,45 Polymers, conductivity and supercon- ductivity in, 5, 95 Polyolefins, commercial, photo-degradation and stabilization of, 4, 533 Polysaccharides, structural studies of, 10,409 Porphyrins and related ring systems, 4, 1 Index Post-Blz problems in corrin synthesis, 5, 377 Potentiometric titrations of aqueouscarbonate solutions, 14, 265 Preparation of organoboranes: reagents for organic synthesis, 3, 443 and properties of isocyanic acid, 11,41 PRESIDENTIALADDRESS1976. Chemistry and the new industrial revolution, 5, 317 Properties and syntheses of sweetening agents, 6,431 Prostaglandins, tomorrow’s drugs,4, 589 , thromboxanes, PGX: biosynthetic products from arachidonic acid, 6,489 Prostanoids, total syntheses of, 2, 29 Protecting ligands, q 5-cyclopentadienyland q6-arene towards platinum metal complexes, 10, 1 Protein production by micro-organ- isms, 8, 143 Proteins, binding of heavy metals to, 6, 139 Proteins, r81e of in nature’s chemistry, 9, 325 Pulse radiolysis, contributions to chem- istry, 7, 235 Purines, isotopic hydrogen exchange in, mechanisms and applications,10, 329 Pyridines as leaving groups, new insights into aliphatic nucleophilic substitution from the use of, 13, 47 Pyrimidine nucleosides and nucleotides, 5-subs tit u ted, 6, 43 Pyro-compounds, inorganic,MoC(X207)b], 5, 269 Quadruple bonds and other multiple metal to metal bonds, 4, 27 Quantitative drug design, 3, 273 Quantum chemistry, computational,growth of from 1950 to 1971, 2, 21 -mechanical tunnelling in chem- istry, 1, 211 Quasielastic laser light scattering, 2,325 Radical cations in condensed phases, 13, 393 Radioactive and toxic wastes: a corn- parison of their control and dis-posal, 4, 90 Radiolysis, pulse, contributions to chemistry, 7, 235 Raman and infrared vibrational spec- troscopy in inorganic chemistry,4, 107 R. A.ROBINSON LECTURE.MEMORIAL Potentiometric titrations of aqueouscarbonate solutions, 14,265 R.A. ROBINSON LECTURE.MEMORIAL Thermodynamics of hydrogen-bonded liquids, 11, 257 Reaction mechanisms, four-membered rings and, 5, 149 -, the conversion of ammonium cyanate into urea, 7,. 1 Reactions involving the triple bond in dimolybdenum and ditungstenhexa-alkoxides and C-C, C-N, C-0 triple bonds, 14, 69 Reactive intermediates in enzyme-catalysed reactions, 13, 97 Reactivities of carbon disulphide,carbon dioxide, and carbonyl sul-phide towards some transition-metal sys tems, 11, 57 Reactivity of organic molecules at phase boundaries, 1, 229 Recent advances in the study of kinetic hydrogen isotope effects, 3, 513 Recent syntheses in the Vitamin D field, 9, 449 Research in chemical education: a reassessment, 1, 27 RESOURCES CONSERVATION BY NOVEL BIOLOGICAL PRO-CESSES I Grow Fats from Wastes, 8, 283 I1 The Use of Microbes in the Petrochemical Industry, 8, 297 I11 Utilization of Agricultural and Food Processing Wastes contain- ing Carbohydrates, 8, 309 Review of chemical education research and development in the U.K., 1972-1 976, 7, 317 Ring, cage, and cluster compounds of the main group elements, 8, 315 ROBERT ROBINSON LECTURE.Post-BI2 problems in corrin synthesis, 5, 377 ROBERTROBINSON The logic LECTURE.of working with enzymes, 2,. 1 ROBERTROBINSONLECTURE.Vitamin BIZ. Retrospect and pros-pects, 9, 125 Rodent control, chemicals in, 1, 381 Role of chemically-induced dynamic electron polarization (CIDEP) in chemistry, 8, 29 Rotationally and vibrationally inelastic scattering of molecules, 3,407 Safety evaluation of natural and synthetic flavourings, 7, 185 Scale insects and aphids, chemistry of, 4, 263 Semistable molecules in the laboratory and in space, 11,435 Silicon compounds in organic synthesis, some uses of, 10, 83 containing carbonyl equiva-lents, 11,493 -in organic synthesis, 7, 15 16 and 18 Electron rule in organometal- lic chemistry and homogeneouscatalysis, 1,337 Small molecules, conformation studies on, 1, 293 Solids, surface energy of, 1, 445 Solute-solvent interactions, spectro-scopic studies of, 5, 297 Solution phenomena, the importance of solvent internal pressure and co- hesion, 4,211 Solutions of metals: solvated elec-trons, 5,337 Solvent internal pressure and cohesion, importance to solution phenomena, 4, 211 Solving chemical problems, 11, 171 Some considerations on the philosophy of chemistry, 5, 203 Some recent developments in chemistry teaching in schools, 1,495 Spectra of stars, absorption bands in, a crystal field approach, 5, 233 Spectral lineshapes, collisional trans- fer of rotational energy with, 7, 219 Spectroscopic studies of solute-solvent interactions, 5, 297 Spectroscopy and chemistry of mixed- valence complexes, 13, 219 Spectroscopy, electron, 1, 355 , Fourier transform, chemical applications of advances in, 4, 569 , laser, of ultra-trace quantities,8, 367 -, rotational and vibrational, com- parison with molecular orbital theory Index in conformational analysis of alco- hols and amines, 5,411 , threshold electron scattering,3,467 Spin-lattice relaxation: a fourth dimen- sion for proton n.m.r.spectroscopy, 4,401 Square-planar complexes, isomeriza-tion mechanisms of, 9, 185 Square-planar molecules, ligandsubstitution reactions of, 14, 197 SRS-A, the synthesis of leukotrienes: a new class of biologically active compounds including, 11, 321 Stability, reactivity, and orbital correla- tions of organo-transition-metalcomplexes, 2,271 Stereochemical choice in enzymic reac- tions, 8,447 Stereoselective synthesis of steroid side- chains, 12,75 Steroid biosynthesis, methyl groupremoval in, 10,435 -, the mechanism of the microbial hydroxylation of, 11, 371 , routes to by intramolecular Diels-Alder reactions of 0-xyly-lenes, 9, 41 side-chains, stereoselective syn- thesis of, 12,75 Sterols, biosynthesis of, 1, 259 Structure and electron-transfer re-activity of the blue copper protein plastocyanin, 14, 283 Structure in solvents and solutions- N.m.r.and vibrational spectroscopic studies, 12, 1 -of aqueous solutions of hydro- phobic solutes, experimental studies on, 2, 203 Substitution and conformation, effects of, on individual CH bond strengths in simple organic compounds,7, 399 Sugars, complex formation with cations, 9,415 Sulphoximides, 4, 189 Sulphoximides-an update, 9, 477 Sulphur compounds, organic, photo- chemistry of, 4, 523 organic compounds of, metal-ion- promoted reactions of, 6, 345 species, homonuclear, chemistry of, 2, 233 Superconductivity and conductivity in polymers, 5, 95 47 1 Index Superoxide, organic chemistry of, 6, 195 Surface chemistry and catalysis, new perspectives, 6, 373 energy of solids, 1,445 modified electrodes, 8, 259 of a liquid, 7, 329 Surfaces, biological, molecular aspects of, 8, 389 , solid, their acidity, 8,475Sweetening agents, properties and syn- theses of, 6,431Sweeter side of chemistry, 14, 357 Syntheses and properties of sweetening agents, 6,431 -of mononuclear cyanocobalt(rI1) complexes, 12, 267 -, recent, in the Vitamin D field, 9,449 -, total, of prostanoids, 2, 29 Synthesis and cycloadditions, cyanok- etenes, 10,289and synthetic utility of halolac- tones, 8, 171 , of corrins, post-BIZ problems in 5, 377 of complex oligosaccharide chains of glycoproteins, 13, 15 of leukotrienes: a new class of biologically active compounds in-cluding SRS-A, 11, 321 of lignans and neolignans,11,75 ___ of polycyclic polysubstitutednatural products, systematic de-velopment of strategy in, 6, 413 -, organic, enzymes in, 3, 387 -, organic, preparation of organo- boranes as reagents for, 3,443 , organic, silicon in, 7, 15 , organic, some uses of silicon compounds, 10, 83 , organic, tervalent phosphoruscompounds in, 3, 87 -, organic, use of inorganic polymer supports in, 3, 65 -, organic, the use of organoborates as synthetic intermediates, 6, 393 , structure, and vibrational spectra of organomethyl compounds, 9, 25 -of substituted benzene compounds from acyclic precursors, 13, 441 Synthetic pyrethroids.A new group of insecticides, 7,473routes to P-lactams, 5, 181 Systematic development of strategy in the synthesis of polycyclic poly-substituted natural products: the aconite alkaloids, 6,413 TATEAND LYLE LECTURE. From carbo- hydrates to enzyme analogues,8, 85 TATEAND LYLE LECTURE. Spin-latticerelaxation: a fourth dimension for proton n.m.r. spectroscopy, 4, 401 TATEAND LYLE LECTURE. Transition-metal oxide chelates of carbohydrate- directed macromolecules, 8, 22 1 Teaching of chemistry in schools, some recent developments in, 1,495Techniques for the kinetic study of fast reactions in solution, 11, 227 Tervalent phosphorus compounds in organic synthesis, 3, 87 Thermal, photochemical, and transi-tion-metal mediated routes to ster- oids by intramolecular Diels-Alder reactions of o-xylylenes (o-quinodi- methanes), 9, 41 Thermodynamics of ion-solvent inter-actions, 9, 381 Thermolysis and photolysis of diazir- ines, 11, 127 Three-dimensional structures and chemical mechanisms of enzymes,1, 319 Threshold electron scattering spectro- SCOPY, 3,467Thromboxanes, prostaglandins, PGX: biosynthetic products of arachidonic acid, 6,489TILDENLECTURE.Alkoxy, amido, hy- drazido, and related compounds of molybdenum and tungsten, 12, 331 TILDENLECTURE.Applications of e.s.r.spectroscopy to kinetics and mech- anism in organic chemistry, 8, 1 TILDEN Carbon-carbon bond LECTURE. formation involving boron re-agents, 11, 191 TILDENLECTURE.Chemistry and Spec- troscopy of mixed-complexes,13, 219 TILDEN LECTURE. Concerning stereo- chemical choice in enzymic reactions, 8, 447 TILDENLECTURE.-q 5-Cyclopentadienyland q6-arene as protecting ligands towards platinum metal com-plexes 10, 1 TILDEN LECTURE. Electrophilic C-nitroso-compounds, 6, 1 TILDEN LECTURE. Initiation of cycliza- tion using 3-methylcyclohex-2-enonederivatives, 9,265 TILDEN LECTURE. Molecular beam reactive scattering, 11, 1 TILDEN New perspectives in LECTURE.surface chemistry and catalysis)6,373 TILDENLECTURE.Semistable molecules in the laboratory and in space,11,435 TILDEN LECTURE. Some uses of silicon compounds in organic synthesis, 10. 83 TILDEN Structural studies on LECTURE. bio-active molecules, 13, 131 TILDEN LECTURE. Structure and elctron-transfer reactivity of the blue copper protein plasto-cy nanin, 14, 283 TILDENLECTURE.The collision dyna- mics of vibrationally excited mole- cules, 14, 141 TILDEN Valence in transition- LECTURE. metal complexes, 1, 431 Time-correlation functions and mole- cular motion, 7, 89 Topological subject--chemistry,2,457 Trace constituents of the diet, chemical aspects, 10, 233 organic constituents of the diet, sources and biogenesis, 10, 280 Transimination, chemical models of enzymic, 12, 309 Transition-metal carbene complexes,chemistry and r81e as reaction intermediates, 2, 99 complexes, containing infinite chains of metal atoms, metal-metal interactions in, 1, 99 complexes of synthetic macro-cyclic ligands, 4,421 -complexes, valence in, 1, 431 -co-ordination compounds,photochemistry of, 1, 241 hydride complexes, 12,415 -systems, reactivities of carbon disulphide, carbon dioxide, and car- Index bony1 sulphide systems towards,11,57 -oxide chelates of carbohydrate- directed macromolecules, 8,221 Transport properties, hard-spheretheories of, 14, 317 Triple bond in dimolybdenum and ditungsten hexa-alkoxides and C-C, C-N, C-0 triple bonds, reactions involving the, 14, 69 Tunable lasers, 3, 293 Unimolecular reactions, current aspects of, 12, 163 Uranyl ion, photochemistry of,3, 139 Use of insoluble polymer supports in organic chemical synthesis, 3, 65 Utilization of agricultural and food processing wastes containing car-bohydrates, 8, 309 Valence in transition-metal complexes, 1,431Valences, bond, a simple structural model for inorganic chemistry,7,359 Very strong hydrogen bonding, 9,91 Vibrational and n.m.r.spectroscopicstudies, structure in solvents and solutions, 12, 1 ) infrared, and Raman spectro-scopy in inorganic chemistry,4, 107 -intensities in electronic transi- tions, 5, 165 spectra, synthesis, and structure of organomethyl compounds, 9,25 Vibrationally and rotationally inelastic scattering of molecules, 3,407 Vibrationally excited molecules, the collision dynamics of, 14, 141 Viologens, electrochemistry of, 10, 49 Vitamin BI2, retrospects and pros-pects, 9, 125 Vitamin D,chemistry of the hormonal calciferols, 6,83 Vitamin D,recent syntheses in, 9,449 Ylides, the chemistry of peroxonium ions and dioxygen, 14, 399

ISSN:0306-0012    

DOI:10.1039/CS9851400457

出版商:RSC    

年代:1985

数据来源: RSC