6 Organometallic Chemistry Part (i) The Transition Elements By A. STEWART D. J. THOMPSON and M. V. TWIGG 1.C.l Corporate Laboratory P.O. Box 1 I The Heath Runcorn Cheshire WA7 4QE 1 Introduction This year we have not included the subsection ‘Synthesis of N-Heterocyclic Compounds’ in order to avoid duplication with the section concerned with hetero- cyclic chemistry. However some advances in this area are reported where appro- pria t e. A number of relevant reviews have been published including one concerned with transition metal clusters.’ Others deal with cyclometalation reactions,2 olefin metathe~is,~ transition metal catalysed cyclizations of acetylene^,^ carbon monox- ide insertion reactions,’ olefin insertion reactions,6 and the use of transition metal derivatives in organic synthesis.’ Amongst new books is the first volume of a series concerned with the application of transition metal organometallic compounds in organic synthesis,’ and the second volume of the well known book by Wender and ~ino.~ 2 Metal-catalysed Hydrogenation Continued progress has been made in improving the selectivity of metal-catalysed reductions and more specifically asymmetric synthesis.The majority of the publications on asymmetric hydrogenation relate to homogeneous rhodium- catalysed reactions and reflect the continuing search for new and more specific chiral ligands. One example for rhodium is the diphosphinite (+) (lS,2S)-mns-1,2-bis(diphenylphosphinoxy)cycIopentane (1) which is conformationally far more rigid than its cyclohexane analogue (2).” In the reduction of a-ethylstyrene the optical yield 60% using (1) was much higher than that found with (2) or (-)-DIOP [(1S,2S)-2,3-0-isopropylidene-2,3 -di hydroxy- 1,4-bis(diphenylp hosp hino)- butane] and the highest so far attained for this substrate by homogeneous catalysts.’ A. K. Smith and J. M. Basset J. Mol. Catalysis 1977 2 229. * M. I. Bruce Angew. Chem. Internal. Edn. 1977,16,73. J. J. Rooney and A. Stewart in ‘Catalysis’ Vol. 1 Specialist Periodical Reports The Chemical Society ed. C. Kemball 1977 Chapter 8. K. P. C. Vollhardt Accounts Chem. Res. 1977,10 1. F. Calderazzo Angew. Chem. Internal. Edn. 1977,16 299. ‘G. Henrici-Olivk and S. Olivt Topics in Current Chem. 1976,67 107. L. Hegedus J.Organometallic Chem. 1977 143 309. ‘Transition Metal Organometallics in Organic Synthesis’ Vol. 1 ed. H. Alper Academic Press 1976. ’‘Organic Synthesis via Metal Carbonyls’ Vol. 2 by I. Wender and P. Pino Wiley-Interscience 1977. lo T. Hayashi M. Tanaka and I. Ogata Tetrahedron Letters 1977,295. 119 A. Stewart D. J. Thompson,and M. V. Twigg However stereoselectivity was lower for unsaturated carboxylic acids but comparable for their esters indicating that (1)is a ligand unique for the asymmetric hydrogenation of olefinic substrates bearing no polar substituents. A new class of ligand has been reported whose chirality is the result of atro-pisomerism and not to an asymmetric centre on phosphorus or carbon." The described example is (- )- l,l-bi-2-naphthylbis(diphenylphosphinite)(3) and two 3 OPPh2 PPh \/ (31 equivalents of this with one of [Rh(cycl~octene)~Cl]~ produced a catalyst system which in the hydrogenation of unsaturated acids and esters gave better optical yields at lower temperatures.In comparison to catalysts derived from ligands with chirality at phosphorus or carbon this system was less active but gave similar optical yields. The preparation of (2S,3S)-bis(diphenylphosphino)butane S,S-chiraphos (4) and its use as a ligand for a Rh' complex have produced remarkably high optical Me Me H->4H PhzP PPh (4) yields in the hydrogenation of a-N-acylaminoacrylic acids to N-acylaminoacids at ambient temperature and hydrogen pressure. ** X-ray evidence indicated that (5) is (51 the preferred conformation of the chelate ring with equatorial methyl groups.With all nine substrates hydrogenated the @)-amino acid derivatives were produced R. H. Grubbs and R. A. DeVries Tetrahedron Letters 1977 1879. l2 M. D. Fryzuk and B. Bosnich J. Arner. Chem. SOC., 1977,99,6262. Organometallic Chemistry-Part (i) The Transition Elements and two of these leucine and phenylalanine were optically pure. The optical yield was sensitive to both the N-acyl and p-vinylic substituents and also to the solvent employed. The authors suggest that the rigidity of the chelated diphosphine with its dissymmetrically orientated phenyl groups is the major source of interaction with the achiral substrates examined and according to their rationalization in the design of (4),they also prepared (R )-1,2-bis(diphenylphosphine)propane,(R)-prophos SO that the naturally-occurring amino acids would result.This in fact was the case and optical yields similar to those obtained with (4)were found. Several ligands (6) derived from L-hydroxylproline have been investigated in Rh-catalysed reductions of certain achiral substrates.l3 Using neutral Rh' complexes with (6b) esters of pyruvic acid were converted to the corresponding lactates with higher optical yields than with (-)-DIOP.13" In the case of (6b) and propyl pyruvate an enantiomeric excess of about 76% was found. When dry aprotic solvents (benzene or THF) were employed optical yields were higher than when methanol was used. Compounds (6b) (6c) and (6d) have been used for synthesis of (R)-and (S)-N-benzyloxycarbonylalaninefrom their olefinic deriva- the (S)-compound being formed when Et3N was present in the reaction.The highest optical yields 59% (R) and 21% (S) were found with (6d) and it was suggested that the N-substituents of PPM (6a) play an important role. In the hydrogenation of substituted cinnamic acids with (6a) and (6b) Et3N had a remark- able effect on optical yields with (6b) but not with 6a),13= indicating some novel interaction which serves to produce 83-9 1% enantiomeric excess in favourable cases. A new lipophilized bisphosphine-rhodium complex allows greater solubility in aliphatic hydrocarbons and increases lipophilic interaction with substrates. 13d Here the new ligand (6e) developed from PPM was applied in the hydrogenation of ethyl and n-butyl pyruvates with optical yields of the (R)-configuration of the product as high as 67.3% in the ethyl ester case in cyclohexane solvent.Application of the same catalyst system to the reduction of olefinic double bonds resulted in complete conversions but low optical yields,'3d certainly lower than systems involv- [ >CH,Ph2 N R (6) a; (PPM) R =H-b; (BPPM) R =t-Bu02C-c; (APPM) R =MeCO-d; (PPPM) R =(Me)3C-CO-e;(CPPM) R =cholesteryloxycarbonyl C27H4502C- A set of phosphines chiral at both phosphorus and carbon have been synthesized though only two of them (S)p-and (R),-menthylmethylphenylphosphine were purified sufficiently well for use as ligands in the reduction of olefinic bonds in a,p-unsaturated carboxylic acids.l4 Reduction of (E)-p-methyl cinnamic acid gave l3 (a)I. Ojima T. Kogure and K. Achiwa J.C.S. Chern. Cornrn. 1977,428; (b)K. Achiwa Chem. Lerters 1977 777; (c)K. Achiwa J. Amer. Chem. SOC.,1976 98 8265; (d) K. Achiwa Tetrahedron Letters 1977,373.5. l4 C. Fisher and H. S. Mosher Tetrahedron Letters 1977 2487. A. Stewart D. J. Thompson andM. V. Twigg the (R) product in 67.1% optical yield. The opposite configuration of the product was formed with the other phosphine. Further work is in progress to improve the purity of the phosphine ligands. Discovery of the catalyst precursor [Rh(COD)L]'BF,- where L = (R,R)-1,2-ethanediylbis(o-methoxyphenyl)phenylphosphine,has allowed Knowles and co-workers to operate at higher temperatures and pressures and as a result they have found great versatility with their ~ystem.'~ Excellent selectivity (about 90% enantiomeric excess) was found in reduction of the a-enol ester (Z)-ethyl-2- acetyloxy-3-phenyl-2-propenoate,the first outstanding result with a nonamide substrate and optical yields of up to 96% were obtained in the hydrogenation of (2)-a-acylaminocinnamic acids but only up to 47% for (E)isomers at much lower rates.A catalyst picture was presented on the basis of an X-ray structure deter- mination and reasons for the discrimination between olefin configurations were discussed. Bulky (2)-adamantyl or bornyl-a -acetamidocinnamate inhibited the reduction of the (2)methyl ester catalysed by a neutral Rh-DIOP complex and underwent (2)to (E)isomerization.'6" Neither in the presence or absence of inhibitors did the methyl ester undergo appreciable isomerization.Further over a range of a-acylamino unsaturated substrates with the grouping NHCOR optical yields of the reaction products decreased as the steric bulk of this group increased from R =Me to R = adamantyl.16b The a-formamido analogue (R = H) although having an even smaller amide group than in the corresponding a-acetamide showed a decrease in optical yield. With a trifluoroacetamido group a reversal in chirality of the major product was observed. The use of chiral ruthenium complexes has been extended to asymmetric induction in catalytic hydrogenation of the olefinic bonds in a,p-unsaturated mono- and di-carboxylic acids using precursor cluster complexes.l7 With reaction -)-DIOPI3 temperatures of 90-120 "C H4R~4(C0)8[( )-DIOPI2 and Ru~(CO)~~[( gave optical yields which varied with substrate and were improved by the presence of Et3N but for (E)-a-methylcinnamic acid the former complex produced up to 68% enantiomeric excess a figure reached by lowering hydrogen pressure. 17" Lower optical yields were found with the corresponding esters and also with substrates without carboxylic groups even at lower reaction temperatures. The authors suggested that since both cluster catalyst precursors gave products with the same predominant configuration and comparable optical purity both complexes may have led to the same catalytic intermediates and that a free carboxylate group in the substrate is required for a substantial asymmetric bias.Regioselectivity and asymmetric induction in catalytic hydrogenation of a$-unsaturated dicarboxylic acids citraconic and mesaconic acids using H4Ru4(C0)8[( -)-DIOP]* gives in addition to (-)(S)-methylsuccinic acid a mixture of y-lactones in ratios which depend on the substrate and the reaction temperat~re.'~~ l5 8. D. Vineyard W. S. Knowles M. J. Sabacky G. L. Bachman and D. J. Weinkauff J. Amer. Chem. SOC.,1977,99 5946. l6 (a)R. Glaser and J. Blumenfeld Tetrahedron Letrers 1977 2525; (b)R. Glaser and S. Geresh ibid.,p. 2527. (a)C. Botteghi S. Gladiali M. Bianchi U. Matteoli P. Frediani P. G. Vergamini and E. Benedetti J. Orgunometullic Chern. 1977 140 221; (b) M.Bianchi F. Piacenti P. Frediani U. Matteoli C. Botteghi S. Gladiali and E. Benedetti J. Orgunometullic Chem. 1977 141 107. Organometallic Chemistry-Part (i) The Transition Elements 123 Two new catalyst systems for reduction of aromatics have appeared. One is the [Rh(q5-CsMes)Cl2I2 complex operating under homogeneous conditions with added base (Et3N) which is required as a cocatalyst.18a All cis isomers are the chief products and some hydrogenolysis of functional groups is observed. The other catalyst system employs salicylaldehyde complexes of Co Ni and Cu though there are doubts about the homogeneity of these systems.lsb When lithium aluminium hydride is used as reductant the catalysts are more active. Rhodium catalysts have been developed for hydrogenation of ketones.l9 The complexes RhC1(CsH12)PPh3 and Rh2H2C12(C8H12)(PPh3)2 in the presence of strong alkali promoted the reduction of several ketones although the dirhodium species gave the best results.19o Pretreatment with sodium borohydride resulted in higher and more reproducible hydrogenation rates. The authors suggested the existence of a hydroxo complex as an active intermediate. An alkaline medium was again required for the reduction of ketones at room temperature and at atmos- pheric pressure catalysed by complexes of the type [Rh(2,2'-bipyridine) (diene)]'PF6-which also act as hydrogenolysis catalysts for molecular oxygen.lg* Consequently hydrogenation activity was not destroyed even if molecular oxygen was present in large amounts.Notable points are that selectivity was observed for reduction of carbon-oxygen double bonds even in the presence of olefinic bonds and the system was also active in the selective reduction of dienes and 2-alkynes to alkenes. In non-co-ordinating solvents such as benzene (in the presence of Et3N) and dichloromethane (with or without Et3N) the complex [Rh(C0D)(PPh3),]PF6 was found to be active in the hydrogenation of ketones as well as alkenes and alkynes to alkanes.''= In contrast the authors found that [Rh(OCOPh)(COD) (PPh,)] and [Rh(COD)(PPh3)2py]PF6 were highly selective in the reduction of 1-alkynes to 1-alkenes if Et3N and benzoic acid were present. Ruthenium and iridium complexes have been used as selective catalysts. A report has appeared showing that RuC~*(PP~~)~ is a highly selective catalyst for hydrogenation of cyclododeca-175,9-trieneand other polyenes to monoenes in the presence of Et3N and for reduction of the olefinic bond in a diene with a terminal and internal double bond.20a The same catalyst was found to be effective in the hydrogenation of aldehydes (both aliphatic and aromatic) but not ketones to alcohols at temperatures of 50-80°C and 10 atmospheres of hydrogen pres-sure.20b Nitro groups were unaffected.An iridium complex ITH~(PP~~)~ in the presence of acetic acid was also selective for the reduction of aldehydes and not ketones.21 A new low-valent cobalt complex H3Co[P(O-Pri),I3 has the notable property of being a soluble selective catalyst for the hydrogenation of @-unsaturated ketones and amides to the saturated ketones and amides.22 Unsaturated aldehydes were not reduced under similar conditions.Catalyst activity was greatly increased (a)M. J. Russell C. White and P. M. Maitlis J.C.S. Chem. Comm. 1977 427; (b)P. Patnaik and S. Sarkar Tetrahedron Letters 1977 253 1. l9 (a) M. Gargano P. Giannoccaro and M. Rossi J. Organometallic Chem. 1977 129 239; (6) G. Zassinovich G. Mestroni and A. Camus J. Mol. Cat. 1977,2,63; G. Mestroni G. Zassinovich and A. Camus J. Urganometal[ic Chem. 1977 140 63; (c)R. H. Crabtree A. Gautier G. Giordano and T. Khan J. Organometallic Chem. 1977 141 113. 2o (a)J. Tsuji and H. Suzuki Chem. Letters 1977 1083; (6)ibid. p. 1085. '' w. Strohmeier and H. Steigerwald J. Organometallic Chem.1977,129 C43. 22 M. C. Rakowski and E. L. Muetterties J. Amer. Chem. SOC..1977 99 739. 124 A. Stewart D. J. Thompson and M. V. Twigg without loss in selectivity by reacting at 70°C. An example of the use of this catalyst is in the reaction of benzalacetone resulting in a 50% conversion to benzylacetone after 1 day. Alkynes and conjugated dienes in mixtures with alkenes are selectively hydro- genated to alkenes without significant reduction of alkene to alkane using hetero- geneous catalysts with Ni Pd or Pt intercalated in gra~hite.’~ The comparison of several homogeneous and heterogeneous olefin hydro- genation catalysts has been made for the metals Rh Co Ni and Pd.24 Phosphine complexes were compared with metal chlorides and bromides supported on phos-phine-modified silica carriers and the heterogenized systems were found to be 2 to 4 orders of magnitude more active than their homogeneous counterparts for the reduction of cyclohexene in THF.Selectivity in the reduction of ap-unsaturated aldehydes can be influenced to give preferential hydrogenation of the carbon-oxygen double bonds.25 With rhodium halide catalyst systems the presence of carbon monoxide and highly basic tertiary amines such as Et,N and N-methylpyrrolidine significantly increased not only catalytic activity but selectivity to cinnamyl alcohol in the reduction of cinnamaldehyde under the 0x0 reaction conditions giving no hydroformylation products; up to 85% selectivity to the unsaturated alcohol was achieved with the best system examined Rh2C12(C0)4 (7).Prereduction of RhC13 3H20 with carbon monoxide suggested that reduction of the trichloride occurred during hydro- genation because activity after this treatment was such that lower reaction temperatures could be employed. Addition of triphenylphosphine to the systems examined resulted in alteration of the selectivity to one of olefin hydrogenation with no reduction of aldehyde groups. Immobilization of (7) was achieved on a cross-linked chloromethylated polystyrene which was functionalized with pyr- rolidine. Rh oc-1R \‘.TO C C 0 0 3 Isomerization Some interesting cases of transition metal catalysed rearrangements and iso- merizations have appeared. Several reactions of which equations (1)and (2) are examples have been reported where double-bond migrations are catalysed by RhCI3,3H2O allowing otherwise 0 0 difficult or impossible exocyclic-endocyclic isomerizations to occur under mild conditions and in good yields.26 Compound (8) readily attainable by isomerization [equation (2)] was previously obtainable only by a three-step synthesis.The iso-23 Ventron Corp. Canad. P. 1,000,306 1976 (Chem.Ah. 1974.80 145 363). 24 K. Kochloefl W. Liebelt. and H. Kndzinger J.C.S. Chem. Comm. 1977 510. 25 T. Mizoroki K. Seki S. Meguro and A. Ozaki Bull. Chem. SOC.Japan 1977 50 2148. ‘‘ J. Andrieux D. H. R. Barton and H. Patin J.C.S. Perkin I,1977 359. Organometallic Chemistry -Part (i) The Transition Elements merization where the migrating double bond moves into conjugation with the aromatic nucleus [equation (3)] was also found to be catalysed by the same metal ---* I 1 (2) \ / am Me Me / \ Me Me (8 ) i3) OR’ &OR2 &OR2 OR’ (9) chloride and the trans isomer predominated in the product in contrast to that found on isomerization of (9) by strong bases.On this evidence even simple transition metal compounds must still have great potential as catalysts in the field of organic synthesis. The application of an iridium catalyst IrCl(CO)(PPhl)2 to migrations of exocy-clic double bonds in some cycloalkanones has been de~cribed.~’ Isoaromatization and disproportionation reactions were also observed with the substrates examined. Nickel and rhodium complexes have been shown to catalyse the rearrangement of ally1 but-3-enoate to hepta-2,6-dienoic (10)or hepta-3,6-dienoic acid (11) in the presence of phosphine or phosphites.’* The isomerization can be driven towards either the 2,6- or the 3,6-isomers through selective hydrogen abstraction.With nickel and tri-o-tolyl phosphite the ratio of (10) to (11) was highest at 14:1 and with triethyl phosphite lowest at 0.8 :1. In anisole solvent stereoselectivity was observed the main product being the truns-2,6-isomer. With Rh(PPh3)3Cl in chloroform the rearrangement gave pre-dominantly the 3,6-isomer. Intermediates such as (1 2) and (1 3) were proposed. -(13) HaYnsfer CH2=CHCH2CH2CH=CHCO*H (10) ’’Z. Aizenshtat M. Hausmann Y. Pickholtz D. Tal and J. Blum J. Org. Chem.1977,42,2386. 28 G.P. Chuisoli G. Salerno and F. Dallatomasina J.C.S. Chem. Comm. 1977 793. 126 A. Stewart D. J. Thompson,and M. V.Twigg Palladium(I1) catalysis of the formation of 3-phenyl indoles from 2,2-diphenyl-2H-azirines has been rep~rted,’~ although as yet the mechanism is not clear. The use of PdC12(PhCN)2at 30°C and then washing with aqueous ammonia afforded the indole product the formation of which normally required thermal rearrange-ment at elevated temperatures. 4 Dimerization Oligomerization and Polymerization Much of the work published in these areas has contributed further to the under-standing of the effects of varying ligands and substrates on the selectivities to certain The palladium-catalysed formation of a 1,4-disilacyclohexa-1,s-diene from a 1-silacyclopropene has been noted3’ and the use of the nitrosyl rhodium complex [Rh(NO)(NCMe),][BF,] as a catalyst for alkene isomerization oligomerization and polymerization has been Stereospecific poly-merization of 1,3-butadiene to truns-l,4-polybutadiene occurred with this rhodium system whereas isoprene gave the oligomers from tetramers up to decamers.Of interest to the organic chemist is the continued work of Vollhardt et ul. in cobalt-catalysed syntheses of polycyclic ring systems.34 The elegant synthesis of the steroid nucleus in 71% yield [equation (4)]34Qillustrates the potential of the general reaction of co-oligomerization of ao-diynes with a monoalkyne equation (S) and the possibility of a wide variety of final structures.X in the triply unsaturated compound (14) can be varied.34bFor X = N the reaction gives annelated pyridines in yields of up to 80%.34c SiMe, I (4) Ill + -+ I SiMe C=CH R I I AR I CECH X n = 3,4 5; R = SiMe3,C02Me,Ph; X = CR,N 29 K. Isomura K. Uto. and H. Taniguchi J.C.S. Chem. Comm. 1977 664. 30 See for example in dimerization and co-oligomerization (a)P. Heimbach A. Roloff and H. Schenk-luhn Angew. Chem. Internat. Edn. 1977 16 252; (b)P. Heimbach B. Hugelin E. F. Nabbefeld D. Reinehr A. Roloff and E. Troxler ibid. 253; (c) K. Kaneda M. Terasawa T. Imanaka and S. Teranishi Tetrahedron Letters 1977 2957; (d)G. Giacomelli A. M. Caporusso and L. Lardicci J.C.S. Perkin I 1977 1333; (e) H. Suzuki K. Itoh Y. Ishii K. Simon and J.A. Ibers J. Amer. Chem. SOC. 1976,98 8494; (f)S. Yoshikawa J. Kiji and J. Furakawa Makromol. Chem. 1977,178 1077; (g)J. Ficini J. d’Angelo and S. Falou Tetrahedron Letters 1977 1645; (h)G. Henrici-OlivC and S. Olivi Transition Met. Chem. 1976 1 109; (i) H. T. Dieck and H. Bruder J.C.S. Chem. Comm. 1977 24. 31 See for example in polymerization C. Carlini R. Nocci and F. Ciardelli J. Polym. Sci. Polymer Chem. 1977 15 767. 32 M. Ishikawa T. Fuchikami and M. Kumada J.C.S. Chem. Comm. 1977,352. 33 N. G. Connelly P. T. Draggett and M. Green J. Orgunometallic Chem. 1977 140 C10. 34 (a)R. L. Funk and K. P. C. Vollhardt J. Amer. Chem. Soc. 1977 99 5483; (b)R. L. Hillard and K. P. C. Vollhardt ibid.,4058; (c)A. Naiman and K. P. C. Vollhardt Angew. Chem. Internut.Edn. 1977 16 708. Organometallic Chemistry-Part (i) The Transition Elements 5 Carbonylation Several interesting reactions involving cobalt carbonyl intermediates have appeared this year. The reaction of cyclic olefins and ethers with hydrosilanes and carbon monoxide catalysed by CO~(CO)~ Cyclic olefins which has been rep~rted.~' were expected to give product (15) by direct analogy with hydroformylation in fact gave the enol silyl ethers (16) in good yield (50-74Y0).~'" Under similar condi- tions cyclic ethers reacted to give the silyl protected hydroxyaldehyde (17) in about OSiEt2Me n C02(CO) / (CHA I I +CO+HSiMeEt2 --+ (CHSC=C 0 II c n +CO+HSiEt2Me CO~ICO)~ ____+ Et2MeSiO(CH2),C//O \ 'H n =2 3 4 (17) 50% yield.356 The intermediate [CO(CO)~]- which was generated by catalytic disproportionation of [CO~(CO)~~] or [Co,(CO),] using free or complexed halide ions reacted with the dihalide (18) to give 2-indanone (19) in 80% yield.36 (18) (19) The use of phase transfer catalysis in carbonylation has been extended to the catalytic carbonylation of benzyl halides using Co,(CO),/CO/NaOH in the presence of benzyltriethylammonium chloride as the phase transfer ~atalyst.~' Yields appear to be good only for benzyl bromides.Carbonylation via iron carbonyl complexes continues to generate interesting reactions. Two groups have reported the synthesis of lactones from dienes via epoxidation followed by carbonylation (Scheme l).38Whereas iron and cobalt catalysts give the a,@-unsaturated lactone (20) rhodium catalysts give the fly-unsaturated lactone (21).3s (a)Y. Seki A. Hidaka S. Murai and N. Sonoda Angew. Chem. Infernut. Edn. 1977,16 174; (6) Y. Seki S. Murai I. Yamamoto and N. Sonoda ibid. p. 789. 36 P. S. Braterman B. S. Walker and T. H. Robertson J.C.S. Chem. Comm. 1977,651. 37 (a) H. Alper and H. D. Abbayes J. Orgunometullic Chem. 1977,134 Cll; (6) L. Cassar and M. Foa ibid. p. C15. (a)R. Aumann and H. Ring Angew. Chem. Internat. Edn. 1977,16,50; (6) G.D. Annis and S. V. Ley J.C.S. Chem. Comm. 1977,581. A. Stewart D.J. Thompson andM. V. Twigg diene -+ R' R' Rh' R5 R R4 R53 R600 Scheme 3 A new synthesis of a-diketones from aldehydes and alkyl halides using Fe(CO)5 has been reported.39 The aldehyde protected as the ethylenedithioacetal is reac- ted with butyl lithium and Fe(CO)5 to generate the acyltetracarbonylferrate (22) which then reacts with the alkyl halide to give the a-diketone (23) in overall yield of around 60%.Reaction of the organotetracarbonylferrate (24) with Michael-type acceptors (25) gives the expected product (26) in high yield (ca. 900/,).40 Z \ II [R'Fe(CO),]@+ C=C-Z + R'CO*C-C-I%(C0)3 / II (24) (25) iH+ 2=C02Et COR2 CN II R'CO-C-C-Z II (26) The direct formylation and acylation of pyridine has been achieved using Fe(CO)S/PhLi. Depending on the work-up conditions a variety of products can be obtained (Scheme 2)." 6 Reaction of Co-ordinated Ligands Whereas a lot of work has been done on the reactions of diene tricarbonyl-iron complexes much less work has been done on the olefin tetracarbonyl-iron complexes.A report4* has appeared this year however on the nucleophilic attack 39 M. Yamashita and R. Suernitsu J.C.S. Chem. Cornrn. 1977 691. 40 M. P. Cooke jun. and R. M. Parlman J. Amer. Chern. SOC.,1977 99 5222. 41 C. S. Giam and K. Ueno J. Amer. Chern. Soc. 1977.99 3166. 42 B. W. Roberts and J. Wong J.C.S. Chem. Comm. 1977,20. Organometallic Chemistry-Part (i) The Transition Elements Scheme 2 on tetracarbonyliron complexes which give after oxidative work up products of type (27) in good yield. HZC=CHR' +[R2C(C02R3)2]-+ (R302C)2CR2CH2CH2R' I Fe(C0)4 (27) An unusual reaction of a cyclopentadienyl ligand occurs in the reaction of thiobenzophenones (28) with dicarbonylcyclopentadienyliron anion.43 The two react together to give the fulvene (29) in yields of up to 82%.This cleavage of a cyclopentadienyl ligand followed by desulphurization gives a mild and potentially useful route to fulvenes. DR Ra -0. c=s +[@F,,c*,.]-Q R (28) (29) R The use of [~'-C5H5Fe(CO)2](Fp) complexes in organic chemistry continues this year with the report of a new synthesis of ,El-la~tarns.~~ Nucleophilic addition of benzylamine to the complex (30) gives the intermediate (31) which is oxidized at -78°C with chlorine to give the ,El-lactam (32) in ca. 34% overall yield. The reaction is stereospecific e.g. trans-2-butene (30; R' R3=Me; R2=H) gave only cis-3,4-dimethylazelidinone(32; R' R3=Me; R2=H).The reaction can also be applied to the synthesis of fused-ring p -1actams. (30) PhCH2' (31) Fp =[T~-C~H~F~(CO)~] 43 H. Alper and H. N. Paik J.C.S. Chem. Comm. 1977 126. 44 P. K. Wong,M. Madhavarao D. F. Marten andM. Rosenblum J. Amer. Chem. SOC.,1977 99,2823. A. Stewart D. J. Thompson and M. V. Twigg Carbanion attack on v-anisole and v-toluene chromium tricarbonyl complexes (33) gives after oxidative work-up rnetu-substituted aromatics as the major pro- With the v-anisole complex the metu-substituted product is obtained with greater than 90% selectivity. The ?r-toluene complex gives mainly the metu- substituted product but with some ortho-substituted product. (33) Intramolecular carbanion attack on arene chromium tricarbonyl complexes followed by oxidative work up leads to the formation of the bicyclic product (34) in good yield [(89'/0) for n = 3].46 By varying the conditions (long reaction time acid work up) spiro compounds e.g.(35) can be obtained in high yield. The use of carbene-chromium complexes in organic synthesis is slowly increas- ing. Reaction of the carbene complex (36) with alkynes occurs in a stereoselective way to give the substituted a-naphthol chromium tricarbonyl complex (37) which can be readily oxidized to the corresponding 1,4-naphthoq~inone.~' The lithium enolate of cyclopentanone reacts with the vinyl-carbene complex (38) to give the complex (39).48 Oxidative work up gives (40) whereas (41) is obtained by treatment of (39) with pyridine.The cobaltacyclopentanone complex (42) reacts with isocyanates to give 2-0x0- 1,2,-dihydropyridines (43) in about 70% yield.49 With unsymmetrical complexes e.g. (42; R' R3=C02Me; R3 R4=C6H5) reaction proceeds regiospecifically to afford only one product (43; R' R3=COzMe; R3,R4=C6&). 45 M. F. Semmelhack and G. Clark J. Amer. Chem. Soc. 1977,99 1675. 46 M. F. Semmelhack Y.Thebtaranonth and L. Keller J. Amer. Chem. Soc. 1977 99 959. 47 K. H. Dotz and R. Dietz Chem. Ber. 1977,110 1555. 48 C. P. Casey and W. R. Brunsvold Inorg. Chem. 1977,16 391. 49 P. Hong and H. Yamazaki Synthesis 1977 SO. Organometallic Chemistry-Part (i) The Transition Elements 0-0 (CO)5Cr5 OMe (39) (38) 82% 0 0 (42) (43) There have been a number of reportsSo this year on the reactions of the organo- manganese complexes (44) which are prepared by reaction of organolithium or organomagnesium compounds with Mn12.These complexes react with a variety of acid chlorides to give the corresponding ketone (Scheme 3).50aThis reaction is very selective no alcoholic bi-products being formed. The ketones formed in this reaction do react further but at a very much slower rate. Aldehydes react much faster than ketones and at low temperatures the organomanganese reagent will selectively attack the aldehyde group even in the presence of an unprotected ketone (Scheme 3).50b The manganese complex (44) reacts with ethyl chloro- formate to the corresponding alkylated ethyl ester (Scheme 3).'OC CIC02Et R1CO.Cl RC02Et RMnI RCO-R' 80% 90% (44) 78% MeCO*(CH2)3CHO I MeCO.(CH&CHOH.R Scheme 3 Organozirconium complexes (45) which are produced by hydrozirconation of olefins or acetylenes using [v5-C5H5Zr(H)Cl] react with aluminium chloride to generate the corresponding organoaluminium dichloride (45)'l i.e.a 50 (a)G. Cahiez D. Bernard and J. F. Norrnant Synthesis 1977 130; (b)G. Cahiez and J. F. Norrnant Tetrahedron Letters 1977 3383; (c) G. Cahiez and J. F. Normant Bull. SOC.Chim. France 1977 570. D. B. Carr and J. Schwartz J. Amer. Chem. SOC.,1977 99 638. 132 A. Stewart D. J. Thompson and M. V. Twigg transmetallation reaction from zirconium to aluminium. The reaction proceeds well only for primary saturated alkyl and alkenyl aluminium dichlorides and the products (46) can be used as mild alkylating agents.[v5-C~Hs]2Zr(R)C1 +[RAIC12], +AlC13 [q5-C5H5]2ZrC12 0°C (45) (46) The alkenylzirconium complex (47) can be coupled with aryl halides in the presence of a catalytic amount of Ni(PPh3)4.52 Yields are generally excellent and the stereochemistry of the product is >98%E. The reaction can tolerate certain functional groups in particular oxy-functional groups that are incompatible with hydroaluminations but it does not work so well for internal alkynes. c1’ (47) 7 Ofefin Metathesis Interest in olefin metathesis continues unabated with the number of papers pub- lished in 1977 being similar to that during the previous year. Some twenty papers were presented at an ‘International Symposium on Metathesis’ held in Noordwij- kerhout Holland.53 Like last year more work was orientated towards mechanistic aspects than to direct application in synthetic organic chemistry.A further exten- sive review has a~peared.~ Mechanistic Studies.-Good evidence for the formation of carbenoid species in metathesis systems comes from the formation of methane and ethylene on mixing [WCI6] with Sn(CH,), or [MO(PP~~)~(NO)~C~~] with A12(CH3)3C13 two typical metathesis catalysts. Production of methane is attributed to metal-carbene forma- tion and ethylene from its dimerization the latter being a possible termination step in the chain mechanism CH,-M‘ CH,-M L,M -L,M-CH3 ___+ LnM=CH2+CH4 2L,M=CH2 -+ CHZ=CH2 When deca-2,8-diene was added propene was the initial product.This and pro- ducts from deuteriated reagents are in accord with the proposed mechanism.54 The results of an extensive examinatior~~~ of product ratio and kinetics of meta- thesis of cyclic and acyclic olefins are also in agreement with the metal carbene chain mechanism as are results of studies on the stereochemistry of metathesis. ” E. Negishi and D. E. Van Horn J. Amer. Chem. SOC.,1977,99 3168. 53 For details see Rec. Trav. Chim. 1977 96 M1-M144. 54 R. H. Grubbs and C. R. Hoppin J.C.S. Chem. Comm. 1977,634. ” T. J. Katz and J. McGinnis J. Amer. Chem. SOC.,1977 99 1903. Organometallic Chemistry -Part (i) The Transition Elements 133 Stereospecificity of the metathesis of cis- and trans-pent-2-ene has been consi- dered in terms of steric interactions in the metallocyclobutane intermediate by two groups of ~orkers.~~.~~ The use of [W(CO),CPh,] as initiator for the metathesis of cis-pent-2-ene leads to much higher yields of cis-products than when the more conventional tungsten catalyst systems containing Lewis acids are employed.It is suggested that low stereoselectivity may result from the Lewis acid facilitating carbon-metal bond cleavage in the metallocyclobutane and formation of a metal- lopropyl cation which would allow bond r~tation.’~ Gassman and have been able to generate significant concentrations of ‘M=CHR’ carbene species in the presence of ‘M=CH2’ by conducting the meta- thesis of a terminal olefin in the presence of ethylcyclopropane.These authors attribute rapid degenerate metathesis of terminal olefins to efficient highly selec- tive capture of ‘M=CHR’ intermediates to which ‘M-CHR’ is a major resonance contributor. The now generally accepted chain mechanism accounts for almost all experi- mental observations but Mango59 has pointed out that given the known free energies of olefins and cyclopropanes the absence of the latter as an olefin meta- thesis product appears to be in conflict with this mechanism. It may be expected that this point will be debated further during 1978. Applications.-The main reason for the limited application of catalytic olefin metathesis in organic synthesis is due to the strong inhibiting effect of many common functional groups.It is therefore of interest that olefins with an amino- group that normally do not undergo metathesis do so when the donor properties of the amine are destroyed by quaternization.60 A convenient heterogeneous catalyst system of rhenium heptoxide on alumina promoted by a small amount of tetramethyl tin is a catalyst for metathesis of methyl penta-4-enoate in CCl,. After an hour the clean conversion of reactant to the diester (48) was 5 1% .61 Similar catalysts have also been used62 for the metathesis 2CH2=CH(CH2)2C02Me$ Me02C(CH2)2CH=CH(CH2)2C02Me+ CH2=CH2 of cis-1-chloro-octadec-9-ene with trans -dec-5-ene which after five hours afforded a mixture of cis- and trans-14-chlorotetradec-5-enein 70% yield. Under similar conditions allylchloride and vinyl chloride failed to react with hex-1-ene.The metathesis of a series of w -0lefinic esters catalysed by WCI6/Sn(CH3) gave the thermodynamic mixture of the cis- and trans-diesters (49). As well as the expected ethylene the chloroester (50) was formed in low yield by the addition of R02C(CH2) CH=CH(CH2),COZR CH3CHCI(CH2),CO2R (49) (50) s6 J. L. Bilhou J. M. Basset R. Mutin and W. F. Graydon J. Amer. Chem. SOC.,1977,99,4083. 57 T. J. Katz and W. H. Hersh Tetrahedron Letters 1977 585. 58 P. G. Gassman and T. H. Johnson J. Amer. Chem. SOC.,1977,99 622. 59 F. G. Mango J. Amer. Chem. SOC.,1977,99,6117. ‘’J. P. Laval A. Lattes R. Mutin and J. M. Basset J.C.S. Chem. Comm. 1977 502. 61 E. Verkuijlen F. Kapteijn J. C. Mol and C. Boelhouwer J.C.S. Chem. Comm. 1977 198.62 R. Nakamura and E. Echigoya Chem. Letters 1977 1227. A. Stewart D. J. Thompson andM. V. Twigg HC1 (derived from the solvent chlorobenzene) across the double bond of the react ant ole fin. 63 It is of considerable interest that complex (51; M=Rh or Ir) which is reminiscent of the proposed metallocyclobutane metathesis intermediate is a powerful catalyst for 1,1,3,3-tetramethyldisiloxane disproportionation into dimethylsiloxane and higher siloxane 01igomers.~~ Here strong Si-0 bonds are broken and reformed under unusually mild conditions and this reaction may provide a route to novel silicone polymers. Ph3P\h/CO !%(Me),\o Ph3P/I\/ H Si(Me) 8 Use of Metal Cluster Complexes in Catalysis The synthesis and characterization of new metal cluster complexes continues and their possible application as catalysts for organic reactions is an area of growing interest.' A number of studies have been concerned with the catalysis of reactions of small molecules such as the methanation of carbon monoxide which are of industrial importance and are at present catalysed by conventional heterogeneous supported metal catalysts.It is possible that mechanistic information for reactions catalysed by metal clusters may lead to an improved understanding of reactions on metal surfaces. Following their work on the slow homogeneous methanation of carbon monox- ide catalysed by [0s3(Cc?),,] or [Ir4(CO)12] in toluene Muetterties and co-workers that [Ir4(CO)12] in molten NaC1,2A1Cl3 converts carbon monoxide and hydrogen to methane and ethane with only minor quantities of other hydrocarbons.At 180"C this relatively fast reaction is homogeneous but unlike conventional Fischer-Tropsch syntheses has the potential of product selectivity. It is appropriate to note a curious reaction of (52) {formed from [ZT(C~H~)~C~,] and AlH'Buz} ,H -AIBu (CsHd2Zr -H \/C1 'H -A1Bu2 (52) which in benzene absorbs two equivalents of carbon monoxide under ambient conditions to give a complex mixture of aluminium alkyls that does not contain simple aluminium alkoxide. On acid hydrolysis linear alcohols in a ratio compar- able to that of a Fischer-Tropsch hydrocarbon synthesis are obtained. The mechanism of this process is thought to be complex involving reduction of carbon monoxide co-ordinated to zirconium insertion and transmetallation of organo-group from zirconium to aluminium.66 " R.Baker and M. J. Crimmin Tetrahedron Letrers 1977,441. 64 J. Greene and M. D. Curtis J. Amer. Chem. SOC.,1977,99,5176. " G. C. Demitras and E. L. Muetterties J. Amer. Chem. Soc. 1977,99 2796. ''L. I. Shoer and J. Schwartz J. Amer. Chem. SOC.,1977,99,5831. Organometallic Chemistry -Part (i) The Transition Elements It is not surprising that (53) and (54) are active catalysts for the hydroformylation of pent- 1-ene and ~ent-2-ene.‘~ During the reaction cluster dissociation does not appear to take place and reaction conditions are milder than those used for the more usual CO~(CO)~ catalyst. These easily prepared clusters are air stable in the solid and in solution and offer the advantage that they can be moderately selective for terminal products.Moreover it is possible to recover the clusters in high yield after reaction. (c0)2c0-=‘CO(CO), ’-’P /fkp0 (CO),Co -R-Co(CO) o=c \\// \C&O) (CO),CO-QFPC 0(co’2 R (53) (54) Carbon monoxide inhibits the isomerization of pent-1-ene and cis-pent-2-ene and [H4R~4(C0)12], catalysed by [H2R~4(C0)13] and the results of labelling experiments suggest the presence of 0-alkyl intermediates. With [HRU3(CO&,Hg] as catalyst a further isomerization path is operative that appears to involve a r-ally1 intermediate.68 The cluster [Ni4{CNC(CH3)3}7] is readily prepared from t-butylisocyanide and [Ni(COD),] but has the disadvantage of being markedly air-sensitive.It is however an active catalyst for a variety of reactions.69 Acetylene is smoothly trimerized to benzene although the corresponding reaction of dialkylacetylenes is slow. Oli-gomerization of butadiene takes place stereoselectively to cyclo-octa- 1,5 -diene in the presence of this cluster which maintains high activity over an extended period. While inactive for the hydrogenation of olefins terminal and internal acetylenes are cleanly hydrogenated to the corresponding cis-olefin without alkane formation. However cluster decomposition products do catalyse olefin hydrogenation. An investigation7’ of the homogeneous methanation of carbon monoxide is the presence of [Ti(C5H5)2(CO)2] showed that the reaction is stoicheiometric and not catalytic in titanium.Methane is produced under an atmosphere of pure hydrogen and a blue solution of the first ‘M~(CSH~)~’ cluster [Ti6(C5HS),O8] is obtained. ‘’I R. C. Ryan C. U. Pittman jun. and J. P. O’Connor J. Amer. Chem. SOC., 1977,99 1986. ‘* G. A. Vaglio D. Osella and M. Valle TransitionMet. Chem. 1977 2 94. 69 M. G. Thomas W. R. Pretzer B. F. Beier F. J. Hirsekorn and E. L. Muetterties J. Amer. Chem. SOC. 1977,99,743. ’O J. C. Huffman J. G. Stone W. C. Krusell and K. G. Caulton. J. Amer. Chem. Soc. 1977,99 5829.