6 Organometal tic Chemistry Part (i)The Transition Elements By H. M. COLQUHOUN and J. HOLTON ICI Corporate Laboratory The Heath Runcorn Cheshire WA7 4QE and M.V. TWlGG ICI Agricultural Division Billingham Cleveland TS23 1LD 1 Introduction This Report deals with applications of transition-metal species in organic synthesis. During 1978 a high level of interest has been maintained and the discovery of useful new procedures continues. There are a large number of publications in this area and in general references have been selected for their practical significance. A number of relevant reviews have been published and some of the more important are given below. A general review on asymmetric synthesis illustrates the growing r81e of organo-transition-metal species in the preparation of optically active compounds.’ Use of metal clusters in catalysis another growth area has been reviewed.2 Reviews dealing with cobalt-catalysed syntheses of pyridines from alkynes and nit rile^,^^ transition-metal 7r-complexes of heterocyclic and reactions of 0-bonded organometallic complexes with ele~trophiles~‘ have also been published.A particularly readable article on mechanistic aspects of olefin metathesis a~peared,~ and several conference reports contain much relevant information.’ 2 Metal-catalysed Hydrogenation and Hydrogen-transfer Reactions Selective reduction of ap-unsaturated carbonyl compounds has been achieved using iridium and lanthanide catalysts. a$-Unsaturated aldehydes are reduced to unsaturated alcohols by the hydridoiridium sulphoxide catalyst [Ir(H)C12(Me2S0)J in propan-2-01 the solvent being the source of hydrogen.6 Under mild conditions ’ D.Valentine and J. W. Scott Synthesis 1978,329; see also A. Nakamura Pure Appl. Chem. 1978,50 37. * C. U. Pittman and R. C. Ryan Chemtech. 1978,170. (a)H. Bonnemann Angew. Chem. Internat. Edn. 1978,17,505;(b)K. H. Pannell B. L. Kalsotra and C. Parkanyi,J. Heterocyclic Chem. 1978,15,1057; (c) M. D. Johnson Accounts Chem. Res. 1978,11,57. T. J. Katz Ado. Organometallic Chem. 1977 16 283. ’‘The Place of Transition Metals in Organic Synthesis’ ed. D. W. Slocum Ann. New York Acad. Sci. 1977 295; ‘Fundamental Research in Homogeneous Catalysis’ ed. M. Tsutsui and R. Ugo Plenum Press London 1977;‘Eighth International Conference on Organometallic Chemistry’ Pure Appl.Chem. 1978 50 677. B. R. James and R.H. Morris J.C.S. Chem. Comm. 1978.929. 97 H. M. Colquhoun J. Holton and M. V. Twigg 90% conversion with >80% selectivity was achieved with cinnamaldehyde a-methylcinnamaldehyde and crotonaldehyde. This may be compared with the rhodium system [RhC1(C0)J2-tertiary amine which gave only 50% selectivity with crotonaldehyde. The reasons for preferential reduction of the carbonyl function are thought to be electronic rather than steric even though reduction of the olefinic bond is observed with unsaturated ketones. Iridium is generally considered to be a less effective hydrogenation catalyst than rhodium and the other platinum metals but this and other reports7 suggest this may not always be the case.ap-Unsaturated ketones are selectively reduced to allylic alcohols using lanthanide chlorides (e.g. samarium chloride hexahydrate) with sodium boro- hydride.* The equimolar reactions take place at room temperature without exclusion of air or moisture giving excellent yields of product (>90%) within minutes. Nearly exclusive selective carbonyl reduction is obtained under conditions which do not affect functional groups such as carboxy- ester and nitro-groups. Linear and branched aliphatic as well as aromatic aldehydes are effectively hydrogenated by molecular hydrogen in the presence of [RuC12(CO),(PPh3),] to the corresponding alcohol.' High catalyst activities and turnover numbers up to 95 000 are observed with yields up to 99%.The facile reduction of substituted benzyl alcohols and styrenes is reported using hydrogen transfer from cyclohexene catalysed by Pd/C and AlCl3.l0* Other hydro- gen donors such as limonene or tetralin may be used. This method offers an alternative to catalytic hydrogenation replacing hydrogen gas by cyclohexene as the hydrogen source. The same catalyst system also reductively cleaves benzylic ethers to give the corresponding alcohols and aryl-alkanes.'Ob Due to the mild conditions used the reduction can be selective; for instance in cholesteryl benzyl ether the isolated double bond is not reduced. The first example of hydrogenation of nitro and nitrile groups using a Pd" catalyst has been reported." PdC12 supported on derivatized polystyrene (1)hydrogenates 0 (1) nitrobenzene to aniline (97%),and the more difficult to hydrogenate benzonitrile to N-benzylbenzamidine (40%)and a-(benzylideneamino)toluene (20%).In general Pd" has only a limited range of hydrogenation activity and this system represents a significant departure from previous Pd chemistry. 'R. H. Crabtree Plutinum Met. Rev. 1978 22 126; W. Strohmeier H. Steigerwalds and M. Lukacs J. Organometullic Chem. 1978 144 135. ' J -L. Luche J. Amer. Chem. SOC. 1978,100,2226. W.Strohmeier and L. Weigelt J. Organometallic Chem. 1978 145 189. lo (a)G. A. Olah and G. K. S. Prakash Synthesis 1978 397; (6)G. A. Olah G. K. S. Prakash and S. C. Narang ibid. p. 825. N. L. Holy J.C.S. Chem. Comm.. 1978 1074. Organometallic Chemistry-Part (i) The Transition Elements 99 A new homogeneous catalyst for the reduction of both benzene and olefins has been reported.[Ru(H)C~(~~~-C~M~~)PP~~] is a stable long-lived catalyst for the hydrogenation of benzene to cyclohexane under mild conditions. l2 No cyclo-hexadienes or cyclohexene were detected suggesting that the hydrocarbon remains strongly co-ordinated to the metal centre throughout all the hydrogenation steps. Olefins are also catalytically reduced and using transfer hydrogenation from secondary alcohols both olefins and diolefins may be reduced with this ruthenium catalyst. Other transfer hydrogenation catalysts [RhH(PPh3L] and [RuH2(PPh3)J will not reduce diolefins. The scope of arene hydrogenation using [Co( T-C~H~){P(OM~)~}~] has been studied using a large selection of substrates.l3 Catalytic hydrogenation has been demonstrated for benzenes with substituent groups that include R OR C02R and NR2 but electron-withdrawing groups e.g. halogen NO2,and CN inhibit the reduction. The matching of ligands to substrate is now an established part of asymmetric hydrogenation and therefore only those ligands of special interest are included in this year's review. One such ligand is (R)-prophos [(R)-l,2-bis(diphenyl- phosphino)propane] (2).14 Whereas the tendancy has been for more complexity in ligand designs this simple ligand has only a single methyl group at a chiral centre to constrain the chirality of the chelate ring. Even so the (R)-prophos-rhodium(1) system [Rh{(R)-prophos)(norbornadiene)] Clod is an excellent catalyst for producing optically active (S)-amino-acids from (2)-a-acylaminoacrylic acids (3).H3C H C02R2 \/ H&-CH2 c=c Ph2P/\PPh2 R3''NHCOR' (2) (3) Optical yields of ca. 90% are obtained and appear to be insensitive to the nature of the substituent on the substrate. Hence neither changes at the amine function (R' =Me or Ph) a change of the acid to an ester (R2 =H or Et) nor the nature of the @-vinyl substituent (R3=e.g. H Ph Pr' or p-hydroxy phenyl) affects the optical yield. Of further interest is that the catalyst is capable of breeding its own chirality. Readily available ethyl pyruvate may be converted into its enol acetate which is rapidly hydrogenated by the (R)-prophos catalyst to ethyl (S)-0-acetyl-lactate in 81% optical purity.By standard procedures the lactate is easily converted into (R)-prophos and hence a small amount of (R)-prophos can produce much larger amounts of itself. (S)-Prophos was prepared in an analogous manner and as may be predicted produces (R)-amino-acids. Further studies have appeared on the use of pyrrolidine-phosphine-rhodium systems in asymmetric hydrogenation. Substrates investigated include @-acetyl-aminoacrylic acid derivatives"" (optical yield C 55%) itaconic (optical yield 12 M. A. Bennett T. Huang A. K. Smith and T. W. Turney J.C.S. Chem. Comm. 1978 582. l3 L. S. Stuhl M. Rakowski Du Bois F. J. Hirsekorn J. R. Bleeke A. E. Stevens and E. L. Muetterties J. Amer. Chem. SOC.,1978,100 2405.l4 M. D. Fryzuk and B. Bosnich J. Amer. Chem. SOC., 1978,100,5491. l5 (u)K. Achiwa and T. Soga Tetrahedron Letters 1978 1119; (b)K. Achiwa ibid. p. 1475;I. Ojima T. Kogure and K. Achiwa Chem. Letters 1978 567; K. Achiwa ibid. p. 561; (c) I. Ojima T. Kogure T. Terasaki and K. Achiwa J. Org. Chem. 1978,43,3444; (d)K. Achiwa Tetrahedron Letters 1978,2583; (e)K. Achiwa Chem. Letters 1978 905. H. M. Colquhoun,J. Holton and M. V. Twigg >go%) ketopantoyl lactone 15' (optical yield 85%),and 2-acetoamido-3-methyl- fumaric acid ester 15d (optical yield -55%). To help overcome problems with catalyst separation from the reaction mixture a supported pyrrolidine-phosphine- rhodium system has been prepared.15= The first examples of chelating chiral ligands derived from a sugar have been reported.Diphosphinites synthesized from (a) D-glucose,'6"*b(6) D-galactose,16b and (c) 1,6-anhydro-~-glucose," in rhodium systems gave optical yields of up to 80% in the asymmetric hydrogenation of a-acetamidoacrylic acid derivatives. The latter (c) required addition of a base triethylamine to achieve high optical yields. A detailed investigation of the mechanism of asymmetric hydrogenation has been performed by a 31P n.m.r. spectroscopic study of rhodium(1) cationic complexes under experimental conditions." These systems with monophosphine ligands were thought to operate by dihydride formation followed by olefin complexation. Results suggest that the actual course of reaction is critically dependent on the structure of the phosphine and that dihydride formation may not always precede olefin complexation.With chelating phosphines it is thought that olefin complexation occurs first. A study of the asymmetric hydrogenation of a-benzamidocinnamic acid using a rhodium(1)-( +)-diop complex leads to the proposal that the stereochemistry is defined in an olefin-binding step via a square-planar chelate intermediate (4). The H H (4) subsequent addition of hydrogen is thought to have only a minor irfluence on the optical yield. 3 Dimerization Oligomerization and Polymerization Although the field of olefin polymerization is characterized by a large volume of experimental results little is known about the nature of active centres or the detailed mechanism.Three papers attempt to correct this situation. Fully characterized Group 3A and lanthanoid metal complexes [M(q-C5H4R)*MeI2 (M =Y Er or Yb) and [M(q-C5H4R)2Me2AlMe2] (M =Y Er Ho or Yb) are homogeneous ethylene polymerization catalysts. l8 A deactivation process involving abstraction of a cyclopentadienyl hydrogen was identified and could be suppressed by using peralkylated derivatives. Comparisons between these catalysts support the suggestion that the cocatalyst species (AlMe,) in Ziegler-Natta catalysis l6 (a)W. R.Cullen and Y. Sugi Tetrahedron Letters 1978 1635; (b)R.Jackson and D. J. Thompson J. Orgunometallic Chem. 1978,159 C29; (c)G. Descotes D. Lafont and D. Sinou ibid. 1978,150,C14. " (a)J. M. Brown and P. A. Chaloner,J.C.S. Chem. Cumm. 1978,321; (b)J.M. Brown P. A. Chaloner and P. N. Nicholson ibid. p. 646; (c)J. M.Brown and P. A. Chaloner Tetrahedron Letters 1978 1877. '*D. G. H. Ballard A. Courtis J. Holton J. McMeeking and R.Pearce J.C.S. Chem. Cumm. 1978,994. Organometallic Chemistry-Part (i) The Transition Elements serves both to alkylate M-Cl and to stabilize co-ordinately unsaturated active centres via alkyl bridges. The reaction mixture of a Nio complex plus the ylide Ph3P=CHCOPh has been described in patent literature as an ethylene oligomeriza- tion catalyst. Complex (5) has been isolated from this mixture and fully charac- terized (X-ray)." This species exhibits an interesting solvent effect. In toluene oligomeric <C30 linear n-olefins are formed whereas a suspension of (5) in hexane produces high-molecular-weight linear polyethylene.(5) A new mechanism for stereospecific olefin polymerization by Ziegler-Natta catalysts has been proposed.20 The key steps involve elimination of an a-hydrogen to form a metal-hydrido-carbene species olefin co-ordination and rearrangement with formation of a metallocyclobutane (Scheme 1). Although the mechanism appears Ill 'CHM~ \ .eMe \ C' CH e-/\ H+W\ /CH2 FC< H Me C. "Me H Scheme 1 very complex (compared with the Cosse mechanism) for what is a very rapid process of polymerization this scheme does present a common mechanism for both olefin polymerization and metathesis. The catalysis of both processes by a number of systems is now simply explained. If the proposed hydrogen-transfer step is slow or if there is a means of removing hydrogen from the metal centre then the catalyst becomes a metathesis catalyst.The Cosse mechanism with slight adaptations has stood the test of time and it will be interesting to see how this new mechanism fares and whether evidence can be found to substantiate what is certainly a novel theory. The metallocyclopentane [Ta(q -C,H,)Cl2C4H8] is a selective catalyst for dimerization of ethylene to but-l-ene.21 This represents a rare example of a w.Keim F. H. Kowaldt R. Goddard and C. Kriiger Angew. Chem. Internat. Edn. 1978,17,466. (a)K. J. Ivin J. J. Rooney C. D. Stewart M. L. H. Green and R. Mahtab J.C.S. Chem. Comm. 1978 604; (b)K. J. Ivin J. J. Rooney and C. D. Stewart ibid. p.603. S. J. McLain and R. R. Schrock J. Amer. Chem. Soc. 1978,100,1315. H. M. Colquhoun J. Holton and M. V.Twigg high-oxidation-state metal complex being involved in olefin oligomerization. Curiously niobium is inactive as a dimerization catalyst; normally second-row metals are better catalysts than their third-row counterparts. The nickel alkoxide system [N~(OR)(V~-C~H~)PP~(NE~~)~] (R= n-CltHZ3 or n-C15H31) oligomerizes isoprene to give up to 60% of the linear trimer trans-p-farnesene (6).22The crude product may then be used to synthesize sesqui- and di-terpenoids in good overall yield based on isoprene. (6) The recently prepared complexes [M(cod),] (M = Pd or Pt) catalyse the dimeriza- tion and telomerization of butadiene at lower temperatury and more selectively than previous cataly~ts.’~ The [Pd(~od)~]-catalysed reactions of butadiene with secondary amines preferably cyclic amines (e.g.morpholine piperidine) afford octa-2,7- dienylamines as pure isomers. With added phosphine this system catalyses the addition of acetaldehyde to butadiene giving 2-methyl-3,6-divinyltetrahydropyran (7) (33% yield) and the addition of phenyl isocyanate gives an isomeric mixture of piperidones (8)and (9). (7) (8) (9) A number of groups have described the telomerization of butadiene in the presence of C02 leading to interesting carboxylated products. Japanese workers using [Pd(Ph2PCH2CH2PPh2)J as catalyst obtained 5.4% of 2-ethylidenehept-5- en-4-olide (lo),but the bulk of the product was butadiene linear dimer.24 However Italian workers using Pdo catalysts derived from non-chelating phosphines report much higher levels of COz incorporation giving three products [(1l),(12) and (13)].The ratio in which (1 1) and (12) are formed depends on the steric requirement of the phosphine ligand bulky phosphines apparently favouring compound (1l),and the overall reaction is markedly accelerated by traces of 22 S. Akutagawa T. Taketomi H. Kumobayashi K. Takayama T. Someya and S. Otsuka Bull. Chem. SOC. Japan 1978 51 1158. 23 M. Green G. Scholes and F. G. A. Stone J.C.S. Dalton 1978 309. 24 Y.Inoue Y. Sasaki and H. Hashimoto Bull. Chem. SOC.Japan 1978,51,2375. ‘’ A. Musco C. Perego and V. Tartiari Inorg. Chim. Acta 1978,28 L147. Organometallic Chemistry-Part (i) The Transition Elements (13) A series of stoicheiometric insertion reactions of CO with ally1 nickel complexes has been described.26 A number of products were characterised by X-ray crystall- ography revealing a strong tendency for such compounds to form polynuclear aggregates [Equation (l)].Me,P-Ni 0 I I (1) 2c0 0 0 The oligomerization of hex-3-yne by [Ni(cod),] plus PPh2CH2CH2PPh2 in the presence of CO, affords a 57% yield of tetraethyl-2-pyrone (14) but when the Et 0 (14) chelating phosphine is replaced by PPh3 incorporation of CO is not observed only the cyclic trimer pentaethylprop-5-enylcyclopentadienebeing obtained in 66% yield.,’ 4 Carbon Monoxide Chemistry Carbony1ation.-A very mild regiospecific synthesis of hydroxybut-2-enolides (15) involves the cobalt-catalysed carbonylation of alkynes by carbon monoxide and methyl iodide under phase-transfer conditions [(Equation (2)].Yields vary from C02(Co),,CTAB RCrCH + Me1 + CO -& 5MdNaOH PhH M~ OH (15) CTAB = Cetyl trimethylammonium bromide 26 P.W. Jolly S. Stobbe G. Wilke R. Goddard C. Kruger J. C. Sekutowski,and Y. H. Tsay Angew. Chem. Internat. Edn. 1978,17,124. 27 Y.Inoue Y.Itoh and H. Hashimoto Chern. Letters 1978,633. H.M. Colquhoun J. Holton and M. V.Twigg 18% (cyclohexylacetylene) to 68% (17-ethynyltestosterone) and carbonylation of dienes under these conditions affords acetyl dienes in modest yield.28 These reac- tions probably proceed by successive insertions of CO and alkyne or diene into the Me-Co bond of [COM~(CO)~ 3 followed by hydrolysis and subsequent ring closure (alkyne) or dehydration (diene).Aromatic nitroso-compounds are carbonylated to isocyanates in the presence of rhodium and iridium carbonyls. The reaction is facilitated both by bulky ortho-substitutents in the substrate and by the presence of n-acceptor ligands in the The palladium-catalysed amidation of o-halogenophenyl alkylamines (16) provides a facile synthesis of otherwise relatively inaccessible benzolactams (17). Five- six- or even seven-membered lactams are obtained in this way in moderate yield.30 Catalytic decarbonylation of aldehydes under mild conditions has been achieved using cationic rhodium and iridium complexes with chelating phosphine ligand~.~~ Turnover numbers >100 000 are reported for the decarbonylation of benzaldehyde at 178 "C by [Rh{PPh2(CH,)2PPh,}2]+Cl-.Acyl-metal hydrides have previously been proposed as intermediates in such decarbonylation reactions and a stable acyl-rhodium(rI1) hydride complex (18) has been isolated from the reaction between 8-quinoline carboxaldehyde and [RhCl(PPh3)3] (Scheme 2).32 Decarbonylation to +[(PPh3)2Rh(CO)Cl] \/ Scheme 2 quinoline occurs only at elevated temperatures perhaps because the intermediate alkyl contains a strained four-membered chelate ring.H. Alper J. K. Currie and H. Des Abbayes J.C.S. Chem. Comm. 1978,311. 29 K.Unverferth C. Rueger and K. Schwetlick J. Prakt. Chem. 1977,319,841. 30 M. Mori K. Chiba and Y.Ban J. Org. Chem. 1978,43 1684. 31 D.H.Doughty and L. H. Pignolet J. Amer. Chem. SOC.,1978 100,7083. 32 J. W.Suggs J. Amer. Chem. SOC.,1978,100,640. 105 Organometallic Chemistry-Part (i) The Transition Elements The reactions of several relatively stable alkyl-metal carbonyls with hydrogen and carbon monoxide at elevated temperature and pressure have been used as models for intermediates in hydr~formylation.~~ However only [CH,Mn(CO),] could be carbonylated and subsequently hydrogenated to the aldehyde. Interestingly the manganese product was not [HMn(CO)5] but [Mn2(CO)lo]. The role of monophosphine ligands in rhodium-catalysed hydroformylation has been extensively studied but little has been done on the effect of cis-chelating pho~phines.~~ This has been corrected by an extensive study using bidentate phosphines including a polymer-bound rhodium 1,2-bis(diphenylphosphino)ethane complex.Catalysts containing bidentate phosphines are more active than those with PPh3. However selectivity for linear aldehyde is considerably lower and isg- merization activity is much higher. A supported catalyst of high activity is obtained by coating phosphinated polystyrene containing [RhCl(CO),P] onto a high-surface- area silica. Activity is approximately proportional to the surface area of the silica.35 The Reppe modification of hydroformylation uses water in place of molecul r hydrogen. In this process iron pentacarbonyl is a good catalyst although in the normal procedure using molecular hydrogen it is a poor catalyst.This is now rationalized in terms of Scheme 3 where the key intermediate [H2Fe(C0)4] is more easily formed from [Fe(CO)5] and water than from molecular hydrogen.36 Scheme 3 Ruthenium and rhodium carbonyl clusters also catalyse this reaction and it is that cluster rather than monomeric species are the catalytically active intermediates. Ruthenium systems have exceptionally high selectivity for linear aldehydes which subsequently undergo aldol condensation in the basic solution used. This does not happen with the less selective rhodium systems because reduction of the first-formed aldehyde to alcohol takes place rapidly. Carbon monoxide similarly reduces aldehyde to alcohol in the presence of rhodium trichloride. However when triethylamine is added aldol condensation takes place more rapidly than reduction with the result that the major product is the branched alcohol from reduction of the aldol reaction Carbon monoxide and water also reduce aromatic nitro-compounds to amines rhodium iridium osmium and iron carbonyls are effective catalysts in the presence of trieth~larnine.~~' The carbon monoxide-water system catalysed by rhodium trichloride will reduce Schiff bases to form N-alkyl-amine~.~~~ 33 R.B. King A. D. King M. Z. Iqbal and C. C. Frazier J. Amer. Chem. Soc. 1978 100 1687. 34 C. U. Pittman and A. Hiras J. Org. Chem. 1978 43 460. 3s H. Arai J. Catalysis 1978 51 135. 36 H.-C. Kang C. H. Mauldin T. Cole W. Slegeir K. Cann and R. Pettit J Amer. Chem. Soc. 1977,W.8323. 3'7 (a)R. M. Laine J. Amer. Chem. Soc. 1978,100,6451; (b)Y. Watanabe K. Takatsuki. and Y. Takegami Tetrahedron Letters 1978 3369; (c)K. Cann T. Cole W. Slegeir and R. Pettit J. Amer. Chem. Soc. 1978,100,3969; (d)Y. Watanabe M. Yamamoto T.-A. Mitsudo and Y. Takegami TetrakedronLetters 1978,1289. 106 H. M. Colquhoun J. Holton and M. V. Twigg Synthesis Gas Chemistry.-Selective conversion of carbon monoxide and hydrogen into alkanes alcohols etc. is an area of growing interest because it may provide a basis for future large-scale processes based on gasified coals and heavy oils. Surface- bound formyl is thought to be an intermediate in many of these heterogeneously catalysed reactions and it is therefore relevant that a number of metal formyl complexes have now been characteri~ed.~~ Several groups are examining the homogeneously catalysed formation of hydrogen from water and carbon monoxide It is apparent that under appropriate conditions most metal carbonyls will catalyse this reaction and not just those of the precious metals.However in spite of much work this year there remain many mechanistic uncertain tie^.^' In dioxan solutions of [Co2(CO),] at 182"C,carbon monoxide is hydrogenated by molecular hydrogen to methanol some higher alcohols and their formate Under the experimental conditions the major cobalt species is [HCo(CO),]. Ano- ther instance of insertion of carbon monoxide into a metal hydride bond provides the first example of a stable metal-carbon bond from this type of reaction.The reaction of [(C,H,),Zr(H)Cl] with carbon monoxide gives [{(C5H5)2ZrC1}2CH20] in which the CH,O group is C- and O-bonded to different metal atoms. Carbon dioxide reacts with [(C5H5),Zr(H)Cl] to form bound methoxide and formaldehyde as in equations (3) and (4); by keeping CO :Zr <1:3 only methoxide is formed.,' 2(C5H&Zr(H)Cl +COz +[(C5H5)2ZrC1]20+CH20 (3) 3(C5H5)2Zr(H)Cl+C02 +[(C5H5)ZrC1I20 +(C5H5)Zr(OMe)Cl (4) The related compounds [(C,Me,),MMe,] (M =Zr Th or U) react with carbon monoxide to form unusual dicarbonylation products. In the case of M =Th reaction at -80 "C (!)quantitatively affords (19) which has been characterized by X-ray Me Me \/ /c=c\ 0 0 '0 0-\/ c=c Me/\ Me (19) Homologation of dimethyl ether to ethyl acetate by reaction with carbon monox- ide and hydrogen is catalysed by [RuI,(CO),] and iodide.Contrary to what is D. A. Slack D. L. Egglestone and M. C. Baird J. Organometallic Chem. 1978 146,71; J. A. Gladysz and J. C. Selover Tetrahedron Letters 1978,319; C. P. Casey and S. M. Neumann J. Amer. Chem. SOC. 1978 100,2544; J. A. Gladysz and W. Tam ibid. p. 2545; J. A. Gladysz and J. H. Merrifield Inorg. Chim. Ada 1978 30 L317. 39 R. B. King C. C. Frazier R. M. Hanes and A. D. King J. Amer. Chem. SOC.,1978 100,2925; T. Yoshida Y. Ueda and S. Otsuka ibid.,p. 3941; P. C. Ford R. G. Rinker C. Ungermann R. M. Laine V. Landis and S. A. Moya ibid. p. 4595 C.-H. Cheng and R. Eisenberg ibid. p. 5968. 40 J. W. Rathke and H. M. Feder J. Amer. Chem.SOC. 1978,100,3623. 41 G. Fachinetti C. Floriani A. Roselli and S. Pucci J.C.S. Chem. Comm. 1978,269. 42 J. M. Manriquez D. R. McAlister R. D. Sanner and J. E. Bercaw I. Amer. Chem. SOC. 1978,100,2716; J. M. Manriquez P. J. Fagan T. J. Marks C. S. Day and V. W. Day ibid.,p. 71 12. Organometallic Chemistry-Part (i) The Transition Elements observed in related systems methyl iodide does not seem to be involved in this homologation and it appears that protonated dimethyl ether reacts with an anionic ruthenium complex to form a methyl-ruthenium intermediate.43 5 Reactions of Co-ordinated Ligands Fischer-type carbene complexes such as [Cr(CO),(MeCOMe) J react with iso- cyanides to give ketenimine complexes which liberate free ketenimines on treat- ment with excess isocyanide.Reactions of the latter complexes with acids methanol and water produce a variety of aminocarbene-chromium derivative~.~~ Methoxycarbene-chromium complexes are known to react with nucleophilic alkynes such as ynamines or ynediamines and it has now been shown4' that the reaction product of [Cr(CO),(PhCOMe)] with Et,NCrCNEt undergoes decar- bonylation and rearrangement on heating to 125 "C to give (20) the chromium tricarbonyl complex of a functionalized indene (Scheme 4). H OMe OMe Scheme 4 Arene ligands may be displaced from chromium tricarbonyl by refluxing in pyridine the complex [Cr(CO)3(py)s J being recovered in high yield.46 This pyridine derivative may itself be used for the synthesis of arene-Cr(CO) complexes and so the potential usefulness of such compounds in organic synthesis is significantly increased.The propargylation of carbanions by cationic propargyl-Co,(CO) complexes gives high yields by avoiding the formation of the allenic elimination and other unwanted by-products produced when propargyl halides or tosylates are used in such reactions. Oxidative demetallation of the products with iron(II1) affords almost quantitative yields of free alk~ne.~' Dimethyl acetylenedicarboxylate reacts with cum-dodecatrienediylnickel (21) to give after demetallation with CO a mixture of 12- and 14-membered-ring products 43 G. Braca G. Sbrana G. Valentini G. Andrich and G. Gregorio J. Amer. Chem. Soc. 1978,100,6238. 44 C.G.Kreiter and R. Aumann Chem. Ber. 1978,111 1223.45 K. H.Dotz and D. Neugebauer Angew. Chem. Internat. Edn. 1978,17,851. 46 G. Carganico P. D. Buttero S.Maiorana and G. Riccardi J.C.S. Chem. Comm. 1978,989. 47 H. D.Hodes and K. M. Nicholas Tetrahedron Letters 1978,4349. H. M. Colquhoun J. Holton and M. V.Twigg in proportions which vary according to the reaction tem~erature.~~ The correspond- ing reaction with methyl propiolate however gives only the 12-membered product (22) regardless of temperature. Phosphine-induced ring-opening of penta-arylcyclobutenylpalladium(rr) complexes proceeds stereospecifically in the expected conrotatory manner to give cr-butadienyl complexes. Cyclobutenylpalladium thiocarbamates however also undergo spontaneous ring-opening to give a mixture of conrotatory and disrotatory ring-opened cr,r-butadienyl derivatives.A mechanism involving the equilibration of these two isomers via a metallocyclopentenyl intermediate has been proposed to account for the appearance of the thermally forbidden disrotatory ring-opened product .49 The synthetic possibilities of homogeneous C-H bond activation continue to be of interest. The reaction of bis(ethylene)(q5-indenyl)rhodium(~) with but-2-yne affords two crystalline dinuclear complexes [(23) and (24)] in one of which an ethylenic C-H bond has been cleaved to generate a vinyl-bridged dirhodium species (24). This complex [but not (23)] is an effective catalyst for alkyne trimerization. It is also readily carbonylated to give dicarbonyl (q’-indeny1)rhodium and the a@-unsaturated ketone (25).” 6 Asymmetric Synthesis of Carbon-Carbon Bonds Catalytic methods are now well established for the creation of chirality by the synthesis of C-H bonds with optical yields of not uncommon.Correspond- ing reactions to form C-C bonds have been much less successful in the past but a number of promising reports have appeared this year. 48 R. Baker P. C. Bevan R. C. Cookson A. H. Copeland and A. D. Gribble J.C.S. Perkin I 1978,480. 49 S.H.Taylor and P.M. Maitlis J.Amer. Chem.SOC.,1978,100,4700; P.M. Bailey S. H. Taylor and P. M. Maitlis ibid. p. 4711. 50 P.Caddy M. Green L. E. Smart and N. White J.C.S. Chem. Comm. 1978 839. Organometallic Chemistry-Part (i) The Transition Elements 109 Nakamura and co-workers have described extensive synthetic and mechanistic studies of olefin cyclopropanation by alkyl diazoacetate esters.’l Chiral oic-dioxi- matocobalt(I1) complexes when used as catalysts produce in high optical yields (up to 88%)approximately equal amounts of cis-and trans-cyclopropane carboxylates [Equation (5)].Kinetic and other mechanistic data indicate that the reaction Ph CO,R 81% Opt. yield Co(a-CQD) PhCH=CH2 + N2CHC02R 4+ (5) HZO &ozR88Y~ Opt. yield R = neopentyl Ph CQD = ( -)camphorquinone-a -dioximate proceeds by C-co-ordination of diazoacetate to cobalt(II) loss of N2 to give a Co” complex and attack by olefin on the co-ordinated carbene with formation of a cobaltacyclobutene followed by decomposition to release the cyclopropane product.The catalytic asymmetric allylic alkylation of carbanions in the presence of Pdo (diop) was first described in 1977 and gave optical yields in the range 35-45% starting from a racemic mixture.52 A related series of reactions has been de~cribed,’~ involving the catalytic transfer of an ally1 group from allylic ethers or esters to ‘active hydrogen’ compounds (see Miscellaneous Section). Using a palladium(0)-diop catalyst optical yields of ca. 10% were obtained. Oxidative ring closure of (26)by palladium(I1) acetate in the presence of a catalytic amount of P-pinene as the source of chirality gives optically active (27) (12% optical yield) together with a small amount of (28). Cyclization is not observed however when P-pinene is present in large excess.54 (26) (27) (28) Palladium-catalysed hydroesterification of a-methylstyrene using chiral diben- zophosphazole ligands in propan-2-01 gives optical yields of ca.40% at up to 83% conversion of olefin [Equation (6); R = Pri]. In Bu‘OH however -69% optical yield is obtained but at only 8% conversion.55 Ph Ph 0 II \C=CH2 + CO + ROH -+ \C*-CH2COR (6) Me/ Me’ ‘H ” A. Nakamura A. Konishi R. Tsujitani M. Kudo and S. Otsuka J. Amer. Chem. SOC. 1978,100,3449; A.Nakamura A. Konishi Y. Tatsumo and S. Otsuka ibid. p. 3443;A. Nakamura Pure Appl. Chem. 1978 50 37. 52 B. M. Trost and P. E. Strege J. Amer. Chem. SOC.,1977,99 1649. 53 J. C.Fiaud A. H. de Gournay M. Larcheveque and H. B. Kagan J. Organometallic Chem. 1978,154 175. ’‘ T.Hosokawa S.Miyagi S. I. Murahashi and A. Sonoda J.C.S. Chem. Comm. 1978,687. 55 T. Hayashi M. Tanaka and I. Ogata Tetrahedron Letters 1978,3925. 110 H. M. Colquhoun J. Holton and M. V.Twigg 7 Metal Clusters In Catalysis Application of metal carbonyl clusters as catalysts for organic reactions continues to be an area of considerable interest. This year most effort has been directed towards methods of preparing heterogeneous systems and reactions of organic nitrogen compounds. Heterogeneous Systems.-Gates and co-workersS6 used poly(styrene4ivinylben- zene) functionalized with PPh2 groups to form polymer-bound catalysts containing characterized iridium and rhodium clusters. Monosubstituted clusters of the type [IT~(CO)~~(P~~P polymer)] were generated by reduction of [Ir(C0)2( p-toluidine)Cl] in the presence of CO and phosphinated polystyrene containing a low concentration of donor groups.The disubstituted derivative was obtained when polymer contain- ing a high concentration of phosphine groups was used. The fomer is an active hydrogenation catalyst but it readily forms aggregated Interaction of phosphinated polymer with a solution of [Rh6(CO)16] in benzene leads to incorpora- tion of the cluster into the which then has an i.r. spectrum similar to that of [Rh6(C0)13(PPh3)3] in the carbonyl region. The polymer-bound cluster is an active hydrogenation catalyst but it is sensitive to oxygen. Oxidation of the phosphine groups to oxide takes place and the unco-ordinated metal agglomerates to form interesting small uniform-sized metal crystallites.Clusters bound to a polymer with a very high ratio of phosphorus to metal have low catalytic activity due to inhibition by excess phosphine groups but this activity remains even after heating in air for several days at 125 "C. Appealingly simple procedures have been devised for anchoring tri-osmium clusters to silica by organic ligands and results of reactivity studies are awaited.s7 Zeolite has been used as a novel support for an incompletely characterized rhodium cluster which was generated in sit~.'~This is a good catalyst for liquid-phase hydroformylation of olefins. Rhodium loss from the catalyst appears to be low. Ichikawas9 obtained active catalysts for methanol synthesis from carbon monoxide and hydrogen by decomposing rhodium carbonyl clusters dispersed on zinc oxide and other supports.Activity depends on the nature of the cluster and significantly the selectivity of these catalysts is higher than that prepared from rhodium tri- chloride which produces mainly methane. Activity and selectivity also depend on the support used. Catalysts prepared from rhodium carbonyl clusters on ZnO MgO BeO and CaO selectively produce methanol. Ethanol and other oxygenated C2 species were obtained with La203 Ce02 Ti02 Zr02 and Tho2. Methane is selectively formed in low conversions over Sn02- V2OS-,and P20s-supported catalysts. Reactions involving Organic Nitrogen Compounds.-Small quantities of [Os,(CO),,] or [H40~4(C0)12] catalyse the conversion of refluxing NMe2Ph into (4-NMe2C6H4)2CH2.Suppression of the reaction by carbon monoxide indicates that co-ordination of the amine to an osmium atom is an important step. A similar " (a)J. J. Rafalko J. Lieto B. C. Gates and G. L. Schrader J.C.S. Chem. Comm. 1978 540; (b) M. S. Jarrell B. C. Gates and E. D. Nicholson J. Amer. Chem. Soc. 1978,100,5727. s7 S. C. Brown and J. Evans J.C.S. Chem. Comm. 1978 1063. E.Mantovani N. Palladino and A. Zanobi J. Mol. Catalysis 1977178,3 285. 59 M.Ichikawa Bull. Chem. SOC.Japan 1978 51,2268 2273. Organometallic Chemistry-Part (i) The Transition Elements reaction takes place with NHMePh but this is complicated by disproportionation to aniline and its dimethyl derivative.60 Nitrobenzene and meta- or para- substituted analogues react with [CO,(CO)~] in refluxing benzene to form azobenzene in moderate yield.61 The cluster [C04(CO)gC6H6] is formed during the reaction.A possible intermediate [Co,(CO),,] also reacts but no [C04(CO)gC6H6] is formed. In contrast to [Co,(CO),] which catalyses the deoxygenation of diphenylketen [Rh4(C0)12] in the presence of carbon monoxide at about 200 "C catalyses the insertion of keten into an arene hydrogen-carbon bond giving Ph,CHCOAr.62 Benzene also reacts under these conditions with para-substituted isocyanates to give benzanilides in moderate yield. Brief details of the first homogeneous catalytic hydrogenation of an isocyanide have been The reaction of [Ni(CNCMe,),] (a non-reactive source of CNCMe3) at 90 "C with hydrogen in the presence of [Ni,(CNCMe,),] gave 99% of Me,CNHMe.Other isocyanides behave similarly and the NiL4-Ni4L7 system can be conveniently generated in situ from [Nqcod),] and isocyanide. The reaction of [H20~3(C0)10] with phenyl isocyanide gives [HOs3(CO),(CHNPh)] whose X-ray structure (29) unfortunately does not locate the bridging hydride ligand. This (29) complex contains an N-phenyl formidoyl ligand bridging three metal atoms. It is possible that this type of bonding pattern represents a step in the cluster-catalysed hydrogenation of is~cyanides.~~~ An interaction involving two carbon atoms and a nitrogen atom with three rhodium metal centres on a face of an octahedral Rh6 unit (30)is proposed to explain Et H I H (30) catalysed specific deuterium exchange in triethylamine.Exchange of methyl hydro- gens and one methylene hydrogen in a single ethyl group takes place when 'O C. Choo Yin and A. J. Deeming J. Organometallie Chem. 1978,144,351. 61 H. Alper and H.-N. Paik J. Organometallic Chem. 1978 144 C18. " P. Hong H. Yamazaki K. Sonogashira and N. Hagihara Chem. Letters 1978 535. " (a)E. L. Muetterties Pure Appl. Chem. 1978,50,941; (6)R. D. Adams and N. M. Golembeski J. Amer. Chem. Soe. 1978,100,4622. 112 H,M. Colquhoun J. Holton and M. V.Twigg triethylamine is heated with D20in the presence of [Rh,(cO),,] under an atmos- phere of carbon monoxide.64 8 Olefin Metathesis The high level of interest in olefin metathesis of previous years has not been maintained. This is largely due to its limited application resulting from failure to devise catalyst systems that are not inhibited by most common functional groups.Most publications this year are concerned with mechanistic aspects and there are indications that the now familiar carbene chain mechanism may be challenged. Few soluble catalyst systems have been successfully supported. However poly(styry1bipyridine) has been used to prepare catalysts for the metathesis of internal ~lefins.,~ In conjunction with AlEtCl, polymer-bound [W(CO),(bipy)] is a better catalyst than the molybdenum compound and both are an order of magnitude more active than the unsupported systems. Moreover they have the advantage of being easily recovered by filtration from reaction mixtures and in some circum- stances they can be re-used.A study of the reaction between [W(CO),L] (L=PPh or PBun3) and A1Br3 in chlorobenzene provides some insight into the role of cocatalyst in methathesis. The first formed 1 1adduct in which aluminium interacts with carbon monoxide trans to the phosphorus ligand subsequently forms a relatively stable zerovalent co-ordina- tively unsaturated species.66 The first example of a stable complex in which the metal is bonded to an olefin and carbene ligands has been rep~rted.~’ It would be of interest to know if this complex (31) is active in olefin metathesis since such a bonding pattern may be a transitory species in the carbene chain mechanism. An intriguing report concerns ring-opening polymerization of cycloalkenes (a special case of metathesis68).Under particular conditions a frequently used cocata- lyst AlEtCl, can induce the formation of ring-opened polymers and oligomers of norbornene similar to those previously observed in the [WCl,]-initiated reaction. In order to account for their observations the authors*’’ suggest the generation of a carbene intermediate (32) and its involvement in a chain reaction. OMe AlCl It has been proposed20a that the carbene/metallocyclobutane chain mechanism might be common to oligomerization Ziegler-Natta polymerization and ole fin 64 R. M. Laine D. W. Thomas L. W. Cary and S. E. Buttrill J. Amer. Chem. SOC.,1978 100 6527. 65 S. Tamagaki R. J. Card and D. C. Neckers .I. Amer. Chem. SOC.,1978,100,6635. 66 Y. B. Taarit J.L. Bilhou M. Lecomte and J. M. Bassett J.C.S. Chem. Comm. 1978 38. 67 W. Priester and M. Rosenblum J.C.S. Chem. Comm. 1978,26. 68 J. J. Rooney and A. Stewart in ‘Catalysis’ ed. C. Kemball (Specialist Periodical Reports) The Chemical Society London 1977 Vol. 1 p. 277. Organometallic Chemistry-Part (i) The Transiiion Elemen, metathesis (see Section 3) and it will be of interest to see whether support for this novel hypothesis is forthcoming during next year. Equilibria between bis-olefin complexes and metallocyclopentanes a key step in a now superseded olefin metathesis mechanism may be more general than previously thought. Labelling experiments have shown that evolution of ethylene from nickel and titanium metallocyclopentanes proceeds via a bis-olefin intermediate.69 The reaction involves initial loss of PPh3 (addition of excess PPh decreases the reaction rate) so that the process does not involve 20-electron intermediates.An appealing application of olefin metathesis from the patent literature” provides a route to styrene from toluene via oxidative coupling of toluene giving stilbene which undergoes metathesis with ethylene to give the desired product [Equation (7)]. lo1 2PhCH3 -+ PhCH=CHPh C2H4 2PhCHzCHZ (7) 9 Isomerization Rhodium-trichloride-catalysed migration of double bonds to form conjugated systems is well established. None-the-less isomerization of the cyclohexenone (33) HOIo^c 0m-. (33) and related molecules to phenols uia remote double-bond migration catalysed by rhodium trichloride in ethanol is surprising.Migration of remote olefinic bonds in the corresponding conjugated imines to form substituted anilines is also catalysed by this reagenf.’l The first stereoselective isomerization of allyl ethers has been Air-stable [Ir(~od)(PMePh~)~]+ after ‘activation’ with hydrogen rapidly catalyses the isomerization of primary allyl ethers to the corresponding trans-propenyl ether at room temperature (Scheme 5). Yields and stereoselectivity are exceptionally high. R2 R2 R2 RI+OR~ - R~+oR’ -+R’,-A,,oR~ ’I‘ H [IrHI H R’,R2 = Hor Me Scheme 5 At high temperatures (180-210 “C),tris(acetylacetonato)ruthenium(m)has been shown to be an effective catalyst for cis-trans isomerization of trisubstituted iso-prenoid olefins.Selectivity is high and no hydrogenation or double-bond migration takes place.” 69 R. H. Grubbs and A. Miyashita,J. Amer. Chem. SOC.,1978,100 1300. 70 P. D. Montgomery R. N. Moore and W. R. Knox U.S. P. 3 965 206 (1976) (Chem. Abs. 1976 85 123 540). 71 P. A. Grieco and N. Marinovic Tetrahedron Letters 1978 2545. 72 D. Baudry M. Ephritikhine and H. Felkin. J.C.S. Chem. Comm. 1978,694. ‘3 Y. Fujita Chem. Letters 1978 533. H. M. Colquhoun J. Holton and M. V.Twigg Incorporation of palladium acetate into poly(styry1)bipyridine followed by reduc- tion with LiAlH4 in THF produces an effective catalyst for the isomerization of quadricyclene to norbornadiene. The reduced species is considerably more active than palladium on charcoal but the unreduced catalyst is ~nreactive.~~ 10 Syntheses involving Iron Carbonyls Useful developments have been made in the use of iron carbonyls in organic synthesis.Italian have demonstrated that the [FeH(C0)4]- anion rapidly and quantitatively exchanges with ion-exchange resin. The exchanged resin in refluxing THF smoothly converts alkyl halides into homologous aldehydes while a-bromo carbonyl compounds aromatic and certain other bromides are dehalo- genated. Work-up of products from these high-yield reactions is simplified by the iron compounds remaining bound to the polymer which is recovered by filtration. A solution of Na[HFe,(CO),] and acetic acid in THF is a mild selective reagent for reduction of olefinic bonds in a range of ap-unsaturated carbonyl compounds.The dimeric carbonyl hydride is prepared in two steps from Na2[Fe(C0)4].76 An almost quantitative one-flask synthesis at room temperature of analytically pure K,[Fe(CO)J from [Fe(CO)J and K[Bu,BH] has been described.77 The potassium salt unlike the more common sodium salt is not spontaneously flammable in air. An important range of carbocyclic compounds with an odd number of carbon atoms rather than the even-numbered units produced by most conventional pro- cedures are obtained from [Fe2(CO)g]-induced reactions of aa’-dibromo-ketones with olefins. Full details of these procedures have been p~blished,~ which are based on trapping the three-carbon oxyallyl-iron intermediate formed from a bromo- ketone and [Fe2(CO)g]. Thus cycloheptenones (34) are obtained from coupling a variety of 1,3-dienes with polybromo-ketones so providing a flexible route to R Ar RA RE (35) Scheme 6 74 R.J. Card and D. C. Neckers J. Amer. Chem. SOC.,1977,99,7733; J. Org. Chem. 1978,43 2985. 75 G.Cainelli F. Maneschalchi A. Umani-Ronchi and M. Panunzio J. Org. Chem. 1973 43 1598. 76 J. P.Collman R. G. Finke P. L. Mallock R. Wahren R. G. Komoto and J. I. Brauman J. Amer. Chem. SOC.,1978,100,1119. 77 J. A. Gladysz and W. Tan J. Org. Chem. 1978 43 2279. 78 Y. Hayakawa F. Shinizu and R. Noyori Tetrahedron Letters 1978,993;R.Noyori Y.Hayakawa H. Takaya S. Murai R. Kobayashi and N. Sonoda J. Amer. Chem. SOC.,1978,100 1759; and following papers. Organometallic Chemistry-Part (i) The Transition Elements 115 tropone alkaloids from readily available materials [Scheme (6)J In a similar fashion aryl-substituted olefins give arylcyclopentanones (35).Variations of these reactions provide a wide range of products many via single-flask procedures and it is to be expected that these methods will be much used in the future. 11 Miscellaneous Catalytic Reductions with Formate Ion.-Reports from several groups are concer- ned with the use of formate ion in selective catalytic hydrodehalogenation reductive coupling of aryl halides to biaryls and reduction of aromatic nitro-compounds to amines. These simple reactions are carried out in an open vessel and if necessary exact amounts of reducing agent (formic acid) can be measured allowing in some instances selective reduction.When a heterogeneous catalyst is employed it is easily isolated from the product mixture and can be re-used. With excess formic acid and a relatively large amount of palladium on charcoal aromatic nitro-compounds are quickly reduced to amine in high yield." No reaction takes place if the nitro-compound contains a halogen other than fluorine. Indeed addition of halide ion prevents reaction and it is suggested that halides poison the catalyst. Presumably for the same reason compounds containing divalent sulphur fail to react. However nitro-groups in chlorine-containing compounds are reduced without removal of halogen by phosphinic or phosphorous acid in the presence of palladium on charc~al.'~ In DMF aryl bromides are dehalogenated by methoxide ion in the presence of [Pd(PPh3)J but this procedure is improved by using formate ion.The proposed" mechanism of the latter reaction involves oxidative addition of aryl bromide to a co-ordinatively unsaturated palladium(0) species exchange of bromide for formate ion subsequent loss of carbon dioxide from co-ordinated formate to give a hydride followed by reductive elimination of ArH (Scheme 7). ArBr -L 'J co2 Scheme 7 When small amounts of palladium on charcoal are used the rate of reaction is markedly enhanced by a large excess of triethylamine.81," With this reagent substituents clearly influence the reaction. For instance the double bond in methyl 4-chlorocinnamate is reduced at a rate comparable with halogen reduction whereas methyl cinnamate does not react.Reduction and cyclization of o-nitrocinnamate gives the saturated lactam (36)in good yield. 79 I. D. Entwistle A. E. Jackson R. A. W. Johnston and R. P. TeIford J.C.S. Perkin I 1977,443. 8o A. Zask and P. Helquist,J.Org. Chem. 1978 43 1619; P. Helquist Tetrahedron Letters 1978 1913. (a)N. A. Cartese and R. F. Heck J. Org. Chem. 1977,42 3491; (6)ibid. 1978,43,3985. H.M. Colquhoun J. Holton and M. V. Twigg The nature of the catalyst also influences the selectivity of the reaction. o-Bromonitrobenzene is dehalogenated by formic acid and triethylamine in the presence of palladium on charcoal whereas reduction of the nitro-group to amine with retention of halogen takes place with platinum on charcoal.81Q The formic acid-triethylamine-palladium system is also effective for reducing ap-unsaturated carbonyl compounds to the saturated carbonyl compound.For example the enone (38)was obtained in 69% yield from the conjugated dienone /3 -ionone (37). Insome instances conjugated dienes and acetylenes can be selectively reduced to mono- enes.'lb Reduction of an aryl halide with aqueous alkaline sodium formate in the presence of palladium on charcoal and a surfactant produces biaryls in moderate yield. Nitro-groups present in the aryl halide are reduced to amine in the biaryl. The role of the surfactant is important since its nature can influence the amount of product.82 It may be expected that further work on these interesting and synthetically important reactions will be reported during the coming year.Palladium-catalysed Reactions.-Palladium is the most commonly used transition metal in organic synthesis particularly in C-C bond-forming reactions. A new general synthesis of ketones from acid chlorides by palladium-catalysed coupling with organotin compounds has been rep~rted.'~ This method [Equation (Sj] is [PdCI(CHzPh)(PPh3)zl R'COCI + RiSn b R1COR2+ R:SnCl (8) synthetically attractive because yields are high sensitive functional groups (e.g. aldehyde) are tolerated no inert atmosphere is required and reaction times are short. A short review on the use of mono-organopalladium(I1)derivatives [Pd(R)L,X] in organic synthesis has a~peared.'~ The most useful reaction [Equation (9)] is vinylic substitution using organic halides (e.g.aryl heterocyclic benzylic and vinylic bromides or iodide^).'^ P. Banfield and P. M. Quan Synthesis 1978,537;see also U.K. P. 1 457 608 (1976) (Chem.Abs. 1977 87,22 732); 1 458 633 (1976) (Chem.Abs. 1976,84 164 376). 83 D. Milstein and J. K. Stille J. Amer. Chem. SOC.,1978 100 3636. 84 R. F. Heck Pure Appl. Chem. 1978 50,691. 85 J. E. Plevyak and R. F. Heck J. Org. Chem. 1978 43 2454; C. B. Ziegler and R. F. Heck ibid. pp. 2941,2949; W. C. Frank Y. C. Kim and R. F. Heck ibid. p. 2947; N. A. Cortese C. B. Ziegler B. J. Hrnjez and R. F. Heck ibid. p. 2952. Organometallic Chemistry-Part (i) The Transition Elements H R3 R1 R3 Common functional groups other than ap-unsaturated carbonyls are unaffected by the reaction.Carbonyls may be protected by ketal or acetal formation. An extensionof this reaction provides convenient syntheses of 3-alkyl-pyridine~,'~"and 2-(3'-0xoalkyl)-furans'~~ from allylic alcohols. Palladium-catalysed allylic alkyl- ation of olefins has been further studied. .rr-Allylpalladium complexes from a variety of olefins have been prepared and characteri~ed,'~" and their reactions with nucleo- phile The effect of ligands (phosphines and phosphites) on product selectivity is also This reaction has been applied in steroid synthesis to obtain stereocontrol in the partial synthesis of 5a -ch~lestanone.'~~ A supported system Pdo on either phosphinated silica gel or phosphinated polystyrene has shown an increase in regioselectivity over homogeneous systems especially for nitrogen nucleophiles (e.g.Et,NH) and may enhance the scope of the Direct catalytic allylic alkylation of olefins uses [PdC12(CH3CN)2] or [Pd(.rr-allyl)C1]2,88 but unfortunately this new reaction is slow catalyst turnover is poor and considerable decomposition of the catalyst occurs during the reaction. Full experimental details have appeared on the palladium-assisted intramolecular amination of olefins to produce indoles quinolines and is~quinolines.'~ The cyclization reaction was achieved catalytically using benzoquinone as oxidant {Pdo-+ Pd"}. Hydrozirconation.-The hydrozirconation/protonationof vitamin D is reported to be superior to previously described hydroboronation procedures for regio-specific reduction of the 10(19)-double bond."" The usefulness of hydrozirconation in organic synthesis is extended by the discovery that alkenyl-zirconium complexes derived from alkynes and [Zr(C,H,),(H)Cl] may be coupled with unsaturated organic halides in the presence of catalytic quantities of Nio or Pdo complexes to give high yields of conjugated dienesgob [Equation (lo)].This coupling reaction is R2 H \/ 86 (a)Y. Tamaru Y. Yamada and 2.Yoshida J. Org. Chem. 1978,43,3396;(6) Y. Tamaru Y. Yamada and Z. Yoshida Chem. Letters 1978 529. *' (a)B. M. Trost P. E. Strege L. Weber T. J. Fullerton andT. J. Dietsche J. Amer. Chem.Soc. 1978,100 3407; (b)ibid.,p. 3416; (c) ibid.,p. 3426; (d)B. M. Trost and T. R. Verhoeven ibid.,p. 3435; (e)B. M. Trost and E. Keinan ibid. p. 7779.88 L. S. Hegedus T. Hayashi and W. H. Darlington J. Amer. Chem. Soc. 1978,100,7747. 89 L. S. Hegedus G. F. Allen J. J. Bozell and E. L. Waterman J. Amer. Chem. Soc. 1978 100 5800. 90 (a)A. W. Messing F. P. Ross A. W. Norman and W. H. Okamura Tetrahedron Letters 1978,3635;(b) N. Okukado D. E. Van Horn W. E. Klima and E. Negishi ibid.,p. 1027; (c)E. Negishi N. Okukado A. 0.King D. E. Van Horn and B. I. Spiegel J. Amer. Chem. Soc. 1978,100 2254. H.M. Colquhoun J. Holton and M. V.Twigg extremely sluggish when a disubstituted alkenyl group is present in the zirconium complex (probably for steric reasons) but addition of catalytic amounts of ZnC12 or CdC12 facilitates the reaction and high yields of coupled products are Alkenyl-zirconium complexes do not react with ap-unsaturated ketones but on addition of a catalytic amount of [Ni(acetylacetonate)J rapid conjugate addition of the terminal vinylic units occurs giving after hydrolysis good yields of a& unsaturated ketones.91a The initial product of conjugate addition is a zirconium 0-enolate and such compounds react with formaldehyde to give a-(hydroxy-methyl)-cycloalkanones [Equation (1l)],a reaction recently used in the synthesis of prostaglandin derivatives." ,Zr(C5H5)2c1 0 0 Aldehydes and ketones are rapidly reduced to alcohols by [Zr(C5H5),(CI)BH4] in benzene whereas carboxylic acids esters nitriles and nitro-compounds react much more slowly so that selective reactions are possible.92 Since the reactivity of this reagent is similar to that of NaBH, its major advantage appears to be the ability to carry out reductions of non-polar substrates which are insoluble in alcoholic media.91 (a)M.J. Loots and J. Schwartz J. Amer. Chem. SOC.,1977,99,8045;(b)M.J. Loots and J. Schwartz Tetrahedron Letters 1978,4381. 92 T. N. Sorrell Tetrahedron Letters 1978 4985.