6 Organometallic Chemistry Part (i) The Transition Elements By R. PEARCE D. J. THOMPSON and M. V. TWlGG ICI Corporate Laboratory PO Box 7 7 The Heath Runcorn Cheshire WA7 4QE 1 Introduction Most recent publications fall into two broad areas the larger concerned with preparations structures and properties of complexes and the smaller with applica- tions in organic synthesis. It is with the latter that we are principally concerned. While advances continue on all fronts the most exciting developments have taken place in transition-metal-catalysed olefin metathesis where we now have a better understanding of the reaction pathway (see Section 7). The more important review articles are given below. Of a general nature publications have included a special editioc of J.Orgunornetdic Chern. (Vol. 100) which is a collection of personal reflections by leading researchers; a comprehensive handbook of data on syntheses physical constants and reactions of complexes that appeared in the literature in the period 1965-1968;’ and a further volume of a multi-author book dealing with a number of applications of transition-metal complexes in homogeneous catalysis.* More specific articles have dealt with trans- ition-metal catalysis of pericyclic reaction^;^ metathesis4 (see also ref. 3);olefin poly- meri~ation;~applications of metal vaporization to organometallic synthesis;6 organo-chromium compound^;^ the roles of nickel’ and palladium’ complexes in catalysis; reactions of hydrido-nickel -palladium and -platinum complexes;” 1 K.Bauer and G. Haller ‘Organometallic Compounds. Methods of Synthesis Physical Constants and Chemical Reactions Second Edition. Volume 1 Compounds of the Transition Metals First Supple- ment’ ed. M. Dub Springer-Verlag Berlin 1975. 2 ‘Aspects of Homogeneous Catalysis’ ed. R. Ugo Reidel Dordrecht Holland Vol. 2 1974. 3 F. D. Mango Coordination Chem. Rev. 1975,15 109. 4 R. J. Haines and G. J. Leigh Chem. SOC.Rev. 1975,4 155; J. C. Mol and J. A. Moulijn Adv. Catalysis 1975 24 131; W. B. Hughes Chemtech. 1975,486; L. Hocks Bull. SOC.chim. France 1975 1893. ‘Co-ordination Polymerisation a Memorial to Karl Ziegler’ ed. J. C. W. Chien Academic Press New York 1975; D. G. H. Ballard J. PoZymer Sci. Polymer Chem. Edn. 1975,13 2191. P. S.Skell and M. J. McGlinchey Angew.Chem. Internat. Edn. 1975,14 195; P. L. Timms ibid. p. 273; E. A. Koerner von Gustorf 0.Jaenicke 0.Wolfbeis and C. R. Eady ibid. p. 278; K. J. Klabunde and T. 0. Murdock Chemtech. 1975,624; K. J. Klabunde Accounts Chem. Res. 1975,8,383. R. P. A. Sneeden ‘Organochromium Compounds’ Academic Press New York 1975. * P. W. Jolly and G. Wilke ‘The Organic Chemistry of Nickel’ Academic Press New York Vol. 2,1975. P. M. Henry Ado. Organometallic Chem. 1975 13 363. lo D. M. Roundhill Adv. Organometallic Chem. 1975 13 274. 119 120 R. Pearce,D.J. Thompson,and M. V.Twigg reactions of Na,Fe(CO),; ''homogeneous catalytic activation of carbon-hydrogen bonds;12 activation of Grignard reagents by transition-metal complexe~;'~ and metal v-complexes (arene cyclopentadienyl and ally1 group^).'^ 2 Hydrogenation Advances in homogeneous catalytic hydrogenation have been in the areas of selective hydrogen addition and asymmetric synthesis Futher details have appeared on [Co(q3-C,H,){P(OMe),},] which among homogeneous catalysts is unique in selectively hydrogenating arenes to cyclohex- anes.15 Selectivity is such that benzene is hydrogenated to cyclohexane at a rate some three to four times greater than is cyclohexene.Neither cyclohexadiene nor cyclohexene are important intermediates. Once transfer of the first hydrogen has taken place the arene must remain tightly bound to the metal centre until the sixth has been added. This tight binding is well demonstrated in the hydrogenation of [2HJbenzene which affords the first synthesis of pure all-cis-[2&]cyclohexane.It was suggested that an important intermediate is the complex [Co(q1-C3H5)-(H),{P(OMe),}(q4-C6H6)].The ally1 group is not lost during reaction and indeed the corresponding hydrido-cobalt complex is inactive. [RuCl,(PPh,),] effects reduction of both aliphatic and aromatic nitro-compounds to amines.I6 For the alphatic compounds this method appears to be the preferred reduction procedure catalyst turnover is high reaction conditions are mild; and aqueous work-up procedures are not necessary.'6a For aromatic compounds high selectivity is observed other substituents e.g. halogen ester or nitrile are not affected and reduction of dinitro-compounds proceeds at a higher rate affording a procedure for the sequential hydrogenation of nitro-amines and diamines.I6' [RuCl,(PPh,),] is an efficient catalyst for the reduction of cyclic carboxylic acid anhydrides to y-lactones the reaction being one of the few examples of transition-metal-catalysed homogeneous hydrogenolysis of a carbon-oxygen bond.17 Other conventional hydrogenation catalysts were ineffective.Carboxylato-rhodium(1) species e.g. [Rh(OCOPh)(cyclo-octa- 1,5-diene)(PPh3)] in the presence of base were highly active in the selective hydrogenation of alk-1-ynes to alk-1-enes." [RhCl(PPh,),] in combination with H202gives an undefined heterogeneous catalyst that is active in the selective conversion of mesityl oxide into methyl isobutyl ketone.l9 Important pointers to the design of ligands particularly those related to diop (1 ; Ar =Ph) for use in asymmetric hydrogenations come from a study of the effect on catalytic activity of changes in the ring size of chelating diphosphine ligands 11 J.P. Collman Accounts Chem. Res. 1975 8 342. 12 G. W. Parshall Accounts Chem. Res. LY7.5 8 113. 13 H. Felkin and C. Swierczewski Tetrahedron 1975 31 2735. l4 W. E. Silverthorne Ado. Organometallic Chem. 1975 13 48; M. A. Bennett Organometallic Chem. 1975 3 290; M. Choke ibid.,p. "!7 W. E. Watts ibid. p. 342. (a)F. J. Hirsekorn M. C. Rakowski and E. L. Muetterties J. Amer. Chem. SOC.,1975,97,237; (6)E. L. Muetterties M. C. Rakowski F. J. Hirsekorn W. D. Larson V. J. Basus andF. A. L. Anet ibid.,p. 1266. l6 (a) J. F. Knifton J.Org. Chem. 1975,40 519; (b)J. F. Knifton Tetrahedron Letters 1975 2163. l7 J. E. Lyons J.C.S. Chem. Comm. 1975,412. Is R. H. Crabtree J.C.S. Chern. Comm.. 1975,647. 19 W. Strohmeier and E. Hitzel J. Orp9iometallic Chem 1975,102 C37. Part (i) The Transition Elements Ph2P(CH2),PPh2 in combination with a Rh’ centre.” Activity was high for n =3 less good for n =4,5,or 6 and poor for n =2 [or in cases where (CH,) was replaced by the groups CH,OCH or cis-CH=CH] (cf. ref. 56). Chiral phosphine-rhodium(1) complexes predominate in reports on catalytic asymmetric hydrogenation. Efficient asymmetric reduction of ketones has been achieved using e.g. [Rh(hexa-Z,5-diene)Cl] +diop with optical yields of up to Using the chiral diphosphine (2) as ligand Knowles has achieved enan- tiomeric excesses of 96% in the reduction of a-acylamido-acrylic acids to a-amino- acids rivalling the stereospecificity observed with enzyme systems.22 Importantly the high optical yields were not markedly sensitive to temperature and pressure.H OMe (1) (2) Chiral diphosphines related to diop (1; Ar = Ph) have been in~estigated.~~ In combination with Rh’ species higher optical yields have been obtained for (1; Ar =C6H4Me-m)23a and (3),23bwhereas when the oxygen-containing ring is substi- tuted by a cyclopentane or a bicyclo[2,2,2]octane ring yields are lower. Using space-filling models the chirality of the major products of asymmetric hydrogena- tion and hydrosilylation using diop-rhodium complexes has been predi~ted.’~ In all but three cases the correct product was predicted.OPPh CI CI n Ru-P P-RU P P = (+)-diop ‘OPPh (3) (4) First reports have appeared on the use of ruthenium complexes in asymmetric hydr~genation.’~ Both mono- and di-nuclear diop complexes of RuII were prepared but of these only the dinuclear (4) was effective. Optical yields up to 60% were [Ru,H,(CO),{( -)-diop},] has been used for the high-temperature and high-pressure hydrogenation of carbon-oxygen and carbon-nitrogen double bonds but optical yields were Further work on the asymmetric hydrogenation catalysed by the bis(dimethylglyoximato)cobalt(II)-chiral amino-alcohol system has shown that asymmeric induction is brought about by amino-alcohol molecules that are not 20 J.-C.Poulin T.-P. Dang and H. B. Kagan J. Organornetullic Chern. 1975,84,87. 21 B. Heil S. Toros S. Vastag and L. Marko J. Organornetullic Chem. 1975 94 C47; J. Solodar Cherntech. 1975 42 1. 22 W. S. Knowles M. J. Sabacky B. D. Vineyard and D. J. Weinkauff J. Amer. Chem. Soc. 1975,97,2567. 23 (a)T. P. Dang J.-C.Poulin and H. B. Kagan J. Organornetullic Chem. 1975,91,105; (b)M. Tanaka and I. Ogata J.C.S. Chern. Cornrn. 1975 735. 24 R. Glaser Tetrahedron Letters 1975 2127. 25 (a)B. R. James D. K. W. Wang and R. F. Voigt J.C.S. Chem. Grnrn.,1975,574; (6) C. Botteghi M. Bianchi E. Benedetti and U. Matteoli Chirniu (Switz.) 1975 29 256. 122 R. Pearce,D.J. Thompson andM. V.Twigg co-ordinated to the cobalt atom.26 This system was compared to oxido-reductases where the catalytic site and stereospecificity-determiningsite are generally sepa- rated.3 Hydrosilylation Rhodium(1) complexes containing chiral phosphines effect catalytic asymmetric hydrosilylation of ap-unsaturated carbonyl c~mpounds.~' The nature of the pro- duct depends on the silane with monohydrosilanes 1,4-addition takes place to give the useful silylenolates which hydrolyse to optically active saturated carbonyl dihydrosilanes undergo 1,2-addition at the carbonyl group to give optically active ap-unsaturated Optical yields were generally low. A problem related to hydrosilylation is the conversion of 'direct process residues' by-products from the direct synthesis of halogenoalkylsilanes containing mixed disilanes into useful products.Two recent disclosures point to a possible solution.28 Nickel palladium and platinum complexes catalyse the reaction of halogenoalkyl- disilanes R,Si2C16-x with alkyl and aryl halides to give monosilanes. For example [Pd(PPh,),] effects essentially quantitative conversion of CIMe2SiSiMe2C1 and PhBr into PhMe,SiCl and Me,SiBrCl. 4 Metal-catalysedHydrogen-transfer Reactions Earlier indications that amines are highly efficient reagents in hydrogen-transfer reactions have been confirmed.29 These reactions generally employ hydrocarbons (e.g. tetralin) or primary or secondary alcohols as donors. With RhC1(PPh3) as catalyst cyclic amines (e.g.piperidine pyrrolidone and indoline) are more reactive than the above donors in transfer hydrogenation to ~ycloheptene.~~" Simple amines (e.g.tri-isopropylamine) are effective in the [RuH,(PPh,),]-catalysed reduction of aldehydes or ketones to Furthermore H2N(CH2)3NH2 or HN(CH,),NHCHEt in combination with a palladium catalyst effect selective reduc- tion of cyclo-octa-l,5-diene to cyclo-octene; this is the first example of a selective reduction of a diene to a monoene via a metal-catalysed hydrogen-transfer reac- ti~n.~~~ Although we are concerned with transition-metal-catalysed reactions it is noteworthy that a metal-free hydrogen-transfer reaction system has been dis- clo~ed.~~ The reagent is dehydrated alumina that has been treated with isopropyl alcohol and it is specific for the reduction of aldehydes to primary alcohols; other functional groups (e.g.ketones esters) are not affected. [Ru(OCOCF,),CO(PPh,),] is reportedly the most efficient homogeneous catalyst for the dehydrogenation of primary and secondary alcohols to aldehydes and z6 Y. Ohgo Y. Natori S. Takeuchi and J. Yoshimura Chem. Letters 1974 1327. z7 (a)T. Hayashi K. Yamamoto and M. Kumada TetrahedronLetters 1975,3;(21) I. Ojima T. Kogure and Y. Nagai Chem. Letters 1975,985. 28 H. Matsumoto S. Nagashima K. Yoshihiro and Y.Nagai J. Organometallic Chem. 1975,85 C1; U.S. P. 3 772 347 (Chem. Abs. l974,80,3625v). 29 (a)T. Nishiguchi K. Tachi and K. Fukuzumi J. Org. Chem 1975,40,237;(6)H. Imai T. Nishiguchi and K. Fukuzumi Chem. Letters 1975,807; (c)S.-I. Murahashi T. Yano and K.-I. Hino Tetrahedron Letters 1975,4235. 3O G. H. Posner and A.W. Runquist Tetrahedron Letters 1975,3601. Part (i) The Transition Elements ketones re~pectively.~’ P-Elimination from an alkoxy-ruthenium intermediate is proposed in the reaction pathway. Rhodium has joined platinum and iridium as an homogeneous catalyst in the form of RhCl, for the isotopic hydrogen-exchange reaction.32 Significantly while it is less active than platinum strong acid is not required to stabilize the catalyst solution. The results from studies of H/D exchange in alkanes and aromatic hydrocarbons point to a common mechanism for all three metals and give added support to the role of n-complexes as intermediates. 5 Oligomerization Oligomerizationof alkenes dienes and alkynes continues to be a productive field. Cyclo-oligomerization to give four- five- or six-membered rings provides the main area of interest.An improved synthesis of benzocyclobutenes indanes and tetralins involves the [Co(q5-C,H,)(C0),]-catalysed cyclo-oligomerizationof a,@-diynes and substituted acetylenes [equation (l)].” The most versatile synthesis involves Me,SiCGCSiMe,. This alkyne shows little tendency to undergo homocyclotrimerization and produces the versatile reagent (5a; R =Me,Si) which isomerizes readily to (5b; R= Me,Si) with acid. The trimethylsilyl groups are readily substituted by electrophiles. (54 (5b) Bonnemann has extended his work on the organocobalt-catalysed 2 1 cyclo-trimerization of alk-l-ynes with nitriles which gives 2,4,6- and 2,3,6-substituted pyridines to the preparation of bipyridyl~.~~ From the readily available cyanopyridines and alk- l-ynes (RC-CH) the corresponding bipyridyls (6a) and (6b) are obtained in almost quantitative yield.The reaction of a,@-dinitriles with alkynes proceeds stepwise to give 2-cyanoalkylpyridines and 2,2‘-dipyridylalkanes. 1,3-Dienes undergo cyclodimerization with nitriles to give dihydropyridines which readily dehydrogenate to the corresponding pyridine. This substitution of alkynes by 1,3-dienes considerably increases the scope and utility of these syntheses and we can expect further useful developments in heterocyclic synthesis with such cobalt- based systems. It is interesting to speculate that in the 1980’s pyridines and bipyridyls may be synthesized industrially in this manner with 2,2’-bipyridyls coming from the inexpensive starting materials butadiene and cyanogen.An interesting five-membered-ring formation has been observed in the [NiCl,(PPh,),]-catalysed telomerization of butadiene with PrMgBr which reacts as a 31 A. Dobson and S. D. Robinson J. Organometallic Chem. 1975,87 C52. 32 M. R. Blake J. L. Garnett I. K. Gregor W. Hannan K. Hoa and M. A. Long J.C.S. Chm. Comm. 1975,930. 33 R. L. Hillard tert. and K. P. C. Vollhardt Angew. Chem. Internat. Edn. 1975 14 712; W. G. L. Aalbersberg A. J. Barkovich R. L. Funk R. L. Hillard tert. and K. P. C. Vollhardt J. Amer.Chem. SOC. 1975,97,5600. 34 H. Bonnemann Seventh Internat. Conf. Organometallic Chem. Venice 1975 abstr. 132. R. Pearce,D.J. Thompson and M. V.Twigg (64 (6b) (7) source of [HMgBr].Both cis-and by thermal isomerization trans-(2-vinylcyclopenty1)methylmagnesium bromide [cis-and trans-(7)]were obtained in high stereochemical X-Ray structures of intermediates have provided further insights into reaction pathways. Confirmation that dienylpalladium complexes are intermediates in [PdClJ-catalysed cyclotrimerization of alkynes comes from the study of (8) an intermediate in the reaction of [PdCl,(PhCN),] with BUT-CH that can be trapped by addition of MeS(CH,),SMe.36 Nickel species are active in the cyclodimerization of 3,3-dimethylcyclopropene. From the reaction with 2,2’-bipyridyl(cyclo-octa-1,5-diene)nickel the active intermediate (9)was is~lated.~’ This is the first example of metallo-ring formation by oxidative dimerization of an olefin at a metal centre presumably related to the ring strain in the cyclopropene which imparts alkyne-type reactivity.(Cf.ref. 38for recent work on the formation of nickelocycles via oxidative dimerization of alkynes). Bu‘ Q Me / Me CI Me (8) (9) Improvements have been made in the selectivity of catalyst systems to the production of linear dimers. [NiBr,(PPh,),] in combination with NaBH, gives high yields of (E,E)-octa-1,3,6-triene in the dimerization of butadiene and overcomes the strong tendency for the production of cyclic products with organonickel cata- lyst~.~~ Isomeric hexenes have been obtained in good selectivity (up to 95% linear products) using the catalyst system [Pd(acac),)-EtAlCl,-PR,.40The reaction rate was however relatively low.Full details have appeared on the catalytic activity towards butadiene oligomeriza- tion shown by diarenetitanium compounds and catalyst systems obtained from condensed vapours of the first-row transition metals Ti V Cr Mn Fe Co and Ni41 3s H. Felkin L. D. Kwart C. Swierczewski and J. D. Umpleby J.C.S. Chem. Comm. 1975 242. 36 B. E. Mann P. M. Bailey and P. M. Maitlis J. Amer. Chem. SOC.,1975 97 1275. 37 P. Binger J. McMeeking U. Schuchardt and M. J. Doyle ref. 34 abstr. 129. 38 J. J. Eisch and C. A. Damasevitz J. OrgunometullicChem.,1975,96 (219; J. J. Eisch and J. E. Galle ibid. p. C23. 39 C. U. Pittman jun. and L. R. Smith J. Amer. Chern. SOC.,1975,97 341. 4O C. Henrici-OlivC and S. Olivk Angew.Chem. Internat. Edn. 1975,14 104. 41 V. M. Akhmedov M. T. Anthony M. L. H. Green and D. Young J.C.S. Dalton 1975 1412. Part (i) The Transition Elements 125 An interesting pointer to the role of metal clusters in catalysis comes from a study of the fluxional tetranickel species [Ni4(CNCMe,),].42 This complex is an active catalyst in the cyclotrimerization of acetylene and in the dimerization of butadiene to cyclo-octadienes (no other cyclo-dimers or linear dimers being produced) whereas the monometallic [Ni(CNCMe,),] is inactive. The authors suggest that more than one metal may be involved in these transformations as in a 'template synthesis'. We look forward to further developments in this field with extensions to the preparation of discrete supported clusters of varying nuclear (M,) sizes.6 Isomerization The conversion of 4-vinylcyclohexene into cyclo-octa- 1,5-diene on reaction with [PdCl,(PhCN),] one of the few reported skeletal rearrangements initiated by a palladium complex has been reinvestigated. Although a variety of experimental conditions were employed no cyclo-octa- 1,5-diene was detected.43 A new process for the cis-trans interconversion of olefins involves formation of an epoxide its ring-opening (with inversion) by [Fe(q '-CsHS)(CO)J and heating the intermediate alkoxide to give the desired olefin. This sequence is not complicated by the presence of an olefin group in the epoxide and is well suited to dialkyl and diary1 epoxides but is less effective for CU,~ -epoxycarbony1 'H N.m.r.of the olefinic molybdenum complexes [MoH(C,H4),(Ph,PCH2CH,PPh,),]' and [MoH(~~-C,H,)(P~~PCH,CH,P~,)~] have provided an insight into reaction pathways in olefin isomerization. For the former rapid exchange of the Mo-H with half of the hydrogens in the bound ethylenes is observed this being the first direct observation of a reversible insertion process of this type. For the latter exchange of the Mo-H with the terminal protons in the co-ordinated ally1 group is observed giving a direct observation of the 1,3-hydride shift mechanism via a hydrido-ally1 intermediate. 7 Olefin Metathesis There has been a high level of interest on all fronts in olefin metathesis this year including the publication of several reviews.374 Much of this work is concerned with elucidating the mechanism of this intriguing reaction and here we concentrate on this aspect.Previously it had been demonstrated that some rhodium-carbene complexes are active in homogeneous metathesis of a number of electron-rich 01efins.~~ The generality of such a mechanism involving metal-carbene intermediates to the metathesis of simple olefins has been questioned. Such doubts have for example been expressed by a reviewer (Mango3) in 1975. Subsequently a number of independent workers have presented evidence4' supporting such a catalytic cycle. 42 V. W. Day R. 0.Day J. S. Kristoff,F. J. Hirsekorn and E. L. Muettertes J. Amer. Chem. SOC.,1975,97 2571. 43 W. T. Wipke and G. L. Goeke J. Org. Chem 1975,40 3242. 44 M. Rosenblum M.R. Saidi and M. Madhavarao Tetrahedron Leners 1975,4009. 45 J. W. Byrne H. U. Blaser and J. A. Osborn J. Amer. Chem. Soc. 1975,97,3871. 46 D. J. Cardin M. J. Doyle and M. F. Lappert J.C.S. Chem. Comm. 1972 927. 47 (a)E. L. Muetterties Znorg. Gem. 1975,14,951; (b)M.F. Farona and W. S. Greenlee J.C.S. Chem. Comm. 1975,759;(c) R. H. Grubbs P. L. Burk and D. D. Carr,J. Amer. Chem. Soc. 1975,97,3265; (d)T. J. Katz and J. McGinnis ibid.,p. 1592. R.Pearce D.J. Thompson and M. V. Twigg Space does not permit a detailed review of all of this evidence but the essential cycle of the carbene mechanism based on the presentation by M~etterties,~~" is shown in Scheme 1. Reagents i R2HC=CHR2; ii R'HC=CHR' Scheme 1 Although intermediates (or transition states) involving cyclobutanes have previ- ously received the major attention five-membered metallocyclic species have also been considered.Unlike the present metal-carbene mechanism these involve the simultaneous interaction of the metal with two olefin molecules. Direct evidence for this scheme comes from initial product analysis of a reaction using an interesting new stable rhenium catalyst system [Re(CO),X]-EtAICl in chloroben~ene,~'~ the ratio of deuteriated ethylenes from specifically deuteriated octa-l,7-diene on treatment with WC1,-LiBu or PhWC13-AlC13,47C and the results of the mixed metathesis of cyclo-octene trans-but-2-ene and trans-oct-4-ene as well as cyclo-octene and hex- 2-ene with [MoCl,(NO),(PPh,),] and Et,AI,CI in chl~robenzene.~~~ It is also pertinent that tungsten-methylene species have been proposed4' as intermediates in other reactions e.g.in the reaction of [W(q-C,H,),Me(C,H,)]+ with phosphines and that the first stable and fully characterized (X-ray) transition- metal-methylene complex [Ta(q5-C5H5)Me(CH,)] has been rep~rted.~' Adsorbed methylene is also thought to be involved in heterogeneous metathesis Last year the WC16-EtAlC1 system originally classified as a homogeneous catalyst was reclassified as a heterogeneous ~atalyst.~' This year further studies have shown that the heterogeneous part of the system is inactive and that the activity resides in the homogeneous solution.52 Two groups53 have examined the unusual catalyst system formed by U.V. irradia-tion of [w(co)6]in CCI,.No activity was observed in hexane chlorobenzene or CHCI,. Although the involvement of [W(CO),Cl] is suggested it is tempting to speculate that a dichlorocarbene complex is the active intermediate. 48 N. J. Cooper and M. L. H. Green J.C.S.Chem. Comm. 1974,761; L. Farrugia and M. L. H. Green ibid. 1975,416. 49 R. R. Schrock,J. Amer. Chem. SOC.,1975,97,6577; L. J. Guggenberger and R. R. Schrock ibid.,p. 6578. J. I. C. Archibald J. J. Rooney and A. Stewart J.C.S. Chem. Comm. 1975 547. 51 E. L. Muetterties and M. A. Busch J.C.S. Chem. Comm. 1974,754. 52 R. Wolovsky and Z. Nir J.C.S. Chem. Comm. 1975,302. 53 P. Krausz F. Granier and J. E. Dubois J. Amer. Chem. SOC.,1975,97,437;A. Agapiou and E. McNelis J.C.S. Chem. Comm. 1975 187. Part (i) The Transition Elements 8 Carbonylation There has been a high level of interest this year in carbonylation using palladium catalysts.Severai groups of workers have reported the catalytic carbonylation of aryl and benzyl halides under mild conditions [equation (2)].54 Yields are generally best for iodides but bromides and chlorides have also been successfully carbonylated. If the reaction is carried out with a primary or secondary amine in place of the methanol the corresponding amide is Pd complex R'X+ CO +MeOH +R:N ______* R'C0,Me +[RzNH]+X-(2) In the carbonylation of styrene using palladium-phosphine complexes it was found that the product depended on the phosphine Using [PdCl,(PPh,),] ethyl 2-phenylpropionate was selectively obtained whereas [PdCl,{Ph,P(CH,),PPh2}] gave ethyl 3-phenylpropionate.The complexes [PdCl,{Ph,P(CH,),PPh,}] (n= 1,2 or 3) were inactive. This variation in catalytic activity with ring size has a parallel with some recently reported hydrogenation studies.20 Mixtures of PdCl and thiourea catalyse in high yield and under mild conditions the carbonylation of appropriate acetylenic alcohols to Q -methylene-lactones e.g. (10)to (11).57 (10) (1 1) Although platinum hydroformylation catalysts are relatively unusual a report has appeared on the use of [PtH(SnCl,)(CO)(PPh,),] for the hydroformylation of pent-1-ene.58 The catalyst is extremely selective giving more than 95% of the straight- chain aldehyde in almost quantitative yield. Pentacarbonyliron has been shown to react with either Q -or p -pinene to give the ring-expanded ketones (12) and (13).59The reaction takes place stereospecifically and represents the first metal-induced ring expansion of a monovinylcyclobutane.s4 A. Schoenberg I. Bartoletti and R. F. Heck J. Org. Chem. 1974,39,3318; J. K.Stille and P. K. Wong ibid. 1975,40 532; M.Hidai T.Hikita Y. Wada Y. Fujikura aild Y. Uchida Bull. Chem. SOC.Japan 1975,48,2075. 55 A. Schoenberg and R. F. Heck J. Org. Chem. 1974,39,3327. 56 Y.Sugi K. Bando and S. Shin Chem. and Ind. 1975,397. 57 J. R.Norton K. E. Shenton and J. Schwartz Tetrahedron Letters 1975,51. 58 C.Y.Hsu and M. Orchin J. Amer. Chem. SOC.,1975,97,3553. 59 A.Stockis and E. Weissberger J. Amer. Chem. SOC.,1975,97,4288. R.Pearce,D.J.Thompson andM. V.Twigg The reaction of NN-dichloro-amines or -amides with Fe,(CO) yields the corre- sponding isocyanate in reasonable yield.60 The reaction is envisaged as going via carbonylation of a nitrene intermediate ta give complexes of the type (14). Oxidative demetallation with Ce4' then liberates the isocyanate. 9 Heterogeneous Catalysts Derived from Known Homogeneous Systems Interest in this area is continuing to grow. Although the majority of the work has been undertaken using cross-linked polystyrene as the heterogeneous support other supports which have been used to anchor catalysts include poly(methally1 and phosphinated (diace tylene) .6 '* Several papers have described the increased activity and selectivity of supported catalysts over their homogeneous counterparts.62 Polymer-supported [IrCl(CO)- (PPh,),] selectively catalyses the hydrogenation of 4-vinylcyclohexene to 4-ethylcyclohexene and the hydrogenation of cyclo-octa- 1,5-diene to cyclo-octene.62u Moreover in the latter case the rate of hydrogenation was significantly greater using the anchored catalyst than using its homogeneous counterpart,62b and it could be recycled without loss of activity or selectivity.The complex [RhH(CO)(PPh,),] was anchored to a polymer support and found to be an active catalyst for the hydroformylation of pent-1-ene giving high yields of both normal and branched aldehyde.62c By varying the phosphine/rhodium ratio the ratio of normal to branched aldehyde could be varied. Butadiene was selectively dimerized to 4-vinylcyclohexene using polymer-supported [Ni(CO),(PPh,),] in high The reduction with sodium borohydride of aromatic nitro-derivatives in the presence of nickel-phosphine catalysts has been studied.62d Whilst the homogene- ous catalyst gives a mixture of products the polymer-supported catalyst shows a high selectivity for the preparation of aromatic azoxy-compounds in good yield.The selective hydrogenation of cup -unsaturated carbonyl and nitrile compounds has been achieved using polymer-supported [Rh6(C0),6].62" Polymeric amines were used as the support and the rate ofhydrogenation was higher for the resin-supported catalyst than for the homogeneous catalyst. This greater rate is attributed to the presence of a higher concentration of Rh6 species the dimerization of the Rh to Rh, species being prevented by polymer attachment.An interesting development of the work of Pittman was the first example of sequential multistep organic reactions carried out by attaching two catalysts to the same Thus the sequential cyclo-oligomerization of butadiene to 4- vinylcyclohexene followed by hydrogenation to ethylcyclohexane was accomplished using a single styrene-divinylbenzene resin to which [Ni(CO),(PPh,),] and [RhCl(PPh,),] had been anchored. 6o H. Suzuki K. Itoh I. Matsuda and Y. Ishii Chem. Letters 1975 641. (u)W. R. Cullen D. J. Patmore A. J. Chapman and A. D. Jenkins J. Orgunomerullic Chem. 1975,102 C12; (b)J. Kiji S. Kadoi and J. Furukawa Angew. mukromol. Chem 1975,46 163.62 (a)S. E. Jacobson and C. U. Pittman jun. J.C.S. Chern. Comm. 1975,187;(b)C. U. Pittman jun. S. E. Jacobson and H. Hiramoto J. Amer. Chem. Soc. 1975,97,4774;(c)C. U. Pittman jun. L. R. Smith and R. M. Hanes ibid. p. 1742; (d) B. Loubinoux J. J. Chanot and P. Caubere J. Orgunometullic Chem. 1975,88 C4; (e)T. Kitamura T. Joh and N. Hagihara Chem. Letters 1975 203. C. U. Pittman jun. and L. R. Smith J. Arner. Chern. Soc. 1975,97 1749. Part (i) The Transition Elements 129 By the reaction of a divinylbenzene-styrene resin with [Cr(CO),] a polymer- anchored tricarbonylchromium moiety bonded to the polymer's phenyl ring was This complex catalysed the selective hydrogenation of methyl sorbate to methyl (Z)-hex-3-enoate. Cyclopentadienyl compounds of titanium have been attached to a styrene-divinylbenzene cop01ymer.~~ The compounds are related to [TiCl,( 17 -C5H5)J and can be reduced apparently without undergoing dimerization to give very active catalysts for the hydrogenation of olefins.The polymer-bound catalysts are much more active than the corresponding reduced non-attached [TiCI,(q-C,H,),] for the reduction of olefins but are not as active for the reduction of dinitrogen. A disadvantage when using insoluble polymers as carriers is the differing accessi- bility of the catalytic functions. With this in mind a series of soluble metal complexes of polymers @ typically a non-crosslinked phosphinated polystyrene was syn- thesized.66 The complexes e.g. [RhCl(CO),@] were active for catalytic hydrogena- tion and hydroformylation but do not appear to be any more active than the normal insoluble polymer-supported catalysts.The soluble macromolecular catalysts were separated from the reaction products by membrane filtration or by precipitation with hexane. An interesting approach to using supported homogeneous catalysts is to conduct the metal-ion-catalysed reactions in the intracrystal space of a swelling layer lattice silicate.67 Using this technique the hydrogenation of hex- 1-ene catalysed by rhodium-phosphine complexes was studied. Since the mineral-bound rhodium complexes showed no activity toward hydrogenation of benzene the reduction is due to metal hydride formation and not to trace amounts of metallic rhodium which is an excellent catalyst for the reduction of aromatics as well as for olefins.10 Reactions of Co-ordinated Ligands Additions and Substitutions at Co-ordinated Ligands.-Interest in the reactions of iron carbonyl complexes has continued. Although the addition of nucleophiles to co-ordinated olefins or dienes normally occurs in an em-fashion it has been shown that the reaction of (tricarbony1)cyclohexadienylironwith MeO- gives the 5-endo- derivative as the ultimate product.68 Work on the reaction of diene-Fe(CO) complexes with Lewis acids has been extended to provide the synthesis of a variety of new ketonic complexes e.g. (16) from (15).69 Although the reaction formally involves the addition of carbon monoxide the yield is independent of the presence of gaseous CO. The synthesis of cis-alk-2-enes by alkylation of tetracarbonyliron complexes using organocadmium reagents has been de~cribed.~' Thus the reaction of the 64 C.U. Pittman jun. B. T.Kim and W. M.Douglas J. Org. Chem 1975 40 590. 65 W. D. Bonds jun. C. H. Brubaker jun E. S. Chandrasekaran C. Gibbons R. H. Grubbs and L. C. Kroll J. Amer. Chem. SOC., 1975,97 2128. E. Bayer and V. Schurig Angew. Chem. Internat. Edn. 1975,14,493. 67 T. J. Pinnavaia and P. K. Welty J. Amer. Chem. Soc. 1975,97 3819. 68 K. E. Hine B. F. G. Johnson and J. Lewis J.C.S. Chem. Comm. 1975,81. 69 B. F. G. Johnson J. Lewis D. J Thompson and B. Heil J.C.S. Dalton 1975,567. 'O A. J. Pearson Tetrahedron Letters 1975 3617. R. Pearce D.J. Thompson,and M. V.Twigg methylallyl-Fe(CO) complex (17) wth Ph2Cd gave the alkene (18)in 60% yield.In all cases the major product was derived by attack at the unsubstituted terminus of 0 (17) (18) The reaction of tropone-Fe(CO) with diazoalkanes has led to a new synthesis of homotropones in good yield (Scheme 2).71 The reaction goes via the stable pyrazoline (19) which is isolated during the reaction. R Fe(COl3 (19) Reagents i R,CN,; ii A; iii Me,NO. Scheme 2 The use of organometallic complexes as protecting groups for olefins and acetylenes has been de~cribed.'~ Olefins co-ordinated to the protecting group [(q-CsHS)Fe(CO),]' are unreactive toward many reagents which attack olefins thus permitting selective transformations at other reactive centres in polyfunctional alkene~.'~~ Moreover the complex selectively co-ordinates to less substituted or strained olefins in a variety of dienes.The free alkene is readily regenerated by the reaction of the complexes with NaI. The CO~(CO)~ group has been used as a protecting group for acetylenes.72b Diarylacetylenes which cannot be acetylated directly undergo nuclear acylation readily as their CO~(CO)~ complexes. Oxidation of the acetylated complex with ceric ammonium nitrate releases the acetylated acetylene in good yield. The regio- and stereo-selective reactions of palladium n-ally1 complexes have been studied.73 In the alkylation of the n-ally1 complex (20) in the presence of 71 M. Franck-Neumann and D. Martha Tetrahedron Letters 1975,1759. 72 (a) K. M. Nicholas J. Amer.Chem. Soc. 1975 9'1 3254; P. F. Boyle and K. M. Nicholas J. Org. Chem. 1975,40,2682; (b)D. Seyferth,M. 0.Nestle and A. T. Wehman J. Amer. Chem. Soc.,1975,97 7417. 73 (a)B. M. Trost and P. E. Strege J. Amer. Chem. Soc. 1975,97,2534; (b)D. N. Jones and S. D. box J.C.S. Chem.Comm. 1975,166. Part (i) The Transition Elements PdC1/2 (20) various ligands it was shown that the nucleophile normally attacks the primary carbon atom but when bulky activating ligands are used the predominant reaction is at the secondary carbon atom.73a Steroidal palladium .rr-allyl complexes are oxidized regiospecifically and with high stereoselectivity to allylic alcohols by 3-chloroperbenzoic acid the hydroxy-group being delivered preferentially to the same diastereotopic face of the .rr-ally1 system as that originally occupied by palladium.736 The use of tricarbonylchromium intermediates in the activation of arenes towards alkylation has been described.74 The readily made arene-Cr(CO) complexes react with a variety of carbanions e.g.LiCH,Cn to give the expected alkylated complexes in high yield.The free alkylbenzene can be liberated in quantitative yield by reaction with iodine or by aerial oxidation. By replacing one of the CO groups in the arene-Cr(CO) complex with other ligands e.g. PPh, the activating power of the Cr(CO) group could be Although a lot of work has been done on metal carbene complexes very little use has been made of them for organic synthesis. A report this year however describes the synthesis of a-methylene- y-butyrolactones uia chromium-carbene complexes of the type (21).75 Although only low to moderate yields were obtained it does indicate the promising nature of these reagents.(21) Generally co-ordinated dinitrogen in metal complexes is inert to chemical attack but this year two reports have appeared on the reaction of co-ordinated dinitrogen in the complex [Mo(N,),(P~,PCH,CH,PP~,),].~~ Alkylation leads to the alkylazo- complexes [MoB~(N~R)(P~~PCH~CH~PP~~)~]~~~ while reaction with trimethylsilyl azide followed by protonation gives the imido-complex [Mo(NH)Cl,-(Ph,PCH,CH2PPh,),].76* Synthesis involving Carbonylferrates.-The application of carbonylferrate species in organic synthesis continues. A number of useful new reactions have been described and earlier work has been reviewed." Two ketone-forming reactions have been reported in which the product contains three additional carbon Ethyl ketones result from treating alkyl halides (or 74 (a)G.Jaouen A. Meyer and G. Simonneaux,J.C.S. Gem. Comm.. 1975,813; (b)M. F. Semmelhack H. T. Hall M. Yoshifuji and G. Clark J. Amer. Chem. Soc.,1975,97 1247. 75 C. P. Casey and W. R. Brunsvoid J. OrgunornefulficChern. 1975,102 175. 76 (a)A. A. Diamantis,J. Chatt G. J. Leigh and G.A. Heath J. OrgunomefuflicChem. 1975,84 C11; (b) J. Chatt and J. R. Dilworth J.C.S. Chem. Comm. 1975,983. 77 (u) M. P. Gmke and R. M. Parlman J. Amer. Chem. Soc. 1975 97 6863; (b) M. Yamashita Y. Watanabe T. Mitsudo and Y. Takegami Tetrahedron Lettern 1975,1867.R. Pearce D.J. Thompson and M. V.Twigg tosylates) with Na,Fe(CO) in the presence of ethylene under ambient conditions [equation (3)].77" This reaction is thought to involve insertion of C2H4 into an C2H4 RX+ Na,Fe(CO) --+RCOCH2CH (3) iron-acyl bond but unfortunately this does not take place with higher olefins. Methyl ketones are obtained from reduction with [HFe(CO),]- of unsaturated pentane-2,4- diones [RCH=C(COMe), products of the reaction of the pentanedione with aldehydes RCHO] under mild conditions.77b The reaction of [RCOCH,Fe(CO),]- with an acid chloride does not produce a 1,3-diketone but the corresponding enol ester.78 By varying the amine reagent ratio selective N-methylation and NN-dimethytation of aliphatic and aromatic amines can be effected by treatment with formaldehyde in the presence of [HFe(C0),]-.79 Substituted aldehydes can be used; for example H0,CCHO gives RNHCH2C02H with [HFe(CO),]- and RNH,.Secondary amines undergo reductive N-alkylation with ketones under an atmos- phere of CO; under these conditions enamines are smoothly reduced to the tertiary amine.80 The reductive amination of glutaraldehyde provides a rapid convenient synthesis of N-alkyl- and N-aryl-piperidines." During an investigation on the facile reduction of nitro- and nitroso-compounds excellent yields of quinoline were obtained from o-nitrocinnamic aldehyde.82 Desulphurizationof thioketones with [HFe(CO),]- gives the parent hydrocarbon; [DFe(CO),]- affords the deuteriated derivative this being a particularly attractive route to geminal dideuterio-species (R'R2C=S -+R'R2CD,).83 Insertion.-Full mechanistic details of insertion reactions into Pt-H bonds in square-planar platinum complexes are not completely resolved.Frequently pro- posed steps are ligand dissociation followed by co-ordination of the incoming molecule (e.g. C,H4) and cis-trans isomerization of intermediates so that the unsaturated ligand becomes cis to the Pt-H bond. Studies on insertion reactions of trans-[(phosphine),PtHL]' have shown that with L = CO the carbonyl group does not leave the co-ordination sphere of the metal during the insertion process with ethylene.84 The complex trans-[(PP)PtH(acetone)J' undergoes ethylene insertion I I PhzP PPh 78 T. Mitsudo Y. Watanabe T.Sasaki H. Nakanishi M. Tamashita and Y. Takegami Tetrahedron Letters 1975,3163. 79 Y. Watanabe T. Mitsudo M. Yamashita S. C. Shim and Y. Takegami ref. 34 abstr. 138. 80 T. Mitsudo Y. Watanabe M. Tanaka S. Atsuta Y. Yamamoto and Y. Takegami Bull. Chem. SOC. Japan 1975,48 1506. 81 Y. Watanabe S. C. Shim T. Mitsudo M. Yamashita and Y. Takegami Chem. Letters 1975,995. 82 Y. Watanabe T. Mitsudo M. Yamashita and Y. Takegami Bull. Chem. SOC.Japan 1975 48 1478. 83 H. Alper J. Org. Chem. 1975,40 2694. 84 H. C. Clark C. R. Jablonski and C. S. Wong Znorg. Chem. 1975,14 1332. Part (i) The Transition Elements 133 into the Pt-H bond even though it is not possible for hydrido and olefin ligands mutually to adopt cis-~rientations.~~ The first fully characterized products from the insertion of an acetylene into an alkynyl carbon-metal bond have been reported;86 earlier work yielded polymeric material [equation (4)].The products isolated are thermally stable and unaffected by dry HCl in ether. Methyl and cr-vinyl complexes undergo similar insertion reactions. [(Et,P),PdCl(C-CPh)] +Me02CC-CC02Me 4 [(Et3P)2PdCIC(C02Me)=C(C02Me)C~CPhl (4) A series of musk compounds were prepared from 12-and 14-membered ring systems obtained from the insertion of MeO,CCECCO,Me into a,w-dodecatrienediylnickel. The metal was removed as [Ni(CO),]."' Schwartz and co-workers8* have found that the readily available [Zr(q- C,H,),HCl] undergoes insertion-isomerization reactions with olefins (alicyclic or cyclic) to form alkylzirconium(1v) complexes.n-Alkyl derivatives are obtained from terminal and internal linear olefins. The subsequent clean high-yield reactions of these complexes with a variety of electrophiles are of potential use in synthesis and have practical advantages over hydroboration and hydroalumination routes. The Zr-R bond is readily cleaved by a variety of electrophiles88" (e.g. H' Br, I, CH,COCl). Since Zr'" is a doelectron system it is possible that these reactions do not proceed by the normally envisioned oxidative addition at the metal but rather by direct attack on the carbon. Alcohols are obtained by reaction with peroxides peracids or even dioxygen though this is a slower process (hours) that involves the formation of [Zr(q -CsHs),C1(OOR)]. Apparently a chiral alkyl complex affords the optically active alcohol on reaction with Bu'OOH but oxygen gives 50% retention and 50% racemization.88b The ready carbonylation (20 p.s.i.room temperature) of [Z~(T~C~H~)~C~R] and subsequent reaction of the acyl complex with electrophiles provides a convenient route to terminal carbonyl derivatives from internal olefins.88c Reactions are summarized in Scheme 3. [Zr(q-C,H,),HCI] also reacts with dialk- ylacetylene to form a mixture of cis-vinylic complexes. After standing for several hours at room temperature an equilibrium mixture containing a high proportion of the sterically favoured complex is obtained. This is the basis of a potential pfepara- tive method for specific tri-substituted olefins since treatment of the equilibrium mixture with N-bromosuccinimide produces a good yield of vinyl bromides with the same stereochemistry and isomer ratio as in the precursor Oxidative Addition.-Halogen-exchange Reactions.Co-ordinatively unsaturated transition-metal complexes catalyse halogen-exchange reactions of alkyl halides.89 The proposed mechanism involved oxidative addition of the organic halide to the metal complex followed by substitution of co-ordinated halide and reductive 85 G. Bracher P. S. Pregosin and L. M. Venanzi Angew. Chem. Internat. Edn. 1975,14,563. 86 Y. Tohda K. Sonogashira and N. Hagihara J.C.S. Chem. Comm. 1975 54. 87 R. Baker P. Bevan and R. C. Cookson J.C.S. Chem. Comm. 1975,752. 88 (a)D. W. Hart and J. Schwartz,J. Amer. Chem. Soc.,1974,% 81 15; (b)T.F. Blackburn J. A. Labinger and J. Schwartz Tetrahedron Letters 1975,3041; (c) C.A. Bertelo and J. Schwartz J. Amer. Chem.Sac. 1975,97 228; (d) D. W:Hart T. F. Blackburn and J. Schwartz ibid. p. 679. Hq J. E. Lyons J.C.S. Gem. Comm. 1975,418. R.Pearce D.J. Thompson and M. V.Twigg ,R COR \ RCOX RX (XU = HCI MeOH-Br, or HONa-H20,) (XU = HCl Br, Cl, C121Ph MeCOCI or H00Bu‘) Reagents i olefin; ii CO; iii XY Scheme 3 elimination of the halogen-exchanged product. However a more recent report” shows that the true catalyst is halide ion formed by quaternization of phosphines derived from the complex. Indeed quaternary ammonium and phosphonium halide salts catalyse exchange in the absence of metal complex. The displacement of iodide from aryl iodides by CN- is strongly catalysed by [Pd(PPh3)4] and in contrast to the alkyl halogen-exchange reactions most probably involves oxidative/reductive elimination Alkyl halides including fluorides also undergo reaction with MoCl in CH2C12 at room temperature to give the corresponding The mechanism of this interesting process is not clear and strangely it is not effective for n-bromoalkanes.Formation of Carbon -Carbon Bonds. Nickel(o) complexes [e.g. Ni(CO),] are re- agents for the production of cyclic systems from a,@-dihalides. They are particularly useful for the production of large-ring Several preparations of biaryls from iodo-aryls have been reported. Ring closure of 1,n -bis-(iodoary1)alkyls to give ortho-bridged biaryls of pharmacological interest can require forcing conditions with the classic copper catalysts.However [Ni(PPh,),] has been found to be a particularly effective catalyst for this A convenient in situ method of generating [Ni(PPh,),] from [NiCl,(PPh,),] zinc and added PPh in DMF has been described.946 This is a particularly useful reagent for coupling aryl vinyl and allylic halides. Biaryls are formed from idobenzenes and catalytic quantities of Pd(OAc) in NEf,.’,‘ The mechanism of this reaction is incompletely understood but it appears to involve the interaction of two arylpalladium species. 90 D. Foster J.C.S. Chem. Cbmm. 1975 917. 91 A. Sekiya and N. Ishikawa Chem. Letters 1975 277. 92 J. San Filippo A. F. Sowinski and L. J. Romano J. Org. Chem. 1975,M.3295. 93 S. Takahashi Y. Suzuki and N. Hagihara Chem.Letters 1974; 1363; E. J. Corey and P. Helquist Tetrahedron Letters 1975,4091. 94 (a)M. F. Semmelhack and L. S. Ryono J. Amer. Chem. Soc. 1975,97 3873; (b) A. S. Kende L. S. Liebeskind and D. M Braitsch Tetrahedron Letters 1975,3375; (c)F. R. S. Clark,R. 0.C. Norman and C. B. Thomas J.C.S. PerkinI 1975 121. Part (i) The Transition Elements Acetylenic hydrocarbons can be substituted by aryl iodides bromoalkenes or bromopyridines under very mild conditions in the presence of [PdCl,(PPh,),] and CuI in Et2NH.95 The active species is thought to be [Pd(PPh,),]. Oxidative addition of organic halide is followed by addition of acetylene with CuI promoting loss of HX in the basic solvent. Subsequent reductive elimination of the monosubstituted acetylene regenerates the active species.K. Sonogashira Y. Tohda and N. Hagihara Tetrahedron Letters 1975,4467.