12 Organometallic Chemistry Part (i)The Transition Elements By M. BOCHMANN and R. A. HEAD ICl New Science Group The Heath Runcorn Cheshire WA7 4QE and M. D. JOHNSON Department of Chemistry University College London 20 Gordon Street London WClH OAJ 1 Introduction The year 1981 saw the publication of an excellent textbook on organometallic chemistry’ that refers substantially to the chemistry of q -complexes. Monographs include surveys of organic synthesis with palladium complexes’ and aspects of homogeneous cataly~is.~ The following reviews refer to organic chemistry or to the reactions of organic ligands; phase-transfer catalysis in organometallic ~hemistry,~ homogeneous asymmetric hydr~genation,”~ the reactions of ClRh(Ph3P)37 and of HCO(CO)~,* heterolytic activation of hydrogen by transition metal^,^ titanocene and vanadocene,” q5-cyclopentadienyl and q6-arenes as protecting ligands towards platinum metal complexes,” and redistribution reactions of silicon catalysed by transition metals1* The literature on the use of transition metals in organic synthesis for 1979 has been reviewed in detail13 and the principal proceedings of the 1st International Symposium on Organometallic Chemistry Directed Towards Organic Synthesis have been p~b1ished.l~ The latter includes papers on palladium-catalysed synthesis of conjugated polyenes bimetallic catalytic systems containing Ti Zr Ni and Pd ’ J.P. Collman and L. S. Hegedus ‘Principles and Applications of Organotransition Metal Chemistry’ University Science Books California 1981.’J. Tsuji ‘Organic Synthesis with Palladium Compounds’ Springer-Verlag Berlin 1980. ’ ‘Aspects of Homogenous Catalysis’ ed. R. Ugo D. Reidel Dordrecht 1981. H. Alper Ado. Organornet. Chem. 1981,19 183. U. Mateoli P. Frediani M. Bianchi C. Botteghi and S. Gladiali J. Mol. Catal. 1981 12. 265. V. G. Comisso V. Caplar and V. Sunjic Synthesis 1981 85. F. H.Jardine Prog. Inorg. Chem. 1981,28.63. M. Orchin Acc. Chem. Res. 1981 14 259. P. J. Brothers Bog. Inorg. Chem. 1981 28 1. G.P. Pez and J. N. Armor Adv. Organomet. Chem. 1981,19,2. l1 P. M. Maitlis Chem. SOC.Rev. 1981 10 1. ’’ M. D.Curtis and P. S. Epstein Adv. Organomet. Chem. 1981,19 213. l3 L.S.Hegedus J. Organornet. Chem. 1981,207 185. Seven contributions in Pure Appi.Chem. 1981,53 2323-2419. 25 5 M.Bochrnann R. A. Head andM. D. Johnson and their application to selective organic synthesis transition-metal templates for selectivity in organic synthesis palladium catalysis in natural product synthesis nucleophilic addition to diene and arene metal complexes the activation of molecular oxygen and selective oxidation of olefins catalysed by Group VIII transition-metal complexes and control in transition-metal-catalysed organic syntheses. Of fundamental interest is the determination of a series of metal-(sp3)carbon metal-(sp2)carbon,metal-hydrogen and metal-oxygen bond dissociation energies for a series of first-row transition-metal complexes MX' (X = CH3 CH2 H or 0)derived from ion-beam experiments." The bond energies correlate with the differences in energy between the metal ion ground-state and the lowest state derived from the 3d"-14s1configuration.This correlation suggests that the forma-tion of the first metal-to-ligand CT-bond involves mainly the 4s orbital on the metal. 2 Reactions of Co-ordinated Ligands Several papers have appeared in which various ligands are reduced and combined into more complex ligands. Thus treatment of a yellow solution of Me2Zr(~-CsHs)2 with H2W(q-C5Hs)2in C6D6 in the presence of carbon monoxide leads to the formation via (3) of (5)which slowly changesinto a mixture of (7)and (8); (Scheme 1). Using 13C0 it is evident that one carbon monoxide has been incorporated R RH co / CPZWH~ /\ CpzZrR +CpzZr ' Cp2Zr (1) R = Me O&C-R 0+wRcp2 I (2)R = Ph (3) R = Me (4) Ph /:$1 R = Me R = Ph %CP,Zm2O + CP2W(V-C*H4) Cp2W=CHPh (7) (8) Cp v-C~HS Scheme 1 because the complex (8) contains the ligand "CH213CH2.The initially formed bimetallic complex (6) from the diphenyl complex (2) is more stable than (5) and can be converted under forcing conditions into the green carbene complex (7-CSH5)2W=CHPh,the structure of which has been confirmed by an X-ray study.16 In the reduction of the cation (9) with NaBH3CN in alcoholic solvents two of the carbon monoxide ligands are converted into two-carbon ligands and subsequently into organic molecules (Scheme 2)." 1s P. B. Armentrout L. F. Halle and J. L. Beauchamp J. Am.Chem.Soc. 1981,103,6501. l6 J. A. Marsella F. Folting J. C. Huffman and K. G. Caulton J. Am.Chem. Soc. 1981,103 5596. '' T. Bodnar G.Coman S. La Croce C. Lambert K. Menard and A. R. Cutler J. Am. Chem. Soc. 1981,103,2471. Organometallic Chemistry -Part (i) The Transition Elements 0 [c~Fe(C0)~]'i,CpFe(C0)2CH20R-% CpFe(CO)(L)C// (9) 'CH20R / I H+ CpFe( CO) (L)CH2CH0 & CpFe( CO) (L)C / CpFe(CO)(L)CH2C02R lH+ CH3CHO CpFe( CO)(L)Et CH3C02R Reagents i NaBH,CN-ROH; ii Ph,P or (MeO),P; iii RiO'; iv R,BH-; v BH; Scheme 2 Just as carbene and q2-alkene complexes are formed in the above reactions so the formation of both types of complex from a single precursor has been postulated to account for the production of the q2-complex (12) and the cation (13) in the reaction of the vinyl complex (11)with acid or of the methoxyethyl complex (10) with acid or the trityl cation (Scheme 3).18The formation of the q2-ethene complex (15) on treatment of (q-C5H5),WMe2'PFz with trityl radicals followed by saturated /OMe \ CpFe(CO)(L)C-H 'Me Ph,C+ (10) [CpFe( CO)( L) =CHMe]+ + [CpFe( CO)(L)(q- CzHJ]+ (12) CpFe(CO)(L)CH=CH2 Y-(11) CpFe(CO)(L)CH26HCH(Me)Fe(CO)(L)Cp (13) (L = Ph3Por CO) Scheme 3 aqueous KOH has been ascribed to the hydrogen-atom abstraction from one of the methyl groups to give the diamagnetic carbene complex (14) which can then undergo insertion of the methylidene ligand into the carbon-tungsten bond.This gives an ethyltungsten cation which is in equilibrium with a hydrido(q2-ethene) cation from which the proton is removed by base to give (15).The intermediate carbene complex can be trapped by PMe2Ph and the equilibrium between the two cationic species was evident from the reaction of (q-C5H5)2W(CD3)2 in protic media (Scheme 4)." In the reaction of the complex (16) with alkylacetylenes the complex formed contains in sequence a carbonyl ligand the acetylene and the carbene ligand. Oxidation of (17)with Ag20 liberates the quinone (Scheme 5); a reaction which Is T. Bodnar and A. R. Cutler J. Organume?. Chem. 1981,213,C31. l9 J. C. Hayes G. D. N. Pearson and N. J. Coope J. Am. Chem. Suc. 1981,103,4648. M. Bochmann R. A. Head and M. D. Johnson H Scheme 4 Ph/\C=Cr(CO) +RCECMe -+ I @:r(co)3 Me0 (16) OMe (17) R = CH2CH=CHCHMeCH2(CH2CH2CHMeCH2)2CH2CH2CHMe2 li aR Me 0 Reagent i Ag,O or CH,C02H-HN03 Scheme 5 is applicable to vitamin K chemistry.'' Carbon dioxide incorporation into a ligand can take place if a negatively charged centre can be generated on that ligand by reversible dissociation of that ligand from the metal.The incorporation shown in reaction (1)is reversible.21 The mechanism of incorporation of carbon dioxide into 2o K. H. Dotz and I. Pruskill J. Orgunomet. Chem. 1981 209 C4. " P.Braunstein D. Matt Y. Dusausoy J. Fisch D. Mischler and L. Richard J. Am. Chem. Soc. 1981 103,5115. Organometallic Chemistry -Part (i) The Transition Elements 259 the titanium complex (18) is not clear but the enoate complex (19) is readily cleaved on hydrolysis to the enoic acid.(Scheme 6). The same reaction has been carried out using chiral cyclopentadienyl ligands and the resulting enoic acid is formed in substantial enantiomeric excess.22 As the titanium(1v) precursor is formed on oxidative hydrolysis in the presence of chloride ion and molecular oxygen the system can be made regenerative (Scheme 6). Cp' = 77-C5H5 or Scheme 6 The reaction of (7-C5H5)2Ti(C0)2 with acetylenes in dry non-polar solvents normally leads to the formation of a metallocycle. However when the reaction is carried out in the presence of a two-fold excess of water (or D20)a hydrogen from the water is incorporated in the formation of the poxobis(dicyc1open-tadienyl)alkenyltitanium(Iv) product (20) in 95%yield [reaction (2)].On hydrolysis the cis-olefin is formed in good yield.23 R2 R' R' RZ Cp2Ti(CO)2+ 2R'CECR' + HzO + HMTi/o\Ti+-(H (2) /\ /\ CP CP CP CP (20) Large ring metallocycles are intermediates in the coupling of ligands on zir- conium(diene) complexes such as (21).Thus (21) reacts with aliphatic aldehydes ketones and nitriles regioselectively at C- 1 to give 2-oxa- or 2-aza-metallocycles which on hydrolysis release the corresponding alcohols or ketone^.'^ Complex (21) also reacts with the terminal or internal alkenes to give highly regiospecific C-C bond formation between C-2 of the terminal olefin and C-4 of the isoprene ligand.Hydrolysis of the intermediate metallocycle liberates the free alkene.The 22 F. Sato S. Iilima and M. Sato J. Chem. SOC.,Chem. Commun. 1981 180. 23 B. Demersman and P. H. Dixneuf J. Chem. SOC.,Chem. Commun. 1981,665. '* H. Yasuda Y. Kajihara K. Mashima K. Nagasura and A. Nakamura Chem. Lett. 1981 671. 260 M.Bochmann R. A. Head and M. D. Johnson \ R 1 II CH,=CHEt E..4 Et Scheme 7 use of an excess of isoprene allows the formation of tail-to-tail dimer~.~~ These diene complexes and the corresponding hafnium analogues are believed from n.m.r. spectroscopy,26 to resemble a metallocyclopent-3-ene as drawn for (21) in Scheme 7. The use of metal complexes to control the course of organic reactions has been elegantly exploited in the oxidative cyclisation of cyclohexadiene(tricarbony1)iron complexes such as (22) having appropriately placed intramolecular hydroxy-groups capable of attacking the transient dienyl cation,27 (Scheme 8)in the cobalt-mediated eo~ M Fe(CO) HO b % Meogoc *OQ Me CO2Me (CO),Fe Me CO&e Me COzMe (22) (23) (24) Reagents i Ac,O-HBF,; ii Ce4' Scheme 8 (2 + 2 + 2)-cycloaddition of a,&-enzynes and alkynes2* and in the cobalt-mediated cycloaddition of several enediynes of the'type shown in Scheme 9.29In each of these cases the organic product a 4,4-disubstituted cyclohexa-2,5-dienone (24) from (23) and the diene (26) from (25) is liberated in high yield by oxidation with cerium(1v).3 StereoselectiveSynthesis The chromium(I1)-mediated reaction between various aldehydes and cis-and trans-1-bromobut-2-ene gives homoallyl alcohols with remarkable threo-selectivity [reac- *' H.Yasuda Y. Kajihara K. Nagasura K. Mashima and A. Nakamura Chem. Lett. 1981 719. 26 H.Yasuda Y. Kajihara K. Mashima K. Lee and A. Nakamura Chem. Lett. 1981,519. " A. J. Pearson and C. W. Ong J. Chem. SOC.,Perkin Trans. 1,1981 1614. '* C.-A. Chang J. A. Kim and K. P. C. Vollhardt J. Chem. SOC.,Chem. Commun. 1981,53. 29 T. R.Gadek and K.P. C. Vollhardt,'Angew. Chem. Int. Ed. Engl. 1981,20,802. Organometallic Chemistry -Part (i) The Transition Elements 261 i - 8% ii/ (26) Reagents CpCo(CO),-A/hv; ii Ce4' Scheme 9 tion A solvent effect also operates with THF giving virjually exclusively the threo-product whereas in N,N-dimethylformamide up to 63% erythro- selectivity is achieved.Preferential erythro -alcohol is obtained regardless of solvent with sterically hindered aldehydes. To account for the pronounced threo-selectivity the chair form of transition state (27) is proposed in which the R-Me interaction is minimized. Epoxidation of the homoallylic alcohols with t-BuO,H gives mainly the erythro-epoxide (28) [(28) (29) = 76 241 using VO(acac), whereas the threo-epoxide predominates [(28) (29) = 18 821 using A~(OBU')~. RCHO +- Br CrC'z '("+ (3) OH OH threo erythro I Me' The important intermediate in the synthesis of Cecropia juvenile hormones (2)-4-methylhex-3-enol is obtained in a single step and under very mild conditions from hex-3-ynol and TiCl,-AlMe3. The reaction is thought to involve methylation at titanium followed by an intramolecular syn-carbotitanation of the yne group to give (30) as the only product on protonolysis (Scheme An interesting stereospecific intramolecular free-radical addition to an alkene occurs in the cyclization of N-methylhex-4-enyl-N- chloramines to 6-functionalized pyrrolidines [reaction (4)].32Whereas several metal salts promote the cyclization only with CuC1-CuC12 is high yield and diastereoisomeric purity combined.30 T. Hiyama K. Kimura and H. Nozaki Tetrahedron Lett. 1981 22 1037. M. D. Schiavelli J. J. Plunkett and D. W. Thompson J. Org. Chem. 1981 46 807. 32 J.-L. Bougeois L. Stella and J.-M. Surzur Tetrahedron Lerr. 1981 22 61. M. Bochmann R. A. Head and M. D. Johnson OH A1 /\ Scheme 10 Vinylsilanes continue to be useful synthetic intermediates and have now been shown to react with acetals to give penta-1,4-diene derivative^.^^ In the presence of Lewis acids especially MoC15 (E)-P-styryltrimethylsilane affords only (E,E)-pentadienes whereas with the (Z)-vinylsilane the (E,Z)-isomer is iso1ate.d.CIC.U7-CI \ 4 Asymmetric Hydrogenation Chiral lactones are from prochiral cyclic anhydrides using Ru2C14(diop) in the presence of triethylamine. For instance 3-methylglutaric anhydride gives (R)-3-methyl-S-lactone under relatively mild conditions (100"C 10 atm. H2). Commercial hydrogenation of invert sugar a 1 1 mixture of D-glucose and D-fructose gives D-mannitol in 25-28'/0 yield. Since D-mannitol is only derived from D-fructose the addition of glucose isomerase (g.i.) increases D-mannitol formation but at the expense of catalyst activity.It has now been found that activities can be increased by at least an order of magnitude using ruthenium- exchanged Y-type zeolite in conjunction with g.i. The enhancement is attributed to both the sieving properties of the zeolite in preventing cell debris from poisoning the catalyst and also an improved dispersion of ruthenium.35 Optically active ditertiary bisphosphines are prepared through a series of high yield steps from the amino-acids (S)-phenylalanine and (S)-valine. Rhodium com- plexes containing these ligand~~~ give excellent optical yields (384%) in the hydro- genation of (Z)-a-acylaminoacrylic acids to the corresponding saturated N-acyl-a-amino-acids.A full paper on the bis(dimethylglyoximato)cobalt(II)-chiral amine alcohol hydro- genation has explained the function of the amino-alcohol and the beneficial effect of adding achiral bases. Under very mild reaction conditions (30 "C 1atm. H,) asymmetric hydrogenation of a-diketones a-ketocarboxylates and 33 T. Hirao S. Kohno J. Enda Y. Ohshiro and T. Agawa Tetrahedron Lett. 1981 22 3633. 34 K. Osakada M. Obana T. Ikariya M. Saburi and S. Yoshikawa Tetrahedron Lett. 1981 22 4297. '' J. F. Ruddlesden and A. Stewart J. Chem. Res. (S),1981 378. 36 W. Bergstein A. Kleemann and J. Martens Synthesis 1981 76. 37 (a) Y. Ohgo S. Takeuchi Y. Natori and J. Yoshimura Bull. Chem. Soc. Jpn 1981 54 2124. (b) S. Takeuchi and Y.Ohgo Bull. Chem. SOC.Jpn 1981 54,2136. Organometallic Chemistry -Part (i) The Transition Elements olefinic compounds gives increased optical yields in less polar solvents and at lower temperatures. A certain amount of preferential hydrogenation is also observed; for instance methyl phenyl diketone affords 88% (S)-a-hydroxy-a-phenylacetone in 56%optical yield. At higher substrate :Co ratios reductively coupled compounds become significant products [reaction (5)]. The new co-catalysts a-aminocar-boxamides (31)and P-aminocarboxamides (32) give poorer optical yields3" in the hydrogenation of benzil and methyl N-acetylaminoacrylate than were found in previous studies. OH C0,Et NMe2 NMe2 I I R'-C-H PhCHCH(0R)CONHCHPh I I CONHCH( R2)Ph Me (31) (32) 5 Nucleophilic Additions Involving r) '-Allyl Complexes The reactions of nucleophiles with 773-allylcomplexes present either in stoicheiometric amounts or as members of catalytic cycles has proved to be an extremely versatile tool for the formation of C-C C-0 and C-N bonds.With palladium complexes in particular the reactions proceed generally under mild conditions and often with a high degree of regio- and stereo-specificity. The palladium-catalysed substitution of allylic substrates often with acetate as a leaving group has found widespread application. An interesting example is provided by the synthesis of 5- 6- and 8-membered azaspirocycles via intramolecular ring closure (Scheme 1l).38Quantitative cyclization is achieved by 5 mol.% Pd(Ph3P) under mild conditions (2 h 70 "C)in the presence of base.This reaction promises to be useful in several natural product syntheses. PdLl OAc-' -2L /H (CH,),-NHR (CH,)" -N R. (L = PPh3 R = CHZPh n = 3 or 4) Scheme 11 38 S.A. Godleski J. A. Meinhart D. J. Miller and S.van Wallendael Tetrahedron Lett. 1981 22 2247. M. Bochmann R. A. Head andM. D. Johnson In some cases it is advantageous to maintain a neutral reaction medium. Since the reaction of allyl halides with alkylating agents such as pentane-2,4-dione liberates acid that may lead to side reactions allylic alcohols and ethers are preferable replacements. Various allylic alcohols have been treated with pentane- 2,4-dione in the presence of a Pd(acac)*-Ph3P catalyst to give good yields of mono- and di-allylated products (Scheme 12).39Benzyl alcohol can also be alkylated but requires more severe conditions.The products include those following a dispropor- tionation of the alcohol into benzaldehyde and toluene. Anhydrous cobalt chloride can replace palladium in these reaction^.^' The cobalt catalyst allows alkylation of benzylic substrates like diphenylcarbinol and bis(diphenylmethy1)ether more smoothly than allyl alcohol. The antibiotics grifolin and neogrifolin have been synthesized in this way. \ H LO R p O H + R' H )=O + 'lz: R/ R.' Scheme 12 Nucleophilic attack of q3-allyl palladium complexes takes place usually on the face of the allyl ligand away from the metal a feature that was elegantly demon- strated in the stereocontrolled synthesis of the side-chain of Vitamin E (35).4' Sodium malonate reacts with complex (33) to give diastereoisomerically pure (34) [5% Pd(Ph3P)4-THF,95%] (Scheme 13).-*-y&OR H RO H N-+ Me \ Me H+ H Pd 0 L' L (33) [N = CH(COOMe)2,L= PPh3] Scheme 13 39 M. Moreno-Maiias and A. Trius Tetrahedron 1981,37 3009. 40 J. Marquet and M. Moreno-Mafias Chem. Lett. 1981 173. '' B.M. Trost and T. P.Klein J. Am.Chem. SOC.,1981,103,1864. Organometallic Chemistry -Part (i) The Transition Elements The presence of palladium catalysts ensures a high degree of regioselectivity in the reactions of 1,3-diene monoepoxides (36) with various nu~leophiles.~~ 3,4-Epoxy-1-alkenes give products of type (37) exclusively whereas 1,2-epoxy-3- alkenes also form minor amounts of 1,2-addition product (38) (Scheme 14).pdLm + R &R1 R &R1 \ (36) PdL N (37) Scheme 14 The cycloaddition of an activated olefin such as cyclopentenone to substituted trimethylenemethyl palladium complexes proceeds with remarkably high regio~electivity.~~ The zwitterionic complexes (40) and (42) are generated from the isomeric precursors (39) and (43) with catalytic amounts of Pd(Ph3P),. Both give an identical addition product (41) with the olefin but different products on proton- ation with dimethylmalonate showing that protonation is faster than equilibration which itself is faster than cycloaddition (Scheme 15). The conversion of (41) into (44) (E = COOMe) Scheme 15 42 J.Tsuji H. Kataoka and Y. Kobayashi Tetruhedron Lett. 1981,22,2575. 43 B. M. Trost and D. M. T. Chan J. Am. Chern.SOC.,1981,103,5972. M. Bochmann R. A. Head and M. D. Johnson (44) demonstrates the value of the method in significantly reducing the number of steps leading to (44) compared with a previous synthesis. Identical trimethylene- methyl intermediates are involved in the reactions of methylenecyclopropanes with olefins in the presence of nickel or palladium catalysts to give mainly methylene- cy~lopentanes.~~ Vinylsilanes are formed regioselectively in the reactions of the allyl acetates (45) and (46) with sodium diethyl mal~nate.~~ No allyl silanes are observed (Scheme 16). (E/Z 22 :78) SiMe (46) Scheme 16 The regiochemistry of the reaction of allyl ethers with Grignard reagents as nucleophiles can be influenced by the choice of metal catalyst (nickel or palladium) and ligand.46 Bis(dipheny1phosphino)ferrocene [47) as ligand gave particularly effective control.The nickel complex of this phosphine generates predominantly (49) whereas palladium leads to (48) (Scheme 17). Ligand control of regioselec- tivity is also apparent in the palladium-catalysed reactions of allylic halides with Me- +PhMgBr 5 Me- Me (48) (E + 2) catalyst Me (49) (47).NiC1212 :88 (47).PdC1296 4 Fe Scheme 17 alkenylzirconium c~mplexes.~’ The use of a phosphine-free catalyst precursor (q3-crotyl)palladium chloride dimer allowed a study of the influence of non-phosphine ligands such as maleic anhydride which promotes coupling at the 44 P.Binger and U. Schuchardt Chem. Ber. 1981 114 3313; P.Binger and P. Bentz J. Organomet. Chem. 1981,221,C33. 45 T. Hirao J. Enda Y. Ohshiro and T. Agawa Tetrahedron Lett. 1981 22 3079. 46 T. Hayashi M. Konishi K.Yokata and M. Kumada J. Chem. SOC.,Chem. Commun. 1981,313. 47 Y. Hayasi M. Riediker J. S. Temple and J. Schwartz Tetrahedron Lett. 1981 22 2629. Organometallic Chemistry -Part (i) The Transition Elements 267 sterically less hindered terminus of the allylic fragment whereas Ph,P has the opposite effect. Maleic anhydride displays a far better selectivity. The- rate-deter- mining step in the catalytic cycle appears to be the oxidative addition of the allylic halide to the Pd(0) intermediate (Scheme 18).This reaction has been applied to the synthesis of 20-(R)-and 20-(S)-cholestan-3-one.J R’ R’ R3 Scheme 18 The attack by nucleophile on a q3-allylmetal complex can occur either cis or trans to the metal. A surprisingly simple method for selecting the stereochemistry of the attack was found in the diacetoxylation of cyclohexa- 1,3-diene.48 The reaction of the diene with HOAc-LiOAc in the presence of benzoquinone (bq) and a palladium catalyst gives trans-(50),but addition of LiCl to the mixture inverts the stereochemistry to give essentially pure cis-(50) (Scheme 19). The reaction was shown to be general for a range of conjugated dienes. HOAc-LiOAc (’ AcO 0- bq-LiC1 cis-(50) Scheme 19 The examples described above deal with the generation of single bonds.Carbon- carbon double bonds are formed in the reaction of aldehydes with allylic alcohols and triphenylphosphine in the presence of a palladium catalyst (Scheme 20).49The equilibrium is shifted by removing water with molecular sieves. The method is quite general and mimics in effect the Wittig reaction. 48 J. E. Backvall and R. E. Nordberg J. Am. Chem. Soc. 1981,103,4959. 49 .M. Moreno-Mafias and A. Trius Tetrahedron Lett. 1981,22 3109. M.Bochmann R. A. Head andM. D. Johnson OH Pdbcac) R'CH=CHCH=CHR2 or + R'CHo + PPh3 dioxanreflux ' + Ph3p=0 + H20 OH (R' = aryl or alkyl; R2 = alkyl) Scheme 20 6 C-C Coupling via Metal-Carbon @-Bonds The facile oxidative addition to low-valent transition-metal complexes by certain organic compounds RX such as vinyl or aryl halides via the generation of intermediates with reactive carbon-metal bonds is now well documented.Reaction of these intermediates can follow two paths both leading to C-C bond formation the first part of this section provides examples of coupling reactions of these intermediates with reagents such as alkenes alkynes and acyl halides and the second part deals with reactions of metal-alkyl-type nucleophiles. Coupling Reactions with Alkenes Alkynes and Acyl Halides.-Con jugated dienals can be prepared by reaction of various vinyl halides with acrolein acetals and amines in the presence of palladium catalysts. With secondary amines 5 -amino-3-enal acetals may also be formed (Scheme 21).Acrylic or maleic acid and their esters &"' -%-& /R1 + HPdLzX H-Pd-L I X 1 OR OR WCHO Scheme 21 Organometallic Chemistry -Part (i) The Transition Elements give the corresponding 2,4-dienoic acid derivative^.^' The reaction of vinyl halides with alkali salts of 3-butenoic acids leads to the regioselective formation of 3,5-dienoic acids. The catalysts are RhCI(Ph,P) or Ni(Ph,P)3 [reaction (6) where RCH=CHBr + CH2=CHCH2COOK c8t,RCH=CHCH=CHCH2COOH + KBr (6) R = Ph or HI. The same product is obtained from phenylacetylene and 3-butenoic acid; the nickel catalyst is inactive in this case.'l The reactive vinyl halide may be part of a wring system as exemplified by the reaction of 2-chlorotropone with styrene [reaction (7)]and Pd(Ph3P)4 as catalyst.The Pd intermediate chloro(2- troponyl)bis(triphenylphosphine)palladium was prepared separately. It reacts with CO in methanol to give 2-methoxycarbonyltropone.52 0 0 The method can be extended to the synthesis of substituted heterocycles. Various N-heterocycles for example 2-bromopyridine react with the commercially avail- able 2-methylbut-3-yn-2-01 as a protected acetylene starting material to give after treatment with NaOH 2-ethynylpyridine. A PdC12(Ph3P)2-CuI catalyst was used.53 The alkylation of halogen-free substrates requires oxidative conditions if the reaction is to be catalytic. Furan and thiophen are alkenylated in their activated 2-positions by electron-deficient olefins such as acrylonitrile and methyl acrylate in the presence of catalytic amounts of Pd(OAc)* and an excess of Cu(OAc) in air.Mono- and di-substituted heterocycles are produced [reaction N-Aroylpyrrole will even react with benzene under these conditions to give mono- and di-phenylated + (8) (X= 0 or S; R = CN Ph or C02Me) The alkylation of orthopalladated compounds with nucleophiles is well known. However it has now been found that complexes of the type (51) also undergo reactions with electrophiles such as acetyl chloride to give ortho-substituted aryl ketones [reaction (9)JS6 'O B. A. Patel J. I. Kim D. D. Bender L. C. Kao and R. F. Heck J. Org. Chem. 1981,46 1061; J. I. Kim B. A. Patel and R. F. Heck. ibid. p. 1067. G. P. Chiusoli G.Salerno. W. Giroldini and L. Pallini I. Orgunornet. Chem. 1981,219 C16. " H. Horino N. Inoue and T. Asao Tetrahedron Lett. 1981,22,741. " D. E. Ames D. Bull and C. Takundwa Synthesis 1981,364. " Y. Fujiwara 0.Maruyama M. Yoshidomi and H. Taniguchi J. Org. Chem. 1981,46,851. " T. Itahara J. Chem. Soc. Chem. Commun. 1981,254. " R. A. Holton and K. J. Natalie Tetrahedron Lett. 1981 22,267. 270 M. Bochmann R. A. Head and M.D. Johnson + CH,COCl -+ + PdC12 (9) 0 Coupling Reactions with Metal-alkyl-type Nucleophi1es.-Palladium phosphine complexes promote the reaction of organic halides with metal hydrocarbyls to give cross-coupling products. The same allylarenes are obtained in a stereo- and regio- selective manner by treating either benzylzinc compounds with vinyl halides or alkenylaluminium compounds with benzyl halides (Scheme 22).Ni(Ph3P) is also an active catalyst but tends to isomerize the ally1 group. No homocoupling was observed. The reaction does not proceed in the absence of Ni or Pd catalysts under comparable c~nditions.~’ The alkenylaluminium compounds of the type used above R’ R2 ArCH,ZnX + X R3 R’ R2 ArCH,X + R2Al R3 (X = halogen) 1 R’ R2 )=.( ArCH R3 Scheme 22 are easily accessible uia the zirconium-catalysed carboalumination of terminal alkynes. Over 90% regioselectivity to the @)-isomer has been achieved and a variety of functional groups can be tolerated [reaction (lO)].” T.t. Z(CH,),CrCH + MeJAl Cp2ZrC1 * Z(CH2)nHH Me AlMe (Z = OH OSiMeBu‘ SPh or I n = 1 or 2) 1,2-And 1,3-transpositions of keto-groups with and without alkylation of the substrate are possible by reaction of enol phosphates with trialkylaluminium in the presence of a palladium catalyst [reactions (11)-(13)].Triethyl- and tri-isobutyl- aluminium give mixtures of alkylated and hydrogenated product~.~’ The modification of nucleic acids and their monomeric units is of interest in the study of enzyme mechanisms. For example 5-arylpyrimidines have been synthe- sized from the 5-chloromercuric derivative and aryl iodides have been synthesized ” E. Negishi H. Matsushita and N. Okukado Tetrahedron Lett. 1981 22 2715. C. L. Rand D. L. van Horn M. W. Moore and E. Negishi J. Org. Chem. 1981,46,4096. 59 M. Sato K. Takai K. Oshima and H.Nozaki Tetrahedron Lett. 1981 22 1609. Organometallic Chemistry -Part (i) The Transition Elements 27 1 AIMe, & +~ 'MSPh' Medph Ph Ph Pd (12) Ph Ph 0 SPh AIMe Phb*+Ph+ 7 X = -OP(OPh)z H II 0 by stoicheiometric transmetallation with Li2PdC14. The yields are generally low [e.g. reaction (14)].60The related 5-allylation of pyrimidine nucleosides has also been reported.61 g'Li,PdCI -HN oAN (14) R I I R (X = H or NO2) The cross-coupling reaction could be significantly simplified if the metal-alkyl component from one of the two organic halides was generated selectively and in situ. This was achieved in the syntheses of benzyl ketones from benzyl bromide and acyl chlorides in the presence of zinc powder and a palladium catalyst.62 The reaction proceeds within 20 minutes at ambient temperature and relies on the preferential oxidative addition of the acyl halide to the palladium catalyst whereas benzyl bromide reacts more readily with zinc to give benzylzinc bromide (Scheme 23).Dibenzyl is the by-product usually in minor amounts. PdLZC12 PdL2 RCoC1+ R-C-PdLC1 ArCH2Br + Zn It 0 / 1 BrZnCH2Ar R-C-CH2Ar + PdLz e R-C-PdL2(CH2Ar) II II 0 0 (R = alkyl or aryl Ar = Ph p-MeC6H4- or p-ClC6H4- L = PPh3) Scheme 23 6o C. F. Bigge and M. P. Mertes J. Org. Chem. 1981,46 1994 D. E. Bergstrom J. L. Ruth and P. Warwick J. Org. Chem. 1981,46 1432. T. Sato K. Naruse M. Enokiya and T. Fujisawa Chem. Lett.. 1981 1135. M.Bochmann R.A.Head and M.D. Johnson 7 Carbonylation In the first example of cyanocarbonylation aromatic iodides are smoothly carbonyl- ated in the presence of potassium cyanide to give good yields of aroyl cyanides (Scheme 24).63" The reaction is conveniently carried out at 100 "C with 20 atm. I CN / / + Pdo + \ Scheme 24 CO pressure in THF with a palladium catalyst and only traces of nitriles are formed as by-products. In contrast when organic chlorides are used the THF acts as a reagent and chlorohydrin esters are formed in 15-35'/0 yield [reaction (15)].63b Yields are greater for smaller cyclic ethers and though higher temperatures (130 "C) are required than for cyanocarbonylation the reaction has been effected at 1atm. CO. The rhodium carbonyl-catalysed synthesis of 5-allyl-2(5H)furanones has been achieved by the carbonylation of alkynes in the presence of an alkene and a hydrogen donor such as ethan01.~~" Reaction (16) using diphenylacetylene at 150- 200 "C 20 atm.ethene and 30 atm. CO gave isolated yields of (52) of 60-73%. Similar products are formed but in lower yield from propene and methyl acrylate R'CECR* + 2CO + R3CH=CH2 + 2[H] R3 63 (a)M.Tanaka Bull. Chem. SOC.Jpn 1981,54,637. (b)M.Tanaka M. Koyanagi and T. Kobayashi Tetrahedron Lett. 1981,22 3875. 64 (a)P. Hong T. Mise,and H. Yamazaki Chem. Lett. 1981,989. (b)T. Mise,P. Hong and H. Yamazaki Chem. Lett. 1981,993. Organometallic Chemistry -Part (i) The Transition Elements whereas with terminal alkynes no furanones are obtained.Interestingly at tem- peratures below 150 "C ethanol rather than the alkene becomes incorporated into the product affording low yields (13% at 100°C) of (53) which can be increased phG: Ph H OEt (53) to 87% in the presence of bases such as sodium The reaction is thought to occur via nucleophilic attack of alkoxide on co-ordinated carbon monoxide to give an alkoxycarbonyl intermediate. Stepwise insertion of the acetylene and CO into the Rh-C bond gives the lactone complex (54) [reaction (17)] from which the furanone is obtained by protonolysis. The carbonylation of methanol to acetic acid using a homogeneous rhodium catalyst has recently been commercialized. It is now found6' that nickel supported on carbon is highly efficient for the vapour-phase carbonylation of methanol principally to acetic acid at 320 "C and 1atm.CO in the presence of an organic halogen compound such as methyl iodide. The catalyst exhibits long life and shows higher selectivity than the supported rhodium analogue and is in addition far less expensive. The high yield synthesis of a,& unsaturated carboxylic esters has been achieved66 by carbonylation of 1-alkenylboranes in the presence of alcoholic sodium acetate under very mild conditions (25 "C 1atm. CO). The reaction proceeds with retention of configuration with respect to the alkenylborane. Thus from (E)-hex-1-enyl-1,3,2-benzodioxaborole methyl (E)-hept-2-enoate (55) was obtained in 92% (g.1.c) yield [reaction (18)]. CH H H )=(-/O* CO-MeOH-PdCI,-NaOAc H 8 Chemistry of Synthesis Gas An interesting approach to synthesis gas chemistry is the use of Lewis acids as solvent media.In a NaC1-AlCl melt Ir4(CO),2 converts synthesis gas (170-180 "C 1atm. CO-H2 1 1 ratio) into a mixture of hydrocarbon^^^" with propane and " T. Inui H. Matsuda and Y. Takegami J. Chem. SOC.,Chem. Commun. 1981,906. '' N. Miyaura and A. Suzuki Chem. Lett. 1981,879. " (a) H. K. Wang H. W. Choi and E. L. Muetterties Inorg. Chem. 1981 20 2661. (6) H. W. Choi and E. L. Muetterties Inorg. Chem. 1981 20 2664. M. Bochmann R. A. Head and M. D. Johnson isobutane as major constituents. Cycloalkanes are also produced with this and with the NaBr-AlBr3 melt although catalyst activities are rather poor. Using the liquid Lewis acid BBr3 and OS~(CO)~~ as catalyst both alkyl bromides and acyclic hydro- carbons are The formation of alkyl halides directly from synthesis gas is quite unique but only occurs with BBr3 and under certain conditions methyl bromide selectivities of up to 69% are observed.Previous studies with homogeneous ruthenium catalysts have shown them to be rather inactive for the conversion of CO-H2 into methanol and methyl formate. Halide promoters are now found6' to give a much faster rate with methanol ethanol and ethane-1,2-diol being formed under more severe conditions (230 "C 850 atm. CO-H2 1:1ratio). While all halides show a beneficial effect over the unpromoted system with iodide ion the effect is particularly pronounced regardless of the cation.Homologation of carboxyfic acids has recently been achieved69 using a soluble ruthenium catalyst promoted by iodide ion. Selectivities to homologated acids are typically 40% with significant amounts of undesirable alkane by-product. The reaction is thought to proceed through acyl iodide formed rapidly in situ which reacts with the catalyst to give an acyl complex. Hydrogenation carbon monoxide insertion followed by attack of water affords the homologated acid (Scheme 25). [Ru] + MeCOI + [Ru]-CMe % [Rul-CH2Me II -HzO 0 HOCCH2Me @ [Rul-CCHzMe I1 II 0 0 Scheme 25 Acetate esters are obtained7' in the absence of iodide ion from CO-H2 in glacial acetic acid. In addition to methyl and ethyl acetates both mono- and di-acetate esters of ethane-1,2-diol are major products.The presence of large cations e.g. Bu,P' both increases the conversion of acetic acid as well as the selectivity to glycol esters. Tracer experiments using Me13C02H have unambiguously identified the source of carbon in the glycol-forming reaction [equation (19)]. 0 II 2CO + 3Hz + 2Me*C02H [Ru? yH20* CMe CH20*CMe II 0 A reinvestigation of benzene alkylation by CO-H2 with transition-metal carbonyls and AlC13 has shown7* that the alkyl group is derived from Lewis acid fragmentation of the arene and that there is no involvement of CO-H or the metal carbonyl. B. D. Dornbek,.L Am. Chem. SOC.,198f 103,6508. ''J. F.Knifton J. Chem. SOC.,Chem. Commun. 1981,41. '' J. F.Knifton J. Chem. SOC.,Chsm. Commun. 1981,188. " L.S.Benner Y.H. Lai and K. P. C. Vollhardt J. Am. Chem. SOC.,1981,103,3609. Organometallic Chemistry -Part (i) The Transition Elements 275 9 Catalysis by Metal Clusters The catalytic behaviour of metal clusters has been reviewed.72 Although there is often little evidence that the active species is a cluster and that no fragmentation has occurred Rh6(C0)16 is found to be exceptionally active for the room-temperature cyclopropanation of alkenes by ethyl diazoacetate [reaction (20)].73 With 2,5-dimethylhexa-2,4-diene~90% isolated yield of ethyl chrysanthemate is obtained which represents turnovers in excess of 2000 per cluster molecule. In addition to higher activities and yields compared with the more conventional Cu and Pd catalysts side reactions are severely reduced.The evidence for Rh6(CO)16 being the actual catalyst comes from its quantitative recovery when the reactions are performed under an atmosphere of carbon monoxide. The recovered cluster shows no loss of activity on re-use. R' R'R~ EtOOCCHN + )=(R3 -N2 b EtOOCq R3R4 (20) R2 R" The same cluster also catalyses the reduction of nitrobenzene to aniline using water as the source of hydrogen.74 Under typical conditions (80°C 1atm. CO) with amine promoters especially 4-dimethylaminopyridine and N,N,N',N'-tetramethyl- 1,3-diaminopropane quantitative formation of aniline and conversion of nitrobenzene occurs. While the same system is also active for the water-gas shift (wgs) reaction (CO + H20 + CO + H2) this is not the source of hydrogen in nitrobenzene reduction.Proof of this is that amines that promote the wgs reaction are virtually inactive for nitrobenzene reduction. Experiments using deuterium oxide give high 2H incorporation in the aniline produced and supports water as the reducing agent. A simple synthesis of from alcohols using Ru3(C0)12 operates by an initial hydrogen transfer from the alcohol to a hydrogen acceptor such as diphenylacetyl- ene. The resulting aldehyde combines with more alcohol to give the ester by a second hydrogen-transfer step. Despite the low activity of the catalyst (turnovers -65) the yields of ester are high (SO%) [equation (21)]. 2RCH20H + 2Phcz~CPh1470c) R-C-OCH2R + 2PhCH=CHPh (21) I1 0 10 Heterocyclic Chemistry Substituted furans are valuable synthetic intermediates but attempts to prepare them by Friedelcrafts alkylation of furan generally result in low yields (-10%) and a significant degree of resinification.A convenient alkylation procedure has 72 E. L. Muetterties Card Rev. Sci. Eng. 1981 23 69; R. Whyman in 'Transition Metal Clusters' ed. B. F. G. Johnson Wiley London 1980 Ch. 8; S. D. Jackson P. B. Wells R. Whyman and P. Worthington in 'Catalysis',ed. C. Kemball and D. A. Dowden (Specialist Periodical Reports) The Royal Society of Chemistry London 1981,Vol. 4,p. 75. 73 M. P. Doyle W. H. Tamblyn W. E. Buhro and R. L. Dorrow Tetrahedron Letr. 1981,22 1783. 74 K. Kaneda M. Hiraki T. Imanaka and S. Teranishi J. Mol. Card. 1981 12 385. 75 Y.Blum D. Reshef and Y. Shvo Tetruhedron Lett.1981,22 1541. M. Bochmann R. A. Head and M. D. Johnson now been using arenemolybdenumtricarbonyl and t-butyl chloride which at 130 "C,affords 2-t-butylfuran with turnovers approaching 153 per molybdenum. Increasing the catalyst concentration increases the amount of 2,5-disubstituted furan at the expense of turnover [reaction (22)]. Substituted pyrroles are in high yield directly from 1-aminoalk-3-yn-2-ols which are themselves synthesized from the conjugated ynone (56) (Scheme 26). In a closely related reaction a range of pyrroles are prepared7' by 1,4- cycloamination of 1,3-dienes in acetic acid (Scheme 27). In the absence of amine high yields of the q3-intermediate (57) are obtained. Nucleophilic attack by the amine on (57) gives the aminoacetate (58).The pyrroline (59) is obtained by intramolecular cyclization of (58) and is oxidized to the pyrrole by Pd" or Cu". 0 OH II (i) Me3SiCN R1-C-C~C-R* -R4-CrC-R2 (ii) LiAIHi I (56) CH2-NH;! I H Scheme 26 (58) Me (57) (59) Scheme 27 An interesting extension to studies on the N-alkylation of amines by alcohols is the synthesis of pyrrolidines from diols and amines [reaction (23)];79 high yields (82%g.1.c.) are obtained. 11 Pyrethroid Synthesis Pyrethroids structurally derived from vinylcyclopropylcarboxylic acid (60),are the most powerful insecticides known to date. The synthesis of the building block has 76 D. J. Milner J. Orgunomet. Chem. 1981 217 199. 77 K. Utimoto H. Miwa and H. Nozaki Tetrahedron Lett. 1981,22,4277.78 Backvall and J.-E. Nystrom J. Chem. Sor. Chem. Cornmun. 1981 59. J.-E. 79 R. Grigg T. R. B. Mitchell S. Sutthivaiyakit and N. Tongpenyai J. Chem. Soc.. Chem. Commun. 1981,611. Organometallic Chemistry -Part (i) The Transition Elements been attempted from very different starting points and synthetic approaches without the aid of transition metals have been reviewed.80 X COOH (60) Cy clopropanation of substituted dienes (61) with diazoacetate gives a mixture of four isomers the ratio of which is influenced by the use of copper complexes of chiral Schiffs bases.'l Cyclopropanation of the diene precursor (62) gives a lower yield but increases the selectivity for the favourable cis-(1R)-isomer (Scheme 28). CI N,CHCOOR cu (61) COOR T COOR Scheme 28 An elegant synthesis of cis- chrysanthemonitrile (63) takes advantage of the high stereo- and regio-selectivity of the palladium-catalysed allylic substitution.82 Alkyla- tion of the allylic acetate (64) occurs at the more highly substituted carbon and gives compound (65) with exclusive E-double bond stereochemistry (Scheme 29; A = COOR or CN; X = COOR S02R' or CN).The stereochemistry of the subsequent ring-closure depends on the nature of the substituents A and X. In the case of A = CN and X = S02Ph only one isomer (66a) is isolated in high yield. The ratio of (66a) :(66b) can be strongly influenced by the presence of a catalyst. If A = CN X = COOEt the presence of Pd(PPh& during the cyclopropanation increases the amount of (66a) from 50 to 95% of the total product.D. Arlt M. Jautelat and R. Lantzsch Angew. Chem. 1981,93,719. " D. Holland D. A. Laidler and D. J. Milner J. Mol. Cutul. 1981 11,119. 82 J. P. Genet and F. Piau J. Org. Chem. 1981,46 2414. M. Bochmann R. A. Head and M. D. Johnson HO Me + NaCHAX PdPPh,) Me>cHAx OH OAc Me Me )=yxVA + Me 'Me Me 'Me Me-Me (A = CN X = SOZPh) Scheme 29 The highly strained bicyclo[ 1.1.O]butane reacts with electron-deficient olefins in the presence of nickel(0) to give allylcyclopropane derivative^.'^ Deuteriation and substitution studies suggest that two geminal u-bonds of the bicycle are cleaved and a nickelallylcarbene complex is formed which cyclopropanates methyl acrylate with retention of the stereochemistry of the olefin (Scheme 30).With bis(1,S-cyclo- octadiene)nickel(O) as catalyst the reaction proceeds smoothly in high yield even at 0 "C.Non-activated olefins do not react. % + 9 O O R D COOR H H Scheme 30 83 H. Takaya T. Suzuki Y. Kumagai M. Hosoya H. Kawauchi and R. Noyori J. Org. Chem. 1981 46,2854. Organometallic Chemistry -Part (i) The Transition Elements (68) (R = H Ph or C02Et) Protection of double bonds can be achieved with iron complexes. Thus the chiral iron complex (67) is cyclopropanated by diazomethane on the unco-ordinated double bond to give after oxidation with Ce4' a cyclopropane with 90% enan-tiomeric excess.84 In contrast the carbene-iron complex (68) reacts with diazo- alkanes to give the diene complex (69) [reaction (24)].85 A.Montpert J. Martelli R. Grke and R. Carrie TetruhedronLett. 1981,22 1961. T.Mitsudo H.Watanabe K. Watanabe Y. Watanabe and Y. Takeganu J. Orgunornet. Chem. 1981 21487.