The Chemistry of Transition-metal Carbene Complexes and their RoIe as Reaction Intermediates By D. J. Cardin, B. Cetinkaya, M. J. Doyle, and M. F. Lappert SCHOOL OF MOLECULAR SCIENCES, UNIVERSITY OF SUSSEX, BRIGHTON BNI 9QJ 1 Introduction The title compounds have the formula (1). They were discovered during the past decade,’ although the Chugaev salts, first prepared in 1915,2 were recently3s4 recognized to contain carbene complexes (e.g. see Figure l).3Nevertheless there have now been more than 200 publications and the topic is one of the fastest growth areas in organometallic chemistry (30 papers in 1971). Initially, interest centred on synthesis and structures, but subsequently much was also learned of the chemistry of the co-ordinated carbene ligands, and of other reactions of carbene complexes.These thenies continue to be elaborated, but a further development is the identification of transition-metal carbene complexes as reactive intermediates in various (organic) syntheses. We may therefore consider two main approaches to the study of transition- metal carbene complexes. One is to examine stable compounds; the other is to investigate those transition-metal systems in which carbene complexes are inter- mediates, including transition-metal-catalysed organic reactions. The former aspect has been comprehensively reviewed5 (and accounts of the contributions from E. 0. Fischer’s laboratory are available6s7) and we now lay more emphasis on the second topic. E. 0.Fischer and A. Maasbol, Angew. Chem.Internat. Edn., 1964,3,580. a L. Chugaev and M. Skanavy-Grigorizeva, J. Russ. Chem. SOC.,1915, 47, 776. a W. M. Butler and J. H. Enemark, Znorg. Chem., 1971, 10, 2146. G. Rouschias and B. L. Shaw, J. Chem. SOC.(A), 1971, 2097. D. J. Cardin, B. Cetinkaya, and M. F. Lappert, Chem. Rev., 1972, 72, 545; see also F. A. Cotton and C. M. Lukehart, Progr. Inorg. Chem., 1972, 16,487. E. 0. Fischer, Rev. Pure Appl. Chem., 1970,24,407; ibid, 1972, 30, 353; C. G. Kreiter and E. 0. Fischer, ‘XXIIIrd International Congress of Pure and Applied Chemistry (Boston)’, Butterworths, 1971, Vol. 6, p. 151. ‘IM. Ryang, Organometallic Chem. Rev. (A), 1970, 5, 67; A. Nakamura, Kagaku No Ryoiki Zokan, 1970,89,285. The Chemistry of Transition-metal Carbene Complexes 2 Stable Transition-metal Carbene Complexes A.Survey of Compounds and their Classification.-Carbene complexes are now known for many of the later transition metals. Metal electron configurations range from d3to dlO,with d5and dgas yet unrepresented, oxidation states from 0 to + 4, and co-ordination numbers from 2 to 7 [taking (r-C5H6)as providing a single co-ordination position]; the corresponding configurations around the metal include linear, square planar, tetrahedral, trigonal bipyramidal, and octahedral. These data are summarized in Table 1 and typical examples are shown in Table 2. Systematic (I.U.P.A.C.)nomenclature for these compounds uses the suffix -ylidene, the ligand being regarded as neutral with respect to the metal oxidation state; thus (OC),Cr-C(0Me)Me is called pentacarbonyl- (1-methoxyethylidene)chromium(O), but trivially is methoxy(methy1)carbene- pentacarbonylchromium(0). The majority of carbene complexes are neutral, mononuclear, and have a single co-ordinated carbene.However, cationic species are known, as are a number of di- and tri-nuclear derivatives. To date no anionic carbene complexes have been reported, although the acyl-metallates (LM-C0R)-are intermediates in a number of syntheses [see Section 2C(i)]. Oligocarbene complexes LM(carbene)n (n = 14) have been prepared (e.g. the mercury compound in Table 2) and also complexes with a chelating dicarbene ligand (e.g. Figure l).a Me H2 H3 ‘1 2/N-N\ yM= H/N-c\ /C-N\, CI/pd\ c1 Figure 1Approximately square-planar around Pd (CW) :Pd-C = 1.86 A, C-l-N-1 = 1.45 A, N-2-N-3, C-1-N-2 = 1.38 A, and Pd-Cl = 2.38 %i (see ref.3) The majority of carbene ligands are terminal and unidentate (e.g. Figures 7and 10) although a few bridging examples (Figures 2--4)8~Bp12are reported. In this review a metal carbene complex, whether terminal or bridging, is defined as a species having the ligand CXY with an approximately spa-hybridized Ccarb, attached to the metal without a formal Cc8r-X or Ccarb-Y multiple bond. Consequently, compounds such as those shown in Figures 4, 5, and 9 are not classified as carbene complexes, whereas Figure 3 represents a bridging carbene (SP2-Ccarb).Generally the co-ordinated ligands (CXY) are ‘tertiary’, neither X nor Ybeing hydrogen atoms, but there are some example^^^^^^^ of secondary carbenes.Stable methylene complexes are unknown at present. trans-cis Isomerism, arising 100 Table 1 The occurrence of carbene complexes" d3 d4 ds d7 d0 d10 CrO(6, n; 4, nC) Mn0(6,n,d) Fe0(5,n; 6, n, d, br; Ni0(4, n, t) 7, n, t, br)MnI(4,n) Cor(4, n;d 5, n) FeIJ(4, n;b 4, nc) NbII(5, n, d, br) MoII(5, n, or cl) Mo0(6,n; 4, nc) UY4, n)RuII(4, cl) WO(6, n; 4, nc) ReI(4, n) PdII(4, n or cl) AuI(2,n) $ IrI"(6, n) PtII(4, n or c1, c2) Hg'I(2,c2) $ PtIV(6, n, cl) Y9 Numbers in parentheses indicate metal co-ordination number [(n-C5H5)taken as occupying a single site], and abbreviations are: n, neutral; cl, s' cationic (+ 1); c*, cationic (+ 2); d, dinuclear; t, trinuclear; br, bridging carbene.*This refers to Fe(CO)(NO),CXY. CThis refers to 8 (n-C,H,)M(CO)(NO)CXY. d This refers to CO(CO)~(NO)CXY. k Note: oligocarbene complexes, LM(carbene)n, are known as follows: n = 2, M = CrO, Wo, FeII, RhI, PdII, PtII, and HgII; n = 3, M = IrI, Rh', Q NiII, and PtII; n = 4, M -Pt. b% k!! % b88 "L The Chemistry of Transition-metal Carbene Complexes Table 2 Typical carbene compZexes Complex Ref. (Me,SiCH2)4Nb 2(CSiMe3),a 6 A 21 17 Ph (.rr-C5H &OC) ,Mn-C(OMe)Me 22 [(~-C,H,)(OC)(Ph,P)Fe-C(OEt)Me] + 23 I(7r-C &)(OC),(R,P)Mo--C( OEt)MeJ 23 cis-(OC),(Ph,P)Mo-C(0Me)Me 24 [(T-C,H,)(OC)(R,P)RU-C(OE~)M~]+-23 I,(OC)Rh-C(Ph)N(Me)C(Ph) :NMeC 9,25(OC) ,W-C(NHMe)Me 16 (.rr-C5H5)(0C),Re-C(OMe)Me 26 CI,(Ph,P),Ir-C(H)NMe, 13 [Mez(F3C)(Me*PhP)*Pt-C()1 Oa 27 I ICIdPt -C(NMeH)NHNHC(NMeH) 28 (OC),Mn,--C(OMe)Ph 29 Me 30 Me [(OC),Fe-C(:O)Ph (OC),Fe-(OC),Fe(H)(CNMe,)Fe(CO), 1la (OC),(ON)Co-C(Et)NMe 31 Ph,Sn(OC),Co-C(0Et)Ph 32 Ph CI(PhiP)7Rh--Cf] N 33 Ph C1 ,(PhNC)Pd-C(NHPh)OMe 34 CI,Pd-C(NMeH)NHNHC(NMeH)e 3 102 Cardin, Cetinkaya, Doyle, and Lappert Complex Ref.Phf N &-and irons -CI,(Et,P)Pt -c: 13,14'3 N Ph trans-[Me(Me,As),Pt-C(OMe)Me] +PF6-35 trans-[(EtNC)(PMe,Ph),Pt-C(SCH,Ph)NHEt 1, + 36 [(OC)Ni--C(OMe)Ph l3 37 ClAu-C(OMe)C ,H ,Me-p 38 2+ 39[+$,I a Figure 2. b Figure 10. C Figure 6. Figure 3. Figure 1. f Figures 7 and 8. 8C.K. Prout, T. S. Cameron, and A. R. Gent, Actu Cryst., 1972, B28, 32; M. L. H. Green and J. R. Sanders, J. Chem. SOC.(A), 1971, 1947. O P. B. Hitchcock, M. F. Lappert, G. M. McLaughlin, and A. J. Oliver, to be published. lo F. Huq, W. Mowat, A. C. Skapski, and G. Wilkinson, Chem. Comm., 1971, 1477. l1 P. F. Lindley and 0. S. Mills, J. Chem. SOC.(A), 1969, 1279. ll@ R. Greatrex, N.N. Greenwood, I. Rhee, M. Ryang, and S. Tsutsumi, Chem. Comm., 1970, 1193. lP A. W.Parkins, E. 0. Fischer, G. Huttner, and D. Regler, Angew. Chem. Internat. Edn., 1970, 9, 633. l3 B. Cetinkaya, M. F. Lappert, and K. Turner, J.C.S. Chem. Comm., 1972, 851. 13a P. M. Treichel, J. P. Stenson, and J. J. Benedict, Inorg. Chem., 1971, 10, 1183. l4 D. J. Cardin, B. Cetinkaya, M.F. Lappert, Lj. ManojloviC-Muir, and K. W. Muir, Chem. Comm., 1971,400. l6 D. J. Cardin, B. Cetinkaya, E. Cetinkaya, M. F. Lappert, Lj. ManojloviC-Muir, and K. W. Muir, J. Organometallic Chem., in the press. E Moser and E. 0. Fischer, J. Organometallic Chem., 1969, 16, 275. l7 H. W. Wanzlick, Angew. Chem. Internat. Edn., 1962, 1, 75. laG. Huttner, S. Schelle, and 0. S. Mills, Angew. Chem. Internat. Edn., 1969,8,515; K. Ofele, ibid., 1968, 7, 950. lo G. N. Schrauzer, H. N. Rabinowitz, J. A. K. Frank, and I. C. Paul, J. Amer. Chem. SOC., 1970, 92, 2 1 2. IDJ. Cooke, W. R. Cullen, M. Green, and F. G. A. Stone, J. Chem. Soc. (A), 1969, 1872. I1 F. A. Cotton and C. M. Lukehart, J. Amer. Chem. SOC.,1971,93,2672. E. 0. Fischer and A. Maasbol, Chem.Ber., 1967, 100, 2445. lS M. L. H. Green, L. C. Mitchard, and M. G. Swanwick, J. Chem. SOC.(A), 1971, 794. s4 E. 0.Fischer and R. Aumann, Chem. Ber., 1969,102, 1495. *5 M. F. Lappert and A. J. Oliver, J.C.S. Chem. Comm., 1972, 274. es E. 0. Fischer and A. Riedel, Chem. Ber., 1968, 101, 156. "M. H. Chisholm and H. C. Clark, J. Amer. Chem. SOC.,1972, 94, 1532. *8 A. L. Balch, J. Organometallic Chem., 1972, 37, C19. *s E. 0. Fischer and E. Offhaus, Chern. Ber., 1969, 102,2449. 30 K. Ofele, Angew. Chem. Internat. Edn., 1969, 8, 916. s1 E. 0.Fischer, F. R. Kreissl, E. Winkler, and C. G. Kreiter, Chem. Ber., 1972, 105, 588.'* D. J. Darensbourg and M. Y. Darensbourg, Inorg. Chem., 1970, 9, 1691. 33 D. J. Cardin, M. J. Doyle, and M. F. Lappert, J.C.S.Chem. Comm., 1972, 927. s4 B. Crociani, T. Boschi, and U. Belluco, Inorg. Chem., 1970, 9, 2021. s6 M. H. Chisholm, H. C. Clark, and D. H. Hunter, Chem. Comm., 1971,809; M. H. Chisholm and H. C. Clark, Inorg. Chem., 1971, 10, 171 1. a8 H. C. Clark and L. E. Manzer, Inorg. Chem., 1972, 11, 503. 57 E. 0. Fischer and H. J. Beck, Angew. Chem. Internat. Edn., 1970, 9, 72. I* F. Bonati and G. Minghetti, Synth. Inorg. Metal-org. Chem., 1971, 1, 299. H. J. Schonherr and H. W. Wanzlick, Chem. Ber., 1970,103, 1037. The Chemistry of Transition-metal Carbene Complexes Figure 2 Nb-1-C-1 = 1.995 A, Nb-2-(2-1 = 1.954 A, arrd Nb-1-Nb-2 = -n2.897 A; Ca-2 = 85.6', Ne-2 = 94.4", Nb-1C-1 Si = 119.8", Aand Nb-2 C-1 Si = 142.4' (Nb-1 and Nb-2 and C-1 and C-2 are related by a centre of symmetry inside the ring)lO Ru2 Figure 3 Both metalshave a distorted octahedral Figure 4 Ru-1, Ru-2, Ru-3, and environment and lie in a crystaliographic mirror C-1 to C-7 alI coplanar, with Ph plane: Fe-24-5 = Fe-245' = 1.945 A, ringperpendicular to this plane, C-54-5 = 1.262A, Fel-0-5 = Fe-14-5' ring 1-6 shows marked bond-= 1.967 A, and Fe-1-Fe-2 = 2.568 A: Iength alternation: Ru-347 n-n = 2.09A (see reJ 12)Fe-2 C-5 0-5 = llQ", C-2 Fe-2 C-5 =.8.41" (see reJ 11) from alternative arrangements of ligands around a central metal, is established for square-planar PdII and PtlI (e.g. Figures 7 and 8)l4Jsand geometrical isomerism due to alternative arrangements within the carbene ligand is known for Cro, Moo, Wo,and PtI1 [e.g.(2) and (3)].'* Cardin, Cetinkaya, Doyle, and Lappert Me. PhC1/"\ \I /C2Ph -Rh-N2 Et0' I \MeI Figure 5 Mo-C-1 = 2.08 81and Mo-N-1 = 2.11 8, (see ref. 8) Figure 7 Square-planar environment around Pt: Pt-C = 2.020A, Pt-C1 = 2.31 1 A, Pt-P = 2.291 8,, Ccarb-N = 1.348 A, and N-Ph = 1.403 8, (see refs. 14 and 15) Ph Fe' (CO), Ph Figure 9 Fe-1-C-1 = 2.089 A, Fe-2-C-1 = 1.96981, Fe2-C-2 = 2.069A, Fe-1-S = 2.243 A, and Fe-1-Fe-2 = 2.533 A (see re$ 19) Figure 6 Approximate octahedral en- vironment around Rh: Rh-C-1 = 1.97 A, Rh-N-2 = 2.05 A, C-1-N-1 = 1.33 A, N-14-2 = 1.43 A, and C-2-N-2 = 1.30 8, (see ref. 9) Figure 8 Square-planar environment around Pt :Pt-C = 2.0098,, Pt-Cl-1 = 2.362 81, Pt-c1-2 = 2.381 81, Pt-P =2.234A, Ccarb-N = 1.327& N-Ph = 1.395 A (see ref.15) Figure 10 Approximate octahedral en- vironmentaroundcr: Cr-C-l= 2.05A, C-2 or C-3-Ph = 1.45 A, Cr-CO = 1.88-1.92A, and C-2-C-3 = 1.35%L (see ref. 17) 105 The Chemistry of Transition-metal Carbene Complexes Me H I I (OC)&r-C /N\ H (OC)&r -c /N\ Me\ \Me Me (2) (3) All the authenticated stable carbene complexes so far described (more than 300 compounds) have X and/or Y capable of conjugating with the electrophilic Ccarb and, except for three compounds with the ligand 2,3-diphenylcyclopropene (e.g.Figure 10),17haveX and/or Y as an oxy-, thio-, seleno-, or amino-substituent ; ligands are listed in Table 3.Hence, existing complexes may be said to originate from nucleophilic carbenes.18 A single electrophilic carbene structure (4) remains to be verified;20 an alternative structure, (Ph2MeP),(0C)IrCC1(CF,),, is possible. Cl Cationi~~~p~~and anionic carbene complexes may alternatively be regarded as metallo-carbonium ions or -carbaniom. For example, in the compound formu- lated as trans-[Me(Me,As),Pt-C(OMe)Me]+PF6-in Table 2, the positive charge may be largely localized on either Pt or Ccsrb. The carbonium ion symbolism has been useful for rationalizing some of the reactions of such complexes.3sA mercury dicationic compound (see Table 2) may more reasonably be formulated74 as shown. Ph Ph Ph Ph Table 3 Carbene ligands* (i) Acyclic carbenes Non-chelated R'(R20)C-R1R2N(R30)C-R1R2C :N(R30)C- R'(R2S)C-R1(R2Se)C-R'(H2N)C-R1(R2NH)C-R' (R2 2N)C- R1NH(R2NH)C-R' ,N(R2NH)C- R1(R2R3C: N)C- (ii) Cyclic carbenes Carbene R' @c-R' 1 R2 c,I c--R2N' CC-R' Cardin, Cetinkaya, Doyle, and Lappert Footnote Bidentate or chelated Footnote a -O(R1)C- m b -CNMe2- n C -C(MeNH)NR1 -(MeNH)C- 0 d -C(MeNH) -NR1 -NH(MeNH)C- p e f -CH SNMe -CH, .NMe CH--NR1C(R2)NHR'(R2)C- r 4 gh i i k I Footnote Carbene Footnote R' S [>c- W R' Fi' t c;c- X N R' R' U N+ "4II c-- Y R' Ph Ph V z References occur on next page 107 The Chemistry of Transition-metal Carbene Complexes * Footnotes show the identity of groups R1, R1, and R8 excepting that simple alkyl and aryl groups [Me, Et, Pr, Bu, and (0, m, P)-C&,.X (X= H, Me, OMe, NMe,, F, C1, Br, or CFJ] are not listed separately in the footnotes, but are denoted by the symbol R; fu = 0(X= 0, S, or NR)., PhCi C,48 CH8:CH,44 I-naphthyl,46 ferr~cenyl,~~ a ~.aap,z R' = C8F5y4' C6C15,41 PhCHzYPz fU,44*46or CH2SiMe,;46 Ra = H,23986 fu,44,46 SiMe3,47 Li,48,40 or CpTiC1. b R.81,50-51 c R.68 d R;54*55 R' = f~4~945or SiMe3.56 8 R.6 f R;54,57 R1 = fu,44,46 0 R;64,68,50 Rl = CHz:(MeO)C Me(O:)C, or Me(MeO),C.609e* R;68,68,68 R1 = H. f R.1~,~~160,66.66f ~-67k ~,682 ~1 = N3s C,H,N, or MeO(0 :)C.46 m R.11946 See ref. 1 la. 0 R.6gP R1 = H or C(:O)NH,.4 Q R.s6 r See ref.13a. R.70 R' = Ph(Me0); R* = C6H11.60~6aSee refs. 21, 27, and 71. U R.l5W R.aots' 2 R.14p16133. fl R.7a R* = OR, NHEt, Ph, or Nn0 .78 W 40 E. 0. Fischer, H. J. Beck, C. G. Kreiter, J. Lynch, J. Miiller, and E. WinkIer, Chem. Ber., 1972,105, 162. J. A. Connor, E. M. Jones, and J. P. Lloyd, J. Organometallic Chem., 1970,24, C20; G. A. Moser, E. 0.Fischer, and M. D. Rausch, ibid., 1971,27, 379. I1M. Y. Darensbourg and D. J. Darensbourg, Inorg. Chim. Actu, 1971, 5, 247. E. 0.Fischer and F. R. Kreissl, J. Organomerallic Chem., 1972, 35, C47. I4J. A. Connor and E. M. Jones, J. Chem. Sac. (A), 1971, 1974. 45E.0. Fischer, C. G. Kreiter, H. J. Kollmeier, J. Muller, and R. D. Fischer, J. Organo-metallic Chem., 1971, 28, 237. 4a 3. A. Connor and E.M. Jones, Chem. Comm., 1971,570; J. Chem. SOC. (A), 1971, 3368. E. Moser and E. 0. Fischer, J. Organometallic Chem.. 1968, 12, P1. aa E. 0.Fischer and V. Kiener, J. Organometallic Chem., 1970, 23, 215. E. Hadicke and W. Hoppe, Acta Cryst., 1971, B27, 760. E. M. Badley, J. Chatt, and R.L. Richards, J. Chem. SOC.(A), 1971, 21 ;E. M. Badley, B. J. L.Kilby, and R. L. Richards, J. Organometallic Chem., 1971, 27, C37. 61 E. 0. Fischer and H. J. Kollmeier, Angew. Chem. Internat. Edn., 1970, 9, 309. 6a U. Schollkopf and F. Gerhart, Angew. Chem. Internat. Edn., 1967, 6, 560. O3 M. F. Lappert and J. McMeeking, to be published. 64 U. Klabunde and E. 0. Fischer,J. Amer. Chem. Soc., 1967,89,7141; R. J. Hoare and 0.S. Mills, J.C.S. Dalton, 1972, 653.S6 E. 0. Fischer, M. Leupold, C. G. Kreiter, and J. Miiller, Chem. Ber., 1972, 105, 150. 56 B. Cetinkaya and M. F. Lappert, to be published. 67 E. 0. Fischer and H. J. Kollmeier, Chem. Ber., 1971, 104, 1339. 68 J. A. Connor and E. 0. Fischer, J. Chem. SOC.(A), 1969, 578. 59 E. 0.Fischer, B. Heckl, and H. Werner, J. Organometallic Chem., 1971, 28, 359. 60 R. Aumann and E. 0.Fischer, Angew. Chem. Internat. Edn., 1967, 6, 879. G. Huttner and S. Lange, Chem. Ber., 1970,103, 3149. 6a R. Aumann and E. 0. Fischer, Chem. Ber., 1968, 101, 954. 68 J. A. Connor and E. 0. Fischer, Chem. Comm., 1967, 1024. 64 P. M. Treichel and W. K. Dean, J.C.S. Chem. Cornm., 1972, 804. 65 F. Bonati, G. Minghetti, T. Boschi, and B. Crociani, J. Organometallic Chem., 1970,25,255; F.Bonati and G. Minghetti, ibid., 24, 251. O6 R. J. Angelici and L. M. Charley, J. Organometallic Chem., 1970, 24, 205. 67 U.Schollkopf and F. Gerhart, Angew. Chem. Internat. Edn., 1967, 6, 970. 68 L. Knauss and E. 0. Fischer, Chem. Ber., 1970,103,3744; J. Organometallic Chem., 1971, 31, C68. 60 J. Muller, A. L. Balch, and J. H. Enemark, J. Amer. Chem., SOC., 1971, 93,4613. '0 K. Ofele, J. Organometallic Chem., 1970, 22, C9. 71 C. P. Casey and R. L. Anderson, J. Amer. Chem. SOC., 1971,93, 3554. K. Ofele and C. G. Kreiter, Chem. Ber., 1972, 105, 529. 73 C. W. Rees and E. V. Angerer, J.C.S. Chem. Comm., 1972,420. 74 C. J. Cooksey, D. Dodd, and M. D. Johnson, J. Chern. SOC.(B), 1971, 1380. Cardin, Cetinkaya, Doyle, and Lappert B.Structureand Bonding.-It has been noted (see Section 2A) that stable metal carbene compIexes are derived from nucleophilic carbenes and that Ccarb is highly electrophilic. This results in multiple bonding with the heteroatoms (X or Y) of the ligand [see (Sa)] and not in (d-ph (back bonding) with the metal. As a ligand,* we can therefore describe the co-ordinated carbene as a strong 0-donor, but a weak n-acceptor. In this context the polarity clearly differentiates it from +-the 'ylide' (e.g., R,P-CH,), structure. The conclusion that (5a) and (5b) are the principal canonical forms implies (i) the absence of a bond order significantly greater than unity in M-Ccarb, (ii) the considerable multiple bond character in Cmrb-x, (iii) the electrophilic character of Ccarb, (iv) the analogy between Ccarb-oR or Ccarb-NR'R' and Cacyi-OR or Cacyl-NRIRa, rather than Cal~l-OR or Ca1e1-NR1R2 organic compounds, and (v) an electronic effect of the carbene ligand on M... .. Hi /x /x-LM-d -LM-CLGI-C \ \ \ v Y Y (5d (5W (5d The clearest evidence for (i), (ii), and (iv) is crystallographic. X-Ray results are now available for more than fifteen compounds. The first such study was on a chromium complex (Figure 11);75some other data are summarized in Figures 1-9. Figure 11Essentially octahedral environment for Cr;Ph at 90"toplane Of Sp2-&b Cr-C-1 = 2.04 A, G1-0 = 1.33 A, 0-Me = 1.46 A, C-l-Ph = 1.47 A, A nCr-C-2 = 1.87 A, andCr-C-3 = 1.86-1.91 A; Cr C-1 0 = 134",Cr C Ph = -122", 0C-1 Ph = 104", and Cme = 121"(see ref.75) It is manifest that z(M-c!carb) (Z denotes bond length) is not particularly short: e.g.,from Figure 11 note that Z(Cr--Ccarb) > Z(Cr-CO); and from Figures 7 and 75 0.S. Mills and A. D.Redhouse, Angew. Chem. Internal. Edn., 1965,4,1802; J. Chem. SOC. (A), 1968, 642. The Chemistry of Transition-metal Carbene Complexes 8 Z(Pt-C) w Z(Pt--C,,a) in trans-[CI(Ph,MeP),Pt--CH,SiMe,] (2.079 A).?, On the other hand, ((Ccarb-x) is significantly shorter than expected for a single bond: e.g. from Figure 11 note that l(Ccarb-0) is shorter even than the Cacyl- OR bond in an ester such as MeC0,Et (1.36 A) and from Figures 7and 8 that /(Ccarb--N) is shorter than in an amide such as MeCONHPh (1.35 A).'? Supporting testimony for (i) is chemical.Thus, there are scarcely any reactions of carbene complexes which suggest M=Ccarb double-bond character (see Section 2C), but insertion reactions with PhSeH or C,HllNC may convenient- ly, although not inevitably, be interpreted as proceeding via such a structure. Nuclear magnetic resonance studies of rotation about Ccarb-NR'R2 or Ccarb-OR bonds show that the energy barriers are considerable and indeed higher than in carboxylic acid amides or esters, thus providing further demonstra- tion of (ii) and (iv). In (OC),Cr-C(OMe)C,H,.OMe-o, -m,or -p, dGt = 13.2, 11.9, or < 8 kcal mol-l, respectively, and in (OC),Cr-C(OMe)C,H, CF3-0, -m,or -p is 13.5, 12.1, or 12.3 kcal mol-l, re~pectively:~~~~~ these trends support the view that the high barrier is due to CO bond multiplicity rather than inversion at oxygen.In (OC) ,Cr-C(OEt)Me, (OC) ,Cr-C(NMe,)Me, (OC) ,Cr-C(0Et)- NMe,, and (OC),Fe-CfNDMe),, dGt values are 13.6 (about CO), > 25 (about CN), 20.8 (about CN;< 8 about CO), and 16.6 (about CN) kcal mol-l, respect-ively., Such data show that barriers to rotation are greater about CN than about CO (and hence probably that N-Ccsrb -?*q occursto a greater extent than Sccarb) and that when both the groupsX and YarecapabIe-of.rr-bondingwith Ccarb, CX and CY bond multiplicities are lower than is the case when only X or Yhas this capacity. Consistent with (ii), (iii), and (iv) are the reactions of the co-ordinated carbene ligand. These have been most clearly demonstrated for alkoxycarbenechromium- (0) compounds (see Figure 13).Nucleophilic substitution reactions at Ccarb and electrophilic substitution at the contiguous carbon in LM-Ccarb(OMe)CH,R are particularly significant. Also relevant to (iii) and (iv) are some n.ni.r. data. 13C chemical shifts, 8(13C), which promise to provide a useful diagnostic tool for metal carbene complexes, show that Ccarb is substantially deshielded. Values of 8(13C) (in p.p.m., relative to Me,% in CDCl,) for various complexes are: (OC),Cr-C(OMe)Me, 362.3 ;79 (OC) ,Cr-C( OMe)Ph, 3 54.5 ; (OC) ,Cr-C(NHMe)Me, 284.8 ; (OC)&r-C(NMe,)Ph, 277.5 ;79 (OC),W-C(OMe)Ph, 322.8 (OC),W--C(OMe)Me, 332.9;81(OC),W-C(SMe)Me, 332.5 ci~-(0C)~Cr- [C(NMeCH,),],, 141.0;s2 (OC),Fe-C(NMeCH ,) ,, 213.O ;82 trans-[Cl,(Bun3P)Pt-C(NMeCH2) ,I, 178.0;s2 '6 M.R. Collier, C. Eaborn, B. JovanoviC, M. F. Lappert, Lj. ManojIoviC-Muir,K. W. Muir, and M. M. Truelock, J.C.S. Chem. Comm., 1972, 61 3. 77 'Tables of Interatomic Distances', Chem. SOC. Special Publication No. 11, 1958; C. J. Brown, Acta Cryst., 1966, 21,442. C. G. Kreiter and E. 0.Fischer, Angew. Chern. Internat. Edn., 1969, 8, 761. 79 L. F. Farnell, E. W. Randall, and E. Rosenberg, Chem. Comm., 1971, 1078. 80 J. A. Connor, E. M. Jones, E. W. Randall, and E. Rosenberg, J.C.S. Dalton, 1972,2419. 81 C. G. Kreiter and V. Formacek, Angew. Chem. Internat. Edn., 1972, 11, 141. 81 D. J. Cardin, B. Cetinkaya, E. Cetinkaya, M. F. Lappert, E. W. Randall, and E. Rosenberg,J.C.S.Dalton, 1973, in the press. Cardin, Cetinkaya, Doyle, and Lappert cis-[C1,(Bun,P)Pt-C(NMeCH2),], 196.5;sz and trans-[Cl(Et,P),Pt-C(NMe-CH,)]+ BF4-, 191.82The values are similar to those found for carbonium ions: e.g., 8(13C)of Me,C+ is 273 p.p.m. to lower field than in Me,CC1.83 Of eighteen organometallic compounds reported in ref. 79,Cmrb in (OC),Cr-C(0Me)Me has by far the lowest 8(13C), although Cacvi in (n-C,H,)(OC),FeCOMe is not far removed. For a range of secondary carbene complexes having the H(Me,N)C- ligand, 8(lH) for C&bH is at T = -1.2 to + 0.9.l3Other more peripheral data (from electric dipole moments, vibrational force constants, ionization potentials, electronic spectra, and other aspects of lH n.m.r. spectra) have been used to discuss the electronic nature of the carbene ligands.g The trans influence (defined as the tendency of a ligand to weaken the bond trans to itself)s4 of several carbene ligands in PtI* complexes is similar to that of a tertiary pho~phine.~~,~~ This may be illustrated by the Z(Pt-Cl) and Z(Pt-P) data of Figures 7 and 8.Supporting evidence comes from v(Pt-CI) and J(195Pt-31P) of such compounds,16,60 and J(1g5Pt-1H) in trans-{PtMe(Y)L2}+PF6-;Y is the trans ligand, including R(RIO)C-, and L is a tertiary phosphine or ar~ine.~~ C. Synthesis and Reactions.-Transition-metal carbene complexes have been obtained from three classes of precursors, (i)-(iii) in Figure 12. (i) Syntheses from Metal-Carbon Compounds. The metal carbonyl route is illustrated in equation (1).The tungsten compounds (6; R = Me or Ph) were the ,OLi LiRW(C0)e +(0C)S w -c,/ [Me4Nlf* I (0C)sW -CORI- INMe41' \R first stable transition-metal complexes to be prepared; methylation then involved diaz0methane.l The synthesis was improved by using oxonium salts,60 and was extended to other transition metals (Cry Mo, Mn, Fe, Ru, and Re)10~22924J6~29948 and other ligands (Table 3). Grignard reagents have been employed, but they are less reactive than the lithium Neutral acyl compounds may likewise be converted into carbene complexes [equations (2)23986and (3)52], and such intermediates, (7) and (8), are probably formed in reactions (4)21and (5).72 83 G. A. Olah, E. B. Baker, J. C. Evans, W. S. Tolgyesi, J.S. McIntyre, and I. J. Bastein, J. Amer. Chem. SOC.,1964, 86, 1360. 84 A. Pidcock, R. E. Richards, and L. M. Venanzi, J. Chem. SOC.(A), 1966, 1707. M. L. H. Green, and C. R. Hurley, J. Organometallic Chem., 1967, 10, 188. 111 The Chemistry of Transition-metal Carbene Complexes 1i,RLi ii,H+-CH2N2 or R30t LMCO LM-CNX 0 II LM-C-X NR' II LM-CX n.-U /xLM-C '* X Y X' /LM-C db 'YY .-L'M-CW \Y '/2CXY=CUY Na2(LM)*' (-2NaHal)Hal2 CXY or L' M (L' Hal,IL) (e.g.Scheme2) IHCXY 1' LMH (-Hq) or LMCl (-HCI) Figure 12 Principal synthetic routes to transition-metal carbene complexes Cardin, Cetinkaya, Doyle, and Lappert (n-C5H ,)(OC),Fe-COMe + HCI -[(n-C,H,)(OC),Fe-C(0H)Me ] +C1- (2) Hg(CONRLR2)2+ 2[Me,0]+[BF4]--[Hg{C(OMe)NR1R2},]2+[BF,],-(3) Co-ordinated isonitriles react with alcohols, primary amines, and sodium bor~hydride~~~ ~e~to yield carbene ~~mpleThe first example is .~ shown in equation (6);50others refer to PdII, PtII, HgII, and FeII.Isonitrile complexes of CrO and Moo as well as PdI2(ButNC), (in contrast to the correspond- ing chloride8') were unreactive.88 The preparation of Chugaev salts is of this as illustrated for (9) in equation (7).*The chelating anionic (mono- carbene ligand in (9) is clearly related to the neutral bidentate (dicarbene) ligand of Figure 1. CiS-Cl,(Et3P)P t-CNPh + EtOH -+ c~s-CI4Et3P)P t-C(0Et)NHPh (6) Neutral imidoyl compounds niay also be converted into carbene com-88 A.Burke, A. L. Balch, and J. H. Enemark, J. Amer. Chem. SOC.,1970,92,2555. G.A. Larkin, R. P. Scott, and M. G. H. Wallbridge, J. Organometallic Chem., 1972.37, C21. J. A. Connor, E. M. Jones, G. K. McEwen, M. K. Lloyd, and J. A. McCleverty, J.C.S. Dalton, 1972, 1246. 113 The Chemistry of Transition-metal Carbene Complexes plexe~,~~, as shown in equation (8).89 The formation of cationic PtII complexes from acetylenes [equation (9)] is critically dependent on the nature of the acetylene, ligands Q, and solvent, and on the reaction ~onditions.~~~~~~~~~~~ “HII+[PFII-trans-I(Ph,P),Pt-C(: trans-[I(Ph,P),Pt-C(NHMe)-NMe)Ph -PhI+(PFsl-(8) i, Ag+[PFJ-trans-ClQ,MePt + R1CfCR2-trans- [MeQ,Pt-C(0Me)- ii, MeOH CHR1R2]+[PFs]- (9) (ii) Syntheses froin Metal-Carbene precursors.Reactions of the co-ordinated carbene ligand have been most widely studied for methoxycarbenechromium(0) complexes, and are shown, with other reactions of such compounds, in Figure 13. Some of these (also found for Mo, W, and Mn) illustrate the analogy mentioned earlier between alkoxycarbenes and carboxylic esters, namely the rections with ammonia, primary and secondary amines, ketimines, and thiols [equation (lo)]. It is noteworthy that not all protic compounds behave similarly [see Figure 13 and Section 2C(iv)]. Both the ~tereochemistry~~~~~~ and the kinetic^^^^^^^ of the aminolysis reaction have been studied. The latter revealed that the reaction proceeds by initial protonation at OMe, [equation (lo)], followed by co-(OC),Cr-C(0Me)R + HA -MeOH + (OC),Cr-C(A)R ordination of a nucleophile and finally reaction of R2NH [= HA in equation (lo)].Deprotonation affords LM-C(NR2)R1. In paraffinic solvents R2NH is capable of acting both as proton donor and acceptor. Clearly Ccarb is an electrophilic centre; this is further demonstrated by the protonic character of the a-hydrogens P. M. Treichel, J. J. Benedict, R. W. Hess, and J. P. Stenson, Chem. Comm., 1970, 1627. D. F. Christian, G. R. Clark, W. R. Roper, J. M. Waters, and K. R. Whittle, J.C.S. Chem. Comm., 1972,458. 91 M. H. Chisholm and H. C. Clark, Inorg. Chem., 1971, 10, 2557. 92 P. E. Baikie, E. 0.Fischer, and 0.S. Mills, Chem. Comm., 1967, 1199. 93 E. 0.Fischer and R. Aumann, Chem.Ber., 1968, 101, 963; Angew. Chem. Internat. Edn., 1967, 6, 181. 9p E. 0.Fischer and V. Kiener, Angew. Chem. Internat. Edn., 1967, 6,961. st, E. 0. Fischer and A. Maasbol, J. Organometallic Chem., 1968, 12, P15. 96 E. 0. Fischer, E. Louis, W. Bathelt, E. Moser, and J. Muller, J. Organometallic Chem., 1968, 14, P9. 97 H. Werner and H. Rascher, Inorg. Chim. Acta, 1968, 2, 181 ;Helv. Chim. Acta, 1968, 51, 1765. E. 0.Fischer and L. Knauss, Chem. Ber., 1969, 102,223. g9 C. G. Kreiter, Angew. Chem. Internat. Edn., 1968, 7, 390. looL. Knauss and E. 0.Fischer, J. Organometallic Chem., 1971, 31, C71. lol J. A. Connor and P. D. Rose, J. Organometallic Chem., 1970, 24, C45. loaE. 0. Fischer and K. H. Dotz, J. Organometallic Chem., 1972, 36, C4.lo3E. 0.Fischer, B. Heckl, K. H. Dotz, J. Muller, and H. Werner, J. Organometallic Chem., 1969, 16,P29. lo4E. 0. Fischer and K. H. Dotz, Chem. Ber., 1970, 103, 1273. lo5 E. Moser and E. 0. Fischer, J, Organometallic Chem., 1968, 15, 147. lo6H.Werner, E. 0.Fischer, B. Heckl, and C. G. Kreiter, J. Organometallic Chem., 1971,28, 367. Cardin, cetinkaya, Doyle, and hppert Ref NH3 NH2Mc NHMe2 * --- Cr-C(NH2)R Cr -C(NHMe)R Cr-C(NMe2)R 54,57 58,92 63 NHPri2 HNZCPh;! MeCHO-NH3 HONH2 H2NNMe2 -e *-Cr-C(NHPr i)R Cr-C( N:CPh*) Cr-C(N:CHMe)R, Cr-NH: C(0Me)R Cr -Ni CMe Cr- C(NH2)R 58,63 68 68 93 93 H0N:CHPh Cr -NH:CHPh, Cr-N ICPh 68 Cr -C(SR')R 54,55 I PhSeH rn Cr -Se/Ph 94 'CH !OMe)R Cr-py, (OC)4Cr(py)2, Et0CH:CH; 95 cr-c,i(oMe)c H+ Cr-C, ,COMe Cr lC(0Me)R CNC6H11 m MeOH NHC6H11 ,C(OMe);!Me 60,62 'N-c6H11 Cr-C, I PX3 (X=Br or I) PH1 ---cis-(OC)~Cr(PH3)2, MeOCH: CH2 cis 40C)~(R13P)Cr-C(OMe)R, cis-(OC)4 Cr(P R1g ) Cr-PX3 NHC6H11 1 96 97 98 HI- "Me&)+ (Cr -1-0)- (NMe4)' 6 McOD -MeONa* Cr -C(OMe)C03 99 Me3OBF4 -MeONa Cr-C(OMe)Et, Cr-C(0Me)CHMez 99 Li[AIH(OBut)i 7 Cr-C(OMe)CH:CHCH:C(OMe)Me 100 EtjSiH Et 3SiCH(OMe)R 101 Ph;! SiHCH(0Me)R 102 Ph( Me0)C :C (0Me)Ph 103 py-HMeC:C(CO7Me)H 104 *In catalytic amount.?Refers to (OC),Cr-C(OEt)Me Figure 13 Reactions of alkoxycarbenechrornium(0) complexes Cr-C(0Me)R (R = Me or Ph), such as (OC),Cr-C(0Me)Me in (OC),Cr-C(OMe)CH,, as shown by the facile conversion (OMe--MeOD) into (OC),Cr-C(OMe)CD, or (OC),Cr-C(OMe)CHnMe,-n (n = 1 or Z).as An interesting reaction of a co-ordinated carbene is shown in equation (11).7s Displacement reactions of either neutral or anionic ligands from transition- 115 5 The Chemistry of Transition-metal Carbene Complexes metal carbene complexes may provide a method of synthesis of further carbene complexes.This is demonstrated by equation (12),13 and has also been used in CrO (e.g. Figure 13), Moo, Wo,RhI, Pd", and PtlI (e.g. Figure 14) chemistry. trans -Br2(Et PIPt-C (NMeCH2)2 frans -Mez(Et3P)P t-C(NMeCH2)Z trans cis -CIz(Et3P)Pt -C(NMeCH2.2 trans -ICI(Et3P)zPt -C(NM~CHZ)~I'[BFLI' trans -CI(H)(Et3P) -Pt -C(NMeCH2)z Me /N\Figure 14 Reactionsllo of trans-Cl,(Et,P)Pt-C I\" Me Additionally, for RhI compounds, it has been possible to displace one carbene Iigand by another [e.g.equation (13)].33Nucleophiles may, however, react in other ways [e.g. Figure 13; for C,H,,NC see also Section 2C(iv)]. Some PtI1 carbene complexes are converted into Ptm derivatives by reaction with chlorine.28 EtaP C13(Ph,P) ,Rh-C(NMeJH +C13(Et3P),Rh--C( N MeJH (12) IC€WG~M*P)CHJ~),C1(PhsP)aRh-C m(Ph)CH,], _______+ Cl(Ph,P),Rh-C [N(C6HI-Me-p)CH,], (13) Two examples of carbene ligand transfer from one metal to another one are known37J07 [e.g. equation (14)].37 It is possible that this proceeds via an electron-rich olefin by analogy with reaction (15).66 WCQ.(?r-CsHs)(ON)(OC)Mo-C(OMe)Ph -+(OC)4Fe-C(OMe)Ph (14) hv Me Me Me N [,;C=C<) FC( COI 5 -(OC),,Fe -C, N Me MU Me K.ofele and M.Herberhold, Angew.Chem. Intwnut. an., 1970,9, 739. Cardin, Cetinkaya, Doyle, and Lappert Square-planar d8complexes trans-Hal,QM-CXY rearrange thermally to &e the thermodynamically more stable cis-isomers [M = Pd or Pt ;Hal = C1 or Br ; NS\Q = RsPorR&; = C(NPhCH&, C(NMeCH,),, or C-MeN--C6H,s](PdIIreacts more readily than PtII).16 (iii) Syntheses from Organic Carbene Precursors. Electron-rich olefins, such as (lo), are good nucleophileslOs and have exceptionally low first ionization poten- tials (ca. 6 eV).lo@They react with certain transition-metal substrates which are responsive to nucleophilic attack to furnish carbene complexes. The first example of this reaction is shown in equation (16) (R = Ph).14 Other carbene complexes to have been made by this procedure are complexes of Cr0,66Feo,6s Rh1,33966 PdI1,l6Jlo and PtII,14~1s~110 and include dicarbene complexes [from Rh,Cl,(CO)4 or Cr(CO), olefins to have been employed are [:CN(R)CH,], (R = Me, Ph, /s\or C6H4.Me-p), [:C-MeN-C,H4-oJ2, and C2(SMe),.Imidazolium salts have been used (Scheme 1) to obtain complexes of CrO, FeO,and Hga+.30,38,72 Electron-rich gem-dichlorides, in which the C4I bonds have appreciable ionic X -CHCr(C0)5]-, R P Me3' / * (OC)5Cr -C(NRCH), Scheme 1 lo' R. W. Hoffmann,Angew. Chem. Internat. Edn., 1968,7, 754; N. Wiberg, ibid., p. 766. meB. etinkaya, G. H. King, S.S.Krishnamurthy, M. F. Lappert, and J.B. Pedley, Chern. Comm., 1971, 1370. B. etinkaya, E. Cetinkaya, and M. F. Lappert, J.C.S. Dalton, in the press. TIe Chemistry of Transition-metal Carbene Complexes character, combine with dianions,as shown in equation (13, the earliest example of such a reaction;,O other reports relate to M%NCHCla and either Na,Cr(CO), Ph Ph + Na2Cr(CO),-(OC)&-C 3-+ 2NaCl (17) Ph Ph or Na2Fe(CO)4.13 Such dichlorides have also been used with co-ordinately unsaturated low-oxidation-state substrates (RhI, IrI, or PtII complexes13 or Pd meta170). This procedure gives carbene complexes by a three-fragment oxidative- addition process, a sequence first postulated in order to account for the reaction products from imidoyl chlorides and RhI complexes (eg.Scheme 2). [Me,NCHCl ]+C1-,ls [(PhNH),CCl]+Cl- (Scheme 2)>, and 2,3-diphenyl-l,1- dichlorocyclopropene have been used. 70 R' N-C-NR' CI3(Ph,P),Rh--C: R R 1NHR' (Ph3P)3RhCl C13( Ph3P)zRh-C, R\\ I(PhNH12CCl)'Cl; 'Cf3( Ph3 P)2Rh- C(NHPh)Z Scheme 2-Three-membered-ring compounds LM-COS [e.g. (n-C,HJ(OC),-Mo-C(NMe,)S and (Et,P)ClPt-C(SMe)S] are known for X = SMeKBand NMe,.64 (iv) Other Reactions. The reactions of transition-metal complexes may be divided into those in which (a) another carbene complex is formed [see Section 2C(ii)], (b) the carbene ligand is transformed, but its constituents remain within the co-ordination sphere of the metal, and (c) the carbene ligand is displaced. Illus- trations are provided in Figures 13 and 14.A number of protic compounds do not behave according to equation (10). These include HONH,, HONHPh, H,NNMe,, PhSeH (Figure 13), and HN, {on [Me4N]+ [(OC),Cr-C(O-)CH,SiMe, ] to give (OC)5Cr-NCMe}.46 All these reagents afford metal-nitrogen co-ordination compounds :the forma- tion of isonitrile complexes may involve an initial methoxy displacement asshown in equation (10) and subsequent rearrangement [e.g. (ll)]. The reactions with PhSeH and CBHl1NC are essentially insertions into the Cr-Cmb bond (Figure Cardin, cetinkaya, Doyle, and Lappert 13). There is a single example of conversion of a co-ordinated carbene into a substituted methyl complex [equation (18)].23 A related reaction is the reversible conversion of Cl,(Ph,P)Pd-C(0Me)NHPh with base into the imidoyl complex [Cl(Ph,P)PdC(OMe)(: NPh) NaBH,-EtOH [(n-C,H,)(Ph,P)(OC)Fe-C(0Et)Me ]+____+ (n-C5H5)(Ph3P)(OC)Fe-CH(OEt)Me (1 8) From Figure 13 it is evident that the carbene ligand may be displaced from chromium by a suitable nucleophile such as pyridine or a phosphine.Similar, but less extensive, results are available for complexes of Moo,Wo, RhI, and HgII: an example is in equation (19).l12Especially noteworthy are those reactions in Hg[C(NPhCH)2]a2++ HZS +HgS + 2[HC(NPhCH),]+ (19) which the carbene ligand is trapped, by dimerization, rearrangement [e.g. Me(Me0)C: 4MeOCH=CH,], or a trapping agent. Because stable metal carbene complexes are derived from nucleophilic carbenes, olefins such as cyclohexene are not particularly good reagents for this purpose, and hence the use of compounds such as $-unsaturated esters.lo4 Silanes and related hydrides are particularly effective: the carbene inserts into the M-H bond101#102(but see Figure 14).3 Metal Carbenes as Reaction Intermediates or Transition States. Several reactions are known for which metal carbene complexes have been postulated as intermediates or transition states. This section describes such reactions, some of which are synthetically important. Figure 15 summarizes details of organic and transition-metal reactants and products for the types of reactions outlined in Sections 3A-E, and Scheme 3 gives a particular example for Section 3D. The evidence in favour of intermediate carbene complexes in the reactions shown in Figure 15 is not equally strong in all cases.Thus, whereas the metal- catalysed decomposition of diazoalkanes (Section A) and the alkylation of carbonylmetallates (Section B) leaves little room for doubt concerning such 111 B. Crociani and T. Boschi, J. Organometallic Chem., 1970, 29, C1. lla H. W. Wanzlick and H. J. Schonherr, Angew. Chem. Internat. Edn., 1968,7,141. The Chemistry of %arition-metal Carbene Complexes XYCN2 M', especially Cur and Pd*' /x LiR RR' CXY Cr Iraq. q.MeCBr2Me Strained various metal catalysts carbocyc'ics 1C.g. Scheme 31 Olefin Dismuted dismutat ion olefins e.g. R'ZN, ,NRZ ,c=c, R: N NR2 Figure 15 Reactions proceeding via carbene-metal complexes products including products -including Scheme 3 Cardin, Cetikaya, Doyle, and Lappert intermediates, the role of the metal in the cyclopropanation reactions (Section C) is rather different.As to Section D, many strained-carbocyclic rearrangements certainly do not involve complexed carbenes, although there is a wealth of circumstantial evidence in favour of such a mechanism in other cases. In terms of Scheme 3 we are here concerned only with reactions proceeding via species analogous to ((12)-(13)] (path b) i.e. carbene complexes or metallo- carbonium ions, and not via metal-substituted carbonium ions, (path a) in which the metal is at a site remote from the carbon with greatest positive charge. Finally, attention is drawn to some reactions which proceed through unstable carbene species but are not of general synthetic utility and are not outlined in Figure 15.These include [braces { } denoting those which have not been iso- lated] {(?r-c,H,)(OC),Fe-CH,+) (see Section C), {(n-C5H5)(OC),Mo--CH2-}, and {(?r-CSH6)(OC), [(C,H, Me-p),CN]Mo-C(C,H, Me-p), 1, which are de- tailed here. The reaction of(T~-C,H,)(OC),MON~with ClCH,SiMe, surprisingly afforded (n-C,H,)(OC),Mo-Me, and not the expected silylmethyl derivative.l13 Deuter- ium labelling studies exclude the possibility of Me migration (from SiMe,). The reaction proceeds via the silyl derivative, as in equation (20), but subsequent (m-C,H,)(OC),Mo-Na+-THF (T~-C~H~)(OC),M~-CH,SiMe3 + room temperature {(?T-C,H,)(OC),M~-~H,-N~+} -+(.rr-C,H,)(OC),Mo-CH, (20) (14) attack by (v-C,H,)(OC),Mo- gives rise to (14); this is presumably because (i) anchimeric assistance by the cyclopentadienyltricrbonylmolybdenum group facilities CH,-Si bond cleavage (unusual at room temperature) and (ii) the negative charge in (14) is substantially delocalized. A metallocarbene inter- mediate (15) has been proposed in the reaction between (n-C6H,)(OC),MoC1 and LiN:C(C,H,* Me-p), (Scheme 4).l14 An intermediate of this type is entirely (IT-C~H~)(OC)~MOCIt 2LiN:CRz-1 R = Ph or p-tolyl Scheme 4 11* M.R.Collier, B.M. Kingston, and M. F. Lappert, Chem. Comm., 1970, 1498. *14 H. R. Keable and M. Kilner, J.C.S. Dalton, 1972, 153. The Chemistry of Transition-metal Carbene Complexes possible; the reaction of a lithium ketimide with co-ordinated carbonyl has been shown to afford carbene complexes [equation (21)],6a but here the subequent reaction is not possible.i, LiNCPh, (OC),Cr -(OQCr-C(0Et)NCPhii, [Et,O]+[BFJ-A. Metal-catalysed Carbene Generation from Diazoalkanes.-The influence of metals in reactions of diazo-compounds XYCN2 has been known for many years,116 particular attention having been paid to catalytic decomposition by copper derivatives. The reactions afford nitrogen, and in many cases the products are those to be expected from the intermediacy of free carbenes. The reactions with metal and metalloid derivatives have been reviewed;lls only those believed to involve carbene-metal species are considered here.As well as transition-metal carbene complexes (LM-CXY), other proposed intermediates include LM(CXYN2) and LM(CXYN,CXY) (LM = catalyst). A few stable compounds having such compositions have been isolated,lls but this does not necessarily imply that they play a role in the catalysed reaction path. Examples of these are (i) (T-C,H,)(OC),W-N: N-CH,SiMe, from (m-C5H6)- (OC),WH and Me3SiCHN2117 and (ii) cis-(Ph,P),Pt [(CF,),C: No N: C(CF,),] from (Ph,P),Pt and (CF3)gCN2.14 Differences in the reactions undergone by the :CXY groups led to the proposal that complexed carbenes were true reaction infermediates,ll* rather than the free carbenes. This proposal has been examined in detail for reaction (22),llS which is homogeneous.It has been observed that, in reactions of this type, the ratios of exolendo products are very different from the photochemical and metal- initiated reactions.12o Since the photochemical mechanism clearly cannot involve metal, these differences were taken as evidence for the intermediacy of copper-carbene complexes. Moser1l9 found that the thermd reaction affords products, the exolendo ratios of which lie closer to those of the metal-modified reactions, implying that these ratios are not sufficient evidence of LM-CXY intermediates. Better evidence for such intermediates has now been obtained by a study of the lls A. Loose, J. puakt. Chem., 1909, 79, 507. M. F. Lappert and J. S. Poland, Adv. Organometallic Chem., 1970,9, 397.117 M.F. Lappert and J. S. Poland, Chem. Comm., 1969, 1061. 11* P. Yates, J. Amer. Chem. SOC.,1952, 74, 5376. lleW.R.Moser, J. Amer. Chem. SOC.,1969, 91, 1135, 1141. lB0P. S. Skell and R. M. Etter, Chem. and Znd., 1958, 624. 122 Cardin, Cetinkuya, Doyle, and Lappert results of changing electronic or steric effects of substituents on the metal catalyst.llB In summary, it was found that increasing the size of the phosphite [in (RO),PCuCl ] favours formation of the endo-isomer (16), additionally electron- withdrawing groups favour a higher proportion of the endo-product (17).Addition-ally, use of the optically active (-)-tribornyl phosphitecopper(1) chloride gave two optically active cyclopropanes with optical yields of 3.2% (18) and 2.6% (19) [equation (23)].From the results, including an Arrhenius treatment of the reaction studied at various temperatures, it was concluded (i) that the ha1 transition state (20 or 21) leading to products involves olefin, metal, and the carboxymethylene, (ii) that the transition state is asymmetric, and (iii) that any intermediate leading to it decomposes unimolecularly to products. A mechanism incorporating these factors has been proposed,lle and is shown in Scheme 5. "2N2HC02Et + I(RO)~PCUCLI~-~(RO)~PCUC~+(RO)3PCuCl*CHC02 Et I I, olefin ii,NZCHCOzEtI-\ )I/J /H (20)8x0 bC02Et (21) endu \ C02Et Scheme 5 123 The Chemistry of Trmition-metal Carbene Complexes The effect of electronegative ligands (favouring formation of the endo-product) has been rationalized in terms of increased steric hindrance in (20) leaving (21) relatively unaffected.The dissociation of the copper phosphite is based on kinetic data.181 Finally, probably the best evidence for copper-carbene complexes as intermediates is the induction of asymmetry at the cyclopropanes, and the linear correlation of exolendo ratios with normal (Hammett) a-constants of substituents in the aromatic ring using triaryl phosphite-copper complexes. Good correlations of this type are not common in catalytic reactions. In a studylea comparing thermaI, photolytic, and metal-initiated decomposition of diazoalkanes with copper or silver salts, both olefins and cyclopropanes were formed [equation (24)] and evidence for metal carbene intermediates emerged.Asymmetric induction in cyclopropanes similar to that using norbornyl phosphite complexes has been demonstrated using an optically active chelate (22) of copper (Scheme In this case the optical yields were rather higher Ph* CHMe H I N=C IH Ph*CHMe (-6%) than with tribornyl phosphitecopper chloride, as one might have predicted with an asymmetric centre closer to the metal. In this case the reaction was inhibited by addition of co-ordinating bases such as pyridine. Scheme 6 also shows asymmetric induction with an oxetan.18a Another chelate, acetylacetonatocopper(n), has been examined with benzoyl-diaz~methane.~~~~~~~Here, metal complexation was believed to account for lS1 A.G. Witenberg, I. A. D’yakorov, and A. Zindel, Zhur. org. Khim., 1966, 2, 1532. W. Kirmse and K. Horn, Chem. Ber., 1967, 100,2698. la*H. Nozaki, S. Moriuti, H. Takaya, and R. Noyori, Tetrahedron Letters, 1966, 5239. lapM. Takebayashi, T. Ibata, H. Kohara, and Bu Hong Kim, Bull. Chem. SOC.Japan, 1967, 40,2392. M. Takebayashi, T. Ibata, H. Kohara, and K. Ueda, Bull. Cham. SOC.Japan, 1969, 42, 2938. Cardin, Cetinkaya, Doyle, and Lappert Ph C02R Ph N2CHC02R + PhCHzCH;! Scheme 6 reduced carbene reactivity. A number of copper salts catalysed a cycloheptatriene synthesis from aromatic In one study,128 with Cu, Hg, or Co catalysts, a mercury intermediate129 was isolated and its subsequent reaction demonstrated. Reactions catalysed by zinc halides, especially ZnI,, are of particular interest because of the possible similarity between intermediate species in this and in the Simmons-Smith reaction (see Section 3C).Kinetic studies with Ph2CN2 show that two intermediates are involved, the first of which may be a carbene complex. In subsequent reactions, ZnI, differs from the chloride and bromide.laO~lal De-composition of the same diazo-compound and analogues by CuBr, in acetoni- trile yields the diary1 ketone and ketazine. Kinetic studies point to the reaction pathway of Scheme 7, which shows only the essential metal ligands. The fast -N2 fastAr2CN2 -t'CuBr; +Ar2CNZ'CuBr-ArZC-CuBr CAr2C*Cu*N2*CAr2+ 6r' Br-/slow 1z:Nz 1::Nz Ar2C:0 Ar&: N * N :CAr2 Scheme 7 kinetic evidence, including spectroscopic, indicates the intermediate formation of CuII-carbene complexes, but the diazoalkane complex is inferred from stopped- flow data on the initial phase of the reaction.lal la6E.Miiller and H. Fricke, Annalen, 1963, 661, 38. la' E. Miiller, H. Kessler, H. Fricke, and W. Kiedaisch, Annalen, 1964, 675, 63. la8T. Saegusa, Y. Ito, T. Shimim, and S. Kobayashi, Bull. Chem. SOC.Japan, 1969, 42, 3535. lagA. N. Nesmeyanov and G. S. Powch, Ber., 1934,67,971. D. Bethell and K. C. Brown, Chem. Comm., 1967, 1266; J. C. S. Perkin 11, 1972, 895. 181 D. Bethel1 and M. Eeeles, personal communication. The Chemistry of Transition-metal Carbene Complexes By contrast with the above catalytic decompositions, ethyl diazoacetate reacts with the organic ligand of bromo-7T-allylnickel(r),affording butadiene derivatives, mainly isomers of (23).lSa A possible reaction scheme involves the carbene intermediate (24). It is proposed that the carbene then inserts into the adjacent Ni-C bond (cf: carbon monoxide) to form, e.g., (23).Decomposition of ethyl CHz=CH -CH=C(H) C02Et (23) diazoacetate by an allylpalladium complex (25) has also been examined;lS3 here reaction with co-ordinated ally1 was not observed. The proposed mechanism (Scheme 8) involves the carbene species (26), an analogue of (24). In this study, Scheme 8 comparison between (25) and copper salts as catalysts was made. Thus for reactions of N,CHCO,Et with but-2-yneY (25) is an effective catalyst at 0-lO”C, whereas copper derivatives required temperatures of 65-l2O0C, and curiously, whereas the former afforded mainly diethyl fumarate in the dimerization reaction, diethyl maleate was the major product in the latter case.In another comparative study, catalytic decomposition of the unusually stable diazotetrachlorocyclo- pentadiene by (25) in acetylenes (as solvents) was examined.I3* When carried out in tolan or 3-hexyne at 75-82°C using copper or copper sulphate, the spiro[2,4]- heptatrienes (27; R = Me or Ph) were obtained. However, with the palladium 13* I. Moritani, Y. Yamamoto, and H. Konishi, Chem. Comm., 1969, 1457. lS3 R.K. Armstrong, J. Org. Chem., 1966, 31, 618. lS4 E. T. McBee, G. W. Calundann, and T. Hodgins, J. Org. Chem., 1966,31,4260.Cardin, Cetinkaya, Doyle, and Lappert complex at 1O-2O0C, low yields of the adducts with two acetylene molecules (28; R = Me or Ph), but no cyclopropane derivatives, were isolated, together with 50-60 %of the mine (29) (not detected with copper catalysts). The proposed CI Cl ‘ CI Nc[@CL Cl R mechanism for the Pd-catalysed system (Scheme 9) involves both a carbene complex(31) and a butadiene complex (30);the latter is postulated to account for the unique feature, namely lack of reaction with solvent acetylene. (30) I (3’) r R valence isomerization Scheme 9 [The decomposition of CHBNBby Ni(CO), is described in the following section, and the use of diazoalkanes in mechanistic studies relevant to carbocyclic rearrangements is described in Section 3D.3 B. Synthesis of Organic Carbonyl Compounds using Metal Carbony1s.-In the syntheses of the Group VIA-metal carbene complexes first used by Fischer and co-workers,Bs7 acylmetallates (32) are intermediates. These may be regarded as anionic carbene complexes, and such a view has been widely accepted. However, the contribution of forms such as (33) cannot be ignored (see Table 3 and ref. 49). In a number of reactions intermediate carbonylmetallates react with organic reagents forming alkyl- or aryl-(carbene) complexes (OC)nM-C(OR)Ph which The Chemistry of Transition-metal Carbene Complexes decompose to products: in this section both ions and neutral species are regarded, formally, as carbenes. As we have seen, the acylmetallates derived from RLI and a Group VIA-metal hexacarbonyl are stable complexes which require rather good alkylating agents [e.g.CH2 then H+, or (Et30)+BFa-] for conversion into neutral carbenes. By contrast the carbonyls Fe(CO), and Ni(CO), are more reactive to organolithium reagents (the latter reacts exothermically at -70 "C), forming rather unstable salts, sensitive to air and moisture. They are, however, useful intermediates in organic syntheses by virtue of their reactions with olefins, alkyl halides, and other organic substances. Such syntheses are exemplified in equations (25)-(34). (ref. 135) (25) (ref. 136 ) (26) (ref. 137) (27) RCO*CH(OH)R (R-aryl) (ref. 138) (28) f H+ Ph&==CH-CHPhLi + M(CO)s -+ (Li [(OC),M-C(O)CHPh*CH=CPh,]}--f PhCH(CHO)CH=CPhs (ref.140) (33) (M = Group VIA metal; routes to unsaturated aldehydes are relatively un- common) 13b M. Ryang, I. Rhee, and S. Tsutsumi, Bull. Chem. SOC.Japan, 1965,38, 330. lacy.Sawa, M. Ryang, and S. Tsutsumi, unpublished work cited in ref. 7; Tetrahedron Letters, 1969, 5189. Is' Y. Sawa, I. Hashimoto, M. Ryang, and S. Tsutsumi, J. Org. Chem., 1968, 33, 2159. lsrY. Sawa, M. Ryang, and S. Tsutsumi, J. Org. Chem., 1970, 35, 4183. la@M. Ryang, S. K. Myeong, Y. Sawa, and S. Tsutsumi, J. Organometallic Chem., 1966, 5, 305. 140 E. 0.Fischer and A. Maasbol, G. P. 1214233/1966. Cardin, Cetinkaya, Doyle, and Lappert HmS LiNMe, + Ni(C0)4 + (LiN(MeaC0 *Ni(CO)3 1 -+ Me,NCO.CO.NMe, + Hg + 2LiCl (ref.141) (34) The intermediate salts of Fe and Ni are too reactive to permit structural studies. They are believed to be mono- and di-nuclear respectively; thus, products from the coupling of two organic groups are formed from the nickel derivatives. The proposed mechanism137 for an acyloin and stilbenediol diester are shown in Scheme 10; the use of this route for direct addition of acyl groups to conjugated enones has been described.lqe The reaction of diphenyldiazomethane with Ni(CO), is extremely vigorous. Catalytic amounts of the carbonyl afford mainly benzophenone azine together with nitrogen, ethylene, and small quantities of other nitrogen-containing com- pound~.~~~With excess nickel, carbonylation takes place. The proposed mechan- ism, equation (39, involves a metal-carbene intermediate.Reaction of CO with free carbene is not known. -+ -co --NS R,C-NrN + Ni(CO), -R2-G-Ni(C0)s -+ R&-Ni(CO)s 3. (35)I+N {Ni(CO),) + Formation of ketazine may well involve the carbene intermediate (and R2CNa), a view in harmony with the dependence on the concentration of metal carbonyl. C. Carbene Transfer Reactions, Especially to 0lefins.-A number of ‘CXY’ transfer reactions (in which the carbene is derived from a diazo-compound) have been described in Section A; others are detailed here, including the synthetically important dihalogenocarbene reactions. Unusually mild conditions (dilute HCl, room temperature) are required for the cleavage of the ether linkage in (34), shown in equation (36).14‘ Similar behaviour is typical of acetals where hydrolysis is favoured by c---O double-bond forma- HCI (7r-C6Ha)(OC)8Fe-CH20CHs-(7r-C5HJ(0C),Fe-CH2C1 (36)MeOH-NaOH (34) (35) tion.An attractive is that reaction (36) is facilitated by carbene formation (stabilization, ‘double-bond’ formation with Fe). Support for this view has been obtained by reaction of (35) with AgBF,: AgCl may be filtered off, after which the filtrate reacts with cyclohexene affording norcarane, presumably 141 S. K. Myeong, Y.Sawa, M. Ryang, and S. Tsutsumi, Bull. Chem. SOC.Japan, 1965,38,330. lP1E. J. Corey and L. S. Hegedus, J. Amer. Chem. SOC.,1969,91,4926. lP3C. Ruchardt and G. N. Schrauzer, Chem. Ber., 1960, 93, 1840. lP4 M. L. H. Green, M.Ishaq, and R. N. Whiteley, J. Chem. SOC.(A), 1967, 1508. 129 The Chemistry of Transition-metal Carbene Complexes Ni(CO),+ LiPh-I PhCH2X/ 0/"\Ph 0 Ph I I0-c c-0I I R' R' Ph >c=c /Ph R'OCO \OCOR' stilbenediol diester Ph 0 LiO CH2Ph PhCOC(0H)PhI CH2Ph (os-benzvlacvloin 1 Scheme 10 Cardin, Cetinkaya, Doyle, and Lappert from the intermediate { [(T-C,H,)(OC)~F~CH~]+BF~-1 (36).14, Without the separation step norcarane yields of 46 % were obtained, and using cis- and trans- but-Z-enes trapping was stereospecific. The compounds isolated in the absence of traps were (T-C,H,)(OC),FeCH, and [(rr-C,H,)(OC),Fe(CH2=CH2)I+, both of which are clearly plausible products from further reaction of (36).Trapping experiments were positive also in the reaction of (n-C,H,)(OC)s-MoCH20Me with acids and in similar but much slower reactions with Re and Mn met hoxymethyl species. 46 Since the discovery of reactive dihalogenocarbenes from halo form^^*^,^^^ and from the Simmons-Smith reagent IZnCH21,148,14e there has been much interest in carbene transfer reactions. A number of studies have suggested a transition state (37) involving methylene and metal, but the species does not come within (37) our definition of a carbene complex (see refs. 146-157). The subject has been authoritatively reviewed.162 The reduction of gem-dihalides by chromium(n) sulphate has been shown to proceed via chromium-carbene intermediate^.^^^ Kinetic data, products, and reactivity sequences support a reduction involving carbenes derived from an initially formed a-halogenomethyl radical : R~R~CX~ *CP+]3R~RZ~X+ cr2+ -+F~R~c(x).-.x*. + CrX*+ (37) followed by R1R2k+ Cra+---+ [R1R2C-CrIa+ -+ R1R2C+ CrXa+ I 3. (38)X [R1R2CCr]*+ (38) 146 P. W. Jolly and R. Pettit, J. Amer. Chem. SOC.,1966, 88,5044. 146 W. von E. Doering and A. K. Hoffinann, J. Amer. Chem. Soc., 1954,76,6162. lQ7 G. Kobrich, H. Buttner, and E. Wagner, Angew. Chem. Internat. Edn., 1970,9, 169. 148 E. P. Blanchard and H. E. Simmons, J. Amer. Chem. SOC.,1964, 86, 1337. 14@ H. E. Simmons, E. P. Blanchard, and R. D. Smith, J. Amer. Chem. SOC.,1964,86, 1347. lSoT. L. Gilchrist and C. W. Rees, ‘Carbenes, Nitrenes, and Arynes’, Nelson, London, 1969.lS1 G. L. Closs and R. A. Moss,J. Amer. Chem. SOC.,1964,86,4042. 16* D. Seyferth, Accounts Chem. Res., 1972, 5, 65. usD. Seyferth and J. M. Burlitch, J. Amer. Chem. SOC.,1964, 86, 2730. lti4W. von E. Doering and W. A. Henderson, J. Amer. Chem. SOC.,1958, 80, 5274. 166 D. Seyferth, J. Y.-P. Mui, and J. M. Burlitch, J. Amer. Chem. SOC.,1967, 89,4953. 16a D. Seyferth, M. E. Gordon, and K. V. Darragh, J. Organometallic Chem., 1968,14,43. lb7D. Seyferth, J. Y.-P. Mui, and R. Damrauer, J. Amer. Chem. SOC.,1968, 90, 6182. 168 C. E. Castro and W. C. Kray, J. Amer. Chem. SOC.,1966, 88,4447. The Chemistry of Transition-metal Carbene Complexes Reduction takes place by proton transfer: [RlRZCCr ?+ Hi-3 [R1RZHC-Cr(OH)I2+ -+R1RaCHCr2++ Cra+ (39)c+ .gR~R~CH+ erg+3.etc. Carbenes have been trapped in this reaction; with 3-butenol and Me,CBr, the cyclopropyl product was obtained in 39% yield [equation (40)].These results indicate a carbene of rather reduced reactivity, presumably species (38). YezCBr;! + Cr2*+ CH2-CH.CH2-CH20H -CHz-CHCH3 +CH3 CH2CH3 1 3*lo 4 OJO CHzCHzOH (40) f Me2CH(OH) + x,.44 '10 Me 39'10 D. Valence Isomerizations of Strained-ring Carbocyclic Compounds.--+) Nature of the Reaction. A number of remarkable a-bond rearrangements in highly strained ring compounds are catalysed by transition-metal ions or their com-plexes. Examples are shown in the equations (41)--(47); the bonds specified are those cleaved. -A9* " (ref.161) (42) (43) (42) 160 L.Cassar, P. E. Eaton, and J. Halpern, J. Amer. Chem. SOC.,1970,92,6366. L. Cassar, P. E. Eaton, and J. Halpem,J. Amer. Chern. Soc., 1970,92,3515. L. A. Paquette and J. C. Stowell, J. Arner. Chern. SOC.,1971,93, 2459. I32 Cardin, Cetinkaya, Doyle, and Lappert*Me Me lelMe \ H Me H Me H# Me + Me# Me tie Me H Me (50) 46% (51) 50% (52) BU' cototysts + 6+ @J \ But But But (57) (59) (tsym-ismr)ro (ref.168) (61) (62) (63) (refs. 165,169) (46b) G. L. Closs and P. E. Pfeffer, J. Amer. Chem. Soc., 1968, 90, 2452. 16a P. G. Gassmann and F. J. Williams,J. Amer. Chem. SOC.,1970,92, 7631. 160 M. Sakai and S. Masamune, J. Amer. Chem. SOC.,1971, 93,4610. 166 P. G. Gassmann, T. J. Atkins, and F.J. Williams, J. Amer. Chem. SOC.,1971, 93, 1812; P. G. Gassmann and T. J. Atkins, ibid., p. 4597; but see also B. S. Solomon, C. Steel, and A. Weller, Chem. Comm., 1969, 927; ref. 181. lee P. G. Gassmann and T. Nakai, J. Amer. Chem. SOC.,1971, 93, 5897. 16' K. L. Kaiser, R. F. Childs, and P. M. Maitlis, J. Amer. Chem. SOC.,1971, 93, 1270. m6 K. B. Wibert and G. Szeimies, Tetrahedron Letters, 1968, 1235. leeL. A. Paquette, G. R. Allen, and R. P. Henzel, J. Amer. Chem. SOC.,1970, 92, 7002; see also L. A. Paquette, R.P. Henzel, and S. E. Wilson, ibid., 1971, 93,2335. The Chemistry of Transition-metal Carbene Complexes (67) (ref. 170) (47) Particular attention has been given to AgI, RhI, and PdII catalysts, among others, and in some cases simple Lewis acids are effective.The Agl catalyses have been reviewed.171 The driving force for these reactions is the relief of the high ring strain initially pre~ent,~~~J~~ and the role of the transition metal is tc provide a low-activation-energy pathway which is otherwise inaccessible, owing to the constraints of orbital ~ymmetry.~~~,~~~ Attention is drawn to the hybrid (69) shown in Scheme 11with both the carbene (69b)and metallo-carbonium ion Me" -MN 1-MN hydrogen shift Scheme 11 170 J. Wristers, L. Brenner, and R. Pettit, J. Amer. Chem. Soc., 1970,92, 7499. 171 L. A. Paquette, Accounfs Chem. Res., 1971, 4, 280. 17' K. B. Wiberg, Adv. Alicyclic Chem., 1968, 2, 185. 17s M. G. Evans, Trans. Faraday SOC., 1939, 35, 824. 17' R. B.Woodward and R. Hoffmann,Angew. Chem. Internat. Edn., 1969,8,781. Cardin, Cetinkaya, Doyle, and Lappert (69a) contributors. Further evidence for (68) and (69) comes from trapping experi- ments with nucleophiles (see below). The derivatives (70) and (71) were produced when the fRh(CO),CI], catalysis of (60) was conducted in methanol, and significantly in the same ratio when sulphuric acid was used in place of the metal ~ata1yst.l~~These experiments support both the stepwise nature of the reaction and the intermediacy of carbonium ions, but do not provide conclusive evidence for (69); some related systems have been rationalized in terms of the parallel with conventional carbonium ion chemistry.17 A similar charge-transfer (cation radical) intermediate has been proposed for the prismane rearrangement [equation (45)].ls7We may contrast conditions of the thermal (unchanged 3 h, 150 "C; 86% recovery after gas-phase pyrolyses 1 s, 500 "C)and catalysed (quan- titative conversion, -c 3 min, 40 "C) reactions analogous to equation (42) for the (saturated) bis(methylcarboxy1ate) of compound (42).Kinetic factors are presumably also responsible for the contrasting behaviour of (64)with AgBF,, the anti-isomer being inert under the same conditions [equation (47)l. Finally we note the synthetic utility of some reactions :equation (44)shows a novel route to azulenes,16s and the first preparations of semibullvalene were based on a bis-homocubyl rearrangement.176 (ii) Mechanisms. A large amount of evidence supports the existence of carbene intermediates or 'metallo-carbonium ions' in many of these reactions, having the structural feature shown.The evidence includes (a)satisfactory product identifica- tion, (b) kinetic data, (c) labelling experiments, (d) trapping experiments both internally and with additives, and (e) studies with model systems. Point (a) is illustrated with reference to equation (46b); the thermal reaction (46a) follows a different course (and, probably, a different mechanism). The product ratios are dependent on the catalyst employed, of which there are many (including Rh, Pd, Cu, Ag, Zn, and Hg derivatives) having the common feature 176 L. A. Paquette,J. Amer. Chem. SOC.,1970,92,5765; R. Askami, Tetrahedron Letters, 1970, 3349; L.Cassar, P. E. Eaton, and J. Halpern, J. Amer. Chem. SOC.,1970,92,6367. 176 M. Sakai, H. H. Westburg, H. Yamaguchi, and S. Masamune, J. Amer. Chem. SOC.,1971, 93, 461 1. 17' J. E. Byrd, L. Cassar, P. E. Eaton, and J. Halpern, Chem. Comm., 1971,40. 77ie Chemistry of Transition-metal Carbene CompIexes of Lewis acidity. A common intermediate (68) has been proposed, which can account easily for the observed products of Scheme 11. Kinetic data (6) relating to bicyclobutane systems [especially equation (46) J have shown that the reactions are not concerted processes and that the derived rate law16g is consistent with the following mechanism: +interrdiate + Ag+ Ccomplexl (48) products + Ag' Kinetic studies with cubane and a norbornene derivative17e also establish stepwise pathways.Point (c) is illustrated by reference to equation (43). Different bonds are cleaved in the thermal reaction162 (43a) from those in catalysed path~ays~~~J~* (43b and c); the ambiguity (C-1-C-3 and C-2-C-3 or C-1-C-4 and C-2-C-3) in pathway (43b) was resolved by a labelling study D( = D; (43b)l. The C-1-C-3 cleavage was also rigorously established for the Ag+-initiated rearrangements18o of exqexo-and endo,endo-l,4-dimethylbicyclo[1,l,O ]butanes which are, respect- ively, largely and highly stereospecific. The methylated analogue (44;X = Me) gives an almost statistical distribution of products [equation (43)], but the isomeric 2,2,4,4-tetramethyl derivative, which has no 2- or 4-hydrogen atom available for migration after the skeletal change, afforded (72) only.lSs This might imply a new bond-breaking sequence, but a more attractive explanation is that Me the unchanged sequence leads to (73) [cf.(69)], which subsequently undergoes a vinyl migration. Studies on migratory aptitudes to carbenoid centres show an order H > vinyl > methyl.l*l Further support for (73) is provided by the low- temperature decomposition of (74) catalysed by the same RhI species, which affords (72) as the only volatile 178 L. Cassar, P. E. Eaton, and J. Halpern, J. Amer. Chem. SOC.,1970,92,3515, 6366. lTeT.J. Katz and S. A. Cerefice, J. Amer. Chem. SOC.,1969, 91, 6520. 180 M. Sakai, H. Yamaguchi, H. H. Westburg, and S. Masamune, J.Amer. Chem. SOC.,1971, 93, 1043. lnl H. Shechter, personal communication quoted in ref. 182; see also D. M. Lemal and K. S. Shim,Tetrahedron Letters, 1964, 323; G. L. Closs and R. B. Larabee, ibid., 1965, 287. 136 Cardin, cetinkaya, Doyle, and Lappert Trapping reactions (d) have been widely used in other systems as evidence for intermediate carbene species. Particularly interesting in this context is the internal benzene into norcaradiene conversion, a typical carbene reaction, believed to take place in the rearrangement of (52) [equation (a)].The proposed reaction scheme is presented in Scheme l2.l" Me H Ph Ph Scheme 12 A (carbonium ion) precursor to the metal carbene complex has been detected in the 1,2,2-trirnethylbicyclo [1,1 ,O]butane system by intermolecular trapping with methanol.las These experiments confirm the stepwise nature of the process, and establish that C-2-C-3 bond rupture precedes C-1-C-3 scission in the reaction (see labelling experiments above), as shown in Scheme 13.The methoxy-deriva- tive was obtained at a rate (97%, 1 min, 25 "C) comparable to that of the Tlte Chemistry of Transition-metal Carbene Complexes Mu Me I (M= Rh) MK MuO Me e Scheme 13 rearrangement. Acid catalysis was eliminated (control experiments) but the possibility that methanol solvent promotes a different mechanism from that in chloroform, although improbable and without experimental foundation, could not be ruled out.le6 Synthesis of the proposed carbene intermediates in these reactions from model compounds and a study of their subsequent reactions have tended to confirm the proposals for some systems only.It is known that the metal-catalysed decompo- sition of diazo-compounds proceeds via carbene complexes (see Section 3A) and this reaction has been used to provide the required intermediates. [One example, (74), has been mentioned above. ] In the palladium-catalysed rearrangements of bicyclobutanes, two bond- breaking pathways are known, as shown in Scheme 14; the intermediate (75) Scheme 14 Cardin, Cetinkaya, Doyle, and Lappert corresponds to (73) in the RhI-catalysed reactions. The percentage distribution of products is very sensitive both to the catalyst and to substituents in the substrate.Diazo-compounds were synthesized, such that their decomposition would lead to corresponding carbonium ion analogues of the general intermediate (75),lS4 Bicyclobutanes with the appropriate diazo-models are shown below. For the palladium-catalysed [(PhCN),PdCI,] reactions of the bicyclobutanes the product distribution was similar or identical to that obtained with the relevant model, strongly supporting a carbene (metallo-carbonium ion) intermediate. However, the silver-catalysed decompositions led to entirely different ~ati0s.l’~ It seems here that an initial C-l-C-2 heterolysis is followed by a cyclopropyl- to allyl-carbinyl rearrangement and loss of metal Evidence for this using traps has been presented above. In conclusion, we may consider for which systems and metals carbene-metal complexes may be intermediates.There is clear evidence for these both from trapping experiments (especially internal insertion into benzenelS6) and from suitable models for several bicyclobutane rearrangements catalysed by both rhodiumls5 and palladium.lS4 For the Ag+ catalyses, evidence for metallo- carbenes is no more than circumstantial, although two aspects have been clearly established: (i) the stepwise nature involving different bonds in the carbon framework from the thermal reactions and (ii) the involvement of carbonium ions, possibly inetal-containing but conceivably at a site remote from greatest positive charge. E. Olefin Dismutation (or Metathesis).-The reaction is illustrated in equation (49), in which X and/or Y are H, alkyl, or some other univalent atom or group.Catalysis may be heterogeneous, the catalyst comprising a ‘promoter’, a metal oxide (e.g. MOO,) and a ‘supporter’, an oxide or phosphate (e.g.Al,O,). We are concerned principally with homogeneous systems : catalysts include WC16- 2BunLi, [(Ph,P),CI,-W(NO),]-(Me,AI,C1,),and (Ph,P),RhCI (for electron-rich olefins). Reviews are a~ailable,l~~-~~~ but these do not consider the role of metal-carbene complexes. There is increasing evidence for the participation of such 188 G. C. Bailey, Catalysis Rev., 1969, 3, 37; M. L. Khidekel’, A. D. Shebaldova, and I. V. Kalechits, Russ. Chem. Rev., 1971,40,669; S. Yoshitomi, Sekiyu Gakkai Shi, 1970,13,92; C. Inoue and K.Hirota, Yuki Gosei Kagaku Kyokai Shi, 1970,28,744; J. Tsuji, Kagaku No Ryoiki Zokan, 1970, 89, 169. F. D. Mango and J. H. Schachtschneider, in ‘Transition Metals in Homogeneous Catalysis’, ed. G. N. Schrauzer, Marcel Dekker, New York, 1971,223. lS4 N. Calderon, Accounts Chem. Res., 1972,5, 127; N. Calderon, E. A. Ofstead, J. P. Ward, W. A. Judy, and K. W. Scott, J. Amer. Chem. SOC.,1968,90,4133. The Chemistry of Transition-metal Carbene Complexes 1 + 2,”0ZF uu species in the reaction, as discussed below and summarized schematically in Figure 16, in which LM represents the transition metal with ancillary ligands. Support for a metal-carbene intermediate comes from kinetic data on the hetero- geneous Co-Mo catalysis of (a)CH2N2 decomposition into N, and C2H4 and (b) the dismutation of C3Hs into C2H4 and CH3CH:CHCH3;186 the rates for (a) and (6) are similar and, as discussed in Section 3A, (a) very probably involves a metal-CH, species. 18s J.J. Rooney and P.P.O’Neill, J.C.S. Chem. Comm., 1972, 104. Cardin,cetinkaya, Doyle, and Lappert 2cx,:cY, +cx,:cx, + CY,:cY, (49) It will be convenient to classify organometallic species according to the number of active M-C sites: the 4-C systems are (76),lS4 (77),lS6 and (78);lS7 the 3-C is (81);33 the 2-C is (79);s3 and the l-C is (80).33 A fou-carbon-metal species is consistent with the results of labelling experi- ments1s8~1sg as exemplified by equation (50).and product characterizati~n,~~~-~~~ However, in several systems products are formed which are not so readily explained(e.g.see refs.189 and 190). For instance, the dismutation of oct-l-ene, catalysed by WCI,-EtAICI,, affords not only the expected ethylene and Me(CH,),CH: CH(CH,),Me but also olefins having odd numbers of carbon atoms (C7-C15), especially at high catalyst con~entration.~~~ It is well knownthat transition-metal complexes often cause the isomerization of olefins, and this affords a possible rationalization of the results [e.g.equation (51)]; however, the possibility of a carbene intermediate has been considered,lgl presumably of type (80). 2CH3-CH=l4CH, +14CH#4CH2 + CH,-CH=CH-CH, (50) CH,=CH(CH&,CHs +C2H4 + CH,(CH,),CH=CH(CHJbCH, 11 CH,CH=CH(CH,),CH, +CHFCH(CH,)~CH~ (51) + CH,(CH,),CH=CHCH, A quasi-cyclobutane intermediate, (76), was first suggested for hetero- geneous but now appears unlikelylS6 because (i) cyclobutanes are not detected in dismutation experiments and (ii) dismutation catalysts do not trans- form cyclobutanes into olefins.These experiments were taken to imply that a so-called ‘tetramethylene complex’ (77) was involved.lss In (77), the four CX, or CY, fragments, formed by simultaneous scission of both 0-and rr-bonds of C2X4 and C2Y4, are co-ordinated to M by the overlap scheme of Figure 17, each carbon utilizing hybrid orbitals approximating to sp3.Thus, (77) is not a metal carbene complex, as defined in Section 2A (sp2-C being required). Further evidence for a four-carbon-metal species, and especially (77), comes from experiments on dismutation of non-4-ene by the d6 complex (n-MePh)- W(CO)3.186In order to form such a species, taking each of CX, or CY, as a two- electron donor to the metal, it is necessary that both toluene and at least one CO ligand be displaced from the metal, unless the metal is to exceed its complement of 18 valence electrons.No dismutation occurred when carbon monoxide loss was prevented, and inhibition was noted when excess of toluene was present. lB6G. S. Lewandos and R. Pettit, J. Amer. Chem. SOC.,1971, 93, 7087. le7 R. H. Grubbs and T. K. Brunck, J. Amer. Chem. Soc., 1972, 94, 2538. J. C. Mol, J. A. Moulijn, and C. Boelhouwer, Chem. Comm., 1968, 633. lag A. Clark and C. Cook,J.Catalysis, 1969,15,420; G. V. Isagulyants and L. F. Rar,Bull. Acad. Sci. U.S.S.R.,1969, 1258. lB0 F. F. Woody, M. J. Lewis, and G. B. Wills, J. Catalysis, 1969, 14, 389. K. Hummel and W. Ast, Naturwiss., 1970, 57, 245. lo*R. L. Banks and G. C. Bailey, Ind. and Eng. Chem. (Proc. Res. and Development), 1964,3, 170; C. P. C. Bradshaw, E. J. Howman, and L. Turner, J. Catalysis, 1967,7,269. The Chemistry of Transition-metal Carbene Complexes Figure 17Orbital overlap for a Vetramethylene complex' (77) The four-carbon metallocyclic species (78) was suggested for systems such as those catalysed by WC16-2BunLi (see ref. 193), as a consequence of the experi- ments on (i) ineso-l,4-dilithio [2,3-2H2]butane, illustrated in Scheme 15,1*' and (ii) the &compound which gave CH2=CHD (88 %), trans-CHD=CHD (6%), and C2H4 (6%).Scheme 15 J. Wang and H. R. Menapace, J. Org. Chem., 1968, 33, 3794. Cardin, cetinkaya, Doyle, and Lappert The metallocycles were not isolated, but as a class such compounds are known. Their interconversion requires a symmetrical transition state or intermediate, e.g. (77), although a [1,3] shift has also been considered.18s The pathways a and 01‘ (for the minor product) were suggested.lS7 However, the possibility of steps /?and p, via two-carbon fragments (79),is now proposed. This allows for alternative competing pathways, such as a and p. Additionally, it provides a plausible route to the origin of the metallocycles in Figure 16. olefin LM + olefin +n-complex (82) +(79) FA(78)-olefin The dicarbene (79) may form via a wolefin complex (82); these are, of course, well-known.As described in Section 2C(iii), electron-rich olefins yield one-carbon complexes (go), probably via (82) and a free carbene.However, it is also possible to isolate a dicarbene complex (79) [equation (52)].56It may be significant that Group VIA hexacarbonyls are considered to be active dismutation catalysts only if a mechanism exists which provides for the loss of two or more CO ligands (e.g. by irradiationlg4). Me Me.. Recently a three-carbon metallocyclic species (81) has been suggested, and definitive evidence for a one-carbon species (80) (a metal-carbene complex) has been presented in the homogeneously catalysed dismutation of the electron-rich olefins (83).ss A mixture of (83a) and (83b) at 140 “C in xylene for 2 h in the presence of a rhodium(1) complex L(Ph,P),RhCl (L = PhsP or CO) underwent a dismutation reaction to produce (83c) in yields approaching the statistical (50%).R1 R2 (83a) R’=Rz-Ph N N = CX2CX2 or CY2CY2 (83b) R’=R~=~-toi“;c=c: N] 1 (83c) R’=Ph R’ R2 = CX2CY2 R2=P’ to[ The suggested mechanism is shown in the lower part of Figure 16. The evidence rests on: (i) the isolation of the monocarbene complexes of type (80a), E.S.Davie, D.A. Whan, and C. Kernball, J. Curakjwis, 1972,24,272. The Chemistry of Transition-metal Carbene Complexes L(PhSP)Rh(CX2)CI, from the reaction of C2X4 with L(Ph3P),RhCI under dismutation conditions; (ii) the demonstration that compounds (80)also catalyse (83b)the dismutation; and (iii) the conversion (80a) -3(80b) for L = Ph,P.Addition- ally, (iv) the oxidative addition step seems plausible because other oxidative addition reactions of Rhl carbene complexes can be demon~trated,~~~ whereas (v), the carbene complex (Et,P)CI,Pt-C~(Ph)CH,],, which is known to be unreactive with regard to oxidative addition, is not a dismutation catalyst under the conditions employed. The possibility of a metal-dicarbene complex (79) in this system is not ruled out (see above). Such a compound has been isolated by reaction of an olefin of type (83) with [Rh(C0)2C1]2.66 At present, whether the dismutation of simple alkenes is related to that of electron-rich olefins remains an open question.4 Addendum Although selective in nature, this section brings the literature coverage up to the end of 1972.The existence of transient anions derived by proton abstraction from co-ordinated carbenesgg has been confirmed by generation at low temperature and affords useful The intramolecular cyclization reaction?' has been extended to cationic and neutral compounds having 2-heteroatom substitu- ents.lg7 Reactions of nucleophiles with the electrophilic Ccarb of :C(OMe)Ph bound to a Group VI metal have been studied: the secondary phosphine HPMe, co-ordinates through phosphorus to Ccarb affording a substituted ylide ~tructure,~~~but phosphonium ylides cleave the carbene and afford a route to vinyl ethers.lSD Two developments in syntheses of carbene complexes from neutral precursors are noteworthy.viz. a general synthesis, particularly of oligo- carbene derivatives using electron-rich olefhs,eoo and diphenylcarbene complexes of rhodium prepared from Ph2CN0 or Ph,C=C=O, which are among the very few co-ordinated carbenes not stabilized by a hetero-substituent on CCBrb.201For recent developments in the mechanism of metal-catalysed rearrangements of strained-ring compounds see references 202 and 203. Wethank Dr. D. Bethell for unpublished data and the S.R.C. for their support. D. J. Cardin, M. J. Doyle, and M. F. Lappert, to be published. lo* C. P. Casey, R. A. Boggs,and R. L. Anderson, J. Amer. Chem. Soc., 1972,94, 8947. M. Green, J. R. Moss,I. W. Nowell, and F. G. A.Stone, J.C.S. Chem. Comm., 1972, 1339. lo*F. R. Kreissl, C. G. Kreittr, and E. 0.Fischer, Angew. Chem. Internat. Edn., 1972,11,643. C. P. Casey and T. J. Burkhardt, J. Amer. Chem. SOC.,1972, 94, 6543. 'O0 B. Cetinkaya, P. Dixneuf, and M. F. Lappert, J.C.S. Chem. Comm., 1973, in the press. *01 P. Hong,N. Nishii, K. Sonogashira, and N. Hagihara, J.C.S. Chem. Comm., 1972,993. *O* L. A. Paquette, R. P. Henzel, and J. E. Wilson, J. Amer. Chem.Soc., 1972, 94, 7780 and references therein. *Oa P. G. Gassman and T. J. Atkins, J. Amer, Chem. Soc., 1972, 94, 7.