Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Jahn–Teller effect wikipedia , lookup
Metalloprotein wikipedia , lookup
Ring-closing metathesis wikipedia , lookup
Metal carbonyl wikipedia , lookup
Spin crossover wikipedia , lookup
Hydroformylation wikipedia , lookup
Evolution of metal ions in biological systems wikipedia , lookup
Vinylidene complexes in organic synthesis" B.GITA AND G.SUNUARARMAN** Depanmenr of Chcmntry. lndian Institute of Technology, Madras 600 036, India. Received on January 21, 1994 Abstract Appl~cationof vmylidenc ~omplcxesin urganic syntbcsis 1s discussed. Mechanism of forma0011from q' alkyrle cornplexea of triu~sitianmetala, preparation from q 2 alkyne, acerylide, carbma, carhyr~zor acyl complexes as well as rhe structure o f thc vinylidme complexes art. outlined. Reactions of vinylidem cuniplexcs with electraphiles and nucieophiles are alao detailed. In addilion, catalyt~crcactiuns invulvmg trans~tionnrelai complexes for the syntheals of a variety of organic compounds and polymcrs from acetylanic compounds which mvuke an in siru formation ot vmylidene intermediates are d~scussed~n detatl. Key words: q2-alkyne-vinylidenen rearrangement, vmylidene complexes 1. Introduction The term vinylidene describes the first member of the homologous series of unsaturated carbenic species of the generic formula C,Hz. The parent vinylidene, CzH2, has never been isolated as it undergoes an exlremely rapid 1,2-hydrogen shift to give acetylene (eqn 1). : C=C<H H . H-C E (1) C-H However, it has been shown that it is possible to statilise vinylidene as ligands in transition metal complexes. Some chemical transformations which relate vinylidene to other monohapto ligands are shown in Scheme 1'. Stcpwise reduction of an ethynyl iron complex to a neopentylidene unit has been reported and here it was concluded by IH-NMR studies that a vinylidene complex is an intermediate (Scheme 2)'. A series of cationic vinylidene complexes ha~rebeen isolatcd containing the 'CpFeL?' fragment (Fig. I)'. A few examples of mono. hi, tri and tetra nuclear metal co~nplexes~-~ are shown in Fig. 2 and review articles are available in literature detailing their synthesis and structure8 lo. * Dedicated to Proi C . N. R. Rao an the occasmn of hls sixtieth binhday. ** Author tor correspondence. ir 9 GiTA %\o C . SUYDARARAIAN Sziit\?i 2 Conieiuon or ethynyl iron to a neopenrylidene ~roncomplex viu a vinylidene colnplch Several reactions of organometallic reagents may also contain intermediacy of vinylidene complexes. Such reactions have proved to he highly useful in the synthesis of organic compounds which otherwise require many cumbersome steps, as shall be shown later. In this paper, the mechanism of vinylidene reamngement, a few preparative methods, thc structure and several reactions of the vinylidene complexes will be discusqed. For the sake of brevity, we restrict ourselves to the chemistry of mononuclear M-C vinylidene complexes, as comprehensive literature is available for heterovinylidenes". 3. Isomerisation of I-alkynes t o vinylidenes o n metal templates Terminal nlkynes lsomerise to the col~espondingvinylidene via two possible mechanistic pathways. In4he first case, a stepwise process, I-alkyne undergoes an oxidative addition at the metal centre (before or after n-coordination) to give a hydrido-alkynyl complex. This can VINYLIDENE a: L,L' b: L,L' c: L,L' COMPLEXES = = = IN ORGANIC SYNTHESIS dppe; R,RS=CH3,X=S03F dppe, R=CH3, R'=H; X=PFs dppe; R,R'=H, X=PF6 Mononuclear Hornobinuclear Heterobinuclear Trinuclear [Mn] = Mn (CO)z (q5-CsHs) [Re] = Re (CO)n (~1~-CsHs!s FIG2 Some erarnples of mono to mulo nuclear complexes w ~ t hvmylidenc ligands further rearrange to the vinyl~denevia a 1.3 hydrogen shift from the metal to Cp (eqn 2). A M [I + R-CEC-H-[M]- / 1 I H I -[M]-C--C-R-[M]=C=C<R (2) 358 B GITA \\D G. SUNDARARAJAN Alrematively, a concerted shift of hydrogen atom from C, to Cp as the metal approaches C, can also be envisaged (eqn 3). R Borh these mechanisms may be valid but which one predominates may depend on the nature of the metal-containing species. Also as a rule of thumb, q2-alkyne complexes of d4 and d' metai centres do not isomerise to the vinylidene analog, while the rearrangement is highly favoured in d6 complexes, possibly due to the unfavourable 4e-2-centre d-rr intera~tion'~.'~. Evidence for acetylene-hydride intermediate can be found in the reaction of the rhodium compiex, l?hCl(~f-HC?Ph)( P R h On treatment with pyridme, the qZ-alkynecomplex is converted to the hydrid*acetylide, a species that rearranges to the vinylidene on addition of cyclopentadienide anion. The intermediate Rh(C2Ph) (Py) (PR3)2 can be isolated if the reaction is carried out at 0°C (Scheme 3)14. SCHEME3. Vinylidene formation I.ra a metal-hydndo-acetyiIde complex On the other hand, in the acetylene-vinylidene transformation in the diiridium complex, the first such example involving a dimetallic complex, the sequence of steps envisaged by the authors could be suggestive of a concerted pathway for the rearrangement (Scheme 4)15. 3. Preparation of vinylidene complexes Mononuclear vinylidene complexes can be obtained by any one of the four methods, viz., from li) $-bound terminal alkynes, (ii) carbene or carbyne complexes, (iii) metal-acetyiides, or (ivl by dehydration of metal-acyl complexes. 3.1. From terminal nlkynes A 1,2-hydrogen shift on the acetylenes attached to a metal centre results in the formation of a vinylidene. As the rearrangement is very facile, detection of q 2 alkyne complex is generally elusive, while in the case of a non-terminal alkyne like Zbutyne, the q2-bound complex has been isolated (eqn 4)M. VlNYLlDENE COMPLEXES IN ORGANIC SYNTHESIS Scirrvr 4 Vmyhdcnc form~rionby a concerted mechanlm m a bismdium complex As a prelude to polyrncrisation reactions involving vinylidene intcmcdiates, like rne?athetical polyrnerisations. more than one alkyne can get incorporated into the vinylidene Ilgand. One such example is given in eqn 5". Cationic vmylidene complexes are mmunc to ligand exchange reactions; howcver, mixed ligand vinylidene complexes can be oblained by performing Lhe exchange reactions on the corresponding acetylides followed by protonation'? Cbiral vinylidene complexes of the formula, [11~-CsHs]Re (NO) (PPhl) (=C=CRR') +X-where R, R'=H, CtI? CsHs have also been synthesised. Geometrical isomerism has been observed in these complexes and various reaction? have also been performed on Anionic vinylidenc complexes have been observed to be formed by the reaction of molybdenum and tungsten alkylidene complexes with a basd9. B. GITA AND G. SUNDARARAJAN 360 3.2. From carbyne or carbene complexes Vinylidene derivatives can be obtained readily from metal-carbyne complexes possessing P-hydrogens when treated with strong bases like 11-butyllithium (eqn 6Y9. Chlorocarbene complexes of Iron(Q porphyrins'" (on reaction with DDT)undergo elimination of HC1 to give the diarylvinylidene complexes (eqn 7)20.21. z . (Por) Fe [CCICH(C,H,CI),] (Po,) Fe = C = C (C,H,CI), (7) 3.3. From metal acecylides An efficient way to obtain disuhstituted vinylidene complexes is by the addition of electrophiles to metal acetylides, accessed by reacting acetylides with the metal halide (Scheme 5)'?. [RU]+=C= C < Y y2 R [Ru] SCHEME 5. Vinylidem complexes from metal acctylides. -C 'x& 3.4. From acyl complexes by dehydration reaction The acetyl complex of rhenium, Ke(NO)(PPhn) q5-C5Hs (COCHzR) on treatment with triflic anhyride resulted in the protonation of the acyl group to yield a mixture of stable VWYLIDENE COMPLEXES IN ORGANIC SYNTHESIS 361 hydroxycarbene and a vinylidene from dehydration of an oxycarbene. Deprotonation of the vinylidene/hydroxycarbene mixture so obtained by potassium tertiary butoxide base led to a 1:1 acetylide/acyl complex mixture which on further reaction with triflic anhydride resulted in protonation of the acetylide to give the vinylidene (Scheme 6)'7,'8. C=0 I C (cF3so2 ) 2 0 CH2 I R I K R + GOH 1 y"' R H [Re] = Re (NO) (PPh,) (11-C, H5) R = H, Me, Ph, 1-ClaH7 SCHEME 6 . Vmylidene complex lrom acyl-metal complex Similarly, the acylate anion from Mn(C0)s (q5-CjHj,)with MeLi, on reaction with a proton sponge like, 1,8-bis(dimethy1amino) naphthalene, gives the vinylidene complex as an intermedi* but it is the binuclear complex that is isolated finally (eqn 8)'?. R GITA n m G SUNUAKARAJAN 362 The pseudo-tetrahedral complexes of (11'-Cp) (acyl) (CO) (PR?) Fe compkxes also react rapidly with niflic anhydride to afford a variety of cationic cornplexes viu the intermediacy of cationic carbene complexes (Fig. 3)". ,R Fe+= oc/ :' C=C L' CF,SO, R '' R = Me; L = PPh3 R = H; L = PPh3 R = H; L = PMenPh Fri.. 3. Some a i rhe vinylidene complexes obtained from (qi-Cp) (acyl) (CO) (PRI) Fe complex 4. Structure o f the vinylidene complexes Free vinylidenes, if they exist, can he either in the singlet or triplet state and may be represented as follows: singlet singlet tnplet In the metal complexes, either of the species is bonded to the metal atom via a ligand to metal 0-donor bond and a metal to ligand n-acceptor bond. Back donation of the electron density ro the ?(*orbitals of the C-C multiple bond systems can thus take place. The M=C=C group is almost linear. The angle at C, is between 1.25 and 1.41 %1 which corresponds to a b n d order between two and three. The M-C bond length is consistent with a bond order of around t w ~ ~ ~ ' . The IR spectra of all these complexes invariably show a characteristic y e c between 1620 and 1680 cm-I. The 'H-NMR spectrum does not give any information on the vinylidene as such. However, I3C-NMR spectral data generally show a highly deshielded carbon at 320380 ppm (i.e., the C,) and Cp between 90 and 140 ppm. Mass spectral data for some of the complexes are also a~ailable*~. UV visible spectroscopic studies and electrochemical studies on such vinylidene complexes have been canied out and are well documentedz9. Structural parameters of several complexes are available and a typical crystal structure of vinylidene complex is shown in Fig. J30. VlNYLlDENE COMPLEXES IN OKGANTC SYNTHESIS FIG 4 ORTEP diagram of bonds. ii vmylidene cotnplcx showing the linear arrangement of the metal-carhan-carbon Compounds having vinylidene ligands which have groups like Ph3Ge or P h 6 n instead of hydrogen atom have been synthesised from the manganese complex, CpMn(CO)(THF). Various acetylenes like H-CrC-Ph, PhsSi-C-C-Ph, and Ph&-C=C-Ph form the vinylidene complex readily from the corresponding q2-alkyrre complexes (Scheme 7)5.This Type of rearrangement is observed in the coordination sphere of rhodium with various trimethylsilyl acetylenes3'. R / CI-Rh=C=C I/ C., L = PPrj R = Me, Ph, COOEt, COOSiMe3 SCHEME7. q2-alkyne to vmylldene rearrangrmenr with tnalkylsdyl groups 5. Reactions of vinylidene complexes It has been shown by theoretical calculations that the clectron deficiency at C, renders it susceptible to mdeophilic attack and the localization of electron density in the metal-carbon double bond and on Cp causes the chemical reactivity to be oriented towards ele~trophiles~~. This prediction is very well borne by experimental results and it may also be conveniently said that barring addition reactions of vinylidenes with olefins or alkynes, their chemistry is B. GITA m o G. SUNDARARAJAN 364 dominated by nucleophilic attack on C , carbon, be it inter- or intra-molecular reactions with the intermediacy of vinylidene complex implied or confirmed. 5.1. Elecrrophilic addition to P-carbon Vinylidenes react with electrophiles at the B-carbon leading to the formation of thc carbyne complex. The fomation of the carbyne complex could follow any of the steps outlincd in Scheme 833. H [Re] = C = c < ~ , - H BH+ - H-[Re] = C = C<ph A B * [Re] B C--C BH+ I - H-[Re] C -C SCHEME8. Reaction of vinylidene complex with electrophiles: possible modcs of formation of the metal carbyne complex 5.2. Nucleophilic addition to the a-carbon One of the most common reactions of the vinylidene is its ability to convert readily to the metalla-carbene complex upon treatment with alcohols (eqn 9)34. R'OH + [MI = C = C H R * [MI = C < OR' CH2 R (9) In some ca$es, a cyclic carbene complex was isolated when the substrates like H e C (CH>),OH react with metal halide complexes. This could occur by a rapid intermolecular addition of the alcohol function to a short-lived vinylidcne complex (cqn (See later pan for more examples with implied vinylidene complex formation.) VINYLIDENE COMPLEXES IN ORGANIC SYNTHESIS 365 Another interesting reaction of the vinylidene complexes is the formation of a Claisen system from R'R2C=C=Fe(CO)(PR3) (qS-CsHs)+and the ally1 alcohol, R3(OH)CHCH=CH(Rd) to give the indicated product (eqn l l ) ? This concept was utilised more effectively in an Ru-mediated coupling of acetylenes and ally1 alcohol and will be dealt with shortly. (11) [Fe] = Fe (CO) (PR'3) (q-CsHs) p-lactam synthesis has been carried out exploiting the formation of the vinylidene intermediate from [NMe4][CrC(O)Me(COs)] and the end product was isolated in 87-100% yield (eqn 12)37. TsCl Me (CO),Cr =6 : H M e (CO),Cr =C = CH2 ArCH = NMe OTs 6. Reactions thought to proceed via vinylidenes With the existence of vinyUdene complexes established and their modes of generation characterized, postulating them as intermediates in reactions involving both a transition metal complex and a terminal alkyne have become acceptable. Thus the following catalytic reactions all have in common the suggestion of a vinylidene intermediate, although their occurrence in the catalytic cycle is only a matter of inference. B. GITA AND G. SUNDARARAJAS 6.1. Vinylcar'bamares Vinylcarbarnates can be synthesised from carbarnates and terminal alkynes and the coupling reaction is catalysed by RuCli(PR4 (q6-CsMea) (eqn 1313'. [Ru] = RuCl (PMe+ (q-Cdi6) 6.2. a .P and!or P , y Unsaturated ketones A general synthetic procedure for obtaining unsaturated ketones was reported recently by Trost where a ruthenium catalyst med~atescoupling of terminal alkynes and allyl alcohols in good yields (eqn 14)39. A variety of acetylenic compounds have been coupled with a series of allyl alcohols and a sample list is given in Table I. The catalytic cycle proposed by the authors involves the formation of a vinylidene intermediate arising out of the interaction betueen the alkyne and the metal catalyst as the key step. The vinylidene, presumably then reacts with the alcohol to give a 'Chisent-type intermediate (vide eqn 11) which by reductive elimination gives the product ketone and the metal catalyst, thus regenerating the catalytic cycle (Scheme 9) *,41. Conventional synthesis of dihydrofurans (cyclic en01 ethers ) normally call for a multi-step procedure4245.Singly decarbonylated (by me thy la mine-N-oxide) Mo(CO)6 induces cyclization of 1-alkyn-4-01s to 2,3-diiydrofurans in a single step process with en route formation of a vinylidene intermediate. The carbonyls of chromium and tungsten, however, give rise to the corresponding metallacasbenes (eqn 15)46. VINYLlDENE COMPLEXES IN ORGANIC SYNTHESIS 367 The authors suggest that the cyclo-isomerisation proceeds the following way. An q2alkyne complex formed initially isomerises to the vinylidene which cyclises intra-molecularly to give an ql-bound cyclic en01 ether. This complex can yield the dihydrofuran by protonation or can isomerise to the corresponding oxycarbene. In the case of chromium and tungsten, according to the authors, the activation banier is high compared to molybdenum for protonation4' and so the formation of dihydrofuran is difficult (vide Scheme 10). 6.4. Polymerisation of alkynes and cyclic olefins It was demonstrated by Katz not long ago that metal carbenes catalyse polymerisation of alkynes and strained cyclic ole fin^“^. AS vinylidene complexes are also structurally, in part, metallacarbenes, they should be able to initiate such polymerisation reactions. It should also be noted that stable vinylidene complexes may not be good catalysts. For instance, the cationic complex containing the phenyl vinylidene ligand, viz., {trans-[FeCI(C2HPh)(depe)z] PF6, depe =1,2-bis (diethyl phosphino)ethane) is very stable and shows no tendency to react with alcohols to form alkoxy c a ~ b e n e s ~Thus, ~ . vinylidene complexes of metal carbonyls containing no other stabilising ligand could very well be good catalysts for polymerisation of alkynes and cyclic olefms. This was demonstrated by Geoffroy who showed that presence of preformed metal carbene is unnecessary for initiating such reactionsSoand can be accomplished by photolysis of metal carbonyls in the presence of a terminal alkyne, the key step suggested Table I Some of the acetylenes and ally1 alcoholr coupled by Ru complex (vide Scheme 9) to yield unsaturated ketones ltaken from ref. 391 AceNIenr Alcohoi Pmducr Time Yield herein being the rearrangement of q2-alkyne to vinylidene preceded by photochemically aided loss of carbon monoxide ligand. In the presence of excess alkyne and extended irradiation addition of alkyne molecule to the vinylidene and propagation of polymerisation VINYLIDENE COMPLEXES IN ORGANlC SYNTHESIS 10. Proposed mechanism for the cydo-isomensation of 1-dkyn-4-01s to 2,3-dihydrofurans by Mo (CO)S SCHEME (NNle?). reaction becomes straightforward (see Scheme 11). It is of pedantic interest to note here that acetylene requires irradiation only for initiation of polymerisation while phenylacetylene when used as the monomer calls for continuous irradiation through the c o m e of the reaction. Ph W (CO), P h - C I C - H ~--------+ hu, -CO . c o w , Ph (~OI~w=c-c<~ -0-+ H Ph Ph-CEC-H Ph b (co),w=c=c< h", -GO Ph 1 ---- --4 (CO) w-c; ; Cl-. - H SCHEME 11. Mechmisrn of polymerisation of rcrminal alkynes by W(C0)6 under photolytic condnions B. GITA 370 AND G . SUNDARARAJAN The need for a vinylidene intermediate for initiating metathetical polymerisation is supported by the observation that internal alkynes and strained cyclic olefins which could be polymerised readily by, say, metallacarbenes, do not react under these conditions to give polymers. However, addition of a small amount of an initiator like phenylacetylene triggers the ring-opening-polymerisation reaction of norbornene, possibly via the initial generation of the vinylidene (Scheme 12)5'. w (CO), P h - C E G H (CO)5W = C = hexane, h i SCHEUE 12. Polynarbomene from W(C0)s hv system with phenylacetylene as initiator. 7. Conclusion From a pristine organometallic reation q2-alkyne to vinylidene rearrangement has grown into a synthetically useful tool. Many reactions need to be unfolded that utilise such vinylidene species because of their simplicity in generation; one needs just a transition metal complex with a vacant coordination site and a terminal alkyne, and its high utility, as a variety of elecbophiles and nucleophiles can add on to the vinylidene as have been shown earlier. Since many such reactions involving vinylidenes are catalytic in nature and as the products obtained are novel and difficult to obtain otherwise, the importance of this rearrangement has been recognised in the recent literaturei2. It is only hoped that many such useful reactions can be uncovered, like, for instance, application of c h i d vinylidenes for synthesis of c h i d compounds. Acknowledgement GB thanks the Department of Science and Technology for a research fellowship. References 1. D ~ v i s o A. ~ . *ND SELEOUE, J. P. J . Am. Chem. Soc., 1978,100.7763-7765. 2. Davisos, A. ruo S n m u ~J. , P. J . Am. Chem. Soc., 1980, 102, 2455-2456. VINYLIDENE COMPLEXES IN ORGANIC SYNTHESIS A ~ 4 s n r o 1 ,Y. N., GREEN, M., WWTE,N. V. AND 371 .I. Chem. Soc , Dalron Trans, 1982, 319-326. TAYLOR, G . E. K o n s w a r , ~ ~R., , ALI, R., SPETH,D. AND ZI~CLER, M. 1. Angew Chem. Inr. Ed Engl., 1981, 20, 1049-1051. ANTONOVA, A. B , K o ~ o s o v ~ N., E., Pernovs~u.P V., LOKSHIN, B. V. AND OBEZWK, N. S J Or~enome?.Chem., 1977, 137, 5 5 4 7 DEEMING, A . I., MASSO,S. AND UNDERHILL, M. J. Chem. Soc , Dalton Trons, 1975, 1614-1620 SEYFERTH, D., ESCHBACH. C. S. AND N E s ~ FM. , 0. I. Organomel. Chem., 1975, 97, CIl-C15. HERRMhNK, W. A. Adv. Orgoriomer. Chem., 1992, 20, 159-263 Houan, I., L A P P ~M. K ~ F., PEARCE, R. AND YARROW, P. I. W. Ckem. Rsv , 1983, 83, 135--201. BRUCE,M. I. Chem. Rev., 1991, 197-257. GEOFFXOY, G L. m u B A ~ N E S. R ,L. Adv Organnmer. Clrrm., 1988, 28, 1-83. T~MYLEION, J. L., WINSTON, P. B. nho W ~ R DB. , C. 3 Am. Chem. Soc., 1981, 103, 7713-7721. BIRDWHISTELL, K. R., BURGMCYER, S. J. N. AND TEMPLETON, I. L. J Am. Ckem Soc , 1983. 105, 7789-7790 WOLF,I.. WERNEK, H., SLKHADLI, 0 . AND ZIEGLER, M. L Angew Chem. int. Ed. Engl., 1983, 22, 414A16. XIAO,J COWIE, M. AND Bnnm, M. I. .AND OrganornernlLcs, 1993, 12, 463472. SWINCER A. G. Aust. J Ckem., 1980, 33, 1471-1483. GLADYFZ, 1. A. J. Am. Chem. S o c , 1982,104, 49484950. SENI*,D. R., WONG,A,, PATTON, A. T., MAKU,M., STROUSE. C. E. AND GLADYSZ. 1. A J. Am. Chem. Soc.. 1988,110, 6096-6109. WONG,A. AND GILL,D. S. AND @am, M. .I Chem Soc, Ckem. Cornmun., 1981, 1037-1038. MANSUY, D., LANOE,M., C H O T T ~ D J. ,C., F u ~ n r P., ~ , M o n i ~ n r ,P., BRAULI, D. AND ROUOEE, M. D., LMGE, M. MANSWY, AND CHOTTARD, 1. C. J Chem. Soc, Chem. Commun., 1977, 648-649. J. Am. Chem. S o c . 1978, 100, 3213-1214. BRUCE, M. I. AND HUMPHREY, M. G. Auri J. Chem.. 1989, 42, 1067-1075. BaL<Nr+Lussm, B. E., C H U R C ~ LM.L ,R., Hutims, R P. AND RHE~FOLD, A. L. Orgonomeralllics. 1982, 1, 628434. B o w , B. E., PAM,S. A. AND HUGHES,R. P. J Organomrr Chem , 1979, 172, C29-C32. BAKER, P. K., BARKER, G.K., GILL,D. S., GREEN,M.. ORPEN,A. G., WLLLIAMS, I. D. AND WELCH,A. . I. J. Ckenr. Soc., Dolton lions., 1989, 1321-1331 BEEVOR, R. G., GREEN,M., OWEN,A. G. WILIIAMS,I. D. J. Chem. Soc., Chem Commlcn., 1983, 673475 Bevon, R G., GREEN.. M., ORWN,A. G. AND WILLIAMS, 1. D. J. Ckem. Soc., Dnllon Trons., 1987, 1319-1328 Bnuca, M. I. Adv. Organornet. Chern., 1983, 22, 59-128. AND AND SWNCER, A. G. B. GITA AND G. SUNDARARAJAN 372 Orgnnomer. Chem , 1988, 332, C17-C20. 29. Lmos, M. A N. D. A. AND POMBQRO, A. J. L. J 30. NESMEYANOV, A. N., ALEKSANDROV, G. G., ANTONOVI\, A. B., AKISIMOV, K.N., K a ~ o e o v N. ~ , E,a m STLUCHKO~, Yu. T. J. Organornet. Chem., 1976, 110, C3gC38. 31. SC~NNDER, D. Axe W ~ E RH., Angew. Chem. Inr. Ed. Engl., 1991, 30, 700-702. 32. WERNER, H., WOLF. J., MULLER, G. *NO KRUOER, C. Angew Chem. Int. Ed Engl., 1984, 23, 431-432. 33. C*RYALHO, M. F. N. N., H ~ D E R S OR.N A., , R. L. Pamuno, A. I. L. a m RICHARDS, J Chem. Soc., Chem. Cornmun., 1989, 1796-1797. Bnuc~,M. I. BRUCE, M. I., Swmcm, A. G., THOMSON, B. J, AND WALLIS, R. C. 36. A. G. M. BARRETT. 37. BARRETT, A. G. M., BROCK.C. P. S ~ n a ~ sM. s , A. AND J. Organornet. Chem., 1978, 161, C1<4 WALUS, R. C. 34. 35. AND Ausr. J . Chem., 1980,33, 2605-2613. C*RPU"ER,N. E. AND Reference 10 and references 148-175 therein. 38. 39. R. J. TROST,B. M., DYKER,G. AND KULAWEIC. J. Am. Chem. Soc., 1990, 112, 7809-7811. Ausf. J. Chem., 1979, 32, 1471-1485. 40. Bniice, M. I 41. M. I., WONO,F. I., BRUCE, SKU-ETON, B. W .WD WHITE, A. H. 42. Darz, K. H., STLIRM. W. 43. Cnsm, C. F. 44. L A ~ A DL.,AL, ~ u h . ~ aE., o, AND AND WALUS,R. C. AL~DA n , H. G. ANDERSON, R. L. J Chem. Soc , Dolron Trans., 1982, 2203-2207 Organometallics, 1987, 6, 1424-1427. J. Chem. Soc., Chem. Comrnun., 1975, 895-896. J. Chem. Soc., Chem. Comrnrm., 1991, 437438. MAIORANA. S. AND PAPAONI, A. 45. H~VEY D., P., Ltim, K. P. AW N m , D. A. J Am. Chem. Soc., 1992, 114, 8424-8434. 46. MCDONALD, P.E., CONNOLLY, C. B.. GLUSON.M. M., Towhr, T. B. AND TREIBBR, K. D. Anovel synthesis of 2.3-dihydmhs; Cycloisomerisation of alkynyl alcohols to endocyclic e n d ethers (personal communication). 47. B~soro,F. 48. IGrz,T.J.,Ho,T.H.,Sm,N.J., YLIO, Y. C. AND STUART, V. I. W. Inorg Chrm. Acm, 1985, 100, 33-39. J. Am. Chem. Soc, 1984, 106, 2659-2668. 49. BUERBY. I. M. 50. L A ~ W NS., J., SW..LMAN, P. M. AND G E O ~ O G. Y ,L. 51. G ~ AB.. AXD SUNDARARYAN, G. SEEBACH, D. 52. AND MAYS,M J. J Organomer. Chem , 1976, 117, C21<22. J. Am. Chem. Soc.. 1985, 107, 6739-6740. Termhedron Lerr., 1993, 34, 61234126. Angew. Chem. Inr. Ed. Engl., 1990, 29, 1320-1367. In this context, it is apt to recall a rather provocative comment made by Prof. Dieter Seebach in this article: 'The discovery of truly new reactions is likely to be limited to the maLn of transition-metal organic chemisuy, which will almost certainly provide us with additional 'maacle reagents' & the y e m to come'.