Carbene-alkyne complexes

Alt has prepared a cationic cis-carbene alkyne derivative, CpW(CO)-(HC=CH)[C(OEt)Me] + (77), but the presence of both an alkyne ligand and a carbonyl ligand in the d4 coordination sphere as well as a heteroatom on the carbene complicates orientational predictions relative to W(CHPh)-(RC=CR)L2X2. No structural data are yet available for this cationic carbene alkyne complex. 

Hydroquinone synthesis (regiospecific) from alkynes and carbonyl carbene chromium complexes... 

Other examples of [2C+2S+1C0] cycloaddition reactions have been described by Herndon et al. by the use of chromium cyclopropyl(methoxy)carbenes. These complexes react with alkynes releasing ethene and forming cyclopenta-dienone derivatives, which evolve to cyclopentenone derivatives in the presence of chromium(O) and water [122] (Scheme 76). This reaction has been extended to intramolecular processes and also to the synthesis of some natural products [123]. These authors have also described another process involving a formal [2C+2S+1C0] cycloaddition reaction. Thus, the reaction of methyl and cyclo-propylcarbene complexes with phenylacetylene derivatives does not afford the expected benzannulated products, and several regioisomers of cyclopentenone derivatives are the only products isolated [124] (Scheme 76). 

An unexpected varying regiochemistry in intramolecular benzannulation has also been observed in the synthesis of cyclophanes. As mentioned above, there are only two possible regiochemical outcomes in the benzannulation reaction, which differ in the direction of alkyne incorporation. / -Tethered vinyl-carbene chromium complexes undergo an intramolecular benzannulation reaction with incorporation of the tethered alkyne with normal regioselectivity to give meta-cyclophanes [28]. 

The species responsible for alkyne polymerization, which is kinetically more facile than eyelotrimerization since only a small fraction of the added alkyne is converted to benzenes, is not yet known. Carbene-metal complexes, both mononuclear (54) and binuclear (y2-CR2) complexes (55,56), have been shown to act as alkyne polymerization initiators and several years ago it was shown that terminal alkynes and alcohols can react to give alkoxycarbene ligands (57) As yet, we have no evidence... 

However, a crucial difference was seen in analogous reactions with the (tert-butyl)alkynylcarbene complex 184.a, carried out under a pressure of carbon monoxide. A [4 + 2] cycloaddition takes place, but in this case the ene fragment is not the alkyne functionality, but the carbene-alkyne bond. The mechanism presented by Park implies that this is due to the preliminary... 

The thermal reaction of vinyl- or aryl(alkoxy)carbene chromium complexes with alkynes can give high yields of 4-alkoxyphenols (Figure 2.24). This elegant synthetic procedure was reported for the first time by Dotz in 1975 [326] and has proven robust and of broad scope. 

Arene(alkoxy)carbene chromium complexes react with aryl-, alkyl-, terminal, or internal alkynes in ethers or acetonitrile to yield 4-alkoxy-1-naphthols, with the sterically more demanding substituent of the alkyne (Rl Figure 2.24) ortho to the hydroxy group. Acceptor-substituted alkynes can also be used in this reaction (Entry 4, Table 2.17) [331]. Donor-substituted alkynes can however lead to the formation of other products [191,192]. Also (diarylcarbene)pentacarbonyl chromium complexes can react with alkynes to yield phenols [332]. 

Aryl(dialkylamino)carbene chromium complexes do not yield aminonaphthols upon treatment with alkynes, but form indene derivatives. Vinyl(dialkylamino)car-bene complexes, however, react with alkynes to yield aminophenols as the main products if solvents of low nucleophilicity are used [335]. (2-Amino-1-vinyl)carbene complexes do not undergo benzannulation when treated with alkynes, but form cyclopentadienes or heterocycles instead [251]. 

The intermediate vinylketene complexes can undergo several other types or reaction, depending primarily on the substitution pattern, the metal and the solvent used (Figure 2.27). More than 15 different types of product have been obtained from the reaction of aryl(alkoxy)carbene chromium complexes with alkynes [333,334]. In addition to the formation of indenes [337], some arylcarbene complexes yield cyclobutenones [338], lactones, or furans [91] (e.g. Entry 4, Table 2.19) upon reaction with alkynes. Cyclobutenones can also be obtained by reaction of alkoxy(alkyl)carbene complexes with alkynes [339]. 

The reaction of alkoxy(alkyl)carbene chromium complexes with alkynes has been reported to give modest yields of cyclopentenones [368] and a few examples of intramolecular carbene C-H insertions of Fischer-type carbene complexes, leading to five-membered heterocycles, have been reported [369,370] (Table 2.22). 

Alkynes react readily with a variety of transition metal complexes under thermal or photochemical conditions to form the corresponding 7t-complexes. With terminal alkynes the corresponding 7t-complexes can undergo thermal or chemically-induced isomerization to vinylidene complexes [128,130,132,133,547,556-569]. With mononuclear rj -alkyne complexes two possible mechanisms for the isomerization to carbene complexes have been considered, namely (a) oxidative insertion of the metal into the terminal C-Fl bond to yield a hydrido alkynyl eomplex, followed by 1,3-hydrogen shift from the metal to Cn [570,571], or (b) eoneerted formation of the M-C bond and 1,2-shift of H to Cp [572]. 

Alkynes coordinated to platinum(O) are susceptible to electrophilic attack. The reaction which has been most fully studied is the protonation of complexes Pt(alkyne)(PPh3)2 to give vinyl platinum(Il) complexes then alkenes. The reaction has been discussed in Section 52. The vinyl complexes formed undergo isomerization in the final step, since the cis vinyl complex yields some tracts-alkene. Carbene intermediates have been proposed in the pathway for this isomerization.848 Platinum(II) alkyne complexes can be converted into carbene complexes, and this reaction has been discussed in Section 52.4,6. This pattern of differential reactivity is apparent in the IR spectra of the two sets of complexes. For alkyne complexes of platinum(O) the C==C stretching frequency is lowered by some 450 cm-1 upon coordination, but with the platinum(II) analogs the difference is only in the region of 200 cm-1. 

An earlier review treated the organometallic derivatives, including alkyl, aryl, vinyl, alkynyl, carbonyl, carbene, alkene and alkyne complexes,16 and these will not be treated here (see Table 1 for other books and reviews on organogold chemistry). Two important articles dealing with [AuCl(CO)], including its structure and catalytic properties, have been published recently.97,381... 

One of the earliest enantioselective carbon-carbon bond-forming processes catalyzed by chiral transition-metal complexes is asymmetric cyclopropanation discussed in Chapter 5, which can proceed via face-selective carbometallation of carbene-metal complexes. Some other more recently developed enantioselective carbon-carbon bond forming reactions, such as Pd-catalyzed enantioselective alkene-CO copolymerization (Chapter 7) and Pd-catalyzed enantioselective alkene cyclization (Chapter 8.7), are thought to involve face-selective carbometallation of acy 1-Pd and carbon-Pd bonds, respectively (Scheme 4.4). Similarly, the asymmetric Pauson-Khand reaction catalyzed by chiral Co complexes most likely involves face-selective cyclic carbometallation of chiral alkyne-Co complexes (Chapter 8,7). 

Bis(adamantylimido) compounds, with monomeric chromium(VI) complexes, 5, 348 Bis(alkene) complexes conjugated, Rh complexes, 7, 214 mononuclear Ru and Os compounds, 6, 401 -02 in Ru and Os half-sandwich rj6-arenes, 6, 538 with tungsten carbonyls and isocyanides, 5, 685 Bis(u-alkenylcyclopentadienyl) complexes, with Ti(II), 4, 254 Bis(alkoxide) nitrogen-donor complexes, with Zr(IV), 4, 805 Bis(alkoxide) titanium alkynes, in cross-coupling, 4, 276 Bis(alkoxo) complexes, with bis-Cp Ti(IV), 4, 588 Bis[alkoxy(alkylamino)carbene]gold complexes, preparation, 2, 288... 

Transition metal-catalysed reactions involving alkenes proceed mostly by the formation of 7r-complexes ( -complexes). Reactions of alkynes can be understood by the formation of three intermediate complexes, namely 7r-alkyne complexes (t]2-complexes) 1, alkynyl complexes 2, and i/ -vinylidene complexes (carbene complexes) 3. 

Enantiopure alkynyl(alkoxy)carbene (37) complexes were produced by formal alkyne insertion into Fisher carbene complexes 39 Reaction of (37) with 1-azadiene gave functionalized 1,4-dihydropyridine (38) with high enantiomeric excess. 

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