Vinylketene Cobalt I Complexes

Although the preparative chemistry of (vinylketene)cobalt(I) complexes is relatively limited in the literature, the methods used include all the major procedures that have been more widely exploited in the analogous chromium and iron systems. There are many similarities between the intermediates involved in the synthesis of vinylketene complexes of iron, chromium, and cobalt, but as the metal is varied the complexes containing analogous ligands often exhibit significant differences in stability and reactivity (see Sections II and VI). Comparison of such species has often been an important aim of the research in this area. The (vinylketene)cobalt(I) complexes have also been shown to be synthetically useful precursors to a variety of naphthols, 2-furanones, ce-pyrones, phenols,6,22,95 8, y-unsaturated esters,51 and furans.51,96a 

In 1986, Wulff reported the first synthesis of an -vinylketene complex from the reaction between a carbene and an alkyne51 the former had of course been postulated as key intermediates in analogous reactions of chromium8 and iron97 carbenes (see Sections II.B and VI.C, respectively). On treatment of the tricarbonylcobalt carbene (84) with 3-hexyne under mild conditions, the yf-vinylketene complex (85) and the y-keto unsaturated 

The 774-vinylketene complex (85) could be oxidatively decomplexed with Ce(IV) to afford the lactone (87). Although no reaction was observed with methanol (unlike a postulated chromium analogue16,18 26), treatment with sodium methoxide produced the expected /3, y-unsaturated ester (88). Thermolysis of complex 85 afforded no trace of the naphthol that one would expect33 from a proposed chromium vinylketene complex with the same syn relationship between the phenyl group and the ketene moiety. Instead, only the furan (89.a) was seen. Indeed, upon exhaustive reaction of tricarbon-ylcobalt carbenes (84 and 90) with different alkynes, the furans (89.a-d) were isolated as the exclusive products in moderate to excellent yields. 

The mechanism of these transformations is proposed to proceed via the initial [2 + 2] addition of the alkyne to the carbene to produce a cobaltacyclobutene species (91), which may then insert carbon monoxide 

The utility of this reaction was demonstrated in the synthesis of bovol-ide (98).51 

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