THE CHEMISTRY OF ETA(3)-VINYLCARBENE COMPLEXES

The chemistry of eta(3)-vinylcarbene transition-metal complexes is rev iewed. The eta(3)-vinylcarbene complexes are prepared by 1) the alkyla tion of (eta(3)-acryloyl)tricarbonylferrate, 2) the treatment of cyclo propenes with [Fe-2(CO)(9)], 3) the insertion of acetylenes into a car bene-metal double bond, 4) the reaction of acetylene with [ReCl3(Me5C5 )](2), 5) the reaction of carbyne complexes with allyl bromides, and 6 ) the thermal decarbonylation of (eta(1)-vinylcarbene)pentacarbonylchr omium. The structures of the complexes, which were determined by X-ray analyses, are represented by the hybrid of the resonance structures ( A) and (B). The contribution of the two structures was very sensitive to the respective MLn fragments. The vinylcarbene complex, [Fe(eta 3-v inylcarbene)(CO)(3)] (1), is a parent complex of a series of various c omplexes. The carbene carbons of the reported Fischer-type eta(3)-viny lcarbene complexes are electrophilic. Carbonylation of the eta(3)-viny lcarbene complexes gives eta(4)-vinylketene complexes. Treatment of th e eta(3)-vinylcarbene(carbonyl)irons with an excess of tertiary phosph ines or bidentate amines gives ferracyclopentenones. Reaction of 1 wit h isocyanides gives the first example of the eta(4)-vinylketenimine co mplex. Treatment of 1 with diazomethane gives (eta(4)-1,3-butadiene)tr icarbonyliron. The complex 1 with a methoxycarbonyl group on the 3-pos ition reacts with carbon monoxide to give a eta(4)-pyrone complex, whi ch thermally rearranges to its isomer. The rearrangement was deduced t o proceed via a eta(4)-methoxyfuran complex. The complex 1 reacts with hydride to give a eta(3)-allyl complex. Reaction of a(3)-2-trifluorom ethylvinylcarbene)tricarbonyliron with K[BH(s-Bu)(3)] or phenyllithium gives ta(4)-difluorotrimethylenemethane)tricarbonyliron, leaving a fl uoride ion. eta(3)-Vinylcarbene complexes react with acetylenes to giv e eta(4)-cyclopentadiene metal complexes. The reaction proceeds via a metallacyclohexadiene. The complex 1 reacts with an unsaturated metal complex (MLn)-L-2 to give a binuclear vinylcarbene complex or (eta(3): eta(1)-allyl)(Fe-M-2). Reaction of 1 with Ru(CO)(3)(COD) provides a me thod for selective preparation of (eta(3):eta(1)-allyl)(Fe-Ru), which reacts with acetylenes to give [Fe(eta(4)-ruthenacyclohexadiene)(CO)(3 )]. Formation of a eta(3)-vinylcarbene complex by the reaction of a ca rbene complex with acetylene, and subsequent carbonylation of this com plex, is the essential pathway of the Dotz reaction.