Abstract :
The focus of this review is transition metal carbenoid reagents, which are considered to be subvalent transition metal compounds having at least one unshared electron pair on the metal center and are thus able to undergo oxidative addition to either σ- or π-bonds of substrate reactants. Illustrative carbenoids are Cp2*Ti and Bpy·Ni, which are contrasted with related carbene complexes, such as Cp2*TiCH2 and Bpy·NiCPh–CPhCHPh. The XRD and IR data for the carbenoid-like adducts of Cp2*Ti with H2CCH2 and of Bpy·Ni with PhCCPh have been analyzed to assess the electron-transfer bonding from the metal to the complexed hydrocarbon. Such considerations lead to the conclusion that such adducts have significant σ-bonding of a typical three-membered metallocycle. The impact of two other characteristics of these metal carbenoids, namely their singlet or triplet state and their electrophilic or nucleophilic reactivity towards π-bonded substrates, is also assessed in exploring their possible modes of reaction. The succeeding discussion of recent research then focuses on non-metallocene early transition metal carbenoids of the type, MtX2, Mt(OR)2 and MtH2−. The utility of alkylative reduction of transition metal salts, MtXm, by RMgX or RLi is shown to be widely applicable and the resulting neutral (MtXm−n) or anionic (CrH2−) carbenoids are shown to insert into or add to a diverse array of linkages, such as C–X, CO, CN, CC and CC bonds leading to C–C bond formation and thus to coupling, oligomerization and/or polymerization. Especially with CE bonds (E=C, N, O) and MtXm−n the initial three-membered metallocycle formed plays a demonstrable role in the organic products generated. Utilization of this metallocycle in selective organic synthesis has already been highly successful in the research groups of Kulinkovich, Sato and Eisch.
