Preparation of Fischer carbene complexes by alkylation of acylmetalates with alkyl iodides

Article abstract of DOI:10.1021/om00031a056

Neutral Fischer carbene complexes bearing alkoxy groups as the donor substituents on the carbene carbon atom have been synthesized by direct alkylation of the lithium acylmetalates (formed by classical addition of organolithium compounds to metal hexacarbonyl) by alkyl iodides. It was shown that the reaction proceeds under normal phase-transfer conditions (CH₂-Cl₂/H₂O) or, preferably, in aqueous medium alone. However, in both cases the presence of a catalytic amount of a tetraalkylammonium salt (n-Bu₄NBr) was required. Attractive features of this methodology are (i) the economy of using alkyl iodides in place of the usual alkylating agents (like Me₃O⁺BF₄^-, magic methyl, or methyl triflate), (ii) the procedural ease of one-pot preparation of the carbene complexes, and (iii) the greater variety of alkyl moieties that can be incorporated as part of the alkoxy substituent using the more readily available alkyl iodides containing nontrivial alkyl groups. The procedure is preparatively useful for accessing chromium-containing carbenes, but yields of the analogous molybdenum- and tungsten-containing species are only ≤10%. Mechanistic considerations suggest that products are formed by competing pathways involving direct S_N2 displacement vs electron-transfer-initiated S_RN1 processes.