Unusual Structure and Reactivity of a Homoleptic Super-Ate Complex of Iron: Implications for Grignard Additions, Cross-Coupling Reactions, and the Kharasch Deconjugation

摘要

Our recent investigations on iron-catalyzed cross-coupling reactions of organomagnesium reagents with various electrophiles were guided by the perception that bare, low-valent iron clusters formed in situ according to Scheme 1 might account for the observed turnover.[1]-[5] Since the analysis of the released gas suggests that this reduction process involves a -hydride elimination/reductive coupling event,[6] it should proceed only with Grignard reagents containing at least two carbon atoms. In line with this notion, MeMgBr essentially fails to react with methyl 4-chlorobenzoate in the presence of [Fe(acac)3] (acac=acetylacetone) or FeCln (n=2, 3) as precatalysts, whereas all higher alkyl Grignard reagents are cross-coupled with exceptional ease under these conditions (Scheme 1).[1] This strikingly different behavior is general; in fact, methyl groups are transferred only to highly activated substrates such as enol triflates, acid chlorides, and very electron-deficient heteroarenes.[7], [8]