Involvement of iron alkyl complexes and alkyl radicals in the Kharasch reactions: probing the catalysis using iron phosphine complexes

Abstract

The X-ray crystal structure of trans-[FeBr₂(depe)₂](depe = Et₂PCH₂CH₂PEt₂) has been determined, and its catalysis of the reactions between MgBrEt and RBr (R = Et, Prⁿ, Buⁿ, PhCH₂ or H₂CCHCH₂) to give alkanes and alkenes investigated. Detailed analysis of the products and the time courses of the reactions demonstrate that the hydrido-species trans-[FeH(Br)(depe)₂]ⁿ⁺(n= 0 or 1) are not intermediates in the reaction, but are products formed in the termination steps. It is shown that the catalysis can proceed by one of two pathways, depending on the alkyl halide. One pathway is proposed to involve the intermediacy of iron(I) species, and the other of iron(III) complexes. Both catalytic cycles contain the same fundamental reaction: the abstraction of a β-hydrogen atom from a [FeEt(Br)(depe)₂]ⁿ⁺ species by alkyl radicals to give the alkane and ethylene. The X-ray crystal structure of trans-[FeH(Br)(depe)₂]BPh₄ is reported.