Elementary Steps of Iron Catalysis: Exploring the Links between Iron Alkyl and Iron Olefin Complexes for their Relevance in CH Activation and CC Bond Formation

Abstract

AbstractThe alkylation of complexes 2 and 7 with Grignard reagents containing β‐hydrogen atoms is a process of considerable relevance for the understanding of C–H activation as well as C–C bond formation mediated by low‐valent iron species. Specifically, reaction of 2 with EtMgBr under an ethylene atmosphere affords the bis‐ethylene complex 1 which is an active precatalyst for prototype [2+2+2] cycloaddition reactions and a valuable probe for mechanistic studies. This aspect is illustrated by its conversion into the bis‐alkyne complex 6 as an unprecedented representation of a cycloaddition catalyst loaded with two substrates molecules. On the other hand, alkylation of 2 with 1 equivalent of cyclohexylmagnesium bromide furnished the unique iron alkyl species 11 with a 14‐electron count, which has no less than four β‐H atoms but is nevertheless stable at low temperature against β‐hydride elimination. In contrast, the exhaustive alkylation of 1 with cyclohexylmagnesium bromide triggers two consecutive C–H activation reactions mediated by a single iron center. The resulting complex has a diene dihydride character in solution (15), whereas its structure in the solid state is more consistent with an η3‐allyl iron hydride rendition featuring an additional agostic interaction (14). Finally, the preparation of the cyclopentadienyl iron complex 25 illustrates how an iron‐mediated C–H activation cascade can be coaxed to induce a stereoselective CC bond formation. The structures of all relevant new iron complexes in the solid state are presented.