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Elementary Steps of Iron Catalysis: Exploring the Links between Iron Alkyl and Iron Olefin Complexes for their Relevance in CH Activation and CC Bond Formation
Author(s) -
Casitas Alicia,
Krause Helga,
Goddard Richard,
Fürstner Alois
Publication year - 2015
Publication title -
angewandte chemie international edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 1433-7851
DOI - 10.1002/anie.201410069
Subject(s) - chemistry , alkylation , alkyne , ethylene , hydride , cycloaddition , propyne , propene , cyclopentadienyl complex , olefin fiber , catalytic cycle , alkyl , medicinal chemistry , allyl bromide , bromide , phenylacetylene , catalysis , electrophile , alkene , photochemistry , hydrogen , organic chemistry
The 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 η 3 ‐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 CC bond formation. The structures of all relevant new iron complexes in the solid state are presented.