Silyl-NHC chelate iron complexes with various substituents on NHC were synthesized and demonstrated to be active catalysts for hydroboration and dehydrogenative borylation of olefins. The selectivity of these borylation reactions is significantly controlled by the steric bulk of a substituent on the NHC nitrogen.

Abstract

Various iron complexes 1, featuring silyl-NHC chelate ligands (NHC = N-heterocyclic carbene) were synthesized and investigated for their catalytic performance in the borylation reactions of olefins. Complexes 1 facilitated both hydroboration and dehydrogenative borylation of olefins with HBpin, yielding borylalkanes and borylalkenes, respectively. The selectivity between the two pathways was found to be governed by the steric effect of a substituent on one nitrogen atom of the NHC moiety. Possible catalytic mechanisms including hydroborato and β-agostic-alkyl complexes as potential resting states or intermediates are proposed based on stoichiometric reactions.