C(spn)−X (n=1–3) Bond Activation by Iron

Abstract

AbstractThe iron‐catalyzed oxidative addition of C(spn)−X bonds (n=1–3 and X=H, CH3, Cl) in archetypal model substrates H3C−CH2−X, H2C=CH−X and HC≡C−X to Fe(CO)4was investigated using relativistic density functional theory at ZORA‐OPBE/TZ2P. The C(spn)−X bonds become substantially stronger going from C(sp3)−X to C(sp2)−X to C(sp)−X, whereas the oxidative addition reaction barrier decreases along this series. Our activation strain and energy decomposition analyses expose that the decreased reaction barrier for the oxidative addition going fromsp3tosp2tospstems from a relief of the destabilizing (steric) Pauli repulsion between the catalyst and substrate. This originates from the decreasing coordination number of the carbon atom that goes from four in C(sp3)−X to three in C(sp2)−X to two in C(sp)−X. In analogy with our previous results on palladium‐catalyzed oxidative additions, this enhances the stabilizing catalyst–substrate interaction, which is able to overcome the more destabilizing strain associated with the stronger C(spn)−X bonds. This work again demonstrates that iron‐based catalysts can resemble the behavior of their well‐known palladium analogs in the oxidative addition step of cross‐coupling reactions.