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

The iron-catalyzed oxidative addition of C(sp(n))-X bonds (n=1-3 and X=H, CH3, Cl) in archetypal model substrates H3C-CH2-X, H2C=CH-X and HC equivalent to C-X to Fe(CO)(4) was investigated using relativistic density functional theory at ZORA-OPBE/TZ2P. The C(sp(n))-X bonds become substantially stronger going from C(sp(3))-X to C(sp(2))-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 from sp(3) to sp(2) to sp stems 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(sp(3))-X to three in C(sp(2))-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(sp(n))-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.

Automated summary

This study investigates the iron-catalyzed oxidative addition of C(sp(n))-X bonds using density functional theory. The results show that the strength of the bonds increases from C(sp(3))-X to C(sp(2))-X to C(sp)-X, while the reaction barrier decreases. The analysis reveals that this decrease in barrier is due to the relief of steric repulsion between the catalyst and substrate, which is caused by the decreasing coordination number of the carbon atom. The study highlights the similarities between iron-based catalysts and palladium analogs in the oxidative addition step of cross-coupling reactions.