Hydrogen-Abstraction Reactivity Patterns from A to Y: The Valence Bond Way

摘要

Give us insight, not numbers" was Coulson's admonition to theoretical chemists. This Review shows that the valence bond (VB) -model provides insights and some good numbers for one of the fundamental reactions in nature, the-hydrogen-atom transfer (HAT). The VB model is applied to over 50 reactions from the simplest H +H2 process, to P450 hydroxylations and PP-transfers among closed-shell molecules; for each system the barriers are estimated from raw data. The model creates a bridge to the Marcus equation and shows that H-atom abstraction by a closed-shell'mo/ecuie requires a higher barrier owing to the additional promotion energy needed to prepare the abstractor for//-abstraction. Under certain conditions, a closed-shell abstractor can bypass this penalty through a proton-coupled electron transfer (PCET) mechanism. The VB model links the HAT and PCET mechanisms conceptually and shows the consequences that this linking has for IP-abstraction reactivity.