Hydrogenation of Alkenes Catalyzed by Rare-Earth Metal Phosphinophosphinidene Complexes: 1, 2-Addition/Elimination Versus σ-Bond Metathesis Mechanism

Abstract

Rare-earth metal complexes have been used as catalysts for many types of reactions. However, the mechanistic studies showed that basically, these various reactions proceeded with σ-bond metathesis of RE-E bond (RE: rare-earth metals; E: main group elements) or RE ions acting as Lewis acid centers to activate the polar functional groups of the substrates. In this article, a lutetium phosphinophosphinidene complex, with Lu=P double-bond character, was synthesized and structurally characterized. This lutetium complex and our previously reported scandium phosphinophosphinidene complexes were able to catalyze the hydrogenation of terminal alkenes under mild conditions, and the lutetium complex showed higher catalytic activity than the scandium ones. More interestingly, isotopic labeling experiments indicated that the catalytic reaction proceeded through an addition/elimination mechanism rather than the traditional σ-bond metathesis mechanism. Density functional theory calculations provided insights into the 1, 2-addi-tion/elimination mechanism and regioselectivities of the H2 addition to the Lu=P double bond of the lutetium phosphinophosphinidene complex and the anti-Markovnikov alkene insertion into the Lu-H bond of the lutetium phosphinophosphido hydride intermediate.