Regio- and Stereoselective Addition of Secondary Phosphine Oxides to Allenoates Catalyzed by Main-Group Lewis Pairs

We report herein a Lewis pair-catalyzed process for the regio- and stereoselective addition of secondary phosphine oxides (SPOs) to allenoates. A Lewis pair composed of B(C 6 F 5 ) 3 and P(4-OMeC 6 H 4 ) 3 dissociates into a free acid and base under reaction conditions, thereby creating key reaction intermediates that enable the atom-economical generation of an array of alkenylphosphorus building blocks. Mechanistic studies indicate that this process proceeds through a catalytic cycle wherein the deprotonation of a P–H bond of a SPO (coordinated to a boron catalyst) by the phosphonium zwitterion (resulting from the nucleophilic addition of a phosphine catalyst to allenoates) is the rate-determining step. Subsequent addition of the phosphinite anion to allenoate substrates furnishes β-addition products, while the reaction between the pair of the phosphinite anion and alkenylphosphonium cation affords γ-addition products.