Generation of Chiral Phosphonium Dialkyl Phosphite as a Highly Reactive P-Nucleophile: Application to Asymmetric Hydrophosphonylation of Aldehydes

The generation of chiral tetraaminophosphonium dialkyl phosphite has been detected by low-temperature NMR analysis, and its synthetic potential as a remarkably reactive P-nucleophile has been successfully demonstrated by its application to the establishment of highly efficient and enantioselective hydrophosphonylation of various aldehydes. A systematic evaluation of the organic-base-dependent generation of the requisite ion pair and its reactivity and selectivity as a P-nucleophile reveals that the structure of the cationic conjugate acid of the base is a key element both for substantial generation of phosphite anions with prominent nucleophilicity and for rigorous stereocontrol. This study provides not only experimental demonstration of the importance of the phosphite tautomer in the P—C bond formation process but also a general yet valuable framework for the molecular design of even superior chiral organic base catalysts.