l‐Menthol‐Assisted Synthesis of P‐Stereogenic Phosphinous Acid Amides and Phosphine‐Boranes

Diastereomerically pure phenylphosphonous acid-borane l-menthyl ester N,N-diethylamide was obtained through the fractional crystallization of a diastereomeric mixture of compounds synthesized from PhPCl2, l-menthol, diethylamine, and BH3THF. Treatment of racemic or diastereomerically pure phenylphosphonous acid-borane derivatives with sodium in liquid ammonia followed by the addition of an electrophile led to the formation of phosphinous acid-amides. Surprisingly, these compounds undergo preferential P-N bond cleavage under Birch reduction conditions.