BF 3 -Promoted Ring Expansion of Iminylphosphiranes and Acylphosphiranes for Divergent Access to 1,2-Azaphospholidines and 1,2-Dihydrophosphetes.

Ring expansion of strained small rings provides an efficient method for the synthesis of various high-value carbocycles and heterocycles. Here we report BF 3 ·Et 2 O as both an activating reagent and fluorine source, enabling ring expansion of phosphirane and P-F bond formation. Treatment of 1-iminylphosphirane complexes with BF 3 ·Et 2 O resulted in 1,2-azaphospholidines, while the reaction of 1-acylphosphirane complexes with BF 3 ·Et 2 O afforded 1,2-dihydrophosphetes. The reaction path was tuned by the nucleophilicity of the N and O atoms toward the intermediate phosphenium cation.