Reactions

First, triphenylphosphine (PPh₃) attacks the diethyl azodicarboxylate (DEAD) to form a phosphonium intermediate and a hydrazide anion. The latter acts as a strong base and deprotonates the nucleophile (NuH), generating a strong nucleophilic species (Nu^-). Then, the oxygen atom of the alcohol attacks the phosphonium intermediate to form an alkoxyphosphonium ion, which is a good leaving group. Finally, the nucleophile (Nu^-) attacks the carbon atom in α position relative to the oxygen atom via an SN2 mechanism, resulting in the formation of the substituted product with inversion of stereochemistry (Walden inversion).