Mechanism of the Mitsunobu Esterification Reaction. 2. The Involvement of (Acyloxy)alkoxyphosphoranes

Article abstract of DOI:10.1021/jo00274a017

A 31P NMR examination of the Mitsunobu reaction using triphenylphosphine or diphenyl-2-pyridylphosphine with diisopropyl azodicarboxylate, an alcohol, ROH, and a carboxylic acid, R'COOH, reveals the presence of a dialkoxyphosphorane, Ar3P(OR)2, in equilibrium with an alkoxyphosphonium carboxylate, Ar3POR⁺ OCOR'^-.The 31P NMR chemical shift of the latter species in exquisitely sensitive to the presence of proton sources and the nature of the solvent, varying over a range of more than 100 ppm.The data are interpreted in terms of a rapid equilibrium between an ion pair, Ar3POR⁺ OCOR'^-, and the corresponding (acyloxy)alkoxyphosphorane, Ar3P(OR)OCOR'.The role of dialkoxyphosphoranes and of (acyloxy)phosphoranes in the mechanism of the Mitsunobu reaction is discussed.