Frontispiece: The Mechanism of Phosphonium Ylide Alcoholysis and Hydrolysis: Concerted Addition of the O−H Bond Across the P=C Bond

Ylide Hydrolysis & Alcoholysis The reactions of phosphonium ylides (Ar3P=CHR′) with alcohols (ROH) and H2O in protic solvents to give alkoxyphosphorane or hydroxyphosphorane, respectively, generally occur through a two‐step pathway involving a phosphonium hydroxide or alkoxide intermediate. In protic solvents, the pKaH of the ylide is typically higher than the pKa of H2O or alcohol. In aprotic media, the pKa values of H2O and alcohols are dramatically higher than in protic media, and yet formation of phosphorane intermediates is still observed. The characteristic high field 31P NMR signal of an alkoxyphosphorane formed in an aprotic solvent is shown in the image. On the basis of this observation, and other substantial evidence in support of their interpretation from 1D and 2D NMR studies, P. A. Byrne and D. G. Gilheany conclude in their Full Paper on page 9140 ff. that concerted formation of phosphorane intermediates from ylide+ROH (R=H or alkyl) occurs in any solvent in which the pKaH of the ylide is lower than the pKa of ROH.