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

The previous work on the hydrolysis and alcoholysis reactions of phosphonium ylides is summarized and reviewed in the context of their currently accepted mechanisms. Several experimental facts relating to ylide hydrolysis and to salt and ylide alcoholysis are shown to conflict with those mechanisms. In particular, we demonstrate that the pKa values of water and alcohols are too high in organic media to bring about protonation of ylide. Therefore, we propose concerted addition of the water or alcohol O−H bond across the ylide P=C bond. In support of this, we provide NMR spectroscopic evidence for equilibrium between ylide and aclohol that does not require the involvement of phosphonium hydroxide. We report the first P‐alkoxyphosphorane to be characterised by NMR spectroscopy that does not undergo exchange on an NMR timescale. Two‐dimensional NMR spectroscopic techniques have been applied to the characterisation to P‐alkoxyphosphoranes for the first time. Concerted, not stepwise: The existing mechanism for phosphonium ylide alcoholysis and hydrolysis is at odds with several experimental facts, for example, H2O and ROH cannot protonate phosphonium ylide in aprotic organic solvents. We propose instead a concerted 4‐centre addition of the O−H bond across the P=C bond. NMR characterisation of alkoxyphosphoranes shows them to be in equilibrium with ylide+alcohol in aprotic solvents.