ESI-MS DETECTION OF IONIC INTERMEDIATES IN PHOSPHINE-MEDIATED REACTIONS

This article describes the application of electrospray ionization mass spectrometry (ESI-MS) to the detection of transient intermediates in the Wittig, Mitsunobu, and Staudinger reactions directly from solution. Ionic intermediates in these reactions could be detected directly and zwitterionic intermediates indirectly upon quenching reaction aliquots with acid. For each reaction, intermediates were identified that correlate with those proposed on the basis of P-31 NMR analysis. Furthermore, ESI-MS spectra were uncomplicated by fragmentation of molecular ions and showed unambiguous single peaks for each species present and, consequently, interpretation of spectra was straightforward. The progress of reactions could be monitored over time by following the gradual appearance and disappearance of peaks corresponding to transient intermediates.