Recognizing that gleaning insight into the present system might enable extension of this catalytic asymmetric strategy to other reactions involving the intermediacy of phosphonium species, we embarked on efforts to elucidate the catalytic mechanism as well as the origins of catalyst enantioinduction and rate acceleration. The canonical Michaelis-Arbuzov reaction proceeds through formation of an intermediate alkylphosphonium ion -in this case a protonated dibenzyl phosphonite -that subsequently undergoes dealkylation through nucleophilic substitution (SN2) by the counterion . Evidence for the intermediacy of a protiophosphonium ion in the catalytic system was obtained by monitoring the reaction of 2a with HCl catalyzed by thiourea 1g by 31 P NMR (Fig. ). At the earliest timepoints the signal due to 2a at δ 159 ppm is not observed, replaced instead by a doublet at δ 52 ppm that diminishes gradually with concomitant appearance of product 3a at δ 24 ppm (see 20 Supplementary Materials, Fig. ). The transient signal at δ 52 is assigned to the phosphonium chloride intermediate 10a based on the 710 Hz (J 1 P-H) coupling constant and the spectrum of an independently prepared phosphonium tetrafluoroborate. These observations not only support the intermediacy of phosphonium chloride 10a, but also indicate that it is the resting state of the substrate under the catalytic conditions.