Monitoring formation of 3a by in-situ infrared (IR) spectroscopy revealed that 1a undergoes rapid conversion to an intermediate (1a•3a-int), which reverts to 1a after the reaction reaches complete conversion (Figure ). The IR features of the intermediate match closely those observed when 1a is combined with an unreactive ammonium chloride salt (tetra-N-butyl ammonium chloride, Figure ). From these observations, it can be concluded that the catalyst resting state under the conditions of catalysis consists of 1a bound to an intermediate N-acyl-azetidinium chloride ion pair. Moreover, observation of a 13 C KIE of 1.053(3) for the carbon that undergoes nucleophilic substitution is consistent with rate-limiting ring opening of the catalyst-bound azetidinium chloride (see SI).