Electrophilic Reactivities of Photo-Generated α,β-Unsaturated Iminium Ions

Significance

To gain insight into the reaction mechanism of iminium catalysis, the Mayr group employed laser flash photolysis to generate the iminium intermediate 2 from 3. Since the rates of reactions with strong nucleophiles to the imidazolidinone-derived iminium intermediate 2 are fast, conventional methods could not be applied. By using the photo-leaving group tri-n-butylphosphine, the iminium intermediate 2 could be observed and showed identical reactivity compared to the isolated iminium salt 2. Then, the in situ generated iminium salt 2 reacts with excess of nucleophile, and the decay of its absorption can be translated to the rate constant. This study supports theoretical and experimental results on the higher reactivity of 1b compared with 1a and 1c.