Intrinsic Reactivity of the Elusive Cyclic N,N-Diacyliminium Ions in Mannich Reactions: The Complete Reactivity Order for Non, N-Mono and N,N-Diacyliminium Ions.

We report the first intrinsic reactivity measurements of four cyclic N,N-diacyliminium ions in Mannich-type reactions. Using MSn experiments (n = 1, 2) performed in the unique, convenient, and gaseous environment of a pentaquadrupole mass spectrometer, we reacted four gaseous five- or six-membered cyclic N,N-diacyliminium ions with allyltrimethylsilane and have established a complete reactivity order, demonstrating that N,N-diacyliminium ions surpass their N-monoacyl counterparts in reactivity. The study also enabled the isolation and characterization of key Mannich intermediates, confirming their bound nature and validating long-standing mechanistic proposals. Calculations have also indicated that the Mannich intermediates for the N,N-diacyliminium ions undergo prompt cyclization, and that therefore the reactions of these N,N-diacyliminium ions with an exocyclic N-acyl group with allyltrimethylsilane form polar [4+ + 2] cycloadducts. These findings provide a fundamental, solvent-free framework for understanding the electrophilic behavior of these key organic ions, completing the reactivity scale for cyclic nonacyl, N-mono-, and N,N-diacyliminium ions.