Intrinsic Reactivity of the Elusive Cyclic N , N -Diacyliminium Ions in Mannich Reactions: The Complete Reactivity Order for Non, N Luiz Alberto B. de Moraes, Adriano Reis, Marcelo G. Montes D’Oca, Cristina M. Schuch, Ronaldo A. Pilli, Rodinei Augusti, Marcos N. Eberlin

Abstract

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.