Catalytic metal-free intramolecular hydroaminations of non-activated aminoalkenes: A computational exploration

Abstract

Frustrated Lewis pairs (FLPs) has been applied to catalytic metal-free hydrogenation. Can the FLP reactivity be used for catalytic hydroamination? Using density functional theory (DFT) calculations, we have explored whether the molecules cat1-cat3, which were previously designed by integrating the dearomatization-aromatization effect and the FLP reactivity, can catalyze the intramolecular hydroaminations of non-activated aminoalkenes to afford nitrogen heterocycles. The study shows that the γ-aminoalkene (am1) hydroamination catalyzed by cat1 proceeds via two steps (aminoalkene N-H bond activation and C-N bond formation) with experimentally accessible energetics, giving the five-membered nitrogen heterocycle product 1,1-dimethylpyrrolidine. The N-H bond activation is reversible. The C-N bond formation step undergoes a concerted mechanism and complies with the Markovnikov addition rule. Possible side reactions which may cause catalyst deactivation were confirmed to be energetically unfavorable. The molecules cat2 and cat3 are less effective than cat1 in catalyzing the am1 hydroamination, but the barriers are not too high. By following the most favorable pathway of the cat1-mediated am1 hydroamination, we further extended the substrate (am1) to other aminoalkenes, including the methyl and phenyl β-substituted am1 (i.e. am2 and am3, respectively), the benzyl-protected primary aminoalkene (am4), and the δ-aminoalkene (am5). The hydroaminations of am2 and am3 have energetics comparable with am1 hydroamination, the am5 hydroamination is energetically less favorable, and the am4 hydroamination is least favorable but could be realizable by elevating the temperature and pressure. We call experimental efforts to synthesize cat1-cat3 or similar new molecules on the basis of the design strategy.