Dual reactivity of B(C6F5)3 enables the silylative cascade conversion of N-aryl piperidines to sila-N-heterocycles: DFT calculations

Abstract

The first catalytic access to bridged sila-N-heterocycles from piperidines via cascade sp³ and sp² C–Si bond formation all mediated by a single catalyst B(C₆F₅)₃ (A) has been recently developed by Park and Chang. Described herein is a theoretical study of the B(C₆F₅)₃-catalyzed silylative cascade conversion of N-aryl piperidines (H) to afford polycyclic azasilaheterocycles with a strong emphasis on the dual reactivity of B(C₆F₅)₃. The DFT calculations show that the catalyzed reaction involves three steps of the cascade reaction: (i) the formation of tetrahydropyridine (I) upon dehydrogenation of the piperidine; (ii) β-selective hydrosilylation of tetrahydropyridine; (iii) an intramolecular sila-Friedel–Crafts reaction to form a bridged sp² C–Si bond. The N-silyl piperidinium borohydride (A′) turns out to be the thermodynamically favored, resting species of the overall reaction, which is consistent with the observation of a species such as A′ during borane catalysis. The DFT calculations suggest that the β-selective hydrosilylation of I is the rate-determining step in the entire catalytic cycle, while the liberated I reacts with B(C₆F₅)₃ to form a zwitterion consisting of iminium and borate units (O). The calculated sila-Friedel–Crafts reactions of a range of presupposed silylated (cyclic)aza intermediates imply the importance of structural integrity on the piperidinyl unit of substrates.