[2.2]Paracyclophane-based N-heterocyclic carbene as efficient catalyst or as ligand for copper catalyst for asymmetric alpha-silylation of N-tosylaldimines

A new series of chiral bicyclic 1,2,4-triazolium salts with various substituents in the pseudo-ortho position of the [2.2]paracyclophane skeleton has been successfully synthesized and applied to enantioselective alpha-silylations of N-tosylaldimines. The dual role of the new carbene precursor as an organocatalyst and as a ligand of the copper catalyst in the asymmetric silylation were investigated. The chiral carbene-copper catalyst has significant advantages over its carbene counterpart as an organocatalyst, affording the corresponding alpha-silyl amines in 77-99% yields over 20 examples and with 76-98% ee. In addition, the transannular electronic effects of the substituents in the pseudo-ortho position of the [2.2]paracyclophane skeleton were also discussed.