[2.2]Paracyclophane-based N-heterocyclic carbene as efficient catalyst or as ligand for copper catalyst for asymmetric α-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 α-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 α-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.