The carbene transfer to strong Lewis acids: copper is better than silver.

A new convenient approach to the synthesis of useful N-heterocyclic carbene complexes of group 13 metals was successfully developed. We demonstrate that air-stable copper(i) diaminocarbene complexes are excellent carbene transfer reagents for the synthesis of (IMes or IPr)MCl3 with high yields (M = Al, Ga). Use of this simple method allowed for the first time to obtain (IPr)AlCl3, inaccessible via a free carbene route. In contrast to copper complexes, under the same conditions the more commonly used silver analog (IPr)AgCl reacts with MCl3 (M = Al, Ga, In) yielding only the homoleptic cationic complexes [(IPr)2Ag]+MCl4-.