Mechanistic Studies on Gold-Catalyzed Direct Arene C–H Bond Functionalization by Carbene Insertion: The Coinage-Metal Effect

Abstract

The catalytic functionalization of the Csp 2-H bond of benzene by means of the insertion of the CHCO2Et group from ethyl diazoacetate (N2=CHCO2Et) has been studied with the series of coinage metal complexes IPrMCl (IPr = IPr = 1,3-bis(diisopropylphenyl)imidazol-2-ylidene) and NaBArF 4 (BArF 4 = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate). For Cu and Ag, these examples constitute the first of such metals toward this transformation, that also provides ethyl cyclohepta-2,4,6-trienecarboxylate as by-product from the so-called Buchner reaction. In the case of methyl-substituted benzenes, the reaction exclusively proceeds onto the aromatic ring, the Csp 3-H bond remaining unreacted. A significant coinage metal effect has been observed, since the gold catalyst favors the formation of the insertion product into Csp 2-H bond whereas copper and silver preferentially induce the formation of the cycloheptatriene derivative. Experimental studies and theoretical calculations have explained the observed selectivity in terms of the formation of a common Wheland intermediate, resembling an electrophilic aromatic substitution, from which the reaction pathway evolves into two separate routes to each product.