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

The catalytic functionalization of the C_sp²–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 = 1,3-bis(diisopropylphenyl)imidazol-2-ylidene) and NaBAr^F₄ (BAr^F₄ = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate). For Cu and Ag, these examples constitute the first use of such metals toward this transformation, which also provides ethyl cyclohepta-2,4,6-trienecarboxylate as a byproduct from the so-called Buchner reaction. In the case of methyl-substituted benzenes, the reaction exclusively proceeds onto the aromatic ring, the C_sp³–H bond remaining unreacted. A significant coinage-metal effect has been observed, since the gold catalyst favors the formation of the insertion product into the C_sp²–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.