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

The
catalytic functionalization of the Csp2–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 NaBArF4 (BArF4 = 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 Csp3–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 Csp2–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.