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Pd-catalyzed C–H arylation of heteorarenes is an important and widely studied synthetic transformation; however, the regioselectivity is often substrate-controlled. Here, we report catalyst-controlled regioselectivity in the Pd-catalyzed oxidative coupling of N-(phenylsulfonyl)indoles and aryl boronic acids using O2 as the oxidant. Both C2- and C3-arylated indoles are obtained in good yield with >10:1 selectivity. A switch from C2 to C3 regioselectivity is achieved by including 4,5-diazafluoren-9-one or 2,2'-bipyrimidine as an ancillary ligand to a "ligand-free" Pd(OTs)2 catalyst system. Density functional theory calculations indicate that the switch in selectivity arises from a change in the mechanism, from a C2-selective oxidative-Heck pathway to a C3-selective C–H activation/reductive elimination pathway.
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