Non-radical pathways control methane sulfonation versus oxygenation C–H functionalization selectivity with Hg(ii) and Au(iii) catalysis

Abstract

Methane C–H functionalization by radical pathways is often unselective and not desirable. Transition metal catalyzed C–H functionalization of methane to methanesulfonic acid (sulfonation) in sulfuric acid has generally been interpreted as resulting from a radical mechanism whereas functionalization to methyl bisulfate (oxygenation) has been proposed to occur by both radical and non-radical pathways. For Hg^II and Au^III catalysis, formation of either methanesulfonic acid or methyl bisulfate depends on whether 98% sulfuric acid or oleum (SO₃ added) is used. Here we report new experiments combined with density functional theory calculations that have revealed that selectivity is determined by non-radical pathways where a Hg^II/Au^III-methyl intermediate can undergo either an electrophilic substitution pathway (S_E2) with SO₃ to form methanesulfonic acid or a nucleophilic substitution pathway (S_N2) with bisulfate to form methyl bisulfate. The favored pathway is determined by the electrophilicity/reduction potential of the metal and the sulfuric acid to SO₃/H₂O equilibrium. Overall, this new selectivity model provides a straightforward understanding of product selectivity and does not require a functionalization mechanism involving radicals.