Non-radical pathways control methane sulfonation versus oxygenation C-H functionalization selectivity with Hg(ii) and
Anjaneyulu Koppaka1, Caz Cullimore1, Jyothish Joy1
1Department of Chemistry and Biochemistry, Brigham Young University Provo UT 84604 USA Anjaneyulu.Koppaka@byu.edu dhe@byu.edu.
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 HgII and AuIII 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 HgII/AuIII-methyl intermediate can undergo either an electrophilic substitution pathway (SE2) with SO3 to form methanesulfonic acid or a nucleophilic substitution pathway (SN2) 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.
Related Concept Videos
02:17Reduction of Alkenes: Asymmetric Catalytic Hydrogenation
The metal catalyst used can be either heterogeneous or homogeneous. When hydrogenation of an alkene generates a chiral center, a pair of enantiomeric products is expected to form. However, an enantiomeric excess of one of the products can be facilitated using an enantioselective reaction or an...
01:25Radical Anti-Markovnikov Addition to Alkenes: Overview
02:13Reduction of Alkenes: Catalytic Hydrogenation
Metals like palladium, platinum, and nickel are commonly used in their solid forms — fine powder on an inert surface. As these catalysts remain insoluble in the reaction mixture, they are referred to as heterogeneous catalysts.
The hydrogenation process takes place on the...
02:24Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
02:36Regioselectivity and Stereochemistry of Hydroboration
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn...
01:26Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride
The bonds formed in this reaction are stronger than the bonds broken, making it energetically favorable. The reaction follows a radical chain mechanism similar to radical halogenation...