Theory and Experiment Demonstrate that Sb(V)-Promoted Methane C–H Activation and Functionalization Outcompete Superacid Protonolysis in Sulfuric Acid

Abstract

Sb­(V) in strong Brønsted acid solvents is traditionally assumed to react with light alkanes through superacid protonolysis, which results in carbocation intermediates, H2, and carbon oligomerization. In contrast to this general assumption, our density functional theory (DFT) calculations revealed an accessible barrier for C–H activation between methane and Sb­(V) in sulfuric acid that could potentially outcompete superacid protonolysis. This prompted us to experimentally examine this reaction in sulfuric acid with oleum, which has never been reported because of presumed superacid reactivity. Reaction of methane at 180 °C for 3 h resulted in very high yields of methyl bisulfate without significant overoxidation. Our DFT calculations show that a C–H activation and Sb-Me bond functionalization mechanism to give methyl bisulfate outcompetes methane protonolysis and many other possible reaction mechanisms, such as electron transfer, proton-coupled electron transfer, and hydride abstraction. Our DFT calculations also explain experimental hydrogen–deuterium exchange studies and the absence of methane carbo-functionalization/oligomerization products. Overall, this work demonstrates that in very strong Brønsted acid solvent, Sb­(V) can induce innersphere reaction mechanisms akin to transition metals and outcompete superacid reactivity.

Reactivity and Mechanisms of Methane, Ethane, and Benzene C–H Amination with an Iodine(III) Bistriflimide Complex

“…We also examined whether HNTf 2 coordinated to the iodine metal center would induce superacid-type reaction steps where [(C 6 F 5 )­I III (NTf 2 )­(HNTf 2 )] + can act as a Bronsted acid and protonate methane to induce carbocation formation. In contrast to Sb V with sulfuric acid that provides a low barrier for protonation of methane, but has a high barrier for generating methyl cation, the reaction energy for [(C 6 F 5 )­I III (NTf 2 )­(HNTf 2 )] + to protonate methane to give [(C 6 F 5 )­I III (NTf 2 )­(NTf 2 )­(CH 5 )] + is relatively high with a Δ G value of 56.8 kcal/mol (50.8 kcal/mol for Δ H ), which indicates that the superacid-type reactivity is unlikely. − …”

Partial Oxidation of Methane Enabled by Decatungstate Photocatalysis Coupled to Free Radical Chemistry

“…However, the energy requirements for the separation of product from oleum and reconcentration of sulfuric acid were a challenge for potential commercialization. 18,19 Molecular iodine has been shown to functionalize light alkanes in oleum, but this approach could not be extended to non-superacidic media. 20,21 Main group compounds, such as Tl(TFA) 3 and Pb(TFA) 4 , and hypervalent iodine, namely, (C 6 F 5 )I III (TFA) 2 , have been shown to functionalize light alkanes in non-superacidic solvent, albeit stoichiometrically.…”

“…The Catalytica process pioneered by Periana and coworkers achieved methane-to-methyl bisulfate conversion with >70% yield and >90% selectivity using the key strategy of protecting the mono-functionalized product from over-oxidation by the electron-withdrawing bisulfate group. However, the energy requirements for the separation of product from oleum and reconcentration of sulfuric acid were a challenge for potential commercialization. , Molecular iodine has been shown to functionalize light alkanes in oleum, but this approach could not be extended to non-superacidic media. , Main group compounds, such as Tl­(TFA) 3 and Pb­(TFA) 4 , and hypervalent iodine, namely, (C 6 F 5 )­I III (TFA) 2 , have been shown to functionalize light alkanes in non-superacidic solvent, albeit stoichiometrically …”

A comprehensive benchmark investigation of quantum chemical methods for carbocations

“…48 Once again, only a limited number of DFAs were evaluated for an extended series of carbocations with all except one DFA from the hybrid family (B3LYP, 31,32 M06, 35 M06-2X, 35 MN15, 49 PBE0, 50 PW6B95, 51 revPBE0, 52 SCAN0 53 and ωB97M-V 54 ). The double-hybrid B2PLYP, 55 which was applied in a recent mechanistic study to verify the involvement of methyl cations, 56 was also considered (Fig. 1).…”