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

Sb(V) in strong Brønsted acid solvents is traditionally assumed to react with light alkanes through superacid protonolysis, which results in carbocation intermediates, H, 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.