Computation Study of Alkane Functionalization by Hypervalent Iodine and Bromine Species

Abstract

Improved technology to convert natural gas into useful and easily transported liquid fuels would enable society to better harness this promising source of energy. In light of impressive experimental findings on hypervalent iodine species’ alkane functionalization reactions, this work set out to employ density functional theory computational methods to understand these results, and to provide insight into potential new classes of reagents. Determination of the free energy profiles for ethane activation by [bis(trifluoroacetoxy)iodo]perfluoroalkanes revealed the Gibbs free energy of activation for ethane C-H bond cleavage varied by 1.0 kcal/mol upon perfluoroalkyl chain extension, while more electron-donating acetate ligands raised this parameter by 8.5 kcal/mol. Based on these findings, reagents hypothesized to be more electrophilic or to have a more reactive initial conformation were explored: [bis(trifluoroacetoxy)bromo]trifluoromethane was among the most successful, with an overall Gibbs free energy of reaction which was 37.6 kcal/mol more exergonic than that of the iodane. The iodosyl cation was found to be a plausible active species for the electrophilic activation of methane, with a Gibbs free energy of activation of 38.6 kcal/mol, and a chloride-assisted deprotonation transition state may explain how this halide has been shown experimentally to increase yield. Overall, this work highlights the promise of hypervalent iodine and bromine reagents for electrophilic functionalization of alkanes, due to the exergonic Gibbs free energy changes of their overall functionalization reactions and low Gibbs free energies leading to their transition states for C-H activation.