A Mechanistic Change Results in 100 Times Faster CH Functionalization for Ethane versus Methane by a Homogeneous Pt Catalyst

The selective, oxidative
functionalization of ethane, a significant
component of shale gas, to products such as ethylene or ethanol at
low temperatures and pressures remains a significant challenge. Herein
we report that ethane is efficiently and selectively functionalized
to the ethanol ester of H2SO4, ethyl bisulfate
(EtOSO3H) as the initial product, with the PtII “Periana-Catalytica” catalyst in 98% sulfuric acid.
A subsequent organic reaction selectively generates isethionic acid
bisulfate ester (HO3S-CH2-CH2-OSO3H, ITA). In contrast to the modest 3–5
times faster rate typically observed in electrophilic CH activation
of higher alkanes, ethane CH functionalization was found to be ∼100
times faster than that of methane. Experiment and quantum-mechanical
calculations reveal that this unexpectedly large increase in rate
is the result of a fundamentally different catalytic cycle in which
ethane CH activation (and not platinum oxidation as for methane) is
now turnover limiting. Facile PtII-Et functionalization was determined to occur via a
low energy β-hydride elimination pathway (which is not available
for methane) to generate ethylene and a PtII-hydride, which
is then rapidly oxidized by H2SO4 to regenerate
PtII-X2. A rapid, non-Pt-catalyzed reaction
of formed ethylene with the hot, concentrated H2SO4 solvent cleanly generate EtOSO3H as the initial
product, which further reacts with the H2SO4 solvent to generate ITA.