A mechanistic explanation for selectivity in the conversion of methanol to 2,2,3-trimethylbutane (triptane): Moderate acidity allows kinetic control to operate

Methanol is converted to hydrocarbons by reaction with ZnI2 or InI3 at 200 °C, with surprisingly high selectivity (yields on the order of 20%) to a single highly branched alkane, 2,2,3-trimethylbutane (triptane). Mechanistic studies demonstrate that the previously proposed mechanism, which proceeds via a carbocation-based route that involves methylation of olefins and hydride transfer to carbocations, can account quantitatively for the selectivity. Differences in product distribution between the Zn- and In-based systems represent quantitative, not qualitative, differences in behavior, and can be attributed to the slightly higher effective acidity of the latter.

The relative rate of olefin homologation by cationic methylation vs. hydrogenation by protonation/hydride transfer increases with the degree of substitution of the olefin, a trend that explains the remarkably high selectivity observed for conversion of methanol to triptane over zinc and indium iodides.Figure optionsDownload high-quality image (57 K)Download as PowerPoint slide