Conversion of Methanol to 2,2,3-Trimethylbutane (Triptane) over Indium(III) Iodide
InI_3 is able to catalyze the conversion of methanol to a mixture of hydrocarbons at 200 °C with one highly branched alkane, 2,2,3-trimethylbutane (triptane), being obtained in high selectivity. The mechanism for InI_3-catalyzed reactions appears to be basically the same as that proposed for the previously studied ZnI_2-catalyzed system in which sequential methylation of olefins is followed by competing reactions of the resulting carbocation: proton loss to give the next olefin vs hydride transfer to give the corresponding alkane. Although the reaction conditions and typical triptane yields achievable with ZnI_2 and InI_3 are quite similar, the two systems behave rather differently in a number of important particulars, including significant differences between the detailed product distributions. Most of the differences in behavior can be ascribed to the stronger Lewis acidity of InI_3, including the ability to activate some alkanes, the higher activity for methylation of arenes, and the fact that methanol conversion can be observed at somewhat lower temperatures with InI_3 than with ZnI_2.