Selective Methylative Homologation: An Alternate Route to Alkane Upgrading
InI3 catalyzes the reaction of branched alkanes with methanol to produce heavier and more highly branched alkanes, which are more valuable fuels. The reaction of 2,3-dimethylbutane with methanol in the presence of InI3 at 180−200 °C affords the maximally branched C7 alkane, 2,2,3-trimethylbutane (triptane). With the addition of catalytic amounts of adamantane the selectivity of this transformation can be increased up to 60%. The lighter branched alkanes isobutane and isopentane also react with methanol to generate triptane, while 2-methylpentane is converted into 2,3-dimethylpentane and other more highly branched species. Observations implicate a chain mechanism in which InI3 activates branched alkanes to produce tertiary carbocations which are in equilibrium with olefins. The latter react with a methylating species generated from methanol and InI3 to give the next-higher carbocation, which accepts a hydride from the starting alkane to form the homologated alkane and regenerate the original carbocation. Adamantane functions as a hydride transfer agent and thus helps to minimize competing side reactions, such as isomerization and cracking, that are detrimental to selectivity.
Additional Information© 2008 American Chemical Society. Received April 24, 2008. Web Release Date: August 13, 2008. We thank Professor Enrique Iglesia for useful discussions. This work was supported by BP through the MC^2 program. Supporting Information Available. A complete listing of experimental results (in Excel format), along with an explanatory guide to the Excel tables and details of a labeling experiment. This material is available free of charge via the Internet at http://pubs.acs.org.
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