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.