Reaction of Carbocations Derived from Alkanes with Carbon Monoxide in HF–SbF5 Superacid

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Abstract
Alkanes readily react with CO and H2O in HF–SbF5 at 30 °C under atmospheric pressure and give considerably different product distributions from those in the usual Koch–Haaf reaction. When using C5-or C6-alkanes with tertiary C–H bonds, a considerable amount of secondary carboxylic acids are produced by the reaction of CO with secondary alkyl cations. Such cations may appear as transient intermediates in the course of the carbon skeletal isomerization of initially formed tertiary cations. For the straight-chain alkanes, the protonolysis at C–C bonds initiated the reaction to give fragment alkyl cations. This resulted in the formation of carboxylic acids with a lower number of carbon atoms than the starting alkanes. Intermolecular hydride shift between the fragment cations and the starting alkanes occurred at the same time to form a large amount of carboxylic acids with the alkyl group of the same number of carbon atoms as the starting alkane. On the other hand, for alkanes with more than seven carbon atoms, β-scission occurs exclusively to produce C4–, C5–, and C6–carboxylic acids.

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