Mechanism of Hydrocarbon Functionalization by an Iodate/Chloride System: The Role of Ester Protection

Abstract

Mixtures of chloride and iodate salts for light alkane oxidation achieve >20% yield of methyl trifluoroacetate (TFA) from methane with >85% selectivity. The mechanism of this C–H oxygenation has been probed by examining adamantane as a model substrate. These recent results lend support to the involvement of free radicals. Comparative studies between radical chlorination and iodate/chloride functionalization of adamantane afford statistically identical 3°:2° selectivities (∼5.2:1) and kinetic isotope effects for C–H/C–D functionalization (k_H/k_D = 1.6(3), 1.52(3)). Alkane functionalization by iodate/chloride in HTFA is proposed to occur through H-atom abstraction by free radical species including Cl• to give alkyl radicals. Iodine, which forms by in situ reduction of iodate, traps alkyl radicals as alkyl iodides that are subsequently converted to alkyl esters in HTFA solvent. Importantly, the alkyl ester products (RTFA) are quite stable to further oxidation under the oxidizing conditions due to the protecting nature of the ester moiety.

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