Selective Monooxidation of Light Alkanes Using Chloride and Iodate

We describe an efficient system for the direct partial oxidation of methane, ethane, and propane using iodate salts with catalytic amounts of chloride in protic solvents. In HTFA (TFA = trifluoroacetate), >20% methane conversion with >85% selectivity for MeTFA have been achieved. The addition of substoichiometric amounts of chloride is essential, and for methane the conversion increases from <1% in the absence of chloride to >20%. The reaction also proceeds in aqueous HTFA as well as acetic acid to afford methyl acetate. ¹³C labeling experiments showed that less than 2% of methane is overoxidized to ¹³CO₂ at 15% conversion of ¹³CH₄. The system is selective for higher alkanes: 30% ethane conversion with 98% selectivity for EtTFA and 19% propane conversion that is selective for mixtures of the mono- and difunctionalized TFA esters. Studies of methane conversion using a series of iodine-based reagents [I₂, ICl, ICl₃, I­(TFA)₃, I₂O₄, I₂O₅, (IO₂)₂S₂O₇, (IO)₂SO₄] indicated that the chloride enhancement is not limited to iodate.