Radical and non-radical mechanisms for alkane oxidations by hydrogen peroxide-trifluoroacetic acid

The oxidation of cyclohexane by the H2O2-trifluoroacetic acid system is revisited. Consistent with a previous report (Deno, N.; Messer, L. A. Chem. Comm. 1976, 1051), cyclohexanol forms initially but then esterifies to cyclohexyl trifluoroacetate. Small amounts of trans-1,2-cyclohexadiyl bis-(trifluoroacetate) also form. Although these products form irrespective of the presence or absence of O-2, dual mechanisms are shown to operate. In the absence of Oz, the dominant mechanism is a radical chain reaction that is propagated by CF (3) over circle abstracting H from C6H12 and S(H)2 displacement of C6H ((11)) over circle on CF3CO2OH. The intermediacy of C6H ((11)) over circle and CF ((3)) over circle is inferred from production of CHF3 and CO2 along with cyclohexyl trifluoroacetate, or CDF3 when cyclohexane-d(12) is used. In the presence of O-2, fluoroform and CO2 are suppressed, the reaction rate slows, and the rate law approaches second order (first order in peracid and in C6H12). Trapping of cyclohexyl radicals by quinoxaline is inefficient except at elevated (similar to 75 degreesC) temperatures. Fluoroform and CO2, telltale evidence for the chain pathway, were not produced when quinoxaline was present in room temperature reactions. These observations suggest that a parallel, nonfree radical,oxenoid insertion mechanism dominates when O-2 is present. A pathway is discussed in which a biradicaloid-zwiterionic transition state is attained by hydrogen transfer from alkane to peroxide oxygen with synchronous O-O bond scission.