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 O₂, dual mechanisms are shown to operate. In the absence of O₂, the dominant mechanism is a radical chain reaction that is propagated by CF₃· abstracting H from C₆H₁₂ and S_H2 displacement of C₆H₁₁· on CF₃CO₂OH. The intermediacy of C₆H₁₁· and CF₃· is inferred from production of CHF₃ and CO₂ along with cyclohexyl trifluoroacetate, or CDF₃ when cyclohexane-d₁₂ is used. In the presence of O₂, 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 (∼75°C) 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 O₂ 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.