Evidence for HOOO Radicals in the Formation of Alkyl Hydrotrioxides (ROOOH) and Hydrogen Trioxide (HOOOH) in the Ozonation of C−H Bonds in Hydrocarbons1

Low-temperature ozonation of cumene (1a) in acetone, methyl acetate, and tert-butyl methyl ether at −70° C produced the corresponding hydrotrioxide, C6H5C(CH3)2OOOH (2a), along with hydrogen trioxide, HOOOH. Ozonation of triphenylmethane (1b), however, produced only triphenylmethyl hydrotrioxide, (C6H5)3COOOH (2b). These observations, together with the previously reported experimental evidence, seem to support the "radical" mechanism for the first step of the ozonation of the C−H bonds in hydrocarbons, i.e., the formation of the caged radical pair (R• •OOOH), which allows both (a) collapse of the radical pair to ROOOH and (b) the abstraction of the hydrogen atom from alkyl radical R• by HOOO• to form HOOOH. The B3LYP/6-311++G(d,p) (ZPE) calculations revealed that HOOO radicals are considerably stabilized by forming intermolecularly hydrogen-bonded complexes with acetone (BE = 8.55 kcal/mol) and dimethyl ether (7.04 kcal/mol). This type of interaction appears to be crucial for the relatively fast reactions (and the formation of the polyoxides in relatively high yields) in these solvents, as compared to the ozonations run in nonbasic solvents. However, HOOO radicals appear to be not stable enough to abstract hydrogen atoms outside the solvent cage, as indicated by the absence of HOOOH among the products in the ozonolysis of triphenylmethane. The decomposition of alkyl hydrotrioxides 2a and 2b involves a homolytic cleavage of the RO−OOH bond with subsequent "in cage" reactions of the corresponding radicals, while the decomposition of HOOOH is most likely predominantly a "pericyclic" process involving one or more molecules of water acting as a bifunctional catalyst to produce water and singlet oxygen (Δ1O2).