Free-radical catalyzed oxidation reactions with cyclohexene and cyclooctene with peroxides as initiators

The free-radical oxidation reaction of cyclohexene is accomplished at 80°C in a 10:1 1,2-dichloroethane:acetonitrile mixture under an air atmosphere using 0.05 molar equivalents of either tBuOOH or H2O2 and results in a 33.7% conversion yielding 23.7% 2-cyclohexen-1-one, 8.0% 2-cyclohexen-1-ol, and 2.0% cyclohexene oxide. With cyclooctene under similar conditions, a 42.3% conversion is obtained, 40.3% cyclooctene oxide, 0.4% of 1,2-cyclooctanediol, and 1.2% of 2-cycloocten-1-one. The reaction is dependent on dioxygen as reduced yields are obtained under a nitrogen atmosphere. It is initiated best with H2O2 and tBuOOH and does not work with ammonium persulfate as a radical initiator. This free-radical reaction was not useful for three other alkenes tested under similar reaction conditions. Transition states and activation energies were calculated with the APFD functional and 6-311 + G(d) basis set for possible reaction pathways involving radicals produced from the homolysis of the peroxide O-O bond followed by abstraction of an alkyl H atom, forming a cyclic alkene radical. This can react rapidly with dioxygen forming a cyclic-alkene peroxide radical. This intermediate may result in ketone products via a unimolecular pathway, whereas the formation of epoxides requires a bimolecular route. This was confirmed experimentally as lower epoxide yields for reaction with cyclooctene were obtained if the reaction solution was diluted. It is suggested that reactions involving cyclohexene where similar yields of products were obtained may also involve free-radical pathways