ABSTRACT
Early and recent theoretical calculations concerning the mechanism of 1O2 ene reaction are notably contradictory. In the mid-1970s, according to semiempirical MINDO/3 calculations, the addition of 1O2 to propene appears to be non-concerted involving the formation of cis-and trans-methylperoxirane as intermediates.20 In 1980, Harding and Goddard predicted (using a combination of ab initio calculation and thermochemical methods) that the title reaction proceeds via a favorable biradical intermediate; the perepoxide is 40 kJ/mol higher in energy.21 One year later, Yamaguchi et al. reported that perepoxides are too high in energy to qualify as viable intermediates (as are zwitterions) in 1O2-mediated allylic oxidations of weakly polar alkenes.22 In the early 1990s, calculations have shown that both concerted and stepwise pathways (involving strained perepoxide-type intermediates), exist on PM3 semiempirical potential-energy surfaces for the addition of 1O2.23 In the mid-1990s, ab initio molecular orbital studies supported a concerted mechanism (involving a non-radical transition state with a perepoxide-like conformation) for the ene reactions of allylic ole ns and enol ethers bearing allylic hydrogen atoms.24 In 2001, the 1O2 addition to 1,3-cyclohexadiene was calculated to occur in a concerted fashion without any intermediates along the reaction coordinate.25
