INTERACTIONS OF SINGLET OXYGEN WITH 2,5-DIMETHYL-2,4-HEXADIENE IN POLAR AND NON-POLAR SOLVENTS EVIDENCE FOR A VINYLOG ENE-REACTION

Article abstract of DOI:10.1016/0040-4020(84)85006-1

2,5-Dimethyl-2,4-hexadiene (1) was studied as a singlet oxygen acceptor in various solvents. 1 undergoes concomitantly the three well-known modes of singlet oxygen reactions: (1) the ene-reaction to give the allylic hydroperoxide 3, (2) the (4+2)-cycloaddition to give the endoperoxide 4, and (3) the (2+2)-cycloaddition to give the dioxetane 2.Beyond that (and in contrast to simple olefins), there are (4) "physical" quenching and (5) a "vinylog ene-reaction" to give the twofold-unsaturated hydroperoxide 5.The latter reaction represents a novel mode of singlet oxygen interaction with a substituted 1,3-diene. - Kinetic analysis shows that "physical" quenching, endoperoxide and vinylog ene-product formations proceed with solvent-independent rates; the rates of dioxetane and ene-product formations, however, are solvent-dependent. - A mechanism (Scheme 3) is proposed, according to which endoperoxide formation is due to a concerted singlet oxygen reaction with the s-cis-conformational isomer 1b; with the s-trans-isomer 1a, "physical" quenching and the vinylog ene-reaction proceed via a non-polar singlet diradical intermediate, whereas the ene-product formation occurs via a perepoxide-like transition state.In aprotic solvents, the dioxetane is mainly formed via a "tight-geometry intermediate", in methanolic solution via a solvent-stabilized zwitterion; the latter is also responsible for the formation of the methanol-addition product 6.