Dye-Sensitized Photooxygenation of 2,3-Dihydrofurans: Competing Cycloadditions and Ene Reactions of Singlet Oxygen with a Rigid Cyclic Enol Ether System

Article abstract of DOI:10.1021/jo00012a040

Singlet oxygen reacts with 2,3-dihydrofuran (1), 5-methyl (7), 4,5-dimethyl- (13), and 4-carbomethoxy-5-methyl-2,3-dihydrofuran (20), 5,6-dimethyl-3,4-dihydro-2H-pyran (26), and 3-methoxy-2-methyl-2-butene (32) in nonpolar and polar aprotic solvents to yield dioxetanes and allylic hydroperoxides, except 32, which gives only allylic hydroperoxides.The dioxetanes were isolated, but decompose slowly with weak chemiluminescence at room temperature to yield the corresponding dicarbonyl compounds.The allylic hydroperoxides produced by the cyclic enol ethers could not be isolated or separated by high vacuum distillation or by chromatography; the endocyclic allylic hydroperoxides arising from the dihydrofurans eliminate H2O2 to yield the corresponding furans while the exocyclic allylic hydroperoxides gives unknown products.Allylic hydroperoxides 28 and 29 and the dioxetane 27 obtained from 26 yield the same dicarbonyl compound 31.The proportion of dioxetanes to allylic hydroperoxides depends on ring size and substitution of the enol ethers and on solvent polarity.Smaller ring size, greater electron-donor substitution, and solvent polarity favor the formation of dioxetanes at the expense of allylic hydroperoxides.It is noteworthy that enol ether 20, an α,β-unsaturated ester, forms appreciable amounts of a dioxetane in polar solvents (44 percent in acetonitrile).Kinetic results show that the rate and product distribution of the ene reaction are independent of solvent polarity, whereas the rate of dioxetane formation increases with solvent polarity.It is suggested that <2+2> cycloadditions and ene reactions occur via different transition states and intermediates, zwitterions and perepoxides, respectively.Furthermore, the remarkable propensity to dioxetane formation of dihydrofurans compared to that of dihydropyrans and the other enol ethers seems to be due to the rigidity of the five-membered ring in the transition state and intermediate zwitterion.