The Mechanism of the Ene Reaction of Singlet Oxygen with Olefins

Ab initio calculations (correlated wave functions using double f basis plus polarization functions) on the reaction of '02 with ethylene are combined with thermochemical methods of estimating substituent effects to predict the energetics for proposed intermediates in the addition of ‘02 to substituted olefins. The results include estimates of peroxy biradical, open 1,4-zwitterion, and perepoxide intermediates. It is concluded that the major reaction pathway involves the biradical intermediate, although certain solvents and substituents can greatly enhance the zwitterion character of this state. Detailed comparisons of the theoretical predictions to experimental results show that many aspects of the stereospecificity and regiospecificity can be understood in terms of the biradical intermediate or transition state.