Historically, four mechanisms have been proposed:[3]
Experimental and computational studies show that the reaction actually proceeds via a two step no intermediate process. One can loosely interpret it as a mix of the perepoxide mechanism and the concerted mechanism. There is no perepoxide intermediate as in the classical sense of reaction intermediates, for there exists no energy barrier between it and the hydroperoxide product.[4]
Such a mechanism can account for the selectivity of the Schenck ene reaction. The singlet oxygen is more likely to abstract hydrogen from the side with more C-H bonds due to favorable interactions in the transition state:[2]
Very bulky groups, like the tertiarybutyl group, will hinder hydrogen abstraction on that side.
Many hydroperoxides derived from fatty acids, steroids, and terpenes are also formed via the Schenck ene reaction. For instance, the generation of cis-3-hexenal from linolenic acid:
It must be noted, however, that this enzyme catalyzed path follows a different mechanism from the usual Schenck ene reaction. Radicals are involved, and triplet oxygen is used instead of singlet oxygen.