Intermediate in the ene reaction of singlet oxygen with 1,4-diphenyl-cis-2-butene and 2-butene

Recent Mechanistic Insights in the Singlet Oxygen Ene Reaction

“…For instance, a very small KIE (average k H /k D = 1.07) was found in the intermolecular competition between cis-15-d 0 and cis-15-d 4 ( Figure 5). 50 However, a significant intramolecular (product) isotope effect (k H /k D = 1.50) in substrate cis-15-d 2 was determined ( Figure 5). More generally, the observed KIEs on the 1 O 2 ene reaction of trisubstituted and cis-disubstituted alkenes were explained by the irreversible formation of PE intermediates.…”

Selective Formation of Allylic HydroperoxidesviaSinglet OxygeneneReaction

“…The phenyl ring of styrene substrates directs a syn selectivity in their ene reactions with singlet oxygen 53 . This effect was demonstrated by the photooxygenation of β,β-dimethyl styrene (20). This substrate, apart from the ene product, produces the 1,2-dioxetane and two diastereomeric diendoperoxides 54,55 , as shown in Scheme 9.…”

“…The ene pathway is less favourable or even absent. For example, β,β-dimethylstyrene (20) affords the ene adduct in approximately 20% yield, benzaldehyde (from a [2 + 2] pathway), and mainly two diastereomeric di-endoperoxides (from a [4 + 2] pathway) in a 2/1 ratio (Scheme 48). Similarly, for 1-(2-methylpropenyl)naphthalene (137), the 1,4-endoperoxide is mainly formed 178 .…”

“…The intrazeolite photooxygenation in a series of isobutenylarenes 179 (20,(137)(138)(139)(140)(141)(142)(143)(144)(145)(146), bearing either electron-withdrawing or electron-deficient substituents on the aryl ring, affords rapidly the ene allylic hydroperoxides as the major or even exclusive products ( Table 22). The relative yield of the ene adduct was always higher than 85%.…”

“…The initially proposed synchronous pathway 15 was challenged by a biradical 16 , zwitterionic 17 or a perepoxide 18 intermediate. Kinetic isotope effects in the photooxygenation of tetrasubstituted 18 , trisubstituted 19 and cisdisubstituted 20 [2 + 2] and ene addition reactions of singlet molecular oxygen to alkenes while for trans-disubstituted alkenes 21,22 a partial equilibration of the intermediate with the reactants was postulated. Consistent with the perepoxide intermediate is also the observation that the ene reaction proceeds as a highly suprafacial process, in which the conformational arrangement of the allylic hydrogen controls the stereochemistry of the product allylic hydroperoxides 23 .…”