Stereoselective Photooxidation of trans-2-Butene to Epoxide by Nitrogen Dioxide Excited with Red Light in a Cryogenic Matrix

Article abstract of DOI:10.1021/ja00196a034

Reaction was induced between trans-2-butene and nitrogen dioxide by exciting trans-butene-NO2 pairs, isolated in solid Ar at red, yellow, and green wavelengths (NO2²B₂ <-- <*>²A₁).The chemistry was monitored by FT-infrared spectroscopy, and Ar ion and cw dye lasers were used for photolysis.Products formed were 2-butene oxide + NO, the former under complete retention of stereochemistry, and an addition product that was identified by (18)O isotopic substitution as a butyl nitrite radical, reported here for the first time.Analysis of the photolysis-wavelengthdependence of the butyl nitrite radical and trans-2-butene oxide (NO) growth kinetics revealed that epoxide + NO is formed along two reaction pathways.The first gives trans-2-butene oxide + NO and butyl nitrite radical upon absorption of a single photon by trans-2-butene-NO2 pairs (one photon path).The second path is formation of trans-2-butene oxide + NO by photodissociation of trapped butyl nitrite radical by a (second) red or shorter wavelength photon (two-photon path).Two alternative transients are proposed for the one-photon path, namely a hot butyl nitrite radical and an oxirane biradical, respectively.The wavelength dependence of the product branching along the one-photon path indicates that branching occurs from a vibrationally unrelaxed transient.This suggests that the observed stereochemical integrity originates from insufficient coupling of the stretching and bending vibrations of the transient with torsion around its central C-C bond on the time scale of reaction to epoxide + NO and its stabilization as butyl nitrite radical.