Stereocontrol of the Red Light Induced Photoepoxidation of 2-Butenes by Nitrogren Dioxide in Solide Ar

Article abstract of DOI:10.1021/j100359a028

Photooxidation of cis-2-butene was initiated in an inerrt gas matrix by exciting cis-2-butene*NO2 pairs at red, yellow, and green wavelengths .Chemical reaction was monitored by FT-IR spectroscopy, and emission from an Ar ion or a tuned CW dye laser was used for photolysis.As in the case of the trans-2-butene + NO2 reaction reported earlier, 2,3-epoxybutane was the only final oxidation product observed upon direct photolysis of reactant pairs.While in the case of the trans-2-butene reaction stereochemical retention was complete, we found in the cis case 85percent of the epoxide with retained configuration when conducting the reaction at low matrix concentration.This fraction decreased with increasing reactant to matrix ratio.Infrared bands of two conformers of a butyl nitrile radical were observed concurrently with the epoxide, one syn, the other anti with respect to conformation (CH3 groups) about the central CC bond.A correlation was found between the syn to anti nitrile radical and the cis to trans epoxide ratios, suggesting a common transient precursor.It is most probably an oxirane biradical, whose conformation determines the stereochemistry of the epoxide product.The photolysis wavelength dependence of the product growth kinetics was studied, and relative reaction efficiencies so obtained are shown to give insight into aspects of the dynamics of the reaction that relate to the observed product and stereospecificity.The two trapped butyl nitrite radical conformers were found to photodissociate under exposure to long-wavelength visible light with complete conformer specificity.The anti conformer gave trans-2-butene oxide and NO at a threshold wavelength of 613 nm, while the syn form was found to decompose to 2-methylpropanal and NO upon 573 nm and shoter wavelength irradiation.