Visible Light-Induced Oxygen Transfer from Nitrogen Dioxide to Ethyne and Propyne in a Cryogenic Matrix. 2. Mechanism and Regioselectivity

The kinetics of visible light-induced oxidation of ethyne and propyne by NO[sub 2] in solid Ar has been monitored as a function of photolysis wavelength. CW dye and Ar ion laser emission was used for irradiation, and FT-infrared spectroscopy for the measurement of product growth. Determination of the wavelength dependence of quantum efficiencies from the kinetic measurements revealed that the photolysis threshold of HC[triple bond]CH + NO[sub 2] lies approximately 6 kcal mol[sup [minus]1] higher than that of CH[sub 3]C[triple bond]CH + NO[sub 2]. This and other parameters derived from the product growth measurements, and the formyl methyl iminoxy radical trapped in the case of the CH[sub 3]C[triple bond]CH + NO[sub 2] reaction are interpreted in terms of a direct O transfer mechanism. The proposed path involves large-amplitude O transfer from photoexcited NO[sub 2] to the C[triple bond]C group to yield a transient ketocarbene. Formation of iminoxy radical is attributed to trapping of the ketocarbene by NO cage coproduct, which is in competition with Wolff rearrangement to yield ketene. From the structure of the iminoxy radical, it is inferred that the photoinduced O transfer from NO[sub 2] to the unsymmetrical C[triple bond]C bond of propyne is completely regioselective, leading exclusively to the carbene transient with oxygen at the terminal carbon. 29 refs., 7 figs.