Triplet-sensitized photolysis of alkoxycarbonyl azides in solution and matrices

Abstract

Photolysis of azides 1– 4 in methanol, which have a built-in intramolecular triplet sensitizer, yields mainly carbamates 5– 8. Laser flash photolysis of 1– 4 shows formation of their triplet-excited ketone, which decays by intramolecular energy transfer to form triplet nitrenes 1n– 4n. Irradiating 1– 3 in matrices yields isocyanic acid, whereas photolysis of 4 forms isocyanate 4i. The depletion rate of the azide bands between 2100 and 2200 cm −1 is different than the rate of formation for the isocyanic acid bands at ∼2265 cm −1; thus, the formation of isocyanic acid is a stepwise process. Irradiating 1 in matrices produces an absorption band due to nitrene 1n ( λ max ∼ 343 nm), which is depleted upon further irradiation, whereas the absorption due to 4-acetyl benzaldehyde ( λ max ∼ 280 nm) increases with prolonged irradiation. We propose that formation of isocyanic acid in matrices must come from secondary photolysis of nitrenes 1n– 3n. This mechanism is further supported by calculation, which show that the estimated transition state for 1n– 4n to fall apart to yield alkoxy and cyanato radicals is only ∼34 kcal/mol above the ground state of the triplet nitrenes and thus the cleavage can take place photochemically. Thus, nitrenes 1n– 4n can be formed selectively, but these intermediates are highly photosensitive and undergo secondary photolysis in matrices.