Photochemical reduction of flavone with NaBH₄ in batch and micro-channel reactors using excimer lasers

Photochemical reduction of a stable enone, flavone (1), was accomplished by excimer laser photolysis using NaBH₄ in ethanol, which formed flavanone (2) as a primary product in contrast to the formation of dimers that predominate in conventional photolysis. Flavanone (2) underwent successive thermal reduction with NaBH₄ leading to flavanol (3) and photolysis to ethyl salicylate (4), in which the formation of 4 was completely suppressed when a single laser shot was used for the photolysis. The photolysis was conducted using quartz batch reactors having different optical paths and a quartz micro-channel reactor, in combination with KrF and XeCl excimer laser irradiation. This showed a considerable acceleration of the reaction with the micro-channel reactor, and the selectivity of the photoproducts 2, 3, and 4 depended on the optical path of the reactors, the laser wavelengths, the concentration of NaBH₄, and the laser repetition rate. An optimal irradiation condition - application of sufficient time interval between the XeCl laser pulses, a low concentration of 1, and high concentration of NaBH₄ - showed a considerable increase in the selectivity of 3 by eliminating the formation of 2 and 4.