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.