Photochemical Transformations of 1-Pyrazolyl-cis-1,2-dibenzoylalkenes. A Laser Flash Photolysis Investigation

Article abstract of DOI:10.1021/jo00198a014

Photorearrangements of several pyrazolyl-cis-1,2-dibenzoylalkenes 8-12 (Scheme III) are reported.Photolysis of 8 in benzene gave a mixture of the butenoic acid 21 and the tetrahydroisoquinoline 33, whereas in methanol, a mixture of the butenoic ester 26 and 33 is formed.Similar results were obtained in the irradiation of 9 both in benzene and methanol.The photolysis of 10-12 in benzene gave exclusively the corresponding butenoic acids 23-25, whereas in methanol, the corresponding butenoic esters 28-30 were formed.In the case of 10 and 11, however, the methanol irradiation gave the corresponding bis(pyrazolyl) derivatives 43 and 44 also.Photolysis of the pyrazolyl fumarate 14 gave the bis(pyrazolyl) succinate 45, whereas the irradiation of 16 gave a mixture of the pyrazole 4a and cis-dibenzoylethylene.Laser flash photolysis (337.1 nm) of 8-12 in benzene and methanol leads to (a) formation of short-lived transients (τ <*> 20 ns), absorbing in the spectral region 400-550 nm and assigned as the triplets of 8-12, (b) pronounced end-of-pulse bleaching in the region of ground-state absorption (300-380 nm), (c) formation of oxygen-insensitive transients (τ = 0.3-33 μs), characterized by dual maxima at ca. 400 and ca. 600 nm, and (d) growth (oxygen insensitive) of a methanol adduct, with absorption maxima at 370-390 nm in methanol and benzene plus methanol (0.5-5 M) mixtures (τ = 0.8-6 μs).Triplet sensitization studies using pulse-generated and γ-radiolytically generated biphenyl triplet photolysis suggest that the formation of the 400/600-nm species is triplet mediated while the methanol related reaction as well as the phototransformation leading to the butenoic ester derivatives occurs via intermediates from the singlet state (S₁).We attribute the 400/600-nm species to a zwitterion (40, Scheme V) and the methanol related growth to the reaction of the ketene 18 with the alcohol (Scheme III).