Photoreaction of conjugated polyacetylenes with olefins

Abstract

Deaerated 2,3-dimethyl-2-butene (DMB) solution of 1-phenyl-1,3-pentadiyne (PPD) was irradiated with 300nm UV light to obtain site-selective 1:1 photoadduct (1). Irradiation of PPD with unsymmetrical olefins such as ethyl vinyl ether (EVE) and acrylonitrile (AN) in deaerated solutions yields site-selective and regioselective [2+2] type 1:1 photoadducts (2 and 3) and 1:2 photoadducts (4-9). In the photolysis of PPD with EVE and AN, the 1:1 photoadducts (2 and 3) is initially formed and prolonged irradiation gives the secondary 1:2 photoadducts (4-9). Photolysis of 1,4-diphenyl-1,3-butadiyne (DPB) with unsymmetrical olefins, AN and EVE, initially yields regioselective [2+2] type photoadducts (12-13) as primary photoproducts and extended irradiation results in the formation of secondary photoadducts (14-17). The secondary photoproduct(s) of 13 are very unstable at room temperature to give rearranged products. The reactive site on the formation of secondary photoadducts is different from each other indicating that the reaction proceeds through polar excited states. All the photoreactions proceed from the triplet excited states. Irradiation of 1,6-diphenyl-1,3,5-hexatriyne (DPH) with electron deficient olefins such as dimethylfumarate (DMF) and fumaronitrile (FN) in deaerated methylene chloride yields site-selective [2+2] type 1:1 photoadducts (22-24). Photolyis of DPH with unsymmetrical olefins such as AN, methyl acrylate (MA), and styrene (ST) in methylene chloride yields siteselective and regioselective [2+2] type 1:1 photoadducts (25-27). Photolysis of deaerated DMB solution of 1-phenyl-1,3,5-heptatriyne (PHT) with 350nm UV light yields very interestingly dicyclopropyl 1:2 photoadducts (28 and 29) while three 1:1 photoadducts (30-38) were obtained with unsymmetrical olefins such as AN, MA, and ST in methylene chloride. All the photoreactions proceed from the triplet excited state. Olefins with electron withdrawing substituents, such as DMF, FN, AN, MA, and ST, ar...