The photochemical reaction of 1,1-dicyano-3-phenylbut-1-ene. Simultaneous occurrence of pi-methane and di-pi-methane rearrangements

Abstract

The direct photolysis of 1,1-dicyano-3-phenylbut-1-ene (3-MDCN) was investigated at room temperature in solvents of different polarities (hexane, dichloromethane and acetonitrile). Cyclopropanes arising from both the di-p-methane and p-methane (1,2-H migration) processes were obtained as photoproducts. The structures of the products were elucidated by 1H-NMR, GC/MS, IR and chromatography. Relative quantum yield determination and GC analysis of sequential irradiations gave evidence that: i) no secondary reactions occur, even at high conversions; ii) the di-p-methane rearrangement is significantly more affected by the solvent variation than the p-methane reaction. Photosensitization with acetophenone or acetone did not yield any observable products. The existence of the simultaneous mechanisms and the observed effects were considered as evidence of a possible differentiation between localized and delocalized excitation on the excited state surface.