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

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