Mechanistic studies on the photocycloaddition of conjugated polialkynes to olefins

Abstract

Irradiation of 1,4-diphenyl-1,3-butadiyne (DPB) in the presence of 9,10-dicyanoanthracene (DCA) yields new methanol photoadducts in addition to normal photoadducts formed in the absence of DCA. Laser flash photolysis studies indicate that new methanol photoadducts are formed through a photoinduced electron transfer mechanism involving DPB radical cation and DCA radical anion. Substitution of one of the two phenyl rings with a p-methoxy group inhibits the formation of radical ion pairs and does not give any methanol photoadduct formed through radical ion pairs, while p-nitro group enhances the formation of radical ion pairs and gives photoadducts solely generated from the radical ion pairs. The photoreaction of DPB with dimethylfumarate (DMFu) gave a primary photoadduct PPCB through [2+2] type cycloaddition reaction. There were two secondary photoadducts that have very interesting oxirane and cyclopropane moieties. The evidence of exciplex of PPCB and DMFu was observed in the laser flash photolysis, and the results show that exciplex is involved in the formation of oxirane and cyclopropane ring in the DPB-DMFu photoreaction rather than a carbene intermediate. Bicyclopropane compounds are obtained via propargylene intermediate in the photoreaction of polialkyne with 2,3-dimethylbutene (DMB). In the photolysis of polialkyne with DMB and methanol to trap carbene intermediates, methanol insertion products were obtained. The formation of methanol insertion products in the photolysis of diazo compound with methanol suggested there were propargylene intermediate in the photoreaction of polialkyne and olefins.