PHOTO-CHEMICAL SERIES .120. UNUSUAL ORGANIC-PHOTOCHEMISTRY EFFECTED BY CYANO AND METHOXY SUBSTITUTION - EXPLORATORY AND MECHANISTIC ORGANIC-PHOTOCHEMISTRY

The photochemistry of 1,1-dicyano-3,3,5,5-tetraphenyl-1,4-pentadiene, 3,3-dicyano-1,1,5,5-tetraphenyl-1,4-pentadiene, and 3,3-dimethoxy-1,1,5,5-tetraphenyl-1,4-pentadiene was investigated. The 1,1-dicyanodiene led to a mixture of 1,1-dicyano-2,2-diphenyl-3-(2,2-diphenylvinyl)cyclopropane, 1,1,2,3-tetraphenyl-2-(2,2-dicyanovinyl)cyclopropane, and 1,1,2,2-tetraphenyl-3-(2,2-dicyanovinyl)cyclopropane. Quantum yields for direct irradiation in benzene were Φ=0.044, Φ=0.059, and Φ=0.051, respectively. In contrast, acetophenone sensitized irradiation afforded just 1,1,2,2-tetraphenyl-3-(2,2-dicyanovinyl)cyclopropane with a quantum yield of Φ=0.79. Singlet excited state reaction rates for formation of the first two products were determined by single photon counting to be kr=4.9 X 1010s-1and kT=6.5 X 1010s-1, respectively. Synthesis of the 3,3-dicyanodiene proved nontrivial and a novel approach was devised. Both direct and acetophenone sensitized irradiations led to 1,1-dicyano-2,2-diphenyl-3-(2,2-diphenylvinyl)cyclopropane in quantum yields of Φ=0.11 and Φ=0.53, respectively. The Si rate of reaction was kr=9.24 X 108s-1. This is severely inhibited (ca. 150 times slower) compared with the 3,3-dimethyl analogue. The triplet efficiency contrasts with the lack of reactivity of the dimethyl analogue as well. Direct irradiation of 3,3-dimethoxy-1,1,5,5-tetraphenyl-1,4-pentadiene led to 1,1-dimethoxy-2,2-diphenyl-3-(2,2-diphenylvinyl)cyclopropane, along with 1,1,4,4-tetraphenylbutadiene as a minor product. The quantum yields were Φ=0.15 and Φ=0.018, respectively. No reaction was observed on sensitization. Direct irradiation of 1,1,2,2-tetraphenyl-3-(2,2-dicyanovinyl)cyclopropane led to isomeric Φ,Φ-dicyano-2,2-diphenyl-3-(2,2-diphenylvinyl)cyclopropane and Φ,Φ-dicyano-3,3,5,5-tetraphenyl-1,4-pentadiene, with quantum yields of Φ=0.12 and Φ=0.34, respectively. These formally derive from a unique acyclic bicycle reaction and reverse di-π-methane rearrangement, respectively. Finally, new photochemical theory was developed to deal with singlet vs. triplet reaction preferences and to treat the di-TT-methane rearrangement. In the former, the role of the exchange integral K between electrons in HOMO and LUMO is shown to be often dominant. © 1979, American Chemical Society. All rights reserved.