PHOTOCHEMISTRY OF 9,10-DICYANOANTHRACENE 1,2-DIARYLCYCLOPROPANE SYSTEMS - PHOTOCYCLOADDITION AND PHOTOISOMERIZATION

The photochemical reactions of 9,10-dicyanoanthracene (DCA)-1,2-diarylcyclopropane (CP) systems have been investigated. In degassed acetonitrile solution, (4-pi + 2-sigma) photocycloaddition between DCA and CP occurred to give cis- and trans-2,4-diaryl-1,5-dicyano-6,7:8,9-dibenzobicyclo[3.2.2]nona-6,8-dienes in a 3:1 ratio in good chemical yields although the quantum yields were not high (PHI = 0.002-0.04). This photocycloaddition did not occur in benzene. The DCA-sensitized photoisomerization of trans- and cis-1,2-bis(4-methoxyphenyl)cyclopropanes (1a and 2a) in acetonitrile efficiently occurred to afford a photostationary mixture containing 1a and 2a in a 95:5 ratio. The photoisomerization was remarkably accelerated by bubbling air, by adding metal salt such as Mg(ClO4)2 and LiBF4, and also by aromatic hydrocarbons such as phenanthrene and biphenyl. The quantum yield for the photoisomerization of 2a to 1a exceeded unity, indicating that this photoisomerization involved a chain process. The DCA-sensitized photoisomerization of optically resolved (-)-1a afforded a racemic mixture of (+)-1a and (-)-1a. These photoreactions are initiated by a one-electron transfer from CP to the excited singlet of DCA. The photocycloaddition proceeds via radical ion pairs in solvent cage and the photoisomerization proceeds via solvent-separated radical cations of CP.