Substituent effect on regioselectivity in the di-π-methane rearrangement: synthesis of disubstituted benzobarrelene derivatives and their photochemistry

2,3- and 2,5-Dicyanobenzobarrelenes 5 and 6 have been synthesized and subjected to triplet-sensitized photoisomerization. 2,3-Dicyanobenzobarrelene 5 gave two di-π-methane rearrangement products 7 and 8 and a [2π+2π]-cycloaddition product 9. However, 2,5-dicyanobenzobarrelene 6 formed a single di-π-methane product 13. The formation of these products is discussed in terms of the radical stabilizing effect of the substituents and the destabilizing effect on the formation of the cyclopropane ring.