The di-?-methane photorearrangement of 2,3-disubstituted benzobarrelenes and benzonorbornadiene - Substituent effects in regioselectivity

2,3-Disubstituted benzobarrelene and benzonorbornadiene derivatives 16, 17, and 18, containing electron-withdrawing and electron-donating substituents, have been synthesized and subjected to triplet-sensitized photoisomerization. Methyl 3-methyl-2-benzobarrelenecarboxylate (16) gave two di-?-methane rearrangement products. However, methyl 3-cyano-2-benzobarrelenecarboxylate (17) underwent an intramolecular [2? + 2?] cycloaddition reaction, whilst methyl 3-cyano-2-benzonorbornadienecarboxylate (18) formed dimer 40. 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 cyclopropane ring