Facile photochemical synthesis of 5,10-disubstituted [5]helicenes by removing molecular orbital degeneracy.

Photocyclodehydrogenation is a key reaction to synthesize helicenes; however, because of overannulation, it is not applicable to the synthesis of [5]helicene. Introduction of a cyano group was found to remove the orbital degeneracy of the low-lying unoccupied MOs; consequently, the lowest excitation comprises a single transition involving the C2-antisymmetric MO. Therefore, the problematic overannulation can be effectively suppressed. Moreover, in combination with the Knoevenagel reaction, a one-pot synthesis of 5,10-dicyano[5]helicene with 67% yield was accomplished.