Substituent effects on the photorearrangements of unsymmetrically substituted diazinobarrelenes

Abstract

A series of diazinobarrelenes 8–15 engendered with alkyl functionalities at the barrelene skeleton were irradiated with 350 nm light under direct and acetone-sensitized reaction conditions. Under these conditions, all the barrelenes except barrelene 14 afforded semibullvalenes with varying degrees of regioselectivity and product distribution. Dicyanopyrazinobarrelenes 8–10 which furnished semibullvalenes 32–41via the aryl–vinyl initial bridging route were strongly controlled by the nitrile functionalities installed at the aromatic sites. Benzoquinoxalinobarrelenes 11–13 which afforded semibullvalenes 42–49, preferentially underwent photorearrangement via vinyl–vinyl bridging even if the compounds were excited at a wavelength where the quinoxaline moiety absorbed most of the light. Zimmerman's bridging hypothesis and the possibility for quinoxalines to undergo intramolecular triplet energy transfer could reasonably account for the observed regioselectivity. Barrelene 14 was insensitive to photorearrangement whereas benzo[f,h]quinoxalinobarrelene 15 preferentially underwent ADPM rearrangement affording semibullvalenes 50–52. Electronic and steric factors of alkyl substituents overwhelmingly controlled the product forming steps whereas localization and minimization of triplet energies greatly influenced the initial bridging interaction.