ABSTRACT
The photochemistry of barrelenes and arene-fused barrelenes has been extensively explored, and several reviews have been published in recent years.
Barrelenes undergo facile photoisomerizations to the corresponding cyclooctatetraenes or semibullvalenes based on the irradiation conditions employed. It has been established
that upon direct irradiation of barrelene (
), cyclooctatetraene (COT) (
) is formed through a singlet-state pathway involving [2 + 2]-cycloaddition to quadricyclane-like intermediate
followed by isomerization, whereas the formation of semibullvalene (SB) (
) involves biradical intermediates
and
under triplet sensitization; this is known as the di-
π
-methane (DPM) rearrangement (Zimmerman rearrangement) (Scheme 1).
