Aryl Insertion vs Aryl–Aryl Coupling in C,C-Chelated Organoborates: The “Missing Link” of Tetraarylborate Photochemistry

The photoreactivity
of 9-borafluorene-based, C,C-chelated organoborates
was investigated. Unlike the related tetraarylborates, the charge-transfer
transitions imparted by the biphenyl chelate lead to selective insertion
of one aryl substituent into the endocyclic B–C bond of the
9-borafluorene moiety, resulting in the formation of boratanorcaradienes.
This photoreaction likely proceeds according to a Zimmerman rearrangement,
which is analogous to one of the initially proposed mechanisms for
tetraarylborates and provides additional insight into these long-debated
photochemical reactions.