Rhodium-Catalyzed B–H Activation of 1,2-Azaborines: Synthesis and Characterization of BN Isosteres of Stilbenes

The
first example of catalytic B–H activation of azaborines
leading to a new family of stilbene derivatives through dehydrogenative
borylation is reported. Ten 1,2-azaborine-based BN isosteres of stilbenes
have been synthesized using this method, including a BN isostere of
a biologically active stilbene. It is demonstrated that BN/CC isosterism
in the context of stilbenes can lead to significant changes in the
observed photophysical properties such as higher quantum yield and
a larger Stokes shift. Direct comparative analysis of BN stilbene 3g and its carbonaceous counterpart 6g is consistent
with a stronger charge-transfer character of the excited state exhibited
by 3g in which the 1,2-azaborine heterocycle serves as
a better electron donor than the corresponding arene.