Synthesis and Lewis Acidic Behavior of a Cationic 9-Thia-10-boraanthracene

As part of our efforts in the chemistry of Lewis acidic organoboron
compounds, we have synthesized a cationic borane ([1]+) featuring a 9-thia-10-boraanthracene moiety substituted
at boron by the cationic anilium group [4-(Me3N)-2,6-Me2-C6H2]+. This new cationic
borane has been fully characterized. Its UV−vis spectrum features
a low-energy band at λmax = 392 nm which, as confirmed
by computational studies, arises from a π−π* transition
of the 9-thia-10-boraanthracene chromophore. As a result of the low
steric bulk present around the boron center and the inductive effects
imparted by the anilium group, [1]+ possesses
unusual Lewis acidic properties and reacts with DMAP to form the corresponding
adduct. This derivative also binds both fluoride and cyanide anions
in THF to afford 1-F and 1-CN whose stability
constants exceed 107 M−1. Although [1]+ is unstable in pure water, it can be used to
selectively extract cyanide under biphasic conditions in nitromethane/water.