Reactions of Imines, Nitriles, and Isocyanides with Pentaphenylborole: Coordination, Ring Expansion, C–H Bond Activation, and Hydrogen Migration Reactions

The reactions of pentaphenylborole
with imines, isocyanides, and
acetonitrile were investigated experimentally and theoretically. On
the basis of literature precedent, we envisioned that the dipolar
substrates would undergo facile ring expansion reactions to yield
new BNC5 heterocycles. For acetonitrile and one particular
imine, this ring expansion process was observed. However, in many
cases, unexpected reactivity occurred. This included hydride migration
of an imine ring expanded product and the ortho C–H
bond activation of an aryl group of an imine if two phenyl groups
were present on the α-carbon. A bulky group on the nitrogen
atom of an imine prevented coordination to the boron center, and no
reaction was observed, indicating that coordination to the borole
is a critical step for any type of reaction to occur. Isocyanides
made coordination complexes, but heating to induce further reactivity
resulted in mixtures. The mechanisms were elucidated via DFT calculations,
which complement the experimental findings.