Ring Opening of Epoxides Induced by Pentaphenylborole

The unsaturated antiaromatic BC4 heterocycle pentaphenylborole
has been shown to have diverse reactivity with a variety of substrates,
including the insertion of polar functional groups into the ring as
a route to conjugated boracycles. This work investigates the reactivity
of a selection of epoxides with pentaphenylborole, both computationally
and experimentally, revealing that the substitution is highly influential
on the reaction outcome. Specifically, isobutylene oxide results in
protodeborylation to a borabutadiene chain attributed to the acidic β-hydrogen
atoms, 1,1-diphenylethylene oxide inserts the C2O unit
to furnish a BOC6 heterocycle, and cyclohexene oxide inserts
two epoxides to form an unusual BC8O2 ring.
The last two species represent rare boron-containing rings of eight
atoms or greater, with the 11-membered species being only the second
reported and the first crystallographically characterized.