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 beta-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.