Mechanistic Studies on the Rearrangement of a Boron Cation: From a nido-Carborane to a Planar Boracycle

The reaction mechanism of the [Et3BH]−-induced conversion of a dicationic boron compound ([Cp*B-IMes]2+, [1]2+) to a planar neutral carbene-coordinated borabenzene (2) is investigated experimentally and theoretically. Owing to the steric congestion around the boron center, bulky [Et3BH]− attacks the less encumbered carbon atom of Cp*, leading to a metastable 5-borabicyclo[2.1.1]hex-2-ene borenium cation ([3-CH]+) at low temperature. Upon raising the temperature, the bicyclic-borenium ion undergoes sequential pericyclic reactions, including suprafacial [1,3]-sigmatropic shift, two-electron electrocyclic ring opening, and then [1,2]-hydrogen shift to a planar cyclic borenium ion, [4-CH]+, which can then be deprotonated to yield the neutral aromatic borabenzene. The attack of hydride at the boron center of [1]2+ was excluded through the preparation and isolation of 6-borabicyclo[3.1.0]hexenylium, [3-BH]+, which could not be transformed into [3-CH]+ or [4-CH]+. The borenium ion rearrangement follows first-ord...