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

摘要

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-borabicydo[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-borabicydo[3.1.0]hexenylium, [3-BH](+), which could not be transformed into [3-CH](+) or [4-CH](+). The borenium ion rearrangement follows first-order kinetics with activation parameters of Delta H double dagger = 19.6 (+/- 0.31) kcal/mol and Delta S double dagger = -2.03 (+/- 1.15) cal/mol.K. The observed kinetic isotope effect of 0.87 at 265 K is consistent with the DFT-calculated reaction mechanism, which predicts an overall KIE value of 0.90. These results show that [1](2+) reacts with [Et3BH](-) as a carbon based electrophile and the formation of an electron-deficient borenium center is responsible for the skeletal rearrangement of the nido-cluster.