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

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• 作者：Ya-Fan Lin ; Chao-Tang Shen ; Yu-Ting Hsiao ; Yi-Hung Liu ; Shie-Ming Peng ; Ching-Wen Chiu
• 刊名：Organometallics
• 出版年：2016
• 出版时间：May 23, 2016
• 年：2016
• 卷：35
• 期：10
• 页码：1464-1471
• 全文大小：483K
• 年卷期：0
• ISSN：1520-6041

文摘

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-order kinetics with activation parameters of ΔH^⧧ = 19.6 (±0.31) kcal/mol and ΔS^⧧ = −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.