Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK.
Chemistry. 2019 Feb 18;25(10):2556-2568. doi: 10.1002/chem.201804808. Epub 2019 Jan 23.
This study details attempts to access N-heterocyclic carbenes (NHCs) featuring the diazaborolyl group, {(HCNDipp) B}, as one or both of the N-bound substituents, to generate strongly electron-donating and sterically imposing new carbene ligands. Attempts to isolate N-heterocyclic carbenes based around imidazolylidene or related heterocycles, are characterized by facile N-to-C migration of the boryl substituent. In the cases of imidazolium precursors bearing one N-bound diazaborolyl group and one methyl substituent, deprotonation leads to the generation of the target carbenes, which can be characterized in situ by NMR measurements, and trapped by reactions with metal fragments and elemental selenium. The half-lives of the free carbenes at room temperature range from 4-50 h (depending on the pattern of ancillary substituents) with N-to-C2 migration of the boryl function being shown to be the predominant rearrangement pathway. Kinetic studies show this to be a first-order process that occurs with an entropy of activation close to zero. DFT calculations imply that an intramolecular 1,2-shift is mechanistically feasible, with calculated activation energies of the order of 90-100 kJ mol , reflecting the retention of significant aromatic character in the imidazole ring in the transition state. Trapping of the carbene allows for evaluation of steric and electronic properties through systems of the type LAuCl, LRh(CO) Cl, and LSe. A highly unsymmetrical (but nonetheless bulky) steric profile and moderately enhanced σ-donor capabilities (compared with IMes) are revealed.
这项研究详细介绍了尝试使用含有二氮杂硼环基的 N-杂环卡宾（NHC），即{(HCNDipp) B}，作为一个或两个 N-键取代基，以生成强供电子和空间位阻较大的新型卡宾配体。尝试分离基于咪唑啉或相关杂环的 N-杂环卡宾，其特点是硼取代基易于发生 N 到 C 的迁移。在具有一个 N 键合的二氮杂硼基和一个甲基取代基的咪唑翁前体的情况下，脱质子化导致目标卡宾的生成，这些卡宾可以通过 NMR 测量原位表征，并通过与金属片段和元素硒的反应捕获。在室温下，游离卡宾的半衰期范围为 4-50 h（取决于辅助取代基的模式），硼基功能的 N 到 C2 迁移被证明是主要的重排途径。动力学研究表明这是一个一级过程，其活化熵接近零。DFT 计算表明，分子内 1,2-迁移在机理上是可行的，计算得到的活化能约为 90-100 kJ mol，这反映了在过渡态中咪唑环保留了显著的芳香性。卡宾的捕获允许通过 LAuCl、LRh(CO)Cl 和 LSe 等系统来评估其空间和电子性质。揭示了一种高度不对称（但仍然庞大）的空间位阻轮廓和适度增强的 σ-供电子能力（与 IMes 相比）。
