N-Heterocyclic Carbene Stabilized Boryl Radicals

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• 作者：M. Sc. Miguel F. Silva Valverde ; Dr. Peter Schweyen ; Daria Gisinger ; Dr. Thomas Bannenberg ; Dr. Matthias Freytag ; Dr. Christian Kleeberg and Prof. Dr. Matthias Tamm
• 刊名：Angewandte Chemie International Edition
• 出版年：2017
• 出版时间：January 19, 2017
• 年：2017
• 卷：56
• 期：4
• 页码：1135-1140
• 全文大小：1205K
• ISSN：1521-3773

文摘

The reaction of the 2-(trimethylsilyl)imidazolium triflate 9 with diarylboron halides (4-R-C₆H₄)₂BX (R=H, X=Br; R=CH₃, X=Cl; R=CF₃, X=Cl) afforded the NHC-stabilized borenium cations 10 a–c. Cyclic voltammetry revealed a linear correlation between the Hammett parameter σ_p of the para substituent and the half-wave potential. Chemical reduction with decamethylcobaltocene, [(C₅Me₅)₂Co], furnished the corresponding radicals 11 a–c; their characterization by EPR spectroscopy confirmed the paramagnetic character of 11 a–c, with large hyperfine coupling constants to the boron isotopes ¹¹B and ¹⁰B, while delocalization of the unpaired electron into the NHC is negligible. DFT calculations of the percentage of spin density distribution between the carbene (NHC) and the boryl fragments (BR₂) revealed for 11 a–c a spin density ratio (BR₂/NHC) of ca. 9:1, which underlines their distinct boryl radical character. The molecular structure of the most stable species 11 c was established by X-ray diffraction analysis.