Neder Marco, Arrowsmith Merle, Endres Lukas, Mihm Cornelius, Nees Samuel, Bertermann Rüdiger, Braunschweig Holger
Institute for Inorganic Chemistry, Julius-Maximilians University of Würzburg, Am Hubland, 97074 Würzburg, Germany.
Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians University of Würzburg, Am Hubland, 97074 Würzburg, Germany.
J Am Chem Soc. 2025 Jul 2;147(26):23034-23048. doi: 10.1021/jacs.5c06017. Epub 2025 Jun 19.
The four-electron reduction of doubly IMes- and SIMes-stabilized 2,3-bis(dibromoboryl)naphthalene (IMes = 1,3-dimesitylimidazol-2-ylidene; SIMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene, mesityl = 2,4,6-trimethylphenyl) yields the first N-heterocyclic carbene (NHC)-stabilized naphtho[2,3-][1,2]diboretes and , which were isolated as dark-blue and red solids, respectively, and analyzed by nuclear magnetic resonance (NMR) and ultraviolet-visible (UV-vis) spectroscopy as well as X-ray crystallography. The synthesis of is systematically accompanied by NHC ring-expanding boron insertion side reactions, absent in that of . Both experimental and computational data show that unlike their biradical cyclic alkyl(amino)carbene (CAAC)-stabilized counterparts, the 1,2-diborete rings in and are closed-shell 4π-antiaromatic compounds. The differences between the IMes and SIMes derivatives lie in the additional delocalization of the π-electron density to one of the SIMes ligands in , which results in an unsymmetrical conformation with lower B═B double bond character and a more planar CB ring. Reactions of and with terminal alkynes (HC≡CR; R = H, TMS (= SiMe), EP (= -(HCC)CH), Fc (= ferrocenyl)) yield the first examples of aromatic 1,4-diboraanthracenes (1,4-DBAs) () by insertion of the alkyne CC unit into the strained diborete B═B bond. These compounds are highly colorful (green-purple), quasi-planar, and aromatic. While most internal alkynes do not react with or , reactions with the highly electron-deficient alkyne DMAD (= dimethyl acetylenedicarboxylate) also yield the corresponding yellow-colored 1,4-DBA. Furthermore, the parent 1,4-DBA undergoes a Diels-Alder reaction with DMAD to yield a naphtho[2,3-][1,4]diborabarrelene, while undergoes a 1,4-addition of the alkynyl C-H functionality of a second equivalent of HC≡C(EP).
双IMes和SIMes稳定的2,3-双（二溴硼基）萘（IMes = 1,3-二甲基咪唑-2-亚基；SIMes = 1,3-二甲基-4,5-二氢咪唑-2-亚基，均三甲苯基 = 2,4,6-三甲基苯基）的四电子还原反应生成了首例N-杂环卡宾（NHC）稳定的萘并[2,3-][1,2]二硼杂环丁二烯 和 ，它们分别以深蓝色和红色固体形式分离出来，并通过核磁共振（NMR）、紫外可见（UV-vis）光谱以及X射线晶体学进行了分析。 的合成过程中系统地伴随着NHC扩环硼插入副反应，而 在合成过程中不存在此类反应。实验和计算数据均表明，与双自由基环烷基（氨基）卡宾（CAAC）稳定的类似物不同， 和 中的1,2-二硼杂环丁二烯环是闭壳层4π-反芳香性化合物。IMes和SIMes衍生物之间的差异在于， 中π电子密度额外离域到其中一个SIMes配体上，这导致了一种不对称构象，具有较低的B═B双键特征和更平面的CB环。 和 与末端炔烃（HC≡CR；R = H、TMS（= SiMe）、EP（= -(HCC)CH）、Fc（= 二茂铁基））反应，通过将炔烃的CC单元插入张力二硼杂环丁二烯的B═B键中，生成了首例芳香性1,4-二硼蒽（1,4-DBAs）（ ）。这些化合物颜色鲜艳（绿紫色）、近乎平面且具有芳香性。虽然大多数内炔烃不与 或 反应，但与高缺电子炔烃DMAD（= 二甲基乙炔二羧酸酯）反应也会生成相应的黄色1,4-DBA。此外，母体1,4-DBA 与DMAD发生狄尔斯-阿尔德反应生成萘并[2,3-][1,4]二硼桶烯，而 与第二当量的HC≡C(EP)的炔基C-H官能团发生1,4-加成反应。
