Isolation of Free Phenylide-like Carbanions with N-Heterocyclic Carbene Frameworks

A series of 1,3-bis(2,6-diisopropylphenyl)-5-methyl-1,3-diaza-4,6-diborabenzenes with methyl, phenyl, and dimethylamino substituents on the ring boron atoms were prepared using the cyclocondensation reaction between N,N′-bis(2,6-diisopropylphenyl)trimethylsilylformamidine and the appropriately substituted 1,1-bis(organochloroboryl)ethane, followed by deprotonation of the cationic ring intermediate. The planar, heterocyclic benzene analogues could be further deprotonated at the other ring carbon using an additional equivalent of potassium hexamethyldisilazide to yield organometallic derivatives akin to the potassium phenylide. The potassium cations could be efficiently sequestered in both solution and solid state using 18-crown-6, and the crystallographic analysis of the reaction products revealed the absence of carbanion−cation contacts in the solid state. The transformation of a planar, tricoordinate sp2 carbon to a tricoordinate, contact ion-pair carbanion and further to a solvent-separated, free dicoordinate carbanion was investigated using solution NMR and single-crystal X-ray diffraction. The first isolation and characterization of free dicoordinate carbanions in the solid state is supported by a charge distribution analysis, and the relationship between phenylide-type carbanions and N-heterocyclic carbenes is discussed through the prism of the results reported herein.

以N,N′-双(2,6-二异丙基苯基)三甲基硅基甲脒与适当取代的1,1-双(有机氯硼基)乙烷发生环缩合反应，随后对阳离子环中间体进行脱质子化，成功合成了一系列环硼原子带有甲基、苯基与二甲氨基取代基的1,3-双(2,6-二异丙基苯基)-5-甲基-1,3-二氮杂-4,6-二硼苯类化合物。该平面杂环苯类似物可通过额外当量的六甲基二硅基氨基钾（potassium hexamethyldisilazide）在另一环碳位点上进一步脱质子化，得到类似苯基化物钾的有机金属衍生物。可利用18-冠-6（18-crown-6）在溶液与固态两种状态下高效络合钾阳离子，对反应产物的晶体学分析显示，固态中不存在碳负离子-阳离子接触相互作用。本研究通过溶液态核磁共振波谱法与单晶X射线衍射技术，对平面三配位sp²杂化碳原子转化为三配位接触离子对碳负离子，再进一步转变为溶剂分离型游离双配位碳负离子的过程展开了探究。本文首次实现了固态游离双配位碳负离子的分离与表征，电荷分布分析为该研究结果提供了有力支撑；同时还基于本次报道的实验结果，探讨了苯基负离子型碳负离子与氮杂环卡宾（N-heterocyclic carbenes）之间的内在联系。