Formation of Trichlorosilyl-Substituted Carbon-Centered Stable Radicals through the Use of π-Accepting Carbenes
Formation of Trichlorosilyl-Substituted Carbon-Centered Stable Radicals through the Use of π-Accepting Carbenes
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DOI：
10.1002/anie.201300668
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发表时间：
2013-11-04
影响因子：
16.6
通讯作者：
Koley, Debasis
中科院分区：
文献类型：
--
作者：
Mondal, Kartik Chandra;Roesky, Herbert W.;Koley, Debasis
带有甲硅烷基取代基的碳或硅自由基是有机金属和有机化学中重要的中间体。 [1]早在 1970 年，Bassindale 等人。报道了持久性三（三甲基硅基）甲基自由基，[2](Me3Si) 3CC，其在 298 K 下的寿命为数天。Ingold、[3] Apeloig、[4] Bravo-Zhivotovskii、[5] Lee、[6] Sekiguchi、[7] 和其他人 [4-6] 的基团表明，自由基的寿命很大程度上取决于取代基的空间体积。 2002年，Sekiguchi等人[7]报道了第一个稳定的硅中心自由基，没有任何 π 共轭。一些报告描述了此类自由基的制备。最成功的反应是通过 Si-Si 键的光解或热裂解进行，并且根据 Apeloig-Sekiguchi 方法，R3Si-SiHCl2 可以与大体积试剂（例如 (tBu)2MeSiLi）反应。主族元素的几种自由基种类[8]，例如PNC+、[8b] P2C+、[8c]氧膦基自由基阳离子、[8d] HBC+、[8e]和具有双自由基特征的烯酮[8f]也被环烷基（氨基）卡宾（cAAC）稳定。我们观察到 cAACs [8a] 对硅烯的化学反应与 N-杂环卡宾 (NHCs) 的化学反应非常不同。最近，我们已经证明了 NHC！ SiCl2 1 在氧化还原反应中与 Me2-cAAC 反应，在双电子氧化还原步骤中形成双自由基 (Me2-cAACC) 2SiCl2 2。[8g] 因此，我们很好奇研究单电子氧化还原过程。因此，我们做出了cAAC反应！ SiCl4 3 与 KC8 在正己烷中以等摩尔比生成稳定的自由基 4，其通式为 (cAACC)–SiCl3。高反应性三氯甲烷自由基 CCl3C[9a] 及其同系物 SiCl3C[9b] 经常通过闪光光解产生。这些物质是许多光化学转化中的活性自由基中间体。[9a，b]最近，探索了TEMPO自由基对SiCl4[9c]和金属离子[9d]的化学反应性，并研究了这些加合物的催化[9d]和流行的磁性[9e，f]性能。尽管处理和控制自由基的化学行为可能非常困难，但自由基的化学性质一直令人着迷。[10, 11] 据我们所知，迄今为止，具有紧邻自由基中心的SiCl3基团的稳定自由基尚未见报道。 NHC 的卡宾碳原子与两个吸σ 和供π 氮原子结合。在 cAAC 中，NHC 的一个氮原子被 σ 供体季碳原子取代。理论计算表明，cAAC 中的 HOMO-LUMO 能隙较小。因此，cAAC 比 NHC 更具亲核性和亲电性。[12a]最近，对许多卡宾-苯基次膦亚膦加合物的 31P NMR 分析表明，cAAC 是比 NHC 更好的 π 受体。[12b]这些固有的差异可能在 cAAC 取代 NHC 及其在反应中的行为中发挥关键作用。国家卫健委！ SiCl4加合物被还原为NHC！ SiCl2、(NHC！SiCl) 2 和 NHC！巳=巳！ NHC 通过使用 KC8。[13]详细地说，是 cAAC 的等摩尔混合物！ SiCl4 3 (1 mmol) 和KC8 (1 mmol) 在正己烷(85 mL) 中最初在788C 下反应。将所得悬浮液缓慢温热至室温以获得澄清的无色溶液和未反应的不溶性KC8沉积物。搅拌24小时，溶液颜色变为澄清浅黄色，并有黑色石墨沉积；过滤后，将溶液浓缩至2–3 mL的体积，以获得(cAACC)-SiCl3自由基4的荧光黄色板/针。在此，我们报告了这两种…的合成、结构相关性、DFT计算和EPR研究。
Carbon or silicon radicals with silyl substituents are important intermediates in organometallic and organic chemistry.[1] Already in 1970, Bassindale et al. had reported the persistent tris (trimethylsilyl) methyl radical,[2](Me3Si) 3CC, which has a lifetime of several days at 298 K. The groups of Ingold,[3] Apeloig,[4] Bravo-Zhivotovskii,[5] Lee,[6] Sekiguchi,[7] and others [4–6] have shown that the lifetime of the radicals largely depends on the steric bulk of the substituents. In 2002, Sekiguchi et al.[7] reported the first stable silicon-centered radical without any π conjugation. Several reports describe the preparation of this class of radicals. The most successful ones proceed through the photolytic or thermal cleavage of a SiÀSi bond, and R3SiÀSiHCl2 can be reacted with bulky reagents, such as (tBu) 2MeSiLi, according to the Apeloig–Sekiguchi method. Several radical species [8] of main-group elements, such as PNC+,[8b] P2C+,[8c] phosphinyl radical cations,[8d] HBC+,[8e] and ketenes with biradical character,[8f] were also stabilized by cyclic alkyl (amino) carbenes (cAACs). We have observed that the chemical response of cAACs [8a] toward silylenes is very different from that of N-heterocyclic carbenes (NHCs). Recently, we have demonstrated that NHC! SiCl2 1 reacts with Me2-cAAC in a redox reaction to form the biradical (Me2-cAACC) 2SiCl2 2 in a two-electron redox step.[8g] Consequently, we were curious to investigate the one-electron redox process. Therefore, we reacted cAAC! SiCl4 3 with KC8 in an equimolar ratio in n-hexane to yield stable radicals 4, of the general formula (cAACC)–SiCl3. The highly reactive trichloromethane radical CCl3C[9a] and its congener SiCl3C[9b] are frequently generated by flash photolysis. These species are active radical intermediates in many photochemical transformations.[9a, b] Recently, the chemical reactivity of the TEMPO radical towards SiCl4[9c] and metal ions [9d] was explored, and the catalytic [9d] and fashionable magnetic [9e, f] properties of these adducts were studied. Although handling and controlling the chemical behavior of radicals can be very difficult, the chemistry of radicals has always been captivating.[10, 11] To the best of our knowledge, stable radicals with the SiCl3 group next to the radical center have not been reported so far. The carbene carbon atom of an NHC is bound to two both σwithdrawing and π-donating nitrogen atoms. In a cAAC, one nitrogen atom of the NHC is replaced by a σ-donating quaternary carbon atom. Theoretical calculations showed that the HOMO–LUMO energy gap is smaller in cAACs. Thus, cAACs are both more nucleophilic and more electrophilic than NHCs.[12a] Recently, 31P NMR analysis of a number of carbene–phenylphosphinidene adducts revealed that cAACs are better π acceptors than NHCs.[12b] These inherent differences may play a pivotal role in the replacement of an NHC by a cAAC and their behavior in a reaction. The NHC! SiCl4 adduct was reduced to NHC! SiCl2,(NHC! SiCl) 2, and NHC! Si= Si! NHC through the use of KC8.[13] In detail, an equimolar mixture of cAAC! SiCl4 3 (1 mmol) and KC8 (1 mmol) in n-hexane (85 mL) was initially reacted atÀ788C. The resulting suspension was slowly warmed to room temperature to obtain a clear colorless solution and an unreacted deposit of insoluble KC8. Upon stirring for 24 h, the color of the solution changed to a clear light yellow with the black deposit of graphite; after filtration, the solution was concentrated to a volume of 2–3 mL to obtain fluorescent yellow plates/needles of the (cAACC)-SiCl3 radical 4. Herein, we report the synthesis, structural correlation, DFT calculations, and EPR studies of the two …