An Imidazolium Salt That Uncharacteristically Avoids the Imminent Deprotonation of Its Acidic 2‑H Proton by KN(SiMe3)2

KN(SiMe3)2, a non-nucleophilic strong base, rarely misses deprotonating an imidazolium-2-H to give its N-heterocyclic carbene (NHC). We report a rare case of a -CH2-linked bifunctional imidazolium-phenol [HO-4,6-tBu2-C6H2-2-CH2{CH(NCHCHNAr)}]Br [LH2Br; Ar = 2,6-iPr2–C6H3 (Dipp)], whose attempted double deprotonation by KN(SiMe3)2 majorly gives an unusual substitution product [(DippImd)K{O-4,6-tBu2-C6H2-2-CH2N(SiMe3)2}] (1; DippImd = CH{(NCHCHN(Dipp)}). Apparently, the second KN(SiMe3)2, instead of deprotonating the imidazolium-2-H, substitutes the whole imidazolium moiety from the benzylic carbon. Control experiments indicate a SN1-type mechanism. But the intermediate LH, seemingly a zwitterionic imidazolium aryloxide, also shows a steady self-fragmentation by releasing free DippImd to suggest a complex formative route for 1. Interestingly, LiN(SiMe3)2 in contrast favors the expected double deprotonation but further undergoes a 1,2-benzyl migration to give trimeric [Li(O-4,6-tBu2-C6H2-2-CH2{C(NCHCHNAr)}]3 [4]3. Two plausible routes for the substitution and deprotonation are proposed using DFT calculations. However, considering the complexities involved, a better clarity of the Li/K divergence would require a more accurate modeling of the reaction species and explicit solvent molecules.六甲基二硅基氨基钾（KN(SiMe3)2）是一种非亲核性强碱，通常可高效完成咪唑鎓-2-H的去质子化以得到对应的氮杂环卡宾（N-heterocyclic carbene, NHC），极少出现无法实现该转化的情况。我们在此报道一例罕见的由-CH₂连接的双官能团咪唑鎓-酚类化合物[HO-4,6-二叔丁基-2-苄基{CH(NCH=CHNAr)}]Br [LH₂Br；Ar = 2,6-二异丙基苯基（2,6-iPr₂–C₆H₃，缩写Dipp）]，当尝试使用六甲基二硅基氨基钾对其进行双重去质子化时，主要得到了一种非常规的取代产物[(DippImd)K{O-4,6-二叔丁基-2-苄基N(SiMe3)2}]（化合物1；DippImd = CH{(NCH=CHN(Dipp))}）。显然，第二当量的六甲基二硅基氨基钾并未对咪唑鎓-2-H进行去质子化，而是从苄基碳上取代了整个咪唑鎓基团。对照实验表明该反应遵循单分子亲核取代（SN1）型机理。不过，看似为两性离子咪唑鎓芳氧化物的中间体LH，同样会通过释放游离的DippImd发生稳定的自碎裂过程，这暗示了化合物1的形成路径较为复杂。有趣的是，六甲基二硅基氨基锂则更倾向于发生预期的双重去质子化，但后续会经历1,2-苄基迁移反应，最终得到三聚体化合物[Li(O-4,6-二叔丁基-2-苄基{C(NCH=CHNAr)}]₃ [4]₃。我们通过密度泛函理论（Density Functional Theory, DFT）计算，提出了取代反应与去质子化反应的两条可行路径。然而，考虑到反应体系涉及诸多复杂因素，要更清晰地阐明锂与钾试剂间的反应差异，需要对反应物种及显性溶剂分子进行更精准的建模。