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

Abstract

KN(SiMe 3 ) 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 -CH 2 -linked bifunctional imidazolium-phenol t Bu 2 -C 6 H 2 -2-CH 2 {CH(NCH�CHNAr)}]Br [LH 2 Br; Ar = 2,6-i Pr 2 −C 6 H 3 (Dipp)], whose attempted double deprotonation by KN(SiMe 3 ) 2 majorly gives an unusual substitution product [Apparently, the second KN(SiMe 3 ) 2 , instead of deprotonating the imidazolium-2-H, substitutes the whole imidazolium moiety from the benzylic carbon. Control experiments indicate a S N 1-type mechanism. But the intermediate LH, seemingly a zwitterionic imidazolium aryloxide, also shows a steady self-fragmentation by releasing free Dipp Imd to suggest a complex formative route for 1. Interestingly, LiN(SiMe 3 ) 2 in contrast favors the expected double deprotonation but further undergoes a 1,2-benzyl migration to give trimeric [Li(O-4,6-t Bu 2 -C 6 H 2 -2-CH 2 {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.