Imidazolium salts, nucleophilic addition

Substituted imidazole 1-oxides 263 upon treatment with dimethyl or diethyl sulfate furnish l-alkoxy-3-subtituted imidazolium salts 283 that were converted to the tetrafluoroborate 283 (A- = BF4 ) or hexafluorophos-phates 283 (A = PF6-) by treatment with sodium tetrafluoroborate or hexa-fluorophosphate (2007ZN(A)295). The tetrafluoroborates 283 (A = BF4 ) reacted with cyanide ion to give 2-cyanoimidazoles 285 (1975JCS(P1)275). The reaction probably follows a mechanism similar to that suggested to be operative in the pyrazole series encompassing O-alkylation succeeded by nucleophilic addition and elimination of methanol (Scheme 85). 

Careful treatment of 1,3-disubstituted imidazolium salts (e.g., 350) with sodium hydride (in some cases addition of a small amount of potassium /< r/-butoxide is also recommended) in tetrahydrofuran gives stable carbenes of type 351 (see also Section 2.4.4.2.3). They behave as rather strong bases (p , 24 in DMSO) and nucleophiles. Thus, carbene 351 and salt 350 form a crystalline bis(carbene)proton complex 352 which is a rare example of an asymmetrical hydrogen bridge between two carbon centers. By the action of BF3, B2H6 or AlH3NMe3 stable mesoionic adducts of type 353 are obtained. 

To decrease the induction period, DMCs have been combined with quaternary ammonium salts, montmorillonite clays modified with quaternary ammonium salts, or imidazolium ionic liquids [10,12,13]. Such additives were found to decrease the induction period by acting as nucleophiles in the catalytic cycles. As such, these compounds (1) facilitate the formation and activation of the zinc-monomer bond and (2) make the zinc-monomer bonds more active for polymerization, and/or they stabilize the polymerization centers and prevent their decomposition [10,13]. 

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