N-acylpyridinium ions

The intermediate N-acylpyridinium salt is highly stabilized by the electron donating ability of the dimethylamino group. The increased stability of the N-acylpyridinium ion has been postulated to lead to increased separation of the ion pair resulting in an easier attack by the nucleophile with general base catalysis provided by the loosely bound carboxylate anion. Dialkylamino-pyridines have been shown to be excellent catalysts for acylation (of amines, alcohols, phenols, enolates), tritylation, silylation, lactonization, phosphonylation, and carbomylation and as transfer agents of cyano, arylsulfonyl, and arylsulfinyl groups (lj-3 ). 

The high efficiency of 4-dimethylaminopyridine (DMAP) as a catalyst in acylation reactions has long been recognized. The reactive intermediates of these acylation reactions are N-acylpyridinium ions [225],... 

As a mechanistic rationale, the pyridine is Ukely to be activated by formation of an N-acylpyridinium ion 61, which undergoes (presumably coordination-directed) C-2-addition of intermediately formed Cu-acetylide 62 providing 1,2-dihydropyridine 60. 

The mechanism of catalysis by DMAP is considered to involve an N-acylpyridinium ion. However, the identity of the anion also affects the reactivity so that a complete formulation requires that the ion pair characteristics of the acylpyridinium ion be taken into account. Interestingly, in the presence of DMAP, acetic anhydride is a more reactive acylating agent than acetyl chlorode. This reversal of the normal reactivity order can be explained if the acetate ion, a stronger base than chloride, is involved in deprotonating the alcohol ... 

The formation of N-acylpyridinium ions from acyl chlorides and pyridines is effectively promoted by (la) and i-Pr3SiOTf. The N-acylpyridinium ions are usable for the synthesis of dihydropyridines and their derivatives by alkylation with Grignard reagents [128]. This reaction sequence has been applied to asymmetric alkylation of pyridines using a chiral acyl chloride [129]. 

Acid chlorides or anhydrides yield N-acylpyridinium salts 6, which are very reactive and sensitive to hydrolysis they are involved in the acylation of alcohols and amine in pyridine as solvent (Einhorn variant of the Schotten-Baumann reaction). Analogously, sulfonyl chlorides react in sulfonylations in pyridine via N-sulfonylpyridinium ions. 

Alkyl halides and related alkylating agents react with pyridines to form N-alkylpyridinium salts (Scheme 2.7). These compounds are much more stable than their 7V-acylpyridinium equivalents and can often be isolated as crystalline solids, particularly if the halide ion is exchanged for perchlorate, tetrafluoroborate or another less polarizable counter anion. 

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