Quaternary salts, reactions with nucleophilic reagents

C. The Quaternary Salts and Their Reactions with Nucleophilic Reagents ... 

From the preparative point of view, reactions of heterocyclic aromatic compounds with nucleophilic reagents are very important, especially the reactions of their quaternary salts containing a formal enamine grouping in the molecule. 

Quaternary pyridinium salts503 having electron-attracting substituents on nitrogen — e.g., l-(2,4-dinitrophenyl)- or l-(4 -pyridyl)-pyridinium salts — can react in two ways with nucleophilic reagents such as amines. Either the quaternary pyridinium nucleus is replaced by the new amine group (reaction... 

Reaction with vatious nucleophilic reagents provides several types of dyes. Those with simple chromophores include the hernicyanine iodide [16384-23-9] (20) in which one of the terminal nitrogens is nonheterocyclic enamine triearbocyanine iodide [16384-24-0] (21) useful as a laser dye and the merocyanine [32634-47-2] (22). More complex polynuclear dyes from reagents with more than one reactive site include the trinuclear BAB (Basic-Acidic-Basic) dye [66037-42-1] (23) containing basic-acidic-basic heterocycles. Indolizinium quaternary salts (24), derived from reaction of diphenylcyclopropenone [886-38-4] and 4-picoline [108-89-4] provide trimethine dyes such as (25), which absorb near 950 nm in the infrared (23). 

Quaternary ammonium salts are known to form ion pairs with nucleophilic anions and to transfer these reagents in organic media. Dou et al.142 have shown that they may also take part in a substitution reaction where the anion is the nucleophile and a tertiary amine is the leaving group. 

The addition-elimination reaction between 2-chlorocarbonyI-lfl-pyr-rolizin-l-one and chlorine has been described (see Section III,B,2,c). The 2,5-dichloro derivative 21b isolated in the same reaction must presumably be derived from an electrophilic substitution on the initially formed 2-chloro-pyrrolizinone.26 The iminopyrrolizine 270 can be methylated, first with Meerwein s reagent and then by methyl iodide to give the quaternary salt 271, which reacts readily with nucleophiles (see Section III,B,5). 

Newkome et al. used nucleophilic aromatic substitution to incorporate the 2,6-pyrido unit into a triazamacrocycle which was precursor to a series of cryptands (Scheme 11) <81TL3039>. The sequence uses a tertiary aminoalcohol as the nucleophile in a sodium hydride-mediated reaction with 2,6-dichloropyridine. In the second step, the tertiary amines function as nucleophiles in a reaction of triethylene glycol diiodide. The tertiary amines are converted into quaternary ammonium salts in the process. The system must be demethylated to form the neutral crown but conditions must be such that reduction of the pyridine residue does not occur. This was accomplished by using the commercial reagents L-Selectride and Super-Hydride. 

Amines are powerful nucleophiles which react under neutral or slightly basic conditions with several electron-accepting carbon reagents. The reaction of alkyl halides with amines is useful for the preparation of tertiary amines or quaternary ammonium salts. The conversion of primary amines into secondary amines is usually not feasible since the secondary amine tends towards further alkylation. 

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