Halopyridinium salts

A closely related method, which often gives higher yields, involves treatment of the hydroxy acids with 1-methyl- or l-phenyl-2-halopyridinium salts, especially l-methyl-2-chloropyri-dinium iodide (Mukaiyama s reagent).6 3 Another method uses organotin oxides.644... 

Li P, Xu J-C. 1-Ethyl 2-Halopyridinium salts, highly efficient coupling reagents for hindered peptide synthesis both in solution and the solid-phase. Tetrahedron 2000 56 8119-8131. 

The formation of esters from carboxylic acids by using Ar-alkyl-2-halopyridinium salts, such as 2-chloro-l-methylpyridine iodide (1) and alcohols, was first reported by Mukaiyama and, thus, the reaction is defined as the Mukaiyama esterification. The active ester (2) forms esters in good yield under mild conditions when subjected to alcohols. 

Esterification.—An alternative to the well-known azodicarboxylate-PhsP esterification procedure has dimethyl mesoxalate as proton acceptor. Yields of esters of benzoic acid are quite good (50—86%), and the reaction results in complete configurational inversion of the alcohol component. Isocyanides have also been shown to be useful dehydrating reagents in this type of reaction. Esterification of acids with one equivalent of an alcohol can be achieved by prior reaction of the acid with 2-halopyridinium salts. cf. refs. 134—140). An overall conversion of amines into acetates and benzoates (Scheme 29) involves conversion... 

Mukaiyama developed l-alkyl-2-halopyridinium salts, which are useful reagents for the preparation of carboxylic esters and lactones in the presence of tertiary amines [22]. After generation of activated onium salts from the corresponding co-hydroxycarboxylic acids, spontaneous lactonization smoothly proceeds to produce various macrolactones. Bartlett first used this lactonization method for the... 

Synthesis of Esters from Carboxylic Acids, Esterification (eq 1) proceeds by activation of the carboxyl component using a 2-halopyridinium salt (2 X = Cl, Y = I or X = F, Y = OTs) and (1) (2,4 equiv) in the presence of an alcohol. The best solvents for this process are CH2CI2 or MeCN, but a range of other solvents may also be used. 

Synthesis of Amides from Carboxylic Acids/ In the corresponding amidation process (eq 2) there is a requirement for the addition of 1 equiv of a tertiary amine (Tri-n-butylamine) to ensure efficient utilization of the amine component (R R NH). Once again, carboxyl activation is achieved using a 2-halopyridinium salt (2) but, unlike the esterification reaction, amidation is best carried out as a two-step one-pot process. 

Reactions of iV -alkylated or arylated azinium compounds with nucleophiles proceed more readily than those of the parent, uncation-ized azines, and the ring tends to open. The iV -substituent may bring into play an accelerative effect from the London forces of attraction. Increased displaceability of the substituent in iV -alkyl-azinium compounds has been noted for 2-halopyridinium (87) 1-haloisoquinolinium, 4-halopyrimidinium, 4-methoxypyrid-inium (88), 4-phenoxy- and 4-acetamido-quinazolinium (89), 3-methylthiopyridazinium, and 2-car boxymethylthiopyrimidi-nium salts (90). The latter was prepared in situ from the iV -alkyl-pyrimidine-2-thione. The activation can be effectively transmitted to... 

Further, thiopyridones and quinaldinethiones can be obtained by the action of alkali hydrogen sulfides on halopyridinium and haloquinaldinium salts 325 326... 

Kosaka Y, Yamamoto HM, Tajima A, Nakao A, Cui H, Kato R (2013) Supramolecular Ni (dmit)2 salts with halopyridinium cations-development of multifunctional molecular conductors with the use of competing supramolecular interactions. CrystEngComm 15 3200-3211... 

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