Alkylpyridines arylation

There is no doubt that in these. reactions the nitrogen atom of the pyridine ring is complexed with either the lithium alkyl or aryl or with the lithium bromide which is usually present in many preparations of organolithium compounds. It has been established that, either in the presence of an excess of lithium bromide or in the total absence of this salt, phenyllithium still gives the same ortho .para ratio on reaction with 3-picoline.229 To account for the predominant formation of the 2,3-isomer in the reaction of CH3Li with 3-alkylpyridines, it was suggested261 that the transition states for these reactions were similar... 

In an analogous manner, the generation of alkyl radicals in benzene solution by the O-acyl oxime method results in the formation of alkylbenzenes with moderate to good yields for simple acids (equation 55). Use of pyridine as solvent leads to the formation of alkylpyridines as mixtures of ortho, meta and para isomers in which the para isomer predominates. The 0-acyI benzophenone oxime chemistry can also be applied to aryl acids in benzene or pyridine, resulting in the formation of mixed biaryls. A closely related method involves photolysis of mixed anhydrides of arenecarboxylic acids with the hydrox-amic acid A -hydroxy-Z-pyridone in benzene solution (equation 56). 

R = H, aikyi, substituted aikyi R = H, alkyl, substituted alkyl, aryl, heteroaryl R = Me, Et, n-Pr base pyridine, alkylpyridine,... 

The reaction of A -phenacylpyridinium bromide with lithium dialkylcuprates (49) results in the isolation of acetophenone. The fate of the heterocyclic moiety has apparently not been determined. It is reasonable to assume, on the basis of the soft nature of these organometallic reagents, that 4-alkylpyridines must have been produced concomitantly. Addition of chloroformate esters to a mixture of pyridine and a cuprate reagent leads predominantly to A -carbo-alkoxy-4-alkyl(aryl)- 1,4-dihydropyridines (50). 

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