Alkylation of Amines with Alcohols and Carboxylate Esters: the Origin of N-Methylpyridinium Cations in the Synthesis of Pyridine-Molybdate(V) Complexes

The formation of N-methylpyridinium salts in the solvothermal synthesis of molybdenum(V) complexes with oxalate in methanol in the presence of pyridine was investigated. It was found that dimethyl oxalate is formed first, which can thereupon alkylate pyridine. Pyridine reacts with a variety of esters, forming N-alkylpyridinium salts. Most reactive in this respect are sulfonates, whereas among carboxylates suitable esters are trifluoroacetates and oxalates. The reactivity of esters is proportional to the Ka of the parent acid. Primary and secondary amines attack an acyl carbon rather than alkyl one, thus forming amides. Sulfonate esters are attacked by all types of amines exclusively at the alkyl carbon.