Formic acid pyridinium salts

Piperideines unsubstituted at the nitrogen atom may be prepared from the corresponding pyridine compounds by partial reduction with sodium and boiling alcohols (the Ladenburg reduction), by electrolytic reduction, or, preferably, by reduction with aluminum hydride. l-Alkyl-3-piperideines are prepared by reduction of quaternary pyridinium salts with formic acid (the Lukes reduction) or with complex hydrides. 

In contrast to aluminum hydride reductions (see Section II, B, 4), no ring openings have been observed in reductions of quaternary pyridinium salts by means of sodium borohydride. Whenever possible, both isomeric tetrahydropyridines are formed, as it may be seen from the following examples (aluminum hydride, electrolytic, and formic acid reductions are included for comparison). 

The reduction of pyridinium species with formic acid in the presence of its salts at elevated temperatures is often referred to as Lukes reduction and has been reviewed. Mixtures of tetrahydropyridines (98) and piperidines (99 Scheme 20) commonly result. The isolated double bond is resistant to further reduction under these conditions. Reductive cleavage and subsequent cyclization have also been observed, producing ketones as by-products. ... 

The iridicycle catalysts also catalyse the TH of A-heterocycles (including quinolines, isoquinolines, indoles and pyridinium salts) in water under mild conditions a solution of formic acid and sodium formate is employed in the reaction. TONs of up to 7500 and catalyst loadings as low as 0.01 mol% were used [206]. 

Reduction of quaternary pyridinium salts by formic acid containing potassium formate, which recalls the Leuckart reaction, is basically a nucleophilic attack by hydride ion ... 

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