Lukes reduction 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. 

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. ... 

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