When N-alkylpyridinium derivatives were reduced with sodium borohydride-nickel (II) chloride reduction system, reductive cleavage occurred at the CN bond in the pyridine ring of N-alkylpyridinium derivatives to give a small amount of reductive cleavage product along with the major perhydrogenated product. It was presumed in the previous report that this reductive cleavage in the pyridine ring proceeded through a complex of nickel ion and 1, 2, 3, 6-tetrahydropyridine derivatives produced with NaBH4 alone reduction. The abundances of these reductive cleavage products arising from N-alkylpyridinium derivatives, i.e., paraquat, diquat and so on, are capable of giving a bad effect on the accuracy of gas chromatographic analysis. For the purpose of inhibition of the reductive cleavage in this reduction system, a suitable catalyst was examined. In addition, we pursued whether borane-1, 2, 3, 6-tetrahydropyridine derivative complexes arose from N-alkylpyridinium derivatives by NaBH4 alone reduction or not, and whether these boraneamine complexes were the precursors of reductive cleavage products or not. N-Alkyl-1, 2, 3, 6-tetrahydropyridine derivatives (III-I, IV-I, VI-I, VII-I and VIII-I) and the corresponding borane-amine complexes (III-II, IV-II, VI-II, VII-II and VIII-II) were synthesized by NaBH4 reduction in aqueous solution of N-alkylpyridinium salts, i.e. I, II, 1, 4-dimethylpyridinium iodide (III), 1-dodecylpyridinium chloride (IV), 1, 1'-diethyl-4, 4'-dipyridinium dichloride (V), 1-methyl-4-phenylpyridinium iodide (VI), 1-n-propylpyridinium iodide (VII) and 1-n-butylpyridinium iodide (VIII). The structure of the borane-amine complexes were proved by the Mass spectrometry and 1H-and 13C-NMR analysis. The NiCl2-NaBH4 reduction of the borane-amine complexes gave the perhydrogenated products alone, but not reductive cleavage products. In conclusion, it was recognized that the precursors of reductive cleavage products were not borane-amine complexes, but 1, 2, 3, 6-tetrahydropyridine. Furthermore, it was found the reductive cleavage at the C-N bond in the pyridine ring of these 1, 2, 3, 6-tetrahydropyridine derivatives was hindered by applying Amberlite-Ni2B, NaBH4 reduction system.
