Reduction of N-Arylpyridinium Compounds by Sodium Borohydride and Dithionite: Regioselectivity and Isomerization of Reduction Products

• Published in: Bulletin of the Korean Chemical Society 1986, Vol.7 (3), p.201-204
• Main Authors: Koh, Park, Kwang-Hee, Han, Du-Hee, Shin, Dae-Hyun
• Format: Article
• Language: Korean
• ISSN: 0253-2964; 1229-5949

Reduction of N-arylpyridinium compounds by $NaBH_4$ gave mixtures of the corresponding 1,2-dihydropyridine(major) and 1,4-dihydropyridine(minor), whereas similar reduction by $Na_2S_2O_4$ produced 1,4-dihydropyridines regioselectively. The proportion of 1,4-isomer in the product by $NaBH_4$ reduction appeared to increase with the electron-donating ability of N-aryl groups. When the N-aryl group is p-methylphenyl, p-ethylphenyl or p-methoxyphenyl, the 1,2-dihydropyridines in ethanol-water (4:1) solutions isomerized to the corresponding 1,4-dihydropyridines. N-(p-methylphenyl)-1,2-dihydropyridine and N-(p-ethylphenyl)-1,2-dihydropyridine in solid state also isomerized to the corresponding 1,4-dihydropyridines. The different behaviors of reduction among N-arylpyridiniums and isomerization of the reduction products depending on the substituent in N-aryl group were explained in terms of difference in the electronic effects of the substituents.