The Reduction of Nitrogen Heterocycles with Complex Metal Hydrides

Publisher Summary This chapter reviews the reduction of nitrogen heterocycles with complex metal hydrides. The chapter describes the reduction of heterocycles containing one nitrogen atom. The reduction of pyridines with borohydride (NBH) occurs only if they carry electron-withdrawing substituents. Most pyridines containing a single electron-withdrawing substituent in the 2 or 4 position are unaffected by NBH. Lithium aluminum hydride (LAH) reductions are carried out in aprotic solvents and give rise to the dihydro- and tetrahydropyridine derivatives. Magnesium and zinc hydrides both form bis (dihydropyridinyl) complexes when dissolved in pyridine. Both have been used as reducing agents for ketones, nitnles, and heterocyclic systems. The reductions of pyridines with aluminum hydrides, pyridinium salts with borohydrides and aluminum hydrides is also described. The reduction of pyridine N-oxide and 1-alkoxypyridinium salts with a variety of reducing agents, including NBH, LAH, and sodium bis (2-methoxyethyl)aluminum hydride, gives a mixture of products. The mixtures contained pyridines, 3-piperidiene, and piperidines. The major product depends on the nature of the substituents present and the reducing agent employed. Titanium tetrachloride and NBH mixtures are employed for the deoxygenation of heteroaromatic amine oxides.