Robert E. Lyle

The Reduction of Nitrogen Heterocycles with Complex Metal Hydrides

Publisher Summary The reactions of complex metal hydrides occur by an attack of the nucleophilic hydride ion on an electrophilic center. Aromatic nitrogen heterocycles in which the nitrogen contributes only one electron to the π-system are electrophilic as compared to benzene, and show a reduction by lithium aluminum hydride, an active reducing agent. Thus, nitrogen heterocycles are more electrophilic than benzene and are susceptible to attack by the hydride ion from a complex metal hydride anion. In protic solvents the intermediate cyclic enamines can undergo further reduction. The proper choice of reducing agent and reaction conditions thus allow the preparation of many partially reduced nitrogen heterocycles unavailable by other routes. These reduction procedures provide a valuable adjunct to catalytic hydrogenation for the syntheses of saturated nitrogen heterocycles. This chapter provides examples of the various types of nitrogen heterocycles which have been investigated to provide a basis for evaluating this method of reduction in the synthesis of reduced and partially reduced nitrogen heterocycles.

THE PARTIAL HYDROGENATION OF PYRIDINES

INTRODUCTION The availability of pyridines from natural sources led to the use of these compounds as a source of piperidines, for the pyridines were shown to undergo a facile conversion by hydrogenation over a variety of catalysts to piperidines.13 The ease of the hydrogenation was increased by converting the amine to a proton salt by acid, but the conversion of the substituted pyridine to a quaternary salt gave a derivative which underwent reduction with even greater ease.4 The anticipated formation of piperidines by the catalytic hydrogenation was not realized under certain conditions, however. A few illustrations will show the importance of these reactions. The hydrogenations of substituted pyridinium iodides are unusual, for the aromatic ring is particularly resistant to saturation in the presence of this anion. Catalytic hydrogenations of pyridinium ions in the presence of bases have been used as a method of production of incipient tetrahydropyridines which are important as synthetic intermediates. The discovery of the unusual course of the reduction of pyridinium ions having an electron-withdrawing group in the 3-position provided a synthetically important, but mechanistically puzzling, partial hydrogenation. Hydrogenation of Quaternary Iodides

Amine-boranes : II. A novel synthesis of the 1,3,2-benzoxazaborine ring system

Abstract The amine—boranes of 3,4-dihydro-sH-1,3-benzoxazines were found to be unstable to heat forming [4H]-1-oxa-3-azonia-2-boratanaphthalenes. The spectra of these compounds showed them to exist in the cyclic form.