Benzene with 1,2,3,6-tetrahydropyridines

The combination of PhMeSiH2 (or Ph2SiH2) and Cp2TiMe2 (10 mol%) reduces pyridines to N-silylated-di- or tetrahydropyridines or the N-silylated piperidines.264,265 With quinoline, only the pyridine ring is reduced preferentially to the benzene ring (Eq. 121). 

Pyridine and its homologs can be reduced completely to hexahydro derivatives, or partially to dihydro- and tetrahydropyridines. Catalytic hydrogenation is faster than with the corresponding benzene derivatives and gives only completely hydrogenated products. Partial reduction can be achieved by different methods (pp. 55, 56). 

Pyridines are more susceptible to reduction than benzenes. Sodium in ethanol or in liquid ammonia evidently reduces pyridine to 1,4-dihydropyridine (or a tautomer) because hydrolysis of the reaction mixture affords glutaric dialdehyde (318 — 317 — 316). Reduction of pyridines with sodium and ethanol can proceed past the dihydro stages to A3-tetrahydropyridines and piperidines (318 — 319 and 320). 

Pyrrolines and Tetrahydropyridines Biltz91 first examined the action of concentrated hydriodic acid on succinonitrile and obtained a product which was claimed to contain four equivalents of hydrogen iodide. The analytical data, however, agree better with the empirical formula C4H4N2 3HI, and undoubtedly this material has the structure 114 in accordance with the recent work of Howard,92 Osborn,93 and Johnson.94 These investigators found that hydrogen bromide causes an immediate precipitation of 115 from a solution of succinonitrile in ether, benzene, or acetic... 

Eisner and Sadeghi described the isomerization of dihydropyridine with RhClj-PhjP complex in benzene. Isomerization of l-alkyl-4-acyl-1,2,5,6-tetra-hydropyridines into l-alkyl-4-acyl-l,4,5,6-tetrahydropyridines has been observed. Pd/C-catalysed rearrangement of iV-acyl-l,2,3,6-tetrahydropyridines to N-acylenamines is described . Besselievre, Beugelmans and Husson reported the photochemical isomerization of cyclic allylaminc 79 to enamine 80. An equilibrium favouring 81 over 82 by 93 7 was established by treating 81 with t-BuOK-DMSO at 90°C. 

Aromatization with migration of carbon-carbon double bonds. A mixture of N-(5-phenylpent-4-enyl)benzamide, POClg, and benzene refluxed ca. 4 hrs. -> 2-phenyl-3-benzal-3,4,5,6-tetrahydropyridine (Y 77.5%) refluxed 1 hr. with 30%-Pd-on-carbon and ethyl cinnamale in a Ng-atmosphere -> 2-phenyl-3-benzylpyridine (Y 70.5%). F. ring closures s. T. Fujisawa and S. Sugasawa, Tetrahedron 7, 185 (1959). 

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