Hydropyridines => pyridines

One Nitrogen Atom (it is self-subdivided into Pyridines, Pyridinium Compounds, Ylides, Pyridine N-Oxides, Applications of Pyridines, Bipyridines and Related Systems, Hydropyridines, Biologically Active Pyridines and Hydropyridines, Pyridines Annulated with Carbocycles, Pyridines Annulated with Heterocycles). 

The initial step in the reaction could be the addition of a Ti-Si species across the C=N bond of the pyridine to give an /V-silyldi hydropyridine. An additional two hydrogen atoms can be transferred to produce the tetrahydropyridine, but complete reduction only occurs in the presence of H2. On the other hand, electron-donating substituents and electron-withdrawing substituents on pyridine derivatives affect the reactions in different ways. 3-Picoline with electron-donating substituents gives two hydrogenation-hydrosilation isomers in a 3 2 ratio in 85% yield [Eq. (24)] whereas ethyl nicotinate with... 

The formation of some pyrroles, pyrrolines, pyrrolidines, pyridines, and tetra-hydropyridines were considered above. Next come the pyrazines, a very important group of odorants. Pyrazines have been reviewed periodically by Maga.231-233 Vemin and Parkanyi216 have tabulated 26 pyrazines, as well as 11 6,7-dihydro-(5//)-cyclopentapyrazines and 9 5,6,7,8-tetrahydroquinoxalines from 15 systems. 

The pyridine ring is easily reducible. Hydropyridines are formed from pyridine and its derivatives, and piperidines by complete hydration. Quinoline and acridine are also easily converted into hydro-compounds. 

Pyridine is not polarographically reducible in aqueous media in MeCN catalytic hydrogen evolution is the main reaction of protonated pyridine at the cathode [224]. Pyridine can, however, be reduced to piperidine at a lead cathode in 10% H2SO4 [225] the yield is very dependent on the purity of the lead and other components. Side products are a,a, a,y-, and y,)/-dipiperidyl. Pyridine may be reduced to a dimer in liquid ammonia [226]. In MeCN pyridine is reduced in a two-electron wave dihydropyridine and cyanomethylated tetra-hydropyridine are formed, the latter by attack of (CH2CN) on the dihydropyridine [227]. Electrochemical synthesis involving pyridine and its derivatives has been reviewed [Ic]. 

The four-electron reduction of derivatives of 3-methoxypicolinic acid may follow different routes, depending on the substitution in the ester or amide. Amides may, after uptake of two electrons, lose an amino or a hydroxyl group, and then be further reduced to a pyridinemethanol or an aminomethylpyridine, or the compound may be reduced in the pyridine ring steric crowding favors the formation of aminomethylpyridines or tetra-hydropyridines [452]. 

Alternatively, reduction of 176 with sodium borohydride (probably to a dihydropyridine, although this intermediate was not characterized) followed by thermolysis in acetic acid or acetic anhydride affords anthracene (93%) and pyridine or hydropyridines. These reaction sequences have been employed for the synthesis of overcrowded anthracene derivatives containing substituents with unfavorable peri interactions100 of particular interest is the conversion of 178 via its 1 2 adduct with benzyne into the pentaphenes 179 and 180. 

The experiments of Zelinsky and Borisov (21) appear to substantiate that piperidine is the intermediate. A high yield of it was obtained in a flow type hydrogenation of pyridine in the presence of palladized or platinized asbestos. A similar flow type reaction with the same catalysts (22) gave not only piperidine, but side products which the investigators suggest might be 1,2,3,4-tetrahydropyridine and 1,2,5,6-tetra-hydropyridine (2-piperideine). The observation, however, needs further confirmation. Reinvestigation of the products of reaction with modern... 

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