Pyridine/alumina ring

Pyridine/alumina 17, 927 Pyridine borane —, reductions with — 16,113 Pyridine derivatives, 1,2,3,6-fefrahydro- 16, 855 Pyridine hydrochloride as reagent 17, 17, 793, 992 Pyridine ring (s. a. Quinoline ring)... 

The alkylation of pyridine [110-86-1] takes place through nucleophiUc or homolytic substitution because the TT-electron-deficient pyridine nucleus does not allow electrophiUc substitution, eg, Friedel-Crafts alkylation. NucleophiUc substitution, which occurs with alkah or alkaline metal compounds, and free-radical processes are not attractive for commercial appHcations. Commercially, catalytic alkylation processes via homolytic substitution of pyridine rings are important. The catalysts effective for this reaction include boron phosphate, alumina, siHca—alurnina, and Raney nickel (122). 

A number of routes are available for the synthesis of 2,2 -bipyridines where one of the pyridine rings is built up from simpler entities. For example, condensation of 2-(aminomethyl)pyridine (31) with acetaldehyde or acetylene over a silicon-alumina catalyst at 450°C gives 2,2 -bipyridine, ° whereas 2-cyanopyridine reacts with acetylene at 120°C in the presence of a cobalt catalyst to afford 2,2 -bipyridine in 95% yield.2-Acetylpyridine with acrolein and ammonia gives 2,2 -bipyridine in the presence of dehydrating and dehydrogenating catalysts, and related condensations afford substituted 2,2 -bipyridines. ° In a similar vein, condensation of benzaldehyde with 2 mol of 2-acetylpyridine in the presence of ammonia at 250°C affords 2,6-di(2-pyridyl)-4-phenylpyridine, ° and related syntheses of substituted 2,2 6, 2"-terpyridines have been described. Likewise, formaldehyde with two moles of ethyl picolinoylacetate and ammonia, followed by oxidation of the product and hydrolysis and decarboxylation, affords a good... 

As in the synthesis of other bipyridines, several routes to 4,4 -bipyridine have been devised where one of the pyridine rings is built up from simpler components. For example, a dimer of acrolein reacts with ammonia and methanol in the presence of boron phosphate catalyst at 350°C to give a mixture of products including 4,4 -bipyridine (3.4% yield), and in a reaction akin to ones referred to with other bipyridines, 4-vinylpyridine reacts with substituted oxazoles in the presence of acid to give substituted 4,4 -bipyridines. ° ° Condensation of isonicotinaldehyde with acetaldehyde and ammonia at high temperatures in the presence of a catalyst also affords some 4,4 -bipyridine, and related processes give similar results,whereas pyran derivatives can be converted to 4,4 -bipyridine (56% conversion), for example, by reaction with ammonia and air at 350°C with a nickel-alumina catalyst. Likewise, 2,6-diphenyl-4-(4-pyridyl)pyrylium salts afford 2,6-... 

A few pyrolytic methods of synthesis are known, leading directly to the formation of pyridine rings by formation of the /3, y-bond. Alkylpyridines are obtained when unsaturated imines (116) or (117) are passed over heated zeolites (80IZV655) or alumina (72IZV2263). More dehydrogenation is achieved by the use of nickel or alumina, as in the synthesis of tetrahydroquinolines (118) or octahydrophenanthridine (119) (78IZV1446). 

Hydrogenations of acetylpyridines were carried out similarly, and yielded various products depending on the reaction conditions and especially on the catalyst used. Thus 4-acetylpyridine was hydrogenated over palladium oxide to form the corresponding alcohol, 4-(l-hydroxyethyl)pyridine, with a small amount of the pinacol (4), but it was converted mainly to the pinacol using palladium on charcoal or rhodium on alumina. However, under different conditions, the pyridyl ring of 3-acetylpyridine was reduced to a mixture of 3-acetyl-l, 4,5,6-tetrahydropyridine and 3-acetylpiperidine. 

Freifelder el al. found rhodium on carbon to be better suited than rhodium on alumina for reduction of the pyridine ring. The poisoning effect of the piperidine base formed can be overcome by use of sufficient catalyst. 

In general the reduction of a pyridine side-chain acid or ester using platinum oxide, Raney Nickel, rhodium-on-carbon, rhodium-on-alumina, or ruthenium oxide as the catalyst gives the piperidine acid or ester. Partial reduction of the pyridine ring to a tetrahydropyridine usually occurred when palladium-on-carbon was employed as the catalyst, although two exceptions were reported. Either a mixture of the piperidine and the tetrahydropyridine ester or the tetrahydropyridine ester alone was formed when sodium borohydride was used at room temperature in the reduction of pyridine side-chain ester salts. When the free bases were employed, reduction of the ester group occurred instead of nuclear reduction. The use of lithium aluminum hydride gave the same results (see Table XI-18). Many acetamides... 

Methanesulfonyl chloride added at 0° to l-methyl-l-(2-hydroxyethyl)-1,2-di-hydronaphthalene and dry pyridine, stirred 18 hrs. at room temp., the soln. of the resulting crude mesylate in wet benzene-petroleum ether chromatographed over basic alumina, and eluted with ether exo-l-methyl-2,3-benzo-bicyclo [3.2.1]-4-octanol. Y 54%.—This is a remarkably facile ring closure through double bond participation. W. Herz and G. Gaple, Am. Soc. 84, 3517 (1962). 

Figure 4.3 Transmission IR spectra of pyridine adsorbed on y-alumina top, MID-IR absorption region bottom, zoom on the ring vibration region, (a) sample before activation (b) activated sample (c) pyridine adsorption and evacuation at room temperature (d) difference between c and b spectra...

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