Platinum oxide quinoline

Reduction. Quinoline may be reduced rather selectively, depending on the reaction conditions. Raney nickel at 70—100°C and 6—7 MPa (60—70 atm) results in a 70% yield of 1,2,3,4-tetrahydroquinoline (32). Temperatures of 210—270°C produce only a slightly lower yield of decahydroquinoline [2051-28-7]. Catalytic reduction with platinum oxide in strongly acidic solution at ambient temperature and moderate pressure also gives a 70% yield of 5,6,7,8-tetrahydroquinoline [10500-57-9] (33). Further reduction of this material with sodium—ethanol produces 90% of /ra/ j -decahydroquinoline [767-92-0] (34). Reductions of the quinoline heterocycHc ring accompanied by alkylation have been reported (35). Yields vary widely sodium borohydride—acetic acid gives 17% of l,2,3,4-tetrahydro-l-(trifluoromethyl)quinoline [57928-03-7] and 79% of 1,2,3,4-tetrahydro-l-isopropylquinoline [21863-25-2]. This latter compound is obtained in the presence of acetone the use of cyanoborohydride reduces the pyridine ring without alkylation. 

Quinoline homologs and derivatives, including those with double bonds in the side chains, were reduced selectively by catalytic hydrogenation over platinum oxide (side chain double bonds), and to dihydro- and tetrahydro-quinolines by sodium in butanol, by zinc and formic acid, and by triethylam-monium formate [319, 472]. Catalytic hydrogenation of quinoline and its derivatives has been thoroughly reviewed [439]. 

Reaction conditions used for reduction of acridine [430,476, partly hydrogenated phenanthridine [477 and benzo f]quinoline [477 are shown in Schemes 38-40. Hydrogenation over platinum oxide in trifluoroacetic acid at 3.5 atm reduced only the carbocyclic rings in acridine and benzo[h]quinoline, leaving the pyridine rings intact [471]. 

Quinoline may be reduced rather selectively, depending on the reaction conditions. Catalytic reduction with platinum oxide in strongly acidic solution at ambient temperature and moderate pressure gives a 70% yield of 5,6,7,8-tetrahydroquinoline. Further reduction of this material with sodium-ethanol produces 90% of fratrr-decahydroquinoline. 

Over ruthenium dioxide quinoline was hydrogenated to tetrahydroquinoline in 97.5% yield at 80°C and 8.2 MPa H2 and to decahydroquinoline in 98% yield at 120°C and 9.3 MPa H2.3 Quinoline was also hydrogenated to tetrahydroquinoline over colloidal platinum in neutral solution or as the hydrochloride over platinum oxide in absolute ethanol.30 Hydrogenation to decahydroquinoline was performed with platinum black (Willstatter) or colloidal platinum (Skita) in acetic acid.73,74 Hiickel and Stepf hydrogenated quinoline under almost the same conditions as used by Skita and Meyer, and obtained the decahydroquinoline consisting of approximately 80% of trans and 20% of cis isomers (eq. 12.46). 

TABLE 12.7 Selective Hydrogenation of Quinolines and Isoquinoline in the Benzene Ring over Platinum Oxide Catalyst... 

Hydrogenation of l,2,3,4-tetrahydrobenzo[/]quinoline over platinum oxide gave also the 1,2,3,4,7,8,9,10-octahydro compound as the major product together with small amounts of cis- and trans-1,2,3, 4,4a,5,6, lOb-octahydro derivative (eq. 12.55). The hydrochloride of the tetrahydro compound was hydrogenated over platinum oxide in ethanol to give the 1,2,3,4,7,8,9,10-octahydro derivative in a better yield of 84% together with 4.5% of m-1,2,3,4,4a,5,6,lOb-octahydro compound.108... 

Benzo[h]quinoline. In contrast to the case of acridine, the hydrogenation of benzo /t quinolinc over platinum oxide in trifluoracetic acid afforded a mixture of two octahydro derivatives together with a small amount of the 7,8,9,10-tetrahydro derivative, although the octahydro compound hydrogenated in the benzene rings was the major product (eq. 12.56).37... 

Catalytic hydrogenation is performed in alcohol solution over Raney nickel at 25° to 100° and 30 atm. or over platinum oxide at room temperature and 1 to 2 atm. The reaction is highly expthermic therefore, precautions should be taken against excessive reaction temperatures. Typical illustrations are found in the preparations of 2-amino- -cymene (90%) and 3,4-diethylaniline (90%). Heterocyclic nitro compounds in the quinoline and dibenzothiophene series also respond favorably to catalytic hydrogenation. 

Witkop (153) could obtain the decahydro derivative by low pressure reduction in acetic acid solution containing excess sulfuric acid only if a large amount of platinum oxide was used. He pointed out that when less than a 100% ratio of catalyst to compound was present or the sulfuric acid was omitted, reduction stopped at the tetrahydro stage. It is of interest that similar to the work of Huckel and Stepf with quinolines (150) the presence of strong acid resulted in a predominance of cis product. Witkop in some later work showed that high pressure re-... 

When ethyl quinoline-2-carboxylate was hydrogenated in the presence of palladium oxide, not only was the desired tetrahydro compound obtained, but in addition, a dimeric tetrahydro compound bridged at 4-position was found (170). When platinum oxide was substituted, a quantitative yield of 1,2,3,4-tetrahydro derivative resulted. 

Selective hydrogenation of quinolines and isoquinolines. Catalytic hydrogenation of quinolines and isoquinolines usually occurs preferentially in the pyridine ring. However, if the hydrogenation is conducted in trifluoroacetic acid, the reverse situation obtains and the benzene ring is reduced more rapidly. The same result can be obtained with mineral acids, but such hydrogenations are much slower. Both 2- and 4-phenylpyiidine can also be reduced preferentially in the benzene ring. Platinum oxide or palladium or rhodium catalysts can be used. Further reduction of 5,6,7,8-tetrahydroquinolines with sodium and ethanol provides a convenient route to rrans-decahydroquinolines. 

Quinolinic acids can be obtained from quinolines and substituted quinolines in good yields by electrolytic oxidation in sulfuric acid with anodes of lead dioxide or platinum. For example, 3-bromoquinoline gives 5-bromoquinolinic acid (34 Scheme 24) (6IJOC8O8). 

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