Platinum catalyst, 3-picoline

Condensation of y-picoline with mesoxalic ester yielded 4-(j8,j8-diethoxycarbonylvinyl)pyridine (35). The unsaturated ester (35) was hydrogenated with platinum catalyst to form 36 which was treated with bromine. 4-(j8-Bromo-j8,j8-diethoxycarbonylethyl)-piperidine (37) was obtained and was cyclized with pyridine to 2,2-die th 0 xycarbony 1 quinuclidine (38). Hydrolysis of 38 and partial decarboxylation gave quinuclidine-2-carboxylic acid (39). 

Palladium on carbon has been used successfully to obtain piperi-dine-2-carboxylic in almost quantitative yield after reduction of picolinic acid in aqueous solution at 60° and 3 atm pressure (53). The time of reduction, however, is much longer than with rhodium or platinum catalyst. Since there is always a difference in reduction time between the 2- and 4-derivatives, hydrogenation of isonicotinic acid should be prohibitively long at low pressure with palladium. However, if pressure equipment is available, reaction time should be decreased considerably. 

The results for the deuteration of 2-, 3-, and 4-picolines are summarized in Table IX. With 2-picoline, specific deuteration in the a-position only is observed on cobalt, whereas platinum is catalytically active for all positions,105 as is nickel chloride which catalyzes deuteration of the methyl group in particular. As with pyridine, borohydride-reduced oxide catalysts are more active than self-activated preparations. The much higher reactivity in 2-picoline compared with pyridine in selfactivation indicates that the methyl group compensates for deactivation from the nitrogen lone pair. Hydrogen for self-activation in the former compound originates predominantly from the methyl group. 

It must be pointed out, however, that physical shielding takes place only when the nitrogen atom is not in an ionized form. Hydrogenations of pyridine and 2-substituted pyridines when carried out in acid solution or as salts show that pyridine is reduced first in comparison with other pyridines. For example, under the conditions used by Skita (9) pyridine was reduced more readily than 2-picoline. The same order of reducibility was observed in hydrogenations in acetic acid in the presence of Adams catalyst (55). In another comparison of the reduction of mixed hydrochloride salts (39) it was found that pyridine reduced first. This is further amplified in the reduction of equimolecular binary mixtures of hydrochloride salts in the presence of platinum oxide (40). 

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