Synthesis of methylpyridines from acetaldehyde and ammonia in the presence of heterogeneous Pd complexes containing S is studied by a pulsed microcatalytic method. The process occurs with selective formation of 4-methylpyridine. The difference in the catalytic activity of the studied complexes is explained on the basis of 13C NMR spectra.
Pyridine and its derivatives are widely used in the synthesis of a wide assortment of chemicals for protecting plants (Lontrel, Chloramp, Tordon), dyes, surface-active substances, and pharmaceutical preparations.
Known catalytic methods of preparing pyridine and its derivatives are based on the use of acetylene and ammonia or aldehydes and ammonia in the presence of oxides and salts of various acidity [1]. However, the nonreaction is nonselective. The mixture formed contains a number of pyridine derivatives, the concentration of each of which is less than 30-40%.
According to one of the schemes, the synthesis of methylpyridines from acetaldehyde and ammonia includes chain-growth up to 6 C atoms resulting from di-and trimerization of starting molecules and a subsequent cyclization step. It is known [2] that transition metal complexes selectively catalyze these reactions. Thus, it seemed interesting to test heterogeneous metal-complex catalysts in the studied process.
In the present study, results are presented for the synthesis of methylpyridines from acetaldehyde and ammonia in the presence of Pd complexes containing S of general formula PdZ2CI 2 [where Z = dimethylsulfoxide (I), 2,2-diphenyl-4-methylthiacyclohexane (II), and 5-butyl-2-thiapyrrolidone (III)] deposited on aluminum oxide from DMF.
The synthesis of methylpyridines from acetaldehyde and ammonia was carried out in the range 80-160~ The amount of deposited complex (0.25 %), the contact time (1 sec), and the acetaldehyde--ammonia--air ratio 1:4:1 were selected on the basis of previous investigations [3] and held constant.
The principal products of the conversion are pyridine, methylpyridines with predominantly 4-methylpyridine (up to 85%), acetonitrile (up to 12%), aldimine (up to 11%), and a series of unidentified oxidation and amination products, the concentration of which is small (1-2%). The activity of the catalyst was compared for the average of the first 5-7 pulses. According to the results obtained (ef. Fig. 1), the degree of conversion of starting acetaldehyde depends little on the properties of the complex used, changing in the range 94-100% (curves 4-6) and decreasing insignificantly on increasing the temperature (94-96%). Ammonia is completely converted regardless of the experimental conditions.
The studied catalysts are placed in the order I > II > III according to content of heterocycles in the catalysates, regardless of the temperature (Fig. 1, curves 1-3). The temperature dependence of formation of 4methylpyridine for all catalysts passes through a maximum at 100~ The reduction of the yield of 4-methylpyridine on increasing the temperature is probably due to the increasing extent of parallel formation reactions of acetonitrile and oxidation and amination products. Between 140-160~ the temperature dependences of heterocycle formation for all three samples approach each other.
The concentration of acetonitrile in the reaction products increases as the temperature is increased from 2-5 % to 10-12% on all studied catalysts, respectively. This agrees with previous results [4] for the synthesis of methylpyridines on catalysts containing N and P, in the presence of which increasing the temperature and contact time resulted in increased formation of acetonitrile.