Aldol reaction with picolines

The reaction of ethanol with ammonia on zeolite catalysts leads to ethylamine. If, however, the reaction is carried out in the presence of oxygen, then pyridine is formed [53]. MFI type catalysts H-ZSM-5 and B-MFI are particularly suitable for this purpose. Thus, a mixture of ethanol, NH3, H2O and O2 (molar ratio 3 1 6 9) reacts on B-MFI at 330 °C and WHSV 0.17 h 1 to yield pyridine with 48 % selectivity at 24 % conversion. At 360 °C the conversion is 81% but there is increased ethylene formation at the expense of pyridine. Further by-products include diethyl ether, acetaldehyde, ethylamine, picolines, acetonitrile and CO2. When applying H-mordenite, HY or silica-alumina under similar conditions pyridine yields are very low and ethylene is the main product. The one-dimensional zeolite H-Nu-10 (TON) turned out to be another pyridine-forming catalyst 54]. A mechanism starting with partial oxidation of ethanol to acetaldehyde followed by aldolization, reaction with ammonia, cyclization and aromatization can be envisaged. An intriguing question is why pyridine is the main product and not methylpyridines (picolines). It has been suggested in this connection that zeolite radical sites induced Ci-species formation. 

Carbonyl reactions— Their aldol reactions with carbonyl compounds are perhaps the most important undergone by primary or secondary alkyl groups attached to the pyridine ring. Ladenburg first observed the reaction of 2-picoline with paraldehyde, which gave, at 250 -260 , 2-propenylpyridine. At lower temperatures, a-(2-picolyl)ethanol is formed, but in low yield 3a, 6-4, Formaldehyde is more reactive. jS-(2-Pyridyl)ethanol has been prepared by heating 2-picoline and formalin or formaldehyde under pressure and this reaction and that with 4-picoline has... 

The pentane-1,5-dione is usually formed in situ by aldol- or Michael-type reactions (203 — 204 — 200). Thus, acetaldehyde (203 R = H, R = Me) and ammonia give 4-picoline and 3-ethyl-4-methylpyridine by formation of the intermediate (205), condensation with another molecule of MeCHO, and subsequent dehydrogenation. The same reaction also yields 2-picoline and 5-ethyl-2-methylpyridine via the intermediate (206). Such reactions are used industrially. 

The gas phase acid-catalyzed synthesis of pyridines from formaldehyde, ammonia and an alkanal is a complex reaction sequence, comprising at least two aldol condensations, an imine formation, a cyclization and a dehydrogenation (9). With acetaldehyde as the alkanal, a mixture of pyridine and picolines (methylpyridines) is formed. In comparison with amorphous catalysts, zeolites display superior performance, particularly those with MFI or BEA topology. Because formation of higher alkylpyridines is impeded in the shape-selective environment, the lifetime of zeolites is much improved in comparison with that of amorphous materials. Moreover, the catalytic performance can be enhanced by doping the structure with metals such as Pb, Co or Tl, which assist in the dehydrogenation. 

The highest pyridine-yd-picoline yields with MFI were found with Si02/Al203 ratios in the 150-400 range [10,11,22]. At such low aluminium concentrations it is likely that Brpnsted acid sites, rather than Lewis sites, assist in the Aldol condensation, cyclization, and hydrogen transfer (or dehydrogenation) reaction steps. Discussion of the mechanism of formation of pyridine bases in the vapor phase and the nature of the acid site dates back to the time of Chichibabin and remains a topic for debate [5,27]. 

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