Pyridines and pyrroles

Schofield, K. (1967). Hetero-Aromatic Nitrogen Compounds Pyrroles and Pyridines. London Burterworths. 

In 1931 Ing pointed out that formula (II) and (III) do not contain methyl or potential methyl groups in j ositions 6 and 8 which they occupy in cytisoline. Further, a partially reduced quinoline ought to oxidise easily to a benzenecarboxylic acid and so far the only simple oxidation, products recorded from cytisine were ammonia, oxalic acid and isovaleric acid. Distillation of cytisine with zinc dust or soda-lime yields pyrrole and pyridine, but no quinoline. On these grounds Ing suggested that cytisine should be formulated without a quinoline nucleus, and that the reactions which indicate the presence of an aromatic nucleus in the alkaloid can be accounted for by an a-pyridone ring. This a-pyridone nucleus can... 

K. Schofield, in Hetero-Aromatic Nitrogen Compounds. Pyrroles and Pyridines, Plenum Press, New York, 1967. 

Some of the factors which affect the relative yields of 2,2 -bipyri-dine and of the organonickel complex are known. For example, the addition of about 1% pyrrole to the pyridine during the reaction causes a 60% increase in the yield of the complex and a 13% decrease in the amount of 2,2 -bipyridine formed (see Table III). When more pyrrole is present the yields of both products are lowered, but the ratio of complex to 2,2 -bipyridine is increased by a factor of about 7. These findings suggest that pyrrole can be incorporated into the complex, presumably by reaction between pyrrole and pyridine to... 

The bonding forms of nitrogen in several Australian coals were determined by XPS and predominantly assigned to pyrrolic and pyridinic forms. Amino forms appear to be absent390. 

Equally simple entries—in fact, one-pot reactions each— lead from D-ftuctose to A -heterocycles of the imidazol, pyrrole, and pyridine type (Scheme 2.11), all of which, due to their hydrophilic substitution patterns, are considered useful building blocks to pharmaceuticals. 

Carbon and nitrogen are the most common elements from the first row of the periodic table to form aromatic compounds, characterized by cyclic electron delocalization. The bonding of these elements in the conjugated systems shows a large variety. Carbon can be a divalent (carbene), sp carbon with one jT-electron, but also sp carbon can be part of hyperconjugate aromatic systems, provided that it is properly substituted. The pyrrole- and pyridine-type nitrogens also allow the formation of cyclic electron delocalization in a large variety of aromatic systems. 

As written, the reaction is endothermic by ca 30 kJmoD. Pyrrole and pyridine are both 6-71 nitrogen-containing heterocycles. However, the former is electron-rich while the latter is electron-deficient and so conjugative stabilization mechanisms are different for the two species. Furthermore, the former can form one more hydrogen bond per molecule than the latter, a feature that may account for pyrrole-2-carboxaldehyde being a solid while pyridine-2-carboxaldehyde is a liquid. We wonder if either difference accounts for the profound lack of thermoneutrality for the above reaction. 

Mention may be made briefly of the studies of Malinovekii and ca-workers,1 . wiw.iw involving addition of ammonia to ethylene oxide, propylene oxide, and styrene oxide under stringent conditions. At 400-450° over an alumina catalyst, for example, ethylene oxide and ammonia are reported to give a moderate yield of pyridine (lfl.6-10.4%). With Btyrene oxide an exceedingly complex mixture of products is formed, among which are various pyrrole and pyridine derivatives, benzene, toluene, ethylbenzeno, benzoldehyde, acetophenone, phenylacetaldehyde, and others. 

K. Schofield, Heteroaromatic Nitrogen Compounds Pyrroles and Pyridines." Butterworth, London, 1967. 

Aelterman, W. De Kimpe, N. Tyvorskii, V. Kulinkovich, O. Synthesis of 2,3-disubstituted pyrroles and pyridines from 3-halo-l-azaallylic anions./. Org. Chem. 2001, 66, 53-58. 

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