Pyrroles autoxidation

The polymeric pyrrolic autoxidation products probably result from the oxidized monomeric systems, which are analogous in structure to those isolated from photooxidation and peroxide oxidation reactions. Thus, for example, analysis of the products of the autoxidation of 1-methylpyrrole (Scheme 47) would suggest that 1 -methyl-A3-pyrrolin-2-one (153) is initially formed from a radical reaction of the pyrrole with triplet oxygen. This reaction sequence should be compared with that proposed for the oxidation of pyrroles with hydrogen peroxide (Scheme 50), which yields (181), (182) and (183) as the major isolable products. The acid-catalyzed reaction of a pyrrole with its oxidation product e.g. 153) also results in the formation of polymeric material and the formation of pyrrole black is probably a combination of oxidation and acid-catalyzed polymerization processes. 

Zhu M, Spink DC, Yan B, Bank S, DeCaprio AP. 1994. Formation and structure of crosslinking and monomeric pyrrole autoxidation products in 2,5-hexane-dione-treated amino acids, peptides, and protein. Chem. Res. Toxicol. 7 551-58... 

Complex reactions occur on the autoxidation of pyrroles (see Section 3.05.1.4) predictably, susceptibility to autoxidation increases with increasing alkyl substitution, llie photosensitized reaction of pyrrole and oxygen yields 5-hydroxy-A -pyrrolin-2-one, probably by way of an intermediate cyclic peroxide (Scheme 28) (76JA802). 

From the preceding examples it can be seen that oxidants and electrophilic reagents attack pyrroles and furans at positions 2 and 5 in the case of indoles the common point of attack is position 3. Thus autoxidation of indoles e.g. 99) gives 3-hydroperoxy-3H-indoles (e.g. 100). Lead tetraacetate similarly reacts at the 3-position to give a 3-acetoxy-3H-indole. Ozone and other oxidants have been used to cleave the 2,3-bond in indoles (Scheme 30) (81BCJ2369). 

Pyrrole itself is very easily converted by acid into intractable and readily autoxidized polymers. In this case and in the case of the alkyl pyrroles, it is important to distinguish between anaerobic acid-catalyzed reactions and autoxidative polymerizations the decomposition of pyrrole and its alkyl derivatives on standing in air belongs to the latter type, this review is concerned only with the former. 

In contrast wdth pyrrole, the polymerization does not appear to go beyond the trimer stage, any amorphous material produced being the product of autoxidation. 

The parent heterocycles (Table 13) display a strong band near 220 nm with one additional band at longer wavelengths for thiophene and selenophene, and two for tellurophene. Analogous weak bands reported in the older literature for furan and pyrrole are now generally accepted as arising from autoxidation products. 

The solution spectrum of pyrrole displays a strong band at ca. 210 nm (Table 23). The weak band at ca. 240 nm reported in the older literature is now generally accepted as arising from autoxidation products. Substitution of methyl for hydrogen generally leads to... 

Some electron-rich arenes or heteroarenes undergo SET even at room temperature when exposed to the air. Such compounds will usually darken quickly, even if only trace amounts of oligomers are formed by autoxidation, because these oligomers can absorb visible light very efficiently and tend to be oxidized even more readily than the monomer. Thus, older samples of aniline, alkoxyanilines, or aminophe-nols are usually dark or black, even if analysis by 1H NMR does not reveal any impurities. Particularly air-sensitive are five-membered heteroarenes (pyrroles, furans, thiophenes) with electron-donating substituents. Some of these compounds polymerize on oxidation to yield materials with good electric conductivity (Scheme 3.19). 

N-Heterocyclics. The reaction of primary amines with the carbonyl products derived from lipid oxidation is a major pathway in lipid-protein interactions. Formation of Schiff s base intermediates followed by cyclization and rearrangement can yield imines, pyridines and pyrroles (5,15,30,31). For example, 2-pentylpyridine may result from the reaction of ammonia with 2,4-decadienal, one of the principle aldehydes from the autoxidation of linoleate (5). 

Pyrroles are unstable in the presence of oxygen. In particular alkyl-pyrroles undergo rapid autoxidation on exposure to air, even in absence of light. Because of its affinity toward oxygen, pyrrole was proposed as an antioxidant by Ziegler and Ganicke.1... 

It is difficult to establish a clear distinction between autoxidation and photo-oxidation, since it is not always reported in the literature whether the uptake of oxygen by pyrroles required the presence of natural and/or artificial light. Therefore, studies carried out without specific use of light sources are reported in this section, despite the fact that, in some cases, the structures of the products seemed to suggest an attack by singlet oxygen. 

Under the same working conditions, pyrroles with electron-withdrawing substituents such as CHO, CN, COR, COOR, and COOH withstand autoxidation 2-methyl-, 2,3-dimethyl-, and 2-methyl-4-ethyl-pyrroles gave no isolated oxidation products.7... 

These are the only two examples of anionic autoxidation in the pyrrole series. Many other organic compounds behave similarly with molecular oxygen in solution of strong bases.15... 

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