Autoxidation of pyrroles

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). 

Hydrogenation of pyrroles to pyrrolidines by Raney nickel proceeds only under pressure and at high temperatures. Autoxidation of pyrroles, as well as oxidation with hydrogen peroxide, can be considered addition reactions. Attack of O2 or H2O2 occurs first at the 2-position and then at the 5-position, resulting finally in the formation of maleimide or A-substitued maleimide ... 

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. 

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... 

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. 

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). 

High yields of hydroperoxypyrrolenines like (30) (Section III) were obtained by photo-oxidation of hindered pyrroles, such as 2,5-di- and 2,3,5-tri-<-butyl-pyrroles.125 The autoxidation of 2,4-dimethyl-pyrrol-5-ethyluretane yielded 50% of the hydroxyderivative (100).126 A series of tetra-arylpyrroles carrying substituents on the 2 and 5 aryl groups were oxidized with lead dioxide and the stability of corresponding free... 

Pyrrole and the simple alkyl-pyrroles are colourless liquids, with relatively weak odours rather like that of aniline, which, also like the anilines, darken by autoxidation. Pyrrole itself is readily available commercially, and is manufactured by alumina-catalysed gas-phase interaction of furan and ammonia. Pyrrole was first isolated from coal tar in 1834 and then in 1857 from the pyrolysate of bone, the chemistry of which is similar to an early laboratory method for the preparation of pyrrole - the pyrolysis of the ammonium salt of the sugar acid, mucic acid. The word pyrrole is derived from the Greek for red, which refers to the bright red colour which pyrrole imparts to a pinewood shaving moistened with concentrated hydrochloric acid. 

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