The mechanism of nucleophilic substitution of alkylpyrroles in the presence of oxygen

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Abstract
The mechanism of oxidation of alkylpyrroles (1a-d) by molecular oxygen in the presence of nucleophiles is explored. Contrary to previous reports, oxidation of these pyrroles resulted in dimers with both the aromatic rings intact. In the presence of additional nucleophiles these pyrroles entered into substitution reactions. With 2-mercaptoethanol the site of substitution on 1a was the 3-position rather than the side chain. The first-order rate constant for this reaction in acetonitrile with excess oxygen was found to be (7.8 +/- 1.2) x 10(-7) s-1. The rate was unaffected by the presence of either BHT or catechol. Replacing hydrogens at all the potential sites of reaction by deuterium (as in 1aD) did not reduce the rate of substitution. However, the product suffered loss of deuterium from all sites. These observations support a mechanism involving the formation of a complex 20 between the pyrrole and triplet oxygen. Electron transfer from the pyrrole to oxygen in the rate-limiting step is followed by the generation of pyrrolylmethyl intermediate 23 that can react with available nucleophiles including unoxidized pyrroles.

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