The mechanism of electrophilic addition of singlet oxygen to pyrrolic ring

Pyrrolic compounds assume an important role in the chemistry of living organisms, coal surrogates and novel drugs. However, literature reports a few studies on their reactivity towards prominent oxidising agents. This contribution presents a comprehensive mechanistic study of the oxidation of unsubstituted pyrrole with singlet oxygen (O₂¹∆_g) by deploying a quantum chemical framework leading to the production of succinimide, as the major products, through a Diels–Alder addition of O₂¹∆_g to the aromatic ring. Other products such as maleimide, hydroperoxide, formamide and epoxide adducts appear to form via insignificant channels. The primary Diels–Alder channel encompasses a barrier of 41 kJ/mol with a fitted rate constant of k(T) = 1.87 × 10⁻¹³ exp(− 48,000/RT) cm³ mol⁻¹ s⁻¹. Furthermore, a kinetic study has been undertaken to investigate the influence of substituents on reaction rate of the Diels–Alder addition of singlet oxygen to a pyrrolic ring. The results clarify that electropositive substituents such as BeH and BH₂ operate as π-acceptors and thus deactivate the ring towards electrophilic attack of singlet oxygen. However, substituents comprising of strong π-donors, e.g., NH₂ and OH, destabilise the ring structure, increasing its oxidation reactivity.