The α‐ Versus β‐Selenylation of Pyrroles

The iodine-mediated selenylation of pyrroles with organic diselenides is proposed as a simple method to prepare the α or β selenylated pyrroles. The α→β intramolecular rearrangement is catalyzed by the released HI, but is prevented by the addition of NaHCO₃, allowing to obtain a pure α-selenylated isomer.

Abstract

The iodine-mediated reaction of a series of pyrroles with diselenides RSeSeR (R = Bu, Ph) affords the products of selenylation to the α- or β-position of the pyrrole ring in good yields. In the presence of NaHCO₃, binding the evolved HI, the α-selenylated products are selectively formed. In the absence of a base, the initial kinetic product of α-selenylation intramolecularly rearranges into the thermodynamic β-selenylated product. Pure α-selenylated products undergo the same rearrangement in the presence of acid, reaching the equilibrium at the α/β ratio of ca. 20:80, as confirmed by theoretical calculations. In the cross-experiment of N-methylpyrrole with N-(2-cyanoethyl)selenylpyrrole, an intermolecular reselenylation was observed as a 10% reaction course with respect to the α→β rearrangement.

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