One-Step Route to 2,3-Diaminopyrroles Using a Titanium-Catalyzed Four-Component Coupling

A simple indolyl titanium complex, Ti(NMe2)(2)(Ind(Me2))(2) (1), can be prepared from commercially available reagents,Ti(NMe2)(4) and 2,3-dimethylindole. Indolyl-containing 1 is a catalyst for the selective generation of unusual 2,3-diaminopyrroles from four-component coupling of 2 equiv of an isonitrile, 1 equiv of an arylamine, and 1 equiv of an alkyne in an atom-efficient process. For this study, both terminal and internal alkynes were used with a selection of arylamines to provide 11 examples of 2,3-diaminopyrroles. Yields for terminal alkyne substrates in this reaction were 75-93%. In all cases, terminal alkynes result in one or the alpha-positions on the pyrrole being unsubstituted. Two reactions with internal alkynes provided pentasubstituted pyrroles in 42% and 88% yield. Catalyst 1, along with six of the pyrrole products, was structurally characterized by single-crystal X-ray diffraction.