Titanium-mediated cross-coupling reactions of 1,3-butadiynes with α-iminonitriles to 3-aminopyrroles: observation of an imino aza-Nazarov cyclization

Ti(OⁱPr)₄/2 ⁿBuLi-mediated highly efficient cross-coupling reactions of 1,3-butadiynes with α-iminonitriles are described. The method provides convenient access to functionalized 3-aminopyrroles with a wide diversity of substituents in a highly regioselective manner. When optically pure α-iminonitrile is employed in this reaction, a chiral pyrrole derivative is obtained without loss of enantiopurity. Mechanistic study indicates that the 3-aminopyrroles are formed upon treatment of the crude hydrolysis products with silica gel or Lewis acid. A novel imino aza-Nazarov cyclization reaction is proposed to account for the formation of the pyrrole products. Employing the titanium-monoyne complex in this reaction resulted in the homo-coupling of α-iminonitriles, leading to the formation of 1,2-diimines.