Pyrroles are structurally important heterocycles. However, the synthesis of polysubstituted pyrroles is often challenging. Here, we report a multicomponent, Ti-catalysed formal [2+2+1] reaction of alkynes and diazenes for the oxidative synthesis of penta- and trisubstituted pyrroles: a nitrenoid analogue to classical Pauson-Khand-type syntheses of cyclopentenones. Given the scarcity of early transition-metal redox catalysis, preliminary mechanistic studies are presented. Initial stoichiometric and kinetic studies indicate that the mechanism of this reaction proceeds through a formally Ti(II)/Ti(IV) redox catalytic cycle, in which an azatitanacyclobutene intermediate, resulting from [2+2] alkyne + Ti imido coupling, undergoes a second alkyne insertion followed by reductive elimination to yield pyrrole and a Ti(II) species. The key component for catalytic turnover is the reoxidation of the Ti(II) species to a Ti(IV) imido via the disproportionation of an η(2)-diazene-Ti(II) complex.

The effect of ZnCl 2 additives on a series of (salicylaldiminato)Ni ethylene polymerization catalysts is reported. While ZnCl 2 acts solely as a pyridine scavenger for simple imine catalyst frameworks such as the biphenylimine 4, in the case of complexes containing a 2,2′-bipyridine pendent group such as 5, ZnCl 2 can coordinate to generate a bimetallic Ni/Zn active species that produces a polymer with significantly higher M n value. 5 is not catalytically active in the absence of ZnCl 2 , and control experiments indicate that Zn coordination of the bpy pocket to generate a heterobimetallic Ni/Zn complex is critical for productive catalysis to occur. A heterobimetallic Ni/ZnCl 2 precatalyst 7 has also been synthesized and structurally characterized and shows activity similar to that of the in situ bimetallic generated from 5 + ZnCl 2 .