title: Catalytic formal [2+2+1] synthesis of pyrroles from alkynes and diazenes via Ti(II)/Ti(IV) redox catalysis.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Minnesota - Twin Cities, 207 Pleasant St SE, Minneapolis, Minnesota 55455, USA.
authors.forename: Zachary W
authors.initials: ZW
authors.lastname: Gilbert
authors.affiliation: Department of Chemistry, University of Minnesota - Twin Cities, 207 Pleasant St SE, Minneapolis, Minnesota 55455, USA.
authors.forename: Ryan J
authors.initials: RJ
authors.lastname: Hue
authors.affiliation: Department of Chemistry, University of Minnesota - Twin Cities, 207 Pleasant St SE, Minneapolis, Minnesota 55455, USA.
authors.forename: Ian A
authors.initials: IA
authors.lastname: Tonks
mesh_terms: D000480:Alkynes / Q000737:chemistry*; D002384:Catalysis; D007094:Imides / Q000737:chemistry*; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011758:Pyrroles / Q000138:chemical synthesis* / Q000737:chemistry; D014025:Titanium / Q000737:chemistry*
chemical_list: D000480:Alkynes; D007094:Imides; D011758:Pyrroles; D014025:Titanium; C038867:diazene