title: Gold-catalyzed cyclization of diynes: controlling the mode of 5-endo versus 6-endo cyclization--an experimental and theoretical study by utilizing diethynylthiophenes.
abstract: Herein, a dual-gold catalyzed cyclization of 3,4-diethynylthiophenes generating pentaleno[c]thiophenes through gold-vinylidenes and C-H bond activation is disclosed. Various new heteroaromatic compounds--substrate classes unexplored to date--exhibiting three five-membered annulated ring systems could be synthesized in moderate to high yields. By comparison of the solid-state structures of the corresponding gold-acetylides, it could be demonstrated that the cyclization mode (5-endo versus 6-endo) is controlled by the electronic and not steric nature of the diyne backbone. Depending on different backbones, we calculated thermodynamic stabilities and full potential-energy surfaces giving insight into the crucial dual-activation cyclization step. In the case of the 3,4-thiophene backbone, in which the initial cyclization is rate and selectivity determining, two energetically distinct transition states could be localized explaining the observed 5-endo cyclization mode by classical transition-state theory. In the case of vinyl and 2,3-thiophene backbones, the theoretical analysis of the cyclization mode in the bifurcated cyclization area demonstrated that classical transition-state theory is no longer valid to explain the high experimentally observed selectivity. Herein, for the first time, the influence of the backbone and the aromatic stabilization effect of the 6-endo product in the crucial cyclization step could be visualized and quantified by calculating and comparing the full potential-energy surfaces.
authors.affiliation: Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg (Germany), Fax: (+49) 6221-54-4205. hashmi@hashmi.de.
authors.forename: Max M
authors.initials: MM
authors.lastname: Hansmann
authors.forename: Svetlana
authors.initials: S
authors.lastname: Tšupova
authors.forename: Matthias
authors.initials: M
authors.lastname: Rudolph
authors.forename: Frank
authors.initials: F
authors.lastname: Rominger
authors.forename: A Stephen K
authors.initials: AS
authors.lastname: Hashmi
mesh_terms: D002384:Catalysis; D003500:Cyclization; D053280:Diynes / Q000737:chemistry*; D006046:Gold / Q000737:chemistry*; D006860:Hydrogen Bonding; D008962:Models, Theoretical; D015394:Molecular Structure; D013237:Stereoisomerism; D013876:Thiophenes / Q000737:chemistry*
chemical_list: D053280:Diynes; D013876:Thiophenes; D006046:Gold
keywords: CH activation; alkynes; bifurcation; density functional calculations; gold