title: Conformations and Threshold Rotational Mechanisms of the (Z)-1,2-Diarylethene Moiety: A Structural Correlation Study.
abstract: The crystal structures of substituted (Z)-1,2-diarylethenes and (Z)-1,2-diarylcyclopropenes, -cyclobutenes, -cyclopentenes, and -cyclohexenes that were reported in the literature were retrieved from the Cambridge Structural Database, and the dihedral angles of the aryl rings with the double bond were plotted in a conformational map. Analysis of the data by the Structural Correlation method together with molecular mechanics calculations suggest that the conformation and the threshold rotational mechanism of the (Z)-1,2-diarylvinyl fragment strongly depend on its steric environment. In 1,2-diarylcyclopropenes the aryl rings are nearly coplanar with the double bond, whereas in 1,2-diarylcycloalkenes with larger rings a propeller conformation is adopted. The threshold rotational mechanism is the one-ring flip for (Z)-1,2-diphenylethene and 1,2-diphenylcyclobutene, and is the two-ring flip for 1,2-diarylcyclopentene and 1,2-diarylcyclohexene. The calculated rotational barriers of the aryl rings in the threshold mechanism for all systems were very low (0.2-2.4 kcal mol(-)(1)). The different conformations and rotational behaviors are dictated by an interplay of conjugation and steric effects.
authors.affiliation: Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel, Department of Chemistry, Technion, Israel Institute of Technology, Haifa 32000, Israel, and The Lise Meitner Minerva Center for Computational Quantum Chemistry, The Hebrew University of Jerusalem and the Technion, Israel.
authors.forename: Erez
authors.initials: E
authors.lastname: Gur
authors.forename: Menahem
authors.initials: M
authors.lastname: Kaftory
authors.forename: Silvio E.
authors.initials: SE
authors.lastname: Biali
authors.forename: Zvi
authors.initials: Z
authors.lastname: Rappoport