Oligoene-Based π-Helicenes or Dispiranes? Winding up Oligoyne Chains by a Multiple
Bastian Milde1, Markus Leibeling1, Alexander Hecht2
1Institut für Organische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106 Braunschweig (Germany).
Abstract:
A domino approach consisting of up to five consecutive steps to access either highly substituted dispiranes or π-helicenes from oligoyne chains is reported. The domino sequence consists of several carbopalladation reactions, a Stille cross-coupling to obtain the helicenes, and, depending on the steric demands of the helicene, a final 6π-electrocyclization to afford the dispiranes. Formally, the latter transformation contravenes the Woodward-Hoffmann rules, as revealed by X-ray crystallography of the dispirane. Additionally, the racemization barrier of the (Z,Z,Z)-triene-based helicene has been determined by a kinetic analysis and compared with results from density functional theory calculations. Characteristic points on the reaction coordinate were further analyzed according to their relaxed force constants (compliance constants).
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