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Thermal C2-C6 cyclization of enyne-carbodiimides: experimental evidence contradicts a diradical and suggests a carbene intermediate
Authors:Schmittel Michael  Steffen Jens-Peter  Rodríguez David  Engelen Bernward  Neumann Elmar  Cinar Mehmet Emin
Affiliation:FB 8 Chemie-Biologie, Universit?t Siegen, Adolf-Reichwein-Strasse, D-57068 Siegen, Germany. schmittel@chemie.uni-siegen.de
Abstract:Mechanistic details of the thermal C2-C6 cyclization of enyne-carbodiimides are investigated. A variety of product and kinetic studies on solvent and substituent effects open the way for a deeper mechanistic understanding. Nonlinear Hammett correlations suggest that a change of mechanism takes place: the thermal C2-C6 cyclization of enyne-carbodiimides with electron-withdrawing substituents may be best described as a coarctate cyclization to a carbene and with electron-donating substituents as a polar cyclization to a carbene with strong zwitterionic character. Theoretical investigations had originally suggested a diradical intermediate. DFT computations and NBO analysis for the parent diazafulvenediyl are in agreement with a carbene intermediate. While any intermolecular trapping of the intermediate failed, the formation of the C-H insertion product 19 strongly supports the carbene hypothesis.
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