Thermal C2-C6 cyclization of enyne-carbodiimides: experimental evidence contradicts a diradical and suggests a carbene intermediate.
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.
Schmittel
Michael
M
FB 8 Chemie-Biologie, Universität Siegen, Adolf-Reichwein-Strasse, D-57068 Siegen, Germany. schmittel@chemie.uni-siegen.de
Steffen
Jens-Peter
JP
Rodríguez
David
D
Engelen
Bernward
B
Neumann
Elmar
E
Cinar
Mehmet Emin
ME
D000466:Alkadienes / Q000737:chemistry; D000586:Amination; D002234:Carbodiimides / Q000737:chemistry*; D002244:Carbon / Q000737:chemistry*; D018360:Crystallography, X-Ray; D003500:Cyclization; D005609:Free Radicals / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008697:Methane / Q000031:analogs & derivatives* / Q000737:chemistry; D015394:Molecular Structure; D013696:Temperature
D000466:Alkadienes; D002234:Carbodiimides; D005609:Free Radicals; C030011:carbene; C025947:propadiene; D002244:Carbon; D008697:Methane