1019841-79-2Relevant academic research and scientific papers
Thermal C2-C6 cyclization of enyne-carbodiimides: Experimental evidence contradicts a diradical and suggests a carbene intermediate
Schmittel, Michael,Steffen, Jens-Peter,Rodriguez, David,Engelen, Bernward,Neumann, Elmar,Cinar, Mehmet Emin
, p. 3005 - 3016 (2008/09/19)
(Chemical Equation Presented) Mechanistic details of the thermal C 2-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-C 6 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.
