Thermal C(2)-C(6) cyclization of enyne-carbodiimides: Experimental evidence contradicts a diradical and suggests a carbene intermediate

Schmittel M., STEFFEN J., RODRIGUEZ D., ENGELEN B., NEUMANN E., Cinar M. E.

JOURNAL OF ORGANIC CHEMISTRY, vol.73, no.8, pp.3005-3016, 2008 (SCI-Expanded) identifier identifier identifier

  • Publication Type: Article / Article
  • Volume: 73 Issue: 8
  • Publication Date: 2008
  • Doi Number: 10.1021/jo701966h
  • Journal Name: JOURNAL OF ORGANIC CHEMISTRY
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus
  • Page Numbers: pp.3005-3016
  • Istanbul Technical University Affiliated: No

Abstract

Mechanistic details of the thermal C(2)-C(6) 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 C(2)-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.