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, cilt.73, sa.8, ss.3005-3016, 2008 (SCI-Expanded) identifier identifier identifier

  • Yayın Türü: Makale / Tam Makale
  • Cilt numarası: 73 Sayı: 8
  • Basım Tarihi: 2008
  • Doi Numarası: 10.1021/jo701966h
  • Dergi Adı: JOURNAL OF ORGANIC CHEMISTRY
  • Derginin Tarandığı İndeksler: Science Citation Index Expanded (SCI-EXPANDED), Scopus
  • Sayfa Sayıları: ss.3005-3016
  • İstanbul Teknik Üniversitesi Adresli: Hayır

Özet

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.