Steric effects in the thermal C-2-C-6 diradical cyclization of enyne-allenes

Schmittel M., STRITTMATTER M., MAHAJAN A. A., VAVILALA C., Cinar M. E., MAYWALD M.

ARKIVOC, ss.66-84, 2007 (SCI-Expanded) identifier

  • Yayın Türü: Makale / Tam Makale
  • Basım Tarihi: 2007
  • Dergi Adı: ARKIVOC
  • Derginin Tarandığı İndeksler: Science Citation Index Expanded (SCI-EXPANDED), Scopus
  • Sayfa Sayıları: ss.66-84
  • İstanbul Teknik Üniversitesi Adresli: Hayır

Özet

The thermal reaction of enyne-allenes 1 carrying bulky substituents ( tert-butyl, trimethylsilyl, triisopropylsilyl) at the acetylene terminus uniquely leads to C-2-C-6 cyclization products in high yields independent of other structural motifs at the enyne-allene ( e. g. benzannulation, substituents at allene). The thermal cyclization can also be set up in a continous process, the latter protocol explored with a Cellular Process Chemistry (CPC) reactor. Bulky groups ( e. g. tBu, TMS, TIPS) at the alkyne terminus stabilize the enyne-allene against cyclization, while the exchange of a hydrogen by a methyl group at the inner locus of the allene unit destabilizes the enyne-allene. DFT calculations at B3LYP/6-311G** level suggest that the observed acceleration is mostly brought about by increasing the equilibrium amount of the reactive s-cis conformer.