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, pp.66-84, 2007 (Journal Indexed in SCI) identifier

  • Publication Type: Article / Article
  • Publication Date: 2007
  • Title of Journal : ARKIVOC
  • Page Numbers: pp.66-84

Abstract

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.