|M.Sc Student||Irena Zouev|
|Subject||New Synthesis of Polysubstituted Allenes|
New Developments in Organozirconium Chemistry
|Department||Department of Chemistry||Supervisor||Full Professor Marek Ilan|
The most popular method for the preparation of allenes is the treatment of propargylic derivatives with organocopper reagents. We wanted to develop a general approach for the preparation of allenes that would avoid the formation of non-polar side products in the reaction mixture. It was found that carbocupration of alkynyl derivative followed by zinc carbenoid homologation and a spontaneous syn β-elimination reaction give the corresponding allene in a one-pot operation. This combined three-step reaction afford mono-, 1,1-, 1,3-, 1,1,3-trisubstituted and functionalized propadienes in excellent isolated yields. The separation of the final allene is very easy, since no isomeric alkynes can be formed as nonpolar products. A thermodynamic equilibration (or deracemization) before the syn β-elimination opens a new route to chiral 1,3-disubstituted allenes.
Reactions of α,β-disubstituted vinyl sulfones with zirconocene derivatives were studied.
It was found that the presence of a phenyl group induces a particular stabilization of zirconacycle intermediate, that leads to complete stereoselective formation of vinylzirconium derivative. In absence of this stabilization (as in the case of two alkyl groups) isomerizations occur. Treatment of α, β-substituted sulfones with remote phenyl group with zirconocene derivative leads to the isomerization towards the most stable alkene with the double bond conjugated with the phenyl group.The double bond isomerization mechanism is supposed to be based on allylic C-H activation, followed by the reduction of the sulfonyl moiety presumably via single electron transfer mechanism.