|M.Sc Student||Helena Chechik|
|Subject||Stereospecific and Stereoselective Preparation of Vinyl|
|Department||Department of Chemistry||Supervisor||Full Professor Marek Ilan|
Although large numbers of methods are known for the preparation of sp2 organometallic derivatives, their preparation from heterosubstituted alkenes was still troublesome. Recently, a new concept, based on an addition-b-elimination sequence with zirconium complexes, was found in our group.
The reaction is stereoselective and whatever the stereochemistry of the hetero-substituted alkene, only the E-metallated vinyl zirconocene is obtained. Stereoselectivity can be explained by isomerization of the zirconacyclopentane intermediate. Since it is well established that sp3 hetero-substituted organometallic geminated to carbamate are configurationally stable, we were interested to investigate the unknown vinyl carbamate into vinyl zirconium transformation. Is this a stereospecific or stereoselective reaction?
First, the zirconation reaction was performed on a mixture of E and Z vinyl carbamate. We found that the reaction proceeds with a good level of stereospecificity. In order to demonstrate the stereospecificity of the zirconation reaction, it should be performed with pure E and Z vinyl carbamates. The preparation of E vinyl carbamate is unknown, in contrary to the preparation of Z vinyl carbamate, which was described in the literature. The synthesis of E vinyl carbamate was developed. It was prepared by a carbocupration reaction of alkynyl carbamate with absolute regio- and stereoselectivity. The zirconation reaction was performed with pure Z and E vinyl carbamates. We found that the reaction is stereospecific in 85% when the zirconation reaction is performed with Z vinyl carbamate. The same reaction, performed with E vinyl carbamate, is absolutely stereospecific.
In order to understand the mechanism of the reaction, some efforts to trap the reaction intermediate has been done. While attempting to trap the intermediate, a new intramolecular cyclization reaction was observed that lead to the formation of metallated cyclobutane. The existence of the metallated cyclobutane was shown by hydrolysis (H+) and iodinolysis (I2) of the reaction.
An additional target of this research, was the transformation of hetero-substituted alkenes into vinylic organometallic species with catalytic amount of zirconocene derivatives.
The catalytic zirconation proceeded smoothly with a wide range of hetero-substituted alkenes and in all cases, after hydrolysis, the protonated olefins were obtained. When iodine was added as an electrophile, no vinyl iodide was observed.