טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentChechik Helena
SubjectStereospecific and Stereoselective Preparation of Vinyl
Zirconocene
DepartmentDepartment of Chemistry
Supervisor Professor Ilan Marek


Abstract

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.