METATHESIS OF FUNCTIONALIZED ACYCLIC OLEFINS

The metathesis of functionally substituted olefins, i.e. olefins containing one or more heteroatoms, affords interesting prospects in the synthesis of valuable organic products. Since the first report on the successful metathesis of certain functional olefins, viz. long-chain fatty acid esters, much research has been done into the possibilities of this reaction. It appears that many types of functionalized olefins undergo metathesis in the presence of a suitable catalyst. Despite the fact that many metathesis catalysts are inactive, interesting results have been obtained in the past with two catalyst systems, viz. the homogeneous system WCl6-Sn(CH3)4 and the heterogeneous system Re2O7/Al2O3-Sn(CH3)4. The scope of the reaction of functionalized olefins has been studied for a number of groups. The functional groups suited for metathesis include -COOR, -OCOR, -COR, -OR, -CN, -OSiR3, -Cl etc. Unsaturated carboxylic acids and alcohols can be metathesized by introducing a protecting group, e.g. a trimethylsilyl group. New homogeneous catalyst systems developed in recent years are nearly all based on tungsten compounds. Generally, however, the heterogeneous rhenium-based catalyst system turns out to be more attractive. The catalyst is active even at room temperature and can be recovered easily. In addition it is active towards certain compounds, especially allyl compounds, that are inactive with the tungsten catalysts. An important development was the access to the metathesis of unsaturated nitriles. Here, the system Re2O7/Al2O3-SnR4 in particular gives good results. Higher activities than those observed for the Re2O7/Al2O3 catalysts can be achieved using modified rhenium catalysts. Thus, noteworthy results were obtained when a third metal oxide was incorporated and when a different support was used, e.g. silica-alumina. These developments are the subject of further study at the present time and will lead to catalysts with activities of practical interest.