Coupling Olefins With Z-Selectivity

Terminal olefins can be coupled to selectively produce the Z-alkene, thanks to highly reactive monoaryloxide-pyrrolide (MAP) olefin metathesis catalysts with molybdenum or tungsten at their core (J. Am. Chem. Soc., DOI: 10.1021/ja908098t). The work is one of a growing number of applications for MAP metathesis catalysts to come from the collaboration of MIT’s Richard R. Schrock and Boston College’s Amir H. Hoveyda. With as little as 0.2% catalyst, the researchers were able to couple a range of terminal olefins to give the Z-stereochemistry in as high as 98% yield of the resulting alkenes. This is the first time that the Z-isomers of homocoupled acyclic products have been formed in such high percentages, according to the researchers. They think the Z-selectivity arises from the steric influence of the catalyst’s large aryloxide ligand. In the course of the metathesis reaction, an intermediate metallacyclobutane is formed. The cramped environment created by the aryloxide allows formation of only metallacyclobutanes with cis substituents, which in turn generate exclusively Z-products.