Olefin Metathesis: A Powerful Reaction Begins to Reach Its Potential

Although olefin metathesis appears to many organic and polymer chemists to be a new reaction, the transformation was first recognized in the 1960s. As were many catalytic reactions, this transformation was discovered by accident while researchers were trying to expand the scope of the Ziegler and related polymerization reactions. Early catalysts were ill-defined mixtures of early transition metal halides and reducing/ alkylating agents, such as alkylaluminum compounds. These catalysts were strongly Lewis acidic and thus were only useful for reactions involving unfunctionalized hydrocarbons. Historically, progress in the development of this reaction has been directly correlated to improvements in catalysts. Only after the Chauvin mechanism involving the intermediacy of metal-carbon double bonds and metallacyclobutanes was established was it possible to explore the development of well-defined catalysts. Chauvin and Katz explored the use of Fisher carbenes as catalysts and demonstrated the basic concept of the activity of metal carbene complexes in metathesis. However, it was the work at Dupont of Tebbe and Schrock that opened the field of alkylidene chemistry and greatly expanded the horizons of metathesis. Although the Tebbe titanocene methylene complexes provided a system that was exploited for use in organic synthesis, allowed the study of metallacyclobutanes and led to the discovery of living ROMP polymerization of strained olefins, its catalytic activity was too low for most practical applications.