Metathesis by Molybdenum and Tungsten Catalysts

Carbon-carbon double bonds are an integral part of the chemical industry and are widely found in natural products, from the small and simple (ethylene) to the large and complex. The ability to manipulate carbon-carbon double bonds to make other carbon-carbon double bonds in a catalytic and stereospecific fashion has revolutionized the way organic molecules and polymers are made today. This article outlines the development of modern molybdenum and tungsten alkylidene catalysts that can be designed at a molecular level to achieve a given result. Carbon-carbon triple bonds also can be manipulated in a similar manner with the appropriate alkylidyne catalyst. Although the 'alkene metathesis' and 'alkyne metathesis' reactions are now fifty to sixty years old, many problems remain that will require an even more detailed understanding of these most intricate, superficially simple reactions.