Catalysts for new tasks: preparation and applications of tunable ruthenium catalysts for olefin metathesis

A continuous survey across structures, made over the past decades, has led to the development of highly active olefin metathesis catalysts for sophisticated synthetic tasks and for polymer technology. In this paper, our efforts toward novel and improved ruthenium complexes with even better performance in olefin metathesis are described. Oxygen ether derivatives 3, pioneered by Hoveyda, exhibit high activity and possess excellent functional group tolerance. We have successfully fine-tuned catalyst 3b to increase its activity and applicability by the introduction of electron-withdrawing groups to diminish the donor properties of the oxygen atom. As a result, the stable and easily accessible nitro-substituted catalyst 6 has found a number of successful applications in various research and industrial laboratories. We were intrigued by the possibility to further fine-tune the Hoveyda-type catalysts by combining two activating effects-steric and electronic-in a single catalyst. This was possible to achieve in so-called scorpio carbenes, which are currently under investigation in our laboratory. These modifications can be used not only to control the catalyst activity, but also to alter its physical-chemical properties, such as solubility in a given medium or an affinity to silica gel. An example of immobilization strategy based on this concept is presented.