Organic Synthesis Using Metal‐Mediated Metathesis Reactions

Abstract

Olefin metathesis is an established reaction, yet it is still growing and diversifying at a remarkable rate. Since 2000, there has been notable progress in a number of areas, especially the development of new catalysts, many of which have N‐heterocyclic carbenes modifying ruthenium. These include phosphine‐free ruthenium catalysts that broaden the scope of efficient ruthenium‐catalyzed olefin metathesis, as well as catalysts with N‐heterocyclic carbenes and electron‐poor phosphines having turnover frequencies >100‐fold faster than the original Grubbs catalyst. In concert with this, some excellent mechanistic experiments and theoretical work have better elucidated the structure‐reactivity relationship and subtle but important points about mechanism relative rates of phosphine. Association, dissociation, and formation of the metallacyclobutane of the ruthenium‐catalyzed reactions are leading the way to further improvements in ruthenium catalysts. Many methodological applications have also been developed, including improvements in the scope of hetereocycle and macrocycle formation. Cross metathesis (CM) has also undergone a transformation to a high yielding and stereoselective reaction that can be performed with predictable outcomes of yield and alkene stereochemistry. Great strides have been made in asymmetric olefin metathesis reactions, especially desymmetrization and kinetic resolution, with the majority of the results reported using molybdenum‐based catalysts with C 2 ‐symmetric chiral bisaryloxy based ligands. In addition, successful ruthenium‐based catalysts with C 2 ‐ and C 1 ‐symmetric ligands have been reported. Numerous reports of Tandem and sequential olefin metathesis reactions have been published, often in one‐pot, and sometimes utilizing the same metal complex for multiple operations, like ring closure/hydrogenation. A number of examples of tandem reactions utilizing a palladium catalyst and ruthenium‐catalyzed olefin metathesis have also been reported as very efficient routes to desirable organic building blocks. One of the areas that exploited olefin metathesis is natural product synthesis. There are an ever increasing number of reports that cite olefin metathesis as a key, often step saving, reaction in the successful completion of target molecules by ring closing or cross metathesis. The synthetic advantages the olefin metathesis reaction have also been employed by materials and polymer chemists as part of design strategies for constructing well‐defined polymers, substrates for combinatorial chemistry, and metal‐containing rings. The olefin metathesis catalysts have been attached to a variety of supports for the purpose of recycling. Many groups have examined methods for tethering both molybdenum and ruthenium‐based catalysts through reversibly coordinating ligands, the alkylidene and the backbone of nonlabile ligands, with reports of high efficiency recycling of up to 10 times.