Development of Catalysts for Metathesis

Several catalysts have been introduced since the discovery of alkene metathesis. Catalysts generated by combinations of MoC15 and WClg with alkylaluminium (RnAlCl3 ) were the first-generation catalysts, which were not suitable for functionalized alkenes. 

Further developments have been made possible by the introduction of third-generation catalysts. These are the commercially available, highly active Schrock-type Mo-based catalyst 19 and W catalyst 20, coordinated by a bulky neopentylidene group and alkoxides [2], They are well-defined, single-component catalysts. The W complex 20 is very active, but the Mo complex 19 is more versatile. These catalysts have some 

Reaction of the complex 24 with terminal alkene 25 generates styrene and the real catalytic species 27 via the ruthenacyclobutane 26. The complex 24 is commercially available, active without rigorous exclusion of O2 and water, and has functional group tolerance. Carbonyl alkenation is not observed with the catalysts 22 and 24. Their introduction has enormously accelerated the synthetic applications of alkene metathesis [11]. 

The new carbene complex 31 can be prepared more easily by reaction of Ru hydride 28 with propargyl chloride 29. Insertion of the triple bond to Ru—H, followed by y-chloro elimination from 30 affords the carbene complex 31 [12], As another method, carbene complex 34 is obtained by oxidative addition of the Ru complex to benzal 

Synthetic reactions via transition metal carbene complexes 

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