The design of new molybdenum and tungsten-based olefin metathesis catalysts has resulted in a number of significant improvements over current systems. The high yield syntheses of bispyrrolide-ligated Mo and W alkylidene complexes have resulted in a versatile precursor for generating new catalysts with enhanced activity. In particular, monoalkoxide pyrrolide (MAP, where the metal is ligated by one alkoxide and one pyrrolide) complexes are highly reactive olefin metathesis catalysts. These catalysts are more active than either their bispyrrolide or bisalkoxide relatives. The MAP catalysts are also attractive for chiral synthesis due to their stereogenicity at the metal center.
Prof. Schrock has recently published an informative review of the synthesis, catalysis, and applications for MAP catalysts. The availability of many imido, pyrrolide, and alkoxide precursors provide a myriad of catalyst composition combinations. In one notable example, judicious selection of ligands/catalyst allows for efficient synthesis of (+)-quebrachamine in high e.e. and yield by a key asymmetric RCM reaction step (catalyst structure shown below). Some proposed decomposition and regeneration pathways have been detailed for these systems. And finally, new applications have been introduced including well defined silica-supported catalysts, alkane metathesis, Z-selective olefin metathesis, endo-enyne metathesis, and tacticity-controlled ROMP.
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