Richard Schrock (2009) - Recent Advances in Olefin Metathesis Catalyzed by Molybdenum and Tungsten Alkylidene Complexes

Richard Schrock (2009)

Recent Advances in Olefin Metathesis Catalyzed by Molybdenum and Tungsten Alkylidene Complexes

Richard Schrock (2009)

Recent Advances in Olefin Metathesis Catalyzed by Molybdenum and Tungsten Alkylidene Complexes

Abstract

In the process of preparing M(NR)(CHCMe2R')(OR")2 (R' = Me or Ph) species (or analogs that contain enantiomerically pure biphenolate or binaphtholate ligands) in situ by treating M(NR)(CHCMe2R')(pyrrolide)2 species with alcohols, we have discovered M(NR)(CHCMe2R')(OR)(pyrrolide) (MonoAlkoxidePyrrolide or MAP) complexes (M = Mo or W)'. MAP complexes are chiral at the metal by virtue of having four different covalently bound ligands. When the alcohol is enantiomerically pure then M(NR)(CHCMe2R')(OR*)(pyrrolide) diastereomers are formed. MAP species are remarkably active olefin metathesis catalysts (as much as 100x more active than the best bisalkoxide or diolate catalysts for a given transformation) and highly enantioselective. Catalyst loadings are typically <1%. A dramatic example of the efficacy of these new catalysts is Mo(NAr)(CHCMe2Ph)(Me2Pyr)(1) (where Ar = 2,6-diisopropylphenyl, Me2Pyr = 2,5-dimethylpyrrolide, and 1 is derived from (R)-1H where R = Si(t-Bu)Me2), which serves as an initiator for an asymmetric ring-closing of an intermediate in the enantioselective synthesis of the Aspidosperma alkaloid, quebrachamine, a reaction that yielded no product when asymmetric Mo(NR)(CHR')(diolate) catalysts were employed². Mo(NAr)(CHCMe2Ph)(Me2Pyr)(1) can be prepared in situ and is effective (95% ee) at relatively low loadings as a 7:1 mixture of diastereomers. New findings include (inter alia) the use of MAP complexes for Z- and enantioselective ring-opening/cross metathesis reactions,³ for the ROMP synthesis of polymers with new structures, and for Z-selective metatheses of internal olefins.

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'Schrock, R. R. Chem. Rev. 2009, 109, online 3/13.
²Malcolmson, S. J.; Meek, S. J.; Sattely, E. S.; Schrock, R. R.; Hoveyda, A. H. Nature 2008, 456, 933-937.
³Ibrahem, I; Yu, M.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131, 3844-3845.

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