Olefin metathesis is an irresistable way to make C=C bonds catalytically in organic molecules and polymers, both as a consequence of its very nature, i.e., the synthesis of C=C bonds from C=C bonds, and because of the control that can be exercised through the use of well-defined alkylidene complexes as catalysts for those reactions. In the last several years we have been especially interested in exploring MonoAryloxide Pyrrolide (MAP) imido alkylidene complexes of Mo and W for Z-selective olefin metathesis reactions, especially complexes that contain sterically demanding 2,6-terphenoxides such as HMTO (O-2,6-(2,4,6-Me3C6H3)2C6H3), HIPTO (O-2,6-(2,4,6-i-Pr3C6H3)2C6H3), DFTO (O-2,6-(C6F5)2C6H3), or related monoprotected biphenolate ligands. Mo(NC6F5)(CHCMe2Ph)(DFTO)2 complexes attracted our attention because analogous bisHMTO or bisHIPTO complexes could not be prepared. We also have begun to explore tungsten oxo alkylidene complexes, which were the first type of high oxidation state alkylidene species to be prepared more than 25 years ago. Highlights of some recent work on Mo- and W-based olefin metathesis catalysts will be discussed.
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 Schrock, R. R. Dalton Trans. 2011, 40, 7484-7495.