Several highly active, recoverable and recyclable Ru-based metathesis catalysts are presented. The
crystal structure of Ru complex 5, bearing a 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene and styrenyl ether
ligand is disclosed. The heterocyclic ligand significantly enhances the catalytic activity, and the styrenyl ether
allows for the easy recovery of the Ru complex. Catalyst 5 promotes ring-closing metathesis (RCM) and the
efficient formation of various trisubstituted olefins at ambient temperature in high yield within 2 h; the catalyst
is obtained in >95% yield after silica gel chromatography and can be used directly in subsequent reactions.
Tetrasubstituted olefins can also be synthesized by RCM reactions catalyzed by 5. In addition, the synthesis
and catalytic activities of two dendritic and recyclable Ru-based complexes are disclosed (32 and 33). Examples
involving catalytic ring-closing, ring-opening, and cross metatheses are presented where, unlike monomer 5,
dendritic 33 can be readily recovered.
A Ru carbene (8, Scheme ) that contains an internal metal−oxygen chelate is an active metathesis
catalyst and is readily obtained by the sequential treatment of Cl2Ru(PPh3)3 with (2-isopropoxyphenyl)diazomethane and PCy3. This Ru-carbene complex offers excellent stability to air and moisture and can be
recycled in high yield by silica gel column chromatography. The structures of this and related complexes have
been unambiguously established by NMR and single-crystal X-ray diffraction studies.
Catalytic olefin metathesis has quickly emerged as one of the most often-used transformations in modern chemical synthesis. One class of catalysts that has led the way to this significant development are the high-oxidation-state alkylidene complexes of molybdenum. In this review key observations that resulted in the discovery and development of molybdenum- and tungsten-based metathesis catalysts are outlined. An account of the utility of molybdenum catalysts in the synthesis of biologically significant molecules is provided as well. Another focus of the review is the use of chiral molybdenum complexes for enantioselective synthesis. These highly efficient catalysts provide unique access to materials of exceptional enantiomeric purity and often without generating solvent waste.
Catalytic olefin metathesis--through which pairs of C = C bonds are reorganized--transforms simple molecules to those that are complex and precious. This class of reactions has noticeably enriched chemical synthesis, which is the art of preparing scarce molecules with highly desirable properties (for example, medicinal agents or polymeric materials). Research in the past two decades has yielded structurally well-defined catalysts for olefin metathesis that are used to synthesize an array of molecules with unprecedented efficiency. Nonetheless, the full potential of olefin metathesis will be realized only when additional catalysts are discovered that are truly practical and afford exceptional selectivity for a significantly broader range of reactions.
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