Recyclable chiral catalyst, olefin metathesis

Hoveyda, Schrock, and coworkers [19] reported using chiral cross-linking compounds immobilized on heterogeneous polystyrene resins. The chiral moiety was then used as a ligand in asymmetric catalyses. In one application, they used the material to prepare a recyclable chiral molybdenum olefin metathesis catalyst that was used in enantioselective ring opening and ringclosing metathesis reactions. The material can be illustrated as follows ... 

The First Polymer-Supported and Recyclable Chiral Catalyst for Enantioselec-tive Olefin Metathesis, K. C. Hultzsch,... 

Hoveyda and Schrock attached (97a) to polymer via attached styrene groups yielding the first reported supported chiral molybdenum olefin metathesis catalyst, (290) (Scheme 27). Supported complex (290) is less active than (97a), but it gives similar ranges of ees for enantioselective transformations like desymmetrization. The catalyst is recyclable and, even though the conversions have eroded, the enantioselectivity is still relatively high. Table 14. 

For a review of asymmetric Mo-catalyzed metathesis, see Catalytic Asymmetric Olefin Metathesis, A. H. Hoveyda, R. R. ScHROCK, Chem. Eur. J. 2001, 7, 945-950 for reports on chiral Ru-based complexes, see (b) Enantioselective Ruthenium-Catalyzed Ring-Qosing Metathesis, T.J. Sei-DERS, D.W. Ward, R.H. Grubbs, Org. Lett. 2001, 3, 3225-3228 (c) A Recyclable Chiral Ru Catalyst for Enantioselective Olefin Metathesis. Efficient Catalytic Asymmetric Ring-Opening/Cross Metathesis In Air, J. J. Van Veldhuizen, S. B. 

Keywords Asymmetric synthesis, Chiral catalysis, Mo-based catalysts, Natural product synthesis, Olefin metathesis, Recyclable catalysts, Ru-based catalysts, Supported chiral catalysts... 

More recently, we have synthesized and studied the activity of 89, the first supported chiral catalyst for olefin metathesis (Scheme 20) [28]. Catalyst 89 efficiently promotes a range of ARCM and AROM processes a representative example is shown in Scheme 20. Rates of reaction are lower than observed with the corresponding monomeric complex, but similar levels of enantioselectivity are observed. Although 89 must be kept under rigorously dry and oxygen-free conditions, it can be recycled. Catalyst activity, however, is notably diminished by the third cycle. As the data and the figure in Scheme 20 show, the product solution obtained by filtration contains significantly lower levels of metal impu-... 

Scheme 20. The first recyclable and supported chiral catalyst for olefin metathesis, 89 delivers reaction products that contain significantly less metal impurity. The two dram vials show unpurified 87 from a reaction catalyzed by 4a (left) and 89 (right)...

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