Recyclable chiral catalyst, olefin

Spectacular achievements in catalytic asymmetric epoxidation of olefins using chiral Mnm-salen complexes have stimulated a great deal of interest in designing polymeric analogs of these complexes and in their use as recyclable chiral catalysts. Techniques of copolymerization of appropriate functional monomers have been utilized to prepare these polymers, and both organic and inorganic polymers have been used as the carriers to immobilize these metal complexes.103... 

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

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 ... 

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)...

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. 

The single-pot biomimetic synthesis of chiral diol 27, mediated by the recyclable trifunctional catalyst, Na2PdCl4-K20s04-Na2W04 embedded in a matrix of layered double hydroxides, resulted in an efficient, low cost process. This protocol provided the desired prochiral olefin from Heck precursors and the in situ recycling of NMM to NMO. The whole process starts with cheap precursors and minimizes the number of unit operations. Other similar broad, interdisciplinary studies which led to an extraordinarily selective solution to the splitting of the amide bond in a mixture of compounds from a natural source will be considered in the next section. 

Many attempts were undertaken to produce chiral epoxides for chemical syntheses. This can be achieved by the use of chiral catalysts. The first applicable and relatively simple procedure of chemical chiral epoxidations was described by Katsuki and Sharpless [2], later called the Katsuki-Sharpless epoxidation. In this reaction, allyl alcohols are epoxi-dized in the presence of tartrate esters, e.g., (—)-diethyl tartrate. This allows the production of either (/ )- or (S)-epoxides depending on the selection of (R)- or (5)-tartrate ester as chir additive. However, the reaction is limited to ally lie alcohols and is somewhat sensitive to steric hindrances. In the meantime, a number of different catalysts have been developed for the epoxidation of cw-alkenes. The Jacobsen-Katsuki reaction allows the epoxidation of fran5-alkenes and terminal olefins [3]. All of these approaches, however, are limited to the epoxidation of activated double bonds like allylic alcohols or require expensive catalysts, and usually the regiospecificity of these reactions is not sufficient for practical applications. Furthermore, the chiral catalysts, although usually they can be recycled, are often very exj nsive. 

To summarize, chiral heterogeneous catalysts were prepared from rhodium-diphosphine complexes and aluminum-containing mesoporous materials. The bonding occurred via an ionic interaction of the cationic complex with the host. These catalysts were suitable for asymmetric hydrogenation of functionalized olefins. The catalysts can be recycled easily by filtration or centrifugation with no significant loss of activity or enantioselectivity. 

In conclusion, chiral heterogeneous catalysts are prepared from chiral Rhodium diphosphine complexes and Al-MCM-41. The bonding supposedly occurs via an ionic interaction of the cationic complex with the host. Also a slight reduction of weak acidic sites of Al-MCM-41 has been observed. These catalysts are suitable for the hydrogenation of functionalised olefins. The organometallic complexes remain stable within the mesopores of the carrier at reaction conditions. The catalyst can be recycled by filtration or centrifugation. 

Good to excellent enantioselectivity was achieved in the epoxidation of mainly cyclic olefins with the chiral salen-catalyst 52 immobilised in [C4Ciim][PF6], but selectivity deteriorated upon catalyst recycling, see Scheme 5.6.[48] Relative to molecular solvents, higher reaction rates were observed even under biphasic conditions when the epoxidation reaction was carried out in the presence of an ionic liquid. UV-VIS spectroscopic1341 and cyclovoltammetric[49] studies suggest that the commonly observed superior reaction rates are a reflection of the solvent s ability to stabilise the active metalla-oxo intermediate. 

A so far still unsolved problem is the direct enantioselective epoxidation of simple terminal olefins. For example the epoxidation of propylene that was achieved with a 41% ee almost twenty years ago by Strukul and his coworkers using Pt/diphosphine complexes is still unsurpassed. Unfortunately such low ee s are of no practical interest. The problem was circumvented by Jacobsen using hydrolytic kinetic resolution of racemic epoxides (Equation 26) and is practised on a multi 100 kg scale at Chirex. The strategy used is to stereose-lectively open the oxirane ring of a racemic chiral epoxide leaving the other enantiomer intact. Reactions are carried out to a 50% maximum conversion. The catalyst belongs to the metal-salen class described above and can be recycled. The products are separated by fractional distillation. 

---