Recyclable Metathesis Catalysts

Synthesis and Metathesis Activity of Ru-based Carbosllane Dendrimers 

As shown in Tab. 11.5, multi-component catalyst (27) matches the activity of its corresponding monomer (4), promoting efficient RCM of (19) in just 15 minutes at 40 °C. The reaction mixture was passed through a short column in methylene chloride to isolate the desired product. Subsequent washing of the silica with diethyl ether led to quantitative recovery of the dendritic catalyst. 400 MHz NMR analysis revealed that 13% of the styrene ligands on the dendrimer were va- 

How can one confirm that there is effective return of the propagating carbene to the polymer support, and that catalysts based on the bidentate styrene ether are truly recyclable  

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The search for even more active and recyclable ruthenium-based metathesis catalysts has recently led to the development of phosphine-free complexes by combining the concept of ligation with N-heterocyclic carbenes and benzyli-denes bearing a coordinating isopropoxy ligand. The latter was exemplified for Hoveyda s monophosphine complex 13 in Scheme 5 [12]. Pioneering studies in this field have been conducted by the groups of Hoveyda [49a] and Blechert [49b], who described the phosphine-free precatalyst 71a. Compound 71a is prepared either from 56d [49a] or from 13 [49b], as illustrated in Scheme 16. 

The reversible nature of cross metathesis is of synthetic importance because, by the use of a sufficiently active metathesis catalyst, it generally ensures the preferential formation of the most thermodynamically stable product. This results in the transformation of terminal olefins into internal ones, and we have seen that undesired self-metathesis products can be recycled by exposing them to a second CM process. 

I 7 7 Polymer-supported Otejin Metathesis Catalysts for Organic and Combinatorial Synthesis Tab. 11.3 Recycling of Ru complex (16) in the ring-closing metathesis of diene (17). 

Easily Recyclable Ru Catalysts for Combinatorial Synthesis 489 Tab. 11.11 Cross metathesis with eiectron-deficient oiefins cataiyzed by (67) and (69). 

Simply Assembled and Recyclable Polymer-Supported Olefin Metathesis Catalysts, L. Jafarpour, S.P. Nolan, Org. Lett. 2000, 2, 4075-4078. 

Immobilization of Olefin Metathesis Catalysts on Monolithic Sol-Gel. Practical, Efficient, and Easily Recyclable Catalysts for Organic and Combinatorial Synthesis, J.S. Kingsbury, S. B. Garber,... 

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

A chiral polymer-bound metathesis catalyst has been developed. The supported chiral complex 75d shows appreciable levels of reactivity and excellent enantios-electivity. " This complex 75d can be recycled and easily removed from unpurified mixtures. In the first and second cycles of the recycle experiment, almost the same reactivity has been shown. In the third cycle, high enantioselection and conversion are still obtained, but catalyst activity is notably diminished ... 

Hoveyda synthesized recyclable ruthenium catalyst 104b for enantioselective olefin metathesis. This catalyst is very effective for AROM-CM and can be recovered after chromatography [Eqs. (6.74)-(6.76)]. The recovered catalyst can be reused without significant loss of enantioselectivity and with similar reactivity ... 

Hoveyda and co-workers immobilized an olefin metathesis catalyst on monolithic sol-gel and claimed that the catalytic material is easily recyclable. Barrett and co-workersprepared a recyclable boomerang polymer supported catalyst for olefin methathesis by grafting the preformed catalyst to a polystyrene... 

The chiral Mo-based catalysts discussed herein are more senstive to moisture and air than the related Ru-based catalysts [1], However, these complexes, remain the most effective and general asymmetric metathesis catalysts and are significantly more robust than the original hexafluoro-Mo complex 1. It should be noted that chiral Mo-based catalysts 4,11, 25, 34 and 77 can be easily handled on a large scale. In the majority of cases, reactions proceed readily to completion in the presence of only 1 mol% catalyst and, in certain cases, optically pure materials can be accessed within minutes or hours in the absence of solvents little or no waste products need to be dealt with upon obtaining optically pure materials. Chiral catalyst 4a is commercially available from Strem, Inc. (both antipodes and racemic). The advent of the protocols for in situ preparation of chiral Mo catalyst 77, the supported and recyclable complex 82 and the debut of a chiral Ru catalyst (83) augur well for future development of practical chiral metathesis catalysts. The above attributes collectively render the chiral catalysts discussed above extremely attractive for future applications in efficient, catalytic, enan-tioselective and environmentally conscious protocols in organic synthesis. 

Rather than modifying the workup conditions, other groups have changed the reaction solvent or catalyst structure. The use of ionic liquids as a reaction solvent allows for easy catalyst and product separation as well as catalyst recycling.79 Alternatively, others have attached the various metathesis catalysts onto solid supports,80 82 which again allows for easy workup and catalyst recycle. 

Ring-opening metathesis polymer sphere-supported seco-porphyrazines efficient and recyclable photooxygenation catalysts. Journal of Organic Chemistry, 71... 

Fuchter, M.J., Hoffman, B.M. and Barrett, A.G.M. (2006) Ring-opening metathesis polymer sphere-supported seco-porphyrazines efficient and recyclable photooxygenation catalysts. Journal of Organic Chemistry, 71 (2), 724-729. 

The expense of ruthenium and the current rise of importance attached to environmentally friendly and cost-effective synthetic protocols gives the search for recyclable alkene metathesis catalysts ever-increasing impetus. Since the... 

As in many other areas of catalysis in ionic liquids, the research of the past years in ionic liquid supported olefin metathesis has been dominated by demonstrating the general feasibility in this reaction medium. With the development of the task-specific complexes new prospects have been opened and the improved recyclability of these compounds relative to the common metathesis catalysts is impressive. Apart from more active and stable catalysts, the development of enantioselective catalysts and of continuous processes are likely to be the next goals. 

A family of phosphine-free ruthenium-based olefin metathesis catalysts has been developed over the last few years. First, work done independently by Hoveyda and Blechert resulted in the H2Mes isopropoxybenzylidene (4b), a highly active air-stable ruthenium (pre)catalyst for olefin metathesis (Scheme 4). Hoveyda described (4b) as a recyclable monomeric catalyst also with high activity for ring-opening, ring-closure, and cross metathesis that tolerates... 

One of the areas gamering attention in catalysis research has been the development of green or enviromnentally benign catalytic systems. For olefin metathesis, the trend has been to develop catalytic systems that can be efficiently recycled. Success in this area has multiple implications for OM processes. First, a recyclable catalyst will give overall more turnovers per catalyst molecule, and thereby be more economical. Second, a catalyst that can be efficiently recycled (low loss of activity over repeated uses) leaches less Ruthenium into the product and thus less expensive processing costs. To this end inunobihzation of the olefin metathesis catalysts on a variety of sohd supports and utilization of nonorganic solvent systems have been explored. 

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. 

Yao, Q., Zhang, Y. Olefin metathesis in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate using a recyclable Ru catalyst Remarkable effect of a designer ionic tag. Angew. Chem., Int. Ed. Engl. 2003,42, 3395-3398. 

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