Olefin Metathesis in Ionic Liquids

ILs were briefly introduced in section Attachment to the Support via SILC or SILP Catalysis where they were employed for the generation of immobilized catalyst complexes via the SILP procedure. ILs can also be used in a more traditional sense, serving as recyclable reaction media. Typically, this approach uses a biphasic mixture of IL along with an organic solvent, where the IL acts as the catalyst carrier, and the organic solvent solubilizes the reaction substrates. The products, free of catalyst contamination, can be easily isolated by phase separation, and the catalyst containing IL can be recycled. 

The first studies examined refer to the transfer of classic CM protocols with Grubbs I and II generation catalysts 64 and 65 in two ionic liquids, [bmim][PF6] and [bmim][BF4]. Compared to the same reactions in CH2CI2 significant enhancements in the reactivity, yield and reaction rate were achieved. The advantage of using ILs is that the catalyst/IL system could be simply recovered and reused for at least four cycles with only a small drop in activity. 

The assistance of microwave heating has been also proposed to accelerate RCM reactions using classic ruthenium-based catalysts. The reaction can be rapidly conducted in either ionic liquids, such as [bmim][BF4], or in a micro-wave transparent solvent such as dichloromethane. 

An alternative solution to ionic tagging to confine a catalyst into an IL phase is that of using a cationic metathesis complex, e.g. ruthenium allenylidene salt (86). Treatment of diallyltosylamide at 80 °C for 5h dissolved in [bmi-m][OTf] in the presence of 2.5 mol.% of pre-catalyst 86 led to the 

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Sledz P, Mauduit M, Grela K (2008) Olefin metathesis in ionic liquids. Chem Soc Rev 37 2433-2442... 

Buijsman RC, vem Vuuren E, Sterrenburg JG (2001) Ruthenium-catalyzed olefin metathesis in ionic liquids. Org Lett 3 3785-3787... 

Audic N, Clavier H, Mauduit M, Giiillemin JC (2003) An ionic liquid-supported ruthenium carbene complex a robust and recyclable catalyst for ring-closing olefin metathesis in ionic liquids. J Am Chem Soc 125 9248-49... 

Wakamatsu H, Saito Y, Masubuchi M, Fujita R (2008) Synthesis of imidazoUum-tagged ruthenium carbene complex remarkable activity and reusability in regard to olefin metathesis in ionic liquids. Synlett 1805-1808... 

Wu B, Wen J, Zhang J, Li J, Xiang YZ, Yu XQ (2009) One-pot Van Leusen synthesis of 4,5-disubstituted oxazoles in ionic liquids. Synlett 3(137) 500-504 Wakamatsu H, Saito Y, Masubuchi M, Fujita R (2008) Synthesis of imidazolium-tagged ruthenium carbene complex remarkable activity and reusability in regard to olefin metathesis in ionic liquids. Synlett 2008 1805-1808... 

Olefin metathesis in ionic liquids 2.1 What are Ionic liquids ... 

To date a number of reactions have been carried out in ionic liquids [for examples, see Dell Anna et al. J Chem Soc, Chem Commun 434 2002 Nara, Harjani and Salunkhe Tetrahedron Lett 43 1127 2002 Semeril et al. J Chem Soc Chem Commun 146 2002 Buijsman, van Vuuren and Sterrenburg Org Lett 3 3785 2007]. These include Diels-Alder reactions, transition-metal mediated catalysis, e.g. Heck and Suzuki coupling reactions, and olefin metathesis reactions. An example of ionic liquid acceleration of reactions carried out on solid phase is given by Revell and Ganesan [Org Lett 4 3071 2002]. 

In addition to the applications reported in detail above, a number of other transition metal-catalyzed reactions in ionic liquids have been carried out with some success in recent years, illustrating the broad versatility of the methodology. Butadiene telomerization [34], olefin metathesis [110], carbonylation [111], allylic alkylation [112] and substitution [113], and Trost-Tsuji-coupling [114] are other examples of high value for synthetic chemists. 

Olefin isomerization, with Claisen rearrangement, 1, 365 Olefin metathesis with alkyllead, 9, 415 in aqueous media, 1, 834 ESI—MS studies, 1, 812 in high-throughput catalyst discovery, 1, 365 in ionic liquids, 1, 869 for polymerization characteristics, 1, 149 Grubbs catalysts, 1, 151 Schrock catalysis, 1, 150... 

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. 

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. 

The reaction is believed to proceed via a mechanism analogous to hydroamina-tion and hydrophosphination. There is experimental evidence for a rate-determining insertion step (Fig. 23). The high oxophilicity of the lanthanide ion results in a high barrier for the olefin insertion and therefore, diminished reactivity of alkenyl alcohols. Rare-earth metal triflates are also capable to catalyze cyclization of alkenyl alcohols in ionic liquids [193], although the mechanism is unlikely to be similar to the o-bond metathesis mechanism discussed above. 

EPR spectroscopy has been applied by Kucherov and coworkers to study the nature of the catalyticaUy active species in [W +(Mo"+) - ionic liquids - olefin] metathesis systems [77]. In this study the key role of isolated paramagnetic complexes of W + and Mo + ions in the catalytic metathesis of 1-hexene could be demonstrated. It... 

Abstract Olefin metathesis has found a tremendous number of application in the past 25 years. Immobilisation of olefin metathesis (pre)catalysts in room temperature ionic liquids (RTILs) offers the opportunity to recover and reuse the catalyst and also to reduce the level of ruthenium (Ru) contaminants in the products. 

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