Olefin Metathesis in Water

The ability to perform olefin metathesis reactions in water is potentially of interest, not only for the environmental concerns or the economic value of using water as a solvent but also for the ability to manipulate and further functionalize molecules of biological importance where aqueous environments are required. This is an important area of research, and it has been the focus of literature reviews [16,100]. As we saw for the case of the supported Ru catalysts described above, the advent of the well-defined catalyst systems also helped to define research in this field. Related strategies to those presented above have been employed to manipulate the catalyst architecture for the purpose of increasing the effectiveness of olefin metathesis catalysis in water. 

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Recent developments include the synthesis of new water-soluble ruthenium alkylidenes and their application to olefin metathesis in water [47, 48]. It is interesting to note that the addition of acid made the polymerization rate up to 10 times faster than without acid (Eq. 21). 

Modification via the Neutral, L-Type Ligand The first examples of well-defined Ru-catalysts for olefin metathesis in water were reported by Grubbs and coworkers [101]. A series of catalysts based on 1-2, composed of phosphine ligands containing either anionic sulfonate or cationic ammonium groups, was prepared and evaluated for each catalyst s ability to perform metathesis in water. The anionic... 

In this chapter I will cover only well-defined or well-characterized compounds. Results will be included that have appeared since reviews in 1991 on alkylidene and metalacyclobutane complexes [41] and in 1993 on ring-opening metathesis polymerization [30], but an overview of prior results that are especially relevant to olefin metathesis in particular will also be included. (An excellent and comprehensive text also has been published recently [1].) The terms well-defined or well-characterized originally were meant to imply that the alkylidene complex is isolable and is essentially identical to that in a catalytic reaction except for the identity of the alkylidene. These terms have been watered down from time to time in the literature, even to the point where they are used to describe a catalyst that is formed from a well-characterized transition metal precursor complex, but whose identity actually is not known. In this article I... 

Water-Soluble Ruthenium Alkylidenes Synthesis, Characterization, and Application to Olefin Metathesis in Protic Solvents, D. M. Lynn, B. Mohr, R. H. Grubbs, et at, J. Am. Chem. Soc. 2000, 722, 6601-6609. 

Rhenium oxide-alumina catalysts are reduced at ambient temperatures and sub-atmospheric pressure by propene and higher alkenes, generating metathesis activity. Ethylene at these conditions did not show any reduction capabilities. Reduction with CO or NH3 at 300-500° C did not result in metathesis activity. At room temperature CO did not adsorb on reduced catalysts however, NO adsorbs and is a poison for the olefin metathesis reaction. Water generated in reducing catalysts with alkenes is mainly associatively adsorbed and, at ambient temperatures, exchanges hydrogen atoms with propene and butene. Activity for double-bond isomerization is partly accounted for by associatively adsorbed water, which generates acidity. ... 

Olefin-metathesis is a useful tool for the formation of unsaturated C-C bonds in organic synthesis.186 The most widely used catalysts for olefin metathesis include alkoxyl imido molybdenum complex (Schrock catalyst)187 and benzylidene ruthenium complex (Grubbs catalyst).188 The former is air- and moisture-sensitive and has some other drawbacks such as intolerance to many functional groups and impurities the latter has increased tolerance to water and many reactions have been used in aqueous solution without any loss of catalytic efficiency. 

The synthesis and olefin metathesis activity in protic solvents of a phosphine-free ruthenium alkylidene bound to a hydrophilic solid support have been reported. This heterogeneous catalyst promotes relatively efficient ring-closing and cross-metathesis reactions in both methanol and water.200 The catalyst-catalyzed cross-metathesis of allyl alcohol in D20 gave 80% HOCH2CH=CHCH2OH. 

Conventionally, organometallic chemistry and transition-metal catalysis are carried out under an inert gas atmosphere and the exclusion of moisture has been essential. In contrast, the catalytic actions of transition metals under ambient conditions of air and water have played a key role in various enzymatic reactions, which is in sharp contrast to most transition-metal-catalyzed reactions commonly used in the laboratory. Quasi-nature catalysis has now been developed using late transition metals in air and water, for instance copper-, palladium- and rhodium-catalyzed C-C bond formation, and ruthenium-catalyzed olefin isomerization, metathesis and C-H activation. Even a Grignard-type reaction could be realized in water using a bimetallic ruthenium-indium catalytic system [67]. 

Ruthenium complexes B are stable in the presence of alcohols, amines, or water, even at 60 °C. Olefin metathesis can be realized even in water as solvent, either using ruthenium carbene complexes with water-soluble phosphine ligands [815], or in emulsions. These complexes are also stable in air [584]. No olefination of aldehydes, ketones, or derivatives of carboxylic acids has been observed [582]. During catalysis of olefin metathesis replacement of one phosphine ligand by an olefin can occur [598,809]. 

Water-soluble mthenium vinyUdene and aUenylidene complexes were also synthetized in the reaction of [ RuCl2(TPPMS)2 2] and phenylacetylene or diphenylpropargyl alcohol [29]. The mononuclear Ru-vinylidene complex [RuCl2 C=C(H)Ph)(TPPMS)2] and the dinuclear Ru-aUylidene derivative [ RuCl(p,-Cl)(C=C=CPh2)(TPPMS)2 2] both catalyzed the cross-olefin metathesis of cyclopentene with methyl acrylate to give polyunsaturated esters under mild conditions (Scheme 7.10). 

Jordan, J. R Grubbs, R. H. Small-molecule N-heterocychc-carbene-containing olefin-metathesis catalysts for use in water. Angew. Chem. Int. Ed. 2007, 46, 5152-5155. 

Although perrhenate/silica is not itself active as an olefin metathesis catalyst, the model reaction shown in Scheme 2 is of interest because the expected product, MeReOs, does not chemisorb onto silica, and can therefore be recovered. To investigate this prediction, perrhenate/silica was prepared according to a literature method (13). A sample of silica was first calcined at 1100 °C for 23 h to generate strained siloxane-2 rings (0.12/nm ), eq 1. This material was treated with Re207 vapor at 250°C under 250 Torr O2, to generate cleanly the silica-supported perrhenate in the absence of water, eq 2. 

Since the discovery of ruthenium and molybdenum carbene complexes that efficiently catalyze olefin metathesis under mild reaction conditions and that are compatible with a broad range of functional groups, olefin metathesis has increasingly been used for the preparation of alkenes on insoluble supports. In particular, the ruthenium complexes Cl2(PCy3)2Ru=CHR, developed by Grubbs, show sufficient catalytic activity even in the presence of air and water [781] and are well suited for solid-phase synthesis. 

The most important discoveries in ruthenium catalysis are highlighted and innovative activation processes, some of which are still controversial, are presented in this volume. They illustrate the usefulness in organic synthesis of specific reactions including carbocyclization, cyclopropanation, olefin metathesis, carbonylation, oxidation, transformation of silicon containing substrates, and show novel reactions operating via vinylidene intermediates, radical processes, inert bonds activation as well as catalysis in water. 

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