Catalysts in metathesis reactions

A bulkyl silsesquioxane has been used to prepare metathetically active Mo complexes [91]. A silsesquioxane mimics the manner in which a metal might be attached to silica in a heterogeneous metathesis system. The conjugate acid of the silsesquioxane appears to have a relatively high pKa, a hint that the silsesquioxane itself may be relatively electron withdrawing. Silsesquioxane complexes were found to be highly active catalysts in metathesis reactions. 

SCCO2 has also been used as a solvent with a silica-immobilized catalyst in metathesis reactions [25]. A heterogeneous catalytic process is developed, in which catalyst leaching is avoided but the reactivity is lower than when using a homogeneous catalyst This application has also been extended to continuous-flow processes for hydrogenation [26], Friedel-Crafts alkylations [27], etherification [28], and hydroformylation [29] reactions. 

The Phillips triolefin process [56] developed at Phillips Petroleum, used a heterogeneous WOj/SiOj catalyst in metathesis reaction to convert propene 127 into mixture of ethene 125 and 2-butene 126. As it is a reversible reaetion (Scheme 9.32) and the price of propene rose high, the reverse reaction of Philips process offered is now by using Lummus teehnology to produce propene known as Olefin Conversion Technology (OCT). 

Because in metathesis reactions with most catalyst systems a selectivity of nearly 100% is found, a carbene mechanism seems less likely. Banks and Bailey ( ) reported the formation of small quantities of C3-C6-alkenes, cyclopropane, and methylcyclopropane when ethene was passed over Mo(CO)6-A1203, which suggests reactions involving carbene complexes. However, similar results have not been reported elsewhere most probably the products found by Banks and Bailey were formed by side reactions, typical for their particular catalyst system. 

Intermolecular enyne metathesis has recently been developed using ethylene gas as the alkene [20]. The plan is shown in Scheme 10. In this reaction,benzyli-dene carbene complex 52b, which is commercially available [16b], reacts with ethylene to give ruthenacyclobutane 73. This then converts into methylene ruthenium complex 57, which is the real catalyst in this reaction. It reacts with the alkyne intermolecularly to produce ruthenacyclobutene 74, which is converted into vinyl ruthenium carbene complex 75. It must react with ethylene, not with the alkyne, to produce ruthenacyclobutane 76 via [2+2] cycloaddition. Then it gives diene 72, and methylene ruthenium complex 57 would be regenerated. If the methylene ruthenium complex 57 reacts with ethylene, ruthenacyclobutane 77 would be formed. However, this process is a so-called non-productive process, and it returns to ethylene and 57. The reaction was carried out in CH2Cl2 un-... 

Numerous catalysts and metathesis reactions have been reported and it is not possible to do justice to all authors, not even to those who have contributed to the development of the mechanistic proofs summarised above. In Figure 16.9 we have collected a schematic overview of the type of reactions that are usually distinguished in metathesis catalysis. 

The M(C0)6 (M = Cr, Mo, W) stable carbonyls have been used to prepare metal supported catalysts of elements of group 6 that have been used as catalysts in several reactions, such as metathesis, water-gas shift, CO hydrogenation and olefin hydrogenation and polymerization [15-24]. Table 8.2 compiles several examples in which M(CO)s (M = Cr, Mo, W) compounds are used as an alternative for preparing chromium-molybdenum or tungsten-based catalysts. 

Trost and Tanoury found an interesting skeletal reorganization of enynes using a palladium catalyst.In this reaction, the second product is derived from a metathesis reaction (Equation (5)). It was speculated that the reaction would proceed by oxidative cyclization of enynes with the palladium complex followed by reductive elimination and then ring opening. To confirm this reaction mechanism, they obtained a compound having a cyclobutene ring, which was considered to be formed by the reductive elimination (Equation (6)). 

The catalytic cycle proposed for the rhodium-porphyrin-based catalyst is shown in Fig. 7.18. In the presence of alkene the rhodium-porphyrin precatalyst is converted to 7.69. Formations of 7.70 and 7.71 are inferred on the basis of NMR and other spectroscopic data. Reaction of alkene with 7.71 gives the cyclopropanated product and regenerates 7.69. As in metathesis reactions, the last step probably involves a metallocyclobutane intermediate that collapses to give the cyclopropane ring and free rhodium-porphyrin complex. This is assumed to be the case for all metal-catalyzed diazo compound-based cyclo-propanation reactions. 

A classical catalyst for metathesis reactions 1 reminiscent of a polymerization Ziegler catalyst it is essentially a combination of a transition metal halide (WCU. MoCl ) and an alkyl metal derivative (AIR). SnK. etc). It is noteworthy that a reduction step occurs during the constitution of the active center, since an elTicieni metathesis of terminal olefins has been achieved under electro-catalytic conditions [40]. (CI4 being the active carbenic entities in... 

The example in Eq. 6.11 demonstrates the compatibility of the catalyst system with a variety of protic and basic groups that were not possible to use in metathesis reactions until the advent of the ruthenium systems. 

Although the synthetic utility of radical anion pericyclic processes is still to be explored, the recently disclosed intermediacy of radical anions in metathesis reactions with Grubb s catalyst [360] should ignite the search for further examples of this interesting class of reactions. 

In the next two sections we limit our analysis to a few recent examples of the use of water or ionic liquids in metathesis reactions using new technical solutions or ionically tagged catalysts. 

One of the more attractive features of this methodology is the ability to form macrocyclic compounds. In Ley s synthesis of (+)-aspicilin,354 435 was treated with 10% of the Grubbs catalyst to give a 73% yield of 437 as a 1.5 1 mixture of Z/E) isomers, along with 26% of a byproduct (438) formed by metathesis across the double bond of the conjugated ester. With other ruthenium catalysts, the metathesis reaction can proceed with... 

Lamaty and coworkers used PEG-bound Ru catalysts for metathesis reactions [71]. In contrast to solid-phase-bound catalysts, the soluble polymeric support allowed for analysis which provided information on the recycling capacities of the catalyst while the activity remained high, the return of the metal on the supported ligand was not total. 

Due in large part to the development of ruthenium catalysts, olefin metathesis reactions can now be carried out on a diverse array of functionalized electron-rich and electron-poor olefins. As we have described, mechanistic analysis was instrumental in the design of more highly active second generation catalysts with expanded substrate scope, which was achieved by proper differentiation of the two L-type ligands within the (L)2(X)2Ru=CHR framework. Further investigations have revealed that these new catalysts display several unexpected features, and mechanistic analysis continues to be an invaluable tool for understanding reactivity patterns and for the development of new catalyst systems. 

---