Hydrovinylation enantioselective

Figure 5.4-3 shows the results of a lifetime study for Wilke s catalyst dissolved, activated, and immobilized in the [EMIM][(CF3S02)2N]/compressed CO2 system. Over a period of more than 61 h, the active catalyst showed remarkably stable activity while the enantioselectivity dropped only slightly. These results clearly indicate - at least for the hydrovinylation of styrene with Wilke s catalyst - that an ionic liquid catalyst solution can show excellent catalytic performance in continuous product extraction with compressed CO2. 

In addition to Rh-catalysed hydroformylation, this special phase behaviour has been successfully applied to other continuous catalytic reactions - such as Ni-catalysed, enantioselective hydrovinylation [66] and the lipase-catalysed kinetic resolution and enantiomer separation of chiral alcohols [67]. 

A further example of a reaction which may be optimised in IL/scC02 by selection of the appropriate anion for the IL is catalytic enantioselective hydrovinylation, a synthetically interesting and truly atom economic C-C bond forming reaction [77-79]. The nickel complex below has been developed by Wilke and co-workers as precursor for a highly active and enantioselective catalyst for this process. 

Figure 5.4-2 Schematic view of the continuous flow apparatus used for the enantioselective hydrovinylation of styrene in the biphasic [EMIM][(CF3S02)2N] system. The components are labeled (alphabetically) as follows C compressor, CT cold trap, D dosimeter, DP depres-surizer, F flow-meter, M mixer, MF metal filter, P HPLC pump, PT pressure transducer and thermocouple, R reactor, S styrene.

Asymmetric hydrovinylation has been pioneered by Bogdanovic [30] and Wilke [31] using nickel catalysts. Of special interest is the reaction between vinylarenes and ethylene, as enantioselective codimerization provides a convenient route to... 

Stability, activity and chemo- and enantioselectivity increased with increasing steric demand of the ortho substituent R. Introduction of the trimethylsilyl group at this position (ligand 38) therefore resulted in an excellent enantioselective system which belongs among the best Pd catalysts described so far for asymmetric hydrovinylation. Almost 70% conversion was observed within 15 min. The product was obtained in 78.5% ee and only a small amount of the isomerization products was detected in the reaction mixture. However, at higher conversions, isomerization of the product to the internal achiral olefin took place. Therefore,... 

The QUINAPHOS Ligand Family/ and its Application in Asymmetric Catalysis 269 Table 2.1.5.6 Enantioselective hydrovinylation of styrene using NaBARF as activator. 

Under particularly mild conditions, a Ni-catalyst based on (] , Sc, Sc)-26 gave quantitative conversion of 11 with 84.9% selectivity for the desired product 23 and an excellent enantioselectivity of 94.8% (S) (entry 5). Moreover, the catalyst system proved extremely efficient and remarkably robust for the hydrovinylation of 11. Almost 90% conversion and perfect chemoselectivity were achieved within 4 h at -65 °C even at a substrate/nickel ratio of 4600 1 (entry 6a). Further... 

This Mulheim chemistry has been highlighted by the discovery of the highly enantioselective hydrovinylation of styrene to produce chiral 2-phenyl-1-butene in 95.2% ee for a 10 kg-scale reaction (Scheme 60) (132). The Ni catalyst is very reactive and contains the unique chiral dimeric aminophosphine ligand derived from (R)-myrtenal and (S)-1-phenylethylamine. Computer simulations suggest that in this chiral Ni complex, the phenyl substituent of the chiral phenylethyl group acts as a windshield wiper across the catalytically active metal center. This... 

For the first time, an enantioselective cobalt-catalysed hydrovinylation of styrene was achieved with a cobalt-based system bearing a chiral bis(phosphine)amide ligand to produce PhC H(Me)CH=CH2.66... 

Spectacular enantioselection has been observed in hydrogenation (cf. Section 2.2) [3] and hydrometallation of unsaturated compounds (cf. Section 2.6) [4], olefin epoxidation (cf Section 2.4.3) [5] and dihydroxylation (cf Section 3.3.2) [6], hydrovinylation (cf Section 3.3.3) [7], hydroformylation (cf Section 2.1.1) [4a, 8], carbene reactions [9] (cf Section 3.1.10), olefin isomerization (cf Section 3.2.14) [10], olefin oligomerization (cf Section 2.3.1.1) [11], organometallic addition to aldehydes [12], allylic alkylation [13], Grignard coupling reactions [14], aldol-type reactions [15], Diels-Alder reactions [12a, 16], and ene reactions [17], among others. This chapter presents several selected examples of practical significance. 

The choice of the anion is also cmcial in systems where the active cationic species is formed from a neutral precursor, as in the case of the nickel allyl chloride catalyst used for asymmetric hydrovinylation (eq. (6) cf. also Section 3.3.3). The previously optimized conditions for this reaction involved the use of highly flammable Al2Et3Cl3 as chloride-abstracting agent and required the use of CH2CI2 at -78 °C. Using NaBARF in compressed CO2, the C-C bond coupling occurs around room temperature with excellent chemo-, stereo-, and enantioselectivity [73]. This example demonstrates nicely that the application of CO2 can have environmental benefits for catalytic processes far beyond the solvent replacement. 

The hydrovinylation reaction has its origin in the observations made in 1963 that propene dimerizes at a quite remarkable rate in the presence of certain organo-nickel catalysts and that the product distribution can be influenced by introducing auxiliary P-donor ligands [1]. In 1967 it was discovered that in the presence of the chiral ligand P( ranx-myrtanyl)3, 2-butene can be co-dimerized with propene to give 4-methyl-2-hexene in an enantioselective manner and the extension of this co-dimerization reaction to ethylene has become known as hydrovinylation. 

The reactions which have been reported are listed in Table 1 along with representative catalysts. In the presence of the appropriate ligand and under suitable conditions, many of the reactions proceed with a surprising chemoselectivity, regioselectivity, and enantioselectivity. The main side reactions are the isomerization of the primary hydrovinylation product or its further reaction with a second molecule of ethylene and the oligomerization or polymerization of the individual alkenes. These side reactions frequently become of significance only after the consumption of one of the reacting alkenes or at elevated temperatures. The hydrovinylation products are presented briefly below and this is followed by a more detailed discussion of the enantioselective control. 

The reaction has been extended to bicycloheptadiene and to bomene. In the former case, monohydrovinylation is the main reaction and is accompanied by isomerization and the formation of Cn-codimers whereas in the latter case only the isomerization product, 3-ethylidenebomane, could be isolated. Of interest in this reaction is the observation of an enantioselective hydrovinylation the (-t-)-en-antiomer of bomene in the racemic starting material reacts preferentially and the unreacted substrate becomes enriched in the (-)-enantiomer [3, 8 c]. 

The main interest in the hydrovinylation reaction lies in the generation of a new asymmetric center (eq. (1)) and considerable effort has been invested in obtaining high enantioselectivity by modifying the metal atom with optically active ligands. Selected results have been brought together in Table 2, in which only those... 

Wegner A, Leitner W. Nickel-catalysed enantioselective hydrovinylation of styrenes in liquid or supercritical carbon dioxide. Chem Commun 1999 1583-1584. 

Since the original discovery of the importance of the hemilabile ligand for high enantioselectivity, we have recognized that such effects are equally important in a number of other monophosphines that promote the hydrovinylation reaction [22]. One example is shown in Eq. (10). 

The hydrovinylation of styrene has been carried out at -60 C on an 8.26 kg (79.6 mol) scale by the Wilke group using the azaphospholene hgand 8. The yield (41%) and enantioselectivity (87.4% ee) are lower than what is observed for small-scale reactions, and further developmental efforts are needed before the reaction can be practiced on an industrial scale. The low temperature and the esoteric nature of the ligand may also limit further apphcations of this chemistry. Discovery of new protocols which yield nearly quantitative yields [12] on a laboratory scale, the use of other metals (especially palladium), and a new generation of ligands that are under investigation in several laboratories may ultimately overcome the current problems. 

The chiral hydrovinylation catalyst could be effectively immobilized for continuous operation in active and selective form by simply dissolving the Wilke catalyst in [EMIM][BTA] (BTA= (CF3S03)2N) and using compressed C02 as the mobile phase (Eq. 3). High conversion and good enantioselectivity were obtained over... 

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