Hydrovinylation of styrene

An example of a biphasic, Ni-catalyzed co-dimerization in ionic liquids with weakly coordinating anions has been described by the author s group in collaboration with Leitner et al. [12]. The hydrovinylation of styrene in the biphasic ionic liq-uid/compressed CO2 system with a chiral Ni-catalyst was investigated. Since it was found that this reaction benefits particularly from this unusual biphasic solvent system, more details about this specific application are given in Section 5.4. 

Figure 5.4-3 Lifetime study of Wilke s catalyst in the hydrovinylation of styrene, activated and...

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. 

Schemes Hydrovinylation of styrene with Wilke s catalyst 13 in an IL/SCCO2 biphasic system...

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.

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

Based on the formal analogy between the intermolecular hydrovinylation and the intramolecular cycloisomerization process, we have chosen catalysts with proven potential for the first reaction type [48, 51] as the starting point of our study. The results are summarized in Table 2.1.5.7 [64]. Despite its excellent performance in the hydrovinylation of styrene [51], the [ Ni(allyl) Br 2]/(Ra, Sc, Sc)-26/NaBARF system led to disappointingly low conversions and selectivities in the cycloisomerization of 27a (entry 1). Similarly, the [ Ni(allyl)Cl 2]/(Ra,Rc)-4cel/Na-BARF system is not effective for the cycloisomerization of 27a (entry 2) even though it is able to promote the hydrovinylation. The other diastereomer, (R ,Sc)-4cel, however, which forms an active nickel catalyst for styrene oligomerization... 

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... 

It was shown earlier that palladium-catalyzed hydrovinylation of styrene using phosphino ester-type ligands leads to isomerization of the external al-kene (kinetic product) to the internal alkene (thermodynamic product) at higher substrate conversion. In this regard, the idea was to suppress this isomerization by running the reaction at lower conversion in a CFMR system in order to minimize the catalyst-substrate contact time. 

Scheme 5 Schematic synthetic pathways and structure of dendritic P,0 ligands applied in the palladium-catalyzed hydrovinylation of styrene...

Effect of Counterions on the Hydrovinylation. In the hydrovinylation of styrene using a ligand with a hemilabile atom... 

Table 2 Effect of counterions on the hydrovinylation of styrene using hemilabile ligand...

A D-allosamine-derived monophosphinite 128 served as a good chiral ligand for the asymmetric Ni(0)-catalyzed hydrovinylation of styrene derivatives 140 with ethylene as shown in O Scheme 36 [126,127,128]. Among some 2-acetamido-2-deoxy-3-0-diarylphosphinyl derivatives prepared from D-hexopyranoses, the ligand 128 revealed a useful level of enantios-... 

For some of them, the use of membrane reactors for their recovery or application in continuously operated reactors has been demonstrated. Examples include the use of dendrimer-bound nickel catalysts for the Kharasch addition [54, 59] and dendritic palladium catalysts for an allylic substitution [73, 60]. The membrane reactor concept has also been transferred to reactions at higher pressure, as shown for the hydrovinylation of styrene (cf. Section 3.3.3) [75]. Modem ultra-and nanofiltration membranes allow an effective recovery of the homogeneously soluble catalyst. However, in some cases the long-term stability of the catalyst under operating conditions has to be improved. 

These results, however, should be contrasted with those observed earlier for the hydrovinylation of styrene using a similar catalyst activated by P(menthyl)2Pr [4] the optical yield was found to decrease in the order... 

Figure 3. A schematic representation of the mechanism of the nickel-catalyzed hydrovinylation of styrene [8h].

This is shown schematically in Figure 3 for the hydrovinylation of styrene the individual steps will be discussed further for the reaction catalyzed by [(7/ -C3H5)NiCl]2-(/ , )-l-EtAlCl2. 

Arguments similar to those presented above for the hydrovinylation of styrene will dictate the stereochemical course of the reactions involving the other alkenes investigated. Thus, interference of the methylene bridge of a complexed bicyclo-heptene molecule (25) with the substituents on the donor atom and with the complex anion will direct the course of the hydrovinylation reaction to the exo-isomer of (+)-( 5,25,4/ )-2-vinylbicycloheptane (eq. (4)) [3], while the arrangement of the five-membered ring with respect to the coordination plane will result in the conversion of cyclopentadiene into (-)-(f )-3- vinylcyclopentane. 

A continuous-flow method for asymmetric catalysis in an SCF/IL system was reported by Leitner s group (144), with the hydrovinylation of styrene [Eq. (31)] as the test reaction. The SCCO2 solution of styrene and ethylene was continuously bubbled up through a column of ionic liquid containing the catalyst. The enantio selectivity was found to be high (in one of the ILs) and catalyst stability was enhanced due to the fact that there was a constant concentration of substrate in the system the catalyst was unstable in ILs in the absence of the olefins. 

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

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. 

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