Hydrovinylations

The utility of this methodology was demonstrated in the development of a two-step procedure for the direct synthesis of (5)-2-arylpropionic acids from 

In addition to the use of NaBArF, another method for activation of the nickel catalyst is the use of a Lewis acid (LA), which can abstract the halide ligand with formation of a corresponding adduct LAX acting as a weakly coordinating anion. In 2009, Leitner et al. investigated a range of available Lewis acids. 

Despite impressive progress that has been achieved in the asymmetric hydrovinylation of styrene and its derivatives, the asymmetric hydrovinylation of (a-alkylvinyl)arenes, which has a potential for being a novel methodology for the construction of chiral all-carbon quaternary centres, has not been well documented. In this context, Zhou et aL reported in 2006 the first 

The asymmetric hydrovinylation of functionalised alkenes remained a challenge until the work reported by Zhou et aL in 2010, in which chiral spiro phosphoramidite Sa,Rfi)-7 previously reported by these authors was applied to the asymmetric hydrovinylation of silyl-protected allylic alcohols. A range 

Since the first enantioselective nickel-catalysed hydrovinylation of eyelo-hexa-l,3-diene reported by Buono et ah in 1985, which was based on the use 

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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. 

Slightly later, and independently of Cole-Hamilton s pioneering work, the author s group demonstrated in collaboration with Leitner et al. that the combination of a suitable ionic liquid with compressed CO2 can offer much more potential for homogeneous transition metal catalysis than only being a new procedure for easy product isolation and catalyst recycling. In the Ni-catalyzed hydrovinylation of... 

In the continuous hydrovinylation experiments, the ionic catalyst solution was placed in the reactor R, where it was in intimate contact with the continuous reaction phase entering from the bottom (no stirring was used in these experiments). The reaction phase was made up in the mixer from a pulsed flow of ethylene and a continuous flow of styrene and compressed CO2. 

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. 

The homo- and cross-addition of alkenes catalyzed by a transition-metal provided another economical way of forming C-C bonds.155 These reactions are carried out by using nickel, palladium, or ruthenium phosphine complexes to yield vinylarenes and some can occur in aqueous media. By using carbohydrate-derived ligands, asymmetric hydrovinylations can be carried out in aqueous conditions.156... 

The retention of the free [Go]-ligand system (without palladium) was 85%, the molecular weight of the actual catalyst is much larger (Mw = 2868 Da versus 1314 Da for the Go-system), which should be sufficient for initial experiments. The result of this continuous hydrovinylation is shown in Figure 4.21. 

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. 

This complex is not the actual catalyst for the hydrovinylation, but needs to be activated in the presence of a suitable co-catalyst. The role of this additive is to abstract the chloride ion from the nickel centre to generate a cationic allyl complex that further converts to the catalytically active nickel hydride species. In conventional solvents this is typically achieved using strong Lewis acids such as Et2AlCl. Alternatively, sodium or lithium salts of non-coordinating anions such as tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate (BARF) can be used to activate hydrovinylation... 

Scheme 8.8. Continuous flow enantioselektive hydrovinylation using the biphasic system IL/scCC>2...

The allylic alcohols 41 derived from the propynols 40 by either a regioselective Pd-catalysed hydroarylation or hydrovinylation are readily cyclised to 4-aryl or 4-vinyl 2//-chromenes in high yield (Scheme 27) <00EJO4099>. 

The transition metal-catalyzed hydrovinylation has been reviewed by RajanBabu who focused mainly on asymmetric reactions, affording chiral compounds.146 The vinylarenes are the most investigated substrates for the hydrovinylation reaction due to the high appeal of the final products in medicinal or polymer chemistry fields.1... 

The palladium-catalyzed hydrovinylation does not seem to be very useful due to the regioselectivity in favor of the linear compound 143 and the formation of a substantial amount of isomerized product 142 (Scheme 39).148 149 Phosphines of basic nature can improve the selective formation of the desired branched product 141 by limiting the isomerization under the experimental conditions.150... 

The proposed mechanism of the hydrovinylation is supported by available evidences, but so far no study has established clearly all the reaction intermediates. RajanBabu has proposed a mechanism of nickel-catalyzed hydrovinylation, which seems to be one of the most efficient processes, involving a cationic nickel hydride species 144 complexed with a weakly coordinated counterion (Scheme 40). The active catalyst species can be generated through... 

Some experimental evidences are in agreement with this proposed mechanism. For example, coordinating solvents like diethyl ether show a deactivating effect certainly due to competition with a Lewis base (149). For the same reason, poor reactivity has been observed for the substrates carrying heteroatoms when an aluminum-based Lewis acid is used. Less efficient hydrovinylation of electron-deficient vinylarenes can be explained by their weaker coordination to the nickel hydride 144, hence metal hydride addition to form key intermediate 146. Isomerization of the final product can be catalyzed by metal hydride through sequential addition/elimination, affording the more stable compound. Finally, chelating phosphines inhibit the hydrovinylation reaction. 

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

Fig. 4 Setup for continuous-flow asymmetric hydrovinylation using an IL/SCCO2 biphasic system. Liquid and gaseous substrates are mixed with the SCCO2 stream before entering the tubular reactor unit and bubbled through the catalyst-containing IL using a capillary. The CO2 flow leaves the reactor on top and the product is collected in a cold trap after controlled expansion to ambient pressure...

The use of soluble dendrimers with phospine ligands experienced a rapid growth and was also transferred to several other dendrimer scaffolds [64—66]. Many reactions, such as hydrogenation, hydrovinylation, Stille coupling, Knoeve-nagel condensation and Michael addition were reported (Tab. 7.2) [73, 74]. 

PPh2 Carbosilane Pd Hydrovinylation 0.01-0.05 mol% 68-99.9 Continuous mem- Negative dendritic 27... 

The hydrovinylation reaction, the codimerization of ethene and styrene (Scheme 2), provides easy access to chiral building blocks from inexpensive hydrocarbon feedstocks, which can be used further for the preparation of fine chemicals. Key problems in this reaction include the selectivity of the reaction and the stability of the catalyst. The main side reactions are oligomerization and isomerization of the product to internal achiral alkenes. The latter reaction can be suppressed by... 

Fig. 9. Hydrovinylation in a CFMR using 6a (a) and 6b (b) Conditions T = 23°C, p = 30 bar, flow rates ethene solution 2.5mLh (lOM), styrene solution 2.5mLh (1.8M), r = 4h, MPF-60 NF membrane (Koch Int., Diisseldorf, Ger.). Y, space time yield (mgL for 6a 0.05mmol Pd,...

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