Hydrovinylation, asymmetric

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 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... 

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...

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. 

SCHEME 60. Asymmetric hydrovinylation. [G. Wilke, Angew. Chem., Int. Ed. Engl., 27, 185 (1988). Reproduced by permission of Verlag Chemie.]... 

Asymmetric hydrovinylation.1 The reaction of ethylene with 1,3-cyclohex-adiene catalyzed by bis(l,5-cyclooctadiene)nickel, diethylaluminum chloride, and 1 gives ( + )-(S)-3-vinyl-l-cyclohexene (2) in quantitative yield and 93% ee. Related ligands prepared from (S)-proline and D-ephedrine are less effective for asymmetric hydrovinylation. 

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-... 

Scheme 7 displays a possibility of the synthesis of chiral 2-arylpropionic acids via the oxidative tranformation of (7 )-3-aryl-l-butenes. The requisite chiral olefins may be obtained by transition metal-catalyzed asymmetric coupling between a benzylic Grignard reagent and vinyl bromide (93 % optical yield) [28] or, more attractively, asymmetric hydrovinylation of an aromatic olefin with ethylene. The asymmetric combination of styrene and ethylene, giving the adduct 25 in 95 % ee, has been performed on a 10-kg scale with a dinuclear Ni catalyst formed from ( -allyl)NiCl2 and a unique chiral dimeric aminophosphine obtainable from (/ )-myrtenal and (5)-l-phenylethylamine [7a],... 

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. 

Figure 4. The nickel catalyst for asymmetric hydrovinylation (see eq. (6)) is activated, tuned, and immobilized in an IL/CO2 continuous-flow system.

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... 

The asymmetric hydrovinylation (cf. Section 3.3.3) of styrene with excellent chemo-, regio-, and stereoselectivity was achieved in SCCO2 using the known Ni" catalyst but - instead of the flammable co-catalyst (C2H5)3Al2Cl3 - the bo-ranate ( BARF ). This was possible because all components are soluble in SCCO2 [111] (cf eq. (6) of Section 3.1.13). 

Transition metal-catalyzed hydrovinylation is one of a few practically useful carbon-carbon bond-forming reactions utilizing feedstock carbon sources for the synthesis of high-value fine chemicals. Asymmetric hydrovinylation has many potential applications in the synthesis of pharmacologically important compounds, such as ibuprofen and naproxen, and has attracted much attention [110]. Recently, chiral monodentate phosphines have proven to be highly efficient ligands for the asymmetric hydrovinylation of a-alkyl vinylarenes [111]. 

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. 

Keywords Asymmetric hydrovinylation. Ethylene, Vinylarene, Diene, Norbornene, 2-Arylpro-panoic acid. Nickel, Palladium, tt-Allylnickel bromide, Phospholane, Ligand timing, Hemila-bile hgand. Catalyzed cychzation, 1,6-Diene... 

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