Ethylene 1.4- hydrovinylation with

Codimerization of styrene and ethylene giving 3-phenyl-l-butene is another example of asymmetrically induced hydrovinylation. With catalyst precursors obtained from (jt-allyl)NiX or (> 3-cyclooct-5-enyl)NiX (X = various anions) modified with bis(menthyl)methylphosphane. ]<)- and (S)-3-phenyl-1 -butene in up to 37% ee is obtained, depending on the counterion X of the catalytic system13. 

In addition to the extensive studies of hydrovinylations using styrene substrates, 1,3-dienes are also an especially useful substrate class. With phosphoramidite ligands, enantioselective diene hydrovinylation with ethylene provides a practical solution to the difficulties associated with the installation of exocyclic side chain stereochemistry, as the representative procedure below illustrates. ... 

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. 

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. 

The mixed coupling of two different alkenes allows the formation of new functional unsaturated products but requires high regioselectivity. A ruthenium hydride complex, generated in situ from the reaction of RuHCl(CO)(PCy3)2 with HBF4.OEt2, was found to be an effective catalyst for the hydrovinylation of alkenes [8]. The reaction of styrene with ethylene produced the hydrovinylation compound 10 in 93% yield (Eq. 5). Initial hydrometallation of the alkene and insertion of ethylene seemed to be a plausible mechanism. 

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 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 nickel-catalyzed hydrovinylation of bicycloheptene has been used as a standard reaction to test the efficacy of a new ligand. The reaction occurs with complete diastereoselectivity to give exo-2-vinylbicycloheptane (16) and none of the endo-isomer is formed. The same species, however, catalyze the isomerization of the primary product to cis- and franv-2-ethylidenebicycloheptane (17) and the codimerization with further ethylene to the butenyl derivatives 18 and 19. The product distribution is dependent upon the nature of the ligand [3, 8 c, 40]. 

The rate of reaction of the other three dienes studied decreases with increasing ring size [27] in the case of cycloocta-1,3-diene, hydrovinylation is accompanied by isomerization or reaction with a second ethylene molecule, and the yield of 3-vinyl-cyclooct-l-ene never exceeds 50%. 

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. 

The dimeric azaphospholines derived from (/ )-myrtenal and (J )-l-phenylethylamine are also active ligands in the nickel catalyzed hydrovinylation of cyclopentadiene with ethylene to give exclusively 3-vinylcyclopentene (7) with 100% chemo- and regioselectivity and up to 76% ee5 (according to recent reports even higher inductions are obtainable53). No isomerization and dimerization of the products is observed. Previously, with other catalytic systems the same... 

Scheme 9.3 Phosphane and phosphinite ligands for the hydrovinylation reaction of styrene with ethylene.

The basic features of the reaction may be summarized as follows (Scheme 3-92). Nickel 7t-allyl dimers are often used as pre-catalysts. Treatment with monodentate phosphines and Lewis acids likely generates a cationic nickel hydride species, which serves as the active catalyst for the reaction. Regioselective hydrometallation of the styrene generates intermediate 26, with the regiochemistry being driven by benzylic stabilization. Insertion of ethylene followed by p-hydride elimination produces the hydrovinylation product and regenerates the active cationic nickel hydride species. 

This reaction has a long history, starting in 1965, when it was found that hydrated Rh and Ru chloride catalysed codimerisations between ethylene and a variety of alkenes. As early as 1972, Wilke and co-workers reported the enantioselective hydrovinylation of 1,3-cyclooctadiene with a system containing Ni(II) and P(i-Pr)(Men)2, which constitutes the second metal-catalysed enantioselective C-C bond-forming reaction ever reported. 

The co-dimerization of ethylene with olefins has also been studied extensively and has been called hydrovinylation. A seminal example, discovered by Bogdanovic and Wilke, involved the co-dimerization of ethylene and norbornene catalyzed by a (TT-aUyl)nickel catalyst (Equation 22.34). This chemistry and more modem versions of these additions of one olefin C-H bond across another were presented in Chapter 16 on the hydrofunctionalization of olefins. 

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