Hydrovinylation nickel-catalyzed

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... 

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]. 

It is frequently assumed that the mechanism of the hydrovinylation reaction is identical for catalysts containing the same metal, irrespective of the nature of the metal precursor. However, it is questionable whether this assumption can be extended to different metals and it should not, for example, be assumed that the nickel-catalyzed reactions have mechanisms identical to those of the palladium-catalyzed reactions. 

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

It is generally accepted that the nickel-catalyzed hydrovinylation of cyclic 1,3-dienes proceeds in an analogous manner to that discussed for styrene with an initial 1,2-addition of the Ni-H species. However, it should be stressed that an initial 1,4-addition has not been excluded. The observation of two isomeric products from the reaction involving hexadeuterocyclopentadiene suggests that the... 

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... 

Continuous, selective hydroformylation in supercritical CO2 using (acac)Rh(CO)2 immobilized on silica as catalyst shows certain advantages. A version of asymmetric hydroformylation in this medium has also been reported,. (Subcritical CO2 gas accelerates solventless synthesis involving solid reactants, including hydrogenation and hydroformylation.) The regioselective and enantioselective nickel-catalyzed hydrovinylation of styrenes in supercritical CO2 make 3-arylpropenes available in an optically active form. " Improvement in the performance of the Pauson-Khand reaction in supercritical media... 

Other C-C bond-forming reactions have been successfully developed using SCCO2 and liquid CO2 as reaction media. Examples include the synthesis of cyclopentenones via cobalt-catalyzed cocyclizations of alkynes with alkenes and carbon monoxide (Pauson-Khand reaction) (Scheme 30) , enantioselective nickel-catalyzed hydrovinylation of styrenes (Scheme 31) , and the palladium-catalyzed hydroarylation of acyclic jS-substimted-o , j8-enones with aryl iodides (formal conjugate addition) (Scheme 32). ... 

Although many notable examples of nickel-catalyzed hydrovinylations have been described, their iterative use in the synthesis of the pseudopterosins provides a clear demonstration of the utility of this strategy (Scheme 3-94). In this impressive illustration, three sequential ligand-controlled hydrovinylation processes enable a highly efficient synthesis of the target molecule. 

More recent key players in this area have been RajanBabu [86, 87] and Leitner [88, 89], Extensive studies by RajanBabu resulted in the identification of the optimized diaryl monophosphite ligand 156 (Equation 22) [87]. In combination with SbF " as the counterion, the nickel-catalyzed hydrovinylation of bromostyrene 155 affords the desired adduct 157 with high selectiv-ities (98% yield, 89% ee). Independent contemporary studies by Leitner,... 

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... 

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. 

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. 

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... 

Similarly, asymmetric metal-catalyzed hydrovinylation of various other olefins has been investigated, usually catalyzed by nickel systems prepared from Ni(IJ) salts or / -allylnickel halides, activated by Lewis acids such as ethylaluminum dichloride or diethylaluminum chloride, and modified by chiral phosphanes. Surveys of these results are found in more general reviews11-12 and reviews dealing preferentially with this subject -5-7-13 - A... 

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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