Nickel catalysts hydrovinylation

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... 

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... 

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... 

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

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. 

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 high activity of the nickel catalysts frequently enables the hydrovinylation reaction to be carried out at low temperatures, thereby allowing full implementation of the small differences in the free activation enthalpy for the formation of the diastereomeric intermediates. The increase in the diastereomeric excess with decreasing reaction temperature for the hydrovinylation of p-divinylbenzene to... 

Fig. 4 Activation and tuning of the nickel catalyst shown in Eq. (3) for hydrovinylation by combination of different imidazolium-based ILs and compressed C02 [49] cv —conversion, sel. = selectivity.

Asymmetric modifications of hydrovinylation are one of the earliest examples of successful asymmetric transition metal catalysis. After optimization of various dimerization and codimerization reactions using phosphane modified nickel catalysts, the first examples of asymmetric olefin codimerization were reported with n-allylnickel halides activated by organoaluminum chloride and modified by chiral phosphanes7. Thus, codimerization of 2-butene with propene using n-allylnickel chloride/A]X, (X = Cl, Br) in the presence of tris(myrtanyl)phosphane gives low yields of (—)-( )-4-methy 1-2-hexene (I) with 3% ee7,7 . 

Recovery of the chiral nickel catalyst using the CESS procedure was only partly successful, because the nickel species rapidly deactivate in the absence of substrate. It was noted, however, that the primary hydrovinylation product is extracted from the reactor very selectively, leaving behind any by-products resulting from double or multiple incorporation of ethylene. Again, this emphasizes the potential of SCCO2 for integrated processes of synthesis and product isolation. 

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... 

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. 

Scheme 5.1 Hydrovinylation of a vinylarene with an in situ generated nickel catalyst from a monophosphine ligand.

Scheme 5.5 Hydrovinylation of styrene with a preformed phosphoramidite chiral nickel catalyst activated by Inlj.

Scheme 5.8 Hydrovinylation of (a-alkylvinyl)arenes with a preformed spiro phos-phoramidite chiral nickel catalyst.

Scheme 5.12 Hydrovinylations of steroid 1,3-dienes with in situ generated nickel catalysts from phosphoramidite ligands.

Asymmetric hydrovinylations using chiral nickel catalysts constitute a powerful method for achieving enantioselective functionalization of olefins [25, 26], Pioneering results in this area were reported by Wilke [25, 84] who in 1987 disclosed the impressively enantioselective catalytic system depicted in Equation 21 [85], The nickel complex of the chiral dimeric azaphospho-lene ligand 153 thus effected the efficient hydrovinylation of styrene (152) with ethene to give the corresponding product 154 in 97 % yield and 95 % ee. 

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... 

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