Nickel hydrovinylation

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... 

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... 

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. 

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... 

Under particularly mild conditions, a Ni-catalyst based on (] , Sc, Sc)-26 gave quantitative conversion of 11 with 84.9% selectivity for the desired product 23 and an excellent enantioselectivity of 94.8% (S) (entry 5). Moreover, the catalyst system proved extremely efficient and remarkably robust for the hydrovinylation of 11. Almost 90% conversion and perfect chemoselectivity were achieved within 4 h at -65 °C even at a substrate/nickel ratio of 4600 1 (entry 6a). Further... 

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... 

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. 

Hydrothermal methods, for molecuarlar precursor transformation to materials, 12, 47 Hydrotris(3,5-diisopropylpyrazolyl)borate-containing acetylide, in iron complex, 6, 108 Hydrotris(3,5-dimethylpyrazolyl)borate groups, in rhodium Cp complexes, 7, 151 Hydrotris(pyrazolyl)borates in cobalt(II) complexes, 7, 16 for cobalt(II) complexes, 7, 16 in rhodium Cp complexes, 7, 151 Hydrovinylation, with transition metal catalysts, 10, 318 Hydroxides, info nickel complexes, 8, 59-60 Hydroxo complexes, with bis-Cp Ti(IV), 4, 586 Hydroxyalkenyl complexes, mononuclear Ru and Os compounds, 6, 404-405 a-Hydroxyalkylstannanes, preparation, 3, 822 y-Hydroxyalkynecarboxylate, isomerization, 10, 98 Hydroxyalkynes, in hexaruthenium carbido clusters, 6, 1015 a-Hydroxyallenes... 

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.

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 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 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... 

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... 

Wegner A, Leitner W. Nickel-catalysed enantioselective hydrovinylation of styrenes in liquid or supercritical carbon dioxide. Chem Commun 1999 1583-1584. 

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... 

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 . 

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