HYDROVINYLATION REACTION

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... 

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. 

The hydrovinylation reaction, the codimerization of ethene and styrene (Scheme 2), provides easy access to chiral building blocks from inexpensive hydrocarbon feedstocks, which can be used further for the preparation of fine chemicals. Key problems in this reaction include the selectivity of the reaction and the stability of the catalyst. The main side reactions are oligomerization and isomerization of the product to internal achiral alkenes. The latter reaction can be suppressed by... 

At present, phosphoramidites represent the most promising ligand class for the hydrovinylation reaction, both for the excellent activities and enanioselectivities. The synthetic potential embraces not only styrene and styrene derivatives but also norbornene [54] and 1,3-dienes [55]. 

Eggeling, E.B., Hovestad, N.J., Jastrzebski, J.T.B.H., Vogt, D. and van Koten, G. (2000) Phosphino carboxylic acid ester functionalized carbosilane dendrimers nanoscale ligands for the Pd-catalyzed hydrovinylation reaction in a membrane reactor. J. Org. Chem., 65, 8857. 

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. 

Bearing in mind that in many cases the active species is believed either to be ionic or to contain a strongly polarized metal-halide bond (through interaction with the Lewis acid), it is not surprising that the preferred solvent for the hydrovinylation reaction is CH2CI2 or CsH Cl. However, examples have been reported where the reaction proceeds satisfactorily in acetone [16], THF [28], dioxane [28], toluene [31, 32, 35] or p-xylene [25]. In two cases, the effect of varying the solvent has been studied [21, 27]. 

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 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. 

The hydrovinylation reaction is suggested to proceed by an extension of the conventional Cossee-type mechanism addition of a Ni-H species to the alkene, insertion of a second alkene molecule into the resulting Ni-alkyl bond followed by P-W transfer with elimination of the product and regeneration of the hydride. 

Arguments similar to those presented above for the hydrovinylation of styrene will dictate the stereochemical course of the reactions involving the other alkenes investigated. Thus, interference of the methylene bridge of a complexed bicyclo-heptene molecule (25) with the substituents on the donor atom and with the complex anion will direct the course of the hydrovinylation reaction to the exo-isomer of (+)-( 5,25,4/ )-2-vinylbicycloheptane (eq. (4)) [3], while the arrangement of the five-membered ring with respect to the coordination plane will result in the conversion of cyclopentadiene into (-)-(f )-3- vinylcyclopentane. 

Hydrovinylation reactions of vinylarenes, Eq. (1), have been investigated most extensively because of the importance of 3-aryl-1-butenes as potential intermediates for widely used anti-inflammatory 2-arylpropanoic acids [4]. Since the first report of a high pressure (1000 atm) ethylene/styrene codimerization in the presence of RhClj [5] various metals such as Ru [6], Co [7],Pd [8],andNi [9,... 

Since the original discovery of the importance of the hemilabile ligand for high enantioselectivity, we have recognized that such effects are equally important in a number of other monophosphines that promote the hydrovinylation reaction [22]. One example is shown in Eq. (10). 

The iridium catalyst was found to be sufficiently soluble in scCOz for catalysis in the form of the substrate complex, but precipitated quantitatively once all the substrate had been consumed. Supercritical fluid extraction at that stage yielded the solvent- and metal-free product in crystalline form, leaving the active and selective catalyst behind for further use. Under batch operation, noticeable deactivation occurred after the fourth cycle this might be avoided by working in a closed system, as demonstrated for the hydrovinylation reaction shown in Eq. (3) [48, 49]. 

Pd complexes of soluble P,0-fimctionalized carbosilane dendrimers (8) were used by Vogt and co-workers for the hydrovinylation reaction (codimerization of ethylene and styrene). This C-C coupling reaction is of great interest, because it opens an easy access to building blocks for fine chemicals as well as pharmaceuticals [Eq. (6)] [16]. 

The asymmetric hydrovinylation reaction was first reported as early as 1972 by the Wilke group. By utilizing the menthol-derived chiral phosphonite LI as the ligand, they realized the Ni-catalyzed asymmetric hydrovinylation of... 

Leitner and co-workers developed ligand L3 derived from 1,2-diazaphos-pholene, which was a real breakthrough in the Ni-catalyzed asymmetric hydrovinylation reaction. Styrene was converted smoothly to 3-phenyl-l-butene 16a in up to 93% ee (Scheme 9.2c). Since L3 contains many stereo-genic centers and suffers from a tedious multistep synthesis, further development based on this catalytic system has been rather limited. ... 

Scheme 9.2 Asymmetric hydrovinylation reactions reported by (a) Wilke, (b) Buono, and (c) Leitner.

Until now, Ni-catalyzed asymmetric hydrovinylation reaction have been intensively studied by the groups of RajanBabu and of Leither. From these studies, one can point out that the most important factor is the proper choice of a chiral ligand. 

In 2002, the RajanBabu group developed a series of sugar-based phosphi-nite ligands for the asymmetric hydrovinylation reaction. As illustrated in Scheme 9.3, Lll was the best ligand, in terms of the yield and enantioselectivity, at that time for the Ni-catalyzed asymmetric hydrovinylation of styrene (89% yield and 81% ee). ... 

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

After continuous development for 30 years, the Ni-catalyzed asymmetric hydrovinylation reactions of alkenes have been proved to be synthetically useful methods and have found application in the total synthesis of natural products. For instance, the syntheses of (f )-(-)-a-curcumene 27 and (i )-(-)-ar-turmerone 28 were accomplished in acceptable overall yields (52% for 27 and 26% for 28), employing asymmetric hydrovinylation of 4-methylstyrene as the first step (Scheme 9.8). ... 

Another important application of Ni-catalyzed asymmetric hydrovinylation reactions was the enantioselective construction of all-carbon quaternary stereocenters. In 2011, the RajanBabu group reported an efficient total synthesis of (-)-desoxyeseroline 29 employing the highly enantioselective hydrovinylation of tetralin derivative 20g as the key step (Scheme 9.9). ... 

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