Alkenes 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... 

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... 

The Heck reaction yields the final product through a p-hydride elimination whereas hydroarylation or hydrovinylation generates the final product via a reductive elimination. Nonetheless, both reactions share a common first step, that is, addition of an aryl or a vinyl palladium species to an alkene, and thus are briefly discussed here. Norbornene 595 is the most studied alkene to evaluate an the... 

It was shown earlier that palladium-catalyzed hydrovinylation of styrene using phosphino ester-type ligands leads to isomerization of the external al-kene (kinetic product) to the internal alkene (thermodynamic product) at higher substrate conversion. In this regard, the idea was to suppress this isomerization by running the reaction at lower conversion in a CFMR system in order to minimize the catalyst-substrate contact time. 

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. 

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. 

Table 1. The hydrovinylation of activated alkenes and cyclic 1,3-dienes. 

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. 

The regiochemistry of the hydrovinylation product (3-phenyl-1-butene) requires the exclusive addition of the Ni atom to the phenyl-substituted olefinic C atom (Ni C2) and of the hydrogen atom to the terminal C atom. The pathway for this addition, which is presumably accompanied by an anticlockwise rotation of the styrene molecule about the Ni-alkene axis in 22, is not clear but has precedence in the preferred Ni—>C2 addition which is observed in the initial step... 

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. 

Hydrovinylations and hydroalkcnyfations are formal additions of vinyl and alkenyl C—H bonds to alkenes and alkynes. These reactions are also called dimerization or codimerization (if carried out with different substrates)1. 

Palladium-catalysed hydroarylations and hydrovinylations of olefins and acetylenes have been reviewed. Vinyl iodide, for instance, reacts with internal acetylenes in the presence of (PPh3)2Pd(OAc)2, triethylamine and formic acid to yield the alkenes 204... 

Hydrovinylation is the transition metal catalyzed co-dimerization of alkenes with ethene yielding 3-substituted 1-butenes [26]. This powerfid carbon-carbon bond forming reaction can be achieved with high enantioselectivity using Wilke s complex as a catalyst precursor [27]. In conventional solvents, this pre-catalyst needs to be activated with a chloride abstracting agent, e.g. Et3Al2Cl3. Leitner et al. reported the... 

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). ... 

In 2009, Yu and co-workers wrote a comprehensive review on diastereo-selective and enantioselective C—H bond functionalization by transition metal catalysts, which presented the historical retrospective of this area in an elegant manner. Therefore, in the following sections, we will focus on the discussion of the latest achievements on the catalytic asymmetric C—H bond functionalization via a transient C—M bond (mainly contributions after 2009). The reactions will be classified according to the metal catalyst as well as the mechanistic scenario. Note that asymmetric hydroacylation and hydrovinylation, two important kinds of methods for the functionalization of aldehyde and alkene C—H bonds, will be covered in two separate chapters of this book (Chapters 8 and 9). 

The RajanBabu group also applied the same catalytic system in the asymmetric hydrovinylation of 1,3-dienes and strained alkenes (Scheme 9.7). Satisfactory results could be obtained typically. 

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). ... 

Catalysts derived from /V-heterocyclic carbenes have also been demonstrated as highly effective catalysts for hydrovinylations involving heterocoupling of aromatic and aliphatic alkenes (Scheme 3-93). In this case, the catalyst is prepared by coupling an aldehyde and aliphatic alkene with a Ni(0) NHC complex in the presence of a silyl triflate. 

The hydrovinylation of alkenes can be defined as the formal addition of ethylene across a double bond or, alternatively, as a codimerisation of ethylene and an activated alkene (Scheme 8.1). ... 

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. 

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