Hydrovinylation palladium-catalyzed

The palladium-catalyzed hydrovinylation does not seem to be very useful due to the regioselectivity in favor of the linear compound 143 and the formation of a substantial amount of isomerized product 142 (Scheme 39).148 149 Phosphines of basic nature can improve the selective formation of the desired branched product 141 by limiting the isomerization under the experimental conditions.150... 

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. 

Scheme 5 Schematic synthetic pathways and structure of dendritic P,0 ligands applied in the palladium-catalyzed hydrovinylation of styrene...

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. 

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). ... 

S. Cacchi, G. Fabrizi, L. Moro, P. Pace, Stereo- and regioselective palladium-catalyzed hydroarylation and hydrovinylation of functionalised alkynes a route to substituted Z-2-cinnamyl esters, 3-chromen-2-ols, and coumarins, S)fnlett (1997) 1367-1370. 

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... 

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... 

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