Regioselectivity organic solvent medium

The term medium engineering , that is the possibility to affect enzyme selectivity simply by changing the solvent in which the reaction is carried out, was coined by Klibanov, who indicated it as an alternative or an integration to protein engineering [5aj. Indeed, several authors have confirmed that the enantio-, prochiral-, and even regioselectivity of enzymes can be influenced, sometimes very remarkably, by the nature of the organic solvent used. 

The cationic nickel complex [ /3-allylNi(PR3)]+, already described by Wilke etal. [21], as an efficient catalyst precursor for alkene dimerization when dissolved in chlorinated organic solvents. It proved to be very active in acidic chloroaluminate ionic liquids. In spite of the strong potential Lewis acidity of the medium, a similar phosphine effect is observed. Biphasic regioselective dimerization of propylene into 2,3-dimethylbutenes can then be achieved in chloroaluminates. However, there is a competition for the phosphine between the soft nickel complex and the hard aluminum chloride coming from the dissociation of polynuclear chloroaluminate anions. Aromatic hydrocarbons, when added to the system, can act as competitive bases thus preventing the de-coordination of phosphine ligand from the nickel complex [22 b]. Performed in a continuous way, in IFP pilot plant facilities, dimerization of propene and/or butenes with this biphasic system (Difasol process) compares... 

Internal olefins could be converted into linear aldehydes if a suitable catalyst is in hand. BeUer and Krauter [26] reported that Co-1 catalyzed hydroformylation of 2-pentene in an aqueous biphasic medium, furnishing the desired aldehydes in good yields with an njiso ratio up to 70 30 at an elevated temperature and pressure. The same group also demonstrated the hydroformylation of internal olefins catalyzed by Rh-2 in an aqueous biphasic system, which afforded the aldehydes with significantly higher regioselectivities (n/iso>99 1), compared to similar catalysts in organic solvents [27]. The control of pH and CO partial pressure was shown to be important for a successful reaction in this case. 

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