Ionic liquids in hydrogenation

The trianionic cobalt catalyst has been successfully employed in the hydrogenation of 1,3-butadiene in [bmim][BF4] [10], The product from this reaction is 1-butene which is formed with 100% selectivity. Unfortunately the catalyst undergoes a transformation to an inactive species during the course of the reaction and reuse is not possible. The cationic rhodium catalyst together with related derivatives have been used for numerous reductions, including the hydrogenation of 1,3-cyclohexadiene to cyclohexane in [bmim][SbF6] [11], 

In the ideal biphasic hydrogenation process, the substrate will be more soluble or partially soluble in the immobilization solvent and the hydrogenation product will be insoluble as this facilitates both reaction and product separation. Mixing problems are sometimes encountered with biphasic processes and much work has been conducted to elucidate exactly where catalysis takes place (see Chapter 2). Clearly, if the substrates are soluble in the catalyst support phase, then mixing is not an issue. The hydrogenation of benzene to cyclohexane in tetrafluoroborate ionic liquids exploits the differing solubilities of the substrate and product. The solubility of benzene and cyclohexane has been measured in 

The dicationic cluster catalyst H4Ru4(r/ -C6H6) [BF4]2, originally developed for biphasic aqueous-organic arene hydrogenation reactions (see Section 8.2.1), has proven to be more effective in ionic liquids presumably due to increased 

Chiral ionic liqnids are also known and the possibility of enantioselective reactions with nonchiral catalysts in snch solvents is of considerable interest. Thns far, however, no enantioselective indnction has been reported. 

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An example of a stereoselective hydrogenation in ionic liquids was recently successfully demonstrated by Drie en-H6lscher et al. On the basis of investigations into the biphasic water/n-heptane system [51], the ruthenium-catalyzed hydrogenation of sorbic acid to cis-3-hexenoic acid in the [BMIM][PFg]/MTBE system was studied [52], as shown in Scheme 5.2-8. 

An unusual arene hydrogenation in ionic liquids was published by Seddon et al. in 1999 [86]. These authors reported a new means of hydrogenating aromatic compounds by dissolving electropositive metals in ionic liquids with HC1 as the... 

Details of the first stereoselective hydrogenation in ionic liquids were published by the group of Chauvin [68], who reported the enantioselective hydrogenation of the enamide a-acetamidocinnamic acid in the biphasic system [BMIM][SbF6]/ iPrOH (ratio 3 8) catalyzed by [Rh(cod) (-)-diop ][PF6]. The reaction afforded (S)-N-acetylphenylalanine in 64% enantiomeric excess (ee) (Fig. 41.4). The product was easily and quantitatively separated and the ionic hquid could be recovered, while the loss of rhodium was less than 0.02%. 

A special example for a regioselective hydrogenation in ionic liquids was reported by our group and by DrieRen-Holscher [96, 97]. Based on investigations in the biphasic system water/n-heptane, the ruthenium-catalyzed hydrogenation of sorbic acid to ds-3-hexenoic acid according to Scheme 41.3 in the system [BMIM][PF6]/MTBE was studied [98],... 

These results demonstrate that the effectiveness of hydrogenation in ionic liquids is not only a parameter of H2 availability. Indeed, many solvent parameters - including polarity, coordinating ability and hydrophobicity - should be taken into account, though these have not yet been studied. 

Enantioselective hydrogenation in ionic liquids is worthy of attention because of the prospects for efficient reuse of metal complexes with expensive chiral ligands. The opportunities and challenges involved in the application of ionic liquids for chiral synthesis have been assessed recently 213). 

Berthold, H., Schotten, T. and HOnig, H., Transfer hydrogenation in ionic liquids under microwave irradiation, Synthesis, 2002, 1607-1610. 

Figure 7.2 Chiral bisphosphine ligands used in asymmetric hydrogenations in ionic liquids without structural modifications.

Frater T, Gubicza L, SzoUosy A, Bakos J (2006) Enantioselective hydrogenation in ionic liquids recyclability of the [Rh(COD)(DIPAMP)]BF4 catalyst in [bmim][BFJ. Inorg Chim Acta 359 2756-2759... 

Transition metal catalyzed hydrogenation in ionic liquids has also been applied to the hydrogenation of polymers. First studies were presented by Dupont s group which investigated the hydrogenation of acrylonitrile-butadiene copolymers [102]. These early studies were later expanded by Rosso and coworkers studying the rhodium catalyzed hydrogenation of polybutadiene (PBD), nitrile-butadiene rubber (NBR) and styrene-butadiene rubber (SBR) in a [BMIM][BF4]/toluene and a... 

Very few examples of imine hydrogenation in ionic liquids have been published so far. Giernoth et al. screened eight different ionic liquids (the cations were [BMIMJ+, l-decyl-3-methylimidazolium ([DMIM]" "), N-butyl-3-methylpyridinium ([BMPy]+) and N-decyl-3-methylpyridinium ([DMPy]" ) combined with the anions [Bp4] and [(CF3 S02)zN] ) and compared them with toluene as solvent in the hydrogenation of trimethylindolenine with the Ir-XYLIPHOS catalyst system (Scheme 5.3-12) [120]. 

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