Complex-catalyzed Hydrogenation in Micellar Media

With the introduction of water-soluble phosphine ligands [41] and their application in complex catalysts in biphasic and phase-transfer systems, the influence of amphiphiles could be expected but the results were not really encouraging [42], Quinn and Taylor [43] reported on selective hydrogenation in phospholipid bilayer membranes and Nuzzo et al. [44] described the rhodium complex of 9 as active exclusively in the presence of SDS. 

It also seems to be noteworthy that in the presence of phospholipids the nitrogen reduction catalyzed by an artificial nitrogenase system is very selectively enhanced [45], Ding et al. [46] gave examples of the use of chiral amphiphilic ligands in hydrogenation reactions but directed attention to their application in biphasic systems. Usually, the presence of water causes loss of activity and, in the case of enantioselective hydrogenation, loss of enantioselectivity [47]. 

Surprisingly, in the hydrogenation of unsaturated amino acid derivatives catalyzed by a chiral rhodium complex in water, these disadvantages are overcome by the addition of micelle-forming surfactants [48]. The mixture was solubilized by 

All types of surfactants promote the reaction but only the hydrogen sulfate was active in the case of the cationic amphiphiles. No influence was found for the anions tetrafluoroborate or triflate. The most favorable chiral ligand was BPPM (10). described by Achiwa [50], but even DIOP (4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl-l,3-dioxolan) [51] and Ph-/i-glup-OH (phenyl 2,3-0-bis(diphenylphos-phino)-/3-D-glucopyranoside) [52] show the effect. A micellar effect was also observed with phosphinites derived from a,a- and /),/(-trehalose as ligands [53]. The increase of activity and enantioselectivity depends on the concentration of the surfactant. There is a sudden increase near the CMC of the surfactant which might be an indication of the existence of micelles [11]. 

The enhancement of enantioselectivity cannot be an effect of solubility because water-soluble substrates gave enhanced activities but no enantioselectivity increase [54], Acylation of PPM ((2S,4S)-4-diphenylphosphino-2-diphenylphosphinomethyl-pyrrolidine) with the chlorocarbonic ester of alkyl polyoxyethylene ethers (Brij 4) 

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