Polymeric surfactant systems in the asymmetric hydrogenation of amino acid precursors with a rhodium complex

Abstract

The synthesis of new types of soluble and insoluble amphiphilic systems is described, being polysoaps, crosslinked polysoaps, an amphiphilised ion exchanger, and ion exchangers loaded with ionic surfactants. It was possible to synthesise insoluble polymeric systems which nevertheless showed micellar properties. The polymers were tested as aids in the stereospecific hydrogenation of (2)-methyl a-acetamidocinnamate to the methyl ester of N-acetylphenylalanine ( R ) by means of an optically active rhodium complex in water. With linear polysoaps the effect on the reaction was dependent on the structure of the soap molecules. The enantioselectivity of the reaction with monomeric surfactants in water was almost attained. Crosslinked polysoaps showed the same effects on the hydrogenation as linear polysoaps with an analogous structure. Macromolecular ion exchangers loaded with amphiphilic counterions gave in hydrogenation almost the same effect as these counterions give when they are not bound to an insoluble carrier. Enantioselectivities and reaction rates were usually slightly lower than with monomeric surfactants. However, there is an advantage in the separation of catalyst and product after reaction. It was shown that the rhodium-containing amphiphilic ion exchangers could be reused for up to 9 times. GeneralMicellar catalysis has been developed into an important field in organic chemistry'-3). Occurring in a colloid medium, some reactions are influenced in rate and stereoselectivity 4* j). Common disadvantages of micellar catalysis, however, are the low capacity of micellar systems and the difficulties in the separation of products and surfactants after the reaction.We are interested in the asymmetric hydrogenation of amino acid precursors such as (2)-methyl-a-acetamidocinnamate by means of optically active rhodium(1) complexes. From previous experiments it was concluded that this hydrogenation may be carried out in organic solvents, e. g. methanol, and in aqueous surfactant dispersions as well6") (Scheme 1).The excellent results with monomeric surfactants led us to the idea to synthesise a polymeric, insoluble surfactant system to be used in hydrogenation in water which could serve as a carrier for the catalyst, filtered off after reaction and reused after separation from the product.Several workers already used insoluble polymers as supports for metal complex catalysts *s9), but the binding of a metal complex in a micelle-analogous insoluble Macromol.

Surface‐Functionalized Nanoporous Catalysts towards Biofuel Applications

“…The activity and selectivity of the immobilized Rh catalyst was comparable with the homogeneous analog (Scheme 15.2 ). Flach et al reported another strategy of entrapping a Rh complex on the surface of immobilized surfactants through hydrophobic interactions ( Figure 15.3 ) [11] . No metal leaching was observed while repeating the hydrogenation experiment ten times in water.…”

Polymerized Ionic Amphiphiles: Synthesis and Effects in the Enantioselective Hydrogenation of an Amino Acid Precursor

“…[16] Sodium 10-undecenylsulfate (7). Dry pyridine (60 mL) was cooled in an ice-bath for 15 min and chlorosulfonic acid (14 g, 0.12 mol) was added dropwise, followed by the addition of 10-undecen-1-ol (17 g, 0.1 mol) in 20 mL dichloromethane.…”

Influence of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers on stereoselective catalysis in water

“…The addition of both monomeric [17][18][19] and polymeric [19][20][21] surfactants significantly enhances the activity and the enantioselectivity. [22] The addition of micelleforming surfactants is necessary for this reaction, because an increase in the solubility of lipophilic substrates in water is required.…”

“…The use of polymeric surfactants which form micelle-like structures could solve this problem. Regen [23] and Janout [24] have already utilized amphiphilic polymers in phase-transfer catalysis, and Flach et al [20,21] have synthesized new types of soluble and insoluble polysoaps for the use in asymmetric hydrogenation.…”

“…The results of the hydrogenation of methyl (Z)-a-acetamidocinnamate catalysed by [BPPM Rh(cod)BF 4 ] in the presence of modified triblock copolymers 3 and 4 as Amphiphile m n % ee R t/2/min none [20] --78 l90 SDS [18] - amphiphiles are shown in Tab. 4.…”