Polymer-supported Phase Transfer Catalysts Research Articles

Polymer-supported Phase Transfer Catalysts in Green Organic Syntheses

This mini review presents recent applications of polymer-supported catalyst in different green synthetic approaches, and is focused on the use of these types of catalysts in phase transfer catalysis processes. Phase transfer catalysis is now a well established method in organic synthesis in both industry and academia, being recognized as a versatile method because of its simple experimental operations, mild reaction conditions, inexpensive and environmentally benign reagents and solvents. The discussions involved only some limited aspects related to the use of polymer-supported catalysts in phase transfer catalysis especially concerning ammonium and phoshonium catalysts and cinchona grafted on organic supports. The illustration of efficiency and reusability but also highlights the scope and limitations associated with the supported catalytic systems from a green chemistry view are presented. Keywords: Polymer-supported, catalyst, green, phase transfer catalysis.

Ultrasound Assisted Synthesis of Diethyl-2,2?-Thiodiacetate with 2-Bromoethylacetate Under a New Polymer-Supported Phase-Transfer Catalyst in Solid-Liquid Condition

In this work, a new polystyrene-bound single-onium phase-transfer catalyst was synthesized, and their catalytic activities were investigated in the synthesis of diethyl-2,2’-thio diacetate derivatives of thioether. The new insoluble polymer anchored mono-site phase-transfer catalyst showed significant high catalytic activity as compared to soluble single site phase-transfer catalysts. The comparative kinetic investigation reveals that the diethyl-2,2’-thio diacetate of ethyl-2-bromoacetate is faster than sodium sulfide in the presence of the new catalyst. This catalyst can be used several times with consistent catalytic activity. Experimental observations support an interfacial-type mechanism.

Synthesis of new polymer-supported multi-site phase transfer catalysts and their catalysis for the C-alkylation of 1-indanone in association with ultrasound: A kinetic study

Three different types of polymer supported multi-site phase transfer catalysts having two, four and six active sites are synthesized by simple routes. The available number of active sites in each catalyst was established based on the results derived from various microscopic and spectroscopic techniques. Similarly, the catalytic ability of each catalyst was studied by determining their pseudo-first order rate constants for C-alkylation of 1-indanone. The comparative catalytic study reveals that the kobs determined under the influence of combination of ultrasound and mechanical stirring showed five and three fold increase in the activity compared to the individual effect. Similarly, the catalytic efficiency was found to be in the order: six-site > four-site > two-site > single-site irrespective of individual effect of ultrasound/mechanical stirring or both, thus confirming the availability of active sites in each catalyst. Furthermore, the detailed kinetics of C-alkylation of 1-indanone was conducted using the superior six-site PSBTTMACEMPE catalyst by varying stirring speed, [substrate], [catalyst], [NaOH], and temperature. Based on the observed kinetic results and activation energy, a suitable mechanism was proposed.

Green Conversion of Phenolic Compound to Benzoate Over Polymer-Supported Phase-Transfer Catalysts

The conversion of sodium phenoxide into benzoates over polymer-supported phase-transfer catalysts with trialkylamines as the functional groups (triphase catalysis) was investigated. The yield of product phenyl benzoate was 100% in 1 h of reaction using a small amount of tributylamine catalyst supported on 1.38% divinylbenzene (DVB) cross-linked copolymer (40–80 mesh) of styrene and chloromethylstyrene. During the reaction, the catalyst stays between the aqueous and organic phases. The overall reaction rate was strongly dependent on the stirring speed and the ionic exchange of Cl− of the catalyst with PhO− in the interfacial region. The reaction rate decreased with increasing the mean particle sizes for 1.38% DVB catalyst. The variation of the catalytic intermediate within the catalyst was determined to verify the reaction mechanism. A kinetic model was also proposed to correlate the reaction rate constants. The efficient conversion of phenolic compounds over polymer-supported catalysts shows that triphase catalysis is an efficient green technology in chemical synthesis.

Poly(ethylene glycol) Grafted onto Dowex Resin: An Efficient, Recyclable, and Mild Polymer-Supported Phase Transfer Catalyst for the Regioselective Azidolysis of Epoxides in Water

In this study, an efficient method was designed to graft poly(ethylene glycol) effectively onto commercial Dowex resin. The catalytic efficiency of the copolymer obtained as a new solid-liquid phase transfer catalyst was studied. It was proved that this organocatalyst is an efficient heterogeneous catalyst for regioselective azidolysis of epoxide in water and gave azidohydrin in excellent yield under mild reaction conditions. The polymeric catalyst was easily recovered by simple filtration and showed no appreciable loss of activity when recycled several times.

Effective Darzen's condensation of 4-nonanolide with 1,6-dibromohexan-2-one using new bead-shaped insoluble multi-site (six-site) phase transfer catalyst and low concentration of aqueous NaOH

New bead-shaped insoluble polymer-supported multi-site (six-site) phase transfer catalyst (BSIMPTC) was synthesized from mesitylene as a starting material and its catalytic efficiency was studied with the kinetics of Darzen's condensation of 4-nonanolide with 1,6-dibromohexan-2-one. The presence of six active-sites in the newly synthesized BSIMPTC, viz., 2,4,6-tris[(4-(2,2′-bis( N-triethylammoniummethylene chloride) eth-1-ene) phenoxy)benzene was characterized through FT-IR, solid 1H NMR, 13C NMR, SEM and [chloride ion] analyses and found that the BSIMPTC contains ≈6 active-site centers. The Darzen's condensation reaction of 4-nonanolide was performed at lower temperature (40 °C) under pseudo-first order rate conditions by taking lower concentration of aqueous NaOH (15%, w/v, 3.75 M) and excess of 1,6-dibromohexan-2-one. The disappearance of 4-nonanolide was quantitatively monitored by a gas chromatograph for the calculation of the pseudo-first order rate constant. The presence of more number of active-sites in BSIMPTC was further confirmed from the comparative study of pseudo-first order rate constant with the rate constants of single-site polymer-supported phase transfer catalyst. The comparative rate constant results reveal that the BSIMPTC is highly active-than the corresponding soluble six-active-site and almost ≈4 times higher active-than with insoluble/soluble single-site PTCs. The effect of various experimental parameters such as [substrate], [catalyst], [NaOH], stirring speed, and temperature on the rate of the reaction have also been studied and found that each variables are influenced the rate of reaction. Based on the obtained kinetic results, a suitable mechanism is proposed.

Synthesis and characterization of novel bead-shaped insoluble polymer-supported tri-site phase transfer catalyst and its efficiency in N-alkylation of pyrrole

Abstract A new bead-shaped insoluble polymer-supported multi-site (tri-site) phase transfer catalyst (PS-MPTC) viz., polymer-supported 4-(2,2,2-tris[triethylammoniummethylene chloride]-1-hydroxy ethyl) phenylether (PSTTEACHPE) has been synthesized by following a simplified procedure. Initially, polystyrene-based cross-linked beads was prepared by allowing the reaction using 2% divinylbenzene (as a cross linking monomer), 25% vinylbenzyl chloride (as a functional monomer) and styrene (as supporting monomer) to proceed through a suspension copolymerization method. The obtained terpolymer beads containing pendant benzyl chloride groups were condensed with 4-(2,2,2-tris[triethylammonium methylene chloride]-1-hydroxy ethyl) phenol, followed by quaternization using triethylamine, resulting in the formation of polymer-supported 4-(2,2,2-tris[triethylammoniummethylene chloride]-1-hydroxy ethyl) phenyl ether (PSTTEACHPE). Then the presence of tri-active site in PSTTEACHPE was examined by FT-IR, 1 H NMR, 13 C NMR, [chloride ion] and SEM analyses. The comparative results of [chloride ion] and the peak intensity of C–N stretching (quaternary onium group) in FT-IR were found to increased three fold in PSTTEACHPE on comparing with their similar types of polymer-supported bead-shaped single-site PTC (PS-BSPTC); thus we confirmed the presence of three active sites. The surface morphology study through SEM shows that the PSTTEACHPE is heterogeneous in nature with tiny nodules; whereas the single-site PS-BSPTC shows smooth homogeneous nature; this is an another piece of evidence for the availability of more numbers of active sites in PSTTEACHPE. Further, the catalytic efficiency of this new PSTTEACHPE was ascertained by N -alkylation of pyrrole with benzylchloride using 30% (w/v) aqueous sodium hydroxide at 60 °C. The pseudo-first order rate constants were calculated by following the disappearance of pyrrole through gas chromatography. The observed rate constants were found to be three-fold higher for PSTTEACHPE than with PS-BSPTC, which again confirmed the presence of three active-sites. Investigations were also made on kinetic variables such as stirring speed, [substrate], [catalyst], [NaOH] and temperature; it is observed that each variable is influenced the rate of reaction. From the Arrhenius plot, the activation energy and thermodynamic parameters were evaluated as 12.44, −7.99, 10.99 and 13.25 kcal mol −1 for E a , Δ S # , Δ H # and Δ G # respectively. Based on the observed kinetic and thermodynamic results, an interfacial mechanism was proposed for the N -alkylation of pyrrole.

Synthesis of dialkyl peroxides in the presence of polymer-supported phase-transfer catalysts

The synthesis of dialkyl peroxides from alkyl hydroperoxides and alkyl bromides in the presence of polymer-supported phase-transfer catalysts has been investigated. It was revealed that polymer-bound quaternary ammonium and polyethylene glycol catalysts were more active than their soluble forms. High yields (60–92%) of the products were obtained. The catalyst could be recycled several times with no appreciable loss of activity.

Phase-Transfer-Catalyzed Etherification of 4,4‘-Bis(chloromethyl)-1,1‘-biphenyl with 1-Butanol by Polymer-Supported Catalysis

The synthesis of 4,4‘-bis(butanoxymethyl)-1,1‘-biphenyl from the reaction of 4,4‘-bis(chloromethyl)-1,1‘-biphenyl and 1-butanol catalyzed by a polymer with quaternary ammonium groups (triphase catalysis) was carried out in a chlorobenzene/aqueous solution of a KOH two-phase medium. The polymer-supported phase-transfer catalysts were prepared using free-radical polymerization of styrene (St) and 4-vinylbenzyl chloride (4-VBC), followed by quaternized copolymer beads with various tertiary amines. The copolymer particles thus produced are uniform without sieving. This work emphasizes the comparison of the reactivity of the various functional groups attached to the copolymer beads. The effects of the various reaction conditions such as the agitation speed, amount of water, triphase catalyst, potassium hydroxide, organic solvents, temperature, and reusability of the triphase catalyst on the conversion and the reaction rate were investigated. Experimental results show that the triphase catalysts with quaternary ammonium groups facilitate the etherification. A comparison of the reactivity between two-phase and triphase catalysis in synthesizing 4,4‘-bis(butanoxymethyl)-1,1‘-biphenyl was made.

A Modified Method for the Synthesis and Characterization of a Novel Surface-Enriched (with Active Site) Polymer-Supported Phase Transfer Catalyst Using Vinyltoluene as a Functionality and Its Catalytic Efficiency in Dichlorocarbene Addition to Olefines—A Kinetic Study

A modified method with respect to an earlier conventional procedure was successfully attempted and we now report the new procedure for the synthesis of surface-grafting technique by the delayed addition of inexpensive vinyltoluene (VT) functionality to the partially polymerized supporting monomers viz., styrene (St) and divinylbenzene (DVB) via suspension polymerization. The resulting copolymer beads containing surface-enriched with methyl group were converted into a catalyst by chlorination followed by a quaternization process. Two groups (I and II) of SE-PSPTC beads were prepared by fixing the crosslinking amount (DVB) as 2% and 6%, respectively, 25% of VT as a common active site functionality and the rest of the contribution was styrene. Each group contains six different catalysts based on the concentration of active sites on the surface which in turn depends upon the partial polymerization time (PPT) of St/DVB i.e., 0, 3, 6, 9, 12, and 15 hrs. These two different groups and 6 different catalysts categories in each were prepared with the objective to learn the influence of higher/lower crosslinking in the process of surface-grafting of poly(VT) on poly(St/DVB) and to determine the optimized partial polymerization time to bring the maximum active site on the surface. The level of enrichment of an active site of all the catalysts were determined through [chloride], FT-IR, SEM, EDAX, and ESCA analyses. The gradual increase of [chloride], C-N peak intensity from FT-IR, rough surface/nodules concentration from SEM, % of surface chloride from EDAX, decreasing/increase trend of surface carbon/surface chloride in ESCA analyses starting with 0 VT to 15 VT irrespective groups, confirms the gradual increase of active site on the surface beads based on the PPT. The results of [chloride], spectral analyses and the rate constants of all the catalysts in dichlorocarbene addition to various olefines ensure that 9 VT-SE-PSPTC was the best catalyst beads in both the groups due to its two-fold enhancement rather than the conventional catalyst (0 VT) of both groups although we had added the same amount of VT in both preparations.

Preparation and spectroscopic characterization of surface‐enriched (with active sites) polymer‐supported phase‐transfer catalysts and their efficiency in organic addition reactions: A kinetic study

AbstractWe prepared two batches of surface‐enriched (with active sites) polymer‐supported phase‐transfer catalysts (SE‐PSPTC) by fixing the crosslinking monomer divinylbenzene (DVB) at 2% (first batch) and 6% (second batch) through a free‐radical suspension copolymerization method with vinylbenzyl chloride (VBC; 25%) as a functionality and with styrene (St) as a supporting monomer, followed by the quaternization of the resulting terpolymer beads with triethylamine. The enrichment of the active sites on the surfaces of the beads was accomplished by a surface‐grafting technique through the delayed addition of the functional monomer (VBC) to the partially polymerized copolymer beads of poly(St/DVB). To bring the active sites fully onto the surfaces, we prepared six different types of terpolymer beads in each batch by varying the partial polymerization time (PPT) of St/DVB—0 h [0 VBC (conventional)], 3 h (3 VBC), 6 h (6 VBC), 9 h (9 VBC), 12 h (12 VBC), and 15 h (15 VBC)—and then gradually adding the functional monomer (VBC) to the partially polymerized poly(St/DVB) system. The resulting terpolymer beads, containing different concentrations of pendant benzyl chloride (CH2Cl) on the surface in each batch, underwent facile quaternization [CH2N+(C2H5)3Cl−] with an increase in the PPT of St/DVB and remained constant at 12 VBC and 15 VBC. To asses the superiority of the catalysts according to the surface enrichment of the active sites, particularly between conventional (0 VBC) catalysts and other PPT‐based SE‐PSPTCs, we characterized all the catalysts by estimating the chloride‐ion concentration, by using Fourier transform infrared (FTIR), scanning electron microscopy (SEM), EDAX, and ESCA, and by carrying out the dichlorocarbene addition to olefins. The chloride‐ion concentration by the Volhard method and the peak intensity of the CN stretching absorbance concentration, that is, the quaternary onium group in the FTIR spectra of both batches, increased with the PPT of St/DVB in both batches of catalysts. In particular, the chloride concentration of a first‐batch catalyst of a representative mesh size (−120 + 140) had a twofold enhancement between the conventional catalyst (0 VBC; 1.88 m equiv g−1) and 9 VBC/SE‐PSPTC (3.74 m equiv g−1), although the same amount of the functional monomer was added in both preparations. These results showed the higher enrichment of the active site on the surface of 9 VBC, and the same trend was also maintained for second‐batch catalysts, regardless of the catalyst mesh size. SEM images of both batches showed that there was a higher concentration of nodules [due to the grafting of poly(VBC)] on the surfaces of the beads of 9 VBC/SE‐PSPTC and the aforementioned PPT catalysts than on the surfaces of the conventional catalysts (0 VBCs), which exhibited smooth surfaces (because of the simultaneous addition of all three monomers). This observation confirmed the enrichment of active sites on the surfaces. In the EDAX analysis, up to a depth of 0.5–1 μm, the surface chloride concentration increased from 0 VBC to 9 VBC/SE‐PSPTC and remained constant in 12 VBC and 15 VBC, first‐batch catalysts of a representative mesh size (−120 + 140). The same trend was also observed in second‐batch catalysts, indicating the enrichment of the onium group more on the surface in 9 VBC/SE‐PSPTCs. The ESCA analysis, to a depth of about 20–30Å, proved that the concentration of covalent chloride on the surface had increased from 0 VBC (15%) to 9 VBC/SE‐PSPTCs (29%) and remained constant thereafter in first‐batch catalyst; the trend was the same for second‐batch catalysts, also confirming the strong evidence of surface enrichment of the active sites. Similarly, the rate constants of different olefin addition reactions catalyzed by both batches of catalysts also increased from 0 VBC to 9 VBC and remained constant with 12 VBC and 15 VBC catalysts. The twofold increase of the rate constants, regardless of the olefins, for conventional catalysts to 9 VBC/SE‐PSPTCs confirmed the enrichment of the active sites on the surfaces. All these experimental observations proved that 50% of the active sites were successfully brought out from inside the poly(St/DVB) networks to the exterior surfaces, although same amount of VBC was added for the preparation of all the catalyst types. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 347–364, 2003

Enhanced activity of polymer-supported phase transfer catalysts

Phase transfer catalysis has been a weapon in the arsenal of synthetic organic chemists for over three decades now. While phase transfer catalysts (PTCs) have found widespread use in the industry, the polymer-supported phase transfer catalysts, also known as triphase catalysts, have not gained industrial acceptance. Triphase catalysts offer many advantages associated with heterogeneous catalysts such as easy separation from the reaction mixture, reusability, and use in continuous reactors. These significant advantages notwithstanding, the main reason for the lack of interest in the industry for the use of triphase catalysts has been its reduced activity in comparison with soluble-phase transfer catalysts. Due to their heterogeneous nature, triphase catalysts, in general, have lower reactivity. In this paper, we address this issue of reduced activity of triphase catalysts for a specific reaction, that is, phase transfer esterification of benzyl chloride with aqueous sodium acetate. It has been found that the triphase catalyst has a higher reactivity than its soluble analogs. The kinetics of the phase transfer reaction using both soluble and polymer-supported catalysts has been determined and the results are presented.

Synthesis and spectral characterization of surface-enriched polymer-supported phase transfer catalysts and their effects on alkylation of phenylacetone

Polymer-supported phase transfer catalysts with active sites mostly on the surface were prepared by suspension copolymerization of styrene (St), divinylbenzene (DVB), and vinylbenzyl chloride (VBC) with AIBN, followed by the quaternization of the resulting copolymer beads with triethylamine. Active sites on the surface were achieved by the delayed addition of functional monomer (VBC) to the partially copolymerized St/DVB. Polymer beads enriched with active sites were characterized by SEM, EDAX, FTIR, and ESCA. The electron micrographs showed that the exterior surface of delayed-addition functional monomer catalysts (type 1) has a large number of nodules attached to the surface compared to the smooth surface exhibited by the conventional type 2 catalyst upon the simultaneous addition of all three monomers. In the EDAX analysis up to a depth of 100 A, the surface chloride of type 1 peak intensity is greater (compared with type 2), indicating the CH2Cl enrichment on the surface. In FTIR, the peak intensities of the CN stretching (quaternary onium group) in type 1 are greater than those of type 2, confirming the evidence of more quaternization on the surface than in the bulk. From ESCA analysis to a depth of about 30 A, it was found that type 1 (beads) contains 26% and type 2 contains 14% of covalent chloride on the surface, which strongly supports the grafting of VBC on St/DVB. In the estimation found by the Volhard method, type 1 has 4.73 m eq g−1 and type 2 has 2.29 m eq g−1 of ionic chloride, thus supporting the surface grafting of VBC. The catalytic activity of these two different catalysts was tested by studying the reaction, that is, the C-alkylation of phenylacetone. The rate constants of this reaction for type 1 are almost twofold greater than those of type 2, a finding that could uphold the preceding experimental observations. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 408–418, 2000

Polymer-supported phase-transfer catalysts: synthesis and high catalytic activity of ammonium and phosphonium salts bound to linear and crosslinked poly(glycidyl methacrylate)

The synthesis of polymer-supported phase-transfer catalysts is described by the polymerization of glycidyl methacrylate (PGMA), either linear or crosslinked. The PGMA polymers were hydrolysed and later modified with chloroacetyl chloride. The chloroacetylated polymers were treated with either triphenylphosphine or triethylamine to produce polymer-supported phosphonium or ammonium salts. The catalytic behavior of the prepared catalysts was tested for some nucleophilic substitution reactions such as thiocyanation, cyanation, and nitration of benzyl bromide as model reactions.

Liquid-liquid-solid reactions: esterification of phenol with benzoyl chloride using a polymer-supported phase transfer catalyst

The reaction of phenol in alkaline solution and benzoyl chloride dissolved in toluene with a polymer-supported tri- n-butylphosphonium ion as phase transfer catalyst has been carried out in a slurry reactor. Mass transfer effects have been analysed with the standard theory of porous catalysts. The intrinsic reaction rate constant and diffusion coefficients within the porous polymer particle are estimated. The rate-limiting step has been identified.

Triphase catalysis for recovery of phenol from an aqueous alkaline stream

Use of a polymer-supported phase-transfer catalyst for the removal/recovery of phenol from an aqueous alkaline stream has been demonstrated. Phenol was extracted into the organic phase as phenyl benzoate via reaction with benzoyl chloride dissolved in toluene using two types of catalysts. Catalyst with a tributylphosphonium ion as the active centre was found to be more effective than Amberlite IRA 401(Cl) resin catalyst. The apparent reaction rates were found to obey pseudofirst-order kinetics under suitable conditions

Anodic oxidation of benzyl alcohol to benzaldehyde in the presence of both redox mediator and polymer-supported phase-transfer catalyst

Using a polymer-supported phase-transfer catalyst (PTC) and a redox mediator, Cl−/OCl−, the anodic oxidation of benzyl alcohol was studied. The polymer support used in this study was synthesized by the method of suspension polymerization. The results revealed that on decreasing the crosslinking and increasing the content of chloromethylstyrene in the polymer support, the number of phase transfer active sites, tri-n-butylphosphine, in the polymer-supported PTC increased. The anodic oxidation of benzyl alcohol in the presence of the redox mediator, Cl−/OCl−, and polymer-supported PTC occurred in the reaction controlled region when the stirring rate exceeded 400 r.p.m. The results also revealed that the current efficiency for the anodic oxidation of benzyl alcohol was mainly affected by the characteristics and weight of the polymer-supported PTC, the pH, and the concentration of benzyl alcohol. However, the temperature affected the current efficiency only slightly.

Hydrophilic recognition by polymer-supported phase transfer catalysts and its effect on reaction activity and selectivity

Cross-linked polystyrene latexes with surface-bound quaternary ammonium groups have been prepared by a two-step method. First the functional monomer vinylbenzyl chloride (VBC) was introduced into a partially polymerized concentrated emulsion of styrene and divinylbenzene (ST/DVB) in water and complete polymerization was carried out. Second the poly(ST/DVB) particles with surface-bound benzyl chloride groups were quaternized. The dual-surface characteristics due to the hydrophilic bound cations and hydrophobic polystyrene can be varied by controlling the partial polymerization time of ST/DVB and the amount of VBC introduced. The phase transfer catalysts thus obtained were employed in the alkylation reaction of isopropylidene malonate (Meldrum acid). The ratio of mono- to dialkylation products was observed to increase with decreasing accessible hydrophilicity of the supported catalysts. When the partial polymerization time is short, a part of VBC copolymerizes with styrene and another part homopolymerizes and is grafted to the surface of the particles. When the partial polymerization time is long, only homopolymers of VBC, which are grafted to the particles, form. The ammonium groups to which the benzyl chloride units are converted via quaternization are more accessible, for steric reasons, to the Meldrum acid and to the monosubstituted Meldrum acid when they are part of the grafted chains than when they are part of the copolymer. As a result, the activity for the conversion of the Meldrum acid increases as the partial polymerization time increases. However, the selectivity for the monosubstituted Meldrum acid increases with decreasing partial polymerization time of the concentrated emulsion, because the fraction of hydrophilic groups that are accessible decreases and the monosubstituted compound has more difficulty, for steric reasons, binding to the onium cations than the unsubstituted compound.

Kinetic studies on an improved Williamson ether synthesis using a polymer-supported phase-transfer catalyst.

Kinetic studies on an improved Williamson ether synthesis using a polymer-supported phase-transfer catalyst.

Some novel polymer-supported optically active phase transfer catalysts: 2. Use in displacement, reduction, epoxidation and addition reactions

The performance of 27 novel polymer-supported chiral phase transfer catalysts in a range of phase separated reactions has been studied. The later include a number of displacement reactions, sodium borohydride reductions of prochiral ketones, the epoxidation of chalcone, the addition of nitromethane to chalcone and the addition of thiophenol to cyclohexenone. Some of the reactions were performed under liquid/liquid conditions and others using solid/liquid. No systematic difference emerged between these. In general many of the catalysts provided fast reactions and good chemical yields of products. However, in no case was any significant enantiomeric excess achieved. The latter observation is discussed in the light of the reported behaviour of analogous non-supported catalytic systems, and suggestions proposed to explain these differences.