Distribution Of Catalytic Sites Research Articles

Chiral Aldehyde Catalysis-Enabled Asymmetric α-Functionalization of Activated Primary Amines.

ConspectusThe development of catalytic activation modes provides a reliable and effective platform for designing new enantioselective reactions and preparing chiral molecules with diverse structures. Chiral aldehyde catalysis is an attractive concept in asymmetric catalysis, which utilizes a chiral aldehyde catalyst to promote the asymmetric hydroamination of allylic amines, the asymmetric α-functionalization of primary amines, or the asymmetric transamination of α-keto esters. Typically, the chiral aldehyde-catalyzed asymmetric α-functionalization of primary amines provides an efficient and straightforward method for the synthesis of α-functionalized chiral amines, which does not require any additional protection or deprotection manipulations of the amine group. However, achieving catalytic stereoselective transformations with high efficiency and enantioselectivity by this strategy has remained an intractable challenge.This Account summarizes our endeavors in the development and application of chiral aldehyde catalysis. Using a chiral aldehyde as a catalyst, we reported the catalytic asymmetric α-C alkylation of 2-aminomalonate with 3-indolylmethanol in 2014, which represents the first chiral aldehyde-catalyzed asymmetric α-functionalization of an activated primary amine. Subsequently, several axially chiral aldehyde catalysts were continuously prepared by using chiral BINOL as the starting material, and their applications in asymmetric synthesis were explored. On the one hand, they were used as organocatalysts to realize the various transformations of α-amino acid esters, such as asymmetric 1,4-addition toward conjugated enones/α,β-unsaturated diesters and cyclic 1-azadienes as well as asymmetric α-arylation/allylation and benzylation with corresponding halohydrocarbons. Notably, taking advantage of the difference in the distribution of catalytic sites between two chiral aldehyde catalysts, we disclosed chiral aldehyde-catalyzed diastereodivergent 1,6-conjugated addition and Mannich reactions. On the other hand, the potential for the cooperative catalysis of a chiral aldehyde with a transition metal has also been demonstrated. Enabled by the combination of a chiral aldehyde, a palladium complex, and a Lewis acid, the enantioselective α-allylation of amino acid esters with allyl alcohol esters was established. Moreover, the ternary catalytic system has been successfully used for the α-functionalization of amino acid esters with 1,3-dienes, allenes, allenylic alcohol esters, 1,3-disubstituted allyl alcohol esters, and arylmethanol esters as well as the asymmetric cascade Heck-alkylation reaction. The combination of a chiral aldehyde and nickel complex allows for the asymmetric α-propargylation of amino acid esters with propargylic alcohol esters and provides excellent enantioselectivities. These transformations provide a large library of optically active amines and amino acids. With those chiral amino acid esters as key building blocks, the synthesis or formal synthesis of multiple natural products and biologically significant unnatural molecules was accomplished. This includes the stereodivergent synthesis of natural pyrrolizidine alkaloid NP25302 and the formal synthesis of natural product (S)-hypoestestatin 1 and manzacidin C, clinical candidate compound (+)-AG-041R, and somatostatin mimetics. It is fully anticipated that chiral aldehyde catalysis will soon witness rapid expansion both in the development of novel asymmetric transformations and in innovative applications for constructing optically active nitrogen-containing molecules with significant values.

Functional Monolayers on a Superatomic Pegboard.

We advance the chemistry of apical chlorine substitution in the 2D superatomic semiconductor Re6Se8Cl2 to build functional and atomically precise monolayers on the surface of the 2D superatomic Re6Se8 substrate. We create a functional monolayer by installing surface (2,2'-bipyridine)-4-sulfide (Sbpy) groups that chelate to catalytically active metal complexes. Through this reaction chemistry, we can create monolayers where we can control the distribution of catalytic sites. As a demonstration, we create highly active electrocatalysts for the oxygen evolution reaction using monolayers of cobalt(acetylacetonate)2bipyridine. We can further produce a series of catalysts by incorporating organic spacers in the functional monolayers. The structure and flexibility of the surface linkers can affect the catalytic performance, possibly by tuning the coupling between the functional monolayer and the superatomic substrate. These studies establish that the Re6Se8 sheet behaves as a chemical pegboard: a surface amenable to geometrically and chemically well-defined modification to yield functional monolayers, in this case catalytically active, that are atomically precise. This is an effective method to generate diverse families of functional nanomaterials.

Significant effect of intra-chain distribution of catalytic sites on catalytic activity in “clickase” single-chain nanoparticles

“Clickase” single-chain nanoparticles (Ck-SCNPs) are folded, enzyme-mimetic unimolecular polymeric nano-objects containing copper (Cu) ions able to catalyze the azide-alkyne Huisgen cycloaddition reaction in water and/or selected organic solvents often in the presence of a reductant. Herein, we investigate the effect of morphology on catalytic activity of identical Ck-SCNPs either containing a homogeneous distribution of Cu catalytic sites (Ck-SCNPs1) or showing intra-chain clustering of Cu ions (Ck-SCNPs2). A model fluorogenic “click” reaction between 9-(azidomethyl)anthracene and phenylacetylene, which was catalyzed either by Ck-SCNPs1 or Ck-SCNPs2, was used to unravel the effect of intra-chain clustering on catalytic activity. This work paves the way to improve the catalytic activity of other metallo-folded SCNPs through control of the intra-chain distribution of catalytic sites.

Identification of the activity source of CO2 electroreduction by strategic catalytic site distribution in stable supramolecular structure system.

Identification of the real catalytic site in CO2 reduction reaction (CO2RR) is critical for the rational design of catalysts and the understanding of reactive mechanisms. In this study, the catalytic activity of pyridine-containing materials was for the first time structurally demonstrated in CO2RR by crystalline supramolecular coordination compounds model system. The system consists of three stable supramolecular coordination compounds (Ni-TPYP, Ni-TPYP-1 and Ni-TPP) with different numbers (4, 2 and 0) of active pyridine groups (i.e. uncoordinated pyridine nitrogen atoms). The electrocatalytic test results show that with the decrease of the number of active pyridine groups, the CO2RR performance is gradually reduced, mainly showing the reduction of highest FECO (99.8%, 83.7% and 25.6%, respectively). The crystallographic, experimental and theoretical evidences prove that the CO2RR activity is more likely derived from uncoordinated pyridine nitrogen than the electrocatalytic inert metal nickel in porphyrin center. This work serves as an important case study for the identification of electrocatalytic activity of pyridine-containing materials in CO2RR by simple supramolecular model system.

High purity H2 production from sorption enhanced bio-ethanol reforming via sol-gel-derived Ni–CaO–Al2O3 bi-functional materials

Catalytic steam reforming of renewable bio-oxygenates coupled with in-situ CO2 capture is a promising option for sustainable H2 production. The current work focuses on high purity H2 production over Ni–CaO–Al2O3 bi-functional materials via sorption enhanced steam reforming of ethanol (SEESR). To ensure the uniform distribution of catalytic sites (Ni), adsorptive sites (CaO) and stabilizer (Al2O3) in the bi-functional materials, a citrate sol-gel synthetic route was employed. These materials were characterized by XRD, N2 physical adsorption, SEM, TG and TPR techniques. It was revealed that the existence of CaO in bi-functional materials could not only in-situ remove CO2, but also play the role of inhibiting the formation of harmful spinel phase. The stabilizing role of Al component against capacity decay was confirmed, whereas the presence of Ni ions had a negative effect on the cycle CO2 uptake. The sample of Ni/Al/Ca-85.5 possessed large specific surface area, abundant porosity with fluffy morphology, and thereby, exhibited the best CO2 sorption capacity during 20 carbonation/calcination cycles. The highest H2 concentration of 96% was obtained through the SEESR during the pre-breakthrough period when the Ni/Al/Ca-85.5 was employed. Over the optimized bi-functional material, the effect of operating conditions on the SEESR was investigated and the results indicated that temperature of 600 °C, reaction liquid space velocity of 0.05 ml/min and steam/ethanol ratio of 4 were the suitable conditions. After 10 cycles, the bi-functional material of Ni/Al/Ca-85.5 also showed the best performance, with a H2 purity of about 90% and pre-breakthrough time of 18 min, conforming the high potential of this material for SEESR process.

Zwitterionic Covalent Organic Frameworks as Catalysts for Hierarchical Reduction of CO2 with Amine and Hydrosilane.

Controllable hierarchical reduction of carbon dioxide (CO2) to selectively afford versatile chemicals with specific carbon oxidation state is important but still remains a huge challenge to be realized. Here, we report new zwitterionic covalent organic frameworks ([BE] X%-TD-COFs), prepared by introducing betaine groups (BE) onto the channel walls of presynthesized frameworks via pore surface engineering methodology, as the heterogeneous organocatalysts for CO2 reduction. The adjustable density of immobilized BE groups as well as good preservation of crystallinity and porosity inherited from their parent COFs endow [BE] X%-TD-COFs with highly ordered catalytic site distribution and one-dimensional mass transport pathway in favor of catalysis. By controlling the reaction temperature and amount of CO2, [BE] X%-TD-COFs present high activity in catalyzing reduction of CO2 with amine and phenylsilane (PhSiH3) to produce formamides, aminals, and methylamines, respectively, with high yield and selectivity. Furthermore, high stability and insolubility bring excellent reusability to [BE]X%-TD-COFs with well-maintained catalytic performance after four cycles of use. Notably, this is a novel example that COFs are developed as heterogeneous catalysts for hierarchical two-, four-, and six-electron reduction of CO2 with amines and PhSiH3 to form C-N bonds as well as afford C+II, C0, and C-II species efficiently and selectively.

Nanostructured Ceramic Photocatalytic Membrane Modified with a Polymer Template for Textile Wastewater Treatment

Photocatalytic ceramic membranes have attracted considerable attention for industrial wastewater treatment. However, morphological control of the membrane surface to improve its photocatalytic reactivity for the degradation of organic pollutants remains a challenge. Herein, we report a new nanostructured TiO2/Al2O3 composite ceramic membrane prepared from a poly(oxyethylene methacrylate) (POEM) template through a sol–gel method and its photocatalytic performance in the treatment of a model dye compound. The POEM polymeric template allowed the homogeneous distribution of catalytic sites, i.e., the TiO2 layer, on the Al2O3 membrane surface, resulting in improved organic dye degradation along with effective fouling mitigation. The immobilization of a TiO2 layer on the Al2O3 membrane support also significantly enhanced the membrane adsorption capacity toward dye organic compounds. An organic removal efficiency of over 96% was achieved with the TiO2/Al2O3 composite membrane under Ultraviolet (UV) irradiation. In addition, the self-cleaning efficiency of the TiO2/Al2O3 composite membrane was remarkably improved by the degradation of organic foulants on the membrane under UV illumination.

Oxo-vanadium (IV) complex supported by microporous organic nanotube frameworks: A high selective heterogeneous catalyst for the oxidation of thiols to disulfides

An efficient oxidation catalyst has been synthesized by anchoring oxo-vanadium (IV) onto the amino-functional microporous organic nanotube frameworks (NH2-MONFs). The amino groups are used to immobilize oxo-vanadium (IV) into the porous organic polymer supporter. The resulting oxo-vanadium (IV) complexes have a high surface area, large pore volume, hierarchically porous structure and robust organic network, which have great potential application in heterogeneous catalysis. The introduction of mesoporous tubular channels in the network units improves mass transfer in the catalytic system and the well distribution of catalytic sites in the channels facilitates the accessibility of active sites, which will be beneficial for good catalytic performance in heterogeneous catalysis. The prepared oxo-vanadium heterogeneous catalyst shows high catalytic activity and excellent stability in the selective oxidation reactions of thiols to disulfides.

A Solvent-Based Strategy for Tuning the Internal Structure of Metallo-Folded Single-Chain Nanoparticles.

Controlling the spatial distribution of catalytic sites in metallo-folded single-chain nanoparticles (SCNPs) is a first step toward the rational design of improved catalytic soft nano-objects. Here an unexplored pathway is reported for tuning the internal structure of metallo-folded SCNPs. Unlike the conventional SCNP synthesis in good solvent (protocol I), the proposed new route (protocol II) is based on the use of amphiphilic random copolymers and transfer, after SCNP formation, from selective to good (nonselective) solvent conditions. The size and morphology of the SCNPs obtained by the two protocols, and the corresponding spatial distribution of the catalytic sites, have been determined by combining results from size exclusion chromatography with triple detection, small-angle X-ray scattering and molecular dynamics (MD) simulations. Remarkably, the use of these protocols allows the tuning of the internal structure of the metallo-folded SCNPs, as supported by MD simulations results. While the conventional protocol I yields a homogeneous distribution of the catalytic sites in the SCNP, these are arranged into clusters in the case of protocol II.

Conversion Reactions in Surface-Functionalized Mesoporous Materials: Effect of Restricted Transport and Catalytic Site Distribution

ABSTRACTWe analyze the interplay between anomalous transport and conversion reaction kinetics in mesoporous materials functionalized with catalytic groups. Of primary interest is functionalized mesoporous silica containing an array of linear pores with diameters tunable from 2-10 nm, although functionalization can produce smaller effective diameters, d. For d < 2 nm, transport and specifically passing of reactant and product species within the pores can be strongly inhibited (single-file diffusion). The distribution of catalytic groups can also vary depending on the synthesis approach. We apply statistical mechanical modeling (utilizing spatially discrete stochastic lattice-gas models) to explore the dependence of reactivity on the extent of inhibition of passing of species within the pore, as well as on the distribution of catalytic sites.

Perturbing Effects of Chiral Stationary Phase on Enantiomerization Second-Order Rate Constants Determined by Enantioselective Dynamic High-Performance Liquid Chromatography: A Practical Tool to Quantify the Accessible Acid and Basic Catalytic Sites Bonded on Chromatographic Supports

Second-order rate constants of the diethylamine-promoted enantiomerization of 2-[2-(1-methyl-1H-pyrrol-2-yl)-2-oxo-1-phenylethyl]-isoindole-1,3-dione, a chiral alpha-substituted ketone endowed with high anti-MAO activity type-A, were measured by dynamic high-performance liquid chromatography (DHPLC), stopped-flow high-performance liquid chromatography (sf-HPLC), and a classical method based on enantioselective HPLC as the monitoring tool. The chiral column used in all determinations was the commercial Chiralpak AD. By comparison of the obtained data, perturbing effects of the stationary phase on the DHPLC and sf-HPLC determinations were highlighted and distinguished in indirect (SP(IPC)) and direct (SP(DPC)) type. It was evidenced that SP(DPC) noise effects may be completely erased by simple mathematical treatment of data obtained at different concentrations of the basic catalyst. Perturbations of type SP(IPC) may instead only be partially kept down by modulating the concentration of the basic catalyst. An estimation of the density distribution of catalytic sites covalently bonded to the stationary phase (SP) of the Chiralpak AD was performed exploiting the quantified SP(DPC) effects. Such an approach might be of general application, supplying a useful way to characterize the attitude of SPs to speed acid- or base-catalyzed equilibria possibly active during chromatographic separations.

Multifunctional Aspects of Three-Way Catalyst: Effects of Complex Washcoat Composition

In this paper multifunctional aspects in operation of Pt/Rh/Ce/γ-Al 2 O 3 three-way catalyst (TWC) are discussed on the basis of computed spatiotemporal patterns of reacting and deposited species on specific active sites distributed in the catalytic washcoat layer. A spatially pseudo-one-dimensional, heterogeneous model of a TWC reactor has been developed, with well-mixed gas phase and description of diffusion in the porous catalytic washcoat. The model includes a detailed non-stationary kinetic scheme for CO and C 2 H 2 oxidation, oxygen storage on CeO 2 , CO 2 adsorption on the support and NO x transformation and reduction. Pt, Rh, Ce and γ-Al 2 O 3 sites are considered with different kinetic parameters. A methodology is developed for the studies of the effects of internal diffusion and washcoat composition (i.e. concentration and distribution of active Pt, Rh and Ce catalytic sites) on the conversion of main pollutants in automobile exhaust gas under transient conditions. Several cases of variable distribution of the catalytic sites in the washcoat layer are discussed with respect to the reactor performance. Effects of oscillations in the inlet oxygen concentration are studied. Complex interactions, particularly between reaction intermediates and the stored oxygen within the washcoat, cause the increase of conversions in the periodic operation when compared with the stationary one.

Development of a Genetic Algorithm for Molecular Scale Catalyst Design

A genetic algorithm has been developed to determine the optimal design of a two-component catalyst for the diffusion-limitedA+B→AB↑ reaction in which each species is adsorbed specifically on one of two types of sites. Optimisation of the distribution of catalytic sites on the surface is achieved by means of an evolutionary algorithm which repeatedly selects the more active surfaces from a population of possible solutions leading to a gradual improvement in the activity of the catalyst surface. A Monte Carlo simulation is used to determine the activity of each of the catalyst surfaces. It is found that for a reacting mixture composed of equal amounts of each component the optimal active site distribution is that of a checkerboard, this solution being approximately 25% more active than a random site distribution. Study of a range of reactant compositions has shown the optimal distribution of catalytically active sites to be dependent on the composition of the ratio ofAtoBin the reacting mixture. The potential for application of the optimisation method introduced here to other catalysts systems is discussed.

Molecular neuroanatomy of human monoamine oxidases A and B revealed by quantitative enzyme radioautography and in situ hybridization histochemistry

Monoamine oxidases are key enzymes in the metabolism of amine neurotransmitters and neuromodulators and are targets for drug therapy in depression, Parkinson's and Alzheimer's diseases. Knowledge of their distribution in the brain is essential to understand their physiological role. To study the regional distribution and abundance of monoamine oxidases A and B in human brain, pituitary and superior cervical ganglion, we used quantitative enzyme radioautography with radioligands [ 3H]Ro41-1049 and [ 3H]lazabemide, respectively. Furthermore, 35S-labelled oligonucleotides complementary to isoenzyme messenger RNAs were used to map the cellular location of the respective transcripts in adjacent sections by in situ hybridization histochemistry. A markedly different pattern of distribution of the isoenzymes was observed. Highest levels of monoamine oxidase A were measured in the superior cervical ganglion, locus coeruleus, interpeduncular nucleus and ventromedial hypothalamic nucleus. The corresponding messengerRNA was detected only in the noradrenergic neurons of the superior cervical ganglion and locus coeruleus. In contrast to rat brain, monoamine oxidase B was much more abundant in most human brain regions investigated. Highest levels were measured in the ependyma of ventricles, stria terminalis and in individual hypothalamic neurons. Monoamine oxidase B transcripts were detected in serotoninergic raphe neurons, histaminergic hypothalamic neurons and in dentate gyrus granule cells of the hippocampal formation. We conclude that [ 3H]Ro41-1049 and [ 3H]lazabemide are extremely useful radioligands for high-resolution analyses of the abundance and distribution of catalytic sites of monoamine oxidases A and B, respectively, in human brain sections. From levels of messenger RNA detected, the cellular sites of synthesis of the isoenzymes are the noradrenergic neurons of the locus coeruleus (for monoamine oxidase A) and the serotoninergic and histaminergic neurons of the raphe and posterior hypothalamus, respectively (for monoamine oxidase B). The combination of quantitative enzyme radioautography with in situ hybridization histochemistry is a useful approach to study, with high resolution, both the physiology and pathophysiology of monoamine oxidases in human brain.

Immobilization of flavin on highly porous polymeric disks: Three routes to a catalytically active membrane

Disks obtained by polymerization of high internal phase emulsions (Polyhipe) had completely open pore structures and were used as a carrier material for the immobilisation of 10-ethyl-isoalloxazine (‘flavinlrs). Three methods for immobilization are described: (1) direct modification of chloromethylated Polyhipe with flavin and triethylamine, (2) deposition of a polyelectrolyte complex of flavin-containing polycations and poly(sodium styrenesulponate) onto the (internal) surface of a Polyhipe; and (3) complexation of flavin-containing polycations on the (internal) surface of a sulphonated Polyhipe. The catalytic activity per flavin moiety in the continuous aerobic oxidation of 1-benzyl-1,4-dihydronicotinamide depends on the method of immobilization and on the loading with catalytic moieties, factors influencing the accessibility and distribution of catalytic sites over the pore surface and the matrix of the Polyhipe. Optimum activity was found in the case of method 3. All methods resulted in excellent stability of the immobilized catalyst in continuous reactions. The linear polycations immobilized by methods 2 and 3 can be decomplexed completely after the continuous reaction, using ternary solvent and analyzed by homogeneous spectroscopic techniques, providing a powerful tool in the study of the fate of catalysts in continuous processes.

Autoxidation of thiols with cobalt(II)phthalocyanine‐tetra‐sodium sulfonate attached to poly(vinylamine), 6. Immobilized catalysts by crosslinking of poly(vinylamine)

AbstractPoly(vinylamine), being both carrier and promotor of the thiol oxidation catalyst cobalt(II)phthalocyanine‐tetra‐sodium sulfonate (CoPc(NaSO3)4), was crosslinked with α,α′‐dichloro‐p‐xylene, thus yielding porous hydrophilic networks. The effects of experimental parameters such as stirring speed, particle size, degree of crosslinking, distribution of catalytic sites (CoPc(NaSO3)4) in the catalyst particles, pH, temperature, and thiol concentration were investigated. Reaction rates observed for the immobilized catalyst system appeared to be 4–30 times lower as compared with the water soluble polymeric catalyst system but still higher than those of the polymer free CoPc(NaSO3)4 catalyst.At a stirring speed around 3 · 103 rpm not the mass transfer from the bulk to the catalyst particles but intra‐particle diffusion limits the reaction rate. Accordingly, an uncrosslinked polymeric catalyst anchored to silica, with catalytic sites situated close to the particle surface, exhibited comparatively high activity, i.e. only four times lower than the soluble polymeric catalyst.In addition, the heterogeneous catalyst systems showed resemblance in kinetic behaviour with the soluble polymeric thiol oxidation catalyst.

Digestive and absorptive functions along dog small intestine: comparative distributions in relation to biochemical and morphological parameters

1. 1. The digestive (hydrolytic enzymes) and absorptive (sugar and amino acid transport) functions of dog small intestine have been evaluated in different segments and analysed in relation to morphometric and biochemical parameters. 2. 2. The dog small intestine is a cylinder of decreasing diameter in which the underlying mueosa thins down from duodenum to ileum, though maintaining its cellular homogeneity as revealed by measuring the mucosal weight, the total DNA and protein content and the protein content of the brush border membrane. 3. 3. Sucrase, gamma-glutamyltranspeptidase, leueylnaphthylamidase and alkaline phosphatase specific activities, measured both in homogenates of the mucosa and purified brush border membrane fractions, were found distributed along proximo-distal gradients of activity. However, different patterns were obtained which are specific for the enzyme considered. 4. 4. Kinetic parameters, V max and K m, were estimated for sucrase and alkaline phosphatase in purified brush border membrane fractions. It appeared that V max correlated well with the observed distribution of catalytic sites along the small intestine. 5. 5. Sugar (glucose) and amino acid (alanine and leucine) transport capacities were also distributed according to specific proximo-distal gradients but passive and facilitated diffusions were not affected. Only the active, Na +-dependent component of transport was sensitive to position along the small intestine and we postulated that this Actaptation should involve variations in carrier densities. 6. 6. It is therefore concluded that absorbo-digestive functions are intrinsic characteristics of the brush border membrane which are regulated according to the position along the small intestine.

Molecular forms of acetylcholinesterase: regulation in a testosterone-sensitive nerve-muscle axis.

We measured the distribution of molecular forms of acetylcholinesterase (AChE) in muscles of a song bird, the zebra finch, and found a pattern similar to those reported in other vertebrates. As in other species, the most rapidly sedimenting form of the enzyme decreases to barely detectable levels following denervation. In the muscles of the syrinx, castration causes a large decrease in AChE activity, but has little or no effect on the relative abundance of AChE forms. This suggests that the number of AChE catalytic sites is changing without affecting the distribution of catalytic sites among the molecular forms. This is in marked contrast with the effect of denervation in the syrinx, which causes changes in the distribution of activity, as well as in total activity.

Stopped-flow Chromatography as a Tool for Studying the Distribution of Reactive Sites on a Catalyst Surface: Methanation of CO on Ru-SiO2

Stopped-flow chromatography may be used to investigate the activity distribution of catalytic sites, provided certain assumptions are made about the catalytic reaction. Oxford applied the method to the methanation of CO on an Ru-SiO/sub 2/ surface and used the distributions obtained as a function of temperature to investigate the distribution of activation energy and pre-exponential factor across the surface.

Influence of spacer chains and percentage ring substitution on phase-transfer catalytic activity of phosphonium salts bonded to polystyrene matrix

In nucleophilic aliphatic substitutions carried out under liquid–liquid phase-transfer conditions, the catalytic activity of tributylphosphonium groups directly bonded to a polystyrene matrix through a methylene bridge decreases by about one order of magnitude on passing from 10 to 60% ring substitution; log kobs. is linearly related to the loading of the catalytic sites. When tributylphosphonium groups are bonded to the polymer matrix by a linear spacer chain of 13 atoms for 10–30% ring substitution, kobs. increases 1.7–3.1 times with respect to directly bonded catalysts. However, for spaced catalysts with 60% ring substitution, the observed rate increase is 4.1–10-fold. This behaviour is explained by the combination of two opposing effects: (i) a polarity increase at the catalytic site by increasing percentage ring substitution leading to a decrease of nucleophilic activity of the anions; (ii) a polarity decrease at the catalytic site by insertion of spacer chains, due to the intrinsic lipophilicity of the alkyl chains as well as the more even distribution of catalytic sites within the polymer matrix. The reactivities of the catalysts, all prepared from chloromethylated polystyrene (200–400 mesh), are independent of the size of particles separated by sieves.