Reaction-controlled phase-transfer catalyst

Commercially available 30% hydrogen peroxide solution can oxidize alkenes readily in the presence of a carbo-diimide promoter <1996SL649, 1998JOC2564, 1998JOC1730> (Equation 67). A method to epoxidize propene using aqueous hydrogen peroxide and a reaction controlled phase-transfer catalysts was developed <20040PD131>. 

Reaction-Controlled Phase-Transfer Catalyst Based on Quaternary Ammonium Phosphotungstates 431... 

REACTION-CONTROLLED PHASE-TRANSFER CATALYST BASED ON QUATERNARY AMMONIUM PHOSPHOTUNGSTATES... 

Catalyst I, containing the small tetrapropyl ammonium ion, was insoluble in the reaction medium during the epoxidation, so both the conversion and selectivity were low, only 60.6% and 60.2%, respectively. Catalyst A is a reaction-controlled phase-transfer catalyst with high catalytic activity and selectivity. Although catalyst 11 also has good catalytic performance, it was totally soluble in the reaction system during and after the epoxidation, because it contains a big octadecyl benzyl methyl ammonium ion. This makes catalyst recovery difficult. 

Recently, considerable effort has been expended on commercializing this technology. A new-generation reaction-controlled phase-transfer catalyst (catalyst D) was developed with lower production cost and a higher recovery yield (>95%) of the catalyst which makes it more economical. 

Since PO is water soluble, a few homogeneous catalytic systems employing aqueous H2O2 as oxidant have been reported [34]. This section report results on the homogeneous catalytic epoxidation of propylene to PO with 52% H2O2 using the reaction-controlled phase-transfer catalyst A [45]. The catalyst is easily... 

M. Li, X. Jian, T. M. Han, L. Liu, Y. Shi, Reaction-controlled phase-transfer catalyst K3PV4024 for synthesis of phenol from benzene. Chin. J. Catal. 25 (2004) 681. 

M. Guo, Catalytic oxidation of cyclohexene to adipic acid with a reaction-controlled phase transfer catalyst. Chin. J. Catal. 24 (2003) 483. 

This chapter will present recent progress made in reaction-controlled phase-transfer catalysis for the epoxidation of olefins, focusing on work with hetero-pol)q5hosphotungstates and quaternary ammonium ions from our group. We have systemically investigated the influence of composition of the heteropoly anion and various quaternary ammonium ions on the catalyst activity. The epoxidation of propylene, allyl chloride, and others olefins and the stability of the catalyst in recycle will be summarized and discussed in detail. 

The investigation of the chemical modification of dextran to determine the importance of various reaction parameters that may eventually allow the controlled synthesis of dextran-modified materials has began. The initial parameter chosen was reactant molar ratio, since this reaction variable has previously been found to greatly influence other interfacial condensations. Phase transfer catalysts, PTC s, have been successfully employed in the synthesis of various metal-containing polyethers and polyamines (for instance 26). Thus, the effect of various PTC s was also studied as a function of reactant molar ratio. 

Following a patented procedure for the conversion of 2,4-dinitrochlorobenzene to 5-chloro-2-nitrophenol, 2,4-difluoronitrobenzene was treated with sodium hydroxide in hot aqueous dioxane containing a phase transfer catalyst. On the small scale, the reaction and isolation of 5-fluoro-2-nitrophenol, including vacuum distillation, were uneventful. On the 20 1 scale, vacuum distillation of combined batches of the crude product led to onset of decomposition at 150°C, which could not be controlled, and the residue erupted with explosive violence and a small fire ensued. Thermal examination of fresh small-scale crude material has shown that it is capable of highly exothermic decomposition, with onset of the exotherm at 150°C (ARC). It was then realised that difficulty in controlling the reaction temperature had been experienced on the 20 1 scale. It is recommended that this procedure and purification should not be attempted on so large a scale. 

Adogen has been shown to be an excellent phase-transfer catalyst for the per-carbonate oxidation of alcohols to the corresponding carbonyl compounds [1]. Generally, unsaturated alcohols are oxidized more readily than the saturated alcohols. The reaction is more effective when a catalytic amount of potassium dichromate is also added to the reaction mixture [ 1 ] comparable results have been obtained by the addition of catalytic amounts of pyridinium dichromate [2], The course of the corresponding oxidation of a-substituted benzylic alcohols is controlled by the nature of the a-substituent and the organic solvent. In addition to the expected ketones, cleavage of the a-substituent can occur with the formation of benzaldehyde, benzoic acid and benzoate esters. The cleavage products predominate when acetonitrile is used as the solvent [3]. 

A degree of stereoselective control of the course of a reaction, which is absent or different from that prevalent when the reaction is conducted in the absence of quaternary ammonium salts, may be achieved under standard phase-transfer catalysed reaction conditions. The reactions, which are influenced most by the phase-transfer catalyst, are those involving anionic intermediates whose preferred conformations or configurations can be controlled by the cationic species across the interface of the two-phase system. For example, in the base-catalysed Darzens condensation of aromatic aldehydes with a-chloroacetonitriles to produce oxiranes (Section 6.3), the intermediate anion may adopt either of the two conformations, (la) or (lb) which are stabilized by interaction across the interface by the cations (Scheme 12.1) [1-4]. 

Stereochemical control of Darzen s reaction of a-chlorophenylacetonitrile with benzaldehyde by phase-transfer catalysts... 

Specific control of the stereochemistry of the chemical reaction is better achieved using chiral phase-transfer catalysts. These catalysts interact specifically with the substrate and sterically hinder the approach of nucleophile to one face of the reactive site. Experimental procedures are essentially the same as those employed in reactions using achiral catalysts where there is no stereochemical control and, in subsequent sections, reference is made back to the appropriate Chapter unless variations in the procedure differ significantly. 

It has been shown that a phase-transfer catalyst could control the oxidation of sulfide to the corresponding sulfoxide. Thus, the oxidation of diaryl sulfides to sulfoxides using Oxone and PTC (equation 54) is in contrast with the reaction in polar solvents without PTC which gives sulfones as a major product. 

At one extreme diffusivity may be so low that chemical reaction takes place only at suface active sites. In that case p is equal to the fraction of active sites on the surface of the catalyst. Such a polymer-supported phase transfer catalyst would have extremely low activity. At the other extreme when diffusion is much faster than chemical reaction p = 1. In that case the observed reaction rate equals the intrinsic reaction rate. Between the extremes a combination of intraparticle diffusion rates and intrinsic rates controls the observed reaction rates as shown in Fig. 2, which profiles the reactant concentration as a function of distance from the center of a spherical catalyst particle located at the right axis, When both diffusion and intrinsic reactivity control overall reaction rates, there is a gradient of reactant concentration from CAS at the surface, to a lower concentration at the center of the particle. The reactant is consumed as it diffuses into the particle. With diffusional limitations the active sites nearest the surface have the highest turnover numbers. The overall process of simultaneous diffusion and chemical reaction in a spherical particle has been described mathematically for the cases of ion exchange catalysis,63 65) and catalysis by enzymes immobilized in gels 66-67). Many experimental parameters influence the balance between intraparticle diffusional and intrinsic reactivity control of reaction rates with polymer-supported phase transfer catalysts, as shown in Fig. 1. 

The affinity of the polymer-bound catalyst for water and for organic solvent also depends upon the structure of the polymer backbone. Polystyrene is nonpolar and attracts good organic solvents, but without ionic, polyether, or other polar sites, it is completely inactive for catalysis of nucleophilic reactions. The polar sites are necessary to attract reactive anions. If the polymer is hydrophilic, as a dextran, its surface must be made less polar by functionalization with lipophilic groups to permit catalytic activity for most nucleophilic displacement reactions. The % RS and the chemical nature of the polymer backbone affect the hydrophilic/lipophilic balance. The polymer must be able to attract both the reactive anion and the organic substrate into its matrix to catalyze reactions between the two mutually insoluble species. Most polymer-supported phase transfer catalysts are used under conditions where both intrinsic reactivity and intraparticle diffusion affect the observed rates of reaction. The structural variables in the catalyst which control the hydrophilic/lipophilic balance affect both activity and diffusion, and it is often not possible to distinguish clearly between these rate limiting phenomena by variation of active site structure, polymer backbone structure, or % RS. 

Functionalized polymers are of interest in a variety of applications including but not limited to fire retardants, selective sorption resins, chromatography media, controlled release devices and phase transfer catalysts. This research has been conducted in an effort to functionalize a polymer with a variety of different reactive sites for use in membrane applications. These membranes are to be used for the specific separation and removal of metal ions of interest. A porous support was used to obtain membranes of a specified thickness with the desired mechanical stability. The monomer employed in this study was vinylbenzyl chloride, and it was lightly crosslinked with divinylbenzene in a photopolymerization. Specific ligands incorporated into the membrane film include dimethyl phosphonate esters, isopropyl phosphonate esters, phosphonic acid, and triethyl ammonium chloride groups. Most of the functionalization reactions were conducted with the solid membrane and liquid reactants, however, the vinylbenzyl chloride monomer was transformed to vinylbenzyl triethyl ammonium chloride prior to polymerization in some cases. The reaction conditions and analysis tools for uniformly derivatizing the crosslinked vinylbenzyl chloride / divinyl benzene films are presented in detail. 

Poly(vinylbenzyl chloride) (VBC) is an ideal starting material onto which a variety of functional groups can be attached through relatively simple reactions and mild reaction conditions. Functionalized polymers are of interest in a variety of applications including but not limited to fire retardants, selective sorption resins, chromatography media, controlled release devices and phase transfer catalysts. An example of the wide applicability of functionalized polymers is provided by trimethyl ammonium functionalized poly(VBC). 

Some organic reactions can be accomplished by using two-layer systems in which phase-transfer catalysts play an important role (34). The phase-transfer reaction proceeds via ion pairs, and asymmetric induction is expected to emerge when chiral quaternary ammonium salts are used. The ion-pair interaction, however, is usually not strong enough to control the absolute stereochemistry of the reaction (35). Numerous trials have resulted in low or only moderate stereoselectivity, probably because of the loose orientation of the ion-paired intermediates or transition states. These reactions include, but are not limited to, carbene addition to alkenes, reaction of sulfur ylides and aldehydes, nucleophilic substitution of secondary alkyl halides, Darzens reaction, chlorination... 

Upon facing the difficulty of stereochemical control in peptide alkylation events, Maruoka and coworkers envisaged that the chiral phase-transfer catalyst should play a crucial role in achieving an efficient chirality transfer, and consequently examined the alkylation of the dipeptide, Gly-L-Phe derivative 57 (Scheme 5.28) [31]. When a mixture of 57 and tetrabutylammonium bromide (TBAB, 2 mol%) in toluene was treated with a 50% KOH aqueous solution and benzyl bromide at 0°C for 4h, the corresponding benzylation product 58 was obtained in 85% yield with the diastereo-meric ratio (DL-58 LL-58) of 54 46 (8% de). In contrast, the reaction with chiral quaternary ammonium bromide (S,S)-lc under similar conditions gave rise to 58 with 55% de. The preferential formation of LL-58 in lower de in the reaction with (R,R)-lc indicated that (R,R)-lc is a mismatched catalyst for this diastereofacial differentiation of 57. Changing the 3,3 -aromatic substituent (Ar) of the catalyst 1 dramatically increased the stereoselectivity, and almost complete diastereocontrol was realized with (S,S)-lg. 

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