Phase-transfer catalysts substitution reactions

As an alternative to the oxidation of sulphides and sulphoxides (see Chapter 10), sulphones can be prepared by the nucleophilic substitution reaction of the sulphinite anion on haloalkanes. In the absence of a phase-transfer catalyst, the reaction times are generally long and the yields are low, and undesirable O-alkylation of the sulphinite anion competes with S-alkylation. The stoichiometric reaction of the preformed tetra-n-butylammonium salt of 4-toluenesulphinic acid with haloalkanes produces 4-tolyl sulphones in high yield [1], but it has been demonstrated that equally good... 

Water-soluble calix[ ]arenes (62 n = 4, 6, and 8) containing trimethylammonio-methyl groups act as efficient inverse phase-transfer catalysts in the nucleophilic substitution reactions of alkyl and arylalkyl halides with nucleophiles in water.126 (Inverse phase-transfer catalysts facilitate reactions between two immiscible reactants via the transport of an organic substrate into an aqueous solution of a second substrate, in which reactions take place.)... 

Nucleophilic Reactions. Useful nucleophilic substitutions of halothiophenes are readily achieved in copper-mediated reactions. Of particular note is the ready conversion of 3-bromoderivatives to the corresponding 3-chloroderivatives with copper(I)chloride in hot /V, /V- dim ethyl form am i de (26). High yields of alkoxythiophenes are obtained from bromo- and iodothiophenes on reaction with sodium alkoxide in the appropriate alcohol, and catalyzed by copper(II) oxide, a trace of potassium iodide, and in more recent years a phase-transfer catalyst (27). 

Compound 1, 2,2-diniethyl-4-pentenal, has been prepared by the Claisen rearrangement route described above and by reaction of isobutyraldehyde with allyl chloride in the presence of aqueous sodium hydroxide and a phase-transfer catalyst. Both routes are applicable to the synthesis of a variety of substituted 4-pentenals. 

Nucleophilic substitution by azide ion on an alkyl halide (Sections 8.1, 8.13) Azide ion is a very good nucleophile and reacts with primary and secondary alkyl halides to give alkyl azides. Phase-transfer catalysts accelerate the rate of reaction. 

Although phase-transfer catalysis has been most often used for nucleophilic substitutions, it is not confined to these reactions. Any reaction that needs an insoluble anion dissolved in an organic solvent can be accelerated by an appropriate phase transfer catalyst. We shall see some examples in later chapters. In fact, in principle, the method is not even limited to anions, and a small amount of work has been done in transferring cations, radicals, and molecules. The reverse type of phase-transfer catalysis has also been reported transport into the aqueous phase of a reactant that is soluble in organic solvents. ... 

The scope of reactions involving hydrogen peroxide and PTC is large, and some idea of the versatility can be found from Table 4.2. A relatively new combined oxidation/phase transfer catalyst for alkene epoxidation is based on MeRe03 in conjunction with 4-substituted pyridines (e.g. 4-methoxy pyridine), the resulting complex accomplishing both catalytic roles. 

Alternatively, the Sn2 nucleophilic substitution reaction between alcohols (phenols) and organic halides under basic conditions is the classical Williamson ether synthesis. Recently, it was found that water-soluble calix[n]arenes (n = 4, 6, 8) containing trimethylammonium groups on the upper rim (e.g., calix[4]arene 5.2) were inverse phase-transfer catalysts for alkylation of alcohols and phenols with alkyl halides in aqueous NaOH solution to give the corresponding alkylated products in good-to-high yields.56... 

Compared with esters, acid halides and anhydrides are more reactive and are hydrolyzed more readily. It is interesting to note that there is a substantial lifetime for these acid derivatives in aqueous media. Acid halides dissolved in PhCl or in PhBr shaken at a constant rate with water shows that hydrolysis occurs at the boundary between the two liquid phases.35 The reaction of benzoyl chloride (PhCOCl) and benzoate ion with pyridine A-oxide (PNO) as the inverse phase-transfer catalyst yields both the substitution product (benzoic anhydride) and the... 

The yield of the potassium pyrrolide reaction is considerably improved by the use of a phase transfer catalyst.20 However, in exploratory work on this substitution reaction we have found that sodium pyrrolide reacts with (NPC 2)3 in tetrahydrofuran at room temperature to produce 1 in quantitative yield in less than 3 hours. The structure of the compound20 is reproduced in Figure 1. 

When reacting two phases that are not very soluble in each other, for example when carrying out nucleophilic substitution reactions, phase transfer catalysts should be considered when scaling down from equipment with poor mixing characteristic, rather than buying new equipment. 

In contrast with aliphatic nucleophilic substitution, nucleophilic displacement reactions on aromatic rings are relatively slow and require activation at the point of attack by electron-withdrawing substituents or heteroatoms, in the case of heteroaromatic systems. With non-activated aromatic systems, the reaction generally involves an elimination-addition mechanism. The addition of phase-transfer catalysts generally enhances the rate of these reactions. 

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]. 

Kenawy 64) immobilized ammonium and phosphonium peripheral functionalized dendritic branches on a montmorillonite supported chloromethylstyrene/methyl methacrylate copolymer (74-75). These polymer/montmorillonite-supported dendrimers were used as phase transfer catalysts (PTC) for the nucleophilic substitution reaction between -butyl bromide and thiocyanate, cyanide, and nitrite anions in a toluene or a benzene/water system. These PT catalysts could be recycled by filtration of the functionalized montmorillonite from the reaction mixture. Generally,... 

This article reports on the synthesis of photosensitive polymers with pendant cinnamic ester moieties and suitable photosensitizer groups by cationic copolymerizations of 2-(cinnamoyloxy)ethyl vinyl ether (CEVE) (12) with other vinyl ethers containing photosensitizer groups, and by cationic polymerization of 2-chloroethyl vinyl ether (CVE) followed by substitution reactions of the resulting poly (2-chloroethyl vinyl ether) (PCVE) with salts of photosensitizer compounds and potassium cinnamate using a phase transfer catalyst in an aprotic polar solvent. The photochemical reactivity of the obtained polymers was also investigated. 

Syntheses of Polymeric Photosensitizers and Self-sensitized Polymers by the Reactions of PCVE. Photosensitizer monomers such as NPVE, NNVE and NPEVE were synthesized by the reaction of excess CVE with potassium salts of the corresponding photosensitizing compounds using TBAB as a phase transfer catalyst as described in the experimental part. Substitution reactions of chloroethyl groups in PCVE with PNP and PNN were also carried out using TBAB as a phase transfer catalyst in DMF at 80 C for 24 h. 

Substitution reactions of the remaining pendant chloroethyl groups in PCVE-NPVE with 20 mol-% excess of potassium cinnamate were carried out using TBAB as a phase transfer catalyst in DMF at 100"C for 24 h. The reaction conditions and results are summarized in Table III. It should be noted that 100 mole % substitution occurred in all cases. 

These results suggest that the reaction conditions for the syntheses of PCEVE-NPVE and PCEVE-NNVE can be accomplished by the reactions of PCVE with any ratio of potassium cinnamate and PNP or PNN in one pot using a phase transfer catalyst. In addition, it is to be expected that PCEVE-NPVE and PCEVE-NNVE prepared from the reactions of PCVE have the same degree of polymerization if no side reactions occur during the substitution reactions. It is also expected that these copolymers are more random compared to the copolymers prepared from the cationic copolymerizations of the monomers, because the former is not affected by the monomer reactivity ratios. 

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