Phase transfer comparisons

Phase Transfer Comparisons. An inhomogeneous mixture of 1-bromohexane (1.5 g, 1.43 x 10 2 m) and an equal volume of a saturated solution of KCN or NaCN in water was heated at 85° in the presence of 8 mole-% of catalyst (based on molecular weight of repeat unit). No stirring was employed. The insoluble polymeric catalyst was suspended at the interface between the two immiscible layers. The reaction was followed with H NMR using the hydrogens adjacent to the bromide and nitrile. Relative rates of reaction were evaluated by comparing reactions carried out simultaneously under the same reaction conditions. 

Montanari and his coworkers used the interesting polypode ligands derived from sym-trichlorotriazine as phase transfer catalysts for a variety of transformations. These catalysts were quite successful and their formation is illustrated below in Eqs. (7.3)— (7.5). Comparisons were also made with certain pentaerythrityl derived polypodes as well. These latter compounds are listed in Table 7.1 as compounds 10—13. 

Table 8.3.1. Comparison of Phase Transfer Catalysts on Yield of Ciamician-Dennstedt reaction...

Gutfelt et al. (1997) have evaluated various ME formulations as reaction media for synthesis of decyl sulphonate from decylbromide and sodium sulphite. The reaction rate was fast both in water-in-oil and in bicontinuous ME based on non-ionic surfactants. A comparison was made with this reaction being conducted in a two-phase. system with quats as phase-transfer catalyst but was found to be much less efficient. However, when two other nucleophiles, NaCN and NaNOj, were used the PTC method was almost as efficient as the ME media. It seems that in the case of decyl sulphonate there is a strong ion pair formation between the product and the PTC. The rate in the ME media could be further increased by addition of a small amount of a cationic surfactant. 

Catalysis at interfaces between two immiscible liquid media is a rather wide topic extensively studied in various fields such as organic synthesis, bioenergetics, and environmental chemistry. One of the most common catalytic processes discussed in the literature involves the transfer of a reactant from one phase to another assisted by ionic species referred to as phase-transfer catalyst (PTC). It is generally assumed that the reaction process proceeds via formation of an ion-pair complex between the reactant and the catalyst, allowing the former to transfer to the adjacent phase in order to carry out a reaction homogeneously [179]. However, detailed comparisons between interfacial processes taking place at externally biased and open-circuit junctions have produced new insights into the role of PTC [86,180]. 

Fig. 6 Formation of a hydrophobic ion pair a comparison with phase transfer catalysis...

It was a result of demand from industry in the mid-1960s for an alternative to be found for the expensive traditional synthetic procedures that led to the evolution of phase-transfer catalysis in which hydrophilic anions could be transferred into an organic medium. Several phase-transfer catalysts are available quaternary ammonium, phosphonium and arsonium salts, crown ethers, cryptands and polyethylene glycols. Of these, the quaternary ammonium salts are the most versatile and, compared with the crown ethers, which have many applications, they have the advantage of being relatively cheap, stable and non-toxic [1, 2]. Additionally, comparisons of the efficiencies of the various catalysts have shown that the ammonium salts are superior to the crown ethers and polyethylene glycols and comparable with the cryptands [e.g. 3, 4], which have fewer proven applications and require higher... 

Arylamines and hydrazines react with tosyl azide under basic conditions to yield aryl azides [1] and arenes [2], respectively, by an aza-transfer process (Scheme 5.25). Traditionally, the reaction of anilines with tosyl azides requires strong bases, such as alkyl lithiums, but acceptable yields (>50%) have been obtained under liquidiliquid phase-transfer catalytic conditions. Not surprisingly, the best yields are obtained when the aryl ring is substituted by an electron-withdrawing substituent, and the yields for the corresponding reaction with aliphatic amines are generally poor (-20%). Comparison of the catalytic effect of various quaternary ammonium salts showed that tetra-/i-butylammonium bromide produces the best conversion, but differences between the various catalysts were minimal [ 1 ]. 

There has been a useful review of phase-transfer catalysis in nucleophilic aromatic substimtion. A comparison has been reported of the reactions with nucleophiles of l-chloro-2,4-dinitrobenzene (substimtion) and 4-nitrophenyl diphenyl phosphate (dephosphorylation) in neutral micelles of dodecyl (10) and (23) polyoxyethylene glycol. In the substimtion reaction considerable amounts of ether may be formed by reaction with alkoxide ions at the micellar surface. Differences in reactivity of the two substrates are probably due to differences in their location in the micellar structures. ... 

The catalytic activity of polyethylene glycol (PEG) phosphonium salts has been evaluated, in phase-transfer dehydrohalogenation reactions, as slightly better than that of the corresponding PEG ammonium compounds886 (reaction 271). By comparison... 

A mixture of eugenol 1 (15 mmol), crushed potassium hydroxide or terbutoxide (33 mmol), the phase transfer catalyst (0.75 mmol) was placed either in a beaker in a domestic oven or in a Pyrex tube introduced into the Maxidigest MX 350 Prolabo microwave reactor filled with a rotational system. Microwave irradiation was carried out in the conditions described in Table 3 and 4. The mixture was cooled to ambient temperature. After elution with diethyl ether (50 mL) and subsequent filtration on Florisil, the organic products were analyzed by GC using an internal standard and characterized by 1H NMR spectroscopy by comparison with authentic samples. 

Dehmlow and coworkers [17] compared the efficiency of monodeazadnchona alkaloid derivatives 14a-c in the enantioselective epoxidation of naphthoquinone 50 with that of cinchona alkaloid-derived chiral phase-transfer catalysts 15a-c (Table 7.7) (for comparison of the alkylation reaction, see Table 7.1). Interestingly, the non-natural cinchona alkaloid analogues 14a-c afforded better results than natural cinchona alkaloids 15a-c. The deazacinchonine derivatives 14a,b produced epoxidation product 51 in higher enantioselectivity than the related cinchona alkaloids 15a,b. Of note, catalyst 14c, which possessed a bulky 9-anthracenylmethyl substituent on the quaternary nitrogen, afforded the highest enantioselectivity (84% ee). 

In Figure 7 a comparison is made of the frequency of the CHj antisymmetric stretching vibration as a function of molecular area for DPPC monolayer films at the A/W and A/Ge interfaces. As described above, the frequency of (his vibration is related to the overall macromolecular conformation of the lipid hydrocarbon chains. For the condensed phase monolayer (-40-45 A2 molecule 1), the measured frequency of the transferred monolayer film is virtually the same as that of the in-situ monolayer at the same molecular area, indicating a highly ordered acyl chain, predominately all-trans in character. For LE films as well as films transferred in the LE-LC phase transition region, however, the measured frequency appears independent (within experimental uncertainty) of the surface pressure, or molecular area, at which the film was transferred. The hydrocarbon chains of these films are more disordered than those of the condensed phase transferred films. However, no such easy comparison can be made to the in-situ monolayers at comparable molecular areas. For the LE monolayers (> ca. 70 A2 molecule 1), the transferred monolayers are more ordered than the in-situ film. In the LE-LC phase transition region ( 55-70 A2 molecule 1), the opposite behavior occurs. 

Use of a microemulsion to overcome reagent incompatibility can be seen as an alternative to the more conventional approach of carrying out the reaction in a two-phase system with the use of a phase transfer catalyst. The latter is usually either a quaternary ammonium salt or a crown ether. There are several examples in the literature of comparisons between the microemulsion concept and phase transfer catalysis. The topic has also recently been reviewed [46]. 

An early example of a comparison between the microemulsion approach and the process of phase transfer catalysis was a study by Menger et al. on the hydrolysis of trichlorotoluene to form sodium benzoate see Fig. 6 [47]. As can be seen from Table 1, hydrolysis in the presence of the cationic surfactant... 

One of the primary uses of modified electrodes has been In the area of electrochemical synthesis. Again the Increased solubilization by micelles and emulsions has been the primary Interest In using cationic surfactants. However, micelle and phase transfer catalysis and the hydrophobic nature of the electrode film has contributed to Increased yields. Table II shows a comparison of yields obtained In the electrooxidation of benz-hydrol in the presence of different surfactants and a comparison of the yields obtained with several other compounds with and without Ify-amlne 2389 (50). It can be seen that without a surfactant there Is no yield In aqueous solutions. Anionic and neutral surfactants which solubilize the compound but do not film the electrode cause only small Increases In yield, but the cationic film forming surfactant causes a sharp Increase in 3rleld. 

In this paper we present a generalized procedure for the calculation of bed-wall heat transfer coefficient in bubble columns on the basis of their hydrodynamic behavior. It has been shown that the high values of heat transfer coefficient obtained in bubble columns, as compared to the single phase pipe flow, can be explained on the basis of the enhanced local liquid velocities in the presence of gas phase. A comparison between the predicted and experimental values of heat transfer coefficient is presented over a wide range of design and operating variables. 

For comparison purposes some substitution reactions were tried under different conditions. While the use of DMSO and sodium hydride or alkali metal hydroxides allowed substitution products to be formed in yields comparable to those employing TEBA, other solvent-base systems failed. These include amide ion in ammonia and sodium hydride in tetrahydrofuran. If any substitution product were isolated using these other methods, the desired product was accompanied by much side-product. Perhaps the reason for successful substitution by methods employing phase transfer catalysis and DMSO solvent lies in the state of aggregation of the carbanion. Free ions and ion pairs are present under the successful conditions but higher aggregates are likely when failure results.39 ... 

It is often difficult to make a comparison between the various results obtained for the same polyenes as different reaction conditions (ratio of reactants, temperature, time) were used in each case. The addition of dichlorocarbene (chloroform/base/phase-transfer catalysis) to straight chain and cyclic unconjugated di- and trienes, carried out under identical conditions but varying the catalysts, showed the peculiar properties of tetramethylammonium chloride. Under precisely tailored conditions, either highly selective mono- or polyaddition of dichlorocarbene to the polyenes is possible tetramethylammonium chloride was the most efficient catalyst for monocyclopropanation. (For the unusual properties of tetramethylammonium salts on the phase-transfer catalyzed reaction of chloroform with electrophilic alkenes see Section 1.2.1.4.2.1.8.2. and likewise for the reaction of bromoform with allylic halides, see Section 1.2.1.4.3.1.5.1.). For example, cyclopropanation of 2 with various phase-transfer catalysts to give mixtures of 3, 4, and 5, ° of 6 to give 7 and 8, ° and of 9 to give 10 and 11. °... 

The monocyclopropanation of conjugated dienes is relatively easily realized, in comparison to unconjugated dienes (Section 1.2.1.4.2.1.3.), since the remaining double bond is deactivated after the addition of the first molecule of dichlorocarbene. It is also possible to prepare diadducts by using excess of carbene source to diene and by the correct selection of method for the generation of the carbene. The chloroform/base/phase-transfer catalyst method is suitable for the synthesis of either mono- or diadducts, while the chloroform/potassium tert-butoxide method allows the monoadducts to be obtained, often in high yields. The examples of the reactions of 16,and 1849,84,85 illustrate the problems. 

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