PTC reactions

Due to process improvements, some benefits have been realized through phase- 

Especially in the manufacture of fine chemicals no catalytic method has made such an impact as PTC. 

PTC technology is used in a wide variety of applications. PTC reactions under neutral conditions include 

More widely applicable are base-catalyzed phase-transfer reactions using aqueous concentrated or solid NaOH, KOH, K2CO3, NaH, etc. These include 

New interesting applications have been in the epoxidation of difficult olefin compounds, side-chain chlorination of substituted toluenes, diazotization, polymer manufacturing and modification, and in organometallic and anal5Tical chemistry. 

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The synthetic procedure used for the chemical modification of PPO involved in the first step the radical bromination of PPO methyl groups to provide a polymer containing bromobenzyl groups. The bromobenzyl groups were then esterified under phase-transfer-catalyzed (PTC) reaction conditions with potassium 4-(4-oxybiphenyl)butyrate (Ph3C00K, Ph3C00-PP0), potassium... 

PTC reactions are perfectly tailored for microwave activation, and combination of solid-liquid PTC and microwave irradiation gives the best results in this area [8] ... 

Similarly to classical PTC reaction conditions, under solid-liquid PTC conditions with use of microwaves the role of catalyst is very important. On several occasions it has been found that in the absence of a catalyst the reaction proceeds very slowly or not at all. The need to use a phase-transfer catalyst implies also the application of at least one liquid component (i.e. the electrophilic reagent or solvent). It has been shown [9] that ion-pair exchange between the catalyst and nucleophilic anions proceeds efficiently only in the presence of a liquid phase. During investigation of the formation of tetrabutylammonium benzoate from potassium benzoate and tetrabu-tylammonium bromide, and the thermal effects related to it under the action of microwave irradiation, it was shown that potassium benzoate did not absorb micro-waves significantly (Fig. 5.1, curves a and b). Even in the presence of tetrabutylammonium bromide (TBAB) the temperature increase for solid potassium benzoate... 

Condensation of salicylaldehyde and its derivatives with a variety of esters of chlo-roacetic acids in the presence of TBAB led to the synthesis of benzo[fo]furans by means of a solid-liquid PTC reaction under the action of microwave irradiation [33], This was a modification of one of the most popular routes to substituted benzo[fo]fu-... 

M. J. O Donnell, Asymmetric PTC Reactions. Part 1 Amino Acids , Phases - The Sachem Phase Transfer Catalysis Review 1998, Issue 4, pp. 5-8. 

Transformation of 2-deoxysugar derivatives into glycosyl xanthates can be performed by the treatment of O-benzyl-protected hemiacetal derivative with diphe-nylphosphoryl chloride, followed by the reaction with O-ethyl potassium xanthate in the presence of a base (NaOH, PTC reaction or NaH in appropriate organic solvent). High yields and selectivities in such reactions were observed when using sodium hydride in anhydrous THF [401],... 

In general, for a successful PTC reaction, the attacking anion should be more lipophilic than the anion that is to be displaced. If this not the case, then a large excess of the inorganic salt is required to push the equilibrium towards the desired product. 

Onium salts, such as tetraethylammonium bromide (TEAB) and tetra-n-butylammonium bromide (TBAB), were also tested as PTCs immobilized on clay. In particular, Montmorillonite KIO modified with TBAB efficiently catalyzed the substitution reaction of a-tosyloxyketones with azide to a-azidoketones, in a biphasic CHCI3/water system (Figure 6.13). ° The transformation is a PTC reaction, where the reagents get transferred from the hquid to the solid phase. The authors dubbed the PTC-modified catalyst system surfactant pillared clay that formed a thin membrane-hke film at the interface of the chloroform in water emulsion, that is, a third liquid phase with a high affinity for the clay. The advantages over traditional nucleophilic substitution conditions were that the product obtained was very pure under these conditions and could be easily recovered without the need for dangerous distillation steps. 

Of particular interest was the reaction of two equivalents of potassium phthalimide with PFB using 18-crown-6 in refluxing acetonitrile. This reaction with either small molecules or the polymeric analogs represents a novel approach to arylimide synthesis via PTC. After 4 hr. under nonoptimized PTC reaction conditions, disubstitution afforded the bisimide 6 in ca. 50% yield. This shows that phthalimide anion, a considerably poorer nucleophile than either the phenoxide or thiophenoxide, is a strong enough nucleophile in the presence of 18-crown-6 to displace aryl fluoride with facility, and demonstrates that the synthesis of polyimides, an important class of thermally stable polymers, is feasible by this PTC polycondensation route. 

It is cavitation in a heterogeneous medium which is the most studied by sonoche-mists. When produced next to a phase interface, cavitation bubbles are strongly deformed. A liquid jet propagates across the bubble towards the interface at a velocity estimated to hundreds of metres per second. At a liquid-liquid interface, the intense movement produces a mutual injection of droplets of one liquid into the other one, i. e. an emulsion (Fig. 3.3). Such emulsions, generated through sonication, are smaller in size and more stable than those obtained conventionally and often require little or no surfactant to maintain stability. It can be anticipated therefore that Phase Transfer Catalysed (PTC) reactions will be improved by sonication. Examples are provided later in this chapter. 

Miscellaneous PTC Reactions The field of PTC is constantly expanding toward the discovery of new enantioselective transformations. Indeed, more recent applications have demonstrated the capacity of chiral quaternary ammonium salts to catalyze a number of transformations, including the Neber rearrangement (Scheme 11.19a), ° the trifluoromethylation of carbonyl compounds (Scheme 11.19b), ° the Mannich reaction (Scheme 11.19c), and the nucleophilic aromatic substitution (SnAt)... 

To facilitate accesses to suitably functionalized sialic acid derivatives and complex sialyloligosaccharides for other usehil neoglycoconjugates, phase transfer catalysis (PTC) has been exploited extensively [for reviews see 42]. This process provided a wide range of carbohydrate derivatives under essentially clean Sn2 transformations. In the case of acetochloroneuraminic acid 1, the PTC reactions always provided inverted a-sialic acid derivatives [43]. para-Formylphenyl sialoside 7 [44], together with many other sialoside derivatives such as 8-10 [43], including thioacetate 12 [45] and azide 14 [46], were thus obtained (Scheme 1). Aldehyde 7 and similar glycosides are of particular interest since they could be directly conjugated to protein by reductive amina-tion after suitable deprotection [44]. 

Figure 6. Suggested PTC reaction mechanism in an omega phase (16).

Crown ethers and cryptates are phase transfer catalysts but the use of these compounds in PTC reactions is limited to cases in which TAA salts are unsuitable.42... 

Usually PTC reactions can be carried out without organic solvents or, if necessary, using solvents in small quantities sufficient to dissolve the reactants. 

The phase transfer catalyzed alkylation reaction of dodecyl phenyl glycidyl ether (DPGE) with hydroxyethyl cellulose (HEC) was studied as a mechanistic model for the general PTC reaction with cellulose ethers. In this way, the most effective phase transfer catalysts and optimum reaction concentrations could be identified. As a model cellulose ether, CELLOSIZE HEC11 was chosen, and the phase transfer catalysts chosen for evaluation were aqueous solutions of choline hydroxide, tetramethyl-, tetrabutyl-, tetrahexyl-, and benzyltrimethylammonium hydroxides. The molar A/HEC ratio (molar ratio of alkali to HEC) used was 0.50, the diluent to HEC (D/HEC) weight ratio was 7.4, and the reaction diluent was aqueous /-butyl alcohol. Because some of the quaternary ammonium hydroxide charges would be accompanied by large additions of water, the initial water content of the diluent was adjusted so that the final diluent composition would be about 14.4% water in /-butyl alcohol. The results of these experiments are summarized in Table 2. 

Symmetrical diaryl sulfides were produced in fair yield from the PTC reaction of sodium sulfide with molten aryl chlorides activated by a cyano, nitro, phthalimido or anhydrido group.193 Typical conditions require use of a 3 1 mole ratio of aryl chloride to Na2S and 10% of catalyst (crown ethers and onium salts) at 200 C for 24 h. 

PTC Reaction Quat Ephedra Quat Crown Ether Other... 

A one-pot PTC reaction procedure for the overall conversion of an alkyl halide into a primary amine via an azide is particularly illustrative.204 Thus the reduction of the azide is effected by the addition of sodium borohydride to a reaction mixture arising from the PTC displacement reaction of an alkyl halide with sodium azide (the preparation of 1-octylamine, Expt 5.193). The reaction appears to be applicable to primary and secondary alkyl halides, alkyl methane-sulphonates and benzylic halides. 

SOLVENT-FREE PHASE TRANSFER CATALYSIS (PTC) REACTIONS 105... 

Solvent-free phase transfer catalysis (PTC) reactions 7.3.1. trans-1,2-Diphenylethylene (trans-stilbene)... 

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