Phase-transfer techniques

The alkene is allowed to react at low temperatures with a mixture of aqueous hydrogen peroxide, base, and a co-solvent to give a low conversion of the alkene (29). These conditions permit reaction of the water-insoluble alkene and minimise the subsequent ionic reactions of the epoxide product. Phase-transfer techniques have been employed (30). A variation of this scheme using a peroxycarbimic acid has been reported (31). 

A solid-liquid phase-transfer technique is used to synthesize aryl difluoro-methyl sulfides and selenides thiophenols dissolved in an aromatic solvent are treated with solid sodium hydroxide in the presence of a catalytic amount of tris(3,6-dioxaheptyl)amine (TDA1) [49] This condensation proceeds by a carbene mechanism (equation 44)... 

Phase-transfer techniques are widely used for the preparation of polymers. For example, potassium fluoride is used to produce poly(etherketone)s under phase-transfer conditions (Scheme 10.18). Use of this reagent allows the chloroaro-matics to be used as starting material as opposed to the more expensive flu-oroaromatics that are usually employed [23]. This method is suitable for the synthesis of high molecular weight semicrystalline poly(ether ketone)s, although the presence of excess potassium fluoride in the reaction mixture can lead to degradation reactions. The use of a phase transfer catalyst can allow the use of water-soluble radical initiators, such as potassium peroxomonosulfate used to promote the free-radical polymerization of acrylonitrile [24],... 

Although the extraction of primary amines from a basic medium with chloroform is an inadvisable procedure, on account of the formation of trace amounts of the pungent isonitriles, the specific synthesis of isonitriles by the two-phase reaction of primary amines with chloroform is unreliable. However, the application of the phase-transfer technique [e.g. 1 -5] for the controlled release of dichlorocarbene facilitates the synthesis of isonitriles in relatively high yields (Table 7.12). 

Several industrial processes use phase-transfer techniques with soluble catalysts, mostly for fine chemical productions (42,43). It is easy to believe that this technology will find a greater application in the near future, perhaps with the use of polymer-supported catalysts. 

There are three commonly used methods [145] to incorporate enzymes in RMs (i) injection of a concentrated aqueous solution, (ii) addition of dry lyophilized protein to a reverse miceUar solution, and (iii) phase transfer between bulk aqueous and surfactant-containing organic phases. Figure 3 shows schematically the three enzyme solubiHzation methods. The injection and dry addition techniques are commonly used in biocatalytic appHcations, the latter being well suited to hydrophobic proteins [146]. The phase transfer technique is the basis for extraction of proteins from aqueous solutions. 

Cardoso et al. [115] using the phase transfer technique studied the driving forces involved in the selective solubilization of three different amino acids having same pi, namely aspartic acid (hydrophilic), phenylalanine (slightly hydrophobic), and tryptophan (hydrophobic) in cationic TOMAC-RMs. The main driving forces involved were found to be hydrophobic and electrostatic interactions. Few other researchers have also identified that the major driving forces involved in the amino acid solubilization were hydrophobic interactions [ 114,159] and amino acid structure as well as its ionization state [160,161]. 

Alkylation of 2-thiopyridone and of 2-thiobenzoxazolone has been described by Dou et al.195 Yields are good, and only S-alkylation is detected. In a later study, A4-thiazoline-2-thione, thiazolidine-2-thione (130) and benzothiazoline-2-thione have been alkylated, using the phase transfer technique.196... 

DHP (182) with DMSO and HC1.278 The compound thus obtained was made lipophilic and was alkylated by PTC in high yield. The DHP was then removed almost quantitatively. The deprotected OH groups were ether-ified by using a phase transfer technique. The overall yield is 92%.180... 

The phase-transfer technique is a simple and efficient tool for the benzylation of carbohydrates. With benzyltriethylammonium chloride or tetrabutylammonium bromide as a catalyst, a mixture of aqueous, 50 % sodium hydroxide and benzyl bromide or chloride in benzene or dichloromethane solution gives a good yield of the fully protected product [103, 104], such as methyl 2,3-di-0-benzyl-4,6-0-benzylidene c-D-glucopyranoside, when stirred at room temperature for several hours. The latter catalyst is slightly more efficient. Dichloromethane has been observed to produce methylene acetals from cis vicinal diols under comparable conditions [103]. 

The phase-transfer technique of partial alkylation introduced by Garegg in the carbohydrate-field (see Sect. 2.2) can be used also for the preparation of partially allylated derivatives. It was found that the reaction of benzyl 4-0-benzyl-a-L-rhamnopyrano-... 

B.R. Langlois, Improvement of the synthesis of aryl difluoromethyl ethers and thioethers by using a solid-liquid phase-transfer technique, J. Fluorine Chem. 41 (2) (1988) 247-261. 

Sodium telluride, prepared from tellurium and sodium borohydride in aqueous medium, reacted with benzoyl chlorides using a phase transfer technique, to produce rather unstable 7e-benzyl tellurobenzoates (benzyl benzoyl telluriums)1 in low yields and not the expected dibenzoyl telluriums. Sodium telluride must reduce benzoyl groups to benzyl groups during these reactions. 

Chromium tricarbonyl complexes of arylacetic esters can be alkylated by the use of phase-transfer catalysis or sodium hydride in Af, /V-dimethyl-formamide (77). However, the yields obtained by the latter method are as good as or superior to those realized using phase-transfer techniques. Both methods give similar stereochemical results (e.g., 68 — 69 and 70). 

In the presence of methyl vioiogen as a catalyst, yields of up to 100% can be obtained. Difluorocarbene can be added to benzenethiols in moderate to good yields. The carbene generation proceeds via a solid-liquid phase-transfer technique."... 

Starting from oxiranes, oxygen-containing macrocycles can also be prepared a good example of this is the formation of the ten-membered oxygen-containing heterocycle 126 via the condensation of chloral hydrate and oxirane using the phase-transfer technique. 

Another methodology applied to the monosubstitution of diols is the use of copper complexation of dianions. The dianion is first formed by reaction of a diol with two equivalents of NaH. The copper complex is then formed by addition of a copper salt. Reaction of the copper complex with various electrophiles (alkyl halides, acyl chlorides) then gives the selectively protected products. As with the phase-transfer technique, very little disubstitution is observed. However, as illustrated in Scheme 3.16, the regioselectivity is reversed (i.e., 4,6-diols give mainly 4-substitution and 2,3-diols give mainly 3-substitution). Using this technique, both alkylations (benzylation, allylation) and acylations (acetylation, benzoylation, pivaloylation) have been carried out. As usual, the degree of selectivity depends on reaction conditions and structural factors [44]. 

The various phase transfer techniques also led to very good results in azide synthesis. Excellent yields of alkyl azides were secured, for instance, when alkyl bromides were treated with alkali metal azides in the presence of catalytic amounts of tetrabutylammonium bromide or aliquat 336 (equation 29). "- The use of crown ethers has been described as well. "... 

The phase-transfer technique has been employed in the mono- and AW -di-phosphorylation of hydrazine. Full papers dealing with the preparation of acyclic and cyclic (perhydro-l,2,4,5-tetra-aza-3-phosphorine) hydrazides, a preliminary report of some of which was given in Organophosphorus Chemistry Vol. 7 (p. 114) have been published of particular interest is the formation of cage compounds (13) and (14) from monocyclic compounds by further reaction with aldehydes or ketones. 

POClj also involves the phase-transfer technique, employing both BnNEtiCl and dibenzo[18]crown-6 as catalyst. 

Sulfides and dithioacetals. Sulfides can be prepared in high yield (85-95%) by reaction of thiols with an alkyl halide using the phase-transfer technique (equation I). The reaction is complete in about 15 min. 

A related solid-liquid phase transfer technique especially suited to hindered esters avoids the use of a solvent and instead simply... 

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