Biphasic systems, phase-transfer catalysi

In the context of biphasic reaction systems, phase transfer catalysis should also be mentioned. It should be noted that it is not limited to aqueous-organic reactions or liquid-liquid systems, but is also sometimes employed in... 

We have exploited this base catalysis of the oxygen exchange process to effect oxygen lability in the less electrophilic carbonyl sites of neutral metal carbonyl species. Because [MCOOH] intermediates are readily decarboxylated in the presence of excess hydroxide ion, in order to observe oxygen exchange processes in neutral metal carbonyl complexes it was convenient to carry out these reactions in a biphasic system employing phase transfer catalysis () (16, 17. 18). Under conditions (eq. 7) the... 

The representative reaction system applied in asymmetric phase-transfer catalysis is the biphasic system composed of an organic phase containing an acidic methylene or methine compound and an electrophile, and an aqueous or solid phase of inorganic base such as alkaline metal (Na, K, Cs) hydroxide or carbonate. The key reactive intermediate in this type of reaction is the onium carbanion species, mostly onium enolate or nitronate, which reacts with the electrophile in the organic phase to afford the product. 

Examples of applying biphasic systems to catalyzed reactions, such as phase-transfer catalysis, overpower the stoichiometric reactions. In a typical catalytic biphasic system, one phase contains the catalyst, while the other phase contains the substrate. In some systems, the catalyst and substrates are in the same phase, while the product produced is transferred to the second phase. In a typical reaction, when the two phases are mixed during the reaction and after completion, the catalyst remains in one phase ready for recycling while the product can be isolated from the second phase. The most common solvent combination consists of an organic solvent combined with another immiscible solvent that, in most applications, is water. However, there are few examples of suitable water-soluble and stable catalysts, and therefore various applications are limited to some extent [192]. Immiscible solvents other than water are recently becoming more applicable in biphasic catalysis because of the better solubility and stability of various catalysts in such solvents. For example, ionic liquids and fluorous solvents have many successful applications in liquid-liquid... 

Phase transfer catalysis involves typically an organic/aqueous biphasic system in the presence of a transfer agent such as a tetraalkylammonium salt which facilitates the exchange of the catalyst between the two phases, while the reactants and the products are usually retained in the organic layer. Almost all types of homogeneously catalyzed reactions can be carried out in this way.173... 

One of the oldest techniques for overcoming these problems is the use of biphasic water/organic solvent systems using phase-transfer methods. In 1951, Jarrouse found that the reaction of water-soluble sodium cyanide with water-insoluble, but organic solvent-soluble 1-chlorooctane is dramatically enhanced by adding a catalytic amount of tetra-n-butylammonium chloride [878], This technique was further developed by Makosza et al. [879], Starks et al. [880], and others, and has become known as liquid-liquid phase-transfer catalysis (PTC) for reviews, see references [656-658, 879-882], The mechanism of this method is shown in Fig. 5-18 for the nucleophilic displacement reaction of a haloalkane with sodium cyanide in the presence of a quaternary ammonium chloride as FT catalyst. 

To determine the mode of operation (see below) it is useful to group these homogeneous catalysts into aqueous biphasic systems (see Section 3.1.1.1) and nonaqueous biphasic systems (see Section 3.1.1.2). Gas-liquid-liquid reactions are also involved in organometallic phase transfer catalysis, e. g., in biphasic carbonylation of benzyl chloride to phenylacetic acid by the catalyst system NaCo(CO)4/Bu4NBr/NaOH [21]. Here, the biphasic system consists of an organic solvent and aqueous alkali. 

Other interesting systems have been employed, such as CO/HjO (water gas) or CO/Hj (syngas) as reducing mixtures [49, 50], phase transfer catalysis [37], and more recently, aqueous [46, 47] and non-aqueous ionic liquid [48] biphasic catalysis which offer more promise for practical uses. Some interesting examples of metal complexes grafted onto oxides [55, 56] or supported metals [38, 39] as arene hydrogenation catalysts have been provided. 

Keywords Biphasic catalysis. Asymmetric phase-transfer catalysis. Micellar catalysis. Vesicles, Microheterogeneous systems... 

Section 7 reviews non-aqueous biphase processes and their variations. Sections 4.5 and 4.6 deal with micellar systems and various applications of phase transfer catalysis in relation to aqueous biphase catalysis. Interestingly, biphase techniques are also being utilized from the other side, that of heterogeneous catalysis [35]. 

In contrast to the case of the water soluble [RhClP3] complexes (P = PTA, TPPMS or TPPTS) which did not promote the reduction of C=0 function in aldehydes or ketones in biphasic systems, [RhCl(PPh3)3] was found an active catalyst for reduction of ketones with aqueous HCOONa (Scheme 3.32). The reaction was aided by phase transfer catalysis using Aliquat-336 and required a large excess of PPh3 to prevent reduction of rhodium into inactive metal. Substrates like acetophenone, butyrophenone, cyclohexanone and dibenzyl-ketone were reduced to the corresponding secondary carbinols with turnover frequencies of 10-40 h 1 [251]. 

Recently, a new aqueous biphasic catalytic system based on the cloud point of nonionic tensioactive phosphine, termed thermoregulated phase-transfer catalysis (TRPTC) has been developed [13]. The concept ofTRPTC as a missing link could not only provide a meaningful solution to the problem of catalyst/product separation, but also extricate itself from the limitation of low reaction rates of water-immiscible substrates. 

There has recently been great interest in the synthesis of dendritic polymers, although applications of these have so far been few [59]. The first report of a reaction where a dendrimer is actually catalytic involved a biphasic system similar to phase transfer catalysis. The quaternary ammonium ion dendrimer (Figure 5.26) has 36 trimethylammonium functions, and catalyses the unimolecular decarboxylation of 6-nitrobenzisoxazole-3-carboxylate, and the hydrolysis of 4-nitrophenyldiphenyl phosphate [60]. 

Activation of the carboxylic acid as the acyl chloride permits direct reaction with azide anion to form the acyl azide substrates for Curtius rearrangement. Sodium azide is commonly used, and the reaction has been used on the process chemistry scale for the synthesis of benzyl-A-vinyl carbamate. Acryloyl chloride was combined with sodium azide in a biphasic system with phase-transfer catalysis (PTC), providing acyl azide 25. Upon heating, Curtius rearrangement provided vinyl isocyanate, which was distilled directly into benzyl alcohol containing phenothiazine (27) to inhibit polymerization of 26 and triethylamine to catalyze addition of the alcohol to the isocyanate. The vinyl carbamate product 28 was isolated by crystallization. As the autiior clearly pointed out, preparation and reaction of acyl azides, particularly on large scales, require appropriate safety precautions. 

Bimetallic phase-transfer-catalysis is a process whereby a reaction that occurs using two different metal complexes, does not proceed in the absence of either metal species, or proceeds only at reduced rate. An apparent system of this class has been reported, in which Co2(CO)g and [RhCl(l,5-hexadiene)]2 mutually increased their reactivity when used as catalysts for the conversion of nitrobenzene to aniline in a biphasic system (benzene, aqueous NaOH, dodecyltrimethylammonium chloride) in a carbon monoxide atmosphere [73]. However, another member of the same research group later showed [74] that the apparent bimetallic promotion was due to the fact that the alkylammonium salt used as a phase-transfer agent actually inhibited the activity of the active rhodium complex (apparently a cluster, which is active in the absence of both the alkylammonium salt and the cobalt compound) by rendering it insoluble. The added Co2(CO)g reacts with the alkylammonium salt to generate... 

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