Phase-transfer catalyst, for

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. 

Interestingly, phase-transfer catalysts including crown ethers have been used to promote enantioselective variations of Darzens condensation. Toke and coworkers showed that the novel 15-crown-5 catalyst derived from d-glucose 33 could promote the condensation between acetyl chloride 31 and benzaldehyde to give the epoxide in 49% yield and 71% A modified cinchoninium bromide was shown to act as an effective phase transfer catalyst for the transformation as well. ... 

Arai and co-workers have used chiral ammonium salts 89 and 90 (Scheme 1.25) derived from cinchona alkaloids as phase-transfer catalysts for asymmetric Dar-zens reactions (Table 1.12). They obtained moderate enantioselectivities for the addition of cyclic 92 (Entries 4—6) [43] and acyclic 91 (Entries 1-3) chloroketones [44] to a range of alkyl and aromatic aldehydes [45] and also obtained moderate selectivities on treatment of chlorosulfone 93 with aromatic aldehydes (Entries 7-9) [46, 47]. Treatment of chlorosulfone 93 with ketones resulted in low enantioselectivities. 

Table 1.12 Cinchona alkaloid-derived phase-transfer catalysts for asymmetric Darzens reactions.

Sulphoxides can be used as phase transfer catalysts, for example, a-phosphoryl sulphoxides (Scheme 24) have been used as phase transfer catalysts in the two-phase alkylation of phenylacetonitrile or phenylacetone with alkyl halides and aqueous sodium hydroxide. However, they are considered to be inefficient catalysts for simple displacement reactions226. 

Recently, it has been reported that methyl 2-pyridyl sulphoxides (10) and related pyridyl derivatives (11) (see Schepie 25) are good phase transfer catalysts for SN2 reactions of various primary or secondary alkyl halides in a two-phase reaction system and for the alkylation of phenylacetonitrile or phenylacetone with alkyl halides in liquid-liquid two-... 

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... 

Crown ethers attached to insoluble polymeric substrates (see the following discussion for examples) have been used as phase transfer catalysts for liquid/liquid systems. In using such systems, the catalyst forms a third insoluble phase the procedure being referred to as triphase catalysis (Regen, 1979). This arrangement has the advantage that, on completion of the reaction, the catalyst may be readily separated from the reaction solution and recycled (Montanari, Landini Rolla, 1982). As... 

Triorganotin fluorides can be prepared by the use of new fluorinating systems. 18-Crown-6 or dibenzo-24-crown-8 can act as solid-liquid phase transfer catalysts for CsF. Trialkyltin mercaptides can be fluorodestannylated by CsF in the presence of crown ethers or alkyl bromides358 ... 

T. Ooi, M Kameda, K Maruoka, Molecular Design of a C2-Symmetric Chiral Phase-Transfer Catalyst for Practical Asymmetric Synthesis of a-Amino Acids , J. Am Chem Soc, 1999,121, 6519-6520. 

Crown ethers have been used successfully as phase-transfer catalysts for liquid-liquid and liquid-solid oxidation reactions. Sam and Simmons (1972) observed that potassium permanganate can be solubilized in benzene by dicyclohexyl-18-crown-6 to yield concentrations as high as 0.06 M. From... 

Compared with primary and secondary amines, tertiary amines are virtually unreac-tive towards carbenes and it has been demonstrated that they behave as phase-transfer catalysts for the generation of dichlorocarbene from chloroform. For example, tri-n-butylamine and its hydrochloride salt have the same catalytic effect as tetra-n-butylammonium chloride in the generation of dichlorocarbene and its subsequent insertion into the C=C bond of cyclohexene [20]. However, tertiary amines are generally insufficiently basic to deprotonate chloroform and the presence of sodium hydroxide is normally required. The initial reaction of the tertiary amine with chloroform, therefore, appears to be the formation of the A -ylid. This species does not partition between the two phases and cannot be responsible for the insertion reaction of the carbene in the C=C bond. Instead, it has been proposed that it acts as a lipophilic base for the deprotonation of chloroform (Scheme 7.26) to form a dichloromethylammonium ion-pair, which transfers into the organic phase where it decomposes to produce the carbene [21]. 

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]. 

Quaternary ammonium and phosphonium halides were used as the phase transfer catalysts. For effective coupling, high-shear mixing and high concentrations of polymer and base were used. 

The utilization of polar polymers and novel N-alkyl-4-(N, N -dialklamino)pyridinium sedts as stable phase transfer catalysts for nucleophilic aromatic substitution are reported. Polar polymers such as poly (ethylene glycol) or polyvinylpyrrolidone are thermally stable, but provide only slow rates. The dialkylaminopyridininium salts are very active catalysts, and are up to 100 times more stable than tetrabutylammonium bromide, allowing recovery and reuse of catalyst. The utilization of b is-dialkylaminopypridinium salts for phase-transfer catalyzed nucleophilic substitution by bisphenoxides leads to enhanced rates, and the requirement of less catalyst. Experimental details are provided. 

K over the range 298-398 K. Partition eoeffieients for HFeCU between water and 1-octanol or dibutyl ether depend markedly on HCl and LiCl concentrations. For dibutyl ether as organic phase there is also a dependence of partition coefficient on Fe " " concentration as there is significant polynuclear complex formation in the ether layer. [FeCU] , in the form of its Et3BzN" salt, acts as a bifunctional or phase transfer catalyst for hydrosilylation of phenylacetylene. ... 

A biphasic system consisting of the ionic liquid [BMIM]PF6 and water was used for the epoxidation reactions of a, 3-unsaturated carbonyl compounds with hydrogen peroxide as an oxidant at room temperature 202). This biphasic catalytic system compared favorably with the traditional phase transfer catalysts. For example, under similar conditions (15°C and a substrate/NaOH ratio of five), the [BMIM]PF6/H20 biphasic system showed a mesityl oxide conversion of 100% with 98% selectivity to oc, 3-epoxyketone, whereas the phase-transfer catalyst with tet-rabutylammonium bromide in a CH2CI2/H2O biphasic system gave a conversion of only 5% with 85% selectivity. 

Preparation of 0-Allyl-N-(9)-anthracenylmethylcinchonidinium Bromide as a Phase Transfer Catalyst for the Enantioselective Alkylation of Glycine Benzophenone Imine tert-Butyl Ester. 

The most straightforward way to obtain polymeric phosphonium salts involves introducing the phosphonio groups on to a suitable polymeric structure, for example by reacting tertiary phosphines with a poly(chloromethylstyrene) (reaction 99). The polymeric phosphonium salts obtained in this way are mostly used as polymer-supported phase-transfer catalysts for nucleophilic substitutions reactions under triphase conditions. 

Solid phase-transfer reactions.2 A strong base ion-exchange resin (Duolile A-109, Diamond Shamrock) can serve as a solid phase-transfer catalyst for synthesis of /1-lactams (3) from oc-methyl-a./l-dibromopropionyl chloride (I) and an amino acid (2) U / 9, 360). 

Cinchona-Derived Chiral Phase-Transfer Catalysts for Amino Acid Synthesis... 

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