Phase transfer systems

Phase-transfer systems for generating fluorohalocarbenes from the corre spending haloforms are simple and effective with nucleophilic olefins [5J, 54 55, 56, 57, 58, 59] (Table 3 and equations 23 and 24), and the process was extended recently to difluorocarbene [60] Chlorofluorocarbene also can be generated from fluorotrichloromethane and utanium(O) produced in situ (T1CI4 + L1AIH4) Yields of gem-chlorofluorocyclopropanes range from excellent (85% for a. meth ylstyrene) to poor (12% for 1-hexene) [61]... 

Figure 4.20. Finite-difference element for the two-phase transfer system.

Chimentao RJ, Cota I, Dafinov A, Medina F, Sueiras JE (2006) Synthesis of silver-gold alloy nanoparticles by a phase-transfer system. Mater Res 21 105-111... 

Heating 101 with aqueous sodium hydroxide in diethyleneglycol monomethyl-ether or with alkali acetate, a reaction which may also be performed in other polar organic solvents, such as dimethylformamide, N-methylpyrrolidone, or di-methylacetamide at 150°C to 210°C, also provides pyranthrone. As an alternative, 101 may be cyclized to form pyranthrone in a two-phase reaction, carried out in the presence of quarternary ammonium salts in a phase-transfer system comprising an aqueous and an organic phase [20]. 

There are a number of advantages to be gained in using a phase transfer system to conduct a reaction, including ... 

Spurred by our desire to avoid use of expensive dipolau aprotic solvents in nucleophilic aromatic substitution reactions, we have developed two alternative phase transfer systems, which operate in non-polar solvents such as toluene, chlorobenzene, or dichlorobenzene. Poleu polymers such as PEG are Inexpensive and stable, albeit somewhat inefficient PTC agents for these reactions. N-Alkyl-N, N -Dialkylaminopyridinium salts have been identified as very efficient PTC agents, which are about 100 times more stable to nucleophiles than Bu NBr. The bis-pyridinium salts of this family of catalysts are extremely effective for phase transfer of dianions such as bis-phenolates. 

The solubility of the components in the solvent must be sufficient. To improve the solubility, cosolvents can be used. Another possibility is the application of a two-phase system or an emulsion in the presence of phase-transfer catalysts. A two-phase system also has advantages in product isolation and continuous electrolysis procedures. A typical example is the synthesis of p-methoxy benzonitrile by anodic substitution of one methoxy group in 1,4-dimethoxybenzene by the cyanide ion (Eq. 22.21). The homogeneous cyanation system (acetonitrile, tetraethylammonium cyanide) [24] can be efficiently replaced by a phase-transfer system (dichloro-methane, water, sodium cyanide, tetrabutylammonium hydrogen sulfate) [71]. 

Microwave-heated Heck reactions in water have also been performed using phase-transfer catalysis. A number of phase-transfer systems were investigated with TBAB, once more giving the best results51 (Scheme 2.18). 

Reactions accomplished by enantioselective phase-transfer catalysis are summarized in Table 10.1 according to type of catalyst and synthetic transformation [9-81], Highest reported enantioselectivities (% ee) or optical purities (% op) are listed to give perspective to the overall field [82]. General aspects of phase-transfer systems, including catalysts are then discussed, followed by particular reaction classes. 

Glycine imine esters, Ph2C=N-CH2-C02R, undergo asymmetric Michael addition to enones using an ether-water phase-transfer system.218 A chiral ammonium salt, in conjunction with cesium carbonate, gives high ees. 

The debromination of 1,2-dibromodiarylethanes to the corresponding trans-diarylethenes, which occurs quantitatively upon sensitized irradiation of alkylviologens (AV2+) in a phase transfer system, involves a different mechanistic pathway209-211. The active reductant here is AV, which is formed upon disproportionation of AV- , produced upon photochemical sensitization of AV2+. 

In this section, om recent approaches based on the use of hydrogen-bonding ionophores [22,23,34,35] are described to achieve the phosphate selectivity in two-phase-transfer systems. 

As described in Section 11.2.1, we have succeeded in a highly efficient transport of H2P04 anions across the NB/water interface based on the formation of the phosphate/ionophore 2 1 complex. However, the observed selectivity for phosphates is still moderate The selectivity, as evaluated from the reversible half-wave potentials of anion transfers, follows in the order of Cl (—0.077 V) > HP04 (—0.17 V) > H2P04 (—0.23V) [22]. Therefore, the further design and synthesis should be required for sufficiently selective ionophores in two-phase-transfer systems. 

Reactions with Sulfur Nucleophiles. The use of sulfur nucleophiles in palladium-catalyzed allylic substitution reactions is less well documented than that of carbon, nitrogen and oxygen nucleophiles. The asymmetric synthesis of allylic sulfones utilizing a catalytic phase transfer system has been used to produce (35)-(phenylsulfonyl)cyclohex-l-ene on a 45 g scale (eq 10). In many cases, it has been reported that allylic carbonates are more reactive than allylic acetates in asymmetric allylic substitution... 

Oxiranes can be opened with periodic acid in a phase-transfer system, in an aqueous medium, a dialdehyde is produced (Eq. 157) in the case of a dienemono-oxirane, the double bond remains unchanged in the reaction. ... 

Potassium superoxide, KO2, when in a phase-transfer system with Aliquat 336 (methyltrioctylammonium chloride), oxidizes ketones containing a-hydrogens to acids at room temperature [792]. 

Benzannulated (9-heterocycles as substrates in aromatic nucleophilic substitution in phase-transfer systems 02MI58. 

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