Gas-liquid phase-transfer catalysis

Figure 4.1 General picture of gas liquid phase transfer catalysis (GL-PTC).

Before the 1990s there was little in the literature on multiphasic L-L-S and L-L-L-S systems used for chemical reactions. There is, however, a relatively large volume of work done on other types of multiphasic systems related to the present topic supported liquid-phase catalysis (SL-PC), and gas liquid phase transfer Catalysis (GL-PTC). The common denominator in both cases is the presence of an interfacial liquid layer of a hydrophilic compound between the catalyst and the bulk of the reaction. 

In fact, since the leaving group, methyl carbonate, decomposes (reaction 3), the base is restored and can be used in truly catalytic amounts. This feature allows utihzation of continuous-flow (c-f) procedures (i.e. gas-liquid phase-transfer catalysis, GL PTC, and continuously stirred tank reactor, CSTR ). 

Angeletti, E., Tundo. P, and Venturello, P., Gas-liquid phase-transfer catalysis. Wittig-Horner reaction in heterogeneous conditions, 7. Chem. Soc., Perkin Trans. 1, 713, 1987. 

Angeletti, E., P. Tundo, P. Venturello, and F. Trotta, Synthetic Opportunities of Gas-Liquid Phase-Transfer Catalysis, BrMsh Pol. ... 

Tundo, P., and M. Selva, Simplify Gas-Liquid Phase Transfer Catalysis, Chemtech, 25(5), 31 (1995). 

Tundo, P., G. Moraglio, and F. Trotta, Gas-Liquid Phase Transfer Catalysis A New Continuous Flow Method in Organic Synthesis, Ind, Eng. Chem. Res., 28, 881 (1989). 

Solid-Liquid and Gas-Liquid Phase-Transfer Catalysis... 

Gas-liquid phase-transfer catalysis (GL-PTC) relies on the use of thermally stable PT catalysts adsorbed onto a solid support, which can also act as a source of the desired nucleophile. Reactions are carried out at a temperature that ensures that the catalyst is in a molten state and that reagents are in the vapor phase, and that the chemical transformation occurs in the organic microphase of molten catalyst. The products are recovered after condensation outside the reaction vessel. Only catalysts having melting points lower then the process temperature <180°C are active, but despite this limitation, GL-PTC is a versatile technique that has been applied to a number of chemical transformations. 

Preparation.— Two procedures for the production of ethers from alky] halides have been mentioned earlier in this Report. From a study of fluoride salts on alumina as reagents for the alkylation of phenols and alcohols, potassium or caesium fluoride on alumina, in acetonitrile or 1,2-dimethoxyethane as the solvent, has been found to be the best combination for general use. A recently reported one-pot synthesis of phenyl ethers from phenol acetates involves their treatment, in solution in acetone, first with potassium carbonate and then with an alkyl halide. Another interesting new procedure for the alkylation of phenols utilizes the gas-liquid phase-transfer catalysis technique that was discussed above. In this case a phenol (or a thiophenol) and an alkyl halide, both gaseous, are passed through a bed of solid K2CO3 (or NaHCOs) at 170°C in the presence of a PEG e,g. Carbowax 6000) as the catalyst. ... 

The 5 -alkylation of aryl thiols by gas-liquid phase-transfer catalysis has been mentioned earlier in this Report/ and a selective S -dealkylation of alkyl aryl sulphides has been reported, using sodium diethylamide in HMPT. ... 

SYNTHETIC REACTIONS BY GAS-LIQUID PHASE-TRANSFER CATALYSIS... 

Gas-liquid phase-transfer catalysis (GL-PTC) is a new synthetic organic method that has similarities both with phase-transfer catalysis (PTC) and with gas-liquid chromatography (GLC) in that anion transfer processes and partition equilibria between gaseous and liquid phases both take place and affect the synthesis. Using GL-PTC, nucleophilic substitution reactions have been so far carried out under operative conditions and with synthetic results, making this method different from the well known liquid-liquid (LL-) and solid-liquid (SL-) phase-transfer catalysis. As regards these latter, phase-transfer catalysts (onium salts, crown ethers and cryptands) transfer the reactive anion from an aqueous liquid (LL-PTC) or a solid salt (SL-PTC) phase into the organic one in which the substitution reaction occurs. In the case of GL-PTC, where no solvent is used, the catalyst always acts as an anion transfer (between solid and liquid phases) but, as it works in the molten state it also constitutes the medium in which the reaction proceeds. 

Dimethyl carbonate can be used for methylation of nitrogen in amine compounds at a temperature of 180°C under continuous-flow gas-liquid phase-transfer catalysis conditions (which involve transfer of organic ionic reactant species between water and an organic phase) as shown below for the methylation of aniline ... 

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