Gas liquid phase transfer catalysis GL-PTC

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 ). 

Trotta, F., P. Tundo, and G. Moraglio, Selective Mono-lV-Alkylation of Aromatic Amines by Dial l Carbonate Under Gas-Liquid Phase-Transfer Catalysis (GL-PTC) Conditions, /. Org. Chem., 52, 7500 (1987). 

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. 

Selva. M. Tundo. P. Gas-liquid phase-transfer catalysis (GL-PTC). Chim. Ind. (Milan) 1995, 77, 411-414. 

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. 

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