Microwaves and Phase-transfer Catalysis

Andr Loupy, Alain Petit, and Dariusz Bogdal 

Liquid-liquid PTC in which the inorganic anions or anionic species generated 

Microwaves in Organic Synthesis. Edited by Andre Loupy 

Copyright 2002 WILEY-VCH Verlag GmbH Co. KGaA,Weinheim 

The organic phase can be a nonpolar organic solvent (e.g., benzene, toluene, hexane, dichloromethane, chloroform, etc.) or a neat liquid substrate, usually the electrophilic reagent, which acts both as a reactive substrate and the liquid phase. 

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A. Loupy, A. Petit, D. Bogdal, Microwave and Phase-Transfer Catalysis. In Microwaves in Organic Synthesis, A. Loupy (Ed.), Wiley-VCH, Weinheim, 2002. 

Loupy, A. Petit, A. Bogdal, D. Microwaves and Phase-transfer Catalysis. InMicrowaves in Organic Synthesis, 2nd ed. Loupy, A. Ed. Wiley-VCH Weinheim, Germany, 2006 Vol 1 Chap 6 pp 278-326. 

A series of polyphosphites, polyphosphates, polythiophosphates, and other polymers containing sulfone functions, based on 1, have also been described [17,119]. An efficient synthesis of polyethers from 1 and 1,8-dibromo or dimesyl octane by microwave-assisted phase transfer catalysis has been reported [120]. 

Although phase-transfer catalysis has been most often used for nucleophilic substitutions, it is not confined to these reactions. Any reaction that needs an insoluble anion dissolved in an organic solvent can be accelerated by an appropriate phase-transfer catalyst. We will see some examples in later chapters. In fact, in principle, the method is not even limited to anions, and a small amount of work has been done in transferring cations," radicals, and molecules." The reverse type of phase-transfer catalysis has also been reported transport into the aqueous phase of a reactant that is soluble in organic solvents." Microwave activated phase-transfer catalysis has been reported." ... 

Microwave irradiation and phase transfer catalysis in C-, O- and N-alkylation reactions 13COS751. 

G. Keglevich, A. Griin, E. Balint, Microwave irradiation and phase transfer catalysis in C-, O- and N-alkylation reactions, Gurr. Org. Synth. 10 (2013) 751-763. 

Loupy and Soufiaoui described a comparative study of the reactivity of diphenylnitri-limine 200 with several dipolarophiles under microwave irradiation in the absence of solvent using a solid mineral support or phase-transfer catalysis (PTC) conditions (Scheme 9.62) [30b]. The results showed that the best yields of adducts were achieved upon impregnating KF-alumina with a mixture of the hydrazynoyl chloride 199 and the dipolarophile followed by irradiation of the mixture in a focused oven. Reaction of this mixture under solid-liquid PTC conditions with KF-Aliquat under microwaves afforded lower yields of cycloadducts, perhaps owing to the partial decomposition of Aliquat at the reaction temperature (140 °C). In all cases, worse yields were obtained by classical heating under comparable reaction conditions (time and temperature). 

This chapter focuses exclusively on microwave heterogeneous catalysis. Microwave homogeneous catalysis by transition metal complexes is treated in Chapt. 11, phase transfer catalysis in Chapt. 5, catalytic reactions on graphite in Chapt. 7, photocataly-tic reactions in Chapt. 14, and catalytic synthesis oflabeled compounds in Chapt. 13. 

The application of phase-transfer catalysis to the Williamson synthesis of ethers has been exploited widely and is far superior to any classical method for the synthesis of aliphatic ethers. Probably the first example of the use of a quaternary ammonium salt to promote a nucleophilic substitution reaction is the formation of a benzyl ether using a stoichiometric amount of tetraethylammonium hydroxide [1]. Starks mentions the potential value of the quaternary ammonium catalyst for Williamson synthesis of ethers [2] and its versatility in the synthesis of methyl ethers and other alkyl ethers was soon established [3-5]. The procedure has considerable advantages over the classical Williamson synthesis both in reaction time and yields and is certainly more convenient than the use of diazomethane for the preparation of methyl ethers. Under liquidrliquid two-phase conditions, tertiary and secondary alcohols react less readily than do primary alcohols, and secondary alkyl halides tend to be ineffective. However, reactions which one might expect to be sterically inhibited are successful under phase-transfer catalytic conditions [e.g. 6]. Microwave irradiation and solidrliquid phase-transfer catalytic conditions reduce reaction times considerably [7]. 

In the main, the original extractive alkylation procedures of the late 1960s, which used stoichiometric amounts of the quaternary ammonium salt, have now been superseded by solid-liquid phase-transfer catalytic processes [e.g. 9-13]. Combined soliddiquid phase-transfer catalysis and microwave irradiation [e.g. 14-17], or ultrasound [13], reduces reaction times while retaining the high yields. Polymer-supported catalysts have also been used [e.g. 18] and it has been noted that not only are such reactions slower but the order in which the reagents are added is important in order to promote diffusion into the polymer. 

Sodium salts of carboxylic acids, including hindered acids such as mesitoic, rapidly react with primary and secondary bromides and iodides at room temperature in dipolar aprotic solvents, especially HMPA, to give high yields of carboxylic esters.679 The mechanism is Sn2. Another method uses phase transfer catalysis.680 With this method good yields of esters have been obtained from primary, secondary, benzylic, allylic, and phenacyl halides.681 In another procedure, which is applicable to long-chain primary halides, the dry carboxylate salt and the halide, impregnated on alumina as a solid support, are subjected to irradiation by microwaves in a commercial microwave oven.682 In still another method, carboxylic acids... 

Solid-liquid solvent-free phase transfer catalysis (PTC) is specific for anionic reactions including numerous alkylations, eliminations and anionic additions. The coupling of PTC conditions and microwave irradiation was applied to numerous alkylations, eliminations, anionic condensations, and... 

Microwave heating and catalysis have been successfully used in the solvent-free synthesis of cosmetic fatty esters (Villa et al., 2003). Two kinds of reaction were performed acid-catalysed esterification (Figure 3.11) and phase-transfer catalysed alkylations, both reactions affording near quantitative yields when microwave heating was used. It should be noted that diethyl ether and water were used in the purification of the product, and alternative purification/separation procedures would be required if this process was performed on an industrial scale, due to the flammability risk of diethyl ether. 

Under microwave irradiation, carbazole reacted remarkably fast with a number of alkyl halides to give Af-alkyl derivatives of carbazole (82) (Bogdal et al., 1997). The reaction was carried out with high yields by simply mixing carbazole with an alkyl halide, which was adsorbed on potassium carbonate. A facile synthesis of a series of A-alkylpyrrolidino fullerenes (83) by phase transfer catalysis without solvent under microwave irradiation has been described by De la Cruz et al. (1998), while Adamczyk and Rege (1998) have illustrated the dramatic rate acceleration for A-sulfopropylation of heterocyclic compounds using 1,3-propane sultone under microwave irradiation affording the A-sulfopropylated compounds in 68-95% yield. 

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