Solid-liquid phase transfer catalysis

Heteropoly catalysts can be applied in various ways (4-10). They are used as acid as well as oxidation catalysts. They are used in various phases, as homogeneous liquids, in two-phase liquids (in phase-transfer catalysis), and in liquid-solid and in gas-solid combinations, etc. The liquid-solid and gas-solid combinations are represented by the classes of catalysis shown in Fig. 1 and described in the following sections. The advantages of heteropoly catalysts stem from the characteristics summarized in Table I. 

Yanagida, S., K. Takahashi, and M. Okahara, Solid-Solid-Liquid Three Phase Transfer Catalysis of Polymer Bound Acyclic Poly(oxyethylene) Derivatives Applications to Organic Synthesis, ... 

One of the major problems in the synthesis of sulphamoyl esters is the low yield of ester from the reaction. Phase transfer catalysis has been used to substantially increase ester yields under mild experimental conditions. Spillane and coworkers211 using liquid/liquid and liquid/solid phase transfer methods (equations 21 and 22) prepared in high yield a wide range of sulphamoyl esters of the type R1R2NS03R3 (165), RNHS03R3 (166) and H2NS03R3 (167). 

Yanagida, S., and Takahashi, K., Losid-solid-liquid three phase transfer catalysis of polymer bound acyclic poly(oxyethylene) derivatives. Application to organic synthesis, J. Org. Chem., 44, 1099, 1979. 

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. 

Perrard T, Plaquevent JC, Desmurs JR, Hebrault D. Enantioselective synthesis of both enantiomers of methyl dihydrojasmo-nate using solid-liquid asymmetric phase-transfer catalysis. Org. Lett. 2000 2(19) 2959-2%2. 

Phase transfer catalysis (PTC) refers to the transfer of ions or organic molecules between two liquid phases (usually water/organic) or a liquid and a solid phase using a catalyst as a transport shuttle. The most common system encountered is water/organic, hence the catalyst must have an appropriate hydrophilic/lipophilic balance to enable it to have compatibility with both phases. The most useful catalysts for these systems are quaternary ammonium salts. Commonly used catalysts for solid-liquid systems are crown ethers and poly glycol ethers. Starks (Figure 4.5) developed the mode of action of PTC in the 1970s. In its most simple... 

Although the use of phase-transfer catalysis (PTC) for manufacturing esters has the merits of a mild reaction condition and a relatively low cost [1], PTC has its limitations, such as the low reactivity of carboxylic ion by liquid-liquid PTC [2], a slow reaction rate by solid-liquid PTC, and the difflculty of reusing the catalyst by both techniques. 

In addition, there are a few examples of heterogeneous nonaqueous sonochemistry, in both liquid-liquid and liquid-solid systems. Two recent reports have utilized ultrasonic agitation in place of or along with phase transfer catalysis for the preparation of dichlorocarbene from aqueous NaOH/CHCl3 (166), and for N-alkylation of amines with alkyl halides (167). Along the same lines, several papers have appeared in which... 

Solvent-free Solid-Liquid Phase-transfer Catalysis (PTC)... 

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). 

Phase transfer catalysis. As well as their use in homogeneous reactions of the type just described, polyethers (crowns and cryptands) may be used to catalyse reactions between reagents contained in two different phases (either liquid/liquid or solid/liquid). For these, the polyether is present in only catalytic amounts and the process is termed phase transfer catalysis . The efficiency of such a process depends upon a number of factors. Two important ones are the stability constant of the polyether complex being transported and the lipophilicity of the polyether catalyst used. 

In phase transfer catalysis of the solid/liquid type, the organic phase (containing dissolved organic reactant and a small amount of the crown) is mixed directly with the solid inorganic salt. Such a procedure enables the reaction to proceed under anhydrous conditions this is a distinct advantage, for example, when hydrolysis is a possible competing reaction. Because of their open structure, crown ethers are readily able to abstract cations from a crystalline solid and are often the catalysts of choice for many solid/liquid phase transfer reactions. 

Phase transfer catalysis can be conducted under liquid-liquid conditions, liquid-solid conditions, or liquid-liquid-solid triphasic conditions. 

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. 

The general concept of phase transfer catalysis applies to the transfer of any species from one phase to another (not just anions as illustrated above), provided a suitable catalyst can be chosen, and provided suitable phase compositions and reaction conditions are used. Most published work using PTC deals only with the transfer of anionic reactants using either quaternary ammonium or phosphonium salts, or with crown ethers in liquid-liquid or liquid-solid systems. Examples of the transfer and reaction of other chemical species have been reported(24) but clearly some of the most innovative work in this area has been done by Alper and his co-workers, as described in Chapter 2. He illustrates that gas-liquid-liquid transfers with complex catalyst systems provide methods for catalytic hydrogenations with gaseous hydrogen. 

The data for the reactions of potassium cyanide with benzyl halides at 85 C and 25 C are summarized in Tables I-III and graphical representations of these data are shown in Figures 1-3. The reactions carried out at 85 C were followed to 70% completion, while those at 25 C were followed to 50% completion. In general, excellent first-order kinetic plots were obtained. Each point on the graphs represents an average of at least three kinetic determinations. It is interesting to note that in the absence of added water (solid-liquid phase transfer catalysis), the rates of benzyl halide disappearance were more accurately described by zero-order kinetics. 

The other approach called upon phase-transfer catalysis (31). The potassium salt of 2,5-furandicarboxylic acid 11a was treated with primary aliphatic dihalides in a typical solid/liquid system in the presence of a crown ether. The resulting furanic-aliphatic polyesters had molecular weights of... 

Solid/Solid/Liquid Phase Transfer Catalysis.92... 

Numerous examples of solid/solid/liquid phase transfer catalysis are now known to be useful synthetically but have not been investigated mechanistically. Poly(ethylene glycol) immobilized on alumina and silica gel is active for reaction of solid potassium acetate with 1-bromobutane 184). Some of the best synthetic results with polymer supports are shown in Table 15. Often use of other solid salts or other catalysts gave poorer yields. It would be valuable to know for the design of future syntheses how these reactions depend on the partial solubility of the inorganic salts in the organic solvents and on the presence of trace amounts of water. 

Table 15. Examples of Solid/Solid/Liquid Phase Transfer Catalysis with Solid Inorganic Reagents Reaction Catalyst Ref.

Tris(l-pyrazolyl)methane was first prepared by Hiickel and Bretschneider by the reaction of sodium pyrazolate with chloroform.17 Trofimenko prepared the tris(3,5-dimethyl-l-pyrazolyl)methane by a similar method.2 More recent preparations have utilized both liquid-liquid18 and solid-liquid12 phase-transfer catalysis. The preparation of tris(l-pyrazolyl)methane (E) given below is a modification of the solid-liquid phase-transfer procedure.12... 

Crown ethers and cryptates represent new classes of heterocyclic catalysts having the ability to complex cations and thereby to promote solid-liquid phase transfer catalysis. A detailed description of their properties is found in the literature.12,21-31... 

Treatment of diethyl malonate and related compounds with 1,2-dihaloethane in the presence of base constitutes a classical method of cyclopropane synthesis296"300. The reaction can be conveniently carried out under PTC conditions. An improved method utilizing solid-liquid phase transfer catalysis has been reported298. The reaction of dimethyl or diethyl malonate with 1,2-dibromoalkanes except for 1,2-dibromethane tends to give only low yields of 2-alkylcyclopropane-l, 1-dicarboxylic esters. By the use of di-tm-butyl malonate, their preparations in satisfactory yields are realized (equation 134)297. The 2-alkylcyclopropane derivatives are also obtained from the reaction of dimethyl malonate and cyclic sulfates derived from alkane-1,2-diols (equation 135)301. Asymmetric synthesis... 

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