Solid-Liquid Phase Transfers

Solid-Liquid Phase Transfer.—Very few examples are known in onium ion chemistry where the anion to be phase transferred is part of a solid phase rather than in aqueous solution. This contrasts with the situation in crown ether chemistry, as will be discussed later in this review. A useful method, which is cheaper than alternatives, for the generation of dichlorocarbene in neutral conditions involves warming solid sodium trichloroacetate in chloroform, in the presence of quaternary salts. Formaldehyde acetals (25) can be synthesized in good yields by the action of solid KOH on alcohols or phenols dissolved in methylene dibromide containing a quaternary ammonium ion, and a phosphonium salt catalyses the alkylation of solid potassium phthalimide in toluene. Solid-liquid phase transfer mediated by polyamines has been reported.  

A further variation is the concept of triphase catalysis where the onium compound is attached to an insoluble polymer support e.g. 26 or 27), and phase transfer between aqueous and organic phases occurs via the polymer surface. Nucleophilic substitutions have been reported under such conditions, and some features of the non-immobilized PT catalysts are retained.  

3 Croivn Ethers, Cryptands and Related Complexing Agents 

Introduction.—The ability of certain molecules, such as the macrocyclic crown ethers, e.g. 18-crown-6 (28), and the macrobicyclic cryptands, e.g. [2,2,2] cryptand (29), to form complexes with metal and ammonium cations has been extensively investigated in recent years. Since the original discovery by Pedersen, in 1967, of the crown group,some reviews and many papers have appeared on the syntheses and complexing properties of different classes of ligands, but it is not the intention here to go into detail concerning these aspects. Laboratory syntheses of the polyether class are dependent on the Williamson ether synthesis (Equation 11), but methods for production of commonly used compounds, such as (28), have been improved, and many representatives of both the crown and cryptand groups are now commercially available. 

Complexes of cations with crown compounds have been shown in many cases to contain the ion in the macrocycle cavity, co-ordinating with the donor heteroatoms of the ring e.g. (30). The cation is thus effectively solvated, and the hydrocarbon portion of the ligand surrounds the ion with a hydrophobic layer, increasing 

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A solid-liquid phase-transfer technique is used to synthesize aryl difluoro-methyl sulfides and selenides thiophenols dissolved in an aromatic solvent are treated with solid sodium hydroxide in the presence of a catalytic amount of tris(3,6-dioxaheptyl)amine (TDA1) [49] This condensation proceeds by a carbene mechanism (equation 44)... 

Far superior yields of l-(arylsulfonyl)-l//-azepines 16 are now available by a one-pot synthesis involving the action of sodium azide on an arylsulfonyl chloride under solid-liquid phase-transfer conditions which prevents the formation of acidic sulfonamides and, hence, the ring-contraction process.75 This procedure also has the advantage of avoiding the use of high pressures and the isolation and handling of the potentially explosive sulfonyl azides. 

Arylsulfonyl)-l//-azepines 16 Using Solid-Liquid Phase-Transfer Conditions General Procedure 75... 

Molecular views of the rates of solid-liquid phase transfer of a pure liquid and a solution at the normal freezing point. The addition of solute does not change the rate of escape from the solid, but it decreases the rate at which the solid captures solvent molecules from the solution. This disrupts the dynamic equilibrium between escape and capture. 

With bifunctional alcohols CDI yields polycarbonates.[5] Syntheses of polycarbonates based on the CDI-method in a solid/liquid phase-transfer catalyzed reaction are described in references [261]—[263],... 

The synthesis of quadrupolar chromophores has also been achieved from 2,6-DTT-dicarboxaldehyde 117. Push-push (i.e., bis-donor) compound 118 was prepared via a double Wittig reaction carried out under solid-liquid phase transfer conditions. Pull-pull (i.e., bis-acceptor) compounds 119 were obtained from a symmetrical bis-aldehydes via a double Horner-Emmons-Wittig condensation (Scheme 9) <2002SM17, 1999CC2055>. 

Malonic acid allylic esters undergo intramolecular cyclization reaction under solid-liquid phase transfer catalytic conditions in the presence of Aliquat 336, potassium carbonate, and iodine (Eq. 60) [84]. Application of microwave irradiation to this procedure enabled 2-3-fold reduction in the reaction time compared with conventional conditions. It was found that use of microwaves affected the exo/endo diastereoi-somers ratio - a linear correlation between microwave power and exo isomer concentration was observed [85]. 

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

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. 

Triorganotin fluorides can be prepared by the use of new fluorinating systems. 18-Crown-6 or dibenzo-24-crown-8 can act as solid-liquid phase transfer catalysts for CsF. Trialkyltin mercaptides can be fluorodestannylated by CsF in the presence of crown ethers or alkyl bromides358 ... 

Herrmann WA, Brossmer C, Reisinger CP, Riermaier T, Ofele K, Beller M (1997) Coordination chemistry and mechanisms of metal-catalyzed C-C coupling reactions. Part 10. Palladacycles efficient new catalysts for the Heck vinylation of aryl halides. Chem Eur J 3 1357-1364 Iyer S, Jayanthi A (2001) Acetylferrocenyloxime palladacycle-catalyzed Heck reactions. Tetrahedron Lett 42 7877-7878 Iyer S, Ramesh C (2000) Aryl-Pd covalently bonded palladacycles, novel amino and oxime catalysts di- x-chlorobis(benzaldehydeoxime-6-C,AT)dipalla-dium(II), di- x-chlorobis(dimethylbenzylamine-6-C,A)dipalladium(II) for the Heck reaction. Tetrahedron Lett 41 8981-8984 Jeffery T (1984) Palladium-catalysed vinylation of organic halides under solid-liquid phase transfer conditions. J Chem Soc Chem Commun 1287-1289 (b) idem,... 

The nucleophilic attack on the bridging carbon results in two different reactivities. Usually ring opening occurs. Malononitrile under hydrolytic solid-liquid phase-transfer conditions attacks the methiodides 26 at the fusion carbon atom, thereby inducing ring opening and the formation of 4-(Ar-aryl-3 -methylisothioureido)-5-(dicyanomethylcnc)-l-... 

The unique ability of crown ethers to form stable complexes with various cations has been used to advantage in such diverse processes as isotope separations (Jepson and De Witt, 1976), the transport of ions through artificial and natural membranes (Tosteson, 1968) and the construction of ion-selective electrodes (Ryba and Petranek, 1973). On account of their lipophilic exterior, crown ether complexes are often soluble even in apolar solvents. This property has been successfully exploited in liquid-liquid and solid-liquid phase-transfer reactions. Extensive reviews deal with the synthetic aspects of the use of crown ethers as phase-transfer catalysts (Gokel and Dupont Durst, 1976 Liotta, 1978 Weber and Gokel, 1977 Starks and Liotta, 1978). Several studies have been devoted to the identification of the factors affecting the formation and stability of crown-ether complexes, and many aspects of this subject have been discussed in reviews (Christensen et al., 1971, 1974 Pedersen and Frensdorf, 1972 Izatt et al., 1973 Kappenstein, 1974). 

Times (h) needed to reach 50% conversion in the reaction of benzyl chloride with metal acetates in acetonitrile at ca. 25 °C under solid-liquid phase-transfer conditions ... 

Yamamura and Murahashi (1977) have studied the crown ether-catalysed cyanation of vinyl halides under solid—liquid phase-transfer conditions (20). The reaction of /rans-/ -bromostyrene [140] with sodium cyanide in benzene,... 

The synthesis of aryloxysulphonyl azides, which can be used as precursors for sulphamates, is improved by the use of tetra-n-butylammonium azide under homogeneous conditions in place of an alkali metal azide [ 1 ]. A stoichiometric amount of the ammonium azide is used and no attempts appear to have been made to conduct the reaction under solid liquid phase-transfer catalytic conditions. 

TV-Aminoethylazoles, which are frequently encountered as intermediates in the preparation of pharmacologically active compounds, are readily obtained by solid liquid phase-transfer catalysed reaction of the azole, diazole or triazole and their benzo derivatives with 2-chloroethylammonium chloride at reflux temperature using a procedure analogous to 5.3.3 [25]. 

It has been reported that alkylation of 2-pyridone and 2-quinolone and other related potentially tautomeric azinones under solid-liquid phase-transfer catalytic conditions generally produces the A-alkylated derivatives exclusively in 65-90% yield [58], although it has been suggested that the yield of the O-alkylated derivative can be increased by the use of long-chain quaternary ammonium salts and when bulky alkylating agents are used [69]. 

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 use of polyethers and quaternary salts as liquid-liquid and solid-liquid phase transfer catalysts has been well-documented in the literature. It has been shown that (1) the catalyst functions as a vehicle for transferring the anion of a metal salt from the aqueous or solid phase into the organic phase where reaction with an organic substrate ensues, (2) the rate of reaction is proportional to the concentration of the catalyst in the organic phase, and (3) small quantities of water have a significant effect on the catalytic process. This Communication specifically addresses the role of cyclic polyethers as phase transfer catalysts and the influence of water with regard to the location of the catalyst. 

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. 

Aromatic Substitution in Condensation Polymerization Catalyzed by Solid-Liquid Phase Transfer... 

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

Polymer-supported polyethylene glycol) analogues 50 145.156.l67-16H> anc[ 5/ 167> were effective catalysts for hydroxide, iodide, and phenoxide displacement reactions, but not for cyanide, chloride, and acetate displacementsI69). These catalysts are highly active for various solid/solid/liquid phase transfer reactions (Sect. 6). 

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.

The dissolution of potassium permanganate involves a solid-liquid phase transfer with the flat macrocycle, presumably, lining up in the correct complexation geometry on the surface of the crystal lattice. The movement of cation from lattice to ligands is then small. 

Jeffery, T. Palladium-Catalysed Vinylation of Organic Halides under Solid-Liquid Phase Transfer Conditions, J. Chem. Soc. Chem. Commun. 1984, 1287-1289. 

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... 

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