Phase-Transfer Catalysis: Fundamentals I

Abstract

A. IntroductionCritical to the success of phase-transfer catalytic (PTC) processes are (1) the maximization of the rate of transfer of reactant anions from the aqueous or solid phase to the organic phase, (2) the maximization of the rate of transfer of product anions from the organic phase to the aqueous or solid phase, and (3) the related equilibrium partitioning of the reactant and product anions between the organic and aqueous or solid phases. The common organic solvents employed in phasetransfer processes are usually relatively nonpolar and usually aprotic. Because anions do not have a great affinity for such solvents and prefer to reside in an aqueous environment, the desired transfer is not a particularly favorable process. The transfer of anions from an aqueous to an organic phase, however, may be achieved by choosing a phase-transfer cation that is not strongly solvated by water and that has organic-like characteristics and is thus compatible with the organic phase. For instance, the volume-to-charge ratio (as well as the organiclike nature) of quaternary ammonium and phosphonium salts can be adjusted over a wide range of values by simply changing the length of the alkyl (or aryl) substituents bonded to the quaternary heteroatom. Tetramethylammonium salts are highly soluble in aqueous media and only slightly soluble in most organic solvents, whereas tetradoecylammonium salts are soluble in most organic media but only slightly soluble in water. The former salt represents a quaternary ammonium ion with a small organic volume-to-charge ratio whereas the latter salt has a large organic volume-to-charge ratio. In a similar manner, macrocyclic multidentate ligands (crown ethers, cryptands, polyethylene oxides, etc.) may be employed to complex metal cations and carry them, along with their anions, from the aqueous or solid phase into the organic phase.The factors that affect the mass transfer and distribution of the phase-transfer 23 C. M. Starks et al., Phase-Transfer Catalysis © Chapman & Hall, Inc. 1994 r >Br->CI-CIO; > BrO; > 10; NO; > NO; CIO; > 10;The divalent anions listed in Table 2-2 have comparatively little affinity for the organic phase. An important measure of the competitive partitioning of anions between the aqueous and organic phases in the presence of a quaternary cation is the selectivity constant, KX1y set, which is defined by Eq. (2-1) and (2-2).(2-1)

Multi-site phase transfer catalyzed radical polymerization of methyl methacrylate in mixed aqueous–organic medium: a kinetic study

“…In this technique, the two mutually insoluble reactants, one being an organic liquid or substrate dissolved in an organic solvent and other being an organic or inorganic salt from a solid or aqueous phase, react with the help of a phase transfer catalyst. It has been applied over 600 processes in variety of industries such as intermediates, dyestuffs, agrochemicals, perfumes, flavors, pharmaceuticals and polymers and value exceeds twelve billion (US$) per year [1][2][3][4]. In polymer chemistry, they have been employed in synthesis of polymers [5][6][7], condensation polymerization [8], anionic polymerization [9,10] and free radical polymerization [11][12][13][14][15][16][17][18].…”

Ionic Liquids Made with Dimethyl Carbonate: Solvents as well as Boosted Basic Catalysts for the Michael Reaction

“…And that it is possible to design, optimise and apply the right ionic liquid for ones specific case. Phosphonium-based ionic liquids, which have been known and used for a long time as phase-transfer catalysts, [6] are particularly appealing [7] on account of their ease of preparation with high purity, their transparency and their higher stability toward thermal and chemical degradation compared with the homologous amAbstract: This article describes 1) a methodology for the green synthesis of a class of methylammonium and methylphosphonium ionic liquids (ILs), 2) how to tune their acid-base properties by anion exchange, 3) complete neat-phase NMR spectroscopic characterisation of these materials and 4) their application as active organocatalysts for base-promoted carboncarbon bond-forming reactions. Methylation of tertiary amines or phosphines with dimethyl carbonate leads to the formation of the halogen-free methyl-onium methyl carbonate salts, and these can be easily anion-exchanged to yield a range of derivatives with different melting points, solubility, acid-base properties, stability and viscosity.…”

Multiphasic heterogeneous catalysis mediated by catalyst-philic liquid phases

“…Liquid-liquid (L-L) systems on the other hand appear a little less obvious as systems for a chemical reaction, because partitioning of the species between the different phases becomes a critical issue. For example, two reactants can be in separate phases, and may need to be brought together by a phase-transfer-catalyst (PTC), 1,2 or by a surfactant. The advantage here lies in the possibility of carrying out a reaction between two species with opposite polarity, without the need for a polar aprotic solvent such as acetone, DMSO or DMF.…”

“…The equilibria that govern the reaction are two-shown in eqn (1) and (2) (1) and the second between the alkyl bromide and the liquid onium iodide to give the desired alkyl iodide product.…”