Phase transfer quaternary ammonium salts

This property of quaternary ammonium salts is used to advantage m an experi mental technique known as phase transfer catalysis Imagine that you wish to carry out the reaction... 

Quaternary ammonium salts as we have seen are useful m synthetic organic chem istry as phase transfer catalysts In another more direct application quaternary ammo mum hydroxides are used as substrates m an elimination reaction to form alkenes... 

Quaternary ammonium salts compounds of the type R4N" X find application m a technique called phase transfer catalysis A small amount of a quaternary ammonium salt promotes the transfer of an anion from aqueous solution where it is highly solvated to an organic solvent where it is much less solvated and much more reactive... 

Halex rates can also be increased by phase-transfer catalysts (PTC) with widely varying stmctures quaternary ammonium salts (51—53) 18-crown-6-ether (54) pytidinium salts (55) quaternary phosphonium salts (56) and poly(ethylene glycol)s (57). Catalytic quantities of cesium duoride also enhance Halex reactions (58). 

Phase-transfer catalysis (Section 22.5) Method for increasing the rate of a chemical reaction by transporting an ionic reactant from an aqueous phase where it is solvated and less reactive to an organic phase where it is not solvated and is more reactive. Typically, the reactant is an anion that is carried to the organic phase as its quaternary ammonium salt. 

It is important to make the distinction between the multiphasic catalysis concept and transfer-assisted organometallic reactions or phase-transfer catalysis (PTC). In this latter approach, a catalytic amount of quaternary ammonium salt [Q] [X] is present in an aqueous phase. The catalyst s lipophilic cation [Q] transports the reactant s anion [Y] to the organic phase, as an ion-pair, and the chemical reaction occurs in the organic phase of the two-phase organic/aqueous mixture [2]. 

Another catalytic system which has been successfully applied to the autoxidation of substituted toluenes involves the combination of Co/Br" with a quaternary ammonium salt as a phase transfer catalyst (ref. 20). For example, cobalt(II) chloride in combination with certain tetraalkylammonium bromides or tetraalkylphosphonium bromides afforded benzoic acid in 92 % yield from toluene at 135-160 °C and 15 bar (Fig. 19). It should be noted that this system does not require the use of acetic acid as solvent. The function of the phase transfer catalyst is presumably to solubilize the cobalt in the ArCH3 solvent via the formation of Q + [CoBr]. ... 

In this method, a catalyst is used to carry the nucleophile from the aqueous into the organic phase. As an example, simply heating and stirring a two-phase mixture of 1-chlorooctane for several days with aqueous NaCN gives essentially no yield of 1-cyanooctane. But if a small amount of an appropriate quaternary ammonium salt is added, the product is quantitatively formed in about 2 h." There are two principal types of phase-transfer catalyst. Though the action of the two types is somewhat different, the effects are the same. Both get the anion into the organic phase and allow it to be relatively free to react with the substrate. 

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... 

In liquid-solid processes reaction takes place between a liquid reactant and an insoluble or sparingly soluble solid which must be finely divided to speed up the process. Another measure to accelerate the process is to use an aqueous solution of a phase-transfer agent (typically a quaternary ammonium salt). The solid can also be a catalyst for reactions between liquid components, e.g. in acylations, carried out both conventionally in the presence of metal chlorides (mostly AICI3) or catalysed by zeolites and Grignard reactions. 

Catalysis, enzymatic, physical organic model systems and the problem of, 11,1 Catalysis, general base and nucleophilic, of ester hydrolysis and related reactions, 5,237 Catalysis, micellar, in organic reactions kinetic and mechanistic implications, 8,271 Catalysis, phase-transfer by quaternary ammonium salts, 15,267 Catalytic antibodies, 31,249... 

Partitioning of carbocations between addition of nucleophiles and deprotonation, 35, 67 Perchloro-organic chemistry structure, spectroscopy and reaction pathways, 25, 267 Permutational isomerization of pentavalent phosphorus compounds, 9, 25 Phase-transfer catalysis by quaternary ammonium salts, 15, 267 Phosphate esters, mechanism and catalysis of nucleophilic substitution in, 25, 99 Phosphorus compounds, pentavalent, turnstile rearrangement and pseudoration in permutational isomerization, 9, 25... 

Arai et al.51 reported that by using a catalytic amount of chiral quaternary ammonium salt as a phase transfer catalyst, a catalytic cycle was established in asymmetric HWE reactions in the presence of an inorganic base. Although catalytic turnover and enantiomeric excess for this reaction are not high, this is one of the first cases of an asymmetric HWE reaction proceeding in a catalytic manner (Scheme 8-20). 

Keywords Phase Transfer m Quaternary Ammonium Salts... 

Cinchona alkaloids now occupy the central position in designing the chiral non-racemic phase transfer catalysts because they have various functional groups easily derivatized and are commercially available with cheap price. The quaternary ammonium salts derived from cinchona alkaloids as well as some other phase transfer catalysts are... 

The first practical and efficient asymmetric alkylation by use of chiral phase-transfer catalysts was the alkylation of the phenylindanone 15 (R1=Ph), reported by the Merck research group in 1984.114-161 By use of the quaternary ammonium salt 7 (R=4-CF3i X=Br) derived from cinchonine, the alkylated products 16 were obtained in excellent yield with high enantiomeric excess, as shown in... 

The asymmetric Darzens condensation, which involves both carbon-carbon and carbon-oxygen bond constructions, was realized by use of the chiral azacrown ether 75als2,s ,ss and the quaternary ammonium salts derived from cinchona alka-loids159"621 under phase transfer catalyzed conditions. The a,p-epoxy ketone 80 (R=Ph) was obtained with reasonable enantioselectivity by the reaction of... 

The catalytic asymmetric Horner-Wadsworth-Emmons reaction was realized by use of the quaternary ammonium salts 7 derived from cinchonine as a phase transfer catalyst.1631 Thus, tert-butylcyclo-hexanone 85 reacted with triethyl phosphonoacet-ate 86 together with RbOH-H20 in the presence of the ammonium salts 7, and then the product 87 was isolated after reesterification by treatment with acidic ethanol, as shown in Scheme 27 Among the... 

E. J. Corey, F. Xu, M. C. Noe, A Rational Approach to Catalytic Enantioselective Enolate Alkylation Using a Structurally Rigidified and Defined Chiral Quaternary Ammonium Salt under Phase Transfer Conditions , J. Am. Chem. Soc, 1997,119,12414-12415. 

Bisphenol-AF-derived poly(carbonate) (2) has been synthesized by the two-phase transfer-catalyzed polycondensation of Bisphenol AF (1) with trichloromethyl chloroformate (TCF) in organic-solvent-aqueous-alkaline solution systems with a variety of quaternary ammonium salts at room temperature (Scheme l).6... 

The following quaternary ammonium salts are used as phase transfer catalyst tetra-K-butylammonium chloride (TBAC), tetra-n-butylammonium bromide (TBAB), benzyltriethylammonium chloride (BTEAC), and benzyltriethylammo-nium bromide (BTEAB). Chlorinated hydrocarbons, such as dichloromethane (DCM), chloroform (CF), tetrachloromethane (TCM), 1,2-dichloromethane (DCE), and nitrobenzene (NB) are used as solvents. The effects of phase-transfer catalyst and solvent on the yield and reduced viscosity are summarized in Table 9.1. 

The authors show that the phase transfer agent exerts a strong influence on the stereochemistry observed in the final product since -E-unsaturaled diacids are the major products in the presence of polyethylene glycol (PEG-400), whereas Z-isomers are obtained when quaternary ammonium salts are added [34]. 

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