Counter-phase Transfer Catalytic Reactions

The biphasic reactions are essentially slow compared with the homogeneous reactions. However, some of the slow biphasic reactions are considerably improved by the use of the counter-PTCs, keeping the advantage of easy separation of the catalysts. 

PdCl2 PPh2(m-C6H4S03Na) 2 (0.05 mmol) PhCH2=CHCH2Cl NaOH = 1 100 1000, heptane  

The hydrophilic palladium complex (2) was also a good catalyst for the carboxylation of benzyl halides under heptane-water two-phase conditions [20]. Benzyl chloride and bromide give phenylacetic acid in high yields under mild conditions (Eq. 5). However, the biphasic carboxylation with PdCI2(PPH,) , is very slow, and gives a considerable amount of benzyl alcohol. The addition of a normal PTC such 

Aryl iodides can also be carboxylated with NaOH and CO at atmospheric pressure in the presence of 2 under biphasic conditions, and give aryl carboxylic acids in high isolated yields [16] (Eq. (6) and Table 5). The normal PTCs are not very effective in accelerating the carboxylation [23]. This may be ascribable to the poor ex-tractability of hydroxy anion [24]. 

It is well known that the normal PTCs are effective for the biphasic cyanation of aryl halides with cyanide salts using hydrophobic catalysts [25]. However, a simple application of hydrophilic catalysts results in failure. The cyanation with 2 requires 

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Mechanism of the Counter-phase Transfer Catalytic Reaction... 

The term Counter Phase Transfer Catalysis (CPTC) was coined by Okano214,215 to describe biphasic reactions catalysed by water soluble transition metal complexes which involve transport of an organic-soluble reactant into the aqueous phase where the catalytic reaction takes place. Similarly, Mathias and Vaidya564,565 gave the name Inverse Phase Transfer Catalysis to describe this kind of biphasic catalysis which contrasts with classical Phase Transfer Catalysis where the reaction occurs in the organic phase and does not involve formation of micelles.389,564... 

The counter-ions of some of the quaternary onium groups were exchanged with an anionic phosphine compound, which was then used to complex palladium. Thus, a polymer material containing phase transfer catalyst and transition-metal catalyst groups was obtained (Fig. 20). The Heck-type vinyla-tion reaction [137] was used to examine the catalytic activity of the heterogeneous system. The polymer-supported catalyst was found to compare favourably with the homogeneous system (Fig. 21). 

The problems encountered in the catalytic transfer of highly hydrophilic anions from aqueous solutions into the organic phase can be countered by the use of anhydrous solid salts the organic reactant is dissolved in the organic solvent or, if liquid, may be used neat. Solid liquid two-phase reactions using ammonium salts have widespread application (see, for example, the many examples cited in later chapters) frequently with shortened reaction times, lower reaction temperatures, and higher yields [e.g. 66, 67] and are generally superior to solidrliquid reactions catalysed by crown ethers [68]. The process is particularly useful in base-initiated reactions with fluorides, hydroxides or carbonates. 

Quaternary ammonium and phosphonium ions bound to insoluble polystyrene present an even more complicated mechanistic problem. Polystyrene beads lacking onium ions (or crown ethers, cryptands, or other polar functional groups) have no catalytic activity. The onium ions are distributed throughout the polymer matrix in most catalysts. The reactive anion must be transferred from the aqueous phase to the polymer, where it exists as the counter ion in an anion exchange resin, and the organic reactant must be transferred from the external organic phase into the polymer to meet the anion. In principle, catalysis could occur only at the surface of the polymer beads, but kinetic evidence supports catalysis within the beads for most nucleophilic displacement reactions and for alkylation of phenylacetonitrile. 

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