Counter Inverse Phase Transfer Catalysis

The term Counter Phase Transfer Catalysis (CPTC) was coined by Okano 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 Vaidya 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. 

Classical Phase Transfer assisted Organometallic Catalysis is a further important field which has found industrial applications e.g. in the carbonylation of benzyl chloride to phenylacetic acid using NaCo(CO)4/Bu4NBr catalysts in aqueous NaOH practiced by Montedison. However, a detailed discussion of classical phase transfer catalysis is beyond the scope of this chapter which is devoted to systems in which the catalytic conversion takes place in the aqueous phase. 

The concept of CPTC has been applied in a large number of catalytic reactions such as reduction of allyl chlorides with HCOONa, carbonylation of aryl and allyl halides, allylation of aldehydes, cyanation of aryl halides etc. For example, Okano et al. reduced l-chloro-2-nonene to afford 1-nonene and 2-nonene (Equation 17) with selectivities of 82% and 17%, respectively using palladium catalysts modified with the water soluble polyether phosphine 76 (Table 4 n=0, x=l, m=3, R=Me) in an heptane/H20 (8/2) two phase system according to CPTC concept. 

With PdCl2(76)2 the conversion of l-chloro-2-nonene was quantitative whereas with the non-polar PdCl2[P(nBu)3]2 the lipophilic substrate hardly came into contact with the hydrophilic reductant HCOONa and the conversion was low (26%).2 5 

Rh/tppts catalysts in the presence of cyclodextrins constitute CPTC systems which are active in the hydroformylation oT various water-insoluble olefins. For example, 1-decene was biphasically hydroformylated with 

2-nonene (Equation 17) with selectivities of 82% and 17%, respectively using palladium catalysts modified with the water soluble polyether phosphine 76 (Table 4 n=0, x=l, m=3, R=Me) in an heptane/H20 (8/2) two phase system according to CPTC concept. 

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Several concepts have been suggested to increases the rates in aqueous-phase catalytic conversion of higher substrates such as addition of conventional surfactants [3, 5] (cf. Sections 4.5 and 6.1.5), counter (inverse)-phase transfer catalysis using /3-cyclodextrins [6] (cf. Section 4.6.1), addition of promoter ligands, e.g. PPh3 [7], or co-solvents (cf. Section 4.3). However, addition of foreign compounds militates against the facile catalyst separation and purification of the products and increase the costs as well. 

It is difficult to verify the counter- or inverse-phase transfer catalysis strictly, because the catalyst more or less acts as a surfactant as well as a normal PTC [13]. However, it should be a positive proof of the counter-PTC to ascertain that the aqueous phase is where the products are formed. 

New tools for such biphasic reactions are inverse- or counter-phase transfer catalysts, which are able to transport lipophilic molecules from the organic phase into the aqueous phase. An advantage of the inverse- or counter-PTC is its applicability to reactions not only with ionic salts but also with non-ionic reagents soluble in water. Such carriers were first reported by Mathias and Vaidya and by us in 1986 [3, 4], and three types of carriers are known at present. Mathias and Vaidya found that pyridine derivatives react with acid halides in the organic phase to form the pyridinium salts, which transfer into the aqueous phase [3]. This catalysis was... 

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