Biphase catalysts

The interest in catalyst recyclability has led to the development of biphasic catalysts for hydro-boration.22 Derivitization of Wilkinson s catalyst with fluorocarbon ponytails affords [Rh(P (CH2)2(CF2)5CF3 3)3Cl] which catalyzes FIBcat addition to norbornene in a mixture of C6FnCF3 and tetrahydrofuran (TF1F) or toluene (alternatively a nonsolvent system can be used with just the fluorocarbon and norbornene) to give exo-norborneol in 76% yield with a turnover number up to 8,500 (Scheme 4). Mono-, di- and trisubstituted alkenes can all be reacted under these conditions. The catalyst can be readily recycled over three runs with no loss of activity.23... 

Renewed interest in this method came recently from its adaptation to the immobilization of water/ organic solvent biphasic catalysts, resulting in the so-called supported aqueous phase catalysts (SAPCs).117 The molecular catalyst is immobilized via water, which is hydrogen bonded to the surface silanol groups reactants and products are in the organic phase (Figure 11)... 

In most cases, the SAPCs are less active than their homogeneous counterparts, but more active than their liquid biphasic catalyst analogue, a phenomenon very simply interpreted as resulting from the increase of the interphase surface area which, in the case of the SAPCs, is similar to that of the silica support. 

Figure 2.3. Water-Organic Biphasic Catalyst System...

Table 16.1 Homogeneous catalysts, tethered single-site catalysts, and biphase catalysts for the hydrogenation of aromatic hydrocarbons.

A fist of the metal complexes that have been claimed to generate catalysts for the hydrogenation of carbocyclic aromatic rings is provided in Table 16.1. This list includes homogeneous catalysts, biphase catalysts, and tethered single-site catalysts. 

Table 41.17 Comparative hydrogenation studies using supported ionic liquid catalysts, biphasic catalyst systems and the classical homogeneous catalyst systems [116].a) ... 

Figure 5 Flow diagram of the Union Carbide Process for hydroformylation of higher olefins catalysed by Rhltppms in a single phase with biphasic catalyst separation.

Horvath, I.T., Kiss, G., Cook, R.A., Bond, J.E., Stevens, P.A., Rabai, J. and Mozeleski, E.J. (1998) Molecular engineering in homogeneous catalysis one-phase catalysis coupled with biphase catalyst separation. The fluorous-soluble HRh(CO) P[CH2CH2(CF2)5CF3]3 3 hydroformylation system. J. Am. Chem. Soc., 120, 3133. 

High turnovers have been reported with gold nanocrystals [13] using an aqueous base and with biphasic catalyst systems [14], bnt the corresponding monoacid and not the aldehyde was the major prodnct in most cases. 

The catalysts that effect the hydrogenation of thiophenes in aqueous-biphase systems (see Section 3.2.13.3.2) are also active for the selective reduction of aromatic heterocycles to cyclic amines using aqueous-biphase catalysts (Structures 6-10). 

From 1974 onwards the scope of different reactions using biphasic catalyst systems, preferably with precious metals, was tested in laboratory-scale experiments. Among these were butadiene hydrodimerization, hydrogenation of acrylonitrile or cyclohexene, hydroformylation of propene, and some other conversions to fine... 

V. S. Nair, Hydroformylation of Olefms Using Homogeneous and Biphasic Catalysts, Ph. D. Thesis, University of Pune, 1999. 

Since the major raw materials for higher plasticizer alcohols are internal alkenes from polygas units (e.g., diisobutene, tripropenes), this marked requires the development of even more efficient biphasic catalyst systems for internal and branched alkenes. 

As can be seen from Figure 2 the resulting aqueous catalyst phase can be further used in the biphase catalyst system (such as the RCH/RP process [5g]) or may be re-immobilized in a regenerator with fresh H2S04/amine for the next catalyst cycle. The amine content of the organic phase can be separated by distillation and recycled again by treatment with recycled sulfonated phosphine and sulfuric acid. 

Although the biphasic catalysts described above appear analogous to those discussed in Section 26.4, it does not follow that the mechanisms by which the catalysts operate for a given reaction are similar. 

The current enthusiasm for biphase catalysts will also benefit work being undertaken with ionic liquids. According to Chauvin et al. [22] olefins in particular can be dimerized with this special form of biphasic reaction in molten salt media (e. g. the Dimersol proeess of dimethyl butenes). 

The specificity of biphase catalyst systems (in conjunction with activities comparable to those anticipated with single-phase systems) has been illustrated in an elegant way by Mortreux et al. [24] In a regioselective hydro-formylation of methyl acrylate to give alpha-formylpropionic acid with the catalyst [HRh(CO)(TPPTS)3], these authors observed an aldehyde yield comparable to that achieved with TPP, as well as an alpha/beta product ratio of the expected order of magnitude and an even higher turnover frequency (TOF) (Scheme 2). 

The catalysts 11 and 12 (Fig. 4) were used for the synthesis of phenylalanine derivatives (Scheme 7) [28]. Besides the SAPC system, both the homogeneous hydrogenation and the aqueous-biphasic system were investigated. The supported complex 11, like the homogeneous analogue, showed poor performance as far asenantio selectivities are concerned. In contrast, the aqueous-biphasic catalyst performed at least moderately enantioselective. In this context enhanced selectivity was achieved with immobilized catalyst 12. The increased ee values (from 16 to 55%) were obtained at the expense of an extended reaction time, which increased from 2 to 40 h. 

Polymers are a general alternative to low molecular weight ligands in always biphasic liquid/liquid systems. Just as the nonpolar or polar polymers above imparted phase-selective solubility to catalysts in thermomorphic systems, the appropriate polymer can impart aqueous or fluorous phase-selective solubility to a catalyst. Several recent examples illustrate this for aqueous, fluorous, and other biphasic catalysts. 

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