Catalyst-binding ligand

Chaudhari, R.V., Bhanage, B.M., Deshpande, R.M. and Delmas, H. (1995) Enhancement of interfacial catalysis in a biphasic system using catalyst-binding ligands. Nature, 373, 501. 

Several modifications of the water-soluble catalysts using co-solvents (cf. Section 4.3 and [14]), micelle forming reagents (Section 4.5 and [15]), super-critical C02-water biphasic system (cf. Section 7.4 and [16]), SAPC (Section 4.7 and [17]), and catalyst binding ligands (interfacial catalysis) [18, 24] have been proposed to overcome the lower rates observed in biphasic catalysis due to poor solubilites of reactants in water. So far endeavors were centered on innovating novel catalyst and development of the existing systems. However, limited information is available on the kinetics of biphasic hydroformylation. 

In a report Chaudhari et al. [18] have shown that the rate of biphasic hydroformylation can be enhanced severalfold by using a catalyst binding ligand which facili-... 

Fig. 10 Effect of catalyst binding ligand on the rate of hydroformylation of 1-octene [18].

It is important to note that the kinetic trend were completely opposite for cases with and without catalyst binding ligand for carbon monoxide. Since, under conditions of interfacial catalysis, a higher CO concentration is accessible to the catalytic species, substrate inhibition is observed. 

This concept has also been demonstrated in reverse for the hydroformylation of a water-soluble olefin (aUyl alcohol) with the organic phase containing a catalyst, H Rh (CO) (P Ph3)3. In this case, the catalyst is present in the organic phase, whereas the catalyst-binding ligand (TPPTS) is added to the aqueous phase. 

Using the dendrimer route, it is possible to prepare supported catalysts not available via traditional routes. Dendrimer derived Pt-Au catalysts having compositions within the bulk miscibility gap can be prepared on several oxide supports. For all the supports studied, the bimetallic catalysts exhibited synergism with respect to mono- and cometallic catalysts for the CO oxidation and hydrocarbon NOx SCR reactions. The bimetallic Pt-Au catalysts also showed evidence of exchanging surface and subsurface atoms in response to strongly binding ligands such as CO. 

Two key issues are essential for the success of the catalytic HAT (CHAT) reaction. First, the two catalytic systems must be compatible. Because early and late transition metals usually bind ligands with opposing donor properties, they should remain mutually unaffected. Also, the Wilkinson catalyst should be stable under the slightly acidic conditions of the titanocene regeneration system (Zn, 2,4,6-collidine hydrochloride). Since it has been reported that Rh(PPh3)3Cl is even stable towards BF3 Et20, this was expected to be the case [44]. 

Why does the (EBTHI)Zr system induce such high levels of enantioselectivity in the C-C bond formation process It is plausible that the observed levels of enantioselection arise from minimization of unfavorable steric and torsional interactions in the complex that is formed between 3 and the heterocycle substrates (Scheme 3). The alternative mode of addition, illustrated in Fig. 1, would lead to costly steric repulsions between the olefin substituents and the cyclohexyl group of the chiral ligand [6]. Thus, reactions of simple terminal olefins imder identical conditions results in little or no enantioselectivity. This is presumably because in the absence of the alkenyl substituent (of the carbon that bonds with Zr in i) the aforementioned steric interactions are ameliorated and the olefin substrate reacts indiscriminately through the two modes of substrate-catalyst binding represented in Fig. 1. 

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