Immobilized Wilkinsons catalyst

In a similar vein, Gelman et al. [25] demonstrated encapsulation by the use of two SiC>2 sol-gel matrices [26,27] to immobilize two catalysts Wilkinson s catalyst and an amine that acts as a base catalyst. The reaction is illustrated in Scheme 5.10. 

The P CP-MAS spectrum of the immobilized Wilkinson s catalyst differs significantly from that of the neat catalyst. It consists of only one broad asymmetric peak visible at 32 ppm, which covers the area of PI and P3. Additionally, there is a broad weak signal at the position of the former P2 signal, which corresponds to less than 5% of the integral intensity and can be assigned most probably to a small amount of not immobilized Wilkinson s catalyst. These results show that P2 has evidently been replaced by an amine group of APTES linker and confirm the successful grafting on the surface. 

However, these results are not yet sufficient for a detailed determination of the structure of immobilized rhodium catalyst. The reason is that the broadening of the signal at 32 ppm can result either from a single phosphorus ligand or from two coalescent P chemical shifts related to the phosphine ligands with P3 and PI phosphorus atoms. In other words, there are two possible ways of binding of the Wilkinson s catalyst to the silica surface, namely a monopodal and a bipodal scenario, (see Fig. 9). In the... 

By standard ID P MAS-NMR experiments it is impossible to distinguish between these two scenarios. For this reason, it is necessary to apply advanced 2D P NMR techniques to characterize in detail the structure of the immobilized Wilkinson s catalyst. 

Figure 7.3 (a) H NMR spectra detected during the in situ (PASADENA) hydrogenation of styrene with pHj in CgDg at 65 °C. The spectra were obtained after bubbiing pHj for 15s either in the presence of poiymer-supported Wiikinson s cataiyst (top) or after the cataiyst was removed (bottom). (b,c) H NMR spectra detected during the gas-soiid hydrogenation of propyiene with pHj catalyzed by immobilized Wilkinson s catalyst... 

Amine catalyzed conversion of P-iodoethyl-benzene to styrene, followed by hydrogenation to ethylbenzene catalyzed by Wilkinson s catalyst Sol-gel immobilization of both catalysts... 

Metal-phosphine complexes can be immobilized on phos-phinated polymers through simple ligand exchange reactions. Wilkinson s Catalyst, Rh(PPh3)3Cl, has been supported on phosphinated polystyrene as shown in equation (9). The supported complex can be used as an alkene hydrogenation catalyst. 

A similar procedure was used for the reduction of cyclohexene in [bmim][Cl]/AlCl3, [bmim][BF4] or [bmim][PF6]. Modest conversions were obtained using Wilkinson s catalyst RhCl(PPh3)3 (40%).Theuseof[Rh(cod)2][BF4] (cod = cyclooctadiene) led to an improvement of the yield (65%), along with a lower turnover. In both cases, catalysts were immobilized in the ionic liquids, allowing clean and easy recovery of the product by decantation. 

Among the large number of catalytically active transition metal complexes, Wilkinson s catalyst (Fig. 3), is one of the most important and probably the most widely studied of all known homogenous hydrogenation catalysts. For this reason, the immobilization of this complex and its deri-vates on solid supports and their application in heterogeneous reactions are intensely investigated. 24,46,149 isi... 

Fig. 8 Central region of the P CP-MAS spectrum of the neat Wilkinson s catalyst and after its immobilization on mesoporous silica SBA-3 (adapted from ref. 171). 

Fig. 9 Two possibilities of the immobilization of the Wilkinson s catalyst at the surface of amine modified silica material SBA-3 (monopodal or bipodal tethering).

Fig. 11 The two dimensional J-resolved MAS slow spinning (2.5 kHz) spectnun of the Wilkinson s catalyst RhCl(PPh3)3, immobilized on mesoporous silica, reveals only a single P ligand. This shows that two Ugands are replaced by the immobilization.

The presented 2D P J-resolved NMR experiments provide clear evidence that Wilkinson s catalyst immobilized on the amine functionalized silica SBA-3 material, possess only one triphenylphosphine ligand. The rhodium centre is coordinated to the silica surface through the two amine ligands. It reveals significant information about the structure of... 

The reaction has also been carried out in the scC02/[bmim][PF5] system with Wilkinson s catalyst RhCl(PPh3)3 [47]. In this biphasic reaction system, the homogeneous transition-metal catalyst was immobilized in the IL phase, and the products could be isolated from the SCCO2 phase. As a result, high selectivity, catalyst recycling, and product recovery could be achieved for the hydrogenation of COj in the presence of dialkylamines. 

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