Non-covalently Supported Catalysts

One of the only examples of a commercial process using immobilised homogeneous catalysts comprises an anionic rhodium complex [RhI2(CO)2] that is bound via ionic interactions to an ion exchange resin [3] and is used for the carbonylation of methanol. 

The metal leached from the support is re-adsorbed to a guard bed of ion exchange material. This shows that the concept of noncovalent anchoring is very attractive, especially if the homogeneous catalyst needs no further modification. 

The grafting procedure is straightforward, which is a major advantage of this method. The silica was pretreated to remove water from the support, after which it was added to a dichloromethane solution containing the catalyst. After 6 hours of mixing the supported catalyst was isolated by filtration. The remaining solvent was colourless 

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Organocatalysts are often nonionic liquid modified-silica, polyelectrolytes, montmorillonite, or P-cyclodextrin. Although bond strength in non-covalently supported catalyst is weak, the catalyst itself is directly used for immobihzation without the need for modification or synthetic steps required for covalent attachment to support [113],... 

Recently, a non-covalently supported iminium-type primary amine catalyst 181 has also been developed using polyoxametalate as the solid acid support (Scheme 5.52) [80]. The catalyst was applied to asymmetric Diels-Alder reactions of a-substituted acroleins with good activity and stereoselectivity and could be recycled and reused for six runs. 

Abstract The immobilization of chiral catalysts through non-covalent methods, as opposed to covalent immobilization, allows an easier preparation of chiral heterogeneous catalysts with, in principle, less influence of the support on the conformational preferences of the catalytic complex. In this review the different possibilities for immobilization without forming a covalent bond between the chiral diazahgand and the support, which can be either solid or liquid, are presented. 

The supramolecular guest—Pd—dendrimer complex was found to have a retention of 99.4% in a CFMR and was investigated as a catalyst for the allylic ami-nation reaction. A solution of crotyl acetate and piperidine in dichloromethane was pumped through the reactor. The conversion reached its maximum ca. 80%) after approximately 1.5 h (which is equivalent to 2—3 reactor volumes of substrate solution pumped through the reactor). The conversion remained fairly constant during the course of the experiment (Fig. 8). A small decrease in conversion was observed, which was attributed to the slow deactivation of the catalyst. This experiment, however, clearly demonstrated that the non-covalently functionalized dendrimers are suitable as soluble and recyclable supports for catalysts. 

The performance of the different catalysts for aerobic oxidation of cinnamyl alcohol is compared under batchwise conditions. The non-covalently modified supports (III, IV) show longer induction periods, implying that the covalently PEG-modified surface (I) provides the best environment for the catalysts formed on basis of the Pd561 cluster. The results of batchwise aerobic alcohol oxidation in... 

Another method for generating an epoxidation catalyst on a solid support is to simply absorb or non-covalently attach the catalyst to the solid support <06MI493>. Epoxidation of olefin 6 with twCPBA and catalyst 8 provides 7 in quantitative yields and with 89% ee. The immobilization of 8 on silica gel improves the enantioselectivity of the reaction providing 7 with 95% ee. Recycling experiments with silica-8 show a decrease in both yield and the enantiomeric excess for each cycle (45% ee after 4 cycles). This is attributed to a leaching of the catalyst from the silica gel. Two other solid supports, a Mg-Al-Cl-LDH resin (LDH) and a quaternary ammonium resin (Q-resin) were also examined. It was expected that ionic attraction between 8 and the LDH or Q-resin would allow the catalyst to remain immobilized through multiple cycles better than with silica gel. Both of these resins showed improved catalytic properties upon reuse of the catalyst (92-95% ee after 4 cycles). 

Of the inorganic supports, best results were reported for a mesoporous MCM-41 [337]. Support on ionic-liquid phases has been studied by different groups with variable results [338, 339], Of the non-conventional organic polymers, non-covalent immobilization on poly(diallyldimethylammonium) is notable [340], Catalysts 133 (15 mol.%) promoted the aldol reaction of acetone and benzaldehydes to afford the corresponding (i-hydroxyketones in 50-98% yields and 62-72% ee, which are clearly lower than those reported for other polymer-supported systems. Recycling of the catalysts was possible at least six times without loss of efficiency. More recently, proline has been attached to one DNA strand while an aldehyde was tethered to a complementary DNA sequence and made to react with a non-tethered ketone [341], To date, the work has focused more on conceptual development than on the analysis of its practical applications in organic synthesis. 

A series of close-to-spherical styrene/DVB resins of varying particle size and pore diameter were employed as supports for non-covalent adsorptive attachment of CALB by hydrophobic interaction. The effect of matrix particle and pore size on CALB i) adsorption isotherms, ii) fraction of active sites, iii) distribution within supports, and iv) catalytic activity for s-CL ring-opening polymerizations and adipic acid/l,8-octanediol polycondensations is reported. Important differences in the above for CALB immobilized on methyl methacrylate and styrene/DVB resins were found. The lessons learned herein provide a basis to others that seek to design optimal immobilized enzyme catalysts for low molar mass and polymerization reactions. 

Keywords Supported catalysts Non-covalent immobilization Heterogeneous catalysis... 

In recent years, supported catalysts have become valuable tools for the simplified separation and recovery of catalysts from reaction mixtures. Commonly, the catalysts are attached covalently to a solid support. This covalent attachment of catalysts may lead to a partial loss of efficacy due to the decreased mobility. Alternatively, catalysts can be immobilized by non-covalent bonding through hydrogen bridges, or ionic, hydrophobic or fluorous interactions. Compared to covalent attachments, such non-covalent approaches increase the flexibility in the choice of the support material, reaction conditions and work-up strategies. 

In this and the following sections we describe the methods which do not need a hydrophilic solvent to retain the catalyst on the surface of the solid support. Utilization of hydrogen bonding for the non-covalent immobilization of Ru and Rh complexes on silica gel was investigated in detail [45-47]. The loading of the support was done without further covalent modification of the silica gel, and there was no need for a solvent film covering the support particles. 

As detailed in this overview, the non-covalent attachment of catalysts on a solid support is an important additional technique for the separation and recovery of catalysts from reaction mixtures. Such non-covalent immobilization strategies bring together a number of advantages of solution-phase chemistry and solid-phase supported chemistry. The catalysts can be separated from reaction mixtures by simple filtration. The pre-catalysts can be prepared and characterized in solution. The underlying principle is partitioning between a solid phase or a supported liquid phase and a liquid reaction phase of different solvating power. 

A non-covalent immobilization of Heck catalyst on silica (SILP concept) has been realized by Hagiwara et al. [217]. They used a silica surface, supported with Pd(OAc)2 dissolved in [BMIM][PFe]. This catalyst was appUed to the Mizoroki-Heck reaction of aryl halides with acrylate without a ligand in n-dodecane as solvent. It was six times reused and the overall TON reached 68 400 (for more details see Section 5.6). 

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