Tethered catalysts

Silica-supported metal (e.g., Pd/Si02) catalysts also have surface silanol groups that can react with the alkoxysilane groups of the complexes. These combination catalysts consist of a tethered complex on a supported metal. A Rh complex was tethered to the surface of a Pd/Si02 catalyst, and the tethered catalyst was more active for the hydrogenation of aromatic compounds than the free complex or the supported catalyst separately.33 It is possible that the H2 is activated on the supported metal and the hydrogen atoms migrate to the silica, where they react with the reactant molecules coordinated by the tethered complex. 

The metal complexes, M +(acac)2 (M = Cu or Co), were observed to be bonded to the surface tether by a Schiff condensation reaction with the primary amine. The metal cations in these tethered complexes were used to activate t-butylhydroperoxide into radicals that reacted with the substrate, cis-pinane, to give mainly 2-pinane hydroperoxide with no observed formation of 2-pinanol. The selectivity to the pinane hydroperoxide was 93% for the immobilized Co(II) and 84% for the immobilized Cu(II) at 91 % substrate conversion. Blank reaction tests using the (1) oxidized carbon and (2) oxidized and hxd-functionalized carbon showed much lower activities ( 10-15% conversion) thus indicating that the tethered metal complexes were largely responsible for the observed activity. The subsequent reuse of the tethered catalysts eventually showed a constant activity ( 60% conversion) with reuse suggesting that the tethering method was effective in firmly attaching the metal ion to the surface. 

The two types of catalysts merge with the development of what are called tethered catalysts, where a homogeneous catalyst is amended so that it is able to be attached covalently to an inert surface, such as silica. This is also called a supported catalyst. Having the active component available as part of a solid can assist processes where carrying the catalyst forward in solution to another stage of the process may lead to contamination or catalyst destruction. Further, surface attachment also can alter catalytic activity favourably in certain cases. 

Chemically tethering the activator component, instead of the metal complex, allows for flexibility in the choice of metal complex while retaining some of the advantages that chemically tethered catalysts exhibit. This technique was one of the earliest sup-... 

Wills and co-workers later suggested (48) that the reaction mechanism of the formic acid decomposition in a HCOOH/EtsN azeotrope to hydrogen and carbon dioxide, using a Rh-TsDPEN (where TsDPEN = AT-(4-toluenesulfonyl)-l,2-diphenylethylenediamine) tethered catalyst, is closely related to that of asymmetric transfer hydrogenations of ketones (Scheme Scheme 1). 

Wills and coworkers reported on the synthesis and application of a series of Ru(II) and Rh(III) tethered catalyst (Fig. 73) related to Noyori s monotosylated-... 

The tethered catalyst 24a proved to be superior over its untethered version 26 in the reduction of functionalised acetylenic ketones 27 with FA-TEA as hydrogen donor (Fig. 8) [59]. High enantioselectivity and substituent tolerance of these ketones indicate that the alkyne group has a dominating stereo-controlling effect through C-Wn interaction. Additionally, an efficient dynamic kinetic resolution was observed with substrates 27 favouring the syn products. The table in Fig. 8 shows selected examples of the reduction of acetylenic p-keto esters. 

Otherwise identical conditions. The origin of the improved catalytic performance of the oxo-tethered catalysts 28 might be ascribed, on the one hand, to the delicate electronic mning of the cooperating hgands (arene moiety with CH2O unit), and, on the other hand, to the enhanced electron density of the secondary amino unit in the TsDPEN-derived ligand. 

A similar reduction towards a primary amine intermediate was reported by scientists at Merck (Fig. 29). The target in this case was the HCV NS5a inhibitor MK-8742, and a key step was a highly enantioselective ATH of a C=N bond. Among the catalysts investigated, the best one proved to be the tethered catalyst reported by Wills et al. The synthesis of the complex target requires simple starting materials and just nine linear steps for completion [100]. 

Fig. 33 Asymmetric transfer hydrogenation of quinolines using a tethered catalyst...

Si02-tethered rhodium complexes, derived from Rh(CO)2(acac) and 3-(mercapto) propyl- and 3-(l-thioureido)propyl-functionalized silica gel, were used as catalysts in the hydroformylation of various vinyl arenes and vinyl acetate. Conversions, che-moselectivity, and regioselectivity obtained with the Si02-tethered catalysts were comparable with those of the well-known homogeneous rhodium catalysts [85]. 

Hydroformylation of 1-hexene in supercritical carbon dioxide was investigated using a rhodium-phosphane catalyst tethered to a silica support. The performance of the tethered catalyst was compared with a homogeneous rhodium-phosphane... 

For encapsulated catalysts, amination reaction was carried out in a high pressure autoclave containing aniline, iodobenzene, catalyst, and potassium ferf-butoxide (KOt-Bu) at 408 K for 14 h. For tethered catalysts, the amination reaction was carried out in a two necked flask under reflux at 385 K for 10-12 h. 

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