Precious metal loaded organically modified silica for organic transformations

Abstract

Precious Metal Loaded Organically Modified Silica for Organic Transformations Highly active heterogeneous silica supported palladium ethylthioglycolate, silica-60-G1- Pd(OAc) and silica supported 1,2-bis(ethylthio)ethane palladium, silica-60-C2-Pd(OAc)2 and 1,2-bis(ethylthio)propane palladium, silica-60-C3-Pd(OAc)2 catalysts were prepared and characterised (using elemental analysis, solid state NMR, electron microscopy and nitrogen sorption porosimetry). The catalysts have been shown to be very active, recyclable and resistant to leaching in a representative selection of Suzuki-Miyaura cross-couplings. Two sets of conditions were employed; those regularly reported in the literature, high temperature with xylene, and much milder reaction conditions, room temperature with isopropanol. In both reaction conditions, catalysts silica-60-G1-Pd(OAc), silica-60-C2-Pd(OAc)2 and silica- 60-C3-Pd(OAc)2 gave cross coupled products in high yields. Microwave-assisted cross coupling reactions of more difficult aryl bromides with substituted phenylboronic acid employing catalyst silica-60-C3-Pd(OAc)2 were also investigated. These reactions proceeded smoothly with excellent yields. S S Pd(OAc)2 S O O Pd(OAc) silica-60-G1-Pd(OAc)2 S S Pd(OAc)2 silica-60-C2-Pd(OAc)2 silica-60-C3-Pd(OAc)2 Catalyst silica-60-C3-Pd(OAc)2 was also assessed for the more difficult Mizoroki-Heck cross coupling and displayed slow conversions. Modification of this catalyst with replacement of Pd(OAc)2 with PdCl2 to give silica-60-C3-PdCl2, gave excellent conversions in which aryl halides were combined with styrene. Reactions were carried out in NMP with K2CO3 as a base. Activity was retained in recycles and the catalyst was again found to be resistant to leaching. Microwave assisted reactions of more difficult substrates were also found to give good conversions. Further application of catalyst silica-60-C3-Pd(OAc)2 was explored in the hydrogenation reaction of alkenes, nitriles and imines. High conversions were found as well as effective reVII use of the catalyst several times without loss in activity. Further studies of the pore size effects in the hydrogenation of nitrobenzene revealed catalysts prepared with smaller pores to be more active possibly as a result of the more confined environment. Interesting examples of one-pot tandem hydrogenation and Suzuki-Miyaura cross-coupling reactions catalysed by silica-60-C3-Pd(OAc)2 were also elaborated. Asymmetric hydrogenations were also explored with palladium chiral cysteine derivatised ligands immobilised on silica, silica-60-Nderivatised- L-cysteine-Pd(OAc). Although high conversions were achieved, the selectivity of these catalysts in the hydrogenation of highly substituted imines was very low. Novel active heterogeneous ethylphosphatrioxaadamantane (PAD) ruthenium catalysts, silica- 110-PAD-Ru-DPEN with chiral amine ligands were also prepared for the asymmetric hydrogenation of ketones. These results gave quantitative conversions to the alcohol but low selectivity (18% e.e) was found. So attention was turned to using these immobilised phosphorus ligands, silica-110-PAD, to complex Rhodium compounds and utilising these materials as catalysts in hydroformylation reactions. P RhClPPh3 silica-110-PAD-RhCl(PPh3)2 Hydroformylation reactions of styrene were carried out using CO:H2 in a 1:1 ratio with toluene as a solvent and with silica-110-PAD-RhCl(PPh3)2 as catalyst. Reactions carried out at room temperature were found to favour the formation of branched aldehydes (73:27). At high temperatures, although activity was improved this was coupled with a decrease in the regioselectivity and in some cases some hydrogenated product was also formed. Interestingly, change of the rhodium salt to Rh(CO)2(acac) was found to give better chemoselectivity with no hydrogenated product. Structure activity behaviour of the catalysts was rationalised against their materials characteristics including immobilised ligand, surface area, pore diameters and volumes, metal loading and oxidation state. The work included a study of the relative metal uptake efficiency of the different materials using ICP-OES and a further study of the relative mobility of the immobilised ligands using 13C CP MAS T1 measurements and effects of dipolar dephasing.