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.
Authors
Qazi, Asma R.Collections
- Theses [3711]