34 research outputs found
Metathesis access to monocyclic iminocyclitol-based therapeutic agents
No full textpeer reviewedBy focusing on recent developments on natural and non-natural azasugars (iminocyclitols), this review bolsters the case for the role of olefin metathesis reactions (RCM, CM) as key transformations in the multistep syntheses of pyrrolidine-, piperidine-and azepane-based iminocyclitols, as important therapeutic agents against a range of common diseases and as tools for studying metabolic disorders. Considerable improvements brought about by introduction of one or more metathesis steps are outlined, with emphasis on the exquisite steric control and atom-economical outcome of the overall process. The comparative performance of several established metathesis catalysts is also highlighted
Synthesis of Castanospermine and Epimers by Metathesis Routes
No full textpeer reviewedThis article provides a fairly comprehensive overview on olefin metathesis reactions applied as pivotal steps in the total syntheses of indolizidine alkaloids belonging to the families of castanospermine, deoxycastanospermine and diastereomers thereof, natural or non-natural compounds that play multiple roles in biological processes. A well-balanced mixture of topics is proposed, covering most relevant traits of their biological activity, therapeutic applications, and structural similarities to, or differences from, other established iminosugar drugs. While mainly focussing on a critical discussion of the metathesis-based processes (RCM, CM, RRM), their promoters and conditions, their advantages and limitations, also shown is how metathesis has contributed to more efficient and shorter protocols for accessing this important class of antiviral, antitumor, antimetastatic, anti-inflammatory and immunoregulating agents
Polyol preparation by liquefaction of technical lignins in crude glycerol
No full textThis work reports a study of polyol synthesis through liquefaction of technical lignins in crude glycerol by means of 1H and 31P NMR spectroscopy. The polyols are intended for preparation of polyurethane foam; thus, it is important to know how different lignin types as well as crude glycerol influence and contribute to the final polyol hydroxyl contents. Polyols prepared from organosolv lignin, kraft lignin and lignosulphonate had hydroxyl numbers suitable for rigid foam of 435, 515 and 529 mgKOH/g, respectively. The polyols differed in composition with glycerol, showing significant variation. During liquefaction the glycerol content was mostly reduced through bonding with lignin, and to a lesser extent monoacylglycerol and diacylglycerol formation through transesterification with fatty acid ethyl esters. It is concluded that crude glycerol can potentially replace petroleum-derived polyols as liquefaction solvent and that different types of technical lignin have a strong impact on the resulting bio-based polyol hydroxyl content
Oligomerisasie van langerketting 1-alkene in die teenwoordigheid van Cp2MCl2-metalloseen-, meer komplekse Zr-metalloseen- en tridentaat bis(imino)piridienyster(II)-katalisatorsisteme
No full text1-alkenes represent a large part of the commercial market and since oligomers thereof are important intermediates for specialty chemicals, they warrant further study. Metallocene catalysts are a promising development, since metallocenes, such as Cp2ZrCl2 (Cp = cyclopentadienyl), are easily obtainable. Examples of the oligomerisation of higher 1-alkenes are limited and there is a shortage of in-depth studies on these types of catalysts [3, 8–39]. It is also very apparent that only low MAO loadings lead to the formation of oligomers, and at higher loadings, polymerisation takes place [3, 18–29]. In an effort to increase the knowledge base of the oligomerisation of 1-alkenes a series of Cp2MCl2-catalysts (M = Zr (1), Ti (2), Hf (3) and Nb(4)), more complex Zr-metallocene (5 to 7) and tridentate bis(imino)pyridine iron(II) (8 to 11) catalyst systems was investigated (Figure 4). The specific aim was to determine the oligomerisation activity and selectivity of these catalysts in the presence of MAO as co-catalyst. Several factors can influence the catalyst activity during the conversion of 1-alkenes, namely activation temperature (Ta), activation time (ta), reaction temperature (Tr), co-catalyst concentration, the transition metal (M), and the monomer (mo) concentration. All these factors were investigated for the metallocene catalysts (1 to 4). The choice of ligand and bridging compound of the metallocene catalysts influenced the properties of the polymers obtained during reactions. The three more complex metallocene catalysts, 5 to 7, were investigated for their ability to oligomerise higher 1-alkenes. An important development in 1-alkene polymerisation catalysts was the discovery of the Group 8 iron catalysts, the so-called neutral tridentate bis(imino)pyridine iron(II) catalysts. Four catalysts of this type, 8 to 11, were identified for investigation of their activity with regard to the oligomerisation of higher chain 1-alkenes. Many factors can influence the catalyst activity during the conversion of 1-alkenes; in this study, only the reaction temperature (Tr) and co-catalyst concentration were varied because they were found to be major factors in determining the catalytic activity. The activity of the catalysts and degree of oligomerisation (n) were investigated with regard to 1-heptene and 1-octeneDie oligomerisasievermoë van ’n reeks Cp2MCl2-metalloseen- (Cp = siklopentadiëniel, η
5
-C5H5;
M = Zr, Ti, Hf en Nb), meer komplekse Zr-metalloseen- en tridentaat bis(imino)piridienyster(II)-
katalisatorsisteme is ondersoek. Verskeie faktore wat die katalisatoraktiwiteit van die Cp2MCl2-
metalloseen-katalisatorsisteme gedurende die omskakeling van 1-alkene kan beïnvloed, is nagegaan. Die
faktore is: aktiveringstemperatuur (Ta), aktiveringstyd (ta), reaksietemperatuur (Tr),
kokatalisatorkonsentrasie, die tipe oorgangsmetaal (M) en die monomeerkonsentrasie (mo). Die
temperatuur, die oorgangsmetaal en die kokatalisatorkonsentrasie het die reaksietempo en die graad van
oligomerisasie dramaties beïnvloed. Die twee faktore wat die reaksietempo en graad van oligomerisasie
dramaties beïnvloed het, naamlik die kokatalisatorkonsentrasie en die reaksietemperatuur (Tr), is verder
met die meer komplekse Zr-metalloseen- en tridentaat bis(imino)piridienyster(II)-katalisatorsisteme
ondersoek. Uit die drie reekse katalisatore wat ondersoek is, is vier geïdentifiseer wat die hoogste
aktiwiteit getoon het, en die potensiaal het om vir die dimerisasie van langerketting 1-alkene gebruik te
kan wor
‐phenylenediamine and its derivatives
No full textAromatic diamine monomers, including o-phenylenediamine (oPD), 4-methyl-o-phenylenediamine (4Me-oPD), 4,5-dimethylo-
phenylenediamine (dMe-oPD) and 4-(tert-butyl)-o-phenylenediamine (tBu-oPD), were polymerized by chemical oxidation
using ammonium persulfate as an oxidant. Aluminium triflate (Al(OTf)3) was also used for the first time as a co-catalyst
under various reaction conditions for the polymerization of oPD derivatives. The polymerization yield was improved when
Al(OTf)3 wasintroduced to thepolymerization reaction for mostpolymers. The solubilityofpoly(4-methyl-o-phenylenediamine)
(P(4Me-oPD)), poly(4,5-dimethyl-o-phenylenediamine) (P(dMe-oPD)) andpoly(4-(tert-butyl)-o-phenylenediamine) (P(tBu-oPD))
polymers was improved compared with the poly(o-phenylenediamine) (P(oPD)) polymers in most common solvents. The
homopolymers obtained were characterized by Fourier transform IR spectroscopy, UV−visible spectroscopy, 1H and 13C NMR,
wide-angle X-ray diffraction, DSC and TGA. The results showed that the yield, solubility and structure of the polymers are
significantly dependent on the polymerization conditions. DSC measurements indicated that the polymers exhibited melting
and crystallization transitions. The polymers also showed good thermal stability and decompose above 400 ◦C in nitrogen.SASOL (South Africa). South African
Department of Science and Technology, THRIP Project, for funding
(TP 2010072200011
Ruthenium Carbene Mediated Metathesis of Oleate-Type Fatty Compounds
No full textThe complexes RuCl2(PCy3)2(=CHPh), 1, and RuCl2(PCy3)(H2IMes)(=CHPh), 2, proved to be active catalysts for the self-metathesis of oleate-type fatty compounds containing the ester, hydroxyl, epoxy and carboxylic acid functional groups. At elevated reaction temperatures 2 showed a higher activity, stability and lower selectivity for primary metathesis products compared to 1. A profound influence of organic functional groups on catalyst activity and selectivity was found and from relative activities and selectivities 2 has proved to be more resistant to deactivation by polar functional groups and more inclined to promote double bond isomerisation than 1. The observed catalyst deactivation by oxygen-containing functional groups could be attributed to a phosphine displacement side reaction
Experimental, DFT and kinetic study of 1-octene metathesis with Hoveyda-Grubbs second generation precatalyst
No full textIn this study we report the catalytic performance, reaction engineering kinetics and elucidation of the reaction mechanism using density functional theory (DFT) for the metathesis reaction of 1-octene in the presence of the Hoveyda-Grubbs 2 [RuCl2(CHoOiPrC6H4)(H2IMes)] precatalyst. The study showed that reaction temperature (30-100 °C), 1-octene/precatalyst molar ratio (5000-14,000) and different solvents had a significant effect on the selectivity, activity and turnover number. Turnover numbers as high as 6448 were observed. Two main reactions were observed, namely: metathesis over the entire temperature range and isomerization above 50 °C. The observed experimental product-time distribution data for the complex parallel reaction system was fairly accurately described by four pseudo-first order reaction rates. The effects of temperature (Arrhenius Equation) and precatalyst concentration were incorporated in the observed rate constant. The primary observed activation energy was approximately 24 kcal mol−1, which is in agreement with the DFT computational values for the proposed Hoveyda-Grubbs mechanism
Technological evaluation of organic solvent nanofiltration for the recovery of homogeneous hydroformylation catalysts
No full textThe prevalence of homogeneous catalysts, to produce high value commodities in the surfactant and detergent range by upgrading inter alia syngas and short chain olefins, through metathesis, hydroformylation and hydrogeneration, is hampered by the expensive, waste generating and destructive thermal separation methods required for the recovery of these catalysts. By using the membrane technology of organic solvent nanofiltration (OSN), the successful recovery of two commercially available homogeneous catalysts, the rhodium-based complex, HRh(CO)(PPh3)3, and the cobalt-based complex, Co(C5H7O2)3, from the solvents representative of the hydroformylation and hydrogenation reactions, with the STARMEM 240 membrane, was performed. The energy-efficiency of OSN for homogeneous catalyst recovery, commonly alluded to in literature, was investigated by performing a technological evaluation between OSN and a conventional downstream recovery process such as distillation. It was found that, using the ST-240 membrane, catalyst concentrations in the permeate stream can be successfully reduced to near negligible amounts (<5 ppm), similar to that of distillation but with significant energy and cost savings of up to 85% and 75%, respectively, for the different reaction system
Functionalising lignin in crude glycerol to prepare polyols and polyurethane
No full textIn this work, crude glycerol liquefaction of lignins produced in the pulp and paper industry, as well
as an organosolv lignin (sugarcane bagasse), was studied with the ultimate aim of preparing biobased
polyols for polyurethane (PU) preparation. This is a proposed strategy to valorise the byproducts
of biodiesel and lignocellulose biorefineries. Size-exclusion chromatography revealed
that the lignins behave differently during liquefaction based on a ranging product molecular weight
(MW). The MW of the liquefaction products was concluded to be related to the phenolic and
aliphatic hydroxyl group content of the respective lignins, as well as the removal of glycerol and
monoacylglycerol during liquefaction. Lignin was modified to yield mostly a solid-phase product.
Fourier transform infrared spectroscopy suggests that crude glycerol constituents like glycerol and
fatty acid esters are bound to lignin during liquefaction through formation of ether and ester bonds.
Liquefaction yield further also varied with lignin type. The liquefaction products were effectively
employed as bio-based polyols to prepare P
The development of a Cu(I)/pyrazolylpyridineamine catalyst system for the hydroxylation of aryl halides
No full textA catalyst system comprising of pyrazolylpyridineamine/Cu(I)/CsOH is reported. for the hydroxylation of aryl iodides and bromides with moderate to outstanding yields, without the use of an inert atmosphere. A comprehensive parameter optimisation study established optimum component concentrations: [Cu(MeCN)4]BF4 and 2-(1H-pyrazol-1-yl)-N-(pyridine-2-ylmethyl)ethan-1-amine (L01) (2 mol %), substrate (1 mmol), CsOH (4 mmol) and DMSO:H2O (1:1, 3 mL). Monitoring substrate conversion as a function of time revealed an induction period of 90 min, which could be eliminated through the initial in situ formation of the proposed [(L01)Cu-OH] intermediate. Eliminating the induction period resulted in complete conversion within one hour, with turnover numbers exceeding that of the benchmark catalyst system operating at an optimal catalyst loading of 0.05 mol
