1,721,014 research outputs found
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
Metathesis access to monocyclic iminocyclitol-based therapeutic agents
Get PDFpeer 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
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
Katalitiese chemie - verlede, hede en toekoms / Hermanus Cornelius Moolman Vosloo
No full textAs samevatting wil ek die volgende aanhaling van Robert Day as boodskap in julle midde laat:
"There are four things that make this world go round:
love, energy, materials and information.
We see about us a critical shortage of the first commodity,
a near-critical shortage of the second,
an increasing shortage of the third,
but an absolute glut of the fourth. "
"We in science, of necessity, must contribute to the glut.
But let us do it with love, .. ,
let us also do it with energy, ... ,
and let us husband our materials, ...
Modellering en sintese van alisikliese dendrimeerligande vir alkeenmetatese
Get PDFThesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2011.Coupling homogeneous catalysts to dendrimers with rigid nuclei is one possible strategy to recycle these compounds in an industrial environment. During this study attempts were made to attach the well-defined ruthenium carbene complex 2 to the alicyclic compound 1A. To achieve this goal attempts were made to functionalise 1A with anime groups to facilitate the attachment of phosphine groups. The catalyst 2 would then be attached through a phosphine exchange reaction. Oximes were used as precursors in attempts to prepare cage amines from 1A. For this purpose, the dioxime 28 was prepared from 1A. Reduction of 28 was unsuccessful. Molecular modelling
showed that the lobes of the LUMOs of the oxime carbon atoms of 28 do not protrude from the total electron density of this molecule. This observation indicates that 28 would probably not react with nucleophiles, such as the hydride ion. Molecular modelling was used to probe the unreactive nature of oxime 28. The probe revealed that the imide ring deactivates the oxime groups in this
compound. Based on these results, attempts were made to change the carbon framework of 1A to eliminate unwanted interaction between the carbonyl groups. Clemmensen reduction of 1A did not yield the expected ketol 40, but gave a mixture of 62 and 63. Extended reaction times yielded 63 only. Reduction of 1A with zinc and acetic acid produced a mixture of 62 and 40 that could not be separated. Consequently, the applicability of 62 as a possible dendrimer nucleus was investigated. Compound 62 was obtained by oxidisation of the diol 63 with sodium periodate. Attempts to synthesise the dioxime 69 from 62 failed and only the mono oxime 71 was obtained. Reduction of 71 with lithium aluminium hydride was unsuccessful. Molecular modelling revealed that the oxime carbon atom does not have a LUMO and that the carbonyl carbon atom would probably be unreactive towards nucleophiles. Several other attempts were made at reducing the
oxime 71 to an amine. None of these attempts met with any success. The reason for the unreactive nature of 71 is less clear than in the case of 28. Failure to produce a cage amine from 1A or derivatives of 1A meant failure in functionalising the cage system with phosphine groups and coupling the Grubbs-I catalyst. It seems that cage compounds based on 1A are generally not suitable as starting materials for amine-functionalised dendrimer nuclei.Master
A DFT computational study of phosphine ligand dissociation versus hemilability in a Grubbs-type precatalyst containing a bidentate ligand during alkene metathesis
No full textUsing density functional theory, the metathesis reaction of 1-octene in the presence of a Grubbs-type ruthenium alkylidene complex bearing a chelating pyridinyl alcoholate ligand, [RuCl(L)(O∧ N)( = CHPh)] (L = H2IMes or PCy3, O∧ N = 1-(2′-pyridinyl)cyclohexan-1-olate)), was investigated. The complete geometry optimisation and activation energy of various activation steps in the dissociative mechanism were performed at the GGA-PW91/DNP level of theory using Accelrys Materials Studio® 4.0. Two possible precatalyst initiations were explored, i.e. the dissociation of the labile N-atom of the O∧ N-ligand as well as the dissociation of ligand L, due to the belief that ruthenium-catalysed metathesis reactions proceed through 14-electron intermediates. The formation of the catalytically active heptylidene species is kinetically favoured for both the first- and second-generation chelating complexes. The computational results are in agreement with the experimental results obtained with NMR for the second-generation system. The computational results suggest that both phosphine ligand dissociation and hemilability may play a role in the metathesis reaction with the first-generation syste
Isomerisation of alkenes using metal carbenes and related transition metal complexes
Get PDFThesis (Ph.D. (Chemistry))--North-West University, Potchefstroom Campus, 2005Different transition metal complexes such as RuCl2(=CHPh)(PCy3,)2, RuCl2(=CHPh)(PCy3)(IMes), RuCl2(=CH2) (PCy3)2, RuCl2(= CHBu)(PCy3)2, RuCl(H)(PPh3)3(CO), RuCl3, RhCl3-3H20, were used to isomerise alkenes. For the majority of experiments, reactions were performed on 1-octene and 4-octene, gas chromatography was used to monitor the consumption of the starting alkenes and the formation of isomers.
The variation of the reaction temperature on the different catalytic systems, the variation of the alkenelcatalyst molar ratio and the variation of the solvents on the isomerisation activity of the different catalytic systems were investigated. The results showed that the best conversion and selectivity are obtained with ruthenium carbenes. lsomerisation was observed at high temperatures ≥ 100°C and at lower temperatures more metathesis products were observed.
Short chain alkenes achieved higher isomerisation activity than long chain alkenes: 4-octene > 4-nonene > 4-decene. lsomerisation favoured more stable internal alkenes with the order of activity being trans-4-octene > trans-3-octene > trans-2-octene > cis-2-octene > 1 -octene.
From the study it is clear that RuCl2(=CHPh)(PCy3)2 is a capable double bond isornerisation catalyst for internal alkenes because metathesis products were not detected in any of the reactions using it. But when it comes to terminal alkenes, the complex gives a mixture of isomerisation and metathesis products. RuCI2(=CHPh)(PCy3)(lMes) is catalytically active for the isomerisation and metathesis of both the internal and terminal alkenes at high temperatures, at room temperature it is selective towards the isomerisation of internal alkenes and metathesis of terminal alkenes. Although RuCI(H)(PPh3)3(CO)/PhCI is catalytically active for the isomerisation of terminal alkenes at room temperature, it does not isomerise internal alkenes at lower temperatures. Interestingly, at high temperatures, especially after prolonged reaction times, isomerisation products were detected. The rhodium carbene and dimer are also catalytically active and selective for the isomerisation of internal and terminal alkenes, metathesis reactions were not observed in reactions involving these two catalysts. RhCl3-3H2O and RuCl3, cannot isomerise alkenes rapidly; longer reaction hours and high temperatures are required to get the isomerisation products. The isomerisation reaction proceeded with first order kinetics in both catalyst and 4-octene which is in agreement with a simple bimolecular interaction between the octene substrate and the ruthenium center. The kinetics data of 1-octene isomerisation did not fit simple first order or second order rate expressions, presumably due to secondary reactions (metathesis). The data showed a reasonably linear second order fit at the initial stages. The reaction mechanisms can be illustrated by the catalytic cycles.Doctora
Synthesis of selected cage alkenes and their attempted ring–opening metathesis polymerisation with well–defined ruthenium carbene catalysts
Get PDFThesis (Ph.D. (Chemistry))--North-West University, Potchefstroom Campus, 2012In this study a number of cage alkenes were synthesised and tested for activity towards ringopening metathesis polymerisation (ROMP) with the commercially available catalysts 55 (Grubbs-I) and 56 (Grubbs-II). The first group of monomers are derivatives of tetracyclo[6.3.0.04,1105,9]undec-2-en-6-one (1). The synthesis of these cage alkenes are summarised in Scheme 7.1. The cage alkene 126b was synthesised by a Diels-Alder reaction between 1 and hexachlorocyclopentadiene (9, Scheme 7.2). The geometry of 126b was determined from XRD data. Knowledge of the geometry of 126b also established the geometry of 127 since conformational changes during the conversion from 126b to 127 are unlikely. Synthesis of the cage alkene 125 by the cycloaddition of 9 to 118 failed. The cage alkene exo-11-hydroxy-4,5,6,7,16,16-hexachlorohexacyclo[7.6.1.03,8.02,13.010,14]hexa-dec-5-ene (124, Scheme 7.3) could therefore not be prepared. Synthesis of 125 by reduction of 126b with various reduction
systems was not successful. Theoretical aspects of these reactions were investigated with molecular modelling. A possible explanation for the unreactive nature of 126b towards reduction is presented, but the lack of reactivity of 118 towards 9 eluded clear explanations. The synthesis of cage alkenes from 4-isopropylidenepentacyclo[5.4.0.02,6.03,10.05,9]-undecane-8,11-dione (23) did not meet with much success (Scheme 7.4). Numerous synthetic methods were investigated to affect the transformation from 134a/134b to 135 (Scheme 7.5). These attempts evolved into theoretical investigations to uncover the reasons for the observed reactivity. Possible explanations were established by considering the differences and similarities between the geometries and electronic structures of reactive and unreactive cage
alcohols. ROMP of cage monomers based on 1 were mostly unsuccessful. Only the cage monomer 127 showed some reactivity. Endocyclic cage monomers with a tetracycloundecane (TCU) framework showed no reactivity. The results from NMR experiments verified the experimental results. Hexacyclo[8.4.0.02,9.03,13.04,7.04,12]tetradec-5-en-11,14-dione (3) exhibited notable ROMP reactivity. Examination of the orbitals of the cage alkenes used in this study suggested that the reactivity of 1 and 3 could possibly be enhanced by removal of the carbonyl groups. Decarbonylation of 1 and 3
yielded the cage hydrocarbons 159 and 175, respectively. ROMP tests revealed that 175 is an excellent monomer, but 159 was unreactive. The results obtained for the ROMP reactions in this study was rationalised by considering aspects such as ring strain, energy profiles, steric constraints, and frontier orbital theory. The concept of
ring strain is less useful when describing the reactivity of cage alkenes towards ROMP and
therefore the concepts of fractional ring strain and fractional ring strain energy (RSEf) were developed. A possible link between RSEf and the ROMP reactivity of cage alkenes was also established. The following criteria were put forth to predict the reactivity or explain the lack of reactivity of cage alkenes towards ROMP reactions with Grubbs-I and Grubbs-II. The criteria for ROMP of cage monomers: 1. Sufficient fractional ring strain energy (RSEf). 2. A reasonable energy profile when compared to a reference compound such as cyclopentene. 3. Ability to form a metallacyclobutane intermediate with reasonable distances between different
parts of the cage fragment. 4. Sufficient ability of the polymer fragment to take on a conformation that exposes the catalytic
site. 5. Sufficient size, shape, orientation and energy of HOMO and/or NHOMO at the alkene functionality of the cage monomer and of the LUMO at the catalytic site.Doctora
Experimental and theoretical investigation of new Grubbs-type catalysts for the metathesis of alkenes
Get PDFThesis (Ph.D. (Chemistry))--North-West University, Potchefstroom Campus, 2007.Ondanks die hoe selektiwiteit van die eentegenerasre Grubbs-prekatallsator (Grl) gedurende die metatese ban terminale alkene, het dit 'n kort leeftyd by verhwgde temperature. Die ontwikkeling van die tweedegenerasie Grubbs-prekatalisator (GR) het die problem tot 'n mate opgelos. Die vervanging van een PCyl ligand met 'n N-hetemsikiiese karbeen het 'n sisteem met verbeterde aktiwiteit en stabiliteit gelewer. Nogtans twn Gr2 lae selektiwiteit by verhoogde temperature waens die vorming van sekondere metateseprodukte gedurende die metatesereaksies. Gedurende die studie is eksperimenlele en teoretiese studies gekombineer om insig te kry in die meganisme van die metatesereaksie en om struktuur- en reaktiwiteitstendense van die katatitlese sisteme te voonpel. 'n Aantal 0.0-, 0,N-. 0.S- and 0-P-bidentate ligande is as moonUike hemilabiele ligande vir inkorporering in Grl en GR geidentifiseer. Die steriese en elektroniese omgewing van die ligande is gevarieer om die invloed van hierdie parameters op die 1-okteenmetateseaktiwileit van die rekatalisatore te bepaal. Dié ondersoek is gemotiveer deur die feit dat hemilabiele ligande 'n vry koördinasieposisie "op aandrang" van 'n inkomende nukleofiele substraat lcan beskikbaarstel tewyl dit andersins beset word. Daar word geglo dat dit die termiese stabiliteit en aktiwileit van die katalitiese sisteme verhoog en dus ontbinding via die vry kotirdinasieposisie vermy. Dit is onlangs as waar vir 'n aantal Gmbbs-karbene in ringopeningmetatesepolimerisasie (ROMP) en ringsluitingsmetatesereaksies (RCM) by verhoogde temperature aangetoon. Molekuulmodellering is as hulpmiddel gebruik om die nuwe Grubbs-tipe
prekatalisabre te ontwerp, wal dan gesinteliseer en vir 14kteen-metateseaMiwite~gt& va)ueer is. Ongelukkig kon 'n aantal van die ligande nie suksesvol in die Gmbbs-karbene geinkorporeer word nie, waarvoor moonUike redes in die prwfsknl bespreek word. Dit is algemeen gevind dat die 0,O-0,s - en karboksiliese 0.P-ligande tot die ontbinding van die Grubbskarbene gelei het. Die opbrengs van hierdie komplekse het algemeen in die gebied van 0 - 10% geval, wat die suiwering en analiseprases bemoeitik het. Die inkorporering van pikoliensuur. 'n 0.N-karboksiliese ligand, in Grl en Gr2 het 'n mengsel van karbene tot gevolg gehad wat nie geisoieer kon word nie. Nogtans kon die 0,N-alkoholaatligande met verskillende steriese volume suksesvol in Grl en GR ge'inkorporeer word met 'n opbrengs wat in die gebied van 40 - 80% geval het met 'n suiwerheid van 98 - 100%. Die inkorporering van die steries gehinderde 0,N-ligande in Grl en GR het die termiese stabiliteit, aktiwiteit, selektiwiteit en leeftyd van hierdie komplekse teenoor die melatese
van 1-okteen verbeter. In vergelyking met Grl is 'n 10 - 30% toename in die vorming van primbre metateseprodukte (PMP) tesarne met 'n 4% toename in isornerisasieprodukte (IP) vrr die eerstegenerasie Grubbs-prekatalisatore waargeneem. Nietemin, alhoewel geen beduidende toename in PMP na 7 h waargeneem is vir die tweedegenerasie anal@ in vergelyking met GtZ nie. is 'n 4 - 10% toename na 20 h waargeneern, tesame met 'n 5 - 15% toename in sekondere metateseprodukle (SMP). 'n Afname in die aktiwiteit van Grl en GR is addisioneel waargeneem nadat 'n hernilabiele 0,N-ligand met twee fenielgraepe in die sisteem geinkorporeer is, terwyl dit hul leeftyd verhwg het. Die 'H-KMR-ondersoek van 'n eerste- en tweedegenerasie sisteem het 'n piridinielalkoholaatligand het voorspel dat die GR-sisteem, maar nie die Gr1-sisteem nie, hernilabiele eienskappe vertoon. Dit het daarop gedui dat twee veskillende meganismes dalk betrokke mag wees gedurende die metatese van I-okteen in die teenwoordigheid van 'n eerste- en tweedegenerasie 0.N-gecheleerde kompleks. Addisioneel is 'n konseptueel-meganistiese model vir die alkeenmetatesereaksie in die
teenwoordigheid van Grl gepostuleer en op die hernilabiele eerste- en tweedegenerasie Grubbsanaloë toegepas. 'n Dieper insig in die KMR-resultate is ook met behulp van molekuulmodellering verkry. Die katalities-aktiewe spesies wat by voorkeur tydens die lokteenmetatese met Gtl en GrZ vorm, is geidentifiseer en eksperimenteel en teoreties geverifieer. Die resultate vir die hernifabiete komplekse is egter nog onbeslis en meer diepgaande studies behoort nog met 'n kombinasie van 'H- en "P-KMR gedoen te word. Dit moet gedoen word om inligting oor die hemilabiliteit van die prekatalisatore sowel as die invloed van die alkeen op die vrystelting van 'n vry koördinasieposisie te verkry. 'n Aantal navorsingsrnoontlikhede is ook geidentifiseer wat verder ondersoek moet word om meer insig in die meganisme van 1-okteenmetatese met 'n hernilabiele kompleks te verkry.Doctora
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
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