77 research outputs found
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
Experimental and DFT investigation of the 1-octene metathesis reaction mechanism with the Grubbs 1 precatalyst
No full textUsing density functional theory the metathesis reactions of 1-octene in the presence of Grubbs 1 [RuCl2(PCy3)2( CHPh)] is investigated. At
the GGA-PW91/DNP level, the complete geometry optimization and the activation energy of various activation steps and catalytic cycles in the
dissociative mechanism are performed. The formation of the catalytically active heptylidene species is kinetically and thermodynamically favoured,
while the formation of trans-tetradecene is thermodynamically favoured. The computational results are in agreement with the experimental results
obtained with NMR and GC/MSD experiments. Grubbs 1 is active for the metathesis of 1-octene at 25 ◦C yielding trans-7-tetradecene as the major
product. The formation of by-products, i.e. styrene isomers and PCy3, is consistent with the dissociative metal carbene mechanism. The rapid
formation of the heptylidene derivative of Grubbs 1 is observed by in situ NM
Synthesis of selected cage alkenes and their attempted ring–opening metathesis polymerisation with well–defined ruthenium carbene catalysts
No full textThesis (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
No full textThesis (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
Modellering en sintese van alisikliese dendrimeerligande vir alkeenmetatese
No full textThesis (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
Separation of Grubbs-based catalysts with nanofiltration
No full textThesis (Ph.D. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2009.This thesis describes the use of organic solvent nanofiltration (OSN) in the field of metathesis for separating homogeneous Grubbs-type catalysts from their post-reaction mixtures for the model metathesis reaction of 1-octene to 7-tetradecene and ethene. The main contributions and objectives of this study were in demonstrating:
(i) the successful separation and re-use of different Grubbs-type catalysts from their
post-reaction mixtures, and (ii) the successful synthesis of a newly developed catalyst, Gr2Ph, that demonstrated a longer catalytic lifetime for re-usability.
The study was twofold in firstly describing the catalytic performances of different Grubbs-type catalysts for the model reaction and secondly in characterizing and describing the separation performances of the 1-octene metathesis system with OSN.
In terms of catalyst performances:
The catalytic performance of different Grubbs-type precatalysts (Gr1, Gr2, HGr1, HGr2 and the newly developed Gr2Ph) was studied for the model reaction by varying operating parameters, such as reaction temperature (30 to 100 °C), catalyst load (1-octene/Ru molar ratio between 1:5000 and 1:14000) and reaction environment (reaction in the presence of various organic solvents). Quantities such as product distribution, selectivity, yield, catalyst lifetime and activity were used in comparing and evaluating the efficiency of these precatalysts with each other.
It was found that all three precatalysts HGM, HGr2 and Gr2Ph showed both metathesis and isomerization activity for the model reaction that was strongly temperature-dependent. Precatalysts HGr2 and Gr2Ph showed significant secondary metathesis activity while precatalyst HGM did not. It was found that the optimal reaction temperature for precatalyst HGM was 30 °C, for precatalyst HGr2 50 °C and for precatalyst Gr2Ph 80 °C. The addition of different solvents to the reaction environment had an overall negative effect towards the formation of the primary metathesis products (PMPs) of 7-tetradecene and ethylene.
In this study it was postulated and demonstrated with molecular modelling, that the metathesis reaction of 1-octene with the different Grubbs-type precatalysts (HGM, HGr2 and Gr2Ph) could accurately be described by a type of release-return dissociative mechanism. It was further found
that the reaction kinetics of the model reaction with the three precatalysts (HGM, HGr2 and Gr2Ph) could fairly accurately be described by a set of three inter-dependent elementary reaction rate-equations.
In terms of separation performances:
Five different Grubbs-type precatalysts (Gr1, Gr2, HGr1, HGr2 and Gr2Ph) and the commercially available STARMEM™ series of OSN membranes were used in this study. Parameters such as feed concentration, feed pressure, membrane pretreat-solvent and catalyst load were varied in a dead-end setup. Quantities such as the permeation rate (flux), catalyst rejection, solvent separation (selectivity), degree of swelling and contact angles were measured.
It was found that the STARMEM™ 228 membrane successfully separated the different Grubbs-type catalysts from their post-reaction mixtures to below 9 ppm with catalysts rejections greater than 99%. Relative moderate fluxes were obtained that ranged from 0.2 to 15 kg.m-².h-¹. It was shown that 7-tetradecene preferentially absorbed in the STARMEM™ 228 membrane. A solvent non-separating system was observed for binary mixtures of 1-octene, 1-tetradecene and 7-tetradecene. It was found that the predominant parameters that influenced the transport of the 1-octene metathesis system through the ST-228 membrane were solvent properties (such as viscosity) » membrane-solvent interaction properties (such as sorption) > solvent-solvent structural properties (such as molar volume or effective molecular volume).
The experimental permeation results for the binary mixtures of 1-octene and 7-tetradecene through the STARMEM™ 228 membrane were described by using pore-flow models, solution-diffusion models and a newly developed model that incorporates structural solvent-solvent interaction. It was found that the newly developed model best described the experimental results.
A coupled reaction-separation process was applied that demonstrated the successful reusability of the in-house synthesized catalyst, Gr2Ph. The turnover number was increased from 1400 for a single pass reaction to 5500 for the overall consecutively coupled reaction-separation steps of four cycles. Catalysts Gr1, Gr2, HGM and HGr2 did not show any catalytic activity after the first separation cycle due to extremely short catalytic lifetimes of less than ten hours compared to catalyst Gr2Ph's three days.
The short catalytic lifetimes of the classical precatalysts such as Gr1, Gr2, HGr1 and HGr2 in the field of alkene metathesis were solved with the synthesizing concept of modifying and binding the dissociating ligand and anionic ligand with bidentate 0,N-chelated Schiff base ligand on the second generation Grubbs-precatalyst.Doctora
Metathesis of alkenes using ruthenium carbene complexes
No full textThesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2005.In this study different ruthenium carbene complexes, Grubbs 1 and 2 were investigated for the reactions of 1-, 2-, 3-octene and 1-tetradecene. All reactions were conducted in glass mini reactors equipped with Mininert® valves under a dry nitrogen atmosphere. A heating block was used to heat the reactors. Gas chromatography was used to determine the product composition of reaction mixtures.
The influence of reaction temperature, solvents and catalyst concentration on the
reactions of 1-octene were investigated. The optimum reaction conditions for activity
and selectivity with Grubbs 1 and 2 were determined. Increased temperature for Grubbs 2 results in decrease in activity in the homometathesis reaction of 1-octene while the product yield increased as a result of isomerisation. Grubbs 2 catalyst displayed a
higher metathesis product yield (65%) at lower temperatures (25°C) with 1-octene and
lower metathesis products (less than 10%) at higher temperatures (100°C). Grubbs 1
displayed higher metathesis products (55-64%) at both lower (25°C) and higher
temperatures (100°C). Secondary metathesis products due to isomerisation were less
than 20% with Grubbs 2 the more active isomerization catalyst. Grubbs 1 and 2 were active for the metathesis of both 2- and 3-octene. With 2-octene high yields of secondary metathesis products were observed. The optimum reaction conditions for the metathesis of 1-octene were used for the reaction of 2-, 3-octene and 1-tetradecene. A higher metathesis yield (70%) was obtained with 2-octene. Secondary
metathesis products (Cg - C13) and oligomers (C16 and longer) were obtained in the presence of Grubbs 2. Grubbs 1 and 2 also catalysed the metathesis of 1-tetradecene. The reaction is very slow in the presence of Grubbs 1 and little metathesis products are observed (less than 20%). In the presence of Grubbs 2 40% primary and less than 10% secondary metathesis products were observed. The reaction mixture of 1- and 2-octene in the presence of Grubbs 1 and 2 produced metathesis products less than 20%. Cross
metathesis was observed from the mixture, C9 and C13 were produced. Both Grubbs 1and 2 showed a high activity and selectivity towards the metathesis reaction of 1-octene. These catalysts showed that they are still active at high alkene/Ru molar ratios and in different solvents that were used. Relatively high turnover numbers (TON) were obtained for both Grubbs 1 and 2. NMR studies confirmed the existence of the original benzylidene complex and the formation of the active heptylidene and methylidene complexes.Master
Synthesis and modeling of alicyclic organic ligands
No full textThesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2005The study of alicyclic cage molecules has in recent years increased due to the possibilities in application of these compounds. The photocyclization of the endo conformation Diels-Alder adduct of p-benzoquinone and cyclopentadiene yields the pentacyclic cage compound derivative pentacyclo[5.4.0.02,6.03,10.05,9]undecane that is a classical starting compound for the synthesis of different alicyclic cage compounds.
An important part in the study of alicyclic cage compounds is the possible use of these compounds as ligands in metal complexes. Literature studies showed that limited research is being done in the field of alicyclic cage ligand activity and catalysis. Marchand et al. and Lee et al. synthesized alicyclic cage compounds that have the ability bind to transition metals to form relative stable alicyclic-metal complexes
Different pentacyclo[5.4.0.02,6.03,10.05,9]undecylamines was identified in literature and synthesised in this study. These ligands was characterised using MS, IR and NMR and investigated by molecular modelling using Accelrys Material Studio and Spartan Pro. The ligand activity of these compounds was investigated by using lithium tetrachloropalladate(II) that lead to the formation of cyclopalladation complexes. Ligand 1,2,3 and 5 were investigated and formed very stable metal-ligand complexes with the palladium metal. These complexes were characterised using FAB+ M.S, 1H and 13C NMR, I.R and ESEM with a INCA 400 EDS system.Master
Sintese en modellering van alisikliese dendrimere
No full textThesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2006Research in the field of dendrimer chemistry has escalated in the past two decades. Dendrimers are macromolecules closely related to polymers but with a much smaller mass distribution. Because of this property, they possess unique three dimensional characteristics. Dendrimers are increasingly used in homogeneous catalysis due to the ease of separation from the reaction mixture with the use of nanofiltration membranes.
Dendritic catalysts are limited to lower generation dendrimers, which unfortunately aren't large and robust enough to be constrained by nanofiltralion membranes. The reason is that higher generation dendrimers sterically hinder access to the active point in the catalyst. Incorporation of rigid. three dimensional alicydic compounds in dendrimers holds a possible solution to this problem. Two objectives were set: 1. Suitable alicyclic compounds were to be syntethised and incorporated into the core of a dendrimer. 2. Suitable alicyclic compounds were to be syntethised and used as periphery molecules for a dendrimer. • Alicyclic compounds 16, 23 and 28 were identified as suitable core molecules for alicyclic dendrimers. Compound 8 was identified as a suitable dendron to be coupled lo an alicyclic compound. • Compound 16 is synthesised through reduction of the p-benzoquinone-cyclopentadiene- adduct 3 by sodiumborohydride in a ceriumtrichloride heptahydrate solution in methanol before irradiation by ultraviolet light. • Coupling alicyclic compound 16 with dendron 8, was done in alkaline dimethyl sulfoxide. Only one hydrogen was substituted with a dendron. Removal of the excess compound 8 proved to be problematic and very little of the pure product could be obtained. • Compound 23 was synthesised by reducing compound 22, which was synthesised as follows. • Coupling alicyclic compound 23 with dendron 8, was done in alkaline dimethyl sulfoxide. The raw product was a yellow oil and couldn't be separated from the excess compound 8. No analytical data could therefore be obtained. The only conclusion that could be made as to the nature of the product was based on semi-empirical calculations performed with Accelrys MS Modeling, which indicated that only one hydrogen would most likely have been substituted by compound 8 and probably at the oxygen indicated below. • The third alicyclic compound synthesised was 28, for which the starting compound is synthesised as follows. • Clemmensen reduction yields a mixture of products of which 28 is the main product. 27 could be separated by steam distillation, after which 28 could be retrieved from the remaining water. • Coupling alicyclic compound 28 with dendron 8, was done in alkaline dimethyl sulfoxide. Compound 32 could be separated from the excess dendron 8 as a colourless crystalline product by flash chromatography using petroleum ether:ethyl acetate:methanol (20:20:1) as eluent. Substitution again only took place at one oxygen. • A possible explanation for the single substitution of dendron 8 on the alicyclic compounds, could be gained from molecular modelling data performed with Accelrys MS Modeling. The moment one aromatic substituent is present in an alicyclic structure, all the HOMO electrons move from the remaining hydroxyl group to the aromatic substituent, leaving no electrons available for nucleophilic attack on another molecule of 8. See example below. • Compound 38 was identified and synthesised as a suitable alicyclic structure for incorporation in the periphery of a dendrimer. • Compound 41 was used as the aromatic core molecule. Both benzylic bromide positions was substituted by molecules of 38, thus obtaining an alicyclic dendrimer 42 with two alicyclic compounds on the periphery. • Compounds 32 and 42 were successfully synthesised as alicyclic dendrimers.Master
Die in situ rutenium(III) chloriedkatalisatorsisteem vir alkeenmetatese in 'n nywerheidsomgewing
No full textThesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2005.In this study, the behaviour of an in situ-prepared RuCI3.xH20 catalytic system, where the active catalyst is generated using RuCI3.xH20 in EtOH, together with an alkyne, a ligand and an alkene substrate in the presence of a small H2 sparge, was investigated. The influence of different reagents on the catalytic system was investigated by looking at metathesis of linear alkenes and NMR studies. A few additives that have an influence on the metathesis catalyst system were identified and the effects of these additives determined. The published mechanism of the catalyst system was verified by the results reported here, and some additional findings about the reaction rate were found. The reaction occurs via a hydride-to-carbene mechanism. It was found that the formation of the hydride is a fast reaction, and the
subsequent formation of the carbene is the rate-determining step. The carbene is the
metathesis active species. The carbene is formed by the reaction of RuCI3.xH20 dissolved in a reducing solvent (EtOH), in
the presence of an alkyne, a ligand and a linear alkene. The reaction with which the EtOH reduces RullI to RuII was not investigated. It was found that the best alkyne is a short-chain, terminal, unfunctionalised alkyne (1-hexyne). BOD was also confirmed to be a good alkyne for
the reaction, although 1-hexyne afforded the highest metathesis of the alkynes investigated. The metathesis reaction is sensitive to the order of addition of the reagents. NMR studies showed that the phosphine ligand coordinates at room temperature to the Ru core, while the
formation of the carbene only took place after heating of the reaction mixture to at least 50 °C. If the order of addition of the reagents is changed, side reactions take place that inhibits the formation of the carbene. A reaction with PIBu3 as ligand showed that the steric bulk of the ligand is important, since it afforded considerably less metathesis than the more bulky PIPr3. In addition to trialkylphosphines, a bicyclic phosphine ligand (EP) was found that afforded metathesis activity in the same order than PCy3. By comparing this ligand with PCy3, it was found that the bicyclic
ligand afforded slower carbene formation and faster deactivation. The overall metathesis yield was still comparable. The optimum Ru:ligand ratio was different for the two ligands, with EP reaching an optimum ratio of 1:1.5, compared to the optimum of 1:4 for PCy3. The metathesis reaction was followed using NMR. This, together with results found when the
time of Hz sparge was varied, showed that the carbene forms continuously during the reaction, but also deactivates continuously. The exact nature of the deactivation process was not determined during this study. The influence of the addition of hydrochloric acid to the reaction was also determined using NMR techniques. It was found that HCI protonates the phosphine as well as the phosphine oxide. Metathesis with other acids showed that acids have a effect on
the metathesis reaction. The addition of strong sulphonic acids suppressed metathesis while promoting isomerisation and, in some cases, hydrogenation. The similar results from metathesis reactions with the addition of acetic acid and trifluoro acetic acid showed that the Bronsted acid strength is not the only factor that influences the reaction. The increased isomerisation using acid addition supports a hydride isomerisation mechanism. The use of chlorinated solvents, especially chloroform and chlorobenzene, for the ligand and
alkyne increased metathesis. The best metathesis conversions were found using EtOH as solvent for the RuC13.xH20. No adverse effect due to ageing of the stock solutions was found, even after 3 months. The in situ catalyst system showed resistance to some oxygenates, but not to all of them. This shows that the presence of an 0-atom is not sufficient for catalyst deactivation. Further studies in this field will be beneficial.
Metathesis of an industrial C7 alkene cut afforded less 6-dodecene than expected from results obtained using 1 -octene. The turnover frequencies were lower using the industrially derived cut than using 1-octene. The reactions using EP as ligand afforded significantly higher yields of 6-dodecene than reactions using PCy3 as ligand, but still less than the yields afforded by reactions using 1 -octene.Master
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