Ruthenium based homogeneous olefin metathesis
| dc.contributor.advisor | Roodt, A. | |
| dc.contributor.advisor | Meyer, W. | |
| dc.contributor.author | Kirk, Megan Michelle | |
| dc.date.accessioned | 2015-09-04T07:53:21Z | |
| dc.date.available | 2015-09-04T07:53:21Z | |
| dc.date.issued | 2005-11 | |
| dc.description.abstract | English: The aim of this study was to investigate the decomposition of the methylidenes 4a [Ru(Cl)2(PCy3)2CH2] and 6 [Ru(Cl)2(PCy3)(NHC)CH2] and alkylidene 5 [Ru(Cl)2(PCy3)2CHR], formed in the homogeneous olefin metathesis reaction using either the first generation Grubbs catalyst 2a [Ru(Cl)2(PCy3)2CHPh] or the second generation Grubbs catalyst 3 [Ru(Cl)2(NHC)(PCy3)CHPh]. (NHC = 1,3-dimesityl-4,5-dihydroimidazole-2- ylidene). Grubbs catalyst di-bromide 2b [Ru(Br)2(PCy3)2CHPh] was found to react with chlorinated solvents with the formation of CDCl2Br and CCl3Br, indicating that there is a direct reaction with the solvent itself (CDCl3) and not HCl in the solvent. A mechanism is proposed for this reaction. The intermolecular halide exchange reaction was observed during this study and used to gain insight into the bimolecular decomposition pathway. A mechanism where Grubbs catalyst 2a acts as a ligand in a reaction with the 14 electron intermediate 7b [Ru(Br)2(PCy3)CHPh] is proposed for intermolecular halide exchange (see scheme below). It is proposed that the intermolecular halide exchange reaction occurs continuously in all solutions of Grubbs catalyst, with the formation of the dimeric intermediate 45 (see below). The decomposition of Grubbs catalyst 2a in CDCl3 results in the formation of paramagnetic species and consequently in the loss of the 31 P NMR signal. It has recently been reported that phenol and SnCl2 dramatically increase the life time of Grubbs catalyst 2a. An understanding of the formation and behaviour of dimeric intermediate 45 lead to the proposal of a mechanism for the inhibition of bimolecular decomposition in the presence of phenol and SnCl2 in CDCl3. To inhibit the formation of the dimeric intermediate 45 above, the use of solid and dissolved salts as additives was studied. 31 P and 1 H NMR studies showed that the presence of Bu4NBr dissolved in CDCl3 results in halide substitution at Grubbs catalyst 2a [Ru(Cl)2(PCy3)2CHPh] to form a mixed halide catalyst 2c [RuBrCl(PCy3)2CHPh] and finally Grubbs catalyst di- bromide 2b [Ru(Br)2(PCy3)2CHPh]. Improved conversion and selectivity were obtained when solid and dissolved salts were added to the reaction mixture in the self metathesis of 1-octene. Molecular modelling has indicated that there is a possibility of ethylene induced decomposition of the Grubbs catalyst methylidene 4a [Ru(Cl)2(PCy3)2CH2] with the formation of propene as shown below. 1 H NMR experiments monitoring the ethylene induced decomposition of methylidenes 4a and 6 in C6D6 overnight clearly showed the formation of α - olefins, supporting the proposed β -hydride decomposition pathway. This pathway includes the formation of a hydride species that is highly likely to be isomerisation active and may lead to the decreased selectivity found experimentally. This study was taken further by carrying out ethylene induced decomposition in CDCl3, where it was found that there is no propene formation but rather addition of CDCl3 across the double bond via the Kharasch reaction. This indicates that the β -hydride pathway is inhibited in chlorinated solvents, which may be part of the reason why better selectivities are obtained in the presence of chlorinated solvents. | en_ZA |
| dc.description.abstract | Afrikaans: Die doel van hierdie studie was om die ontbinding van die metilideenspesies, 4a [Ru(Cl)2(PCy3)2CH2], 6 [Ru(Cl)2(PCy3)(NHC)CH2] en die alkielideen 5 [Ru(Cl)2(PCy3)2CHR], wat gedurende enige homogene olefienmetatese waarin die Grubbs-katalisator 2a [Ru(Cl)2(PCy3)2CHPh], of die tweede generasie Grubbs katalisator 2a [Ru(Cl)2(NHC)(PCy3)CHPh] (NHC = N heterosikliese karbeen) gebruik word, te bestudeer. Die studie het getoon dat die dibroomkompleks, Grubbs katalisator 2b [Ru(Br)2(PCy3)2CHPh], direk met gechlorineerde oplosmiddels (soos CDCl3, en nie met die HCl daarin) reageer om CDCl2Br en CCl3Br as newe-produkte te lewer. ‘n Meganisme is vir hierdie reaksie voorgestel. Die intermolekulêre halieduitruiling wat tydens hierdie studie waargeneem is, het addisionele inligting ten opsigte van die bimolekulêre ontbindingsroete gelewer. 'n Meganisme waarin Grubbs-katalisator 2a as 'n ligand in die reaksie met die 14-elektron tussenproduk 7b [Ru(Br)2(PCy3)CHPh] optree, is gevolglik vir die intermolekulêre halieduitruiling voorgestel (sien skema hieronder). Dit is verder voorgestel dat hierdie halieduitruiling in alle Grubbskatalisatoroplossings plaasvind via die dimeriese intermediêre spesie, 45. Die ontbinding van Gubbs-katalisator 2a in CDCl3 vorm 'n paramagnetiese spesie en gevolglike verlies van die 31P KMR-sein. Die onlangsgerapporteerde stabilisering (betekenisvole verlenging van leeftyd) van Grubbs-katalisator 2a deur die byvoeging van fenol en SnCl2, en 'n begrip van die vorming en algemene gedrag van die dimeriese intermediêr, 45, het gelei tot die voorstelling van die inhibering van die bimolekulêre ontbinding in CDCl3 in teenwoordigheid van hierdie bymiddels. Ten einde die vorming van die dimeriese intermediêr, 45, te verhoed, is die byvoeging van vaste en opgeloste soute bestudeer. 31P en 1 H KMR-studies het getoon dat in teenwoordigheid van (Bu4N)Br, opgelos in CDCl3, haliedverplasing op Grubbs katalisator 2a [Ru(Cl)2(PCy3)2CHPh] induseer en die vorming van die gemengde haliedspesie, 2c [RuBrCl(PCy3)2CHPh] en uiteindelik die dibromospesie, Grubbs katalisator 2b [Ru(Br)2(PCy3)2CHPh]. Verhoogde omskakeling en selektiwiteit is gevind wanneer vaste of opgeloste haliedsoute by die reaksiemengsel in die selfmetatesereaksie van 1-okteen gevoeg is. Modellering het aangetoon dat daar 'n moontlikheid van 'n eteengeïnduseerde ontbinding van die Grubbs-katalisator metilideen 4a [Ru(Cl)2(PCy3)2CH2], met gepaardgaande propeenvorming, bestaan. 1 H KMR eksperimente in C6D6 (oornag) het duidelik die vorming van alfaolefiene aangedui, wat die voorgestelde β-hidried ontbindingsroete bevestig het. Hierdie roete sluit ook die vorming van 'n hidriedspesie in wat moontlik isomerisasie kan ondergaan, en dus die afname in selektiwiteit wat eksperimenteel verkry word, kan verklaar. 'n Verdere deel van die studie het die eteengeïnduseerde ontbinding in CDCl3 ingesluit, waar addisie van CDCl3 oor die dubbelbinding geskied het (Kharasch-reaksie) in plaas daarvan dat propeen gevorm is. Hierdie toon dat die β-hidriedroete deur gechlorineerde oplosmiddels inhibeer word, wat moontlik kan verklaar hoekom beter selektiwiteite in hierdie oplosmiddles waargeneem word. | af |
| dc.identifier.uri | http://hdl.handle.net/11660/1161 | |
| dc.language.iso | en | en_ZA |
| dc.publisher | University of the Free State | en_ZA |
| dc.rights.holder | University of the Free State | en_ZA |
| dc.subject | Metathesis (Chemistry) | en_ZA |
| dc.subject | Ruthenium compounds | en_ZA |
| dc.subject | Dissertation (M.Sc. (Chemistry))--University of the Free State, 2005 | en_ZA |
| dc.subject | Alkenes | en_ZA |
| dc.title | Ruthenium based homogeneous olefin metathesis | en_ZA |
| dc.type | Dissertation | en_ZA |