'n Ondersoek van alkynmetatese deur middel van molekuulmodellering en laboratoriumeksperimentering
Van der Merwe, Anna Catharina
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In this study, the metathesis reaction of alkynes was investigated with the classic Mo(CO)6/PhOH-catalyst system by utilizing both laboratory experimentation and molecular modelling. The reaction was optimized by using the alkyne used in the initial study, namely ptolylphenylacetylene. The progress of the reactions was monitored by gas chromatography. As p-tolylphenylacetylene is not available commercially, a synthesis had to be developed. An uncatalysed, organic method and a palladium catalysed method were investigated. The catalysed method proved to be the fastest and gave a higher, better quality, yield. The products were characterized by IR, NMR, MS and SEM techniques. During the optimisation of the reaction, various factors that could have an influence on the activity of the system, were investigated. These included reaction temperature, cocatalyst, solvent, activation period, Mo(CO)6 content, PhOH content as well as the alkyne that was used as substrate. No reaction was observed below 100 °C and an increase in activity was displayed as the temperature increased until an optimum was reached at 160 °C. Elevation of the temperature above this resulted in diminished activity. The influence of three cocatalysts namely phenol, resorcinol and α-naphtol, was tested and their characteristics were compared to those determined for optimum reactivity in the metathesis polymerisation reaction using the same catalyst system. As the difference between the cocatalysts that were tested was very small, PhOH was used as the cocatalyst in the remaining optimisation reactions. The optimum PhOH content for the reaction was determined to be 25:1 molar ratio with regard to Mo(CO)6. The optimum Mo(CO)6 content of the reaction was determined by using the alkyne content. A molar ratio of 25:1 alkyne substrate:Mo(CO)6 was found to be the most active. Higher concentrations of the alkyne led to deactivation of the catalyst system. The alkyne substrate was also investigated for maximum activity – alkynes with different substituents were used and the results compared. The results showed that internal alkynes were most suitable for metathesis, but that terminal alkynes could undergo the reaction if suitable steric substituents were present. OPSOMMING/SUMMARY 194 While phenylacetylene underwent only polimerisation, t-butylacetylene underwent both polimerisation and metathesis. 1-Phenyl-substituted alkynes were also investigated and it was found that a main chain length of four carbon atoms resulted in optimum conversion with regard to the alkynes that were tested. A mechanistic postulate, by which the hydrogen of the hydroxyl group on the phenol coordinates with the alkyne triple bond to render it more olefinic in nature, was subjected to testing by the investigation of a possible activation period. This was conducted by heating the reaction mixture without the catalyst at 70 °C for a specified time. After this period had elapsed, the catalyst was added and the reaction allowed to proceed normally. Small differences in conversion were noted between the samples that were activated and those that had had no activation period. Solvents of low polarity showed the best results for metathesis. The solvent, however, still had to be able to stabilise any polar species that formed during the reaction. Solvents that yielded metathesis products were tetralin, decalin, indane, chlorobenzene and to a lesser extent, cyclohexanol. Molecular modelling studies, during which the formation of the proposed metallacyclobutadiene complex was investigated, was compared with results of studies in the literature and it was found that the DMol3 DFT-code results (Accelrys Materials Studio (2002)) correlated very well with these studies. Molecular modelling was also used to determine and confirm the influence of electron withdrawing ligands on the metal centre. Dissociation of the metallacyclobutadiene complex to form metathesis products was shown to be the rate-determining step by the calculation of the minimum required activation energies. A postulate for the metathesis of 1-phenyl-1-propyne in the presence of the Mo(CO)6/PhOH catalyst system was proposed. A puckered metallacyclobutadiene was found to be the most stable conformation and alternative intermediates, in the form of metallatetrahedranes and cyclopropenyl complexes, were identified. Further study is necessary to elucidate the mechanistic pathway of this reaction.
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