'n Modelleringsondersoek na die meganisme van die homogene alkeenmetatesereaksie
Du Toit, Jean Isabelle
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Alkene metathesis is the substitution of parts of two substances through the breaking and rearranging of the carbon-carbon double bonds. Chauvin proposed a metal carbene catalysed mechanism for alkene metathesis. Fischer-type metal carbenes, known then, have low metathesis activity. Fischer-type metal carbenes however, played a significant role in the proving and acceptance of this mechanism for alkene metathesis reactions. The discovery of the highly active Schrock- and Grubbs-type metal carbenes, based on tungsten, rhenium, molybdenum or ruthenium, also supported the metal carbene mechanism so that it is now generally accepted and metal carbenes are confirmed as alkene metathesis catalysts. In this study a molecular modelling investigation into the mechanism and products of the alkene metathesis reaction is done in an attempt to explain die general metathesis activity of metal carbene catalysts. Fischer- and Grubbs-type metal carbenes are investigated in this regard. The results of the investigation indicate that for the formation of metathesis products the LUMO orbital must be concentrated on the metal. It is also clear that the lower the energy of the LUMO orbital, the more favourable the formation of the alkene-catalyst bond. The hypotheses is formulated that for metal carbenes to show alkene metathesis activity, the metal carbene must be a triplet carbene, die LUMO orbital must be centred on the metal atom and the metal atom must have a positive Mulliken charge. This is confirmed by Tebbeand Schrock-type metal carbenes by additionally testing for these characteristics. The proposed general activity of alkene metathesis catalysts that increases from Fischer to Tebbe and then to Grubbs and Schrock is strongly supported by the formulated lowest- LUMO-energy-hypothesis. This study can forthwith play a significant role in the development of more active alkene metathesis catalysts.
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