A mechanism for alkene metathesis has been proposed by Chauvin, wherein metal
carbenes act as catalysts for the reaction. The use and discovery of Fischer-, Tebbe-,
Grubbs- and Schrock-type metal carbenes have to a certain extent proven the general
mechanism. These metal carbenes showed different activity for alkene metathesis.
Only Grubbs- and Schrock-type carbenes proved to be highly active for metathesis. A
lot of studies have been done on the reasons for the activity, but still the main factors
are unknown.
In this study a molecular modelling investigation into the mechanism of the alkene
metathesis reaction is done in an attempt to identify a factor(s) that can predict
activity. By defining and knowing factors that contribute to activity, new catalysts can
be designed that are truly active and selective. Fischer-, Tebbe-, Grubbs- and Schrocktype
metal carbenes are investigated in this regard.
The results of the investigation indicate that the frontier molecular orbital theory
shows a possibility for prediction of alkene metathesis activity. By observing the size
and location of the atomic orbital coefficients of the molecular orbital, the site of
primary overlap for formation of metathesis products could be identified. The largest
atomic orbital coefficient of the LUMO should be located on the metal atom. An
atomic orbital coefficient should also be present on the carbene carbon for secondary
overlap for formation of the metallacyclobutane intermediate. By exchanging the
ruthenium in the second generation Grubbs catalyst framework the effect of the metal
could be elucidated. The results clearly showed the important influence the metal
atom has on the electronic properties of the catalyst complex. The results of frontier molecular orbital calculations supported the general activity
trend of the four main types of metal carbenes for the metathesis of linear alkenes. By
changing the metal in known catalyst frameworks a deeper understanding can be
gained for the design of new alkene metathesis catalysts. / Thesis (PhD (Chemistry))--North-West University, Potchefstroom Campus, 2013
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/8733 |
| Date | January 2012 |
| Creators | Du Toit, Jean Isabelle |
| Publisher | North-West University |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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