Abstract:
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.
