Abstract
Ruthenium carbene complexes, with the general structure, [LL’Ru=CHR], are
commonly known as Grubbs type catalysts, named after the discoverer of
these metathesis catalysts. The discovery was quite revolutionary, since the
catalysts proved to be easy to handle, tolerant towards various functional
groups and more stable with regard to air and water than previous transition
metal catalysts. Another important advantage was that all types of olefin
metathesis reactions could be initiated without the help of co-catalysts or
promoters.
Today Grubbs type catalysts find wide application in especially organic and
synthetic chemistry. A well-known example is the SHOP-process which
produces long chain -olefins, while other important applications include the
synthesis of macro-cyclic and cyclic olefins.
The current study involved experimental and theoretical work to investigate
various aspects comprising synthetic procedures, reactivity, kinetics,
geometry and electronic properties of the complexes. Results are discussed
briefly in the following paragraphs.
The first aim of the project was to synthesise a Grubbs type catalyst. Initial
efforts were focused on the preparation of a first generation catalyst through
various methods. This included modifying the reported method for the
synthesis of [(PPh3)2Cl2Ru=CH-CH=CMe2] to yield [(PPh2Cy)2Cl2Ru=CHCH=
CMe2] instead; a phosphine exchange reaction with the complex
[(PPh3)2Cl2Ru=CH-CH=CMe2] and free phosphine PPh2Cy; and utilising the
analogue arsine ligand, AsPh3, to synthesise [(AsPh3)2Cl2Ru=CHCH=CMe2];
but unfortunately no success was achieved.
However, it was possible to synthesise a novel second generation Grubbs
type catalyst, [(IMesH2)(PPh2Cy)Cl2Ru=CHPh], through the phosphine
exchange reaction of [(IMesH2)(NC5H5)2Cl2Ru=CHPh] and PPh2Cy. The new complex was tested in kinetic reaction studies and phosphine
exchange reactions. Results showed that [(IMesH2)(PPh2Cy)Cl2Ru=CHPh]
was catalytically active for the ring closing metathesis of commercial diethyl
diallylmalonate. The reaction was first order with regard to the olefin, contrary
to the second order kinetic results reported for similar reactions catalysed by
first generation Grubbs catalysts.
The phosphine exchange reactions were very successful and a rate constant
could be determined. The rate constant was independent of the free
phosphine concentration and activation parameters had relatively large,
positive values; results indicative of a dissociative mechanism. These findings
are in correlation with literature reports.
A kinetic investigation was done on the catalyst-olefin coordination involving
the functionalized olefins vinyl acetate, allyl acetate and allyl cyanide; and the
first generation Grubbs catalyst, [(PCy3)2Cl2Ru=CHPh]. A two-step rate law,
similar to an interchange mechanism, was determined.
Phobcat, [(PhobCy)2Cl2Ru=CHPh], is modified first generation Grubbs type
catalyst with rigid bicyclic phosphine rings which was recently developed by
the Sasol Homogeneous Metathesis Group. In the current study Phobcat was
compared to Grubbs1-PCy3 in the cross metathesis reaction of 1-octene.
Results showed that Phobcat was up to 60% more active and had a 5 hour
longer lifetime than Grubbs 1-PCy3.
Theoretical studies were done on the three functionalized olefins of the earlier
experimental study to gain fundamental understanding of steric and electronic
influences on these catalyst-olefin systems. Without exception, coordination
via the heteroatom of the olefin was significantly more favourable than
coordination via the double bond functionality. This result indicates that
metathesis of these olefins is highly unlikely, since the stable heteroatom
coordination will suppress the parallel Ru=C/C=C interaction which is
compulsory for the metathesis reaction. Orbital studies highlighted the difference between coordination of acetate and cyanide, but no trend of an
electronic nature could be recognised.
Prof. A. Roodt