Chemistry Reference
In-Depth Information
Scheme 6.25
Dissociative (a) and associative (b) ligand exchange mechanisms.
also believed that a mono-phosphine complex was responsible for more than 95%
of observed metathesis activity. It was therefore suggested that more sterically
bulky phosphines promote metathesis by promoting phosphine dissociation [36].
Magnetization transfer
31
P NMR experiments established that degenerate phos-
phine exchange (phosphine dissociation and rebinding in the absence of olefin)
for the whole family of complexes is dissociative rather than associative. This was
somewhat of a counter-intuitive result, since the intermediate derived from disso-
ciation of a phosphine ligand,
I
d
, is a coordinatively unsaturated, 14-electron spe-
cies, whereas the intermediate expected from associative ligand exchange is a pre-
sumably octahedral 18-electron complex (Scheme 6.25). Intermediate
I
d
has not
been spectroscopically observed [101].
In support of the existence of
I
d
, the 14-electron complex 12 and partially fluori-
nated analogues have been isolated and characterized [102]. Complex
12
adopts a
slightly distorted trigonal pyramidal geometry in the solid state, with a planar P-
Ru-C
-H
unit (C
-H
refers to the carbene carbon and proton). Thus, the carbene
has rotated approximately 90
relative to the parent complex
6
. No agostic interac-
tions were present. This complex is much more sterically bulky than
I
d
, but the
structure may provide some insight into the nature of this highly metathesis-ac-
tive intermediate. The structural similarity of
12
to the Schrock catalysts
1
and
2
is particularly intriguing.
Search WWH ::
Custom Search