Chemistry Reference
In-Depth Information
1.4.2 Oxidative Addition and Reductive Elimination
Oxidative addition is the most important method for the formation of a metal-carbon single bond, although
it is not limited to just this. In oxidative addition, a transition-metal fragment, which must have less than
eighteen electrons, inserts into the X Y bond, and the oxidation state of the metal increases by 2 (Scheme 1.7).
Usually X is an organic group and Y is a leaving group, such as a halide. There are, however, many other
possibilities, including the simple one where both X and Y are hydrogen. The reverse process is reductive
elimination in which the metal fragment is expelled by formation of an X Y bond, and the oxidation state of
the metal drops by 2.
oxidative
addition
reductive
elimination
X
M
M + X-Y
Y
Scheme 1.7
1.4.3 Transmetallation
Oxidative addition is often followed by transmetallation in which an organic group on a second metal,
usually a main group metal, is transferred in exchange for a group such as a halide (Scheme 1.8). This is
another important method for formation of a transition metal-carbon bond. There is no change in oxidation
state.
M
1
X
M
1
R
X
M
2
transmetallation
RM
2
Scheme 1.8
1.4.4 Alkene and Alkyne Insertion
A fundamental process for coordinated alkenes and alkynes is insertion (also called migratory insertion),
usually into a metal-carbon or metal-hydrogen bond (Scheme 1.9). This is a stereospecifically
syn
process,
so insertion of alkynes results in
cis
-vinyl complexes. There is no change of oxidation state.
MR
insertion
M
R
MR
insertion
M
R
Scheme 1.9
The insertion of ethylene into a carbon-cobalt bond was carefully studied as part of work on the mechanism
of Ziegler-Natta polymerization (Scheme 1.10).
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