Chemistry Reference
In-Depth Information
X
X
R
X
R
X
isomerization
L
n
Pd
II
Pd
II
L
n
PdL
n
R
R
cis
trans
Scheme 7.1 The oxidative addition reaction.
heterolytic cleavage via a concerted, three-centered transition state
(Scheme 7.1).
9-11
Overall, palladium donates two non-bonding electrons to
form two new metal-ligand bonds with a cis relationship, which can undergo
rearrangement to the thermodynamically favored trans product.
12
The clas-
sical order of reactivity with respect to aryl halides is I 4Br 4Cl44F, where
C-F bonds are essentially unreactive. However, recent efforts in the area aim
to shift this longstanding paradigm.
13-15
Oxidative addition is generally promoted by strongly s-donating ligands
that increase the electron density at palladium, thus making it more electron
rich for breaking C-X bonds.
16,17
Conversely, p-accepting ligands counteract
this process by rendering palladium less electron rich. Additionally, more
electron-deficient carbon-halogen bonds are increasingly susceptible to
undergo oxidative addition with electron-rich Pd(0) sources. Steric effects also
have an important but less straightforward role in oxidative addition. While
this process involves an increase in steric bulk around the metal center, large
and sterically hindered ligands are typically used to favor challenging oxidative
additions. This behavior has been attributed to the formation of highly active
PdL
1
or PdL
2
species readily in situ, which have open coordination sites to
allow oxidative addition to take place.
18
7.4 Reductive Elimination from Transition Metal
Complexes
Reductive elimination is the microscopic reverse of oxidative addition and
can be promoted by various factors, including steric and electronic per-
turbations of the substrate, product, and/or ancillary ligand(s). These trends
are generally opposite to those seen for oxidative addition reactions. For
example, electron-poor metal centers are better suited to undergo reductive
elimination than more electron-rich metal centers possessing similar steric
properties, since there is an increase in electron density at the metal center
over the course of the reaction. Additionally, metal complexes possessing
more sterically encumbering ligands will have faster rates of reductive
elimination compared to less bulky ligands with similar electronic prop-
erties. These general trends correspond well to the essence of the overall
process, in which both reduction and a decrease in coordination number
occur at the metal center.
19
These fundamental effects have been studied
with respect to both carbon-hydrogen
20
and, in rare instances, carbon-
halogen bonds.
21,22
The reductive elimination of carbon-halogen bonds
from transition metal complexes represents an interesting extension,
as it is typically not thermodynamically favored (compared to the case with
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