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L
Pd
X
reductive elimination
X
O
O
7.50
7.52
Δ
G
‡
(kcal/mo l)
24 .9
27 .4
27 .9
BDE
(kcal/mol)
Pd
BDE
(kcal/mol)
C
−
I:
C
65.7
75.0
91.0
−
I:
72. 8
−
Br:
Pd
Br:
81.1
−
C
Cl:
−
Pd
−
Cl:
91.9
Figure 7.2 Bond dissociation energies (BDE) and activation energies (DG
z
) associ-
ated with carbon-halogen reductive elimination. Adapted from Ref. 100
with permission.
7.7.3 Carbon-Halogen Reductive Elimination: Role of the
Ligand
To understand how the nature of the ligand affects the rate of reductive
elimination, catalytic cycles were computed for a set of phosphine ligands
with varying steric and electronic properties (Table 7.5). To reduce the
computational cost, FcP
t
Bu
2
, a truncated analog of QPhos, was used for the
calculations. The experimental results demonstrating that bulky phosphines
are the most ecient ligands for carbohalogenation were corroborated by
the computations. The lowest barriers to reductive elimination are observed
for P
t
Bu
3
(24.9 kcal mol
1
) and FcP
t
Bu
2
(23.7 kcal mol
1
) (Table 7.5, Entries 1
and 2), whereas a higher barrier is demonstrated for the slightly less bulky
P
i
Pr
3
(26.3 kcal mol
1
) (Entry 3). Significantly smaller ligands, such as
P(OMe)
3
,PMe
3
and PH
3
(Entries 4-6), are believed to follow a different
mechanism for carbohalogenation. Owing to reduced steric crowding, the
alkylpalladium(II) halide intermediate can accommodate two ligands, which
results in more stable and lower energy tetracoordinated intermediates of
the type 7.53 and 7.54 [eqn (7.8)] Endergonic ligand dissociation to form a
tricoordinated species 7.50a is required prior to reductive elimination,
therefore increasing the barrier to reductive elimination with these smaller
ligands.
Although P(OMe)
3
and PMe
3
possess similar degrees of steric bulk, re-
ductive elimination is favored with P(OMe)
3
, suggesting that, in addition to
steric effects, electronic factors can also have a significant impact. It is well
known that electron-withdrawing ligands facilitate reductive elimination by
decreasing the electron density at palladium.
The overall results of this study suggest that steric effects in the
ligand play a significant role in promoting carbohalogenation by both
preventing the formation of stable tetracoordinate Pd species and causing
destabilization of the neopentylpalladium halide ground state. Although
electron-poor phosphine ligands also facilitate reductive elimination, ligand
steric effects seem to have a more pronounced influence on the observed
reactivity.
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