Chemistry Reference
In-Depth Information
Since the reaction rates using Pd(I) dimer as catalyst have been observed
to be much higher than those using the corresponding L
2
Pd(0) as pre-
catalyst, the activation of the Pd(I) precatalyst must be a more facile process
than the substrate-assisted ligand displacement process that is presumed to
activate L
2
Pd(0) to the catalytically active LPd(0) species. Recent computa-
tional studies suggested that the precatalyst was converted into the active
species via a reductive pathway,
106
which is a lower energy pathway than the
previously assumed disproportionation mechanism (Scheme 3.32).
The exact mechanism of the reduction to catalytically active LPd(0) species
is currently not known. It should be noted that the same LPd(0) species is
generated from (t-Bu
3
P)Pd(crotyl)Cl, which is discussed in Section 3.7.2. This
precatalyst is air stable and convenient to handle, compared with air-sen-
sitive [(t-Bu
3
P)Pd(m-Br)]
2
.
In addition, it was demonstrated that under Suzuki-Miyaura cross-
coupling reaction conditions, the Pd(I) dimer underwent precatalyst
deactivation to form (t-Bu
3
P)
2
Pd and Pd black.
106
This observation could
explain the incomplete conversions when using [(t-Bu
3
P)Pd(m-Br)]
2
as pre-
catalyst in Suzuki-Miyaura couplings of aryl chlorides, based on the fact that
(t-Bu
3
P)
2
Pd on its own provided the desired product in very low yield.
106
Schoenebeck's group further demonstrated the importance of the cataly-
tically active species to the outcome of a chemoselective reaction. Based on
the finding that the actual catalyst formed in situ from Pd
2
dba
3
and t-Bu
3
P
could be fine-tuned by the choice of solvent (polar solvents favored the
formation of [LPdX]
and non-polar solvents the formation of neutral
PdL),
11,107
[(t-Bu
3
P)Pd(m-Br)]
2
was investigated for chemo- and regio-
selectivity in Suzuki-Miyaura cross-coupling reactions (Scheme 3.33).
106
Br
disproportionatio
n
Pd
(II)
P
t
-Bu
3
+
Pd
0
P
t
-Bu
3
Br
Br
12-electron species
t-
Bu
3
PPd
Pd P
t-
Bu
3
Br
16 electron species
reduction
[Pd
0
P
t
-Bu
3
Br]
-
or/and
[Pd
0
P
t
-Bu
3
]
12-electron species
Scheme 3.32 Disproportionation versus
reduction pathway
to LPd(0)
from
[(t-Bu
3
P)Pd(m-Br)]
2
.
Ar
1.5 mol% [(
t
-Bu
3
P)Pd(
μ
-Br
)]
2
MeCN, 30 min
Cl
OTf
Me
78%
+KF
+
Ar
Cl
B(OH)
2
1.5 mol% [(
t
-Bu
3
P)Pd(
μ
-Br)]
2
THF, 30 min
TfO
76%
Scheme 3.33 Chemoselective
Suzuki-Miyaura
cross-coupling
reactions using
[(t-Bu
3
P)Pd(m-Br)]
2
.
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