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dormant for a remarkable 8 years until van den Hoogenband et al. described
the cross-coupling of cyclopentyl-BF
3
K derivatives using RuPhos as the lig-
and in combination with Pd(OAc)
2
to yield the corresponding coupling
products in moderate to good yields, [eqn (11.11)].
40
Pd(OAc)
2,
n
BuPAd
2
BF
3
K
25
+
Ar-Cl
Ar
Cs
2
CO
3
toluene/H
2
O
100°C
(11
:
9)
26
27
Pd(OAc)
2,
t
Bu
3
P
B(OH)
2
+ Tolyl-Cl
CH
3
KF, THF
100°C
(11
:
10)
28
29
30
75% yield
Pd(OAc)
2
, RuPhos
BF
3
K
+
Ar
Ar-Br
(11
:
11)
K
3
PO
4
,115°C
toluene/water
31
32
33
Note that in both of these cases, symmetrical nucleophiles are employed
and therefore b-hydride elimination/reinsertion events will not be visible.
Closely following after the van den Hoogenband report,
40
Molander and co-
workers showed clearly that these events do occur [eqn (11.12)],
41
and they
result in chain walking of the Pd and subsequently aryl incorporation
throughout the entire structure of the organoborane. This sequence of
events can be minimized by the use of bulky electron-rich phosphine ligands
such as PhP
t
Bu
2
and, for symmetrical organoborates such as 39, high yields
can be obtained [eqn (11.13)].
41
Ar
CH
3
CH
3
CH
3
CH
3
ArCl
,Pd(OAc)
2
PhP
t
Bu
2
,Cs
2
CO
3
100°C, 72% yield
Ar
BF
3
K
+
+
+
(11
:
12)
Ar
Ar
34
35
36
37
38
ratio = 72 : 4 : 3 : 21
Pd(OAc)
2
,PhP
t
Bu
2
Ar
BF
3
K
Cs
2
CO
3
toluene/water
100°C
(11
:
13)
39
40
up to 94% yield
Interestingly, for the simple isopropyl trifluoroborates (41), the amount of
direct coupling versus b-hydride elimination followed by coupling is
dependent on the structure of the aryl halide, illustrating the sensitivity of
this reaction to sterics (Scheme 11.3). Note that these elimination/addition
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