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the impact of steric shielding on Suzuki coupling was found to be
opposite to that observed with normal NHCs.
58
Indeed, complexes bearing
small alkyl groups were more active. In the case of the allyl-type complexes
(23a-e), the influence of the substitution on the allyl group was studied.
59
The congener bearing a cinnamyl group (23e) was found to be the most
active, allowing the coupling of a range of aryl chlorides at room tempera-
ture. This is in agreement with previous studies carried out by Nolan and
co-workers.
54
4.3.1.1.6 Suzuki Coupling in Water. In recent years, several groups were
also interested in developing catalysts active in water. Roy and Plenio not-
ably described the preparation of complex 24 (Figure 4.15),
60
a sulfonated
version of the highly active and previously reported [Pd(IPr)(cin)Cl]
54
(6,
Figure 4.6). The new precatalyst was active for the Suzuki reaction in a
mixture of water and nBuOH (1 : 1) at 100 1C using KOH as the base. Both
aryl chlorides and bromides were suitable coupling partners. Godoy and
co-workers also used the sulfonate functionality to prepare the bis-NHC
precatalyst 25 as a water-soluble complex (Figure 4.15).
61
The use of 25 in
pure water at 110 1C in the presence of K
2
CO
3
notably allowed the coup-
ling of 4-bromoacetophenone and phenylboronic acid with a TON of
100 000 (Scheme 4.10). Karimi and Akhavan developed the ''nanocenti-
pede-like'' Pd-NHC polymer 26 (Figure 4.15).
62
This promoted the Suzuki
reaction in water at room temperature with catalyst loadings as low as
0.005 mol% for the coupling of aryl bromides and 0.05 mol% for the
coupling of aryl chlorides. Interestingly, the catalyst was also recyclable
and could be used up to 17 times.
Nechaev and co-workers reported on the use of [Pd(NHC)(cin)Cl] complexes
bearing expanded-ring NHCs for the Suzuki coupling of heteroaryl chlorides
3
3
3
3
O
O
O
O
N
N
NN
N
N
Br
N
N
N
N
NaO
3
S
SO
3
Na
Pd
Br
2
Pd
Pd
PdBr
2
Pd
N
N
N
N
I
I
Br
Cl
n
SO
3
K
KO
3
S
Ph
O
O
O
O
3
3
3
3
24
25
26
Figure 4.15
Structures of the water-soluble precatalysts 24, 25 and 26.
25
(0 .001 mol%)
K
2
CO
3
H
2
O, 110°C
MeOC
Br
+
B(OH)
2
MeOC
>9 9%, TON = 100 000
Scheme 4.10
Suzuki coupling using 25 in water.
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