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Table 4.22 Activity and recyclability of 49a and 49b in Heck alkynylation.
Br
49a-b (0.2 mol%)
+
H
Ph
MeOC
Ph
NBu
4
OAc
DMF, 110°C
MeOC
49a
49b
Yield (%)
a
[isolated yield]
Yield (%)
a
[isolated yield]
Cycle
Time (h)
Time (h)
1
1
90 [71]
1
87 [73]
2
4
76
4
78
3
4
79
4
82
4
4
84
4
83
5
5
62
4
86
a
By GC.
Br
N
N
N
Pd
N
N
N
N
Br
65
Ph
Ph
Figure 4.30
Structure of precatalyst 65.
4.3.3.4 Development of New Pd-NHC Systems
As for other cross-coupling reactions, the modulation of the steric and
electronic properties of the NHCs and also the impact of the throwaway
ligand were investigated in alkylation reactions. In an attempt to improve
the catalytic activity by increasing the bulk of the ligand, Straub and co-
workers showed that the use of IPr** was not advantageous in copper-free
cross-coupling.
55
Using previously established successful reaction con-
ditions
102
for the coupling of phenyl iodide with phenyl acetylene, complex
20 yielded only 27% of coupling product (Scheme 4.42).
In 2012, Xu et al. reported the preparation of NHC-based cyclopalladated
ferrocenylchloropyrimidine for Heck alkynylation.
138
Of three new com-
plexes, the IPr-bearing complex was found to be the most active in the
coupling reaction (66, Figure 4.31). The latter proved to be ecient under
copper- and amine-free conditions, allowing the coupling of aryl bromides
and activated aryl chlorides with phenyl- and p-tolylacetylenes. Optimized
reaction conditions were the use of CsOAc as the base in DMAc at 120 1C
using catalyst loadings in the range 0.1-2 mol%. Cao and co-workers
139
prepared novel chiral Pd-NHC complexes (67, Figure 4.31) and investigated
their reactivity in the copper-free Sonogashira reaction. Complex 67a ex-
hibited the highest catalytic activity. However, despite success with activated
bromides, the reaction did not work with chlorides. Interestingly, a similar
complex (67e) was successfully used in a tandem Sonogashira-hydroarylation
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