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Na
2
PdCl
4
(0.25 mol% )
64
R'
(0.5 mol%)
n
X
n
+
H
R'
Y
H
2
O:i PrOH(1:1)
90 - 95°C, 12 h
Y
X=Br,Cl
Y=N,S
n=1,2
H
2
N
S
N
N
X=Br:99%
X=Cl:94%
X = Br: 99%
X=Br:99%
S
N
N
4
OMe
X=Br:99%
X=Cl:99%
X=Cl:76%
NN
SO
3
Na
NaO
3
S
Cl
64
Scheme 4.41 Alkynylation of heterocycles in water using ligand 64.
4.3.3.3 Development of Recyclable Systems
As with other coupling reactions, the development of recyclable systems for
the Sonogashira coupling and Heck alkynylation is important. The hybrid
silica-based catalysts of Pleixats and co-workers,
113
which were already active
in the Heck reaction (49, Figure 4.24), also found application in Heck
alkynylation. The systems were used to promote the coupling between
4-bromoacetophenone and phenylacetylene under copper-free conditions at
110 1C in DMF and in the presence of nBu
4
OAc as the base. Initial experi-
ments revealed complete reactivity within 1 h using 0.2 mol% of palladium.
The reaction time had to be increased to 4 h after the first cycle, but the
catalysts could be used up to five times (Table 4.22). Further experiments
allowed the reduction of the catalyst loading to as low as 0.01 mol%, af-
fording high TONs (4800-5400) and TOF (3200-3600 h
1
).
Huynh and Lee also developed pincer-type di(1,2,4-triazolin-5-ylidene)-
palladium complexes for Heck alkynylation (Figure 4.30).
137
Precatalyst 65
was found to be active for copper- and amine-free coupling between aryl
bromides and phenylacetylene. The catalyst loading could be decreased to as
low as 0.01 mol% at 120 1C, affording TONs of up to 8000 for a single run.
Since pincer-type complexes are known for their high thermal stability, re-
cycling of the catalyst was investigated. More substrates were added in turn
to the reaction medium. This could be done up to six times, affording an
overall TON of 56 000.
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