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aryl triflates because of the formation of potassium halide precipitates
during the reaction. Third, in certain cases it was found that potassium
carboxylates precipitated from the reaction mixture. This was solved by re-
placing the potassium tert-butoxide base with tetraethylammonium hy-
droxide. Good results were obtained for the preparation of a variety of biaryls
(Figure 13.23). Compared with batch operation, better yields, fewer side re-
actions (less protodecarboxylation and homocoupling) and shorter resi-
dence times (1 h versus 16 h) were obtained.
Another example involves a combination of a decarboxylative coupling of
propiolic acid followed by a Heck-Cassar coupling to produce unsymmetrical
diarylalkynes (Figure 13.24).
54
In the first capillary, propiolic acid, aryl iodide
and Pd(PPh
3
)
2
Cl
2
(5 mol%)-dppb (10 mol%) were combined and heated to
CuNO
3
(phen)(PPh
3
)
2
(5 mol%)
Pd(OAc)
2
(2 m ol%)
OTf
Ar
R
ArCO
2
H
R
+
KO
t-
Bu or Et
4
NOH
NMP, 170 °C, 1h
Me
S
O
S
N
NO
2
Me
Me
Me
Me
Me
70%
60%
67%
54%
Figure 13.23 Decarboxylative cross-coupling of aryl triflates and arylcarboxylic acids
in flow.
Br
R
Pd (PPh
3
)
2
Cl
1
dppf
I
R
R
+
O
R
OH
50 °C
4h
120 °C
4h
Decarboxylative
coupl ing
Heck-Cassar
coupling
N
OMe
N
S
Me
Me
75%
68%
70%
98%
Figure 13.24
Synthesis of diarylalkynes via a decarboxylative coupling-Heck-Cassar
coupling sequence in flow.
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