Chemistry Reference
In-Depth Information
F
F
Pd(OAc)
2
(1-5 mol%)
P
t
Bu
2
Me-HBF
4
(2-10 mol%)
K
2
CO
3
(1.1 equiv)
DMA, 120 °C
F
F
F
Br
F
+
R
F
F
H
R
F
F
yield = 29-98%
Scheme 12.11 Cross-coupling of bromoarenes with polyfluoroarenes.
F
F
Ar
L
n
Pd
0
ArBr
F
F
F
PdL
n
(Ar)Br
mechanism 1
K
2
CO
3
F
F
F
Pd(L
n
)Ar
F
KBr
F
F
F
F
R
3
P
Pd
F
H
F
Br
HBr
PR
3
K
2
CO
3
Pd
O
O
mechanism 2
Ar
KHCO
3
+KBr
KHCO
3
O K
F
F
F
F
H
F
F
R
3
P
Pd
F
H
F
F
F
O
O
O
Figure 12.5 Proposed mechanism for
cross-coupling of bromoarenes with
polyfluoroarenes.
proposed: (a) concerted palladation and loss of HBr to afford a diarylpalla-
dium(II) intermediate; and (b) exchange of the bromide ion with carbonate,
which can allow for a related palladation-deprotonation process
(Figure 12.5). In the case of substrates with more than one C-H bond
available, products corresponding to di-and triarylation were also observed.
Yu's group has also contributed significantly to intermolecular direct
arylation for C-C bond formation via C-H activation. They used the carb-
oxylic acid group as a directing group for C-H activation/C-C coupling
(Scheme 12.12).
29
Optimization studies revealed that carboxylates as sub-
strates are much more effective than simple benzoic acids. A possible ex-
planation for this is that the O-anion of the carboxylate has an impact on the
C-H activation step where Pd
II
inserts in the C-H bond.
This method was applicable to simple benzoic acids but bis-arylated
compounds were also formed in significant amounts. Notably, meta-substi-
tuted benzoic acids led to regioselective arylation (Scheme 12.13).
Yu and co-workers also reported the direct arylation of benzoic and phe-
nylacetic acids with potassium aryltrifluoroborates as the coupling partner
(Schemes 12.14 and 15).
30
Coordination of the carboxylate moiety to K
+
was
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