Chemistry Reference
In-Depth Information
B(OH)
2
F
[
2
H
4
]-
3
F
3
C
Br
F
3
C
F
D
4
+
D
0/4
BF
3
K
6
CF
3
CF
3
5
[
2
H
0
]-
4
D
4
in advance of D
0
F
D
0
Scheme 8.12 Kinetic analysis of a competition between [
2
H
4
]-boronic acid 3 and
[
2
H
0
]-trifluoroborate 4 for limiting aryl bromide 5.
F
[Pd
II
L
n
]
[Pd
0
L
n
]
"F
-
"
BF
3
K
B(OH)
2
I
F
4
3
Slow
F
F
OH
II
[O]
F
F
[Pd
0
L
n
]
III
F
OH
O
2
O
O
Pd
F
Scheme 8.13 Degradation pathways (I, II and III) for 4-fluorophenylboronic acid (3)
that are all reduced through its slow formation from trifluoroborate 4
and the fluoride that is co-liberated.
18
O-labelling studies of organotrifluoroborate SM coupling indicated that
the fluoride liberated upon hydrolysis of the organotrifluoroborate led to
this palladium(II) activation pathway.
65
The same study also found that
organotrifluoroborates exhibited a quenching effect of common oxidants
that are found in ethereal solvents.
The final mechanism by which organotrifluoroborates were shown to
reduce side reactions was in a slow release of the active boronic acid
species.
65,68
This slow hydrolysis allows the reactive species to be maintained
at a low concentration. This was shown to lead to a favourable partitioning
between cross-coupling and oxidative homocoupling, whereby a lower
concentration of boronic acid decreased the catalytic flux through the non-
productive cycle. The absolute rate of protodeboronation is also slowed
because of this effect, which is highly useful for the coupling of unstable
substrates.
69
The slow-release effect was verified by a slow syringe pump
addition of boronic acid to the reaction mixture conducted in air, where very
low levels of homocoupling were observed.
65
A survey of reaction conditions used to couple R-BF
3
K salts showed that,
for every class of reagent there is a unique set of optimized conditions.
49
This analysis alone suggests that there is no single mechanistic regime for
hydrolysis to the transmetallating species. Optimization of the reaction
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