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i) B(OR)
3
ii) H
+
/ H
2
O
1 mol% [Pd]
B
2
(OH)
4
M
B(OH)
2
X
i) KHF
2
,ii) H
2
O / "fluorophile"
or
i) NH(C
2
H
4
OH)
2,
ii) H
+
/ H
2
O
or
i) NaIO
4
M = Li or MgX
O
B
O
Scheme 8.7 The three general routes to prepare arylboronic acids.
3 H
2
O
R
R
B
B
O
O
O
O
B(OH)
2
3
R
B
B
B
B
R
O
R
R
O
R
Aromatic (4
n
+ 2)
Scheme 8.8 Entropically favourable dehydration of boronic acids to form the par-
tially aromatic boroxine species.
as active transmetallating species, e.g. with rhodium complexes,
42
it is not
yet clear whether they are more or less reactive than boronic acids in the
transmetallation of palladium(II) complexes. However, if their formation
does reduce the rates of transmetallation, it could be an additional reason as
to why water is a common component in SM coupling reactions. In addition,
setting the correct stoichiometry of the boronic acid can sometimes be dif-
ficult, as establishing the degree of dehydration to the boroxine in the solid
material is not straightforward. To account for this situation, it is common
practice to add an excess of the reagent.
The mechanism of transmetallation of boronic acids to Pd(II) intermedi-
ates has been the subject of a range of computational studies.
43,44
However,
owing to the ready formation of the ''ate'' complex ([RB(OH)
3
]
) under basic
SM coupling conditions, most studies have tended to investigate only the
reaction of this species with the halide complex, i.e. the boronate pathway.
Maseras and co-workers,
43
however, calculated a lower energy pathway for
the oxo-palladium pathway but could not locate any reasonable mechanism
for the formation of the key oxo-palladium(II) species, despite their being
precedent for such species.
45
They therefore concluded that the boronate
pathway dominates.
43
Nevertheless, three experimental studies published in
2011 all found kinetic evidence in favour of the oxo-palladium pathway.
Jutand and co-workers
22
carefully examined the role of the base by
studying voltammograms arising from generation and decay of palladium
species. The concentrations of the electroactive species determined the
reduction or oxidation currents and this then allowed kinetic analysis.
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