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In-Depth Information
Direct Transmetallation
Prior Activation Necessary
Ambiguous Modes of Activation
Catechol-
boronic ester
Organotrifluoroborate
salt
9-BBN borane
BF
3
K
R
O
BR
B
O
Z
Pinacol boronic
ester
Disiamylborane
MIDA boronate
O
N
B
O
O
O
O
O
B
B
Z
R
Trihydroxyborate
Z
O
H
Na
+
Boronic acid
1,8-diaminonaphthyl
boronamide
OH
B
-
OH
B(OH)
2
Z
HN
Z
B
Triisopropylborate
H
O
O
B
-
Z
O
Li
+
Z
Cyclictriol borate
O
O
K
+
B
-
O
Z
Figure 8.5 Categorization of boron reagents in SM coupling into three groups: those
that transmetallate directly, those that require prior ligand substitution
and those whose activation mechanisms are ambiguous.
transmetallation category. MIDA boronates undergo activation under mild
aqueous basic conditions [NaOH
(aq)
, THF]. The reaction rate is rapid but can
be slowed by changing the base and solvent system [K
3
PO
4
, dioxane-water
(5 : 1)]. This rate variation between R groups is very much less sensitive than
with organotrifluoroborate salts and these reagents are therefore amenable
to the cross-coupling of a wide range of unstable boronic acids using a
simple and reliable method. Complementary to MIDA boronates are DAN
boronamides, which are stable to basic conditions but rapidly liberate the
boronic acid under acidic conditions.
The final category of reagents includes boronic esters and borate salts and
here there is currently ambiguity surrounding whether activation occurs
prior to transmetallation. Despite numerous clues throughout the chemical
literature, insucient mechanistic studies have been conducted to conclude
accurately which category they belong to. Borate salts have the capacity to be
included in either category and it may depend on the nature of the particular
ligand on boron. Indeed, there may also be variation within a class of boron
reagent, e.g. some especially electron-poor aromatic pinacolboronic esters
may undergo direct transmetallation with an oxo-palladium(II) species due
to the increased Lewis acidity at boron, whereas other electron-rich aromatic
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