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reaction conditions that are employed. This originates from the relatively
stable, innocuous and environmentally benign nature of the boron-
containing reagent. Nonetheless, transmetallation of organoboron com-
pounds with the appropriate palladium(II) complex proceeds rapidly and
eciently. These combined features contribute to the practical upscaling of
the reaction and, together with the relatively low cost of the reagent, explain
its value to the fine chemical, pharmaceutical and agrochemical industries.
The hydroboration of terminal alkynes allows ready access to
alkenylboranes and catecholboronic esters,
17,18
a process that provided the
reagents employed by Suzuki and Miyaura for their initial discovery and
investigation of the SM coupling. However, part of the power of the SM
coupling lies in its diversity of reagents, as differentiated not just by the
organic fragment but also by the substitution patterns at boron. In the
1980s, the borane and catecholboronic ester reagents initially explored were
generally replaced by the more reactive and atom-economical boronic
acids,
19
especially for aryl couplings. The scope was further expanded with
the introduction of pinacolboronic esters in the 1990s, made available by the
recently developed Miyaura borylation protocol. The last decade has
witnessed a substantial expansion with a very wide variety of reagents for SM
coupling now having been reported (Figure 8.2), with characteristics tailored
for more specific tasks, such as distal manipulation or slow-release.
The outer-shell bonding electrons of neutral boron reagents (2s
2
,3p
1
)
results in a tricoordinate sp
2
boron centre with trigonal planar geometry and
a non-bonding vacant p-orbital orthogonal to this plane. The vacant p-orbital
dominates the reactivity patterns and characteristics of these species as the
Catechol
boronic ester
9-BBN borane
Boronic acid
R
O
B(OH)
2
B
BR
O
Z
MIDA boronate
Organotrifluoroborate
Pinacol boronic
ester
N
BF
3
K
O
O
O
O
O
B
B
O
Z
Z
Z
1,8-Diaminonaphthyl
boronamide
Triisopropylborate
Cyclictriol borate
O
O
O
O
K
+
HN
B
-
B
-
B
O
O
H
Li
+
Z
Z
Z
Figure 8.2
Some of the most popular boron reagents used directly or indirectly in
SM coupling reactions.
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