Chemistry Reference
In-Depth Information
2 H
2
O
OH
O
O
KOH
- H
2
O
K
+
O
49 - 99%, 11 examples
R-B(OH)
2
B
-
O
B
R
R
O
OH
OH
OH
O
O
OH
Li
+
B
-
O
Br
B(Oi-Pr)
3
Li
n
-BuLi,
B(O
i
-Pr)
3
OH
OH
82%
N
N
N
Scheme 8.29 Methods for preparing the potassium and lithium salts of cyclic triol
borates.
Ar-X
Pd(OAc)
2
/JohnPhos
DMF:H
2
O (5:1)
Base free
O O
K
+
B
-
Ar
80-99%
14 examples
O
Scheme 8.30
SM coupling of cyclic triol borates with aryl halides.
parent boronic acid, with 2 equiv. of liberated water being removed azeo-
tropically. Pyramidalization is then achieved by treatment with KOH to
furnish the potassium salt of the borate. The lithium salt is readily accessed
by the organometallic pathway (Scheme 8.29).
Despite the stability gained from blocking the vacant p-orbital on boron,
the nucleophilicity of the organic moiety is the highest of all the SM coupling
boron reagents. This was illustrated by comparing the rates of their metal-
free reaction with the benzhydrylium ion electrophile.
129
The cyclic triol-
borate salt reacted 11 orders of magnitude faster than MIDA boronates,
which were the least nucleophilic. This analysis may indicate that hydrolysis
prior to transmetallation is not necessary, as the boron reagent is reactive
enough to proceed through the boronate pathway. However, although cross-
coupling was achieved under base-free conditions (Scheme 8.30), the aque-
ous DMF solvent mixture employed still has the potential for prior hydrolysis
to liberate potassium hydroxide and thus reaction could proceed through an
oxo-palladium intermediate or through a tethered palladium alkoxy species.
Other sterically congested systems were prepared using these coupling
partners, also under base-free conditions.
130,131
Optimized conditions used
for the tetra-ortho-substituted biaryl preparation involved an anhydrous/
base-free DMF system with a copper(I) co-catalyst. Under these conditions, it
is dicult to envisage significant prior hydrolysis of the cyclic triol borate
salt. However, formation of an arylcopper species with the nucleophilic
boronate may be facile, which then undergoes reaction with palladium(II).
Cyclic triol borates have been compared with boronic acids in the SM
coupling of dibromoarenes and found to provide superior yields of the
double-arylation product (Scheme 8.31).
131
The presence of a copper salt
(CuCl) was again found to be key for the generation of the sterically
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