702
Published on the web June 4, 2011
Direct Conversion of Pinacol Arylboronic Esters to Aryl Triolborates
Gao-Qiang Li, Shunsuke Kiyomura, Yasunori Yamamoto,* and Norio Miyaura*
Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University,
Sapporo, Hokkaido 060-8628
(Received April 21, 2011; CL-110341; E-mail: yasuyama@eng.hokudai.ac.jp)
B2pin2
HO
H
Conversion of pinacol arylboronic esters
3 to aryl
or
HO
FG
FG
HBpin
M+
triolborates 5 via transesterification with 1,1,1-tris(hydroxy-
methyl)ethane (4) was established with the advantages of
tolerance to various functional groups. Transesterification was
carried out at 30-60 °C in dioxane in the presence of MOH
(M = Na and K) and H2O. High yields were achieved for stable
aryl triolborates 5.
O
B
O
HO
B-
O
Ir catalyst
1
2
4
O
O
FG
FG
B2pin2
MOH,
or
HBpin
X
dioxane/H2O
3
5
M = Na, K
Pd catalyst,
base
X = I, Br, Cl, OTf, N2BF4
Over the past three decades, it has become increasingly clear
that organoboron compounds are valuable reagents capable of
undergoing many catalytic C-C bond formations in organic
synthesis.1-4 Boronic acids are convenient reagents that are
generally thermally stable and are inert to water and oxygen. The
C-B bond of organoboronic acids is totally covalent and inert to
ionic reaction, but nucleophilicity of organic groups on a boron
atom is significantly enhanced by quaternerization by an anionic
ligand. Thus, tetracoordinated ate-complexes are key species that
have been successfully used for addition and coupling reactions
of organoboron compounds, including metal-catalyzed reactions
of organoboronic acids. Recently, air- and water-stable trifluoro-
borates are typical ate-complexes that are advantageous over
boronic acids in preparation and handling of pure and water-
stable crystalline materials.5 However, their metal-catalyzed
bond-forming reactions are very slow in the absence of bases
because of the low nucleophilicity of organic groups due to the
high electronegativity of fluorine. We have reported novel cyclic
triolborates that have exceptionally high levels of stability in air
and water and higher solubility in organic solvents than that
of potassium trifluoroborates.6-9 High performance of lithium
or potassium triolborates for transmetalation has been demon-
strated in palladium- and copper-catalyzed C-C7,8 and C-N9
bond-forming reactions and rhodium-catalyzed addition reac-
tions.10-12
We have developed methods for the synthesis of aryl
triolborates.6 The azeotropic removal of water upon treatment of
organoboronic acids with the 1,1,1-tris(hydroxymethyl)ethane
(4) gave boronic esters, which were readily converted into
triolborates by treatment with KOH. The corresponding lithium
salts were synthesized by the alkylation of B(OMe)3 or
B(Oi-Pr)3 with RLi, followed by the removal of MeOH or
i-PrOH though ether exchange with triol 4. Recently, synthesis
of pinacol boronic esters has been achieved by palladium-,13-15
nickel-,16 and copper-catalyzed17 coupling reactions between
aryl halides or triflates 2 and pinacolborane13 or diborons such as
B2pin2 (pin: pinacol).14,15 More recently, direct borylation of
C-H bonds by HBpin or B2pin2 is a convenient, economical, and
environmentally benign process for the synthesis of aromatic
boron compounds.18
Scheme 1.
Table 1. Reaction conditionsa
HO
HO
+ O
Na
O
B-
Cl
Cl
O
O
O
HO
4
B
Cl
NaOH
3a
5a
Cl
Entry Solvent NaOH/equiv Temp/°C H2O/equiv Yield/%b
1
2
3
4
5
6
7
8
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
THF
0.9
0.9
0.9
1.0
1.0
1.0
1.0
1.0
60
60
60
60
30
90
30
30
3
72
27
trace
76
84
trace
48
1
none
3
3
3
3
3
DME
46
aReaction conditions: 2-(3,5-dichlorophenyl)-4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane (3a: 1.0 mmol), triol, and NaOH
in solvent (5 mL) was stirred for 16 h. Yields of the isolated
b
product.
boronic esters have been converted to arylboronic acids by
oxidative cleavage or displacement of pinacol by diethanol-
amine or KHF2.19,20 Matteson and co-workers reported the
hydrolysis of 1,2-dicyclohexyl ethanediol (DIECHED) boronic
ester to boronic acids with sodium hydroxide and tris(hydroxy-
methyl)methane derivative in a two-phase system.21 We exam-
ined transesterification of pinacol boronic ester 3a to sodium
triolborate 5a. There is a strong accelerating effect of water
(Entries 1-3, Table 1). The reaction required the presence of
3 equivalents of water and proceeded smoothly in dioxane but
was very slow in other solvents such as THF and DME (Entries
5, 7, and 8). Finally, the reaction took place smoothly at 30 °C
in the presence of 1.0 equivalent of NaOH with 84% yield
(Entry 5).
We tested the generality of this conversion of pinacol
boronic esters to sodium aryl triolborates (Table 2). High yields
were easily achieved in most aromatic boronic esters possessing
halogens and ester substituents at para and meta carbons (Entries
Herein, we report a convenient method to directly convert
pinacol boronic esters 3 to aryl triolborates 5 (Scheme 1). Aryl-
Chem. Lett. 2011, 40, 702-704
© 2011 The Chemical Society of Japan
Li, Gao-Qiang
