A new catalytic method for the synthesis of boroxanes

Article abstract of DOI:10.1016/j.tetlet.2010.09.094

A new Ru-H complex catalyzed O-borylation of boronic acids with vinylboronates leading to boroxane bond formation with evolution of ethylene is described. Under the optimum conditions, when [Ru(CO)Cl(H)(PPh₃) ₃] and excess vinylboron

Full text of DOI:10.1016/j.tetlet.2010.09.094

Tetrahedron Letters 51 (2010) 6177–6180
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Tetrahedron Letters
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A new catalytic method for the synthesis of boroxanes
⇑
Je˛drzej Walkowiak, Bogdan Marciniec
´
Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 June 2010
Revised 25 August 2010
Accepted 20 September 2010
Available online 26 September 2010
A new Ru–H complex catalyzed O-borylation of boronic acids with vinylboronates leading to boroxane
bond formation with evolution of ethylene is described. Under the optimum conditions, when
[Ru(CO)Cl(H)(PPh3)3] and excess vinylboronate are used, the reaction gives exclusively the corresponding
boroxane in high yield. The proposed mechanism of catalysis is supported by the results of the equimolar
reaction of Ru–B complex with 1,3,2-dioxaborinan-2-ol.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Homogenous catalysis
Boronic acids
Vinylboronates
Ruthenium coupling reactions
We have previously developed a new type of transition metal
(TM) catalyzed reaction of vinyl substituted organosilicon com-
pounds with a variety of olefins, a process known as silylative cou-
pling (SC), which occurs in the presence of complexes containing
or generating M-H and M-Si (silicometallic) bonds.1,2 This mode of
catalytic reactivity has been recently found to be general and was
also exhibited by vinyl derivatives of other p-block elements (e.g.,
boron3 and germanium4). It has also been extended to catalytic acti-
vation of Csp–H,5 Caryl–H,6 as well as by the O–H bond, of silanols7
and alcohols8 indicating a new general role of vinyl metalloid com-
pounds as metallation agents and hydrogen acceptors. Such reac-
tions proceed according to the following general equation (Eq. 1).1
Compounds with covalently linked B–O–B bonds can be used as
precursors for self-assembly linear polymers. This type of self-
repairing polymer is typically synthesized via condensation reac-
tions of boronic acids with polyols.11
Boroxane chemistry is used to initiate the formation of linkages
in polymeric materials or anion acceptors for battery applications.
This strategy is also used for the synthesis of porous thermally sta-
ble boronate networks with high surface area.12,13 The presence of
boron atoms in the molecular structures of compounds also has a
strong influence on the mechanical and thermal resistance of
new materials, for example, ceramics.10–13
To check the efficiency of the O-borylation process a number of
boronic acids were tested. The reaction with vinylboronate was
catalyzed by ruthenium hydride complexes and proceeded via acti-
vation of the H–O bond and elimination of ethylene (Eq. 2).
TM
+
E E'
E
+
H
E'
ð1Þ
E = SiR3, GeR3, BR2
E'-H = Csp2-H, Csp-H,
HO
O
B
,
C
O H
Si O H
O
O
O
O
B
B
-
All these reactions lead to the synthesis of a variety of linear and
cyclic organometallic (Si, B, Ge) compounds as potential reagents
in organic synthesis9 and as precursors of optoelectronic and cera-
mic materials.10
O
O
O
[RuHCl(CO)(PPh3)]
B
O
O
O
O
O
(HO)2B
-
R
B
B
B
O
R
O
In view of our very recent reports on the new role of vinylbor-
onates as hydrogen acceptors for the O-borylation of silanols7a and
alcohols,8a the aim of this work was to make use of this general fea-
ture in the O-borylation of boronic acids. These processes can be
used as a versatile method for blocking the hydrogen atom in boro-
nic acids as well as in borylations of inorganic and organometallic
materials.
ð2Þ
This process can be accompanied by vinylboronate homocou-
pling, also catalyzed by the same complexes, as previously re-
ported (Eq. 3).3
O
O
[Ru-H]
-
O
B
B
ð3Þ
2
B
O
O
O
⇑
Corresponding author. Tel.: +48 618291366.
E-mail address: marcinb@amu.edu.pl (B. Marciniec).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.09.094

6178
J. Walkowiak, B. Marciniec / Tetrahedron Letters 51 (2010) 6177–6180
Table 2
When [Ru(CO)Cl(H)(PCy3)2] (I), the most active catalyst for
metallative coupling of olefins,1–5 alkynes,6 and silanols7 with
vinylmetaloids was used as the catalyst, the homocoupling of vinyl-
boronates giving bis(boryl)ethenes was also observed (see Table 1).
Higher chemoselectivity was achieved with the ruthenium cat-
alyst with coordinated triphenylphosphine as the ligand [Ru(-
CO)Cl(H)(PPh3)3] (II), the most efficient catalyst for O-borylation
of alcohols and silanols.7a,8 A two-fold excess of vinylboronate for
each hydroxy group in the boronic acid was necessary to obtain
the desired products. It is worth noting that under such conditions,
when the concentration of vinylboronate was much higher than
that of the boronic acid, homocoupling of the vinylboronate was
not observed, even in trace amounts (see Tables 2 and 3).
The reactions proceed in very high, almost quantitative conver-
sions. All products were isolated and the structures were deter-
mined by 1H, 13C and 11B NMR spectroscopy.14 The 11B NMR
spectra of the products showed two signals assigned to the boron
atom bonded to three oxygen atoms (ꢀ30 ppm) and the boron
atom with one B–C and two B–O bonds (ꢀ37–42 ppm).
The O-borylation of boronic acids with 2-vinyl-1,3,2,-dioxaborinane catalyzed by
[Ru(CO)Cl(H)(PPh3)3] (II)
a
B–OH reagent
Conversion of B–OH (%)b
92
Yield (%)c
71
O
B
OH
OH
O
B
100
100
100
77
74
71
OH
OH
OH
Me
Br
B
OH
B
B
OH
OH
89
92
74
72
OH
OH
OH
For this new O-borylation of boronic acids with vinylboronates,
on the basis of the obtained results and the previously reported
mechanisms of the O-borylation of alcohols8 and silanols7a with
vinylboronates, an analogous insertion-elimination mechanism is
proposed. The insertion of vinylborane into the ruthenium–hy-
dride bond and the generation of a ruthenium–boron bond with
simultaneous elimination of ethylene was previously reported.3
This experiment, monitored by 1H NMR and GC–MS, confirmed
clearly the first-half of the catalytic cycle. For the O-borylation
reaction of boronic acids the process analogous to that for the cou-
pling of vinylboranes with silanols and alcohols occurred. Deuter-
ated DMSO and THF were used as reaction solvents, because the
boronic acids were insoluble in deuterated toluene or benzene,
the solvents typically used in such experiments.
Initially, the catalytic reaction of vinylborane and 1,3,2-diox-
aborinan-2-ol catalyzed by [Ru(CO)Cl(H)(PPh3)3] (II) was moni-
tored by 1H NMR spectroscopy. After a few hours the evolution
of ethylene was apparent from the appearance of a singlet at
5.25 ppm, which provided evidence of the formation of a ruthe-
nium–boryl complex.
B
a
Reaction conditions: [Ru]:[B–OH]:[vinylboronate] = 2 ꢂ 10ꢁ2:1:2; 1,4-dioxane
(0.5 M), T = 80 °C, t = 24 h.
b
Determined by GC analysis.
Yield of isolated product.
c
Table 3
The O-borylation of boronic acids with 2-vinyl-4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lane catalyzed by [Ru(CO)Cl(H)(PPh3)3] (II)a
B–OH reagent
Conversion of B–OH (%)b
90
Yield (%)c
73
O
B
OH
OH
O
B
96
96
65
60
OH
OH
OH
The stoichiometric reaction of the ruthenium–boryl complex
[Ru(BO2C6H4)(CO)Cl(PCy3)2] (III) with 1,3,2-dioxaborinan-2-ol
was carried out at 60 °C in THF-d8 and the progress of the reaction
was monitored by 1H NMR spectroscopy.15 Regeneration of the
Br
B
OH
OH
87
59
B
a
Reaction conditions: [Ru]:[B–OH]:[vinylboronate] = 2 ꢂ 10ꢁ2:1:2; 1,4-dioxane
(0.5 M), T = 80 °C, t = 24 h.
Table 1
b
Determined by GC analysis.
Yield of isolated product.
The O-borylation of boronic acids with 2-vinyl-1,3,2-dioxaborinane catalyzed by
c
[Ru(CO)Cl(H)(PCy3)2] (I)a
c
B–OH reagent
Conversion of B–OH (%)b
100
Selectivity (%)b,
90/10
O
B
OH
OH
ruthenium hydride bond was manifested as a low intensity triplet
at ꢁ24.3 ppm [J (P-H) = 20.2 Hz], which appeared after 96 h. This
was a clear evidence of the O-borylation process. When the reac-
tion was carried out in DMSO-d6 regeneration of the Ru–H bond
was not observed in the 1H NMR spectrum, even after a few days.
This was probably due to the fact that DMSO shows a high propen-
sity to coordinate with the metal center and reduces the activity of
the ruthenium complex. A similar stoichiometric reaction occurred
with phenylboronic acid as the reagent. The process was also
monitored by 11B NMR spectroscopy, which confirmed the
generation of a product with the B–O–B moiety. After a few hours,
a signal due to the boron connected to three oxygen atoms
occurred (33.6 ppm), which was the evidence of product formation
(Eq. 4).
O
B
100
100
86/14
89/11
OH
OH
Br
B
B
OH
OH
95
83/17
OH
Reaction conditions: [Ru]:[B–OH]:[vinylboronate] = 2 ꢂ 10ꢁ2:1:2; 1,4-dioxane
a
(0.5 M), T = 80 °C, t = 24 h.
b
Determined by GC analysis.
Selectivity: [O-borylation/homocoupling].
c

J. Walkowiak, B. Marciniec / Tetrahedron Letters 51 (2010) 6177–6180
6179
O
O
O
O
B
B
B
R
O
O
[Ru]-H
O
B
O
O
H
H
O
O
O
O
O
B
R
OH
H
B
O
[Ru]
B
R
H
[Ru]
B
[Ru]
O
B
O
O
R
B
H
B
H
O
OH
O
O
B
O
HO
R
B
-
OH
O
O
[Ru]-
B
Scheme 1. Mechanism of the O-borylation of boronic acids with vinylboronates.
PCy3
PCy3
O
O
O
O
O
O
Cl
O
THF-d8
Cl
B
B
Ru
H
+
Ru
B
+ HO B
T=60 oC
4 d
ð4Þ
OC
OC
O
O
PCy3
PCy3
(III)
(I)
On the basis of these experiments and the previously proposed
this article can be found, in the online version, at doi:10.1016/
j.tetlet.2010.09.094.
mechanism for the O-borylation of silanols7a and alcohols8 with
vinylboronates, a catalytic mechanism for this new O-borylation
reaction of boronic acids is proposed (Scheme 1) based on the acti-
vation of the B–C bond in the vinylborane and the H–O bond in the
boronic acid.
References and notes
1. Marciniec, B. Acc. Chem. Res. 2007, 40, 943–952.
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borylation process. The product with one hydroxy group connected
to the boron atom may undergo subsequent oxidative addition to
the ruthenium–boryl complex and then reductive elimination giv-
ing a diboryloxy-substituted boronic acid derivative, which opens a
new route to linear inorganic–organometallic polymers with boron
atoms in the structure.
This coupling reaction allows the synthesis not only of symmet-
ric, but also of non-symmetric boroxanes, which are difficult to ob-
tain via typical synthetic processes. The much lower reactivity of
vinylboronates compared to that of vinylsilanes and vinylgerm-
anes is responsible for this high chemoselectivity, which is in
agreement with previously published results.7,8a
In conclusion, we have reported a new catalytic O-borylation of
boronic acids with vinylboronates. This process extends the role of
vinylboronates as hydrogen acceptors to form borylated products
and indicates that such a strategy can also be used for the boryla-
tion of other heteroatom–hydrogen bonds. The boroxanes obtained
may be used as monomers for the synthesis of polymers with bor-
yl–oxygen–boryl bridges or substrates for new ceramic materials.
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Acknowledgments
This work was supported by the Ministry of Science and Higher
Education (Poland) (NN204265538).
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determined by GC. After the reaction, the solvent and excess borane were
removed under vacuum and the crude product was purified by column
chromatography (silica gel modified with HMDS) with hexane/EtOAc as eluent.
14. Representative experimental procedure for the O-borylation of boronic
acid by vinylboronates: the ruthenium catalyst [Ru(CO)Cl(H)(PPh3)3)] (II)
(2 mol%) was dissolved in 1,4-dioxane and placed in a glass ampoule. Dodecane
(used as internal standard, 5% by volume all components), boronic acid
(0.5 mmol), and 2-vinyl-1,3,2-dioxaborinane (1–2 mmol) were added. The
ampoule was heated at 80 °C for 24 h. The conversion of boronic acid was
15. Experimental
procedure
for
the
stoichiometric
experiment:
[Ru(BO2C6H4)(CO)Cl(PCy3)2] (III) (0.012 mmol), 1,3,2-dioxaborinan-1-ol
(0.016 mmol) and THF-d8 (0.8 mL) were placed in an NMR tube under argon.
The reaction was carried out at 80 °C and the reaction course was monitored by
1H NMR spectroscopy.