Dihydroxylation of styrene by sodium chlorite with scandium triflate

Article abstract of DOI:10.1002/poc.3619

Dihydroxylation of styrene by chlorite (ClO₂ ^?) with scandium triflate [Sc(OTf)₃] occurs efficiently to produce 1-phenylethane-1,2-diol under the ambient conditions. Scandium triflate acting as a strong Lewis acid is found to accelerate the disproportionation of ClO₂ ^? to produce ClO^? and ClO₃ ^?. A scandium-chlorine dioxide complex (Sc³⁺ClO₂ ^?) is generated by the reaction of ClO^? with ClO₂ ^? in the presence of Sc^{3 +}. The binding of Sc^{3 +} to ClO₂ ^? was detected by the electron paramagnetic resonance measurements, enhancing the reactivity and selectivity of styrene hydroxylation.

Full text of DOI:10.1002/poc.3619

Received: 28 April 2016
DOI 10.1002/poc.3619
Revised: 10 June 2016
Accepted: 15 June 2016
S H O R T C O M M U N I C A T I O N
Dihydroxylation of styrene by sodium chlorite with scandium
triflate
Kei Ohkubo1,2,3,4 | Kensaku Hirose4 | Takekatsu Shibata5 | Kiyoto Takamori5 |
*
Shunichi Fukuzumi3,6
*
1 Division of Applied Chemistry, Graduate School
Abstract
of Engineering, Osaka University, Suita, Osaka,
Japan
Dihydroxylation of styrene by chlorite (ClO2−) with scandium triflate [Sc(OTf)3]
occurs efficiently to produce 1‐phenylethane‐1,2‐diol under the ambient conditions.
Scandium triflate acting as a strong Lewis acid is found to accelerate the dispropor-
2 dotAqua Inc., Suita, Osaka, Japan
3 Department of Chemistry and Nano Science,
Ewha Womans University, Seoul, Korea
4 Department of Material and Life Science,
Graduate School of Engineering, Osaka University,
Suita, Osaka, Japan
tionation of ClO2 to produce ClO− and ClO3−. A scandium‐chlorine dioxide
−
complex (Sc3+ClO2 ) is generated by the reaction of ClO− with ClO2 in the
•
−
presence of Sc3+. The binding of Sc3+ to ClO2 was detected by the electron
•
paramagnetic resonance measurements, enhancing the reactivity and selectivity of
styrene hydroxylation.
5 Acenet Inc., Tokyo, Japan
6 Faculty of Science and Technology, Meijo
University, SENTAN, Japan Science and
Technology Agency (JST), Nagoya, Aichi, Japan
KEYWORDS
Correspondence:
chlorine dioxide, chlorite, dihydroxylation, electron paramagnetic resonance, Lewis
acid
Kei Ohkubo, Division of Applied Chemistry,
Graduate School of Engineering, Osaka University,
Suita, Osaka, Japan.
Email: ookubo@chem.eng.osaka‐u.ac.jp
Shunichi Fukuzumi, Department of Chemistry and
Nano Science, Ewha Womans University, Seoul
120‐750, Korea.
Email: fukuzumi@chem.eng.osaka‐u.ac.jp
Sodium chlorite (NaClO2), which is a nontoxic and cheap
oxidizing reagent, has been used as a precursor of chlorine
NaClO2 as a precursor of ClO2• has been examined; however,
no 1,2‐diol product(s) was obtained because the oxidizing
dioxide radical (ClO2 ).[1–4] ClO2• is known to be a reactive
ability of ClO2 is not strong enough to oxidize the olefin
•
•
•
and stable radical; however, ClO2 is a yellow explosive gas
to the diol without acid.[14–18] Activation of the Cl=O double
•
•
at room temperature. ClO2 can be experimentally prepared
bond of ClO2 is a key for the 1‐step and selective
by the oxidation of NaClO2 with Cl2 and the reaction of
dihydroxylation of olefin.
potassium chlorate (KClO3) with oxalic acid.[5] These
We report herein that ClO2• is readily produced from ClO−
in the presence of scandium triflate [Sc(OTf)3] as a Lewis
methods also cause serious problems such as toxicity of Cl2
−
•
and explosibility of ClO3
.
acid and that the binding of Sc3+ to ClO2 has made it
Olefin oxidation to epoxidation and 1,2‐diol are key
industrial processes for the production of precursors of a
variety of fine or specialty chemicals such as resins, pharma-
ceuticals, drugs, dyestuffs, pesticides, and perfume composi-
tions. Several methods for olefin oxygenation to the
corresponding epoxide and alcohol have so far been reported
using inorganic metal‐oxo complexes and heavy atom metal
oxide. A high‐valence OsVIIIO4 is an effective and selective
oxidizing reagent for olefin to 1,2‐diol[6–13]; however, the
toxicity and sublimability of osmium compounds and their
waste cause serious problems. Epoxidation of olefin using
possible to dihydroxylate styrene under ambient condi-
tions.[19,20] The dihydroxylation mechanism is clarified on
the basis of the detection of the radical intermediate by the
electron paramagnetic resonance and UV‐visible absorption
spectroscopies.
No dihydroxylation of styrene occurred in the reaction of
styrene (2.0 mM) with NaClO2 (20 mM) in an aqueous
acetonitrile solution (CD3CN/D2O 1:1 v/v) at room tempera-
ture (Equation 1) (see the supporting information Figure S1,
entry 1 in Table 1). When the temperature was increased to
333 K, the epoxidation occurred without formation of
J. Phys. Org. Chem. 2016; 1–5
wileyonlinelibrary.com/journal/poc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
OHKUBO ET AL.
TABLE 1 Reaction conditions and yields for the reactiona of styrene with NaClO2
Yieldb (%)
Entry
Additive
None
Temperature (K)
Conversion (%)
1
2
0
3
0
1
2c
298
333
298
298
298
3
61
0
0
None
0
44
0
3
CF3COOD
Sc(OTf)3
Sc(OTf)3
100
100
100
15
51
0
69
30
82
4
5d
0
0
aConditions: styrene (2.0 mM), NaClO2 (20 mM), additive (30 mM), CD3CN/D2O (1:1 v/v), 17 h.
b1H nuclear magnetic resonance yield.
cNaClO2 (200 mM), CD3CN/D2O (4:1 v/v), 24 h.
dConditions: styrene (2.0 mM), NaClO (6.0 mM), additive (10 mM), CD3CN/D2O (1:1 v/v), 6 h.
dihydroxylation product (entry 2 in Table 1, Figure S2 in the
supporting information).[14] In contrast, when trifluoroacetic
acid (CF3COOD, 30 mM) as a Brønsted acid was added as
an additive, no epoxide was formed after mixing for 17 hours
but 1‐phenylethane‐1,2‐diol (1) and 2‐chloro‐1‐phenylethanol
(2) were produced with the yields of 15% and 69%, respec-
accompanied by an increase in a new band with vibronic
structure at 358 nm, which is assigned to ClO2 . A similar
•
absorption spectral change was also observed in the presence
of CF3COOH.[25,26] The time course of the appearance of the
band at 358 nm is shown in Figure 2a, being well fitted by the
second‐order plot (Figure 2B). Thus, formation of ClO2• with
1
−
tively (entry 3 in Table 1), which were measured by H
Sc(OTf)3 involves 2 molecules of ClO2 in the rate‐deter-
nuclear magnetic resonance spectroscopy (Figure S3 in the
supporting information).[21] When trifluoroacetic acid was
replaced by a strong Lewis acid, Sc(OTf)3 (30 mM), the yield
of the diol (1) is significantly increased to 51% (entry 4 in
Table 1, Figure S4 in the supporting information).[21,22]
Ultraviolet‐visible absorption spectroscopy was employed
to clarify the reaction mechanism and the detection of reactive
intermediate. NaClO2 exhibits an absorption band at 260 nm
in an aqueous solution as shown in Figure 1. The band
disappeared by the addition of Sc(OTf)3 (10 mM),[23,24]
mining step (vide infra). The bimolecular rate constant was
determined to be 0.16M−1 s−1 from the slope of the linear
line. ClO2 is stabilized with Sc3+ as a Sc3+ClO2 complex
in solution (vide infra).
•
•
•
No decay of absorbance at 358 nm due to ClO2 gene-
rated by NaClO2 with Sc(OTf)3 was observed in MeCN in
the absence of substrate at 298 K. The decay rates obeyed
pseudo–first‐order kinetics under the conditions in the
presence of excess of styrene (Figure 3a). The observed
pseudo–first‐order rate constant (kobs) for the dihydroxylation
increases linearly with increasing styrene concentration
(Figure 3b). The bimolecular rate constant for the con-
•
sumption of ClO2 with styrene was determined to be
1.9 × 10−2M−1 s−1 [27]
.
Electron paramagnetic resonance measurements were
•
performed to clarify the radical structure. The pure ClO2
was generated by the reflux of an MeCN solution
containing NaClO2 at 353 K for 1 hour, where a small
amount of ClO2 is generated at high temperature.[14] The
•
electron paramagnetic resonance spectrum was measured
after cooling down to 298 K, showing the characteristic
isotropic signal at g = 2.0151 ( 0.0002) with 4 hyperfine
lines due to the unpaired electron on the Cl nucleus
(I = 3/2 for 35Cl and 37Cl, which have similar magnetic
moments of 0.821 and 0.683, respectively) (Figure 4a).[28]
The g value was significantly changed by the addition of
CF3COOH (g = 2.0106) and Sc(OTf)3 (g = 2.0103)
(Figure 4b and c). The hyperfine coupling constant of
FIGURE
1
Ultraviolet‐visible absorption spectra of NaClO2 (5.0 mM)
•
ClO2 (a(Cl) = 16.26 G) was decreased in the presence of
taken at 0, 4, and 16 h after mixing with Sc(OTf)3 (10 mM) in an aqueous
solution at 298 K
CF3COOH (15.78 G) and Sc(OTf)3 (15.56 G).[29] This

OHKUBO ET AL.
3
FIGURE 2 A, UV‐visible absorption time profiles at 358 nm for formation of Sc3+ClO2• in the reaction of NaClO2 (5.0 mM) with Sc(OTf)3 (10 mM) in an
aqueous solution (0.20 M acetate buffer pH 2.9) at 298 K. B, Second‐order plot
FIGURE 3 A, UV‐visible absorption time profiles at 358 nm for the
consumption of Sc3+ClO2 in the presence of styrene (30‐90 mM) in an
•
MeCN/H2O (1:1 v/v) solution at 298 K. B, Plot of pseudo–first‐order
rate constant vs [styrene]
indicates that proton and Sc3+ strongly bind to ClO2 to
•
form H+ClO2 and Sc3+ClO2 as reactive intermediates for
the dihydroxylation of styrene.[30]
•
•
Density functional theoretical (DFT) calculations of
ClO2 , H+ClO2 , and Sc3+ClO2 were examined to predict
the reaction mechanism for the dihydroxylation as shown in
Figure 5. The structural optimization was performed at the
DFT CAM‐B3LYP/6‐311+G(d,p) level of theory. The bond
lengths of Cl–O double bonds are calculated to be 1.502 Å
for ClO2• (Figure 5a). The Cl–O bond length is elongated to
•
•
•
1.643 Å in the case of H+ClO2 (Figure 5b). Figure 5c
•
shows that the bond strength of Sc3+ClO2 is also signifi-
•
•
FIGURE 4 Electron paramagnetic resonance spectra: A, ClO2 generated
cantly weakened (Cl–O: 1.818 Å) compared with that of
by the reaction of NaClO2 (0.10 mM) at 353 K for 1 h; B, H+ClO2 gene-
•
• [31]
ClO2 .
The Cl–O bond cleavage may be favorable in
rated by the reaction of NaClO2 (0.10 mM) with CF3COOH (10 mM); C, Sc3
the presence of a substrate to produce ClO• as a strong
oxidant.
+ClO2 generated by the reaction of NaClO2 (0.10 mM) with Sc(OTf)3
•
(10 mM) in MeCN. T = 298 K
On the basis of the above‐mentioned results, the
dihydroxylation mechanism of styrene with the Sc3+ClO2
•
(Equation 4). The overall stoichiometry is given by Equation
5. ClO2 is activated by binding of acids such as H+ and
•
complex is shown in Scheme 1. The disproportionation reac-
tion of NaClO2 occurs in the presence of H+ or Sc3+ to form
Sc3+. In the case of H+, no Cl–O bond cleavage occurred
ClO− and ClO3 (Equation 2).[32] ClO− readily reacts with
−
on the basis of the DFT calculations (vide supra). The styrene
−
oxidation with H+ proceeds via addition of ClO2 to styrene
•
ClO2 and proton to generate Cl2O2 (Equation 3). Then,
Cl2O2 is reduced by ClO2− with Sc3+ to form the Sc3+ClO2
•
double bond. In contrast, the styrene dihydroxylation with
Sc3+ occurred via addition of ClO• and Sc3+O•, generated
complex as a reactive species for the hydroxylation of styrene

4
OHKUBO ET AL.
when NaClO2 was replaced by NaClO as an oxidant in the
reaction of styrene with Sc(OTf)3, the chlorinated product
(2) was selectively observed without formation of a
dihydroxylated product (entry 5 in Table 1, Figure S5 in the
supporting information).
Sc3þ
2ClO−2
ClO− þ ClO−3
(2)
(3)
ClO− þ ClO−2 þ 2Hþ
Cl2O2 þ ClO−2 þ 2Sc3þ
Cl2O2 þ H2O
2Sc3þClO2• þ Cl− (4)
4ClO−2 þ 2Hþ þ 2Sc3þ
2Sc3þClO2• þ H2O þ 2Cl−
(5)
In conclusion, the Sc3+ClO2 complex is produced
•
−
from ClO2 in the presence of Sc(OTf)3, which binds to
ClO2 . The binding of Sc3+ to ClO2 resulted in an effec-
•
•
−
tive dihydroxylation of styrene with ClO2 in the presence
of Sc(OTf)3. This study provides a unique dihydroxylation
pathway of olefin without toxic waste such as heavy
metal.
FIGURE 5 Density functional theoretical optimized structures with bond
lengths (Å) of the following: A, ClO2•; B, H+ClO2•; C, Sc3+ClO2•, calcu-
lated at the CAM‐B3LYP/6‐311+G(d,p) level of theory
CONFLICT OF INTEREST
by the homolytic cleavage of the Sc3+O–Cl bond in the Sc3
+ClO2 complex, to the styrene double bond as shown in
•
The authors declare no competing financial interests.
Scheme 1. Then, the scandium complex is hydrolyzed to give
the final diol product and Sc3+ClO• (Scheme 1). Sc3+ClO•
can be recycled by the oxidation with a large excess of
ACKNOWLEDGMENTS
This work was supported by Grants‐in‐Aid (no. 16H02268
to S.F. and nos. 16K13964, 26620154, and 26288037 to
K.O.) from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT) and SENTAN project
from JST, Japan (to S.F.).
ClO2− to form Sc3+ClO2 . ClO− is also recovered with
ClO2 as shown in Equation 2. The chlorinated product
2‐chloro‐1‐phenylethanol (2) as a side product is given by
the reaction of styrene, with ClO− as an intermediate. In fact,
−
•
SCHEME 1 Plausible catalytic cycle for hydroxylation of styrene

OHKUBO ET AL.
5
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SUPPORTING INFORMATION
Additional Supporting Information may be found online in
the supporting information tab for this article.
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How to cite this article: Ohkubo, K., Hirose, K.,
Shibata, T., Takamori, K., and Fukuzumi, S. (2016),
Dihydroxylation of styrene by sodium chlorite with
scandium triflate, J. Phys. Org. Chem., doi: 10.1002/
poc.3619
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• [4b]
[20] The reactions of transition metal ions with ClO2
.
[21] No styrene epoxide as an intermediate was formed during the reaction with
CF3COOD or Sc(OTf)3, confirmed by 1H nuclear magnetic resonance.
[22] Sc(OTf)3 and solvent are recyclable by abstraction from the reaction mixture.