Asymmetric oxidation of 1,2-diols using N-bromosuccinimide in the presence of chiral copper catalyst

Article abstract of DOI:10.1016/j.tetlet.2007.10.014

Asymmetric oxidation of 1,2-diols using N-bromosuccinimide (NBS) in the presence of copper(II) triflate and (R,R)-Ph-BOX has been exploited. This oxidation was applicable to asymmetric desymmetrization of meso-hydrobenzoin and kinetic resolution of dl-hydrobenzoin and racemic-cycloalkane-cis-1,2-diols to afford optically active α-ketoalcohols with good to high enantiomeric excess.

Full text of DOI:10.1016/j.tetlet.2007.10.014

Available online at www.sciencedirect.com
Tetrahedron Letters 48 (2007) 8668–8672
Asymmetric oxidation of 1,2-diols using N-bromosuccinimide
in the presence of chiral copper catalyst
Osamu Onomura,* Hitomi Arimoto, Yoshihiro Matsumura and Yosuke Demizu
Graduate School of Biomedical Sciences, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan
Received 10 September 2007; revised 29 September 2007; accepted 4 October 2007
Available online 6 October 2007
Dedicated to the memory of the late Professor Yoshihiko Ito
Abstract—Asymmetric oxidation of 1,2-diols using N-bromosuccinimide (NBS) in the presence of copper(II) triflate and (R,R)-Ph-
BOX has been exploited. This oxidation was applicable to asymmetric desymmetrization of meso-hydrobenzoin and kinetic resolution
of dl-hydrobenzoin and racemic-cycloalkane-cis-1,2-diols to afford optically active a-ketoalcohols with good to high enantiomeric
excess.
ꢀ 2007 Elsevier Ltd. All rights reserved.
The oxidation of a hydroxyl group into a carbonyl
group is a basic and important organic reaction.1 Selec-
tive oxidation of 1,2-diols to the corresponding
a-ketoalcohols was reported in 1974 by utilizing a stoi-
chiometric amount of dibutyltinoxide (Bu2SnO) which
forms dibutylstannylenes followed by brominolysis,2
and the method has been applied to fine chemistry as
exemplified by the synthesis of (+)-spectinomycin3 and
the oxidation of unprotected sugars.4,5 From the stand-
point of green chemistry, we have recently reported an
efficient oxidation of 1,2-diols 1 by electrochemical
method using a catalytic amount of Bu2SnO and bro-
mide ion to afford a-ketoalcohols 2 in high yield without
1,2-diketones 3 (Eq. 1).6 Also, chemical oxidation of 1
using N-bromosuccinimide (NBS) (1 equiv) and Bu2SnO
(0.1 equiv) in the presence of K2CO3 (1 equiv) proceeded
to afford 2.7 To the best of our knowledge, catalytic
asymmetric oxidation8 of 1 to 2 has not been known
except for two examples using semi-catalytic amount
of chiral dioxiranes9 or chiral hypervalent iodine.10
-2e
R
R
OH
OH
R
R
OH
O
R
R
O
O
Bu2SnO (0.05 equiv)
+
Et4NBr (0.2 equiv)
MeOH
3
1
2
up to 96% yield
~0%
ð1Þ
We wish to report herein a catalytic asymmetric oxida-
tion of (meso or dl)-1,2-diols meso- or dl-1, or cis-1,2-
diols 4 to afford the corresponding optically active
a-ketoalcohols chiral-2 or chiral-6 in good to high yield
and enantioselectivity, which is based on the recognition
R2
O
O
R2
*
R2
H
O
R1
R1
OH
OH
R1
R1
OH
O
Cu2+
L*
R1
R1
NBS
N
N
Cu2+ L*
K2CO3
Ph
Ph
ð2Þ
O
H
2+
-
L*
Cu
meso-1 (R2=H),
dl-1 (R2=H), or
L* (R,R)-Ph-BOX
chiral-2 (R2=H)
chiral-6 (R2=alkyl etc)
5
cis- 4 (R2=alkyl etc)
Keywords: Asymmetric oxidation; vic-Diol; a-Ketoalcohol; Copper complex.
*
Corresponding author. Tel.: +81 95 819 2429; fax: +81 95 819 2476; e-mail: onomura@nagasaki-u.ac.jp
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.10.014

O. Onomura et al. / Tetrahedron Letters 48 (2007) 8668–8672
8669
of the diol-moiety by a copper(II) ion associated with
(R,R)-Ph-BOX complex11,12 to form the activated inter-
mediates 5 followed by oxidation with NBS13 as an oxi-
dant (Eq. 2).
Next, we investigated the effect of solvents and bases so
as to optimize reaction conditions for the asymmetric
oxidation of meso-1a (Eq. 5).15 The results are summa-
rized in Table 1. CHCl3 is the best solvent for the reac-
tion in terms of enantiomeric excess (entry 1). CH2Cl2,
THF, CH3CN and AcOEt give high yield of product
(R)-2a although the enantioselectivity is very low or
sometime racemic mixture (entries 2–5). MeOH gives
very low yield with moderate enantioselectivity (entry
6). In the case of bases, K2CO3 emerged as the best base
especially when used in combination with CHCl3 (entry
1). Na2CO3 and NaHCO3 give comparable results to that
of K2CO3 (entries 8 and 9). Other bases fall short in
terms of yield or enantioselectivity (entries 7, 10 and 11).
We began by trying an oxidation of meso-hydrobenzoin
(meso-1a) using NBS as an oxidant to see whether meso-
1a was recognized by the Cu(II)–(R,R)-Ph-BOX com-
plex under the above stated oxidation condition or
not. The oxidation of meso-1a in the presence of
Cu(OTf)2 and (R,R)-Ph-BOX predominantly afforded
mono-oxidized product 2a (83% yield) along with a
small amount of di-oxidized product 3a (17% yield),
while there was almost no oxidation in the absence of
Cu(OTf)2 and (R,R)-Ph-BOX (Eq. 3). These results sug-
gested that meso-1a is recognized by the Cu(II)–(R,R)-
Ph-BOX complex under these oxidation conditions.
Acceleration of the oxidation was also observed in the
presence of Cu(OTf)2 without (R,R)-Ph-BOX to afford
2a in high yield (91%).14
Utilizing the conditions optimized in Table 1, we
screened other halogen compounds as oxidants in this
reaction (Eq. 6). The results are shown in Table 2. In
addition to NBS, N-bromophthalimide (entry 4) was
usable for asymmetric oxidation, while other oxidants
NBS (2.0 equiv)
Ph
Ph
OH
OH
Ph
Ph
OH
O
Ph
Ph
O
O
K2CO3 (2.0 equiv)
+
CHCl3, rt, 3 h
3a
meso-1a
2a
ð3Þ
in the presence of Cu(OTf)2 (0.1 equiv) and
(R,R)-Ph-BOX (0.1 equiv)
83%
17%
9%
91%
4%
in the presence of Cu(OTf)2 (0.1 equiv)
in the absence of Cu(OTf)2 and
(R,R)-Ph-BOX
0%
Then, we tried competitive reaction between diol meso-
1a and monool 7 (Eq. 4). In the absence of Cu(OTf)2
and (R,R)-Ph-BOX, meso-1a and 7 were oxidized to 2a
and 8 with almost same ratio. On the other hand, in
the presence of Cu(II)–(R,R)-Ph-BOX, 2a was predom-
inantly obtained. This result indicates that meso-1,2-diol
was more preferentially recognized with the Cu(II)–
(R,R)-Ph-BOX catalyst than monool.
(entries 1–3) were less effective. The use of 1.5 equiv of
NBS or N-bromophthalimide gave (R)-2a in high yield
and moderate enantioselectivity, respectively (entries 6
and 8). Using 1.5 equiv of NBS, 0.05 or 0.2 equiv of
Cu(OTf)2 and (R,R)-Ph-BOX afforded almost similar
results to that using 0.1 equiv of chiral Cu(II) catalyst
(entries 9 and 10). Whereas using 0.01 equiv of chiral
Cu(II) catalyst slightly reduced the enantioselectivity
NBS (0.125 mmol)
Ph
Ph
OH
OH
Ph
Ph
Ph
Ph
Ph
Ph
OH
O
K2CO3 (0.125 mmol)
+
+
CHCl3 (5 mL)
rt, 3 h
OH
O
meso-1a
7
2a
8
ð4Þ
ð5Þ
ratio
:
0.25 mmol
0.25 mmol
2a
8
in the presence of
Cu(OTf)2 (0.025 mmol), (
:
:
84
57
16
43
R,
R)-Ph-BOX (0.025 mmol)
in the absence of Cu(OTf)2 and (R,R)-Ph-BOX
Ph
OH
Ph
Ph
OH
O
Ph
Ph
O
O
NBS (2.0 equiv)
+
Cu(OTf)2 (0.1 equiv)
OH
Ph
(R,R)-Ph-BOX (0.1 equiv)
base (2.0 equiv)
solvent, rt, 3 h
meso-1a
(R
)-2a
3a

8670
Table 1. Asymmetric oxidation of meso-hydrobenzoin (meso-1a)a
Entry Solvent Base (R)-2a 3a
Yield (%) eeb (%) Yield (%)
O. Onomura et al. / Tetrahedron Letters 48 (2007) 8668–8672
on the yields and the enantioselectivities (entries 13 and
14).
Then, we applied this methodology to the kinetic resolu-
tion of cis-cyclohexane-1,2-diol derivative 4ap (Eq. 7).
Compound 4ap was enantioselectively oxidized with
NBS and the Cu(II)–(R,R)-Ph-BOX complex to afford
a-ketoalcohol (S)-6ap17 with moderate yield (30%) and
selectivity (s) value of 14.19
1
2
3
4
5
6
7
8
CHCl3
CH2Cl2 K2CO3
THF K2CO3
CH3CN K2CO3
K2CO3
83
92
87
88
87
9
88
96
91
79
72
26
29
0
23
46
59
66
67
29
15
17
8
13
12
13
25
12
4
9
21
3
AcOEt
MeOH
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
K2CO3
K2CO3
Li2CO3
Na2CO3
NaHCO3
KOH
Asymmetric oxidation of other cycloalkane-cis-1,2-diols
4bp-at is summarized in Table 3 (Eq. 8).20 The chemical
yield of 6bp-dp and s value varied significantly depend-
ing on the ring size. That is, the larger the ring size,
the better the yield and s value obtained (entries 1–3).
R substituent also influenced the s value (entries 4–7).
Compound 4at with a cyclohexyl substituent was asym-
metrically oxidized to afford 6at in higher enantioselec-
9
10
11
2,6-Lutidine 97
a meso-1a (0.5 mmol), Cu(OTf)2 (0.05 mmol), (R,R)-Ph-BOX (0.05
mmol), NBS (1.0 mmol), base (1.0 mmol) in a solvent (5.0 mL) at rt
for 3 h.
b Determined by HPLC.
Oxidant (1-2 equiv)
Ph
Ph
OH
OH
Ph
OH
O
Ph
Ph
O
O
Cu(OTf)2 (0.01-0.20 equiv)
+
(R,R)-Ph-BOX (0.01-0.20 equiv)
K2CO3 (1-2 equiv)
CHCl3, rt, 3 h
ð6Þ
Ph
(
3a
meso-1a
R)-2a
(entry 11), use of the same amount of Cu(OTf)2 and
slightly excess amount of (R,R)-Ph-BOX was effective
(entry 12). In the case of using 0.1 equiv of Cu(OTf)2,
varying the amounts of (R,R)-Ph-BOX showed no effect
tivity (85% ee, entry 7) than 6aq with a methyl
substituent (5% ee, entry 4), 6ar with an isopropyl sub-
stituent (74% ee, entry 5) and 6as with a benzyl substitu-
ent (48% ee, entry 6).
NBS (0.5 equiv)
Ph OH
Ph OH
Ph OH
OH
OH
O
Cu(OTf)2 (0.1 equiv)
(R,R)-Ph-BOX (0.1 equiv)
+
K2CO3 (0.5 equiv)
CHCl3, rt, 3 h
ð7Þ
s
= 14
(
S
)-6ap
(R,R)-4ap
70% yield,
28% ee
4ap
30% yield,
82% ee
Table 2. Oxidation of meso-1a by some oxidantsa
Entry
Oxidant
Equiv of oxidant
Equiv of Cu(OTf)2
Equiv of (R,R)-Ph-BOX
(R)-2a
Yield (%)
3a
eeb (%)
Yield (%)
1
2
3
4
5
6
7
8
NCS
NIS
Br2
2.0
2.0
2.0
2.0
1.0
1.5
1.0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.2
0.05
0.01
0.01
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
26
88
59
72
63
94
91
97
96
94
96
94
93
95
6
9
2
10
5
28
2
6
0
3
4
6
4
6
7
43
71
67
70
69
76
69
68
63
69
70
69
N-Bromophthalimide
NBS
NBS
N-Bromophthalimide
N-Bromophthalimide
NBS
NBS
NBS
NBS
NBS
NBS
9
0.2
10
11
12
13
14
0.05
0.01
0.012
0.12
0.2
2
a meso-1a (0.5 mmol), oxidant (0.5–1.0 mmol), Cu(OTf)2 (0.005–0.1 mmol), (R,R)-Ph-BOX (0.005–0.1 mmol), K2CO3 (1.0 equiv to oxidant) in
CHCl3 (5.0 mL) at rt for 3 h.
b Determined by HPLC.

O. Onomura et al. / Tetrahedron Letters 48 (2007) 8668–8672
Table 3. Asymmetric oxidation of cis-1,2-diols (4bp-at)a
8671
Entry
n
R
a-Ketoalcohol
Recovered diol
s
Yield (%)
eeb (%)
Yield (%)
eeb (%)
1
2
3
4
5
6
7
4bp
4cp
4dp
4aq
4ar
4as
4at
1
3
4
2
2
2
2
Ph
Ph
Ph
Me
iPr
Bn
Cye
6bp
6cp
6dp
6aq
6ar
6as
6at
23
35
42
32
30
33
29
76
84
82
5
74
48
85
77
65
58
68
70
67
67
24
30
58
9
15
18
—
10
4
NDc
45d
32
48d
19
a 4bp-at (0.5 mmol), Cu(OTf)2 (0.05 mmol), (R,R)-Ph-BOX (0.05 mmol), NBS (0.25 mmol), K2CO3 (0.25 mmol) in CHCl3 (5.0 mL) at rt for 3 h.
b Determined by HPLC using chiral columns: Daicel Chiralcel OJ-H for 4bp, 4cp, 4as, 6bp, 6cp, 6dp; Chiralpak AS for 4dp, 4ar,d 6ar; Chiralpak AD
for 4at,d 6as, 6at; Chiralcel OC for 6ap.
c Not determined.
d Ee of the corresponding 2-phenylcarbamoylated compound.
e Cyclohexyl.
OH
OH
OH
NBS (0.5 equiv)
R
R
R
R
Cu(OTf)2 (0.1 equiv)
OH
O
OH
+
(R,R)-Ph-BOX (0.1 equiv)
K2CO3 (0.5 equiv)
CHCl3, rt, 3 h
ð8Þ
n
n
n
4bp-at
(
S
)-6bp-at
(
,
R)-4bp-at
This method was then applied to the kinetic resolution
of dl-hydrobenzoin (dl-1a), where (S)-benzoin ((S)-2a)
was obtained with 43% yield and 73% ee (Eq. 9).
Although the activated intermediate (R,R)-5a might be
formed more easily than (S,S)-5a, Br+ predominantly
NBS (0.5 equiv)
Ph
Ph
OH
OH
Ph
Ph
OH
O
Ph
Ph
OH
OH
Cu(OTf)2 (0.1 equiv)
+
(
R,R)-Ph-BOX (0.1 equiv)
K2CO3 (0.5 equiv)
CHCl3, rt, 3 h
ð9Þ
dl-1a
(
S
)-2a
(R,R)-1a
43% yield,
73% ee
55% yield,
60% ee
s
= 12
Scheme 1 shows our proposed mechanism for asymmet-
ric oxidation of meso-1a catalyzed by Cu(II)–(R,R)-Ph-
BOX. Possibly, Br+ approaches the less crowded
alkoxide OA compared with OB of the activated interme-
diate meso-5a which is generated from 1a with Cu(II)–
(R,R)-Ph-BOX, to afford (R)-2a.
Br+
O
N
OH
O
N
S
O
O
S
S
O
Scheme 2 shows our proposed mechanism for kinetic
resolution of dl-1a catalyzed by Cu(II)–(R,R)-Ph-BOX.
Br+
Br+
5a
2a
(S)-
(S,S)-
Br+
O
Br+
Br+
O
OH
O
N
O
N
R
OH
O
O
N
O
N
O
R
Cu
S
OB
Br+
Br+
OA
R
R
R
5a
(R,R)-
2a
(R)-
Br+
meso-5a
(R)-2a
Scheme 1. Plausible stereochemical course for desymmetrization of
meso-1a.
Scheme 2. Plausible stereochemical course for kinetic resolution of dl-
1a.

8672
O. Onomura et al. / Tetrahedron Letters 48 (2007) 8668–8672
10. Ladziata, U.; Carlson, J.; Zhdankin, V. V. Tetrahedron
Lett. 2006, 47, 6301–6304.
approaches the less crowded intermediate (S,S)-5a to
afford (S)-2a.
11. Some of asymmetric reactions catalyzed with Cu(II)–Ph-
BOX reported by us: Monobenzoylation of 1,2-diols: (a)
Matsumura, Y.; Maki, T.; Murakami, S.; Onomura, O. J.
Am. Chem. Soc. 2003, 125, 2052–2053; Monocarbamoy-
lation of 1,2-diols: (b) Matsumoto, K.; Mitsuda, M.;
Ushijima, N.; Demizu, Y.; Onomura, O.; Matsumura, Y.
Tetrahedron Lett. 2006, 47, 8453–8456; Monosulfonyl-
ation of 1,2-diols: (c) Demizu, Y.; Matsumoto, K.;
Onomura, O.; Matsumura, Y. Tetrahedron Lett. 2007,
48, 7605–7609; Benzoylation of 2-aminoalcohols: (d)
Mitsuda, M.; Tanaka, T.; Tanaka, T.; Demizu, Y.;
Onomura, O.; Matsumura, Y. Tetrahedron Lett. 2006,
47, 8073–8077; Alkylation into iminium ions: (e) Mat-
sumura, Y.; Minato, D.; Onomura, O. J. Organomet.
Chem. 2007, 692, 654–663.
The results presented in this Letter are novel for asym-
metric oxidation of 1,2-diols to afford enantiomerically
enriched a-ketoalcohols. Its synthetic application and
mechanistic study are underway.
Acknowledgement
This study was supported by a Grant-in-Aid for Scien-
tific Research (B) (No. 17350051) from Japan Society
for the Promotion of Science.
12. A recent review of chiral bis(oxazoline) ligands: Desimoni,
G.; Faita, G.; Jørgensen, K. A. Chem. Rev. 2006, 106,
3561–3651.
13. Recently, oxidation of 1,2-diols to 1,2-diketones using
NBS as an oxidant has been reported: Khurana, J. M.;
Kandpal, B. M. Tetrahedron Lett. 2003, 44, 4909–4912.
14. The activated intermediates 5 are transformed by K2CO3
to the copper(II) alkoxide,11a which might be more easily
oxidized than the corresponding diols.
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15. A typical procedure for asymmetric oxidation: Under an
aerobic atmosphere, a solution of Cu(OTf)2 (18.1 mg,
0.05 mmol) and (R,R)-Ph-BOX (16.7 mg, 0.05 mmol) in
CHCl3 (5 mL) was stirred for 10 min. Into the solution
were added 1a (0.5 mmol), potassium carbonate (138 mg,
1.0 mmol) and NBS (178 mg, 1.0 mmol). After stirring for
3 h at rt, the solution was poured in 10% aqueous Na2S2O3
and extracted with AcOEt (20 mL · 3). The combined
organic layer was dried over MgSO4 and the solvent was
removed under reduced pressure. The residue was purified
by silica gel column chromatography (n-hexane/
AcOEt = 15:1) to afford (R)-2a (83% yield, 72% ee) as a
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26
white solid. Mp 135–137 ꢁC. ½aꢀD ꢁ77.7 (c 1.5, acetone)
22
[lit.16 (S)-2a (>98% ee); ½aꢀD +111.9 (c 1.5, acetone)]. The
7. Reaction conditions: in CHCl3, at rt, for 3 h; from meso-
hydrobenzoin (1a) to benzoin (2a) in 65% and benzil (3a)
in 18% yield.
optical purity of (R)-2a was determined by chiral HPLC:
Daicel Chiralcel OJ-H column (4.6 mm B, 250 mm),
n-hexane/isopropanol = 10:1, wavelength: 210 nm, flow
rate: 1.0 ml/min, retention time: 20.5 min ((R)-(+)-2a),
24.5 min ((S)-(ꢁ)-2a).
8. Representative literatures for catalytic asymmetric oxida-
tion of 1,3- and/or 1,4-diols: (a) Ferreira, E. M.; Stoltz, B.
M. J. Am. Chem. Soc. 2001, 123, 7725–7726; (b) Tanaka,
H.; Kawakami, Y.; Goto, K.; Kuroboshi, M. Tetrahedron
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Commun. 2003, 114–115; (d) Mandal, S. K.; Sigman, M. S.
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K.; Matsuo, Y.; Hiroi, K. Tetrahedron Lett. 2003, 44,
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Katsuki, T. J. Am. Chem. Soc. 2005, 127, 5396–5413; (g)
Nakamura, Y.; Egami, H.; Matsumoto, K.; Uchida, T.;
Katsuki, T. Tetrahedron 2007, 63, 6383–6387.
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comparing with specific rotation of authentic sample:
¨
Demir, A. S.; Sßeßsenoglu, O. Org. Lett. 2003, 5, 2047–
2050.
17. The absolute stereoconfiguration of recovered (R,R)-4ap
was determined by comparing with specific rotation of
25
authentic sample. Compound (R,R)-4ap: ½aꢀD ꢁ3.5 (c 1.2,
25
EtOH). [lit.18 (R,R)-4ap (>99% ee); ½aꢀD ꢁ7.1 (c 1.2, 95%
EtOH)].
18. Huang, J.; Corey, E. J. Org. Lett. 2003, 5, 3455–3458.
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Eliel, E. L., Ed.; Wiley & Sons: New York, 1988; Vol. 18,
pp 249–330.
20. Absolute stereoconfiguration of 6bp-at shown in Eq. 8 and
Table 3 was deduced on the basis of that of 6ap.
9. (a) Jakka, K.; Zhao, C.-G. Org. Lett. 2006, 8, 3013–3015;
Stoichiometric method: (b) D’Accolti, L.; Detomaso, A.;
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