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5 B. M. Stoltz, Chem. Lett., 2004, 33, 362; M. S. Sigman and D.
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the (S)-enantiomers in > 99% ee, respectively (Table 3, entries 8,
11 and 12). The OKR of 6,7,8,9-tetrahydro-5H-benzocyclohep-
ten-5-ol proceeded well and the unreacted alcohol was recovered
in good yields with high enantioselectivity under the same reac-
tion conditions (Table 3, entry 9). Surprisingly, several aliphatic
sec-alcohols, such as ( )-3,3-dimethyl-2-propanol or ( )-1-
cyclopropylethanol, resulted in low ee using the NBS protocol.7b
The reported oxidations of alcohols with NBS are usually
carried out using either anhydrous solvents or in acidic or basic
media at varied temperatures.11,12 In acidic media, NBS may
react with water to form positive bromine species HOBr.13
Based on NMR studies the behavior of NBS and D2O, only a
trace amount of succinimide was observed even in the presence
of KOAc.14 These findings indicate that the forming of HOBr
from NBS and water in aqueous media is very slow. It seems
that NBS directly acts as the oxidant in the OKR reaction instead
of HOBr in the present system.12 As a result, the Mn(III)-salen–
NBS–H2O system can be extended to the OKR of the ortho-sub-
stituted benzylic alcohols.
Conclusions
7 (a) W. Sun, H. W. Wang, C. G. Xia, J. W. Li and P. Q. Zhao, Angew.
Chem., Int. Ed., 2003, 42, 1042; (b) Z. Li, Z. H. Tang, X. X. Hu and C.
G. Xia, Chem.–Eur. J., 2005, 11, 1210; (c) W. Sun, X. M. Wu and C.
G. Xia, Helv. Chim. Acta, 2007, 90, 623; (d) Q. G. Cheng, F. G. Deng, C.
G. Xia and W. Sun, Tetrahedron: Asymmetry, 2008, 19, 2359.
8 (a) M. L. Kantam, T. Ramani, L. Chakrapani and B. M. Choudary, J.
Mol. Catal. A: Chem., 2007, 274, 11; (b) K. Pathak, I. Ahmad,
S. H. R. Abdi, R. I. Kureshy, N. H. Khan and R. V. Jasra, J. Mol. Catal.
A: Chem., 2007, 274, 120; (c) R. I. Kureshy, I. Ahmad, K. Pathak, N.
H. Khan, S. H. R. Abdi, J. K. Prathap and R. V. Jasra, Chirality, 2007,
19, 352; (d) F. Han, J. Zhao, Y. Zhang, W. Wang, Y. Zuo and J. An, Car-
bohydr. Res., 2008, 343, 1407; (e) S. Sahoo, P. Kumar, F. Lefebvre and S.
B. Halligudi, Tetrahedron Lett., 2008, 49, 4865.
9 M. K. Brown, M. M. Blewett, J. R. Colombe and E. J. Corey, J. Am.
Chem. Soc., 2010, 132, 11165.
10 krel = ln[(1 − conv)(1 − ee)]/ln[(1 − conv)(1 + ee)] where conv is the
conversion and ee is the enantiomeric excess. For a leading reference,
see: H. B. Kagan and J. C. Fiaud, Kinetic resolution, Top. Stereochem.,
1988, 18, 249.
In conclusion, this communication has demonstrated the viability
of enantioselective oxidative of racemic secondary alcohols cata-
lyzed by chiral Mn(III)-salen complexes using NBS as stoichio-
metric oxidant. The mild reaction conditions and
enantioselectivity of the catalyst system provide access to a range
of secondary alcohols including the ortho-substituted benzylic
alcohols in excellent enantioselectivity. Additionally, the utiliz-
ation of cheap, easily available NBS as oxidant makes the proto-
col more practical toward the synthesis of enantiomerically pure
secondary alcohols. Efforts are currently under way in our group
to further expand the scope and synthetic utility of the asym-
metric oxidation.
This work was funded by the Chinese Academy of Sciences
and the National Natural Science Foundation of China
(20873166, 21073210 and 21133011).
11 R. Filler, Chem. Rev., 1963, 63, 21.
12 N. S. Krishnaveni, K. Surendra and K. R. Rao, Adv. Synth. Catal., 2004,
346, 346.
13 (a) N. Venkatasubramanian and V. Thiagarajan, Tetrahedron Lett., 1968,
39, 3349; (b) N. Venkatasubramanian and V. Thiagarajan, Can. J. Chem.,
1969, 47, 694; (c) P. F. Kruse, K. L. Grist and T. A. McCoy, Anal. Chem.,
1954, 26, 1319.
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14 See the ESI for details†.
2732 | Org. Biomol. Chem., 2012, 10, 2730–2732
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