Enantioselective baeyer-villiger oxidation: Desymmetrization of meso cyclic ketones and kinetic resolution of racemic 2-arylcyclohexanones

Article abstract of DOI:10.1021/ja309262f

Catalytic enantioselective Baeyer-Villiger (BV) oxidations of racemic and meso cyclic ketones were achieved in the presence of chiral N,N'-dioxide-Sc ^III complex catalysts. The BV oxidations of prochiral cyclohexanones and cyclobutanones afforded series of optically active μ- and γ-lactones, respectively, in up to 99% yield and 95% ee. Meanwhile, the kinetic resolution of racemic 2-arylcyclohexanones was also realized via an abnormal BV oxidation. Enantioenriched 3-aryloxepan-2-ones, whose formation is counter to the migratory aptitude, were obtained preferentially. Both the lactones and the unreacted ketones were obtained with high ee values.

Full text of DOI:10.1021/ja309262f

Communication
pubs.acs.org/JACS
Enantioselective Baeyer−Villiger Oxidation: Desymmetrization of
Meso Cyclic Ketones and Kinetic Resolution of Racemic
2‑Arylcyclohexanones
Lin Zhou, Xiaohua Liu, Jie Ji, Yuheng Zhang, Xiaolei Hu, Lili Lin, and Xiaoming Feng*
Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu
610064, P. R. China
S
* Supporting Information
Scheme 1. Baeyer−Villiger (BV) Oxidation:
Desymmetrization and Kinetic Resolution
ABSTRACT: Catalytic enantioselective Baeyer−Villiger
(BV) oxidations of racemic and meso cyclic ketones were
achieved in the presence of chiral N,N′-dioxide−ScIII
complex catalysts. The BV oxidations of prochiral
cyclohexanones and cyclobutanones afforded series of
optically active ε- and γ-lactones, respectively, in up to 99%
yield and 95% ee. Meanwhile, the kinetic resolution of
racemic 2-arylcyclohexanones was also realized via an
abnormal BV oxidation. Enantioenriched 3-aryloxepan-2-
ones, whose formation is counter to the migratory
aptitude, were obtained preferentially. Both the lactones
and the unreacted ketones were obtained with high ee
values.
he Baeyer−Villiger (BV) oxidation1 is highly valuable for
ketones using molecular catalysts. The excellent catalytic
performance of chiral N,N′-dioxide−metal complex catalysts
is attributed to the appropriate adjustment of the stereo-
environment for lots of asymmetric transformations7a−d and
the efficient chiral recognition of enantiomers.7e Thus, we
expected that they could serve as good catalysts for asymmetric
BV oxidations. Herein we describe asymmetric BV oxidations of
prochiral cyclohexanones and cyclobutanones (desymmetriza-
tion process) as well as racemic cyclohexanones (kinetic
resolution process). In the presence of a chiral N,N′-dioxide−
ScIII complex catalyst, series of optically active ε- and γ-lactones
were obtained with excellent outcomes. Especially, the latter
reaction gave the “abnormal” lactone (AL) from a preferential
migration of a CH2 group with high enantioselectivity (Scheme
1b); such reactivity has not been reported hitherto.
In the initial study, 4-phenylcyclohexanone (1a) and m-
chloroperoxobenzoic acid (m-CPBA) were chosen as the model
substrate and oxidant,8 respectively. When the reaction was
performed in EtOAc at 0 °C without a catalyst, the racemic ε-
lactone 2a was isolated in 19% yield (Table 1, entry 1). Next,
various complex catalysts prepared in situ from a metal salt and
the chiral N,N′-dioxide L1 were surveyed. The complex
catalysts formed from L1 and Cu(OTf)2, Mg(OTf)2, and
Ni(OTf)2 afforded 2a in low yields (entries 2−4). A good yield
was obtained in the presence of the L1−In(OTf)3 complex but
with only 6% ee (entry 5). We were delighted to find that the
T
the synthesis of esters or lactones. Especially, the
asymmetric BV oxidation of racemic or prochiral cyclic ketones
provides a simple and attractive route for the synthesis of
optically active lactones.2 Impressive results have been achieved
for the BV oxidation of racemic cyclic ketones and 3-substituted
cyclobutanones with biocatalysts, chiral metal complexes, or
organocatalysts.3,4 As is often the case, ring-strained cyclo-
butanones are more reactive in BV oxidations than other cyclic
ketones, and they have been well-studied. For example, the BV
oxidation of meso-cyclobutanones was successfully realized by
the Ding group using a chiral Brønsted acid catalyst.4i Though
biocatalysts have shown excellent enantiocontrol in the BV
oxidation of 4-alkylcyclohexanones,5 the desymmetrization of
meso-cyclohexanones is less explored at present (Scheme 1a).
To date, only one nonenzymatic catalytic system has been
shown to afford the BV products in low yields or moderate ee
values.6
In regard to the kinetic resolution of racemic cyclic ketones
through BV oxidation, the stereochemistry is affected not only
by stereoelectronic control but also by chiral recognition. The
first two examples of BV oxidation of racemic cyclic ketones
were independently reported by the groups of Bolm3a and
Strukul.3b In general, the normal CHR-group-migrated product
[i.e., the “normal” lactone (NL)] distribution, which depends
on the migratory aptitude (tertiary > sencondary > primary),
was observed (Scheme 1b).1d,e
As far as we know, it is still difficult to perform a highly
sophisticated catalysis for both the desymmetrization of meso
cyclic ketones and the kinetic resolution of racemic cyclic
Received: September 18, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja309262f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
Table 1. Optimization of the Reaction Conditions
Table 2. Substrate Scope for the Desymmetrization of meso-
Cyclohexanones
a
b
c
entry
R
product
yield (%)
ee (%)
1
2
Ph
2a
2b
2c
2d
2e
2f
86
90
84
81
87
71
84
82
81
80
84
87
76
72
75
81
78
95
95
94
95
94
92
94
94
94
94
95
94
4-MeC6H4
3-MeC6H4
4-MeOC6H4
3-MeOC6H4
4-FC6H4
3-FC6H4
4-ClC6H4
3-ClC6H4
4-PhC6H4
1-naphthyl
2-naphthyl
Me
3
4
5
b
c
entry
L
metal
yield (%)
ee (%)
6
7
2g
2h
2i
1
2
3
4
5
6
7
8
−
−
19
42
39
42
91
84
91
86
86
0
0
8
L1
L1
L1
L1
L1
L2
L3
L3
Cu(OTf)2
Mg(OTf)2
Ni(OTf)2
In(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
9
0
10
11
12
13
14
15
16
2j
0
2k
2l
6
68
86
88
95
d
d
d
d
2m
2n
2o
2p
2a
84 (R)
85 (R)
92 (R)
94 (R)
93
Et
iPr
tBu
d
9
a
Unless otherwise noted, the reactions were performed with L/metal
e
(5 mol %) and 1a (0.10 mmol) in EtOAc (0.5 mL) at 35 °C for 30
17
Ph
b
a
min followed by addition of m-CPBA (0.12 mmol) at 0 °C. Isolated
Unless otherwise noted, the reactions were performed with L3/
c
d
yields. Determined by chiral HPLC. The reaction was performed at
Sc(OTf)3 (5 mol %) and 1 (0.1 mmol) in EtOAc (1.0 mL) at 35 °C
−20 °C in 2.0 mL of EtOAc.
for 30 min followed by addition of m-CPBA (0.12 mmol in 1.0 mL of
b
c
EtOAc) at −20 °C. Isolated yields. Determined by chiral HPLC and
d
L1−Sc(OTf)3 complex gave 2a in 84% yield with 68% ee
(entry 6). Next, the efficiency of other N,N′-dioxide ligands
with Sc(OTf)3 was explored. When ligand L2 containing
bulkier isopropyl groups at the ortho and para positions of the
aniline moiety was employed, 86% ee and 91% yield were
achieved (entry 7). As for the chiral backbone moiety, the
N,N′-dioxide L3 derived from (S)-ramipril exhibited a slight
superiority in enantiocontrol toward this reaction compared
with ligand L2 derived from (S)-pipecolic acid (entry 8 vs 7).
Performing the reaction at −20 °C and lowering the reaction
concentration benefited the enantioselectivity (entry 9).
chiral GC. The absolute configuration was confirmed by comparison
of the optical rotation with the literature value.5a 1a (5.0 mmol), m-
e
CPBA (6.0 mmol), 36 h.
methyl-substituted adipic acid 5 (Scheme 2), which is an
important intermediate in the synthesis of 6, an effective
inhibitor of acetylcholinesterase.9
Scheme 2. Synthesis of Adipic Acid 5
Under the optimal conditions (Table 1, entry 9), various
meso-cyclohexanones 1 afforded the corresponding ε-lactones
with excellent enantioselectivities. As shown in Table 2, the
enantiocontrol of the reaction was sensitive to neither the
electronic properties nor the steric hindrance of substituents on
the phenyl ring of 4-aryl-substituted cyclohexanones. Generally,
the desired chiral 5-aryl-substituted ε-lactones 2 were isolated
with excellent enantioselectivities (up to 95% ee) in good yields
(up to 90%) (Table 2, entries 1−10). Moreover, fused-ring-
substituted cyclohexanones 2k and 2l were also tolerated,
giving the desired products with 95% and 94% ee, respectively
(entries 11 and 12). 4-Alkyl-substituted cyclohexanones also
gave good enantioselectivities (84%−94% ee; entries 13−16).
The enantioselectivity increased gradually along with the
improvement of the steric hindrance of the alkyl group,
implying that the stable conformation of the prochiral
cyclohexanone could probably benefit the enantiocontrol.
Additionally, treatment of 5.0 mmol of 1a under the optimal
reaction conditions smoothly produced 2a in 78% yield (0.74
g) with 93% ee in the scaled-up version (entry 17).
Next, the use of this catalytic system for BV oxidation of a
variety of meso-cyclobutanones was explored, and the desired γ-
lactones were obtained in good yields (up to 99%) with good
enantioselectivities (up to 91% ee). As shown in Table 3, the
electronic nature of the substituents in cyclobutanones 3 had
nearly no effect on the efficiency and enantioselectivity of the
reaction (entries 1−6). For 3-alkyl-substituted substrates 3g
and 3h, the reaction using ligand L1 instead of L3 at −40 °C
generated the desired products in quantitative yields with 80%
ee (entries 7 and 8).
Another challenge to the catalytic enantioselective BV
oxidation is its application to the kinetic resolution of racemic
cyclic ketones. It was thus significant that the BV oxidation of
racemic 2-phenylcyclohexanone [( )-7a] catalyzed by the
chiral N,N′-dioxide−Sc(OTf)3 complex was exemplified. Only a
The synthetic importance of this desymmetrization was
exemplified by a transformation of one of the optically active ε-
lactone products. Product 2m was easily converted into β-
B
dx.doi.org/10.1021/ja309262f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Al(OiPr)3 as an additive, the conversion reached 50.0% (entry
2). In this case, an abnormal lactone distribution was also
observed, with the CH2 group migrating preferentially. The
abnormal/normal lactone ratio was AL-8a/NL-9a = 17/1. Both
AL-(R)-8a and unreacted (S)-7a were obtained with 94% ee
and a high s value (entry 2). It is premature to provide a clear
rationalization of the role of Al(OiPr)3 and the unprecedented
regioselectivity using this catalyst system. Al(OiPr)3 might
benefit the release of the product from the chiral catalyst and
then accelerate the catalytic cycle. The steric nature of the
catalyst has a critical effect on the migratory aptitude. Because
of the particular environment generated by the chiral N,N′-
dioxide−Sc(OTf)3 catalyst, the less-hindered CH2 group can
achieve the antiperiplanar migration more easily than the
bulkier CHR group, resulting in generation of AL-(R)-8a from
(R)-7a prior to the other isomer.
Table 3. Substrate Scope for the Desymmetrization of meso-
Cyclobutanones
a
The kinetic resolution of a series of racemic 2-aryl-
cyclohexanones 7 was then examined11 (see the SI for details
about the experimental procedure). As shown in Table 4,
compounds AL-8 were obtained as the major products.
Generally, lactones AL-(R)-8 and unreacted ketones (S)-7
were isolated with high conversion and s values and good to
excellent AL-(R)-8/NL-9 ratios (5.6/1 to >19/1) (entries 2−
7). The reaction efficiency and enantiocontrol were sensitive to
the electronic properties of the substituent on the phenyl group
of the substrate (entries 3−6). Substrates with an electron-
withdrawing substituent (Cl, Br) gave the unreacted ketone
(S)-7 and lactone AL-(R)-8 with higher ee values than those
with electron-donating ones (Me, OMe) (entries 3 and 4 vs 5
and 6). Moreover, 2-fused-ring-substituted ketones 7 were also
tolerated, giving the products and unreacted ketones with good
ee values (entries 8−10). Especially, 2-(naphthalen-1-yl)-
cyclohexanone [( )-7g] achieved the best results of the kinetic
resolution via BV oxidation, affording unreacted ketone (S)-7g
in 99% ee and AL-(R)-8g in 98% ee with 50.3% conversion. A
a
The procedure was similar to that in Table 2 except the reaction
b
c
temperature was −60 °C. Isolated yields. Determined by chiral
d
HPLC. The absolute configurations were confirmed by comparison
of the optical rotations with the literature values.4i Using ligand L1 (5
e
mol %) at −40 °C.
trace amount of product (R)-8a was observed under the
optimal conditions shown in Table 1, entry 9. It is worth
pointing out that this product was generated from the
migration of the less-substituted group, which is contrast to
the previous reports.3 Further prolonging the reaction time did
not improve the yield. Fortunately, the ee value of the abnormal
lactone AL-8a was excellent (Table 4, entry 1). To improve the
conversion of this reaction, the efficiency of additives was then
examined [see the Supporting Information (SI) for details].
Interestingly, when the reaction was performed with 20.0 mg of
a
Table 4. Substrate Scope for the Kinetic Resolution of Racemic 2-Aryl-Substituted Cyclohexanones
c
d
ee (%)
yield (%)
8 + 9
b
e
f
entry
7: R
7a: Ph
T (°C)
t (h)
conv. (%)
7
8
7
8/9
s
g
1
−20
−40
−40
−40
−40
−40
−40
−20
−20
−20
−20
18
36
41
41
43
48
48
27
15
27
48
8.3
50.0
51.1
51.3
49.4
47.7
50.0
50.3
52.3
57.6
47.9
9
99 (R)
94 (R)
93 (R)
92 (R)
91 (R)
90 (R)
90 (R)
98
−
46
47
47
51
44
44
49
45
39
53
trace
51
53
53
48
56
53
51
54
61
47
18/1
17/1
−
2
3
7a: Ph
94 (S)
97 (S)
97 (S)
89 (S)
82 (S)
90 (S)
99
115
113
102
114
48
7b: 4-ClC6H4
7c: 4-BrC6H4
7d: 4-MeC6H4
7e: 4-MeOC6H4
7f: 4-PhC6H4
7g: 1-naphthyl
7h: 2-naphthyl
7h: 2-naphthyl
7g: 1-naphthyl
>19/1
>19/1
12/1
h
4
5
6
5.6/1
16.5/1
>19/1
9/1
7
58
8
481
17
9
90
82
10
98
72
6.5/1
>19/1
27
i
11
91
99
624
a
Unless otherwise noted, the reactions were performed with L3/Sc(OTf)3 (5 mol %), 7 (0.1 mmol), and Al(OiPr)3 (20.0 mg) in EtOAc (1.0 mL),
b
c
d
to which m-CPBA (0.10 mmol in 1.0 mL of EtOAc) was added. Conv. = (ee7)/(ee7 + ee8). Determined by chiral HPLC. Isolated yields.
e
f
g
h
Determined by 1H NMR analysis. s = ln[(1 − conv.)(1 − ee7)]/ln[(1 − conv.)(1 + ee7)]. Reaction without Al(OiPr)3. The absolute
configuration of 8c was determined to be R by X-ray analysis10 (for details about the absolute configurations of the other compounds, see the SI).
i
Using 5.0 mmol of 7g as the substrate.
C
dx.doi.org/10.1021/ja309262f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
X.; Ding, K. Eur. J. Org. Chem. 2011, 110. (i) Schulz, F.; Leca, F.;
Hollmann, F.; Reetz, M. T. Beilstein J. Org. Chem. 2005, 1, 10.
(j) Bocola, M.; Schulz, F.; Leca, F.; Vogel, A.; Fraaije, M. W.; Reetz, M.
T. Adv. Synth. Catal. 2005, 347, 979. (k) Hollmann, F.; Taglieber, A.;
Schulz, F.; Reetz, M. T. Angew. Chem., Int. Ed. 2007, 46, 2903.
(l) Reetz, M. T.; Wu, S. Chem. Commun. 2008, 5499. (m) Reetz, M.
T.; Wu, S. J. Am. Chem. Soc. 2009, 131, 15424.
(4) For selected examples, see: (a) Bolm, C.; Luong, T. K. K.;
Schlingloff, G. Synlett 1997, 1151. (b) Sugimura, T.; Fujiwara, Y.; Tai,
A. Tetrahedron Lett. 1997, 38, 6019. (c) Shinohara, T.; Fujioka, S.;
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Cosp, A.; Palazzi, C. Synlett 2001, 1461. (e) Uchida, T.; Katsuki, T.
Tetrahedron Lett. 2001, 42, 6911. (f) Murahashi, S.-I.; Ono, S.; Imada,
Y. Angew. Chem., Int. Ed. 2002, 41, 2366. (g) Bolm, C.; Frison, J.-C.;
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X.; Zhang, X.; Ding, K. Angew. Chem., Int. Ed. 2008, 47, 2840. (j) Xu,
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(5) (a) Taschner, M. J.; Black, D. J.; Chen, Q. Z. Tetrahedron:
Asymmetry 1993, 4, 1387. (b) Stewart, J. D.; Reed, K. W.; Martinez, C.
A.; Zhu, J.; Chen, G.; Kayser, M. M. J. Am. Chem. Soc. 1998, 120, 3541.
(c) Mihovilovic, M. D.; Chen, G.; Wang, S.; Kyte, B.; Rochon, F.;
Kayser, M. M.; Stewart, J. D. J. Org. Chem. 2001, 66, 733. (d) Rial, D.
V.; Bianchi, D. A.; Kapitanova, P.; Lengar, A.; van Beilen, J. B.;
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Brunner, B.; Schneider, T.; Schulz, F.; Clouthier, C. M.; Kayser, M. M.
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(6) (a) Paneghetti, C.; Gavagnin, R.; Pinna, F.; Strukul, G.
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sufficiently high s value (up to 481) and 8g/9g ratio (>19/1)
were obtained (entry 8). Furthermore, treatment of 5.0 mmol
of ( )-7g smoothly afforded the product 8g in 47% yield
(0.564 g) with 99% ee, and the unreacted ketone was recovered
in 53% yield (0.593 g) with 91% ee (entry 11).
In summary, we have successfully developed highly
enantioselective BV oxidations of both meso and racemic
cyclic ketones using chiral N,N′-dioxide−ScIII complex catalysts
in a common solvent (ethyl acetate). The desymmetrization of
both prochiral cyclohexanones and cyclobutanones afforded the
corresponding lactones in up to 99% yield with up to 95% ee.
Moreover, unlike the system described previously, kinetic
resolution of racemic 2-arylcyclohexanones was realized via an
abnormal BV oxidation, giving ε-lactones with a reversal of
migratory aptitude and high levels of enantioselection. To the
best of our knowledge, there is no precedent for the
asymmetric BV oxidation with an unusually broad array of
cyclic ketones. This work also represents the first example of
asymmetric BV oxidation catalyzed by rare-earth metal
complexes. Additional research on the mechanism of these
processes, extension of the methodology to inert cyclic
ketones,11 and the switch of selectivity for normal and
abnormal lactones is underway in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details and analytical data (NMR, HPLC and HR-
ESI-MS). This material is available free of charge via the
Internet at http://pubs.acs.org.
(7) (a) Liu, X. H.; Lin, L. L.; Feng, X. M. Acc. Chem. Res. 2011, 44,
574. (b) Shen, K.; Liu, X. H.; Lin, L. L.; Feng, X. M. Chem. Sci. 2012,
3, 327. (c) Feng, X. M.; Liu, X. H. In Scandium: Compounds,
Productions and Applications; Greene, V. A., Ed.; Nova Science: New
York, 2011; p 1. (d) Huang, S.; Ding, K. Angew. Chem., Int. Ed. 2011,
50, 7734. (e) He, X.; Zhang, Q.; Liu, X. H.; Lin, L. L.; Feng, X. M.
Chem. Commun. 2011, 47, 11641.
(8) When H2O2 or TBHP was used as the oxidant, no BV product
was observed.
(9) Pisoni, D. d. S.; da Costa, J. S.; Gamba, D.; Petzhold, C. L.;
AUTHOR INFORMATION
Corresponding Author
xmfeng@scu.edu.cn
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(21021001 and 21172151), the Ministry of Education
(20110181130014), and the National Basic Research Program
of China (973 Program; 2010CB833300) for financial support.
Borges, A. C. d. A.; Ceschi, M. A.; Lunardi, P.; Gonca
̧ lves, C. A. S. Eur.
J. Med. Chem. 2010, 45, 526.
(10) CCDC 897036 (8c) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
(11) Under the optimal reaction conditions, only a trace amount of
BV product was observed for 2-phenylcyclopentanone, and no BV
products were observed for 2-alkylcyclohexanones and 2-phenyl-
cycloheptanone.
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dx.doi.org/10.1021/ja309262f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX