Charge-transfer effect on chiral phosphoric acid catalyzed asymmetric Baeyer-Villiger oxidation of 3-substituted cyclobutanones using 30% aqueous H2O2 as the oxidant

Article abstract of DOI:10.1002/cjoc.201090292

The intermolecular charge-transfer effect has been employed for the first time as a modulating approach to affect the enantioselectivity in asymmetric catalysis by taking the chiral phosphoric acid catalyzed asymmetric Baeyer-Villiger oxidation of 3-aryl

Full text of DOI:10.1002/cjoc.201090292

FULL PAPER
Charge-Transfer Effect on Chiral Phosphoric Acid Catalyzed
Asymmetric Baeyer-Villiger Oxidation of 3-Substituted
Cyclobutanones Using 30% Aqueous H2O2 as the Oxidant†
Xu, Senmiaoa(徐森苗)
Wang, Zhenga(王正)
Zhang, Xumua,b(张绪穆)
Ding, Kuiling*,a(丁奎岭)
a State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, Shanghai 200032, China
bDepartment of Chemistry and Chemical Biology, Center of Molecular Catalysis, Rutgers, The State University of
New Jersey, 610 Taylor, Piscataway, New Jersey 08854, USA
The intermolecular charge-transfer effect has been employed for the first time as a modulating approach to af-
fect the enantioselectivity in asymmetric catalysis by taking the chiral phosphoric acid catalyzed asymmetric
Baeyer-Villiger oxidation of 3-aryl cyclobutanones as the reaction prototype. It was found that the electron acceptor
additives were able to effectively tune the enantioselectivity via donor-acceptor interaction with the catalyst and up
to 9% enhancement of ee value was observed in a favorable case.
Keywords asymmetric catalysis, Baeyer-Villiger oxidation, Bronsted acid, charge-transfer effect, hydrogen per-
oxide, organocatalysis, phosphoric acid
Introduction
we will report our results of investigations on the first
use of charge-transfer effect5 as a modulating approach
to affect the enantioselectivity in asymmetric catalysis
by taking asymmetric Baeyer-Villiger oxidation of
3-aryl cyclobutanones catalyzed by chiral phosphoric
acids as the reaction prototype.
In our previous report1 on the asymmetric Baeyer-
Villiger oxidation2 of 3-substituted cyclobutanones (2)
with 30% aqueous H2O2 catalyzed by the BINOL
(1,1'-binaphthyl-2,2'-diol)-derived chiral phosphoric
acids (1) (Scheme 1), it was found that the 3,3'-substi-
tuents (Ar group) of the catalyst plays a key role in en-
antioselectivity control.3 A prominent feature of the
catalyst with high asymmetric induction is that the
backbone of chiral phosphoric acid bears fused conju-
gated aromatics such as pyrenyl groups, which are rigid,
planar, and electron-rich in nature. On the other hand,
some BINOL derivatives have also been reported to
function as electron donor units to form intermolecular
charge-transfer complexes with various electron accep-
tors.4 However, the use of such kind of effect for tuning
the enantioselectivity in asymmtric catalysis has never
been reported to the best our knowledge. Inspired by
these facts, we envisaged that an appropriate elec-
tron-deficient molecule might influence the mi-
cro-environment of the catalyst through intermolecular
charge-transfer interaction with the binaphthyl back-
bone of catalyst and its electron rich Ar groups at
3,3'-positions, and as a result, to further fine-tune the
enantioselectivity of the catalysis. In the present work,
Scheme 1 BINOL-derived chiral phosphoric acid-catalyzed
enantioselective B-V reaction of 3-substituted cyclobutanones
Results and discussion
It has been observed in our previous report that
chiral phosphoric acid 1a featuring electron-rich pyren-
1-yl groups at the 3,3'-positions of the backbone cata-
*
E-mail: kding@mail.sioc.ac.cn
Received August 9, 2010; revised and accepted August 17, 2010.
Project supported by the National Natural Science Foundation of China (Nos. 20821002, 20923005, 20972176), the Chinese Academy of Sciences,
the Major Basic Research Development Program of China (No. 2010CB833300) and the Science and Technology Commission of Shanghai Munici-
pality.
† Dedicated to the 60th Anniversary of Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences.
Chin. J. Chem. 2010, 28, 1731— 1735
© 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Xu et al.
FULL PAPER
lyzed the BV oxidation of 2a in up to 71% ee at room
temperature.1 The investigation of impact of steric and
electronic properties of the 3,3'-substituents and the ef-
fect of the backbone of phosphoric acid scaffold on the
enantioselectivities of the reaction shows that fine tun-
ing of the chiral environment around phosphoric acid is
critical for the enantioselectivity of the reaction.1,2 We
speculated that in the case of chiral phosphoric acid
catalysts bearing electron-rich aromatic motifs on their
backbones (e.g. pyren-1-yl), charge-transfer complex
formation by self-assembly with an electron-deficient
molecule (such as tetracyanobenzene) would result in a
subtle change in the chiral environment nearby the reac-
tive site, and hence may lead to a variation in the enan-
tioselectivity of the catalysis (Figure 1). On the basis of
working hypothesis shown in Figure 1, we decided to
employ a variety of electron acceptor molecules (A1—
A7, Figure 2) as additives for examining their impact on
the phosphoric acid 1a catalyzed B-V reaction of 2a
with 30% H2O2 as the terminal oxidant at room tem-
perature.
an orange or dark green solution was obtained, respec-
tively, when catalyst 1a and electron acceptor additive
A1 or A7 were mixed in chloroform (Figure 2). This
clearly indicated the charge-transfer interaction exists
between 1a and A1 or A7.
Figure 2 (a) 1a in CHCl3, (b) 1a and A1 in CHCl3, (c) 1a and
A7 in CHCl3.
To further estimate this charge-transfer interaction in
1
a molecular level, the H NMR spectra of catalyst 1a,
A1 and their mixture were measured in CDCl3. As
shown in Figure 3, the signal of A1 at δ 8.26 shifted to
δ 6.45 as a broad singlet after mixing with catalyst 1a in
a molar ratio of 2∶1. This apparent up-field shift of the
signal for acceptor is obviously caused by the electron
transfer from electron rich catalyst 1a to electron-poor
acceptor additive A1. Although the UV absorption
spectrum of catalyst 1a in CHCl3 does not undergo evi-
dent change after mixing with A1 (Figure 4a), the maxi-
mum fluorescence emmision band of A1 completely
disappeared after combination with catalyst 1a in CHCl3,
indicating a strong fluorescence quenching effect of 1a
on A1. All the spectral evidence clearly indicated the
charge transfer interaction is indeed present in the sys-
tem, which might provide an excellent opportunity for
reaction optimization via intermolecularly charge-
transfer interaction between the catalyst and the addi-
tive.
As expected, the charge-transfer interaction indeed
resulted in a remarkable effect on the enantioselectivity
of the reactions performed at room temperature, as evi-
denced by the results shown in Table 1. The reaction of
2a in the presence of 20 mol% of A1 with 10 mol% of
1a as the catalyst in chloroform at room temperature
affords the product with 99% yield in 76% ee (Table 1,
Entry 2), which is obviously higher than those without
additive under otherwise identical experimental condi-
tions (Table 1, Entry 1).1 Under the otherwise identical
conditions, the impact of a variety of additives A1—A7
was further examined on the control of enantioselectiv-
ity in the catalysis of B-V oxidation of 2a in chloroform
at room temperature, leading to the nearly quantitative
formation of lactone 3a with an enhancement of ee val-
ues from 71% (Entry 1) up to 78% (Entries 2—4, 8—
13). Among the solvents screened for the 1a/A1 combi-
nation, chloroform turns out to be the best in terms of
both the catalytic efficiency and enantioselectivity (En-
tries 5—7 vs .2).
Figure 1 (a) Schematic representation for the hypothesis of
donor-acceptor interaction on enantioselective control of the ca-
talysis by formation of charge-transfer complex. (b) Electron ac-
ceptor additives employed for affecting the enantioselectivity of
asymmetric B-V reaction catalyzed by chiral phosphoric acid 1a.
Indeed, intensely colorful solutions developed im-
mediately upon the mixing of catalyst 1a with the ac-
ceptor A in chloroform. For examples, both catalyst 1a
and acceptor A1 or A7 are colorless in CHCl3, however,
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Chin. J. Chem. 2010, 28, 1731— 1735

Charge-Transfer Effect on Asymmetric Baeyer-Villiger Oxidation
Figure 3 Comparison of 1H NMR spectra of A1, 1a and 1a＋A1 in CDCl3.
Figure 4 (a) Comparison of UV absorption spectra of 1a, A1 and 1a＋A1; (b) comparison of fluorescence emission spectra of 1a, A1
and 1a＋A1 (excitation wavelength of 275 nm).
An important advantage of the strategy using elec-
tron-acceptor additives is that the enantioselectivity can
be fine-tuned by simply changing the catalyst/additive
combinations without additional synthesis, which has
never been explored previously to the best of our
knowledge.6 In an effort to further evaluate the potential
of the donor-acceptor intereaction for the stereocontrol,
a variety of chiral phosphoric acid catalysts7 with dif-
ferent fused-ring aromatic substituents were examined
in combination with A3 in the asymmetric B-V oxida-
tion of 2a. As shown in Table 2, the ee values of 3a can
be enhanced appreciably (up to 9%) relative to that of
using phosphoric acid 1e alone by appropriate combina-
tions of A3 and 1e at room temperature (Entry 5). Sur-
prisingly, the effect diminished (or essentially disap-
peared) as the reaction was carried out at a much lower
temperature. As shown in Table 3, for 1a-catalyzed B-V
reactions of 3-aryl substituted cyclobutanones per-
formed at －40 ℃, there is no much difference in ee
values for the reactions performed in the presence or
absence of the electron-acceptor additive A1. Although
the exact reason is still not clear at the present stage, it
is indeed reflecting the complexity in the control of the
stereoselectivity in the catalysis of this type reaction.3
Conclusions
In summary, we have clearly demonstrated that the
intermolecular charge-transfer effect can be used as an
effective strategy for tuning the enantioselectivity of the
asymmetric cataysis, as shown in asymmetric BV oxi-
dation of 3-aryl cyclobutanones performed at room
temperature catalyzed by chiral phosphoric acid. Up to
9% enhancement of ee value was observed in a case
with the combined use of the chiral phosphoric acid 1a
catalyst and an appropriate electron-acceptor additive. It
Chin. J. Chem. 2010, 28, 1731— 1735
© 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
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Xu et al.
FULL PAPER
Table 1 Additive effect in the phosphoric acid 1a catalyzed
asymmetric B-V oxidation of 3-phenyl cyclobutanone 2aa
Table 2 Chiral phosphoric acids catalyzed asymmetric B-V
reaction of 3-phenyl cyclobutanone in the presence of electron
acceptor A3a
Entry Additive/mol% Solvent Time/h Yieldb/% eec/%
1
2
None
CHCl3
CHCl3
CHCl3
CHCl3
DCM
12
12
12
12
12
12
12
12
12
12
12
12
11
99
99
99
99
88
88
93
99
99
98
99
99
99
71
76
76
76
69
72
53
75
78
75
72
75
75
A1 (20)
A1 (10)
A1 (40)
A1 (20)
A1 (20)
A1 (20)
A2 (20)
A3 (20)
A4 (20)
A5 (20)
A6 (20)
A7 (20)
3
4
5
6
DCE
Entry
Catlyst
1a
Time/h
12
Yieldb/%
eec/%
7
Toluene
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
1
2
3
4
5
6
99
80
90
83
86
96
78 (71)
51 (50)
67 (65)
65 (57)
63 (54)
46 (44)
8
1b
24
9
1c
24
10
11
12
13
1d
24
1e
24
1f
24
a
The reaction was carried out at room temperature with [2a]＝
a The reaction was carried out at room temperature with [2a]＝0.1
mo/L and 1.5 equiv. 30% H2O2 to give the lactone 3a. b The yield
of isolated product. c The enantiomeric excess of 3a was deter-
mined by HPLC analysis on a chiral column (Chiralpak AS-H).
0.1 mol/L and 1.5 equiv. 30% H2O2 to give the lactone 3a. b The
yield of isolated product. c The enantiomeric excess of 3a was
determined by HPLC analysis on a chiral column (Chiralpak
AS-H), the data in the parentheses were obtained in the absence
of additive under the otherwise identical experimental conditions.
is noteworthy that this work represents the first example
of unequivocal demonstration on the use of the do-
nor-acceptor interaction for fine-tuning the enantiose-
lectivity in the area of the asymmetric catalysis to the
best of our knowledge. This conceptually new strategy
might stimulate future efforts to explore its new appli-
cation in other catalytic systems.
Table 3 Chiral phosphoric acid 1a catalyzed asymmetric B-V
reaction of 3-phenyl cyclobutanone in the presence of electron
acceptor A1a
Experimental
General methods
Entry
R in 3
Time/h
18
Yieldb/%
88
eec/%
1
2
C6H5 (a)
90 (88)
93 (93)
83 (83)
82 (82)
86 (84)
87 (86)
84 (88)
88 (88)
NMR spectra were recorded on a Varian Mercury
300 (1H: 300 MHz; 13C: 75 MHz) or Varian 400-MR
(1H: 400 MHz; 13C: 100 MHz) spectrometer in CDCl3.
Chemical shifts are expressed in δ with TMS as an in-
4-MeC6H4 (b)
4-BrC6H4 (c)
4-ClC6H4 (d)
4-FC6H4 (e)
2-Nphthyl (f)
C6H5 (a)
24
95
3
24
99
4
24
99
1
ternal standard (δ＝0) for H NMR data. Coupling con-
5
24
99
stants, J, are listed in hertz. 13C NMR data are reported
in terms of chemical shift (δ) with CDCl3 (δ＝77) as
internal standard. EI-MS and HRMS-EI data were ob-
tained on the instrument Shimadzu GCMS-QP2010 and
Waters Micromass, GCT respectively. UV and fluores-
cence spectra were recorded on a CARY 100 Conc.
UV-Visible spectrophotometer and a F-4500 FL spec-
trophotometer respectively. HPLC analysis was carried
out on a JASCO PU 2089 or JASCO 1589 instrument.
Unless stated otherwise, all solvents were purified and
dried according to standard methods prior to use.
6
24
91
7d
8e
18
99
C6H5 (a)
28
99
a
All the reactions were carried out at －40 ℃ with [2]＝0.1
mol/L and 1.5 equiv. 30% H2O2. b The yield of isolated product.
c
The enantiomeric excesses of 3 were determined by HPLC
analysis on a chiral column. The data in the parentheses were
obtained in the absence of any additive under the otherwise iden-
tical conditions. and Additives A3 and A7 were used, respec-
d
e
tively, in the reactions.
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Chin. J. Chem. 2010, 28, 1731— 1735

Charge-Transfer Effect on Asymmetric Baeyer-Villiger Oxidation
General procedure for asymmetric B-V reaction of
3-arylcyclobutanones catalyzed by chiral phosphoric
acid in the presence of electron acceptor A1— A7
lates or recycling asymmetric catalysts via formation of
charge transfer complexes, for examples, see:
(a) Chuzel, O.; Magnier-Bouvier, C.; Schulz, E. Tetrahe-
dron: Asymmetry 2008, 19, 1010.
A 5-mL Schlenk tube was charged with 1 (0.01
mmol), additive A (0.02 mmol), and CHCl3 (1 mL). The
mixture was stirred at r.t. for 30 min before 3-arylcyclo-
butanone 2 (0.1 mmol) and 30% H2O2 (17 µL, 1.5
equiv.) were added, respectively. The resulting mixture
was stirred for 12—24 h at the defined temperature. The
residual hydrogen peroxide was quenched with an
aqueous solution of Na2SO3, and the lactone product 3
was purified by column chromatography on silica gel
with petroleum ether/ethyl acetate (V/V＝6/1) as the
eluent. The enantiomeric excess was determined by
HPLC on a Chiralcel AS-H column.
(b) Chollet, G.; Rodriguez, F.; Schulz, E. Org. Lett. 2006, 8,
539.
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(d) Kampen, D.; Reisinger, C. M.; List, B. In Asymmetric
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2010, pp. 395—456.
For selected recent examples, see:
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