Aerobic photooxidative direct asymmetric aldol reactions of benzyl alcohols using water as the solvent

Article abstract of DOI:10.1039/c5ra05155j

We report an aerobic photooxidative direct asymmetric aldol reaction using water as the solvent. In this reaction, primary benzyl alcohols are oxidized into benzaldehydes under an oxygen atmosphere using anthraquinone-2-sodium sulfonate monohydrate as an organophotocatalyst. Stereoselective aldol reactions then proceed using a proline-type organocatalyst.

Full text of DOI:10.1039/c5ra05155j

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Air, h (VIS)
cat. AQNs
OH
O
OH
O
R
cat. organocatalyst
+
R
H2O
Oxidative direct asymmetric aldol reaction
· H2O as solvent
· Molecular oxygen as oxidant
· Visible light
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DOI: 10.1039/C5RA05155J

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DOI: 10.1039/C5RA05155J
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COMMUNICATION
Aerobic Photooxidative Direct Asymmetric Aldol Reactions
of Benzyl Alcohols Using Water as the Solvent
Received 00th January 20xx,
A. Fujiya, a T. Nobuta, a E. Yamaguchi, a N. Tada, a T. Miura, b and A. Itoh*a
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report an aerobic photooxidative direct asymmetric aldol derivative), and the final product have to be able to survive the
reaction using water as the solvent. In this reaction, primary oxidative reaction conditions. Recently, Kudo and coworkers
benzyl alcohols are oxidized into benzaldehydes under oxygen described highly stereoselective sequential oxidation/ asymmetric
atmosphere
using
anthraquinone-2-sodium
sulfonate aldol reactions of several benzyl alcohols using resin-supported
monohydrate as an organophotocatalyst. A stereoselective aldol catalysts, but the sequential addition of the ketone of interest was
reactions are then proceeded by proline-type organocatalyst.
required after the oxidation reaction had been performed because
the ketones used could not tolerate the oxidative conditions.9
We have been investigating mild oxidation reactions using
molecular oxygen and irradiation with fluorescent lamps.10
Recently, we reported oxidation reactions catalyzed by
anthraquinones, which acted as organophotocatalysts.11
Furthermore, we have also been studying organocatalyzed
asymmetric aldol reactions using water or brine as the reaction
solvent.12 It occurred to us that using water as the solvent would
allow the selective oxidation of a benzyl alcohols (which would be
relatively soluble in water) to be achieved without the other
compounds in the reaction mixture (such as aldehydes, ketones,
organocatalysts, and products) being oxidized and react with
enamine generated by ketone with organocatalyst to give oxidative
DAA reactions (Scheme 1). Here, we describe a detailed study of an
oxidative DAA reaction using water as the solvent.
The aldol reaction is a fundamental and reliable method for forming
carbon-carbon bonds. The products of this reaction, β-hydroxy
carbonyl compounds, are contained in many natural products and
are used in many medicines.1 Direct asymmetric aldol (DAA)
reactions catalyzed by organocatalysts have been actively
developed in recent decades because such reactions offer
advantages, such as stability in the presence of air and water and
reactants with low toxicities, over alternative reactions.2 A great
deal of attention has been paid to the use of water as a solvent for
these reactions because it is
a
safe, inexpensive, and
environmentally benign solvent.3 Various organocatalytic DAA
reactions using water as the solvent have been reported.4
Catalytic tandem reactions, in which a series of multistep reactions
are performed in a single vessel, can be powerful tools from an
environmental viewpoint because they save energy and waste.5, 6
Alcohols are readily accessible, low cost, and easy to handle, so
some researchers have reported the reactions in which alcohols are
used as starting materials to produce aldehydes in situ.7, 8 To the
best of our knowledge, no oxidative DAA reactions using alcohol
have yet been developed. There are some difficulties in performing
such a reaction: (1) the alcohol has to be selectively oxidized to the
aldehyde and (2) the aldehyde that is generated, the ketone
reactant, the asymmetric organocatalyst (such as
a proline
Scheme 1 Oxidative direct asymmetric aldol reaction from benzyl alcohol
First, we examined the asymmetric organocatalyzed reaction of 4-
bromobenzyl alcohol (1a) and cyclohexanone (2a) in the presence
of anthraquinone-2-sodium sulfonate monohydrate (AQN-2-
SO3Na·H2O) using water under an air atmosphere (i.e., open) with
irradiation from a fluorescent lamp for 12 h. Among the proline
a.Gifu Pharmaceutical University 1-25-4, Daigaku-nishi, Gifu 501-1196, Japan E-
mail: itoha@gifu-pu.ac.jp
b.Tokyo University of Pharmacy and Life Sciences, 1432-1 Horinouchi, Hachioji,
Tokyo 192-0392, Japan
† Electronic Supplementary Information (ESI) available. See
DOI: 10.1039/x0xx00000x
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derivatives (4a–d) that were used as organocatalysts, only trans-4-
tert-butyldiphenylsiloxy L-proline (4a) gave the β-hydroxy ketone
3a, and it gave a 28% yield with high stereoselectivity (Table 1).4c
Table 3 Study of further experiments to optimize the reaction conditions
Table 1 Study of organocatalyst
a Isolated yields. b Determined by 1H NMR spectroscopic analysis. c Determined by
HPLC. d 0.2 equiv of AQN-2-SO3Na·H2O was used.
The results of the tests using different solvents and
organophotocatalysts are shown in Table 2. We found that a
combination of H2O and AQN-2-SO3Na·H2O that is soluble in water
gave the photooxidative DAA reaction product 3a. General organic
solvents gave none of the desired product 3a because AQN-2-
SO3Na·H2O is insoluble in these solvents.
a Isolated yields. b Determined by 1H NMR spectroscopic analysis. c Determined by
HPLC.
We then conducted further experiments to optimize the reaction
conditions (Table 3). After several investigations, a prolonged
reaction time (48 h) and the addition of AcOH (entry 10) which may
accelerate the formation of enamine as an active species derived
from 2a and 4a gave the best result.
Table 2 Study of AQN derivatives and solvents
We examined the scope and limitations of the reaction using the
optimized conditions. As is shown in Table 4, the corresponding
aldol adducts were obtained in moderate to good yields with strong
enantioselectivities (entries 1–7). Reactions in which para- or meta-
nitrobenzyl alcohols were used were conducted under an O2
atmosphere to promote the oxidation of alcohols (entries 8 and 9).
However, substrates with an ortho-substituted or without electron-
withdrawing groups on the benzene rings or heterocyclic rings gave
poor results (entries 10–15). It seems that electron-withdrawing
group on the benzene ring promote the aldol reaction by making
electron poor at the carbonyl carbon of the corresponding
benzaldehyde. We changed the ring size of cycloalkanone and
found that cycloheptanone is not applicable to the reaction, even
though cyclopentanone is (entries 16–21). Unfortunately, though
we investigated the reactions with acyclic ketones, we could just
get corresponding aldol adduct in low yields (entries 22 and 23).
a Isolated yields. b Determined by 1H NMR spectroscopic analysis. c Determined by
HPLC.
2 | J. Name., 2012, 00, 1-3
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Table 4 Scope and limitation
Air, h (VIS)
(0.1 equiv)
4a
AQNꢀ2ꢀSO3Na—H2O (0.1 equiv)
AcOH (0.5 equiv)
+
product
3
OH
(0.3 mmol)
ketone
Ar
H2O, rt, time
(5 equiv)
2
1
yield (%) a
anti:synb
%eec
H2O
(mL)
time
(h)
product
entry
Ar1 = 4ꢀBrC H (
)
)
3a
48
48
48
48
60
60
1
3
3
2
3
3
3
71
60
63
65
62
58
13 : 1
16 : 1
14 : 1
17 : 1
12 : 1
7.4 : 1
15 : 1
94
>99
95
6
4
3ꢀBrC H (
3b
2
6
4
3 d
4
4ꢀClC H (
)
3c
6
4
3ꢀClC6H4
(
3d
)
97
5
4ꢀCF3C6H4 (3e)
4ꢀCNC H (
98
6
)
3f
>99
>99
94
6
4
OH
O
7 d,e
8 d,f
4ꢀCO Me H (
)
3g
2
48
73
61
54
2
6 4
Ar1
4ꢀNO2C6H4 (3h)
3ꢀNO C H (
0.5
0.5
120
120
9.7 : 1
9 d,f
10d,f
11d,f
12 f
13
)
8.5 : 1
>99
3i
2
6 4
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2ꢀNO2C6H4 (3j)
C6F5
120
120
120
48
1
(14)
(
)
3k
0.5
0.5
0.5
(0)
(7)
4ꢀOMeC H (
)
3l
6
4
(4)
(7)
ꢀ
ꢀ
ꢀ
ꢀ
2ꢀnaphthyl (
)
3m
4ꢀpyridyl (
)
3
3
48
3n
14
ꢀ
ꢀ
(0)
2ꢀfuryl (
)
3o
15
48
16d
17
18 e,f
Ar2 = 4ꢀClC6H4
(
)
2
3
48
60
2.4 : 1
2.2 : 1
3.2 : 1
>99
>99
>99
OH
OH
50
3p
O
53
55
4ꢀCNC H (
)
3q
Ar2
6
4
0.2
120
4ꢀNO C H (
)
)
3q
2
6 4
O
19 d
20
3s
4ꢀCNC H ( t)
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Ar3 = 4ꢀClC6H4
(
)
(2)
(7)
(8)
2
3
48
60
3
4
Ar3
6
21 e,f
0.5
120
4ꢀNO C H (
3u
2
6 4
OH
O
22 d,f,g
(
)
0.5
0.5
120
120
ꢀ
ꢀ
ꢀ
ꢀ
(5)
3v
O2N
OH
O
23 d,f,g
(
)
3w
5
(trace)
O2N
a Isolated yields. Numbers in parentheses are yields determined by 1H NMR spectroscopic analysis of crude. b Determined by 1H NMR spectroscopic analysis. c
Determined by HPLC. d Without AcOH. e 10 equiv of ketone was used. f Reactions were carried out under oxygen atmosphere. g 27 equiv of ketone was used.
Next, we performed some experiments to reveal the reaction scavenger, the photooxidation of the alcohol is suppressed to a
mechanism (Scheme 2). When we conducted the reaction under Ar, certain extent. Thus, we suppose that the oxidation step involves
the yield of the desired product is very low (eq. 1). If there is no radical steps.
irradiation from fluorescent lamp, the reaction is suppressed
perfectly (eq. 2). In eq. 3, we added sodium L-ascorbate as a radical
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Conclusions
In conclusion, we have described a photooxidative stereoselective
aldol reaction starting from an alcohol substrate. This method uses
molecular oxygen and visible light with AQN-2-SO3Na·H2O to oxidize
the alcohol. We found that this photooxidation is not what
interrupts the asymmetric aldol reaction because high
enantioselectivity is preserved. The reaction can be performed
using water as the solvent. We believe that this method should be
applicable to other reactions starting from aldehydes. Further
investigations into photooxidative reactions are now in progress in
our laboratory.
Notes and references
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R. Mahrwald, in Modern Aldol Reactions, Wiley-VCH,
Weinheim, Germany, 2004, vols. 1 and 2.
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According to the results mentioned above, we depicted the
plausible path of this reaction (Scheme 3). To begin with AQN-2-
-
SO3 is excited by the irradiation of fluorescent lamp and H atom at
the benzylic position of alcohol 1 is abstracted to produce benzyl
radical. Then, this intermediate traps molecular oxygen to generate
aldehyde via forming peroxy radical and hydroperoxide. Finally, the
aldehyde react with ketone 2 assisted by proline-type catalyst and
desired aldol adduct 3 is obtained.
2010, 8, 4043.
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Scheme 3 Plausible Path
4 | J. Name., 2012, 00, 1-3
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