Catalytic asymmetric Meerwein-Ponndorf-Verley reduction of glyoxylates induced by a chiral N,N′-dioxide/Y(OTf)3 complex

Article abstract of DOI:10.1039/c7cc00273d

An asymmetric Meerwein-Ponndorf-Verley (MPV) reduction of glyoxylates was for the first time accomplished via an N,N′-dioxide/Y(OTf)₃ complex with aluminium alkoxide and molecular sieves (MSs) as crucial additives. A variety of optically active α-hydroxyesters were obtained with excellent results. A possible reaction mechanism was proposed based on the experiments.

Full text of DOI:10.1039/c7cc00273d

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DOI: 10.1039/C7CC00273D
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Catalytic Asymmetric Meerwein-Ponndorf-Verley Reduction of
Glyoxylates Induced by Chiral N,N′-Dioxide/Y(OTf)3 Complex
Received 00th January 20xx,
Accepted 00th January 20xx
Wangbin Wu,a Sijia Zou,a Lili Lin,*,a Jie Ji,a Yuheng Zhang,a Baiwei Ma,a Xiaohua Liua and Xiaoming
Feng*,a,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
An asymmetric Meerwein-Ponndorf-Verley (MPV) reduction of
glyoxylates was firstly accomplished by N,N'-dioxide/Y(OTf)3
complex with aluminium alkoxide and molecular sieve (MS) as
crucial additives. A variety of optical active α-hydroxyesters were
obtained with excellent results. A possible reaction mechanism
was proposed based on the experiments.
There is no doubt that enantioselective reductions occupy
important synthetic position in both academic and industry
fields.1 Among them, the enantioselective reduction of
carbonyl compounds converts the prochiral ketones into chiral
secondary alcohols.2 Chiral transition metal catalyst-directed
asymmetric hydrogenations (AH) using H2 as reductant have
been well studied and myriad efficient catalytic systems have
emerged.3 Another powerful and convenient method is the
asymmetric transfer hydrogenation (ATH), which possesses the
advantages of easy operation, avoidance of high hydrogen
pressure and homogeneous reaction manner.4
The Meerwein-Ponndorf-Verley (MPV) reduction, a transfer
hydrogenation method for reducing carbonyl compounds, was
discovered in 1920s.5 The classical reaction pathway involves a
coordination of both reductive alcohol and carbonyl
compound with aluminium alkoxides, followed by an
intermolecular hydride transfer through a six-membered ring
transition state.6 Because of its carbonyl-specific
chemoselectivity and mild reaction conditions, various
catalytic systems have been developed for the MPV reaction to
synthesize racemic secondary alcohols.7 The first asymmetric
MPV reaction induced by chiral alcohols was reported by
Doering and Young in 1950 with the use of aluminium
alkoxides as catalyst.8 In catalytic asymmetric versions, chiral
aluminium catalytic systems were mainly focused.10 The most
a.Key Laboratory of Green Chemistry & Technology, Ministry of Education, College
of Chemistry, Sichuan University, Chengdu 610064, P. R. China. Fax: +86 28
85418249; Tel: +86 28 85418249; E-mail: lililin@scu.edu.cn; xmfeng@scu.edu.cn.
b.Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, P.
R. China
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Scheme 1 Asymmetric Meerwein-Ponndorf-Verley (MPV) reaction
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Table 1 Optimization of the reaction conditions.
efficient one might be the BINOL/AlMe3 system, developed by
Nguyen group. Chiral α-phenylethanol was achieved in high
yield with moderate to good ee value (Scheme 1a).10d As far as
we know, the only example where the central metal salt is not
aluminium alkoxides was reported by Evans and co-workers in
1993. Chiral amino-diol/samarium(III) iodide was developed
and efficiently catalyzed the reduction of aryl methyl ketones
(Scheme 1b).9 It can be seen that the catalytic asymmetric
Meerwein-Ponndorf-Verley (MPV) reduction of simple ketones
has achieved some progresses, but, the catalytic asymmetric
reaction of other ketones has not been explored.
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DOI: 10.1039/C7CC00273D
The asymmetric reduction of glyoxylates can construct
optically active α-hydroxyesters, which are not only found to
be the structural units of numerous natural products but also
drug molecules and important intermediates in organic
synthesis.11 For example, α-hydroxy carboxylic acid can act as
Brønsted acid catalyst.11e Moreover, α-hydroxy carboxylic acid
derived 2-(aziridin-1-yl)-1-phenylethanol and α-hydroxy-2-
oxazoline were reported as chiral ligands.11f,g Mirabegron,
derived from mandelic acid and approved by the FDA, provide
better treatment options for the management of overactive
bladder (OAB).11h Herein, we report our efforts on developing
Yieldb
(%)
0
0
0
Eec
(%)
-
-
-
Entrya
Metal
T (oC)
t (h)
Additive
1
2
3
4
5
6d
7d,e
Al(OiPr)3
Sc(OTf)3
Zn(OTf)3
Y(OiPr)3
Y(OTf)3
Y(OTf)3
Y(OTf)3
30
30
30
30
30
30
30
24
24
24
24
24
24
14
3 Å MS
3 Å MS
3 Å MS
3 Å MS
3 Å MS
-
Al(OiPr)3
3 Å MS,
Al(OiPr)3
3 Å MS,
Al(OiPr)3
3 Å MS,
Al(OtBu)3
3 Å MS,
Al(OtBu)3
3 Å MS,
Al(OtBu)3
3 Å MS,
Al(OtBu)3
0
-
82
trace
99
73
n.d.
73
8f
Y(OTf)3
Y(OTf)3
Y(OTf)3
Y(OTf)3
Y(OTf)3
Y(OTf)3
30
30
30
0
42
3
64
99
99
87
99
89
69
73
75
83
87
92
12
an N,N'-dioxide/Y(OTf)3 complex system for MPV reaction of
aryl and alkyl glyoxylates.
9e
We initiated our investigation with iso-propyl
phenylglyoxylate (1a) as standard substrate to optimize the
reaction conditions. Firstly, various metals by complexing with
10g
11g
12g,h
13g,h
3
24
24
72
o
L-RaPr3 were investigated in the presence of 3 Å MS at 30 C
(Table 1, entry 1‒5). Many metal salts, such as Al(OiPr)3,
Sc(OTf)3, Zn(OTf)2 and Y(OiPr)3 cannot promote the reaction
(Table 1, entries 1‒4). To our excitement, when Y(OTf)3 was
applied, the desired product iso-propyl mandelate (2a) was
obtained in 82% yield with 73% ee after 24 h (Table 1, entry 5).
Without 3 Å MS, only trace amount of product was detected
(Table 1, entry 6), which suggested that the addition of 3 Å MS
was crucial for the reaction. The addition of Al(OiPr)3 had an
positive impact on the reaction rate, and quantitative yield
was obtained in 12 h (Table 1, entry 7). When iso-propanol was
excluded from the reaction, the target product 2a could also
be obtained in 64% yield with 69% ee after 42 h with Al(OiPr)3
in toluene (Table 1, entry 8), which suggested that aluminium
alkoxide might participate in the reduction step. Introduction
of both 3 Å MS and Al(OiPr)3 into the system caused higher
reactivity, and quantitative yield was obtained in 3 h with the
ee maintained (Table 1, entry 9). Elaborate screening of
aluminium alkoxide showed that Al(OtBu)3 offered a little
better enantioselectivity (75% ee, Table 1, entry 9). It might
because that the larger steric hindrance of tBu group in
Al(OiPr)(OR)2, which was generated through anion exchange, is
beneficial for the enantioselectivity. When the reaction
temperature was decreased to 0 oC, the reactivity was
decreased (87% yield after 12 h), but the enantioselectivity
was promoted (83% ee, Table 1, entry 10). When the reaction
was performed in mixed solvent of iPrOH/CHCl3 (1/2 volume
ratio), 99% yield and 85% ee were obtained in 24 h (Table 1,
entry 11). In order to further improve the enantioselectivity,
the temperature was dropped to -10 oC, and the ee value was
0
-10
-10
then
0
72
then
48
3 Å MS,
Al(OtBu)3
14g,h
Y(OTf)3
99
90
aUnless otherwise noticed, the reactions were performed with L-RaPr3-Metal
(1:1, 10 mol%), 1a (0.10 mmol) and 3Å MS (25 mg) in iPrOH (0.1M) in air. bYields
of the isolated products. cDetermined by HPLC analysis using a chiral stationary
phase. dWithout addition of 3 Å MS. eAl(OiPr)3 (50 mol%) was added. fThe
reaction was perform in toluene (0.1 M) without iPrOH. gAl(OtBu)3 was used
instead of Al(OiPr)3. hThe reaction was performed in iPrOH/CHCl3 (v/v = 1/2, 0.1
M).
promoted to 92% with the yield decreased to 89% after 72 h
(Table 1, entry 12). After optimization, we found that after a
further stirring at 0 C for 48 h, quantitative yield and 90% ee
o
could be acquired (Table 1, entry 13). Therefore, the optimized
reaction conditions were established with L-RaPr3/Y(OTf)3 as
catalyst, 3 Å MS and Al(OtBu)3 as additives in a mixture of
o
o
iPrOH and CHCl3 at -20 C for 72 h and then at 0 C for 48 h
without extrusion of air.
With the optimized reaction conditions in hand (Table 1,
entry 13), the substrate scope was examined. As listed in Table
2, aryl and alkyl glyoxylates underwent the asymmetric MPV
reaction well, delivering the corresponding mandelates in
excellent yields with good ee values (>98% yields, 84–92% ee).
Generally, ortho- and para-substituted aryl glyoxylates gave
2 | J. Name., 2012, 00, 1-3
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higher enantioselectivities than the meta-substituted ones through a six-membered ring transition state, where metal iso-
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(Table 2, entries 1–16). The reaction also worked well with
Table 2 Substrate scope of glyoxylates for the catalytic asymmetric MPV reaction.
propoxide coordinate with ketone substrDaOteI:s1.0T.1h0e3r9e/Cfo7CreC,0a0273D
Entrya
R
Yieldb (%)
Eec(%)
90(S)
91(S)
85(S)
90(S)
91(S)
89(S)
89(S)
Scheme 2 Gram-scale experiment
1
2
3
4
5
6
7
Ph (1a)
98
99
98
98
99
99
99
2-MeC6H4 (1b)
3-MeC6H4 (1c)
4-MeC6H4 (1d)
4-tBuC6H4 (1e)
4-PhC6H4 (1f)
4-MeOC6H4 (1g)
8
99
90(S)
(1h)
9
2,4-(MeO)2C6H3 (1i)
2-FC6H4 (1j)
99
99
98
99
98
99
98
99
99
99
99
99
90(S)
92(S)
91(S)
91(S)
90(S)
89(S)
90(S)
85(S)
86(S)
86(S)
90
10
11
12
13
14
15
16
17
18
19
20
4-FC6H4 (1k)
2-ClC6H4 (1l)
4-ClC6H4 (1m)
2,3,4-Cl3C6H2 (1n)
4-BrC6H4 (1o)
4-F3CC6H4 (1p)
1-naphthyl (1q)
2-naphthyl (1r)
n-butyl (1s)
1-adamantyl (1t)
84
aUnless specified, the reactions were performed with L-RaPr3/Y(OTf)3 (1:1,
10mol%), 1 (0.1mmol), 3 Å MS (25 mg), Al(OtBu)3 (50 mol%) in iPrOH/CHCl3
(v/v=1/2, 0.1 M) at -20 oC for 72 h then at 0 oC for 48 h in air. bYield of the isolated
products. cDetermined by HPLC analysis using a chiral stationary phase.
Scheme 3 Proposed mechanism for asymmetric MPV reaction.
fused-ring substrates. Both 1-naphthyl and 2-naphthyl
substrates (1q and 1r) transformed to the corresponding α-
hydroxy ester in good enantioselectivities (86% ee, Table 2,
entries 17–18). Notably, linear and branched alkyl glyoxylates
possible catalytic cycle was proposed on the basis of the
experiments and previous work.12 As shown in Scheme 3, in
the presence of iso-propanol, Al(OtBu)3 can gradually
converted into aluminium salt
complex coordinate with the oxygen atoms in both carbonyl
substrate 1a and aluminium salt , forming a six-membered
3. Then, N,N'-dioxide/Y(OTf)3
1s–1t performed well in this catalytic system, giving α-hydroxy
aliphatic esters in quantitative yields with good
enantioselectivities (84%–90% ee, Table 2, entries 19–20).
Simple ketones as well as a variety of other ketones were
also investigated in the catalytic system (for details, see ESI).
Only α-bromoacetophenone gave 95% yield and 81% ee with 3
Å MS and Al(OtBu)3 after 120 h, and the others gave low
enantioseletivity or no reactivity. This might because only
ketone esters and α-bromoacetophenone can bind to metals
in two-point as proposed by Katz.10i
To testify the utility of the methodology, the reduction of
1a was performed in a gram scale. 7 mmol of 1a transformed
well under the optimized reaction conditions, and 1.18 g (87%
yield) of the target alcohol 2a was obtained with 90% ee
(Scheme 2).
3
ring transition state. In such transition state, the
intermolecular hydrogen transfer happened between 1a and
aluminium iso-propoxide salt
3. Also, it was proposed that the
ester group on the gloxylates (or the bromo atom on 1v
)
serves as another binding point besides the carbonyl group
and coordinates with the aluminium alkoxide as former
initiated.7u,10i Because Si face of 1a was shielded by the
neighboring 2,6-diisoproylphenyl group on the ligand, the 1,5-
hydride transfer took place on the Re face of 1a. In the
presence of iso-propanol, (S)-2a and N,N'-dioxide/Y(OTf)3 was
released, and the catalyst would realized cyclic catalysis.
In summary, we have developed an N,N'-dioxide/Y(OTf)3
complex catalytic system for asymmetric MPV reaction with
excellent yields and good to excellent ee value for both aryl
and alkyl glyoxylates. It was found that the additions of both
molecular sieve and Al(OtBu)3 were crucial for the reaction.
Besides, a possible catalytic mechanism was proposed to
Along with the phenomenon in table 1 (entries 6 and 8),
HNMR analysis and kinetic informations (for details, see ESI) all
suggested that aluminium alkoxide may serve as reductant in
the hydride transformation step. As previously put forward,6d,e
the MPV reaction is an intermolecular hydride transfer
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We appreciate the National Natural Science Foundation of
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