Biomimetic asymmetric hydrogenation: In situ regenerable Hantzsch esters for asymmetric hydrogenation of benzoxazinones

Article abstract of DOI:10.1021/ja208073w

A catalytic amount of Hantzsch ester that could be regenerated in situ by Ru complexes under hydrogen gas has been employed in the biomimetic asymmetric hydrogenation of benzoxazinones with up to 99% ee in the presence of chiral phosphoric acid. The use of hydrogen gas as a reductant for the regeneration of Hantzsch esters makes this hydrogenation an ideal atom economic process.

Full text of DOI:10.1021/ja208073w

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pubs.acs.org/JACS
Biomimetic Asymmetric Hydrogenation: In Situ Regenerable Hantzsch
Esters for Asymmetric Hydrogenation of Benzoxazinones
Qing-An Chen, Mu-Wang Chen, Chang-Bin Yu, Lei Shi, Duo-Sheng Wang, Yan Yang, and Yong-Gui Zhou*
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road,
Dalian 116023, P. R. China
S Supporting Information
b
ABSTRACT: A catalytic amount of Hantzsch ester that
could be regenerated in situ by Ru complexes under hydro-
gen gas has been employed in the biomimetic asymmetric
hydrogenation of benzoxazinones with up to 99% ee in the
presence of chiral phosphoric acid. The use of hydrogen gas
as a reductant for the regeneration of Hantzsch esters makes
this hydrogenation an ideal atom economic process.
educed nicotinamide adenine dinucleotide (NADH) and
nicotinamide adenine dinucleotide phosphate (NADPH)
R
are recognized as a couple of the most important coenzymes
found in living cells that play vital roles in reductionÀoxidation
(redox) metabolism (Figure 1A).1 The reduced form of the
coenzyme NAD(P)H acts as reductant for most of the reductases
in the cell that need to reduce their substrates. Because coenzyme
NAD(P)H is too expensive to be used in stoichiometric
amounts, the oxidized form of the coenzyme has to be trans-
formed into a reduced form for the next redox cycle in vivo
through cellular respiration, such as glycolysis or the citric acid
cycle (TCA cycle), which convert biochemical energy from
nutrients into adenosine triphosphate (ATP).2
Figure 1. (A) NAD(P)H-mediated reduction reaction (E1: regenera-
tion enzyme; E2: production enzyme). (B) Biomimetic hydrogenation
process (Cat. 1: regeneration catalyst; Cat. 2: production catalyst).
Table 1. Generation of Hantzsch Ester 2 from Pyridine 1a
Owing to their importance in metabolism, NAD(P)H mimics
have been a central point in biomimetic chemistry over the past
few decades. As the simplest NAD(P)H model, Hantzsch esters
(HEHs)3 have been widely and successfully used as a hydrogen
source in the biomimetic asymmetric transfer hydrogenation of
CdC, CdN, and CdO bonds in the presence of organo-
catalysts4,5 and metal catalysts.6 Despite much progress having
been achieved, most of the current research focuses on the
hydride transfer ability and selectivity in redox reactions rather
than the regeneration of Hantzsch ester.7 Developing a biomi-
metic asymmetric hydrogenation system that simultaneously
involves an asymmetric reduction process and the regeneration
of Hantzsch ester is still a great challenge in the field of NAD-
(P)H mimics.
Hydrogen gas (H2) is recognized as the most cost-effective
reductant and yields no byproducts. Therefore, asymmetric
hydrogenation processes that involve the use of transition metals
in conjunction with hydrogen gas are ubiquitous methods to
access various biologically active compounds.8 Developing a
biomimetic asymmetric hydrogenation reaction using H2 as the
reductant for the regeneration NAD(P)H models is of great
interest in the field of NAD(P)H mimics (Figure 1B). Herein, we
report an efficient method for in situ regeneration of Hantzsch
ester from Hantzsch pyridine under hydrogen gas for chiral
conv.
(%)b
entry
cat.
solvent
T (°C)
1
RuCl2(PPh3)3
toluene
toluene
toluene
toluene
toluene
CH2Cl2
THF
25
25
25
25
50
50
50
50
70
70
---
---
---
4
2
Grubbs’ cat. I
3
Grubbs’ cat. II
4
[Ru(p-cymene)I2]2
[Ru(p-cymene)I2]2
[Ru(p-cymene)I2]2
[Ru(p-cymene)I2]2
[Ru(p-cymene)I2]2
[Ru(p-cymene)I2]2
[Ru(p-cymene)I2]2
5
10
29
39
62
64
69
6
7
8c
THF
9
10d
THF
THF
a 1 (0.02 mmol), Ru(II) (10 mol %), H2 (600 psi), solvent (2 mL), 16 h.
b Determined by 1H NMR. c H2 (1000 psi). d 2 (0.02 mmol) was added
instead of 1. After reaction, mixture 2/1 (69/31) was obtained.
phosphoric acid catalyzed biomimetic asymmetric hydrogena-
tion of benzoxazinones.9À11
Received: August 26, 2011
Published: September 20, 2011
r
2011 American Chemical Society
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|

Journal of the American Chemical Society
COMMUNICATION
Table 2. In Situ Regeneration of Hantzsch Ester 2 for
Table 3. Biomimetic Asymmetric Hydrogenation of Ben-
zoxazinones 3 Using Catalytic Amount of Hantzsch Ester 2a
Biomimetic Asymmetric Hydrogenation of Benzoxazinone 3aa
entry
R in 3
Ar in 3
yield (%)b
ee (%)c
1
H
Ph
93 (4a)
96 (4b)
96 (4c)
98 (4d)
96 (4e)
95 (4f)
94 (4g)
86 (4h)
59 (4i)
81 (4j)
83 (4k)
90 (4l)
68 (4m)
98 (S)
98 (S)
99 (S)
98 (S)
99 (S)
99 (S)
99 (S)
98 (S)
92 (R)
98 (S)
96 (S)
94 (S)
98 (S)
2
H
4-MeOC6H4
4-MeC6H4
3,4-Me2C6H3
4-ClC6H4
4-BrC6H4
4-FC6H4
3-FC6H4
2-Thienyl
Ph
H2
conv.
(%)b
ee
(%)c
3
H
entry
solvent
(psi)
4
H
1d
2e
THF
THF
THF
600
600
600
600
600
600
600
800
800
1000
1000
1000
1000
----
<5
28
----
89
95
97
96
97
98
96
98
98
98
98
98
98
5
H
6
H
3
23
7
H
4
CH2Cl2
18
8
H
5
THF/CH2Cl2 (3:1)
THF/CH2Cl2 (1:1)
THF/CH2Cl2 (1:3)
THF/CH2Cl2 (1:1)
THF/CH2Cl2 (1:3)
THF/CH2Cl2 (1:3)
THF/CH2Cl2 (1:3)
THF/CH2Cl2 (1:3)
THF/CH2Cl2 (1:3)
THF/CH2Cl2 (1:3)
26
9
H
6
40
10
11
12
13
6-Cl
6-Me
7-Me
6-tBu
7
38
Ph
8
47
Ph
9
52
Ph
a 3 (0.20 mmol), 2 (10 mol %), [Ru(p-cymene)I2]2 (1.25 mol %), (S)-5
(2 mol %), H2 (1000 psi), THF/CH2Cl2 1/3 (2 mL), 48 h, 50 °C.
b Isolated yields. c Determined by HPLC.
10
11f
12f,g
13f,h
14f,i
64
>95
>95
>95
>95
Scheme 1. Proposed Mechanism for Biomimetic Asym-
metric Hydrogenation of Benzoxazinones 3 Using Catalytic
Amount of Hantzsch Ester 2
a 3a (0.10 mmol), 1 (20 mol %), [Ru(p-cymene)I2]2 (2.5 mol %), (S)-5
(2 mol %), Solvent (2 mL), 16 h, 50 °C. b Determined by H NMR.
1
c Determined by HPLC. d No addition of 1. e 70 °C. f 48 h. g 2 (20 mol %).
h 3a (0.20 mmol), 2 (10 mol %), [Ru(p-cymene)I2]2 (1.25 mol %), (S)-5
(2 mol %). i 2 (120 mol %), with no addition of [Ru(p-cymene)I2]2.
The key point to realize in the above-mentioned idea is the
high efficiency and selectivity for regenerating Hantzsch ester 2
from pyridine 1 which belongs to the partial hydrogenation of
aromatic heterocycle. Based on our efforts in asymmetric hydro-
genation and transfer hydrogenation of heteroaromatic com-
pounds,12,13 Ru(II) complexes were chosen as the catalysts for
the reduction of pyridine 1 under hydrogen gas (Table 1). From
the survey of some commercially available Ru(II) complexes,
[Ru(p-cymene)I2]2 provided Hantzsch ester 2 with 4% conver-
sion (entries 1À4). The primary screening of solvents showed
that CH2Cl2 and THF were relatively good choices for the
generation of Hantzsch ester 2 (entries 5À7). With increasing
reaction temperature or pressure of hydrogen gas, an improve-
ment in reaction conversion was observed (entries 8À9).
Dehydroaromatization occurred in the controlled trial (entry
9 vs 10).
Further investigations were carried on the feasibility of in situ
regeneration of Hantzsch ester 2 for biomimetic asymmetric
hydrogenation in the presence of a Brønsted acid (Table 2). The
inhibition of a background reaction of a substrate promoted by
achiral Ru complexes is another challenge. Benzoxazinone 3a
emerged as a suitable model substrate owing to its inactivity
under hydrogen gas without the addition of Hantzsch pyridine 1
(entry 1). Initially, a promising result (28% conversion, 89% ee)
was obtained (entry 2).11a Examination of the solvent effects
indicated that the solvent mixture DCM/THF was the best
choice for the regeneration of Hantzsch ester 2 and transfer
hydrogenation of benzoxazinone 3a (entries 3À7). A moderate
conversion (67%) was obtained when the hydrogen pressure was
increased to 1000 psi (entries 8À10). Prolonging the reaction
time was good for achieving complete conversion (entry 11).
The adoption of Hantzsch ester 2 instead of Hantzsch pyridine 1
gave the same result with respect to conversion and enantios-
electivity (entry 12 vs 11). To our delight, reducing the amount
of Hantzsch ester 2 to 10 mol % leads to no deterioration with
regard to conversion and enantioselectivity (entry 13). Notably,
this catalytic efficiency is identical to that of a pure organocata-
lytic process which consumed a stoichiometric amount of
Hantzsch ester 2 (entry 13 vs 14).11a
Having established the optimized conditions, the scope of
enantioselective synthesis of dihydrobenzoxazinones 4 using a
catalytic amount of Hantzsch ester 2 was explored (Table 3). In
general, good yields (86À98%) and excellent enantioselectivities
(98À99% ee) were obtained in this biomimetic asymmetric
hydrogenation regardless of the electronic properties of the phenyl
ring of benzoxazinones 3 (entries 1À8). For heteroaromatic
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Journal of the American Chemical Society
COMMUNICATION
benzoxazinone 3i, a moderate yield (59%) but high enantioselec-
tivity (92% ee) was observed (entry 9). The reduction of 6- or
7-position substituted 2-phenyl benzoxazinones 3 gave dihydro-
benzoxazinones 4 with excellent enantioselectivities (94À98% ee)
and moderate to good yields (68À90%, entries 10À13).
Fernandez-Lafuente, R. Curr. Org. Chem. 2010, 14, 1000. (d) Hollmann,
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Dufour, J.; Schoepke, F. R. Green Chem. 2011, 13, 1084.
A proposed mechanism is depicted in Scheme 1 to account for
the biomimetic asymmetric hydrogenation of benzoxazinones 3
promoted by a metal/Brønsted acid relay catalyst14,15 with a
catalytic amount of Hantzsch ester 2. It is assumed that chiral
phosphoric acid (S)-5 catalyzes the asymmetric transfer hydro-
genation of benzoxazinones 3 with Hantzsch ester 2 first
affording optically active products 4 and Hantzsch pyridine 1.
Subsequently, the undesirable product pyridine 1 undergoes
hydrogenation to regenerate Hantzsch ester 2 for the next
catalytic cycle in the presence of Ru complexes under hydrogen
gas. The excellent enantioselectivities achieved in this enantio-
selective transfer hydrogenation is attributed to the fact that the
reaction rate of this principal reaction, k2, is faster than that of the
undesired side reaction, k3, which generates dihydrobenzoxazi-
nones 4 in racemic form (Scheme 1).16
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Fonseca, M. T.; List, B. Angew. Chem., Int. Ed. 2004, 43, 6660. (b) Ouellet,
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In summary, we have successfully developed an efficient
method for the regeneration of Hantzsch ester from Hantzsch
pyridine with Ru complexes as the catalyst under hydrogen gas.
A catalytic amount of Hantzsch ester regenerated in situ has been
employed in the chiral phosphoric acid promoted biomimetic
asymmetric hydrogenation of benzoxazinones with up to 99% ee.
The use of hydrogen gas as the reductant for the regeneration of
Hantzsch ester makes this biomimetic asymmetric hydrogena-
tion an ideal atom economic process. Further investigations on
the application of the developed strategy and detailed mechan-
istic studies of the catalytic cycle are currently ongoing in our lab.
’ ASSOCIATED CONTENT
S
Supporting Information. Complete experimental proce-
b
dures and characterization data for the prepared compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
ygzhou@dicp.ac.cn
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drogenation with Hantzsch esters, see: (a) Zehani, S.; Gelbard, G.
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Asymmetry 2007, 18, 1103.
’ ACKNOWLEDGMENT
(7) For selected work on regeneration of other NAD(P)H models,
see: (a) Lo, H. C.; Fish, R. H. Angew. Chem., Int. Ed. 2002, 41, 478.
(b) Wagenknecht, P. S.; Penney, J. M.; Hembre, R. T. Organometallics 2003,
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This work was supported by by the National Natural Science
Foundation of China (21032003 and 20921092) and the Na-
tional Basic Research Program of China (2010CB833300). We
also thank Prof. Xumu Zhang of Rutgers University and Prof.
Shu-Li You of Shanghai Institute of Organic Chemistry for very
helpful discussions. This paper is dedicated to Prof. Christian
Bruneau on the occasion of his 60th birthday.
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(16) Moderate conversion (60%) and enantioselectivity (75% ee)
were obtained when 2-phenylquinoline was tested in this reaction. The
lower enantioselectivity is due to the existence of a background reaction
in the hydrogenation of quinoline.
16435
dx.doi.org/10.1021/ja208073w |J. Am. Chem. Soc. 2011, 133, 16432–16435