Convergent asymmetric disproportionation reactions: Metal/Bronsted acid relay catalysis for enantioselective reduction of quinoxalines

Article abstract of DOI:10.1021/ja200723n

A convergent asymmetric disproportionation of dihydroquinoxalines for the synthesis of chiral tetrahydroquinoxalines using a metal/Bronsted acid relay catalysis system has been developed. The use of hydrogen gas as the reductant makes the convergent dispr

Full text of DOI:10.1021/ja200723n

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Convergent Asymmetric Disproportionation Reactions: Metal/Brønsted
Acid Relay Catalysis for Enantioselective Reduction of Quinoxalines
Qing-An Chen, Duo-Sheng Wang, Yong-Gui Zhou,* Ying Duan, Hong-Jun Fan,* Yan Yang, and Zhang Zhang
State Key Laboratory of Catalysis and Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of
Sciences, 457 Zhongshan Road, Dalian 116023, P. R. China
S Supporting Information
b
Scheme 1. Convergent Asymmetric Disproportionation
ABSTRACT: A convergent asymmetric disproportionation of
dihydroquinoxalines for the synthesis of chiral tetrahydroqui-
noxalines using a metal/Brønsted acid relay catalysis system has
been developed. The use of hydrogen gas as the reductant makes
the convergent disproportionation an ideal atom-economical
process. A dramatic reversal of enantioselectivity was observed in
the reduction of quinoxalines because of the different steric
demands in the 1,2- and 1,4-hydride transfer pathways.
eductionÀoxidation (redox) reactions constitute one of the
R
most fundamental reaction types in chemistry. They result
in changes in the oxidation numbers of the participating chemical
species.1 Addition of hydrogen or electrons or removal of oxygen is
reduction, and removal of hydrogen or electrons or addition of oxygen
is oxidation. Both oxidation and reduction occur simultaneously and
in equivalent amounts during any reaction involving either process
(eq 1 in Scheme 1). A redox reaction is called a disproportionation
(or dismutation) reaction when the starting element is both oxidized
and reduced (eq 2 in Scheme 1 ). In general, the atom economy of
disproportionation reactions is not very good because the reaction
simultaneously gives an oxidation product and a reduction product.
If the reduction product is the target compound, the atom economy
would be significantly improved if the starting material could be
regenerated from the oxidation product with the aid of a reductant.
Thus, a convergent disproportionation reaction would be realized: (1)
initially, the oxidation product would deliver the starting material in
the presence of the reductant; (2) subsequently, the starting material
would undergo the disproportionation reaction to give the desired
reduction product and regenerate the oxidation product for next
redox cycle (eq 3 in Scheme 1). H2 is a preferred reductant for the
generation of starting materials because it is the cheapest reducing
agent and yields no requisite byproducts.2 Herein we report metal/
Brønsted acid relay catalysis for the asymmetric hydrogenation of
quinoxalines through a convergent disproportionation of dihydroqui-
noxalines (eq 4 in Scheme 1).3À5 A dramatic reversal of enantios-
electivity was also observed for hydrogenation relative to transfer
hydrogenation of quinoxalines promoted by chiral phosphoric acid.6
In our studies of asymmetric hydrogenation and transfer hydro-
genation of heteroaromatic compounds,7À9 a serendipitous dis-
proportionation of dihydroquinoxaline 4a was observed. In spite of
its sensitivity to air, dihydroquinoxaline 4a showed a greater
tendency to undergo the unexpected self-transfer hydrogenation
at room temperature (rt) (eq 5 in Scheme 2). To our delight, the
self-transfer hydrogenation of 4a delivered 3a with excellent 93% ee
inthepresenceofchiral phosphoric acid (S)-1b (eq6inScheme2).
Scheme 2. Self-Transfer Hydrogenation of
Dihydroquinoxaline
Recently, Xiao10aÀc and Rueping10d developed highly enantio-
selective hydrogenations of acyclic imines and quinolines employing
combined Brønsted acid and chiral diamine-ligated Ir(III) catalysts.
Rueping and co-workers reported a highly enantioselective Brønsted
acid-catalyzed transfer hydrogenation of quinoxalines with Hantzsch
esters(HEH).8i,11 Inspired by Xiao and Rueping’s work, we envisaged
that hydrogen gas activated by achiral metal complexes could reduce
Received: January 25, 2011
Published: April 05, 2011
r
2011 American Chemical Society
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Scheme 3. Hydrogenation of Quinoxalines
Table 1. Metal/Brønsted Acid-Catalyzed Asymmetric
Reduction of 2-Phenylquinoxalinea
entry
solvent
1
X
conv. (%)b
ee (%)c
1
2
3
4
CHCl3
(S)-1a
(S)-1a
(S)-1a
(S)-1a
(S)-1a
(S)-1a
(S)-1a
(S)-1a
(S)-1b
(S)-1c
(S)-1b
(S)-1b
(S)-1b
(S)-1b
(S)-1b
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
1.25
0.5
0.25
0.05
67
95
>95
>95
85
87
28
>95
78
58
45
2
CH2Cl2
MeOH
THF
10
79
80
81
77
90
À
5
toluene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
6
7d
8e
2a to dihydroquinoxaline 4a, ultimately affording chiral 3a with
complete conversion through convergent asymmetric disproportio-
nation of 4a in the presence of chiral phosphoric acid 1 (eq 7 in
Scheme 3). The key point to warrant our above hypothesis is the
difference between the reaction rates for self-transfer hydrogenation
of 4a (k2) promoted by chiral phosphoric acid and the background
hydrogenation of 4a (k3) catalyzed by metal complexes that generate
3a in racemic form (eq 7 in Scheme 3). From a survey of achiral metal
precursors, [Ru(p-cymene)I2]2 emerged as a suitable catalyst for
in situ generation of dihydoquinoxaline 4a because of its low efficiency
in giving 3a (eq 8 in Scheme 3).
9
10f
11g
12g
13g
14g
15g
À
>95
>95
>95
>95
91
86
88
90 (R)
89
84
a Conditions: 2a (0.05 mmol), [Ru(p-cymene)I2]2 (2.5 mol %), (S)-1
(6 mol %), H2 (600 psi), solvent (2 mL), 16 h, rt. b Determined by 1H
NMR analysis. c Determined by HPLC. d H2 (400 psi). e 40 °C. f (S)-1c
(11 mol %). g 2a (2.0 mmol), 48 h.
Initially, moderate conversion (67%) and ee (58%) were obtained
in CHCl3 under 600 psi hydrogen at ambient temperature using
[Ru(p-cymene)I2]2/(S)-1a as the catalyst (Table 1, entry 1). Further
investigations of the solvent effect suggested that benzene was the best
solvent for the hydrogenation of 2-phenylquinoxaline with respect to
enantioselectivity (80% ee; entries 2À6). Reducing the pressure of
hydrogen slightly improved the enantioselectivity (81% ee) but
deteriorated the catalytic activity (entry 7). A small decrease in ee
was observed with increasing reaction temperature (entry 8). The
evaluation of chiral phosphoric acids showed that (S)-1b was the best
acid for this transformation in terms of enantioselectivity (90% ee;
entry 9). Tetrahydroquinoxaline 3a was not observed upon the
addition of silver phosphate/(S)-1c, which undergoes an exchange
of counterion with Ru(II) complex; thus, a chiral-counterion-oriented
ruthenium-catalyzed asymmetric hydrogenation process was ex-
cluded (entry 10). Complete conversion was obtained by prolonging
the reaction time, albeit with a slight decrease in ee (entry 11).
Interestingly, the best results with respect to reactivity and enantios-
electivity were achieved when the catalyst loading was reduced to 1
mol % (entry 13). Notably, 91% conversion was obtained at
substrate-to-catalyst ratios of 1000 that the catalyst loading was
relatively low in the asymmetric transformation involving chiral
phosphoric acids (entry 15). Therefore, the optimal conditions were
established to be [Ru(p-cymene)I2]2 (0.5 mol %)/(S)-1b (1.2 mol
%)/H2 (600 psi)/benzene/rt.
With the optimized conditions in hand, we explored the scope of
the enantioselective synthesis of tetrahydroquinoxalines through
convergent disproportionation of dihydroquinoxalines (Table 2). In
general, excellent yields and enantioselectivities were obtained in the
asymmetric reduction of various quinoxalines promoted by self-
transfer hydrogenation of dihydroquinoxalines (entries 1, 3, 5À7,
and 9À13). A moderate yield (51%) but excellent enantioselectivity
(93%) were obtained in the hydrogenation of 6-chloro-2-phenylqui-
noxaline (entry 15). Interestingly, an unexpected reversal of enantio-
selectivity for the convergent disproportionation of dihydroquino-
xalines promoted by H2 relative to the pure organocatalytic transfer
hydrogenation of quinoxalines with Hantzsch esters developed by
Table 2. Asymmetric Reduction of Quinoxalines through
Convergent Disproportionation of Dihydroquinoxalinesa
entry
Ar in 2
Ph
R in 2
yield (%)b
ee (%)c
1
H
H
H
H
H
H
H
H
H
H
H
H
H
H
6-Cl
96 (3a)
98 (3a)
98 (3b)
98 (3b)
98 (3c)
98 (3d)
97 (3e)
88 (3e)
95 (3f)
98 (3g)
96 (3h)
97 (3i)
98 (3j)
82 (3j)
51 (3k)
90 (R)
90 (S)
83 (R)
94 (S)
89 (R)
87 (R)
87 (R)
96 (S)
86 (R)
90 (R)
93 (R)
94 (R)
94 (R)
90 (S)
93 (R)
2d
3
Ph
4-MeC6H4
4-MeC6H4
4-FC6H4
4-ClC6H4
4-BrC6H4
4-BrC6H4
4-CF3C6H4
3-FC6H4
3-ClC6H4
3-BrC6H4
2-naphthyl
2-naphthyl
Ph
4d
5
6
7
8d
9
10
11
12
13
14d
15
a Conditions: 2 (0.20 mmol), [Ru(p-cymene)I2]2 (0.5 mol %), (S)-1b (1.2
mol %), H2 (600 psi), benzene (2 mL), 48 h, rt. b Isolated yields.
c Determined by HPLC. d Data quoted from ref 8i. (R)-1b was used in
Rueping’s work for the formation of (R)-3a. For a simple discussion, (S)-1b
was employed in this table. Reaction conditions: 2 (0.1 mmol), Hantzsch
ethyl ester (0.24 mmol), (S)-1b (10 mol %), 35 °C, CHCl3 (0.5 M), 24 h.
Rueping and co-workers (entries 1 vs 2, 3 vs 4, 7 vs 8, and 13 vs 14)
was observed.8i 2-Alkyl-substituted quinoxalines were also tested but
resulted in low enantioselectivities because of enhancement of the
background reaction.
Further experiments were focused on elucidation of the role played
by the N-H in intermediate 4a in regard to enantioselective control in
the convergent disproportionation process (Scheme 4). As predicted,
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Scheme 4. Hydrogenation of Dihydroquinoxalines 4a and 4b
Scheme 5. Origin of Enantioreversal in the Asymmetric
Reduction of Quinoxalines Resulting from Different
Hydride Transfer Paths
Figure 1. Transition states for the self-transfer hydrogenation. Bond
lengths are given in Å, and relative electronic energies in kcal/mol are
given in parentheses. Hydrogen atoms, except those forming hydrogen
bonds, have been omitted for clarity.
Scheme 6. Proposed Mechanism for the Convergent
Asymmetric Disproportionation of Dihydroquinoxalines
the hydrogenation of intermediate 4a gave tetrahydroquinoxaline 3a
with the same enantioselectivity as in the hydrogenation of 2a (eq9in
Scheme 4). Poor ee (7%) and low conversion (24%) were observed
in the hydrogenation of N-methyl-3,4-dihydroquinoxaline 4b because
of its inability to undergo self-transfer hydrogenation (eq 10 in
Scheme 4).12 These results suggest that the N-H in intermediate 4a
is crucial for the realization of convergent disproportionation.
The origin of enantioreversal in the asymmetric reduction of
quinoxalines can be explained by the stereochemical model illu-
strated in Scheme 5. For convergent disproportionation of dihy-
droquinoxalines, the hydrogenation of 2a first delivers intermediate
3,4-dihydroquinoxaline 4a, which subsequently interacts with chiral
phosphoric acid (S)-1b through two hydrogen bonds (eq 11 in
Scheme 5). These two hydrogen bonds with the phosphate and the
effect of steric hindrance build up a “three-point contact model” that
determines the stereoselectivity in the disproportionation of 3,4-
dihydroquinoxaline 4a. In the pure organocatalytic process, 4a/(S)-
1b/HEH form another “three-point contact model” leading to Re-
face reduction on the basis of Goodman and Himo’s calculation
(eq 12 in Scheme 5).13 The reversal of enantioselectivity perhaps
lies in the different steric demands of the 1,2-hydride transfer
pathway in the self-transfer hydrogenation of 4a and the 1,4-hydride
transfer pathway using HEH (Scheme 5).
The theoretical studies provided additional evidence to support the
self-transfer hydrogenation mechanism proposed in Scheme 5.
Concerning the mechanism, we assume that the hydrogenation of
quinoxalines 2 first generates dihydroquinoxalines 4 with ruthenium-
(II) as the catalyst (Scheme 6). Subsequently, the intermediates 4
undergo self-transfer hydrogenation to deliver primary starting
materials 2 and final products 3 in the presence of Brønsted acid
(S)-1b. The detection of intermediate 4a in the hydrogenation of 2a
without the addition of Brønsted acid (S)-1b suggests that the first
hydrogenation process catalyzed by ruthenium(II) is the rate-deter-
mining step (eq 8 in Scheme 3). Therefore, the excellent enantios-
electivities observed in this transformation are attributed to the fact
that the reaction rate of this principal reaction (k2) is higher than that
of the undesired side reaction (k3) (Scheme 6).
In summary, we have successfully developed an efficient metal/
Brønsted acid relay catalysis system for the highly enantioselective
hydrogenation of quinoxalines through convergent asymmetric dis-
proportionation of dihydroquinoxalines with up to 94% ee. Employ-
ing hydrogen gas as the reductant makes this convergent dispropor-
tionation an ideal atom-economical process. A dramatic reversal of
enantioselectivity was observed for the hydrogenation relative to the
Density functional theory calculations at the B3LYP/6-31G** level
were carried out to validate this proposal (Figure 1). The transition
state TS-R for the formation of (R)-3a is more favorable than TS-S by
1.01 kcal/mol, in good agreement with the experimental observations.
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under hydrogenation conditions.14 Further study will be directed
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’ ASSOCIATED CONTENT
S
Supporting Information. Complete experimental pro-
b
cedures 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; fanhj@dicp.ac.cn
’ ACKNOWLEDGMENT
This work was supported by the National Natural Science
Foundation of China (21032003 and 20921092), the National
Basic Research Program of China (2010CB833300), and the
Dalian Institute of Chemical Physics (K2010F1).
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