The first general, highly enantioselective lewis base organocatalyzed hydrosilylation of benzoxazinones and quinoxalinones

Article abstract of DOI:10.1002/adsc.201000274

The first general, highly enantioselective hydrosilylation of benzoxazinones and quinoxalinones has been developed. The chiral Lewis base organocatalysis that are readily accessible from (1S,2R)-ephedrine and (1R,2S)-ephedrine promoted the title reaction to afford various chiral dihydrobenzoxazinones and dihydroquinoxalinones with good yields as well as good enantioselectivities.

Full text of DOI:10.1002/adsc.201000274

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DOI: 10.1002/adsc.201000274
The First General, Highly Enantioselective Lewis Base Organo-
catalyzed Hydrosilylation of Benzoxazinones and Quinoxalinones
Zhou-Yang Xue,a,b Yan Jiang,a,b Xiao-Zhi Peng,a,b Wei-Cheng Yuan,a
and Xiao-Mei Zhanga,*
a
Key Laboratory for Asymmetric Synthesis and Chiraltechnology of Sichuan Province, Chengdu Institute of Organic
Chemistry, Chinese Academy of Sciences, Chengdu 610041, Peopleꢀs Republic of China
Fax : (+86)-28-8522-9250; e-mail: xmzhang@cioc.ac.cn
Graduate School of Chinese Academy of Sciences, Beijing 100049, Peopleꢀs Republic of China
b
Received: April 8, 2010; Published online: September 1, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000274.
Abstract: The first general, highly enantioselective
hydrosilylation of benzoxazinones and quinoxali-
nones has been developed. The chiral Lewis base
organocatalysts that are readily accessible from
(1S,2R)-ephedrine and (1R,2S)-ephedrine promoted
the title reaction to afford various chiral dihydro-
benzoxazinones and dihydroquinoxalinones with
good yields as well as good enantioselectivities.
in asymmetric synthesis. Many efficient chiral Lewis
bases have been developed to catalyze the enantiose-
lective hydrosilylation of ketimines with trichlorosi-
lane.[8] This mild and economical organocatalytic
methodology has become a promising alternative to
transition metal catalysis in the synthesis of chiral
amines. However, up to now, a very limited number
of examples of chiral Lewis base-catalyzed enantiose-
lective hydrosilylation of benzoxazinones were report-
ed and the results are not satisfactory yet.[9] Herein
we describe the first general, highly enantioselective
Lewis base organocatalyzed hydrosilylation of ben-
zoxazinones and quinoxalinones.
Keywords: dihydrobenzoxazinones; dihydroquinox-
alinones; enantioselective hydrosilylation; Lewis
bases; organocatalysis
During our continuing research on the enantiose-
lective Lewis base organocatalyzed hydrosilylation of
C=N double bond compounds, a series of chiral 2-pi-
Dihydrobenzoxazinones and dihydroquinoxalinones colinamide Lewis base organocatalysts (Figure 1, 1b–
represent the important structural motif of clinically 1h) was developed by us to promote the enantioselec-
significant pharmaceuticals and biologically active tive hydrosilylation of several kinds of substrates,
molecules.[1] These heterocyclic compounds are also such as ketimines, b-enamino esters and a-imino es-
pivotal intermediates for the synthesis of many phar-
maceutical compounds.[2] Hence, synthesis of these
heterocyclic compounds has attracted much atten-
tion.[3] However, up to now, only a few examples con-
cerning the enantioselective synthesis of these com-
pounds have been described. Lectka and co-workers
reported the enantioselective [4+2] cycloaddition of
o-benzoquinone imides or o-benzoquinone diimides
and ketene enolates to generate dihydrobenzoxazi-
nones or dihydroquinoxalinones with excellent enan-
tioselectivities.[4] Rueping et al.[5] and MacMillan
et al.[6] employed chiral phosphoric acids in promoting
enantioselective transfer hydrogenation of benzoxazi-
nones and quinoxalinones, respectively, and excellent
enantioselectivities were demonstrated.
Recently the strategy of metal-free chiral Lewis
Figure 1. Chiral Lewis base organocatalysts evaluated in this
base activation of Lewis acids[7] has been widely used study.
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Adv. Synth. Catal. 2010, 352, 2132 – 2136

The First General, Highly Enantioselective Lewis Base Organocatalyzed Hydrosilylation
Table 2. Optimization of the reaction conditions.[a]
ters.[9b,10] Due to the significance of chiral dihydroben-
zoxazinones and dihydroquinoxalinones in pharma-
cology, we are also interested in extending the meth-
odology to cyclic benzoxazinones and quinoxalinones
through which chiral dihydrobenzoxazinones and di-
hydroquinoxalinones can be easily prepared.
Hence, catalysts 1a[11]–1h were first investigated in
the hydrosilylation of 3-phenyl-2H-benzo[b]
azin-2-one (2a) in chloroform at 08C. The results
were summarized in Table 1. Proline-derived catalyst
1a gave moderate enantioselectivity (Table 1,
ACHTUNGTRENN[UNG 1,4]ox-
ACHTUNGTRENNUNG
a
Entry Solvent
Additive
[equiv.]
t
Yield
ee
entry 1). When catalysts 1b–1f which bear bulky
groups at C-4 of the pyrrolidine rings were employed,
much higher ee values were observed (Table 1, en-
tries 2–6). However, these results were still unsatisfac-
tory. Therefore, acyclic catalysts 1g–1h were tested.
Catalyst 1g delivered a very poor enantioselectivity
(Table 1, entry 7). To our delight, ephedrine-derived
catalysts 1h and (ent)-1h displayed good reactivities as
well as good ee values (Table 1, entries 8 and 9), pro-
viding the products with opposite configurations in
high enantioselectivities.
Therefore, 1h and (ent)-1h were determined as the
optimal catalysts in this study. Catalyst 1h was em-
ployed in the further investigations. Consequently,
various solvents were screened. Chlorinated solvents
and THF delivered good ee values (Table 2, entries 1–
3 and 5). Toluene resulted in inferior reactivity and
enantioselectivity (Table 2, entry 4). Among the
chlorinated solvents, dichloromethane gave the best
ee value, whereas the yield was not satisfactory. It is
gratifying that prolonging the reaction time to 48 h
[h] [%][b]
[%][c]
1
CHCl3
none
none
24 91
24 57
24 66
24 27
24 57
48 94
48 84
35 77
74 86
69 98
69 89
88
91
85
83
88
90
90
87
91
97
93
2
CH2Cl2
3
ClCH2CH2Cl none
4
5
toluene
THF
none
none
none
4ꢂ MS
H2O (0.10)
H2O (0.11)
H2O (0.12)
H2O (0.13)
6
7
8
9
10
11
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
[a]
Unless specified otherwise, the reaction was carried out
with 2a (0.5 mmol), trichlorosilane (1.0 mmol) and cata-
lyst 1h (0.05 mmol) in 2 mL of solvent with or without
additive at 08C for the specified time.
[b]
[c]
Isolated yield.
The ee was determined by chiral HPLC (Chiralcel OD).
provided a high yield of the product without erosion
of the ee value (Table 2, entry 6). Thus dichlorome-
thane was selected as the optimal solvent. However,
addition of 4ꢂ molecular sieves in the reaction mix-
ture resulted in an obvious decrease in yield (Table 2,
entry 7). It suggested that a trace of water in the mix-
ture promoted the reaction to some extent. In order
to elucidate the effect of water in the reaction, the
amount of water in the reaction system was carefully
checked. The solvent and the reaction tube were
dried severely. Then several quantities of water were
added in the reaction system. It was demonstrated
that the amount of water affected the yield as well as
the enantioselectivity. When 0.10 equiv. of water was
added, the reaction gave a moderate yield of the
product with decreased ee value (Table 2, entry 8).
0.11 equiv. of water delivered a superior yield and
enantioselectivity (Table 2, entry 9). To our surprise,
when 0.12 equiv. of water were added, a quantitative
product yield with an excellent enantioselectivity of
97% ee was obtained (Table 2, entry 10). We reasoned
that part of the trichlorosilane decomposed in the
presence of water to generate HCl which activated
the benzoxazinone so that the reactivity and enantio-
selectivity both increased. Further increasing the
Table 1. Screen of chiral Lewis base catalysts in hydrosilyla-
tion of benzoxazinone 2a.[a]
Entry
Cat*
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
83
57
73
74
71
52
91
91
89
57
76
72
75
72
72
11
88
88
ACHTUNGTRENNUNG(ent)-1h
[a]
Unless specified otherwise, the reaction was carried out
with 2a (0.5 mmol), trichlorosilane (1.0 mmol) and cata-
lyst (0.05 mmol) in 2 mL of chloroform at 08C for 24 h.
Isolated yield.
The ee value was determined by chiral HPLC (Chiralcel
OD).
[b]
[c]
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Zhou-Yang Xue et al.
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amount of water to 0.13 equiv. led to a decrease in
Furthermore, this enantioselective hydrosilylation
yield and ee value (Table 2, entry 11).
procedure was extended to quinoxalinones 4. Rueping
With the optimized conditions in hand, we exam- and co-workers employed 1-unsubstituted quinoxali-
ined the scope of the reaction with a variety of ben- nones as substrates in chiral phosphoric acid-cata-
zoxazinones (Table 3). Generally, for 3-substituted lyzed enantioselective transfer hydrogenation.[5b] Al-
phenylbenzoxazinones, para or meta substituents on though excellent enantioselectivities were observed,
the phenyl groups exerted no deleterious effect on re- the reactions gave only moderate yields of the prod-
activity but produced an obvious decrease in enantio- ucts due to the poor solubility of the substrates. To
selectivity, no matter whether the substituents were our delight, we found that this problem can be cir-
electron-donnating or electron-withdrawing (Table 3, cumvented by introduction of a substituent on 1-N of
entries 2, 3, 5–8). ortho Substitution on the phenyl quinoxalinone. Thus, readily soluble 1-methyl-3-phe-
group resulted in dramatic drops in yield as well as nylquinoxalin-2(1H)-one 4a was selected as the stan-
enantioselectivity (Table 3, entry 4). Perhaps this is dard substrate in search of the optimal reaction condi-
due to the steric hindrance of the ortho substituent tions. Screening of the catalysts revealed that 1h and
which made it difficult for the catalyst to act on the (ent)-1h derived from (1S, R)-ephedrine and (1R,2S)-
substrate. 3-Heteroarylbenzoxazinone 2i underwent ephedrine proved to be the catalysts of choice once
the reaction smoothly to afford the corresponding again (Table 4, entries 6 and 7). Examination of sever-
product with high yield and good ee value (Table 3, al chlorinated solvents displayed that chloroform was
entry 9). Finally, two 3-alkylbenzoxazinones were also the most appropriate solvent. Dichloromethane gave
subjected to the enantioselective hydrosilylation to the product with the same ee value but lower yield
provide the desired products with good yields but (Table 4, entry 8). On the other hand, 1,2-dichloro-
poor ee values (Table 3, entries 10 and 11).
ethane afforded quantitative yield but inferior enan-
tioselectivity (Table 4, entry 9).
Having established the optimal reaction conditions,
the generality of this reaction was examined. In the
presence of 10 mol% of catalyst 1h, a wide variety of
quinoxalinones was reduced in chloroform at 08C
with trichlorosilane. The results are summarized in
Table 5. Generally, 3-para- or meta-substituted phe-
nylquinoxalinones displayed high reactivity as well as
high enantioselectivity, no matter whether the sub-
Table 3. Enantioselective hydrosilylation of benzoxazinones
2a–2k catalyzed by 1h.[a]
Table 4. Screen of chiral Lewis base catalysts and solvents in
hydrosilylation of quinoxalinones 4a.[a]
Entry 2 (R4)
t
Yield ee
Conf.[d]
[h]
[%][b] [%][c]
1
2
3
4
5
6
7
8
2a (Ph)
2b (4-Me-C6H4)
2c (4-MeO-C6H4)
69
65
69
98
98
95
97
86
80
48
89
89
87
90
86
8
(+)
(+)
(+)
(À)
(+)
(+)
(+)
(+)
R (+)
(À)
(À)
2d (2-MeO-C6H4) 120 60
Entry[a] Cat*
Solvent
Yield [%][b] ee (%)[c]
2e (4-F-C6H4)
2f (3-F-C6H4)
2g (4-Cl-C6H4)
2h (4-Br-C6H4)
2i (2-thienyl)
2j (C6H5CH2CH2) 45
2k (Me) 94
61
98
58
78
72
94
90
95
97
95
96
86
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1h
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
92
86
68
77
81
96
94
86
66
83
82
81
9
10
11
75
16
À92
A
N
92 (>99)[d]
[a]
Unless specified otherwise, the reaction was carried out
with 2 (0.5 mmol), trichlorosilane (1.0 mmol), catalyst
(0.05 mmol) and water (1.1 mL, 0.12 equiv.) in 2 mL of di-
chloromethane at 08C for the specified time.
Isolated yield.
The ee was determined by chiral HPLC (Chiralcel OD,
AD or OJ).
The absolute configuration of 3i was determined by com-
parison of its optical rotation value with the literature
data.[5a]
92
90
AHCTUNGTRE(GUNNN ent)-1h ClCH2CH2Cl 97
[a]
Unless specified otherwise, the reaction was carried out
with 4a (0.3 mmol), trichlorosilane (0.6 mmol) and cata-
lyst (0.03 mmol) in 2 mL of solvent at 08C for 24 h.
Isolated yield.
The ee was determined by chiral HPLC (Chiralcel AD).
The ee value in parentheses was obtained after a single
recrystallization.
[b]
[c]
[b]
[c]
[d]
[d]
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The First General, Highly Enantioselective Lewis Base Organocatalyzed Hydrosilylation
Table 5. Enantioselective hydrosilylation of quinoxalinones 4a–4q catalyzed by 1h.[a]
Entry
4
R5
R6
t [h]
5
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
MOM
allyl
Bn
24
28
34
40
30
60
32
33
28
60
28
45
28
72
33
33
33
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5n
5o
5p
5q
96
95
96
93
92
98
97
97
98
46
89
trace
98
95
90
97
94
92
89
91
89
90
74
92
4-Me-C6H4
3-Me-C6H4
3,4-diMe-C6H3
4-MeO-C6H4
2-MeO-C6H4
4-F-C6H4
3-F-C6H4
4-Cl-C6H4
2-Cl-C6H4
4-Br-C6H4
1-naphthyl
2-thienyl
Me
91
9
88 (>99)[d]
10
11
12
13
14
15
16
17
37
94
–
93
8
91
91
91
Ph
Ph
Ph
[a]
Unless specified otherwise, the reaction was carried out with 4 (0.3 mmol), trichlorosilane (0.6 mmol) and catalyst
(0.03 mmol) in 2 mL of chloroform at 08C for the specified time.
Isolated yield.
The ee was determined by chiral HPLC (Chiralcel OD, AD or OJ).
The ee value in parentheses was obtained after a single recrystallization.
[b]
[c]
[d]
stituents were electron-donnating or electron-with- action to afford high yields of corresponding products
drawing (Table 5, entries 2–5, 7–9, 11). 3-ortho-Me- in high eantioselectivities (Table 5, entries 15–17).
thoxyphenylquinoxalinone 4f also gave high yield of
In conclusion, we have developed a general, highly
the product after a prolonged recation time, although enantioselective hydrosilylation of benzoxazinones
the ee value was inferior (Table 5, entry 6). However, and quinoxalinones promoted by chiral Lewis base or-
3-ortho-chlorophenylquinoxalinone 4j exhibited poor ganocatalysts that are readily accessible from (1S,2R)-
reactivity and enantioselectivity (Table 5, entry 10) ephedrine and (1R,2S)-ephedrine. This transformation
suggesting an unfavourable steric effect of the ortho enables the straightforward and mild synthesis of vari-
substitution. Moreover, 1-methyl-3-(naphthalen-1-yl)- ous chiral dihydrobenzoxazinones and dihydroquinox-
quinoxalin-2(1H)-one 4l gave only a trace of the alinones in high yields (up to 98%) and enantioselec-
product (Table 5, entry 12). This might result from the tivities (up to 97% ee). The absolute configuration of
bulkiness of the naphthyl group causing it to twist out 3i has been determined as R. Further work is in prog-
of the conjugate plane, which made the catalyst hard ress to extend this reaction in the construction of
to approach the substrate. 3-Heteroarylquinoxalinone pharmaceutic active candidates.
4m underwent the reaction smoothly to generate the
corresponding product with high yield and good ee
value (Table 5, entry 13). Reaction of 3-methylquinox-
alinone 4n proceeded for a prolonged time to provide
Experimental Section
a quantitative yield of product, however, with very
poor ee value (Table 5, entry 14). Finally, we found
that varying the 1-substituent did not affect the reac-
General Procedure for Enantioselective
Hydrosilylation of Benzoxazinones 2a–2k (Table 3)
tivity as well as enantioselectivity. 1-MOM-, 1-allyl-
and 1-benzylquinoxalinones 4o–4q underwent the re- 0.3 mL of CH2Cl2) was added to a solution of the catalyst
Trichlorosilane (102 mL 1.0 mmol, 2.0 equiv., dissolved in
Adv. Synth. Catal. 2010, 352, 2132 – 2136
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Zhou-Yang Xue et al.
COMMUNICATIONS
(0.05 mmol) and the corresponding benzoxazinone
(0.50 mmol) in 2 mL of dry CH2Cl2 and H2O (1.1 mL,
0.12 equiv.) at 08C. The reaction mixture was stirred at 08C
until the reaction stopped. Then the reaction was quenched
with saturated aqueous solution of NaHCO3. The mixture
was extracted with EtOAc and the combined extracts was
washed with brine and dried over anhydrous MgSO4. Con-
centration in vacuum followed by flash chromatography on
silica gel with hexane/ethyl acetate as the eluent afforded
the pure product. The ee values were determined using es-
tablished HPLC techniques with chiral stationary phases.
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General Procedure for Enantioselective
Hydrosilylation of Quinoxalinones 4a–4q (Table 5)
Trichlorosilane (102 mL, 0.6 mmol, 2.0 equiv., dissolved in
0.3 mL of CHCl3.) was added to a solution of the catalyst
(0.03 mmol) and the corresponding quinoxalinone
(0.3 mmol) in 2 mL of dry CHCl3 at 08C. The reaction mix-
ture was stirred at 08C until the reaction stopped. Then the
reaction was quenched with saturated aqueous solution of
NaHCO3. The mixture was extracted with EtOAc and the
combined extracts was washed with brine and dried over an-
hydrous MgSO4. Concentration in vacuum followed by flash
chromatography on silica gel with hexane/ethyl acetate as
the eluent afforded the amine. The ee values were deter-
mined using established HPLC techniques with chiral sta-
tionary phases.
[5] a) M. Rueping, A. P. Antonchick, T. Theissmann,
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J. Am. Chem. Soc. 2006, 128, 84–86.
[7] For reviews on Lewis base activation of Lewis acids,
see: a) S. Rendler, M. Oestreich, Synthesis 2005, 1727–
1747; b) Y. Orito, M. Nakajima, Synthesis 2006, 1391–
1401; c) S. E. Denmark, G. L. Beutner, Angew. Chem.
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Acknowledgements
We are grateful for the financial support from National Sci-
ences Foundation of China (20772122, 20972155) and Na-
tional Basic Research Program of China (973 Program)
(2010CB833301).
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