Enantioselective cyanosilylation of α,α-dialkoxy ketones catalyzed by proline-derived in-situ-prepared N-oxide as bifunctional organocatalyst

Article abstract of DOI:10.1021/jo062336i

Bifunctional N,N′-dioxide catalysts have been developed for highly enantioselective cyanosilylation of α,α-dialkoxy ketones. This process, catalyzed by in-situ-prepared proline-derived N,N′-dioxide 2b, produced the corresponding cyanohydrin trimethylsilyl ethers in excellent yields (up to 99%) with high enantioselectivities (up to 93% ee). A reasonable mechanism was proposed according to the observation of the linear effect, ¹H NMR spectra, isolated cyanohydrin, and the roles of the NH and N-oxide moieties of the catalyst.

Full text of DOI:10.1021/jo062336i

Enantioselective Cyanosilylation of r,r-Dialkoxy Ketones Catalyzed
by Proline-Derived in-Situ-Prepared N-Oxide as Bifunctional
Organocatalyst
Bo Qin,† Xiaohua Liu,† Jian Shi,† Ke Zheng,† Haitao Zhao,† and Xiaoming Feng*,†,‡
Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan
UniVersity, Chengdu 610064, China and State Key Laboratory of Biotherapy, West China Hospital,
Sichuan UniVersity, Chengdu 610041, China
xmfeng@scu.edu.cn
ReceiVed NoVember 13, 2006
Bifunctional N,N′-dioxide catalysts have been developed for highly enantioselective cyanosilylation of
R,R-dialkoxy ketones. This process, catalyzed by in-situ-prepared proline-derived N,N′-dioxide 2b,
produced the corresponding cyanohydrin trimethylsilyl ethers in excellent yields (up to 99%) with high
enantioselectivities (up to 93% ee). A reasonable mechanism was proposed according to the observation
of the linear effect, 1H NMR spectra, isolated cyanohydrin, and the roles of the NH and N-oxide moieties
of the catalyst.
Introduction
cyanosilylation of ketones, such as chiral oxazaborolidinium
ion,7 thiourea catalyst,8 and amino acid salt.9 Cyanosilylation
of R,R-dialkoxy ketone (acetal ketone) attracted great concern
owing to the pivotal importance of the acetal group as precursor
to many functionalities.10 In 2003, Deng and co-workers
The enantioselective cyanosilylation of carbonyl compounds
represents an effective approach for the synthesis of optically
active cyanohydrins, which are important intermediates for the
preparation of R-hydroxy and R-amino acids.1 Considerable
efforts have been devoted to the development of efficient
catalytic asymmetric cyanosilylation.2 Recently several chiral
metal complexes3-6 have been identified to be highly efficient
catalysts for cyanation of ketones. However, there are only a
few organocatalytic systems succeeding in the asymmetric
(4) (a) Hamashima, Y.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc.
2000, 122, 7412-7413. (b) Yabu, K.; Masumoto, S.; Yamasaki, S.;
Hamashima, Y.; Kanai, M.; Du, W.; Curran, D. P.; Shibasaki, M. J. Am.
Chem. Soc. 2001, 123, 9908-9909. (c) Hamashima, Y.; Kanai, M.;
Shibasaki, M. Tetrahedron Lett. 2001, 42, 691-694. (d) Yabu, K.;
Masumoto, S.; Kanai, M.; Curran, D. P.; Shibasaki, M. Tetrahedron Lett.
2002, 43, 2923-2926. (e) Masumoto, S.; Suzuki, M.; Kanai, M.; Shibasaki,
M. Tetrahedron Lett. 2002, 43, 8647-8651. (f) Suzuki, M.; Kato, N.; Kanai,
M.; Shibasaki, M. Org. Lett. 2005, 7, 2527-2530.
† College of Chemistry.
‡ West China Hospital.
(1) For reviews on enantioselective construction of quaternary stereo-
centers, see: (a) Fuji, K. Chem. ReV. 1993, 93, 2037-2066. (b) Corey, E.
J.; Guzman-Perez, A. Angew. Chem., Int. Ed. 1998, 37, 388-401. (c)
Christoffers, J.; Mann, A. Angew. Chem., Int. Ed. 2001, 40, 4591-4597.
(2) For reviews on enantioselective synthesis of cyanohydrin trimeth-
ylsilyl ethers and their derivatives, see: (a) Gregory, R. J. H. Chem. ReV.
1999, 99, 3649-3682. (b) Shibasaki, M.; Kanai, M.; Funabashi, K. Chem.
Commun. 2002, 1989-1999. (c) North, M. Tetrahedron: Asymmetry 2003,
14, 147-176. (d) Brunel, J. M.; Holmes, I. P. Angew. Chem., Int. Ed. 2004,
43, 2752-2778. (e) Chen, F. X.; Feng, X. M. Curr. Org. Synth. 2006, 3,
77-97. (f) North, M., Ed. Synthesis and Applications of Non-Racemic
Cyanohydrins and alpha-Amino Nitriles. Tetrahedron 2004, 60, 10371-
10568.
(5) Belokon, Y. N.; Green, B.; Ikonnikov, N. S.; North, M.; Tararov, V.
I. Tetrahedron Lett. 1999, 40, 8147-8150.
(6) (a) Xiong, Y.; Huang, X.; Gou, S. H.; Huang, J. L.; Wen, Y. H.;
Feng, X. M. AdV. Synth. Catal. 2006, 348, 538-544. (b) Chen, F. X.; Zhou,
H.; Liu, X. H.; Qin, B.; Feng, X. M., Zhang, G. L.; Jiang, Y. Z. Chem.
Eur. J. 2004, 10, 4790-4797. (c) Chen, F. X.; Feng, X. M.; Qin, B.; Zhang,
G. L.; Jiang, Y. Z. Org. Lett. 2003, 5, 949-952. (d) Shen, Y. C.; Feng, X.
M.; Li, Y.; Zhang, G. L.; Jiang, Y. Z. Eur. J. Org. Chem. 2004, 129-137.
(e) He, B.; Chen, F. X.; Li, Y.; Feng, X. M.; Zhang, G. L. Eur. J. Org.
Chem. 2004, 4657-4666.
(7) Ryu, D. H.; Corey, E. J. J. Am. Chem. Soc. 2005, 127, 5384-5387.
(8) Fuerst, D. E.; Jacobsen, E. N. J. Am. Chem. Soc. 2005, 127, 8964-
8965.
(3) Deng, H.; Isler, M. P.; Snapper, M. L.; Hoveyda, A. H. Angew. Chem.,
Int. Ed. 2002, 41, 1009-1012.
(9) Liu, X. H.; Qin, B.; Zhou, X.; He, B.; Feng, X. M. J. Am. Chem.
Soc. 2005, 127, 12224-12225.
10.1021/jo062336i CCC: $37.00 © 2007 American Chemical Society
Published on Web 03/06/2007
2374
J. Org. Chem. 2007, 72, 2374-2378

Cyanosilylation of R,R-Dialkoxy Ketones
TABLE 1. N,N′-Dioxide-Catalyzed Asymmetric Cyanosilylation of
developed the first highly enantioselective cyanosilylation of
acetal ketone catalyzed by only 2 mol % (DHQ)2AQN as
commercially available and fully recyclable catalyst.10b,c
Ketonea
catalyst
loading
yield
ee
entry
ketone
acetophenone 2a
N,N-dioxide (mol %) time (h) (%)b (%)c
1
20
20
20
20
10
10
10
10
10
48
17
10
10
10
10
10
10
10
28
n.d.e
99
99
99
95
43
23
82
30
2d
3
4
1
4
2a
54
60
60
40
75
72
72
4
4
2b
As a versatile catalyst, the N-oxide plays a significant role
in many asymmetric reactions of organosilicon chemistry.11
Encouraged by the success of the biquinoline N,N′-dioxide with
the axial chirality (S)-1 in the catalysis of the asymmetric
allylation12 and Strecker reactions,13 we developed a novel C2-
symmetric chiral N,N′-dioxide 2a to catalyze the asymmetric
cyanosilylation of aldehydes successfully.14
5f
6f
7f
8f
9f
4
5
6a
7a
7b
3a/m-CPBA
3a/m-CPBA
3a/m-CPBA
3a/m-CPBA
3a/m-CPBA
a Conditions: the reaction was performed with ketone (0.1 mmol) and
TMSCN (2.0 equiv) in CH2Cl2. b Isolated yield. c Determined by HPLC
analysis. d The reaction was at 0 °C. e Not detected. f N,N′-Dioxide was
generated in situ from diamide 3a (10 mol %) and m-CPBA (20 mol %).
TABLE 2. Effect of Solvent on Asymmetric Cyanosilylation of
Acetal Ketone 6aa
During the course of our study a new method in which the
chiral N-oxide was generated in situ from N-alkyl prolinamide
and m-CPBA (m-chloroperbenzoic acid) was developed to
resolve the sensibility of N-oxide compounds toward moisture.15
Herein, we wish to report the asymmetric cyanosilylation of
acetal ketones catalyzed by in-situ-prepared N,N′-dioxide with
excellent reactivity and enantioselectivity.
entry
solvent
Et2O
t-BuOMe
THF
toluene
CH3CN
hexane
CH2Cl2
CHCl3
yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
9
60
92
99
90
99
99
43
trace
99
64
64
65
57
37
35
75
79
75
Results and Discussion
For the preliminary study, cyanosilylation of acetophenone
with trimethylsilyl cyanide (TMSCN) proceeded sluggishly
(entry 1, Table 1). When acetophenone was replaced with R,R-
diethoxy-1-phenylethanone 4 in the presence of 20 mol %
biquinoline N,N′-dioxide 1, no reaction was observed (entry 2,
Table 1). N,N′-Dioxide 2a bearing the amide moiety was applied
to the reaction, resulting in a high yield and moderate ee (entry
3, Table 1). Further study showed that in-situ-generated 2b from
the corresponding amide and m-CPBA retained the same
asymmetric catalytic activity and enantioselectivity as well as
isolated 2b (entry 5 vs 4, Table 1). For the acetal, the bulkier
dibenzyl acetal 6a was superior to the dimethyl and diethyl ones
(entry 7 vs entries 5 and 6, Table 1). When the acetal group
ClCH2CH2Cl
a Conditions: the reaction was performed with ketone 6a (0.1 mmol)
and TMSCN (2.0 equiv) in solvent (0.5 mL); N,N′-dioxide was generated
in situ from diamide 3a (10 mol %) and m-CPBA (20 mol %). b Isolated
yield. c Determined by HPLC analysis.
was iPr in aliphatic ketone, increased reaction rate and
comparable enantioselectivity were detected (entry 9 vs 8,
Table 1).
To optimize this process, several reaction conditions were
investigated using compound 6a as the model substrate. Initially,
we examined the effect of solvents using 10 mol % of N,N′-
dioxide prepared in situ as the catalyst and 2.0 equiv of TMSCN.
As shown in Table 2, the cyanosilylation reaction usually
proceeded better in halogenated solvents than in the others. Of
the conditions screened, 1,2-dichloroethane was the optimal
solvent. Under this condition, 99% yield and 75% ee of 8a was
obtained (entry 9, Table 2).
(10) (a) Tian, S. K.; Deng, L. J. Am. Chem. Soc. 2001, 123, 6195-
6196. (b) Tian, S. K.; Hong, R.; Deng, L. J. Am. Chem. Soc. 2003, 125,
9900-9901. (c) Tian, S. K.; Chen, Y. G.; Hang, J. F.; Tang, L.; Mcdaid,
P.; Deng, L. Acc. Chem. Res. 2004, 37, 621-631.
(11) For a review on N-oxide in the asymmetric synthesis. see: Malkov,
A. V.; Kocovsky, P. Eur. J. Org. Chem. 2007, 29-36.
With these leading results in hand we switched our effort to
further improving the enantioselectivity by steric modifications
of the N,N′-dioxide. Accordingly, a series of diamides 3 were
prepared (Figure 1, see Supporting Information) and evaluated
in the asymmetric cyanosilylation of acetal ketone 6a (Table
3). The results showed that L-proline-derived diamide 3a was
superior to L-piperidinamide derivative 3g in both yield and ee
(entry 1 vs 7, Table 3). As a bulky alkyl group, tert-butyl could
(12) (a) Nakajima, M.; Sasaki, Y.; Shiro, M.; Hashimato, S. Tetrahe-
dron: Asymmetry 1997, 8, 341-344. (b) Nakajima, M.; Saito, M.; Shiro,
M.; Hashimoto, S. J. Am. Chem. Soc. 1998, 120, 6419-6420.
(13) (a) Liu, B.; Feng, X. M.; Chen, F. X.; Zhang, G. L.; Chui, X.; Jiang,
Y. Z. Synlett 2001, 1551-1554. (b) Jiao, Z. G.; Feng, X. M.; Liu, B.; Chen,
F. X.; Zhang, G. L.; Jiang, Y. Z. Eur. J. Org. Chem. 2003, 3818-3826.
(14) Wen, Y. H.; Huang, X.; Huang, J. L.; Xiong, Y.; Qin, B.; Feng, X.
M. Synlett 2005, 2445-2448.
(15) Huang, J. L.; Liu, X. H.; Wen, Y. H.; Qin, B.; Feng, X. M. J. Org.
Chem. 2007, 72, 204-208.
J. Org. Chem, Vol. 72, No. 7, 2007 2375

Qin et al.
FIGURE 1. Chiral catalyst-precursor employed for asymmetric
cyanosilylation of acetal ketone.
TABLE 3. Effect of Catalyst Structure on the Cyanosilylation of
Acetal Ketone 6aa
FIGURE 2. Relationship between the ee of product 8a and that of
the chiral in-situ-prepared N,N′-dioxide 2b.
entry
diamide 3
yield (%)b
ee (%)c
1
2
3
4
5
6
7
3a
3b
3c
3d
3e
3f
99
99
99
99
99
70
50
75
74
69
70
50
66
70
3g
a Conditions: the reaction was performed with ketone 6a (0.1 mmol)
and TMSCN (2.0 equiv) in ClCH2CH2Cl (0.5 mL); N,N′-dioxide was
generated in situ from diamide 3 (10 mol %) and m-CPBA (20 mol %).
b Isolated yield. c Determined by HPLC.
FIGURE 3. 1H NMR spectra of the NH group of N,N′-dioxide 2b in
diverse reaction stages: (a) 2b, (b) 2b and TMSCN (ratio 1/1), and (c)
2b, TMSCN, and 6a (ratio 1/1/1).18
TABLE 4. Effect of Cosolvent on the Cyanosilylation of Acetal
Ketone 6aa
entry
solvent
cosolvent
ratio (vol.) yield (%)b ee (%)c
1
2
3
4
5
6
7
8
9
ClCH2CH2Cl
58
63
96
97
97
72
86
80
88
85
90
ClCH2CH2Cl CH2Cl2
ClCH2CH2Cl acetone
ClCH2CH2Cl Et2O
3:2
3:2
3:2
3:2
3:2
3:2
3:2
2:3
4:1
ClCH2CH2Cl THF
ClCH2CH2Cl t-BuOMe
ClCH2CH2Cl ethyl acetate
ClCH2CH2Cl CH3CN
ClCH2CH2Cl t-BuOMe
ClCH2CH2Cl t-BuOMe
95
n.d.d
n.d.d
99
89
92
10
99
a Conditions: the reaction was performed with ketone 6a (0.1 mmol)
and TMSCN (2.0 equiv) in the solvent (0.5 mL). b Isolated yield. c Deter-
mined by HPLC. d Not detected.
FIGURE 4. Proposed catalytic cycle.
increase the enantiomeric excess compared to n-butyl (entry 4
vs 5, Table 3). Cyclic groups were superior to the (S)-
phenylethyl group in the enantioselectivity (entries 1-3 vs 6,
Table 3). Bearing a moderate cyclic moiety, diamide 3a was
selected for further optimization.
Generally, decreasing the reaction temperature can improve
the enantioselectivity. However the system could not tolerate
such an operation because of the comparatively high freezing
point of ClCH2CH2Cl (entry 1, Table 4). To resolve this
problem, cosolvent was examined. The product was produced
in higher enantioselectivity (86% ee) with 63% yield in the
mixed solvent of CH2Cl2 and ClCH2CH2Cl (ratio 2:3, entry 2,
Table 4). When acetone as a cosolvent was added to the reaction,
the yield was increased to 96% (entry 3, Table 4). Using
t-BuOMe as cosolvent, the desired product 8a was obtained with
90% ee (entry 6, Table 4), whereas in the presence of ethyl
acetate and CH3CN as cosolvent, the reaction could not proceed
(entries 7 and 8, Table 4). Furthermore, the best ee value was
in the 4:1 mixture of ClCH2CH2Cl and t-BuOMe (entry10,
Table 4).
As demonstrated in Table 5, the aromatic, R,â-unsaturated,
heteroaromatic, and aliphatic acetal ketones 6 and 7 were tested.
Generally, excellent yield and high enantioselectivities of
cyanohydrin trimethylsilyl ethers 8 were obtained with 2.0 equiv
of TMSCN and 10 mol % in-situ-prepared N,N′-dioxide at
2376 J. Org. Chem., Vol. 72, No. 7, 2007

Cyanosilylation of R,R-Dialkoxy Ketones
TABLE 5. Enantioselective Addition of TMSCN to Acetal Ketones Catalyzed by in-Situ-Prepared N,N′-Dioxidea
a Conditions: the reaction was performed with ketone 6 and 7b (0.2 mmol) and TMSCN (2.0 equiv) in the indicated conditions. b Isolated yield. c Determined
by HPLC analysis as described in the Supporting Information.
-45 °C in the 4:1 mixed solvent of ClCH2CH2Cl and t-BuOMe.
Aromatic ketones as well as heterocyclic ketones reacted
smoothly to give the corresponding derivatives 8 in high
enantioselectivities (entries 1-11, Table 5). In particular, acetal
ketone 6c and 6j gave the best enantioselectivities (entries 3
and 10, Table 5). However, the acetal ketones with electron-
donating groups such as Me and MeO generally showed lower
reactivity (entries 7-9, Table 5). The enones 6l-p gave the
1,2-adducts with complete regioselectivity and high enantiose-
lectivities (entries 12-16, Table 4). Notably, product 8q, which
could be converted to a natural product (+)-bisorbicillinolide,
was obtained in 92% ee (entry 17, Table 4).16
In the study of the activation of TMSCN by N,N′-dioxide
2b, the relationship between the product 8a and the catalyst 2b
in enantioselectivity was examined and showed a strong
linear correlation of this reaction, implying that the Si atom of
TMSCN might be activated by one N,N′-dioxide molecule
(Figure 2).17
(16) Hong, R.; Chen, Y.; Deng, L. Angew. Chem., Int. Ed. 2005, 44,
3478-3481.
(17) For reviews on the nonlinear effect in asymmetric synthesis, see:
(a) Girard, C.; Kagan, H. B. Angew. Chem., Int. Ed. 1998, 37, 2922-2959.
(b) Avalos, M.; Babiano, R.; Cintas, P.; Jimenez, J. L.; Palacios, J. C.
Tetrahedron: Asymmetry 1997, 8, 2997-3017.
(18) See Supporting Information for details.
J. Org. Chem, Vol. 72, No. 7, 2007 2377

Qin et al.
TABLE 6. Effect of N,N′-dioxide 2b and N-oxide 9 on the
is shown in Figure 4. In the first stage S1, the hypervalent silicon
intermediate A was formed through a combination of TMSCN
and N,N′-dioxide 2b, enhancing the nucleophilicity of the CN
group; in the second stage S2, acetal ketone 6a was activated
by the catalyst combined through the hydrogen bonding between
CdO and NH to produce B; in the third stage S3, the activated
CN group attacked the “prochiral center” to generate the key
intermediate C, and then the TMS group shifted smoothly to
the cyanohydrin (stage S4) to complete the catalytic cycle and
regenerate the catalyst.
Cyanosilylation of Acetal Ketone 6aa
entry
catalyst
time (h)
temp (°C)
yield (%)b
ee (%)c
1d
2e
2b
9
10
10
-45
-45
99
49
92
7
a Conditions: the reaction was performed with ketone 6a (0.1 mmol)
and TMSCN (2.0 equiv) in the solvent (0.4 mL ClCH2CH2Cl, 0.1 mL
t-BuOMe). b Isolated yield. c Determined by HPLC. d N,N′-Dioxide 2b was
generated in situ from diamide 3a (10 mol %) and m-CPBA (20 mol %).
e 10 mol % N-oxide 9 was used in the reaction.
Conclusions
In summary, we developed a new class of C2-symmetric
bifunctional organocatalysts for the asymmetric cyanosilylation
of R,R-dialkoxy ketones in excellent yields (up to 99%) with
good to high enantioselectivities (up to 93% ee) in mild
conditions. The catalytic system could be tolerant of air and
moisture with the convenient in-situ generation of N,N′-dioxide.
Future efforts will be devoted to investigations of the mecha-
nistic features, scope, and synthetic application of this novel
approach and search for new amide N-oxide catalysts.
1H NMR spectroscopy was recorded to obtain preliminary
insight into the function of the NH moiety of N,N′-dioxide 2b
(Figure 3). The NH proton showed a strong deshielding effect
at 10.67 ppm due to the characteristic strong hydrogen bonding
between N-oxide and the NH proton (Figure 3a). However, upon
combination of TMSCN and N,N′-dioxide 2b in a ratio of 1:1,
a new chemical shift was observed at δ 4.50 ppm (Figure 3b),
suggesting that the NH proton which has been released was no
longer bonding to N-oxide. It confirmed the interaction between
N-oxide and TMSCN, which has been observed in a 29Si NMR
study.13b,15 Upon addition of acetal ketone 6a, the resonance
for the NH moiety significantly shifted downfield to δ 7.89 and
8.23 ppm, indicating that the intermolecular hydrogen bonding
between CdO of acetal ketone and the NH moiety of N,N′-
dioxide existed in the catalytic process (Figure 3c). These results
clearly showed that N,N′-dioxide 2b was a bifunctional orga-
nocatalyst containing N-oxide as a Lewis base to activate
TMSCN and amide hydrogen as a Bro¨nsted acid to activate
the carbonyl group of the substrate.
Compared with N,N′-dioxide 2b, N-oxide 9 with one dipolar
was prepared and evaluated in the reaction (Table 6). However,
only a 7% ee with 49% yield was observed in the cyanosilylation
of acetal ketone 6a. This important piece of evidence proved
that TMSCN was synchronously activated by two dipolars of
N,N′-dioxide 2b and the hexacoordinated silicon was formed
in the reaction. Furthermore, cyanohydrin 10 was detected by
TLC and successfully separated during the course of the reaction
but disappeared after complete conversion of the substrate,
meaning that complex C might be the key intermediate of this
transformation (Figure 4).
Experimental Section
Typical Procedure for the N,N′-Dioxide-Catalyzed Asym-
metric Cyanosilylation of Acetal Ketones. A mixture of chiral
diamide 3a (8.0 mg, 10 mol %) and m-CPBA (7.0 mg, 20 mol %)
in ClCH2CH2Cl (0.4 mL) was stirred at -20 °C for 10 min; then
t-BuOMe (0.2 mL) was added to the mixture. Subsequently,
TMSCN (56 µL, 0.4 mmol) and acetal ketone 6a (66 mg, 0.2 mmol)
in ClCH2CH2Cl (0.4 mL) were added sequentially at -45 °C. After
stirring for 10 h, the reaction mixture was directly purified by
column chromatography on silica gel eluting with ether/petroleum
ether (1/40) to give 3,3-bis(benzyloxy)-2-phenyl-2-(trimethylsily-
loxy)propanenitrile 8a: colorless oil, 99% yield, 92% ee, [R]25
)
D
1
+10.2 (c ) 0.92, CH2Cl2); H NMR (300 MHz, CDCl3) δ 7.56-
7.60 (m, 2H), 7.35-7.40 (m, 8H), 7.19-7.22 (m, 3H), 6.87-6.90
(m, 2H), 4.76 (d, J ) 15.7 Hz, 2H), 4.58 (s, 1H), 4.52 (d, J ) 12.3
Hz, 1H), 4.33 (d, J ) 12.3 Hz, 1H), 0.16 (s, 9H); 13C NMR (75
MHz, CDCl3) δ 137.5, 137.3, 136.7, 128.9, 128.4, 128.2, 128.1,
127.9, 127.8, 127.7, 127.7, 126.7, 119.1, 104.0, 78.7, 71.8, 70.7,
0.9; ESI-HRMS calcd for (C26H29NO3Si + Na+), 454.1809; found,
454.1795.
Acknowledgment. This work was financially supported by
the National Natural Science Foundation of China (nos.
20225206, 20390055, and 20372050), the Ministry of Education,
P. R. China (no. 104209), and the Student Innovation Fund of
Sichuan University (no. 2006L011). We also thank the Sichuan
University Analytical and Testing Center for NMR spectra
analysis.
On the basis of our investigations and previous results on
the hydrogen bonding of N-oxide,19 the catalytic cycle proposed
Supporting Information Available: Experimental procedures
(19) (a) Taylor, M. S.; Jacobsen, E. N. Angew. Chem., Int. Ed. 2006,
45, 1520-1543. (b) Derdau, V.; Laschat, S.; Hupe, E.; Ko¨nig, W. A.; Dix,
I.; Jones, P. G. Eur. J. Inorg. Chem. 1999, 1001-1007. (c) Aurich, H. G.;
Soeberdt, M.; Harms, K. Eur. J. Org. Chem. 1999, 1249-1252. (d) O’Neil,
I. A.; Miller, N. D.; Peake, J.; Barkley, J. V.; Low, C. M. R.; Kalindjian,
S. B. Synlett 1993, 515-518.
1
and characterization of products for catalysts and racemates, H
NMR, 13C NMR spectra, HRMS and HPLC conditions, etc. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
JO062336I
2378 J. Org. Chem., Vol. 72, No. 7, 2007