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Functionalized Chiral Ionic Liquids as Highly Efficient Asymmetric Organocatalysts for Michael Addition to Nitroolefins.

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Angewandte
Chemie
employed as chiral additives indeed helped to induce
moderate enantioselectivity in some cases, such as in photoisomerization,[5] the Baylis–Hillman reaction,[6] and Michael
additions,[7] the full potential of CILs in asymmetric synthesis
still remains undisclosed. Compared to the extensive works
on the achiral ionic-liquid catalysts,[8] essentially no highly
efficient catalyst of the CIL type has been reported up to now.
Here, we present our recent discovery of a new class of
distinctly functionalized CILs that act as highly efficient
asymmetric organocatalysts.
This work was inspired by the extraordinary success of
chiral pyrrolidines and imidazolines as highly enantioselective
organocatalysts (Scheme 1, left and middle).[9] Since the
Chiral Ionic Liquids
DOI: 10.1002/ange.200600048
Functionalized Chiral Ionic Liquids as Highly
Efficient Asymmetric Organocatalysts for
Michael Addition to Nitroolefins**
Scheme 1. Privileged organic catalysts.
Sanzhong Luo,* Xueling Mi, Long Zhang, Song Liu,
Hui Xu, and Jin-Pei Cheng*
Ionic liquids, especially functional ionic liquids (FIL; or taskspecific ionic liquids, TSIL), have received growing attention
recently due to their tuneable features for various chemical
tasks and their advantages as reusable homogeneous supports, reaction media, and reagents with “green” credentials.[1] In general, FILs are designed and synthesized by
attachment of functional groups onto the side chains of ionic
liquids. Such chemical functionalization usually enhances the
versatility of ionic liquids, thereby leading to a large number
of diverse FILs with improved properties.[1i, 2] Important
progress along this line has been made with the development
of chiral ionic liquids (CILs).[3] Following the initial work of
Seddon and co-workers,[4] a number of CILs were employed
as chiral-resolution reagents, chiral solvents, or chiral additives.[3c] Although it has been shown that chiral ionic liquids
[*] Dr. S. Luo, H. Xu, Prof. Dr. J.-P. Cheng
Centre for Molecular Science
Institute of Chemistry
Chinese Academy of Sciences
Beijing, 100080 (P.R. China)
Fax: (+ 86) 10-6255-4449
E-mail: luosz@iccas.ac.cn
chengjp@mail.most.gov.cn
X. Mi, L. Zhang, S. Liu, Prof. Dr. J.-P. Cheng
Department of Chemistry and State-key Laboratory of Elementoorganic Chemistry
Nankai University
Tianjin, 300071 (P.R. China)
Fax: (+ 86) 10-5888-1878
[**] This work was supported by the Natural Science Foundation of
China (grant nos.: NSFC 20452001, 20421202, and 20542007), the
Ministry of Science and Technology (MoST), and the Institute of
Chemistry, Chinese Academy of Sciences (ICCAS). We thank Prof. D.
Wang, Prof. Q. H. Fan, and Dr. L. Liu for the HPLC instrument.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. 2006, 118, 3165 –3169
seminal works of List et al.[10] and MacMillan and co-workers,[11] a great number of pyrrolidine- and imidazoline-type
asymmetric organocatalysts have been reported. The cyclic
five-membered secondary amine structure of these compounds is now regarded as one of the “privileged” backbones
for asymmetric catalysis.[9a] Thus, our design for functionalized CILs (for example, 2 a in Scheme 1) comprises such a
“privilieged” chiral pyrrolidine unit covalently tethered to an
IL moiety, so that the former can serve as a catalytic site and
the latter as both the phase tag and a chiral-induction group.
Previous studies have shown that the use of ionic liquids as
reaction media or phase tags for organocatalysts facilitated
catalyst recycling and offered enhanced reactivity and enantioselectivity.[12] In this work, we envisaged that ionic-liquidtype organocatalysts such as 2 a would still maintain the
unique properties of an ionic liquid and would also serve as an
efficient catalyst for judiciously selected reactions. The role of
imidazolium as a chiral-induction group may be rationalized
by considering that 1) the bulky and planar organic cation
may impart space shielding to the reaction intermediate and
2) the proximity of the ionic-liquid unit to the active site may
create a microenvironment that is favorable for the reaction.
In fact, the high polarity and ionic character of the ILs exert
synergistic effects on many organic reactions.[1i, 13]
A series of the pyrrolidine-type CILs were synthesized
from the “chiral pool” using l-proline as a starting material
(Scheme 2). The synthetic procedures were quite straightforward and afforded the product (for example, 2 a) in 45 % total
yield from l-proline. All the CILs obtained are viscous liquids
at room temperature and are soluble in moderately polar
solvents, such as chloroform, dichloromethane, and methanol,
but insoluble in less polar solvents, such as diethyl ether, ethyl
acetate, and hexane. These properties, together with the
straightforward synthesis, suffice for practical applications in
asymmetric synthesis.
We next examined the applications of these CILs in
asymmetric catalysis, where the asymmetric Michael reaction
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Table 1: The effect of chiral ionic-liquid catalysts in asymmetric Michael
additions of cyclohexanone and trans-b-nitrostyrene.[a]
Scheme 2. Synthesis of functionalized chiral ionic liquids. Conditions:
a) LiAlH4, THF, 75 %; b) 1. Boc2O, NaOH; 2. TosCl, pyridine, 90 % for
2 steps; c) NaH, imidazole, 83 %; d) nBuBr, toluene, 70 8C, 93 %;
e) HCl/EtOH; then sat. NaHCO3, 90 %; f) NaX, acetone/acetonitrile,
room temperature. Boc = tert-butoxycarbonyl, THF = tetrahydrofuran,
Tos = toluene-4-sulfonyl.
was selected as our initial focus. As one of the most important
carbon–carbon bond-forming reactions, Michael addition has
been the subject of considerable research efforts aimed at
developing efficient asymmetric catalysts, especially environmentally friendly, metal-free organocatalysts.[14] Proline
derivatives such as pyrrolidinyltetrazole,[15] aminomethylpyrrolidine,[16] and 2,2’-bipyrrolidine[17] have been shown to serve
as useful asymmetric catalysts for the Michael addition of
ketones or aldehydes. Recently, Kotsuki and co-workers
developed a pyrrolidine–pyridine conjugate base as a highly
selective catalyst for Michael reactions of ketones.[18] Wang
et al.[19] and Hayashi et al.[20] have independently reported
excellent catalysts for the asymmetric Michael reaction of
aldehydes. Intrigued by the success of chiral pyrrolidines as
asymmetric Michael addition catalysts, we investigated pyrrolidine-type CILs in the Michael addition of cyclohexanone
to trans-b-nitrostyrene.
As can be seen from the results summarized in Table 1, all
synthesized CILs catalyzed the asymmetric Michael addition.
The corresponding catalytic and enantioselective activities
varied significantly with different ionic-liquid units. With
regard to the ionic-liquid-cation moiety, CILs with an
imidazolium core (2 a–c and 4 a) are superior to their 2’methyl counterparts (3 a–c and 4 b) in terms of both activities
and enantioselectivities (Table 1, entries 2–13). Incorporation
of a protic group in the side chain of the cation, as in 4 a and
4 b, led to a decrease in both the catalytic activity and
selectivity (Table 1, entries 12 and 13). As for the anion, while
the swap of Br to BF4 provided comparable catalysts with
slightly enhanced activities (Table 1, entry 2 versus 4 and
entry 9 versus 10), the exchange of Br for PF6 had a
detrimental effect on both the activities and selectivities
(Table 1, entries 8 and 11). The functionalized CILs tested in
this work performed much better than previously reported
chiral pyrrolidine catalysis in ionic liquids as the reaction
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Entry
Catalyst
t [h]
Yield [%][b]
syn/anti[c]
ee [%][d]
1
2
3[e]
4
5[f ]
6[g]
7[h]
8
9
10
11
12
13
1
2a
2a
2b
2b
2b
2b
2c
3a
3b
3c
4a
4b
18
10
20
8
8
24
48
12
20
16
12
18
18
97
99
99
100
97
99
96
86
97
100
40
86
25
97:3
99:1
99:1
99:1
97:3
96:4
97:3
98:2
97:3
96:4
96:4
97:3
94:6
91
98
97
99
94
91
93
87
97
94
82
89
70
[a] TFA = trifluoroacetic acid. [b] Yield of isolated product. [c] Determined by 1H NMR spectroscopy. [d] Determined by HPLC analysis
(chiralcel AD-H column). [e] 10 mol % of catalyst was used. [f]–
[h] Second, third, and fourth reuses of the catalyst, respectively.
media.[12d, h] For example, 40 mol % of l-proline was required
to catalyze cyclohexanone addition to nitrostyrene in ionic
liquids with 75 % yield after 60 h (d.r. = 95:5 and 75 % ee for
the syn diastereomer).[12h] These observations, together with
the comparatively lower efficiencies of non-ionic-liquid
pyrrolidine–imidazole conjugate 1 (Table 1, entry 1), clearly
indicate the critical role of the ionic-liquid moieties for
asymmetric catalysis. Overall, pyrrolidine–imidazolium bromide and tetrafluoroborate, 2 a and 2 b, respectively, demonstrate the best performances with nearly quantitative yields
and high diastereoselectivity (syn/anti = 99:1) and enantioselectivity (98 % ee).
In general, the reactions were carried out in neat mixtures
with 15 mol % of catalyst and 5 mol % of TFA as the cocatalyst. (These were observed to be the optimal conditions;
see the Supporting Information.) A reduction in the amount
of catalyst (2 a) to 10 mol % resulted in a relatively slow
reaction but still gave quantitative yield and 97 % ee within
20 h (Table 1, entry 3). This result is comparable to that of the
experiment by Kotsuki and co-workers with a pyrrolidine–
pyridine catalyst (24 h, 95 %, syn/anti 98:2, 99 % ee).[18] In
agreement with previous reports,[16, 17] the use of an acidic cocatalyst was essential in our catalytic system. In the absence of
co-catalyst, the reaction became very sluggish and only
afforded 33 % yield after 40 h.
The synthetic functionalized CILs still maintained the
biphasic property of ionic liquids and could be easily recycled
by precipitation with diethyl ether. Recycled CIL 2 b was
directly used in the next run and demonstrated identical
activity with slightly decreased selectivities (Table 1, entry 5;
Table 2, entry 1). Loss of activity was observed for the third
and fourth reuse of the CIL, but excellent yields and ee values
could still be achieved (Table 1, entries 6 and 7; Table 2,
entry 1).
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 3165 –3169
Angewandte
Chemie
Table 2: CIL-catalyzed asymmetric Michael addition to nitroalkenes.
CIL
t [h]
Yield [%][a]
syn/anti[b]
ee [%][c]
1
2a
2b
2 b[d]
2 b[e]
12
10
10
24
92
100
92
93
98:2
99:1
96:4
97:3
95
99
94
93
2
2a
2b
12
12
99
100
99:1
99:1
3
2a
2b
15
10
76
94
4
2a
2b
16
12
5
2a
2b
6
2a
2b
Entry
Product
CIL
t [h]
Yield [%][a]
syn/anti[b]
7
2a
2b
42
24
99
99
98:2
99:1
94
97
96
99
8
2a
2b
80
60
61
87
75:25
63:37
83/80
79/82
98:2
98:2
96
96
9
2a
2b
12
12
77
83
–
–
94
99
99:1
99:1
92
95
10
2a
2b
24
24
57
92
83:17
85:15
20
12
94
99
99:1
99:1
94
95
11
2a
2b
96
96
51
70
–
–
89
86
12
10
99
99
99:1
99:1
95
97
12
2a
2b
60
60
73
100
90:10
90:10
77
72
Entry
Product
ee [%][c]
45
43
79/81
76/80
[a] Yield of isolated product. [b] Determined by 1H NMR spectroscopy. [c] Determined by HPLC analysis (chiralpak AD-H or AS-H columns). [d] and
[e] Second and third reuses of the catalyst, respectively.
We next probed the scope of the reaction for a variety of
Michael donors and nitroalkenes with the identified optimal
catalysts 2 a and 2 b (Table 2). The reactions worked
extremely well with cyclohexanone to generate Michael
adducts in nearly quantitative yields, high diastereoselectivities (d.r. 97:3), and excellent enantioselectivities (92–99 %
ee). Both electron-rich and electron-deficient nitrostyrenes
were excellent Michael acceptors for cyclohexanone (Table 2,
entries 1–7). With regard to the Michael donors, the reaction
with cyclopentanone occurred smoothly and showed moderate diastereoselectivity and enantioselectivity for both diastereomers (Table 2, entry 8). Acetone also served as an
efficient Michael donor to produce the desired adduct with
good yield and moderate enantioselectivity (Table 2, entry 9).
Angew. Chem. 2006, 118, 3165 –3169
In the presence of 2 a, the reaction of acetone with cyclic
nitroolefin afforded the desired Michael adducts with high
yields and good enantioselectivities (Table 2, entry 9, syn:
76 % ee, anti: 80 % ee).
Our preliminary studies demonstrated that CILs 2 a and
2 b could also catalyze the Michael addition of aldehydes
(Table 2, entries 11 and 12). Under the optimized conditions,
the addition of isobutyraldehyde to trans-b-nitrostyrene gave
the desired adduct in good yields and up to 89 % ee (Table 2,
entry 11). Valeraldehyde also worked well to afford the
desired product with quantitative yield and moderate selectivity (Table 2, entry 12, syn/anti = 90:10, 72 % ee). In most of
the cases examined, CIL tetrafluoroborate 2 b performed
slightly better than CIL bromide 2 a.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3167
Zuschriften
The relative and absolute configurations of the Michael
adducts were determined by 1H NMR spectroscopic analysis
and comparison of the optical rotations with those of known
compounds. The high diastereoselectivities and excellent
enantioselecitivities of CIL catalysis in the Michael additions
reported here may be explained by the concept of an acyclic
synclinal transition state, as proposed by Seebach and
Golinski[21] and in other previous studies.[15–20] In this model,
the ionic-liquid moiety would effectively shield the Si face of
the enamine double bond in the ketone donor and the
reaction would occur through a Re–Re approach
(Scheme 3).[22] The ionic and highly polar nature of the
imidazolium group may also contribute in the transition state
and this feature is currently under investigation.
Scheme 3. Transition-state model for ketone donors.
In summary, we have developed a novel asymmetric
catalytic system with functionalized CILs and demonstrated
their potential as highly efficient asymmetric organocatalysts
for Michael reactions. The pyrrolidine–ionic-liquid conjugates have several noteworthy features: 1) they can very
efficiently catalyze the Michael additions for a broad range of
Michael donors (both ketones and aldehydes) and Michael
acceptors (nitroolefins) with high yields (up to 100 %),
excellent enantioselectivity (up to 99 % ee), and very good
diastereoselectivity (syn/anti up to 99:1); 2) the ionic-liquid
moiety can not only act as a phase tag to facilitate recycling
and reuse of the catalyst but can also function as an efficient
chiral-induction group to ensure high selectivity; and 3) the
modular and tuneable features of ionic liquids promise
further developments. Further improvements of the present
catalysts (reusability) and the application of the CILs to other
types of reactions are currently underway in our laboratory.
Experimental Section
Typical experimental procedure: Nitrostyrene (37 mg, 0.25 mmol)
and 2 b (12 mg, 15 mol %) were mixed with cyclohexanone (0.5 mL,
5 mmol) at room temperature. After stirring at room temperature for
8 h, the homogeneous reaction mixture was diluted with ethyl ether to
precipitate the catalyst. The organic layer was separated and loaded
onto a silica gel column for purification, to afford the Michael adduct
(61 mg, 99 %) as a white solid: syn/anti 99:1, 99 % ee (by HPLC on a
chiralpak AD-H column, l = 254 nm, eluent iPrOH/hexane (10:90),
flow rate = 0.5 mL min 1; tR = 21.4 (minor), 27.2 min (major)). The
catalyst was used directly for the next run after removing the residual
solvent.
Received: January 5, 2006
Published online: April 4, 2006
3168
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.
Keywords: asymmetric catalysis · ionic liquids ·
Michael addition · nitroalkenes · organocatalysis
[1] For reviews, see: a) J. H. Davis, Chem. Lett. 2004, 33, 1072 – 1077;
b) P. Wasserscheid, W. Keim, Angew. Chem. 2000, 112, 3926 –
3946; Angew. Chem. Int. Ed. 2000, 39, 3772 – 3789; c) T. Welton,
Chem. Rev. 1999, 99, 2071 – 2083; d) R. Sheldon, Chem.
Commun. 2001, 2399 – 2407; e) J. Dupont, R. F. de Souza,
P. A. Z. Suarez, Chem. Rev. 2002, 102, 3667 – 3692; f) J. S.
Wikes, J. Mol. Catal. A 2004, 214, 11 – 17; g) T. Welton, Coord.
Chem. Rev. 2004, 248, 2459 – 2477; h) C. E. Song, Chem.
Commun. 2004, 1033 – 1043; i) N. Jain, A. Kumar, S. Chauhan,
S. M. S. Chauhan, Tetrahedron 2005, 61, 1015 – 1060.
[2] Z. Fei, T. J. Geldbach, D. Zhao, P. J. Dyson, Chem. Eur. J. 2006,
12, 2122 – 2130.
[3] For reviews, see: a) C. Baudequin, J. Baudoux, J. Levillain, D.
Cahard, A.-C. Gaumont, J.-C. Plaquevent, Tetrahedron: Asymmetry 2003, 14, 3081 – 3093; b) J. Ding, D. W. Armstrong,
Chirality 2005, 17, 281 – 292; c) C. Baudequin, D. BrJgeon, J.
Levillain, F. Guillen, J.-C. Plaquevent, A.-C. Gaumont, Tetrahedron: Asymmetry 2005, 16, 3921 – 3945.
[4] M. J. Earle, P. B. McCormac, K. R. Seddon, Green Chem. 1999,
1, 23 – 25.
[5] J. Ding, V. Desikan, X. Han, T. L. Xiao, R. Ding, W. S. Jenks,
D. W. Armstrong, Org. Lett. 2005, 7, 335 – 337.
[6] B. PJgot, G. Vo-Thanh, A. Loupy, Tetrahedron Lett. 2004, 45,
6425 – 6428.
[7] Z. Wang, Q. Wang, Y. Zhang, W. Bao, Tetrahedron Lett. 2005, 46,
4657 – 4660.
[8] For achiral ionic-liquid catalysts, see: acidic catalysts: a) A. Cole,
J. Jensen, I. Ntai, K. Loan, T. Tran, K. Weaver, D. Forbes, J.
Davis, J. Am. Chem. Soc. 2002, 124, 5962 – 5963; b) Y. Gu, F. Shi,
Y. Deng, Catal. Commun. 2003, 4, 597 – 601; c) D. C. Forbes, K. J.
Weaver, J. Mol. Catal. A 2004, 214, 129 – 132; d) Y. Gu, F. Shi, Y.
Deng, J. Mol. Catal. A 2004, 212, 71 – 75; e) J. Gui, H. Ban, X.
Cong, X. Zhang, Z. Hu, Z, Sun, J. Mol. Catal. A 2005, 225, 27 –
31; f) J. Gui, Y. Deng, Z. Hu, Z. Sun, Tetrahedron Lett. 2004, 45,
2681 – 2683; g) D. Li, F. Shi, J. Peng, S. Guo, Y. Deng, J. Org.
Chem. 2004, 69, 3582 – 3585; h) S. Kitaoka, K. Nobuoka, Y.
Ishikawa, Chem. Commun. 2004, 1902 – 1903; i) Z. Fei, D. Zhao,
T. J. Geldbach, R. Scopelliti, P. J. Dyson, Chem. Eur. J. 2004, 10,
4886 – 4893; basic catalysts: j) B. C. Ranu, S. Banerjee, Org. Lett.
2005, 7, 3049 – 3052; k) S. AbellN, F. Medina, X. RodrOguez, Y.
Cesteros, P. Salagre, J. E. Sueiras, D. Tichit, B. Coq, Chem.
Commun. 2004, 1906 – 1907; l) A. Zhu, T. Jiang, D. Wang, B.
Han, L. Liu, J. Huang, J. Zhang, D. Sun, Green Chem. 2005, 7,
514 – 517; nucleophilic catalysts: m) D. W. Kim, C. E. Song, D. Y.
Chi, J. Am. Chem. Soc. 2002, 124, 10 278 – 10 279; n) D. W. Kim,
C. E. Song, D. Y. Chi, J. Org. Chem. 2003, 68, 4281 – 4285;
o) D. W. Kim, Y. S. Choe, D. Y. Chi, Nucl. Med. Biol. 2003, 30,
345 – 350; p) D. W. Kim, D. Y. Chi, Angew. Chem. 2004, 116,
489 – 491; Angew. Chem. Int. Ed. 2004, 43, 483 – 485; q) L.-W.
Xu, Y. Gao, J.-J. Yin, L. Li, C.-G. Xia, Tetrahedron Lett. 2005, 46,
5317 – 5320; r) B. C. Ranu, S. S. Dey, Tetrahedron 2004, 60, 4183 –
4188; s) L.-W. Xu, J.-W. Li, S.-L. Zhou, C.-G. Xia, New J. Chem.
2004, 28, 183 – 184.
[9] For selected reviews of organocatalysis, see: a) P. I. Dalko, L.
Moisan, Angew. Chem. 2004, 116, 5248 – 5286; Angew. Chem.
Int. Ed. 2004, 43, 5138 – 5175; b) A. Berkessel, H. Groger,
Asymmetric Organocatalysis, Wiley-VCH, Weinheim, 2005; c) J.
Seayad, B. List, Org. Biomol. Chem. 2005, 3, 719 – 724; d) S. T.
Handy, Chem. Eur. J. 2003, 9, 2938.
[10] B. List, R. A. Lerner, C. F. Barbas III, J. Am. Chem. Soc. 2000,
122, 2395.
[11] K. A. Ahrendt, C. J. Borths, D. W. C. MacMillan, J. Am. Chem.
Soc. 2000, 122, 4243 – 4244.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 3165 –3169
Angewandte
Chemie
[12] a) P. Kotrusz, I. Kmentova, B. Gotov, Š. Toma, E. SolčQniovQ,
Chem. Commun. 2002, 2510 – 2511; b) T.-P. Loh, L.-C. Feng, H.Y. Yang, J.-Y. Yang, Tetrahedron Lett. 2002, 43, 8741 – 8743;
c) N. S. Chowdari, D. B. Ramachary, C. F. Barbas III, Synlett
2003, 1906 – 1909; d) P. Kotrusz, S. Toma, H.-G. Schmalz, A.
Adler, Eur. J. Org. Chem. 2004, 1577 – 1583; e) H. M. Guo, L. F.
Cun, L. Z. Gong, A. Q. Mi, Y. Z. Jiang, Chem. Commun. 2005,
1450 – 1452; f) P. Kotrusz, S. Alemayehu, T. Toma, H. G.
Schmalz, A. Adler, Eur. J. Org. Chem. 2005, 4904 – 4911; g) A.
CNrdova, Tetrahedron Lett. 2004, 45, 3949 – 3952; h) M. S.
Rasalkar, M. K. Potdar, S. S. Mohile, M. M. Salunkhe, J. Mol.
Catal. A 2005, 235, 267 – 270; i) X. L. Mi, S. Z. Luo, J.-P. Cheng, J.
Org. Chem. 2005, 70, 2338 – 2341; j) R. T. Dere, R. R. Pal, P. S.
Patil, M. M. Salunkhe, Tetrahedron Lett. 2003, 44, 5351 – 5353.
[13] a) S. T. Handy, Curr. Org. Chem. 2005, 9, 959 – 988; b) J.
Habermann, S. Ponzi, S. V. Ley, Mini-Rev. Org. Chem. 2005, 2,
125 – 137.
[14] For a review, see: a) J. Christoffers, A. Baro, Angew. Chem. 2003,
115, 1726 – 1728; Angew. Chem. Int. Ed. 2003, 42, 1688 – 1690.
For imidazoline-type organocatalysts for Michael additions, see:
b) N. Halland, P. S. Aburel, K. A. Jørgensen, Angew. Chem.
2004, 116, 1292 – 1297; Angew. Chem. Int. Ed. 2004, 43, 1272 –
1277, and references therein; c) M. T. H. Fonseca, B. List,
Angew. Chem. 2004, 116, 4048 – 4050; Angew. Chem. Int. Ed.
2004, 43, 3958 – 3960; d) Y. Hayashi, H. Gotoh, T. Tamura, H.
Yamaguchi, R. Masui, M. Shoji, J. Am. Chem. Soc. 2005, 127,
16 028 – 16 029.
[15] a) A. J. A. Cobb, D. A. Longbottom, D. M. Shaw, S. V. Ley,
Chem. Commun. 2004, 1808 – 1809; b) A. J. A. Cobb, D. M.
Shaw, D. A. Longbottom, J. B. Gold, S. V. Ley, Org. Biomol.
Chem. 2005, 3, 84 – 96; c) C. E. T. Mitchell, A. J. A. Cobb, S. V.
Ley, Synlett 2005, 611 – 614.
[16] a) B. List, P. Pojarliev, H. J. Martin, Org. Lett. 2001, 3, 2423 –
2425; b) J. M. Betancort, C. F. Barbas III, Org. Lett. 2001, 3,
3737 – 3740; c) N. Mase, R. Thayumanavan, F. Tanaka, C. F.
Barbas III, Org. Lett. 2004, 6, 2527; d) J. M. Betancort, K.
Sakthivel, R. Thayumanavan, F. Tanaka, C. F. Barbas III, Synthesis 2004, 1509 – 1521.
[17] a) A. Alexakis, O. Andrey, Org. Lett. 2002, 4, 3611 – 3614; b) O.
Andrey, A. Alexakis, G. Bernardinelli, Org. Lett. 2003, 5, 2559 –
2561; c) O. Andrey, A. Alexakis, A. Tomassini, G. Bernardinelli,
Adv. Synth. Catal. 2004, 346, 1147 – 1168.
[18] T. Ishii, S. Fujioka, Y. Sekiguchi, H. Kotsuki, J. Am. Chem. Soc.
2004, 126, 9558 – 9559.
[19] W. Wang, J. Wang, H. Li, Angew. Chem. 2005, 117, 1393 – 1395;
Angew. Chem. Int. Ed. 2005, 44, 1369 – 1371.
[20] Y. Hayashi, H. Gotoh, T. Hayashi, M. Shoji, Angew. Chem. 2005,
117, 4284 – 4287; Angew. Chem. Int. Ed. 2005, 44, 4212 – 4215.
[21] D. Seebach, J. Golinski, Helv. Chim. Acta 1981, 64, 1413 – 1423.
[22] For aldehyde donors, anti-enamine intermediates would be
formed and the reactions would occur through a Si–Si
approach.[15–17]
Angew. Chem. 2006, 118, 3165 –3169
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.de
3169
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chiral, efficiency, asymmetric, michael, nitroolefins, ioni, functionalized, additional, organocatalytic, liquid, highly
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