Catalytic enantioselective Passerini three-component reaction

Article abstract of DOI:10.1002/anie.200704315

(Chemical Equation Presented) Make it enantioselective: The [(salen)-Al^IIICl] complex 1 catalyzes the title reaction of an aldehyde, a carboxylic acid, and an isocyanide to afford α-acyloxyamides 2 with good to excellent enantioselectivity. A v

Full text of DOI:10.1002/anie.200704315

Communications
DOI: 10.1002/anie.200704315
Asymmetric Catalysis
Catalytic Enantioselective Passerini Three-Component Reaction**
Shi-Xin Wang, Mei-Xiang Wang,* De-Xian Wang, and Jieping Zhu*
The Passerini three-component reaction (P-3CR) involves the
condensation of a carbonyl compound 1, a carboxylic acid 2,
and an isocyanide 3 with the concurrent generation of a
d-threitol) promoted the P-3CR to afford a-acyloxyamides
with low to moderate enantioselectivity.[8] Schreiber and co-
workers demonstrated that an indan-pybox–CuII complex
(pybox = pyridinebis(oxazoline)) could catalyze the P-3CR.[9]
Nevertheless, the enantiomerically enriched Passerini adduct
was obtained only when a chelating aldehyde was used.
The development of a truly catalytic enantioselective
three-component Passerini reaction of wide application scope
remains a significant challenge, in sharp contrast to the
formidable progress made in the field of asymmetric synthesis
in general. Several pitfalls exist that make this task partic-
ularly challenging: 1) the complexity of the reaction mecha-
nism, 2) the competitiveness of the uncatalyzed background
reaction, 3) the potential of the three components, all of
which are Lewis bases, to coordinate to or deactivate the
catalyst, and 4) the problem of catalyst turnover as a result of
product inhibition. Indeed, when a nonchelating aldehyde is
used, the reaction produces an imidate intermediate A that is
bidentate in nature. Furthermore, the P-3CR adduct itself is
also a bidentate ligand and can therefore compete with the
substrate to coordinate to the catalyst (Scheme 1). We
proposed recently to use a chiral catalyst with a single
coordination site for the enantioselective a-addition of
isocyanides to aldehydes. As a proof of concept, we described
an enantioselective synthesis of 2-(1-hydroxyalkyl) 5-amino-
oxazoles by the Lewis acid catalyzed condensation of an
aldehyde with an a-isocyanoacetamide.[10b] As a continuation
of this research, we report herein that the presence of a
carboxylic acid is tolerated well in the [(salen)AlIIICl]-
catalyzed a-addition of isocyanides to aldehydes and docu-
ment an efficient catalytic enantioselective three-component
Passerini reaction that is applicable to a wide range of
nonchelating aliphatic aldehydes.
The P-3CR of 2-methylpropanal (1a), benzoic acid (2a),
and benzyl isocyanide (3a) in toluene was used as a standard
reaction for the screening of possible chiral Lewis acid
catalysts (catalyst loading: 0.1 equiv). In a control experi-
ment, the reaction proceeded even at À408C in the absence of
a catalyst to afford the racemic adduct 4a in 37% yield
(Table 1, entry 1). As the carboxylic acid itself catalyzed the
P-3CR, we designed a protocol involving the slow addition
over 1 h of the carboxylic acid to the solution of the catalyst,
1a, and 3a to minimize or suppress the undesired background
reaction. Representative results obtained by varying the
ligand structure, the metal source, the temperature, and the
concentration of the reaction mixture are summarized in
Table 1. When N,N’-bis(3,5-di-tert-butylsalicylidene)-(R,R)-
cyclohexane-1,2-diamine (5a) was used as the supporting
ligand[13] in association with Et2AlCl,[14] the adduct 4a was
produced with 63% ee (Table 1, entry 2). The enantioselec-
tivity dropped significantly when Et3Al was used instead of
Et2AlCl (Table 1, entry 3). Other salts, such as MnCl3, CrCl3,
stereogenic center to afford an a-acyloxyamide
4
(Scheme 1).[1] Together with the Ugi four-component reaction
Scheme 1. Passerini three-component reaction.
(U-4CR),[2] the P-3CR has been investigated intensively
during the past two decades. Many innovative variations
have been uncovered and have led to the facile synthesis of a
large collection of diverse heterocyclic scaffolds in a step- and
atom-economic manner.[3] Although a number of diastereo-
selective P-3CRs[4] and U-4CRs[5] are known, only limited
success has been attained in the development of enantiose-
lective P-3CRs[6–11] and U-4CRs.[12] Denmark and Fan devel-
oped an asymmetric a-addition of isocyanides to aldehydes
catalyzed by a chiral Lewis base with good to excellent
enantioselectivity.[7] The protocol is applicable to nonchelat-
ing aldehydes, but it is a bimolecular transformation without a
carboxylic acid substrate. Dömling and co-workers screened a
large number of metal–ligand combinations in a parallel
fashion and found that a stoichiometric amount of a Ti–taddol
complex (taddol = 1,1,4,4-tetraphenyl-2,3-O-isopropylidene-
[*] S.-X. Wang, Prof. M.-X. Wang, Dr. D.-X. Wang
National Laboratory for Molecular Sciences
Laboratory of Chemical Biology, Institute of Chemistry
Chinese Academy of Sciences
Beijing 100080 (China)
Fax: (+33)861062564723
E-mail: mxwang@iccas.ac.cn
Homepage: http://www.mxwang.iccas.ac.cn/
Dr. J. Zhu
Institut de Chimie des Substances Naturelles, CNRS
91198 Gif-sur-Yvette Cedex (France)
Fax: (+33)1-6907-7247
E-mail: zhu@icsn.cnrs-gif.fr
Homepage: http://www.icsn.cnrs-gif.fr/article.php3?id_arti-
cle=122
[**] We gratefully acknowledge the National Science Foundation of
China (NSFC), the Chinese Academy of Sciences, and the CNRS
(France) for financial support.
Supporting information for this article, including experimental
procedures, product characterization, and the 1H NMR spectra and
HPLC traces (chiral phase) of 4a–p, is available on the WWW under
http://www.angewandte.org or from the author.
388
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Table 1: Survey of reaction conditions for the enantioselective P-3CR of
1a–3a.[a]
with a reaction time of 48 h at À408C, and with slow addition
of the acid), good to excellent enantioselectivity was generally
observed with representative aliphatic aldehydes, carboxylic
acids, and isocyanides (Scheme 2 and Table 2). As might be
Entry
Catalyst
T [8C]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
none
5a+Et2AlCl
5a+Et3Al
À40
À40
À40
À40
À40
À40
À40
À20
À60
À40
À20
À40
37
70
32
36
54
trace
35
51
26
50
71
66
rac.
63
8
19
24
n.d.[d]
0
53
80
59
47
51
5a+MnCl3
5a+CrCl3
5a+Ti(OiPr)4
5a+Et2Zn
5a+Et2AlCl
5a+Et2AlCl
5a+Et2AlCl
5a+Et2AlCl
5b+Et2AlCl
10[e]
11[f]
12
[a] General conditions: 1a/2a/3a 1:1:1, 48 h, c=0.33m, toluene.
[b] Yield of the analytically pure product. [c] Determined by HPLC
analysis on a chiral phase. [d] Not determined. [e] The reaction was
performed at a concentration of 0.1m. [f] The reaction was performed in
CH2Cl2. Bn=benzyl.
Scheme 2. Starting materials used in the enantioselective P-3CR.
expected, the enantioselectivity depended on the structures of
the isocyanide and the aldehyde. The selectivity of the
reaction increased when the aliphatic isocyanide substrate
was exchanged for
a less-reactive aromatic isocyanide
(Table 2, entries 1 and 2 versus 3 and 4), presumably owing
to the suppression of the uncatalyzed background reaction.[15]
However, no clear-cut tendency was discernable with respect
to the electronic effect of substituents on the aromatic ring
(Table 2, entries 7 and 11–16). 4-Methoxy-2-nitrophenyl iso-
cyanide (3 f), which was developed by Martens and co-
Ti(OiPr)4, and Et2Zn, were found to be inefficient (Table 1,
entries 4–7). When the reaction was
Table 2: Generality of the [(salen)AlIIICl]-catalyzed enantioselective Passerini reaction.[a]
carried out at the higher temper-
ature of À208C, the ee value of the
product decreased, probably as a
result of a more pronounced back-
ground reaction (Table 1, entry 8).
At À608C, the reaction afforded the
adduct 4a with 80% ee, albeit in
lower yield (Table 1, entry 9). A
decrease in the concentration of
Entry
Aldehyde
Acid
Isocyanide
Product
Yield [%]
ee [%]
1
1a
2a
3a
70
63
2
3
1a
1a
2b
2a
3a
3b
59
63
63
84
the reaction mixture led to
a
decrease in both the reaction rate
and the ee value of the product
(Table 1, compare entries 2 and
10). The reaction proceeded well
in CH2Cl2 but with slightly
decreased
enantioselectivity
(Table 1, entry 11). Finally, the
salen compound 5b derived from
1R,2R-diphenylethylenediamine
was less efficient as a ligand than 5a
(Table 1, compare entries 2 and 12).
Under the optimized conditions
(that is, with presynthesized [5a–
AlIIICl] (0.1 equiv) at a reagent
concentration of 0.33m in toluene,
4
5
1a
1a
2b
2c
3b
3b
60
62
84
80
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Communications
Table 2: (Continued)
ing a-hydroxy acid under basic
conditions.[16] The use of both
linear and a-branched aliphatic
aldehydes provided the correspond-
ing adducts with excellent enantio-
selectivity. The reaction of 2-meth-
ylpropanal (1a) with 2e and 3b
afforded the corresponding adduct
4g in 64% yield with an exceptional
99% ee (Table 2, entry 7). How-
ever, the reaction of pivalaldehyde
with 2e and 3b afforded the race-
mic adduct in 74% yield, and aro-
matic aldehydes failed to partici-
pate in this condensation reac-
tion.[17] The aromatic acid 2a, the
a,b-unsaturated acid 2c, and ali-
phatic acids, such as acetic acid (2b)
and the functionalized substrates a-
thioacetic acid (2d) and a-chloro-
acetic acid (2e), participated in the
reaction to afford the correspond-
ing a-acyloxyamides with good to
excellent enantioselectivity (75–
99% ee; Table 2, entries 3–7). The
presence of a chloroacetyl function-
ality in 4g–p provides an interesting
handle for subsequent functionali-
zation. The observation that the
structure of the acid influenced the
enantioselectivity of the reaction
may indicate that this component
Entry
Aldehyde
Acid
Isocyanide
Product
Yield [%]
51
ee [%]
6
1a
2d
3b
75
7
8
1a
1b
1c
1d
2e
2e
2e
2e
3b
3b
3b
3b
64
66
67
59
>99
87
9[b]
73
10
87
11[b]
12[b]
13
1e
1a
1a
2e
2e
2e
3b
3c
3d
68
68
61
71
93
81
À
is involved directly in the key C C
bond-forming process even in the
presence of a Lewis acid catalyst.
Comparison of the sign of opti-
cal rotation of the a-hydroxyamides
6 and 7 obtained by saponification
of the corresponding esters 4a and
4c with that of the authentic sam-
ples derived from (S)-2-hydroxy-3-
methylbutyric acid enabled us to
assign the S configuration to 4a and
4c. The observed S enantioselectiv-
ity indicates that the isocyanide
attacks predominantly the Re face
of the aldehyde.
14[b]
15[c]
16
1a
1a
1a
2e
2e
2e
3e
3 f
3g
52
66
64
88
75
68
In summary, we have described
an efficient enantioselective Passer-
ini three-component reaction cata-
lyzed by a readily available and
stable Lewis acid catalyst. Good to
excellent enantioselectivity was
[a] General conditions: 0.33m solution in toluene, 1/2/3/catalyst 1:1:1:0.1, slow addition of 2 to the
premixed solution of the catalyst, 1, and 3. [b] Catalyst loading: 20%. [c] The reaction was performed in
toluene/dichloromethane (3:1) owing to the low solubility of 3 f in toluene.
workers[16] as a convertible isocyanide, also participated in this
reaction to afford 4o with 75% ee (Table 2, entry 15).
Compound 4o can be hydrolyzed readily to the correspond-
390
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Chemie
[4] a) S. Marcaccini, D. Miguel, T. Torroba, M. Garcia-Valverde, J.
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catalyst with one coordination site is essential for the
development of enantioselective Passerini- and Ugi-type
reactions.
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[12] To date, no enantioselective U-4CR has been reported.
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Experimental Section
General procedure: The [(salen)AlIIICl] complex (30.3 mg,
0.05 mmol) derived from N,N’-bis(3,5-di-tert-butylsalicylidene)-
(R,R)-cyclohexane-1,2-diamine (5a) was added with dry toluene
(0.3 mL) to a 25-mL flame-dried round-bottomed flask equipped with
a stir bar under argon. The mixture was stirred until the catalyst had
dissolved completely. The aldehyde (0.5 mmol) was then added as a
solution in toluene (0.1 mL), and the resulting mixture was stirred at
room temperature for 0.5 h. The mixture was then cooled to À408C, a
solution of the isocyanide (0.5 mmol) in toluene (0.1 mL) was added,
and the mixture was stirred for a further 10 min. A solution of the acid
(0.5 mmol) in toluene (1 mL) was then added slowly with a syringe
pump (addition time: 1 h). The reaction mixture was stirred at À408C
for 48 h, then quenched with saturated aqueous NaHCO3 solution,
stirred at room temperature for 0.5 h, and extracted with EtOAc. The
combined organic phases were washed with brine, dried over
anhydrous sodium sulfate, filtered, and concentrated. The crude
product was purified by flash chromatography on silica gel to give the
corresponding a-acyloxyamide 4.
Received: September 18, 2007
Published online: November 15, 2007
Keywords: aluminum · asymmetric catalysis · isocyanides ·
.
multicomponent reactions · salen ligands
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3577; Angew. Chem. Int. Ed. 2006, 45, 3495 – 3497.
[2] a) A. Dömling, I. Ugi, Angew. Chem. 2000, 112, 3300 – 3344;
Angew. Chem. Int. Ed. 2000, 39, 3168 – 3210; b) for an oxidative
Ugi-type reaction, see: T. Ngouansavanh, J. Zhu, Angew. Chem.
2007, 119, 5877 – 5880; Angew. Chem. Int. Ed. 2007, 46, 5775 –
5778.
[15] The reaction of tert-butylisocyanide with 1a and 2a afforded the
P-3CR adduct in 53% yield. However, to date we have been
unable to determine the ee value of this adduct.
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[17] We have no clear-cut explanation for these observations,
although aromatic aldehydes are known to be less reactive in
the Passerini reaction. Note that aromatic aldehydes are better
substrates than aliphatic aldehydes in the procedure of Denmark
and Fan.[7]
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