Enantioselective Iron-catalyzed azidation of β-keto esters and oxindoles

Article abstract of DOI:10.1021/ja402082p

The first example of Fe-catalyzed enantioselective azidations of β-keto esters and oxindoles using a readily available N₃- transfer reagent is reported. A number of α-azido-β-keto esters were obtained with up to 93% ee, and this methodology also generates 3-substitued 3-azidooxindoles with high enantioselectivities (up to 94%).

Full text of DOI:10.1021/ja402082p

Communication
pubs.acs.org/JACS
Enantioselective Iron-Catalyzed Azidation of β‑Keto Esters and
Oxindoles
Qing-Hai Deng,† Tim Bleith,‡ Hubert Wadepohl,‡ and Lutz H. Gade*,†,‡
†Catalysis Research Laboratory (CaRLa), Im Neuenheimer Feld 584, 69120 Heidelberg, Germany
‡Anorganisch-Chemisches Institut, Universitat Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
̈
S
* Supporting Information
Table 1. Optimization of the Azidation of 4a
ABSTRACT: The first example of Fe-catalyzed enantio-
selective azidations of β-keto esters and oxindoles using a
readily available N3-transfer reagent is reported. A number
of α-azido-β-keto esters were obtained with up to 93% ee,
and this methodology also generates 3-substitued 3-
azidooxindoles with high enantioselectivities (up to 94%).
H-
t
yield
ee
a
b
entry
MXn
Fe(OAc)2
Lig AgX solvent
(h) (%)
(%)
1
2
2a
2a
2a
2a
2a
2a
2a
2a
2b
2c
−
−
−
−
−
−
−
−
−
−
−
−
−
−
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
36
6
87
90
88
76
80
85
53
86
56
86
61
86
85
73
87
88
75
15
35
15
11
55
66
76
44
81
69
83
66
53
91
90
93
86
90
53
c
Fe(ClO4)2·xH2O
Fe(BF4)2·6H2O
Fe(OTf)2
rganic azides are valuable intermediates in organic
3
24
72
72
48
72
72
72
72
72
48
72
72
48
48
72
72
O
synthesis, in particular as powerful precursors for a wide
range of nitrogen-containing synthetic targets.1 During the past
two decades, their utility has expanded dramatically in medicine,
biology, and materials sciences.1,2 Despite the extensive studies
and significant advances in this field, the catalytic stereoselective
introduction of an azido group into organic compounds remains
comparatively rare.3,4
4
5
Fe(OOCEt)2 (3)
6
3
7
3
−
−
toluene
Et2O
8
3
9
3
−
Et2O
10
11
12
13
14
15
16
17
3
−
Et2O
6a
6a
6a
6a
6a
6a
6a
−
Et2O
α-Azido-β-keto esters and 3-azidooxindoles are attractive
targets because they are transformed smoothly into the
corresponding amino derivatives.5,6 However, except for several
non-enantioselective syntheses,7,8 only Shibatomi and co-
workers have reported the stereospecific SN2 substitution of
optically active α-chloro-β-keto esters to generate the corre-
sponding enantiopure α-azido-β-keto esters.9 To the best of our
knowledge, the direct catalytic asymmetric azidation of β-keto
esters and oxindoles remains unexplored. On the other hand,
efficient and direct stereoselective C−N bond formation is an
attractive topic in organic synthesis, and only a few compounds
such as azodicarboxylates10 and nitroso derivatives11,12 have been
used as electrophilic sources of nitrogen. Exploring other
electrophilic nitrogen sources is still a challenge.
7a
7b
7c
7d
7e
7f
7d
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
d
18
6a
Et2O
a
b
c
Isolated yields. Determined by HPLC analysis. x is defined to be 6.
1b was used instead of 1a.
d
We recently developed a class of chiral pincer ligands (“boxmi”
ligands, 2)13 that were used, inter alia, in the asymmetric Cu-
catalyzed trifluoromethylation of β-keto esters.13c This reaction
employed 3-dimethyl-1-(trifluoromethyl)-1,2-benziodoxole
(Togni’s reagent)14 as a trifluoromethylating agent. In this
work, we exploited a similar strategy to achieve highly
enantioselective Fe-catalyzed azidation of β-keto esters and
oxindoles using the T-shaped iodine(III) compound 1a15 as an
azido-transfer reagent.
We began by using the reaction of β-keto ester 4a with 1a as a
model reaction to investigate different kinds of metal salts [see
the Supporting Information (SI)] and found iron(II) salts to be
the most suitable catalyst precursors in combination with the
boxmi ligands (Table 1). Various iron salts were tested in the
reaction, and iron(II) propionate, Fe(OOCEt)2 (3), was found
to be optimal (entries 1−5). The choice of solvent markedly
influenced both the yield and enantioselectivity, and the use of
diethyl ether generated the product in 86% yield with 81% ee
(entries 5−8). Further screening of ligands showed that ligand 2c
slightly improved the ee (entries 8−10).
Notably, we observed that iron(II) carboxylates gave rise to
the highest enantioselectivities, prompting us to examine the
effect of different carboxylate counterions in the reaction. First,
the isolated Fe complex 6a catalyzed the reaction to afford the
Received: February 27, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja402082p | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
product with 66% ee (entry 11). Next, we exchanged the anionic
ligand of the catalyst in situ by mixing equimolar amounts of 6a
and silver carboxylate. Screening of a series of silver salts showed
that silver arylcarboxylates gave better ee values. Moreover, using
silver benzoates with an electron-withdrawing group (7d and 7e)
shortened the reaction time to 48 h, and silver 4-nitrobenzoate
(7d) improved the ee to 93% (entries 12−17). Finally, using the
alternative azido-transfer reagent 1b led to much lower activities
(entry 18). It should be noted that these reactions are
operationally convenient and can be performed without removal
of silver chloride.
Scheme 1. Enantioselective Azidation of β-Keto Esters
Although the Fe(II) catalysts employed in this study were
generated in situ, Fe complexes bearing boxmi ligands 2 were
readily isolable. Direct complexation of the R,R enantiomer of
protioligand 2c (the S,S enantiomer of the ligand was used in the
catalyses) with Fe(OAc)2 in methanol at room temperature gave
the corresponding Fe(II) acetate complex 6b. The molecular
structure of 6b(py), a pyridine adduct of 6b, was established by
X-ray diffraction (Figure 1) and revealed a strongly distorted
a
Isolated yields and ee’s determined by HPLC are shown. Absolute
configurations of the products were determined on the basis of the
single-crystal structure of 11 and evidenced by comparison with
literature αD values (for 5b and 5n).
cyclic β-keto esters under the conditions described above (Table
2, entry 1). However, with the catalyst generated in situ from 10
mol % 3 and 12 mol % 2c and optimization of the reaction
conditions, the ee value increased to 91% (entry 2).
Table 2. Optimization of the Azidation of 8a
Figure 1. Structure of 6b(py) (H atoms omitted for clarity).
octahedral coordination environment involving four N donors
and an η2-coordinated acetate. The meridional coordination
mode of the pincer ligand (R,R)-2c was confirmed, with the
metal in the plane spanned by the N-donor atoms [as shown by
the N(2)−Fe(1)−N(3) angle of 175.0(2)°].
H-
a
b
entry MXn
Lig
AgX solvent
T
t (h) yield (%) ee (%)
1
2
3
4
5
6a
3
−
7d
−
−
−
−
Et2O
Et2O
THF
Et2O
Et2O
rt
rt
rt
rt
36
36
36
36
48
86
87
84
84
51
78
91
84
90
69
Under the optimized reaction conditions described above, we
explored the generality of the protocol for different cyclic β-keto
esters. As summarized in Scheme 1, all of the tested indanone-
derived tert-butyl β-keto esters were converted to the
corresponding products 5a−g in high yields with high
enantioselectivities (90−93% ee), except for the methylthio-
substituted substrate, which afforded 5h with 81% ee. It was
found that a bulky ester substituent is essential for obtaining high
selectivity (compare 5g with 5i). Three differently functionalized
tert-butyl esters of cyclopentenone were converted to the
corresponding products 5j−l with high enantioselectivities.
Moreover, a cyclopentanone-derived β-keto ester was also
successfully employed in the process, generating 5m with 69 ee
%. Finally, the cyclic six-membered-ring derivatives 5n and 5o
were obtained with only moderate enantioselectivities. Unfortu-
nately, acyclic ketoesters proved to be unreactive under these
reaction conditions.
2c
2c
2a
3
3
3
2c
b
0 °C
a
Isolated yields. Determined by HPLC analysis.
Having established the optimal reaction conditions for this
type of substrate, we explored the oxindole scope of the azidation
(Scheme 2). 3-Phenyloxindoles generated the corresponding
products in high yields with excellent enantioselectivities (91−
94% ee) regardless of the nature and the positions of the
substituents on the oxindole framework (9a−d). On the other
hand, the presence of an electron-donating substituent on the
phenyl group at the C3 position slightly decreased the ee value
(9h vs 9a and 9g).
To highlight the utility of this enantioselective azidation, we
undertook further transformations of the resulting azides
(Scheme 3). α-Azido ester 5a was smoothly converted into α-
amino ester 10 by Pd-catalyzed hydrogenolysis,5a providing a
The methodology described above could be extended to the
azidation of 3-aryloxindoles. Oxindole 8a was azidated with
somewhat lower enantioselectivity (78% ee) compared with the
B
dx.doi.org/10.1021/ja402082p | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
Scheme 2. Enantioselective Azidation of Oxindoles
Figure 2. Structure of triazole 11 (H atoms omitted for clarity).
physiologically active molecules,6 this approach provides a
strategy for the generation of bioconjugates with enantiopure
3-substitued 3-azidooxindoles.2f,g,k
In conclusion, by employing the boxmi system as stereo-
directing ligand, we have developed an efficient protocol for
enantioselective Fe-catalyzed azidation of cyclic β-keto esters and
3-aryloxindoles using a readily available and stable azidoiodinane
as an N3-transfer reagent. Cyclic β-keto esters were converted to
the corresponding products in high yields with up to 93% ee
catalyzed by the combination of an iron(II) chlorido complex
and silver carboxylate. 3-Azido-3-aryloxindoles were obtained
with up to 94% ee using the catalyst prepared from iron(II)
propionate and the ligand in situ. Further studies of synthetic
applications of this transformation as well as the reaction
mechanism are underway in our laboratory.
a
Isolated yields and ee’s determined by HPLC are shown. Absolute
configurations of the products were determined on the basis of the
single-crystal structure of 13.
Scheme 3. Further Transformations of the Azide Products
ASSOCIATED CONTENT
* Supporting Information
Methods, additional data, and CIFs. This material is available free
of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
lutz.gade@uni-hd.de
■
Notes
The authors declare no competing financial interest.
a
ACKNOWLEDGMENTS
CaRLa is cofinanced by the University of Heidelberg, the state of
Conditions: H2 (1 atm), Pd/BaSO4 (10 mol %), MeOH, rt.
■
b
Conditions: 1-Bromo-4-ethynylbenzene (1.2 equiv), CuSO4 (20 mol
t
%), TBTA (1 mol %), sodium ascorbate (0.4 equiv), BuOH/H2O
Baden-Wurttemberg, and BASF SE. Support from these
̈
(2:1), rt. TBTA = tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine.
institutions is gratefully acknowledged. We also thank the
c
The absolute configuration was determined by X-ray structure
́
Fonds der Chemischen Industrie for the Kekule doctoral
fellowship (T.B.) and the Studienstiftung des Deutschen Volkes
for a doctoral fellowship (T.B.).
d
analysis. Conditions: CF3COOH (5 equiv), CH2Cl2, rt.
useful method for the synthesis of highly substituted α-amino
acid derivatives. On the other hand, the Cu-catalyzed azide−
alkyne 1,3-dipolar cycloaddition (CuAAC) “click” reaction,
which has been established as a powerful coupling technology,2e,j
was used to transform α-azido ester 5c into the corresponding
triazole 11 in high yield.7b
The absolute configuration of the optically active 11 was
established to be R by single-crystal X-ray structure analysis
(Figure 2). On this basis, we determined the absolute
configurations of α-azido esters 5. Furthermore, 3-azidooxindole
9f also generated triazole 12 in 94% yield; subsequent removal of
the Boc group by treatment with trifluoroacetic acid afforded the
corresponding product 13, for which an X-ray diffraction study
established the S configuration (see the SI). Since oxindoles are
common structural motifs that are present in a variety of
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