Cinchonidinium acetate as a convenient catalyst for the asymmetric synthesis of cis-stilbenediamines

Article abstract of DOI:10.1016/j.tetlet.2014.12.105

Inexpensive and readily available cinchonidinium acetate is an effective catalyst for the syn-selective aza-Henry reaction of arylnitromethanes and aryl imines. The resulting masked cis-stilbenediamine products are produced in excellent diastereoselectivi

Full text of DOI:10.1016/j.tetlet.2014.12.105

Tetrahedron Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
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Cinchonidinium acetate as a convenient catalyst for the asymmetric
synthesis of cis-stilbenediamines
⇑
Ryan R. Walvoord, Marisa C. Kozlowski
Department of Chemistry, Roy and Diana Vagelos Laboratories, University of Pennsylvania, Philadelphia, PA 19104, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Inexpensive and readily available cinchonidinium acetate is an effective catalyst for the syn-selective aza-
Henry reaction of arylnitromethanes and aryl imines. The resulting masked cis-stilbenediamine products
are produced in excellent diastereoselectivity and good enantioselectivity, and enantiopure material can
be achieved via recrystallization. The features of the cinchona catalyst needed for selectivity are dis-
cussed, with specific emphasis on formation of a kinetically controlled syn-product without epimeriza-
Received 23 October 2014
Revised 16 December 2014
Accepted 17 December 2014
Available online xxxx
tion of the highly acidic a-nitro stereocenter.
Keywords:
Ó 2014 Elsevier Ltd. All rights reserved.
Aza-Henry
Asymmetric organocatalysis
Cinchona alkaloid
Nitro-Mannich
Vicinal diamine
Introduction
While considerable success has been achieved for the asymmet-
ric aza-Henry reaction using nitromethane and its alkyl congen-
ers,6 examples using arylnitromethanes are limited, and largely
reveal orthogonal reactivity and stereocontrol when utilized with
otherwise well-behaved catalyst systems.7 In addition to identify-
ing suitable conditions for arylnitromethane reactivity, a key chal-
lenge lay in constructing and preserving the highly acidic b-nitro
stereocenter. Notably, Johnston and coworkers have reported an
elegant process for accessing the syn b-nitroamine aza-Henry
adducts with high diastereo- and enantiocontrol utilizing a bisami-
dine-quinoline catalyst, highlighted by the synthesis of the p53-
MDM2 inhibitor Nutlin-3.8 The same group recently revealed a
The vicinal diamine moiety is a central structural feature of
many biologically significant molecules, ranging from pharmaceu-
ticals to molecular imaging tools.1 Moreover, this array serves as
the critical stereocontrol element in a substantial percentage of
both organo- and metal-based asymmetric catalysts.2 Despite their
utility, asymmetric methods for the synthesis of several classes of
vicinal diamines remain challenging, often relying on chiral resolu-
tions.3 In particular, access to cis-stilbenediamines, possessing a
syn-1,2-diaryl unit, has largely remained limited to symmetric
compounds via dimerization methods to form the meso diamine.4
Many of these processes suffer from poor diastereoselection, and
all require subsequent chiral resolution to afford enantioenriched
diamines. Recently, Seidel has elegantly demonstrated the desym-
metrization of meso-stilbenediamines via thiourea catalysis, pro-
viding monobenzoylated diamines in good ee.5
H2N
NH2
reduction
An attractive approach for the enantioselective synthesis of
both symmetric and unsymmetric cis-stilbenediamines exploits
the use of the aza-Henry, or nitro-Mannich, reaction for the union
of two nitrogen containing fragments. Specifically, addition of an
arylnitromethane nucleophile (presumably as the nitronate) into
an aryl imine provides the b-nitroamine adduct, which upon
reduction and deprotection reveals the free diamine (Scheme 1).
R1
R2
deprotection
cis-stilbenediamine
stereoselective
asymmetric
aza-Henry
PG
PG NH NO2
NO2
N
R2
+
R1
this work
R1
cis-nitroamine adduct (3)
R2
aryl imine (1) arylnitromethane(2)
⇑
Corresponding author. Tel.: +1 215 898 3048; fax: +1 215 573 7165.
E-mail address: marisa@sas.upenn.edu (M.C. Kozlowski).
Scheme 1. Proposed synthesis of cis-stilbenediamines via aza-Henry reaction.
http://dx.doi.org/10.1016/j.tetlet.2014.12.105
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
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2
R. R. Walvoord, M. C. Kozlowski / Tetrahedron Letters xxx (2015) xxx–xxx
Table 1
modified monoamidine-amide catalyst to be effective for this
transformation on a variety of substrates, although poorer stereo-
control was observed with electron-deficient arylnitromethanes.9
Herein, we report a complimentary and convenient method for
the asymmetric synthesis of cis-stilbenediamines using readily
available, inexpensive cinchonidinium acetate as the catalyst. This
convergent approach provides exceptional diastereocontrol for
unsymmetric products as well as good enantiocontrol, and enan-
tiopure material is readily obtained via recrystallization.
Survey of chiral amine and Brønsted acid catalysts
Entry
Catalyst
dra (syn:anti)
eea (%)
1
2
3
4
5
6
4
5
88:12b
89:11b
45:55
93:7
—
93:7
0(0)
1(0)
3(1)
1(11)
—
Results and discussion
6
7c
8c
b
Initial investigations of the aza-Henry reaction with arylnitrom-
ethanes were conducted using BINOL-salen catalysts previously
developed in our laboratories.10 While these systems proved unse-
lective, a survey of Brønsted acids (Fig. 1) led to the discovery of
the cinchona alkaloid cinchonidinium acetate as a potential stereo-
control platform. Although proline salts 4 and 5 as well as bisamid-
inium 7 exhibited moderate diastereocontrol (Table 1), they
provided essentially racemic nitroamine adduct. Notably,
3,30-substituted BINOLphosphoric acid 8, effective in several asym-
metric Brønsted-acid catalyzed transformations,11 yielded only
trace product. In contrast, cinchonidinium acetate, formed via pro-
tonation of the quinuclidine nitrogen (pKa AcOH = 4.76, pKa cin-
chonidinium–H+ = 8.40),12 smoothly provided the desired syn
product13 3aa with a high 93:7 dr and moderate 61% ee.
9ꢁHOAc
61
a
Determined by HPLC. Numbers in parentheses refer to ee of the anti
diastereomer.
b
Trace product formed.
25 mol % catalyst used.
c
Table 2
Optimization of acid additive and temperature
Reasoning that the acetate anion plays a role in the asymmetric
step, chiral counteranions were examined (Table 2, entries 1 and
2). The similar levels of enantioselection offered by the salts from
both enantiomers of 10 pointed away from the counterion being
a key controller of the asymmetric environment.8a A survey of
acids of varying strengths revealed a rather narrow window for
optimal selectivity centered on acetic acid. In particular, stronger
sulfonic acids, p-TsOH and TfOH, inhibited the reaction. Greater
success was found by decreasing the reaction temperature, result-
ing in nearly complete diastereocontrol when performing the reac-
tion at ꢀ30 °C. Colder temperatures further increased
enantioselectivity, but conversion became problematic, with no
product formation at ꢀ78 °C after several days. Increasing the ace-
tic acid/catalyst stoichiometry to 3:1 hindered reactivity, whereas
a 1:3 ratio resulted in slightly poorer diastereo- and enantiocon-
trol. Alternative imine protecting groups, including N-Ts and
N-Cbz exhibited diminished reactivity, and afforded the corre-
sponding products in lower selectivity in comparison with N-Boc
imines 1.
a
Entry Acid
pKa (H2O) T (°C) drb (syn:anti) eeb (%)
1
(R)-10
1.29c
1.29c
ꢀ14d
–2.8f
0.23
0
0
88:12
88:12
21(44)
37(45)
—
2
3
4
5
(S)-10
TfOH
p-TsOH
TFA
e
ꢀ45
—
e
0
—
—
0
0
86:14
93:7
96:4
93:7
95:5
97:3
99:1
33(25)
31(1)
31
61
68
71
74
—
—
6
BzOH
4.20
7
p-Aminobenzoic acid 4.89
ꢀ45
8
AcOH
4.76
4.76
4.76
4.76
4.76
4.76
4.76
0
9
AcOH
ꢀ15
ꢀ30
ꢀ45
ꢀ78
ꢀ45
ꢀ45
10
11
12
13
14
AcOH
AcOH
AcOH
AcOH (150 mol %)
AcOH (17 mol %)
e
—
—
e
98:2
70
a
Ref. 11.
b
Determined by HPLC. Numbers in parentheses refer to ee of the anti
diastereomer.
c
Value for diphenylphosphate.
Ref. 14.
Trace product formed.
Ref. 15.
d
e
f
Effects of catalyst structure were explored using other members
of the cinchona alkaloid family, as well as synthetically modified
Figure 1. Catalyst structures investigated for the aza-Henry reaction of arylnitromethanes.
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R. R. Walvoord, M. C. Kozlowski / Tetrahedron Letters xxx (2015) xxx–xxx
3
Table 3
was postulated to enhance ee, catalyst 15 provided negligible
enantioinduction. The pseudo-meta position of the 6-substituent
only slightly alters the basicity of the quinoline nitrogen (cf. quin-
olinium pKa of 9 = 4.17, 12 = 4.32).12 Consequently, a steric influ-
Investigation of cinchona structure on product selectivity
ence of the substituent may play
a significant role in the
enantiodetermining step of the mechanism, rather than an effect
on the basicity of the quinoline nitrogen. Alternative binding
modes with the basic atoms of the substituent may also be inter-
vening. For the phenolic catalyst 14, a separate activation mecha-
nism involving hydrogen-bonding activation of the imine from
the phenol may predominate. Based on the results using silyl ether
16 or dimeric catalysts 17 and 18, the free secondary hydroxyl
group is important for high enantioselectivity. The role of the bicy-
clic amine was explored with N-benzylated catalyst 19, which was
poorly reactive, forming only trace amounts of product in low ee.16
This result suggests the quinuclidinium N–H acts as an acid or
hydrogen bond donor in the active catalyst.
Catalyst
dra (syn:anti)
eea (%)
9 (cinchonidine)
11 (cinchonine)
12 (quinine)
13 (quinidine)
99:1
99:1
98:2
98:2
95:5b
96:4
99:1
91:9
76:24
88:12
74 (R,S)
71 (S,R)
44 (S,R)
37 (R,S)
12 (S,R)
1 (R,S)
15 (R,S)
4 (S,R)
13 (R,S)
13 (R,S)
14
15c
16
17d
18d
19e
Performing the reaction at ꢀ30 °C with lowered catalyst and
acetic acid loadings (10 mol %) provided reasonable rates and a
minimal decrease in selectivity. The generality of the method
was explored under these optimized conditions as illustrated in
Table 4.17
a
b
c
Determined by HPLC.
Trace product formed.
Run with 25 mol % catalyst and AcOH.
Run at 0 °C with 100 mol % AcOH.
Run at 0 °C without AcOH.
d
e
The arylnitromethane substrate tolerated both electron-with-
drawing and electron-donating substitution, providing trifluoro-
methyl and methoxy products 3ab and 3ac in high yield with
excellent diastereoselectivity and moderate enantioselectivity. Ste-
ric hindrance on the nucleophile led to deleterious results, as
exhibited by 1-naphthyl substrate 2d. Although only the syn dia-
stereomer was observed, the product formed with poor conversion
and selectivity. The method proved general for a variety of aryl
imines, including electron rich, electron poor, and naphthyl sub-
strates. Based on the results observed for trifluoromethyl- and
chloro-substituted products 3ba (79% ee) and 3ca (77% ee), higher
enantioselection is afforded with electron-deficient imines.
Accordingly, para-methoxy adduct 3da formed with lower
selectivity (61% ee). Notably, ortho-substituted imine 1f performed
well in the reaction. In contrast, meta-trifluoromethyl product 3ga
structures, as shown in Table 3. The four canonical cinchona struc-
tures, 9 and 11–13, exhibited near perfect diastereocontrol. As
anticipated, pseudoenantiomer 11 afforded the product with sim-
ilar enantioselectivity but for the opposite absolute configuration.
Catalysts 12 (quinine) and 13 (quinidine) are isostructural to 9
and 11, respectively, except for the methoxy substitution at the
6-position of the quinoline ring. However, these catalysts provided
significantly lower enantioselection compared to their unsubsti-
tuted analogs, along with an unanticipated change in product
absolute configuration. This effect was further probed via synthetic
manipulation to afford 14 and 15, which contain phenol and meth-
anesulfonyl substitution, respectively. Although replacement of
the donating methoxy group with withdrawing sulfonyl group
Table 4
Substrate scope of the cinchonidinium acetate-catalyzed aza-Henry reactiona
a
Reaction performed on a 0.2 mmol scale using 1.2 equiv imine. Yields refer to isolated yields after purification. Diastereomeric ratio and ee
determined by chiral HPLC or SFC analysis.
Performed on a 2.3 mmol scale.
After a single recrystallization from CH2Cl2.
b
c
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4
R. R. Walvoord, M. C. Kozlowski / Tetrahedron Letters xxx (2015) xxx–xxx
Table 5
was obtained in high diastereoselectivity, but with diminished
enantioselectivity.
Epimerization studies with amine bases
A key feature of the method is the simplicity of both the catalyst
system as well as product purification. Eluting the reaction mixture
through a small plug of silica with dichloromethane removes the
catalyst, and subsequent trituration with hexanes removes unre-
acted arylnitromethane to afford the pure product as a white
solid.18 The optical purity is easily enhanced via recrystallization
of the solid products. As an example, the model reaction was per-
formed on a 2.3 mmol scale, providing adduct 3aa in 70% ee. One
recrystallization from dichloromethane afforded nearly enantio-
pure product (98% ee), and only syn diastereomer detectable by
HPLC. In addition, crystallization by slow evaporation yielded crys-
talline product suitable for X-ray crystallographic analysis,19 secur-
ing both relative and absolute stereochemistry (Fig. 2).
a
Base
pKa
drb (syn:anti)
eeb syn (%)
eeb anti (%)
Pyridine
DMAP
DBU
5.4
9.9
11.6
98:2
44:56
62:38
97
97
97
—
97
97
a
Refers to dissociation constants of the protonated amine in H2O. Ref. 21.
Determined by HPLC.
b
Considering the highly acidic nature of the b-nitro benzylic pro-
ton, the exceptional diastereocontrol of the cinchonidine-acetic
acid catalyst system is of particular note. Indeed, during the course
of these investigations, it was observed that subjecting syn product
3aa to weakly basic conditions produced significant amounts of the
anti diastereomer. While a retro process (Scheme 2, path a), occur-
ring via deprotonation of the carbamate proton, could account for
this transformation, treatment of enantioenriched syn-3aa with
amine bases produced the anti-isomer with complete retention
of stereochemical information at the b-carbon (Table 5) thus con-
firming an epimerization mechanism (path b). Favorable deproto-
nation of the b-nitroalkyl proton is also rationalized based on pKa
values, despite the kinetic barrier associated with nitronate
formation.20
Ostensibly, the active catalyst is selective for the deprotonation
of the primary arylnitromethane nucleophile and formation of the
kinetic syn product while avoiding epimerization to the thermody-
namic mixture of diastereomers. Based on the inactivity of pyridine
in the study above (Table 5), the quinoline nitrogen of the cinchona
catalyst appears unable to effect product epimerization. Similarly,
the acetic acid additive may act as an internal buffer to prevent
deprotonation by the more basic quinuclidine nitrogen.
Conclusion
In conclusion, we have demonstrated the utility of catalytic cin-
chonidinium acetate for the facile synthesis of cis-stilbene diamine
derivatives with excellent diastereocontrol and good enantioselec-
tivity. The high levels of diastereocontrol across a broad range of
substrates are of particular note, highlighting the ability of the cat-
alyst to form one isomer via kinetic control while preventing epi-
merization to the thermodynamic mixture of syn and anti
products. The only other asymmetric, catalytic method for this
transformation achieves higher enantioselectivity, but uses a more
costly catalyst.8,9 In contrast, the report herein defines a simple
catalyst useful in situations, such as large-scale applications, where
catalyst cost is
a primary driver and crystallization can be
employed. Additionally, this method achieves excellent diastere-
oselectivities that do not erode with more acidic products.
Acknowledgments
We are grateful to the National Institutes of Health (GM-
087605) and the National Science Foundation (CHE0911713) for
financial support of this research. Partial funding for the High
Throughput Experimentation Laboratory is acknowledged from
the NSF (GOALI CHE-0848460) and NIH (S10 OD011980). Partial
instrumentation support was provided by the NIH for MS
(1S10RR023444) and NMR (1S10RR022442) and by the NSF for
an X-Ray Diffractometer (CHE 0840438). The assistance of Dr.
Patrick Carroll (UPenn) in obtaining the crystal structure is grate-
fully acknowledged. Dr. Simon Berritt (UPenn) is acknowledged
for assistance in an optimization screen.
Figure 2. X-ray structure of product 3aa.
Boc
N
NO2
Ar'
+
Ar
path a
path b
retro-aza-Henry
epimerization
Boc
NO2
BocHN
Ar
NO2
Ar'
H
N
BocHN
Ar
NO2
Ar'
B:
:B
Supplementary data
H
Ar'
Ar
Supplementary data (experimental procedures, full character-
ization, including 1H NMR and 13C NMR spectra, for all new com-
pounds, HPLC and SFC chromatograms, and experimental
procedures for epimerization experiments) associated with this
article can be found, in the online version, at http://dx.doi.org/10.
1016/j.tetlet.2014.12.105.
racemization
anti syn
syn
stereoretention
anti syn
kinetic
thermodynamic
equilibrium
thermodynamic
equilibrium
Scheme 2. Potential base-mediated isomerization pathways of b-nitroamine
products.
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R. R. Walvoord, M. C. Kozlowski / Tetrahedron Letters xxx (2015) xxx–xxx
5
12. Jencks, W. P.; Regenstein, J. Ionization Constants of Acids and Bases 3rd ed. In
Handbook of Biochemistry and Molecular Biology; Fassman, G. D., Ed.; CRC Press:
Ohio, 1975; vol. 1, pp 305–351.
13. The syn-configuration here is defined as shown in the product of Table 1.
Opposite nomenclature is used in certain references, notably including
Shibasaki’s reports.
References and notes
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(0.020 mmol, 0.2 M in CH2Cl2) were added, and the solution was cooled to
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Please cite this article in press as: Walvoord, R. R.; Kozlowski, M. C. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2014.12.105