Catalytic enantioselective iodoaminocyclization of hydrazones

Article abstract of DOI:10.1021/ol5013982

The first catalytic enantioselective iodoaminocyclization of β,γ-unsaturated hydrazones has been developed with the help of a trans-1,2-diaminocyclohexane-derived bifunctional thiourea catalyst and allows for the direct access to δ²-pyrazolines containing a quaternary stereogenic center in high yield with good enantioselectivity (up to 95% yield and 95:5 er).

Full text of DOI:10.1021/ol5013982

Letter
pubs.acs.org/OrgLett
Catalytic Enantioselective Iodoaminocyclization of Hydrazones
Chandra Bhushan Tripathi and Santanu Mukherjee*
Department of Organic Chemistry, Indian Institute of Science, Bangalore 560 012, India
S
* Supporting Information
ABSTRACT: The first catalytic enantioselective iodoaminocycli-
zation of β,γ-unsaturated hydrazones has been developed with the
help of a trans-1,2-diaminocyclohexane-derived bifunctional
thiourea catalyst and allows for the direct access to Δ2-pyrazolines
containing a quaternary stereogenic center in high yield with good
enantioselectivity (up to 95% yield and 95:5 er).
tereospecificity associated with electrophilic halogen-in-
duced reactions of unactivated olefins has established them
halofunctionalization reaction. Encouraged by the success of this
S
iodoetherification reaction, we reasoned that an analogous
iodocyclization could be achieved with the closely related ketone
derivative, namely hydrazones (Scheme 1B). Herein we disclose
an efficient catalytic enantioselective iodoaminocyclization of
β,γ-unsaturated hydrazones.10
Pyrazolines are privileged substructure in many natural
products and also possess diverse biological activities.11 A series
of studies have recently established N-acetyl 3,5-diaryl Δ2-
pyrazolines containing a quaternary stereogenic center as potent
kinesin spindle protein (KSP) inhibitors.12 Although a number
elegant routes to enantioenriched pyrazolines has been
reported,13 enantioselective synthesis of pyrazolines containing
a quaternary stereogenic center remained challenging.14 Our
method enables direct access to enantioenriched Δ2-pyrazolines
containing a quaternary stereogenic center.
Considering the similarities between the OH group of oximes
and the acidic NH of protected hydrazones, we anticipated the
compatibility of similar bifunctional thiourea catalysts15 for this
iodoaminocyclization. Consequently, the protecting group on
the nitrogen (PG in Scheme 1B) was expected to play a key role,
both in deciding the reactivity as well as enantioselectivity.
With this premise in mind, we began our investigation by
studying the iodoaminocyclization of phenyl-substituted β,γ-
unsaturated 4-nitrobezenesulfonyl (4-Ns) hydrazone 1a with N-
iodosuccinimide (NIS, 2a) as the electrophilic iodine source
(Table 1). This protecting group was chosen to render sufficient
acidity to NH. For efficiently evaluating the catalyst influence, we
had to suppress the strong background reaction (entry 1), which
was possible by lowering the reaction temperature to −80 °C.
Under these conditions, no product formation was detected,
even after 48 h (entry 2). The subsequent catalyst screenings
were therefore conducted at −80 °C.16 As with our previously
studied oxime iodoetherification reaction,9b the necessity of
bifunctional catalyst for this iodoaminocyclization reaction was
as a general strategy for olefin trans-heterodifunctionalization.1
Compared to other heterodifunctionalizations, this class of
reactions holds greater synthetic potential due to the presence of
an easily modifiable halogen handle. Tremendous advancement
has been made in the area of asymmetric olefin halofunction-
alization during the past few years.2,3 Particularly noteworthy are
the intramolecular versions, often referred to as halocyclization.4
With the development of various strategies and diverse range of
catalysts, synthesis of a wide variety of enantioenriched
heterocycles consisting of any of the four halogens has been
accomplished.5−8 However, while diversification with respect to
the electrophilic component has been realized to include
dienes,8b enynes,4d allenes,7d and even alkynes,6c the choice of
nucleophilic component has remained mostly restricted to acids,
alcohols, amines, and their derivatives. In contrast, aldehyde- or
ketone-derived nucleophiles have largely been overlooked.9
We have recently reported an enantioselective iodoether-
ification of β,γ-unsaturated oximes, a ketone derivative, using a
dihydrocinchonidine-derived bifunctional thiourea catalyst,
resulting in the formation of Δ2-isoxazolines containing a
quaternary stereogenic center with good to excellent er’s
(Scheme 1A).9b With this reaction, a ketone derivative was
used for the first time as a nucleophile in enantioselective
Scheme 1. Ketone Derivatives in Asymmetric Halocyclization
Received: May 15, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ol5013982 | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
iodoetherification reaction, no positive effect of I2 was observed
(entry 14). Finally, the best result in terms of enantioselectivity
was obtained with catalyst VI containing a pentafluorophenyl
ring (entry 15).
Table 1. Catalyst Evaluation and Reaction Conditions
Optimization for Enantioselective Iodoaminocyclization of
Hydrazone 1a
The compatibility of other protecting groups on hydrazone
was tested with catalyst VI under the optimized reaction
conditions (Table 2). These protecting groups include 3-
Table 2. Effect of Hydrazone Protecting Group on the
Catalytic Enantioselective Iodoaminocyclization
a
b
entry
PG
time (h)
product
yield (%)
er
1
2
3
4
4-Ns (1a)
3-Ns (4)
p-Ts (5)
48
48
48
48
3a
7
95
91
97
90
94:6
84:16
67:33
86:14
8
a
b
entry
cat. I+ source
additive
time (h) conv (%)
er
4-Bs (6)
9
c
c
c
c
c
c
,d
1
2
3
4
5
6
7
8
9
2a
2a
1
48
48
12
36
36
30
30
48
45
48
48
48
48
48
>95
<5
a
b
Isolated yield. Enantiomeric ratio (er) was determined by HPLC
analysis using a stationary-phase chiral column (see the Supporting
Information).
I
2a
2a
2a
2a
2a
2a
2b
2c
2c
2c
2c
2c
2c
<5
II
III
IV
IV
V
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
50:50
53:47
67.5:32.5
79:21
79:21
55:45
85:15
89:11
92.5:7.5
91:9
nitrobezenesulfonyl (3-Ns), p-tolylsulfonyl (p-Ts), and 4-
bromobezenesulfonyl (4-Bs). Although poor enantioselectivity
was observed for p-Ts hydrazone, both 3-Ns and 4-Bs
hydrazones returned with good er. However, 4-Ns remained
the protecting group of choice under our catalyst and reaction
conditions.
V
10
11
12
13
14
15
V
V
MS 3 Å
MS 4 Å
MS 5 Å
MS 4 Å
MS 4 Å
Having identified the optimum catalyst and reaction
conditions, we set out to demonstrate the generality of our
enantioselective iodoaminocyclization protocol. However, our
initial attempt toward this end with catalyst VI was severely jolted
as we failed to replicate the same level of enantioselectivity for
other substrates.16 To our relief, when catalyst V was employed
instead, high level of enantioselectivity was ensured for a wide
range of β,γ-unsaturated (4-Ns)-hydrazones (1) under otherwise
identical reaction conditions (Table 3). Electron-deficient aryl
substituents on either end of the substrate (R1 and R2) are
generally tolerated, and the resulting Δ2-pyrazoline derivatives
(3) were obtained with good er. However, products with
significantly reduced enantioselectivities were obtained for the
highly electron-rich aryl substituent on olefin (entry 7) and a
sterically hindered o-chlorophenyl substituent on the hydrazone
carbon (entry 10). Electron-rich or heteroaryl substituents on
the hydrazone carbon, on the other hand, affored the products
with fairly good level of enantioselectivity (entries 18 and 19).
β,γ-Unsaturated (4-Ns)-hydrazones with aliphatic substituents
showed good reactivities, but poor enantioselectivities (entries
20−22), leaving room for further improvement. It must be noted
that irrespective of the nature of the substrate, a uniform reaction
time (86 h) was followed in all cases to ensure complete
conversion as reaction monitoring (by TLC) proved challenging.
Single-crystal X-ray diffraction analysis of the pyrazoline
derivative 3a established its absolute configuration to be R
(Figure 1).17 The configurations of the other products reported
herein were tentatively assigned as the same assuming that a
similar catalytic mechanism was followed.
V
V
e
V
90:10
94:6
VI
a
Conversion as determined by 1H NMR analysis of the crude reaction
mixture. Enantiomeric ratio (er) was determined by HPLC analysis
using a stationary phase chiral column (see the Supporting
Information). Reaction in CH2Cl2 at 0.1 M concentration. Reaction
b
c
d
e
at 25 °C. Reaction with 5 mol % of I2.
confirmed by the lack of catalytic activity of the bisthiourea
derivative I (entry 3). The cinchonidine-derived thiourea II, a
highly efficient catalyst for the enantioselective oxime iodoether-
ification, showed high catalytic activity but failed to induce any
enantioselectivity in this reaction (entry 4). Therefore, we turned
our focus toward bifunctional catalysts derived from trans-1,2-
diaminocyclohexane. Whereas the urea derivative III resulted
Δ2-pyrazoline derivative 3a with poor enantioselectivity (entry
5), promising results were obtained with the corresponding
thiourea derivative IV, commonly known as Takemoto catalyst
(entry 6).15a At this point, an extensive solvent and concentration
optimization revealed a mixture of toluene and CH2Cl2 (1:1) as
the optimum reaction medium, and at 0.01 M concentration, 3a
was obtained with an er of 79:21 (entry 7).16 Catalyst V
containing a pyrrolidine ring at the Brønsted basic nitrogen
proved to be equally efficient as IV (entry 8). The nature of I+
source was found to have a considerable influence on the
enantioselectivity of this reaction: while N-iodophthalimide 2b
led to a drastic drop in er, product with improved er was obtained
with N-iodopyrrolidinone 2c (entries 9 and 10). The presence of
molecular sieves improved the er, with 4 Å MS emerged as the
optimal (entries 11−13). However, unlike in the case of oxime
In conclusion, we have developed the first catalytic
enantioselective haloaminocyclization of hydrazones using a
bifunctional thiourea catalyst. Starting from easily accessible β,γ-
B
dx.doi.org/10.1021/ol5013982 | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 3. Scope of Catalytic Enantioselective
Iodoaminocyclization of β,γ-Unsaturated Hydrazone
ASSOCIATED CONTENT
* Supporting Information
■
a
S
Experimental procedures, characterization data, and crystallo-
graphic data (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
b
c
R1
R2
3
yield (%)
er
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
3a
3b
3c
3d
3e
3f
95
88
79
93
82
78
79
73
92.5:7.5
91.5:8.5
90.5:9.5
94:6
*E-mail: sm@orgchem.iisc.ernet.in.
Notes
4-FC6H4
4-ClC6H4
4-MeC6H4
3-MeC6H4
2-MeC6H4
The authors declare no competing financial interest.
93:7
ACKNOWLEDGMENTS
90:10
75:25
92:8
■
4-OMeC6H4 3g
This work was supported by DST, New Delhi, and CSIR, New
Delhi. C.B.T. thanks CSIR for a doctoral fellowship. C.B.T. is
also a recipient of a Bristol-Myers Squibb fellowship. We thank
Mr. Vijith Shetty and Mr. Pavan M.S. (SSCU, IISc, Bangalore)
for their help with the X-ray structure analysis.
4-ClC6H4
3-ClC6H4
2-ClC6H4
4-BrC6H4
3-BrC6H4
3-FC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
Ph
3h
3i
d
e
78 (95)
94:6
10
11
12
13
14
15
16
17
18
19
20
21
22
3j
80
85
80
75:25
92:8
3k
3l
90:10
91:9
f
e
4-MeC6H4
Ph
3m
3n
3o
3p
3q
3r
3s
3t
61 (92)
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90:10
92:8
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