Catalytic, enantioselective 1,3-dipolar cycloadditions of nitrile imines with methyleneindolinones

Article abstract of DOI:10.1039/c3ob41815d

Catalytic, enantioselective 1,3-dipolar cycloadditions of nitrile imines with methyleneindolinones are reported. The spiro[pyrazolin-3,3′-oxindole] products are formed in good yields (up to 98%) and high enantioselectivity (up to 99% ee).

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Catalytic, Enantioselective 1,3-Dipolar
Cycloadditions of Nitrile Imines with
Methyleneindolinones
Cite this: DOI: 10.1039/x0xx00000x
Anthony L. Gerten,a Michael C. Slade,b Kelsie M. Pugha and Levi M. Stanley*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Catalytic, enantioselective 1,3-dipolar cycloadditions of nitrile an ortho-substituted aryl group. Cycloadditions of nitrile imines with
imines with methyleneindolinones are reported. The methyleneindolinones substituted with smaller β-alkyl (e.g. iso-propyl)
spiro[pyrazolin[3,3ʹ-oxindole] products are formed in good groups or β-aryl (e.g. Ph, 4-Cl-C6H4) groups lacking ortho-substitution
yields (up to 98%) and high enantioselectivity (up to 99% ee).
occur with lower enantioselectivity.
Ph
1,3-Dipolar cycloadditions of nitrile imines with alkenes represent an
attractive strategy to generate pyrazolines. Reactions of nitrile imines
with a wide variety of olefinic dipolarophiles have been developed to
provide an array of compounds containing a pyrazoline motif.1,2 Despite
efforts to develop the utility of nitrile imine cycloadditions, controlling
the enantioselectivity of nitrile imine cycloadditions remains difficult to
achieve. The high reactivity of nitrile imines renders catalytic,
enantioselective cycloadditions of this class of dipole challenging due
to the high rates of uncatalyzed background cycloaddition. To date,
only a select few examples of catalytic, enantioselective cycloadditions
of nitrile imines have been reported. Sibi et al. showed that 1,3-dipolar
cycloadditions of nitrile imines with α,β-unsaturated oxazolidinone and
pyrazolidinone imides occur in high yields with high enantioselectivity
in the presence of chiral, non-racemic magnesium catalysts.3 However,
the scope of dipolarophiles in enantioselective nitrile imine
cycloadditions remains narrow.
R
β
N
N
Ph
L1/Mg(ClO4)2
(10 mol %)
R
α
NH
Ph
+
O
N
(a)
O
iPr2NEt, 60 °C
N
N
Ph
Cl
ref. 8
O
O
t-BuO
t-BuO
R = Ph (77% ee), 4-Cl-C6H4 (63% ee),
2-Br-C6H4 (84% ee), t-Bu (96% ee)
Ph
R
β
N
N
Ar
L2/Mg(NTf2)2
(10 mol %)
R
α
NH
Ar
(b)
O +
Ph
N
NEt3, -78 °C
this work
O
N
N
Br
O
Ar = 4-Br-C6H4
O
Ph
R = Ph (92% ee), 4-Cl-C6H4 (99% ee),
2-Br-C6H4 (0% ee), t-Bu (99% ee)
Ph
The prevalence of pyrazoline motifs in bioactive compounds4 and
the growing importance of spirocyclic oxindole derivatives5,6 led our
group and others to investigate synthetic approaches to spiro[pyrazolin-
3,3ʹ-oxindoles]. During the course of our studies, Roth et al.7 and Feng
et al.8 reported the first cycloadditions of nitrile imines with
methyleneindolinones to generate spiro[pyrazolin-3,3ʹ-oxindoles].
Roth and co-workers reported the first racemic synthesis of
spiro[pyrazolin-3,3ʹ-oxindoles] by uncatalyzed cycloadditions of nitrile
imines with methyleneindolinones.7 Shortly thereafter, Feng and co-
workers reported the first catalytic, enantioselective cycloadditions of
nitrile imines generated from hydrazonoyl chlorides with
methyleneindolinones (Scheme 1a).8 These cycloadditions occur with
high enantioselectivity (up to 99% ee) provided the β- substitutent of
the methyleneindolinone substrate is a bulky alkyl (tert-butyl) group or
O
O
N
N
N
N
O
O
O
O
N
N
H
H
L1
L2
Ar
Ar
Scheme 1 Catalytic, Enantioselective Cycloadditions of Nitrile Imines with
Methyleneindolinones.
Herein, we report catalytic, enantioselective cycloadditions of
nitrile imines generated from hydrazonoyl bromides with a variety of
methyleneindolinones (Scheme 1b). The nitrile imine cycloadditions
occur in the presence of a catalyst generated in situ from Mg(NTf2)2 and
an aminoindanol-derived bisoxazoline ligand L2. The spiro[pyrazolin-
3,3ʹ-oxindole] products are formed in good to high yields with high
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 1

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DOI: 10.1039J/oC3uOrnB4a1l8N1a5Dme
enantioselectivities. Furthermore, the nitrile imine cycloadditions that
The identity of the Mg(II) salt and the oxindole nitrogen substituent
occur with high enantioselectivity encompass methyleneindolinones proved important to the yields and/or selectivities of the cycloaddition
containing β-aryl groups lacking substitution at the ortho position. This reactions (entries 5, 7-11). Cycloadditions catalyzed by complexes of
methodology expands the breadth of spiro[pyrazolin-3,3ʹ-oxindoles] Mg(ClO4)2 or MgI2 and L2 occurred with low enantioselectivity
that can be accessed in highly enantioselective fashion and is relative to the cycloaddition catalyzed by a complex of Mg(NTf2)2 and
complementary to the methodology reported by Feng.
L2 (compare entries 7 and 8 with entry 5). Cycloadditions of N-acetyl
Our initial studies focused on reactions of the nitrile imine and N-Boc arylidine methyleneindolinones 1b and 1d with the nitrile
generated from hydrazonoyl bromide 2a with methyleneindolinones 1a- imine generated from 2a formed spiro[pyrazolin-3,3ʹ-oxindoles] 3b and
e. Table 1 summarizes the effect of Lewis acid identity, Lewis acid 3d in 94% and 98% ee, but the yields were substantially lower than the
loading, temperature, and substitution at the oxindole nitrogen on these corresponding reaction with 1a (entries 9 and 11). In contrast, the
reactions. We chose to begin our investigation by conducting reaction of N-pivaloyl methyleneindolinone 1c generated
cycloadditions catalyzed by 30 mol % loading of a complex prepared in spiro[pyrazolin-3,3ʹ-oxindole] 3c in high yield, but the
situ from Mg(NTf2)2 and bisoxazoline ligand L2. The cycloaddition of enantioselectivity was poor (40% ee, entry 10).
the nitrile imine generated at room temperature from 2a with 1a (R =
Table 2 Scope of Nitrile Imine Cycloadditions with Various
Ph) formed spirocycle 3a in high yield but with poor enantioselectivity.
Methyleneindolinonesa
While lowering the temperature of the cycloaddition from room
Ph
temperature to -78 °C did not significantly impact the yield of 3a, this
R
modification led to a dramatic increase in enantioselectivity (entries 1-
3). The cycloaddition of the nitrile imine generated from 2a with 1a
formed 3a in 88% and 90% ee when the reaction was conducted at -78
°C (entry 3). Reducing the catalyst loading from 30 mol % to 10 mol %
did not have a significant impact on the
N
N
Ar
L2 (11 mol %)
Mg(NTf2)2 (10 mol %)
R
NH
Ar
+
O
N
O
Et3N, 4Å MS
N
N
CH2Cl2, -78 °C
Ph
Br
O
O
Ph
1e-m
Ph
3e-m
Table 1 Identification of Catalyst Precursors and Reaction Conditionsa
2a: Ar = 4-Br-C6H4
entry
1
2
3
4
1
1e
1f
1g
1h
1i
1j
1i
1k
1l
1m
R
3
3e
3f
3g
3h
3i
3j
3j
3k
3l
3m
Yield (%)b
ee (%)c
99
99
94
80
94
61
91
4-MeO-C6H4
4-Cl-C6H4
4-Br-C6H4
4-CF3-C6H4
3-MeO-C6H4
3,4,5-(MeO)3-C6H2
3,4,5-(MeO)3-C6H2
2-F-C6H4
81
86
63d
81
90e
91
88
84
43
70
O
O
N
N
Ph
Ph
5
L2
N
N
Ar
6
(11-33 mol %)
MgX2 (10-30 mol %)
Ph
7f
8
NH
Ar
+
O
N
92
0
99
O
Et3N, 4Å MS
N
9
10
2-Br-C6H4
(E)-tert-butyl
N
CH2Cl2, temperature
Ph
Br
O
O
R
a For detailed reaction conditions, see the Supporting Information. b Isolated
yield of 3 with >95:5 diastereomeric and regioisomeric ratios. c Determined
by chiral HPLC. d Isolated yield of 3g as a 10:1 mixture of diastereomers and
an 11:1 mixture of regioisomers. e Isolated yield of 1i as a 9:1 mixture of
diastereomers. f The reaction was conducted in the presence of 100 mol % of
the Mg catalyst.
R
1a-d 2a: Ar = 4-Br-C6H4
R (1)
3a-d
entry
MgX2 (mol %)
temp
(°C)
rt
3
yield ee (%)c
(%)b
1
2
3
4
5
6
7
8
9
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Me (1b)
t-Bu (1c)
Ot-Bu (1d)
Mg(NTf2)2 (30)
3a
3a
3a
3a
3a
3a
3a
3a
3b
3c
3d
82
88
88
91
98
96
98
95
76
97
37
29
55
90
91
92
89
44
64
94
40
98
Mg(NTf2)2 (30)
Mg(NTf2)2 (30)
Mg(NTf2)2 (20)
Mg(NTf2)2 (10)
Mg(NTf2)2 (5)
Mg(ClO4)2 (10)
MgI2 (10)
Mg(NTf2)2 (10)
Mg(NTf2)2 (10)
Mg(NTf2)2 (10)
-20
-78
-78
-78
-78
-78
-78
-78
-78
-78
The scope of the Mg-catalyzed cycloaddition of the nitrile imine
generated from 2a with
a
variety of methyleneindolinones is
The reactions of an electron-rich
summarized in Table 2.
methyleneindolinone and halogenated methyleneindolinones generated
products 3e-g in moderate to good yields with high enantioselectivities
(entries 1-3). The absolute configuration of spiro[pyrazolin-3,3ʹ-
oxindole] 3h was determined to be (3S,4ʹS) by X-ray crystallographic
analysis (Figure 1). In contrast, the reaction of an electron-deficient
dipolarophile 1h furnished the corresponding cycloadduct 3h with
modest enantioselectivity (entry 4). The reaction of a meta-substituted
methyleneindolinone 1i formed spiro[pyrazolin-3,3ʹ-oxindole] 3i in
high yield with high enantioselectivity (entry 5).
10
11
a Reactions conditions: 1 (0.20 mmol), 2a (0.24 mmol), L2 (0.022-0.066
mmol), MgX2 (0.020-0.060 mmol), Et3N (0.30 mmol), 4Å MS (100 mg),
CH2Cl2 (2 mL). b Isolated yield of 3 with >95:5 diastereomeric and
regioisomeric ratios. c Determined by chiral HPLC.
enantioselectivity of the cycloaddition (entries 3-5). In fact, the
reaction performed with 10 mol % catalyst occurred in slightly higher
yield (98%) and enantioselectivity (92% ee) than the reactions
performed with 20 and 30 mol % catalyst. The reaction conducted in
the presence of 5 mol % catalyst formed 3a in high yield, but the
enantioselectivity was marginally lower (89% ee, entry 6).
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C3OB41815D
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substituent at the 2-position of the C-terminal aryl ring: whereas
reaction with a smaller fluoro substituent afforded product 3q in 90%
yield and 90% ee (entry 3), the reaction to afford product 3r with a
larger bromo substituent only proceeded in 81% yield with 62% ee
(entry 4).
Ph
Br
N
N
≡
Table 3 Scope of 1,3-Dipolar Cycloadditions of Methyleneindolinone 1a
Br
with Nitrile Iminesa
O
N
Ar1
O
Ph
Ph
N
N
Ar2
NH
(3S,4!S)-3g
Ph
L2 (11 mol %)
Mg(NTf2)2 (10 mol %)
Ar2
+
O
N
O
Et3N, 4Å MS
N
Figure 1 Absolute stereochemistry and structure of 3g based on X-ray diffraction
N
Ar1
Br
CH2Cl2, -78 °C
O
data.
O
Ph
1a
Ph
3o-t
2b-g
The reaction of a highly electron-rich dipolarophile proved more
challenging. The reaction of methyleneindolinone 1j occurred with
61% ee in the presence of 10 mol % catalyst (Table 2, entry 6). The
observed decrease in enantioselectivity likely results from product
inhibition instead of poor facial selection, as product 3j was formed
with 91% ee in the presence of 100 mol % catalyst (entry 7).
entry
2
Ar1
Ar2
3
yield
ee (%)c
(%)b
86
1
2
3
4
5
6
2b
2c
2d
2e
2f
4-CF3-C6H4
3-Br-C6H4
2-F-C6H4
2-Br-C6H4
Ph
4-Br-C6H4
4-Br-C6H4
4-Br-C6H4
4-Br-C6H4
4-F-C6H4
3o
3p
3q
3r
3s
3t
94
84
90
62
91
90
72
90
81
83
Reactions of methyleneindolinones containing an ortho-substituted
aryl group were sensitive to the size of the ortho substituent. The
reaction of ortho-fluorinated dipolarophile 1k formed cycloadduct 3k in
high yield with high enantioselectivity (entry 8), but the reactivity of
ortho-brominated dipolarophile 1l was poor and the corresponding
product 3l was generated as a racemic mixture (entry 9).
2g
Ph
4-CF3-C6H4
87
a For detailed reaction conditions, see the Supporting Information. b Isolated
yield of 3 with >95:5 diastereomeric and regioisomeric ratios. c Determined
by chiral HPLC.
Nitrile imines with electron-deficient aryl groups at the N-terminus
of the dipole were also well tolerated. Reactions of nitrile imines
substituted with N-terminal 4-fluorophenyl and 4-trifluoromethylphenyl
groups formed the corresponding cycloadducts 3s and 3t in 83% and
87% yields with 91% and 90% ee, respectively (entries 5 and 6).
The scope of cycloadditions with methyleneindolinone 1a
encompasses a range of sterically and electronically distinct nitrile
imines. However, the hydrazonoyl bromide precursors to nitrile imines
with electron-rich aryl groups at either the C- or N-terminus were too
unstable in our hands to generate reproducible results. In addition,
attempts to utilize hydrazonoyl chlorides as nitrile imine precursors
resulted in either low reactivity or poor enantioselectivity.
Although aryl groups with large ortho-substituents are not well
tolerated by the current catalyst, bulky β-alkyl substituents are well
tolerated. The reaction of (E)-methyleneindolinone 1m (R = t-Bu)
furnished cycloadduct 3m in 70% yield and 99% ee (Table 2, entry 10).
However, the enantioselectivity of the cycloaddition is sensitive to the
geometry of the alkene unit. The reaction of (Z)-methyleneindolinone
1n (R = t-Bu) furnished the diastereomeric cycloadduct 3n with only
66% ee (eq 1).
Ph
t-Bu
N
L2 (11 mol %)
Mg(NTf2)2 (10 mol %)
t-Bu
N
In summary, we have developed
cycloadditions of nitrile imines to form spiro[pyrazolin-3,3ʹ-oxindole]
in high yields with high enantioselectivity. This process is
a method for 1,3-dipolar
Ar
(1)
+
2a
O
Et3N, 4Å MS
O
N
CH2Cl2, -78 °C
N
O
O
complementary to extant methods, allowing for the formation of
spiro[pyrazolin-3,3ʹ-oxindole] from that could not previously be
accessed in highly enantioenriched form. Specifically, spiro[pyrazolin-
3,3ʹ-oxindole] cycloadducts are formed with high enantioselectivity
from methyleneindolinones lacking an ortho-substituted aryl group at
β-position of the dipolarophile. Studies to develop asymmetric
cycloadditions of nitrile imines with additional classes of dipolarophiles
are ongoing in our laboratory.
Ph
1n
64% yield, 66% ee
Ph
3n
The scope of the Mg-catalyzed cycloaddition of 1a with a selection
of nitrile imines is summarized in Table 3. As illustrated in Table 1,
entry 5, the nitrile imine generated from benzaldehyde-derived
hydrazonoyl bromide 2a furnished cycloadduct 3a in high yield with
excellent enantioselectivity. The reaction of a nitrile imine bearing an
electron-deficient aryl group (Ar1 = 4-CF3C6H4) at the C-terminus of
the dipole also generated the corresponding spiro[pyrazolin-3,3ʹ-
oxindole] 3o in high yield with high enantioselectivity (Table 3, entry
1). The reaction of a nitrile imine containing a 3-bromophenyl group at
the C-terminus furnished cycloadduct 3p in 72% yield with 84% ee
(entry 2). The reaction also appears sensitive to the size of the
We thank Iowa State University and the ISU Institute for Physical
Research for financial support of this work. We thank Dr. Arkady
Ellern and the Molecular Structure Laboratory (Iowa State University)
for X-ray diffraction data collection and structure determination. We
thank Professor Emeritus Richard Larock (Iowa State University) for
instrumentation and chemicals. M.C.S. thanks the HHMI program for
improving STEM education at Iowa State University for funding as a
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DOI: 10.1039J/oC3uOrnB4a1l8N1a5Dme
Postdoctoral Science Teaching Fellow (HHMI grant P.I.: C. Ogilvie).
Wu, W. Sun, G. Zhu, G. Li, X. Yao and R. Wang, Chem. Commun.,
2013, 49, 6713.
L.S. thanks the NIH for
(GM95697).
a Pathways to Independence Award
7
8
A. Singh, A. L. Loomer and G. P. Roth, Org. Lett., 2012, 14, 5266.
G. Wang, X. Liu, T. Huang, Y. Kuang, L. Lin and X. Feng, Org.
Lett., 2013, 15, 76.
Notes and references
a
Department of Chemistry, Iowa State University, Ames, Iowa 50011,
United States. E-mail: lstanley@iastate.edu
b
Current Address: Department of Chemistry, University of Evansville,
Evansville, Indiana 47722, United States.
† Electronic Supplementary Information (ESI) available: Experimental
procedures, characterization data for new compounds, and
crystallographic
data
(CIF)
for
compound
3g.
See
DOI: 10.1039/c000000x/
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