L[e3tte+r 2] cycloaddition of aziridines and alkenes catalyzed by a cationic manganese porphyrin

Article abstract of DOI:10.1055/s-0033-1340012

A formal [3+2] cycloaddition between aziridines and alkenes to give the corresponding pyrrolidines was successfully carried out in the presence of a cationic manganese porphyrin catalyst. The use of the porphyrin catalyst allowed, for the first time, styrene derivatives to react with aziridines. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1340012

LETTER
▌2763
letter
[3+2] Cycloaddition of Aziridines and Alkenes Catalyzed by a Cationic
Manganese Porphyrin
[3+2] Cycloaddition of Aziridines and Alkenes
Takuya Ozawa,a Takuya Kurahashi,*a,b Seijiro Matsubara*b
a
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan
b
JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
Fax +81(75)3832438; E-mail: kurahashi.takuya.2c@kyoto-u.ac.jp; E-mail: matsubara.seijiro.2e@kyoto-u.ac.jp
Received: 03.09.2013; Accepted after revision: 16.09.2013
ethers and allylsilanes.6 Here, we report a [3+2] cyclo-
addition of aziridines with alkenes in the presence of a cat-
ionic manganese porphyrin catalyst to afford pyrrolidines.
The use of the cationic manganese porphyrin catalyst al-
lows, for the first time, styrene derivatives to react with
aziridines.
Abstract: A formal [3+2] cycloaddition between aziridines and al-
kenes to give the corresponding pyrrolidines was successfully car-
ried out in the presence of a cationic manganese porphyrin catalyst.
The use of the porphyrin catalyst allowed, for the first time, styrene
derivatives to react with aziridines.
Key words: cycloadditions, manganese, porphyrins, heterocycles,
catalysis, alkenes, ring expansion
We initially surmised that N-sulfonylated aziridines might
be activated by a cationic manganese porphyrin complex
to generate a zwitterionic 1,3-dipole precursor that would
undergo formal [3+2] cycloaddition with alkenes to give
pyrrolidines (Scheme 1).7–12 Indeed, the cycloaddition of
2-(4-tolyl)-1-tosylaziridine (1a) with styrene (2a) in
the presence of cationic 5,10,15,20-tetraphenylporphyrin
(TPP) manganese hexafluoroantimonate catalyst (5
mol%) in 1,2-dichloroethane at 100 °C for twelve hours
gave the corresponding pyrrolidine 3aa in 75% yield as a
diastereomeric mixture (Table 1, entry 1).13 In attempts to
optimize the counteranion of the manganese porphyrin
catalyst, pyrrolidine 3aa was obtained in 65% and 54%
yield when triflate and tetrafluoroborate ions, respective-
ly, were introduced as counteranions (entries 2 and 3);
however, the reaction was retarded when a chloride ligand
was present in the manganese porphyrin catalyst (entry 4).
Other solvents, such as toluene, 1,4-dioxane, or acetoni-
trile, gave reduced yields of 3aa (entries 5–7), as did the
use of silver hexafluoroantimonate instead of the cationic
manganese porphyrin catalyst (entry 8). Note that the use
Aziridines are three-membered nitrogen-containing het-
erocyclic compounds that are stable but inherently reac-
tive, because of their ring-strain energy.1 As a result,
aziridines have attracted considerable attention as syn-
thetic building blocks and, through ring opening, they
have been used as 1,3-dipoles in formal [3+2] cycloaddi-
tion reactions with unsaturated compounds or carbonyl
compounds to form five-membered heterocyclic scaf-
folds, such as pyrrolidines, imidazolines, and oxazoli-
dines.2–5 In this context, Lewis acids have been
demonstrated to activate aziridines efficiently, generating
1,3-dipoles by ring opening, thereby effecting stereo- and
regioselective [3+2] cycloadditions. The related Lewis
acid catalyzed reaction in which aziridines are catalytical-
ly activated in situ to generate 1,3-dipoles for [3+2] cyclo-
addition is of great significance in heterocycle synthesis;
however, such catalytic reactions have been successful
only with a limited range of co-reactants, such as vinyl
Scheme 1 Manganese porphyrin catalyzed [3+2] cycloaddition of aziridines and alkenes
SYNLETT 2013, 24, 2763–2767
Advanced online publication: 05.11.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
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2
0
9
6
DOI: 10.1055/s-0033-1340012; Art ID: ST-2013-U0851-L
© Georg Thieme Verlag Stuttgart · New York

2764
T. Ozawa et al.
LETTER
of a stoichiometric amount of boron trifluoride etherate ed pyrrolidines 3 in good yields as diastereomeric mix-
instead of the manganese catalyst for the reaction of 1a tures (entries 4–6). Furthermore, the cycloaddition of
with 2a at –78 °C resulted in the formation of 3aa in 20% aziridine 1a with (1-cyclopropylvinyl)benzene (2h) gave
yield. In other words, the use of a conventional Lewis acid pyrrolidine 3ah in 70% yield (entry 7); no side-reaction
catalyst resulted in polymerization of 2a in preference to involving cleavage of the cyclopropane ring was ob-
[3+2] cycloaddition with 1a, even at low temperatures.
served. Even exocyclic alkenes, such as 1-methylene-
1,2,3,4-tetrahydronaphthalene (2i) or methylenecyclo-
heptane (2j) reacted with aziridine 1a to give the corre-
sponding bicyclic pyrrolidines 3ai and 3aj (entries 8 and
9).
Table 1 Manganese-Catalyzed Cycloaddition of 1a and 2a.
Ts
N
Ts
N
Ph
catalyst (5 mol%)
+
Table 2 Manganese-Catalyzed Cycloaddition of Aziridines 1b–g
with Styrene (2a)
Ph
solvent, 100 °C, 12 h
2a
Ts
N
Ts
N
[Mn(TPP)]SbF6
(5 mol%)
1a
Ph
3aa
+
Ph
DCE
100 °C, 12 h
R1
2a
Entry
Catalyst
Solvent
Yield (%)a
R1
1
3
1
2
3
4
5
6
7
8
[Mn(TPP)]SbF6
[Mn(TPP)]OTf
[Mn(TPP)]BF4
[Mn(TPP)]Cl
[Mn(TPP)]SbF6
[Mn(TPP)]SbF6
[Mn(TPP)]SbF6
AgSbF6
DCE
75
60
54
<1
45
38
<1
39
Entry Aziridine
Product
Yield (%)a
DCE
Ts
N
DCE
Ts
Ph
N
DCE
1
69
toluene
1,4-dioxane
MeCN
DCE
1b
3ba
Ts
Ts
N
N
Ph
a Yield determined by NMR spectroscopy; mixture of two diastereo-
mers.
2
63
Cl
Next, we examined the cycloaddition of various aziridines
1 with styrene (2a); the results are summarized in Table 2.
The reactions of styrene with 2-arylaziridines containing
electron-donating or electron-withdrawing substituents
gave the corresponding pyrrolidines 3ba–fa in good to
moderate yields (entries 1–5). 2-(1-Naphthyl)-1-tosyl-
aziridine (1g) similarly reacted with styrene (2a) to give
pyrrolidine 3ga (entry 6). However, 2-alkylaziridines
such as 2-hexylaziridine or 2-cyclohexylaziridine did not
undergo cycloaddition with styrene. Furthermore, neither
N-alkylated aziridines nor N-tert-butoxycarbonyl aziridi-
ne reacted with styrene. These results suggest that the sul-
fonamide moiety is necessary to generate a zwitterionic
intermediate, because of the greater stability of the nitro-
gen-centered anion.
Cl
1c
3ca
Ts
N
Ts
Ph
N
3
65
MeO
MeO
1d
3da
Ts
N
Ts
N
Ph
4
67
Next, we examined the cationic manganese porphyrin-
catalyzed formal [3+2] cycloaddition of aziridine 1a with
various alkenes 2 (Table 3). The reaction of aziridine 1a
with halostyrenes 2b and 2c, gave the corresponding sub-
stituted pyrrolidines 3ab and 3ac in 71% and 69% yield,
respectively (entries 1 and 2). Likewise, the trimethylsilyl
styrene derivative 2d reacted with aziridine 1a to give
pyrrolidine 3ad in 72% yield (entry 3). 1,1-Disubstituted
styrene derivatives such as α-methylstyrenes 2e–g reacted
with aziridine 1a to give the corresponding polysubstitut-
1e
3ea
Synlett 2013, 24, 2763–2767
© Georg Thieme Verlag Stuttgart · New York

LETTER
[3+2] Cycloaddition of Aziridines and Alkenes
2765
Table 2 Manganese-Catalyzed Cycloaddition of Aziridines 1b–g
with Styrene (2a) (continued)
Table 3 Manganese-Catalyzed Cycloaddition of Aziridine 1a and
Alkenes 2b–j (continued)
Ts
Ts
R1
Ts
N
[Mn(TPP)]SbF6
(5 mol%)
N
N
Ts
N
[Mn(TPP)]SbF6
(5 mol%)
Ph
R1
R2
+
Ph
DCE
100 °C, 12 h
R1
2a
+
R2
R1
DCE
100 °C, 12 h
1
2
3
Yield (%)a
1a
Entry Aziridine
Product
3
Ts
Entry Alkene
Product
Yield
(%)a
Ts
N
N
Ph
Ts
N
5
72
TIPSO
SiMe3
TIPSO
1f
3
72
3fa
SiMe3
Ts
N
2d
Ts
N
Ph
3ad
3ae
3af
Ts
N
6
59
1g
3ga
4
87
82
79
70
a Isolated yield of mixture of two diastereomers.
2e
Table 3 Manganese-Catalyzed Cycloaddition of Aziridine 1a and
Alkenes 2b–j
Ts
N
Ts
R1
N
R1
Ts
N
[Mn(TPP)]SbF6
(5 mol%)
OMe
R2
5
+
R2
MeO
DCE
2
100 °C, 12 h
2f
1a
3
Ts
N
Entry Alkene
Product
Yield
(%)a
6
Ts
N
Cl
2g
3ag
1
71
Cl
Ts
N
2b
3ab
7
Ts
N
Br
2h
3ah
2
69
Br
2c
3ac
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 2763–2767

2766
T. Ozawa et al.
LETTER
Table 3 Manganese-Catalyzed Cycloaddition of Aziridine 1a and
Alkenes 2b–j (continued)
Ts
R1
N
R1
Ts
N
[Mn(TPP)]SbF6
(5 mol%)
R2
+
R2
DCE
100 °C, 12 h
2
1a
3
Entry Alkene
Product
Yield
(%)a
Ts
N
Scheme 2 [3+2] Cycloaddition of chiral aziridine 1b with methy-
lenecycloheptane (2j)
8
86
ies to elucidate the mechanism underlying the unique
reactivity of the cationic metalloporphyrin catalyst and ef-
forts to improve the diastereoselectivity of the cycloaddi-
tion are underway.
2i
3ai
Ts
N
Acknowledgment
9
48b
This work was supported by JST and ACT-C, and by Grants-in-Aid
from the Ministry of Education, Culture, Sports, Science, and Tech-
nology, Japan. T.K. also thanks the Asahi Glass Foundation, the Ue-
hara Memorial Foundation, Tokuyama Science Foundation, and
Kurata Memorial Hitachi Science and Technology Foundation.
2j
3aj
a Isolated yields of mixture of two diastereomers.
b Single isomer.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
To gain further insight into the manganese porphyrin-cat-
alyzed cycloaddition, we examined the reaction of a chiral
aziridine 1b with an alkene (Scheme 2). The reaction of
1b (99% ee) with methylenecycloheptane (2j) in the pres-
ence of the cationic manganese porphyrin gave cycload-
duct 3bj in 37% yield as a racemate; unreacted aziridine
1b was recovered in 45% yield and 27% ee. The enantio-
meric excess of the starting aziridine 1b therefore de-
creased from 99% to 27% under the reaction conditions.
These results suggest that the cycloaddition proceeds by a
stepwise pathway that involves (1) the formation of the
zwitterionic 1,3-dipole intermediate 4 through C–N bond
cleavage of aziridine 1 by the cationic manganese porphy-
rin catalyst with loss of chirality, (2) nucleophilic addition
of the π-bond of alkene 2j to the positively charged ben-
zylic position of intermediate 4 to give intermediate 5, and
(3) cyclization to afford cycloadduct 3bj with regenera-
tion of the active cationic manganese porphyrin catalyst.
References and Notes
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porphyrin catalyzed [3+2] cycloaddition of aziridines
with alkenes to afford pyrrolidines. The readily available
metalloporphyrin complex, which contains a Lewis acidic
cationic manganese center and a large π-conjugated pla-
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Synlett 2013, 24, 2763–2767
© Georg Thieme Verlag Stuttgart · New York

LETTER
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[3+2] Cycloaddition of Aziridines and Alkenes
2767
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A screw-cap vial was charged sequentially with
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[Mn(TPP)][SbF6] (0.02 mmol, 19 mg), aziridine 1 (0.2
mmol), alkene 2 (0.6 mmol), and anhyd DCE (0.8 mL) in a
dry box. The vial was sealed and the mixture was stirred at
100 °C for 12 h. The mixture was then diluted with 10:1
hexane–EtOAc (3 mL) and passed through a short pad of
silica gel, which was washed with 1:1 hexane–EtOAc
(2 × 10 mL). The mixture was then concentrated in vacuo
to give a crude product that was purified by flash column
chromatography [silica gel, (20 g, 2 × 15 cm), hexane–
EtOAc (5:1)].
2-Phenyl-4-(4-tolyl)-1-tosylpyrrolidine (3aa)
Colorless oil; yield: 58 mg (75%); TLC: Rf = 0.40 (hexane–
EtOAc, 5:1). IR (neat): 3028, 2954, 2923, 2870, 1599, 1494,
1348, 1338, 1182, 1027, 814, 662 cm–1. 1H NMR (500 MHz,
CDCl3): δ = 7.74–7.73 (m, 1.8 H) 7.65–7.64 (m, 2 H), 7.42–
7.24 (m, 13.3 H), 7.11–7.02 (m, 5.8 H), 6.93–6.92 (m, 1.8
H), 5.06 (d, J = 8.0 Hz, 0.9 H), 4.81 (dd, J = 6.5, 10 Hz, 1 H),
4.17–4.14 (m, 1 H), 4.02 (dd, J = 7.5, 9.0 Hz, 0.9 Hz), 3.52–
3.41 (m, 1.9 H), 3.28 (dd, J = 9.5, 10.5 Hz, 0.9 H), 2.97–2.89
(m, 1 H), 2.69–2.65 (m, 1 H), 2.46 (s, 2.7 H), 2.44 (s, 3 H),
2.32 (s, 3 H), 2.30 (s, 2.7 H), 2.18–2.14 (m, 0.9 H), 2.11–2.00
(m, 1.9 H). 13C NMR (125.7 MHz, CDCl3): δ = 143.4, 143.2,
142.9, 142.6, 136.7, 136.6, 136.5, 135.9, 135.8, 134.8,
129.6, 129.5, 129.3, 129.2, 128.4, 128.3, 127.6, 127.4,
127.2, 127.1, 126.8, 126.8, 126.4, 126.1, 64.5, 63.0, 55.9,
55.1, 44.4, 43.3, 42.1, 41.0, 21.5, 21.4, 20.9, 20.9. HRMS
(ESI+): m/z [M + H]+ calcd for C24H26NO2S: 392.1679;
found: 392.1663.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 2763–2767

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