Iodobenzene diacetate/tetrabutylammonium iodide-induced aziridination of N-tosylimines with activated methylene compounds under mild conditions

Article abstract of DOI:10.1002/adsc.200800157

Aziridination of N-tosylimines with activated methylene compounds induced by iodobenzene diacetate [PhI(OAc)₂] and tetrabutylammonium bromide [Bu₄NBr] afforded the corresponding 2,2-difunctionalized aziridines in good yields with the aid of a catalytic amount of base. The reaction is hypothesized to proceed via a tandem nucleophilic addition-oxidative cyclization pathway.

Full text of DOI:10.1002/adsc.200800157

FULL PAPERS
DOI: 10.1002/adsc.200800157
Iodobenzene Diacetate/Tetrabutylammonium Iodide-Induced
Aziridination of N-Tosylimines with Activated Methylene
Compounds under Mild Conditions
Renhua Fana,* and Yang Yea
a
Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, Peopleꢀs Republic of China
Fax : (+86)-216-510-2412; e-mail: rhfan@fudan.edu.cn
Received: March 15, 2008; Revised: May 2, 2008; Published online: June 23, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Aziridination of N-tosylimines with acti- pothesized to proceed via a tandem nucleophilic ad-
vated methylene compounds induced by iodoben- dition-oxidative cyclization pathway.
zene diacetate [PhI(OAc)2] and tetrabutylammonium
G
bromide [Bu4NBr] afforded the corresponding 2,2-di- Keywords: cycloaddition; hypervalent compounds;
functionalized aziridines in good yields with the aid imines; methylene compounds; nitrogen heterocy-
of a catalytic amount of base. The reaction is hy- cles; oxidation
Introduction
compounds, if applicable, would be desirable in many
applications. As a part of our development of the syn-
Aziridines are versatile intermediates in organic syn- thetic application of sulfonamides in the presence of
thesis. They not only appear as a three-membered hypervalent iodine,[11] we report herein the results re-
heterocyclic substructure in many biologically impor- garding the aziridination of N-tosylimines with meth-
tant natural products, but also can undergo ring open- ylene compounds activated by the combination of io-
ing reaction to afford other building blocks in synthe- dobenzene diacetate [PhI(OAc)2] and tetrabutylam-
A
sis.[1,2] The addition of a carbon to a C=N bond is the monium bromide (Bu4NBr).
most attractive approach in ring formation. The reac-
tion between imines and ylide,[3,4] along with the aza-
Darzens reaction,[5] is well-documented to synthesize Results and Discussion
functionalized aziridines. Compared to the extensive
studies on the synthesis of 2-monofunctionalized aziri- Although the synthetic application of b-dicarbonyl io-
dines,[6] the preparation of 2,2-difunctionalized aziri- donium ylides in the transition metal-catalyzed cyclo-
dines has, however, received much less scrutiny. Aziri- propanation of C=C bond has been well devel-
dine-2,2-dicarboxylates, for example, have recently oped,[12,13] the aziridination of C=N bond with b-dicar-
been identified as protease inhibitors.[7] As a conse- bonyl iodonium ylides remains to be investigated. An
quence, they are considered as potentially attractive intermolecular aziridination of N-tosylimine 1a with
starting materials for biologically active substances.[8] phenyliodonium ylide was initially tested, but no azir-
Cardillo et al. reported the synthesis of aziridine-2,2- idine ring was formed under the typical cyclopropana-
dicarboxylates via a two-step reaction involving the tion conditions (Scheme 1).
conjugated addition of hydroxylamino derivatives
To avoid the decomposition of the phenyliodonium
(TMS-NH-OTMS) to alkylidene malonates, followed ylide,[14] a one-pot reaction was tested but failed
by cyclization under basic conditions.[9] Pellacani et al. (Table 1, entry 1). When Bu4NI was added as a phase-
demonstrated that aziridine-2,2-dicarboxylates could
be prepared by the aziridination of electron-deficient
C=C bonds with nosyloxy carbamates (NsO-NH-
COOR) in the presence of CaO.[10] This procedure,
however, cannot be extended to 2-benzylidenemalo-
nates. The synthesis of 2,2-difunctionalized aziridines
by the direct cycloaddition of imines with methylene Scheme 1.
1526
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Adv. Synth. Catal. 2008, 350, 1526 – 1530

Iodobenzene Diacetate/Tetrabutylammonium Iodide-Induced Aziridination
FULL PAPERS
Table 1. Exploration of the one-pot aziridination of N-toysl-
imine 1a with diethyl malonate.
Table 2. Optimization of reaction conditions.
ACHTREUNG
Entry PhI
A
2a
A
G
G
[%][a]
Entry
CuCl
[equiv.]
Base
[equiv.]
Additive
[equiv.]
2a
[%][a]
1
2
3
4
5
6
7
8
1.0
2.0
3.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.0
1.0
1.0
2.0
3.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2
2
2
2
2
1
0.5
0.25
0
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
toluene
ClCH2CH2Cl 62
THF
EtOAc
t-BuOH
DMSO
DMF
17
G
G
ACHTREUNG
25
24
52
51
62
83
73
0
1
2
3
4
5
6
7
0.1
0.1
0.1
0.1
0.1
0.1
0
KOH (2)
KOH (2)
KOH (2)
KOH (2)
KOH (2)
t-BuOK (2)
t-BuOK (2)
0
2
0
8
0
15
17
Bu4NI (1)
18-crown-6 (1)
Bu4NBr (1)
Bu4NCl (1)
Bu4NBr (1)
Bu4NBr (1)
9
10
11
12
13
14
15
16
17
73
[a]
Isolated yield based on imine 1a.
56
38
37
29
25
0
transfer catalyst aziridination occurred and product
2a was isolated in 2% yield (entry 2). As a compari-
son, the control experiment with 18-crown-6 as phase-
transfer catalyst failed to achieve aziridination
(entry 3). The yield of 2a increased to 8% using
Bu4NBr (entry 4), while no aziridination product was
detected in the presence of Bu4NCl (entry 5). Because
H2O
[a]
Isolated yield based on imine 1a.
a ring opening product of 2a attacked by a hydroxy matic imines with diethyl malonate, most of them pro-
group was isolated as one of the by-products, KOH ceeded well to afford the corresponding aziridine-2,2-
was replaced by the less nucleophilic base t-BuOK. dicarboxylates in good yields (entries 1–10). The elec-
As a result a higher yield of 2a was obtained tron-rich imines 1k and 1l were found to be poor sub-
(entry 6). It was, however, surprising to find that aziri- strates for the aziridination. Their reactions were slug-
dination still occurred even in the absence of a metal gish, and the expected products were not stable
catalyst (entry 7).
enough to be purified, hence the formation of aziri-
1
Further optimization of the reaction conditions is dine was detected by H NMR spectroscopy of the
shown in Table 2. The best ratio of imine 1a, crude product (entries 11 and 12). Dimethyl malonate
CH
N
N
1:1.2:2:2:0.5, with which the yield of 2a increased to (entry 13). The reaction of sterically hindered di-tert-
83% (entry 7). It was noteworthy that a catalytic butyl malonate required a higher temperature and a
amount of base could drastically promote the aziridi- longer time (entry 14). Moreover, the reactions with
nation. The reaction still proceeded smoothly even ethyl acetoacetate and 2,4-pentanedione also gave the
using 0.25 equiv. of t-BuOK (entry 8). Control experi- corresponding aziridines in good yields with an extra
ment showed that no aziridine was formed in the ab- requirement of lower reaction temperature (entries 15
sence of t-BuOK (entry 9). In processes involving, and 16). No corresponding aziridine was formed when
e.g., the reaction between ylides and imines and the ethyl 4-chloro-3-oxobutanoate, malononitrile, ethyl 2-
aza-Darzens reaction, a stoichiometric amount of cyanoacetate, and ethyl 2-nitroacetate were em-
base is normally required. The aziridination of N-to- ployed, and the corresponding Knoevenagel conden-
sylimine with malonate could proceed with varied ef- sation products were isolated (entries 17–20). When
ficiency in various solvents except water, in which no alkanesulfonylimines were used as substrates, only
aziridine was isolated (entries 10–17). The major by- moderate yields were obtained (entries 21–23).
product of the reaction was diethyl 2-benzylidenemal-
A nucleophilic addition product of imine with mal-
onate. This was assumed to be generated from the onate 3a was detected during the reaction, and disap-
Knoevenagel condensation of imine 1a with diethyl peared after the reaction. A reaction pathway was ini-
malonate.[11c]
tially hypothesized as shown in Figure 1. After the
(OAc)2/Bu4NBr- generation of 3a, the enolate anion of 3a attacks
PhI(OAc)2 to yield an intermediate A. After an intra-
The generality of the present PhI
induced aziridination of N-tosylimines with activated
ACHTREUNG
A
ACHTREUNG
methylene compounds was further explored under op- molecular nucleophilic replacement and a reductive
timized conditions (Table 3). For the reactions of aro- elimination of PhI, an aziridine ring is formed.[12]
Adv. Synth. Catal. 2008, 350, 1526 – 1530
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asc.wiley-vch.de
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Renhua Fan and Yang Ye
FULL PAPERS
Table 3. PhI
A
of N-tosylimines with activated methylene compounds.
Entry
R
P
CH2E2
2 [%][a]
1
2
h
A
2a (83)
2b (77)
2c (78)
2d (74)
2e (73)
2f (78)
2g (82)[b]
2h (68)[b]
2i (89)[b]
2j (73)
2k[c]
p-CH3C6H4
m-CH3C6H4
p-ClC6H4
o-ClC6H4
o-BrC6H4
p-CNC6H4
p-FC6H4
m-NO2C6H4
1-Naphthyl
p-CH3OC6H4
p-(CH3)2NC6H4 CH2
Ph
Ph
Ph
Ph
Ph
Ph
Ph
CH2(COOEt)2
CH2(COOEt)2
CH2(COOEt)2
CH2(COOEt)2
CH2(COOEt)2
CH2(COOEt)2
CH2(COOEt)2
CH2(COOEt)2
CH2(COOEt)2
CH2(COOEt)2
(COOEt)2
CH2(COOCH3)2
CH2(COOBu-t)2
ACHTREUNG
AHCTREUNG
Scheme 2.
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
ACHTREUNG
ACHTREUNG
ACHTREUNG
dine 2a was obtained in 52% yield. More notably, the
reaction proceeded better in the absence of t-BuOK,
with an increased yield of aziridine (88%). We have
ACHTREUNG
ACHTREUNG
ACHTREUNG
reported a PhI(OAc)2/Bu4NBr-induced intramolecular
N
ACHTREUNG
oxidative cyclization of homoallylic sulfonamides.[11b]
Acetyl hypobromite (AcOBr) was supposed to be the
reaction intermediate. Further investigation showed
that aziridine 2a could be formed but in a lower yield
using a mixture of Br2/AcONa, and no reaction oc-
curred if only Br2 was used.
AHCTREUNG
A
2l[c]
G
2m (83)
2n (75)[d]
2o (81)[e,f]
2p (77)[f]
ACHTREUNG
CH3COCH2COOEt
CH3COCH2COCH3
ClCH2COCH2COOEt
CH2(CN)2
NCCH2COOEt
O2NCH2COOEt
[g]
-
A tentative mechanism for the aziridination is
shown in Figure 2. The reaction might proceed via a
tandem nucleophilic addition-oxidative cyclization
pathway. After the nucleophilic addition of malonate
with imine to yield 3a, the enolate anion of 3a reacts
with AcOBr, which is generated from the ligand ex-
[g]
-
[g]
-
[g]
Ph
-
i-Bu
t-Bu
n-C7H15
CH2
CH2(COOEt)2
CH2(COOEt)2
(COOEt)2
2q (35)
2r (43)
2s (37)
AHCTREUNG
AHCTREUNG
change between PhI(OAc)2 and Bu4NBr and their
A
[a]
Isolated yield.
0.25 equivalent of t-BuOK was used.
The formation of aziridines was detected by the H NMR
of the crude product.
The reaction was carried out at 0–408C.
The ratio of trans:cis is 50:50 as determined by H NMR.
The reaction was carried out at 0–158C.
The Knoevenagel condensation product was isolated.
subsequent reductive elimination of PhI, to form an
intermediate B. Intermediate B undergoes an intra-
molecular nucleophilic attack by the a-amide, and the
further elimination of a bromine anion provides the
aziridine.
The nucleophilic addition of alkyl N-tosylimines
with diethyl malonate in the presence of 1 equiv. t-
BuOK proceeded more sluggishly, and the corre-
sponding addition products 3q, 3r and 3s were isolat-
[b]
[c]
1
[d]
[e]
[f]
1
[g]
However, the experiments indicated that 3a could ed in low yields (35-45%). However, 3q, 3r and 3s are
not be converted into aziridine 2a in the presence of good substrates for the ring cyclization in the pres-
PhI(OAc)2 and t-BuOK (Scheme 2). The ring cycliza-
ACHTREUNG
tion occurred with the addition of Bu4NBr, and aziri-
Figure 1.
Figure 2.
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Iodobenzene Diacetate/Tetrabutylammonium Iodide-Induced Aziridination
FULL PAPERS
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Scheme 3.
ence of PhI(OAc)2/Bu4NBr. The corresponding aziri-
A
dines were obtained in the near quantitative yields
(Scheme 3). This indicates that the lower yields ob-
tained in the one-pot aziridination of alkyl N-Ts
imines were due to their instability and low reactivity
in the nucleophilic addition step.
Conclusions
In summary, we have developed an efficient method
for the synthesis of 2,2-difunctionalized aziridines by
the aziridination of N-tosylimines with activated
methylene compounds induced by PhI(OAc)2 and
A
Bu4NBr in the presence of a catalytic amount of base.
The potential of this reaction system has been evalu-
ated for its simple procedure, mild reaction condi-
tions, and adaptability to a wide variety of substrates.
The further development of the asymmetric reactions
is ongoing and will be reported in due course.
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General Experimental Procedure
A solution of imine (0.25 mmol) and CH2E2 (0.3 mmol) in
anhydrous CH3CN was cooled to 08C, and treated with
A
N
and t-BuOK (14 mg, 0.125 mmol). The resultant mixture was
warmed up and stirred at 308C. After the imine had disap-
peared (determined by TLC), the mixture was concentrated,
and directly purified by flash column chromatography (10–
20% ethyl acetate in hexane) to provide the corresponding
aziridine. The characterization data are available in the Sup-
porting Information.
Acknowledgements
Financial support from National Natural Science Foundation
of China (20702006), the Shanghai Rising-Star program
(07QA14007), and Fudan University are gratefully acknowl-
edged.
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FULL PAPERS
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