A practical method for the aziridination of α,β-unsaturated carbonyl compounds with a simple carbamate utilizing sodium hypochlorite pentahydrate

Article abstract of DOI:10.1039/d1ra04297a

The efficient formation oftert-butylN-chloro-N-sodio-carbamate by the reaction of simpletert-butyl carbamate with sodium hypochlorite pentahydrate (NaOCl·5H₂O) would be a practical and green method for the aziridination of α,β-unsaturated carbo

Full text of DOI:10.1039/d1ra04297a

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A practical method for the aziridination of a,b-
unsaturated carbonyl compounds with a simple
carbamate utilizing sodium hypochlorite
pentahydrate†
Cite this: RSC Adv., 2021, 11, 22120
*
Takehiro Umeda and Satoshi Minakata
The efficient formation of tert-butyl N-chloro-N-sodio-carbamate by the reaction of simple tert-butyl
carbamate with sodium hypochlorite pentahydrate (NaOCl$5H2O) would be a practical and green
method for the aziridination of a,b-unsaturated carbonyl compounds. The process described herein is
transition-metal free, all of the materials are commercially available, the byproducts (NaCl and H2O) are
environmentally benign and the reaction is stereoselective. The resulting aziridines are potential
precursors of amino acids.
Received 3rd June 2021
Accepted 17th June 2021
DOI: 10.1039/d1ra04297a
rsc.li/rsc-advances
only sodium chloride as the by-product. By taking advantage of
the poor solubility of chloramine salts, the use of solid–liquid
Introduction
phase transfer catalyst, optically active quaternary ammonium
salts, realized the stereoselective aziridination.5,6 Since chiral azir-
idines derived from enones are valuable building blocks,7 tremen-
dous efforts have been devoted to the development of the
stereoselective aziridination of enones.8,9 Among these, our proto-
cols5,6 are promising for producing enantioenriched aziridines. Our
nitrogen sources, chloramine salts, however, are prepared by the
reaction of a carbamate (benzyl carbamate or tert-butyl carbamate)
with tert-butyl hypochlorite followed by treatment with sodium
hydroxide, in which tert-butanol is by-product, and the efficiency is
low. In addition, the chloramine salts are not sufficiently stable to
allow them to be stored in a freezer (Scheme 1).
To overcome these problems and make it a more sustainable
reaction, we envisaged the in situ generation of the reactive
nitrogen source, a chloramine salt, using a suitable oxidant for
the desired aziridination. From these points of view, we report
herein on the practical aziridination of a,b-unsaturated
carbonyl compounds with a simple carbamate and an ideal
oxidant in the presence of quaternary ammonium salt as
a phase transfer catalyst. The optimal conditions using the
oxidant were applied to the stereoselective aziridination devel-
oped by our group.5,6
Aziridines containing a carbonyl functionality at the 2-position
represent potential precursors of natural and unnatural a- or b-
amino acids by ring-opening reactions with suitable nucleo-
philes.1,2 Among the possible synthetic methods for preparing
such aziridines, the one-step installation of a nitrogen unit into
an a,b-unsaturated carbonyl compound would be a practical
approach, and catalytic and stereoselective variants have been
developed.3,4 The preparation of activated aziridines that
contain electron-withdrawing groups on the nitrogen, which
would be readily ring-opened, from a,b-unsaturated carbonyl
compounds would be useful route to the synthesis of amino acid
derivatives.4 The nitrogen sources used for synthesizing these
aziridines are highly oxidized, and include hypervalent iodine
reagents (ArSO2N]IPh)4a,b,d and nitrogen-oxygenated agents
(ArSO3–NHCO2R).4c,f,g These reagents require transition-metal
catalysts or bases for the desired aziridination to be successful,
and they have drawbacks such as difficulties associated with their
preparation and instability. When these nitrogen sources are used
in aziridination reactions, the by-products are iodobenzene and
aryl sulfonates, thus making such reactions not atom-economical.
N-Chloro-N-sodio-carbamates (chloramine salts) as an alternative
nitrogen source were developed by our group for the aziridination
of a,b-unsaturated carbonyl compounds.5
This aziridination involved the 1,4-addition of a chloramine
salt to an a,b-unsaturated carbonyl compound, followed by
cyclization of the resulting enolates accompanied by liberating
Department of Applied Chemistry, Graduate of Engineering, Osaka University,
Yamadaoka 2-1, Suita, Osaka 565-0871, Japan. E-mail: minakata@chem.eng.
osaka-u.ac.jp
Scheme 1 Inefficient formation and instability of chloramine salt of
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra04297a
carbamate.
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Table 1 Optimization of reaction conditions (1)a
Scheme 2 Initial assumption and actual result for efficient formation
of chloramine-Boc.
Yieldb
(%)
Results and discussion
In order to nd an ideal oxidant, considering minimum
combination of atoms required for the transformation of
simple carbamate into N-chloro-N-sodio-carbamate, we
concluded that sodium hypochlorite would be the ideal oxidant,
because the by-product molecule is only water (Scheme 2). We
recently found that sodium hypochlorite pentahydrate in the
solid form10 and not aqueous solution of sodium hypochlorite
was the ideal oxidant for the transformation of sulfonamides
into the corresponding N-chloro-N-sodiosulfonamides.11 As
a simple carbamate, we focused on tert-butyl carbamate because
it contains a Boc group that can be readily removed. In fact,
when tert-butyl carbamate was treated with NaOCl$5H2O in
MeCN at room temperature for 3 h, tert-butyl N-chloro-N-sodio-
carbamate (chloramine-Boc)12 was obtained in high yield
(Scheme 2).
Entry
Additive
PTC (mol%)
Time (h)
2a
3a
1
2
3
4
5
6
7
8
None
10
10
10
None
5
5
5
5
5
12
12
12
24
18
18
24
21
24
18
24
24
15
74
80
58
82c
60
73
69
33
11
9
40
25
9
7
1
12
3
1
15
35
61
47
MS3A (800 mg)
K2CO3 (9 equiv.)
K2CO3 (9 equiv.)
K2CO3 (9 equiv.)
K2CO3 (6 equiv.)
Cs2CO3 (9 equiv.)
K3PO4 (9 equiv.)
Et3N (9 equiv.)
Na2CO3 (9 equiv.)
KHCO3 (9 equiv.)
NaHCO3 (9 equiv.)
9
10
11
12
5
5
5
9
a
b
Reactions were performed on a 0.5 mmol scale. Determined by 1H
NMR analysis of the crude product. c Isolated yield.
The efficient formation of chloramine-Boc with the ideal
oxidant prompted us to investigate the aziridination of a,b-
unsaturated carbonyl compounds using the simple tert-butyl
carbamate. Although chloramine-Boc is formed in 90% yield,
the use of 1.4 equivalents of H2NBoc and NaOCl$5H2O relative
to the olen substrates was found to be suitable for the desired
aziridination. When benzyl acrylate (1a) was treated with tert-
butyl carbamate in the presence of NaOCl$5H2O as an oxidant
and benzyltriethylammonium chloride under conditions of as
a phase transfer catalysis, the desired aziridine 2a was obtained
in 15% yield (path a) along with the b-aminated ester 3a as the
main product (Table 1, Entry 1). Since the enolate derived from
the 1,4-addition of in situ generated chloramine-Boc to 1a would
be protonated with water leading to 3a (path b), we screened
a series of dehydrating agents. Although the addition of MS3A
drastically improved the yield of 2a (Entry 2), the formation of
3a could also be effectively reduced by using K2CO3 (Entry 3).
The rate of aziridination was very slow without the addition of
the PTC (Entry 4) but when the reaction time was extended,
5 mol% of PTC was sufficient to allow the aziridination to
proceed (Entry 5). Decreasing the amount of K2CO3 resulted in
a decrease in the yield of aziridine 2a and an increase in the
protonated product 3a (Entry 6). Other additives were not
effective for the aziridination (Entries 7–12).
(NCS), and aqueous NaOCl were employed in the reaction, but the
efficiencies were low compared to the solid state NaOCl (Table 2,
Entries 1–4). Aer screening other solvents, MeCN was found to be
the optimal solvent for the aziridination (Entries 5–9).
Because the Boc group on the nitrogen of the resulting
aziridine can be readily detached under mild conditions, we
concluded that BocNH2 should be a useful nitrogen source. To
examine the scope of the aziridination, other simple carba-
mates and amides as a nitrogen source were subjected to the
conditions. Unfortunately, only benzyl carbamate could be used
for the aziridination (2a0), albeit in low yield (Scheme 3). Tri-
uoroacetamide, benzamide, methanesulfonamide, benzen-
sulfonamide, and tosylamide had no effect on the process.
Carboxamides such as triuoroacetamide and benzamide failed
to react with NaOCl$5H2O under the conditions because of the
low acidity of N–H protons. Sulfonamides were converted to
chloramine salts by the reaction with NaOCl$5H2O, but their
low nucleophilicity did not induce the 1,4-addition to 1a.
With the optimized conditions (Table 1, Entry 5) in hand, the
scope of the a,b-unsaturated esters were investigated (Table 3).
Methyl and ethyl acrylates were transformed into the corre-
sponding aziridines (Entries 1 and 2). Although, under the same
conditions, the yield for the reaction of tert-butyl acrylate was
low, increasing the amount of the catalyst used and the reaction
In order to examine the superiority of NaOCl$5H2O, related
oxidants, including tert-BuOCl, Ca(ClO)2, N-chlorosuccinimide
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Table 2 Optimization of reaction conditions (2)a
Yieldb (%)
Entry
Oxidant
Solvent
Time (h)
2a
3a
1
2
3
4
5
6
7
8
9
tert-BuOClc
Ca(ClO)2
NCS
MeCN
MeCN
MeCN
MeCN
CH2Cl2
Toluene
THF
18
24
48
18
18
18
18
24
6
51
44
0
42
66
66
44
8
23
0
0
21
5
0
0
0
0
d
Scheme 4 Comparison of the rate of the epoxidation of an a,b-
unsaturated ester and ketone with NaOCl$5H2O.
NaOCl aq.e
NaOCl$5H2O
NaOCl$5H2O
NaOCl$5H2O
NaOCl$5H2O
NaOCl$5H2O
In order to further expand the scope of substrates, an a,b-
unsaturated ketone, phenyl vinyl ketone (1g), which was not
examined in our previous studies,5,6 was employed in the
present process, unexpectedly affording epoxide 4g as a main
product as well as the desired aziridine 2g (Scheme 4).
DMF
EtOH
0
a
b
Reactions were performed on a 0.5 mmol scale. Determined by 1H
c
d
NMR analysis of the crude product. K2CO3: 10.4 equiv. Ca(ClO)2:
0.7 equiv. e 13 wt% (360 ml).
To clarify the difference between the reaction of a,b-unsat-
urated esters and that of the ketone, considering that the
oxygen source of the epoxide is NaOCl$5H2O, the olens 1a and
1g were treated with NaOCl$5H2O. The rapid epoxidation of 1g
was observed compared with that of 1a (Scheme 4). These
Table 4 Scope of electron-deficient olefins
Scheme 3 Formation of N-Cbz aziridine.
temperature improved the efficiency (Entries 3–5). Methyl cin-
namate was completely unreactive, and it was fully recovered
(Entry 6). Arylmethyl acrylates were converted into the corre-
sponding aziridines in good yields (Entries 7 and 8).
Table 3 Scope of a,b-unsaturated estersa
Entry
1
EWG
Solvent
CH2Cl2
Time (h)
1
Yielda (%)
1g
1h
87 (2g)
2
3
MeCN
MeCN
1
1
85 (2h)
96 (2i)
Temp.
1i
Entry R1
R2
(ꢀC)
Time (h) Yieldb (%)
1
2
3
4
Me
H
H
H
H
H
Ph
H
H
1b r.t.
18
18
24
20
18
20
18
18
60 (2b)
55 (2c)
12 (2d)
27 (2d)
35 (2d)
0
Et
1c r.t.
1d r.t.
1d 40
1d 80
r.t.
tBu
4
1j
CH2Cl2
MeCN
1
2
76 (2j)
66 (2k)
tBu
5c
6d
7
tBu
Me
5
1k
2-MeOC6H4CH2
9-Anthracenylmethyl
1e r.t.
1f r.t.
75 (2e)
57 (2f)
8
a
a
Reactions were performed on a 0.5 mmol scale. b Isolated yield. c PTC:
Isolated yields.
10 mol%. d Recovery of olen: 100%.
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the cinchonine-derived anthracenylmethylated ammonium salt
CN1 as a chiral PTC induced asymmetric aziridination in good yield
and enantioselectivity. The resulting aziridine containing dime-
thylpyrazole was readily transformed into the methyl ester-
substituted aziridine 2b with no change in optical purity.
Regarding the diastereoselectivity of the reaction, the
combination of an L-menthopyrazole chiral auxiliary and a cin-
chonidine-derived chiral ammonium catalyst is essential for
achieving a high diastereoselectivity. When tert-butyl carbamate
was treated with NaOCl$5H2O in the presence of CD1 and
K2CO3 in CH2Cl2 at room temperature for 3 h, followed by the
addition of the enone 1l containing a chiral auxiliary at ꢁ40 ꢀC
for 80 h, the corresponding aziridine was produced in good
yield with complete diastereoselectivity. The auxiliary was easily
deprotected without epimerization giving the methyl ester
derivative, with the efficient recovery of L-menthopyrazole
auxiliary which can be recycled.
Conclusions
In conclusion, a practical and efficient process for the synthesis
of valuable aziridines is reported. Although this basic trans-
formation was previously established by our group, the use of
a potential oxidant, NaOCl$5H2O allowed it to be an efficient,
sustainable and robust reaction with the stereoselectivity of the
aziridination being retained. The reaction is metal free and
organocatalysts can be used with a high enantio- and diaster-
eoselectivity, and the resulting aziridines represent potential
precursors of a- or b-amino acids.
Conflicts of interest
There are no conicts to declare.
Scheme
5 Enantio- and diastereoselective aziridination and
deprotection.
Acknowledgements
˜
In memory of Prof. Dr Kilian Muniz (1970–2020). This project
results indicate that NaOCl$5H2O reacts with a,b-unsaturated
ketones faster than with H2NBoc.
was partially supported by the Nippon Light Metal Company,
Ltd., and a Grant-in-Aid for Science Research from the Japan
Society for the Promotion of Science (Grant No. JP19H02716).
We acknowledge the donation of NaOCl$5H2O from the Nippon
Light Metal Company, Ltd.
To avoid epoxide formation, when H2NBoc was rst treated
with NaOCl$5H2O in the presence of the PTC and K2CO3 at
room temperature for 3 h, followed by the addition of phenyl
vinyl ketone (1g), the desired aziridine 2g was produced in 87%
yield (Table 4, Entry 1). Changing the solvent from MeCN to
CH2Cl2 improved the reaction efficiency in this case. Since the
electron-decient olens shown in Table 4 reacted rst with
NaOCl$5H2O, these conditions were employed in reactions of
these substrates. Enones containing pyrazole, oxazolidinone,
and carbazole moieties were smoothly transformed into the
corresponding aziridines in good to excellent yields (Entries 2–
4). The present method was applicable to a vinyl sulfone 1k,
affording the aziridine 2k in good yield (Entry 5).
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22124 | RSC Adv., 2021, 11, 22120–22124
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