Preparation of N-arylamines from 2-oxo-7-azobicyclo[4.1.0]heptanes

Article abstract of DOI:10.1016/j.tet.2012.05.054

A wide range of N-phenylated secondary amines were prepared directly from 2-oxo-7-azobicyclo[4.1.0]heptanes using 4-nitrobenzoic acid as acid catalyst. The intermediate enol esters could also be isolated under similar conditions. A catalytic cycle is proposed.

Full text of DOI:10.1016/j.tet.2012.05.054

Tetrahedron 68 (2012) 6263e6268
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journal homepage: www.elsevier.com/locate/tet
Preparation of N-arylamines from 2-oxo-7-azobicyclo[4.1.0]heptanes
,
c
,
b
M. Teresa Barros a, Suvendu S. Dey a, Christopher D. Maycock b , Paula Rodrigues
*
ˇ
a REQUIMTE, CQFB, Departamento de Química, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
b Instituto de Tecnologia Química e Biologia, Universidade Nova de Lisboa, 2780-157 Oeiras, Portugal
ˇ
c Departamento de Química e Bioquímica, Faculdade de Ciencias, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
A wide range of N-phenylated secondary amines were prepared directly from 2-oxo-7-azobicyclo[4.1.0]
heptanes using 4-nitrobenzoic acid as acid catalyst. The intermediate enol esters could also be isolated
under similar conditions. A catalytic cycle is proposed.
Received 12 March 2012
Received in revised form 8 May 2012
Accepted 15 May 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Available online 23 May 2012
Keywords:
Organocatalysis
4-Nitrobenzoic acid
2,3-Aza-1-cyclohexanones
Aliphaticearomatic transformation
N-Phenylated secondary amines
1. Introduction
at carbonyl group resulting in the corresponding tertiary cyclo-
hexanol (e.g., allyl anions, hydride nucleophiles)9 or sub-
The importance of nitrogen-containing compounds,1 as is evi-
dent from their wide occurrence in nature and broad application in
chemistry, biology, and material sciences, has for a long time in-
duced chemists to actively investigate methods to form CeN bonds.
Commonly used methods, such as reductive amination of carbonyl
groups,2 Ullmann reaction and its modifications,3 Pd-catalyzed
BuchwaldeHartwig amination of aryl halides,4 and direct CeH
activation using transition-metal catalysis5 are very efficient. Some
of these processes depend upon the availability and use of precious
metal catalysts some of which are predicted to reach critical levels
or soon be exhausted.6 The challenge for chemists is to find new
methods using readily available metal catalysts, or even metal-free
alternatives. Few methods for the generation of aryl amines
through aliphaticearomatization pathway are known.7 Herein we
wish to report a metal free pathway leading to arylamines through
the breakage of C(sp3)eN bond followed by the regioselective for-
mation of a C(sp2)eN bond within the same molecule.
stitutioneelimination reaction at C-2 and C-3 leading to 2-
substituted cyclohex-2-en-1-ones (e.g., azide, halides, thiols as
nucleophiles)10 as shown in Scheme 1. While investigating the
synthetic utility of 2,3-aza-1-cyclohexanones for the synthesis of
natural products, such as (þ)-bromoxone,11 it was found that these
aziridines were selectively opened with the loss of the amine
function to produce new 2-substituted cyclohexenones.12 In-
terestingly in the present work the fate of such aziridines did not
always stop at this stage. Under certain conditions this free amine
rejoined to produce the aryl amine (Scheme 2).
O
O
Nu
Nu
Z
Z = O, NR, NTs
Owing to the ring strain and the electronegativity of nitrogen,
aziridines exhibit fascinating and diverse reactivity. The ease and
regioselectivity of the cleavage of the aziridine ring are highly de-
pendent on the nature of the substituents on the three-membered
heterocyclic ring, the nucleophiles, and the reaction conditions
employed.8 Conventionally 2,3-aza-1-cyclohexanones and 2,3-
epoxy-1-cyclohexanones undergo nucleophilic attack either
Substitution-elimination reaction of 2,3-aza-1-cyclohexanone
Scheme 1.
2. Results and discussion
Focusing on the opening of the aziridine ring of 2,3-aza-1-
cyclohexanone, CeCl3$7H2O was chosen as the Lewis acid, as it
also contains a chloride ion source, to open the aziridine ring in
* Corresponding author. E-mail address: maycock@itqb.unl.pt (C.D. Maycock).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.05.054

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M.T. Barros et al. / Tetrahedron 68 (2012) 6263e6268
acidity with 4-nitrobenzoic acid (p-NBA), the desired aryl amine 2
O
H
N
R1
R2
was formed in excellent yield (Table 1, entry 13). In this case we
isolated only trace amounts of 2-(4-nitrobenzoate) (4). Good yields
of phenylated amine were obtained with just 75 mol % of p-NBA
(Table 1, entry 16). Further decreases in the quantity of p-NBA
proved detrimental to the yield (Table 1, entry 17).
R
N
R
R1
R3
R3
R2
Aromatisation of 2,3-aza-1-cyclohexanones
With the optimal reaction conditions in hand, the scope of this
domino reaction was explored, and the results are summarized in
Table 2. The experimental method is very simple. A mixture of
aziridine and 75 mol % p-NBA in acetonitrile was heated at 80 ꢀC for
the required time under argon. After complete reaction (TLC)
a saturated solution of sodium carbonate was added and stirred for
a few minutes followed by extraction with ethyl acetate. Evapora-
tion of the organic layer followed by silica gel column chromatog-
raphy gave the desired product in moderate to excellent yields.
N-phenylated aminoacid esters (2h, 2i) could also be prepared
easily. During the course of the reaction, aziridines 1k and 1q,
which produce low boiling allylamine, afforded the corresponding
amines 2k and 2q but with lower yields, enol esters 5 and 6 being
the major products. Due to steric effects caused by the bulky tert-
butyl group, 1s afforded enol ester 5 in 90% yield (Scheme 3) and
not the expected phenylated amine. This must be due to the diffi-
culties in forming the intermediate imine.
In order to gain some insight into the reaction pathway, enol
ester 4 was reacted with benzylamine under the conditions in-
dicated in Scheme 4 this resulted in a mixture of products including
amine 7 (32%), enamine 8 (18%), and amide 9 (40%). In order to
simulate the normal reaction conditions aziridine 1t was reacted
with benzylamine using p-NBA as promoter. The result was again
inconclusive amine 7 (51%) and enol ester 11 (30%) being formed.
These experiments indicated that under the normal conditions the
quantity of enol ester available at any one time was probably small
and that perhaps the aziridine carbonyl was also reacting with
available amine.
Scheme 2.
a model reaction, on substrate 1 (Table 1). In acetonitrile13 at room
temperature, 2-chlorocyclohex-2-en-1-one was quantitatively
obtained (Table 1, entry 1). Under the same conditions and after the
full conversion of aziridine 1 to 2-chlorocyclohex-2-en-1-one, the
reaction mixture was heated at 80 ꢀC. We observed the appearance
of the secondary aryl amine 2, which indicated that the free ben-
zylamine released from the aziridine had reassociated with the
chloroenone and aromatization had occurred. However the for-
mation of aryl amine stops after some time, leaving unreacted
2-chlorocyclohex-2-en-1-one. The same result was obtained when
a mixture of CeCl3$7H2O and 1 was heated at 80 ꢀC in acetonitrile
but with a faster conversion rate. The reaction appears to stop after
4 h and further heating for 12 h did not improve the yield (Table 1,
entry 2). Addition of more CeCl3$7H2O or benzylamine did not alter
the result. Experiments with different solvents or metal chlorides
(Table 1, entries 3e6) were in vain. We assume that the free amine
in the reaction system may have formed complexes with the metal
salts reducing their Lewis acidity; hence no further progress of the
reaction was taking place. To explore our hypothesis we turned our
attention toward the organic salts containing halide anions as nu-
cleophile, such as tetrabutylammonium halides (Table 1, entries 7
and 8), room temperature ionic liquid, such as N-butylmethylimi-
dazolium bromide [BMIm]Br (Table 1, entry 9), and the acid func-
tional ionic liquid acylmethylimidazolium chloride [AcMIm]Cl
(Table 1, entry 9).14 Although the yield of aryl amine 2 increased
with the acidity of the reagent, they were insufficient for full con-
version of the aziridine. Using carboxylic acids we achieved better
results. In the presence of benzoic acid, 2 was obtained at 62% yield
(Table 1, entry 12) along with 30% of 2-benzoate (3). Increasing the
O
O
O
O
O
O
O
O
O
O
O
O
5
( )
6
( )
(3)
(4)
Table 1
NO2
NO2
NO2
Optimization of reactiona
O
H
N
On the basis of these observations, a plausible catalytic cycle for
the reaction is proposed in Scheme 5. Acid catalyzed nucleophilic
attack of p-NBA on the aziridine ring leads to the formation of the
ester II. Elimination of a primary amine affords the enol ester III. In
the presence of acid, the free amine released into the system con-
denses with the carbonyl group resulting in imines IV and V. An H-
shift affords VII, which undergoes elimination to regenerate p-NBA
with the formation of the desired aryl amine VIII. The regenerated
p-NBA continues the catalytic cycle. In principle, a sub stoichio-
metric amount of p-NBA should be enough for the progress of re-
action but the formation of the amine reduces its acidity and
obliges its use in excess.
Reagent
N
(1)
Solvent, 12 h
(2)
Temp (ꢀC)
Entry
Reagent (equiv)
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
CeCl3$7H2O (1)
CeCl3$7H2O (1)
CeCl3$7H2O (1)
CeCl3$7H2O (1)
LaCl3$7H2O (1)
FeCl3 (1)
MeCN
MeCN
THF
rt
80
Reflux
Reflux
80
80
80
80
80
80
80
80
80
100
120
80
dc
50
45
35
55
45
d
CH2Cl2
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
Toluene
DMF
TBAB (1)
TBAI (1)
d
[BMlm]Br (1)
[AcMlm]CI (1)
p-TsOH (1)
PhCO2H (1)
p-NBA (1)
p-NBA (1)
p-NBA (1)
p-NBA (0.75)
p-NBA (0.50)
10
30
10
62
80
30
35
82
45
3. Conclusion
In summary, we have developed an organo-catalytic domino
aliphaticearomatic transformations of 2,3-aza-1-cyclohexanones
to N-arylamines. The reaction is catalyzed using inexpensive p-
NBA with regioselective formation of aryl amines with moderate to
excellent yields. Further studies will be directed toward the de-
velopment of this catalytic system to synthesize N-aryl function-
alized natural products efficiently.
MeCN
MeCN
80
a
Reactions are carried under argon.
Isolated yield.
2-Chlorocyclohex-2-en-1-one is obtained quantitatively.
b
c

M.T. Barros et al. / Tetrahedron 68 (2012) 6263e6268
6265
Table 2
Synthesis of arylamines from aziridinesa
O
H
N
R1
R2
0.75 eq. p-NBA
MeCN, 80oC
R
N
R3
(1)
R
R1
R3
R2
(2)
Entry
1
Aziridine
Product
Time (h)
10
Yieldb (%)
O
H
N
62
75
N
OMe
OMe
2a
1a
OMe
OMe
O
H
N
2
3
4
10
10
5
N
1b
2b
OMe
OMe
OMe
H
N
O
73
64
N
1c
OMe
2c
O
H
N
N
1d
2d
O
H
N
5
6
12
12
80
78
N
1e
2e
F
F
H
N
O
N
1f
2f
O
H
N
7
8
12
8
75
78
71
75
N
1g
2g
O
H
N
COOEt
COOEt
COOEt
N
1h
2h
O
H
N
9
8
N
1i
COOEt
2i
O
H
N
10
11
10
3
N
1j
2j
O
H
N
30
N
2k
1k
(continued on next page)

6266
M.T. Barros et al. / Tetrahedron 68 (2012) 6263e6268
Table 2 (continued )
Entry
Aziridine
Product
Time (h)
12
Yieldb (%)
70
O
NH(CH2)7CH3
12
13
N
(CH2)7CH3
2l
1l
O
H
N
12
12
60
63
N
1m
2m
O
H
N
14
15
N
2n
1n
OMe
OMe
O
H
N
10
70
N
2o
1o
OMe
OMe
OMe
O
H
N
16
10
68
N
OMe
2p
1p
O
H
N
17
5
40
72
N
1q
2q
O
H
N
18
10
N
1r
2r
a
Mixture of 0.50 mmol of aziridine and 0.38 mmol 4-nitrobenzoic acid in 1.0 ml of CH3CN was heated at 80 ꢀC.
Isolated yield.
b
spectrophotometer and are in cmꢁ1. Exact Masses were measured
using ESI-FIA-TOF on a Brucker Microtof.
O
O
O
O
1.0 eq. p-NBA, 12 h
N
MeCN, 80oC
4.2. General experimental procedure for the synthesis of aryl
amines
90%
1s
(
)
5
( )
NO2
In a screw capped 5.0 ml vessel, a mixture of aziridine
(0.50 mmol) and p-NBA (63 mg, 0.38 mmol) in 1.0 ml of acetonitrile
was heated at 80 ꢀC for the required time under argon. After
complete reaction (TLC), 5.0 ml saturated solution of sodium car-
bonate was added and stirred for few minutes. The solution was
extracted with ethyl acetate (20.0ꢂ2 ml) and washed with brine.
Evaporation of organic layer followed by small silica gel column
chromatography (eluted with hexanes/ethylacetate) to afford pure
aryl amine.
Scheme 3.
4. Experimental
4.1. General
All chemicals used were of reagent grade and all reactions were
carried out under argon. All solvents were dried by established
methods. Flash chromatography was performed on Kieselgel 60,
particle size 0.032e0.063 mm. NMR spectra were obtained at
Brucker 400 MHz (1H NMR) and 100 MHz (13C NMR) using CDCl3 as
solvent unless noted otherwise. Chemical shifts are reported in
parts per million relative to TMS (1H) or solvent (13C). Infrared (IR)
spectra were obtained using a Mattson Research Series FT-IR
4.2.1. N-Allyl-3-methylaniline (2k). Colorless oil; IR
3405, 3043, 2919, 2869, 1702, 1671, 1606, 1509, 1492, 769; 1H
NMR:
7.08 (t, J¼7.6 Hz, 1H), 6.57 (d, J¼7.6 Hz, 1H), 6.51e6.48 (m,
(nmax):
d
2H), 5.97 (m, 1H), 5.28 (dd, J¼17.2, 1.4 Hz, 1H), 5.16 (dd, J¼10.2,
1.4 Hz, 1H), 3.77 (d, J¼5.5 Hz, 2H), 2.28 (s, 3H); 13C NMR:
d 147.3,
139.1, 135.1, 129.1, 119.2, 116.6, 114.4, 110.8, 47.1, 21.6. Anal. Calcd

M.T. Barros et al. / Tetrahedron 68 (2012) 6263e6268
6267
Ph
NH
NO2
O
O
H
O
N
Ph
+
50 mol% p-TsOH
O
NH
Ph
+
1.5 eq PhCH2NH2
MeCN, 80 oC, 12 h
O
O2N
7
8
9
4
NO2
O
O
1.0 eq PhCH2NH2
O
7
+
N
1.5 eq PNBA
O
MeCN, 80 oC, 12 h
1t
10
Scheme 4.
NO2
O
O
H
O
N
R
O
NHR
(II)
(I)
O
O
O
O
OH
O
RNH2
R
(III)
HN
N
R
NO2
NO2
R
(VIII)
N
R
N
R
N
O
O
(IV)
O
O2N
OH
NH
R
(V)
NH
R
NO2
O
O
O
O
H-shift
NO2
(VI)
NO2
(VII)
Scheme 5. Plausible mechanism of the reaction.
for C10H13N: C, 81.59; H, 8.90; N, 9.51. Found: C, 81.30; H, 8.71;
N, 9.40.
6.46 (d, J¼1.7 Hz, 1H), 6.42 (dd, J¼8.2, 1.7 Hz, 1H), 4.22 (s, 2H), 3.82
(s, 3H), 3.78 (s, 3H), 2.22 (s, 3H); 13C NMR:
160.2, 158.4, 145.9,
d
129.8, 129 (2C), 126.8, 119.8, 113.6 (2C), 103.9, 98.6, 55.4, 55.3, 43.7,
4.2.2. 3-Methyl-N-octylaniline (2l). Colorless oil; IR (nmax): 3410,
20.4. HRMS [MþH] calcd: 258.1486; found: 258.1489.
3052, 3020, 2957, 2930, 2871, 1604, 1506, 1478, 748, 692; 1H NMR:
d
7.06 (t, J¼7.5 Hz, 1H), 6.51 (d, J¼7.5 Hz, 1H), 6.45e6.37 (m, 2H),
Acknowledgements
3.09 (t, J¼7.1 Hz, 2H), 2.27 (s, 3H), 1.60 (m, 2H), 1.43e1.20 (m,
ˇ
10H), 0.88 (t, J¼7.0 Hz, 3H); 13C NMR:
d
148.5, 139.0, 129.1, 118.2,
S.D. and P.R. are grateful to Fundac¸ ao para
Tecnologia, Portugal for grants (BPD/66763/2009) and (SFRH/BD/
2742ˇ 3/2006). This work has been supported by Fundac¸ ao para
a Ciencia e a Tecnologia through grants no. PEst-OE/EQB/LA0004/
2011 and PEst-C/EQB/LA0006/2011 and project PTDC/QUI-QUI/
a
Ciencia
e
~
113.6, 110.0, 44.2, 31.8, 29.5, 29.4, 29.3, 27.2, 22.7, 21.7, 14.1.
Anal. Calcd for C15H25N: C, 82.13; H, 11.49; N, 6.39. Found: C, 81.98;
H, 11.20; N, 6.21.
~
4.2.3. N-(2,4-Dimethoxybenzyl)-4-methylaniline (2p). Colorless oil;
IR (nmax): 3415, 2835, 1604, 1507, 1244, 1175, 1032, 749; 1H NMR:
104056/2008. The National NMR Network (REDE/1517/RMN/
ˇ
~
2005), is supported by POCI 2010 and Fundac¸ ao para a Ciencia e
d
7.19 (d, J¼8.2 Hz, 1H), 6.97 (d, J¼8.2 Hz, 2H), 6.59 (d, J¼8.2 Hz, 2H),
a Tecnologia.

6268
M.T. Barros et al. / Tetrahedron 68 (2012) 6263e6268
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Supplementary data related to this article can be found online at
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