Metal-Free Amidation of Ethers with N, N -Dibromosulfonamides

Article abstract of DOI:10.1055/s-0035-1561373

A new metal-free amidation of ethers with N,N-dibromosulfonamides has been developed. A series of hemiaminal ethers or imines were prepared with moderate to good yields.

Full text of DOI:10.1055/s-0035-1561373

SYNLETT
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©
Georg Thieme Verlag Stuttgart · New York
2015, 26, A–E
A
letter
Synlett
Y.-Y. Wang et al.
Letter
Metal-Free Amidation of Ethers with N,N-Dibromosulfonamides
Yuan-Yuan Wang
Ming-Hui Sun
Ning-Ning Zeng
Jie Chen*
O
H
O
2
NHSO R
K2CO3, CH2Cl2
+
RSO2NBr2
40 °C, 4 h
1
6 examples, 21–84% yield
R = Ph, 4-MeC6H4, 4-ClC6H4, 4-O2NC6H4
Ling Zhou*
Key Laboratory of Synthetic and Natural Functional Molecule
Chemistry of Ministry of Education, College of Chemistry &
Materials Science, Northwest University, 1 Xuefu Ave.,
Chang’an District, Xi’an, 710127, Shaanxi Province,
P. R. of China
chmchenj@nwu.edu.cn
zhoul@nwu.edu.cn
Received: 04.12.2015
Accepted after revision: 15.01.2016
Published online: 08.02.2016
under transition-metal-free conditions (Scheme 1, eq. 2).10
Yet, despite Ochiai’s environmentally benign method using
the active organonitrenoid specie, the metal-free amidation
of ethers by nitrene-insertion strategy is sparse in the liter-
ature. Here we wish to report a metal-free amidation of
ethers by using readily available N,N-dibromosulfonamides
as nitrene sources under mild conditions (Scheme 1, eq. 3).
DOI: 10.1055/s-0035-1561373; Art ID: st-2015-w0942-l
Abstract A new metal-free amidation of ethers with N,N-dibromosul-
fonamides has been developed. A series of hemiaminal ethers or imines
were prepared with moderate to good yields.
Key words amidation, amides, N,N-dibromosulfonamides, ethers,
hemiaminal ethers, nitrene, regioselectivity
Previous work:
O
O
O
H
PhI=NTs or
NHTs
NHTf
Metal catalyst
Metal-free
TsNH2/PhI(OAc)2
(1)
(2)
The regioselective construction of C–N bonds is consid-
ered to be an important approach due to the ubiquitous
presence of amines in many biologically active natural
+
+
or TsNClNa
1
O
products and pharmaceutical compounds. Therefore,
H
–
+
TfN-Br-Ar
many remarkable endeavors have been made to develop ef-
ficient and general methods for the amination reactions.2
Amidation of ethers by transition-metal-catalyzed nitrene
insertion into C–H bonds has been shown to be the most ef-
ficient and synthetic useful method for the construction of
hemiaminal ether frameworks.3,4 For example, Che and co-
workers developed amidation of THF with PhI=NTs or
TsNH /PhI(OAc) catalyzed by ruthenium and manganese
This work:
O
H
O
NHTs
Metal-free
(3)
+
TsNBr2
2
2
Scheme 1 Amidation of ethers
5
porphyrins (Scheme 1, eq. 1). Díaz-Requejo, Pérez, and co-
workers developed copper–homoscorpionate complex cata-
lyzed amidation of cyclic ethers by using PhI=NTs or TsNClNa
During the course of our studies on the bromoform re-
action of tertiary amines with N,N-dibromo-4-methylben-
6
(chloramine-T) as a nitrene source. Then Yu and co-work-
ers developed simple copper salt [Cu(OTf) ] catalyzed ami-
zenesulfonamide (TsNBr ) in THF, we noticed that small
2
2
dation of cyclic ethers by using PhI=NTs or TsNH /PhI(OAc)
amounts of α−(N-tosylamino)tetrahydrofuran (3a) were
2
2
7
11
as a nitrene source. On the other hand, photoinduced radi-
cal strategy and oxidative cross-dehydrogenative-coupling
strategy have also been developed for the amination of
formed when TsNBr was employed (Scheme 2). As active
2
reagents, N,N-dibromosulfonamides can be used as bro-
mine sources, as amine sources, as sources of both amine
8,9
12
ethers. Notably, metal-free amidation of cyclic ethers by
nitrene insertion have been reported by Ochiai and co-
and bromine, or as others. However, amidation of ethers
using N,N-dibromosulfonamides under metal-free condi-
tions has never previously been reported. Additionally,
hemiaminal ether group is generally presented in many
3
workers, in which hypervalent N-triflylimino-λ -bromane
was used as a nitrene source to realize amidation of ethers
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, A–E

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Synlett
Y.-Y. Wang et al.
Letter
natural products and some biologically active pharmaceuti-
cal compounds. Then we decided to study this metal-free
amidation reaction on both of the mechanism and its scope.
Table 1 Amidation of THF with TsNBr a
2
13
O
O
NHTs
conditions
+
TsNBr2
K2CO3 (3.0 equiv)
THF, r.t.
O
NHTs
1a
2
3a
Temp (°C) Solvent
THF
Et3N
+
TsNBr2
+
N
NTs
(
1.0 equiv) (2.5 equiv)
Entry
1a/2
Base (equiv)b
Yield (%)c
18%
3a 10%
1
2
3
4
5
6
7
8
9
120:1
K
K
K
K
K
2
2
2
2
CO
CO
CO
CO
3
3
3
3
3
(3.0)
(3.0)
(3.0)
(3.0)
(3.0)
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
40
40
40
40
40
40
80
40
9
Scheme 2 Bromoform reaction of triethylamine (Et N) with TsNBr in
3
2
10:1
5:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
CH
CH
CH
2
2
2
Cl
Cl
Cl
2
2
2
26
25
24
6
THF
The results of the optimization studies are summarized
in Table 1. Simply mixing TsNBr and K CO in THF at room
temperature for four hours gave 3a in 9% yield (Table 1, en-
2CO
MeCN
EtOAc
CH Cl
2
2
3
K CO (3.0)
2
3
3
try 1), indicating that Et N is unrelated to this reaction. De-
3
Na CO (3.0)
7
2
3
2
2
2
2
2
2
creasing the amount of THF from 120 to 10 equiv afforded a
6% yield of the amidation product 3a (Table 1, entry 2). To
our delight, stoichiometric amount of THF resulted in a
comparable yield of 3a (Table 1, entry 4 vs. entry 2). Screen-
ing of solvents was carried out with a mixture of THF and
TsNBr in a ratio of 1:1 in the presence of 3.0 equivalents of
K CO (Table 1, entries 4–6), and we found that CH Cl is a
suitable solvent. Other bases such as Na CO , NaHCO ,
NaOAc, NaOH, and KOH were also screened, however, no
further improvement was achieved (Table 1, entries 7–12).
Next, changing the ratio of the reactants resulted in signifi-
cant difference in the reaction yields. When the reaction
NaOAc (3.0)
NaHCO (3.0)
CH
CH
CH
CH
Cl
Cl
Cl
Cl
2
2
2
2
2
2
2
2
2
2
2
2
8
2
3
trace
trace
trace
28
44
84
trace
trace
61
–
10
NaOH (3.0)
KOH (3.0)
11
12
13
2
1:1.2
K CO (3.0)
2
3
CH Cl
2
2
2
3
2
2
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
K CO (5.0)
CH Cl
2
3
2
3
3
14
15
16
K
2
CO
3
(5.0)
CH
CH
CH
CH
2
2
2
2
Cl
Cl
Cl
Cl
Cs
2
CO
3
(5.0)
KOt-Bu
Na PO
4
17
18
3
(5.0)
was carried out in a ratio of 1:1.5 (THF/TsNBr ) in the pres-
2
CH Cl
2
DBU (5.0)
ence of 5.0 equivalents of K CO , the desired product 3a was
9d
2
3
1
2
K
K
K
2
2
2
CO
CO
CO
3
3
3
(5.0)
(5.0)
(5.0)
CH
DCE
CH Cl
2
Cl
62
27
–
obtained in 44% yield (Table 1, entry 13). Satisfactorily, a
dramatic increase in yield was detected when the reaction
temperature was raised to 40 °C (Table 1, entry 14). Further
increasing the reaction temperature in 1,2-dichloroethane
0
2
1e
2
2
a
Reactions were carried out with 1a (0.2 mmol for entries 4–21), TsNBr
0.2 mmol for entries 1–3), base in solvent (2.0 mL) for 4 h.
Conditions: Based on 1.0 equiv of 1a (entries 4–21) or 2 (entries 1–3).
Isolated yield.
Conditions: 0.02 mmol NaI was used.
2
Replaced TsNBr with chloramine-T.
2
(
(
DCE) diminished the reaction (Table 1, entry 20). Notably,
b
c
replacement of TsNBr with chloramine-T gave no desired
2
d
e
amidation product (Table 1, entry 21).1
4
With the optimized reaction conditions in hand,15 the
substrate scope of this metal-free amidation reaction was
explored. The results are summarized in Table 2. Both tetra-
hydropyran (1b) and 1,4-dioxane (1c) resulted in the de-
sired α-amidation products with good yields when excess
cyclic ethers were used (Table 2, entries 2, 3). Isochroman
ether such as diethyl ether, methyl tert-butyl ether did not
result in the desired amidation product. However, acyclic
benzyl ethers were shown to be suitable substrates for this
metal-free amidation reaction. In these cases, the hemiami-
nal ethers products could not be isolated, but the imine
compounds were obtained (Table 2, entries 6–8), a result of
(
1d) bearing two different α-methylene groups gave only
the benzylic α-amidation product 3d in 88% yield under the
optimized conditions (Table 2, entry 4). Notably, nitrene in-
sertion into benzylic C–H bonds has been reported previ-
elimination of an alcohol. This phenomenon has been ob-
served previously.7
b,10b
Finally, performing the reaction on a
12
ously, here the sole regioselectivity was achieved, indicat-
ing the position is more active for the nitrene insertion. Ox-
epane was also suitable substrate for this amidation
reaction, and the desired hemiaminal 3e was obtained in
large scale showed no change in efficiency (Table 2, entry
1).
Besides TsNBr , other N,N-dibromosulfonamides are
2
also suitable for this reaction. N,N-Dibromobenzenesulfon-
amide, N,N-dibromo-4-chloro-benzenesulfonamide, and
N,N-dibromo-4-nitrobenzenesulfonamide all returned the
desired hemiaminal ethers in 55%, 56%, and 27% yields, re-
spectively (Table 3, entries 2–4). It seemed that a strong
electron-withdrawing group on the benzene ring of N,N-di-
53% yield (Table 2, entry 5).
Interestingly, treatment of morpholine under the opti-
mized conditions gave no desired product, but diaziridine
15
3
f was obtained in 71% yield (Table 2, entry 6). Acyclic
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, A–E

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Synlett
Y.-Y. Wang et al.
Letter
Table 2 Amidation of Ethers Using TsNBr2a
Table 3 Amidation of Ethers Using N,N-Dibromosulfonamidesa
O
O
O
O
H
K CO , CH Cl
NHTs
H
NHSO R
2
3
2
2
K2CO3, CH2Cl2
40 °C, 4 h
2
+
TsNBr2
+ RSO NBr
2
2
40 °C, 4 h
1
2a
3
1
2
3
Entry
1
Substrate
Product
Yield (%)b
83 (84)c
Yield (%)b
Entry
Substrate
R
Product
O
O
NHTs
NHTs
1
2
3
4
5
6
7
1a
1a
1a
1a
1b
1c
1d
1e
4-MeC
6
H
4
3a
84
55
56
27
49
54
82
21
Ph
3ab
3ac
3ad
3bb
3cb
3db
3eb
1
1
a
3a
4-ClC
6 4
H
O
O
4-O NC H
2
6 4
2d
76
72
4-ClC
4-ClC
6
H
4
b
3b
6
H
4
4
4
O
O
O
NHTs
6
4-ClC H
3d
8
6
4-ClC H
O
a
Reactions were carried out with 1 (0.2 mmol), RSO
CO (1.0 mmol) in CH Cl (2.0 mL) at 40 °C for 4 h.
Isolated yield.
2 2
NBr (0.3 mmol),
1
1
c
3c
K
2
3
2
2
b
NHTs
O
O
4
88
To probe the mechanism, control experiments were
conducted. Using TsNH and NBS as reagents was found to
be ineffective in offering any desired product (Scheme 3, eq.
). However, replacing TsNBr with TsNBrNa showed similar
d
2
3
d
O
O
NHTs
1
2
5
6
53
71
reactivity as the TsNBr /K CO system (Scheme 3, eq. 2).
2 2 3
These results demonstrated that the TsNBr ion maybe play
an important role in this amidation reaction. A possible re-
action pathway for this novel metal-free amidation reaction
is depicted in Scheme 3 (eq. 3). Firstly, reaction of TsNBr2
with K CO leads to TsNBr ion intermediate (I), then reac-
1
e
3e
O
O
N
H
N
2
3
NTs
1
1
f
3f
tion of I with THF gives an intermediate II, in which the
electrophilic Br may interact with the Lewis basic oxygen,
O
NTs
16
as a result of Lewis base activation of Lewis acid. Thus the
7
8
66
63
87
α-C–H bonds of THF are activated by the oxonium interme-
diate; meanwhile, the potential active nitrenoid specie is
also activated, as the N–Br bond is more polarized. Further-
more, the intermolecular nitrene insertion reaction is
transformed to an intramolecular reaction (generally intra-
molecular reactions are entropically favored).
g
3g
O
NTs
O
O
1
h
3h
OBn
9
3g
K2CO3
1
a
+
+
TsNH2 + NBS
3a
3a
(1)
(2)
1
i
trace
a
Reactions were carried out with 1a–g (0.2 mmol), TsNBr
CO (1.0 mmol) in CH Cl (2.0 mL) at 40 °C for 4 h.
Isolated yield.
2
(0.3 mmol),
K2CO3
75%
K
2
3
2
2
1a
TsNBrNa
b
c
4
The reaction was conducted on a 10.0 mmol scale.
d
2 3
The ratio of 1/2a/K CO is 8:1:5.
Br
K2CO3
–
O
TsNBr2
TsNBr
–NTs
3a (3)
H
I
II
bromosulfonamide is harmful to the reaction yield (Table 3,
entry 1 vs. entry 4), the reason may be ascribed to the de-
creased nucleophilicity of the corresponding nitrene inter-
mediate. Reaction of N,N-dibromo-4-chlorobenzenesulfon-
amide with 1b, 1c, 1d, and 1e all gave the desired products
with moderate to good yields (Table 3, entries 5–8).
Scheme 3 Control experiments and proposed reaction pathway
In summary, we have developed a new metal-free ami-
dation of ethers with N,N-dibromosulfonamides. A series of
hemiaminal ethers and imines were prepared with moder-
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, A–E

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Synlett
Y.-Y. Wang et al.
Letter
ate to good yields. Control experiments indicated that the
TsNBr ion is responsible for this amidation reaction. Further
investigations to better understand the mechanism and to
develop new applications of this reaction are under way.
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(
4) For selected examples, see: (a) Torimoto, N.; Shingaki, T.; Nagai,
T. Bull. Chem. Soc. Jpn. 1977, 50, 2780. (b) Ozaki, S.; Tamaki, A.
Bull. Chem. Soc. Jpn. 1978, 51, 3391. (c) Breslow, R.; Gellman, S.
H. Chem. Commun. 1982, 1400. (d) Breslow, R.; Gellman, S. H.
J. Am. Chem. Soc. 1983, 105, 6728. (e) Mahy, J. P.; Bedi, G.;
Battioni, P.; Mansuy, D. Tetrahedron Lett. 1988, 29, 1927.
(
(
(f) Nägeli, I.; Baud, C.; Bernardinelli, G.; Jacquier, Y.; Moran, M.;
Müller, P. Helv. Chim. Acta 1997, 80, 1087. (g) Wehn, P. M.; Du
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J.; Du Bois, J. J. Am. Chem. Soc. 2006, 128, 3926. (j) Bhuyan, R.;
Nicholas, K. M. Org. Lett. 2007, 9, 3957. (k) Huard, K.; Lebel, H.
Chem. Eur. J. 2008, 14, 6222. (l) Ton, T. M. U.; Tejo, C.; Tiong, D. L.
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©
Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, A–E

E
Synlett
Y.-Y. Wang et al.
Letter
H.; Han, J. L.; Li, G. G.; Pan, Y. Tetrahedron Lett. 2010, 51, 2745.
j) Saikia, I.; Kashyap, B.; Phukan, P. Chem. Commun. 2011, 47,
967. (k) Ghorbani-Vaghei, R.; Shahbazi, H.; Veisi, H. Tetrahe-
°C for 4 h, and the reaction mixture was cooled to room tem-
(
2
perature. Then the reaction was quenched with sat. aq Na SO3
2
(2.0 mL) and H O (2.0 mL). The mixture was extracted with
2
dron Lett. 2012, 53, 2325. (l) Borah, A. J.; Phukan, P. Chem.
Commun. 2012, 48, 5491. (m) Nagamachi, T.; Takeda, Y.;
Nakayama, K.; Minakata, S. Chem. Eur. J. 2012, 18, 12035.
CH Cl (3 × 5.0 mL), and the extracts were combined, washed
2
2
with brine (10.0 mL), dried over Na SO , filtered, and concen-
2
4
trated in vacuo. The residue was purified by flash column chro-
(
2
n) Fan, G.-T.; Sun, M.-H.; Gao, G.; Chen, J.; Zhou, L. Synlett 2014,
5, 1921. (o) Qi, J.; Fan, G.-T.; Chen, J.; Sun, M.-H.; Dong, Y.-T.;
matography (silica gel, PE–EtOAc) to give 3a as a white solid;
1
yield 40.5 mg (84%). H NMR (400 MHz, CDCl ): δ = 7.79 (d, J =
3
Zhou, L. Chem. Commun. 2014, 50, 13841.
8.0 Hz, 2 H), 7.29 (d, J = 8.0 Hz, 2 H), 5.35 (ddd, J = 9.2, 6.4, 3.6
Hz, 1 H), 5.14 (d, J = 6.4 Hz, 1 H), 3.72–3.68 (m, 2 H), 2.42 (s, 3
H), 2.20–2.15 (m, 1 H), 1.91–1.75 (m, 3 H). C NMR (100 MHz,
(
13) (a) Subramaniam, G.; Hiraku, O.; Hayashi, M.; Koyano, T.;
Komiyama, K.; Kam, T.-S. J. Nat. Prod. 2007, 70, 1783. (b) Bonnac,
L. F.; Mansky, L. M.; Patterson, S. E. J. Med. Chem. 2013, 56, 9403.
14) For metal-catalyzed amination using chloramine-T as an amine
source, see ref. 3 and 4.
13
CDCl ): δ = 142.8, 138.4, 129.2, 126.6, 84.6, 66.7, 32.0, 23.6, 21.2.
3
+
(
(
ESI-HRMS: m/z calcd for C11H15NO SNa [M + Na] : 264.0665;
3
found: 264.0672.
15) 4-Methyl-N-(tetrahydrofuran-2-yl)benzenesulfonamide
(16) (a) Gutmann, V. The Donor-Acceptor Approach to Molecular
Interactions; Plenum Press: New York, 1978. (b) Denmark, S. E.;
Wynn, T. J. Am. Chem. Soc. 2001, 123, 6199. (c) Denmark, S. E.;
Wynn, T.; Beutner, G. L. J. Am. Chem. Soc. 2002, 124, 13405.
(3a); Typical Procedure
TsNBr2 (2, 98.7 mg, 0.30 mmol) was added in one portion to a
stirred solution of THF (1a, 16 μL, 0.20 mmol), and K CO (137.9
2
3
mg, 1.0 mmol) in CH Cl (2.0 mL). The mixture was stirred at 40
2
2
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, A–E