DDQ-mediated direct oxidative coupling of amides with benzylic and allylic sp3 C-H bonds under metal-free conditions

Article abstract of DOI:10.1016/j.tetlet.2012.03.137

A simple and efficient method for the direct oxidative coupling of amides with benzylic and allylic sp³ C-H bonds using DDQ as an oxidant is described. A range of amides including benzamide, benzyl carbamate, and substituted sulfonamides reacted efficiently with various benzylic and allylic substrates under metal free conditions to afford amidation products in good to excellent yields.

Full text of DOI:10.1016/j.tetlet.2012.03.137

Tetrahedron Letters 53 (2012) 2904–2908
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
DDQ-mediated direct oxidative coupling of amides with benzylic and allylic sp3
C–H bonds under metal-free conditions
D. Ramesh a,b, U. Ramulu a, K. Mukkanti b, Y. Venkateswarlu a,
⇑
a Division of Natural Products Chemistry, Indian Institute of Chemical Technology, Hyderabad 500 007, India
b Chemistry Division, Institute of Science and Technology, JNT University, Kukatpally, Hyderabad 500 072, India
a r t i c l e i n f o
a b s t r a c t
A simple and efficient method for the direct oxidative coupling of amides with benzylic and allylic sp3
C–H bonds using DDQ as an oxidant is described. A range of amides including benzamide, benzyl carba-
mate, and substituted sulfonamides reacted efficiently with various benzylic and allylic substrates under
metal free conditions to afford amidation products in good to excellent yields.
Ó 2012 Elsevier Ltd. All rights reserved.
Article history:
Received 18 February 2012
Revised 30 March 2012
Accepted 31 March 2012
Available online 6 April 2012
Keywords:
C–N bond formation
Metal-free
Oxidative coupling
DDQ
Amides
Introduction
available for C–N bond formation, reactions under metal-free con-
ditions are scarce and restricted to only sulfonamides, benzylic
Selective and efficient activation of C–H bonds to generate func-
tional compounds with minimal energy, toxicity, cost, and with
less environmental problems is one of the challenging areas in syn-
thetic chemistry.1,2 Direct conversion of unmodified C–H bonds
into C–N bonds is an important strategy for the synthesis of valu-
able nitrogen containing compounds,3 which are prevalent in phar-
maceuticals, fine chemicals, and natural products. In most of the
methods, the C–N bond was constructed by various functional
group interconversions.4,5 The reaction may be further enhanced
from the view point of atom efficiency6 and green chemistry.7
The development of amidation reactions of prior unfunctionalized
substrates is an important alternative,8 which makes the synthetic
path simpler, shorter, and atom economical. In this perspective,
considerable achievements have been made toward the direct
utilization of benzylic and allylic sp3 C–H bonds.9,10
substrates and require more than one catalyst or reagent.
To the best of our knowledge, there is no protocol for the direct
oxidative coupling of amides with both benzylic and allylic sp3 C–H
bonds using single oxidizing reagent under metal-free conditions.
Furthermore, benzyl carbamate (Cbz-NH2) can be directly installed
at benzylic and allylic position, which is a very common protecting
group for amines and amino acids and is also very useful in multi-
step synthesis. DDQ (2,3-dichloro-5,6-dicyano-p-benzoquinone) is
an inexpensive and readily available oxidizing reagent. Recently
our group,20 Bao,21a Li21b,c and Dong21d groups have reported some
oxidative coupling reactions using DDQ.
The initial explorations were carried out by the intermolecular
coupling of diphenyl methane (1a) with benzyl carbamate (2a) as
a prototype reaction under various reaction conditions, including
optimization of oxidants, solvents, and temperature to afford cor-
responding product (3a). The results are summarized in Table 1.
The reaction was examined by using various oxidants, such as ceric
ammonium nitrate (CAN), chloranil (2,3,5,6-tetrachloro-p-benzo-
quinone), and DDQ. When CAN was used as the oxidant, only a
trace amount of product was observed (Table 1, entry 1), whereas
reaction with chloranil gave the product in low yield (Table 1, en-
try 2). However the reaction with DDQ gave good yield. Further, we
investigated for the suitable solvent such as chloroform, dichloro-
methane, carbon tetrachloride, dichloroethane, dioxane tetrahy-
drofuran, hexane, and nitromethane. Chlorinated solvents gave
good yields (Table 1, entries 3–6), whereas in hexane no reaction
was observed (Table 1, entry 7), dioxane and tetrahydrofuran gave
In the reported methods, majority of amidation reactions are
based on nitrene derivatives as the primary nitrogen source11 or
hypervalent iodine reagents.12
Additionally, metal catalysts such as Cu,13 Ru,14 Rh,15 Fe,16 Ag,17
Pd,18 and Co19 were essential for the activation of C–H bond for the
C–N formation. In view of the increased awareness toward envi-
ronmental problems, transition metal-free methods are preferable
to the amidation reactions. Even though various methods are
⇑
Corresponding author. Tel.: +91 40 27193167; fax: +91 40 27160512.
E-mail address: luchem@iict.res.in (Y. Venkateswarlu).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.03.137

D. Ramesh et al. / Tetrahedron Letters 53 (2012) 2904–2908
2905
Table 1
Optimization of reaction conditionsa
Cbz
HN
O
Oxidant
O
NH2
+
Solvent, temp, time
2a
3a
1a
Entry
Oxidant
Solvent
Temp (°C)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
CAN
CH3CN
ClCH2CH2Cl
CHCl3
CH2Cl2
CCl4
ClCH2CH2Cl
Hexane
Dioxane
THF
CH3NO2
CH3NO2
CH3NO2
CH3NO2
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
rt
12
10
5
5
5
Trace
31
78
76
69
Chloranil
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
5
80
12
12
10
12
4
No reactionc
30
36
10
9
10
11
12
13
80
80
80
86
4
4
75d
83e
a
b
c
Conditions: 1a (1.5 mmol), 2a (0.3 mmol) and oxidant (0.36 mmol) in the presence of indicated solvent, temperature, and time.
Isolated yield.
No reaction based on TLC analysis.
Compound 1a (0.6 mmol).
Compound 1a (0.9 mmol).
d
e
moderate yield (Table 1, entries 8 and 9). The best result was
achieved in nitromethane (Table 1, entry 11). The reaction in nitro-
methane was sluggish at room temperature (Table 1, entry 10) and
proceeded well at 80 °C (Table 1, entry 11). When the reaction was
carried out using 2, 3, and 5 equiv of 1a, the best result was ob-
tained with 5 equiv (Table 1 entries 12, 13, and 11). This was noted
in earlier reports where benzylic substrates were taken in excess
equivalents (5 and 12 equiv).22 Under optimal conditions the ami-
dation reaction was carried out with 0.3 mmol of amide,
0.36 mmol of DDQ, and 1.5 mmol of benzylic substrate in nitro-
methane as the solvent at 80 °C.
Further we examined the scope of DDQ-mediated amidation be-
tween amide N–H bonds and various allylic and benzylic sp3 C–H
bonds. Diphenylmethane was reacted with various amides to af-
ford amidation products in excellent yields (Table 2, entries 1, 4,
10, 16, and 17). Sulfonamides bearing aromatic ring with elec-
tron-donating group in the para position gave good yields (Table
2, entries 10–15 and 17–20), whereas sulfonamides bearing aro-
matic ring electron-withdrawing group in the para position gave
relatively low yields (Table 2, entry 16), which has been correlated
with the earlier oxidative coupling reactions.21a,23 Benzamide also
reacted well with diphenylmethane to afford amidation product in
85% yield (Table 2, entry 4). Substrates having less activated ben-
zylic sp3 C–H bond such as indane, 1,2,3,4-tetrahydro naphthalene,
and ethyl benzene were reacted with various amides to give ami-
dation products in good yields (Table 2, entries 2, 5–7, 11–13, 18,
and 19).
range of amides including benzamide, benzyl carbamate, and
substituted sulfonamides can be utilized, (iii) un-functionalized
benzylic and allylic sp3 C–H substrates can be used directly, (iv)
no additional reagents are required for the reaction, (v) DDQ is
an inexpensive and readily available oxidizing reagent.
General experimental procedure
To a solution of amide 2 (0.3 mmol), allylic or benzylic sp3 C–H
compound 1 (1.5 mmol) in CH3NO2 (3 mL) was added DDQ
(0.36 mmol). The resulting mixture was heated at 80 °C while stir-
ring for indicated time. After completion of the reaction as evi-
denced by TLC, the reaction mixture was cooled to room
temperature and filtered. Filtrate was diluted with water (10 mL)
and extracted into ethyl acetate (3 Â 10 mL). The combined organic
layer was washed with saturated NaHCO3 solution, dried over
anhydrous Na2SO4 and concentrated under reduced pressure to
give crude residue, which was purified on silica gel column chro-
matography using hexane and ethyl acetate as the eluent to give
the pure product. All products were characterized by 1H, 13C, IR,
and mass spectral data.
Spectral data for selected compounds
Benzyl benzhydrylcarbamate (3a)
IR (KBr): 3304, 3027, 2927, 2850, 1684, 1528, 1244, 1055,
750 cmÀ1 1H NMR (300 MHz, CDCl3): d 7.35–7.19 (m, 15H), 5.96
;
(br d, 1H, J = 6.8 Hz), 5.30 (br s, 1H), 5.09 (s, 1H); 13C NMR
(75 MHz, CDCl3): d 155.4, 141.7, 136.3, 128.6, 128.5, 128.2, 127.5,
127.3, 66.9, 58.8; MS (ESI) m/z: 340 [M+Na]+.
To study the scope of the amidation, we further carried out the
reaction with allylic sp3 C–H substrates such as cyclopentene and
cyclohexene with different amides that is benzyl carbamate, benz-
amide, and substituted sulfonamides to afford the amidation prod-
ucts in good yields (Table 2, entries 3, 8, 9, 14, 15, and 20). We have
carried out the reaction with primary and tertiary benzylic sub-
strates under same reaction conditions without success (Table 2,
entries 21 and 22).
Benzyl cyclopent-2-enylcarbamate (3c)
IR (KBr): 3312, 3027, 2948, 2850, 1682, 1332, 1253, 1052,
747 cmÀ1 1H NMR (300 MHz, CDCl3): d 7.36–7.27 (m, 5H), 5.93–
;
In conclusion, we have developed a simple metal-free direct
amidation of benzylic and allylic sp3 C–H bonds using DDQ as an
oxidizing reagent. This method provides numerous merits, specifi-
cally: (i) reaction proceeds without metal or metal catalyst, (ii) a
5.87 (m, 1H), 5.72–5.66 (m, 1H), 5.06 (s, 2H), 4.83–4.57 (m, 2H),
2.49–2.23 (m, 3H), 1.64–1.52 (m, 1H); 13C NMR (75 MHz, CDCl3):
d 155.5, 136.6, 134.1, 131.3, 128.3, 128.0, 127.8, 66.3, 57.2, 31.4,
30.9; MS (ESI) m/z: 240 [M+Na]+.

2906
D. Ramesh et al. / Tetrahedron Letters 53 (2012) 2904–2908
Table 2
Oxidative coupling of amides with various benzylic and allylic C–H bondsa
Entry
Substrate 1
Substrate 2
Product 3
Time (h)
Yieldb (%)
O
Cbz
HN
Ph
Ph
Ph
O
O
O
NH2
NH2
NH2
1
4
86
1a
2a
3a
O
Cbz
HN
2
3
4
2
78
1c
2a
3b
O
Cbz
HN
81c,d
1f
2a
3c
O
O
NH2
HN
Ph
1a
4
5
3
3
85
80
2b
3d
O
O
HN
NH2
Ph
1b
1c
2b
3e
O
O
NH2
HN
Ph
6
7
4
4
69
71
2b
3f
O
O
Me
NH2
HN
Ph
1d
Me
2b
3g
O
O
O
NH2
HN
Ph
Ph
1e
1f
8
1
73c
2b
O
3h
NH2
HN
9
1
2
77c,d
2b
3i
O
S
O
Ts
HN
NH2
10
84
1a
Me
3j
2c

D. Ramesh et al. / Tetrahedron Letters 53 (2012) 2904–2908
2907
Table 2 (continued)
Entry
Substrate 1
Substrate 2
Product 3
Ts
HN
Time (h)
Yieldb (%)
O
O
S
S
NH2
11
12
13
14
15
3
81
1b
Me
Me
3k
2c
O
O
Ts
HN
NH2
3
3
2
2
63
1c
2c
3l
O
O
Ts
Me
HN
S
S
NH2
Me
68
1d
Me
Me
3m
2c
O
O
Ts
HN
HN
NH2
64c
1e
1f
2c
3n
O
O
Ts
S
NH2
71c,d
Me
Br
2c
O
3o
O
O
O
S
S
NH2
HN
Br
O
1a
1a
16
17
4
4
60
87
2d
3p
O
O
O
S
O
S
NH2
HN
Me
Me
Me
O
O
2e
3q
O
O
O
O
S
S
NH2
HN
18
19
20
4
4
1
73
O
Me
1b
2e
2e
3r
O
O
O
O
O
S
S
S
NH2
HN
64
Me
Me
Me
1c
O
O
O
3s
O
O
O
S
NH2
HN
77c,d
1f
Me
O
2e
3t
Me
O
O
H
N
S
Ts
NH2
21
12
0
Me
1g
2c
3u
(continued on next page)

2908
D. Ramesh et al. / Tetrahedron Letters 53 (2012) 2904–2908
Table 2 (continued)
Entry
Substrate 1
Me Me
Substrate 2
Product 3
Time (h)
Yieldb (%)
O
O
H
Me
N
S
Me
Ts
NH2
22
Me
12
0
2c
1h
3v
a
Conditions: 1.5 mmol of 1, 0.3 mmol of 2, and 0.36 mmol of DDQ in nitromethane at 80 °C for indicated time.
b
c
Isolated yield.
3 mmol of 1.
At 50 °C.
d
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IR (KBr): 3306, 3048, 2916, 2840, 1635, 1529, 1275, 749 cmÀ1
;
1H NMR (300 MHz, CDCl3): d 7.79–7.76 (m, 2H), 7.51–7.20 (m,
7H), 6.33 (br d, 1H, J = 6.8 Hz), 5.71–5.65 (m, 1H), 3.06–2.99 (m,
1H), 2.97–2.88 (m, 1H), 2.74–2.66 (m, 1H), 1.97–1.88 (m, 1H);
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128.6, 128.1, 127.0, 126.9, 124.9, 124.2, 55.2, 34.2, 30.4; MS (ESI)
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N-(2,3-Dihydro-1H-inden-1-yl)-4-methylbenzene sulfonamide
(3k)
IR (KBr): 3280, 3027, 2923, 2850, 1595, 1440, 1327, 1159,
749 cmÀ1 1H NMR (300 MHz, CDCl3): d 7.82 (d, 2H, J = 8.1 Hz),
;
7.32 (d, 2H, J = 7.9 Hz), 7.22–7.05 (m, 4H), 4.85–4.73 (m, 2H),
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1H), 1.81–1.62 (m, 1H); 13C NMR (75 MHz, CDCl3): d 143.4,
142.8, 142.0, 138.1, 129.7, 128.2, 127.1, 126.8, 124.7, 124.0, 58.6,
34.6, 29.9, 21.5; MS (ESI) m/z: 310 [M+Na]+.
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1.45 (m, 1H); 13C NMR (75 MHz, CDCl3): d 162.8, 134.9, 132.8, 130.6,
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Acknowledgments
Authors are thankful to the CSIR New Delhi, India, for financial
assistance and to the Director of IICT for his encouragement.
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