Di-tert-butyl peroxide (DTBP) promoted dehydrogenative coupling: An expedient and metal-free synthesis of oxindoles via intramolecular C(sp2)-H and C(sp3)-H bond activation

Article abstract of DOI:10.1039/c5ra09055e

An efficient di-tert-butyl peroxide (DTBP) promoted synthesis of oxindole has been developed. This methodology involves C(sp³)-H and C(sp²)-H bond activation under metal-free conditions. This synthetic approach towards oxindole synth

Full text of DOI:10.1039/c5ra09055e

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Di-tert-butyl peroxide (DTBP) promoted dehydrogenative coupling: An
2
expedient and metal-free synthesis of oxindoles via intramolecular C(sp )-H
3
and C(sp )-H bond activation
Biplab Mondal and Brindaban Roy*
Department of Chemistry, University of Kalyani, Kalyani, Nadia, West Bengal, India
E-mail: broybsku@gmail.com; Tel.: +913325828750; Fax: +913325828282
Abstract: An efficient di-tert-butyl peroxide (DTBP) promoted synthesis of oxindole has been
3
2
developed. This methodology involves C(sp )-H and C(sp )-H bond activation under metal-free
condition. This synthetic approach towards oxindole synthesis avoids base and hazardous iodine
reagent unlike other methodologies developed so far. This metal- and base-free protocol is
operationally simple and ecofriendly. It provides an expedient approach to access oxindoles in
o
moderate to very good yields in DCE at 110 C.
3
2
Key words: Di-tert-butyl peroxide (DTBP), radical reaction, C(sp )-H and C(sp )-H bond
activation, metal-free intramolecular C-C coupling, oxindole synthesis.
1

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Oxindoles are important heterocyclic scaffolds with unique biological activity found in a wide
1
2,
3
variety of bioactive compounds, natural alkaloids and pharmaceutically active molecules. As
for example, oxindole derivatives have been demonstrated to have significant potential for use in
4
a wide range of biological applications such as NMDA antagonist and calcium channel
5
6
7
8
blockers as well as antiangiogenic, anticancer agent (sunitinib 1, Figure 1), the vasopressin
9
10
V2 receptor antagonist (satavaptan 2, Figure 1). Oxindole derivatives show analgesic effect,
1
1
and antimalarial effect (3, Figure 1) also.
Fig 1. Some biologically and pharmaceutically important oxindole derivatives
Efficient methodologies to switch C-H bonds directly to other functionalities remains a key
1
2
challenge to modern synthetic organic chemists. Therefore, new and efficient methods that
could be carried out under milder and eco-friendly conditions always demand special importance
to the field of synthetic organic chemistry. In this context, creation of carbon-carbon (C-C) bonds
via oxidative coupling of carbon-hydrogen (C-H) bonds has gained acute attention in the
1
3
development of different new synthetic methods.
1
4
Since last decade, palladium-catalyzed C-C coupling reaction was shown to be useful for the
1
5a-g
synthesis of oxindole derivatives. Recently, copper-catalysed
oxindole synthesis via
1
5
2
intramolecular oxidative coupling (intramolecular dehydrogenative coupling, IDC) of Csp -H
3
and Csp -H bond have been developed by different groups. Besides IDC, oxindole synthesis via
1
4j, 16, 17
cyclizations of N-arylacrylamides have received special attention in recent years.
1
8
Nowadays transition-metal-free organic transformation is the topic of growing interest. Very
recently transition metal and iodine (molecular and hypervalent)-free synthesis of oxindole have
2

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9
gained more importance due to their low toxicity and more eco-friendly nature. These ground-
breaking efforts led to the synthesis of a broad range of 2-oxindoles. Our interest has focused on
2
0
the development of transition metal-free methodologies for carbon-carbon bond formation. In
this perspective, we planned a transition-metal-free protocol for the intramolecular
2
3
dehydrogenative C(sp )-C(sp ) coupling of β-N-arylamido nitrile and esters to generate
1
5a-d
substituted oxindoles. Pioneering work
in this field by Taylor and Künding showed that Cu-
catalysed IDC reaction follows a radical pathway (SET), hence we envisioned a metal-free
radical source to initiate the reaction. Thus we performed the reaction in presence of di-tert-butyl
o
peroxide (DTBP) in 1,2-dichloroethane (DCE) at 110 C in a sealed tube, without any base and
iodine or metal based oxidant to establish the hypothesis (scheme 1).
Scheme 1. Base-, metal- and iodine-free synthesis of oxindoles.
We started our initial studies with β-N-arylamido nitrile (1a) as the model substrate (Table 1). To
our delight, a 43 % yield of product 2a was obtained using di-tert-butyl peroxide (DTBP) (2
o
mmol) in toluene (Table 1, entry 1) for 10 h at 110 C under nitrogen atmosphere. Lower yields
of 2a were isolated when DTBP was replaced by other peroxides such as tert-butyl
peroxybenzoate (TBPB) and benzoyl peroxide (BPO) (28 % and 32 %, respectively) (Table 1,
entry 2 and 3). Trace amounts of 2a were found in the presence of H O (35 % aqueous solution)
2
2
and tert-butyl hydroperoxide (TBHP, 5.0-6.0 M in decane) (Table 1, entry 4 and 5). Solvent
effect on the model reaction was also examined (Table 1, entries 6-11). Relatively lower yields
were observed when the reactions were performed in actonitrile, ethyl acetate and under solvent-
free condition (14 %, 24 % and 36 % respectively) (Table 1, entries 6-8). No sign of 2a was
found in THF and dioxane, total recovery of starting material (1a) was observed (Table 1, entry 9
3

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and 10). Trace amount of 2a was found in DMF and most of the starting material remained
unchanged (Table 1, entry 11).
a
Table 1. Optimization of Reaction Conditions
Me
CN
O
radical initiator, solvent
CN
O
N
temperature, time
N
Me Me
1
a
Me
2
a
b
Entry
oxidant
Solvent
t (h)
Yield (%)
1
2
3
4
5
6
7
8
9
DTBP
TBPB
BPO
toluene
toluene
toluene
toluene
toluene
10
10
10
10
10
10
10
10
10
10
10
10
15
20
15
15
15
15
15
15
43
28
32
H O
2
trace
trace
14
2
TBHP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
CH CN
3
EtOAc
-
24
36
THF
Dioxane
DMF
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
NR
NR
trace
64
1
1
1
1
1
1
1
1
1
1
2
0
1
2
3
4
5
6
7
8
9
0
73
71
c
83
d
78
e
80
f
78
g
58
h
76
a
Reaction conditions: 1a (1.0 mmol), radical initiator (2.0 mmol) (DTBP = di-tert-butyl
peroxide, TBHP = tert-butyl hydroperoxide, TBPB = tert-butyl peroxybenzoate, BPO =
b
benzoyl peroxide,), solvent (4.0 mL), under nitrogen atmosphere, sealed tube. Isolated
c
d
e
f
yield. NR = No reaction. DTBP (3.0 mmol). DTBP (4.0 mmol). Under air. Reaction
o
g
o
h
performed at 120 C. Reaction performed at 100 C. Reaction performed in 6 mL DCE.
Bold row implies optimized reaction conditions.
4

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A steep rise in yield (64 %) of 2a with full consumption of starting material was observed when
the reaction was performed in 1, 2-dichloroethane (DCE) (Table 1, entry 12). The reaction
afforded 73 % yield of desired product 2a on prolonged heating for 15 h, but gave 71 % yield
when heated for 20 h under the same reaction conditions (Table 1, entry 13 and 14). On
increasing DTBP loading to 3 mmol the yield of 2a increased to 83 % (Table 1, entry 15), while
somewhat lower yield (78 %) was obtained with 4 mmol of DTBP (Table 1, entry 16). Finally,
change of reaction atmosphere from nitrogen to air (Table 1, entry 17) and different reaction
temperature showed that nitrogen and 110 °C were the optimal reaction atmosphere and
temperature (Table 1, entry 18 and 19). With increasing the volume of solvent from 4 mL to 6
mL yield of this coupling reaction decreased slightly (Table 1, entry 20).
Table 2: Substrates scope for intramolecular dehydrogenative coupling (IDC).
Me
Et
N
Et
Me
CN
CN
CN
CN
O
O
O
O
N
N
N
Et
Et
Me
2
b (79%) entry 2
Me
entry 4
2
d (77%)
2
c (81%) entry 3
entry 1
2
a (83%)
iPr
Me
Et
CN
Me
CN
Cl
CN
O2N
Cl
CN
O
O
O
N
O
N
N
N
Me
h (77%) entry 8
Me
g (59%) entry 7
Me
f (71%) entry 6
Me
e (68%) entry 5
2
2
2
2
Et
Bn
nBu
CN
NC
CO Et
2
CN
O
O
O
O
N
N
N
N
Et
l (76%) entry 12
Me
k (63%) entry 11
Et
Me
j (80%) entry 10
2
2
2
2
i (76%) entry 9
Me
Et
Bn
CO Et
Me
2
CO Et
CO Et
2
CO Et
2
2
O
O
O
N
O
N
N
N
Me
Me
o (83%) entry 15
Me
n (73%) entry 14
Et
2p (86%) entry 16
2
2
2
m (81%) entry 13
CN
Me
Me
NC
MeO
PO(OEt)2
3
O
O
O
N
H
N
N
2
s (0 %) entry 19
decomposition of starting material)
Me
r (67 %) entry 18
Me
q (63 %) entry 17
(
2
5
2

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With the optimized reaction conditions in our hand, we explored the scope and validity of this
methodology with different β-N-arylamido nitrile and esters (Table 2). A range of oxindoles (2a-
r, Table 2) were synthesized in moderate to very good yields (59-86 %).
When the phenyl ring was substituted with a chloro group which has a mild electron
withdrawing effect went smoothly in this reaction conditions and gave 68 % and 71 % yield of
2e and 2f respectively (Table 2, entry 5 and 6). A lower yield (59 %) of 2g was isolated when the
phenyl ring was substituted with strong electron withdrawing nitro-group (Table 2, entry-7).
This reaction equally responded with a long chain alkenyl group (2q, 63 %) and phosponate ester
(2r, 67 %). The reaction failed for the substrate with NH functionality (2s, substrate decomposed
under this reaction conditions). However, the substrate 1a' (Scheme 2, Eq. 3) remained
unchanged under this reaction conditions.
Some control experiments were also performed to establish the mechanism, (Scheme 2). The
reaction did not proceed at all in absence of radical initiator DTBP (Scheme 2, Eq. 1). When 2
mmol of 2,2,6,6-tetramethylpiperidine N-oxide (TEMPO) and butylated hydroxytoluene (BHT)
were added under optimized reaction conditions separately and a trace amount of coupling
product (2a) was isolated (Scheme 2, Eq. 2). These facts support that the reaction follows a
radical pathway. The reaction did not occur with substrate 1a' (Scheme 2, Eq. 3), this experiment
suggest that the reaction goes through the generation of a more stable tertiary radical.
Scheme 2. Control experiments
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1
Based on literature and above experimental results, a plausible mechanism is postulated in
Scheme 3. Initially, on heating, homolytic cleavage of DTBP produces tert-butoxy radical A.
This tert-butoxy radical (A) abstracts one H atom from α-position of nitrile or ester group of β-
N-arylamido nitrile or esters (1) to give B, which then produces resonance stabilized aryl radical
C' which eliminates one hydrogen radical and rearomatization leads to the final product 2.
t
(
BuO)2
heat
BuO (A)
Me
R
H
Me
R
Me
R
H
t
O
N
Me
N
O
N
B)
Me
O
1
(C)
(
Me
R = CN, CO Et
2
Me
Me
R
R
H
-
H
O
O
N
N
2
C' Me
Me
Scheme 3. Plausible mechanismistic pathway
In conclusion, we have developed a di-tert-butyl peroxide (DTBP) promoted transition-metal-
2
free, atom economical and ecofriendly intramolecular dehydrogenative coupling (IDC) of Csp -
3
H and Csp -H bond. This protocol provides a simple approach for the synthesis of a variety of
oxindoles. This methodology avoided base and hazardous iodine reagent unlike other metal-free
oxindole synthesis strategy. Further development of transition-metal-free strategy for the
synthesis of bioactive heterocycles is being explored in our laboratory.
Acknowledgement:
We thank DST (FIST, PURSE New Delhi, Govt. of India), CSIR (New Delhi, Govt. of India)
and University of Kalyani for financial assistance.
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