DIAD as a promoter for cyanomethylation of alkenes by Cu catalysis

Article abstract of DOI:10.1016/j.tetlet.2015.01.175

A novel and practical DIAD-promoted oxidative cyanomethylation of alkenes with acetonitrile by copper catalysis was developed, in which oxindoles bearing a nitrile moiety at the 3-position were synthesized efficiently via dual C-H bond cleavage of an arene and acetonitrile. This protocol demonstrated that DIAD served as a new promoter instead of commonly encountered AgF or base for the C_sp3-H functionalization of acetonitrile for the first time.

Full text of DOI:10.1016/j.tetlet.2015.01.175

Tetrahedron Letters 56 (2015) 1423–1426
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
DIAD as a promoter for cyanomethylation of alkenes by Cu catalysis
a
a
a
b,
Shi Tang a, , Shu-Hua Li , Zhi-Hao Li , Dong Zhou , Rui-Long Sheng
⇑
⇑
a Department College of Chemistry and Chemical Engineering, Jishou University, Jishou 416000, China
b Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel and practical DIAD-promoted oxidative cyanomethylation of alkenes with acetonitrile by copper
catalysis was developed, in which oxindoles bearing a nitrile moiety at the 3-position were synthesized
efficiently via dual C–H bond cleavage of an arene and acetonitrile. This protocol demonstrated that DIAD
served as a new promoter instead of commonly encountered AgF or base for the Csp3–H functionalization
of acetonitrile for the first time.
Received 25 December 2014
Revised 19 January 2015
Accepted 27 January 2015
Available online 3 February 2015
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
DIAD
Cu catalysis
C–H cyclization
Alkene
Cyanomethylation
Introduction
AgF to prepare cyano-substituted oxindoles (Fig. 1, Eq. 1). Just
recently, You7c developed an elegant copper-catalyzed cyanome-
Over the past years, the C–H bond activation of acetonitrile by a
stoichiometric amount of transition-metal (e.g., Ir, Rh, Ni, Fe, etc.)
has been well documented.1 Moreover, recent years also witnessed
thylation of alkene using a catalytic combination of CuCl with
tBuOOtBu. Regardless of substantial advances in this field, apparent
limitation remains: either the combination of the Pd catalyst with
stoichiometric AgF or an elevated temperature (P120 °C) possibly
resulting in side-reactions6k was required for high reaction effi-
ciency. Diisopropyl azodicarboxylate (DIAD) represents a kind of
environmental-friendly and readily available aliphatic azo com-
pound, which could be act as a promoter in some transformations
(e.g., Mitsunobu reaction). As a continuation of our interest in
oxindole synthesis,8 we herein want to demonstrate a novel
DIAD-promoted cyanomethylation of N-arylacrylamides with
actetonitrile via dual C–H bond cleavage by Cu catalysis, which
substantial advances on catalytic a-C–H functionalization of aceto-
nitriles.2 Nevertheless, success on catalytic C–H functionalization of
acetonitrile (e.g., cyanomethylation) is limited mainly to these cases
requiring a strong base for the deprotonation (pKa (Me3CN) ꢀ 31.3)
or stoichiometric Lewis acid AgF as a promoter. Therefore, the
development of new environmental-friendly promoters instead of
commonly encountered strong bases or Ag reagents for the C–H
functionalization of acetonitrile is still of high interest. Due to
economic and environmental benefits, copper has recently gained
much popularity as a catalyst in oxidative cross-dehydrogenative
coupling and difunctionalization of alkenes.3,4 However, base-
free copper-catalyzed oxidative cyanomethylation of alkenes via
the Csp3–H functionalization of acetonitrile was still rarely reported.2
It had been known that oxindole is a common structural motif in
pharmaceutical agents and natural products.5,6 Given that cyano-
containing molecules exhibit important functions in materials and
pharmaceuticals, the incorporation of the cyano group into the
oxindole scaffold through base-free metal-catalyzed cyanomethy-
lation of alkene with acetonitrile is of great interest, which is still
rare.7 In earlier studies, Liu7a reported a pioneering Pd-catalyzed
oxidative alkylarylation of alkene with the aid of stoichiometric
[7a]
Liu's work
a)
(Pd/Ag catalysis)
CN
R3
Pd(OAc)2
R3
2,2'-bipyrimidine
R1
R1
(1)
CN
O
+
PhI(OCOtBu)2
AgF (4 eq.)
H
N
N
O
R2
R2
c) This work ( Cu catalysis using DIAD as promoter)
R3
CN
(2)
DIAD
CuI, DTBP
95 o
R3
CN
R3
R1
CuI, DTBP
95 oC
R1
O
N
O
R1
O
R2
N
C
+
N
R2
R2
< 10%
CN
H
⇑
Corresponding authors. Tel./fax: +86 743 856 3911.
E-mail address: stang@jsu.edu.cn (S. Tang).
Figure 1. Base-free oxidative cyanomethylation of alkene to form oxindoles.
http://dx.doi.org/10.1016/j.tetlet.2015.01.175
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

1424
S. Tang et al. / Tetrahedron Letters 56 (2015) 1423–1426
could allow the preparation of cyano-containing oxindoles via cya-
nomethylation reaction using DIAD as a novel promoter instead of
commonly encountered AgF under mild reaction conditions (at
95 °C) (Eq. 2).
inconsistent with previously reported reactions that occur through
Pd-catalyzed oxidative cyanomethylation of N-arylacrylamides
(entries 16–18).7a Lowering the reaction temperature to 80 °C
retarded the reaction to some extent, and afforded 3a in 71% yield
(entry 19). Sequential screenings revealed that a 20 mol % loading
of CuI was beneficial to achieve high reaction efficiency (entries
20 and 21). The optimal amount of DIAD was also examined, and
using 2.0 equiv of DIAD was beneficial to obtain good yield (entries
22 and 23).
Results and discussion
In our initial studies, the attempt to introduce the ethoxylcar-
bonyl moiety into oxindole scaffold by using DEAD did not afford
the desired product 46a,9 efficiently (<5% yield by GC/MS) but led
to the oxindole 3a involving Csp3–H functionalization of acetonitrile
(Table 1). With these results in mind, we wondered whether the
DEAD really involved this cyclization via dual C–H bond cleavage.
Therefore, the reaction between N-methyl-N-phenylacrylamide 1a
and acetonitrile was initially employed as the model reaction to
exploit optimal reaction conditions. After brief screenings,
the azodicarboxylate reagents were observed to be crucial for the
formation of the desired cyano-substituted oxindole 3a, and the
reaction failed to give 3a at current temperature conditions
(95 °C) in the absence of the azodicarboxylate reagent (entries
1–4).10 Notably, the reaction using DIAD (2 equiv) afforded 3a in
85% yield via the catalytic combination of CuI (20 mol %) and DTBP
(4 equiv) at 90 °C. Among screening various inexpensive copper(I)
and iron(II) salts as the catalyst, use of metal catalyst was beneficial
and CuI proved to be the best choice (entries 6–10). Other oxidants
including TBHP, TBPB, and PhI(OAc)2, as well as the absence of
oxidant, resulted in unparalleled yields (entries 12–15). Note that
the use of N-ligands retarded the reaction somewhat, which is
After the standard reaction conditions to hand, we set out to
investigate the scope of N-arylacrylamides in the cyanomethyla-
tion reactions (Scheme 1). Initial screening revealed that N-substit-
uents of acrylamides had an obvious effect on the reaction. For
example, N-arylarylamides with a benzyl or ethyl group on the
N-atom were found to be compatible with the reaction conditions,
whereas unprotected N-arylacrylamide (R2 = H) was less efficient
in the cyclization (3b, 3c, and 3d). Next, we embarked upon inves-
tigating the substitution effects of N-aryl moiety in the reaction.
Delightedly, a wide variety of substituents, such as Me, MeO, Cl,
F, CF3, and CO2Et, at the 4-, 3-, or 2-position of the aromatic ring
generally displayed good reactivity irrespective of the steric and
electronic character of the substituent groups (3e–n). Note that
the reactive order is: poor electron-withdrawing and electron
donating groups>strong electron-withdrawing groups, which is
compatible with previously reported reactions that occur through
cascade radical addition/C–H cyclization of N-arylacrylamides.6m
The reaction of meta- methyl substituted N-arylacrylamide affor-
ded a mixture of two regioselective isomers 3k and 3k0, whereas
3,4-dichloro substituted N-arylacrylamide regioselectively under-
went the C–H cyclization and gave the oxindole isomer 3o specif-
ically. While phenyl groups at the 2-position of the acrylamide
moiety were compatible with the optimal conditions to afford
3p, the mono-substituted olefins (R3 = H) were inefficient for the
cyclization (3q).
Table 1
Optimization of reaction conditionsa
CN
Me
Metal/[O]
RCO2-N=N-CO2R
CO2R
O
Me
+
O
N
O
Of more importance, this protocol could also be utilized to pre-
pare complex and pharmaceutically interesting oxindoles. For
CH3CN
N
Me
N
Me
Me
1a
3a
Oxidant
4
Entry
Metal (mol %)
Additive
Yield of 3ab
CN
Me
1
2
3
4
6
7
8
9
10
11
12
13
14
15
16c
17d
18e
19f
20
21
22g
23h
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuBr (20)
CuCl (20)
FeSO4ꢁ7H2O (20)
FeS (20)
FeBr2 (20)
None
CuI
CuI
CuI
CuI
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuI (10)
CuI (30)
CuI (20)
CuI (20)
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
TBHP
TBHP
TBPB
PhI(OAc)2
none
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
None
DMAD
DEAD
DIAD
DIAD
DIAD
DIAD
DIAD
DIAD
DIAD
DIAD
DIAD
DIAD
DIAD
DIAD
DIAD
DIAD
DIAD
DIAD
DIAD
DIAD
DIAD
<10
35
43
85
77
conditions[a]
R1
R1
+
O
O
H
CN
N
N
Me
Me
3
1
CN
Me
CN
CN
73
Me
Me
R
<10
<10
Trace
Trace
36
O
O
O
N
N
N
Et
Me
Me
R = Et (3b), 75%
R = Me (3e), 87%
85%
Bn (3c), 63%
MeO (3f),83%
F (3g), 77%
H
(3d), < 5%
66
CN
Cl (3h), 81%
CF3 (3i), 71%
EtO2C (3j), 52%
CN
Trace
Trace
32
24
73
71
56
82
46
Me
Me
Me
O
O
+
Me
N
N
Me
Me
3k + 3k', 82%
CN
Me
Me
Cl
CN
CN
Me
Me
O
O
O
N
Me
N
N
17
Me
Me
Me
OMe
3m, 79%
a
3n, 63%
Reaction Conditions: 1a (0.5 mmol), metal (10–30 mol %), oxidant (4 equiv),
3l, 73%
RO2C–N@N–CO2R (2 equiv), and CH3CN (2 mL) at 95 °C for 20 h. DTBP = Di-tert-
butyl peroxide, TBHP = tert-butyl hydrogen peroxide (70% aqueous solution).
TBPB = tert-butylperoxy benzonate.
CN
CN
Me
CN
H
Cl
Cl
b
Yield of the isolated product.
0.2 Equiv of 1,10-phenanthroline was added.
0.2 Equiv of 1,4-diazabicyclo[2.2.2]octane (DABCO) was added.
0.2 Equiv of 2,20-bipyrimidine was added.
At 80 °C.
DIAD (1 equiv) is added.
DIAD (0.5 equiv) is added.
O
O
c
N
O
N
Me
51%
Me
3q, < 5%
d
N
Me
3o,
e
3p, 77%
f
g
a
Scheme 1. Scope of N-arylacrylamides. Reaction conditions: 1 (0.5 mmol), DIAD
(2 equiv), DTBP (4 equiv), CuI (20 mol %), and CH3CN (2 mL) at 95 °C for 20 h.
h

S. Tang et al. / Tetrahedron Letters 56 (2015) 1423–1426
1425
O
example, the polycyclic oxindole 3r could be synthesized through
the cyclization of N-aryl acrylamides 1r (Eq. 3). In addition, the
N-pyridyl acrylamides 1s was also a viable substrate for the reac-
tion, and afforded synthetically important heterocycle 3s (Eq. 4).
Next, we examined the compatibility of other nitriles as C–H reac-
tants under the standard conditions (Eq. 5). n-Butyl nitrile was
observed to be viable C–H reactants to involve the cyanomethyla-
tion to N-methyl-N-phenylacrylamide 1a, but the yield deceasing
to some extent (3t).
N2
N
OiPr
O
CH3CN
iPrO
N
iPrO
CH2CN
O
DIAD
tBuOOtBu
1a
CN
Cu(II)
Cu(I)
H
CN
O
3a
N
N
O
A
B
CuI (20 mol%)
DTBP (4 equiv.)
(3)
Scheme 3. Proposed mechanism for the formation of oxindoles.
DIAD (2 equiv.)
N
+
H
CN
95 oC, 20 h
O
Me
3r
O
NC
(62%)
1r
Based on these experimental results and previously reported
mechanism,6,7 a possible mechanism was proposed as outlined in
Scheme 3 for the cyanomethylation of N-arylacrylamides. Firstly,
CN
CuI (20 mol%)
Me
Me
N
DTBP (4 equiv.)
(4)
DIAD (2 equiv.)
+
H
O
CN
i
DIAD undergoes homolysis to generate radical PrO2CÅ under heat,
95 oC, 20 h
N
O
N
N
Me
i
and then PrO2CÅ abstracts a hydrogen of acetonitrile to generate
1s
Å
ÅCH3CN.12 The resulting CH3CN adds onto C@C bond of 1a to give
3s
(49%)
intermediate A, followed by cyclization of intermediate A generat-
ing B. Finally, the abstraction of an aryl hydrogen by [CuII]-species
produced by the oxidative reaction13 of the [CuI] salt with tBuOOtBu
takes place to give the corresponding oxindole 3a.
CuI (20 mol%)
Me
CN
O
DTBP (4equiv.)
DIAD a(2equiv.)
95 oC, 20 h
+
O
Bu
(5)
CN
N
N
Me
Me
1a
3t
43%, d.r. = 1:1.1( by 1H NMR)
Conclusion
In summary, we have developed a novel DIAD-promoted oxida-
tive radical cyanomethylation of activated alkenes by inexpensive
CuI, which represents one of the most practical protocols for the
preparation of oxindoles with an alkyl nitrile moiety to date. Inter-
estingly, this protocol discovered that DIAD could be used as a
novel promoter instead of commonly encountered Ag reagents or
To investigate the mechanism of the cascade addition/Csp2–H
cyclization process, the inter- and intra-molecular kinetic isotope
control experiments were performed (Scheme 2). No kinetic isoto-
pic effect (the intramolecular KH/KD = 1.0 and intermolecular KH/
KD = 1.0) was observed in the C–H cyclization process, which sug-
gested that either the SEAr mechanism or free radical mechanism
was involved.11 When the reaction of 1a was conducted in a 1:1
mixture of CH3CN/CD3CN using DIAD, alarge primary isotope effect
(KH/KD = 4.5) was obtained (Eq. 6), which suggested that a C(sp3)–H
bond cleavage of acetonitrile contributed to the rate-determining
step. Notably, the free radical mechanism was supported by the
control experiment: a stoichiometric amount of TEMPO (4 equiv),
a well-known radical scavenger, was used in the cyanomethylation
reaction of 1a with acetonitrile, and the formation of the corre-
sponding oxindole 3a was suppressed seriously (Eq. 7).
a strong base for the
a-Csp3–H functionalization of acetonitrile
for the first time, which would open a new door for the application
of the azo compound in the C–H functionalization of nitriles. The
use of environmental-friendly and readily available azo com-
pounds and inexpensive copper as the catalyst, as well as the sim-
plicity of preparation and handling, make this protocol a highly
attractive complement for the construction of pharmaceutically
interesting cyano-substituted oxindoles. The detailed mechanism
and application of the novel reaction to more complex targets
are currently under investigation in our laboratory.
Acknowledgments
Intermolecular KIE Experiment
Intramolecular KIE Experiment
D
Financial support from the National Natural Science Foundation
of China (Nos. 21462017, 21372251), China Postdoctoral Science
Foundation funded project (No. 2014M560649), the Scientific
Research Fund of Hunan Provincial Education Department (No.
13B094), The fund of Science and Technology Innovation and
Entrepreneur for Hunan Young Talents, and the Project Sponsored
by the Scientific Research Foundation for the Returned Overseas
Chinese Scholars, State Education Ministry.
D
D
D
+
N
O
N
O
N
O
D
D
1a
[D1]-1a
[D5]-1a
73% yield (24 h),
K
H / KD = 1.0
31% yield (2 h), KH / KD = 1.0
CN
CN
D
CuI (20 mol%)
DIAD (2 equiv.),
DTBP (4 equiv.)
Me
Me
D
O
+
O
(6)
N
O
CH3CN:CD3CN(1:1)
N
N
95 oC, 20 h
References and notes
76% yield, KH /KD
=
1a
3a
-3a
[D2]
4.5
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12. In view of GC/MS results, DIAD was completely consumed after the reaction
proceeds under standard conditions over 20 h, and some amount of side-
products such as iPrOCOOiPr, iPrOCOOtBu was also observed. These results
could suggest that DIAD is subject to decompose to form radical iPrO2CÅ which
is responsible for the generation of radical ÅCH3CN under current reaction
conditions.
13. (a) Gephart, R. T., III; McMullin, C. L.; Sapiezynski, N. G.; Jang, E. S.; Aguila, M. J.
B.; Cundari, T. R.; Warren, T. H. J. Am. Chem. Soc. 2012, 134, 17350–17353; (b)
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