A facile access for the C-N bond formation by transition metal-free oxidative coupling of benzylic C-H bonds and amides

Article abstract of DOI:10.1007/s11426-015-5381-2

Using 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) as the oxidant, we communicate an efficient oxidative C-N coupling of benzylic C-H bonds with amides to afford a series of amination products in good yields. A wide range of functional groups as well as various sulfonamides and carboxamides are well tolerated. Moreover, this reaction involves both the challenging C-H functionalization and C-N bond formation.

Full text of DOI:10.1007/s11426-015-5381-2

SCIENCE CHINA
Chemistry
June 2015 Vol.58 No.6: 1–6
doi: 10.1007/s11426-015-5381-2
• ARTICLES •
· SPECIAL ISSUE · CH bond activation
doi: 10.1007/s11426-015-5381-2
A facile access for the C–N bond formation by transition
metal-free oxidative coupling of benzylic C–H bonds and amides
Jie Liu1, Heng Zhang1, Hong Yi1, Chao Liu1* & Aiwen Lei1,2*
1College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China
2National Research Centre for Carbohydrate Synthesis, Jiangxi Normal University, Nanchang 330022, China
Received January 14, 2015; accepted January 27, 2015
Using 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) as the oxidant, we communicate an efficient oxidative C–N coupling
of benzylic C–H bonds with amides to afford a series of amination products in good yields. A wide range of functional groups
as well as various sulfonamides and carboxamides are well tolerated. Moreover, this reaction involves both the challenging
C–H functionalization and C–N bond formation.
oxidative coupling, C–N bond formation, amination, C–H functionalization
aliphatic C–H amination reactions. Due to the presence of
1 Introduction
transition- metal impurities in the final products and/or the
costly metal catalysts, a strategy involving metal-free pro-
tocol would be an attractive approach for introducing nitro-
gen functional groups.
The direct N-functionalization of C–H bonds has become an
attractive tool for the construction of valuable nitrogen-
containing compounds in recent decades [1–7]. Cross-
dehydrogenative coupling (CDC) reactions, pioneered by Li
and co-workers [8,9], indicate a new path to construct C–C
and C-heteroatom bonds. This approach has the advantage
of avoiding the pre-functionalization of the simple sub-
strates to the corresponding organic (pseudo) halides or or-
ganometallic species, which is an atom-economic and envi-
ronmentally benign strategy for C–N bond formation. These
skeletons of containing C–N bond compounds existed in
many natural products and drug molecules (Scheme 1). Par-
ticularly, the amination of Csp3–H bonds has led to signifi-
cant results since the discovery of metal nitrenoids insertion
into the C–H bonds [10–13]. Various transition-metal com-
plexes including rhodium [14–17], ruthenium [18–20], pal-
ladium [21–25], manganese [26–28], silver [29,30], gold
[31], copper [32–39], cobalt [40–42] and iron [43,44] as
catalysts or mediators are essential to the success of these
Recently, transition-metal-free coupling protocols for
biaryl synthesis have drawn widespread attention [45–50].
However, to the best of our knowledge, the metal-free direct
oxidative C–H amination of Csp3–H bonds was mostly lim-
ited via hypervalent halogen compounds [51–53]. In spite of
the significant progress offered by these methods, disad-
vantages such as the usage of unstable hypervalent halogen
reagents and the generation of ArX as the byproducts still
remain prominent concern. As a result, the development of
other complementary methods is strongly desired. Recently,
Scheme 1 Representative natural products and drug molecules.
*Corresponding authors (email: chao.liu@whu.edu.cn; aiwenlei@whu.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2015
chem.scichina.com link.springer.com

2
Liu et al. Sci China Chem June (2015) Vol.58 No.6
Bolm’s group [43] has developed an efficient oxidative sul-
foximines with diarylmethanes to N-alkylated sulfoximines
using iron as catalyst (Eq. (1) in Scheme 2). Though good
selectivity and relative mild condition was presented, it is
still necessary to seek wider substrate scope and more sim-
ple condition to realize C–N bond formation. Therefore,
following our persistent interest in metal-free oxidative
coupling, we communicate a 2,3-dichloro-5,6-dicyano-p-
benzoquinone (DDQ) mediated intermolecular benzylic
C–H amination reaction (Eq. (2) in Scheme 2).
cooled to room temperature and filtered with ethyl acetate
(2×10 mL) to remove the solid residue. The combined solu-
tion was concentrated by the rotary evaporator, and then
purified by column chromatography on silica gel using pe-
troleum ether/ethyl acetate as eluent to give the desired
1
product. Yield: 82%. H NMR (300 MHz, CDCl3):  7.57–
7.55 (m, 2H), 7.26–7.20 (m, 6H), 7.15–7.10 (m, 6H), 5.56
(d, J=6.9 Hz, 1H), 5.10 (d, J=6.6 Hz, 1H), 2.37 (s, 3H); 13
C
NMR (75 MHz, CDCl3):  140.8, 129.6, 128.8, 127.8, 127.6,
127.5, 61.6, 21.8.
2.3 Procedure for radical trapping experiments
2 Experimental
A 20 mL Schlenk tube equipped with a stir-bar was charged
with diphenylmethanes (0.5 mmol), TsNH2 (1.0 mmol),
DDQ (0.6 mmol), 2,2,6,6-tetramethylpiperidinooxy (TEM-
PO) or 1,1-dibenzenethene (1.0 mmol) and 200.0 mg 3 Å
molecular sieve. The reaction tube was purged with nitrogen.
4.0 mL DCE was then added to the reaction tube via a sy-
ringe. The Schlenk tube was placed in an oil-bath and heat-
ed to 100 °C for 24 h. Then it was quenched. The conver-
sion and yield were determined by GC.
2.1 General experimental section
All manipulations were carried out using standard Schlenk
techniques. Unless otherwise stated, analytical grade sol-
vents and commercially available reagents were used as
received. Thin layer chromatography (TLC) employed glass
0.25 mm silica gel plates. Flash chromatography columns
were packed with 200–300 mesh silica gel in petroleum
ether (bp. 60–90 °C). Gradient flash chromatography was
conducted eluting with a continuous gradient from petro-
leum ether to the ethyl acetate, which are listed below as
volume/volume ratios. All new compounds were character-
2.4 Procedure for kinetic isotopic effect experiments
1
ized by H NMR, 13C NMR and HRMS. The known com-
2.4.1 Procedure for intermolecular kinetic isotopic effect
1
1
pounds were characterized by H NMR. The H and 13C
NMR spectra were recorded on a Varian Mercury 300, 400
or 600 MHz NMR spectrometer (USA). The chemical shifts
() were given in part per million relative to internal tetra-
A 20 mL Schlenk tube equipped with a stir-bar was charged
with di-deuterated diphenylmethane (0.25 mmol), diphe-
nylmethane (0.25 mmol), TsNH2 (1.0 mmol), DDQ (0.6
mmol) and 200.0 mg 3 Å molecular sieve. The reaction tube
was purged with nitrogen. 4.0 mL DCE was then added to
the reaction tube via a syringe. The Schlenk tube was placed
in an oil-bath and heated to 100 °C for 1.5 h. The reaction
mixture was then cooled to room temperature and filtered
with ethyl acetate (2×10 mL) to remove the solid residue.
The combined solution was concentrated by the rotary
evaporator and then purified by column chromatography on
silica gel using petroleum ether/ethyl acetate as eluent to
given the desired product. The reaction conversion is 19%.
1
methylsilane (TMS, 0 ppm for H), CDCl3 (77.3 ppm for
13C) and (CD3)2SO (39.5 ppm for 13C).
2.2 Procedure for the transition metal-free oxidative
coupling of of diphenylmethanes with TsNH2
A 20 mL Schlenk tube equipped with a stir-bar was charged
with substituted diphenylmethanes (0.5 mmol), TsNH2 (1.0
mmol), DDQ (0.6 mmol) and 200.0 mg 3 Å molecular sieve.
The reaction tube was purged with nitrogen. 4 mL 1,2-di-
chloroethane (DCE) was then added to the reaction tube via
a syringe. The Schlenk tube was placed in an oil-bath and
heated to 100 °C for 24 h. The reaction mixture was then
2.4.2 Procedure for intramolecular kinetic isotopic effect
A 20 mL Schlenk tube equipped with a stir-bar was charged
with mono-deuterated diphenylmethane (0.5 mmol), TsNH2
(1.0 mmol), DDQ (0.6 mmol) and 200.0 mg 3 Å molecular
sieve. The reaction tube was purged with nitrogen. 4.0 mL
DCE was then added to the reaction tube via a syringe. The
Schlenk tube was placed in an oil-bath and heated to 100 °C
for 1.5 h. The reaction mixture was then cooled to room
temperature and filtered with ethyl acetate (2×10 mL) to
remove the solid residue. The combined solution was con-
centrated by the rotary evaporator, and then purified by
column chromatography on silica gel using petroleum
ether/ethyl acetate as eluent to given the desired product.
The reaction conversion is 24%.
Scheme 2 Oxidative C–N coupling reaction of benzylic C–H bonds.

Liu et al. Sci China Chem June (2015) Vol.58 No.6
3
Table 2 Transition metal-free oxidative C–N coupling of substituted
diphenylmethanes 1 with 2a a)
3 Results and discussion
3.1 Optimizing the conditions
Initially, the C–H amination of diphenylmethane (1a) with
4-methylbenzenesulfonamide (2a) was selected as model
reaction. After optimizing various reaction parameters, we
could obtain 82% yield of amination product (3aa) with 3 Å
molecular sieve as additive in the solvent of DCE using
DDQ as the oxidant (Table 1, Entry 1). Moreover, the reac-
tion yields decreased slightly in the presence of metal cata-
lysts (Entries 2 and 3). The use of other oxidants as hydro-
gen acceptors led to lower yields (Entry 4) and even shut
down the reaction completely (Entries 5–7). Additionally,
the solvents had obvious effect on the efficiency of this re-
action. Non-polar solvents afforded the product 3a in mod-
erate to good yields (Entries 8 and 9), while polar solvents
inhibited this transformation (Entries 10 and 11). We also
found that the molecular sieve (MS) improved the reaction
selectivity. Small amount of benzophenone would be gener-
ated as the by-product without the molecular sieve as the
additive (Entries 1 and 12) (for more detailed mechanism
discussion, please see Supporting Information online).
3.2 Substrates scope
With the above-optimized conditions in hand, we firstly
explored the substrate scope of substituted diphenylmeth-
anes 1 and received moderate to good yields of the corres-
ponding amination products (Table 2). As shown in Table 2,
the reaction condition is successfully amenable to a wide
range of substrates: no matter they have electron-rich
(Entries 2–5) or electron-deficient functional groups
a) Reaction conditions: 1 (0.5 mmol), 2a (1 mmol), DDQ (0.6 mmol)
and 3 Å MS (200 mg) in 4 mL of DCE for 24 h; b) isolated yields.
(Entries 6 and 7) in diversified positions. Moreover, there
was no reaction for 1-benzyl-4-nitrobenzene 1h indicating
that it is probably a radical process because this reaction
was completely inhibited by the nitro group (Entry 8). It is
noteworthy that the substrates bearing steric-hindered
substituted groups underwent this transformation smoothly
(Entries 5 and 9). In addition, di-substituted diphenylme-
thanes showed good reactivity and delivered excellent
yields (Entries 10 and 11). However, lower yield was
obtained for the amination reaction of ethylbenzene (Entry
12).
Next, a series of sulfonamide and carboxamide substrates
2 were also subjected to the standard reaction conditions
and the results are shown in Table 3. We were pleased to
find that both primary and secondary sulfonamides
underwent this transformation smoothly (Entries 1–4) as
well as steric-hindered secondary sulfonamides (Entries 3,
4). In addition, we also tried a variety of substituted carbo-
xamide (Entries 5–12). Various functional groups were
tolerated as well, including aryl-OMe, Me, Br and tBu
groups. To our delight, the electron deficient (Entry 10) and
more steric hindered carboxamide (Entry 11) also gave
satisfying yields while electron-rich substrates gave good to
excellent results (Entries 5–9). Particularly, benzylic
Table 1 Impact of reaction parameters on the efficiency of DDQ medi-
ated oxidative C–N coupling of 1a with 2a a)
Entry
Catalyst
Oxidant
DDQ
Solvent
DCE
Yield b)
82%
77%
76%
70%
0
1
2
–
10 mol% FeCl2
DDQ
DCE
3
10 mol% CuCl
DDQ
DCE
4
–
–
–
–
–
–
–
–
–
NBS
DCE
5
BQ
DCE
6
tBuOOH
tBuOOtBu
DDQ
DCE
0
7
DCE
0
8
PhCl
80%
56%
20%
0
9
DDQ
toluene
EtOAc
DMF
DCE
10
11
12 c)
DDQ
DDQ
DDQ
69%
a) Reaction conditions: 1a (0.5 mmol), 2a (1 mmol), oxidant (0.6 mmol)
and 3 Å MS (200 mg) in 4 mL of solvent for 24 h; b) isolated yields; c)
standard conditions in the absence of 3 Å MS.

4
Liu et al. Sci China Chem June (2015) Vol.58 No.6
Table 3 Transition metal-free oxidative C–N coupling of 1a with
sulfonamides and carboxamides 2 a)
1,1-dibenzenethene. These results gave us an indication that
this transformation probably involved radical intermediates.
In addition, experiments focusing on the kinetic isotopic
effect were also carried out to gain additional insights into
this reaction (Scheme 5). The intramolecular and intermo-
lecular KH/KD ratios were 3.2:1 and 1.7:1 respectively sug-
gesting that C–H bond cleavage could be the rate-deter-
mining step.
3.4 Proposed pathway
The proposed mechanism is shown in Scheme 6. The reac-
tion is initiated by single electron transfer (SET) oxidation
to form the benzyl radical, which could be further oxidized
to the benzyl cation and DDQH- anion. A subsequent ab-
straction of the proton of amides 2a by DDQH generated
the C–N coupled product 3a and reduced hydroquinone
DDQH2.
4 Conclusions
In summary, we have developed a simple and efficient
DDQ mediated oxidative amination of benzylic C–H bonds.
This transformation has accomplished both C–H function-
alization and C–N bond formation. A wide range of
Scheme 3 Amidation Reactions of diphenylmethane 1a with electron-
deficient anilines.
a) Reaction conditions: 1a (0.5 mmol), 2 (1 mmol), DDQ (0.6 mmol)
and 3 Å MS (200 mg) in 4 mL of DCE for 24 h; b) isolated yields.
carboxamide 2l and acetanilide 2m also afforded moderate
yields (Entries 11–13).
Furthermore, an interesting result was observed when
electron-deficient aniline derivatives were employed as ni-
trogen nucleophile. Diphenylmethyl imines 4 were obtained
instead of desired C–N coupling products, which presuma-
bly generated from the further oxidation of the active ami-
nation products, benzyl amines (Scheme 3).
Scheme 4 Radical trapping experiments.
3.3 Mechanistic insights
In order to confirm the participation of radical intermediate
in this oxidative C–N coupling reaction, we introduced rad-
ical scavengers, such as TEMPO and 1,1-dibenzenethene in
the reaction (Scheme 4). It was found that 2 equiv. of
TEMPO completely shut down the reaction while the reac-
tion conversion was reduced to 13% in the presence of
Scheme 5 Kinetic isotopic effect experiments.

Liu et al. Sci China Chem June (2015) Vol.58 No.6
5
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method to construct C–N bond under transition metal-free
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Supporting information
The supporting information is available online at chem.scichina.com and
link.springer.com/journal/11426. The supporting materials are published as
submitted, without typesetting or editing. The responsibility for scientific
accuracy and content remains entirely with the authors.
This work was supported by the National Basic Research Program of Chi-
na (2011CB808600, 2012CB725302), the National Natural Science Foun-
dation of China (21390400, 21272180, 21302148), the Research Fund for
the Doctoral Program of Higher Education of China (20120141130002)
and the Program for Changjiang Scholars and Innovative Research Team
in University (IRT1030). The Program of Introducing Talents of Discipline
to Universities of China (111 Program) is also appreciated.
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