Synergistic catalysis of Cu+/Cu0 for efficient and selective N-methylation of nitroarenes with para-formaldehyde

Article abstract of DOI:10.1016/j.jcat.2019.06.022

In this paper, an inexpensive heterogeneous copper nanoparticles catalyst derived from CuAl-layered double hydroxide via an in situ topotactic transformation process was developed. Cu nanoparticles with uniform size were homogeneously dispersed on amorphous Al₂O₃ with strong metal-support interaction. Characterization results reveals that the Cu⁰ and Cu⁺ were simultaneously formed with Cu⁺ species as the dominant sites on the surface during the reduction process. The resultant catalyst Cu/Al₂O₃ demonstrates high catalytic activity, selectivity and durability for the reductive N-methylation of easily available nitroarenes in a cost-efficient, environmentally friendly and cascade manner. A broad spectrum of nitroarenes could be efficiently N-methylated to their corresponding N,N-dimethyl amines with good compatibility of various functional groups. The protocol is also applicable for the late-stage functionalization of biologically and pharmaceutically active nitro molecules. A structure-function relationship discloses that Cu⁰ and Cu⁺ sites on the surface pronouncedly boosts the reaction efficiency in a synergistic manner, in which Cu⁰ could facilitate H₂ production and N-methylation of anilines, while Cu⁺ is considerably more active and participates in the overall process of the selective N-methylation of nitroarenes. Moreover, the catalyst also showed a strong stability and could be easily separated for successive reuses without an appreciable loss in activity and selectivity.

Full text of DOI:10.1016/j.jcat.2019.06.022

Journal of Catalysis 375 (2019) 304–313
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
+
0
Synergistic catalysis of Cu /Cu for efficient and selective N-methylation
of nitroarenes with para-formaldehyde
a
a
b
a,
⇑
Xiaosu Dong , Zhaozhan Wang , Youzhu Yuan , Yong Yang
a
Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, PR China
State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, College of Chemistry
b
and Chemical Engineering, Xiamen University, Xiamen 361005, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, an inexpensive heterogeneous copper nanoparticles catalyst derived from CuAl-layered
Received 26 February 2019
Revised 9 June 2019
Accepted 11 June 2019
double hydroxide via an in situ topotactic transformation process was developed. Cu nanoparticles with
2 3
uniform size were homogeneously dispersed on amorphous Al O with strong metal-support interaction.
0
+
+
Characterization results reveals that the Cu and Cu were simultaneously formed with Cu species as the
dominant sites on the surface during the reduction process. The resultant catalyst Cu/Al demonstrates
2 3
O
high catalytic activity, selectivity and durability for the reductive N-methylation of easily available
nitroarenes in a cost-efficient, environmentally friendly and cascade manner. A broad spectrum of
nitroarenes could be efficiently N-methylated to their corresponding N,N-dimethyl amines with good
compatibility of various functional groups. The protocol is also applicable for the late-stage functionaliza-
tion of biologically and pharmaceutically active nitro molecules. A structure-function relationship dis-
Keywords:
N-methylamines
Nitroarenes
CuAl-layered double hydroxide
Cu nanoparticles
Para-formaldehyde
0
+
closes that Cu and Cu sites on the surface pronouncedly boosts the reaction efficiency in a
0
2
synergistic manner, in which Cu could facilitate H production and N-methylation of anilines, while
+
Cu is considerably more active and participates in the overall process of the selective N-methylation
of nitroarenes. Moreover, the catalyst also showed a strong stability and could be easily separated for suc-
cessive reuses without an appreciable loss in activity and selectivity.
Ó 2019 Elsevier Inc. All rights reserved.
1
. Introduction
formic acid [10,11], (para)formaldehyde [12,13], methanol
[
2
14,15], CO [16–18] and dimethyl carbonate [19,20] were
N-methylation is one of the most fundamental transformation
employed. A number of homogeneous and heterogeneous metal
catalysts have been developed for the transformation with consid-
erable progress [21]. However, in these cases, harsh reaction condi-
tions, such as high reaction temperature and pressure together with
varying reductants, e.g., hydrosilanes, boranes, or molecular hydro-
gen are usually required to achieve satisfactory results accompany-
ing with low atom efficiency and massive waste production,
thereby significantly impeding their practical applications. Further-
more, aryl amines are most frequently used as starting materials for
the synthesis of N-methylamines among these well-developed
methods, which are generally prepared form reduction of readily
available and inexpensive nitroarenes catalyzed by transition met-
als under high pressure of hydrogen. As such, from both economic
and environmental perspectives, the development of a straightfor-
ward one-pot selective synthesis of N-methylamines from easily
available nitroarenes with greener C1 source in a tandem and
cost-effective manner is of great practical significance.
to form N-methylamines in organic chemistry, which represent
important platform chemicals for bulk and fine chemicals as well
as materials [1–3]. Besides, N-methylation, as a powerful synthetic
method, allows for the late-stage functionalization by incorporat-
ing into a magic methyl group to regulate the biological and phar-
maceutical properties of advanced life science molecules [4–6].
Traditional procedures for N-methylation typically employ the
reactions of amines with powerful but carcinogenic methylating
reagents [7–9], such as methyl iodide, dimethyl sulfate or dia-
zomethane, which are operationally problematic and generally suf-
fer from narrow scopes of amines and generation of a large amount
of waste. In recent years, with the intensive awareness of environ-
mental protection and the prevalence of ‘‘green chemistry”, transi-
tion metal-catalyzed N-methylation of amines with more
environmentally benign and safer methylating reagents, such as
To date, only a handful of examples catalyzed by metal cata-
lysts for one-pot direct N-methylation of nitroarenes to
⇑
Corresponding author.
E-mail address: yangyong@qibebt.ac.cn (Y. Yang).
https://doi.org/10.1016/j.jcat.2019.06.022
021-9517/Ó 2019 Elsevier Inc. All rights reserved.
0

X. Dong et al. / Journal of Catalysis 375 (2019) 304–313
305
N-methylamines using formic acid/H
2
, (para)formaldehyde/H
2
,
In this work, we developed a new heterogeneous catalyst with
Cu nanoparticles supported on amorphous Al (denoted as Cu/
Al derived from CuAl-layered double hydroxide (LDH)
precursor via an in situ topotactic transformation process. Cu
nanoparticles were homogeneously and uniformly dispersed on
methanol, CO /H as C1 sources have been reported. In this regard,
2
2
2 3
O
heterogeneous metal catalysts are mostly frequently employed
owing to their advantages, such as catalyst recyclability and easy
of separation from the reaction media (Scheme 1). In 2009, Li
and co-workers reported the synthesis of N,N-dimethylanilines
from nitroarenes with methanol catalyzed by a pretreated Raney-
2 3
O )
amorphous Al
with the catalyst ip-Cu/Al
2 3
nation method. The catalyst Cu/Al O
2
O
3
with strong support-metal interaction compared
prepared using conventional impreg-
exhibited efficient and selec-
2 3
O
Ni catalyst under high temperature and N
2
pressure (170 °C,
3
MPa Ar) [22]. In 2013, Rong and co-workers developed a skeletal
tive N-methylation of nitroarenes with para-formaldehyde in the
absence of external molecular hydrogen under milder conditions.
A broad set of nitroarenes including biologically relevant sub-
strates were efficiently and selectively converted to their corre-
sponding N,N-dimethyl amines in high to excellent yields with
good tolerance of various functional groups. A combination study
including comprehensive characterizations and control experi-
Cu (also as known as Raney-Cu) as chemoselective catalyst for one-
pot synthesis of N,N-dimethylanilinesfrom nitroarenes with
formaldehyde in the presence of molecular hydrogen (1.5 MPa)
[
23]. Later on, Shi and co-workers developed two heterogeneous
catalysts, e.g., CuAlO and Pd/CuZrO , for direct and selective
N-methylation of nitro compounds with CO /H , respectively,
while both required harsh conditions and longer reaction times
30–100 bar, 150–170 °C, 30–48 h) [24,25]. The same group subse-
quently reported TiO supported nano-Pd catalyst (Pd/TiO ) could
x
x
2
2
2 3
ments discloses that: (1) the catalyst Cu/Al O derived from
(
CuAl-LDH results in a strong interaction between Cu nanoparticles
+
0
2
2
and amorphous Al
2
O
3
with formation of Cu and Cu active species
+
enable the N-methylation of nitro compounds with methanol
under UV irradiation at room temperature [26], and also allows
for a kinetically controlled synthesis of N-monomethylamines from
nitroarenes with formaldehyde in the presence of molecular
hydrogen (1.0 MPa) [27]. Meanwhile, Cao and co-workers devel-
oped a Au/rutile catalyst for the synthesis of N,N-dimethlanilines
from nitroarenes using formic acid (FA) as a benign C1 source,
with Cu as the major sites on the surface upon in situ topotactic
transformation; (2) Cu and Cu sites synergistically boost the cat-
0
+
alytic efficiency for the reductive N-methylation of nitroarenes,
0
+
while Cu and Cu have their individual role in the entire process.
2. Experimental section
2
but high pressure of H (4 MPa) and high temperature (140 °C)
were required [28]. Remarkably, in 2017, Beller and co-workers
2.1. Preparation of catalysts
made a great achievement and developed a non-noble iron
oxide-based nanocatalyst (Fe
2
O
3
/NGr@C), which enables the effi-
CuAl-layered double hydroxide (LDH) with the mole ratio of Cu:
Al = 2:1 were synthesized by co-precipitation method. Typically,
two aqueous solutions, a solution of the copper and aluminum
nitrates (mole ratio of Cu:Al = 2:1) and a mixed solution of NaOH
cient synthesis of a series of functionalized and structurally diverse
N-methylamines from nitroarenes with para-formaldehyde as both
methylating and reducing agent [29]. In this case, no necessity of
any specialized equipment and use of additional reducing agents
makes this protocol operationally simple and practical.
Apparently, there is a strong desire to develop a heterogeneous
non-noble metal catalyst, which can enable efficient and selective
N-methylation from inexpensive and readily available nitroarenes
with green methylating agents in simple operation process
under milder reaction conditions. Layered double hydroxides
2 3 2 3
and Na CO (mole ratio of NaOH:Na CO = 5:1) precipitant, were
simultaneously added dropwise to deionized water (100 mL)
under vigorous stirring at room temperature. The pH during pre-
cipitation was kept at a constant value of 10 ± 0.2. The precipitate
was then aged at 60 °C for 16 h followed by filtering and washing
thoroughly with deionized water. The filtration cakes were dried
for 10 h at 50 °C in vacuum oven overnight. The obtained CuAl-
À1
(
LDHs) are a class of two-dimension (2D) anion-intercalated
LDH was reduced in a flow of 5% H
2
/Ar (60 mL mim ) for 6 h at
À1
materials, which can be generally expressed by the formula
300 °C, and the heating rate was 2 °C min . The resulting catalyst
2
+
3+
nÀ
2+
3+
[
M
1-x
M
x
(OH)] À (A
)
x/n mH
2
O. M and M metal cations are dis-
2 3 2 3
was denoted as Cu/Al O . The catalyst cal-Cu/Al O was prepared
nÀ
tributed at atomic level in the hydroxide layers, and A is an anion
located in the interlayer region [30,31]. Owing to their versatility in
composition, morphology as well as unique structural topotactic
transformation properties, LDHs have been extensively applied in
photocatalysis [32–35], heterogeneous catalysis [36–41], and
adsorption/separation processes [42–44]. As far as catalysis is con-
cerned, the most attractive feature of LDHs is that the LDH precur-
sors containing transition metal cations, e.g., Ni [45–47], Co [48–
from firstly calcined of CuAl-LDH at air atmosphere at 500 °C for
À1
6 h with 2 °C min in tube furnace (labelled as cal-CuO/Al
2
O
3
cat-
À1
alyst), followed by a reduction in a flow of 5% H
2
/Ar (60 mL mim
)
for 6 h at 300 °C. Other layered double hydroxides, e.g., NiAl-LDH
and CoAl-LDH were prepared following by the same method as
described above. Their corresponding derived catalysts Ni/Al
2 3
O
and Co/Al were also prepared similarly by in situ structural
2 3
O
topotactic transformation process.
50], Fe [51], and Cu [52–54] can be in situ reduced to catalytically
2 3
For comparison, the catalyst denoted as ip-Cu/Al O as a control
active metal nanoparticles supported to a mixed metal oxides with
a high dispersion, strong metal-support interaction, and good stabil-
ity in a morphologically control manner.
sample was prepared by a conventional impregnation method. The
neutral Al
(NO
Á3H
2
O
3
(10.0 g) was dispersed into an aqueous solution of Cu
À1
3
)
2
2
O (0.0157 mol L , 10 mL). The suspension was stirred
Scheme 1. Illustration of discovery of heterogeneous catalysts for direct one-pot N-methylation from nitroarenes.

3
06
X. Dong et al. / Journal of Catalysis 375 (2019) 304–313
at 70 °C overnight, and the precipitate was then dried at 80 °C for
1
6
0 h. The obtained solid sample was calcined in air at 500 °C for
h (labelled as ip-CuO/Al catalyst) followed by H reduction
2
O
3
2
at 300 °C for 6 h.
.2. Characterization methods
The X-ray diffraction (XRD) patterns of all the catalysts were
2
obtained on a Bruker D8 Advance X-ray diffraction diffractometer
equipped with Cu Ka radiation (k = 1.5147 Å). The morphology of
catalysts was examined by a H-7600 transmission electron micro-
scopy (TEM), a Tecnai G2 F30 high-resolution TEM (HRTEM). Nitro-
gen adsorption-desorption data were obtained on a Micromeritics
ASAP 2020 static volumetric sorption analyzer. The specific surface
area of the samples was calculated by the Brunauer-Emmet-Teller
(
BET) method. Temperature programmed reduction (TPR) was per-
formed in a quartz reactor with a thermal conductivity detector to
record the consumption of H
of 5 vol% H
with Ar. The exposed copper surface area and the dispersion of
Cu was determined by dissociative N O adsorption and operated
2
. The samples were reduced in a flow
/Ar at a heating rate of 10 K min after pretreatment
À1
2
Fig. 1. XRD pattern for (a) CuAl-LDH precursor, (b) Cu/Al
(d) ip-Cu/Al
2 3 2 3
O , (c) cal-Cu/Al O , and
2 3
O .
2
on Micromeritics AutoChem 2920 instrument [55]. Irreversible
CO adsorption isotherms were collected with a chemisorption
module of the Micromeritics ASAP 2020 instrument [56–58]. Cata-
o
at 2h = ꢀ12 , 24°, 32°, 36°, 40°, and 48°, respectively, which can
be indexed to the (0 0 3), (0 0 6), (0 1 2), (0 0 9), (0 1 5) and
0012) planes, a characteristic feature of layered CuAl-LDH phase
(
2
lysts were firstly reduced in situ with 5 vol% H /Ar at 573 K for 4 h
2
À
with CO
3
located in the interlayer region. A thin plate-shaped
and then cooled to 300 K. A total adsorption isotherm was col-
lected using 200–500 torr CO and the reversible adsorption iso-
therm was repeated at the same conditions after evacuating for
crystals morphology with a layer structure and good crystallization
for the CuAl-LDH was confirmed by SEM image (Fig. 2A). Upon
reduction or calcination process, the structure of CuAl-LDH was
destroyed and decomposed to mixed metal and/or metal oxides.
Consequently, the characteristic reflections of CuAl-LDH disappear,
accompanied by the observation of five peaks centered at ꢀ 35°,
3
0 min. The difference between the total and reversible values
was used to determine the irreversible isotherm. The X-ray photo-
electron spectroscopy (XPS) data was collected on an ESCALAB
2
50Xi (Thermo Scientific, UK) instrument equipped with a
ꢀ
43°, ꢀ51°, ꢀ63° and ꢀ 74°, attributing to the characteristic reflec-
tions of metallic Cu (JCPDS# 04-0836, pink lines) and Cu
JCPDS# 05-0667, blue lines) (Fig. 1b), respectively. However, no
reflection of Al is observed, clearly indicating an amorphous
phase. Meanwhile, the catalyst cal-Cu/Al prepared from the cal-
cination of CuAl-LDH in air followed with H reduction, also
demonstrates similar diffraction peaks to that of Cu/Al , but with
monochromatized Al Ka line source. All the binding energies
obtained were calibrated based on the C 1s peak at 284.8 eV.
Inductively coupled plasma atomic emission spectroscopy (ICP-
AES) was conducted on a PerkinElmer Optima 5300 DV instrument.
2
O
(
2 3
O
2 3
O
2
2.3. One-pot direct N-methylation of nitroarenes
2 3
O
a relatively stronger diffraction of metallic Cu (Fig. 1c). The average
sizes of Cu NPs calculated by Cu (1 1 1) diffraction peak shows that
In
equipped with a magnetic stir bar was charged with the catalyst
Cu/Al (6 mg, 27 mol% of Cu), p-nitrotoluene (0.25 mmol), para-
formaldehyde (3.75 mmol), and Na CO (0.5 mmol). Then, DMSO-
water (3 mL, 1:1 v/v) was injected into the tube and the reaction
was tested under stirring at 130 °C for 15 h. After completion of
the reaction, the reaction mixture was extracted with ethyl acetate
and then filtered. The liquid was analysed by GC and GC-MS on an
Agilent HP-7890 instrument with a flame ionization detector (FID)
a typical procedure, a sealable Schlenk reaction tube
d
(
Cu of Cu/Al
34.2 nm). As a reference sample, the catalyst ip-Cu/Al
pared via a conventional impregnation method from dispersion of
Cu(NO on Al followed by calcination and H reduction, in
which XRD pattern is different from that of Cu/Al or cal-Cu/
. Apart from the appearance of Cu and Cu weak diffaraction
peaks, the diffraction of a Al phase was clearly observed
Fig. 1d).
TEM measurements were performed to investigate the mor-
phology and distribution of Cu species in the catalyst Cu/Al . It
was observed that Cu NPs with an average size of 22.5 nm are uni-
formly dispersed on amphorous Al with a high density (Fig. 2B).
HR-TEM images (Fig. 2C and D) clearly show two crystalline phases
disperse on Al with crystal lattice fingers of 0.209 and 0.246 nm
corresponding to Cu (1 1 1) and Cu O (1 1 1) planes, respectively
2
O
3
with 20.0 nm is smaller than that of cal-Cu/Al
2 3
O
2 3
O
2 3
O
was pre-
2
3
3
)
2
2
O
3
2
2 3
O
0
+
2 3
Al O
2 3
O
(
and an HP-5MS capillary column (30 m, 0.25 mm i.d., 0.25 lm film
thicknesses) using nitrogen as the carrier gas to determine the con-
version and selectivity using mesitylene as an internal standard.
The desired N,N-dimethylated amines were purified by column
chromatography using an appropriate eluent and structurally con-
2 3
O
2 3
O
1
13
firmed by H/ C NMR and HR-MS.
. Results and discussion
.1. Structure and morphology characterizations of Cu/Al
2 3
O
2
3
[59]. These observations are in good consistent with the XRD
results and also reveal that Cu species were simultaneously pre-
o
+
3
2
O
3
derived
sent in the form of Cu and Cu on the surface of the catalyst Cu/
Al
from CuAl-LDH
2 3
O .
To further investigate the reducibility and the interaction
between Cu phase and amorphous Al of over catalysts prepared
by different methods, H -TPR experiment was performed (Fig. 3).
The TPR profile of cal-CuO/Al (Fig. 3a) exhibits a broad and
strong hydrogen consumption peak ranging from 220 to 350 °C,
The Cu nanoparticles (NPs) supported on Al
2
O
3
(Cu/Al
2
O
3
) was
2 3
O
prepared on the basis of the in situ structural topotactic transfor-
mation of CuAl-layered double hydroxoide (CuAl-LDH) precursor
according to the previously reported method [52–54]. The XRD
pattern of CuAl-LDH precursor (Fig. 1a) shows diffraction peaks
2
2 3
O
2
+
0
+
accounting for the reduction of Cu to Cu and Cu , as evidenced

X. Dong et al. / Journal of Catalysis 375 (2019) 304–313
307
0
+
by the coexistence of Cu and Cu in XRD and HR-TEM characteri-
CuAl-LDH, the Cu 2p3/2 peak at ꢀ 935.2 eV and Cu 2p1/2 peak
at ꢀ 955.0 eV along with their satellite peaks are detected
(Fig. 4A (a)), implying that Cu species in CuAl-LDH exist in the form
+
zation. Cu species on the surface may result from the reduction of
2
+
2 3
O
, while Cu0 spe-
Cu species having strong interaction with Al
cies are from the readily reduced CuO [55,60]. In contrast, the
reduction behavior over ip-CuO/Al (Fig. 3b) had two hydrogen
of Cu(OH)
sphere (that is, Cu/Al
2 3
followed by H reduction (that is, cal-Cu/Al O
2
. Upon in situ topotactic transformation under H
) or pretreatment with calcination in air
), the reductive for-
2
atmo-
2
O
3
2 3
O
consumption peaks: the one starting from 180 °C to 280 °C is
2
+
assigned to the reduction of Cu2 species with weak interaction
+
mation of Cu and Cu0 species occurs, as observed by the disap-
pearance of the Cu 2p satellite peak at 941–944 eV together with
a blue shift of binding energy (Fig. 4A (b and c)). Two overlapping
peaks at 916.6 and 918.6 eV were observed in the Cu LMM Auger
electron spectroscopies (XAES), further confirming the formation
with Al
due to the Cu species located into the channel of Al
nificant difficulty to be reduced [61]. Comparatively, the higher
reduction temperature for the catalyst cal-CuO/Al strongly indi-
cates the interaction between Cu NPs and Al is considerably
2
O
3
, and another broad peak at 300–450 °C is possibly
2 3
O
with of sig-
2 3
O
+
0
2
O
3
of Cu (916.6 eV) and Cu (918.6 eV) (Fig. 4B) on the catalyst sur-
face. These observations are in well consistent with the results of
XRD and HR-TEM. Furthermore, deconvoluation analysis of two
enhanced via the in situ topotactic transformation from CuAl-
LDH, which could make Cu NPs much harder for agglomeration
during the preparation and reaction process as previous reports
45,62].
XPS analysis was performed to study the surface chemical state
and distribution of Cu species in the catalysts. For the sample of
+
0
XAES peaks discloses that the composition ratio of Cu /Cu in the
catalysts Cu/Al and cal-Cu/Al is 2.8 and 1.6, respectively.
[
2
O
3
2 3
O
+
To further quantitatively measure Cu species on the surface of
these two catalysts, irreversible CO adsorption experiments were
executed. It is well-established that Cu species can irreversibly
+
0
bind with CO molecule, while CO is easily desorbed from Cu and
2
+
Cu sites [56]. As shown in Fig. 5, the catalyst Cu/Al
a significantly larger value of CO uptake than that of cal-Cu/Al
76.1 vs 24.8 mol/g), evidencing the exposure of a much higher
2 3
O exhibited
2 3
O
(
l
+
2 3
content of Cu species on Cu/Al O surface.
2 3
Fig. 2. (A) SEM image of CuAl-LDH, (B–D) HR-TEM images of the catalyst Cu/Al O ;
the inset shows the Cu nanoparticles size distribution.
Fig. 4. (A) Cu 2p XPS spectra for (a) CuAl-LDH, (b) Cu/Al
2 3 2 3
O , and (c) cal-Cu/Al O
Fig. 3. H
2
-TPR profiles of (a) cal-CuO/Al
2
O
3
and (b) ip-CuO/Al
2
O
3
.
and (B) Cu LMM Auger electron spectra for Cu/Al , and (c) cal-Cu/Al O .
2
O
3
2 3

3
08
X. Dong et al. / Journal of Catalysis 375 (2019) 304–313
2 3 2 3
Fig. 5. Irreversible CO adsorption isotherms for (A) Cu/Al O and (B) cal-Cu/Al O .
3.2. Selective N-methylation of nitroarenes with para-formaldehyde
conversion of 1a toward 2a, while Cu powder showed no reactivity
at all (Fig. 6). Such results strongly imply that Cu sites are primar-
+
We initiated our attempts for the one-pot selective
N-methylation of p-nitrotoluene with para-formaldehyde as a
ily responsible for the reductive N-methylation of nitroarene. How-
ever, Cu as a possible active sites involving in this cascade process
0
benchmark reaction to investigate the catalytic performance of
can’t be ruled out. Next, a set of control experiments were further
performed to elucidate the individual role of Cu and Cu .
0
+
the catalyst Cu/Al
2
O
3
. A rapid screening of various parameters
including solvents, bases, amounts of para-formaldehyde and reac-
tion times shows that the maximum outcomes could be obtained
We firstly performed the products distribution as a function
of reaction time using the benchamrk reaction under the opti-
mized conditions to disclose the possible reaction pathway. As
shown in Fig. 7, at the early stage of the reaction (within 1 h),
p-nitrotoluene (1a) was rapidly consumed and converted into
using the following optimal conditions (6 mg of Cu/Al
equivalent of para-formaldehyde with respect to 1a, 2 equivalent
of Na CO as a base, DMSO/H O (1:1, v/v) at 130 °C) (see
Table S2, ESI). Under the optimized conditions, full conversion of
a with exclusive selectivity to N,N-dimethyl amine 2a was
2 3
O , 15
2
3
2
a
mixture of products, including p-methylaniline (4a),
1
N-p-tolylmethanimine (3a), N-monomethyl-p-toluidine (5a), and
N,N-dimethyl-p-toluidine (2a). With an elapse of the reaction times,
the formation of the desired 2a started to dominate the reaction till
complete formation of 2a, whereas 3a, 4a and 5a were simultane-
ously and gradually consumed. As such, the reaction profiles clearly
demonstrate the N-methylation of nitroarenes with para-
formaldehyde proceeded in an one-pot, direct and stepwise
pathway. More specifically, the reaction pathway most likely
involves a release of molecular hydrogen from para-formaldehyde
for selective reduction of nitroarene and the corresponding imine
after condensation with formaldehyde to afford the mono-N-methy-
lated amine. A subsequent second condensation with formaldehyde
followed by reduction again finally yields the N,N-dimethylamine.
achieved after 15 h (Table 1, entry 1). In the absence of either a
catalyst, or para-formaldehyde, or a base, no reaction took place
at all, indicating a combination of all of them is prerequisite for
the success of the reaction (Table 1, entries 2–4).
2 3 2 3
For comparison, the catalysts cal-Cu/Al O and ip-Cu/Al O with
co-existence of Cu+ and Cu0 species on the surface were catalyti-
cally active for the reaction, while 46.3 and 26.4% conversion of
1
a were merely achieved, respectively, albeit with perfect selectiv-
ity to 2a under identical conditions (entries 6 and 7). In sharp con-
trast, CuAl-LDH was also employed as the catalyst for the reaction
but failed to give any product under otherwise identical conditions
+
(
entry 5), suggesting that Cu2 species are not active for the reac-
tion. In addition, other layered double hydroxide precursors, such
as NiAl-LDH, CoAl-LDH, and even their derived corresponding cat-
2
Obviously, the in situ generation of H for reduction of nitroarene
towards aniline and its subsequent N-methylation with formalde-
hyde are two key reaction steps in the entire process.
After awareness of the reaction pathway, we next investigated
2
the two key reaction steps in detailed. The release of H from para-
alysts upon a H
for N-methylation of 1a with no reactivity (entries 8–11), clearly
indicating that the key role of Cu/Al in catalysis.
Such a significant difference in catalytic performance, especially
between Cu/Al and cal-Cu/Al , inspires us to further distin-
guish the individual role of Cu and Cu sites for N-methylation.
To this end, firstly, the metallic Cu powder and Cu O were sub-
jected to N-methylation of 1a under the optimized conditions. It
was found that Cu gave substantial reactivity with 38%
2
reduction process, all demonstrated inert feature
2
O
3
formaldehyde over various catalysts was carried out under the
optimized conditions, and the results are compiled in Table S3.
2
O
3
2 3
O
0
+
0
+
Either Cu or Cu species could facilitate the generation of molecule
hydrogen from para-formaldehyde in the presence of H O [63]. In
were detected as confirmed by GC-TCD anal-
2
2
each case, H
2
and CO
2
2
O
ysis (Fig. S2). Cu
2
2
O gave higher H release capacity than Cu powder

X. Dong et al. / Journal of Catalysis 375 (2019) 304–313
309
Table 1
Catalyst screening in one-pot N-methylation of p-nitrotoluene a.
Entry
Catalyst
(CH
2
O)
n
(eq.)
Base
Conversion (%)b
Selectivity (%)b
1
2
3
4
5
6
7
8
9
Cu/Al
–
Cu/Al
Cu/Al
CuAl-LDH
cal-Cu/Al
ip-Cu/Al O
2 3
NiAl-LDH
CoAl-LDH
2
O
3
15
15
15
–
15
15
15
15
15
15
15
Na
Na
–
2
CO
CO
3
100
–
–
–
–
46.3
26.4
–
–
–
100
–
–
–
–
100
100
–
–
–
2
3
2
2
O
O
3
3
Na
2
Na
2
Na
2
Na
2
Na
2
Na
2
Na
2
Na
2
CO
3
CO
3
CO
3
CO
3
CO
3
CO
3
CO
3
CO
3
2
O
3
c
1
1
0
1
Ni/Al
Co/Al
2
O
3
2
O
3
–
–
a
b
c
2 n 2 3 2
Reaction conditions: p-nitrotoluene, (0.25 mmol); catalyst, (6 mg); (CH O) (15 eq.); Na CO (2 eq.); H O/DMSO = 1:1, (3 mL).
Conversion and selectivity were obtained by GC using mesitylene as an internal standard.
The catalyst dosage was 40 mg (27 mol% Cu).
did (0.237 vs 0.107 mL/mg), clearly indicating that Cu+ sites are
preferential and more active for in situ generation of molecular
hydrogen from para-formaldehyde. Similarly, Cu/Al
relatively larger amount than that of cal-Cu/Al
2
O
3
generated
2
O
3
(0.775 vs
+
0
.57 mL/mg) under identical conditions, due to the a larger Cu
content on the surface, further confirming the more active feature
+
of Cu for H
2
generation. Subsequently, we investigated the reac-
for N-methylation of
tivity of Cu powder and Cu
2
O
p-methylaniline (4a) with para-formaldehyde under the standard
conditions. The results revealed that either Cu0 or Cu+ sites are
+
effective for the direct N-methylation of anilines, whereas Cu sites
are more reactive than that of Cu0 to a certain extent, along with
the formation of a mixture of mono- and di-methyl amines with
the desired 2a being the major product (Scheme 2, Eqs. (4) and
0
+
(5). Likewise, the catalyst Cu/Al
N-methylation of p-methylaniline with para-formaldehyde in
comparison with cal-Cu/Al under the standard conditions
2 3
O also outperformed the
Fig. 6. Synergistic role of Cu /Cu for the benchmark reaction under the optimized
conditions. Reaction conditions: p-nitrotoluene, (0.25 mmol); Cu powder (10 mg) or
Cu
2
O (10 mg); (CH
2
O)
n
(15 eq.); Na
2
CO
3
2
(2 eq.); H O/DMSO = 1:1, (3 mL).
2 3
O
2 3
Fig. 7. The products distribution as a function of reaction times for the benchmark reaction over Cu/Al O under the optimized conditions.

3
10
X. Dong et al. / Journal of Catalysis 375 (2019) 304–313
(
Scheme 2, Eqs. (1) and (2), mainly accounting for its larger content
+
2 3
of Cu sites on the surface. Besides, the catalyst Cu/Al O could effi-
ciently convert 5a to 2a under identical conditions (Scheme 2,
Eq. (3)).
0
Given the inert feature of Cu for the N-methylation of 1a with
para-formaldehyde, the benchmark reaction in the presence of a
mixture of Cu powder and Cu O was performed under otherwise
2
identical conditions. Surprisingly, a pronounced enhancement in
activity was observed compared with the reaction in the only pres-
2
ence of Cu O (Fig. 6). Therefore, taken all control experimental
0
+
results together, a synergistic role of Cu and Cu responsible for
this direct and efficient reductive N-methylation of nitroarenes
with para-formaldehyde is proposed. In the entire process, Cu0
2
could facilitate both the generation of H and N-methylation of ani-
+
lines, while Cu is highly active for participating in each individual
reaction as shown in Scheme 3.
After identifying the optimized conditions and the plausible
reaction pathway, we next investigated the general applicability
of this reductive N-methylation protocol. Generally, the catalyst
Scheme 3. Reaction pathway for stepwise reductive N-methylation of nitroarenes
with para-formaldehyde over Cu/Al
2 3
O .
2 3
Cu/Al O proceeded the N-methylation reaction efficiently for var-
ious nitroarenes bearing with electron-donating or electron-
withdrawing substituents on the phenyl ring, affording their
respective N,N-dimethyl amines in decent to high yields, as shown
in Table 2. The nitroarenes incorporating with electron-donating
that N-methylation reaction of nitroarenes is a typical nucleophilic
feature. Apparently, the substituents in the para, meta, and ortho of
phenyl ring are compatible with reaction conditions. The sub-
strates with ortho position appeared to be less reactive than their
para or meta substituted analogues, indicating the ortho-
substituted groups have a negative influence on the reaction
3 3 2
groups, such as -CH , -OH, -OCH , -NH groups, could give
corresponding N,N-dimethyl amines in excellent yield (2b-2h).
However, the nitroarenes substituted with electron-withdrawing
groups dramatically decreased the efficiency (2o-2q), indicating
Scheme 2. Control experiments.

X. Dong et al. / Journal of Catalysis 375 (2019) 304–313
311
Table 2
a.
Substrate scope of functionalized nitroarenes
a
2 3 2 n
Reaction conditions: nitroarenes (0.5 mmol), Cu/Al O (12 mg, 27 mol% Cu), (CH O)
(
2
15 eq.), H O/DMSO (6 mL, 1:1 v/v), 130 °C. Yields of isolated products are given.
efficiency, most likey due to the steric effect. Importantly, halogen-
substituted nitroarenes were converted to corresponding
N,N-dimethyl amines without observation of dehalogenation
biologically active nitro-substituted molecules (Scheme 4). To
our delight, fluorenone, nimisulide, and nimodipine were
successfully incorporated with dimethyl groups in good yields
without affecting other functionalities or the core-structure of
the molecules, further implying the practical utility of this
protocol.
(2i-2n), which can be used for further functionalization via a cou-
pling reaction. Gratifyingly, nitroarenes bearing readily reducible
functional groups, such as nitrile (1o), aldehyde (1p) and ester
(
1q), were successfully dimethylated and the reducible groups
remained untouched, highlighting the excellent chemoselectivity
of the catalyst Cu/Al under the present conditions. In addition,
2 3
The catalyst Cu/Al O also showed good stability. After ten
successive reuses, the conversion of p-nitrotoluene (1a) pre-
sented a slight decrease, while the selectivity to N,N-dimethyl-
p-toluidine (2a) maintained a high value for the benchmark
reaction under the optimized conditions (Fig. 8). The decrease
of catalytic activity mainly accounts for the mass loss of the cat-
2 3
O
nitroarenes containing heteroatom were also applicable to this
protocol. For example, 3-nitropyridine and 6-nitroquinoline were
smoothly dimethylated to give their respective 2r and 2s in 81
and 72% yields, respectively.
2
alyst in each cycle and the growth of Cu/Cu O particles in the
To demonstrate the potential application of this methodol-
ogy, we performed the reductive N-methylation of several
catalyst upon recycle as evidenced by XRD characterization
(Fig. S3).
Scheme 4. Synthesis of pharmaceutical N, N-dimethylamines from the corresponding nitro molecules.

3
12
X. Dong et al. / Journal of Catalysis 375 (2019) 304–313
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Acknowledgment
The authors would like to acknowledge the financial support
from QIBEBT and the 13th-Five Key Project of the Chinese Academy
of Sciences (Grant No. Y7720519KL). Y.Y also thanks for the sup-
port from Royal Society-Newton Advanced Fellowship (NAF-R2-
1
80695).
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