N-Methylation of amines and nitroarenes with methanol using heterogeneous platinum catalysts

Article abstract of DOI:10.1016/j.jcat.2019.01.027

We report herein the selective N-methylation of amines and nitroarenes with methanol under basic conditions using carbon-supported Pt nanoparticles (Pt/C) as a heterogeneous catalyst. This method is widely applicable to four types of N-methylation reactions: (1) N,N-dimethylation of aliphatic amines under N₂, (2) N-monomethylation of aliphatic amines under 40 bar H₂, (3) N-monomethylation of aromatic amines under N₂, and (4) tandem synthesis of N-methyl anilines from nitroarenes and methanol under 2 bar H₂. All these reactions under the same catalytic system showed high yields of the corresponding methylamines for a wide range of substrates, high turnover number (TON), and good catalyst reusability. Mechanistic studies suggested that the reaction proceeded via a borrowing hydrogen methodology. Kinetic results combined with density functional theory (DFT) calculations revealed that the high performance of Pt/C was ascribed to the moderate metal–hydrogen bond strength of Pt.

Full text of DOI:10.1016/j.jcat.2019.01.027

Journal of Catalysis 371 (2019) 47–56
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
N-Methylation of amines and nitroarenes with methanol using
heterogeneous platinum catalysts
Md.A.R. Jamil a, Abeda S. Touchy a, Md. Nurnobi Rashed a, Kah Wei Ting a, S.M.A. Hakim Siddiki a,
,
⇑
Takashi Toyao a,b, Zen Maeno a, Ken-ichi Shimizu a,b,
⇑
a Institute for Catalysis, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan
b Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura, Kyoto 615-8520, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
We report herein the selective N-methylation of amines and nitroarenes with methanol under basic con-
ditions using carbon-supported Pt nanoparticles (Pt/C) as a heterogeneous catalyst. This method is widely
applicable to four types of N-methylation reactions: (1) N,N-dimethylation of aliphatic amines under N2,
(2) N-monomethylation of aliphatic amines under 40 bar H2, (3) N-monomethylation of aromatic amines
under N2, and (4) tandem synthesis of N-methyl anilines from nitroarenes and methanol under 2 bar H2.
All these reactions under the same catalytic system showed high yields of the corresponding methylami-
nes for a wide range of substrates, high turnover number (TON), and good catalyst reusability.
Mechanistic studies suggested that the reaction proceeded via a borrowing hydrogen methodology.
Kinetic results combined with density functional theory (DFT) calculations revealed that the high perfor-
mance of Pt/C was ascribed to the moderate metal–hydrogen bond strength of Pt.
Received 2 November 2018
Revised 12 January 2019
Accepted 21 January 2019
Keywords:
Amines
Borrowing hydrogen
N-Methylation
Platinum nanoparticles
Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction
pesticides [53], where reductive amination of toxic formaldehyde
still constitutes a major approach [3]. As a greener alternative
Methanol is primarily produced from natural gas or coal.
Methanol from CO2 and solar hydrogen could be a promising can-
didate as a sustainable carbon feedstock for the production of a
variety of chemicals [1–3]. In previous catalytic studies on metha-
nol conversion, fuels and bulk chemicals have been the main target
products rather than fine chemicals. Potentially, methanol can be a
C1 source in fine chemical synthesis as an alternative to the con-
ventional toxic methylation reagents (methyl iodide, dimethyl sul-
fone, diazomethane, and formaldehyde), which can be sources of
genotoxic impurities in pharmaceutical manufacturing [4]. In this
context, borrowing hydrogen (or hydrogen auto-transfer) reactions
[5–11] for construction of CAN [12-46], CAC [44-51] and CAO
bonds [52] by methanol have attracted recent attention in catalysis
and organic synthesis since it can be applied to the atom-economic
manufacture of fine chemicals from methanol.
method, catalytic N-methylation of amines with methanol has
been investigated mainly using homogeneous catalysts including
Ir [12–14], Ru [15–21], Mn [22–24], Fe [25,26] or Re [27] catalysts,
in which N-monomethylation of aromatic primary amines (aniline
derivatives) was mainly investigated. Recently, Seayad et al.
reported that a Ru complex catalyst, which was effective for N-
methylation of aromatic amines, promoted N,N-dimethylation of
primary aliphatic amines [17]. The tandem transformation of nitro
compounds into N-methylated amines was demonstrated by
Kundu et al. where in situ generated amines by reduction of nitro
compounds reacted with methanol in a borrowing hydrogen
manner [19]. Very recently, Hong’s group reported the selective
N-monomethylation of aliphatic primary amines by methanol
using a homogeneous Ru catalyst. The key to achieving high selec-
tivity was the use of H2 as a reducing agent in combination with a
relatively low reaction temperature, which can retard the
dehydrogenation steps of methanol [37]. Although various kinds
of N-methylation reactions have been achieved by metal complex
catalysts, these methods principally suffer from difficulties in
catalyst product separation and catalyst reuse.
N-Methylation of amines is one of the most important transfor-
mations in the industrial production of fine chemicals including
surfactants, polymers, dyes, pharmaceuticals, agrochemicals and
⇑
In contrast, heterogeneous catalysts have advantages in terms
of simplicity in handling, catalyst product separation and reuse.
However, only a few heterogeneous systems using solid acid/base
Corresponding authors at: Institute for Catalysis, Hokkaido University, N-21,
W-10, Sapporo 001-0021, Japan.
E-mail addresses: hakim@cat.hokudai.ac.jp (S.M.A. Hakim Siddiki), kshimizu@
cat.hokudai.ac.jp (K.-i. Shimizu).
https://doi.org/10.1016/j.jcat.2019.01.027
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

48
Md.A.R. Jamil et al. / Journal of Catalysis 371 (2019) 47–56
[28–30] and photocatalysts (Pt, Ag, or Pd-loaded TiO2) [31–33] for
N-methylation of amines with methanol have been reported to
date. Additionally, these systems require high temperatures
(230–425 °C) [28–30] or photo-irradiation [31–33] and still have
problems such as low selectivity for di- or mono-methylated ami-
nes [29,31,34] and limited substrate scopes [30,31,34,35], which
are disadvantageous from a practical viewpoint. Accordingly, a
heterogeneous catalytic system that can selectively promote vari-
ous kinds of N-methylations of aromatic and aliphatic amines
under milder reaction conditions is strongly required.
Herein, we report a simple and versatile heterogeneous cat-
alytic method for various types of N-methylation reactions with
methanol. The developed catalytic system, Pt/C with base, was
demonstrated for selective N,N-dimethylation of aliphatic amines
and N-monomethylation of anilines under N2 atmosphere. Inspired
by Hong’s method [37], the selective N-monomethylation of
aliphatic primary amines under the same catalytic system was
developed using pressurized H2. Moreover, the one-pot synthesis
of N-methylanilines from nitroarenes and methanol [19] was car-
ried out under H2 atmosphere. Kinetic and computational studies
were conducted to discuss the catalytic pathway and the factors
controlling the catalytic activity of transition-metal nanoparticles
for the N-methylation of amines by methanol.
(NO3)3 or Pd(NH3)2(NO3)2 or an aqueous solution of metal nitrates
(for Ni, Cu), IrCl3ꢁnH2O, RuCl3 or NH4ReO4.
2.3. Characterization
Transmission electron microscopy (TEM) observations were
conducted using a JEOL JEM-2100F TEM (200 kV). Pt L3-edge X-
ray absorption near-edge structures (XANES) and extended X-ray
absorption fine structure (EXAFS) experiments were carried out
in transmittance mode at the BL14B2 with a Si(1 1 1) double crys-
tal monochromator in SPring-8 operated at 8 GeV. The Pt/C catalyst
pre-reduced in a flow of H2 (20 cm3 minꢀ1) for 0.5 h at 300 °C was
cooled to room temperature under a flow of H2 and was sealed in
cells made of polyethylene under N2, and then the EXAFS spectrum
was taken at room temperature. The spectra of Pt foil and the Pt/C
catalyst just after the 5th reaction cycle were recorded without the
pre-reduction treatment. The EXAFS analysis was performed using
the REX ver. 2.5 program (RIGAKU) using the parameters for Pt–O
and Pt–Pt shells provided by the FEFF6.
2.4. Typical procedures of catalytic reactions
After the reduction under a flow of H2 at 300 °C for 0.5 h, we
carried out catalytic tests without exposing the catalyst to air as
follows. Methanol (30 mmol) was injected to the reduced catalyst
inside the glass tube through a septum inlet, thus the catalyst was
covered with a layer of methanol to restrict it from air exposure.
After removal of the septum under air, amine (1 mmol), solid NaOH
(1 mmol), n-dodecane (0.25 mmol) and a magnetic stirrer bar were
placed in the tube. The tube was inserted into a stainless-steel
autoclave (28 cm3) and purged with N2 gas. Finally, the resulting
mixture was heated at 150 °C and stirred under 1 bar N2. For the
model reaction of n-octylamine, the catalyst screening, optimiza-
tion of reaction conditions, kinetic studies and control reactions,
the conversion of n-octylamine and yields of products were deter-
mined by GC analyses, using n-dodecane as an internal standard by
applying the GC sensitivity of the isolated or commercial products.
For the substrate scope study, the products were isolated by col-
2. Experimental
2.1. General
We used commercial chemicals (Tokyo Chemical Industry,
Wako Pure Chemical Industries, Kishida Chemical, or Mitsuwa
Chemicals) without purification. GC-FID (Shimadzu GC-2014) and
GC-MS (Shimadzu GCMS-QP2010) analyses were performed using
an Ultra ALLOY capillary column UA+-1 (Frontier Laboratories, Ltd.)
with N2 or He as the carrier gas. 1H and 13C NMR measurements
were conducted using a JEOL-ECX 600 spectrometer operating at
600.17 and 150.92 MHz, or JEOL-ECX 400 operating at 399.78
and 100.52 MHz, respectively, with tetramethylsilane as the inter-
nal standard.
umn chromatography with silica gel 60 (spherical, 60–100 lm,
Kanto Chemical Co., Ltd.) using hexane/ethyl acetate or ethyl acet-
ate/methanol as the eluting solvent. The yields of the isolated
amine derivatives were determined and identified by 1H and 13C
NMR and GC-MS methods.
2.2. Catalyst preparation
The carbon support (296 m2 gꢀ1, Kishida Chemical) was com-
mercially obtained.
c-Al2O3 was prepared by calcining c-AlOOH
(Catapal B Alumina kindly supplied by Sasol) at 900 °C (3 h).
CeO2 (JRC-CEO3, 81 m2 gꢀ1), MgO (JRC-MGO-3), and TiO2 (JRC-
TIO-4) were supplied by the Catalysis Society of Japan. ZrO2 was
prepared by calcination of hydrous zirconia at 500 °C (3 h) [51].
Nb2O5 was prepared by calcination of Nb2O5ꢁnH2O (kindly supplied
by CBMM) at 500 °C (3 h). SiO2 (Q-10, 300 m2 gꢀ1) was kindly sup-
plied by Fuji Silysia Chemical, Ltd.
2.5. Computational methods
Adsorption energies of a H atom on various metal surfaces
reported in our recent study were used to rationalize the catalytic
performance in the N-methylation reaction [51]. Brief computa-
tional methods are described as follows. All the calculations were
performed with the Vienna ab-initio simulation package (VASP)
[54,55] using projector-augmented wave potentials [56] and the
Perdew–Burke–Ernzerhof (PBE) functional [57]. The dispersion-
corrected DFT-D2 method was employed to account for van der
Waals interactions [58]. The Ni(1 1 1), Cu(1 1 1), Rh(1 1 1), Pd
(1 1 1), Ir(1 1 1), Pt(1 1 1), Ru(0001), and Re(0001) surfaces were
modeled by a supercell slab consisting of a 3 ꢂ 3 surface unit cell
with four atomic layers. It should be noted that the most stable
and common planes were used for each metal. The (1 1 1) surface
(Ni, Cu, Rh, Pd, Ir, and Pt) presents a face-centered-cubic (fcc) struc-
ture, while the (0001) surface (Ru and Re) has a hexagonal-close-
packed (hcp) structure. The slab was separated in the vertical
direction by a vacuum void (height: 15 Å). An energy cut-off of
400 eV and a 5 ꢂ 5 ꢂ 1 point mesh were used for the slab model
calculations. The energy of the isolated H atom was obtained using
the same parameters as those in the free-surface slab calculations.
The Pt/C catalyst with Pt loading of 5 wt% was prepared by the
same wet impregnation method as reported in our earlier paper
[51]: a mixture of carbon (10 g), a 10.62 g of an aqueous HNO3
solution of Pt(NH3)2(NO3)2 that contained 4.96 wt% of Pt (Furuya
Metal Co., LTD.), and 50 mL of deionized water was added to a
round-bottom flask (500 mL). The mixture was then stirred
(200 rpm) for 30 min at 50 °C, followed by subsequent evaporation
to dryness at 50 °C at reduced pressure and finally dried in an oven
at 100 °C under ambient pressure for 12 h. The obtained powder
was reduced in a Pyrex tube under a flow of H2 (20 cm3 minꢀ1
)
at 300 °C for 0.5 h and then used for the reaction.
Other supported Pt catalysts with Pt loading of 5 wt% were also
prepared by the same method. M/C (M = Rh, Ir, Ru, Pd, Re, Cu, Ni)
catalysts with metal loading of 5 wt% [51] were prepared by a sim-
ilar method as that for Pt/C using an aqueous HNO3 solution of Rh

Md.A.R. Jamil et al. / Journal of Catalysis 371 (2019) 47–56
49
The geometry-convergence criterion was set at Fmax < 0.03 eV Åꢀ1
where Fmax is the maximum force acting on a mobile atom. The
adsorption energy (Eads) was calculated as the difference in energy
between the molecule absorbed on the surface (Emolecule/surface), the
,
were used for the subsequent reactions without exposing them
to air. The reactions with metal oxides-supported Pt catalysts
(entries 5–11) gave mixtures of 2a, 3a, 4a, 5a and 6a and the yields
of 2a (0–56%) were lower than that of Pt/C. Note that the kind of
carbon support did not affect the catalysis as revealed by control
experiments using several Pt/C catalysts (See Table S1). We also
tested various transition-metal nanoparticles loaded on the carbon
support (entries 3,12–18). Unlike Pt/C, other metal catalysts gave a
mixture of various products. The yield of 2a decreased in the fol-
lowing order: Pt > Pd > Rh > Ir > Ni > Ru > Re > Cu. In summary,
the results in Table 1 show that Pt/C (entry 3) is the most suitable
catalyst for the selective N,N-dimethylation of 1a with methanol
into 2a in the presence of 1 equiv. NaOH.
individual adsorbate molecule (Emolecule), and the surface (Esurface
)
according to:
Eads ¼ Emolecule=surface ꢀ ꢀEmolecule þ Esurface
ð1Þ
3. Results and discussion
3.1. N,N-Dimethylation of aliphatic amines
Using the most effective catalyst (Pt/C), we optimized the reac-
tion conditions (Table 2). The N-methylation reaction did not occur
in the absence of a base (entry 1). This indicates that Pt/C alone
does not catalyze the reaction and the presence of a base is
crucial. The results with different bases (entries 2–12) indicate that
the yield of 2a depends strongly on the base and changes in
the order of NaOH > KOH > KOtBu > NaOMe > NaOtBu > Cs2CO3 >
NaOEt > Na2CO3 > K2CO3 > DBU > Et3N. The yield of 2a also
depends on the amount of NaOH (entries 2, 13–15); the yield
increases with the amount of NaOH up to 1 equiv. with respect
to 1a, but further addition of base gives lower yields. The yield of
2a also depends on the amount of methanol (5–60 mmol, entries
2, 16–20); the optimal amount of methanol is 30 mmol (entry 2).
The lowest amount of methanol (entry 16) results in the highest
yield of the imine intermediate 4a, which suggests that methanol
acts as a hydrogen source as well as a methylation reagent. The
reaction with too much methanol (60 mmol) results in a low yield
of 2a (27%), which may be due to the lower concentration of the
substrate 1a than the standard conditions, leading to lower reac-
tion rates.
We selected the N-methylation of n-octylamine (1a) in metha-
nol as a model reaction to optimize various parameters, including
catalyst composition and reaction conditions. First, catalyst screen-
ing was carried out for the reaction of 1a (1 mmol) and methanol
(30 mmol) with NaOH (1 mmol) at a temperature of 150 °C for
36 h. Since the reaction was tested in a closed stainless-steel auto-
clave, the temperature of the reaction mixture could be higher than
the boiling point of methanol. We screened a series of Pt-loaded
(5 wt%) heterogeneous catalysts (39 mg in total; 1 mol% Pt with
respect to 1a). Table 1 lists the yields of N,N-dimethyloctylamine
(2a) as the main product, together with the yields of minor prod-
ucts, i.e., N-methyl-n-octylamine (3a), n-octylmethanimine (4a),
dioctylamine (5a) and N-methyl-dioctylamine (6a). The substrate
1a was not consumed in the absence of a transition-metal catalyst
(entry 1). Platinum oxide did not catalyze the reaction; the product
yields were negligible in the presence of carbon-supported plat-
inum oxide, PtOx/C (entry 2). In contrast, carbon-supported Pt
metal nanoparticles (Pt/C, entry 3) gave 2a in 96% yield. Exposure
of the as-reduced Pt/C catalyst to air at room temperature for 0.5 h
decreased the yield of 2a to 58% (entry 4), possibly due to oxidation
of the surface Pt0 species. Under the standard conditions (entries
3,5–18), supported metal nanoparticle catalysts reduced under
H2 at 300 °C (for Pd, Rh, Ir, Ru, and Cu) or 500 °C (for Ni and Re)
Under the optimized conditions, we studied the performance of
the Pt/C-catalyzed N,N-dimethylation of aliphatic amines with
methanol. Scheme 1 shows the scope of various aliphatic amines.
Table 1
Catalyst screening for N,N-dimethylation of n-octylamine by methanol.
Entry
Catalysts
1a conv. (%)a
GC yield (%)a
2a
3a
4a
5a
6a
1
2
3
4
5
6
7
8
blank
PtOX/C
Pt/C
0
3
0
0
96
58
9
0
0
2
3
44
39
18
35
7
36
8
7
11
14
42
43
39
2
0
0
0
0
4
0
12
1
33
13
22
0
1
0
0
0
0
0
0
0
0
0
2
2
18
29
0
20
9
100
65
85
86
96
96
85
94
73
97
98
94
92
84
49
5
Pt/C-air
Pt/Al2O3
Pt/CeO2
Pt/MgO
Pt/ZrO2
Pt/Nb2O5
Pt/TiO2
Pt/SiO2
Pd/C
Rh/C
Ir/C
Ni/C
Ru/C
7
7
12
10
0
22
7
9
3
5
6
3
2
0
0
56
18
0
27
3
69
62
55
14
10
7
9
10
11
12
13
14
15
16
17
18
1
1
10
1
12
31
16
1
Re/C
Cu/C
0
2
1
0
a
Conv. and yields were determined by GC analyses.

50
Md.A.R. Jamil et al. / Journal of Catalysis 371 (2019) 47–56
Table 2
Optimization of the conditions for Pt/C-catalyzed N,N-dimethylation of n-octylamine.
Entry
x (mmol)
Base (y)
1a conv. (%)a
GC yield (%)a
2a
3a
4a
5a
6a
1
2
3
4
5
6
7
8
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
5
None
NaOH (1.0)
KOH (1.0)
33
100
96
68
56
70
86
91
94
76
41
36
83
92
100
92
95
97
100
100
0
3
2
0
0
0
19
18
7
0
0
0
0
0
1
0
3
0
0
1
0
0
0
0
0
2
0
0
0
3
2
0
2
2
0
0
0
1
1
2
0
0
0
1
1
3
0
0
1
1
0
96
82
16
12
32
61
78
67
30
0
12
23
14
13
21
12
23
19
4
Na2CO3 (1.0)
K2CO3 (1.0)
Cs2CO3 (1.0)
NaOtBu (1.0)
KOtBu (1.0)
NaOMe (1.0)
NaOEt (1.0)
Et3N (1.0)
9
0
10
11
12
13
14
15
16
17
18
19
20
16
21
17
0
0
0
23
4
0
0
12
DBU (1.0)
4
9
5
NaOH (0.25)
NaOH (0.5)
NaOH (1.5)
NaOH (1.0)
NaOH (1.0)
NaOH (1.0)
NaOH (1.0)
NaOH (1.0)
74
78
68
27
76
90
64
47
13
24
31
15
5
10
20
40
60
23
29
a
Yields were determined by GC analyses.
Linear aliphatic primary amines with different chain lengths
(C6–C16) were selectively methylated to give the corresponding
N,N-dimethyl amines in excellent isolated yields (88–98%). It is
noteworthy that the products, N,N-dimethylated aliphatic amines,
are an important class of chemicals for the industrial production of
surfactants, germicides, and softeners [53]. 2-Phenylethylamine, a
branched aliphatic primary amine, 1,5-dimethyl hexylamine, and
cyclic aliphatic amines were converted to the corresponding
N,N-dimethylated amines in 72–89% yields. A typical diamine,
1,5-pentanediamine, was transformed to the tetramethylated pro-
duct in 76% yield. A hydroxyl group in 5-aminopentan-1-ol was
tolerated, with the N,N-dimethylated product obtained in 86%
yield. These results demonstrate the wide applicability of this
heterogeneous catalytic method for the N,N-dimethylation of the
–NH2 group in various aliphatic amines.
of the products, analyzed by GC, for the standard reaction of 1a.
During the initial period (t < 5 h), n-octylamine (1a) is consumed,
accompanied by the formation of n-octylmethanimine (4a) and
N-methyl-n-octylamine (3a) as the main products. Next, 4a and
3a are consumed, accompanied by an increase in the amount of
Among the previous reports on the N-methylation of amines by
methanol [12–36] by homogeneous and heterogeneous catalysts,
the N,N-dimethylation of aliphatic amines by methanol is rare
[12,17,18,28,32]. One of the most successful methods was reported
by Seayad and co-workers [17] in which the 0.5 mol% Ru-complex-
catalyzed reaction of undecylamine amines with methanol gave
the N,N-dimethylated amine in 83% yield, corresponding to a
TON of 166. We tested gram scale reactions of C8, C11 and C12 linear
aliphatic amines with methanol using 10 mg of Pt/C containing
0.0025 mmol (0.025 mol%) of Pt (Table 3). The yields of N,N-
dimethylated amines from C8, C11 and C12 amines were 91%, 88%
and 92% respectively, corresponding to TONs of 3640 (C8), 3520
(C11) and 3680 (C12). For the N,N-dimethylation of n-octylamine,
the TON of our method is one order of magnitude higher than that
of the previous method [17].
According to the previously proposed mechanism for this type
of reaction [17], the present reaction proceeds via the borrowing
hydrogen mechanism (Scheme 2). Fig. 1 shows the time course
Scheme 1. Pt/C-catalyzed N,N-dimethylation of aliphatic amines by methanol.

Md.A.R. Jamil et al. / Journal of Catalysis 371 (2019) 47–56
51
Table 3
Comparison of TON of Pt/C with a previous catalyst.
Catalyst
Pt/C
R
TONa
Ref.
C8H17
C11H23
C12H25
3640
3520
3680
166
This work
Ru complex
C11H23
[17]
a
TON per number of total metal atoms in the catalyst.
Scheme 2. Proposed catalytic cycle for the N,N-dimethylation of amines.
the final product, N,N-dimethyloctylamine (2a). This indicates that
2a is produced by consecutive reactions via 4a and 3a. Based on
this result, we proposed a reasonable mechanism as shown in
Scheme 2. Methanol is first dehydrogenated to give two hydrogen
atoms on the Pt surface and formaldehyde, which reacts with the
amine to afford an imine. NaOH would assist this dehydrogenation
through deprotonation [51]. Next, the imine intermediate is hydro-
genated by the hydrogen atoms on the catalyst to yield mono-
methyl amine. Consecutive borrowing hydrogen methylation of
the monomethyl amine with methanol, probably via an enamine
intermediate [12], gives the N,N-dimethyl amine. Intermediacy of
the enamine is supported by a control experiment shown in Eq.
(1). We synthesized the enamine, (E)-N,N-dimethyl-2-
phenylethen-1-amine, which was isolated and used for transfer
hydrogenation by methanol as a hydrogen donor in the presence
of Pt/C and NaOH. The C@C bond of the enamine underwent trans-
fer hydrogenation with methanol, affording N,N-dimethyl-2-
phenylethan-1-amine in 95% yield.
3.2. N-Monomethylation of aliphatic amines
The selective synthesis of N-monomethyl amines provides var-
ious functional chemicals but has been a challenging target in
organic synthesis due to the difficulties in preventing over-
methylation. Classical and less atom-efficient methods are still
utilized for the synthesis of N-monomethyl amines with a conven-
tional methylating agent such as methyl halides [59,60], generally
by a protection/deprotection methodology [59]. Heterogeneous
catalysis for the selective monomethylation of aliphatic primary
amines by methanol has not been achieved previously. Our results
shown above also indicate that the reactions of aliphatic primary
amines with methanol under N2 afforded dimethyl amines. This
may be due to the higher nucleophilicity of secondary amines than
primary amines, which results in over-methylation of the mono-
methyl amines. Inspired by the first report by Hong’s group [37]
for the selective monomethylation of aliphatic primary amines
by a homogeneous Ru catalyst under 40 bar H2, we studied the
selective monomethylation of aliphatic primary amines by the
heterogeneous Pt/C catalyst.
First, we optimized the conditions by changing the reaction
temperature (120–180 °C), reaction time (12–40 h), H2 pressure
(2.5–40 bar) and catalyst loading (0.5–5 mol%) for the reaction of
the model primary amine 1a with methanol using Pt/C and 1 mmol
of NaOH (Fig. 2). The reactions under 40 bar H2 (36 h) at different
temperatures (Fig. 2A) show that
a moderate temperature
(130 °C) is suitable for selective monomethylation; higher temper-
atures resulted in higher yields of dimethyl amine 2a. Fig. 2B
shows the time dependence of the reaction under 40 bar H2 at
Fig. 1. Time course for methylation of 1a with methanol: conditions are shown in
Scheme 1.

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Md.A.R. Jamil et al. / Journal of Catalysis 371 (2019) 47–56
Fig. 2. Optimization of the conditions for monomethylation of 1a under H2.
with different chain lengths (C8, C12) were selectively transformed
to the corresponding monomethyl amines in high yields (84–86%).
2-Phenylethylamine and cyclohexyl amine were converted to the
corresponding monomethyl amines in good yields.
The question arises as to how H2 addition increases the selectiv-
ity for monomethyl amine 3a. As shown in Scheme 2, dehydro-
genation of methanol to formaldehyde is the key step of this
reaction. It is reasonable to assume that H2 addition decreases
the amount of the formaldehyde intermediate [37], which in turn
decreases the formation rates of the imine and enamine intermedi-
ates. We studied controlled kinetic experiments for methylation of
the primary amine 1a and monomethyl amine 3a under the same
conditions. Table 4 compares the initial formation rates of mono-
methyl amine 3a (V3a) and dimethyl amine 2a (V2a) under 1 bar N2
and 40 bar H2 at 130 °C. Regarding the methylation of the primary
Scheme 3. Pt/C-catalyzed N-monomethylation of aliphatic amines by methanol.
the optimal temperature (130 °C). A reaction time of 36 h gives the
highest yield of monomethyl amine 3a. After 36 h, the mono-
methyl amine 3a was converted to the dimethyl amine 2a. We next
optimized the H2 pressure (Fig. 2C). It is shown that higher H2 pres-
sure results in a higher yield of monomethyl amine 3a. Finally, the
catalyst loading was optimized (Fig. 2D). The use of 1 mol% Pt/C
was found to be optimal, and higher catalyst loading resulted in
a drastic decrease of the yield of 3a. These results indicate that
we can maximize the yield of monomethyl amine 3a by kinetic
control of the reaction. The best conditions for the selective
monomethylation of 1a are as follows: 130 °C for 36 h under
40 bar H2 with 1 mol% of Pt/C.
amine 1a, the V3a value under H2 (12.7 mol mol hꢀ1) is higher
ꢀ1
Pt
than that under N2 (5.6 mol mol hꢀ1), which indicates that H2
ꢀ1
Pt
addition promotes the monomethylation of the primary amine
1a. Considering the fact that the imine 4a was not detected under
H2 atmosphere, it is suggested that the promotion of the
monomethylation of the primary amine is caused by hydrogena-
tion of the imine 4a by H2 to give 3a. On the other hand, for the
methylation of the monomethyl amine 3a, the V2a value under
H2 (2.0 mol mol hꢀ1) is lower than that under N2 (10.0 mol molP-t
ꢀ1
Pt
ꢀ1hꢀ1), which indicates that H2 addition suppresses the methyla-
With the optimized conditions in hand, we tested the substrate
scope of this method (Scheme 3). Linear aliphatic primary amines
tion of the monomethyl amine 3a. The reason for the opposite

Md.A.R. Jamil et al. / Journal of Catalysis 371 (2019) 47–56
53
Table 4
Initial formation rates (mol mol hꢀ1) of 3a (V3a) and 2a (V2a).
ꢀ1
Pt
Under 1 bar N2
Under 40 bar H2
Amine
V3a
V2a
V3a
V2a
R-NH2
R-NH-CH3
5.6
–
0.9
10
12.7
–
0
2
impact of H2 on the methylation rates of primary and monomethyl
amines is not clear.
donating and electron-withdrawing groups, aminopyridines, 6-m
ethyl-1,2,3,4-tetrahydroquinoline, and indoline were converted to
the corresponding N-monomethylated products in high yields
(81–98%). A gram scale reaction of aniline (10 mmol) with metha-
nol (100 mmol) using 0.025 mol% of the Pt/C catalyst gave N-
methylaniline in 88% yield, corresponding to a TON of 3500 (Eq.
(2)).
3.3. N-Monomethylation of aromatic amines
Similar to the reported results [12–36], the N-methylation of
aromatic amines under N2 with Pt/C and NaOH resulted in N-
monomethylation. We initially optimized the catalyst and reaction
conditions for the N-monomethylation of aniline 1b. Based on the
results (not shown), we standardized the following optimized reac-
tion conditions: amine (1 mmol), methanol (10 mmol), 1 mol% Pt/
C, 10 mol% NaOH, 140 °C, and 15 h. Scheme 4 summarizes the sub-
strate scope of our catalytic method for the N-monomethylation of
aromatic amines. Aniline and its derivatives with electron-
Reusability of the Pt/C catalyst was tested for the reaction of
aniline 1b. After the reaction of 1b for 15 h, 2-propanol (3 mL)
was added to the reaction mixture, and the Pt/C catalyst was col-
lected by filtration and was washed with acetone (3 mL), water
(3 mL) and acetone (3 mL). After drying at 110 °C (12 h), the cata-
lyst underwent H2 reduction (300 °C, 0.5 h) in a test tube. The tube
was then charged with the reaction mixture according to the stan-
dard procedure. Applying this recycling procedure, the spent cata-
lyst was reused four times, maintaining high 2b yields (81–92%) as
shown in Fig. 3. In a separate series of catalyst recycling experi-
ments, the initial rate of 2b formation was estimated for each cycle.
The result (black bars in Fig. 3) indicates that the Pt/C catalyst is a
reusable catalyst for this reaction.
The structure of the fresh and the 5th cycled Pt/C catalysts were
characterized by TEM (Fig. 4) and Pt L3-edge XANES/EXAFS (Fig. 5).
The characterization results for the fresh Pt/C were reported in our
previous study [51,61]. The particle size distribution, estimated by
TEM analysis (Fig. 4), shows that the volume-area mean diameter
(D) of Pt particles in the 5th cycled Pt/C (5.2 1.1 nm) is close to
that in the fresh Pt/C (4.7 1.0 nm) [51] within experimental error.
The XANES result (Fig. 5A) shows the metallic state of the fresh Pt/
C [61]. The white line intensity for Pt/C was increased by the cata-
lyst recycling treatment, indicating that Pt is partly oxidized. How-
ever, after reduction of the recycled Pt/C sample under H2 at
300 °C, its white line intensity was again close to that of the fresh
Pt/C. The EXAFS results (Fig. 5B) were used for the curve fitting
analysis to give the structural data in Table 5. After the reduction
treatment (under H2 at 300 °C), the EXAFS features of the recycled
Pt/C sample (10.9 Pt–Pt bonds at 2.74 Å) were close to those of
fresh Pt/C (9.9 Pt–Pt bonds at 2.75 Å). Based on the TEM and
XANES/EXAFS results, we can conclude that the structure of the
Pt species in the recycled catalyst is essentially unchanged after
the 5th cycle, but the recovered catalyst should be re-reduced in
order to regenerate the metallic state of the Pt nanoparticles.
Scheme 4. Pt/C-catalyzed N-methylation of aromatic amines by methanol.
3.4. Synthesis of N-methylanilines from nitroarenes
Fig. 3. Catalyst reuse for the N-monomethylation of aniline with methanol using Pt/
C catalyst with NaOH under the standard conditions shown in Scheme 4: (blue bars)
2b yields after 15 h and (black bars) initial rates of 2b formation.
Recently, Kundu et al. reported the first example of the tandem
transformation of nitro compounds into N-methylated amines by

54
Md.A.R. Jamil et al. / Journal of Catalysis 371 (2019) 47–56
Fig. 4. Pt particle size distributions of Pt/C (B) before and (A) after the recycling test in Fig. 3. A typical TEM image for Pt/C after the recycling test is also shown.
reaction is not clear. Considering the recent report on the Pd
nanoparticle-catalyzed tandem transformation of nitroarenes into
N-methylated amines by formaldehyde under H2 [62], it is most
likely that the in situ formed formaldehyde and anilines are the
key intermediates in this tandem reaction of nitroarenes.
3.5. Factors affecting the catalytic activity
Finally, we discuss an essential factor affecting the catalytic
activity of different metals supported on carbon. We carried out
kinetic experiments for N-monomethylation of aniline 1b by
methanol under the conditions in Scheme 4 using various metal-
loaded carbon catalysts. By controlling the reaction time, the initial
Fig. 5. Pt L3-edge (A) XANES spectra and (B) EXAFS Fourier transforms of Pt/C before
and after the recycling test in Fig. 3. Spectra of a reference compound (Pt foil) are
shown as gray lines.
methanol under H2 [19]. Upon successful development of the
selective N-methylation protocols for anilines, we aimed to
extend the scope of our method to the tandem synthesis of methyl
anilines from nitroarenes under H2. Optimization studies (not
shown) on a model substrate, nitrobenzene, produced the follow-
ing conditions: nitrobenzene (1 mmol), methanol (30 mmol),
1 mol% Pt/C, 1 equiv. KOtBu, 150 °C, under 2 bar H2. Scheme 5
shows the substrate scope for the conversion of nitroarenes to
N-methyl amines. Nitrobenzene and its derivatives with electron-
donating and electron-withdrawing groups, 3-nitropyridine,
3,4-ethylenedioxynitrobenzene, and 1-nitronaphthalene were
transformed to the corresponding N-methylated amines in good
to high yields (76–93%). The detailed mechanism of this one-pot
Scheme 5. Pt/C-catalyzed synthesis of N-methylanilines from nitroarenes and
methanol under H2.
Table 5
Curve-fitting analysis of Pt L3-edge EXAFS of Pt/C.
Sample
Shell
Na
R (Å)b
r
(Å)c
Rf (%)d
Pt/C
Pt
Pt
Pt
9.9
9.0
10.9
2.75
2.73
2.74
0.079
0.069
0.071
3.9
2.1
2.7
Pt/C (after 5th cycles)
Pt/C (H2 at 300 °C)
a
b
c
Coordination numbers.
Bond distance.
Debye-Waller factor.
Residual factor.
d

Md.A.R. Jamil et al. / Journal of Catalysis 371 (2019) 47–56
55
Table 6
TOF calculations for carbon-supported metal catalysts and DFT calculations.
c
ꢀ1
f
Metal
Da (nm)
Nb (nmꢀ2
)
NS/NT
V0d(mol hꢀ1 mol
)
TOFe (hꢀ1
)
EHad (eV)
metal
Cu
Ir
Pt
Rh
Ni
Pd
Ru
Re
9.3
4.9
4.4
3.8
7.0
8.3
6.4
3.9
17.7
14.6
14.6
15.6
18.8
14.8
16.3
15.1
0.14
0.26
0.31
0.35
0.17
0.17
0.21
0.35
0.12
6.31
18.25
6.99
8.45
9.51
3.98
1.17
0.9
ꢀ2.60
ꢀ2.74
ꢀ2.81
ꢀ2.91
ꢀ2.92
ꢀ2.99
ꢀ2.99
ꢀ3.17
24.2
58.3
19.9
48.7
57.6
18.7
3.3
a
b
c
d
e
f
Volume mean diameter of supported metal particles estimated by TEM analysis.
Number of metal atoms per unit area (nm2) on the assumed surface.
Dispersion of supported metals, NS/NT, where NS is the number of surface metal atoms and NT is the total number of metal atoms in the supported metals.
Initial rate of 2b formation per total metal atoms in the catalyst.
TOF per surface metal atom defined as TOF = V0 (mol hꢀ1 molꢀ1) ꢂ NS/NT.
Adsorption energy of a H atom on the assumed metal surface [51].
dimethylation of aliphatic amines, monomethylation of aliphatic
and aromatic amines, and monomethylation of nitroarenes by tun-
ing the reaction conditions such as temperature and atmosphere.
Compared to the previously reported methods with homogeneous
and heterogeneous catalysts, our method has the following advan-
tages: (1) easy catalyst/product separation, (2) catalyst reusability,
(3) high TON, and (4) broad applicability. Kinetic studies show the
moderate metal–hydrogen bond strength of Pt is the main reason
for its high performance over other carbon supported metal
catalysts.
100
50
Ni
Rh
Pt
Ir
Pd
Ru
10
5
Re
Cu
1
Acknowledgement
0.5
-3.2
-3
-2.8
-2.6
This study was supported by JSPS KAKENHI grants (Nos.
17H01341, 18K14051 and 18K14057) from the Japan Society for
the Promotion of Science (JSPS) and by the Japanese Ministry of
Education, Culture, Sports, Science, and Technology (MEXT) within
the projects ‘‘Integrated Research Consortium on Chemical
Sciences (IRCCS)” and ‘‘Elements Strategy Initiative to Form Core
Research Center”, as well as by the JST-CREST project JPMJCR17J3.
The authors are indebted to the technical division of the Institute
for Catalysis (Hokkaido University) for manufacturing experimen-
tal equipment. X-ray absorption measurements were performed
at the BL-01B1 or BL-14B2 facility of SPring-8 at the Japan Syn-
chrotron Radiation Research Institute (JASRI) (Nos. 2017B1382,
2018A1757).
Adsorption energy of H (eV)
Fig. 6. TOF (per surface metal atoms) for N-monomethylation of aniline by metal-
loaded carbon catalysts as a function of the adsorption energy of hydrogen on the
metal surfaces (Table 6).
rates of N-methylaniline (2b) formation (V0) were measured under
the conditions where conversions of 1b were below 30% (Table 6).
In our previous work, we estimated the number of the surface
metal atoms by using the volume-area mean diameter of the metal
particles, assuming the (1 1 1) surface for fcc structures (Cu, Ir, Pt,
Rh, Ni, Pd) and the (0001) surface for hcp structures (Ru and Re)
[63]. The initial rates were divided by the number of the surface
metal atoms to give the turnover frequency (TOF) per surface metal
atom. We also reported the adsorption energies of a H atom on Pt
(1 1 1), Ir(1 1 1), Pd(1 1 1), Rh(1 1 1), Cu(1 1 1), Ni(1 1 1), Ru(0001),
and Re(0001) surfaces obtained by DFT calculations [51]. The TOF
per surface metal atom was plotted as a function of the adsorption
energy of the H atom in Fig. 6. The result shows the metal species
with favorable absorption energy exhibit higher TOF values. It is
reasonable that metals with moderate metal–hydrogen bond
strength are suitable for this N-methylation reaction because it
proceeds through a borrowing hydrogen mechanism where the
catalysts should dehydrogenate the substrate and formally transfer
the H atoms to an unsaturated intermediate. The high performance
of Pt/C over other metal catalysts is ascribed to its moderate
metal–hydrogen bond strength.
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2019.01.027.
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