Pt-Sn/γ-Al2O3-catalyzed highly efficient direct synthesis of secondary and tertiary amines and imines

Article abstract of DOI:10.1002/chem.201101725

Versatile syntheses of secondary and tertiary amines by highly efficient direct N-alkylation of primary and secondary amines with alcohols or by deaminative self-coupling of primary amines have been successfully realized by means of a heterogeneous bimetallic Pt-Sn/γ-Al₂O₃ catalyst (0.5 wt % Pt, Pt/Sn molar ratio=1:3) through a borrowing-hydrogen strategy. In the presence of oxygen, imines were also efficiently prepared from the tandem reactions of amines with alcohols or between two primary amines. The proposed mechanism reveals that an alcohol or amine substrate is initially dehydrogenated to an aldehyde/ketone or NH-imine with concomitant formation of a [PtSn] hydride. Condensation of the aldehyde/ketone species or deamination of the NH-imine intermediate with another molecule of amine forms an N-substituted imine which is then reduced to a new amine product by the in-situ generated [PtSn] hydride under a nitrogen atmosphere or remains unchanged as the final product under an oxygen atmosphere. The Pt-Sn/γ-Al₂O ₃ catalyst can be easily recycled without Pt metal leaching and has exhibited very high catalytic activity toward a wide range of amine and alcohol substrates, which suggests potential for application in the direct production of secondary and tertiary amines and N-substituted imines.

Full text of DOI:10.1002/chem.201101725

DOI: 10.1002/chem.201101725
Pt–Sn/g-Al2O3-Catalyzed Highly Efficient Direct Synthesis of Secondary and
Tertiary Amines and Imines
Wei He,[a] Liandi Wang,[a] Chenglin Sun,[a] Kaikai Wu,[a] Songbo He,[a] Jiping Chen,[a]
Ping Wu,[a] and Zhengkun Yu*[a, b]
Abstract: Versatile syntheses of secon-
dary and tertiary amines by highly effi-
cient direct N-alkylation of primary
and secondary amines with alcohols or
by deaminative self-coupling of pri-
mary amines have been successfully re-
alized by means of a heterogeneous bi-
The proposed mechanism reveals that
an alcohol or amine substrate is initial-
ly dehydrogenated to an aldehyde/
ketone or NH-imine with concomitant
formation of a [PtSn] hydride. Conden-
sation of the aldehyde/ketone species
or deamination of the NH-imine inter-
mediate with another molecule of
amine forms an N-substituted imine
which is then reduced to a new amine
product by the in-situ generated [PtSn]
hydride under a nitrogen atmosphere
or remains unchanged as the final
product under an oxygen atmosphere.
The Pt–Sn/g-Al2O3 catalyst can be
easily recycled without Pt metal leach-
ing and has exhibited very high catalyt-
ic activity toward a wide range of
amine and alcohol substrates, which
suggests potential for application in the
direct production of secondary and ter-
tiary amines and N-substituted imines.
metallic
Pt–Sn/g-Al2O3
catalyst
(0.5 wt% Pt, Pt/Sn molar ratio=1:3)
through a borrowing-hydrogen strategy.
In the presence of oxygen, imines were
also efficiently prepared from the
tandem reactions of amines with alco-
hols or between two primary amines.
Keywords: alcohols · alkylation ·
amines
imines
· borrowing hydrogen ·
Introduction
pounds with primary amines,[3] oxidation of secondary
amines,[4] or by other methods.[5] Recently, transition-metal-
catalyzed direct N-alkylation of amines[6–8] and ammonia[9]
with alcohols has been reported as an alternative route to
Amines[1] and imines[2] are important intermediates that
have been extensively applied in organic synthesis and the
chemical industry. The reaction between amines and organic
halides has been the major method utilized to prepare sec-
ondary and tertiary amines but this method is often associat-
ed with environmental problems, uses expensive starting
materials, and exhibits low selectivity for the desired prod-
ucts. Reductive amination of aldehydes or ketones with
amines by means of high-pressure hydrogenation is another
known route to higher-order amines. Imines are often syn-
thesized by acid-promoted condensation of carbonyl com-
amines through
a borrowing-hydrogen (BH) strategy.
Through a BH process, amines,[6–9] imines,[10] and amides[11]
can be formed by variation of the transition-metal-complex
catalyst and reaction conditions. Deaminative N-alkylation
between two amines was also documented for the prepara-
tion of secondary and tertiary amines.[12] However, examples
of the BH strategy in amine/imine synthesis with supported
catalysts are only scattered.[10e,13] In a BH process, imines
are usually formed as the intermediates, and can be further
reduced to amine products by the hydrogen species generat-
ed in-situ, or occasionally isolated as the major products.[10]
In principle, catalysts suitable for a BH process should be:
[a] W. He, L. D. Wang, Prof. C. L. Sun, K. K. Wu, Dr. S. B. He,
Prof. Dr. J. P. Chen, P. Wu, Prof. Dr. Z. K. Yu
Dalian Institute of Chemical Physics
Chinese Academy of Sciences
1) highly active for dehydrogenation of an alcohol or amine
substrate into an aldehyde/ketone or NH-imine species.
2) effective to promote condensation of the in-situ generat-
ed aldehyde/ketone species with an amine to form an
N-substituted imine intermediate, or effective for deami-
native coupling of the in-situ generated NH-imine with a
primary amine substrate.
457 Zhongshan Road
Dalian 116023 (P. R. China)
Fax : (+86)411-8437-9227
E-mail: zkyu@dicp.ac.cn
Homepage: http://www.omcat.dicp.ac.cn
[b] Prof. Dr. Z. K. Yu
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
354 Fenglin Road
3) highly active for reduction of the resultant N-substituted
imine intermediate to form a new amine product.
4) readily available and recyclable, in view of industrial ap-
plication.
Shanghai 200032 (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101725.
13308
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Chem. Eur. J. 2011, 17, 13308 – 13317

FULL PAPER
During the search for a readily available, applicable, and
highly active catalyst for a BH process, we found that heter-
ogeneous bimetallic Pt–Sn/g-Al2O3 catalysts meet these re-
quirements. Pt–Sn/g-Al2O3 catalysts have usually been ap-
plied to alkane dehydrogenation,[14] reforming processes,[15]
and hydrogenation[16] in the petroleum industry. Moreover,
Pt nanoparticles on TiO2 can mediate photoirradiation of o-
arylenediamines in alcohol to form benzimidazoles.[8e] In the
gas phase at 200–2908C, primary amines can interact with
methanol over g-Al2O3 to yield a mixture of amines.[17]
Herein, we report the first versatile synthesis of amines and
imines from the direct N-alkylation of amines with alcohols
or amines by means of a heterogeneous bimetallic Pt–Sn/g-
Al2O3 catalyst (Scheme 1).
Screening of conditions for direct N-benzylation of aniline
(2a) with benzyl alcohol (1a): In our initial studies, a heter-
ogeneous Pt/g-Al2O3 catalyst that contained 0.5 wt% Pt was
tested in the N-benzylation of aniline (2a) with benzyl alco-
hol (1a). In a sealed reactor under nitrogen atmosphere, the
reaction was heated at reflux in o-xylene for 24 h and af-
forded N-phenylbenzylamine (3aa, 14%) and imine 4a
(19%) (Table 1, entry 1). Incorporating a second metal, spe-
cifically, tin, improved the catalyst activity remarkably;
amine 3aa was obtained in 80% yield within 8 h by using a
bimetallic Pt–Sn/g-Al2O3 catalyst (catalyst I, Pt/Sn=1:1,
Table 1, entry 2). Increasing the Pt/Sn molar ratio from 1:1
to 1:9 further improved the yield of 3aa (91–97%, Table 1,
entries 3–7). The best result was obtained with catalyst III
(Pt/Sn=1:3); the reaction formed 3aa in 95% isolated yield
(Table 1, entry 4). The high tin content in catalysts IV–VI
(Pt/Sn=1:5–1:9) may partially block exposure of the Pt0
metal surface required for the BH process, which leads to
lower reaction efficiency (Table 1, entries 5–7). Use of ex-
cessive alcohol (1.1 equiv) was not necessary (Table 1,
entry 10), and the reaction proceeded less efficiently either
with an atmospheric nitrogen balloon or at a lower tempera-
ture (1108C) (Table 1, entries 11 and 12). An air atmosphere
favored formation of imine 4a (46%) as the major product
(Table 1, entry 13). The Pt–Sn/TiO2 catalyst only exhibited a
moderate catalytic activity, clearly suggestive of a support
effect[20] (Table 1, entry 14). Somehow, PtCl2 did not show
any catalytic activity for the reaction (Table 1, entry 15).[21]
Scheme 1. Pt–Sn/g-Al2O3-catalyzed direct N-alkylation of amines with al-
cohols and amines through a BH strategy.
Results and Discussion
Catalyst preparation: The Pt–Sn/g-Al2O3 catalyst has been
revealed as a useful heterogeneous bimetallic catalyst for
the dehydrogenation of long-chain paraffins and was pre-
pared by our previously reported method;[14] mono-
metallic Pt/g-Al2O3 and bimetallic Pt–Sn/TiO2 cata-
lysts were made in a similar fashion. The platinum
N-Benzylation of amines 2 with benzyl alcohol (1a): Next,
catalyst III was applied in the N-benzylation of amines 2
Table 1. Screening of conditions for direct N-benzylation of 2a with 1a.[a]
content was fixed at 0.5 wt% in all of the heteroge-
neous catalysts and the tin content was varied
(Table 1). Prior to use, these heterogeneous cata-
lysts were reduced in a hydrogen flow at 4008C for
2 h, cooled to room temperature under a H2 atmos-
phere, and then kept for use in a desiccator under
an air atmosphere. It has been known that PtSn
alloy, SnOx (x=1, 2), and g-Al2O3 phases coexist in
a Pt–Sn/g-Al2O3 catalyst.[14–16,18] In this bimetallic
catalyst, tin acts as a spacer to disperse the plati-
num particles over the Al2O3 support surface, which
enhances the activity and stability of the catalyst re-
markably.[19] In addition, the presence of tin in the
bimetallic Pt–Sn/g-Al2O3 catalyst modifies the elec-
tronic state of the supported Pt particles by electron
transfer from tin to platinum. This results in reduc-
tion of the binding energy of the intermediate spe-
cies and products thus, suppresses further reactions
of some chemical bonds.[16c] The g-Al2O3 support
usually shows Lewis acidity, highly desired in our
case, to facilitate condensation of the aldehyde/
ketone intermediates with amines to generate
imines.
Entry
Catalyst
Pt/Sn
molar
ratio[b]
Pt
T
[8C]
t
3aa[c]
[%]
4a[c]
[%]
U
[h]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Pt/g-Al2O3
Pt–Sn/g-Al2O3 (I)
Pt–Sn/g-Al2O3 (II)
–
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.50
0.10
0.25
0.25
0.25
0.25
0.25
0.25
145
145
145
145
145
145
145
145
145
145
145
110
145
145
145
24
8
14
80
94
19
14
6
3
6
7
9
3
10
2
1:1
1:2
1:3
1:5
1:7
1:9
1:3
1:3
1:3
1:3
1:3
1:3
1:3
–
8
8
8
8
8
8
Pt–Sn/g-Al2O
3 A
97 (95)[d]
Pt–Sn/g-Al2O3 (IV)
Pt–Sn/g-Al2O3 (V)
Pt–Sn/g-Al2O3 (VI)
94
93
91
97
III
III
III
III
III
8
65
8[e]
98 (96)[d]
8[f]
90
67
5
10
10
46
9
24
III
8[g]
8
Pt–Sn/TiO2
PtCl2
55
24
n.d.[h]
n.d.
[a] Conditions: 1a (1 mmol), 2a (1 mmol), o-xylene (5 mL), mesitylene (20 mg, inter-
nal standard), 0.1 MPa N2, in a sealed 15 mL Pyrex glass screw-cap tube. [b] The Pt
content in all of the heterogeneous catalysts is 0.5 wt%. [c] Yield determined by GC
analysis. [d] Isolated yield in parentheses. [e] Alcohol 1a (1.1 mmol). [f] Reaction per-
formed under an atmospheric N2 balloon. [g] In air. [h] None detected.
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Z. K. Yu et al.
with 1a under the optimized conditions (Table 2). A variety
of anilines and pyridylamines reacted with 1a to form the
desired secondary amines 3aa–ar in 90–99% yields, and
only in the cases of sterically hindered 2,6-dimethylaniline
(2 f) and 4-PhNH-aniline (2o) were the products obtained
in relatively low yields (82 and 74%, respectively; Table 2,
entries 1–18). Various substituents, such as methyl, chloro,
fluoro, ethoxy, and amino groups, can be tolerated under the
Table 2. Direct N-benzylation of amines 2 with alcohol 1a.[a]
Entry RNH2 or R2NH
t
RNHBn or R2NBn
Yield[b]
[%]
Entry
RNH2 or R2NH (2)
t
RNHBn or R2NBn (3)
Yield[b]
[%]
(2)
[h]
(3)
[h]
1
2
3
4
8
8
95
90
93
91
14[d]
15
48[c]
48[c]
48[c]
48[c]
95
74
92
97
8
16
24
17
5
6
10
94
82
18
19
24
8
99
95
48[c]
7
8
10
24
92
95
20
21
8
80
72
24[c]
9
24[c]
93
22
24
80
10
11
12
13
36[c]
91
97
96
91
23
24
25
26
24
97
97
83
90
24
24
24
24[c]
36[c]
24[c]
[a] Conditions: 1a (1 mmol), 2 (1 mmol), catalyst III (98 mg, 0.25 mol% Pt), o-xylene (5 mL), 1458C, 0.1 MPa N2 atmosphere. [b] Yield of isolated prod-
uct. [c] Pt (0.5 mol%). [d] Amine 2n (0.5 mmol).
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Pt–Sn/g-Al2O3-Catalyzed Synthesis of Amines
FULL PAPER
reaction conditions. Benzylamine (2s) reacted with 1a to
give dibenzylamine (3as) in 95% yield, whereas cyclohexyl-
amine (2t), 2-(aminomethyl)pyridine (2u), and benzhydryla-
mine (2v) exhibited relatively low reactivity (Table 2, en-
tries 20–22), which revealed unfavorable electronic and
steric effects from the amine substrates. However, the reac-
tions of aliphatic secondary amines 2w–y and 3as gave the
corresponding tertiary amines in good to excellent yields
(83–97%, Table 2, entries 23–26). It should be noted that di-
phenylamine and benzamide did not undergo N-benzylation
Scheme 3.
Deaminative self-coupling of aliphatic primary amines 2 to
form secondary amines 3: N-Alkylation of 2s with nonben-
zylic alcohol n-heptanol (1u), formed a mixture of two sec-
ondary amines (Scheme 4). The production of 3as reveals a
deaminative self-coupling of 2s.[7c,12h] Thus, intermolecular
À
with 1a due to the weak nucleophilicity of their N H
bonds.[22]
N-Alkylation of aniline (2a) with alcohols 1: N-Alkylation
of 2a with alcohols 1 was then explored (Table 3). Benzyl
and 1-naphthylmethyl alcohols 1b–i acted as very efficient
alkylating reagents for 2a; N-phenyl, N-benzyl, and
N-phenyl-N-(1-naphthyl)methylamines 3ba–bh were ob-
tained in 90–98% yields (Table 3, entries 1–8). Both increas-
ing the steric hindrance and introduction of a strong elec-
tron-withdrawing group on the benzyl alcohol lessened gen-
eration of the desired products, or led to an intermediate
product, such as imine 4k (Table 3, entries 10–12). Heteroar-
ylmethyl alcohols 1n–q and aliphatic alcohols 1r–x also effi-
ciently underwent N-alkylation with 2a (Table 3, entries 13–
23). Inter- and intramolecular N-alkylation of 2a with diols
successively occurred to form cyclic tertiary amines in 54–
95% yields (Table 3, entries 24–29). In the case of 1,7-hepta-
nediol (1y3), eight-membered cyclic tertiary amine 3bz was
obtained in 30% yield with concomitant formation of dia-
Scheme 4.
deaminative coupling of benzylamines 2s and 2s1–9 was car-
ried out under the same conditions and the corresponding
dibenzylamines were afforded in 87–96% yields (Table 4,
entries 1–10). Electron-donating and electron-withdrawing
substituents methyl, methoxy, chloro, and fluoro did not
affect formation of the desired products. With long-chain
(2s10, 2s11) and cyclic (2t) aliphatic nonbenzyl primary
amines as substrates, dialkylamines were also obtained in
excellent yields (93–95%) (Table 4, entries 11–13). Notably,
the reaction of an aliphatic primary amine with 2a formed a
mixture of two secondary amines due to easy deaminative
self-coupling of aliphatic primary amines under the stated
conditions (Scheme 5).
mine PhNH
ACHTUNGTRENNUNG(CH2)7NHPh (3bzꢀ, 22%) and aminoalcohol
PhNH(CH2)7OH (3bzꢀꢀ, 23%) as the minor products
ACHTUNGTRENNUNG
(Scheme 2); an unusual example of N-alkylation of amines
with long-chain diols.[7d,11b,23]
Scheme 5.
Scheme 2.
Unexpectedly, the present synthetic protocol worked
more efficiently under solvent-free conditions. For example,
the reactions of amines 2a and 2b with alcohol 1a in a 1:1
molar ratio on a 10 mmol scale catalyzed by III (0.05 mol%
Pt) afforded the desired products 3aa and 3ab in 95 and
97% yield, respectively, within 8–24 h. Moreover, N-alkyla-
tion of aniline with excessive alcohol or treatment of a pri-
mary or secondary amine in benzyl alcohol solvent efficient-
ly produced tertiary amines (Scheme 3). These results sug-
gest the potential application of the present synthetic meth-
odology in the preparation of secondary and tertiary amines.
Direct synthesis of imines 4 from the tandem reactions of
amines 2 with alcohols 1 in the presence of oxygen: As ob-
served under an air atmosphere, catalyst III promoted the
reaction of 2a with 1a to form imine 4a (Table 1, entry 13).
This revealed that oxygen, as an oxidant, protected the
imine intermediate from further reduction to amine 3aa in
the catalytic cycle. The conditions for direct imine synthesis
were screened in the reaction of 2a with 1a (Table 5). Imine
4a was formed in a considerably good yield when the reac-
tion mixture was heated at reflux in o-xylene under an air
Chem. Eur. J. 2011, 17, 13308 – 13317
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Z. K. Yu et al.
Table 3. Direct N-alkylation of 2a with alcohols 1.[a]
Entry ROH or HORꢁOH (1) t [h] PhNHR or PhNR’ (3) Yield[b] [%] Entry ROH or HORꢁOH (1) t [h]
PhNHR or PhNR’ (3) Yield[b] [%]
1
2
3
24
24
24
96
95
98
16
17
18
36[c]
24
73
88
95
93
36[c]
4
24
90
19
36[c]
5
6
7
24
24
24
93
90
93
20
21
22
24
36
24
93
89
90
8
9
24
92
87
84
23
24
25
24
92
95
80
24[c]
36[c]
24[c]
48[c]
10
HOACHTUGNTRENNNUG
11
12
48[c]
48[c]
70
64
26
27
24[c]
77
30
HOACHTUGNTRENNNUG
48[c,e]
13
24[c]
88
28
29
36[c]
54
57
14
15
24
87
77
36[c]
36[c]
[a] Conditions: 2a (1 mmol), 1 (1 mmol), catalyst III (98 mg, 0.25 mol% Pt), o-xylene (5 mL), 1458C, N2 atmosphere. [b] Isolated yields. [c] Pt
(0.5 mol%). [d] 4-NO2C6H4CH=NPh (4k). [e] 1758C.
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Pt–Sn/g-Al2O3-Catalyzed Synthesis of Amines
FULL PAPER
Table 4. Deaminative self-coupling of aliphatic primary amines 2 to form
secondary amines 3.[a]
Table 5. Screening of conditions for direct imine synthesis from the
tandem reaction of 2a with 1a.[a]
Entry
1
RNH2 (2)
R2NH (3)
Yield[b] [%]
95
Entry Pt [mol%] Solvent Atmos.[b] t [h] Conv.[c] [%] Selec.[c]
4a/3aa
1
2
3
4
5
6
0.25
0.25
0.25
0.05
0.25
0.25
0.25
0.25
0.50
0.25
0.30
0.25
0.30
o-xylene
o-xylene
o-xylene
o-xylene
o-xylene air flow
o-xylene O2 flow
PhMe
PhEt
PhEt
PhEt
PhEt
PhEt
PhEt
N2
air
air
air
8
8
24
42
8
99
51
81
60
99
99
95
83
90
3:97
90:10
81: 19
93: 7
30: 70
51: 49
99: 1
90: 10
92: 8
2
3
4
5
6
93
92
93
94
95
8
7[e]
8[f]
9[f]
10[f]
11[f]
12[f]
13[f]
O2
air
air
O2
O2
24
24
24
24
24
24
24
99
>99
95
99: 1
>99:<1
99:1
O2 flow
O2 flow
>99
>99:<1
[a] Conditions: 1a (1 mmol), 2a (1 mmol), solvent (5 mL), 0.1 MPa O2,
air, or N2, catalyst III (98 mg, 0.25 mol% Pt or 120 mg, 0.30 mol% Pt),
1458C, in a sealed 25 mL Pyrex glass screw-cap tube. [b] Atmosphere.
[c] Conversion was determined by GC analysis. [d] Selectivity. [e] 1108C.
[f] 1388C.
7[c]
93
94
96
8
atmosphere—an oxygen gas flow was not necessary—to pro-
duce imine 4a.[10c] An array of benzyl alcohols 1 were react-
ed with various anilines 2; in most cases, nearly complete
conversion was reached and 90–99% selectivity for the de-
sired imine products 4 was obtained (Table 6). Only trace or
small amounts of secondary amines (RNHRꢁ) were detected
as the minor products. The steric hindrance in substrate 1k
led to lower selectivity for the desired product 4z1 (Table 6,
entry 27). Aerobic couplings between aliphatic amine 2s
and alcohols 1a or 1r also occurred to give 4x and 4z3, re-
spectively (Table 6, entries 24 and 29).
9
10[c,d]
87
11[c]
12
93
95
Tandem synthesis of imines 4 from the deamination of ben-
zylamines 2 in the presence of oxygen: Due to the potential
for deaminative coupling of aliphatic primary amines to
form secondary amines (Table 4), aerobic deaminative cou-
pling of benzylamines 2s and 2s1–8 was carried out and effi-
ciently afforded dibenzylimines 4x and 4x1–8 in 86–96%
yields (Table 7, entries 1–9). Steric hindrance in substrate
2s9 lessened formation of the desired product 4x9 (Table 7,
entry 10), and aliphatic nonbenzylic amines 2s10 and 2t un-
derwent complicated conversion or showed no reactivity
(Table 7, entries 11 and 12). Note that the reaction of ben-
zylamine 2s3 with 2a formed a mixture of two imines
(Scheme 6), which revealed aerobic deaminative self- and
cross-couplings between primary amines.
13[d]
95
[a] Conditions: 2 (1 mmol), catalyst III (98 mg, 0.25 mol% Pt), o-xylene
(5 mL), 1458C, 24 h, 0.1 MPa N2. [b] Isolated yield. [c] Pt (0.5 mol%).
[d] 36 h.
atmosphere for an extended reaction time (Table 5, entries 2
and 3). However, an air or oxygen flow led to more forma-
tion of the secondary amine product 3aa (Table 5, entries 5
and 6). Under an oxygen atmosphere, in toluene at reflux
temperature, the desired imine product 4a was selectively
generated in a high yield (Table 5, entry 7) but ethylbenzene
seemed to be the most suitable solvent for the reaction
(Table 5, entries 8–13). Thus, it was found that catalyst III
(0.3 mol% Pt) efficiently catalyzed the reaction of 2a with
1a at reflux temperature in ethylbenzene under an oxygen
Catalyst recycling: Catalyst III was recycled for use in the
N-benzylation of 2a with 1a (Table 8). For the first three
runs, catalytic activity remained the same (Table 8, en-
tries 1–3). In the fourth and fifth cycles, the catalytic activity
was gradually decreased but 100% conversion and high
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Z. K. Yu et al.
Table 6. Direct synthesis of imines 4 from the tandem reactions of
amines 2 with alcohols 1.[a]
Table 7. Tandem synthesis of imines 4 from the deamination of benzyla-
mines in the presence of oxygen.[a]
Entry RCH2OH (1)
R’NH2 (2)
t [h] Conv.
4
Selec.
[%][b]
Entry RCH2NH2 (2) Conv.
RCH=NCH2R (4)
Selec.
[%][b]
[%][b]
[%][b]
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1 f
1g
1h
1i
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
24
30
30
30
30
30
30
30
24
30
30
>99
95
97
4a 99
4b 93
4c 87
4d 92
4e 97
4 f 96
4g 97
4h 96
4i 86
4j 96
4k 96 (94)[c]
4l 90
1
2
3
4
2s
97[c]
99
96
86
88
94
99
2s1
2s2
2s3
>99
>99
>99
>99
>99
>99
>99
99
10
11
12
1j
1m
1p
99
30[d] >99
13
2a
30
>99
4m 83
5
6
2s4
2s5
97
97
90
95
14
15
16
17
18
19
1a
1a
1a
1a
1a
1a
2b
2e
2g
2h
2i
26
30
30
30
30
30
>99
99
>99
>99
>99
>99
4n 96 (94)[c]
4o 96
4p 97
4q 99
4r 96
7
2s6
99[d]
86
2k
4s 97
20
21
22
1a
1a
1a
30
30
30
>99
>99
>99
4t 99
8
9
2s7
2s8
99[d]
99
94
96
2l
4u 98 (97)[c]
4v 90
23
24
25
26
27
28
29
1a
1a
1b
1e
1k
1r
1r
2m
2s
26
26
30
48
93
>99
99
95
62
4w 98
4x 90
4y 95
4z 96
4z1 60
4z2 60
4z3 90
10
2s9
91[d]
71
2b
2g
2a
2a
2s
30[d]
48[d]
30
11
12
2s10
2t
99
0
27[e]
–
70
99
–
[a] Conditions: 1 (1 mmol), 2 (1 mmol), catalyst III (120 mg, 0.3 mol%
Pt), PhEt (5 mL), 0.1 MPa O2, in a sealed 25 mL Pyrex screw-cap tube.
[b] Determined by GC analysis. [c] Isolated yields in parentheses. [d] Pt
(0.5 mol%).
[a] Conditions: 2 (1 mmol), catalyst III (120 mg, 0.30 mol% Pt), PhEt
(5 mL), 0.1 MPa O2, 30 h, in a sealed 25 mL Pyrex screw-cap tube.
[b] Determined by GC analysis. [c] Pt (0.15 mol%). [d] Pt (0.5 mol%).
[e] Complicated mixture.
Table 8. Recycling catalyst III from the N-benzylation of 2a with 1a.[a]
Cycle
t [h] Conv. 2a[b] [%] Yield of 3aa [%][b] Yield of 4a [%][b]
1
2
3
8
8
8
8
24
8
100
100
100
86
100
57
97 (95)[c]
3
4
3
8
6
96
97 (95)[c]
78
94
47
95
4
5
Scheme 6.
10
5
30
100
[a] Conditions: 1a (1 mmol), 2a (1 mmol), catalyst III (98 mg, 0.25 mol%
Pt), o-xylene (5 mL), 1458C, 0.1 MPa N2, in a 15 mL sealed Pyrex glass
screw-cap tube. [b] Determined by GC analysis. [c] Isolated yield in pa-
rentheses.
yields for the desired product were still reached by exten-
sion of the reaction time (Table 8, entries 4 and 5). After
each reaction was complete, the catalyst was removed by
centrifugation. Analysis of the supernatant by ICP-AES
technology showed that no Pt was leaching into the liquid
phase during the reaction. Furthermore, the supernatant did
not exhibit any catalytic activity for the same reaction, con-
firmed by addition of equimolar amounts of 2a and 1a to
the supernatant and treatment of the resultant mixture
13314
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 13308 – 13317

Pt–Sn/g-Al2O3-Catalyzed Synthesis of Amines
FULL PAPER
between two amines. This catalyst can be easily recycled and
has exhibited very high catalytic activity toward a wide
array of amine and alcohol substrates, which suggests its po-
tential for application in the production of secondary and
tertiary amines and imines.
Experimental Section
General considerations: All manipulations of air- and/or moisture-sensi-
tive compounds were carried out under a nitrogen atmosphere by using
standard Schlenk techniques. Reaction solvents were dried and distilled
1
prior to use by literature methods. H and 13C NMR spectra were record-
ed on a Bruker DRX-400 spectrometer and all chemical shift (d) values
13
are referenced to TMS (d=0.00 ppm) or CDCl3 (1H: d=7.26 ppm; C:
d=77.16 ppm). HRMS analyses were measured on a Waters GC-TOF
CA156 mass spectrometer. All the melting points are uncorrected. Ana-
lytical TLC plates (Sigma–Aldrich silica gel 60F200) were viewed under
UV light (254 nm). Chromatographic purifications were performed on
SDZF silica gel 160. The known compounds were identified by compari-
son of their NMR spectra with the reported data or of their GC traces
with those of authentic samples. The spectroscopic features of these
known compounds are in good agreement with those reported in the lit-
erature.
Typical procedure for the direct N-alkylation of amines 2 with alcohols 1
(3aa): Under nitrogen atmosphere, 1a (108 mg, 1 mmol), 2a (93 mg,
1 mmol), catalyst III (98 mg, 0.25 mol% Pt), and o-xylene (5 mL) were
added to a 15 mL Pyrex glass screw-cap tube. The resultant mixture was
stirred in the sealed tube at 1458C for 8 h. The mixture was cooled to
ambient temperature, then the catalyst was removed by centrifugation
and washed with Et2O (2ꢂ5 mL). The combined supernatant was con-
centrated to approximately 0.5 mL under reduced pressure and then sub-
ject to purification by silica gel column chromatography (petroleum
Figure 1. TEM micrographs of catalyst III: a) fresh catalyst prior to use;
b) the catalyst after five cycles.
ether (60–908C)/EtOAc=20:1) to afford 3aa as
a colorless liquid
(174 mg, 95%).
under the reaction conditions. The decrease in the catalytic
activity after recycling is attributed to the aggregation of Pt
nanoparticles on the g-Al2O3 support during the reaction,
which reduces the available bare surface of Pt particles cata-
lytically active for the BH process. Transmission electron
microscopy (TEM) images of fresh catalyst III and the used
catalyst revealed that Pt particle size increased as the cata-
lyst was recycled, from 3–5 nm in the fresh catalyst to
7–9 nm in the used catalyst after five cycles (Figure 1). Pulse
chemisorption of CO experiments[24] demonstrated that Pt
particle dispersion was decreased from 88.2% in the fresh
catalyst to 78.3% in the used catalyst after five cycles, which
also suggests aggregation of Pt particles during the course of
the reaction. This is in agreement with the TEM observation
and variation of the catalyst activity as shown in Table 8.
Typical procedure for N-alkylation of amines 2 in alcohol solvents (3ay):
Under nitrogen atmosphere, 1a (5 mL, 47.8 mmol), 2s (107 mg, 1 mmol),
and catalyst III (98 mg, 0.25 mol% Pt) were added to a 15 mL Pyrex
glass screw-cap tube. The resultant mixture was stirred in the sealed tube
at 1458C for 8 h. The mixture was cooled to ambient temperature, then
the catalyst was removed by centrifugation and washed with Et2O (2ꢂ
5 mL). The combined supernatant was concentrated to approximately
0.5 mL under reduced pressure and then subject to purification by silica
gel column chromatography (petroleum ether (60–908C)/EtOAc=20:1)
to afford tribenzylamine 3ay as a white solid (273 mg, 95%). M.p. 91–
928C.
Typical procedure for deaminative self-coupling of aliphatic primary
amines 2 (3as): Under nitrogen atmosphere, 2s (107 mg, 1 mmol), cata-
lyst III (98 mg, 0.25 mol% Pt), and o-xylene (5 mL) were added to a
15 mL Pyrex glass screw-cap tube. The resultant mixture was stirred in
the sealed tube at 1458C for 8 h. The mixture was cooled to ambient tem-
perature, then the catalyst was removed by centrifugation and washed
with Et2O (2ꢂ5 mL). The combined supernatant was concentrated to ap-
proximately 0.5 mL under reduced pressure and then subject to purifica-
tion by silica gel column chromatography (petroleum ether (60–908C)/
EtOAc=20:1) to afford dibenzylamine 3as as a colorless liquid (94 mg,
95%).
Conclusion
Typical procedure for synthesis of imines from the reactions of amines 2
Efficient synthesis of secondary and tertiary amines from
direct N-alkylation of primary and secondary amines with
alcohols or deaminative self-coupling of primary amines has
been successfully realized by using a heterogeneous bimetal-
lic Pt–Sn/g-Al2O3 catalyst in a borrowing-hydrogen strategy.
In the presence of oxygen, imines were also efficiently pre-
pared from the tandem reactions of amines with alcohols or
with alcohols
1 under oxygen atmosphere (4a): Benzyl alcohol 1a
(108 mg, 1 mmol), aniline 2a (93 mg, 1 mmol), catalyst III (120 mg,
0.3 mol% Pt), and ethylbenzene (5 mL) were added to a 25 mL Pyrex
glass screw-cap tube. The atmosphere was replaced by 0.1 MPa oxygen
and the reactor was sealed. The resultant mixture was then stirred at
1388C for 24 h. The mixture was cooled to ambient temperature, then
the catalyst was removed by centrifugation and washed with Et2O (2ꢂ
5 mL). The combined supernatant was subject to GC analysis: conversion
Chem. Eur. J. 2011, 17, 13308 – 13317
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13315

Z. K. Yu et al.
of 1a>99%; selectivity for imine 4a=99%. To isolate 4a, the combined
supernatant was concentrated to approximately 0.5 mL under reduced
pressure and then subject to purification by silica gel or alumina column
chromatography (petroleum ether (60–908C)/EtOAc=20:1) to afford
imine 4a (177 mg, 97%).
G. W. Lamb, A. C. Maxwell, H. C. Maytum, A. J. A. Watson, J. M. J.
Williams, J. Am. Chem. Soc. 2009, 131, 1766; e) O. Saidi, A. J. Black-
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2009, 121, 7511; Angew. Chem. Int. Ed. 2009, 48, 7375; f) A. J. Black-
er, M. M. Farah, M. I. Hall, S. P. Marsden, O. Saidi, J. M. J. Williams,
Org. Lett. 2009, 11, 2039.
Typical procedure for catalyst recycling: Under nitrogen atmosphere, 1a
(108 mg, 1 mmol), 2a (93 mg, 1 mmol), catalyst III (98 mg, 0.25 mol%
Pt), and o-xylene (5 mL) were added to a 15 mL Pyrex glass screw-cap
tube. The resultant mixture was stirred in the sealed tube at 1458C for
8 h. The mixture was cooled to ambient temperature, then centrifuged in
the reaction tube and decanted. The precipitate was washed with Et2O
(2ꢂ5 mL) and n-hexane (3ꢂ1 mL) with repeated centrifugation/decanta-
tion after each washing. The combined supernatant was subject to GC
analysis and then concentrated to approximately 0.5 mL under reduced
pressure. Purification by silica gel column chromatography (petroleum
ether (60–908C)/EtOAc=20:1) afforded amine 3aa as a colorless liquid
(174 mg, 95%). The recovered catalyst was left in the tube reactor, dried
at 608C/2 mmHg for 2 h, and then reused in a subsequent reaction of 2a
with 1a.
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Acknowledgements
We are grateful to the National Basic Research Program of China
(2009CB825300), the National Natural Science Foundation of China, 863
Program of China (2009AA063903), and the Knowledge Innovation Pro-
gram of the Chinese Academy of Sciences (DICP K2009D04).
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 13308 – 13317

Pt–Sn/g-Al2O3-Catalyzed Synthesis of Amines
FULL PAPER
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Received: June 7, 2011
Published online: October 13, 2011
Chem. Eur. J. 2011, 17, 13308 – 13317
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13317