Highly efficient heterogeneous gold-catalyzed direct synthesis of tertiary and secondary amines from alcohols and urea

Article abstract of DOI:10.1002/cssc.201100581

Urea, the white gold: The efficient synthesis of tertiary and secondary amines is achieved by heterogeneous gold-catalyzed direct amination of stoichiometric alcohols with urea in good to excellent yields. Via a hydrogen autotransfer pathway, the reactions of primary alcohols with urea give tertiary amines exclusively, while secondary alcohols selectively afford secondary amines.

Full text of DOI:10.1002/cssc.201100581

DOI: 10.1002/cssc.201100581
Highly Efficient Heterogeneous Gold-catalyzed Direct Synthesis of Tertiary
and Secondary Amines from Alcohols and Urea
Lin He, Yue Qian, Ran-Sheng Ding, Yong-Mei Liu, He-Yong He, Kang-Nian Fan, and Yong Cao*[a]
The transformation of simple and readily available raw materi-
als into chemically complex and high-value-added molecules is
one of the central priorities in chemistry. In this context, am-
monia is one of the basic feedstocks of the industrial chemis-
try, and its low cost and widespread availability make it
a highly attractive, environmentally benign nitrogen source in
organic synthesis. For example, the use of NH3 as a coupling
partner in catalytic amination reactions allows the direct syn-
thesis of substituted amines from simple substrates such as
alkyl halides.[1] However, there are practical difficulties associat-
ed with the use of NH3, including its storage, handling, and
transportation. Urea, which is one of the most abundant (over
100 million tons per year) and least expensive organic com-
pounds,[2] is an appealing alternative to avoid these drawbacks,
and is currently being considered as a convenient carrier of
NH3 in industry (e.g., in applications involving control of auto-
motive emissions).[3] Given its high nitrogen content of (46%
of its total weight), urea offers great potential to establish new
sustainable procedures for the synthesis of valuable nitrogen-
containing compounds.
ed to this work, Ru(OH)x supported on TiO2 proved to be an ef-
ficient catalyst for the direct amination of alcohols with urea.
One critical limitation associated with this process, however, is
that a large excess of relatively expensive alcohols (alcohol/
urea=10:1) was required to obtain high yields of the desired
substituted amines. From a synthetic and economic point of
view, an atom-efficient catalytic transformation that can be car-
ried out using stoichiometric amounts of alcohol and urea is
preferred.
On the basis of our ongoing interest in the unique catalytic
properties of supported gold nanoparticles (NPs)[10] and their
applications to green and sustainable organic synthesis,[11] we
have recently discovered that very small Au NPs (mean size ca.
1.8 nm) supported on acid-tolerant TiO2 (Au/TiO2-VS, see Sup-
porting Information) can promote selective N-alkylation of
amines with alcohols under mild conditions.[12] A wide range of
secondary amines were obtained with high atom efficiency by
the employment of equimolar amounts of starting materials. In
view of the excellent performance of the gold system for the
alkylation of amines with alcohols, we reasoned that the heter-
ogeneous Au-mediated hydrogen-borrowing strategy could
afford a sustainable protocol for the amination of alcohols
with urea. Herein, we report for the first time that gold cataly-
sis enables the clean and selective formation of tertiary or sec-
ondary amines from the direct amination of stoichiometric
amounts of alcohols with urea. By using this method, the reac-
tions of primary alcohols with urea exclusively gave tertiary
amines, while those of secondary alcohols selectively afforded
secondary amines.
Tertiary and secondary amines are important building blocks
in organic synthesis and routinely serve as synthons for phar-
maceuticals, herbicides, agricultural chemicals, and functional-
ized materials.[4] The synthesis of tertiary and secondary amines
is traditionally carried out with (mostly multistep) procedures
such as N-alkylation of amines with alkyl halides,[5] reductive
amination of carbonyl compounds,[6] or hydroamination of un-
saturated hydrocarbons with amines.[7] In spite of their utility,
clear drawbacks of these methods include the use of expensive
starting materials, tedious workup procedures, low selectivities,
and the concomitant formation of large amounts of wasteful
salts. Alternatively, transition-metal-catalyzed alkylation of
amines (including ammonia) by alcohols via a facile hydrogen-
borrowing (also known as hydrogen autotransfer) strategy[8]
has been attracting considerable attention, since alcohols are
inexpensive, readily available, nontoxic, and theoretically water
is the only byproduct; thus, the overall reaction is intrinsically
environmentally friendly.
We initially investigated the direct coupling of stoichiometric
amounts of benzyl alcohol (1a) and urea (6:1 molar ratio) in
the presence of Au/TiO2-VS (1.5 mol% of Au, see Supporting
Information) for 10 h at 1408C under N2 atmosphere. To our
delight, the corresponding tertiary amine, tribenzylamine (2a),
was obtained in an excellent yield of ca. 92%, (Table 1,
entry 1). This result underscores the significantly greater perfor-
mance of supported gold relative to Ru(OH)x/TiO2 (35% yield
of 2a when stoichiometric amounts of 1a and urea were em-
ployed),[9b] and thus highlights the synthetic potential of this
new urea-mediated alcohol amination protocol. After the reac-
tion, the Au/TiO2-VS catalyst could be easily separated by fil-
tration; inductively coupled plasma (ICP) analysis confirmed
that the Au content of the filtrate was below the detection
limit (<0.10 ppm). Furthermore, the recovered Au/TiO2-VS
could be used in at least three runs without appreciable loss
of the original catalytic activity (Table 1, entry 2). These results
rule out any contribution to the observed catalysis from gold
species that had leached into the reaction solution, and show
that the observed catalysis is heterogeneous.
Compared to the impressive progress being made in the al-
kylation of organic amines with alcohols, reports dealing with
the direct synthesis of tertiary or secondary amines from alco-
hols and urea are only scarcely available.[9] In a precedent relat-
[a] L. He, Y. Qian, R.-S. Ding, Dr. Y.-M. Liu, Prof. Dr. H.-Y. He, Prof. K.-N. Fan,
Prof. Dr. Y. Cao
Department of Chemistry
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials
Fudan University
Handan Road 220, Shanghai 200433 (PR China)
Fax: (+86)21-65643774
E-mail: yongcao@fudan.edu.cn
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621

age particle size is shown in
Table S1. Clearly, Au NPs with
Table 1. Direct stoichiometric amination of benzyl alcohol (1a) with urea (alcohol/urea=6:1), using various
catalysts.[a]
a
smaller particle size have
a higher intrinsic activity.
To demonstrate the usefulness
of this Au-mediated amina-
tion system, we explored the
reactions of various alcohols
(Table 2). Structurally diverse pri-
mary alcohols, including benzyl-
ic, aliphatic linear, and aliphatic
cyclic ones reacted with urea (al-
cohol/urea=6:1) to give the de-
sired tertiary amines in good to
excellent yields. Benzyl alcohols
with electron-donating groups
reacted smoothly, while substi-
tution of electron-withdrawing
groups on the benzene ring de-
creased the reactivity (entries 1–
9). Notably, halogenated alcohols
including 4-fluoro-, 4-chloro-,
and 3-chloro benzyl alcohols re-
sulted in excellent yields of the
corresponding tertiary amines
without any dehalogenation (en-
tries 5–7). Aliphatic primary alco-
hols, which are difficult to react,
Entry
Catalyst
Conv. [%][b]
Yield. [%][b]
4a
2a
3a
other[c]
1
2[d]
3
4
5
6
7
8
9
10
11
12
13
14
15
Au/TiO2-VS
Au/TiO2-VS
Au/Fe2O3
Au/Al2O3
Au/CeO2
Au/ZnO
Au/C
TiO2
HAuCl4
Au2O3
Au0 powder
Pt/TiO2
98
97
23
94
40
8
35
5
n.r.
n.r.
n.r.
9
92
90
12
50
22
2
3
2
n.d.
4
n.d.
n.d.
1
–
–
1
2
3
2
8.5
44
8
8
35
2
–
–
–
3
0.5
n.d.
6
n.d.
n.d.
2
–
–
–
2
n.d.
n.d.
n.d.
–
–
–
1
n.d.
n.d.
–
3
6
n.d.
Pd/TiO2
Ag/TiO2
Ru/TiO2
32
9
41
17
6
4
9
3
7
19
11
[a] Reaction conditions: benzyl alcohol (0.6 mmol), urea (0.1 mmol), toluene (1.5 mL), catalyst (metal:
1.5 mol%), 1408C, 0.5 MPa N2, 10 h, n.r.=no reaction, n.d.=not detected. [b] Conversion and yield based on
benzyl alcohol consumption. Determined by GC using n-dodecane as the internal standard. [c] The main by-
products were benzaldehyde, benzylamine, and benzyl carbamate. [d] Third run.
The high efficiency of Au/TiO2-VS for the direct amination of
1a with urea relative to other supported gold catalysts was
clear (Table 1). Au/Fe2O3, Au/CeO2, Au/ZnO, and Au/C gave
poor yields, whereas Au/Al2O3 afforded a moderate yield (en-
tries 3–7). In control experiments with Au-free supports as cat-
alysts under similar conditions, the decomposition of urea to
NH3 was promoted by TiO2.[13] This result suggests that the in-
herent nature of the underlying support is critical in mediating
the in situ formation of NH3 involved in the direct amination
process. On the other hand, 2a was hardly observed in the
presence of pure TiO2 (entry 8), further confirming that the
highly dispersed gold NPs are indispensable for the desired
transformation. In addition, the use of the catalyst precursor
HAuCl4, and other Au species including Au2O3 and bulk Au0
powder (average particle size, ca. 150 nm), did not promote
the reaction at all (entries 9–11). Moreover, other metals (Pt,
Pd, Ag, and Ru) supported on TiO2 showed only very limited
activity and selectivity towards amination of 1a under present
conditions (entries 12–15).
could also be successfully converted into the corresponding
aliphatic tertiary amines in good yields (entry 10).
Secondary amines were obtained from reactions with secon-
dary alcohols (alcohol/urea=4:1). Various types of secondary
alcohol, including benzylic, aliphatic cyclic, and aliphatic linear
ones, reacted smoothly with urea (Table 2, entries 11–14). Even
with the alcohol/urea ratio set at 6:1, the corresponding secon-
dary amines still formed selectively, and tertiary amines were
detected in very small amounts,[16] which is likely due to steric
hindrance of the secondary alcohols or secondary amines pro-
duced. This result agrees very well with previous reports on
homogenous iridium-catalyzed N-alkylation of NH4BF4 with sec-
ondary alcohols[17] and Ru(OH)x/TiO2-catalyzed direct amination
of alcohols with urea.[9a]
The applicability of the synthetic protocol was highlighted
by a 60 mmol-scale amination of stoichiometric amounts of 1a
with urea in the presence of Au/TiO2-VS (0.03 mol% Au) under
solvent-free conditions at 1808C (see Scheme 1). After comple-
tion of the reaction, an impressive yield (93%) was achieved,
and 5.28 g of the corresponding 2a could be isolated in high
purity after simple manipulation of the reaction mixture (isolat-
ed yield ca. 92%).
To obtain an insight into the origin of the superior perfor-
mance achieved with Au/TiO2-VS as catalyst, we examined
a series of Au/TiO2 samples with different average Au particle
sizes. As determined by transmission electron microscopy
(TEM), the mean diameter of the Au NPs ranged from 1.8 to
8.0 nm (see Supporting Information, Figure S1). By assuming
that the Au NPs can be modeled as hemispherical particles,[14]
the number of surface Au atoms for each catalyst was estimat-
ed according to the method developed by Abad et al.[15] The
turnover frequency (TOF) per surface Au site versus the aver-
The time–conversion plot for the Au/TiO2-VS-catalyzed direct
amination of benzyl alcohol (1a) with urea provides useful in-
formation on the reaction pathway. As depicted in Figure 1,
benzylamine (5a, <0.5% yield) could be detected, and initially
formed imine (4a) was converted into secondary amine (3a)
with subsequent N-alkylation to produce the desired tribenzyl-
amine (2a). Moreover, it was confirmed in a separate experi-
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ChemSusChem 2012, 5, 621 – 624

Table 2. Au/TiO2-VS-catalyzed direct amination of alcohols with urea.[a]
Entry Substrate
1
Product
t [h] Conv. [%][b] Yield [%][b]
10 98
92 (91)
95 (93)
90 (86)
2
3
6
8
99
99
4
9
99
92 (90)
Figure 1. Time–conversion plot for the transformation of 1a to 2a in the
5
6
20 96
12 99
88 (83)
91 (85)
~
&
*
presence of Au/TiO2-VS. : Conversion of 1a, : yield of 2a, : yield of 3a,
!
: yield of 4a, N: Yield of 5a.
7
8
9
22 97
10 94
18 91
86 (82)
85 (80)
86 (80)
10
21 90
20 93
81 (79)
87 (82)
11[c]
12[c]
13[c]
14[c]
12 98
14 91
18 99
80 (74)
85 (81)
86 (81)
Scheme 2. Direct synthesis of tertiary and secondary amines through a cas-
cade of N-alkylation reactions.
tion of the alcohol initially gives the corresponding carbonyl
compound, the reaction of which with the NH3 produced by
urea hydrolysis leads to the formation of an imine intermedi-
ate. Subsequent hydrogen transfer of the resultant imine yields
the corresponding primary amine. In the overall reaction, the
second and third N-alkylations proceed by similar processes.
During this domino reaction sequence, the dehydrogenation
of alcohols could be involved in the rate-determining step,
based on the negative Hammett 1 value (1=À0.51, R2 =0.99)
found for the competitive amination of benzyl alcohol against
substituted benzyl alcohols with urea (see Figure S2).[18]
In summary, we demonstrate a new, versatile, and environ-
mentally benign catalytic route for the direct synthesis of terti-
ary or secondary amines by direct amination of alcohols with
inexpensive and readily available urea as a convenient nitrogen
source. A preliminary study using NH4HCO3 and (NH4)2CO3 as
alternative surrogates shows that the present Au-mediated
protocol is not limited to the amination of alcohols with urea
(see Table S2). Compared to protocols known from literature,
the main advantage of the heterogeneous gold-catalyzed strat-
egy, besides its recyclability, is its high atom efficiency by the
employment of stoichiometric amounts of safe and convenient
starting materials.
[a] Reaction conditions: alcohol (0.6 mmol), urea (0.1 mmol), toluene
(1.5 mL), 1.5 mol% Au, 1408C, 0.5 MPa N2. [b] Conversion and yield based
on alcohol consumption. Numbers in parenthesis refer to yields of isolat-
ed products. [c] Alcohol (0.6 mmol), urea (0.15 mmol), toluene (1.5 mL),
1.5 mol% Au, 1408C, 0.5 MPa N2.
Scheme 1. Au/TiO2-VS-catalyzed 60 mmol-scale direct amination of benzyl al-
cohol with urea under solvent-free conditions.
ment that the direct conversion of 5a and 3a with 1a over
Au/TiO2-VS resulted in high yields of 2a. These results indicate
that the direct synthesis of tertiary or secondary amines pro-
ceeds through a cascade of three (or two) N-alkylation reac-
tions (see Scheme 2). In the first N-alkylation, the dehydrogena-
ChemSusChem 2012, 5, 621 – 624
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623

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General procedure for the direct amination of alcohols with urea:
A mixture of alcohol (0.6 mmol), urea (0.1 mmol for primary alco-
hol, 0.15 mmol for secondary alcohol), metal catalysts (1.5 mol%
metal), toluene (1.5 mL), and n-dodecane (10 mL) as internal stan-
dard were placed into an autoclave (10 mL). The mixture was vigo-
rously stirred at 1408C under N2 atmosphere (0.5 MPa) for a given
reaction time. The product was identified by GC–MS, and the ob-
tained spectra were compared to standard spectra. Conversion and
product selectivity were determined by GC-17A gas chromato-
graph equipped with a HP-FFAP column (30 mꢁ0.25 mm) and
a flame ionization detector (FID). For isolation, the product was ex-
tracted with dichloromethane, the resulting combined organic
layers were dried over anhydrous Na2SO4, concentrated, and puri-
fied by silica gel column chromatography (eluent: n-hexane until
elution of toluene, then ethyl acetate).
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (20873026 and 21073042), New Century Excellent
Talents in the University of China (NCET-09-0305), National Basic
Research Program of China (2009CB623506), and Science & Tech-
nology Commission of Shanghai Municipality (08DZ2270500)
programmes.
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Keywords: alcohols · amination · amines · gold · nanoparticles
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Published online on March 13, 2012
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