Amination of aliphatic alcohols with urea catalyzed by ruthenium complexes: effect of supporting ligands

Article abstract of DOI:10.1080/00958972.2020.1804058

In the present study, ruthenium-catalyzed amination of alcohols by urea as a convenient ammonia carrier in the presence of free diphosphine ligands has been described. A number of ruthenium-phosphine complexes have been studied among which, [(Cp)RuCl(dppe)] was found as an efficient catalyst for alcohol amination reaction. The crystal structures of two new half-sandwich ruthenium complexes, [(Cp)RuCl(dppe)] and [(C₆H₆)RuCl₂(PHEt₂)], were determined by X-ray crystallographic analysis. Also the effect of using different supporting phosphines, ratio of raw materials and reaction temperature on conversion and selectivity was investigated. Under optimum reaction conditions high conversion (98percent) and chemo-selectivity toward secondary amines were obtained.

Full text of DOI:10.1080/00958972.2020.1804058

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Amination of aliphatic alcohols with urea
catalyzed by ruthenium complexes: effect of
supporting ligands
Sara Dindar & Ali Nemati Kharat
To cite this article: Sara Dindar & Ali Nemati Kharat (2020) Amination of aliphatic alcohols with
urea catalyzed by ruthenium complexes: effect of supporting ligands, Journal of Coordination
Chemistry, 73:13, 1954-1966, DOI: 10.1080/00958972.2020.1804058
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JOURNAL OF COORDINATION CHEMISTRY
020, VOL. 73, NO. 13, 1954–1966
2
https://doi.org/10.1080/00958972.2020.1804058
Amination of aliphatic alcohols with urea catalyzed by
ruthenium complexes: effect of supporting ligands
Sara Dindar and Ali Nemati Kharat
School of Chemistry, College of Science, University of Tehran, Tehran, Iran
ABSTRACT
ARTICLE HISTORY
Received 3 May 2020
Accepted 14 July 2020
In the present study, ruthenium-catalyzed amination of alcohols
by urea as a convenient ammonia carrier in the presence of free
diphosphine ligands has been described. A number of ruthenium-
phosphine complexes have been studied among which,
KEYWORDS
Amination reaction; urea;
half-sandwich ruthenium
complex; diphosphine
ligands; homogeneous
catalyst; borrowing
hydrogen method
[
(Cp)RuCl(dppe)] was found as an efficient catalyst for alcohol ami-
nation reaction. The crystal structures of two new half-sandwich
ruthenium complexes, [(Cp)RuCl(dppe)] and [(C )RuCl (PHEt )],
6
H
6
2
2
were determined by X-ray crystallographic analysis. Also the effect
of using different supporting phosphines, ratio of raw materials and
reaction temperature on conversion and selectivity was investigated.
Under optimum reaction conditions high conversion (98%) and
chemo-selectivity toward secondary amines were obtained.
1. Introduction
Amines are essential intermediates in large scale production of industrial chemicals.
Lower aliphatic amines (C –C ) are important intermediates in chemical, pharmaceut-
1
6
ical and petrochemical industries and have many applications as corrosion inhibitors
in lubricating oils, greases and fuel oil as sludge dispersants and stabilizers [1, 2].
CONTACT Ali Nemati Kharat
alnema@khayam.ut.ac.ir
School of Chemistry, College of Science, University of
Tehran, Tehran, Iran
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2020.1804058.
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

JOURNAL OF COORDINATION CHEMISTRY
1955
Higher aliphatic amines (fatty amines) and their derivatives also are useful as fabric
softeners, corrosion inhibitors and emulsifiers [3]. The most significant method known
for production of aliphatic amines is the reductive amination of the corresponding car-
bonyl compounds [4, 5]. Utilizing stoichiometric amounts of toxic and expensive
reagents, low selectivity, low atom-economy and large amounts of wasteful salts have
been recognized as the main problematic issues in these reactions [6–8]. Given this
drawback as well as the benefits which could be brought up with green chemistry,
the development of methods which are economically and environmentally accepted
and more efficient for the synthesis of amines is still a challenge for researchers to be
taken into account. A green approach of catalytic alkylation of amines using alcohols
instead of aldehydes or ketones has been done via a facile strategy known as
“
Borrowing Hydrogen” method which is an attractive candidate for synthesis of amines
since alcohols are inexpensive, readily available, non-toxic and theoretically water is
the only by-product [9, 10]. The Borrowing Hydrogen, also called hydrogen auto-trans-
fer, catalytic cycle involves initial metal catalyzed dehydrogenation to form an inter-
mediate carbonyl compound which undergoes condensation with the amine to form
corresponding imine and water. Hydrogen generated in the dehydrogenation step
reduces the imine to reach the desired alkylated amine product [11].
Reactions of simple alcohols (e.g. methanol and ethanol) with ammonia are used
for commercial production of amines in the presence of heterogeneous catalysts,
severe reaction conditions (relatively high temperatures and pressures), also lower
selectivity is consequences of utilizing these types of catalysts [12]. The first homoge-
neously catalyzed reaction of this type was reported by Grigg and Watanabe in 1981
[13, 14], in which a number of examples have been catalyzed by mainly ruthenium-
and iridium-based complexes and less commonly by other transition metal complexes.
The scope of ruthenium-catalyzed Borrowing Hydrogen amination could be signifi-
cantly improved by the development of catalyst systems that require lower catalyst
loading, shows broader substrate scope, avoid the need for a large excess of either
the alcohol or the amine and operate at more moderate temperatures and with
shorter reaction times [15]. Recently, great contributions have also been made by the
groups of Beller [16, 17], Williams [18, 19], Milstein [20], Zhao [21], Fujita [22] and
Bruneau [23]. Catalytic systems based on ruthenium precursors have proved to be effi-
cient for N-alkylation of amines by alcohols in the preparation of higher amines.
Despite the recent progress in catalytic aminations, the chemo-selective synthesis of
amines using ammonia is a highly challenging goal. Owing to their nucleophilicity, pri-
mary amines are in general more reactive than ammonia, and the sequential formation
of secondary and tertiary amines results [24]. Although low cost and convenient avail-
ability of ammonia make it a proper source of nitrogen, there are some difficulties
regarding its storage, handling and transportation. Applying ammonium related com-
pounds such as ammonium salts or urea are an attractive alternative to tackle these
issues [25, 26]. Despite its great potential to synthesize amines by reaction with alco-
hols, there are only several reports in the literature, for example, synthesis of second-
ary and tertiary amines from NH OAc and NH BF have been reported by a
4
4
4
ꢀ
homogenous iridium (Cp Ir) catalyst [27, 28]. Urea as abundant and inexpensive
material that provides high nitrogen content can be used for preparation of valuable

1
956
S. DINDAR AND A. N. KHARAT
nitrogen containing compounds. In 2009, synthesis of tertiary and secondary amines
directly from alcohols and urea was reported by Mizuno [29]. However, Ru(OH) /TiO2
x
performed as an efficient heterogeneous catalyst; obtaining of the desired substituted
amines required large excess of alcohols (alcohol/urea ¼ 10:1). Cao et al. obtained the
best activity and selectivity for preparation of tertiary amine using an expensive gold
catalyst (Au/TiO ) [30].
2
In this article, half-sandwich ruthenium complexes have been synthesized and
tested homogeneously for preparation of aliphatic amines from different linear ali-
phatic alcohols using urea as nitrogen source. Secondary amines were prepared with
acceptable chemoselectivity from several aliphatic alcohols with urea through the use
of catalytic amounts of ruthenium complexes. Also the effect of a number of phos-
phine ligands as supporting ligands on activity and selectivity of the reaction were
investigated. Several experiments were performed with different ratios of urea and
alcohol to find optimum conditions.
2. Experimental
2.1. Materials and instrumentation
All the used chemicals and reagents in this study were Sigma-Aldrich and
StremChemical Companies’ products and utilized without purification. The solvents
1
were of analytical grade and purified prior to use based on standard methods. H
NMR spectra were recorded at room temperature in CDCl3 on a Bruker Avance
400 MHz instrument. Single-crystal X-ray diffraction data were collected on a MAR345-
dtb diffractometer with an image plate detector graphite-monochromated Mo Ka radi-
ation at room temperature using Stoe X-AREA software. A Gas Chromatograph (GC),
Agilent Technologies 7890 A Instrument (equipped with a HP-1 capillary column, a FID
detector, and a mass spectroscope model 5975 C with a triple-axis detector), were
used for monitoring of reaction products and their identity and octane considered as
6
the internal standard. [(Ru(cymene)Cl ] , [RuCl (g –C H )] , [RuH (CO)(PPh ) ],
2
2
2
6
6
2
2
3 3
[
RuCl(OAc)(PPh ) ] and [(Cp)RuCl(PPh ) ] were prepared according to literature meth-
3 3 3 2
ods [31–33]. Ruthenium diphosphine complexes were synthesized by published meth-
ods [34, 35].
2.2. Catalyst preparation
2.2.1. [(Cp)RuCl(dppe)]
This complex was prepared according to the method reported by Ashby [36].
Diphenylphosphinoethane (dppe) (160 mg, 0.4 mmol) was added to a solution of
[
RuCpCl(PPh ) ] (290 mg, 0.4 mmol) in toluene (50 mL) (Scheme 1, (a)). The mixture was
3 2
refluxed for 10 h, and then the solution was cooled to room temperature. The solution
was then concentrated in vacuum to approximately 1 mL and the product precipitated
1
with diethyl ether. H NMR (500 MHz, CDCl ) d (ppm): 7.14–7.19 (m, 20H, Ph), 4.56 (s,
5
3
31
H, C H ), 2.41–2.65 (m, 4H, P-CH ). P NMR (CDCl ), d (ppm): 79.60.
5 5 2 3

JOURNAL OF COORDINATION CHEMISTRY
1957
Figure 1. ORTEP view of (A) [(Cp)RuCl(dppe)] (1) and (B) [(C H )RuCl (PEt H)] (2) with atom num-
6
6
2
2
bering scheme. The thermal ellipsoids are drawn at the 50% probability level. Orientation disorder
was observed for benzene ring in 2 at 298 K. The hydrogen atom attached to P is shown in 2; all
1
other hydrogens are not shown.
2
.2.2. [(C h )RuCl (PEt H)]
6 6 2 2
In a 50-mL round bottom flask, 300 mg (0.6 mmol) of [(C H )RuCl ] was dissolved in
6
6
2 2
5
0 mL toluene, then an excess amount of diethylphosphine (PEt H) (1 mL) was added
2
to the solution. The reaction mixture was heated under reflux for 8 h (Scheme 1, (b)).
The solution was cooled, and the solvent was removed under reduced pressure after-
wards, and the residual brown solid was recrystallized from dichloromethane-ether to
1
give red crystals of the complex [37]. H NMR (400 MHz, CDCl ) d (ppm): 5.8 (s, 6H,
3
1
31
C H ), 4.65 (dm, 1H, H-P, J 381.2), 2.17 (m, 6H, CH ), 1.29 (dt, 4H, CH ). P NMR
6
6
PH
2
3
(
CDCl ) d (ppm): 23.03.
3
2.3. General catalytic procedure for amination
The autoclave was charged with 1-hexylalcohol (2.5 mmol) and urea (2.5 mmol) in 1 mL
diglyme. [(Cp)RuCl(dppe)] (3% mol) as catalyst, diphenylphosphinopropane (dppp)
(
0.07 mmol) as supporting ligand and 0.04 g K CO as base was added. The autoclave
2 3
was closed, flushed with argon for 5 min and then temperature was adjusted at
ꢁ
1
60 C. After 12 h, the autoclave was cooled to room temperature and the products
were analyzed by gas chromatography.
3
. Results and discussion
.1. Crystal structures
The molecular structures of [(Cp)RuCl(dppe)] (1) and [(C H )RuCl (PHEt )] (2) are illus-
3
6
6
2
2
trated in Figure 1. These complexes are air-stable, soluble in most organic solvents
and crystallize in the monoclinic space group P2 . Suitable crystals of the complexes
1
were obtained by slow diffusion of diethyl ether in DCM solution of 1 and CH CN
3
solution of 2. The structures of 1 and 2 are similar to previously reported half-sand-
wich ruthenium(II) complexes and single-crystal structure of complexes confirmed
three-legged piano-stool geometry for Ru(II) in both complexes with chlorine and

1
958
S. DINDAR AND A. N. KHARAT
Table 1. Crystallographic and structure refinement data of 1 and 2.
[
(Cp)RuCl(dppe)]
29ClP Ru
600.0
[(C
6
H
6
)RuCl
2
(PEt
l2PRu
340.17
2
H)]
Formula
C
31
H
2
10 16
C H C
Fw
k/Å
T/K
0.71073
290(1)
0.71073
290(1)
Crystal system
Space group
Monoclinic
P12
Monoclinic
P12
1
1
1
1
a/Å
b/Å
c/Å
a/˚
b/˚
c/˚
9.4949(19)
15.042(3)
9.983(2)
90
110.16(3)
90
7.4980(15)
15.538(3)
11.681(2)
90
100.969(3)
90
3
V/Å
1338.4(5)
1.489
1336.1(5)
1.691
ꢃ3
D
Z
calc/Mg.m
2
4
ꢃ1
l (mm
F(000)
)
0.824
612
1.656
680
2
h (˚)
52.0
0.0387
1.093
0.0328
0.0905
2000008
49.962
0.0566
1.168
0.0365
0.0847
2000009
R (int)
GOOF
R
wR (I > 2r(I))
2
CCDC No.
a
1
(I > 2r(I))
b
a
R
wR
1
¼ RjjF
o
j ꢃ jF
c
jj/RjF
o
j.
b
2
2 2
2
1
=
2
2
¼ [R(w(F
o
ꢃ F
c o
) )/Rw(F )2] .
Table 2. Selected geometric parameters for 1 and 2.
Parameters
1
Parameters
2
Bond distances
Ru–Cl
1
2.441
2.286
2.268
1.865
Ru–Cl
Ru–Cl
Ru–P
1
Ru–CCent
1 3
P –C
1
2.401
2.416
2.310
1.673
1.820
Ru–P
1
2
Ru–P
2
Ru–CCent
Bond angles
P
P
P
P
P
1
2
1
1
2
–Ru–Cl
–Ru–Cl
1
82.36
84.48
82.57
98.74
98.87
91.39
111.32
107.91
P
P
Cl
P
1
–Ru–Cl
–Ru–Cl
1
86.86
81.69
87.17
100.90
87.80
90.28
90.98
1
1
2
–Ru–P
2
1
–Ru–Cl
–Ru–C
2
–Ru–C13
–Ru–C19
1
2
P
1
–Ru–C12
–Ru–C14
–Ru–C
Cl–Ru–C18
Cl
Cl
1
P
P
1
–C
–C
7
–C
–C
4
2
4
2
4
7
phosphine ligands. Crystallographic data and selected bond lengths and angles are
listed in Tables 1 and 2, respectively.
Complex 1 consists of one bidentate phosphine ligand (dppe) coordinated to Ru(II)
center by bond distances of Ru–P(1) ¼ 2.286(2) Å and Ru–P(2) ¼ 2.268(2) Å and bite
ꢁ
angle of 82.57 . The Ru-bonded Cp are essentially planar with C–C bond length, giving
a mean value of 1.403 Å and Ru-CpCent bond distance of 1.865 Å. There are two mole-
cules of 1 in each unit cell that connected each other by C–Hꢂꢂꢂp interaction
i
ii
[
C(9)–H(9) … C (C21–C26) ] ¼ 2.857 Å (i ¼ 1 ꢃ x, ꢃ1/2 þ y, 1 ꢃ z and ii ¼ x, y, z).
cent
Furthermore, a weak hydrogen bond exists between the C–H bond of phenyl group of
phosphine ligand and chloride ions of the next molecule [C(27)–H(27) …Cl(1) ]¼ 2.935 Å
iii
ii

JOURNAL OF COORDINATION CHEMISTRY
1959
Figure 2. View of C–H … Cl hydrogen bonding (red line) in [(C H )RuCl (PEt H)].
6
6
2
2
ꢁ
(
iii ¼ 1 þ x, y, 1 þ z) with angle of 133.01 that is stabilizing the structure of
the complex.
(C H )RuCl (PEt H)] has four molecules in packing and it has same conformation
[
6
6
2
2
with 1 consisting of a monodentate phosphine ligand (PEt H) and two chlorine atoms
2
with bond distances of Ru–P ¼ 2.310 Å, Ru–Cl(1) ¼ 2.401 Å and Ru–Cl(2) ¼ 2.416 Å,
comparable with reported bond lengths in similar ruthenium complexes [38]. The ben-
zene ring that is connected to the ruthenium center shows a disorder position which
was caused by an internal rotation around the central axis of the molecule. This disor-
dered configuration is normal for benzene ring in room temperature and upon cool-
6
ing, the benzene ring becomes ordered [39]. Distance from -coordinated benzene
centroid and Ru atom is 1.673 Å. The C–HꢂꢂꢂCl interactions play an important role in
assembly of this complex and contribute to stabilize the crystal structure. One of the
chlorine atoms of each molecule (Cl1) interacted by two C–H bonds,
iv
ii
v
ii
[
C(12)–H(12) … Cl(1) ] ¼ 2.723 Å and [C(3)–H(3A) … Cl(1) ] ¼ 2.932 Å of next molecule.
Another chlorine atom is connected by C–H bond of benzene ring in two separate
vi
ii
vi
ii
molecules, [C(1)–H(1) … Cl(2) ] ¼ 2.788 Å and [C(13)–H(13) … Cl(2) ] ¼ 2.934 Å; these
two interactions are shown in Figure 2 (iv ¼ 1 ꢃ x, 1 ꢃ y, 1 ꢃ z, v ¼ ꢃ1 þ x, y, z and
vi ¼ ꢃ1/2 þ x, 1/2 ꢃ y, ꢃ1/2 þ z).
3.2. Catalytic studies
Herein, secondary amines were prepared with good chemo-selectivity from reaction of
aliphatic alcohols with urea in the presence of catalytic amounts of ruthenium com-
plexes. In this context, a series of ruthenium phosphine complexes have been synthe-
sized and tested in the reaction of hexylalcohol and urea in ethylene glycol dimethyl
ether as solvent. As shown in Table 3, octahedral complexes (entries 7–12) showed
less reactivity compared to four-coordinate complexes. It shows the importance of free
coordination sites on metal centers to increase activity of complexes [40]. This is con-
firmed by the least activity of octahedral diphosphine complexes in these series.
Complex 1 showed higher catalytic activity than 2 (entries 3 and 4 in Table 3). It
was proposed that catalytic dehydrogenation of alcohols can be facilitated by an

1
960
S. DINDAR AND A. N. KHARAT
Table 3. Amination of hexyl alcohol with urea in the presence of different ruthenium catalysts.
Selectivity
ꢁ
ꢁ
ꢁ
Entry
Complex
Conversion
1 Amine
2 Amine
3 Amine
1
2
3
4
5
6
7
8
9
No catalyst
–
–
2
9
10
9
12
19
12
24
12
7
–
–
4
1
RuCl
[(Cp)RuCl(dppe)]
[(C )RuCl (PHEt
[Ru(cymene)Cl
[(Cp)RuCl(PPh
[RuH (CO)(PPh
[RuCl(OAc)(PPh
[RuCl (dppe)
(dppm)
3
2
ꢂxH O
61
99
95
91
88
83
79
65
60
62
59
96
94
90
90
90
86
81
87
61
76
82
85
83
6
H
6
2
2
)]
2
]
)
)
2
1
2
–
3
2
]
2
3
3
]
]
3
)
3
1
2
2
]
15
12
11
9
10
11
12
13
[RuCl
[RuCl
[RuCl
2
2
]
2
(dppp)
(dppb)
2
]
]
2
2
6
15
a
[(Cp)RuCl(dppe)]
2
Reaction conditions: 2.5 mmol hexyl alcohol, 2.5 mmol urea, 3 mol% catalyst, 0.07 mmol (dppp) supporting ligand,
ꢁ
1
6
.5 mL diglyme, 12 h and 160 C.
mol% of catalyst was used.
a
electron-rich metal center. Therefore, bidentate phosphine ligand has been found to
have a stronger effect, perhaps due to the higher basicity and stronger coordination
ability compared to mono phosphine ligand, PEt H. Among the ruthenium complexes
2
with diphosphine ligands, the selectivity was likely controlled by the strict environ-
ment around the metal center (entries 9–12 in Table 3). cis-[RuCl (dppp) ] showed the
2
2
best activity in this group of catalysts. The selectivity of the complexes depends on
the ligand’s size, as smaller diphosphines (dppm and dppe) showed higher selectivity
for primary amines while increasing the bridge length for bulky diphosphine (dppp
and dppb) leads to higher selectivity for secondary amines. From the experimental
results we chose 3 mol% of catalyst as the optimum value for proper conversion,
while, with increasing the amount of catalyst, no positive effect on the reaction con-
version was obtained (entry 13).
In this reaction, only with careful adjustment of reaction parameters, such as the
relative proportion of the reactants, reaction temperature and the use of proper addi-
tives one can tune conditions, so that the desired product becomes predominant.
Encouraged by the results obtained by [(Cp)RuCl(dppe)] (1), this complex was used for
further investigation to find the effect of different parameters on reaction conversion
and selectivity (Table 4). In this regard, the effect of reaction temperature was exam-
ꢁ
ined from 120 to 170 C. Excellent results were obtained when the reactions were per-
ꢁ
formed at 160 C, during which a total conversion of over 90% was obtained under
our mentioned experimental conditions. A complete hydrolysis of urea did not occur
ꢁ
ꢁ
during the reactions from 120 to 130 C, so after 12 h of reaction at 130 C, unreacted
urea remained in the autoclave. As shown in Table 4, for reaction of aliphatic alcohols
even a higher temperature is required. The reaction was considerably accelerated by
ꢁ
increasing temperature and for reaction of hexylalcohol with urea at 160 C proper
yield for secondary amine was obtained.
Effect of alcohol to urea mole ratio on n-alkylation reaction is summarized in
Table 5. The hydrolysis of urea leads to formation of two moles of ammonia and one
mole of carbon dioxide, resulting in a rise in alkalinity of the mixture as the reaction
proceeds. Alkali conditions can expedite dehydrogenation of alcohols and formation

JOURNAL OF COORDINATION CHEMISTRY
1961
Table 4. Effect of temperature on amination of aliphatic and aromatic alcohols with urea.
Selectivity
ꢁ
ꢁ
ꢁ
ꢁ
Entry
Alcohol
Temperature ( C)
Conversion
1 Amine
2 Amine
3 Amine
1
2
3
4
5
6
7
8
9
Benzylalcohol
Benzylalcohol
Benzylalcohol
Benzylalcohol
Benzylalcohol
Benzylalcohol
Benzylalcohol
Benzylalcohol
Hexyl alcohol
Hexyl alcohol
Hexyl alcohol
Hexyl alcohol
120
130
140
150
155
160
165
170
150
155
160
170
20
28
50
86
97
100
100
100
55
85
94
89
84
60
59
54
51
48
44
40
25
21
9
11
16
40
39
46
44
45
44
60
74
78
83
–
–
–
–
5
7
12
–
1
1
8
1
1
1
0
1
2
96
Reaction conditions: 2.5 mmol alcohol, 2.5 mmol urea, 3 mol% complex 1, 0.07 mmol (dppp) supporting ligand,
.5 mL solvent, 12 h. Yields were determined by GC.
1
Table 5. Effect of alcohol to urea mole ratio on amination of alcohol with urea.
Selectivity
ꢁ
ꢁ
ꢁ
Entry
Substrate
Ratio alcohol/urea
Conversion
1 Amine
2 Amine
3 Amine
1
2
3
4
5
6
7
8
9
Benzylalcohol
Benzylalcohol
Benzylalcohol
Hexyl alcohol
Hexyl alcohol
Hexyl alcohol
2-Ethylhexanol
2-Ethylhexanol
2-Ethylhexanol
4
2
1
4
2
1
4
2
1
88
97
100
81
93
98
73
77
88
1
2
8
1
3
10
48
55
61
77
81
81
83
95
88
52
45
39
22
17
11
16
2
2
–
–
–
ꢁ
Reaction conditions: 2.5 mmol alcohol, 3 mol% complex 1, 0.07 mmol (dppp) ligand, 1.5 mL diglyme, 12 h and 160 C.
Yields were determined by GC.
Scheme 1. Synthesis route for preparation of ruthenium complexes [36].

1
962
S. DINDAR AND A. N. KHARAT
Scheme 2. Hydrogen-borrowing mechanism of amination reaction of aliphatic and aromatic alco-
hols. (a) dehydrogenation, (b) condensation, (c) hydrogenation.
Scheme 3. Direct synthesis of cyclohexylamine and dicyclohexylamine from cyclohexanol and urea
catalyzed by 1 [37].
of related aldehydes [41]. On the basis of reaction mechanism, the direct synthesis of
amine by alcohol proceeds through three steps (Scheme 2). In the first step (a), the
dehydrogenation of the alcohol initially gives the corresponding carbonyl compound,
the reaction of which with the NH produced by urea hydrolysis leads to the forma-
3
tion of an imine intermediate (b) and then subsequent hydrogen transfer to the result-
ant imine yields the corresponding primary amine (c). In the overall reaction, the
second and third N-alkylations proceed by similar processes and lead to secondary
and tertiary amines [20, 27].
After several experiments we found that 2 moles of alcohol per mole of urea is the
optimum amount for having the best reactivity and we reached 93% conversion with
9
8
5% selectivity toward secondary amines. By changing this ratio to 4, conversion was
1% but when stoichiometric amounts of urea and alcohol were used 98% conversion
with lower selectivity for secondary amine was obtained. The same trends were seen
for other aliphatic alcohols. In addition, when a higher mole ratio of urea respect to
alcohol was used a large amount of unknown by-products were produced. The alcohol
to urea mole ratio of 2 was chosen as the optimum ratio afterwards.

JOURNAL OF COORDINATION CHEMISTRY
1963
Table 6. Effect of supporting ligand on amination of alcohols with urea.
Selectivity
ꢁ
ꢁ
ꢁ
Entry
Supporting ligand
Conversion
1 Amine
2 Amine
3 Amine
1
2
3
4
5
6
7
8
No ligand
Xantphos
30
98
73
50
82
98
98
93
50
24
10
20
12
8
50
76
79
70
81
91
73
66
–
–
11
10
6
1
–
–
PPh
3
dppe
dppm
dppp
dppb
dpppe
26
34
Reaction conditions: 2.5 mmol 1-hexylalcohol, 0.12 mmol urea, 3 mol% complex 1, 0.07 mmol phosphine supporting
ꢁ
ligand, 1.5 mL diglyme, 12 h and 160 C.
Table 7. Amination of primary and secondary alcohols with urea.
Selectivity
ꢁ
ꢁ
ꢁ
Entry
1
Substrate
Conversion
98
1 Amine
2 Amine
3 Amine
12
40
88
60
–
2
3
90
92
–
62
38
–
4
5
87
90
14
52
75
48
1
–
6
89
17
83
–
Reaction conditions: 2.5 mmol alcohol, 2.5 mmol urea, 3 mol% complex 1, 0.07 mmol (dppp) ligand, 1.5 mL solvent,
ꢁ
1
2 h and 160 C.

1
964
S. DINDAR AND A. N. KHARAT
A range of phosphine ligands have been used to investigate the importance of sup-
porting ligands in the course of the reaction and the results are shown in Table 6.
Supporting ligand has a very important effect on this reaction and in the absence of
free supporting ligand very low conversion will be obtained (entry 1, Table 6). For hav-
ing high conversion, using an excess amount of free ligand is crucial. Phosphine
ligands may displace the complex aldehyde to facilitate its reaction in solution with
amine or it may accelerate the catalytic hydrogenation of imine [27]. Under experi-
mental reaction conditions a common phosphine ligand, such as triphenylphosphine,
showed low conversion. Enhancement of the product yield with suitable chemo-select-
ivity was observed using diphenylphosphinopropane (dppp), while the medium yield
was the only outcome of using structurally similar diphenylphosphinomethane (dppm)
and diphenylphosphinoethane (dppe). Moreover, by bulky diphenylphosphinobutane
(dppb), diphenylphosphinopentane (dpppe) and xantphos satisfying results were
obtained. Under these conditions primary amines did not proceed to secondary
amines so the selectivity has been decreased. This ruthenium catalytic system was
tested for preparation of amines from other alcohols under the optimum conditions.
Good yields were obtained for the amination of alcohols which can be seen in Table
7
. For sterically hindered 2-ethylhexanol and 2-methylbutanol, the percentage of pri-
mary amine increased in comparison with octanol and butanol. These results show
that steric hindrance has affected selectivity of amines and led to decrease in selectiv-
ity toward secondary amines.
Also we tested the applicability of this method for the amination of a secondary
alcohol, cyclohexanol (Scheme 3). Due to difficulty in dehydrogenation step as well as
the hydrogenation of the iminium or enamine intermediates, secondary alcohols are
more challenging in amination reaction using hydrogen borrowing strategy. With the
current catalytic system cyclohexanol results in a mixture of primary and secondary
amines with 90% conversion.
4. Conclusion
A simple and environmentally benign homogeneous catalytic route introduced for the
direct synthesis of amines by reaction of alcohols with inexpensive and readily avail-
able urea as a convenient nitrogen source. [CpRuCl(dppe)] was selected as an active
catalyst and it showed proper activity and selectivity for secondary amine under the
experimental reaction condition.
Acknowledgement
The authors gratefully acknowledge the financial support from Iran National Science
Foundation (INSF).
Disclosure statement
No potential conflict of interest was reported by the authors.

JOURNAL OF COORDINATION CHEMISTRY
1965
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