A practical and highly efficient transfer hydrogenation of aryl azides using a [Ru(p-cymene)Cl2]2 catalyst and sodium borohydride

Article abstract of DOI:10.1016/j.crci.2018.07.004

Various aniline derivatives were synthesized by selective reduction of aryl azides in the presence of a dichloro(p-cymene)ruthenium(II) dimer ([Ru(p-cymene)Cl₂]₂) via hydrolysis of sodium borohydride. The hydrogenation reactions were carried out in aqueous media at room temperature. Most of the reactions were completed within 10 min with quantitative yields.

Full text of DOI:10.1016/j.crci.2018.07.004

C. R. Chimie xxx (2018) 1e4
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Full paper/M eꢀ moire
A practical and highly efficient transfer hydrogenation of aryl
azides using a [Ru(p-cymene)Cl ] catalyst and sodium
2
2
borohydride
Benan Kilbas a
,
b
,
*
, Yunus Emre Yilmaz , Sinem Ergen a
a
a
Department of Chemistry, Faculty of Sciences, Duzce University, 81620 Duzce, Turkey
Moltek A. S. Gebze Organize Sanayi, 41400 Gebze, Kocaeli, Turkey
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Various aniline derivatives were synthesized by selective reduction of aryl azides in the
Received 22 May 2018
Accepted 17 July 2018
Available online xxxx
2 2
presence of a dichloro(p-cymene)ruthenium(II) dimer ([Ru(p-cymene)Cl ] ) via hydrolysis
of sodium borohydride. The hydrogenation reactions were carried out in aqueous media at
room temperature. Most of the reactions were completed within 10 min with quantitative
yields.
Keywords:
©
2018 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
2 2
[Ru(p-cymene)Cl ]
Homogeneous catalyst
Transfer hydrogenation
Aryl azide
1. Introduction
unsaturated functional system, is a more convenient
method for the reduction of diverse functional groups
Reduction of azidoarenes to the aniline derivatives is an
bearing mainly carbonyls and imines [15,16]. This method
has several important advantages such as lack of explosive
free hydrogen, chemoselectivity, inexpensive and readily
available hydrogen sources and also the use of readily
accessible, recyclable and reusable catalysts [17e19]. A
plenty of homogeneous and heterogeneous catalysts have
been already reported in the literature for the TH process.
Although heterogeneous catalysts are recyclable and reus-
able, homogeneous catalysts are usually preferred for TH
due to their more activity and selectivity. Recently,
considerable attention has been focused on arene ruthe-
nium complexes as TH catalysts due to their unique prop-
erties, inherent catalytic ability and versatile building
blocks for coordination cages and macrocyles [20]. Arene
rings provided sterical hindrance of the ruthenium center,
which prevent rapid oxidation, and they are also resistant
to the substitution reactions. Besides, there are three
available coordination sites located on the opposite side of
the arene ligand can be easily replaced by donor ligands to
employ more catalytically active Ru(II) complexes [21,22].
industrially important process, since the aniline derivatives
are crucial intermediates for the synthesis of dyes, poly-
mers, pharmaceuticals, agrochemicals and biologically
active molecules [1e5]. Various synthetic approaches for
the reduction of azidoarenes proceed through catalytic
hydrogenation [6,7], NaBH
4
/phase-transfer catalysis [8],
$OEt /EtSH [10], metal medi-
eZn [11], FeCl eZn [12], Sm/I
triphenylphosphine [9], BF
ated reductions such as NiCl
3
2
2
3
2
[
13] and tellurium metal [14]. These traditional methods
have some disadvantages such as high temperature, high
pressure, long reaction time and tedious work-up process.
Furthermore, desired transformation is more complicated
by using polyfunctional azide substrates in which poor
chemoselectivity can be observed. Transfer Hydrogenation
(
TH), hydrogen abstraction from the reagent by catalyst
contribution followed by hydrogen addition to the
*
Corresponding author.
E-mail address: bkilbas@gmail.com (B. Kilbas).
https://doi.org/10.1016/j.crci.2018.07.004
631-0748/© 2018 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1
Please cite this article in press as: B. Kilbas, et al., A practical and highly efficient transfer hydrogenation of aryl azides using a
Ru(p-cymene)Cl catalyst and sodium borohydride, Comptes Rendus Chimie (2018), https://doi.org/10.1016/j.crci.2018.07.004
[
2 2
]

2
B. Kilbas et al. / C. R. Chimie xxx (2018) 1e4
Although exponential growth has been observed for TH of
carbonyl and imine polar bonds by the aid of arene ruthe-
nium (II) complexes, the usage of these catalysts for azide
hydrogenation is rare.
Herein we report for the first time that various azido
arenes were catalytically converted to aniline derivatives
by the contribution of the [Ru(p-cymene)Cl ] [23] dimeric
2 2
structure and sodiumborohydride in the aqueous medium
with shorter reaction times and high yields.
As it is known that, azido compounds are more reactive,
2 2
therefore we initially utilized the [Ru(p-cymene)Cl ]
dimer to check the completion of the reduction process.
The careful GC analysis indicated that absolute conversions
of azidoarenes to corresponding anilines were achieved
within 10 min at room temperature. These results
demonstrate that there is no need for an extra reaction step
for the generation of a new catalyst to afford reduction.
2 2
Furthermore, [Ru(p-cymene)Cl ] is a promising candidate
for the chemoselective TH as previously published mono-
meric arene ruthenium (II) complexes. On the other hand, a
hydrogen source played another crucial role in TH. Appar-
ently, a compound which has lower oxidation potential can
act as a versatile hydrogen carrier, because these unique
properties enable hydrogen transfer from the donor to the
substrate in the presence of the catalyst. Alcohols, formic
acid, hydrazine and 1,4-dihydropyridine derivatives have
been extensively used as hydrogen carriers [25]. Even they
are more accessible, eco-friendly and safe, TH requires
harsh reaction conditions such as high temperature and
long reaction time. The nature of the hydrogen carriers also
influences the chemoselectivity and activity of TH [26,19c].
2
. Results and discussion
Monomeric piano-stool type arene ruthenium (II)
complexes are generally derived by the cleavage of the
Ru(p-cymene)Cl dimeric structure with eN, eS, eP
[
2 2
]
type strong donor ligands to provide efficient performance
and catalytic activity for TH [16]. Recently, Cetinkaya et al.
described novel monomeric 3,4-dihydroquinazoline ruth-
enium(II)complexes for the reduction of acetophenone to
the corresponding secondary alcohol with high yield after
1
h [24]. They claimed that only 15% conversion was
2 2
observed when [Ru(p-cymene)Cl ] was used as a catalyst.
Table 1
2 2 3
Ru(p-cymene)Cl ] catalyzed reduction of various ReN
compounds.a
[
Entry
ReN
3
Product
Conversion (%)
Yieldb (%)
1
2
3
4
5
6
7
8
9
Phenyl-
Aniline
100
100
100
100
100
100
100
100
100
100
100
100
100
100
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
o-Chlorophenyl-
p-Chlorophenyl-
o-Bromophenyl-
p-Bromophenyl-
p-Iodophenyl-
o-Tolyl-
m-Tolyl-
p-Tolyl-
o-Methoxyphenyl-
m-Methoxyphenyl-
p-Methoxyphenyl-
o-(Trifluoromethyl)phenyl-
p-(Trifluoromethyl)phenyl-
o-Chloroaniline
p-Chloroaniline
o-Bromoaniline
p-Bromoaniline
p-Iodoaniline
o-Toluidine
m-Toluidine
p-Toluidine
o-Methoxyaniline
m-Methoxyaniline
p-Methoxyaniline
o-(Trifluoromethyl)aniline
p-(Trifluoromethyl)aniline
10
11
12
13
14
a
Reaction conditions, substrate (0.25 mmol), NaBH
4
(0.5 mmol) and [Ru(p-cymene)Cl
]
2 2
(10 mg) was used with 1.5 ml of water/methanol (v/v¼2/1) at
room temperature.
b
Isolated yield.
Table 2
a
Use of different commercially available catalysts for the reduction of 1-azido-2-chlorobenzene to 2-chloroaniline.
Entry
Catalyst
Amount of
catalyst, mg
Time (min)
Yieldc (%)
TON (molproduct
molmetal(s)
/
TOF (molproduct
molmetal(s)/h
/
1
2
3
4
Pd/Cb
Pd(OAc)
3.4
6.7
5.3
10.0
50
60
60
10
70
72
35
>95
109.3
6.0
2.9
131.2
6.0
2.9
2
PdCl
[Ru(p-cymene)Cl
2
2
]
2
28.8
172.7
a
Reaction conditions, substrate (0.25 mmol), NaBH
4
(0.5 mmol) and catalyst (0.03 mmol metal content) was used with 1.5 ml of water/methanol (v/v¼2/
1
) at room temperature.
b
5
wt % Pd.
C
isolated yield.
Please cite this article in press as: B. Kilbas, et al., A practical and highly efficient transfer hydrogenation of aryl azides using a
Ru(p-cymene)Cl catalyst and sodium borohydride, Comptes Rendus Chimie (2018), https://doi.org/10.1016/j.crci.2018.07.004
[
2 2
]

B. Kilbas et al. / C. R. Chimie xxx (2018) 1e4
3
Table 3
Comparison of the designed catalytic system with previously published studies about the reduction of aryl azide.
ꢀ
Catalyst
Conditions
Temp.
C
Time
1 h
Yield, %
86
None [33]
Phenyl azide (1.33 mmol), NaBH
methanol (0.18 ml)
4
(0.89 mmol), THF (4 ml),
reflux
Hexadecyltributylphosphonium
bromide [8]
Phenyl azide (1 mol), catalyst (0.1 mol), NaBH
toluene (2 ml)
4
(1.1 mol),
Cl
20
1 h
92
95
75
87
90
61
98
80
94
>95
BF
Tetrathiomolybdate [30]
Sm/I [13]
Zn [31]
RhCl .3H
PANF-QAS (A-Hp-Br) [32]
CuSO [34]
LiCl [35]
Ru(p-cymene)Cl
3
.OEt
2
/EtSH [10]
Phenyl azide (0.32 mmol), catalyst (0.8 mmol), CH
5 ml), ethanethiol (1.6 mmol)
2
2
25
1.5 h
6 h
(
Phenyl azide (2 mmol), catalyst (1. 1 mmol), acetonitrile/
25
water (v/v¼20/1)
2
Phenyl azide (1 mmol), Sm (1.0 mmol), I
2
(0.2 mmol),
Cl (0.07 mol),
25
6 h
2
methanol (5 ml), N atm
Benzyl azide (0.03 mol), Zn (0.04 mol), NH
ethyl alcohol (80 ml), water (27 ml)
4
90
10 min
4 h
ꢁ
2
3
2
O [28]
Phenyl azide (40 mmol), catalyst (4.3.10 mmol), benzene
20 ml), CO (20 atm), water (1.5 ml)
Phenyl azide (4 mmol), catalyst (10 mol %), NaBH
0.5 mmol), Na HPO /NaH PO
(pH¼12)
Phenyl azide (3.4 mmol), catalyst (0.034 mmol), NaBH4
1 mmol), methanol (10 ml)
Phenyl azide (1 eq), catalyst (1 eq), NaBH4 (1 eq), THF
15 ml)
Phenyl azide (0.25 mmol), catalyst (0.016 mmol), NaBH
0.5 mmol), water/methanol (v/v¼2/1)
150
50
(
4
4 h
(
2
4
2
4
4
0e5
25
1 h
(
30 min
10 min
(
[
2
]
2
(this study)
4
25
(
Sodium borohydride was utilized as a hydrogen carrier,
because it donates four moles of hydrogen to the substrate
According to the catalytic cycle illustrated in Scheme 1,
free hydrogen gas was released by the hydrolysis of sodium
borohydride with the aid of a ruthenium (II) dimer fol-
lowed by the oxidative addition of hydrides to the metal
center. Coordination of the azide group and subsequent
reductive elimination of hydrides remarkably furnished
aniline derivatives.
2 2 4
[27]. In this study, the [Ru(p-cymene)Cl ] coupled NaBH
system successfully employed the azide reduction process
with absolute conversions and high yields (over 95%)
within 10 min (Table 1). Although the TH process was rapid,
halosubstituted azido arenes were exposed to selective
hydrogenation where halogen substituents preserved their
2 2
functionality (Table 1). Actually, the [Ru(p-cymene)Cl ]
homogeneous catalyst is more soluble in organic solvents;
however catalytic performance was effectively observed in
the aqueous medium without any loss of activity where
water is the main component. Azidoarenes were also sub-
mitted to the reduction reaction under catalyst free con-
ditions, the results indicated that no aniline derivatives
were produced after 5 h of the reaction.
2 2
A comparison test of [Ru(p-cymene)Cl ] with palladium-
based commercially available catalysts was also performed.
The 1-azido-2-chlorobenzene served as a substrate for the
comparison test. Under the same reaction conditions, 1-
azido-2-chlorobenzene was subjected to a transfer hydro-
genation reaction to provide 2-chloroaniline in the presence
of Pd/C, Pd(OAc)
dium borohydride. The results with TON/TOF values exhibi-
ted that [Ru(p-cymene)Cl has distinct priority such as high
2 2 2 2
, PdCl and [Ru(p-cymene)Cl ] with so-
2 2
]
yield and shorter time for aniline formation (Table 2).
Furthermore, it is more effective and more stable in the water
component of the solvent system. Pd(II) [28] and Pd(0) [29]
assisted reduction reactions are not selective, whereas the
chlorine group preserved its functionality in the presence of
the [Ru(p-cymene)Cl
Aniline formation was compared to previously pub-
lished studies. The [Ru(p-cymene)Cl -assisted aniline
2 2
] coupled hydrogen donor system.
2 2
]
formation was successfully achieved with high yield at
room temperature. In addition, the reaction was performed
in a short time with respect to the previous studies as
depicted in Table 3.
Scheme 1. Proposed mechanism for the hydrogenation of aryl azide.
Please cite this article in press as: B. Kilbas, et al., A practical and highly efficient transfer hydrogenation of aryl azides using a
Ru(p-cymene)Cl catalyst and sodium borohydride, Comptes Rendus Chimie (2018), https://doi.org/10.1016/j.crci.2018.07.004
[
2 2
]

4
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4
Please cite this article in press as: B. Kilbas, et al., A practical and highly efficient transfer hydrogenation of aryl azides using a
Ru(p-cymene)Cl catalyst and sodium borohydride, Comptes Rendus Chimie (2018), https://doi.org/10.1016/j.crci.2018.07.004
[
2 2
]