Water-soluble superbulky (η6- p -cymene) ruthenium(ii) amine: An active catalyst in the oxidative homocoupling of arylboronic acids and the hydration of organonitriles

Article abstract of DOI:10.1039/c8nj03715a

A phosphine free water-soluble superbulky amine-ruthenium-arene complex (2) encompassing 2,6-bis(diphenylmethyl)-4-methylaniline was synthesised in good yield. 2 was characterized by FT-IR, ¹H NMR, and ¹³C NMR spectroscopies, TGA and elemental analyses. The structure of 2 was confirmed by a single-crystal X-ray diffraction study. The ruthenium centre in 2 adopts the pseudo-octahedral geometry due to the η⁶-p-cymene ring and bulky aniline ligand along with two chloro groups. Besides, complex 2 was efficaciously employed as a catalyst in the hydration of organonitriles to amides. This reaction proceeds efficiently for a wide range of substrates in an environmentally benign medium and is an economically reasonable synthetic route to amides in good yields. In addition, 2 acts as an excellent catalyst in the oxidative homocoupling of arylboronic acids in water. A range of arylboronic acids undergo a homocoupling reaction in the presence of catalyst 2 to yield symmetrical biaryls in reasonable to good yields.

Full text of DOI:10.1039/c8nj03715a

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J. Chem., 2018, DOI: 10.1039/C8NJ03715A.
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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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ARTICLE
Water-soluble Superbulky (η6-p-cymene) Ruthenium(II) Amine:
Active Catalyst in Oxidative Homocoupling of Arylboronic Acids
and Hydration of Organonitriles
Received 00th January 20xx,
Accepted 00th January 20xx
Muthukumaran Nirmala,a Mannem Adinarayana,a Karupnaswamy Ramesh,a Mannarsamy
Maruthupandi ,a Moulali Vaddamanu,a Gembali Rajua and Ganesan Prabusankar*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Phosphine free water-soluble superbulky amine-ruthenium-arene complex (2) encompassing 2,6-bis(diphenylmethyl)-4-
methylaniline was synthesised in good yield. 2 was characterized by FT-IR, 1H NMR, 13C NMR spectroscopy, TGA and
elemental analyses. Structure of 2 was confirmed by single-crystal X-ray diffraction study. The ruthenium centre in 2
adapts in the pseudo-octahedral geometry by η6-p-cymene ring, bulky aniline ligand along with two chloro groups. Besides,
the complex 2 was efficaciously employed as catalyst in hydration of organonitriles to amides. This reaction proceeds
efficiently for a wide range of substrates in environmentally benign medium and economically reasonable synthetic route
to the amides in good yield. In addition, 2 acts as excellent catalyst in oxidative homocoupling of arylboronic acid in water.
A range of arylboronic acids undergoes a homocoupling reaction in the presence of catalyst 2 to yield the symmetrical
biaryls
in
reasonable
to
good
yield.
demonstrate the structural activity relationship [18]. So, the
structural adjustments of these organometallic compounds are
probed currently, in order to endow a promising relationship
between their structural and catalytic activities. Based on the
above fact, the expansion of water-soluble arene-ruthenium
complexes containing aniline ligands has aroused interest in
recent years for various organic transformations [19a-c].
Moreover, these arene-ruthenium-aniline complexes have the
capability to execute organic reactions in water, which is
inexpensive, harmless and most importantly environment-
friendly solvent [20-22]. Typically, anilines are deliberated as a
weakly coordinating ligand, wields a considerable role to tune
the activity of the catalyst. For instance, the phosphine-free
[(η6-arene)RuCl2(aniline)] complexes exhibits similar or even
higher activity than their structurally akin phosphine
containing complex [(η6-arene)RuCl2(PPh3)] [23-25,19a]. Sanjay
et al., have intended that being a simple σ-donor ligand,
aniline ligands can form a stable mononuclear ruthenium
Introduction
Over the last few years, the organometallic compounds have
been the subject of profound research due to their affirming
applications in catalysis. The most classical compounds of this
class, are the so-called “piano-stool” complexes encompassing
the dimeric chloro bridges and arene ligands, such as
cyclopentadienyl or p-cymene, have been explored by many
research groups [1-4]. In particular, the half-sandwich
complexes of ruthenium have engrossed a reputed position in
organometallic chemistry due to their distinctive properties,
the lenitive reaction conditions requisite for the synthesis and
readiness to form stable complexes with an array of ligands
under aqueous conditions [5,6]. Among the half-sandwich
complexes, arene ruthenium complexes of the three-legged
piano stool type have been recently acknowledged as building
blocks for the construction of anticipated architectures due to
their versatile applications in different fields of biological
science as medicines as well as material science as catalysts [7-
17]. Considering these points, the nature of chelating ligands
and the leaving groups in these half-sandwich complexes
greatly impact their chemical and biological activities and can
complex, which is
a
structural analogue of [(η6-
arene)RuCl2(PPh3)], but in contradictory to PPh3, aniline can be
simply ectopic by incoming coordinating reactant molecule
under catalytic reaction conditions [19c]. Inspirited by these
findings, various aniline based ruthenium-arene complexes
were accounted [19a-c]. These findings were indeed
interesting, as electronic and steric properties of the aniline
ligand can be easily fine-tuned by applying several substituents
on the phenyl ring. Moreover, the subsequent complexes
strategically showed high aqueous solubility and hence
accorded an opportunity to accomplish the water-based
catalytic reaction [19a].
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thermograms. From the TGA curve, it is clear that the complex
is stable up to 180 ˚C. After 180 ˚C the complex begins to
decompose. The decomposition ends at 700 °C with 14.5 % of
initial mass, which corresponds to residual ruthenium metal
(Calcd. mass =101.01 g mol-1; mass found = 108 g (calculated
for 1 mol)). Moreover, the complex is air stable and no
decomposition or structural change of this complex was
observed on exposure to air.
Me
Me
N
N
Me
NH2
Ru
Ru
L
Cl
Cl
Cl
Cl
L = PPh3 & Py
This work
Nolan et al., 2012
Cl
Me
Ru
Cl
Scheme 1 Known super bulky ruthenium(II) supported by neutral donor ligands.
Cl
Ru
Cl
NH2
Ru
Motivated by the high activity of arene-Ru(II) complexes for
innumerable organic transformations, we engrossed our
efforts on developing a new highly flexible yet hindered
neutral donor ligand. For example Nolan et al., has isolated
two different ruthenium catalysts, [RuCl2(IPr*)(Py)(3-
NH2
Me
MeOH/CH3CN,
RT, 8 h
Cl
Cl
phenylindenylidene)]
and
[RuCl2(IPr*)(PPh3)(3-
1
2
phenylindenylidene)] from 1,3- bis(2,6-bis(diphenylmethyl)-4-
methylphenyl)imidazol-2-ylidene for ring closing and cross-
metathesis (Scheme 1) [26]. As of our knowledge
[RuCl2(IPr*)(Py)(3-phenylindenylidene)] is the only example to
be demonstrated towards the effect of steric hindrance at four
coordinated Ru(II) to assess influence of sterically encumbered
phosphine free neutral donor ligand in catalysis. We believe
that these sorts of hefty ligands may attune themselves
against the entering substrates in the catalytic cycle [19d].
Thus, in this paper we have studied the impact of super bulky
aniline ligand in Ru(II) mediated catalytic reactions.
Surprisingly, the new water soluble superbulky 2,6-
Scheme
2
Synthesis
of
ruthenium-arene
complex
[(η6-p-cymene)-
RuCl2(C6H2(C6H5)4CH3NH2)] (2).
bis(diphenylmethyl)-4-methylaniline
ruthenium(II)-arene
complex acts as an efficient catalyst towards the oxidative
homocoupling of arylboronic acids and hydration of nitriles in
water.
Results and discussion
Synthesis and characterization of 2
The superbulky 2,6-bis(diphenylmethyl)-4-methylaniline ligand
(
1
) was synthesized in excellent yield as previously described
Fig. 1 TGA curve 2 from 100 to 880 °C under a nitrogen atmosphere with a heating rate
of 10 °C min−1
[28]. Treatment of latter (1) with [RuCl2(p-cymene)2]2 in a
.
mixture of methanol and acetonitrile at room temperature
afforded the targeted pseudo-octahedral bulky 2,6-
The 1H NMR spectrum of complex
2 was recorded in
bis(diphenylmethyl)-4-methylaniline
complex of general formula
RuCl2(C6H2(C6H5)4CH3NH2)] in good yield (Scheme 2). The
complex was highly soluble in polar solvents such as water,
ruthenium(II)-arene
deuterated chloroform. The NMR spectrum contains sharp
signals in the expected regions, thus ruling out molecular
(2)
[(η6-p-cymene)-
functionality.
As the aniline ligand is coordinated to
2
ruthenium, the complex displayed a downfield shift for the
protons of aniline ligand as expected. Signals corresponding to
methanol ethanol, chloroform and dichloromethane, whereas
insoluble in hexane and petroleum ether. The identity of deep
aromatic protons of complex
ppm which can be compared to the signals appeared from
7.34 6.48 ppm for the bulky 2,6-bis(diphenylmethyl)-4-
methylaniline ( ). Moreover, a doublet is present for the
ruthenium bound η6-p-cymene ring in the range of 5.41
5.26
2 were observed from 7.30−6.72
red complex
NMR and 13C NMR spectroscopy. Stability of the
accomplished by thermal gravimetric analysis (TGA)
2
was confirmed by elemental analyses, IR, 1H
2
was
−
1
experiments (Fig. 1). The complex
2 exhibits four distinct
−
2 | J. Name., 2012, 00, 1-3
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ppm. The isopropyl methyl groups resonate at 1.20-1.17 ppm.
ARTICLE
Analogously, the NH2 protons resonate at 4.87 ppm as a broad It is clear that the bulky 2,6-bis(diphenylmethyl)-4-
singlet. The -CH3 protons were observed at 2.11–2.07 ppm, methylaniline coordinated to the ruthenium by the nitrogen
respectively. In the 13C NMR of complex
2
, η6-p-cymene atom, as akin to the formerly reported arene-ruthenium(II)
resonates in the region 128.09-126.31 ppm (CDCl3). The peak amine complexes [19a-19c, 27, 29]. As shown in figure 1, the
appeared between 142.60 and 120.81 ppm was attributed to ruthenium centre in 2 is tailored to the pseudo-octahedral
aromatic carbons. The C-N carbon appears at 143.43 ppm and geometry. The η6-p-cymene ring engaged in the pinnacle
-CH(Ph2) carbon emerged at 51.80 ppm. In addition, the position and the bulky aniline ligand along with two chloro
groups engrossed the three legs, correspondingly. The η6-p-
methyl carbons are resonating in the region 22.16-18.96 ppm.
cymene ring in 2 remains planar and the ruthenium centre is at
ectopic by 1.66 Å from the centroid of the η6-p-cymene ring.
X-ray crystal structure description of complex 2
The molecular structure of the adumbrative arene-
Table 2 Selected bond lengths (Å) and bond angle (ᵒ) for complex 2.
ruthenium(II) amine complex (
2), was firmly established by the
Bond lengths (Å)
single-crystal XRD analysis of the most congruous crystals
Ru(1)-Cl(1)
Ru(1)-Cl(2)
Ru(1)-N(1)
Ru(1)-C(33)
Ru(1)-C(34)
2.411(6)
2.408(7)
2.193(15)
2.199(2)
2.187(2)
Ru(1)-C(35)
Ru(1)-C(36)
Ru(1)-C(37)
Ru(1)-C(38)
N(1)-C(1)
2.193(2)
2.202(2)
2.172(2)
2.157(2)
1.446(2)
developed
methanol−acetonitrile saturated soluꢀon of the complex
ray crystallography analyses proclaim
from
a
sluggish
evaporation
of
2
. X-
2
crystallize in the
triclinic crystal system with Pī space group and two complex
Bond Angle (o)
units residing at the unit cell. The molecular structure, space
filling model and packing diagram of
2 are portrayed in figure
Cl(2)-Ru(1)-Cl(1)
N(1)-Ru(1)-Cl(2)
86.47(3)
81.31(4)
N(1)-Ru(1)-Cl(1)
81.94(4)
2. An elaborated crystal refinement data is collated in table 1,
and the selected bond distances and angles of
table 2.
2 were listed in
Table 1 Crystal data and structure refinement parameters for complex 2.
2
Empirical formula
C43H43Cl2NRu
745.80
295
Formula weight
T (K)
Wavelength (Å)
0.71073
Triclinic
Pī
Crystal system
Space group
Unit cell dimensions
a (Å)
7.8768(12)
13.917(2)
b (Å)
c (Å)
17.713(3)
α (o)
104.199(4)
93.373(4)
β (o)
γ(o)
Volume (Å3)
105.059(4)
1802.1 (5)
2
Z
Density (calculated) Mg m-3
Absorption coefficient mm-1
F(000)
1.3743
0.615
770.3401
Scan range for data collection (deg)
Index ranges
3.3831 to 71.0327
-10< = h< = 10
-17< = k< = 17
-22< = l< = 22
46323/7965/0.0299
1
Reflections collected/unique, Rint
Completeness to thetamax
Data/restraints/parameters
Goodness-of-fit on F2
7965/0/4298
1.0488
Fig. 2 (I) The solid state structure of 2. (II) The coordination environment of Ru(II) in 2.
(III) The space filling model of 2.
Final R indices [I >2σ(I)]a
R1 = 0.0299
wR2 = 0.0827
R1 = 0.0346,
wR2 = 0.0871
Noteworthy that the ruthenium-nitrogen bond length (2.193
Å) is moderately higher for this sterically crowded 2,6-
R indices (all data)
bis(diphenylmethyl)-4-methylaniline encompassing complex 2,
aStructures were refined on Fo : wR2 = [ ∑[w(Fo2− Fc2)2]/ ∑w(Fo2)2]1/2, where w−1 = [ than those for earlier reported similar type complexes [19a-
2
∑(Fo2) + (aP)2 + bP] and P = [max(Fo2, 0) + 2Fc ]/3.
2
19c]. The phenyl ring of bulky 2,6-bis(diphenylmethyl)-4-
methylaniline ligand was located beyond to the metal centre,
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as described from the C(1)-N(1)-Ru(1) bond angle of 121.83°
for the complex [19c, 30, 31, 32]. Angles between the legs,
Cl(2)-Ru(1)-Cl(1) (86.47°), N(1)-Ru(1)-Cl(1) (81.94°), and N(1)-
Ru(1)-Cl(2) (81.31°) for the complex are nearest to those of
proclaimed earlier [19a, 19c, 27, 19b, 29]. Notably, the space-
filling model of confirms the presence of substituted bulky
2
2
2
phenyl rings in the ortho position, which in principle should
allow the complex to acclimate themselves for the incoming
catalytic substrates without much hurdle (Fig. 2) [19d]. In
addition, complex 2 contains intermolecular hydrogen bond
through C−H· ···Cl and C−H···Ru, respecꢀvely (Fig. 3).
Catalytic hydration of organonitriles
Hydration of organonitriles is the most straightforward
approach to produce primary amides with a good atom
economy. Generally, the hydration reaction was performed
under harsh reaction condition and tedious workup
procedures. Several transition metal complexes (e.g. Ni, Pt, Rh,
Ir, Mo, Au and Ru) have been demonstrated as good catalyst
for hydration of nitrile [33-47]. Among them, ruthenium based
metal complexes are highly active for nitriles hydration [43-
47,48]. Therefore, the ligand effect on the catalytic hydration
nitriles has been studied [49,50]. Recently, water was used as
the reaction medium by the replacement of organic solvents
[40,47,51,52]. Gimeno and his co-workers have reported the
half-sandwich ruthenium(II) species [RuCl2(η6-arene)(L)],
containing the hydrosoluble phosphorus donor ligands PTA
(1,3,5-triaza-7-phosphatricyclo[3.3.1.13,7]decane), PTABn (1-
benzyl-3,5-diaza-1-azonia-7-phosphatricyclo[3.3.1.13,7]decane
chloride)
and
DAPTA
(3,7-diacetyl-1,3,7-triaza-5-
phosphabicyclo[3.3.1]nonane) are acts as an excellent
catalysts for the selective hydration of organonitriles to amides
[53-55]. Later, the Gimeno et al., reported the chloro-bridged
bis(allyl)-ruthenium(IV)dimer
(C10H16 2,7-dimethylocta-2,6-diene-1,8-diyl) catalyst to
[{RuCl(μ-Cl)(η3:η3-C10H16)}2]
=
catalyze the selective hydration of organonitriles [56]. Oshiki
and Breit and their co-workers have proposed the cooperative
effect on hydration process through a bifunctional catalysis
mechanism, in which, metal acts as a Lewis acid to activate the
nitrile, while the nitrogen-containing ligand acts as a Lewis
base for the attack of water through hydrogen bonding
[57,50]. Therefore, in light of the above, we report here the
phosphine-free water soluble bulky 2,6-bis(diphenylmethyl)-4-
methylaniline ruthenium(II)-arene complex. Interestingly the
catalyst is highly active for the hydration of organonitriles to
corresponding amides at a lower temperature, under aqueous-
aerobic conditions.
Fig. 3 (I) The solid state packing of 2. (II) The hydrogen bonding network in 2.
For the better optimizing condition, the hydration of
benzonitrile was chosen as the model reaction. Figure 4
summarizes the influence of catalyst loading and reaction time
on the hydration reaction. The reactions were performed by
changing the quantity of the catalysts ranging from 1 to 5 mol
%. The results revealed that the rate of the catalytic reaction is
influenced by the catalyst loading, which is in connection with
the reaction time and the production yield. The catalytic
loading of 4 mol % is found to be the optimized amount of
catalyst to perform the catalytic reaction. No significant
increase in the rate and thereby the efficiency of the reaction
is observed on further raise of catalyst loading upto 5 mol %
and even after a prolonged time (Fig. 4).
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ARTICLE
benzonitriles incorporating electron withdrawing groups
furnished the desired amides with good yield (Chart 1, Entries
3, 6,
7 and 8). The 2-cyanonaphthalene gives the
corresponding amide in moderate yield around 74 % (Chart 1,
Entry 9). Hydration of acrylonitrile to acrylamide has been
efficiently emanated, which is a very significant conversion in
industrial point of view (Chart 1, entry 10). The present
protocol also effectively hydrates the aliphatic nitriles (Chart 1,
Entries 11 and 12). Notably, the TOF of present system is
better than the reported TOF for ruthenium complexes [58].
In addition, we have examined the current catalytic system for
hydration of heteroaromatic nitriles under the prior optimized
conditions. The formation of amide product for this type was
quite interesting, and no such reports exist in the water
soluble bulky ruthenium-arene-amine catalysis. The results are
Fig. 4 Effect of catalyst mol % (1 – 5 mol %) with respect to time (1 – 4 h) on hydration summarized in chart 2. Surprisingly, the heteroaromatic
of benzonitrile to benzamide.
nitriles containing sulphur, oxygen and nitrogen has been
efficaciously transformed to the corresponding amides within
4 h at ambient temperature, and no concomitant carboxylic
acids were identified (Chart 2, Entries 1-7). The hydration of
O
Cat. 2, 4 mol %, H2O
R
O
C
N
R
C
4 h, RT
NH2
isonicotinonitrile
and
nicotinonitrile
afforded
the
corresponding amides in 97-99 % yield (Chart 2, Entries 1 and
2). Even, 3-quinolinecarbonitrile was hydrated within 4 h, and
gives the corresponding 3-quinolinecarboxamide in 93 % yield
(Chart 2, Entry 3), which is usually carried out in 12 h to 24 h
[59a]. Noteworthy that the pyrazine carboxamide, a drug
against tuberculosis was produced in 96 % yield (Chart 2, Entry
4). Subsequently, the hydration of 2-furancarbonitrile and 2-
thiophenecarbonitrile underwent smoothly to provide the
amide product in 96-98 % yield (Chart 2, Entries 5 and 6). To
our delight, the sterically hindered ethyl 2-chloro-3-cyano-6-
methyl-5-nitrosoisonicotinate hydrated well under our
O
O
NH2
NH2
NH2
O
Cl
(1, 98 %, 12 h-1 b
)
(2, 82 %, 10 h-1
)
(3, 99 %, 12 h-1
)
O
O
O
NH2
NH2
NH2
H3C
HO
O2N
(4, 87 %, 11 h-1
)
(5, 81 %, 10 h-1
)
(6, 99 %, 12 h-1
)
O
O
optimized condition and furnishes 79
% of 2-chloro-4-
NH2
NH2
NH2
(ethoxycarbonyl)-6-methyl-5-nitroso-3-pyridine carboxamide
(Chart 2, Entry 7). Interestingly, the isolated yield under the
present synthetic methodology is more convenient and energy
efficient than the most efficient (isolated 94 %) catalytic route
demonstrated using MnO2 under very high pressure (100 psi)
and high temperature (60 oC) [59b].
Br
OHC
(8, 95 %, 12 h-1
O
(7, 97 %, 12 h-1
)
(9, 74 %, 9 h-1
)
)
NH2
O
NH2
O
NH2
O
(10, 71 %, 9 h-1
)
(11, 96 %, 12 h-1
)
(12, 99 %, 12 h-1
)
Chart
1 Hydration of organonitriles catalyzed by bulky arene-ruthenium(II)-amine
complex 2. aReaction Conditions: Nitrile (1 mmol), water (5 mL), catalyst loading (4 mol
%), time (4 h) at room temperature. bEntry number, isolated yield (%) and TOF (h-1) was
given in brackets. Turnover frequency ((mol product/mol Ru)/time).
In order to extend the scope of the present catalytic system,
the hydration reaction was carried out under optimized
conditions on different nitriles possessing various
functionalities. Chart 1 summarizes the catalytic activity of 4
mol %
aqueous and aerobic condition. The reaction time is fixed for 4
for all the nitriles under investigation. Benzonitriles
2 on various nitriles at ambient temperature under
h
encompassing electron donating groups gets hydrated
efficiently to furnish the corresponding amides in excellent
yield (Chart 1, Entries 2,
4 and 5). Furthermore, the
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Oxidative homo-coupling of arylboronic acids
H2N
O
O
O
For our initial catalytic investigation, the homo-coupling of
NH2
NH2
phenylboronic acid in the presence of
2 was fixed as the model
N
N
N
reaction in water at room temperature and the results are
depicted in figure 5. In order to assess the crucial function of
base in advancing the generation of intermediates all along the
catalytic cycle, the effect of bases on the desired
homocoupling reaction was studied by various bases such as
K2CO3, Na2CO3, K3PO4, Cs2CO3 and NaOAc. Among the bases
employed, K3PO4 was found to be the best and led to higher
yield for this reaction at room temperature. To make sure the
catalytic role, a control experiment was executed without the
base or the catalyst. As expected, there was no reaction even
after an extended reaction time. The catalytic loading of 4 mol
% is found to be the optimized amount of catalyst to perform
the catalytic reaction. No significant increase in the rate and
thereby the efficiency of the reaction is observed on further
raise of catalyst loading to 5 mol %. In the latest reports, akin
catalytic reaction was executed at 70 °C using K2CO3 base in
water/methanol mixture and Cu(OAc)2 as additive [19a]. In
order to decreases the reaction temperature, we treated the
phenylboronic acid with K3PO4 base and Cu(OAc)2 additive in
water at room temperature and without any inert gas
(3, 93 %, 12 h-1
)
(2, 99 %, 12 h-1
)
(1, 97 %, 12 h-1 b
)
O
O
S
O
O
N
N
NH2
NH2
NH2
(4, 96 %, 12 h-1
)
(6, 96 %, 12 h-1
)
(5, 98 %, 12 h-1
)
O
O
O
N
NH2
O
N
Cl
(7, 79 %, 10 h-1
)
Chart 2 Hydration of heteroaromatic nitriles catalyzed by bulky arene-ruthenium(II)-
amine complex 2. aReaction Conditions: Heteroaromatic nitrile (1 mmol), water (5 mL),
catalyst loading (4 mol %), time (4 h) at room temperature. bEntry number, isolated
yield (%) and TOF (h-1) was given in brackets. Turnover frequency ((mol product/mol
Ru)/time)
On the basis of the above results and also in conformity with
previous literature reports [60], we trust that the catalytic
nitrile hydration of bulky 2,6-bis(diphenylmethyl)-4- fortification and in the presence of 4 mol % of catalyst
2.
methylaniline ruthenium(II)-arene catalyst follows three steps
(Scheme 3). Step 1: coordination of the nitrile to the
ruthenium centre. Step 2: Then, formation of α-hydroxyimide
by the intermolecular nucleophilic attack of water on the
nitrile and step-3: dissociation of the amide product.
Ru
Cl
Cl
L =
L
NH2
R
C
N
Cl
O
H2O
Fig. 5 Effect of the base (NaOAc, K2CO3, Cs2CO3, K3PO4 and Na2CO3) on homocoupling of
R
NH2
Ru
biphenyl.
Cl
N
L
C
R
In order to decreases the reaction temperature, we treated
the phenylboronic with K3PO4 base and Cu(OAc)2 additive in
Cl
Cl
the presence of 4 mol % of catalyst
2 in water at room
temperature under aerobic condition. In order to explicate the
role of Cu(OAc)2, we did the controlled experiments by varying
Ru
L
Ru
L
Cl
NH2
Cl
NH
the amount of Cu(OAc)2 (0.1−1.5 equiv.) (Table 3, Entries 1-
O
OH
11). Results inferred that a lower content of Cu(OAc)2 gave
poor yield of biphenyl (Table 3, Entries 1, 2, 3). There is no
conversion of biaryls was observed for the reaction performed
in the absence of base (Table 3, Entry 7). Notably, the
reactions performed without additive Cu(OAc)2 resulted in
R
R
Scheme 3 Proposed general mechanism for 2 catalyzed hydration of nitriles.
6 | J. Name., 2012, 00, 1-3
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moderate yield (61 %, Table 3, Entry 8). Moreover, the
Cu(OAc)2 additive in the absence of O2 (under N2 atm) did not
give better results (Table 5, Entry 10). Awfully, the complex
2
endows a good yield (92 %) of biphenyl, implying the vital role
of Cu(OAc)2 as an oxidant to restore the Ru(II) species and
impede catalyst decomposition during the catalytic cycle
(Scheme 5) [61]. However, the Cu(OAc)2 is an efficient oxidant
in this transformation. Additionally, the results indicate that
the aerial oxygen is a very active partner to render an efficient
Cu/O2 oxidative system. Complex
2 shows excellent stability in
an aerobic environment. This may be due to the presence of
super bulky aniline at ruthenium centre. Hence, we carried out
all the catalytic reactions under aerobic circumstances. With
these novel features, the present catalytic system is much
better than the known protocols documented in the literature
[19a, 62, 61i, 61j, 61n].
With a dynamic catalyst in hand and unswerving reaction
conditions are identified. Consequently, we examined the
scope of various arylboronic acids that undergoes the
oxidative homocoupling reactions. The results are collated in
chart 3 from which we can find that all the reactions take place
gently and furnished the corresponding biphenyls in moderate
to good yield (Chart 3, Entries 1-10). Results confirmed that
Chart 3 Homocoupling of arylboronic acids catalyzed by bulky arene-ruthenium(II)-
amine complex 2. aReaction conditions: arylboronic acid (1 mmol), K3PO4 (2 mmol),
Cu(OAc)2 (1.5 mmol), water (5 mL), catalyst loading (4 mol %), time (4 h) at room
c
temperature. bEntry number, isolated yield (%) and TOF (h-1) was given in brackets.
Turnover frequency ((mol product/mol Ru)/time).
having sterically bulky 2,6-diphenylmethane aniline ligand,
2
afforded the moderate to good yield of biarlys in 4 h as
compared to the earlier reported catalytic systems containing
With the purpose of ensuring the flexibility of our catalyst, we
have studied the chemoselectivity of coupled products
between two different arylboronic acids (Scheme 4). Under
the optimized reaction condition, the reaction was carried out
using an equimolar amount of two different arylboronic acids.
The selectivity towards the cross- and homocoupled biaryls
was confirmed by 1H NMR. Results proved that the reaction of
4-methoxyphenylboronic acid with 4-methylphenylboronic
shows the higher selectivity of 4-methyl-4’-methoxy-1,1’-
aniline ligand [19a]. Catalyst
2 is able to endure the range of
functional groups and substitutions like methyl, ethyl,
methoxy, chloro, bromo, trifluoro methyl, and thiophene
(Chart 3, Entries 1-10). Conversely, electron withdrawing group
substituted phenyl boronic acids were fruitfully transformed to
the corresponding products in excellent yields (Chart 3, Entries
2,
3 and 8). The p-C2H5-substituted phenylboronic acid
furnishes 82 % yield for 4,4'-diethyl-1,1'-biphenyl (Chart 3,
Entry 7). The electron donating phenylboronic acids afforded
the desired biphenyls in good yields (Chart 3, Entries 4, 5 and
6). Similarly, the (4-trifluoromethyl)phenylboronic acid
homocoupled gave the expected biaryl in very good yield
(Chart 3, Entry 9). Thiophen-3-yl boronic acid also fruitfully
achieved the moderate yield of desired biaryl in 4 h (Chart 3,
Entry 10).
biphenyl cross-coupled product (48
% for 4-methyl-4’-
methoxy-1,1’-biphenyl) together with the homocoupled
products of both of the arylboronic acids.
Scheme 4 C−C coupling reacꢀons of different arylboronic acids by 2.
Table 3 Effect of additive and oxidant on Homocoupling of
arylboronic acids catalyzed by bulky arene-ruthenium(II)-
amine complex
2
Entry Base
Additive
Oxidant Time Yield (%)g
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1a
2a
3a
4a
5a
6a
7b
K3PO4 Cu(OAc)2
(0.1 equiv.)
O2
O2
O2
O2
O2
O2
O2
8 h
8 h
8 h
8 h
4 h
17 %
28 %
NH2
K3PO4 Cu(OAc)2
(0.4 equiv.)
L =
K3PO4 Cu(OAc)2
(0.8 equiv.)
46 %
K3PO4 Cu(OAc)2
(1.2 equiv.)
77 %
92 %
B(OH)2
K3PO4 Cu(OAc)2
(1.5 equiv.)
R
Cu(I)
O2
Ru
K3PO4 Cu(OAc)2
(1.5 equiv.)
12 h
4 h
94 %
K3PO4
HO
L
Cl
OH
Cl
[Ru]
B
-
Cu(OAc)2
No
Cu(II)
OH
(1.5 equiv.)
reaction
61 %
8c
9d
K3PO4
K3PO4
-
-
O2
O2
4 h
8 h
No
[Ru]0
Ru
Cl
L
reaction
58 %
10e
K3PO4 Cu(OAc)2
(1.5 equiv.)
-
4 h
4 h
R
B(OH)2
R
11f
K3PO4 Cu(OAc)2
(1.5 equiv.)
O2
32 %
K3PO4
Ru
L
R
OH
a
HO
Arylboronic acid (1 mmol), water (5 mL), K3PO4 (2 mmol),
Cu(OAc)2 (1.5 mmol), catalyst loading (4 mol %), time (4-12 h)
b
at room temperature. Without base. Without additive.
B
R
OH
R
c
d
R
e
Scheme 5 Proposed general mechanism for 2 catalyzed homocoupling of arylboronic
Without catalyst and additive. Arylboronic acid (1 mmol),
water (5 mL), K3PO4 (2 mmol), Cu(OAc)2 (1.5 mmol), catalyst
loading (4 mol %), time (4 h) at room temperature under N2
acids.
f
atm. Without catalyst, arylboronic acid (1 mmol), water (5
Experimental
mL), K3PO4 (2 mmol), Cu(OAc)2 (1.5 mmol), time (4 h) at room
temperature. g Isolated yield.
Materials and methods
All reactions involving the synthesis of metal complex were
carried out in oven dried glassware with magnetic stirrer and
without inert gas protection. All commercial chemicals were
used as purchased. 2,6-bis(diphenylmethyl)-4-methylaniline
[28] and [(p-cymene)RuCl2]2 were prepared according to the
earlier proclaimed methods [63]. FT-IR measurement (neat)
was carried out on a Bruker Alpha-P Fourier transform
spectrometer. Microanalyses of carbon, hydrogen and
nitrogen were carried out using a Euro EA - CHNSO Elemental
Analyzer. NMR spectra were recorded on Bruker Ultrashield-
On the basis of earlier literature report [19a], we believe that
the bulky ruthenium(II)-arene-amine complex forms a σ-aryl-
Ru(II) intermediate by the reaction of aryl boronic acid
(Scheme 5). We consider that this may be due to the presence
of the bulkier diphenylmethane substituents on the ortho
positions of phenyl ring (of amine ligand) pushes out the
chloride ligand and thereby easily forms the vacancy on the
ruthenium complex. This vacancy allowed the complex to
accommodate the catalytic substrate to initiate the catalytic
o
400 spectrometers at 25 C unless otherwise stated. Chemical
cycle. Afterwards, the second molecule of arylboronic acid
coordinates with the latter intermediate to form a new
transmetalated di(σ-aryl)-Ru(II) intermediate. In the final step,
the biaryl was produced via the reductive elimination of di(σ-
shifts are given relative to TMS and were referenced to the
solvent resonances as internal standards.
aryl)-Ru(II) intermediate with the generation of the Ru0 Synthesis of of [(η6-p-cymene)-RuCl2(C6H2(C6H5)4CH3NH2)] complex
(2)
species, which can be reoxidized to restore the active
ruthenium species RuII by the Cu(II)/aerial O2 oxidative system
to complete the catalytic cycle [19a].
The methanolic solution of [RuCl2(p-cymene)2]2 (0.044 g, 0.1
mmol) was added to the solution of
1 (0.0612 g, 0.1 mmol)
made in CH3CN (4 mL). The mixture was stirred for 8 h at room
temperature. The resulting deep red solution was filtered and
kept for crystallization. Single crystals of
diffraction were obtained in a mixture of CH3OH and CH3CN
(v/v, 1:1). Yield: 76% (based on ). Anal. Calcd for
2 suitable for X-ray
1
C43H43NCl2Ru: C, 69.25; H, 5.81; N, 1.88 %. Found: C, 69.49; H,
5.56; N, 1.68 %. FT-IR (cm-1, neat): 3039(w), 2958(m), 2148(m),
8 | J. Name., 2012, 00, 1-3
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2004(w), 1734(w), 1445(m), 1256(s), 1080(m), 1014(s), 868(m), were integrated with the Bruker SAINT Software package using
793(s), 691(m). 1H NMR (400 MHz, DMSO-d6): δ 7.30-7.28 (d, J
narrow-frame algorithm. All data were corrected for
= 7.2 Hz, 2H, Ar-H), 7.21 (s, 2H, Ar-H), 7.15-7.11 (m, 2H, Ar-H), absorption effects using the multi-scan method (SADABS) [64].
7.07-7.05 (d, J = 8 Hz, 2H, Ar-H), 7.04-7.01 (d, J = 10 Hz, 4H, Ar- The molecular structure of was solved by direct method
a
2
H), 7.006-6.99 (d, J = 4.8 Hz, 2H, Ar-H), 6.98-6.97 (d, J = 6 Hz, using the Olex program [65] and refined via full-matrix least-
2H, Ar-H), 6.76 (s, 2H, Ar-H), 6.74-6.72 (t, 2H, Ar-H), 5.41-5.39 squares method based on F2 using the SHELXL-97 program [66,
(d, J = 9.6 Hz, 2H, Ar-H), 5.33 (s, CH, 2H), 5.28-5.26 (d, J = 8 Hz, 67].
2H, Ar-H), 5.05 (bs, 2H, NH2), 2.87-2.80 (s, CH, 1H, CH(CH3)3),
2.11 (s, 3H, CH3), 2.07 (s, 3H, CH3), 1.20-1.17 (d, 6H, (CH(CH3)2).
13C NMR (100 MHz, DMSO-d6) δ: 143.43 (C-N), 142.60 (Ar-C),
141.49 (Ar-C), 139.31 (Ar-C), 133.76 (Ar-C), 130.90 (Ar-C),
130.27 (Ar-C), 130.04 (Ar-C), 129.76 (Ar-C), 128.29 (Ar-C),
128.09 (Ar-C), 126.93 (Ar-C), 126.50 (Ar-C), 126.44 (Ar-C),
126.31 (Ar-C), 120.81 (Ar-C), 51.80 (CH(Ph2)) 30.77 (CH), 22.16
((CH3)2), 18.96 (CH3).
Conclusions
In conclusion, we have conferred the synthesis and structural
characterization of phosphine free water-soluble bulky amine-
ruthenium-arene complex (
complex was confirmed by single crystal XRD studies which
unveiled that the complex adopts pseudo-octahedral
2). The structural identity of the
2
a
geometry. The 2,6-bis(diphenylmethyl)-4-methylaniline ligand
interacts with the metal through the amine group, the pseudo-
octahedral geometry being compensated by a η6-coordinated
General procedure for the homocoupling of arylboronic acids to
form biaryls
p-cymene ring and by two chloride ligands. The complex
2 was
The arene-ruthenium- catalyzed homocoupling of arylboronic
acids was carried out according to the previously reported
method [19a]. To a mixture of arylboronic acid (1 mmol), tri-
potassium phosphate (0.425 g, 2 mmol) and Cu(OAc)2 (0.300 g,
successfully employed for the hydration of organonitriles to
amides and oxidative homocoupling of arylboronic acids to
corresponding biaryls in water under moderate reaction
conditions and short reaction time. The present catalytic
system bestowed the hydration of a broad scope of nitriles,
together with aromatic, heteroaromatic and aliphatic
substrates. In the catalytic activity of homo coupling of aryl
boronic acids, an incredible effect of the coordinating bulky
aniline ligands was noted on the selectivity toward biaryls,
where moderate to good yields of the corresponding biaryls
were achieved. Indeed, the outcomes of the current work will
significantly endow towards the exploration and growth of
highly active water-soluble new catalytic system for the
synthesis of biaryls and amides. Further explorations in this
direction are in progress.
1.5 mmol), [Ru]-catalyst
2 (0.029 g, 4 mol %) in water (5 mL)
was added. The reaction mixture was stirred for the preferred
reaction time at room temperature. The procession of reaction
was examined by TLC. After completion of the reaction, it was
extracted with ethyl acetate (3 × 10 mL). The organic layer was
alienated and dried with anhydrous Na2SO4 to expel moisture.
The solvent was evaporated under reduced pressure to get the
desired product. The product formation was identified by 1H
NMR. Isolated yield was computed by using column
chromatography with Hexane: EtOAc (99:1 or 95:5 v/v) as
eluent.
General procedure for the hydration of nitriles
To a stirred solution of ruthenium(II) complex [(η6-p-cymene)-
Conflicts of interest
RuCl2(C6H2(C6H5)4CH3NH2)] (2) (0.029 g, 0.04 mmol, 4 mol %,
There are no conflicts to declare.
dissolved in 5 ml of water) in a round bottom flask, was added
a nitrile (1 mmol) under open air conditions. The reaction
mixture was stirred at room temperature for a pertinent
period of time. After culmination of the reaction, the reaction
mixture was extracted with dichloromethane. The organic
layer was alienated and dried with sodium sulfate to expel
moisture. The product was filtered and evaporated under
reduced pressure. The crude reaction mixture was purified by
column chromatography using diethyl ether as eluent. The
identity of the resulting amides was assessed by 1H NMR
spectroscopy.
Acknowledgements
GP gratefully acknowledge the Council of Scientific and
Industrial Research (No: 01(2884)/17/EMR-II) for financial
support. NM thank DST-NPDF (PDF/2016/001834) for
fellowship. RK thank UGC-JRF for the fellowship. MA thank
CSIR-JRF for the fellowship. We acknowledge the Department
of Physics, Bharathiar University, TN, India for extending their
single crystal XRD facility established under the DST-PURSE
programme.
Crystallography
Notes and references
The crystal structure of
2 was measured on a Bruker D8 QUEST
1
‡ H and 13C NMR spectra of the synthesized complex. Catalysis
diffractometer with graphite monochromated radiation of Mo
Kα (λ= 0.71073 Å) from a micro focus source at 273 K. The X-
ray generator was operated at 50 kV and 1 mA and the data
collection was monitored by the APEX2 program. The frames
protocols and characterization data of catalytic products. CCDC
1855045. These data can be obtained free of charge from the
Cambridge Crystallographic
www.ccdc.cam.ac.uk/data_request/cif.
Data
center
via
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Table of Contends
The synthesis and catalytic applications of water-soluble superbulky (η6-p-cymene) ruthenium(II) amine
complex in oxidative homocoupling of arylboronic acids and hydration of organonitriles are reported.