Hemilability-Driven Water Activation: A NiII Catalyst for Base-Free Hydration of Nitriles to Amides

Article abstract of DOI:10.1002/chem.201700816

The Ni^II complex 1 containing pyridyl- and hydroxy-functionalized N-heterocyclic carbenes (NHCs) is synthesized and its catalytic utility for the selective nitrile hydration to the corresponding amide under base-free conditions is evaluated. The title compound exploits a hemilabile pyridyl unit to interact with a catalytically relevant water molecule through hydrogen-bonding and promotes a nucleophilic water attack to the nitrile. A wide variety of nitriles is hydrated to the corresponding amides including the pharmaceutical drugs rufinamide, Rifater, and piracetam. Synthetically challenging α-hydroxyamides are accessed from cyanohydrins under neutral conditions. Related catalysts that lack the pyridyl unit (i.e., compounds 2 and 4) are not active whereas those containing both the pyridyl and the hydroxy or only the pyridyl pendant (i.e., compounds 1 and 3) show substantial activity. The linkage isomer 1′ where the hydroxy group is bound to the metal instead of the pyridyl group was isolated under different crystallization conditions insinuating a ligand hemilabile behavior. Additional pK_a measurements reveal an accessible pyridyl unit under the catalytic conditions. Kinetic studies support a ligand-promoted nucleophilic water addition to a metal-bound nitrile group. This work reports a Ni-based catalyst that exhibits functional hemilability for hydration chemistry.

Full text of DOI:10.1002/chem.201700816

A Journal of
Accepted Article
Title: Hemilability Driven Water Activation: A Ni(II) Catalyst for Base-
Free Hydration of Nitriles to Amides
Authors: Jitendra K. Bera, Kuldeep Singh, Abir Sarbajna, Indranil
Dutta, and Pragati Pandey
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To be cited as: Chem. Eur. J. 10.1002/chem.201700816
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Hemilability Driven Water Activation: A Ni(II) Catalyst for Base-
Free Hydration of Nitriles to Amides
Kuldeep Singh, Abir Sarbajna, Indranil Dutta, Pragati Pandey and Jitendra K. Bera*[a]
Abstract: A Ni(II) complex 1 containing pyridyl- and hydroxy-
functionalized N-heterocyclic carbenes (NHC) is synthesized and its
catalytic utility for selective nitrile hydration to amide under base-free
condition is evaluated. The title compound exploits hemilabile pyridyl
unit to interact with a catalytically relevant water molecule via
hydrogen-bonding and promote nucleophilic water attack to the
nitrile. A wide variety of nitriles are hydrated to the corresponding
amides including pharmaceutical drugs Rufinamide, Rifater and
Piracetam. Synthetically challenging α-hydroxyamides are accessed
from cyanohydrins under neutral conditions. Related catalysts that
lack the pyridyl unit (2 and 4) are not active whereas those
containing both the pyridyl and the hydroxy or only the pyridyl
pendant (1 and 3) show substantial activity. A linkage isomer 1’
where hydroxy group is bound to the metal instead of the pyridyl was
isolated under different crystallization conditions insinuating ligand
hemilabile behavior. pKa measurements reveal accessible pyridyl
unit under catalytic conditions. Kinetic studies support a ligand-
promoted nucleophilic water addition to a metal-bound nitrile. This
work reports a Ni based catalyst that exhibits functional hemilability
for hydration chemistry.
catalysts have been developed. These ‘metal-ligand cooperative
catalysts’ operate on the principle that a heteroatom present in
the ligand backbone polarizes a water molecule via hydrogen-
bonding interactions and promote hydration reaction.10 Oshiki,
Takai and co-workers demonstrated that the pyridyl unit of
Ph2Ppy ligand enhances nitrile hydration activity of the catalyst
cis-Ru(acac)2(Ph2Ppy)2 (acac = acetylacetonate, Ph2Ppy =
diphenyl-2-pyridylphosphine) (Scheme 1a).11 We earlier
exploited double hydrogen-bonding interactions of 1,8-
naphthyridine with a water molecule to hydrate organonitriles
efficiently and selectively using the catalyst [Rh(COD)(PIN)Br]
(PIN = 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazol-
2-ylidene, COD = 1,5-cyclooctadiene) at room temperature
(Scheme 1b).12 Cadeirno group has made remarkable progress
in recent years using homogeneous ruthenium/rhodium catalysts
containing pyridyl-phosphines, aminoaryl-phosphines, thiazolyl-
phosphines, 1,3,5-triaza-7-phosphaadamantane (PTA), amino-
phosphines (Scheme 1c) and related auxiliary ligands.13
Introduction
Nitrile hydration is a highly convenient and a desired route for
amide synthesis.1 It prevails over the limitation of poor atom
economy and avoids the use or generation of toxic chemicals
associated typically with traditional amide synthesis techniques.2
Most hydration catalysts involve precious second and third row
transition metals.3 There is a pressing demand to develop 3d
metal-based catalysts for obvious reasons- lower down the
production cost, reduce the environmental impact and eliminate
the metal ion toxicity.4 Although catalysts involving earth
abundant 3d metals are known for nitrile hydration reaction,5
only a few Ni compounds are reported under both homogeneous
and heterogeneous conditions.6 However, they exhibit limited
substrate scope, require additives and, in most cases, are not
molecularly well defined.7
Scheme 1. Water activation strategies for nitrile hydration.
All these catalysts employ a heteroatom in the ligand
backbone to direct water attack to the nitrile carbon. Catalyst
featuring a free basic unit attached to the ligand architecture
presents several problems. Protic impurities may diminish the
water activating ability of the catalyst. The possibility of a stable
dimer formation via coordination of the donor group to the metal
from a second molecule compromises catalyst activity. Bulky
ancillary ligands are usually employed to impede aggregation
though it has detrimental consequences for sterically hindered
substrates. Further, building such catalysts imposes a significant
synthetic challenge. A likely solution is to use a hemilabile basic
group for water activation.14 The hemilability would allow the
passage of a water molecule to the vicinity of the metal and the
basic group would engage the water molecule via hydrogen-
bonding interaction promoting its nucleophilic attack to the nitrile
(Scheme 2). Besides water activation, such hemilability might
improve the selectivity, the reversibility and hence the overall
turnover values.15
Several metalloenzymes are credited to hydrate a host of
substrates.8 One such enzyme is carboxypeptidase where a
divalent Zn and a glutamate carboxylate act in unison to
promote nucleophilic water attack to a peptide bond.9 Inspired by
the design strategies of natural systems, several synthetic
[a]
K. Singh, A. Sarbajna, I. Dutta, P. Pandey and Prof.J. K. Bera*
Department of Chemistry and Center for Environmental Sciences
and Engineering, Indian Institute of Technology Kanpur, Kanpur
208016, India. *E-mail: jbera@iitk.ac.in
Supporting information for this article is given via a link at the end of
the document.
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Scheme 2. Hemilabile ligand-promoted nitrile hydration reaction.
1’
1
We herein report a Ni(II) catalyst bearing functionalized N-
heterocyclic carbene (NHC) ligands, which exploits hemilabile
pyridyl groups to catalyze nitrile hydration reactions without
additives or base. A wide range of nitriles is hydrated efficiently
and selectivity to the corresponding amides including several
Figure 1. Molecular structures (40% probability thermal ellipsoids) of 1 (left)
and 1’ (right) with important atoms labeled. Hydrogen atoms except the
hydroxy group are omitted for the sake of clarity.
pharmaceutical
drugs.
Synthetically
challenging
α-
Compound 1 was obtained when crystallized from a methanol
solution layered with diethyl ether. Interestingly, a linkage isomer
1’ was isolated from an acetonitrile/methanol (1:1) mixture
layered with diethyl ether at −20°C (Scheme 3). X-ray structure
reveals a chelate structure similar to that of 1 but here the
hydroxy groups are bound to the metal instead of pyridines
(Figure 1). Isolation of two isomers under different crystallization
conditions hints at hemilabile nature of the ligand.
hydroxyamides are accessed under base-free conditions.
Control experiments demonstrate the significance of the activity-
enhancing hemilabile pyridyl group. The accessibility of the
pyridyl unit under catalytic conditions is corroborated by pKa
experiments. Kinetic studies support
nucleophilic water reaction with
a
ligand-promoted
metal-bound nitrile.
a
Hemilability driven nitrile hydration to amide by a Ni based
catalyst is reported in this work.
Catalysis
Results and Discussion
Nitrile Hydration
Catalyst 1 was evaluated for the hydration of benzonitrile at 2
mol% catalyst loading in H2O/iPrOH (1:3 v/v) at 70°C. After 6 h,
91% amide conversion was observed (Table S2). No over-
hydrated acid/ester or any other side products were detected
Synthesis of Hemilabile Catalyst
Reaction between a pyridyl and hydroxy functionalized NHC-
ligand precursor [L1H]Cl and NiCl2 (2:1 molar ratio) in presence
n
of excess BuLi in THF provided the dicationic compound 1 in
t
during the course of the reaction. Addition of 10 mol% BuOK
85% yield (Scheme 3). Molecular structure of 1 depicts two NHC
ligands bound to nickel via carbene carbons in cis arrangement
and the remaining sites in the square planar geometry are
occupied by pyridine nitrogens (Figure 1). The Ni1−C7 and
Ni1−C26 bond distances are 1.847(7) and 1.845(6) Å whereas
the corresponding values for Ni1−N1 and Ni1−N4 are 1.943(5)
marginally improved the amide formation (97%, Table S2).
Solvent optimization was done over a range of combinations
including H2O/THF, H2O/MeOH, H2O/EtOH, H2O/DMSO,
H2O/acetone, H2O/1,4-dioxane. Significant conversion could be
achieved only for H2O/MeOH combination (68%, Table S2).
Notably, catalyst 1 was active when pure water was used as
solvent. However, a higher temperature (100°C) and a longer
reaction time (24 h) were needed to achieve good conversion
(82%, Table S2). Lowering the catalyst loading or temperature
had a detrimental effect on the reaction rate (Table S2). Hence,
all subsequent reactions were carried out at the standard
conditions: 2 mol% catalyst loading in H2O/iPrOH (1:3 v/v) at
70°C. Compound 1’ showed very similar hydration activity under
identical reaction conditions.
and 1.950(5)
Å respectively. The relatively longer Ni−N
distances reflect the trans effect of the carbene carbons. The cis
angles around the metal range from 87.7(3) to 93.0(3)° revealing
a slightly distorted environment from a perfect square planar
structure. The carbene carbon appears at δ 173.3 ppm in the
13C NMR spectrum.
Substrate scope for catalyst 1 was evaluated using different
organonitriles under optimized conditions (Table 1). Electron
deficient
nitriles
such
as
3,5-dinitrobenzonitrile,
4-
nitrobenzonitrile and 4-bromobenzonitrile were converted to the
corresponding amides in high yields (Table 1, entries 2-4).
Presence of electron donating group had an adverse effect on
the yields. 4-methylbenzamide and 4-methoxybenzamide were
isolated from the corresponding nitriles in 72% and 65% yields
respectively
(entries
5,
6).
Only
15%
4-
(dimethylamino)benzamide was obtained after 24 h for entry 7.
Increasing the steric bulk from 2-naphthalenecarbonitrile to 9-
anthracenecarbonitrile drastically decreased the product
Scheme 3. Synthesis of 1 or 1’ depending upon different crystallization
conditions: (a) methanol/diethyl ether, rt; (b) acetonitrile/methanol/diethyl ether,
−20°C.
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formation from 88% to <10% (entries 8, 9). Catalyst 1 was
successfully employed to obtain o-substituted pyridyl amide
(82%) from the corresponding nitrile (entry 10). An excellent
yield (97%) was also obtained for heterocyclic 2-furonitrile (entry
11) proving that the presence of a donor site on the substrate
did not impede the catalytic effectiveness of 1. A range of
aliphatic nitriles were also tested. Cinnamonitrile showed 62%
conversion to its corresponding amide (entry 12).
Diphenylacetonitrile showed moderate conversion (37%)
presumably because of increased steric crowding at nitrile
position (entry 13). Aliphatic cyclohexanecarbonitrile was
converted to cyclohexanecarboxamide in moderate yield 38%
(entry 14). Industrially important acrylamide was synthesized in
high yield of 92% without any over oxidized product (entry 15).
For butyronitrile, the reaction took 12 h to show 52% yield (entry
16). Aromatic and aliphatic dintriles were also included as
substrates and selective amidation of only one functional group
was achieved (entries 17, 18).
Cyanohydrin Hydration
Hydration of cyanohydrins is synthetically challenging as it
rapidly decomposes in solution to produce ketones and HCN.16
Tyler
cymene){P(NMe2)3}]
demonstrated catalyst deactivation by generated cyanide.17
and
co-workers,
using
catalysts
[RuCl2(η6-p-
and
[RuCl2(η6-p-cymene)(PMe2OH)],
A
practical solution for hydrating cyanohydrins is to design catalyst
which operates under neutral conditions and has high catalytic
activity to disrupt the equilibrium of the cyanohydrin/ketone-HCN
reaction. The ability of the catalyst 1 to hydrate nitrile under
base-free conditions prompted us to check its effectiveness for
the hydration of typical cyanohydrins (Scheme 4). Only 22%
amide was obtained from mandelonitrile after 6 h in presence of
2 mol% 1. The yield substantially increased to 68% after 48 h at
100°C when catalyst loading was adjusted to 10 mol%. The
reaction was extended to 4-methoxymandelonitrile (52%) and 4-
chloromandelonitrile (70%) to produce corresponding α–
hydroxyamides in modest yields.
Table 1. Catalytic hydration of nitriles by 1[a]
Scheme 4. Synthesis of α–hydroxyamides.
1, 6h, 91%
4, 6h, 94%
2, 2h, 99%
3, 2h, 99%
Bioactive Amides
Once an optimum number of substrates were tested for
hydration, the scope of catalyst 1 was expanded to synthesize
commercial pharmaceuticals (Scheme 5). Pyrazinamide
(commercially sold as Rifater®), a widely recommended drug
during short-course treatment of pulmonary tuberculosis,18 was
synthesized from pyrazinecarbonitrile in quantitative yield.
Nicotinamide has anti-inflammatory actions and benefits patients
with inflammatory skin conditions.19 Quantitative yields of
nicotinamide were achieved using catalyst 1. 2-(2-oxopyrrolidin-
1-yl)acetonitrile under catalytic conditions produced 2-(2-
5, 24h, 72%
6, 24h, 65%
7, 24h, 15%[b]
10, 6h, 82%
8, 6h, 88%
9, 24h, <10%[b]
12, 12h, 62%
oxopyrrolidin-1-yl)acetamide, also known as Piracetam,
nootropic agent which improves the function
neurotransmitters via muscarinic cholinergic receptors.20
a
of
11, 6h, 97%
The catalytic utility of 1 was further exploited for the
synthesis of Rufinamide, an antiepileptic drug. Rufinamide
(marketed as Inovelon® in Europe) is prescribed as an
adjunctive seizure medicine for children and for adults with
Lennox-Gastaut syndrome.21 The nitrile precursor was
synthesized from commercially available 2-(bromomethyl)-1,3-
difluorobenzene and 2-chloroacrylonitrile (Scheme S1).22
Subsequent hydration by 10 mol% 1 at 100°C in 24 h produces
the target drug molecule in 70% yield. At room temperature,
product crystallizes out of the reaction mixture that was filtered
off and purified by column chromatography. The isolated
compound was characterized by NMR spectroscopy and X-ray
crystallography (Scheme 5, inset). Energy dispersive X-ray
(EDX) analysis experiments confirmed no Ni incorporation in the
product (Figure S8).
13, 12h, 37%
16, 12h, 52%
14, 24h, 38%[b]
15, 6h, 92%
18, 6h, 45%
17, 6h, 72%
[a] Organic nitrile (1 mmol) and distilled water (1 mL) were
sequentially added to a 3 mL iPrOH solution of the catalyst
1 (0.02 mmol) and the reaction mixture was heated at
70ºC. Yields are calculated based on isolated products
after work-up. [b] GC yields are reported relative to
mesitylene as an internal standard.
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Hemilability and pKa
working hypothesis for the catalyst activity involves
A
decoordination of the pyridyl unit from the metal center under
catalytic conditions to pave the way for an incoming water and
then polarizes it by hydrogen-bonding interaction. To establish
ligand hemilability, pKa measurement of the putative unbound
pyridyl unit was attempted. Complex 3 was chosen since it is
devoid of protic hydroxy groups. When 1:2 (m/m) mixture of 3
and TfOH in acetonitrile/water at room temperature was titrated
against a standard 0.01 M NaOH solution, a gradual increase in
pH was observed. This implies that the pyridine ring is bound to
the metal and hence not accessible for protonation. Interestingly,
when the same mixture was stirred at 70ºC for 1 h and then
titrated against NaOH, a sigmoidal curve was observed from
which pKa of 2.51 was estimated (Figure 2a). In a separate
experiment, an 1:1 (m/m) mixture of [L3H]Br and TfOH in
acetonitrile/water mixture at room temperature when titrated
against a standard NaOH solution gave pKa of 2.99 attributed to
the pyridynium hydrogen (Figure 2b). The closeness of the pKa
values of [L3H]Br and 3 suggests that the pyridyl unit in the
latter complex is accessible under the reaction conditions.
Scheme 5. Synthesis of pharmaceutical drugs by 1. X-ray structure of
Rufinamide is depicted inset.
Mechanistic Investigation
Control Experiments
In order to recognize the key role played by the hemilabile units,
several related catalysts were synthesized and their catalytic
activities were examined. First, a ligand precursor [L2H]Br that is
devoid of the pyridyl unit but contains the hydroxy side group
n
was synthesized. Reaction between [L2H]Br, excess BuLi and
NiCl2 (0.5 equivalents) in THF provided complex 2 in 72% yield.
The molecular structure of 2 shows a cis arrangement of two
carbene ligands and the two remaining sites in the square planar
geometry are occupied by the hydroxy groups (Scheme 6;
Figure S2 for X-ray structure). Under optimized conditions, 2
gave less than 10% benzamide in 6 h that could be increased to
t
30% in the presence of catalytic amount of BuOK (10 mol%).
This experiment strongly suggests that the hemilabile pyridyl unit
is indispensable for the hydration activity. Subsequently, a new
ligand [L3H]Br was synthesized which bears the pyridyl group
but the hydroxy appendage was masked. Molecular structure of
3 revealed a dicationic structure analogous to 1 (Scheme 6;
Figure S3 for X-ray structure). Under standard conditions, 3
afforded benzamide in 85% yield (vs 91% for catalyst 1). This
indicates that the absence of hydroxy appendages does not
impede the hydration activity significantly. The peripheral alcohol
groups in 1 may enhance the catalyst solubility in aqueous-
alcohol binary mixture and consequently afford marginally better
yield. Finally, a Ni complex 4, devoid of both the pyridyl and the
hydroxy side group, was synthesized (Scheme 6; Figure S4 for
X-ray structure). The molecular structure of 4 shows two
monodentate NHCs bound to the metal centre in trans geometry
along with two bromides. Catalyst 4 gave less than 5% amide
formation under catalytic conditions. In summary, catalysts that
lack the pyridyl component (2 and 4) are not active whereas
those containing both the pyridyl and the hydroxy or only the
pyridyl pendant (1 and 3) show substantial activity. Systematic
tuning of the ligands reveals that a hemilabile pyridyl unit is
essential for the hydration activity.
(a)
(b)
Figure 2. Titration curves obtained by titrating a) 1:2 (m/m) of 3 and TfOH in
acetonitrile/water 3:1 (v/v) at 70ºC for 1 h against 0.01 M NaOH; b) 1:1 (m/m)
of [L3H]Br and TfOH in acetonitrile/water 3:1 (v/v) against 0.01 M NaOH.
Kinetic Studies
Kinetic experiments were performed to gain insight on the
hydration mechanism. Kinetic Isotope Effect (KIE) was
determined by carrying out nitrile hydration of benzonitrile with
H2O and D2O, and a kH/kD value of 1.38 ± 0.21 was obtained
(Figure 3a). This observation is consistent with other nitrile
hydration catalysts.23 The effect of temperature on the rate of the
reaction of
1 with 4-nitrobenzonitrile was evaluated. The
activation parameters were determined from ln(k/T) versus 1/T
plot which is linear over the temperature range studied (333–353
K) (Figure 3b). The estimated entropy of activation (ΔS⧧) is
−33.39 ± 0.56 cal mol–1 K–1, and the enthalpy of activation (ΔH⧧)
is 11.35 ± 0.77 kcal mol–1. Such high negative ΔS⧧ value is
indicative of an organized transition state involving both
substrates (water and nitrile) and the catalyst.
Scheme 6. Catalysts 2, 3 and 4.
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(a)
(b)
(c)
Figure 3. a) Reaction rates for amide formation in H2O and D2O; b) Arrhenius plot for hydration of 4-nitrobenzonitrile catalyzed by 1 over 333−353 K; c) Hammett
plot for competitive hydration of benzonitrile and p-substituted derivatives catalyzed by 1 at 343 K.
The relationship between the relative rates and the Hammett
parameter for the hydration of p-substituted benzonitrile by 1
was also examined. For p-substituted benzonitriles, their
hydration reactivity follows the trend: p-Br >p-Cl >p-H >p-Me >p-
OMe. The relative rates (log(kX/kH)) were plotted against the
substituent constant (σ) yielding a fairly good linear relationship.
From the slope of the Hammett plot, ρ = +2.47 ± 0.19 was
determined (Figure 3c). A positive ρ value suggests that the
reaction should be favored by electron deficient substrates,
which is in complete agreement with the experiments (vide
supra). Further, it suggests a transition state in the rate-
determining step (rds) in which negative charge is developed at
α-carbon atom adjacent to the phenyl ring.24 However, a linearity
could not be achieved when log(kX/kH) was plotted against
standard σ– values.25 This implies that the negative charge
developed at α-carbon in the transition state is of lesser extent
and hence a nucleophilic water attack to the metal-bound nitrile
is more likely over a hydroxide (vide infra).
with σ– values. Thus, kinetic experiments do not support a
hydroxide attack. For hydration reaction that is carried out in
neutral conditions, direct water addition to the nitrile carbon
appears to be favored over a hydroxide attack.28
Proposed Mechanism
A hemilability driven water addition mechanism is proposed for
nitrile hydration to amide (Scheme 7). The pyridyl units being
trans to the carbene centers (long Ni-N distances, vide supra)
are detached from the metal to allow the approach of a water
molecule and a substrate nitrile towards the metal centre
forming A. pKa measurements indicated accessibility of the
pyridyl group under catalytic conditions. Hydrogen-bonding
interaction with one of the pyridyl nitrogen atom polarizes the
water molecule. Such interaction also aids in bringing a
Scheme 7. Proposed mechanism for nitrile hydration (for clarity,
benzimidazole is replaced with imidazole).
Conclusions
A Ni(II) catalyst bearing NHC-derived hemilabile ligands is
reported. It hydrates a wide range of organonitriles to the
corresponding amides, including several amide-based
pharmaceuticals. Cyanohydrins, which are otherwise destroyed
in basic medium, are hydrated by this catalyst. Control
experiments reveal the activity-enhancing roles for the
hemilabile pyridyl group. pKa measurements validate ligand
hemilability under catalytic conditions. Kinetic studies suggest a
ligand-promoted nucleophilic water addition to the nitrile carbon.
Hemilability driven nitrile hydration by a 3d-metal complex opens
up new avenues for further development of water-activation
catalysts.
catalytically relevant water molecule from the bulk solvent to the
12
vicinity of the metal center.9b,
Subsequent ligand-promoted
nucleophilic water attack leads to the formation of an iminol
intermediate B. This is followed by a rapid tautomerization to
yield the amide coordinated intermediate C. Finally amide is
released and catalyst 1 is regenerated completing the cycle.
An alternative pathway is water proton abstraction by the
pyridyl nitrogen and simultaneous hydroxide attack to the nitrile
carbon (Scheme S2).12, 26 However, the pKa values reveal that
the pyridyl nitrogen is not sufficiently basic to abstract water
hydrogen.27 The absence of primary KIE also does not support a
water dissociation mechanism. Furthermore, a linear Hammett
plot was obtained with σ parameters of the substrates but not
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(CCH3), 27.3 (CCH3); ESI-MS (CH3CN): m/z 282.1606 [M-Cl]+; Anal. Calcd
for C17H20N3Cl1O1: C, 64.33; H, 6.36; N, 13.25. Found: C, 64.12; H, 6.11;
N, 12.99.
Experimental Section
General Procedures. All reactions were carried out under nitrogen
atmosphere with the use of standard Schlenk–line techniques unless
stated otherwise. Glasswares were flame–dried under vacuum prior to
Synthesis of 1. In a flame dried Schlenk flask [L1H]Cl (220 mg, 0.70
mmol) was dissolved in 15 mL THF and cooled to 0°C. nBuLi (1.75 mmol,
1.6 M in hexane, 2.5 equivalents) was added to this solution followed by
addition of anhydrous NiCl2 (44 mg, 0.35 mmol, 0.5 equivalents). The
mixture was allowed to attain room temperature and then refluxed for 24
h. The solution was cooled and the solvent was evaporated under
reduced pressure. The crude solid obtained was redissolved in 15 mL
methanol and the orange mixture was filtered through a small pad of
celite. The solution was concentrated under reduced pressure, and 15
mL diethyl ether was added to induce precipitation. The supernatant
solution was discarded by cannula filtration, and the precipitate was
further washed with diethyl ether (3x10 mL). Finally, the precipitate was
dried under vacuum to afford 1 as a yellow solid. Crystals suitable for X-
ray diffraction were grown by layering diethyl ether over a concentrated
methanolic solution of 1 inside an 8 mm o.d. vacuum−sealed glass tube.
Yield: 412 mg (85%). 1H NMR (400 MHz, CD3OD) δ 8.45 (d, J = 4.56 Hz,
1H, Py), 8.04 (d, J = 8.24 Hz, 1H, Py), 7.87 (t, J = 7.80 Hz, 1H, Py), 7.80
(d, J = 8.24 Hz, 1H, Ph), 7.65-7.60 (m, 3H, Ph), 7.37-7.34 (m, 1H, Py),
5.91 (s, 2H, CH2py), 4.53 (s, 2H, CH2), 2.01 (s, 1H, OH), 1.28 (s, 6H,
CH3); 13C NMR (100 MHz, CD3OD) 173.3 (CIm), 154.2 (CPy), 151.7 (CPy),
148.8 (CPy), 140.3 (CPh), 137.0 (CPh), 132.3 (CPy), 126.0 (CPy), 111.2
(CPh), 109.8 (CPh), 69.0 (CCH2), 55.9 (CCOH), 54.8 (CCH2Py), 25.1 (CCH3),
24.4 (CCH3); ESI-MS (CH3CN): m/z: 310.1205 [M-2Cl]2+; Anal. Calcd for
C34H38N6O2Cl2Ni1: C, 59.12; H, 5.55; N, 12.17. Found: C, 58.95; H, 5.37;
1
use. H, 13C and 19F NMR spectra were obtained on JEOL JNM–LA 500
MHz and JEOL JNM–LA 400 MHz spectrometers. Chemical shift values
were referenced to the residual signals of the deuterated solvents. ESI–
MS were recorded on a Waters Micro mass Quattro Micro triple–
quadruple mass spectrometer. Elemental analyses were performed on a
Thermoquest EA1110 CHNS/O analyzer. The crystallized compound was
washed several times with dry diethyl ether, powdered and dried in
vacuum for at least 48 h prior to elemental analyses. GC–MS experiment
was performed on an Agilent 7890A GC and 5975C MS system. EDX
experiment was performed on
a JEOL EDS-LA 6510 analyzer.
Potentiometric titration was carried out at 25ºC using Metrohm 794 Basic
Titrino instrument connected to a Metrohm AG 9101 Herisau pH glass-
electrode and a ground-joint diaphragm. Prior to the experiment, the
standardization was carried out with aqueous buffer solutions at pH 4.00
and 7.00. The ionic-strength of the medium was maintained at I = 0.1 M
NaNO3.
Materials. Solvents were dried by conventional methods, distilled under
nitrogen and deoxygenated prior to use. Anhydrous NiCl2 and
organonitriles were purchased from Sigma-Aldrich. nBuLi was bought
from Acros Organics.
X-ray data collections and refinement. Single crystal X–ray structural
studies were performed on a CCD Bruker SMART APEX diffractometer
equipped with an Oxford Instruments low–temperature attachment. Data
were collected at 100(2) K using graphite–monochromated Mo–K
radiation ( = 0.71073 Å). The frames were indexed, integrated and
scaled using SMART and SAINT software package,29 and the data were
corrected for absorption using the SADABS program.30 The structures
were solved and refined using SHELX 2014 suite of programs.31 The
hydrogens of the O–H groups in complex 1, 1’ and 2 were located in the
difference Fourier map and were refined with restraints. All other
hydrogen atoms were included in the final stages of the refinement and
were refined with a typical riding model. All non–hydrogen atoms were
refined with anisotropic thermal parameters. The “SQUEEZE” option in
the PLATON program was used to remove any disordered solvent
molecule, if present from the overall intensity data.32 Crystallographic
data and pertinent refinement parameters for compounds are
summarized in Table S1 in the Supporting Information. The
crystallographic figures used in this manuscript have been generated
using Diamond 3.1e software.33 CCDC 1457346, 1479483-1479487,
1539691 contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
N, 11.99. Compound 1’ was crystallized from
CH3CN/CH3OH layered with Et2O solution at -20°C. Spectroscopic
details of 1' are identical to 1.
a 1:1 mixture of
Synthesis of [L2H]Br. Benzimidazole (1.00 g, 8.46 mmol) and isobutylene
oxide (0.7 mL, excess) were taken in a pressure tube and were heated at
50ºC for 6 h in neat condition. This was followed by the addition of benzyl
bromide (1.59 g, 9.31 mmol) in 5 mL THF. The resulting solution was
refluxed for another 24 h. A white precipitate appeared which was filtered,
subsequently washed with diethyl ether and finally dried in vacuum.
Yield: 2.50 g (82%). 1H NMR (400 MHz, CDCl3) δ 10.90 (s, 1H, Im), 7.89
(d, J = 7.80 Hz, 1H, Ph), 7.57-7.46 (m, 5H, Phen), 7.36-7.31 (m, 3H, Ph),
5.84 (s, 2H, CH2), 4.54 (s, 2H, CH2), 4.18 (s, 1H, OH), 1.26 (s, 6H, CH3);
13C NMR (100 MHz, CDCl3) δ 145.2 (CIm), 133.0 (CPh), 132.2 (CPh), 131.2
(CPh) 127.1 (CPhen), 126.8 (CPhen), 124.2 (CPhen), 123.3 (CPhen), 114.3
(CPh), 113.2 (CPh), 79.9 (COH), 69.6 (CCH2), 52.8 (CCH2), 27.5 (CCH3), 27.3
(CCH3).
Synthesis of 2. In a flame dried Schlenk flask [L2H]Br (252 mg, 0.70
mmol) was dissolved in 15 mL THF and cooled to 0°C. nBuLi (1.75 mmol,
1.6 M in hexane, 2.5 equivalents) was added to this solution followed by
addition of anhydrous NiCl2 (44 mg, 0.35 mmol, 0.5 equivalents). The
mixture was allowed to attain room temperature and then refluxed for 36
h. The solution was cooled and the solvent was evaporated under
reduced pressure. The crude solid obtained was redissolved in 15 mL
dichloromethane and the yellow mixture was filtered through a small pad
of celite. The solution was concentrated under reduced pressure, and 15
mL petroleum ether was added to induce precipitation. The supernatant
solution was discarded by cannula filtration, and the precipitate was
further washed with petroleum ether (3x10 mL). Finally, the precipitate
was dried under vacuum to afford 2 as a yellow solid. Crystals suitable
for X-ray diffraction were grown by layering petroleum ether over a
concentrated dichloromethane solution of 2 inside an 8 mm o.d. vacuum-
sealed glass tube. Yield: 311 mg (72%). 1H NMR (400 MHz, CDCl3) δ
7.79-7.78 (m, 2H, Ph), 7.57-7.44 (m, 5H, Phen), 7.40-7.32 (m, 2H, Ph),
5.84 (s, 2H, CH2), 4.80 (s, 2H, CH2), 4.82 (s, 1H, OH), 1.28 (s, 6H, CH3);
Synthesis of [L1H]Cl. Benzimidazole (1.00 g, 8.46 mmol) and isobutylene
oxide (0.7 mL, excess) were taken in a pressure tube and were heated at
50ºC for 6 h in neat condition. This was followed by the addition of
neutralized picolyl chloride (1.19 g, 9.31 mmol) in 5 mL THF. The
resulting solution was refluxed for another 24 h. A pink precipitate
appeared which was filtered, subsequently washed with diethyl ether and
finally dried in vacuum. Yield: 2.42 g (90%). 1H NMR (500 MHz, CDCl3) δ
10.89 (s, 1H, Im), 8.49 (d, J = 5.15 Hz, 1H, Py), 7.76 (d, J = 8.60 Hz, 1H,
Py), 7.72-7.69 (m, 2H, Py), 7.61 (d, J = 7.45, 1H, Ph), 7.59-7.55 (m, 1H,
Ph), 7.53-7.47 (m, 1H, Ph), 7.25-7.22 (m, 1H, Ph), 5.86 (s, 2H, PyCH2),
4.67 (s, 2H, CH2), 4.23 (s, 1H, OH), 1.29 (s, 6H, CH3); 13C NMR (125
MHz, CDCl3) δ 152.3 (CPy), 149.9 (CPy), 144.6 (CIm), 137.8 (CPy), 132.5
(CPh), 131.1 (CPh), 127.0 (CPh), 126.9 (CPh), 124.1 (CPy), 123.3 (CPy),
114.2 (CPh), 113.3 (CPh), 69.6 (CCH2), 56.5 (CCOH), 52.8 (CCH2Py), 27.5
This article is protected by copyright. All rights reserved.

10.1002/chem.201700816
Chemistry - A European Journal
FULL PAPER
13C NMR (100 MHz, CDCl3) δ 163.7 (CIm), 137.2 (CPh), 129.1 (CPh), 128.2
(CPh), 127.9 (CPhen), 126.7 (CPhen), 125.0 (CPhen), 118.9 (CPhen), 110.9
(CPh), 110.6 (CPh), 71.9 (CCOH), 68.7 (CCH2), 52.4 (CCH2), 26.5 (CCH3) 26.3
(CCH3).
at reflux for 24 h. Obtained white precipitate was filtered and washed with
diethyl ether. Finally, the precipitate was dried under vacuum to afford
[L4H]Br. Yield: 2.50 g (85%). 1H NMR (400 MHz, CDCl3) δ 11.35 (s, 1H,
Im), 7.81 (d, J = 7.80 Hz, 1H, Ph), 7.56-7.47 (m, 5H, Phen), 7.33-7.30 (m,
3H, Ph), 5.84 (s, 2H, CH2), 4.82 (t, J = 4.56 Hz, 2H, CH2), 3.93 (t, J =
5.04 Hz, 2H, CH2), 3.32 (s, 3H, OCH3); 13C NMR (100 MHz, CDCl3) δ
143.2 (CIm), 132.7 (CPhen), 132.3 (CPh), 131.1 (CPh), 129.5 (CPhen), 129.3
(CPhen), 128.4 (CPhen), 127.2 (CPh), 114.2 (CPh), 113.5 (CPh), 70.2
(CCH2OCH3), 59.2 (COCH3), 51.6 (CCH2Phen), 48.0 (CCH2CH2OMe); Anal. Calcd
for C17H19N2O1Br1: C, 58.95; H, 5.53; N, 8.09. Found: C, 58.72; H, 5.29; N,
7.84.
Synthesis of [L3H]Br. Benzimidazole (1.00 g, 8.46 mmol), picolyl chloride
(1.19 g, 9.31 mmol), and KOH (1.04 g, 18.62 mmol, 2.2 equivalents)
were dissolved in 15 mL THF and heated at reflux for 6 h. The resulting
mixture was cooled to room temperature, and solvent was evaporated
under vacuum. Water was added to the residue and extracted three
times with 30 mL of dichloromethane. After washing with water, the
combined organic phases were dried over anhydrous MgSO4, filtered,
and volatiles were removed under reduced pressure. Yellow solid 1-
benzyl-1H-benzimidazole was formed. Subsequently, 1-benzyl-1H-
benzimidazole, 2-bromoethyl methyl ether (0.87 mL, 9.31 mmol, 1.1
equivalents) and 5 mL dry THF were taken in a pressure tube and
refluxed for 24 h. Obtained white precipitate was filtered and washed with
diethyl ether. Finally, the precipitate was dried under vacuum to afford
[L3H]Br. Yield: 2.42 g (82%). 1H NMR (500 MHz, CDCl3) δ 11.35 (s, 1H,
Im), 8.48 (d, J = 4.05 Hz, 1H, Py), 7.88 (d, = 8.00 Hz, 2H, Ph), 7.78 (d, J
= 8.60 Hz, 1H, Py), 7.74-7.70 (m, 1H, Py), 7.59-7.53 (m, 2H, Ph), 7.25-
7.22 (m, 1H, Py), 6.00 (s, 2H, CH2), 4.78 (t, J = 4.85 Hz, 2H, CH2), 3.94 (t,
J = 4.85 Hz, 2H, CH2), 3.35 (s, 3H, CH3); 13C NMR (125 MHz, CDCl3) δ
152.5 (CPy), 149.7 (CIm), 143.2 (CPy), 137.8 (CPy), 132.1 (CPh), 131.6 (CPh),
127.1 (CPy), 127.1 (CPy), 124.1 (CPh), 124.0 (CPh), 114.3 (CPh), 113.8
(CPh), 70.2 (CCH2OCH3), 59.3 (COCH3), 52.6 (CCH2Py), 48.0 (CCH2CH2OCH3);
Anal. Calcd for C16H18N3O1Br1: C, 55.32; H, 5.23; N, 12.10. Found: C,
55.14; H, 4.97; N, 11.84.
Synthesis of 4. In a flame dried Schlenk flask [L4H]Br (242 mg, 0.70
mmol) was dissolved in 15 mL THF and cooled to 0°C. nBuLi (1.05 mmol,
1.6 M in hexane, 1.5 equivalents) was added to this solution followed by
addition of anhydrous NiCl2 (45 mg, 0.35 mmol, 0.5 equivalents). The
mixture was allowed to attain room temperature and then refluxed for 36
h. The volatiles were evaporated and crude solid was redissolved in 15
mL dichloromethane and the yellow mixture was filtered through a small
pad of celite. The filtrate was concentrated and 15 mL of hexane was
added with stirring to induce precipitation. The supernatant solution was
discarded by cannula filtration, and the precipitate was further washed
with petroleum ether (3×10 mL). Finally, the precipitate was dried under
vacuum to afford 4 as a yellow solid. Crystals suitable for X-ray diffraction
were grown by layering petroleum ether over
a concentrated
dichloromethane solution of 4 inside an 8 mm o.d. vacuum−sealed glass
tube. Yield: 429 mg (82%). 1H NMR (400 MHz, CDCl3,) δ 7.32-7.29 (m,
5H, Phen), 7.11-6.96 (m, 3H, Ph), 6.84 (d, J = 7.76 Hz, 1H, Ph), 5.06 (s,
2H, CH2), 4.09 (t, J = 5.52 Hz, 2H, CH2), 3.70 (t, J = 5.52 Hz, 2H, CH2),
3.33 (s, 3H, OCH3); 13C NMR (100 MHz, CDCl3) δ 154.6 (CIm), 136.4
(CPhen), 130.0 (CPh), 128.8 (CPh), 127.7 (CPhen), 127.6 (CPhen), 121.4
(CPhen), 121.3 (CPh), 108.5 (CPh), 108.3 (CPh), 70.7 (CCH2OCH3), 59.0
(COCH3), 45.0 (CCH2Phen), 41.4 (CCH2CH2OCH3); Anal. Calcd for
C34H38Br2N4O2Ni1: C, 54.39; H, 5.11; N, 7.47. Found: C, 54.11; H, 4.91;
N, 7.25.
Synthesis of 3. In a flame dried Schlenk flask [L3H]Br (174 mg, 0.50
mmol) was dissolved in 15 mL THF and cooled to 0°C. nBuLi (0.75 mmol,
1.6 M in hexane, 1.5 equivalents) was added to this solution followed by
addition of Ni(COD)2 (69 mg, 0.25 mmol, 0.5 equivalents). The mixture
was allowed to attain room temperature and then refluxed for 36 h. The
volatiles were evaporated and crude solid was again dissolved in 15 mL
dichloromethane and the red mixture was filtered through a small pad of
celite. The filtrate was concentrated and 15 mL of hexane was added
with stirring to induce precipitation. The supernatant solution was
discarded by cannula filtration, and the precipitate was further washed
with petroleum ether (3x10 mL). Finally, the precipitate was dried under
vacuum to afford 3 as a red solid. Crystals suitable for X-ray diffraction
were grown by layering diethyl ether over a concentrated methanol
solution of 3 inside an 8 mm o.d. vacuum−sealed glass tube. Yield: 319
mg (85%). 1H NMR (400 MHz, CDCl3) δ 8.55 (d, J = 4.56 Hz, 1H, Py),
7.59-7.56 (m, 1H, Py), 7.18-7.14 (m, 2H, Py), 7.11-7.04 (m, 2H, Ph),
7.00-6.92 (m, 2H, Ph), 5.19 (s, 2H, PyCH2), 4.10 (t, J = 5.48 Hz, 2H,
CH2O), 3.70 (t, J = 5.48 Hz, 2H, CH2), 3.33 (s, 3H, CH3); 13C NMR (100
MHz, CDCl3) δ 156.3 (CIm), 154.5 (CPy), 149.5 (CPy), 137.1 (CPy), 130.0
(CPh), 129.3 (CPh), 122.7 (CPy), 121.8 (CPy), 121.6 (CPh), 121.4 (CPh),
108.6 (CPh), 108.5 (CPh), 70.6 (CCH2OCH3), 59.1 (COCH3), 47.0 (CCH2Py),
41.5 (CCH2CH2OCH3); Anal. Calcd for C32H34N6Ni1O2Br2: C, 51.20; H, 4.57;
N, 11.20. Found: C, 50.94; H, 4.29, N, 10.95.
Nitrile Hydration. An oven dried Schlenk tube was charged with
i
compound 1 (0.02 mmol), nitrile (1 mmol) and PrOH/H2O (4mL, 3:1 v/v).
The tube was placed in a pre-heated oil bath at 70°C with stirring for the
specified time. After the reaction was over, the solvent was evaporated
under vacuum. The residue was purified by column chromatography on
silica gel (petroleum ether/ethyl acetate) to give desired amide. Yields
are calculated based on isolated products. GC yields are reported in
presence of internal standard mesitylene (for selected entries) in the
manuscript.
Hydration of cyanohydrins. An oven dried Schlenk tube was charged
i
with compound 1 (0.1 mmol), cyanohydrin (1 mmol) and PrOH/H2O (4
mL, 3:1 v/v). The tube was placed in a pre-heated oil bath at 100°C with
stirring for 48 h. After the reaction was over, the solvent was evaporated
under vacuum. The residue was purified by column chromatography on
silica gel (CH2Cl2/CH3OH) to give desired α–hydroxyamide. Yields are
calculated based on isolated products.
Synthesis of [L4H]Br. Benzimidazole (1.00 g, 8.46 mmol), benzyl
bromide (1.11 mL, 9.31 mmol, 1.1 equivalents) and KOH (1.04 g, 18.62
mmol, 2.2 equivalents) were dissolved in 15 mL THF and refluxed for 6 h.
The resulting mixture was cooled to room temperature, and solvent was
evaporated under vacuum. Water was added to the residue and
extracted three times with 30 mL of dichloromethane. After washing with
water, the combined organic phases were dried over anhydrous MgSO4,
filtered, and volatiles were removed under reduced pressure. Yellow solid
1-benzyl-1H-benzimidazole was formed. Subsequently, 1-benzyl-1H-
benzimidazole, 2-bromoethyl methyl ether (0.87 mL, 9.31 mmol, 1.1
equivalents) and 5 mL dry THF were taken in a pressure tube and heated
Kinetic Isotope Effect Studies. An oven dried Schlenk tube was
charged with compound 1 (0.02 mmol), benzonitrile (1 mmol), internal
standard mesitylene (0.5 mmol) and PrOD-d8/D2O (3:1 v/v). The tube
was placed in a pre-heated oil bath at 70°C with stirring. After stipulated
time intervals, small aliquots of 0.2 mL were taken out with a hypodermic
needle. KIE was calculated to be the ratios for the two reaction rates.
i
Arrhenius Plot. An oven dried Schlenk tube was charged with
compound 1 (0.02 mmol), benzonitrile (1 mmol), internal standard
mesitylene (0.5 mmol) and iPrOH/H2O (3:1 v/v). The tube was placed in a
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10.1002/chem.201700816
Chemistry - A European Journal
FULL PAPER
pre-heated oil bath with stirring and after stipulated time intervals, small
aliquots of 0.2 mL were taken out with a hypodermic needle. The
activation parameters were determined from the ln(k/T) versus 1/T plot
which is linear over the temperature range studied (333–353 K).
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1 (0.02 mmol), benzonitrile (1 mmol), p-substituted benzonitriles (1 mmol),
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Acknowledgements
[4]
Financial support from the Department of Science and
Technology (DST) of India, the Department of Atomic Energy
(DAE) and the Council of Scientific and Industrial Research
(CSIR) is gratefully appreciated. J.K.B. thanks the DAE India for
an SRC-OI fellowship. We thank Dr. Biswajit Saha and Dr.
Tapas Ghatak for their help. K.S. and A.S. thank the UGC India
for fellowships. I.D. and P.P. thank the CSIR India for
fellowships.
Keywords: Water Activation • Cooperative Catalysis •
Hemilability • 3d-Metal Catalysis • Amide Synthesis
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Entry for the Table of Contents
FULL PAPER
Water Addition: A hemilabile pyridyl
unit on a Ni catalyst bearing
functionalized NHC ligands, guides a
water molecule to the metal, polarizes
it through hydrogen-bonding
Kuldeep Singh, Abir Sarbajna, Indranil
Dutta, Pragati Pandey and Jitendra K.
Bera*
Page No. – Page No.
interaction and promotes nucleophilic
attack to the nitrile carbon for nitrile
hydration to amide under base-free
condition.
Hemilability Driven Water
Activation: A Ni(II) Catalyst for
Base-Free Hydration of Nitriles to
Amides
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