Nickel catalysed construction of benzazoles: Via hydrogen atom transfer reactions

Article abstract of DOI:10.1039/d0cy00748j

Herein we report a homogeneous, phosphine free, inexpensive nickel catalyst that forms a wide variety of benzazoles from alcohol and diamines by a reaction sequence of alcohol oxidation, imine formation, ring cyclization and dehydrogenative aromatization. A reversible azo/hydrazo couple, that is part of the ligand architecture steers both the alcohol oxidation and dehydrogenation of the annulated amine under fairly mild reaction conditions. Interestingly, both the alcohol oxidation and amine dehydrogenation steps are directly mediated by hydrogen atom transfer (HAT), which is greatly facilitated by the reduced ligand backbone. The kH/kD for the amine dehydrogenation step, measured at 60 °C is 5.9, fully consistent with HAT as the rate determining factor during this step. This is a unique scenario where two consecutive oxidation steps towards benzazole formation undergo HAT, which has been substantiated via kinetic studies, KIE determination and intermediate isolation. This journal is

Full text of DOI:10.1039/d0cy00748j

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Nickel catalysed construction of benzazoles via
hydrogen atom transfer reactions†
Cite this: DOI: 10.1039/d0cy00748j
Amreen K. Bains, Dhananjay Dey, Sudha Yadav, Abhishek Kundu and
*
Debashis Adhikari
Herein we report a homogeneous, phosphine free, inexpensive nickel catalyst that forms a wide variety of
benzazoles from alcohol and diamines by a reaction sequence of alcohol oxidation, imine formation, ring
cyclization and dehydrogenative aromatization. A reversible azo/hydrazo couple, that is part of the ligand
architecture steers both the alcohol oxidation and dehydrogenation of the annulated amine under fairly
mild reaction conditions. Interestingly, both the alcohol oxidation and amine dehydrogenation steps are
directly mediated by hydrogen atom transfer (HAT), which is greatly facilitated by the reduced ligand
backbone. The kH/kD for the amine dehydrogenation step, measured at 60 °C is 5.9, fully consistent with
HAT as the rate determining factor during this step. This is a unique scenario where two consecutive
oxidation steps towards benzazole formation undergo HAT, which has been substantiated via kinetic
studies, KIE determination and intermediate isolation.
Received 14th April 2020,
Accepted 6th May 2020
DOI: 10.1039/d0cy00748j
rsc.li/catalysis
handful of base metal representatives for this important
transformation, although cobalt, nickel and manganese
Introduction
Benzimidazoles and its derivatives are ubiquitously present in
natural products and employed widely as antibacterial,
antitumor, antihistaminic, and antiviral drugs.1 Furthermore,
substituted benzimidazoles have found applications in
pharmaceuticals, agrochemicals, dye industries as well as in
material science as thermostable membranes for fuel cells.2
Previous synthetic strategies for the construction of
benzimidazole involved condensation of 1,2-diaminobenzene
with a carboxylic acid derivative or formic acid, which
required harsh reaction conditions and corrosive acids.3 To
this end, it is highly desirable to find a milder, operationally
simple and sustainable method which can be simultaneously
environmentally benign.
catalysts appeared promising in dehydrogenative coupling of
diamine and alcohols to realize benzimidazole derivatives.8 A
major drawback of these systems is the requirement of
relatively harsh reaction conditions and long reaction time.
Additionally, rational design of catalysts is often challenging
because of the lack of precise mechanistic understanding.
Hence, development of a molecularly-defined, air-stable,
base-metal catalyst which can synthesize benzimidazoles or
other benzazoles under mild conditions yet in a controllable
fashion, is very much desirable.
Herein we communicate a well-defined, phosphine-free,
homogeneous
nickel
catalyst
that
can
construct
benzimidazoles under fairly mild conditions in the absence
Toward this direction, the first synthesis of benzimidazole
formation via dehydrogenative coupling was showcased by
Watanabe which required noble metal catalyst ruthenium at
a very high temperature, 215 °C.4 Recent examples from
Kempe involve Ir-based catalysts which synthesize
benzimidazoles and substituted benzimidazoles at 110 °C.5
Some heterogeneous systems6 have also been developed for
the synthesis of benzimidazoles but those comprise of
precious metals Ir and Ru.7 On the contrary, there are only a
Department of Chemical Sciences, Indian Institute of Science Education and
Research (IISER) Mohali, Sector-81, Knowledge City, Manauli-140306, India.
E-mail: adhikari@iisermohali.ac.in
† Electronic supplementary information (ESI) available: Detailed synthetic
procedure, Control experiments, characterization details, NMR spectra. See DOI:
10.1039/d0cy00748j
Scheme 1 2e− redox in azo–hydrazo couple driven hydrogen atom
transfer towards the synthesis of benzazoles.
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of any sacrificial hydrogen acceptor. A thorough literature
survey reveals that our reaction conditions are much milder
even compared to few well-performing noble metal catalysts.
Encouragingly, the catalyst can also regulate the formation of
2-substituted benzimidazole or their 1,2-disubstituted
analogues. The precatalyst 1 (Scheme 1), used in this work is
an isolable, bench-stable and molecularly-defined entity
which helps us to sketch a thorough mechanistic scenario,
demonstrating that our reaction pathway is governed by the
hydrogen atom transfer (HAT) to the redox active backbone.
Fascinatingly, both key steps in the overall reaction rely on
HAT, which is unprecedented for dehydrogenative
aromatization of N-heterocycles.
neopentyl alcohol offers high yield (81%) of the 2-substituted
benzimidazole (5u) under the same reaction conditions.
Furthermore, when 1-hexanol was (2w) employed as the
coupling partner, product 2-pentyl benzimidazole (5w) was
synthesized in 65% yield. Similarly, for 1-pentanol (2x), the
corresponding benzimidazole product (5x) was also obtained
in good (70%) yield. This is much improved yield under
milder conditions than many base metal catalyzed
2-benzimidazole formation where aliphatic alcohols with
longer chain were used as a coupling partner.8b,c We further
expanded the scope of our synthetic protocol by having a
diverse choice of bis-amine. Accordingly, when –Br, –Cl, −Me,
–OMe substituted o-phenylenediamines were coupled to
benzyl alcohol, all generated the appropriate benzimidazole
(5q–5t) in good to very good yield (75–85%). We were further
delighted to observe that 1,2 disubstituted benzimidazoles
can be elegantly synthesized simply by changing the
stoichiometry of the alcohol. Accordingly, when 2 was used
twice in amount to 4, corresponding 1,2-disubstituted
products formed smoothly under the same reaction
conditions with a tiny amount of mono-substituted product
Results and discussion
Our preliminary exploration commenced with 1,2-phenylene
diamine coupling with benzyl alcohol in presence of the
putative square planar nickel complex, 1 synthesized from
azophenolate ligand and NiIJOAc)2 (Scheme 1).9 To our
advantage, the catalyst molecule is air-stable and the entire
reaction can be operated under aerobic condition, unlike
many catalysts containing phosphine ligands. Optimization
of the reaction conditions revealed that the reaction can be
performed in presence of 5 mol% of 1, 0.50 equivalent of
base KOtBu, and the reaction is completed in 10 h at 80 °C
giving 90% yield of the desired benzimidazole. A set of
control experiments disclose no product formation in
absence of the catalyst, or KOtBu. It has been proved earlier
that KOtBu behaves as a crucial reductant parallel to its
reactivity as a base.10 Furthermore, control experiments prove
that the reaction can be considerably expedited by adding an
oxygen-filled balloon to the reaction flask whose reason will
be clear during mechanistic evaluation. Notably, our
conditions are much milder compared to the existing noble
and base metal catalysts reported for benzazole formation.
With the optimized conditions in hand, we expanded the
scope of the reaction with a diverse set of benzyl alcohol. As
described in Table 1, p-substituted benzyl alcohols with
alkyls, halides (2b–d, 2f, 2n) all result in high yield of the
corresponding benzimidazoles. Similarly, o-substituted benzyl
alcohols with halides (2e, 2h) or –Me group (2p) all provided
the product in 80–88% yield. Presence of electron-donating
or -withdrawing groups (2c, 2l) in the benzyl alcohol were
well tolerated under the optimized reaction conditions and
the yield of the final products were similar. Notably,
heterocyclic alcohols such as 2-pyridinemethanol (2g),
2-furanmethanol (2j) smoothly furnished the respective
benzimidazole products in moderate to high yields (65–81%).
Moreover, during the benzimidazole formation, no
N-alkylated product was observed to form as a detrimental
byproduct.
Table 1 Scope of alcohols and diamines: synthesis of benzimidazole
5a (90%)
5d (75%)
5b (79%)
5e (80%)
5c (84%)
5f (76%)
5g (65%)
5j (81%)
5m (76%)
5h (82%)
5k (62%)
5n (82%)
5q (82%)
5t (75%)
5w (65%)
5i (77%)
5l (86%)
5o (78%)
5r (85%)
5u (81%)
5p (88%)
5s (81%)
After successfully using many derivatives of benzyl alcohol
for the dehydrogenative cyclization reactions, we tested
whether our catalyst can be equally efficient if aliphatic
alcohols are chosen as the coupling partner. Gratifyingly,
5x (70%)
5v (60%)
Reaction conditions: 1 (5 mol%), benzyl alcohol (1 mmol), diamine
(1 mmol), KOtBu (0.5 mmol), toluene (2 mL), 80 °C, O2, 10 h.
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(∼13%) formation. The developed protocol was tested for six
different alcohols, including heterocyclic 2-furanmethanol
and corresponding benzimidazoles were isolated in 66–85%
yields (Table 2). Encouragingly, simple control of benzyl
alcohol stoichiometry to furnish di- over mono-substituted
products add considerable modularity to the developed
process. Such tunability prompted us further to expand the
scope of this reaction to other benzazoles. Along this
direction, we changed the o-phenylenediamine with o-amino
phenol and o-amino thiol. Gratifyingly, the corresponding
benzoxazoles and benzthiazoles were synthesized in good to
excellent yields (Table 3). Furthermore, the diols like quinol
and resorcinol offer the disubstituted products in 67 and
56% respectively under the same reaction conditions
(Table 4).
cyclization yields 2,3-dihydrobenzimidazole, 3 (Fig. 2a). In
fact, this is the precursor to benzimidazole upon
dehydrogenation. The dehydrogenation of different saturated
heterocycles is considered to be an important class of
reaction, since these heterocycles often serve as liquid
organic hydrogen storage material, in commercial fuel cell
etc.11 A set of control experiments strongly suggest that the
dehydrogenation reactions are very inefficient in absence of
1, under our reaction conditions. Henceforth, the same
catalyst plays a crucial role for the amine dehydrogenation
reaction parallel to its function in alcohol oxidation. We
posited the dehydrogenation of 3, being a 2e− process, may
undergo a similar cycle of HAT, taking help from the mono
reduced azo motif. To examine whether the binding of 3 is
necessary during the dehydrogenation process, we performed
saturation kinetics experiment with this substrate and
Mechanistically, the benzimidazole formation reaction can
be dissected into three major parts: alcohol oxidation,
condensation to form imine followed by cyclization, and
oxidative dehydrogenation. The plausible intermediates along
the reaction sequence have been fully characterized upon
their isolation (ESI,† Scheme S1 and S2). Importantly,
reaction with d-labelled benzyl alcohol, resulted in
d-incorporation (96%) only at the benzylic position of the 1,2-
disubstituted benzimidazole (ESI,† Scheme S4 and Table S6).
This fact unequivocally establishes that the oxidized alcohol
is a source of alkyl group in the final product benzimidazole.
The formation of di-substituted benzimidazole upon using
two equivalents of alcohols can be easily explained by
bisimine formation followed by a ring cyclization and a
hydride migration (ESI,† Scheme S2). We have indicated in
our previous study that the alcohol oxidation comprises a
HAT step where a H-atom from the β-position of the bound
alcohol migrates to the mono-reduced azo motif in the ligand
backbone.9
observed
a clear saturation of rate (kobs) when the
concentration of 3 was exceeding 0.8 M (Fig. 1a). An
equilibrium constant (K) of 231 M−1 and k of 4.5 × 10−4 s−1 was
derived from the non-linear curve fitting of the saturation plot
at 80 °C following the rate law kobs = kKij3]/1+Kij3]. This finding
implies that the pathway of dehydrogenation likely follows very
similar steps of alcohol oxidation. Furthermore, with a
deuterium isotopologue of 3 (labelled isotopically at the
benzylic position), a KIE of 5.9 (Fig. 1b) was measured at 60 °C.
This considerably large value of KIE again strongly indicates
that HAT to the reduced azo backbone is rate- determining for
the amine dehydrogenation reaction.
This mode of aromatization is rare and to our knowledge
there is not much precedence of HAT based dehydrogenation
of N-heterocycles.12 To unequivocally establish that a radical
pathway is involved during this oxidative dehydrogenation
step, we used a radical quencher TEMPO and found the
reaction is thwarted considerably (ESI,† section 2, III). Very
encouragingly, the radical generated upon HAT can cleanly
be intercepted by administering TEMPO and can be
The resulting aldehyde undergoes condensation with
o-phenylenediamine to form an imine, which upon ring
Table 2 Scope of alcohols: synthesis of 1,2 di-substituted benzimidazole
Table 3 Scope of alcohols: synthesis of benzthiazole, benzoxazoles
7a (71%)
7d (73%)
7b (84%)
7e (81%)
7h (40%)
7c (75%)
7f (53%)
7i (55%)
6c (72%)
6f (66%)
6a (85%)
6d (84%)
6b (81%)
6e (68%)
7g (42%)
Reaction conditions: 1 (5 mol%), benzyl alcohol (1 mmol), o-amino
phenol/ o-amino thiol (1 mmol), KOtBu (0.5 mmol), toluene (2 mL),
80 °C, O2, 10 h.
Reaction conditions: 1 (5 mol%), benzyl alcohol (2 mmol), diamine
(1 mmol), KOtBu (0.5 mmol), toluene (2 mL), 80 °C, O2, 10 h.
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Table 4 Scope of di-alcohol: synthesis of benzimidazole
unreduced azo arm, and readily becomes converted to the
benzimidazole. In the context of alcohol oxidation, it is
important to note that aliphatic alcohols are difficult to
oxidize due to its greater C–H bond strength. The oxidation
for this class of substrates is likely facilitated by the
reduction of this putative bond strength due to the electronic
effect of the alkoxide lone pair.16
Overall, the present example is distinctly different from a
large body of reports for dehydrogenative benzannulations
which are simply directed by traditional metal–ligand
cooperativity.17 As can be easily anticipated, the ligand part
of the catalyst traps the borrowed hydrogen both in alcohol
oxidation and dehydrogenative aromatization steps and
stores in the form of a hydrazo moiety. Interestingly, using
O2 as a terminal oxidant, the catalyst can be revived very
easily by formation of H2O2. As a conclusive evidence, the
formation of H2O2 as a byproduct was further traced by
iodometry (ESI,† Fig. S3). Indeed, ease of catalyst revival
under neat oxygen condition helped the reaction to operate
under fairly mild conditions (80 °C, 10 h) that rivals all other
catalysts for benzazole formation. As anticipated, the hydrazo
form of the catalyst can also be oxidized with simple aerobic
oxygen, however, that will make the process sluggish.
8a (67%)
8b (56%)
Reaction conditions: 1 (5 mol%), benzyl alcohol (0.5 mmol), diamine
(1 mmol), KOtBu (0.5 mmol), toluene (2 mL), 80 °C, O2, 10 h.
characterized by high-resolution mass spectrometry. The
observed mass of the trapped intermediate, 351.2301 amu
and its isotopic distribution match very well with the
simulated pattern for the molecule (Fig. 2c). During this
dehydrogenative reaction, supposedly
a relatively stable
carbene radical forms where the stability of the radical is
gained by delocalization through the -Ph ring and parallel
stability conferred by heteroatoms in the near vicinity. To
substantiate this HAT step being operative during
dehydrogenation, we further computed the pertinent
intermediate and transition state (TS) at M06/lanl2dz (6-31g*
for C, H, N, O) level of theory on the model catalyst.13 In the
bound intermediate (9, Fig. 2a), the radical character on azo
and its proximity to the benzylic hydrogen atom of the
substrate makes the H-atom abstraction mechanism very
feasible. At the TS, the N—H and C—H distances are 1.38
and 1.30 Å respectively (Fig. 2b). The computed distance
between two termini of the HAT was found to be 2.62 Å, that
has a close resemblance to many other HAT processes in
enzyme catalysis.14 Not surprisingly, since the reduced azo
motif in the ligand backbone is responsible for both HAT
steps, the TS for dehydrogenation of 3 is reminiscent of the
TS for alcohol oxidation step.9 Further scrutiny also discloses
that the binding of 3 to catalyst 1 is only facilitated when the
proton transfer to one phenolate arm leads to detachment of
the arm and nickel maintains a distorted square planar
geometry (Fig. 2a). This also closely resembles the
protonation of an axial tyrosinate arm from the CuII-bound
alcohol in galactose oxidase.15 Akin to aldehyde or ketone
formation from transient ketyl radical the carbene radical
also does an intramolecular reduction to the second
Conclusions
In summary, we report an inexpensive nickel catalyst that
forms a variety of benzazoles in a tunable manner, utilizing
hydrogen borrowing from alcohols. The redox-active azo
backbone present in the catalyst system helps both alcohol
oxidation and
a dehydrogenative aromatization via a
common hydrogen atom transfer pathway. The success of the
catalyst hinges on the easy reversible 2e−-redox of the azo–
hydrazo couple, which is part of the ligand utilized in this
system. Both the alcohol oxidation and aromative
dehydrogenation reactions are governed by a radical housed
in the azo backbone, that makes this class of catalyst entirely
different from previously evaluated systems relying on metal–
ligand cooperativity. We hope this catalytic process will be
considered as
a new avenue toward the formation of
heterocycles and will be a nice addition to the repertoire of
HAT catalysis. Our current effort is focused on furnishing a
wide variety of heterocycles using this methodology to prove
the prowess of the catalyst to promote HAT-based
dehydrogenations.
Experimental section
General considerations
All the starting compounds employed in this study were
procured from commercial suppliers and were used without
further purification. Potassium tert-butoxide, potassium
hydroxide and o-phenylene diamine were purchased from
Sigma Aldrich. Glassware were dried overnight at 160 °C
prior to use. Solvents such as methanol, ethanol and toluene
were used as received from the suppliers. For thin layer
Fig. 1 a) Saturation kinetics plot of kobs vs. [3], b) kinetic isotope effect
for amine dehydrogenation at 60 °C by 1.
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Fig. 2 a) Plausible mechanistic scheme for HAT promoted amine dehydrogenation reaction. b) DFT optimized transition state for HAT. All
hydrogens except chemically important ones are removed for clarity (tBu groups in the model ligand are truncated to Me). c) Mass spectrum of
trapped radical intermediate by TEMPO.
chromatography (TLC), aluminium foils coated with silica
and fluorescent indicator (from Merck) were used. Column
chromatography was performed using SD fine silica gel 60–
120 mesh using a gradient of ethyl acetate and hexane as
mobile phase. High-resolution mass spectra were recorded
on a Waters QTOF mass spectrometer. IR spectra were
recorded on a Perkin–Elmer FT IR spectrometer using KBr
pellet, and presented as νmax in inverse centimetres. 1H and
13C NMR spectra were recorded on a 400 MHz Bruker Biospin
Advance III FT-NMR spectrometer. NMR shifts are reported
as delta (δ) units in parts per million (ppm) and coupling
constants (J) are reported in Hertz (Hz). The following
abbreviations are utilized to describe peak patterns when
appropriate: s = singlet, d = doublet, t = triplet, q = quartet
and m = multiplet. Chemical shifts (δ) are quoted to the
nearest 0.01 ppm relative to tetramethylsilane (δ 0.00 ppm) in
CDCl3 (δ 7.26 ppm) or (CD3)2SO (δ 2.50 ppm). To improve
solubility of the probe molecule, sometimes small amount of
(CD3)2SO has been mixed in the primary solvent CDCl3.
Carbon chemical shifts are internally referenced to the
deuterated solvent signal of CDCl3 (δ 77.1 ppm).
After completion of the reaction, the solution was filtered to
obtain dark brown coloured product in 82% yield. The
1
desired product was fully characterized by H, 13C NMR and
IR spectroscopies.
General procedure for the synthesis of benzazoles
In a typical reaction 5 mL vial was charged with alcohols (1
mmol),
o-phenylenediamine/2-aminothiophenol/2-
aminophenol (1 mmol), KOtBu (0.5 mmol), 1 (5 mol%) in 2
mL toluene and was closed with rubber septum. The
resulting solution was purged with O2. The reaction mixture
was stirred at 80 °C for 10 h. The reaction mixture was cooled
to room temperature upon completion and concentrated in
vacuo. The residue was purified by column chromatography
using ethyl acetate/petroleum ether (5–10%) as eluent to
afford pure products. The desired coupling products were
1
fully characterized by H, 13C NMR spectroscopies.
General procedure for the synthesis of 1,2 di-substituted
benzimidazole
In a typical reaction 5 mL vial was charged with alcohols (2
mmol), o-phenylenediamine (1 mmol), KOtBu (0.5 mmol), 1 (5
mol%) in 2 mL toluene and was closed with rubber septum.
The resulting solution was purged with O2. The reaction mixture
was stirred at 80 °C for 10 h. The reaction mixture was cooled
to room temperature upon completion and concentrated in
vacuo. The residue was purified by column chromatography
using ethyl acetate/petroleum ether (5–10%) as eluent to afford
pure products. The desired coupling products were fully
characterized by 1H, 13C NMR spectroscopies.
Synthesis of 1
The synthesis of L was performed following a reported
procedure in our previous work.9 A methanolic solution of
0.1 mmol of L was taken in a 100 mL round-bottom flask
and to it 0.1 mmol of KOH was added and the mixture was
stirred at rt for 30 min. Then, 0.05 mmol of NiIJOAc)2 was
added to the reaction mixture and refluxed for another 30
min. Immediate precipitate was obtained during the reflux.
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Conflicts of interest
8 (a) P. Daw, Y. Ben-David and D. Milstein, ACS Catal., 2017, 7,
7456–7460; (b) A. Bera, M. Sk, K. Singh and D. Banerjee,
Chem. Commun., 2019, 55, 5958–5961; (c) K. Das, A. Mondal
and D. Srimani, J. Org. Chem., 2018, 83, 9553–9560; (d) S.
Das, S. Mallick and S. De Sarkar, J. Org. Chem., 2019, 84,
12111–12119; (e) Z. Xu, D.-S. Wang, X. Yu, Y. Yang and D.
Wang, Adv. Synth. Catal., 2017, 359, 3332–3340.
There are no conflicts to declare.
Acknowledgements
We thank SERB (DST), India (Grant No. ECR/2017/001764) for
financial support and IISER Mohali for seed funding. AKB
and DD thank IISER Mohali for a research and postdoctoral
fellowship respectively. SY thanks DST for an Inspire
fellowship. AK thanks CSIR for a research fellowship.
9 A. K. Bains, A. Kundu, S. Yadav and D. Adhikari, ACS Catal.,
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10 J. P. Barham, G. Coulthard, K. J. Emery, E. Doni, F. Cumine,
G. Nocera, M. P. John, L. E. A. Berlouis, T. McGuire, T. Tuttle
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