Photocatalysis Enabling Acceptorless Dehydrogenation of Diaryl Hydrazines at Room Temperature

Article abstract of DOI:10.1021/acscatal.8b01579

Aromatic azo compounds are privileged structural motifs, and they exhibit a myriad of pharmaceutical as well as industrial applications. Here, we report a catalytic acceptorless dehydrogenation of diarylhydrazine derivatives to access a wide variety of aryl-azo compounds with the removal of molecular hydrogen as the sole byproduct. This distinctive reactivity has been achieved under dual catalytic conditions by merging the visible-light active [Ru(bpy)₃]²⁺ as the photoredox catalyst and Co(dmgH)₂(py)Cl as the proton-reduction catalyst. The reaction proceeds smoothly under very mild and benign conditions and operates at ambient temperature. This dual catalytic approach is highly compatible with many different functional groups and has a broad substrate scope. We have also demonstrated the reversible hydrogen storage and release phenomenon on hydrazobenzene/azobenzene couple to show the utility of these compounds as hydrogen storage materials. Further diversification of azobenzene was shown by a transition-metal-catalyzed azo-group-directed ortho-C-H bond functionalization.

Full text of DOI:10.1021/acscatal.8b01579

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Letter
Photocatalysis Enabling Acceptorless Dehydrogenation
of Diaryl hydrazines at Room Temperature
Manoj Kumar Sahoo, Krishnasamy Saravanakumar, Garima Jaiswal, and Ekambaram Balaraman
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01579 • Publication Date (Web): 12 Jul 2018
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ACS Catalysis
Photocatalysis Enabling Acceptorless Dehydrogenation of Diaryl
hydrazines at Room Temperature
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Manoj K. Sahoo, Krishnasamy Saravanakumar, Garima Jaiswal, and Ekambaram Balaraman*
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Organic Chemistry Division, Dr. Homi Bhabha Road, CSIRꢀNational Chemical Laboratory (CSIRꢀNCL), Pune ꢀ 411008,
India.
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KEYWORDS: Azo benzene, acceptorless dehydrogenation, hydrogen, photoredox catalysis, dual catalysis.
ABSTRACT: Aromatic azo compounds are privileged structural motifs, and they show myriad pharmaceutical as well as industrial
applications. Here, we report an unprecedented catalytic acceptorless dehydrogenation of diarylhydrazine derivatives to access wide
verity of arylꢀazo compounds with the removal of molecular hydrogen as the sole byproduct. This distinctive reactivity has been
achieved under dual catalytic conditions by merging the visibleꢀlight active [Ru(bpy)3]2+ as the photoredox catalyst and
Co(dmgH)2(py)Cl as the protonꢀreduction catalyst. The reaction proceeds smoothly under very mild and benign conditions and
operates at room temperature. This dual catalytic approach is highly compatible with many different functional groups and has a
broad substrate scope. We have also demonstrated the reversible hydrogen storage and release phenomenon on hydrazobenꢀ
zene/azobenzene couple to show the utility of these compounds as hydrogen storage materials. Further diversification of azobenꢀ
zene was shown by a transitionꢀmetal catalyzed azo group directed ortho CꢀH bond functionalization.
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Aromatic compounds with the N=N motif are highly valuable
products as they show profound applications1ꢀ6 in dyes and
pigment industry,1 therapeutic agents,2 photoꢀresponsive
switches and chemosensors, polymers, soft materials, radical
initiators, bioactive ligands, and food additives.3ꢀ6 In recent
times, several methods have been reported for the synthesis of
aromatic azo derivatives (Figure 1).7ꢀ11 These methods include
oxidative dehydrogenative couplings of anilines,7 reductive
coupling of aromatic nitro compounds,8 the coupling of aroꢀ
matic compounds with aryl diazonium salts,9 and Mills reacꢀ
tion.10 There are few reports on direct oxidative dehydrogenaꢀ
tion (ODH) of hydrazobenzene derivatives to azobenzenes.11ꢀ12
These synthetic methods suffer from harsh reaction conditions
such as high catalyst loading, need of stoichiometric reagents
or strong oxidants and thus, generate an equivalent amount of
waste.
the most important key reactions in contemporary science.13
The catalytic acceptorless dehydrogenation reactions (ADH)
obviate the requirement of external oxidants and produce less
toxic waste. The ADH method with the liberation of hydrogen
gas is much superior over the classical oxidative
dehydrogenation methods in view of atomꢀeconomy, and
environmental sustainability.14 Removal of hydrogen atoms
from adjacent atomic centers of an organic molecule is a
thermodynamically uphill process; however, recent
experimental and computational studies showed that the
presence of nitrogen atom makes the removal of hydrogen
atom more easier.15 Herein, we report ADH of
hydrazobenzene derivatives using a dual catalytic system
comprising photoredox catalysis coupled with proton reducꢀ
tion catalysis. This unprecedented reaction operates under very
mild, benign conditions. A consequence of this finding with
only molecular hydrogen as the byꢀproduct makes this process
more ecoꢀbenign and sustainable for the effective synthesis of
a wide range of aromatic azo compounds for various applicaꢀ
tions. Interestingly, the reversible hydrogen storage/release
phenomenon on hydrazobenzene /azobenzene system is also
demonstrated.
Jiao and coꢀworkers reported a Cuꢀcatalyzed oxidative
dehydrogenation of anilines7d to azobenzenes using air as the
oxidant. Notably, due to the competing selfꢀcoupling of
anilines, the unsymmetrical azo compounds were obtained in
poor yields. Of late, Suib and his group reported the synthesis
of symmetrical azobenzene derivatives by oxidative
dehydrogenation of anilines using mesoꢀMn2O3.7e The reaction
operates at elevated temperature with very high catalyst
loading. Very recently, Lin and coꢀworkers reported a method
to prepare azo compounds by oxidative dehydrogenation of
anilines using a stoichiometric amount of NBS and DBU unꢀ
der cryogenic conditions.7a Ma and coꢀworkers described the
synthesis of diaryl hydrazines using heterogeneous graphene
oxide system with high catalyst loading (10 weight%) under
oxidative conditions.11c Gozin and coꢀworkers reported oxidaꢀ
tive dehydrogenation of hydrazobenzene using the toxic
TiCl3/HBr system.11e Recently, Hashimoto and coꢀworkers
reported oxidative dehydrogenation of hydrazobenzene using a
catalytic amount of strong KOtBu.11a Although this method has
a broad substrate scope, the use of liquid NH3 as solvent meꢀ
dium and strong base limited the practical applicability of the
reaction.
Figure 1. Previous reports on catalytic approches to synthesis
of azobenzene derivatives.
We began optimization of the ADH reaction using diphenyl
hydrazine 1a as a model substrate. We screened a number of
photoredox catalysts merged with different proton reduction
In recent times, catalytic dehydrogenation of organic
molecules with the liberation of molecular hydrogen is one of
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catalysts (I-III) in various solvent systems (Tableꢀ1). We
Page 2 of 7
action in presence of [Ru(bpy)3]Cl2 (1 mol%), protonꢀ
reduction cobalt catalyst (III) (2 mol%) under visibleꢀlight
irradiation in EtOH at room temperature. As shown in Table 2,
the present ADH strategy shows a wide substrate scope and
high functionalꢀgroup tolerance and gives dehydrogenated
azobenzene derivatives in good to excellent yields (up to
97%). Electronꢀdonating as well as electronꢀwithdrawing
groups at different position in the phenyl ring of the
hydrazobenzene derivatives did not affect the reactivity and
yielded the corresponding dehydrogenated products (Tableꢀ2).
Among symmetrical hydrazobenzene derivatives, substrates
bearing electronꢀwithdrawing group at the para position of the
phenyl ring, such as fluoro, chloro, bromo and trifluoromethyl
proceeded smoothly to give the corresponding azobenzenes in
89%ꢀ97% isolated yields (products 2b-2d, and 2f) under the
optimized conditions. Also substrates with electronꢀdonating
groups such as pꢀmethyl gave 2e in 92% isolated yield. Like
symmetrical hydrazobenzenes, unsymmetrically substituted
hydrazobenzenes showed a similar trend in their reactivity.
Thus, substrates with electronꢀwithdrawing groups such as 4ꢀ
F, 4ꢀCl, 4ꢀBr, 4ꢀCF3, 3ꢀBr, and 3ꢀCF3 were well tolerated and
afforded the corresponding azobenzenes in good to excellent
yields of 86% to 94% (Table 1, products 2g-2i, 2m-2n, and
2p). The diaryl hydrazines with electronꢀdonating groups such
as 4ꢀMe, 4ꢀMeO, 4ꢀEtO, and 3ꢀMe were well tolerated and
offered the corresponding dehydrogenated products in excelꢀ
lent yields (Table 2, products 2j-2l, and 2o). Similarly, a hetꢀ
erocyclic azo derivative was obtained with 91% isolated yield
under the optimized condition (Table 2, product 2q).
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found that 1 mol% of photoredox catalyst, 2 mol % of Coꢀ
based proton reduction catalyst, in 0.1 M ethanol gave the
desired dehydrogenated azobenzene 2a in 93% isolated yield
(Table 1, entry 1). Among the various photoredox catalysts,
[Ru(bpy)3]2+ catalyst was found to be optimal for this reaction
(Table 1, entries 1ꢀ4). Simillary, Co(dmgH)2(py)Cl (III)
proton reduction catalyst was found to give superior activity,
and gave the maximum yield of 2a (Table 1, entries 1, 5ꢀ6).
The liberated hydrogen gas was qualitatively analyzed on gas
chromatography (GCꢀTCD).16 Among a number of solvents
screened, EtOH was found to be the most suitable solvent for
this ADH reaction (Table 1, entries 1, 7ꢀ8). A series of control
experiments were carried out to show the necessity of each of
the reaction components such as photoredox catalyst, proton
reduction catalyst, and light source (Table 1, entries 9ꢀ11). The
product 2a formation was not observed in the absence of either
photoredox or proton reduction catalyst (Table 1, entries 9ꢀ
10). Similarly, in the absence of the light source, no formation
of 2a was observed (Table 1, entry 11). Notably, reducing the
photoredox catalyst loading to 0.5 mol% gave the desired
product 2a in moderate yield (Table 1, entry 12). Gratifyingly,
this unprecedented catalytic ADH reaction proceeds very
smoothly at ambient temperature under neutral conditions in
presence of visibleꢀlight source with no generation of waste
except hydrogen gas.
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Table 1. Optimization of the reaction conditions.[a]
Table 2. Substrate scope for room temperature dual catalyzed
ADH of hydrazobenzene.[aꢀb]
[a] General reaction conditions: Substrate 1a (0.25 mmol), photoꢀ
redox catalyst (1.0 mol%), protonꢀreduction (I-III) catalyst (2.0
mol%), solvent 2.5 mL, 28 oC, visibleꢀlight, 12 h. [b] Based on 1H
NMR yield. [c] Isolated yield. [d] Dark conditions. [e] 0.5 mol%
of photoredox catalyst.
[a] Reaction conditions: 1 (0.25 mmol), [Ru(bpy)3]Cl2 (1 mol%),
Coꢀcatalyst (III) (2 mol%), EtOH (2.5 mL), Ar atm, 36W blue
With an optimal reaction conditions in hand, several
hydrazobenzene derivatives 1 were subjected to the ADH reꢀ
LED, 28 oC, 12 h. [b] Isolated yields
.
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ACS Catalysis
The progress of the present ADH reaction was monitored by
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UVꢀVisible absorption spectroscopy (Figure 2). From UVꢀ
Visible absorption spectroscopic analysis, it is observed that
the rate of the reaction decreases as the concentration of prodꢀ
uct (2a) increases. This is because azo products are typically
absorbing in a visible range, and as more and more azo prodꢀ
uct is generated in the reaction mixture, it obscures the activity
of the photocatalyst. The reaction kinetics was measured by
applying the Beer Lambert’s law the concentration of azobenꢀ
zene was determined at a different time interval of the reacꢀ
tion, and a plot was drawn between azobenzene yield vs
time.16
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A
B
Scheme 2. Diversification of azobenzene derivatives.
To better understand the reaction mechanism, a series of conꢀ
trol experiments were carried out. In the absence of the cataꢀ
lysts or under the dark conditions, no formation of product
was observed. This clearly confirms that all the key reaction
components are essential for the success of the present ADH
reaction. Performing the reaction under air/oxygen atmosꢀ
phere, 66% yield of 2a was observed. Importantly, there is no
formation of dihydrogen was observed on GC, and indeed the
product formation is mainly due to the oxidative pathway.11ꢀ12
Cyclic voltammetry experiment of 1a and 1c shows the oxidaꢀ
tion potential of the two compounds are +0.58 V vs. SCE, and
Figure 2. UVꢀVisible spectra of reaction mixture at different
time intervals.
In recent times, liquid organic hydrogen carriers (LOHCs) as
efficient hydrogen storage have been paid much attention in
contemporary science.15aꢀc,17ꢀ18 However, the viability of azoꢀ
benzene as hydrogen storage system is only possible when
both the dehydrogenation of hydrazobenzenes and hydrogenaꢀ
tion of azobenzenes become feasible under mild reaction conꢀ
ditions. Gratifyingly, the reversible hydrogen storage/release
phenomenon on hydrazobenzene/azobenzene couple is sucꢀ
cessfully demonstrated. In this context, we have developed an
efficient method for hydrogenation of azobenzene to hydrazoꢀ
benzene under very mild conditions using a commercially
available heterogeneous catalyst. The hydrogenation reaction
was carried out using catalytic amount of Pd/C (5 weight% of
Pd) and catalytic amount of pyridine as additive under 1 atm
pressure of hydrogen at ambient temperature (Scheme 1).16
+0.66 V vs. SCE, respectively. The reduction potentials EIII/II
+2
and EII*/I of tris(2,2′ꢀbipyridine)ruthenium, [Ru(bpy)3
]
were
determined to be +1.29 V vs. SCE, and +0.77 V vs. SCE, reꢀ
spectively.20 The reduction potential EIII/II (III) of
Co(dmgH)2(py)Cl and the oxidation potential EIII/II* of
+2
[Ru(bpy)3
]
were reported as ꢀ0.67 V vs. SCE,21 and ꢀ0.81 V
vs. SCE, respectively.20 Thus, it is expected that the excited
state of *[Ru(bpy)3]2+ could transfer an electron to CoIII while
hydrazobenzene 1a could transfer an electron to [Ru(bpy)3]3+
and in this way the catalytic cycle will be completed, and thus
may follow the oxidative quenching mechanism.
Based on the above experimental observation, and recent literꢀ
ature precedents20 on dual catalysis21 and hydrogen evolving
reactions by proton reduction cobalt complexes,22 we have
proposed a plausible mechanism as follows (Scheme 3). Under
the visibleꢀlight irradiation the photocatalyst, [Ru(bpy)3]2+
generates the excited state *[Ru(bpy)3]2+ which can reduce
CoIII to CoII via the single electron transfer (SET) through oxiꢀ
dative quenching and lead to [Ru(bpy)3]3+. Then, the formed
[Ru(bpy)3]3+ is reduced by 1a to regenerate [Ru(bpy)3]2+ and
an amine radical cation 1a’. This radical cation 1a’ can underꢀ
go hydrogenꢀatom transfer with CoII to produce the intermediꢀ
ate 2a’ and a highly reducing CoI or can directly produce CoIIIꢀ
H intermediate. Subsequently, the highly reducing CoI species
immediately reacts with a H+ ion (released from 1a’) to give
CoIIIꢀH intermediate. This CoIIIꢀH intermediate may react with
the second proton (released from 2a’) releasing a molecule of
H2 and CoIII or may undergo reduction to CoIIꢀH followed by
protonation to release a H2 molecule and CoII. However, the
homolytic cleavage that involving the two CoIIIꢀH intermediate
to evolve hydrogen gas could also not be ruled out.22ꢀ23
Reaction conditions: 2a (0.25 mmol), Pd/C (1.0 mg), pyridine (10
mol%), methanol (5.0 mL), H2 (1 atm), rt, 15 min.
Scheme 1. Catalytic hyderogenation of azobenzenes to
hydrazobenzenes.
Next, we have shown the diversification of azobenzene (acts
as a good directing group19 in the CꢀH bond activation chemisꢀ
try) and illustrated in Scheme 2. The Ruꢀcatalyzed ortho CꢀH
bond arylation of azobenzene gave 3a in 47% isolated yield.19a
Similarly, Pdꢀcatalyzed orthoꢀCꢀH bond acylation was
achieved using toluene as the acyl equivalent and TBHP as the
oxidant gave 4a in 65% isolated yield.19b The Pdꢀcatalyzed
orthoꢀCꢀH bond sulfonylation of azobenzene (2a) proceeded
smoothly in the presence of catalytic amount of Pd(OAc)2 and
K2S2O8 as the oxidant yielding 5a in 79% isolated yields.19c
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Scheme 3. Plausible reaction mechanism for ADH of hyꢀ
drazobenzene (1a)
.
In conclusion, we have developed a dual transitionꢀmetal cataꢀ
lyzed acceptorless dehydrogenation of hydrazobenzene derivaꢀ
tives to the corresponding azobenzenes with the liberation of
hydrogen gas. This unprecedented ADH strategy was achieved
under environmentally benign conditions by merging the visiꢀ
bleꢀlight photoredox catalysis with Coꢀbased proton reduction
catalysis at room temperature under baseꢀ, and oxidantꢀfree
conditions. Interestingly, reversible hydrogen storage/release
phenomenon on hydrazobenzene/azobenzene couple is also
demonstrated.
(7). (a) John, A. A.; Lin, Q. Synthesis of Azobenzenes Using Nꢀ
Chlorosuccinimide
and
1,8ꢀDiazabicyclo[5.4.0]undecꢀ7ꢀene
ASSOCIATED CONTENT
Supporting Information
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Lin, C. H.; Takeda, Y.; Minakata, S. Oxidative Dimerization of
(Hetero)aromatic Amines Utilizing tꢀBuOI Leading to (Hetꢀ
ero)aromatic Azo Compounds: Scope and Mechanistic Studies. J.
Org. Chem. 2013, 78, 12090ꢀ12105. (c) Takeda, Y.; Okumura, S.;
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tBuOI: Entry to Unsymmetric Aromatic Azo Compounds. Angew.
Chem., Int. Ed. 2012, 51, 7804ꢀ7808. (d) Zhang, C.; Jiao, N.
CopperꢀCatalyzed Aerobic Oxidative Dehydrogenative Coupling
of Anilines Leading to Aromatic Azo Compounds using Dioxyꢀ
gen as an Oxidant. Angew. Chem., Int. Ed. 2010, 49, 6174ꢀ6177.
(e) Dutta, B.; Biswas, S.; Sharma, V.; Savage, N. O.; Alpay, S. P.;
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The Supporting Information is available free of charge on the
ACS Publications website.
Experimental and spectroscopic data, copies of 1H, and 13C NMR
spectra (PDF).
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: eb.raman@ncl.res.in
ORCID
E. Balaraman: 0000ꢀ0001ꢀ9248ꢀ2809
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This research is supported by the EMR/2015/000030 (under the
SERBꢀGreen Chemistry programme), and INPROTICS
(HCP0011ꢀA). MKS thanks the CSIR and GJ thanks the UGC for
the research fellowships.
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