Synthesis of Azobenzenes Using N-Chlorosuccinimide and 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU)

Article abstract of DOI:10.1021/acs.joc.7b01530

A convenient method for the synthesis of symmetrical azobenzenes is reported. This one-step procedure involves treatment of anilines with N-chlorosuccinimide (NCS) and organic base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). A wide range of commercially available substituted anilines readily participate in this reaction to produce the corresponding azobenzenes in moderate-to-excellent yields in minutes.

Full text of DOI:10.1021/acs.joc.7b01530

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Note
Synthesis of Azobenzenes Using N-Chlorosuccinimide and DBU
Alford Antoine John, and Qing Lin
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b01530 • Publication Date (Web): 28 Aug 2017
Downloaded from http://pubs.acs.org on August 29, 2017
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Synthesis of Azobenzenes Using N-Chlorosuccinimide and DBU
Alford Antoine John, and Qing Lin*
Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14260-3000,
United States
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RECEIVED DATE (automatically inserted by publisher)
E-mail: qinglin@buffalo.edu
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Abstract: A convenient method for the synthesis of symmetrical azobenzenes is reported. This one-step
procedure involves treatment of anilines with N-chlorosuccinimide (NCS) and organic base 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU). A wide range of commercially available substituted anilines
readily participate in this reaction to produce the corresponding azobenzenes in moderate-to-excellent
yields in minutes.
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Azobenzenes have long attracted interests from organic materials community due to their rapid,
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reversible photoswitching properties upon irradiation with UV or visible light. Because the cis and trans
isomers exhibit distinct structural and photophysical properties,1 azobenzenes have been extensively
employed in the design of photoresponsive materials including polymers,2 crystals,3,4 and bioactive
ligands.5,6 From a synthetic point of view, azobenzenes can be readily accessed using a number of
methods.7 A classic method involves oxidation of anilines by potassium permanganate and copper sulfate
or ferrous sulfate in refluxing methylene chloride for two days, which limits the type of functional groups
that can be attached to the benzene ring 8 (Eq. 1 in Chart 1). To avoid this problem, a number of milder
reaction conditions have been developed recently. For example, Jiao and coworker9 reported the copper-
catalyzed aerobic oxidative dehydrogenative coupling of anilines to generate the azobenzenes (Eq. 2 in
Chart 1). Minakata and coworkers10,11 reported oxidative dimerization of anilines using organic oxidant,
tert-butylhypoiodite (tBuOI), at room temperature to generate a variety of aromatic azo compounds with
excellent tolerance of the functional groups10,11 (Eq. 3 in Chart 1).
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Chart 1. Oxidative coupling to form azobenzenes
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In our pursuit of harnessing the photoswitching properties of azobenzenes for photoregulation of
protein function in live cells, we were interested in synthetic methods that allow facile synthesis of red-
shifted, highly functionalized azobenzenes carrying functional groups that can be readily incorporated
into biological systems. To this end, we previously reported the synthesis of a series of red-shifted, amino
acid-functionalized azobenzenes by reacting the appropriately substituted phenylhydrazines with the
amino acid spirolactone in the presence of catalytic amount of ceric ammonium nitrate.12 During the
preparation of some of the substrates, we serendipitously discovered a simple method for direct synthesis
of symmetric azobenzenes. Herein, we report the rapid synthesis of symmetric azobenzenes through
oxidative coupling of anilines using N-chlorosuccinimide (NCS) and 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) (Eq. 4 in Chart 1), and propose a mechanism to explain this unprecedented transformation.
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Our initial observation was made during the preparation of 4-bromo-2,6-difluoroaniline using NBS
and pyridine, where we detected trace amount of the corresponding symmetric azobenzene. We surmised
it should be possible to use NBS as an organic oxidant for oxidative homo-coupling of anilines to form
the azobenzenes. Interestingly, Takeda and coworkers have examined NBS as an organic oxidant for
homo-dimerization of toluidine without much success.11 In our own hands, when toluidine was treated
with NBS and pyridine in methylene chloride at room temperature, the brominated aniline product, instead
of the azobenzene, was generated exclusively. To distinguish the aryl bromination reactivity from the
nitrogen oxidation reactivity of NBS, we were encouraged by an earlier literature report in which
combination of NBS with a base at low temperature gave rise to a predominant oxidative bromine
species.13 Later work by Keillor and coworkers showed that treatment of aryl carboxamides with
NBS/DBU produced the Hofmann rearrangement products in excellent yields, suggesting that the
combination of NBS with DBU serves principally as an oxidant.14
With these results in mind, we screened several organic and inorganic bases in combination with NBS
and found DBU gave the highest yield in azobenzene formation (entries 1-4, Table 1). We then examined
various solvents including polar, nonpolar, protic, and aprotic solvents and found dichloromethane gave
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the highest yield (entries 4-8, Table 1). Subsequently, we investigated other electrophilic halogen sources,
including N-bromophthalimide (NBP), N-iodosuccinimide (NIS), and N-chlorosuccinimide (NCS).
Compared to NBS, NCS gave a higher yield while NBP and NIS gave lower yields (compare entries 9-
11 to entry 4, Table 1). The reactivity trend of NCS > NBS > NIS suggests that complete dissociation of
the electrophilic halogen cation is not conducive to oxidative coupling of toluidine. We also assessed the
reactivity of two other electrophilic N-chloroimide reagents, trichlorocyanuric acid (TCCA) and 1,3-
dichloro-5,5-dimethylhydantoin (DCDMH); both gave slightly lower yields than NCS (compare entries
12-13 to 11, Table 1), presumably due to their more rapid ionization to form a free Cl+ that participates
competing aromatic chlorination reactions. Yield remains the same when the amidine base DBU was
changed to 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) (entry 14, Table 1). However, the use of a stronger
organic base such as KOtBu led to a lower yield (entry 15, Table 1), suggesting that the higher yields
afforded by the amidine bases are not a result of their higher basicity.
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Table 1. Condition Optimization a
entry
solvent
oxidant
base
yield (%)b
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DCM
DCM
DCM
DCM
THF
NBS
NBS
NBS
NBS
NBS
NBS
NBS
NBS
NBP
pyridine
Cs2CO3
Et3N
NR
NR
24
DBU
80
DBU
NR
15
EtOAc
EtOH
MEK
DCM
DBU
DBU
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DBU
12
DBU
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DCM
DCM
DCM
DCM
DCM
DCM
NIS
NCS
DBU
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85
73
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DBU
DBU
DBU
DBN
KOtBu
TCCAc
DCDMHd
NCS
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NCS
a The reaction was set up by incubating 1 equiv of aniline and 2 equiv of base in appropriate solvent
followed by addition of 2 equiv of oxidant. b Yields are isolated yields and include both cis and trans-2a.
c 0.67 mmol trichlorocyanuric acid (TCCA) was used to generate 2 equiv of electrophilic Cl+ species.
d
1.0 mmol 1,3-dichloro-5,5-dimethylhydantoin (DCDMH) was used to generate 2 equiv of electrophilic
Cl+ species. NR, no reaction. MEK = methyl ethyl ketone.
Since NCS is commercially available and inexpensive, we decided to use NCS as organic oxidant in
the substrate scope study. Under the optimized conditions, a wide range of mono-substituted anilines
carrying alkyl (1a, 1b), ethynyl (1c), electron-withdrawing groups (1d-1g), and halogens (1h-1m) were
smoothly converted into their azobenzene products with 56-93% yields (entries 1-12, Table 2). Since
tetra-ortho-fluoroazobenzenes have shown robust photoswitching properties under visible light,15 we
assessed whether our condition is suitable for making this type of highly substituted azobenzenes. To this
end, five poly-substituted fluorine-containing anilines were treated with NCS/DBU in DCM for 2-5 min
and the desired tetra-ortho-fluoroazobenzenes were obtained uneventfully in 58-70% yields (entries 13-
17, Table 2). Compared to KMnO4/FeSO4·7H2O-mediated oxidative coupling that gave rise to the
azobenzenes in 10-25% yield after overnight reflux,15 the NCS/DBU method produces the desired
azobenzene products in higher yields and shorter time.
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Table 2. Substrate Scope
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entry
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aniline
p-Me (1a)
yield (%) a
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p-iPr (1b)
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p-acetylene (1c)
p-CF3 (1d)
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p-CN (1e)
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p -CO2Et (1f)
m-CN (1g)
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p–F (1h)
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p-Cl (1i)
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p-Br (1j)
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p-I (1k)
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m-F (1m)
75
2,4,6-F3 (1n)
4-Br-2,6-F2 (1o)
4-I-2,6-F2 (1p)
4-CO2Et-2,6-F2 (1q)
2,3,4,5,6-F5 (1r)
70
64
63
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a Yields are isolated yields and include both cis and trans-azobenzene.
Based on the previous report of using the halogen-containing oxidants for oxidative dimerization of
aromatic amines,10,11 a plausible mechanism for NCS/DBU-mediated homo-coupling of anilines is put
forward in Scheme 1. The combination of NCS and DBU would allow formation of the chlorinated DBU
(DBU-Cl+), an organic oxidant whose balanced reactivity and selectivity may be responsible for the two
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key chlorination/oxidation steps: with aniline in step (i) to generate N-chloro-aniline 3 after proton transfer
in step (ii); and with hydrazine 4 in step (v) to generate N-chloro-hydrazine 5 after proton transfer in step
(vi). The use of DBU in facilitating chloronium ion transfer is catalytic as DBU is regenerated after N-
chlorination (steps i and v). However, two equivalents of DBU (relative to one-half of the azobenzene
structure) are consumed as a base during the proton transfer in step (iv) after nucleophilic substitution and
during the final elimination step (vii). Therefore, for symmetric azobenzenes only one equivalent of NCS
and DBU (relative to aniline starting materials) are needed to complete the overall transformation. To
confirm this, we varied the amount of the NCS in the reaction from 0.5 equiv to 3.0 equiv and obtained
73% isolated yield when 1.0 equiv of NCS was used (Table 3), consistent with our mechanistic hypothesis.
Furthermore, when hydrazine 4, which was prepared separately using a literature procedure,16 was
exposed to NCS and DBU, quantitative conversion to azobenzene 2a was observed, indicating that
hydrazine 4 is a competent intermediate and likely involved in the reaction. This mechanism is reminiscent
of the use of NBS/DBU for generation of more electrophilic bromine species for allylic amination
reaction17 as well as tandem bromoamination/debromination reaction18 reported in the literature.
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Scheme 1. Plausible reaction mechanism.
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Table 3. Effect of NCS Equivalency a
NCS equiv.
yield (%)
0.5
1.0
2.0
3.0
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a The amount of DBU was kept constant at 2 equivalent relative to toluidine.
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In summary, we have discovered a new oxidative homo-coupling method for one-step synthesis of
the symmetric azobenzenes from the substituted anilines. This method employs stable and inexpensive
NCS and DBU reagents, which may produce an electrophilic chloro-DBU species likely responsible for
selective N-chlorination/oxidation of the anilines. The substrate scope study revealed that the tetra-ortho-
substituted azobenzene analogs can be rapidly accessed in good yields using this method.
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Experimental Section
General methods: All reactions were performed under ambient atmosphere. The commercially available
reagents were used without further purification. The NMR spectra were recorded in CDCl3 or DMSO-d6
on a Varian Mercury-300 or Inova-400 MHz instrument. NMR data are presented as follows: chemical
shift (ppm); multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet); coupling constants
in hertz (Hz). Flash column chromatography was performed with SiliCycle P60 silica gel (40-63 µm, 60
Å).
Synthetic procedure: To a solution of aniline (1 mmol) dissolved in 15 mL DCM was added DBU (2
mmol). The solution was stirred at room temperature for 5 min before cooled down to a -78°C. The oxidant
(2 mmol) was added as a solid to the reaction mixture. The orange solution was stirred for 10 min at -78
°C before quenching by addition of saturated bicarbonate solution. The organic layer was separated,
washed sequentially with 50 mL water and 50 mL 1 N HCl, dried over anhydrous sodium sulfate, and
concentrated to dryness in vacuo. Compounds with compatible functional groups were treated with 5 mL
TFA at room temperature for 20 min to maximize the conversion to the trans-azobenzene, and TFA was
removed in vacuo. The residue was purified by silica gel flash chromatography using DCM-hexanes as
eluent.
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1,2-Di-p-tolyldiazene (2a).9 Orange solid; 91 mg, 87% yield: 1H NMR (300 MHz, CDCl3) δ 7.81 (d, J =
8.3 Hz, 4H), 7.30 (d, J = 8.5 Hz, 4H), 2.43 (s, 6H).
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1,2-Bis(4-isopropylphenyl)diazene (2b).19 Orange solid; 88 mg, 66% yield: 1H NMR (400 MHz, CDCl3)
δ 7.85 (d, J = 8.4 Hz, 4H), 7.41 – 7.32 (m, 4H), 3.10 – 2.92 (m, 2H), 1.31 (d, J = 6.9 Hz, 12H).
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1,2-Bis(4-ethynylphenyl)diazene (2c).20 Orange solid; 101 mg, 88% yield: 1H NMR (300 MHz, CDCl3) δ
7.89 (s, 4H), 7.64 (s, 4H), 3.24 (s, 2H).
1,2-Bis(4-(trifluoromethyl)phenyl)diazene (2d).21 Yellow solid; 147 mg, 93% yield: 1H NMR (300 MHz,
CDCl3) δ 8.04 (d, J = 8.1 Hz, 4H), 7.81 (d, J = 8.1 Hz, 4H).
4,4'-(Diazene-1,2-diyl)dibenzonitrile (2e).11 Orange-yellow solid; 98 mg, 85% yield: 1H NMR (300 MHz,
CDCl3) δ 8.04 (d, J = 8.3 Hz, 4H), 7.85 (d, J = 8.5 Hz, 4H).
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Diethyl 4,4'-(diazene-1,2-diyl)-dibenzoate (2f).11 Red-orange solid; 134 mg, 82% yield: H NMR (400
MHz, CDCl3) δ 8.21 (d, J = 8.4 Hz, 4H), 7.98 (d, J = 8.4 Hz, 4H), 4.42 (q, 4H), 1.43 (t, 6H).
3,3'-(Diazene-1,2-diyl)dibenzonitrile (2g).22 Yellow solid; 65 mg, 56% yield: 1H NMR (400 MHz, CDCl3)
δ 8.49 – 7.96 (m, 4H), 7.81 (dd, J = 7.7, 1.1 Hz, 2H), 7.69 (t, J = 7.9 Hz, 2H).
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1,2-Bis(4-fluorophenyl)diazene (2h).11 Orange-yellow solid; 90 mg, 83% yield: H NMR (400 MHz,
CDCl3) δ 7.94-7.91 (m, 4H), 7.22-7.18 (m, 4H).
1,2-Bis(4-chlorophenyl)diazene (2i).11 Orange solid; 108 mg, 86% yield: 1H NMR (400 MHz, CDCl3) δ
7.86 (d, J = 8.7 Hz, 4H), 7.49 (d, J = 8.7 Hz, 4H).
1,2-Bis(4-bromophenyl)diazene (2j).11 Orange solid; 101 mg, 59% yield: 1H NMR (300 MHz, CDCl3) δ
7.79 (d, J = 8.4 Hz, 4H), 7.65 (d, J = 8.3 Hz, 4H).
1,2-Bis(4-iodophenyl)diazene (2k).11 Orange-red solid; 164 mg, 76% yield: 1H NMR (400 MHz, CDCl3)
δ 7.87 (d, J = 8.4 Hz, 4H), 7.64 (d, J = 8.4 Hz, 4H).
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1,2-Bis(3-fluorophenyl)diazene (2m).23 Orange solid; 82 mg, 75% yield: 1H NMR (400 MHz, DMSO-d6)
δ 7.79 (d, J = 7.9 Hz, 2H), 7.69-7.61 (m, 4H), 7.55 – 7.35 (m, 2H).
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1,2-Bis(2,4,6-trifluorophenyl)diazene (2n).24 The reaction was stirred at -78°C for 5 min. The solvent was
removed in vacuo and the residue was loaded directly onto silica gel for flash chromatography. Orange-
yellow solid; 102 mg, 70% yield: 1H NMR (300 MHz, CDCl3) δ 6.84 (s, 4H).
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1,2-Bis(4-bromo-2,6-difluorophenyl)diazene (2o). Red solid; 132 mg, 64% yield: H NMR (400 MHz,
DMSO-d6) δ 7.77 (d, J = 9.1 Hz, 4H).
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1,2-Bis(2,6-difluoro-4-iodophenyl)diazene (2p). Red solid; 160 mg, 63% yield: H NMR (400 MHz,
DMSO-d6) δ 7.81 (d, J = 8.9 Hz, 4H).
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Diethyl 4,4'-(diazene-1,2-diyl)-bis(3,5-difluorobenzoate) (2q). Red solid; 124 mg, 62% yield: H NMR
(400 MHz, CDCl3) δ 7.75 (d, J = 9.0 Hz, 2H), 4.43 (q, J = 7.1 Hz, 2H), 1.43 (t, J = 7.1 Hz, 3H).
1,2-Bis(perfluorophenyl)diazene (2r) The reaction was stirred at -78°C for 5 min. The solvent was
removed in vacuo and the residue was loaded directly onto silica gel for flash chromatography. Orange-
yellow solid; 104 mg, 58% yield: 19F NMR (282 MHz, CDCl3) δ -147.46 – -149.35 (m, 6F), -160.72 – -
162.12 (m, 4F).
ASSOCIATED CONTENT
Supporting Information
NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*qinglin@buffalo.edu
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ACKNOWLEDGMENTS
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We gratefully acknowledge the NIH (GM085092) for financial support.
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