Electrochemical Synthesis of Benzazoles from Alcohols and o-Substituted Anilines with a Catalytic Amount of CoII Salt

Article abstract of DOI:10.1002/chem.201505074

An electrochemical synthesis of benzazoles directly from alcohols and o-substituted anilines has been developed. The reaction conditions have been optimized by varying the composition of the electrolyte and the metal salt used as catalyst. The cyclization proceeds smoothly with a catalytic amount of a cobalt salt under air at room temperature to afford 2-substituted benzimidazoles, benzothiazoles, and benzoxazoles in good to excellent yields with a wide substrate scope.

Full text of DOI:10.1002/chem.201505074

DOI: 10.1002/chem.201505074
Full Paper
&
Benzazole Electrosynthesis
Electrochemical Synthesis of Benzazoles from Alcohols and
o-Substituted Anilines with a Catalytic Amount of CoII Salt
Yin-Long Lai, Jian-Shan Ye, and Jing-Mei Huang*[a]
Abstract: An electrochemical synthesis of benzazoles direct-
ly from alcohols and o-substituted anilines has been devel-
oped. The reaction conditions have been optimized by vary-
ing the composition of the electrolyte and the metal salt
used as catalyst. The cyclization proceeds smoothly with
a catalytic amount of a cobalt salt under air at room temper-
ature to afford 2-substituted benzimidazoles, benzothiazoles,
and benzoxazoles in good to excellent yields with a wide
substrate scope.
Introduction
stituted anilines in the presence of a catalytic amount of
a cobalt salt under air at room temperature.
Benzazoles (benzimidazoles, benzothiazoles, and benzoxazoles)
and their derivatives are important targets among heterocyclic
compounds. Such substructures are widely encountered in
pharmaceutical and agrochemical agents.[1] Classical methods
for the preparation of benzazoles include metal-catalyzed in-
tramolecular cyclization of o-haloanilides or their analogues[2]
and the condensation of o-substituted anilines with either car-
boxylic acids, acyl esters, chlorides, amides, nitriles, and such
like[3] or aromatic aldehydes.[4] Since alcohols are abundantly
available,[5] the development of efficient reactions whereby al-
cohols are converted into useful classes of compounds is an at-
tractive goal.[6] Direct oxidative cyclization to form benzazoles
from alcohols and o-thio/hydroxy/aminoanilines is thus espe-
cially attractive. However, alcohol-to-heterocycle reactions re-
quire high selectivity to achieve dehydrogenation of the CÀH
and NÀH bonds in one pot. Recently, progress has been made
by employing alcohols directly to react with o-thio/hydroxy/
aminoanilines to afford benzazoles.[7]
Results and Discussion
To start our investigation, benzyl alcohol 1a (0.5 mmol), 1,2-
phenylenediamine 2a (0.75 mmol), and CF3COOH (1.0 mmol)
were added to a one-compartment cell containing an electro-
lyte of 0.1m LiClO4 in CH3CN (5 mL) and Pt foils (1.0”1.5 cm2)
as electrodes, and electrolysis was carried out at constant cur-
rent (20 mA) at room temperature. Benzimidazole 3a was ob-
tained in 28% yield after the reaction mixture had been stirred
for 150 min (Table 1, entry 1). In this reaction, 1,2-diphenyl-
benzimidazole was obtained as a by-product. Transition metal
salts, specifically NiSO4·6H2O, FeSO4·7H2O, CoSO4·7H2O,
CoCl2·6H2O, and Co(acac)2, showed promising activities in pro-
moting this reaction (Table 1, entries 2–6). Among them,
CoSO4·7H2O proved to be the best transition metal salt addi-
tive, and the desired product was obtained in 86% yield with-
out any detectable 1,2-diphenylbenzimidazole formation.
When the reaction was carried out in anhydrous CH3CN using
anhydrous CoSO4, only a 79% yield of 3a was obtained
(Table 1, entry 7). It was interesting to observe that by adding
a small amount of water, the yield increased to 93% (Table 1,
entry 9). It was also noteworthy that when the Pt foil electro-
des were replaced by graphite rods, the product yield was
85% (Table 1, entry 10). This implied that the Pt electrode did
not serve as a catalyst or promoter for this reaction (Table 1,
entries 9 and 10). Increasing or decreasing the current both led
to a decrease in the product yield (Table 1, entries 11 and 12).
A nitrogen atmosphere also led to a lower chemical yield
(Table 1, entry 13). No reaction occurred when the reaction
mixture was stirred for 150 min without applying a potential
difference (entry 14). Among the solvents examined, CH3CN
was found to be the most suitable for this reaction (Table 1,
entries 9, 15–17).
Electrochemistry has been found to be a powerful method
for the synthesis of organic compounds and electrosynthetic
processes constitute green chemistry, wherein the electric cur-
rent takes the place of a stoichiometric amount of a redox
agent.[8] Recently, Zeng and co-workers reported an indirect
electrochemical synthesis of 2-substituted benzoxazoles from
pre-formed Schiff-bases using sodium iodide as a mediator.[9]
As part of our ongoing activities in exploring electrochemical
reactions,[10] we report here the first example of the direct elec-
trochemical synthesis of benzazoles from alcohols and o-sub-
[a] Y.-L. Lai, Prof. Dr. J.-S. Ye, Prof. Dr. J.-M. Huang
School of Chemistry and Chemical Engineering
South China University of Technology
Guangzhou, Guangdong 510640 (P. R. China)
E-mail: chehjm@scut.edu.cn
Having established the optimal reaction conditions (Table 1,
entry 9), we proceeded to examine the reactions of various
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201505074.
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Table 1. Optimization of the reaction conditions for the electrochemical
synthesis of benzimidazoles.[a]
Table 2. Substrate scope of alcohols for electrochemical synthesis of ben-
zimidazole derivatives.[a,b]
Entry
Solvent
Catalyst
Yield 3a[b] [%]
1
2
3
4
5
6
7
8
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN (anhydrous)
CH3CN/H2O (96:4)
CH3CN/H2O (98:2)
CH3CN/H2O (98:2)
CH3CN/H2O (98:2)
CH3CN/H2O (98:2)
CH3CN/H2O (98:2)
CH3CN/H2O (98:2)
THF/H2O (98:2)
DMF/H2O (98:2)
DMSO/H2O (98:2)
–
28
62
71
86
84
66
79
83
93
85
88
75
72
N.R.
68
82
74
NiSO4·6H2O
FeSO4·7H2O
CoSO4·7H2O
CoCl2·6H2O
Co(acac)2
CoSO4
CoSO4·7H2O
CoSO4·7H2O
CoSO4·7H2O
CoSO4·7H2O
CoSO4·7H2O
CoSO4·7H2O
CoSO4·7H2O
CoSO4·7H2O
CoSO4·7H2O
CoSO4·7H2O
9
10[c]
11[d]
12[e]
13[f]
14[g]
15
16
17
[a] Unless otherwise noted, a mixture of 1a (0.5 mmol), 2a (0.75 mmol),
CF3COOH (1.0 mmol), and metal salt (20 mol%) in solvent (5.0 mL) with
0.1m LiClO4 was electrolyzed with a pair of Pt foils (1.0”1.5 cm2) at con-
stant current (20 mA) in an undivided cell at r.t., 150 min, air. N.R.: no re-
action. [b] The yield of the product was determined by 1H NMR spectros-
copy. [c] Graphite rod electrodes (diameter: 0.5 cm, height: 1.78 cm).
[d] The current was 30 mA. [e] The current was 10 mA. [f] N2 was used in-
stead of air. [g] No electrolysis.
[a] Unless otherwise noted, 1 (0.5 mmol), 2a (0.75 mmol), CoSO4·7H2O
(20 mol%), CF3COOH (1.0 mmol), 0.1m LiClO4, CH3CN/H2O (98:2; 5.0 mL),
a pair of Pt foils (1.0”1.5 cm2), constant current (20 mA), undivided cell,
r.t., air, 150 min. [b] Isolated yield. [c] 508C, t=6 h. [d] 458C, t=6 h.
[e] Constant current (5 mA), t=12 h.
aryl alcohol substrates. As presented in Table 2, the reaction
proceeded smoothly with a series of primary alcohol substrates
to afford moderate to excellent yields of the corresponding
1,2-benzimidazoles (3a–n).
It was noteworthy that halogen and nitro groups were toler-
ated under the electrochemical conditions. Electron-donating
(Me, OMe; 3b–d) and electron-withdrawing groups (F, Cl, Br;
3e–h) on the aryl ring had minimal effects on the yield. Only
the strongly electron-withdrawing NO2 group (3i,j) led to
a slight decrease in yield. An alcohol bearing a naphthyl group
(3k) also participated in this reaction with high reactivity to
afford the desired product in 83% yield. Heterobenzylic alco-
hols also proved to be good substrates for this process, giving
good yields of benzazoles under similar conditions (3l–n). It
was also noteworthy that aliphatic alcohols (n-butanol and n-
heptanol) were compatible with the reaction, and the desired
products were obtained in yields of 37% and 38%, respectively
(3x,y).
Table 3. Substrate scope of o-aminoanilines for electrochemical synthesis
of benzimidazole derivatives.[a,b]
To further examine the scope of this reaction, various substi-
tuted 1,2-diaminoarenes were employed. As shown in Table 3,
several electron-rich and electron-deficient o-diaminobenzenes
2 proved to be good reaction partners with benzyl alcohol 1a,
and the corresponding benzimidazoles (3o–w) were obtained
in good to excellent yields. Similarly, this reaction showed satis-
factory tolerance of halogen and nitro groups.
[a] Unless otherwise noted, 1a (0.5 mmol), 2 (0.75 mmol), CoSO4·7H2O
(20 mol%), CF3COOH (1.0 mmol), 0.1m LiClO4, CH3CN/H2O (98:2; 5.0 mL),
a pair of Pt foils (1.0”1.5 cm2), constant current (20 mA), undivided cell,
r.t., air, 150 min. [b] Isolated yield. [c] 458C, t=6 h.
Having successfully achieved the electrochemical synthesis
of benzimidazole derivatives, we expanded the reaction
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system to the synthesis of benzothiazoles and benzoxazoles,
which are also found in numerous pharmaceutical agents. By
reacting o-aminothiophenol with various aryl alcohols, benzo-
thiazoles were produced in moderate to excellent yields
(Table 4, 6a–g). Furthermore, n-butanol also reacted with o-
aminothiophenol to give the corresponding product in a mod-
erate yield of 40% (Table 4, 6i). 2-Phenyl-benzoxazole could
also be obtained in 40% yield from o-aminophenol (Table 4,
6h).[11b]
Table 4. Electrochemical synthesis of benzothiazole and benzoxazole de-
rivatives.[a,b]
Figure 1. Cyclic voltammograms of the relevant compounds in CH3CN/H2O
(98:2), using a Pt disk as working electrode, a Pt wire as counter electrode,
and a saturated calomel electrode (SCE) as a reference electrode, at a scan
rate of 100 mVsÀ1: (a) 0.1m LiClO4; (b) 0.1m LiClO4, CF3COOH (20 mmolLÀ1);
(c) 0.1m LiClO4, benzyl alcohol (20 mmolLÀ1); (d) 0.1m LiClO4, CF3COOH
(20 mmolLÀ1), benzyl alcohol (20 mmolLÀ1); (e) 0.1m LiClO4, CoSO4·7H2O
(2 mmolLÀ1), CF3COOH (20 mmolLÀ1), benzyl alcohol (20 mmolLÀ1); (f) 0.1m
LiClO4, CoSO4·7H2O (2 mmolLÀ1), CF3COOH (20 mmolLÀ1).
could have protonated the 1,2-phenylenediamine, making it
more difficult to oxidize.[16]
To gain more insight into the reaction mechanism, several
other control experiments were carried out (Scheme 1). When
a mixture of benzoic acid 7a and o-diaminobenzene 2a was
subjected to the standard conditions, a trace of benzimidazole
3a was detected [Eq. (1)]. When benzaldehyde 8a was added
dropwise to the reaction system under the standard condi-
tions, benzimidazole 3a was produced in 76% yield [Eq. (2), a].
Furthermore, the product 3a was obtained in 78% yield when
imine 9a was used as starting material under the standard
conditions [Eq. (3), a]. These results suggested that benzalde-
hyde 8a and imine 9a might be intermediates in this reaction.
Next, the effects of oxygen and CoII were studied, and it was
found that reactions under a nitrogen atmosphere or in the
[a] Unless otherwise noted, 1 (0.5 mmol), 5 (0.75 mmol), CoSO4·7H2O
(20% mol), CF3COOH (1.0 mmol), 0.1m LiClO4, CH3CN/H2O (98:2; 5.0 mL),
a pair of Pt foils (1.0”1.5 cm2), constant current (20 mA), undivided cell,
r.t., air, 150 min. [b] Isolated yield. [c] 508C, t=6 h. [d] Constant current
(10 mA), 508C, t=6 h. [e] Constant current (5 mA), t=12 h.
Cyclic voltammograms (CV) were then studied (see the SI). It
is convenient that the oxidation process of benzyl alcohol can
be elucidated by CV analysis. As shown in Figure 1,[11] the elec-
trochemical response of benzyl alcohol in an electrolyte of
0.1m LiClO4 in CH3CN/H2O (98:2) exhibited its oxidation peak
at Ep =1.78 V vs SCE (curve c). However, when CF3COOH was
added to the reaction mixture, the oxidation peak of benzyl al-
cohol disappeared (curve d). Furthermore, in the presence of
CoSO4·7H2O, benzyl alcohol, and CF3COOH, the oxidation peak
of CoII (Ep =2.04 V vs SCE) was observed, but the oxidation
peak of benzyl alcohol was not detected (curve e).[11a] The
above CV results indicated that the acidic reaction conditions
inhibited the anodic oxidation of benzyl alcohol to form ben-
zaldehyde. This was also indicated by previous work.[12] We
propose that, under our acidic reaction conditions, CoIII gener-
ated on the anode reacts chemically with benzyl alcohol to
give benzaldehyde.[13] In CV studies in the absence of
CF3COOH, the oxidation peak of 1,2-phenylenediamine was ob-
served at Ep =0.45 V vs SCE (SI, Figure S1k), whereas in the
presence of CF3COOH, it was observed at Ep =0.86 V vs SCE (SI,
Figure S1e). This implies that in our reaction system the acid
Scheme 1. Control experiments. For reaction (2): 8a was added dropwise to
the reaction system a) at standard condition, 3a=76%; b) at standard con-
ditions but without CoSO4·7H2O, 3a=34%. For reaction (3): a) at standard
conditions, 3a=78%; b) using anhydrous solvent and reagents, N2,
3a=20%; c) at standard conditions but without CoSO4·7H2O, 3a=45%;
d) using anhydrous solvent and reagents, O2 (balloon), 3a=70%.
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Scheme 2. A possible mechanism for the benzazole synthesis.
(petroleum ether/ethyl acetate=3:1) to afford the pure product
3a.
absence of cobalt(II) sulfate gave much lower yields. This con-
firmed that molecular oxygen and CoII played crucial roles in
this transformation (entry 13 in Table 1; [Eq. (2)], b; [Eq. (3)],
b,c). Moreover, it was observed that water enhanced this trans-
formation (entries 7 and 9 in Table 1; [Eq. (3)], a,d).[14]
Acknowledgements
On the basis of our control experiments, the cyclic voltam-
mograms, and salient literature,[12–15] a possible mechanism for
this reaction is proposed, as depicted in Scheme 2. Benzyl alco-
hol 1a is oxidized to benzaldehyde 8a by CoIII generated on
the anode. Benzaldehyde 8a quickly condenses with protonat-
ed diamine 2a to give the intermediate 11 through cyclization
of intermediate 10. Subsequently, rapid oxidative dehydrogen-
ation of benzazoline 11 generates the target compound benzi-
midazole 3a by the combined effects of CoIII, anodic oxidation,
and O2.
We are grateful to the National Natural Science Foundation of
China (Grant Nos. 21172080 and 21372089) for financial sup-
port.
Keywords: alcohols · benzazoles · cobalt · electrochemistry ·
o-substituted anilines
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Conclusion
In conclusion, an electrochemical strategy for the synthesis of
benzazoles directly from benzylic alcohols and o-substituted
anilines in the presence of a catalytic amount of a cobalt salt
has been developed. This method is atom-economic and is car-
ried out in a one-compartment cell at room temperature
under air. It is enhanced by water, and gives 2-substituted ben-
zimidazoles, benzothiazoles, and benzoxazoles in good to ex-
cellent yields with a wide range of functional groups. Further
extension of this electrochemical protocol to the synthesis of
other heterocycles is underway.
Experimental Section
Synthesis of 2-phenylbenzimidazole (3a)
An electrolyte of 0.1m LiClO4 in CH3CN/H2O (98:2; 5.0 mL) was
added to a round-bottomed flask. Benzyl alcohol (52 mL, 0.5 mmol,
1.0 equiv), 1,2-phenylenediamine (81.1 mg, 0.75 mmol, 1.5 equiv),
CoSO4·7H2O (28.1 mg, 0.1 mmol, 20 mol%), and CF3COOH (74 mL,
1.0 mmol, 2.0 equiv) were then added. The resulting solution was
electrolyzed with a pair of Pt foils (1.0”1.5 cm2) as electrodes at
constant current (20 mA) at room temperature for 150 min
(3.73 FmolÀ1). Thereafter, the reaction mixture was extracted with
ethyl acetate (3”15 mL). The combined organic layers were
washed with brine (10 mL) and dried over MgSO4. The concentrat-
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Received: December 17, 2015
Published online on February 25, 2016
Chem. Eur. J. 2016, 22, 5425 – 5429
www.chemeurj.org
5429
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