Copper-catalyzed solvent-free redox condensation of benzothiazoles with aldehydes or benzylic alcohols

Article abstract of DOI:10.1016/j.tetlet.2014.01.120

An efficient and practical method for the construction of 2-aryl-and 2-alkyl-substituted benzothiazoles via a copper-catalyzed redox condensation process between benzothiazoles and aldehydes or benzylic alcohols has been developed. The reaction proceeded under mild reaction conditions using environmentally benign tert-butyl hydroperoxide (TBHP) as the oxidant. A reaction mechanism involving the ring-opening of benzothiazoles initiated by the attack of acyl radical on the thiazole ring and intramolecular condensation is proposed based on the isolation of an anilide disulfide intermediate.

Full text of DOI:10.1016/j.tetlet.2014.01.120

Accepted Manuscript
Copper-Catalyzed Sovent-Free Redox Condensation of Benzothiazoles with
Aldehydes or Benzylic Alcohols
Mingliang Zhang, Wen-Ting Lu, Wenqing Ruan, Hui-Jun Zhang, Ting-Bin Wen
PII:
S0040-4039(14)00166-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.01.120
TETL 44153
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
9 November 2013
19 January 2014
27 January 2014
Please cite this article as: Zhang, M., Lu, W-T., Ruan, W., Zhang, H-J., Wen, T-B., Copper-Catalyzed Sovent-Free
Redox Condensation of Benzothiazoles with Aldehydes or Benzylic Alcohols, Tetrahedron Letters (2014), doi:
http://dx.doi.org/10.1016/j.tetlet.2014.01.120
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Copper-Catalyzed Solvent-Free Redox
Condensation of Benzothiazoles with
Aldehydes or Benzylic Alcohols
Mingliang Zhang, Wen-Ting Lu, Wenqing Ruan, Hui-Jun Zhang* and Ting-Bin Wen*

1
Tetrahedron Letters
journal homepage: www.els evier.com
Copper-Catalyzed Sovent-Free Redox Condensation of Benzothiazoles with
Aldehydes or Benzylic Alcohols
Mingliang Zhang, Wen-Ting Lu, Wenqing Ruan, Hui-Jun Zhang∗ and Ting-Bin Wen∗
Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, P. R. China. Fax: (+86) 592-218-6295;
Tel: (+86) 592-218-6295; E-mail: meghjzhang@xmu.edu,cn, chwtb@xmu.edu.cn
ARTICLE INFO
ABSTRACT
Article history:
Received
An efficient and practical method for the construction of 2-aryl- and 2-alkyl-substituted
benzothiazoles via a copper-catalyzed redox condensation process between benzothiazoles and
aldehydes or benzylic alcohols has been developed. The reaction proceeded under mild reaction
conditions using enviromentally benign tert-butyl hydroperoxide (TBHP) as the oxidant. A
reaction mechanism involving the ring-opening of benzothiazoles followed by amidation and
intramolecular condensation is proposed based on the isolation of an anilide disulfide
intermediate.
Received in revised form
Accepted
Available online
Keywords:
copper
2009 Elsevier Ltd. All rights reserved.
benzothiazoles
aldehydes
benzylic alcohols
C-C bond formation
1. Introduction
conditions, aliphatic aldehydes are not suitable substrates perhaps
due to their easy oxidation to carboxylic acids.[13a,b] Recently,
Itoh and Ma’s groups developed a general and operationally
Benzothiazole
derivatives,
especially
2-substituted
benzothiazoles, are of particular interest due to their important
biological activities and diverse applications in pharmaceutical
chemistry and material science.[1,2] Until now, a large number of
synthetic routes have been developed for the formation of 2-
substituted benzothiazoles, including the condensation of 2-
aminothiophenol with aldehydes (Scheme 1, pathway a),[3,4]
carboxylic acids,[5] orthoesters,[6] or nitriles,[7] and the
intramolecular cyclization of thioanilides[8] (Scheme 1, pathway
b) etc. However, these methods suffered from several limitations,
such as the use of readily oxidized 2-aminothiophenols,[9] the
difficulties involved in the preparation of functionalized
thioamides,[10] and the relatively harsh reaction conditions.
Therefore, several pathways starting directly from benzothiazoles
were also developed.[11-13] Transition metal-catalyzed cross-
coupling of benzothiazoles with different kinds of arylating
reagents appeared as a powerful route for the preparation of 2-
arylsubstituted benzothiazoles (Scheme 1, pathway c).[12]
Recently, the research groups of Li, Tan and Wu respectively
reported the facile and direct oxidative condensations of 2-
unsubstituted benzothiazoles with aromatic aldehydes or aryl
ketones for the construction of 2-aryl- or 2-acylbenzothiazoles
(Scheme 1, pathway d).[13] In these transformations, a ring-
opening process involving the formation of aldimine
intermediates followed by their intramolecular cyclization was
simple route for the construction of benzothiazole moieties via
the coupling of 2-haloanilides with thiol surrogates followed by
intramolecular condensation of the anilide intermediates (Scheme
1, pathway e).[15] Inspired by these results, we envisioned that
amide intermediate II, which could be obtained through the
transition-metal catalyzed oxidative amidation of various
aldehydes with 2-aminobenzenethiols
I
derived from
benzothiazoles,[14,16] might undergo further intramolecular
condensation to form the corresponding 2-aryl- and 2-alkyl-
substituted benzothiazoles (Scheme 2).
proposed.[13,14] However, under both Li and Tan’s reaction
Scheme 1. Major routes to 2-substituted benzothiazoles.
———
∗ Hui-Jun Zhang. Tel.: +86-592-218-6295; fax: +86-592-218-6295; e-mail: meghjzhang@xmu.edu,cn
∗ Ting-Bin Wen. Tel.: +86-592-218-6085; fax: +86-592-218-6295; e-mail: chwtb@xmu.edu.cn

2
Tetrahedron
reaction of 4-nitrobenzaldehyde 2e with benzothiazole 1a
afforded the desired product 3ae in only 24% yield. Notably,
aliphatic aldehydes are also favorable substrates for this reaction
process (3ai-ak), which represent an advantage of this catalytic
system. Moreover, substituents on the phenyl ring of
benzothiazoles had some effects on the reaction yields (3ba-ga).
Table 2. Investigation of the substrate scope.a)
Scheme 2. A new route to 2-substituted benzothiazoles.
Herein, we report the efficient and solvent-free CuCl2/TBHP
promoted redox condensation of benzothiazoles with both
aromatic and aliphatic aldehydes or benzylic alcohols under mild
reaction conditions.
We initially explored the reaction of benzothiazole 1a with p-
methylbenzaldehyde 2a in the presence of 0.3 equiv of CuCl2 and
1.4 equiv of TBHP.[16] To our delight, the condensation reaction
between 1a and 2a did occur at 80 oC in toluene affording the 2-
aryl benzothiazole 3aa in 62% yield after 36 h (Table 1, entry 1).
Screening with different solvents suggested that the reaction
proceeded much faster without solvent, and 3aa was obtained in
71 and 80% yields after 12 h and 24 h respectively (Table 1,
entry 2-5). Control experiments showed that no reaction took
place in the absence of either CuCl2 or TBHP (Table 1, entry 6-
7). Increasing the reaction temperature to 100 oC led to the
formation of 3aa in only 39% yield (Table 1, entry 8). Several
other transition metal catalysts, such as Cu(OAc)2, CuCl, and
NiCl2, could also promote the same transformation, whereas they
proved to be much less efficient than CuCl2 (Table 1, entry 9-11).
In addition, the amount of CuCl2 is crucial for the process, and
the reaction in the presence of either 0.2 or 0.4 equiv of CuCl2
gave 3aa in lower yield.
a)
Reaction conditions: 1 (1.86 mmol), 2 (1.69 mmol), 70%
TBHP (2.37 mmol), and CuCl2 (0.51 mmol), 24 h, 80 oC. b) t-
BuOH (2 mL).
It has been reported that, under oxidative conditions, alcohols
could be directly used instead of aldehydes in several
reactions.[4,17] Consequently, the direct reactions of benzylic
alcohols 4 with benzothiazole 1a were conducted, which led to
the formation of 3aa, 3ab, and 3al-ao in moderate yields (Table
3).
Table 1. Optimization of the reaction conditions.a)
Table 3. Arylation of benzothiazoles with several benzylic
alcohols.
Entry Cat.
t (h) Solvent
Yield (%)b)
CuCl2
toluene
chloroform
62
74
59
71
80
0
1
2
3
36
36
36
12
24
24
24
24
24
24
24
CuCl2
CuCl2
CuCl2
CuCl2
-
DCE
-
-
-
-
-
-
-
-
4
5
6
7c)
8d)
9
CuCl2
CuCl2
Cu(OAc)2
CuCl
0
39
8
33
10
11
NiCl2
39
a) Reaction conditions: 1a (1.86 mmol), 2a (1.69 mmol), 70%
a)
Reaction conditions: 1a (1.86 mmol), 2 (1.69 mmol), 70%
o
TBHP (4.06 mmol), CuCl2 (0.51 mmol), without solvent, 80 C,
TBHP (2.37 mmol), catalyst (0.3 equiv), solvent (2 mL) or
o
without solvent, 80 C. Yield of 3aa. Reaction in the
b)
c)
24 h. b) chloroform (2 mL).
absence of TBHP. d) Reaction at 100 oC.
To understand the reaction mechanism, several experiments
were performed. Initially the reaction between 4-
With the optimized reaction conditions in hand, we then
investigated the scope of this transformation with various
aldehydes and several benzothiazoles (Table 2). Both electron-
rich and electron-deficient aromatic aldehydes 2b-d and 2f-h
reacted with benzothiazole 1a producing the desired products
3ab-ad and 3af-ah in moderate to excellent yields. However, the
methylbenzoaldehyde 2a and 2-aminothiophenol
5 under
standard reaction conditions was conducted giving rise to the
formation of 3aa in 77% yield (eq 1), which indicated that the
transformation might proceed through a ring-opening pathway.13
Furthermore, a careful study on the reaction between 1a and 2a
suggested that a disulfide intermediate 6 was formed in the

3
beginning and could be separated in 55% yield after 40 min (for
the crystal structure of 6, see supporting information), which was
then converted to 3aa in 88% yield in the presence of 0.3 equiv
of CuCl2 and 1.4 equiv TBHP in t-BuOH under N2 (Scheme 3).
Notably, control experiments showed that, in the absence of 4-
methylbenzoaldehyde 2a or using TBHP in decane instead of
TBHP aqueous solution, no ring-opening products from
benzothiazole 1a were detected at all. Further investigation on
the direct reaction of 2-aminopenyl disulfide 7 with 2a under
similar reaction conditions (eq 2) indicated that CuCl2/TBHP
could also be an efficient catalytic system for the construction of
2-substituted benzothiazoles starting from stable 2-aminophenyl
disulfides and aldehydes. Based on the above observations, a
plausible mechanism can be proposed as shown in Scheme 4.
Initially, the attack of acyl radical on the thiazole ring led to the
formation of intermediate A which then reacted with water to
form intermediate B.[16,18] The ring-opening of B followed by the
oxidation of intermediate C gave the disulfide intermediate
D.[9,18,19] The cleavage of the disulfide and subsequent
intramolecular condensation of intermediate D were expected to
afford the desired 2-substituted benzothiazole.[19-21]
benzothiazoles. Anilide disulfides instead of aldimine were
formed as intermediates, which might explain the efficient
condensation between even aliphatic aldehydes and
benzothiazoles. Moreover, CuCl2/TBHP proved also efficient in
the condensation of 2-aminophenyl disulfides with aldehydes.
Further investigation on the scope, exact mechanism and
applications of this method are underway in our laboratory.
Experimental Section
Typical procedure
Benzothiazole 1 (1.86 mmol, 1.1 equiv), aldehyde 2 (1.69 mmol)
or benzylic alcohol 4 (1.69 mmol), TBHP (2.36 or 4.06 mmol,
1.4 or 2.4 equiv, 70% solution in water), CuCl2 (0.51 mmol, 0.3
equiv) were placed in a 25-mL reaction vessel fitted with a
magnetic stirring bar under nitrogen. The reaction mixture was
o
first stirred in an ice bath for 0.5 h, and then stirred at 80 C for
24 h. After cooling to room temperature, the crude product was
purified by column chromatography to afford the 2-substituted
benzothiazoles 3.
Acknowledgements
This work was financially supported by Natural Science Foundation
of China (Nos. 21072161 and 21302157), National Basic Research
Program of China (973 Program, No. 2012CB821600), Program for
Changjiang Scholars and Innovative Research Team in University
(PCSIRT), and the Fundamental Research Funds for the Central
Universities.
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H2N
S
S
NH2
CuCl2 (30 mol%)
N
7
70%TBHP (1.4 equiv)
Me (2)
80 oC
2
+
CHCl , N ,
3
S
Me
CHO
3aa, 76%
2a
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10. For low functional group tolerance in the preparation of
thioanilides due to the use of Lawesson’s reagent, see: a)
Perregaad, J.; Scheibye, S.; Meyer, H. J.; Thomsen, I.; Lawesson,
S. O. Bull. Soc. Chim. Belg. 1977, 86, 679; b) Cava, M. P.;
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18. Wang, L.; Ren, X.; Yu, J.; Jiang, Y.; Cheng, J. J. Org. Chem.
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19. Two months after the submission of our manuscript, we noticed
that a similar transformation promoted by KI/TBHP was just
appeared, see: Gao, Y.; Song, Q.; Cheng, G.; Cui, X. Org. Biomol.
Chem. 2014, DOI: 10.1039/c3ob42318b.
11. For several general pathways, see: a) del Amo, V.; Dubbaka, S. R.;
Krasovskiy, A.; Knochel, P. Angew. Chem. Int. Ed. 2006, 45, 7838;
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2958; d) Naka, H.; Uchiyama, M.; Matsumoto, Y.; Wheatley, A. E.
20. It was observed that the addition of TEMPO prevented the
transformation from 6 to 3aa.
21. a) Antonello, S.; Daasbjerg, K.; Jensen, H.; Taddei, F.; Maran, F.
J. Am. Chem. Soc. 2003, 125, 14905; b) Jiang, Y.; Qin, Y.; Xi, S.;
Zhang, X.; Dong, J.; Ma, D. Org. Lett. 2009, 11, 5250.

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