Cu-Catalyzed Aerobic Oxidative Sulfuration/Annulation Approach to Thiazoles via Multiple Csp3-H Bond Cleavage

Article abstract of DOI:10.1021/acs.orglett.8b00840

A novel and practical Cu-catalyzed aerobic oxidative synthesis of thiazoles was developed. This chemistry for the first time achieved thiazole construction from simple aldehydes, amines, and element sulfur through multiple Csp³-H bond cleavage processes. Molecular oxygen was used as a green oxidant in this oxidative protocol. The substrate scope is broad with the tolerance of aliphatic amines. The mechanistic study might promote the reaction design for a new sulfuration/annulation reaction with readily available element sulfur.

Full text of DOI:10.1021/acs.orglett.8b00840

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cu-Catalyzed Aerobic Oxidative Sulfuration/Annulation Approach to
Thiazoles via Multiple Csp3−H Bond Cleavage
Xiaoyang Wang,† Xu Qiu,† Jialiang Wei,† Jianzhong Liu,† Song Song,† Wen Wang,*,† and Ning Jiao*,†,‡
†State Key Laboratory of Natural and Biomimetic Drugs, and Medical and Health Analysis Center, Peking University, Xue Yuan Road
38, Beijing 100191, China
‡State Key Laboratory of Organometallic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: A novel and practical Cu-catalyzed aerobic
oxidative synthesis of thiazoles was developed. This chemistry
for the first time achieved thiazole construction from simple
aldehydes, amines, and element sulfur through multiple Csp3−
H bond cleavage processes. Molecular oxygen was used as a
green oxidant in this oxidative protocol. The substrate scope is
broad with the tolerance of aliphatic amines. The mechanistic
study might promote the reaction design for a new sulfuration/annulation reaction with readily available element sulfur.
hiazoles are very important compounds and ubiquitous
structural motifs found in biologically active molecules,
Scheme 1. Hypothesis for the Thiazole Construction
T
through Multiple Csp3−H Sulfuration/Annulation
pharmaceuticals, and functional materials (Figure 1).1 In the
Figure 1. Thiazoles as core structures in compounds.
past decades, by using the cheap, stable, nontoxic, and
nonsmelly element sulfur reagent, the sulfurations of function-
alized substrates for the synthesis of sulfur-containing hetero-
cycles have been well developed.2 Recently, with the develop-
ment of C−H bond functionalization,3 the direct C−H
sulfuration has been an attractive strategy for the synthesis of
sulfur-containing compounds.4 So far, the C−H sulfuration and
annulation with element sulfur reagent is still a challenging
issue and therefore highly desired.
multiple Csp3−H bond cleavages from readily available
substrates remains unknown and is urgently desired (Scheme
1b). Notably, Nicolaou and co-workers reported a novel one-
pot tandem strategy for the introduction of sulfur into cyclic
2,5-diketopiperazines for the synthesis of epidithiodiketopiper-
azines via two Csp3−H bond sulfurations (Scheme 1c).6
Inspired by these reactions, we envisioned that Csp3−H
bonds of both amine and acetaldehyde would undergo
sulfuration to form C−S bonds for the construction of thiazoles
(Scheme 1d). However, there are two main challenges in this
multiple Csp3−H dehydrogenative sulfuration/annulation
hypothesis: (1) Sulfur might poison the transition metal
More recently, by using the aryl or heteroaryl Csp2−H bond
sulfuration strategy (Scheme 1a), several sulfur-containing
heterocycles such as thiophenes, benzothiazoles, benzoisothia-
zolones, and thienoindoles were significantly constructed
through an intra- or intermolecular sulfuration/annulation
reaction by the groups of Itami,5a Deng,5b Shi,5c Deng,5d−f and
Li.5g In contrast to the significance of Csp2−H sulfuration, the
Csp3−H sulfuration was rarely achieved.6 To the best of our
knowledge, the important thiazole construction through
Received: March 15, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00840
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Organic Letters
Letter
a
Table 1. Optimization of the Reaction Conditions
entry
[Cu]
CuBr2
additive
ligand
atmosphere
t (°C)
4a yield (%)
1
-
-
-
-
-
-
-
-
-
air
air
air
air
air
air
O2
O2
O2
O2
O2
O2
O2
O2
80
80
80
80
80
80
80
100
80
80
80
80
80
80
40
34
29
trace
28
52
65
64
60
61
58
76
78
0
2
Cu(OTf)2
Cu(OAc)2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
-
-
3
-
4
PivOH
DABCO
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
5
6
7
8
9
TMEDA
Py
10
11
12
bPy
1,10-phen
1,10-phen
1,10-phen
b
13
14
a
Reaction conditions: 1a (0.3 mmol), 2a (0.6 mmol), 3 (0.16 mmol), CuBr2 (20 mol %), 1,10-phen (20 mol %), DBU (2.0 equiv), and DMSO (3.0
b
mL), 16 h. 1.0 equiv of 1,10-phen was used.
a
catalysts, which further blocks the catalytic process and leads to
the failure in synthesis of thiazoles. (2) An oxidant is required
for this dehydrogenative annulation strategy.7 Since the amine
and aldehyde substrates are unstable under strong oxidative
conditions, a weak oxidant, which does not cause the
destruction of unstable substrates but can enable the
dehydrogenative process, should be found in this protocol.
Herein, by using the green and sustainable O2 as the oxidant,8
we report a novel Cu-catalyzed thiazole construction through
multiple Csp3−H sulfuration/annulations from readily available
amines, aldehydes, and elemental sulfur (Scheme 1d).
Scheme 2. Construction of Thiazoles with Various Amines
Recently, we reported a copper-mediated oxazole synthesis
via the oxygenation process with O2, but 1.5 equiv of CuBr2 is
required for this transformation.9a With our constant endeavor
in transition metal catalyzed aerobic oxidation and oxygenation
reactions,9 our initial efforts commenced with the reactions of
benzylamine, acetaldehyde, and element sulfur utilizing CuBr2
as catalyst in DMSO at 80 °C under air. Fortunately, the
desired 2,5-diphenylthiazole 4a was formed in 40% yield with
only a trace amount of oxazole as the byproduct (Table 1, entry
1). Other copper salts were tested and exhibited similar or
lower efficiencies (entries 2 and 3). Only a trace amount of
product was obtained when the reaction was carried out in the
presence of an acid additive (entry 4). In contrast, when an
organic base DBU was added, diphenylthiazole was formed in
52% yield (entry 6). The reaction under dioxygen atmosphere
afforded the product 4a in 65% yield (entry 7). Notably, when
1,10-phen (1,10-phenanthroline) was employed as a ligand, the
reaction performed well with 76% yield (entry 12). The
reaction in the absence of a Cu catalyst did not work (Table 1,
entry 14). These results demonstrate that this reaction
undergoes a copper-catalyzed aerobic oxidative sulfuration/
annulation process.
a
Reaction conditions: 1 (0.3 mmol), 2a (0.6 mmol), 3 (0.16 mmol),
CuBr2 (20 mol %), 1,10-phen (20 mol %), DBU (2.0 equiv), DMSO
(3.0 mL), O2 (1 atm), 16 h.
and o-substituted methyl benzylamines. The efficiency was
unaffected with the formation of 4b, 4c, and 4d, in 67%, 63%,
and 65% yields, respectively. Naphthalen-1-yl methanamine
showed good reactivity giving 4f in 66% yield. In addition,
fluoro-, chloro-, and bromo-substituted benzylamines produced
the halo-substituted products 4g−4i in good yields. Those
products could be used for further coupling transformations as
coupling partners.
The scope of the reaction was then investigated by
employing various substituted amines with phenylacetaldehyde
(Scheme 2). It is noteworthy that the substituents at the phenyl
ring, regardless of the electron-rich or electron-deficient nature,
did not hinder the efficiency, producing the corresponding
thiazoles in moderate to good yields. The position effect of
substituents on the phenyl ring was investigated by using p-, m-,
B
DOI: 10.1021/acs.orglett.8b00840
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Organic Letters
Letter
Notably, alkyl-substituted amines also proceeded well in this
protocol, while the benzylic C−H bonds of phenethylamines
were further oxidized into carbonyl groups in the correspond-
ing products. For example, substituted phenethylamines
worked well to produce the benzoyl thiazole derivatives 4l−
4o in moderate yields. Furthermore, the aliphatic amines also
performed well leading to the corresponding alkyl-substituted
thiazoles 4r−4t in moderate yields. Unfortunately, α-methyl-
substituted amine was not compatible in the present protocol.
In subsequent studies, the applicability was further
investigated by using various phenylacetaldehydes bearing
different substituents on the aryl rings (Scheme 3). The
Scheme 4. Kinetic Study
Scheme 3. Construction of Thiazoles with Different
a
Aldehydes
concentration of CuBr2 was tested, a first-order dependence
was established (Scheme 4B). This result suggested that in this
reaction the copper catalyst might be the catalytically active
species.10 For further understanding of this process, different p-
substituted benzylamine derivatives were utilized to establish a
Hammett equation. We were pleased to find a linear
relationship between different electron-withdrawing groups
and electron-donating groups (Scheme 4C). This plot of
Hammett equation is linear with a negative slope. In this
process, the electron-donating groups accelerated the reaction.
The reason for this phenomenon is probably due to the
oxidative activity of the Csp3−H bonds in electron-rich
benzylamines.
Moreover, some control experiments were conducted to
interpret the reaction pathway. The reaction of benzothioamide
6 and 2-phenylacetaldehyde 2a did not give the desired thiazole
product under the standard conditions (eq 2). N-Phenethyl-
a
Reaction conditions: 1a (0.3 mmol), 2 (0.6 mmol), 3 (0.16 mmol),
CuBr2 (20 mol %), 1,10-phen (20 mol %), DBU (2.0 equiv), DMSO
(3.0 mL), O2 (1 atm), 16 h.
corresponding thiazole products 5a−5d were produced in good
yields (58%−67%). No significant position effect was observed.
Phenylacetaldehydes bearing an electron-donating methoxy
group produced 5e and 5f in yields of 52% and 54%,
respectively. However, for 2-(3,4-dimethoxyphenyl)-acetalde-
hyde the yield of 5g decreased to 30%. This substrate might be
allergic to the oxidation system and degrade faster than forming
the desired product. The fluoro-substituted phenylaldehydes
were also tolerated in this protocol (5h−5j). In addition, 2-(4-
chlorophenyl)acetaldehyde and 2-(4-bromophenyl)-
acetaldehyde worked well producing 5k and 5l in 51% and
62% yields, respectively. Moreover, this protocol was applied to
gram-scale reaction with the formation of 4a in 52% yield (eq
1).
benzothioamide 7 did not work either (eq 3). The 1,4-
diphenyl-2-azabutadiene 8 was prepared, but it did not react
with element sulfur under the standard conditions (eq 4). In
addition, the atom exchange between oxygen and the sulfur
atom was not detected in 2,5-diphenyloxazole 9 (eq 5). Those
results demonstrate that the compounds 6−9 are not the
intermediates involved in this transformation.
Although the detailed mechanism is not completely clear yet,
a plausible mechanism is outlined in Scheme 5 on the basis of
the above results and previous reports.2j,5a Initially, the
condensation of anilines and aldehydes yields imines A11 and
B with the tautomerization. Then intermediate B reacts with an
electrophilic element sulfur to form species C.12 Afterward,
further aerobic oxidation assisted by Cu catalysis would
produce D. Elimination of element sulfur and the intra-
Furthermore, the kinetic study of this process was studied
with the model reaction of 1a and 2a (Scheme 4). The effect of
concentration on phenylacetaldehyde was tested by utilizing 0.2
and 0.4 M 2a. The reaction rate rises together with the
concentration of 2a (Scheme 4A). The increasing concen-
tration might promote a dehydration process between anilines
and aldehydes in forming an imine intermediate. When the
C
DOI: 10.1021/acs.orglett.8b00840
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Organic Letters
Letter
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Scheme 5. Proposed Mechanism
molecular nucleophilic addition of intermediate D generate
intermediate E,13 which undergoes a further oxidative process
under the Cu catalysis and affords the target molecule 4a.
In summary, we developed a novel Cu-catalyzed aerobic
oxidative approach to thiazoles. Simple aldehydes, amines, and
element sulfur were employed to construct thiazoles for the
first time by this protocol through a novel multiple Csp3−H
bond cleavage process. The substrate scope is broad with the
tolerance of aliphatic amines. Inexpensive Cu catalysts,
commercially available substrates, and green oxidants were
used, which makes this protocol economical, step-efficient, and
environmentally friendly. Further studies on the bioactivity
screening of these products are ongoing with the collaborators.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00840.
Experimental procedures, full characterization of prod-
ucts, and copies of NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: jiaoning@pku.edu.cn.
*E-mail: wangwenmm@bjmu.edu.cn.
ORCID
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Song, J.; Liang, Y.; Deng, G. Chem. Commun. 2017, 53, 11917.
(c) Chen, F.; Liao, G.; Li, X.; Wu, J.; Shi, B. Org. Lett. 2014, 16, 5644.
(d) Li, G.; Xie, H.; Chen, J.; Guo, Y.; Deng, G.-J. Green Chem. 2017,
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Ning Jiao: 0000-0003-0290-9034
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Basic Research Program of China (973
Program) (No. 2015CB856600) and the National Natural
Science Foundation of China (Nos. 21632001, 21772002) for
financial support of this work. We thank Tongyu Huo in this
group for reproducing the results of 4t and 5l.
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