Exogenous Photosensitizer-, Metal-, and Base-Free Visible-Light-Promoted C-H Thiolation via Reverse Hydrogen Atom Transfer

Article abstract of DOI:10.1021/acs.orglett.8b03679

Visible-light-driven, intramolecular C(sp²)-H thiolation has been achieved without addition of a photosensitizer, metal catalyst, or base. This reaction induces the cyclization of thiobenzanilides to benzothiazoles. The substrate absorbs visible light, and its excited state undergoes a reverse hydrogen-atom transfer (RHAT) with 2,2,6,6-tetramethylpiperidine N-oxyl to form a sulfur radical. The addition of the sulfur radical to the benzene ring gives an aryl radical, which then rearomatizes to benzothiazole via RHAT.

Full text of DOI:10.1021/acs.orglett.8b03679

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Exogenous Photosensitizer‑, Metal‑, and Base-Free Visible-Light-
Promoted C−H Thiolation via Reverse Hydrogen Atom Transfer
Ze-Ming Xu,† Hong-Xi Li,*,†,‡ David James Young,§ Da-Liang Zhu,† Hai-Yan Li,†
and Jian-Ping Lang*,†,‡
†College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, Jiangsu, China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
§College of Engineering, Information Technology and Environment, Charles Darwin University, Northern Territory 0909, Australia
S
* Supporting Information
ABSTRACT: Visible-light-driven, intramolecular C(sp2)−H thiola-
tion has been achieved without addition of a photosensitizer, metal
catalyst, or base. This reaction induces the cyclization of
thiobenzanilides to benzothiazoles. The substrate absorbs visible
light, and its excited state undergoes a reverse hydrogen-atom transfer
(RHAT) with 2,2,6,6-tetramethylpiperidine N-oxyl to form a sulfur
radical. The addition of the sulfur radical to the benzene ring gives an
aryl radical, which then rearomatizes to benzothiazole via RHAT.
he utilization of direct C(sp2)−H activation to form
intramolecular C−X (X = N, O, S) bonds1−3 is an
Scheme 1. C−H Thiolation of Substituted Thioamides
T
attractive strategy for the synthesis of a variety of benzohetero-
cycles.4 Particularly promising is intramolecular C(sp2)−H/S−
H cross-coupling for the construction of benzothiazole
moieties, which are found in bioactive natural products,
pharmaceuticals, and organic optoelectronic materials.5 These
intramolecular cyclizations have been achieved using transition
metal catalysts with oxidants at high temperatures (Scheme
́
1).6 Pen
enory et al. reported the cyclization of thioformanilides
̃ ̃
with chloranil as a photosensitizer under a medium pressure
Hg lamp (λmax = 365 nm) at 80 °C.7 Visible-light-driven
aromatic C−H thiolation is of great interest because sunlight is
a clean and abundant energy source. Li et al. employed
Ru(bpy)3(PF6)2 (bpy = 2,2′-bipyridine) as a photocatalyst for
the aerobic, visible-light-driven photoredox synthesis of 2-
substituted benzothiazoles using molecular oxygen as the
terminal oxidant.8a Lei et al. developed a novel ruthenium
photoredox and cobalt dual catalytic system for intramolecular
C−S coupling to yield 2-substituted benzothiazoles with the
release of H2.8b Despite considerable advances in the
methodology of oxidative cross-couplings, most procedures
suffer from one or more limitations. The use of O2 or inorganic
oxidants can lead to the formation of amide byproducts by
desulfurization of the thioamide substrate.6−8 The use of
transition metal catalysts or noble photocatalyst necessitates
the removal of residual metal impurities and thus entails
additional processing and cost. The presence of functional
groups such as halogen atoms on substrates can result in
dehalogenated contaminants under metal-catalyzed, thermal,
or light-irradiation conditions.6a,8 For example, the visible-
light-driven photoredox cyclization of N-(2-iodophenyl)-
benzothioamide only afforded dehalogenated product 2-
phenylbenzothiazole using Ru(bpy)3(PF6)2 as the photoredox
catalyst.8a Hence, developing metal-free methodology operat-
ing under mild conditions and with high functional group
compatibility would be a valuable extension to the synthetic
chemist’s repertoire.
Received: November 18, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03679
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Organic Letters
Letter
Hydrogen-atom transfer (HAT) and proton-coupled elec-
tron transfer (PCET) are basic mechanisms used in a wide
variety of chemical and biological transformations.9 Visible-
light-mediated HAT offers the opportunity for direct C−H/
X−H bond activation, avoiding the use of transition metal
catalysts and high temperatures.10 The activated photocatalyst
(PC*) generally behaves as a HAT catalyst to abstract a
hydrogen atom from a substrate RX−H (X = C, N, O, S) to
form a radical RX•, which then reacts to form the desired
product.11 Herein, we report a photoinduced C(sp2)−H/S−H
activation with intramolecular C−S bond formation through a
reverse hydrogen-atom transfer (RHAT). Activated thiobenza-
nilide (*TBA) formed by visible-light irradiation undergoes a
RHAT process with 2,2,6,6-tetramethylpiperidine N-oxide
(TEMPO) to form a sulfur radical, which then cyclizes to
generate benzothiazole. This photochemical cyclization does
not require extra photoredox catalyst, transition-metal catalyst,
or base (Scheme 1).
Scheme 2. Cyclization of N-(Aryl)benzothioamides to 2-
Phenylbenzothiazoles
a
We initially chose N-phenylbenzothioamide (1aa) as the
substrate for optimization studies. The combination of
TEMPO and Ru(bpy)3Cl2·6H2O in CHCl3 provided the
desired cyclization product 2aa exclusively in >99% yield under
irradiation using a 45 W household fluorescent bulb at room
temperature (Table S1, entry 1; Figure S1). To our delight, the
reaction without Ru(bpy)3Cl2·6H2O also afforded 2aa in an
almost quantitative yield (entry 2). A strong solvent depend-
ence of the coupling reaction was observed with chloroform
proving to be the most suitable solvent (entry 2). The same
reaction gave lower yields in toluene, THF, CH2Cl2, i-PrOH,
EtOH, MeOH, DMF, and DMSO (entries 3−11). Excellent
yields could also be obtained with a shorter reaction time of 12
h (entry 12). Product 2aa could be obtained in 90% yield after
8 h of irradiation (entry 13). The additive TEMPO proved
more effective (entry 8) than 2,3-dicyano-5,6-dichlorobenzo-
quinone (DDQ), tetrachloro-p-benzoquinone (p-chloranil), or
phenanthrenequinone (entries 14−16). However, di-t-butyl
peroxide and diphenylmethanone were almost inactive (entries
17 and 18). When the reaction was carried out in air, the yield
of 2aa was decreased to 70% yield and was contaminated by
byproducts N-phenylbenzamide (15%) and N-phenylbenzimi-
dic dithioperoxyanhydride (5%) (entry 19). The critical roles
of light and TEMPO in this C−H thiolation were
demonstrated using control experiments (entries 20 and 21).
When the reaction was carried out at 80 °C in the dark, none
of the desired product was detected (entry 22). It is
noteworthy that 1aa reacted smoothly to afford 2aa in 90%
after 8 h of sunlight irradiation (entry 23).
a
1a (0.2 mmol), TEMPO (0.4 mmol), 6 mL of CHCl3, room
temperature, 45 W CFL, N2, and isolated yield.
corresponding products in high yields. When the meta-
substituted thioanilides (1an−1ap) were used, two regioiso-
meric products were obtained. 2,6-N-(4-Iodo-2-
methylphenyl)benzothioamide also reacted to afford the
expected product 6-iodo-4-methyl-2-phenylbenzothiazole
(2aq) in 87% yield. N-(Naphthalen-1-yl)benzothioamide
reacted to produce 2ar in 92% yield under the optimized
reaction conditions. The cyclization of 1aa was performed on a
gram scale (1.07 g, 5 mmol) over 24 h to generate 2aa in 78%
yield.
Encouraged by the high efficiency for the reaction of N-
(aryl)benzothioamides described above, we investigated the
effect of the substituents on the 2-aryl group (Scheme 3). Both
electron-donating and -withdrawing groups on the phenyl ring
Scheme 3. Cyclization of N-(Aryl)benzothioamides to 2-
Phenylbenzothiazoles
a
Having identified the optimal reaction conditions, we
investigated the electronic and steric effects of the N-aryl
group. Good to excellent yields could be achieved for a variety
of N-(aryl)benzothioamides bearing methyl, methoxy, halide
(F, Cl, Br, I), trifluoromethyl, and methylthio groups (Scheme
2). Intramolecular C−H thiolation was satisfactorily accom-
plished for N-(aryl)benzothioamides (2aa−2ai) with electron-
rich, -neutral, and -withdrawing groups at the 4-position with
over 84% isolated yields. Importantly, iodo-substituted
derivative 1ai reacted to provide the desired product 2ai in
88% yield. Successful reaction of similar substrates has rarely
been described in the literature.8a N-(2-(Methylthio)phenyl)-
benzothioamide and N-(2-fluorophenyl)benzothioamide re-
acted to generate 2ak and 2al in satisfactory yields. Ortho-
positioned substituents such as methyl and bromo (1aj, 1am)
did not hamper this oxidative C−H thiolation, giving the
a
b
Reaction conditions: as for Scheme 2, isolated yields. Reaction
conditions: as for Scheme 2, but with 5 mol % of 9,10-
phenanthrenequinone.
B
DOI: 10.1021/acs.orglett.8b03679
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
were tolerated. Good to high yields were obtained under our
standard conditions. 2,2-Dimethyl-N-arylpropanethioamide
underwent cyclization to yield the corresponding 2-alkylben-
zothiazole derivative in a relatively low yield, presumably due
to low visible-light absorbance (Figure S2). The yield of 2-
(tert-butyl)-6 methylbenzothiazole increased to 94% in the
presence of 5 mol % of phenanthrenequinone. Some
substituted 2-alkylbenzothiazoles could be isolated in high
yields under similar reaction conditions.
The cyclization of substrates bearing a halogen atom,
including iodine, is appealing because of the potential for
further functionalizations. Under metal-catalyzed reaction
conditions, modest yields of the desired product are usually
obtained, accompanied by some level of dehalogenation of the
substrate.6−8 In our case, a variety of thiobenzanilides
possessing halogen atoms underwent smooth cyclizations to
yield the desired products (2af−2ai, 2al−2aq, 2bd−2bg, 2bi−
2bk, 2bn, 2bo, 2br−2bt) without contamination by the
corresponding dehalogenated species. The cyclization of N-(2-
fluorophenyl)-3,4-dimethoxybenzothioamide yielded 2-(3,4-
dimethoxyphenyl)-4-fluorobenzothiazole (2bv), which is a
potent antitumor agent.12
Their excited-state redox potentials were thus estimated to be
in the range −1.00 ∼ −1.19 V vs Ag/AgCl based on both the
electrochemical and spectroscopic data (Figure S4 and Table
S2). The CV of TEMPO had one reversible oxidation wave at
E1/2 = +0.57 V vs Ag/AgCl in MeCN and one reduction wave
at −1.57 V vs Ag/AgCl (Figure S3). A single electron transfer
process from *1aa to TEMPO was unfavorable.
Light on/off experiments revealed that the cyclization
progressed smoothly under visible-light irradiation, but no
further conversion was observed when the light source was
removed (Figure S5), indicating that the reaction did not go
through a radical chain propagation. The oxidative cyclization
of 1aa over time was monitored using HPLC (Figure S6a).
Substrate 2aa got accumulated at the same rate as the
consumption of 1aa. Substrate 1a was totally converted in 12
h. A logarithmic plot of the percentage of residual substrate (ln
c/c0) with irradiation time (t) (Figure S6b) showed that the
kinetics of this reaction obeyed a first-order rate law.
The kinetic isotope effect (KIE, kH/kD) was measured using
monodeuterated 1aa (1aa-d) (Figure S7) and pentadeuterated
1aa (1aa-d5) (Figure S8). The intramolecular KIE value was
0.61 (see Supporting Information) with a 99% combined yield
(Figure S9). The reaction of 1aa (0.1 mmol) and deuterated
1aa-d5 (0.1 mmol) for half an hour provided a mixture of the
products 2aa and pentadeuterium-2aa (2aa-d5) in 15% overall
yield (Figures S10 and S11) and a ratio of 2aa:2aa-d5 of 1.27
(Scheme 4). These intra- and intermolecular KIE values
indicated that the cleavage of the C−H bond was not the rate-
determining step.
The absorption spectra of substrates 1aa, 1ac, 1ae, and 1bd
indicated a photoabsorption range from UV to visible
wavelengths with the absorption edge at about 470 nm
(Figure S2). Cyclization of 1aa was conducted with various
light sources, visible light, blue LED, and green LED and
afforded 2aa in >99%, 85%, and 28%, respectively (Scheme 4).
Scheme 4. Control and Deuterium Experiments
Substrate 1aa was treated with TEMPO under standard
conditions for 2 h, and the positive-ion ESI-MS spectrum of
the reaction mixture was obtained (Figure S12). The peaks at
m/z = 212.0539 and 158.1557 were assigned to be the [2aa +
H]+ and [TEMPOH + H]+ ions, respectively. The peak at m/z
= 369.1958 was consistent with the [SO-(2,2,6,6-tetramethyl-
piperidin-1-yl)-N-phenylbenzimido(thioperoxoate) + H]+ ion.
Therefore, we proposed that the cyclization involved a sulfur-
centered radical. Moreover, reaction of N-phenyl-4-
(trifluoromethyl)benzimidic dithioperoxyanhydride (3bc, Fig-
ure S13) with TEMPO in CHCl3 under visible-light irradiation
also gave rise to the desired product 2bc in 90% yield. In the
absence of light, the reaction of 3bc and TEMPO in CHCl3 at
room temperature or at 80 °C afforded 2bc in about 6% yield.
On the basis of this mechanistic data and on literature
precedents,7,8,12 we propose the pathway outlined in Scheme 5.
Substrate 1aa accepts a photon to generate excited *1aa, which
then undergoes a reversible HAT process with TEMPO to
form a thiyl radical A and TEMPOH. The resulting A can be
trapped by TEMPO to afford the adduct B, which undergoes
Scheme 5. Proposed Mechanism
The results correlate with the absorption spectrum of 1aa. A
temperature of 80 °C could not promote this reaction in the
dark, confirming that this cyclization is a visible-light-driven
process. The cyclic voltammogram (CV) measurements of 1aa,
1ac, 1ae, 1ai, and 1bd revealed that these substrates possessed
a half wave oxidation potential of 1.35, 1.25, 1.49, 1.43, and
1.39 V (vs Ag/AgCl), respectively, in MeCN (Figure S3).
C
DOI: 10.1021/acs.orglett.8b03679
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
homolytic cleavage to regenerate A and TEMPO, facilitated by
the relative weakness of the S−O bond.12 The 1,5-homolytic
radical cyclization of A produces aryl radical C. The
aromatized product 2aa is then obtained through the
photomediated RHAT process between activated *C and
TEMPO under visible light. This mechanism is different from
that for TEMPO-catalyzed electrochemical intramolecular C−
H thiolation. In the latter, thioamide is oxidized by the
electrochemically generated TEMPO+ through an inner-sphere
electron transfer to afford a thioamidyl radical, which
undergoes homolytic aromatic substitution to form the C−S
bond.13
To summarize, we have developed methodology for visible-
light-driven, intramolecular C−S bond formation of aromatic
substrates that does not require addition of a photosensitizer,
metal-based catalyst, or base. Thioamide derivatives in the
presence of TEMPO smoothly cyclize to give benzothiazoles
through two RHAT events. This cyclization is compatible with
a wide range of functional groups and will therefore be suitable
for constructing a variety of other aromatic heterocyclic
compounds.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03679.
Experimental procedures, 1H and 13C NMR spectra, and
characterization data for all products (PDF)
Accession Codes
CCDC 1874831 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: lihx@suda.edu.cn.
*E-mail: jplang@suda.edu.cn.
ORCID
Jian-Ping Lang: 0000-0003-2942-7385
Notes
The authors declare no competing financial interest.
́
ello, J. E.; Penenory, A. B.
̈ ̃ ̃
ACKNOWLEDGMENTS
■
The authors thank the National Natural Science Foundation of
China (21471108, 21531006, 21771131, and 21773163), the
Natural Science Foundation of Jiangsu Province
(BK20161276), the State Key Laboratory of Organometallic
Chemistry of Shanghai Institute of Organic Chemistry
(2018kf-05), and the “Priority Academic Program Develop-
ment” of Jiangsu Higher Education Institutions and Scientific
and Technologic Infrastructure of Suzhou (SZS201708). We
are grateful to the useful comments of the editor and the
reviewers.
D
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