A Novel Fluorescent Skeleton from Disubstituted Thiochromenones via Nickel-Catalyzed Cycloaddition of Sulfobenzoic Anhydrides with Alkynes

Article abstract of DOI:10.1021/acs.orglett.9b02161

Nickel-catalyzed cycloaddition of alkynes and 2-sulfobenzoic anhydrides gives highly functionalized thiochromenones. The reaction undergoes a deoxygenative rather than decarbonylative pathway and shows advantages of an excellent isolated yield (up to 95%), high reaction efficiency, and high regioselectivity. As one of the resulted products, 2,3-di(triphenylamine)-thiolchromenone possesses a typical aggregation-induced emission property and emits efficient near-infrared fluorescence.

Full text of DOI:10.1021/acs.orglett.9b02161

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
A Novel Fluorescent Skeleton from Disubstituted Thiochromenones
via Nickel-Catalyzed Cycloaddition of Sulfobenzoic Anhydrides with
Alkynes
Pengbing Mi,†,§ Lirong He,†,§ Tanxiao Shen,‡ Jing Zhi Sun,*,‡ and Hui Zhao*,†
†Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054,
China
‡MOE Key Laboratory of Macromolecular Synthesis & Functionalization, Department of Polymer Science & Engineering, Zhejiang
University, Hangzhou 310027, China
S
* Supporting Information
ABSTRACT: Nickel-catalyzed cycloaddition of alkynes and 2-sulfobenzoic anhydrides gives highly functionalized
thiochromenones. The reaction undergoes a deoxygenative rather than decarbonylative pathway and shows advantages of an
excellent isolated yield (up to 95%), high reaction efficiency, and high regioselectivity. As one of the resulted products, 2,3-
di(triphenylamine)-thiolchromenone possesses a typical aggregation-induced emission property and emits efficient near-
infrared fluorescence.
Scheme 1. Transition-Metal-Catalyzed Decarbonylative
Cycloaddition of Anhydrides or Their Derivatives with
Alkynes
lkyne-based addition reactions have received great
Aattention because of their broad applications in organic
synthesis and polymer preparation.1 For example, the alkyne−
azide “click” reaction finds promising applications in
bioorthogonal chemistry and “click” polymerization.2,3 Re-
cently, metal-free addition reactions between alkynes and
thiols, phenols, or amines were also reported for the facile
synthesis of functional heterocyclic compounds.4 For the
synthesis of highly functionalized heterocyclic compounds
featuring six-member rings, the transition-metal-catalyzed
cycloaddition of an aromatic substrate with an unsaturated
CC or CC bond is considered to be a versitile and
powerful method.5 Alkynes are the central synthons in all these
reactions. As shown in Scheme 1, alkynes undergo a nickel-
catalyzed decarbonylative cycloaddition reaction with phthalic
anhydrides,6 thiophthalic anhydrides,7 or phthalimides.8
Consecutively decarbonylative and decarboxylative addition
of cyclic anhydride to alkyne can be achieved by a Pd-catalyst.9
In a previous work, we demonstrated the synthesis of
benzosultams by using nickel-catalyzed decarbonylative cyclo-
addition of saccharins with alkynes.10
By carefully analyzing the chemical structure of these
compounds, it is found that the chemical structure of
sulfobenzoic anhydrides is similar to that of phthalic
anhydrides, thiophthalic anhydrides, and phthalimides. Thus,
it is trivial to consider that Ni-based cycloaddition of
saccharins with alkynes could be extendable to other saccharin
anhydride analogues through the decarbonylative cyclo-
addition (upward arrow, Scheme 1). From this perspective,
we examined Ni-catalyzed cycloaddition of sulfobenzoic
anhydride with 3-hexyne. Surprisingly, we found that the
reaction underwent an unexpected deoxygenative pathway to
give the 2,3-disubstituted-thiochromenone as a sole product
instead of the decarbonylative product.11 This accidental
Received: June 21, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02161
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
finding triggered our interest to carry out a systematic
investigation on this unique reaction.
Under the optimized reaction conditions, we examined the
scope of alkynes (Scheme 2). The symmetrical alkyl- or aryl-
Enlightened by the primary result, we commenced the
condition optimization with the reaction of sulfobenzoic
anhydride 1a with 3-hexyne 2a as the model (Table 1).
a b
,
Scheme 2. Scope of Thiochromenones 3
a
Table 1. Optimization of the Reaction Conditions
b
entry
catalyst
additive
solvent
toluene
yield
c
1
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(OTf)2
NiCl2
Ni(acac)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
(n-Bu)3P
(n-Bu)3P
(n-Bu)3P
PMe3
PPh3
P(OMe)3
Dppe
(n-Bu)3P
(n-Bu)3P
(n-Bu)3P
(n-Bu)3P
(n-Bu)3P
(n-Bu)3P
(n-Bu)3P
(n-Bu)3P
(n-Bu)3P
(n-Bu)3P
(n-Bu)3P
(n-Bu)3P
22%
32%
50%
N.R.
N.R.
N.R.
N.R.
18%
12%
8%
45%
43%
N.R.
11%
29%
N.R.
22%
77%
93%
d
2
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
benzene
PhCF3
3
4
5
6
7
8
9
10
11
12
13
14
15
1,4-dioxane
THF
MeCN
toluene
toluene
toluene
toluene
e
16
f
17
18
g
h
19
a
Reaction conditions: 1a (1.0 mmol), 2a (1.5 mmol), [Ni] (10 mmol
%), and additive (4.0 mmol) in toluene (4 mL) under an argon
b
atmosphere. Isolated yield of chromenones 3a after column
c
d
chromatography. (n-Bu)3P (0.8 mmol instead of 4.0 mmol). (n-
e
Bu)3P (2.0 mmol instead of 4.0 mmol). 80 °C instead of 120 °C.
150 °C instead of 120 °C. 1b instead of 1a. 1c instead of 1a.
f
g
h
Initially, the reaction of sulfobenzoic anhydride 1a, with 3-
hexyne in the presence of Ni(cod)2 (10 mol %), and (n-Bu)3P
(80 mol %) at 120 °C in toluene overnight afforded 3a in 22%
yield, and tri-n-butylphosphine oxide was detected as a
byproduct (Table 1, entry 1). Consistent with the hypothesis,
the yields improved when the amount of (n-Bu)3P was
gradually increased (entries 2 and 3). Further exploration of
additives revealed that the use of PMe3, PPh3, P(OMe)3, or
DPPE is inefficient (entries 4−7). The use of other nickel
catalysts such as Ni(OTf)2, NiCl2, or Ni(acac)2 resulted in
lower yields of 3a (entries 8−10). Among the various solvents
that were screened, benzene and PhCF3 are found to be
suitable for the reaction, whereas MeCN, 1,4-dioxane, and
THF are ineffective (entries 11−15). Further investigation
revealed that decreasing or increasing the temperature
decreased the efficacy (entries 16 and 17). Notable to
mention, when 1a was replaced with its lower valence stage
analogues 1b or 1c, the Ni-based cycloaddition reaction gave
the same product 3a with excellent yields (entries 18 and 19).
a
Reaction conditions: 1 (1 mmol), 2a (1.5 mmol), Ni(cod)2(oct)4
(10 mol %), (n-Bu)3P (4.0 mmol), toluene (3 mL) at 120 °C, under
b
N2. Isolated yield.
alkynes reacted smoothly with 1c to afford the corresponding
products (3b, 3d−3j) in good yields (83−95%). Regarding
unsymmetrical alkynes, the reaction generated regioselective
products (the ratio between 3c and its isomer is >99 to 1) in
good yield (81%). Other unsymmetrical alkynes, such as alkyl/
alkyl or aryl/aryl alkynes, only give nonselective products. The
regioselectivity of the reaction can be rationalized in terms of
the different steric repulsive interaction between alkyl/aryl and
ligands.5m,6b Noticeably, the bulky aryl-substituted alkynes
such as phenyl-carbazole, triphenylamine, and thiophenyl-N,N-
diphenylaniline were also amenable to the reaction conditions
affording the thiochromenones 3k−3n in high yields (67−
B
DOI: 10.1021/acs.orglett.9b02161
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
83%). Unfortunately, the terminal alkynes are unsuitable for
this reaction, which only gives trace product and unidentified
mixtures.
To gain deep insight into the reaction mechanism, control
experiments were conducted. First, we verified that sulfoben-
zoic anhydride 1a cannot be converted to 1b or 1c under the
standard conditions (Scheme S1, reaction 1). Based on the
high resolution mass spectrum (HRMS) data of the reaction
mixture, no products other than 3a were found (Scheme S1,
reaction 2). This result indicated that the deoxygenation
process has occurred even before the cycloaddition process.
We also excluded the way of deoxygenation of diphenyl
substituted sulfobenzoic anhydride 8 to give the thiochrome-
nones 3d as the product (Scheme S1, reaction 3). Based on
these results, a plausible mechanism is proposed as shown in
Scheme 3. Initially, the nickel-catalyst (Ni(II)) inserts into the
Scheme 3. Proposed Mechanism
Figure 1. (a) Photographs of fluorescence (FL) emission of 3k in
THF−water mixtures with different water fractions (fw). (b) FL
spectra of 3k in THF−water mixtures with different fw. (c) Plot of
relative (I/I0) FL intensity vs fw in THF/water mixtures, inset shows
the chemical structure of 3k. Concentration of 3k: 1.0 × 10−5 mol/L;
Excitation wavelength: 382 nm.
quantum yield of 3k film is 10.2%, which was enhanced to
around 60 times that of 3k in pure THF (0.17%). The
thiochromenones substituted with TPA derivatives (3k, 3l,
3m) except 3n all showed AIE active properties. When the
substituted groups were replaced by substituted phenyls, all the
compounds showed aggregation caused quenching properties.
These results suggest that the emission property of the
didisubstituted-thiochromenones highly depend on the sub-
stitution groups.
2,3-Disubstituted-thiochromenones could be converted into
oxidized thiochromenones after postsynthesis oxidation of
sulfur to sulfone of 3k by 3-chloroperoxybenzoic acid (m-
CPBA) under mild conditions and with high yield.15 As shown
in Figure 2a, the oxidation of thiochromenone derivative 3k
was conducted in one step with a good yield (81%) (Figure
2b). Its structure was confirmed by NMR, HRMS, and X-ray
single crystal analysis. Since the topological feature of 4 is
similar to that of 3k, it is reasonable to think that 4 is an AIE-
active molecule; that is, 4 will show emission enhancement
when aggregates of the solute formed in solvent/nonsolvent
mixtures. The experimental results demonstrated that the FL
intensity of 4 enhanced about 9 times when fw (%) proceeded
from 0% to 90% (Figure S2). It is noticeable that 4 displays
dramatically red-shifted fluorescence emission. The solid film
of compound 4 showed a PL peak at 720 nm, which is an
around 200 nm red shift in comparison with 3k film. The large
spectral shift can be attributed to the much stronger electron-
withdrawing ability of sulfone compared to sulfur. More
importantly, the fluorescence quantum yield of the solid film of
4 is measured to be 7.6%, which was enhanced to more than
12 times that of 4 in pure THF solution (0.6%). The near-
infrared emission with high efficiency in aggregation states is
suitable for biological applications such as fluorescent imaging
for cell clusters and thick tissues.
C−O bond of anhydrides to form the intermediate A. Then,
deoxygenation occurs to give the complex B, which is followed
by the migratory insertion of an alkyne to the Ni−S bond and
gives rise to D as an intermediate.5m,l,6b,8a Complex B is
deoxygenated via intermediate C,12 with the use of 1a or 1b as
substrate. Finally, reductive elimination of nickel from D
generates the final product 3.
To confirm the chemical structure of product 3 and
analogues, single crystals were cultivated, and some of them
can be difracted to gave structural details. In the structure of
2,3-disubstituted-thiochromenones, for example molecule 3k,
there is an ethenyl group linked with two propeller-shaped
triphenylamine (TPA) moieties (Figure 2a), which is similar to
the structure of well-known tetraphenylethylene (TPE), a
representative luminogen with an aggregation-induced emis-
sion (AIE) property.13,14 Accordingly, we examined the
emission behavior of these disubstituted-thiochromenones.
As shown in Figure 1, compound 3k shows typical AIE
phenomena. The solution of 3k showed very weak emission in
pure THF; the fluorescent quantum yield was only 0.17%. But
an emission peak at around 540 nm appears on the
fluorescence (FL) spectra after addition of nonsolvent water
into the THF solution when the volume fraction of water (fw)
is increased to 70%. As fw increases from 0% to 90%, the FL
intensity of 3k in the mixture solution grows to more than 45
times the previous level (Figure 1b and 1c). The fluorescent
In summary, we have reported a novel reaction path to
access 2,3-disubstituted-thiochromenones from commercially
available alkynes and 2-sulfo(sulfur)benzoic anhydrides.
Distinct from traditionally Ni-based cycloaddition of anhy-
C
DOI: 10.1021/acs.orglett.9b02161
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Organic Letters
Letter
Accession Codes
CCDC 1935447−1935448 contain the supplementary crys-
tallographic 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
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: zhaoh@uestc.edu.cn.
*E-mail: sunjz@zju.edu.cn.
ORCID
Pengbing Mi: 0000-0002-3771-386X
Jing Zhi Sun: 0000-0001-5478-5841
Author Contributions
§P.M. and L.H. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
For financial support, we thank National Natural Science
Foundation of China (21490571 and 51273175).
■
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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.9b02161.
Experimental procedures and copies of spectra (PDF)
D
DOI: 10.1021/acs.orglett.9b02161
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