Oxy-sulfonylation of terminal alkynesviaC-S coupling enabled by copper photoredox catalysis

Article abstract of DOI:10.1039/d1gc00736j

We report the first literature example using visible light-induced trimethylsilyl azide (TMS-N₃)-assisted copper-catalyzed oxy-sulfonylation of terminal C-C bonds to form β-keto sulfones (C-S bond formation). TMS-N₃promotes the reaction by facilitating the formation of sulfonyl radicals, which later decompose into N₂gas upon light irradiation. This method involves the use of commercially available and stable starting materials. Also, a wide range of functional groups have been well-tolerated under the current photoredox process, evading the side product formation. Potent biologically active compounds, such as CES1, 11β-HSD1 inhibitors, anti-analgesic agents, and reactive synthesis intermediates were synthesized to demonstrate the synthetic utility of the current methodology. Moreover, green chemistry metrics and Eco-scale evaluation for the current photochemical method show that the protocol is eco-friendly and highly efficient.

Full text of DOI:10.1039/d1gc00736j

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Oxy-sulfonylation of terminal alkynes via C–S
coupling enabled by copper photoredox catalysis†
Cite this: DOI: 10.1039/d1gc00736j
V. Kishore Kumar Pampana,
Deb Kumar Das, Chun-Cheng Lin,
Vaibhav Pramod Charpe, Arunachalam Sagadevan,
Jih Ru Hwu and Kuo Chu Hwang
Received 26th February 2021,
Accepted 19th April 2021
*
DOI: 10.1039/d1gc00736j
rsc.li/greenchem
We report the first literature example using visible light-induced redox catalysts an ecofriendly and economically feasible
trimethylsilyl azide (TMS-N )-assisted copper-catalyzed oxy-sulfo- alternative for the synthesis of new chemical molecules. In
nylation of terminal CuC bonds to form β-keto sulfones (C–S recent years, our group has reported various C–C, C–N, C–O
bond formation). TMS-N promotes the reaction by facilitating the coupling reactions, cyclization, and hydrogen atom transfer
formation of sulfonyl radicals, which later decompose into N gas (HAT) reactions using a unique in situ generated copper(I) phe-
3
3
2
4
upon light irradiation. This method involves the use of commer- nylacetylide complex as a key light-absorbing photocatalyst.
cially available and stable starting materials. Also, a wide range of In this work, we present the first literature example using
3
functional groups have been well-tolerated under the current visible-light-induced trimethylsilyl azide (TMSN )-assisted
photoredox process, evading the side product formation. Potent copper-catalyzed oxy-sulfonylation of terminal Cu bonds to
biologically active compounds, such as CES1, 11β-HSD1 inhibitors, form new C–S bonds (synthesis of β-keto sulfones) under blue
anti-analgesic agents, and reactive synthesis intermediates were LED irradiation at room temperature.
synthesized to demonstrate the synthetic utility of the current
Sulfones are ubiquitous and versatile functional groups
methodology. Moreover, green chemistry metrics and Eco-scale that are found in a wide range of biologically active moieties
evaluation for the current photochemical method show that the and pharmaceutical drugs and are considered to be valuable
protocol is eco-friendly and highly efficient.
synthesis intermediates as they can be further functionalized
with ease. Traditionally, β-keto sulfones were synthesized by
5
Photoredox catalysis has emerged as a promising technique
over the past decade for the activation of a wide range of small
molecules to construct new chemical bonds in the field of
refluxing acyl halides and sodium sulfonate. However, this
6
method is not an atom economical method, as it requires pre-
functionalization of ketones (with halides) and tedious syn-
thesis of sulfinic acid precursors which are thermally unstable
and decompose easily to form thiosulfonates and sulfinic
1
organic chemistry. The recent renaissance of photocatalysis is
due to the selective excitation of light-absorbing photocatalysts
(Ru, Rh, or Ir polypyridyl ligands), which upon excitation
7
acids. Later in 2013, Lei and co-workers reported the synthesis
produce long lived triplet excited states and open-shell reactive
intermediates via a single electron transfer process (SET).
Furthermore, these reactive intermediates act as oxidants/
reductants in the reaction (photoredox catalysis) that is not
of β-keto sulfones via di-functionalization of terminal
CuC bonds using oxygen (O ) and sulfinic acids under
2
thermal conditions using stoichiometric amounts of the base
8
(
Scheme 1a). Next, various thermal and photochemical
2
common in thermal reactions.
methods were developed for the synthesis of β-keto sulfones,
such as (a) a two-step thermal method reported by Jiang et al.
to synthesize β-keto sulfones by oxidative coupling of oxime
acetates and sodium sulfinate at high temperatures using a
In the past few years, copper photocatalysts have come to
the forefront in the realm of photocatalysis and act as a light-
absorbing species, which are further involved in the bond
3
making and breaking process. Their low cost, earth-abundant,
copper catalyst via hydrolysis of vinyl amino sulfones
and environmentally benign nature make these copper photo-
9
(
Scheme 1b); (b) AgNO
3
/K
2
S
2
O
8
catalyzed aerobic oxysulfony-
lation of alkenes, which needs long reaction time and a strong
1
0
external oxidant;
(c) visible light-promoted Ru(II)-based
Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan, Republic
of China. E-mail: kchwang@mx.nthu.edu.tw; Fax: (+886)35711082
photoredox catalyzed oxysulfonylation of terminal CuC bonds
using tosyl hydrazides, bases, and additives at RT
†
Electronic supplementary information (ESI) available. CCDC 2057129 and
008686. For ESI and crystallographic data in CIF or other electronic format see
1
1a
(
Scheme 1c);
(d) visible light-mediated rearrangement of
2
DOI: 10.1039/d1gc00736j
vinyl tosylates for the synthesis of β-keto sulfones using an
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a
Table 1 Optimization of reaction conditions
Cu
b
Entry [catalyst] Ligand
Solvent
[Yield%]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
CuI
CuBr
CuCl
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
None
CuI
CuI
CuI
2-Picolinic acid
2-Picolinic acid
2-Picolinic acid
2-Picolinic acid
2-Picolinic acid
2-Aminopyridine
1,10-Phenanthroline MeOH
2,2′-Bipyridine
None
2-Picolinic acid
2-Picolinic acid
2-Picolinic acid
2-Picolinic acid
2-Picolinic acid
2-Picolinic acid
CH
CH
CH
CH
MeOH
MeOH
3
3
3
3
CN–MeOH 70
CN–MeOH 64
CN–MeOH 53
CN
0
84
0
62
41
0
80
10
n.r
n.r
n.r
n.r
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
c
d
e
f
0
1
2
3
4
5
Scheme 1 Different synthesis approaches for β-keto sulfones.
g
h
i
1
1b
organic photosensitizer (9-fluorenone);
and (e) recently, the
a
Unless otherwise noted, reaction conditions are as follows: 1a
electrochemical and histidine catalyzed synthesis of β-keto sul-
(
0.50 mmol), 2a (0.5 mmol), [Cu] catalyst (5 mol%), ligand (20 mol%),
1
2a
fones, respectively, reported by S. Shaabanzadeh
and A. TMSN (1.1 equiv.), and solvent (6 mL). The reaction mixture was irra-
3
−
2
1
2b
diated with blue LEDs (40 mW cm at 460 nm) under O
2
(1 atm) at
Kumar . However, these methods use excess amounts (2.0
equiv.) of sodium sulfinate, which in turn generates large
b
c
d
RT. Yield of the isolated product. Absence of a ligand. Under 1 atm
air. In the absence of TMSN
amounts of waste. Thus, the overall limitations of all the the absence of a copper catalyst. Under N2 atm (absence of O ). In
e
f
3
, addition of triflic acid (1.0 equiv.). In
g
h
2
i
the dark (absence of light). n.r = no reaction. Reaction conducted in
the dark at 60 °C.
above-mentioned methods include the use of (a) pre-syn-
thesized starting materials, (b) high reaction temperature, (c)
excess amounts of external oxidants and additives, (d) expen-
sive catalysts/photocatalysts such as ‘Ru-photo-catalysts’ which
1
3
are known for their high price and toxicity, (e) organic photo- 3a (Table 1). First, copper iodide (5 mol%) and 2-picolinic acid
sensitizers, which may have poor photostability and may (20 mol%) in CH CN–MeOH (1 : 1 v/v) afforded the product 3a
3
involve photo-bleaching, (f) excess amounts of starting in 70% yield after 4 h irradiation (entry 1, Table 1). By screen-
materials (sodium sulfinate), and (g) electrochemical cells (in ing various CuX (X = Cl, Br) compounds, we found that CuI
electrochemical synthesis), which require sophisticated instru- was a good catalyst for this process (entries 2 and 3, Table 1).
mentation and involve additional cost, and (h) the formation In solvent screening, we observed that in pure MeOH as a
of unwanted waste (leading to higher E-factors).
solvent, one could obtain the product 3a in a good yield (84%),
The key features of the current oxidative coupling reaction whereas using ACN as a solvent did not afford the product 3a
are as follows: (a) the use of bench stable commercially avail- (entries 4 and 5). Later, the effect of other ligands, such as
able starting materials (sodium sulfinate salts and alkynes); 2-amino pyridine, 1,10-phenanthroline, and 2,2′-bipyridine or
(
b) TMSN assists in the reaction (copper catalysts and sodium the absence of ligands was examined. The results showed the
3
sulfinate salts) to form β-keto sulfones without forming the poor formation of the product 3a (entries 6–9). By using the
1
4a,b
14c
hydroazidation products
tion); (c) the in situ-generated copper complex undergoes yield under 1 atm air (entry 10). When using triflic acid to
photoexcitation and catalyzes the formation of a new C–S bond replace TMSN , the product 3a was formed in only 10% yield.
via the SET process with O (sustainable oxidant); and (d) no Overall, optimization experiments showed that CuI, O , and
formation of side products, such as sulfonic acids or homo- blue LED light all play a crucial role in the current photoredox
or triazole products (click reac- current protocol, the compound 3a could be formed in 80%
3
2
2
coupling products of terminal alkynes (1,3-diynes).
protocol (entries 9–15).
Recently, we reported a regioselective acetamidation of a
Under the optimal conditions (Table 1, entry 5) for β-keto
4
e
terminal alkyne via C–N coupling under a similar photo- sulfone 3, the scope of various sodium sulfinates with phenyl-
induced electron transfer (PET) process. By anticipating a C–S acetylene 2a was examined to check the efficacy of the present
coupling reaction, we commenced our initial study by using photoredox system (Table 2). Sodium p-toluene sulfinate and
sodium p-toluenesulfinate 1a, trimethylsilyl azide (TMSN3), benzene sulfinates showed decent reactivity with 2a, furnish-
and phenylacetylene 2a as substrates with copper halides as a ing the products 3a and 3b in good yields of 84% and 82%,
catalyst, ligands, solvent, and photo-irradiation (blue LEDs), in respectively. Also, various halogens (–F, –Cl, and –Br), strong
the presence of O at room temperature to form β-keto sulfone electron-withdrawing group (–CF , –OCF , –CN, –NO )-bearing
2
3
3
2
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a
a
Table 2 Substrate scope of sodium sulfinates
Table 3 Substrate scope of terminal alkynes
a
Standard reaction conditions. Isolated yield after purification by
2
column chromatography on SiO .
sulfinates and naphthalene sulfinate reacted smoothly under
the standard reaction conditions to form β-keto sulfone pro-
ducts (3c–3k) in very good yields. Interestingly, thiophene sul-
fonate also reacted well with 2a to form the product 1-phenyl-
2
-(thiophen-2-ylsulfonyl) ethanone (3l) in 81% yield.
Next, the scope of terminal alkynes was explored with 1a.
Various electron-donating, withdrawing, and halo functional
group substituted terminal alkynes underwent reaction with
1
a smoothly to afford β-keto sulfones (Table 3). With common
electron-donating substituents such as –Me, –tBu, –OMe, and
OH, 1,3 di-alkyne 2e reacted well with 1a to form the corres-
a
Standard reaction conditions. Isolated yield after purification by
b
column chromatography on SiO
scale.
2
. Reaction performed on a 2.0 mmol
–
ponding sulfone analogs (4a–4e) in excellent yields without
forming any possible side products, such as Glaser homocou-
15
7
pling and other over-oxidation products. Interestingly, when
1
-ethynyl-4-(phenylethynyl) benzene 2f was used to react with for 24 h. Pleasingly, bulky terminal alkynes, such as 1-ethynyl-
1
a, the reaction occurred exclusively at the terminal alkyne site naphthalene and 9-ethynylphenanthrene also formed the
with no reaction at the internal alkyne site, showing good corresponding products (4q–4s) in high (85–90%) yields.
chemo-selectivity to produce 4f in 86% yield. Furthermore, Moreover, heterocyclic terminal alkynes, such as 3-ethynylpyri-
halogen-substituted (–F, –Cl, –Br, and –CF ) and moderate to dine, 3-ethynylthiophene, and 2-ethynylthiophene, showed
3
strong electron-withdrawing functional groups (–CN, –NO
Ac, and –COOMe) showed exceptional functional group toler- 4v) in 80–84% yields. The structures of 3d (CCDC 2057129†)
ance in forming the corresponding β-keto sulfones (4g–4p) in and 4v (CCDC 2008686†) were confirmed by single-crystal XRD
good to moderate yields. (Fig. S5 and S6 ESI†). Unfortunately, the aliphatic alkynes
2
,
good reactivity to form the corresponding β-keto sulfones (4t–
–
Furthermore, a gram-scale reaction of 1a and 2q success- failed to furnish the corresponding sulfones, and the reason is
fully afforded 4q in a moderate yield of 63%, after irradiation not clear.
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The synthetic utility of β-keto sulfones was demonstrated by
synthesizing some important intermediates/precursors
(Scheme 2). First, the reaction of 3a with hydroxylamine
afforded tosylethanone oxime 3aa, which is a potent anti-
6
inflammatory agent. Then, imidazole compound 3ab was syn-
1
6b
thesized by de-sulfonylation
the compound 3ac
of β-keto sulfones, and also,
1
6a
was synthesized by iodination on the
active methylene group of β-keto sulfones. Moreover, the com-
6
pounds 4i, 4j, and 4q are found to be anti-analgesic agents,
1
6d
1
1 β-hydroxysteroid dehydrogenase type I inhibitors,
and
1
6c
carboxylesterase 1 (CES1), respectively.
Furthermore, to demonstrate the green side of the reaction,
we have evaluated the green chemistry metrics and Eco-scale
for the compound 4j. The gram-scale reaction of 1a (2.0 mmol)
with 2j (2.0 mmol) under the standard reaction conditions
and photoirradiation for 24 h at room temperature produces
0
.43 g of product 4j. Next, we evaluated Green chemistry
4
c,e,f,g
metrics
(Table S1, ESI†) for the current photochemical
method, and the evaluation showed an E factor of 17.55,
1.4% atom economy, 45% atom efficiency, 84.2% carbon
Scheme 3 Mechanistic control studies.
7
efficiency and 62.5% reaction mass efficiency. Moreover,
wecompared the green chemistry metrics of the current proto-
4
col with that of a reported thermal method (Table S2, ESI†), generated in situ, might be the key light absorbing species.
and the results show that the current photochemical process is On the other hand, replacing O with N
.8 times better than the thermal process.
presses the formation of 3a, indicating that molecular O
Furthermore, we also calculated the Eco-scale (Table S4, key oxidant participating in the reaction. The addition of a
2
2
completely sup-
is a
1
2
4
g
ESI†) value for this photochemical protocol, and the calculated radical scavenger TEMPO (1.0 equiv.) into the reaction mixture
value of 58.5 (on the scale of 100) indicates that this method is does not lead to the formation of 3a (see eqn (2)), suggesting
an acceptable synthesis procedure for the preparation of β-keto the involvement of radical intermediates in the reaction. We
sulfones.
In mechanistic investigations, a series of control experi- azide intermediate 5 by hydro-azidation on terminal alkynes.
ments were carried out (Scheme 3, eqn (1)–(9)). β-keto sulfone To confirm this possibility, we allowed 5 (instead of 2a) to
a was obtained in 55% yield when CuI and phenylacetylene react with 1a. However, product 3a was not formed in both
were replaced by a pre-synthesized copper phenylacetylide 2a′ cases in the presence and absence of TMSN under the stan-
surmised that this reaction might be going through the vinyl
1
4
3
3
complex and reacted with 1a under standard reaction con- dard conditions. Therefore, the pathway involving the for-
ditions after 30 h blue LED irradiation (see eqn (1)). This mation of intermediate 5 was ruled out (see eqn (3) and (4)).
result suggests the copper(I)-phenyl acetylide 2a, which was
Next, we carried out a reaction using tetraethylammonium
phenyl sulfonate 6 to replace sodium phenyl sulfonate.
Tetraethylammonium phenyl sulfonate 6 has a bulky cation
and can ionize faster than 1a. In the absence of TMS-N
and dark), the reactions failed to produce 3a, suggesting that
TMS-N plays a key role in facilitating the formation of sulfonyl
3
(light
3
radicals (see eqn (5) and (6)). However, the reaction of tetra-
ethylammonium phenyl sulfonate 6 with 2a in the presence of
TMS-N
3
(under standard reaction conditions) under light
irradiation yields the product 3a in 77% yield (eqn (7)). Also,
the reaction of 2a with tosyl azide 7 failed to produce 3a,
suggesting that tosyl azide 7 is not a potential intermediate in
the current photoredox system (see eqn (8)). Finally, to
confirm the source of the O atom, we conducted the experi-
ment under the standard reaction conditions and in the pres-
1
8
16
ence of labelled
O
2
gas (97%) (instead of
2
O ) (see, eqn (9)).
This experiment formed the product 3a in 76% yield with 96%
1
8
incorporation of
2
O in the product (Scheme S3, ESI†), con-
firming that the source of the O atom in the product originates
from molecular O2.
Scheme 2 Late-stage synthesis of medicinal β-keto sulfones.
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mediate 13) and re-generated the LCu(I) complex.
Furthermore, intermediate 13 abstracts the proton and forms
β-keto sulfones 3.
Conclusions
In conclusion, we report a unique copper-catalyzed photoredox
method for oxy-sulfonylation (di-functionalization) of terminal
alkynes (CuC bond) by low energy visible light irradiation
using commercially available inexpensive starting materials to
synthesize β-keto sulfones at room temperature. Experimental
evidence suggested that a SET process occurs from the in situ-
generated triplet state excited copper complex to molecular O2
3
to generate a superoxide radical anion. TMSN promotes the
reaction by forming a sulfonyl radical, leading to the for-
mation of a new C–S bond. Also, the current photoredox proto-
col does not form hyper-oxidized products, namely, the Glaser
homocoupling side products or unwanted waste, which is in
contrast to the reported thermal procedures. Besides, the
current photoredox approach can be used to synthesize
Scheme 4 Proposed mechanism.
A plausible mechanism (Scheme 4) was proposed based on various potent biologically active compounds, such as anti-
the results of the above control studies and former literature analgesic agents (4i), 11β-HSD1 (4j), and CES1 inhibitors (4q)
4
f
reports. The in situ generated Cu(I)-phenyl acetylide 2a′ from simple, inexpensive, and readily available starting
undergoes photo-excitation (λabs = 476 nm) and forms a long- materials. In addition, the E-factor (the crucial parameter for
4
f
lived triplet excited state Cu(I)-phenylacetylide 8 (τ = 15.9 μs).
green synthetic chemistry) of the current photochemical
This photo-excited complex 8 undergoes a single electron method is 1.8 times better than that of the reported thermal
transfer (SET) process by donating an electron to molecular method. Moreover, the Eco-scale calculations scale the current
O , as the reduction potential (E = −2.048 VSCE in CH CN) is protocol of 58.5 on a scale of 0–100 (shows an acceptable syn-
2
1/2
3
4
f
2
adequately greater than that of O (E1/2 = +0.98 VSCE). The thesis). Thus, overall, the present photochemical method is a
SET process generates a superoxide radical anion, as evidenced green, highly efficient method and follows the various prin-
by EPR measurements using DMPO as a radical spin trapping ciples of green chemistry synthesis.
4
f
agent (Fig. S2, ESI†). The SET process generates a superoxide
radical anion (as evidenced by the EPR measurements using
DMPO as a radical spin trapping agent) (Fig. S2, ESI†) and
Conflicts of interest
II
4f
Cu -phenylacetylide 9. The as-formed superoxide radical
II
anion could further react with Cu -phenylacetylide 9 and form There are no conflicts to declare.
I
a Cu -superoxo radical complex 10, which further undergoes
an intramolecular radical attack on CuC to form a five-mem-
bered carbon-centered radical cyclic intermediate 11.
Acknowledgements
Meanwhile, sodium sulfinate 1a readily reacts with TMS-N in
3
the presence of a copper(II) catalyst and oxidant O
with O and forms a Cu(II)-superoxo complex (λmax
30–510 nm)
generate a sulfonyl radical intermediate.
2
(CuI reacts We are grateful to the Ministry of Science and Technology,
Taiwan for financial support. We also thank Dr. Hsin-Ru Wu
as evidenced by EPR, see Fig. S4, ESI†) to of the Instrumentation Center of National Tsing Hua
2
,
=
4
g,17
4
1
8a–d,19,20
The struc- University for HRMS measurements.
ture of the metal-superoxo complex could have side-on and
end-on geometries. Many factors can influence the oxidation
ability and chemical reactivities of metal-superoxo complexes,
including the types of ligands, the types of central metal ions
Notes and references
(
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2
1
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an O–O bond led to the formation of carbonyl carbon (inter-
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