Continuous-flow electro-oxidative coupling of sulfides with activated methylene compounds leading to sulfur ylides

Article abstract of DOI:10.1039/d1gc00226k

An unprecedentedly straightforward electro-oxidative coupling of sulfides with activated methylene compounds to synthesize sulfur ylides has been developed. Good to excellent yields can be obtained under catalyst- and oxidant-free conditions at room tempe

Full text of DOI:10.1039/d1gc00226k

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Continuous-flow electro-oxidative coupling of
sulfides with activated methylene compounds
leading to sulfur ylides†
Cite this: Green Chem., 2021, 23,
2956
Received 20th January 2021,
Accepted 23rd March 2021
Chengkou Liu,a Yang Lin,a Chen Cai,a Chengcheng Yuan,a Zheng Fanga and
a,b
DOI: 10.1039/d1gc00226k
Kai Guo
*
rsc.li/greenchem
An unprecedentedly straightforward electro-oxidative coupling of generated from the active methylene compounds and PIDA
sulfides with activated methylene compounds to synthesize sulfur (Scheme 1, eqn (b)). Maulide and co-workers found that bis
ylides has been developed. Good to excellent yields can be [α,α-bis(trifluoromethyl)benzyloxy]diphenylsulfur
(Martin’s
obtained under catalyst- and oxidant-free conditions at room sulfurane) could be used as an efficient ylide transfer reagent
temperature. Owing to the use of continuous-flow electro- to yield the desired sulfur ylides (Scheme 1, eqn (c)).15 The
chemical setups, this green, mild and practical electrosynthesis relatively costly hypervalent iodine reagent or ylide transfer
features high efficiency and excellent functional group tolerance reagent was required, which led to undesired byproducts and
and is easy to scale up.
low atom-economy. Thus, it is still desirable to develop more
green, economical and efficient methodologies to access sulfur
ylides.
Electrosynthesis employs traceless electrons to perform
redox reactions obviating the need for an excess of chemical
oxidizing and reducing reagents, which leads to low atom-
economy, environmental pollution, potential danger and
product contamination.16 In this context, we have been inter-
Sulfur ylides are important precursors in synthetic chemistry
for the construction of C–C and C–hetero bonds1 through
epoxidation,2 aziridination,3 cyclopropanation,4 [2,3]-sigmatro-
pic shift,5 Stevens rearrangement,6 and allkylation7 and the
synthesis of complex architectures through multi-step cascade
cyclizations.8 Recently, sulfonium and sulfoxonium ylides have
been widely used as metal carbene precursors9 instead of
α-diazocarbonyl derivatives,10 which are hazardous and toxic.
Moreover, sulfur ylides have increasingly attracted the interest
of medicinal chemists because of their antimicrobial
properties.11
Traditionally, sulfur ylides are prepared from the deproto-
nation of sulfonium salts using strong bases (Scheme 1, eqn
(a)).12 In addition, metal carbenes can be transformed into the
corresponding sulfur ylides,13 which are usually prepared from
diazo compounds (Scheme 1, eqn (b)). However, a tedious
preparation procedure of the activated precursors (diazo com-
pounds) and the use of stoichiometric amounts of metal salts
compromise the atom-economy. Most recently, Bayer and co-
workers developed a rhodium-catalyzed synthesis of sulfur
ylides using phenyliodonium diacetate (PIDA).14 Rh-Carbenes
were formed from iodonium ylide intermediates, which were
aCollege of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University,
Nanjing, 211816 China. E-mail: guok@njtech.edu.cn
bState Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech
University, Nanjing, 211816 China
†Electronic supplementary information (ESI) available: Experimental procedures
and spectral data. CCDC 2056830. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/d1gc00226k
Scheme 1 Synthesis of sulfur ylides.
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ested in electro-redox reactions, especially the electro-initiation out (Table 1). It was found that a higher constant current led
of stable radical or radical-cation intermediates. Recently, we to a decrease of the yield (Table 1, entries 1–3). The isolated
have reported an electrochemical heteroaryl migration using yield increased to 81% when the electrolysis time was extended
10-phenyl-10H-phenothiazine (PTH) as the efficient redox cata- from 2.25 min to 9 min (Table 1, entries 1 and 4–6). Further
lyst via an in situ generated S-radical-cation intermediate.17 extending the electrolysis time to 15 min resulted in a dra-
Thus, we envisioned that the generated S-radical-cations via matic reduction in yield with the formation of a sulfoxide
electrooxidation may react with active methylene compounds byproduct (Table 1, entry 7). To our delight, the yield was
to give sulfur ylides.
increased to 85% at a higher concentration (0.075 M based on
Moreover, several continuous-flow electro-oxidative coup- 1, Table 1, entries 8–10). The further investigation of constant
ling reactions to construct C–hetero bonds or cyclic com- current was carried out (Table 1, entries 11 and 12). A 94% iso-
pounds were previously reported by us.18 Compared with tra- lated yield was obtained at a constant current of 11 mA
ditional batch electrolysis, a larger ratio of the electrode (Table 1, entry 12).
surface to reactor volume and higher heat and mass transfers
In addition, a sharp drop in yield was observed when the
lead to higher selectivity and productivity.19 Besides, a less amount of active methylene compound or electrolyte was
supporting electrolyte is needed and it is easy to scale up. reduced, which was similar to the reaction in batch (Table S4†).
Herein, we report the synthesis of sulfur ylides through unpre-
Then, the optimized conditions of electrosynthesis of sulfur
cedented continuous-flow electro-oxidative coupling of sulfides ylides in both batch and continuous-flow were obtained.
with active methylene compounds at room temperature under
catalyst- and oxidant-free conditions.
It was noteworthy that compared with the electrolysis yield
in batch, the yield using a continuous-flow reactor was boosted
The oxidative coupling of PTH with diethyl malonate was to 94% at room temperature (Table 2, entries 1 and 2).
chosen as the model reaction to explore the optimal electroly- However, only trace amounts of the desired sulfur ylide
sis conditions in batch (Tables S1–S3†). To our delight, excel- product were detected when the electrolysis was performed in
lent yield (89% isolated yield) was obtained after electrolysis in a batch reactor at 25 °C (Table S2,† entry 7). Besides, a higher
an undivided cell using a mixed solvent of DMSO/TFE (7 : 1) at concentration in batch led to a dramatic reduction in yield
a constant current of 10 mA and at 45 °C, with a platinum (Table S3,† entries 6–8). The reaction in continuous-flow was
plate cathode, a carbon cloth anode, KH2PO4 as a base and fully tolerant to higher concentrations, which led to solvent-
nBu4NOAc as an electrolyte. No reaction occurred without elec- economy (Table 2, entry 3). Moreover, lower charge was needed
tric current (Table S1,† entry 12).
in a continuous-flow reactor (Table 2, entry 4).
Based on the optimized electrolysis conditions in batch,
With the optimized conditions of electrosynthesis of sulfur
further investigation in a continuous-flow reactor was carried ylides in continuous-flow being obtained, the versatility of this
reaction was probed (Scheme S1,† and Tables 3, 4). Firstly, the
coupling of
2
with various sulfides was evaluated
(Scheme S1†). Interestingly, the results revealed that the phe-
nothiazine ring and aromatic ring on the linking nitrogen are
indispensable in this electrosynthesis of sulfur ylides, probably
because of the higher stability of S-radical-cation intermedi-
ates. The reaction was furtherly explored by varying the aro-
matic rings on the linking nitrogen, phenothiazine rings, and
the active methylene compounds (Tables 3 and 4). It was
found that this electrosynthesis reaction demonstrated broad
tolerance toward both electron-donating and electron-with-
drawing groups at the phenyl ring A, including H (3), alkyl
groups (4–6), halogens (7, 8, 16), phenyl group (9), OMe (10,
17–19), SMe (15), CN (11), NO2 (12), CF3 (13) and CO2Et (14).
In particular, the phenyl ring A bearing highly electron-with-
drawing NO2 or highly electron-donating OMe or SMe at the
para position was converted into the corresponding sulfur
Table 1 Optimization of the electrosynthesis of sulfur ylides in con-
tinuous-flowa
Current
(mA)
Residence time
(min)
Concentration
(based on 1, M)
Yieldb
(%)
Entry
1
2
3
4
5
6
7
8
10
15
20
10
10
10
10
10
10
10
9
2.25
2.25
2.25
3
4.5
9
15
9
9
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.025
0.0375
0.075
0.075
0.075
69
63
56
71
77
81
63
69
76
85
91
94
9
Table 2 Comparison of optimized conditions of batch and continuous-
flow
10
11
12
9
9
9
11
Entry
Parameters
Batch
Continuous-flow
a Reaction conditions:
1 (0.4 mmol), 2
(0.8 mmol), nBu4NOAc
(0.8 mmol), KH2PO4 (0.8 mmol), DMSO/TFE (7 mL/1 mL), a C (carbon
filled PPS, 5.0 × 4.0 cm) anode, a Pt (SS 316L platinum coated, 5.0 ×
4.0 cm) cathode, volume (0.225 mL), residence time, current, rt.
b Isolated yield.
1
2
3
4
Isolated yield (%)
Temperature (°C)
89
45
0.025
4.67
94
rt (about 25–27)
0.075
3.65
Concentration (M, based on 1)
Charge (F mol−1
)
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Table 3 Scope of the sulfides
Table 4 Scope of the active methylene compounds
Reaction conditions, see Table 3, isolated yield.
1,3-Dicarbonyl compounds including malonic esters (3–29),
keto esters (30, 34), and diketones (31–33, 38) exhibited excel-
lent reaction compatibility and furnished the desired products
in good to excellent yields. Remarkably, cyclic diketone was
also a viable substrate (38). Besides, cyanoacetate (35), phenyl-
sulfonylacetic acid ester (36) and phosphonate ester (37)
underwent electro-oxidative coupling to afford the corres-
ponding sulfur ylides smoothly.
Furthermore, we performed gram-scale experiments in
batch and continuous-flow (Table 5). A slight reduction in
yield was observed when the reaction was applied on a deca-
gram scale in batch (from 89% to 79%). In contrast, a stable
Table 5 Gram-scale synthesis in batch and continuous-flow and deri-
vatization of α-carbonyl sulfonium ylide 3
Reaction conditions: sulfide (0.6 mmol), 2 (1.2 mmol), nBu4NOAc
(1.2 mmol), KH2PO4 (1.2 mmol), DMSO/TFE (7 mL/1 mL), a C (carbon
filled PPS, 5.0 × 4.0 cm) anode, a Pt (SS 316L platinum coated, 5.0 ×
4.0 cm) cathode, volume (0.225 mL), residence time (9 min), total reac-
tion time (320 min), 11 mA, rt, 3.65 F mol−1
.
ylide in good yield (10, 12). A slightly lower yield was obtained
when the A was substituted at the ortho or meta position
(16–18). In addition, this reaction was compatible with sub-
strates with a multisubstituted phenyl ring A (19), naphthalene
(20) and a heterocyclic ring such as quinoline (21) and pyrimi-
dine (22).
No obvious inhibitory effect on the reactivity was observed
by the electronic and steric properties of the B ring (23–28).
Even though the B ring was substituted with highly electron-
donating OMe or SMe, this electrolysis proceeded smoothly
affording good yield (26, 28). Meanwhile, the scope by varying
the active methylene compounds was explored (Table 4).
a Gram-scale synthesis in batch. b Gram-scale synthesis in continuous-
flow. c t-BuOH, KOH, 80 °C, 58%. d AlCl3, AcCl, 43%. e Aniline,
Cu(OAc)2, 37%.
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and excellent yield was obtained using a continuous-flow
reactor. Next, further derivatizations of 3 were carried out via
transesterification and acetylation to give the corresponding
ester (39) and aryl methyl ketone (40). In addition, the
α-tertiary amino ester (41) was generated from the protona-
tion–amination of α-carbonyl sulfonium ylide 3.
Intrigued by the efficient continuous-flow electrosynthesis
of sulfur ylides, a series of mechanistic studies were conducted
to unravel its reaction mechanism (Scheme 2 and Fig. 1). The
results of the H/D exchange experiments provided strong
support for the existence of the fast cleavage of the C–H bond
of activated methylene compounds (Scheme 2a). A minor KIE
(kinetic isotope effect, kH/kD ≈ 1.36) further substantiated the
conclusion above and revealed that the C–H cleavage of acti-
vated methylene compounds was not the rate-determining
step (Scheme 2b). At the same time, this electro-oxidative coup-
ling reaction was probed by voltammetric analysis (Fig. 1). An
obvious oxidation peak of 1 was observed at 1.16 V (Fig. 1a,
blue and green lines).
While diethyl malonate 2 revealed no redox activity (Fig. 1a,
pink and indigo lines), which revealed that 1 was possible to
be oxidized firstly to give the corresponding radical-cation
intermediate. Nevertheless, the oxidation peak of 1 dis-
appeared with the generation of a new oxidation peak at 1.49 V
in the presence of 2 and base. Compared with substrate 1, the
oxidation potential of the new peak was shifted to a less posi-
tive potential. This indicated that the existence of the inter-
reaction between 1 and 2, which affected the electro-oxidative
process of 1. This result was similar to the proton-coupled
electron transfer process.20
The oxidation potentials of TEMPO and BHT were higher
than the oxidation potential of 1 (TEMPO: 1.68 V, Fig. 1b, blue
line; BHT: 2.10 V, pink line), which indicated that TEMPO and
BHT were possible not to interfere with the oxidation of 1.
Moreover, no desired product was detected when a radical sca-
venger TEMPO or BHT was introduced (Scheme 2c). The above
Fig. 1 Cyclic voltammetry. (a) cyclic voltammetry of reactants; (b) cyclic
voltammetry of additives.
results further substantiated the existence of a radical-cation
intermediate. Moreover, a radical cation signal (g = 2.00737,
AN = 13.18 G, AH = 10.27 G) was observed in the electron para-
magnetic resonance (EPR) spectrum (Fig. 2).
On the basis of the mechanistic studies and previous
reports,17,21 a plausible catalytic cycle of electro-synthesizing
sulfur ylides was proposed (Scheme 3). The stable radical-cation
intermediate 1-1 was generated via single electron oxidation at
the anode, which reacted with the enol intermediate 2-1 to give
the radical intermediate 1-2. Further oxidation of 1-2 at the
anode led to the generation of cation intermediate 1-3. Finally,
the deprotonation of 1-3 delivered the desired product.
Scheme 2 Key mechanistic findings.
Fig. 2 The electron paramagnetic resonance spectrum of 1.
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Conclusions
In summary, we have developed an unprecedented ambient
temperature electro-oxidative coupling of sulfides with active
methylene compounds to give sulfur ylides in a continuous-
flow reactor under catalyst- and oxidant-free conditions. Owing
to the use of continuous-flow equipment, this method features
mild and green reaction conditions, high efficiency and a
broad substrate scope and is easy to scale up, which shows tre-
mendous application potential in industrial chemistry.
Furthermore, the products generated with phenothiazine
scaffolds are used as potential organic catalysts in photoredox
reactions and electrosynthesis.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The research was supported by the National Natural Science
Foundation of China (grant no. 21776130, 22008118 and
22078150).
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