Hantzsch Ester as a Visible-Light Photoredox Catalyst for Transition-Metal-Free Coupling of Arylhalides and Arylsulfinates

Article abstract of DOI:10.1002/chem.201905281

Diethyl 2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate (HEH) has been utilized as a visible-light photoredox catalyst for the cross coupling of arylhalides and arylsulfinates without transition metal, sacrificial agent, and mediator. This method is compatible with various functional groups and provides diaryl sulfones in good to high yields. Mechanistic studies indicate that this reaction undergoes the stepwise light irradiation of HE^?, single electron transfer (SET) in donor–acceptor complex (DAC) from *HE^? to arylhalide, trapping of aryl radical with sulfinate, and SET oxidation of sulfone radical anion by HE^. to sulfone by the DAC method.

Full text of DOI:10.1002/chem.201905281

A Journal of
Accepted Article
Title: Hantzsch Ester as a Visible-light Photoredox Catalyst for
Transition-Metal-Free Coupling of Arylhalides and Arylsulfinates
Authors: Hong-Xi Li, Da-Liang Zhu, Qi Wu, Hai-Yan Li, and Jian-Ping
Lang
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To be cited as: Chem. Eur. J. 10.1002/chem.201905281
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Hantzsch Ester as a Visible-light Photoredox Catalyst for
Transition-Metal-Free Coupling of Arylhalides and Arylsulfinates
Da-Liang Zhu, Qi Wu, Hai-Yan Li, Hong-Xi Li,* and Jian-Ping Lang
Abstract: We firstly use diethyl 2,6-dimethyl-1,4-dihydropyridine-3,5-
dicarboxylate (HEH) as a visible-light photoredox catalyst for cross
coupling of arylhalides and arylsulfinates without transition-metal,
sacrificial agent and mediator. This method is compatible with
various functional groups and provides diaryl sulfones in good to
high yields. Mechanistic studies indicate that this reaction undergoes
the stepwise light irradiation of HE-, single electron transfer (SET) in
donor–acceptor complex (DAC) from *HE- to arylhalide, trapping of
aryl radical with sulfinate, SET oxidation of sulfone radical anion by
HE• to sulfone via the DAC method.
above methods necessitate expensive Ir/Ru organometallic
complexes and transition-metal catalysts, which bring about
some limitations, such as toxicity and metal contamination. Very
recently, ultraviolet (UV) light-induced-coupling of sulfinates and
arylhalides was reported.[23] The high-energy UV light is
incompatible with some functional groups and leads to side
reactions. Therefore, the strategy for the systhesis of sulfones
under nonmetallic catalysis and greener conditions is still a
challenging and fascinating project. Herein we report the first
discovery of HEH as a visible-light photoredox catalyst for
coupling of arylsulfinates and arylhalides to sulfones in absence
of transition-metal, mediator and sacrificial reductant.
Hantzsch esters (HEs) are bio-inspired hydride donors and have
been widely employed as terminal proton sources and electron
donors in transfer hydrogenations[1] and in photoredox
catalysis.[2-4] 4-Alkyl- or acyl-Hantzsch esters can be used as
alkyl and acyl radical precursors in C–C formations.[5,6] Recently,
the excited-state of HEs have also been identified as strong
photoreductants [Eox(*HEH) = -2.28V vs. SCE; HEH = 2,6-
dimethyl-1,4-dihydropyridine-3,5-dicarboxylate].[7a]
Under
photoirradiation, the excited HEs have acted as electron and
hydride donors in reduction of phenacyl onium salts,[8]
debromination of alkyl bromides[7] and vicinal dibromo
compounds,[9] reductive cleavage of C-P bond in difluorinated
phosphonium salts,[10] reduction of cyanopyridines[11] and aryl tri-
/difluoroethanones.[12] After the single-electron-, or hydrogen-,
alkyl-transfer, the resulting radicals or radical cations are highly
active and readily aromatized into pyridine derivatives (Scheme
1(a)).[13] So HEs in cases above were used in stoichiometric or
excess amounts. To date, the utilizing of HEs as photoredox
catalysts remains unexplored. In this context, we report the first
example of visible-light-induced HEH-catalyzed C-S formation.
Diaryl sulfones are not only highly important organic
complexes, but also widely exist in natural products, bioactive
and pharmaceutical molecules.[14] Various protocols have been
developed to synthesize these scaffolds.[15,16] Palladium- and
copper-catalyzed coupling of sulfinates with aryl halides or
oxidation of diarylsulfides at high temperature have been well
investigated (Scheme 1(b)).[17,18] In recent years, visible-light
photoredox catalysis has emerged as a versatile platform for
various transformations.[19] Although progress has been made in
the field of visible-light-mediated C-S bond formation,[20] few
references reported arylations of sulfinates using aryl halides as
arylating reagents.[21] In 2018, Rueping, Manolikakes, Molander
groups independently demonstrated Ru- or Ir-based
photosensitizer facilitated nickel-catalyzed cross-couplings of
sulfinates with (hetero)aryl and vinyl iodides/bromides.[22] These
Scheme 1. Applications of Hantzsch Esters and Synthesis of Sulfones.
With the strong photoredox potential of excited *HEH[7a] in
mind, we hypothesized that *HEH might undergo single-electron
reduction of aryl halides to deliver aryl radical and HEH•+. The
complementary single electron transfer (SET) from sulfinate (Eox
= +0.45 V vs. SCE for 4-methylbenzenesulfinate in MeCN)[22a] to
HEH•+ (Ered = +0.79 V vs. SCE in DMF)[7a] affords sulfonyl radical.
[*]
D. L. Zhu, Q. Wu, Prof. H. Y. Li, Prof. Dr. H. X. Li, Prof. Dr. J. P.
Lang
College of Chemistry, Chemical Engineering and Materials Science
Soochow University
Suzhou 215123, China
E-mail: lihx@suda.edu.cn
Supporting information for this article is given via a link at the end of
the document.
The radical-radical coupling would yield
demonstrate our hypothesis, we carried out a preliminary
investigation of reaction conditions using 4-bromobenzonitrile
a sulfone. To
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(1a) and sodium benzenesulfinate (2a) as model substrates in
DMSO under 10 W blue light-emitting diodes (LEDs) (λ = 435-
455 nm) at ambient temperature (Table 1 and S1). The
combination of HEH (20 mol%) provided the desired product 4-
(phenylsulfonyl)benzonitrile (3aa) in 20% yield (Table 1, entry 1).
Fortunately, the addition of Cs2CO3 led to a full conversion of 1a
and could deliver 3aa in a good yield of 84% (entry 2). Other
applied bases (entries 2-7) and solvents (entries 8-15) did not
improve this coupling further. Performing the reaction in air led
to much lower yield (entry 16). Upon 445-465 nm, 455-480 nm,
460-485 nm, or 495-550 nm LEDs irradiation, the same reaction
afforded 3aa in 88%, 90% and 93%, and 89% yields (entries 17-
20). Only trace of 3aa was detected in absence of HEH (entry
21). No desired product was observed when the reaction was
carried out in dark (entry 22). A temperature of 60 °C did not
promote this reaction in the dark (entry 23).
Bromobenzonitrile
Bromobenzonitrile
delivered
3ja
in
63%
yield.
2-
or (2-bromophenyl)(phenyl)methanone
reacted with 2a to afford 3ka and 3la in 51% and 63% yields.
Aryl bromides containing groups at 3,5- or 3,4-positions of
phenyl rings gave 3ma-3pa in 82-89% yields. 2-
Bromonaphthalene underwent this coupling to offer 3qa in 60%
yield. The current photoredox catalytic conditions tolerated
heteroaryl halides, and 47-85% yields of 3ra-3xa were obtained.
Noteworthy, inexpensive and less toxic aryl chlorides
participated in the reaction efficiently to give 3aa, 3ea, 3ga, 3la,
and 3va in 40-83% yields. Treatment of 8-chloro-3-iodoquinoline
(1y) with 2a gave 8-chloro-3-(phenylsulfonyl)quinoline (3ya) in
83% yield. The subsequent Pd-catalyzed amination with
piperazine would afford the medicinal molecule 3-
(phenylsulfonyl)-8-(piperazin-1-yl)quinoline,
which
has
a
potential application in treatment of Alzheimer’s disease.[22a]
Table 1. Optimization of reaction conditions.[a]
Table 2. Coupling of arylhalides and arylsulfinates.[a]
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), HEH (0.04 mmol),
Cs2CO3 (0.3 mmol), solvent (1 mL), N2, 10 W LEDs with cooling by fan for 24 h.
HPLC conversion and yield. [b] In air. [c] Without HEH. [d] At room
temperature. [e] At 60 °C.
[a] Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol), HEH (0.04 mmol), Cs2CO3
(0.3 mmol), in 1 mL DMSO, 10 W LEDs (λ = 460–485 nm) with cooling by fan,
in N2, for 24 h and isolated yield.
The established photocatalytic protocol was then tested for a
family of reactions of various arylhalides and 2a. As shown in
Table 2, arylbromides substituted with electron-withdrawing
groups at para-position of phenyl (3aa-3ga) were suitable
substrates, giving expected products in good to excellent yields
We further examined the arylation of various sulfinates with 1a.
Electron-deficient aryl sulfinates were more active than electron-
rich ones. Reactions of sulfinates 2b, 2c, 2i and 2j with 1a
proceeded with high yields of 78-89%. The moderate yields
were observed for sulfinates (2d-2h and 2k) with electron-
(60-92%).
1-Bromo-4-methylbenzene
(1h),
1-bromo-4-
methoxybenzene (1i) gave rise to 3ha and 3ia in 37-40% yields.
Their yields could be enhanced to 65-76% using 1-iodo-4-
methoxybenzene and 1-iodo-4-methylbenzene as partners. 3-
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donating methyl, methoxyl, phenyl, tert-butyl groups. 2-Naphthyl
sulfonate was compatible with the cross-coupling in 57% yield
(3al). The photoreaction of 1a and 2a could also be performed
under sunlight (the maximum power density is about 8.14
mW·cm-2) for 8 h to give 3aa in 98% HPLC yield. The
preparative-scale experiment was carried out using 1.09 g of 1a
and 1.97 g of 2a. Satisfactorily, the yields of 75% under the
irradiation of a blue LED soft rope light (λ = 435-455 nm) and
80% under sunlight were achieved (Figure S1).
To gain some mechanistic information, the UV-vis absorption
and emission spectra of HEH solution were recorded at room
temperature. HEH in DMSO shows strong absorption in the UV
region, but weak visible-light absorption tailing to 400-435 nm
regions. The addition of Cs2CO3 deepened the color (Figure S2).
A new broad peak (420-550 nm) centred at 475 nm is observed.
Similar peaks appear with the addition of other strong or weak
bases (Figure S3). This new peak is attributed to the absorption
of HE- anion. The hardly absorption of HEH/Cs2CO3 solutions in
other solvents at λ = 475 nm is due to the low solubility of
Cs2CO3 in toluene, n-pentane, CHCl3, THF, 1,4-dioxane or the
difficulty of HE- formation in MeOH (Figure S3c). HEH shows no
photocatalytic activity in these solvents (Table 1, entries 10-15).
Upon excitation at 383 nm, the DMSO solution of HEH displays
of reductive quenching via SET from 2a to *HE-. As predicted, no
obvious changes in the emission spectra were observed when
2a was added into HE- solution (Figure S10). The 1H NMR
titration experiments have been performed between HE- and 1a
or 2a (Figure S11). When 1a or 2a is added, the resonance of
HE- protons at δ 3.98, 2.03 and 1.15 ppm shows remarkable
downfield shifts. So an electron donor–acceptor (EDA) complex
is formed between HE- and arylhalide (or arylsulfinate) in
photocatalytic process.[24] The light on/off experiments showed
that a smooth transformation under the light irradiation, but no
further conversion was observed when the mixture was kept in
dark (Figure S12). The coupling reactions is carried out at
100 °C to result in the formation of 3aa in about 25% yield with
or without 2,2'-azobis(2-methylpropionitrile) (AIBN) (Table S1,
entries 1 and 2). Taken together, the coupling reaction does
proceed through a photoredox catalytic pathway.
The addition of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)
(2 equiv.) under optical conditions largely inhibited the model
reaction, giving 3aa in 2% yield. Reaction of 1a, HEH (1 equiv.),
and TEMPO (2 equiv.) with Cs2CO3 (1.5 equiv.) under optical
conditions
gave
aryl-TEMPO
adduct
4-((2,2,6,6-
tetramethylpiperidin-1-yl)oxy)benzonitrile in 29% isolated yield
coupled with diethyl 2,6-dimethylpyridine-3,5-dicarboxylate in
79% HPLC yield (details in supporting information). These
results demonstrated the involvement of aryl radical species in
this photocatalytic coupling system. The SET from *HE- to 1a
gives aryl radical (Ar•) and HE•. The SET from 2a to HE• is
excluded due to that Eox(2a) of +0.540 V vs. SCE is higher than
Eox(HE-) of −0.186 vs. SCE. The subsequent coupling of aryl
radical with 2a affords sulfone radical anion (3aa•-), which can
transfer one electron to HE• [Ered(3aa) = -1.566 V vs. SCE
(Figure S6i)]. In addition, Ered(1a) is lower than Ered(3aa). So the
SET process from 3aa•- to 1a is unfavorable (Scheme S1).
Based on all above data, the proposed catalytic cycle via
oxidative quenching progress is proposed in Scheme 2. First,
the EDA complex [HE-···ArX] (I) is excited by visible light to
provide [*HE-···ArX] (II) species. The SET occurs in II to yield
aryl radical (Ar•) and HE• [E(HE•/*HE-) = −2.490 V vs. SCE in
DMSO, E(Ar-X) ≈ -2.0 V vs. SCE[25]]. Then, carbon radical Ar•
reacts with 2 to form EDA intermediate III, which undergoes an
SET to offer 3 along with regeneration of HE-. [Ered(3aa) = -1.566
V; E(HE•/HE-) = −0.186 V vs. SCE in DMSO].
an emission at 450 nm (Figure S4a). The broad band at λmax
=
538 nm arises from luminescence of HE- solution in DMSO
excited at 469 nm (Figure S4b). The photoluminescence
lifetimes and quantum yields of HEH and HE- in DMSO are
0.419 ns/0.031, and 4.8 ns/0.325, respectively (Figure S5). The
quantum yield of reaction system was decreased to 0.012. As
discussed above (Table 1, entries 2, 17-20), the yield of 3aa is
related to the spectral overlap between the radiation light and
the absorbance of HE- to a certain extent, which means more
overlap would cause higher yield (Figure S2). These results
suggest that HE- anion plays a key role in the cross coupling.
The electrochemical properties of HEH and HE- anion were
measured by cyclic voltammetry (CV). Two irreversible redox
waves of HEH in DMSO at E = +0.882, −1.202 V vs. SCE are
assigned to HEH+/HEH and H+/H2 couples. The two other
irreversible peaks are observed at −0.173, −2.379 V vs. SCE, in
which HE- is oxidized to HE• and reduced to HE2- (Figure S6a).
The CV of HE- contains two waves at E = −0.186 V and −2.372
V vs. SCE, being assumed to one electron oxidation and
reduction of HE- ion (Figure S6b). The wave at E = −2.004 V
belongs to the reduction of diethyl 2,6-dimethylpyridine-3,5-
dicarboxylate (Figure S6c). Based on the electrochemical and
spectroscopic data, the redox potentials of excited-states *HEH
and *HE- are estimated to be +1.554/−1.874
V
and
−0.067/−2.490 V vs. SCE. So HE2- dianion, *HEH, *HE- are
powerful reductants (Table S2).
The luminescence responses of HEH or HE- ion in DMSO to
substrates were investigated. As shown in Figures S7 and S8,
the addition of 1a or 2a into the HEH solution did not cause
significant variations in the emission spectra (λex = 383 nm). In
contrast, 1a did reduce the photoluminescent intensity (λex = 469
nm) of the HE- solution (Figure S9). Under photocatalytic
conditions, the reduction potential of *HE- (−2.490 V vs. SCE) is
large enough to reduce 1a [−2.043 V vs. SCE (Figures S6e)].
The relationship between Eox(2a) [+0.540 V vs. SCE in DMSO
(Figure S6d)] and Ered(*HE-) definitively excludes the possibility
Scheme 2. Proposed mechanism
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In summary, we have developed the new application of cheap,
commercially available Hantzsch ester (HEH) as a visible-light
photoredox catalyst for the synthesis of sulfones. This
transformation allows the cross-coupling of various sodium
sulfinates with a wide range of aryl and heteroaryl bromides,
iodides and chlorides. This conversion does not need transition
metal, external oxidant, sacrificial reductant or mediator under
blue LEDs or sunlight. This newly developed photoredox
catalysis based on Hantzsch ester is also applicable to new C-X
(X = C, N, O, S) bond transformations, which are currently under
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Acknowledgements
The authors thank the National Natural Science Foundation of
China (grant numbers 21771131 and 21971182), the "Priority
Academic Program Development" of Jiangsu Higher Education
Institutions, Scientific and Technologic Infrastructure of Suzhou
(grant numbers SZS201708, SZS201905). We highly appreciate
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Keywords: Hantzsch ester • transition-metal-free • C-S bond
formation • arylhalides • arylsulfinates
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Entry for the Table of Contents
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Da-Liang Zhu, Qi Wu, Hai-Yan Li, Hong-
Xi Li,* and Jian-Ping Lang
Page No. – Page No.
Hantzsch Ester as
a Visible-light
Photoredox Catalyst for Transition-
Metal-Free Coupling of Arylhalides
and Arylsulfinates
Hantzsch Ester as a Photocatalyst: We firstly use diethyl 2,6-dimethyl-1,4-
dihydropyridine-3,5-dicarboxylate (HEH) as a visible-light photoredox catalyst for
coupling of aryl sulfinates and arylhalides without transition metal, external oxidant,
sacrificial agent and mediator.
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