Syntheses of diarylethenes by perylene-catalyzed photodesulfonylation from ethenyl sulfones

Article abstract of DOI:10.1246/cl.200046

Diarylethenes were obtained from the corresponding ethenyl sulfones by photocatalyzed desulfonylation using UV or blue LEDs. When perylene and i-Pr2NEt were used as a photocatalyst and a sacrificing reagent, respectively, this desulfonylation proceeded smoothly to afford the desired ethenes with the functional groups such as chloro, alkoxy and heteroaromatic rings remaining untouched. The use of a flow photoreactor enabled this desulfonylation to proceed more rapidly to finish in an hour of residence time.

Full text of DOI:10.1246/cl.200046

Advance Publication Cover Page
Syntheses of Diarylethenes by Perylene-Catalyzed Photodesulfonylation
from Ethenyl Sulfones
Hikaru Watanabe, Mai Takemoto, Kazumasa Adachi, Yasuhiro Okuda,
Aki Dakegata, Takahide Fukuyama, Ilhyong Ryu,
Kan Wakamatsu, and Akihiro Orita*
Advance Publication on the web February 13, 2020
doi:10.1246/cl.200046
© 2020 The Chemical Society of Japan
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Publication manuscripts.

1
Syntheses of Diarylethenes by Perylene-Catalyzed Photodesulfonylation
from Ethenyl Sulfones
Hikaru Watanabe,1 Mai Takemoto, 1 Kazumasa Adachi,1 Yasuhiro Okuda,1 Aki Dakegata,2 Takahide Fukuyama,2 Ilhyong Ryu,2
Kan Wakamatsu3 and Akihiro Orita*1
1Department of Applied Chemistry and Biotechnology, 3Department of Chemistry,
Okayama University of Science, 1-1, Ridai-cho, Kita-ku, Okayama 700-0005
2Department of Chemistry, Graduate School of Science, Osaka Prefecture University,
Sakai, Osaka 599-8531, JAPAN
E-mail: orita@dac.ous.ac.jp
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Diarylethenes were obtained from the corresponding
47 syntheses of the diarylethynes by taking advantage of
48 benzyl sulfones and arylaldehydes as starting compounds.6
49 In the synthetic protocols for acetylenes were involved two
50 reactions: (i) the formation of ethenyl sulfones from benzyl
51 sulfones and arylaldehydes and (ii) the subsequent base-
52 promoted elimination of sulfinic acid from the ethenyl
53 sulfones to provide the desired diarylethynes (path 1,
54 Scheme 1). We envisioned that if the reductive
55 desulfonylation could be photochemically promoted in the
56 ethenyl sulfones, the desired diarylethenes would be
57 obtained without the use of environmentally hazardous
58 reductants (path 2, Scheme 1). We report herein that the
59 reductive desulfonylation proceeds smoothly by using
60 perylene photocatalyst under the irradiation of UV or blue
61 light.7 We also exemplify the usability of this
62 desulfonylation in the syntheses of functionalized ethenes
63 and in the flow reaction using a microreactor as well.
64
ethenyl sulfones by photo-catalyzed desulfonylation using
UV or blue LEDs. When perylene and i-Pr2NEt were used
as a photocatalyst and a sacrificing reagent, respectively,
this desulfonylation proceeded smoothly to afford the
desired ethenes with the functional groups such as chloro,
alkoxy and heteroaromatic rings remaining untouched. The
use of a flow photo-reactor enabled this desulfonylation to
proceed more rapidly to finish in an hour of residence time.
10 Keywords: Photochemistry, Perylene, Desulfonylation
11 Carbon-carbon double bond is one of the fundamental
12 functional groups which could be often observed in
13 bioactive compounds1a and organic materials.1b For
14 syntheses of these olefinic compounds, a number of
15 protocols involving carbon-carbon bond formations have
16 been developed: for instance, Wittig olefination and Hornor-
17 Wadsworth-Emmons olefination.1a Julia olefination is one
18 of the synthetic protocols for olefins and has been widely
19 applied in organic synthesis because it can be carried out in
20 the presence of a variety of functional groups.2,3 Julia
21 olefination is composed of consecutive two-step reactions:
22 (i) aldol-type carbon-carbon bond formation between
23 sulfone and aldehyde followed by acetylation of the
24 resulting alkoxide and (ii) Na(Hg)-promoted reductive
25 desulfonylation of the acetate to form the C-C double bond.4
26 In order to prevent from utilizing the hazardous amalgam,
27 several variations of Julia olefination were developed. For
28 instance, Keck showed SmI2-promoted desulfonylative
29 olefination although some parts of the reaction mechanism
30 were ambiguous.3a Kocienski succeeded to avoid utilization
31 of the hazardous reductants by applying heteroaromatic
32 sulfones to one-flask olefination.4b Although the one-flask
33 synthetic protocol served well as environmentally benign
34 olefination, it was often required to add less nucleophilic
35 base like LDA to a mixture of the heteroaromatic sulfone
36 and aldehyde to suppress the undesired ipso-substitution of
37 aromatic ring with the base and/or the self-condensation of
38 the sulfones.2a In order to avoid the tedious experimental
39 procedure and to achieve a wider usability of the olefination,
40 further development of alternative Julia olefination is still in
41 demand.
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71 Scheme 1. Syntheses of ethynes and ethenes from benzyl
72 sulfone.
73
Requisite ethenyl sulfones 1 were prepared from the
74 corresponding benzyl sulfones and arylaldehydes in a
75 stepwise manner (Scheme 2). When benzyl phenyl sulfone
76 was successively treated with BuLi, benzaldehyde and
77 acetic anhydride, -acetoxy sulfone 2a was obtained.
78 Consecutive treatment of 2a with t-BuOK provided ethenyl
79 sulfone 1a (79% x 95% yield). Bromo-substituted ethenyl
80 sulfones 1b and 1c were similarly synthesized in good
81 yields.
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Our group has been involved in syntheses of
43 diarylethynes and their applications to organic materials
44 such as light-emitting compounds5a,b and organic
45 semiconductors.5c,d In these researches, we achieved one-
46 shot and one-flask double elimination protocols for
89
90 Scheme 2. Stepwise syntheses of ethenyl sulfones 1.
91

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The two-step synthesis of 1 could be compacted in
one-flask or one-shot process (Scheme 3). For instance,
when a THF solution of benzyl sulfone, benzaldehyde and
diethyl chlorophosphate was treated with 2.2 equivalents of
LiHMDS (lithium hexamethyldisilazide), 1a was obtained
in 56% yield by column chromatography. By changing a
combination of sulfones and aldehydes, a variety of
diarylethenyl sulfones 1 were synthesized.8
60
In order to shed light on a reaction mechanism of
61 this photo-desulfonylation, we performed DFT calculations
62 on the B3LYP/6-31+G* level. The simulation proposed a
63 tentative reaction pathway as shown below (Scheme 4).
64 The ethenyl sulfone 1a was transformed to radical anion 4
65 by single electron transfer (SET) from radical anion of
66 perylene which would be produced by electron abstraction
67 of photo-excited perylene from i-Pr2NEt.16 The DFT
68 calculations showed that in 4, the minus charge was located
69 mainly on -carbon of the ethenyl sulfone moiety while the
70 radical spin was mainly on -carbon as depicted in A.10
71 Consecutive protonation of A and SET to the resulting
72 radical B provided -sulfonyl carbanion 5, and 5 underwent
Perylene is a photocatalyst of choice in the reduction
10 of organic compounds under mild reaction conditions.9
11 Because in the electrochemical analyses using cyclic
12 voltammogram (CV), perylene indicated the higher LUMO
13 level (-2.69 eV) than ethenyl sulfone 1a (-2.75 eV),10 we
14 expected that perylene would serve well as photocatalyst for
15 reduction of 1a.11 When UV-light was irradiated to an
73 spontaneous desulfonylation to give 3a.17
74 photocatalytic reaction, the sacrificial reagent i-Pr2NEt
75 provided two electrons and one proton to serve as hydride
76 equivalent.
In this
One-flask synthesis
16
17
(1) BuLi / THF
(2) PhCHO
Ph
SO2Ph
1a [50%]
(3) Ac2O
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(4) t-BuOK
LiHMDS
(2.2 equiv)
One-shot synthesis
Ph
SO Ph
ClP(O)(OEt)2
Br
+
PhCHO
+
1a 56%
2
THF
PhSO2
o-Br: 1d [62%]
m-Br: 1b [62%]
p-Br: 1c [84%]
p-Cl: 1e [73%]
m-I: 1f 47%
X X=
1k [86%]
Ph
X
PhSO2
Ph
N
PhSO2
Ph
p-MeO: 1g 63%
PhSO2
Ar= 2-naphthyl: 1h 58%
2-furyl: 1i 61%
X= Me: 1l 58%
Ts: 1m 59%
Ar
Ph
3-thienyl: 1j 34%
In the brackets are shown yields of one-flask synthesis.
29 Scheme 3. One-flask and one-shot syntheses of 1.
30
31 MeCN/THF solution12 of 1a in the presence of perylene (5
32 mol%) and sacrificial reagent i-Pr2NEt (8 equiv) with UV
33 LED (light-emitting diodes) at 50-60 °C13 for 9 h, the
34 desired desulfonylation proceeded smoothly to provide
35 trans-stilbene (3a) in 97% yield (Table 1).14 In 4 h of UV-
36 light irradiation, TLC analysis indicated that 1a still
37 remained, and 3a was obtained in only 63% yield. The
38 desulfonylation proceeded sluggishly in DMF, DMSO and
39 toluene/THF to provide 3a in 76%, 38% and 15% yields,
40 respectively, while no reaction did in THF, 1,4-dioxane and
90
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93
Scheme 4. Plausible mechanism of reductive
desulfonylation of 1a.
In order to investigate usability of the photo-
94 promoted desulfonylation, we applied this protocol to other
95 ethenyl sulfones by using UV and blue LEDs (Scheme 5).
96 When (para-chlorophenyl)ethenyl sulfone 1e was subjected
97 to the desulfonylation, the desired ethene 3e was obtained in
98 91% NMR yield. For bromo- and iodo-substituted
99 derivatives, the desulfonylation was carried out at 25-30 °C
41 CHCl3. It is noteworthy to mention that the perylene- 100 in order to suppress dehalogenation. The desulfonylation of
42 catalyzed desulfonylation also proceeds by use of blue LED 101 meta-bromo-sulfones 1b and 1k, afforded the desired 3-
43 to afford 3a in 97% yield.15
102 bromostilbene (3b) in 72% and 51% yields, respectively. It
103 was disappointing that the desulfonylation of para-bromo-
104 sulfone 1c provided 4-bromostilbene (3c) in only 20% yield
105 (trans/cis 90 : 10) together with the debrominated product
106 1a in 44% yield. In the desulfonylation of ortho-bromo-
107 sulfone 1d, the desired ortho-bromostilbene 3d was
108 obtained in 25% yield together with cyclized products,
109 phenanthrenes 6 (9%) and 7 (36%)(Eq 1, Scheme 6).18 The
110 photo-reduction of meta-iodo-sulfone 1f also proceeded
111 sluggishly to provide a mixture of the desired product 3f
112 (22% yield, trans/cis 77 : 23) and 3a (25% yield, trans/cis
113 92 : 8) together with deiodinated starting sulfone 1a (48%
114 yield). para-Methoxyphenyl 1g and 2-naphthyl derivatives
115 1h were smoothly converted to the corresponding
116 diarylethenes 3g19 and 3h in 75% and 93%, respectively.
117 Heteroaromatic compounds were also tolerant in this
118 desulfonylation, and 2-furyl 3i and 3-thienylethenes 3j were
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Table 1. Solvent effect in reductive desulfonylation

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50 synthesized in 82%, 54% and 81% yields, respectively.
51 Fortunately, we succeeded to achieve further compaction of
52 alternative Julia olefination which could transform a pair of
53 benzyl sulfone and aldehyde to the desired ethene in the
54 sequential two-flasks reaction (Scheme 8). When benzyl
55 sulfone was treated successively with BuLi, benzaldehyde,
56 Ac2O and t-BuOK, the NMR analysis indicated the
57 formation of ethenyl sulfone 1a in 58% NMR yield. The
58 consecutive subjection of the crude ethenyl sulfone 1a to the
59 photo-desulfonylation provided 3a in 56% yield in 2 steps.
60 In the sequential two-flasks reactions, 3b, 3i and 3j were
61 also obtained in moderate yields.
62
Scheme 5. Photo-reductive desulfonylation of 1.
Although N-
19 synthesized in 99% and 90% yields.
20 methylindolyl derivative 1l provided desulfonylation
21 products 3l (8%) and 8 (14%) together with the remaining
22 starting sulfone (39%) (Scheme 6, Eq 2), N-tosyl sulfone
23 1m did a complicated mixture.20
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63 Scheme 8. Sequential two-flasks transformation of 1 to 3.
64
65
We also tackled to shorten the reaction time in the
66 desulfonylation by invoking a microreactor (Scheme 9).21
67 For photoreaction, the shorter optical path length of
68 microchannel affords efficient light penetration compared
69 with conventional macro photoreaction system, therefore,
70 the reactions run in the flow reactor would be expected to
71 terminate in considerably shortened irradiation time. A
72 MeCN/THF solution of 1a (100 mM) containing perylene
73 and i-Pr2EtN was irradiated by blue LED using a flow
74 photo-reactor (MiChS L-1; width = 2 mm, length = 1.6 m,
75 depth = 0.5 mm, quartz). After a survey of residence time,
76 we were delighted to find that the reaction proceeded
77 smoothly in 60 min residence time (flow rate = 1.6 mL/h) to
78 give 3a in 89% GC yield.
79
80
36
37
Scheme 6. Photo-reductive desulfonylation of 1d and 1l.
38 Although in all these desulfonylation processes were used
39 ethenyl sulfones 1, we also achieved one-flask
40 transformation of -acetoxy sulfones 2 to the desired
41 ethenes 3; the consecutive transformation of the -acetoxy
42 sulfones 2 to 1 and the reductive desulfonylation of 1 were
43 successfully carried out in one-flask manner (Scheme 7).
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90 Scheme 9. Flow synthesis of 3a by use of microreactor.
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46
Scheme 7. One-flask synthesis of 3 from 2
92
Finally, we synthesized fluorescent -expanded
47 When 2a, 2i and 2j were successively subjected to t-BuOK-
48 promoted elimination of acetic acid and perylene-catalyzed
49 desulfonylation, the desired ethenes 3a, 3i and 3j were
93 enyne 9 by Sonogashira coupling of 3-bromostilbene 3b
94 which was prepared from the desulfonylation protocol.
95 Although 3b was obtained as a mixture with a trace amount

4
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a) M. Julia, J. M. Paris, Tetrahedron Lett. 1973, 483. b) P. J.
Kocienski, B. Lythgoe, S. Ruston, J. Chem. Soc., Perkin Trans. 1,
1978, 829.
1
2
3
4
5
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7
8
of inseparable perylene photocatalyst, Sonogashira coupling
of 3b with 4-methoxyphenylethyne proceeded smoothly,
and the desired enyne 9 was isolated in a pure form (52%
yield) by column chromatography. The enyne 9 showed max
at 293 nm ( 6.29 x 104 L/molcm) in UV-vis absorption
spectrum (CHCl3, 10-5 mol/L), and 9 emitted fluorescence at
371 nm (F 0.20) in CHCl3 (CHCl3, 10-5 mol/L) and at 390
nm (0.57) in the powdery states.
a) F. Xu, T. Nishida, K. Shinohara, L. Peng, M. Takezaki, T.
Kamada, H. Akashi, H. Nakamura, K. Sugiyama, K. Ohta, A.
Orita, J. Otera, Organometallics, 2017, 36, 556. b) G. Mao, A.
Orita, L. Fenenko, M. Yahiro, C. Adachi, J. Otera, Mater. Chem.
Phys. 2009, 115, 378. c) D. Matsuo, X. Yang, A. Hamada, K.
Morimioto, T. Kato, M. Yahiro, C. Adachi, J. Otera, Chem. Lett.
2010, 39, 1300. d) T. Oyamada, G. Shao, H. Uchiuzou, H.
Nakanotani, A. Orita, J. Otera, M. Yahiro, C. Adachi, Jpn. J.
Appl. Phys. 2006, 45, L1331.
One-flask protocol: a) A. Orita, D. Hasegawa, T. Nakano, J.
Otera, Chem. Eur. J. 2002, 8, 2000. One-shot protocol: b) A.
Orita, H. Taniguchi, J. Otera, Chem. Asian J. 2006, 1, 430.
Several photoreductive desulfonylations of -sulfonylketones
were reported: for instance, M. Fujii, K. Nakamura, H. Mekata, S.
Oka, A. Ohno, Bull. Chem. Soc. Jpn. 1988, 61, 495. See also refs
20 for desulfonylation of N-sulfonylamine.
Although the ratios of geometric isomers (E/Z) in the ethenyl
sulfones 1 were determined by NMR analyses, neither isomer
was characterized in geometry. All the ethenyl sulfones were
recrystallized from the proper solvents before desulfonylation.
a) S. Okamoto, H. Tsujioka, A. Sudo, Chem. Lett. 2018, 47, 369.
b) S. Okamoto, K. Kojiyama, H. Tsujioka, A. Sudo, Chem.
Commun. 2016, 52, 11339.
6
7
9
10 Scheme 10. Synthesis of -expanded compound 9 from 3a.
11
8
9
12
In summary, we developed a new methodology for
13 desulfonylation of diarylethenyl sulfones using perylene
14 redox-catalyst under irradiation with UV/blue LEDs. This
15 protocol could be applied to ethenyl sulfones bearing
16 halogens, alkoxy and heteroaromatic rings. We also
17 succeeded to synthesize fluorescent -system expanded
18 enyne by Sonogashira coupling of bromostilbene which was
19 obtained in the desulfonylation. Further application of this
20 desulfonylation protocol to syntheses of expanded systems
21 and further investigation of the mechanism in the
22 photoreduction are under investigation.
80 10
81 11
82
See Supporting Information for details.
Although the SOMO level in radical anion of perylene should be
investigated for more precise prediction, we determined the
LUMO level for simplicity.
Because of low solubility of 1a in MeCN, the mixed solvents
MeCN/THF were used in the reductive desulfonylation.
The reaction temperature was kept at 50-60 °C by heat radiation
from the LEDs.
The yields of stilbenes 3 were determined by internal standard
method because it was difficult to separate 3 from the perylene
catalyst due to their similar Rf values.
The UV and blue LEDs emit at the centers of 398 nm and at 447
nm, respectively. Because perylene indicated the absorption band
at 350-450 nm, perylene catalyst would be photo-excited
efficiently by use of either UV or blue LED.
It is ambiguous that photo-excited perylene or perylene radical
anion would serve as reductant of 1a at this stage. See also
references 9.
It is possible that PhSO2 radical would eliminate from A or B to
provide 3a, and further investigation of the mechanism would be
required.
83
84 12
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86 13
87
23
88 14
89
24 This work was supported by the Grant-in-Aid from JSPS
25 (JP18K05134 to A.O. and JP19K15574 to Y.O.), the Grant-
26 in-Aid for Scientific Research on Innovative Areas 2707
27 Middle Molecular Strategy (JP15H05850 to A.O.),
28 Okayama Prefecture Industrial Promotion Foundation
29 (A.O.), Okayama Foundation of Science and Technology
30 (Y.O.), Promotion and Mutual Aid Corporation for Private
31 Schools of Japan (Y.O.), OUS Research Project (OUS-RP-
32 29-1 to A.O. and Y.O., OUS-RP-19-4 to A.O. and Y.O.) and
33 OUS Faculty of Engineering Exploratory Research (Y.O.).
34 The authors thank Research Instruments Center, Okayama
35 University for the measurements of 300MHz NMR (LA300),
36 400MHz NMR (JNM-ECS400 and JNM-ECZ400S) and
37 MALDI-TOF MS (autoflex speed) measurements.
90
91 15
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95 16
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98 17
99
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101 18 F. B. Mallory, C. S. Wood, J. T. Gordon, J. Am. Chem. Soc. 1964,
102
86, 3094.
103 19 High polarity of methoxy-substituted ethene 3g enabled its easy
104
105
separation from the perylene redox-catalyst by column
chromatography.
106 20 This result is ascribable to preferential photo-induced elimination
38
107
108
109
110
111
112
113
114
of the tosyl group. a) E. Hasegawa, Y. Nagakura, N. Izumiya, K.
Matsumoto, T. Tanaka, T. Miura, T. Ikoma, H. Iwamoto, K.
Wakamatsu, J. Org. Chem. 2018, 83, 10813. b) E. Hasegawa, N.
Izumiya, T. Miura, T. Ikoma, H. Iwamoto, S-y. Takizawa, S.
Murata, J. Org. Chem. 2018, 83, 3921. c) Y. Li, B. Hu, W. Dong,
X. Xie, J. Wan, Z. Zhang, J. Org. Chem. 2016, 81, 7036. d) C.
Liu, B. Zhang, RSC Adv. 2015, 5, 61199. e) L. Zheng, C. Yang,
Z-Z. Xu, F. Gao, W. Xia J. Org. Chem. 2015, 80, 5730.
39 Supporting
40 http://dx.doi.org/10.1246/cl.******.
Information
is
available
on
41 References and Notes
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1
For a review of polyene, see: a) Carotenoids, Vol. 2. Synthesis,
ed. by G. Britton, S. Liaaen-Jensen, H. Pfander, Birkhäuser
Verlag, Boston, 1996. For a review of poly(arylene vinylene)s
(PAVs), see: b) A. G. Grimsdale, in Organic Light-Emitting
Devices, Synthesis Properties, and Applications, ed. by K.
Müllen and U. Scherf, Wiley-VCH, Weinheim, 2006, p. 215.
For reviews, see: a) P. R. Blakemore, J. Chem. Soc., Perkin
Trans. 1, 2002, 2563. b) D. Gueyrard, Synlett 2018, 29, 34.
a) Reductive desulfonylation by use of SmI2; G. E. Keck, K. A.
Savin, M. A. Weglarz, J. Org. Chem. 1995, 60, 3194. b) Julia-
Lythgoe olefination using sulfoxides; J. Pospíšil, T. Pospíšil, I. E.
Markó, Org. Lett. 2005, 7, 2373.
115 21 For reviews on photoreaction using flow microreactors, see: a) D.
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Cambie, C. Bottecchia, N. J. W. Staathof, V. Hessel, T. Noel,
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