Photoredox-Catalyzed Alkenylation of Benzylsulfonium Salts

Article abstract of DOI:10.1002/asia.201801732

Visible light-mediated radical alkenylation of benzylsulfonium salts was achieved by means of fac-Ir(ppy)₃ as a photocatalyst, giving allylbenzenes as products. A variety of functional groups, such as halogen, ester, and cyano, were well tolera

Full text of DOI:10.1002/asia.201801732

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Accepted Article
Title: Photoredox-Catalyzed Alkenylation of Benzylsulfoniums
Authors: Shinya Otsuka, Keisuke Nogi, Tomislav Rovis, and Hideki
Yorimitsu
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Photoredox-Catalyzed Alkenylation of Benzylsulfoniums
Shinya Otsuka,[a,b] Keisuke Nogi,[a] Tomislav Rovis,[b] and Hideki Yorimitsu*[a]
Abstract:
benzylsulfoniums was achieved by means of fac-Ir(ppy)3 as a
photocatalyst, giving allylbenzenes as products. A variety of
Visible
light-mediated
radical
alkenylation
of
benefits, Zhang’s group and our group recently succeeded in
developing palladium-catalyzed Mizoroki-Heck-type reactions of
arylsulfoniums.[5] Unfortunately, however, these methods were
applicable only to arylsulfoniums, and alkylsulfoniums did not
functional groups such as halogen, ester, and cyano groups were well
tolerated in this transformation. Starting benzylsulfoniums could be
readily prepared from benzyl alcohols via an acid-mediated
substitution, increasing the synthetic utility of this transformation.
react at all.
In order to convert alkylsulfoniums to the
corresponding alkene products, we then turned our strategy to
photoredox-catalyzed radical-mediated alkenylation.[6] Recently
Lei and co-workers reported photoredox-catalyzed[7] radical-
mediated alkenylation of benzyl bromides under very mild
reaction conditions.[8] We envisioned that Lei’s alkenylation would
be applicable to benzylsulfoniums. Our working hypothesis is
shown in Scheme 1: A) photoexcited catalyst reduces
benzylsulfonium to generate benzyl radical and dialkyl sulfide, B)
addition of the benzyl radical to alkene takes place, C) single
electron transfer from the radical adduct to the oxidized
photocatalyst generates cation I, and D) deprotonation affords
alkenylation product. Indeed, various benzylsulfoniums and
alkenes could be coupled under visible light irradiation. It is worth
Organosulfur compounds widely exist in natural compounds,
living bodies, pharmaceuticals, and functional materials.[1] From
the viewpoint of organic synthesis, organosulfur compounds can
be regarded as important synthetic intermediates, due to their
unique reactivities that organohalogen compounds do not have.
In order to utilize such characteristics for organic synthesis,
recently,
transition
metal-catalyzed
transformations
of
organosulfur compounds with cleavage of their C–S bonds have
been actively explored.[2]
+
SR2
mentioning
that
electrochemical,[9]
chemical,[10]
and
X–
Ar
photosensitized[11,12] formations of aryl or alkyl radicals from
sulfonium salts have been well studied, while its application to
organic synthesis has been limited to α-carbonyl or α-
PC*
A
SR2
R
X–
+
SET
+
Ar
cyanosulfoniums,[10,12]
triarylsulfoniums,[13]
and
1-
trifluoromethyldibenzothiophenium (Umemoto reagent).[14]
Table 1. Optimization of reaction conditions.[a]
2 mol% fac-Ir(ppy)3
B
PC+·
hν
Ar
R
SET
C
PC
Ph
Ph
2 equiv base
S
Ph
–H+
+
+
solvent, blue LED, 16 h
Ph
Ar
R
Ar
R
Br
1a
D
I
2a
3aa
Scheme 1. Working hypothesis of this work.
entry
catalyst
base
solvent
NMR yield
39%
1
2
3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
Na2CO3
K2CO3
NaOAc
DMF
DMF
DMF
Among various transition metal-catalyzed transformations,
the Mizoroki–Heck alkenylation is one of the most important
reactions in organic synthesis.[3,4] However, Mizoroki-Heck-type
alkenylation of organosulfur compounds has been left unexplored
due to the strongly coordinating and potentially catalyst-poisoning
nature of leaving anionic sulfur moieties. In order to overcome
these problems, utilization of arylsulfoniums instead of aryl
sulfides is beneficial because 1) oxidative addition of
arylsulfoniums is easier than that of aryl sulfides due to their
electron-deficient nature; and 2) leaving neutral sulfides would be
less debilitating to the catalyst. Taking advantage of these
34%
16%
4
5
fac-Ir(ppy)3
fac-Ir(ppy)3
[Ir(ppy)2(dtbpy)](PF6)
fac-Ir(ppy)3
fac-Ir(ppy)3
–
Cs2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
DMF
DMF
DMF
NMP
NMP
NMP
NMP
0%
52%[b]
0%[b]
64%[b]
71%[b,c]
0%[b,c]
0%[b,c,d]
6
7
8
9
10
fac-Ir(ppy)3
[a] 1a (0.50 mmol), 2a (0.50 mmol), fac-Ir(ppy)3 (0.010 mmol), base (1.0
mmol), solvent (2.5 mL). [b] 2a (1.0 mmol). [c] NMP (1.0 mL). [d] In the
dark.
[a]
[b]
S. Otsuka, Dr. Keisuke Nogi, Prof. Dr. H. Yorimitsu
Department of Chemistry, Graduate School of Science
Kyoto University
Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: yori@kuchem.kyoto-u.ac.jp
S. Otsuka, Prof. Dr. T. Rovis
In the presence of 2 mol% of fac-Ir(ppy)3 as a photocatalyst,
the reaction of 1-benzyltetrahydrothiophenium bromide (1a) with
1,1-diphenylethylene (2a) was tested under blue LED irradiation.
By using Na2CO3 as a base and DMF as a solvent, a 39% yield
of the alkenylated product 3aa was obtained (Table 1, entry 1).
The use of K2CO3 instead of Na2CO3 gave the product 3aa in a
slightly lower yield (entry 2). NaOAc was found to be inferior, and
3aa was not obtained when Cs2CO3 was used[15] (entries 3 and
Department of Chemistry, Columbia University
3000 Broadway, New York, NY 10027 (USA)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/asia.201801732
Chemistry - An Asian Journal
COMMUNICATION
4). Organic bases such as Et3N, iPr2EtN, and DABCO resulted in
no or negligible formations of the desired product (See Table S1
in the Supporting Information). By increasing the amount of 2a to
2 equivalents, the yield of 3aa increased to 52% (entry 5). Since
the reduction potential of the sulfonium salt 1a is –1.48 V (vs SCE,
see Figure S1 in the supporting information), strongly reducing
fac-Ir(ppy)3 (IrIII*/IrIV = –1.73 V)[16] was crucial. As another
photocatalyst, [Ir(ppy)2(dtbpy)](PF6) having a higher reduction
potential (IrIII*/IrIV = –0.96 V)[17] did not give 3aa at all (entry 6).
The result of solvent screening revealed that NMP was the best
solvent (entry 7 and Table S1). Eventually, the reaction in higher
concentration (0.5 M) furnished 3aa in 71% yield (entry 8). In the
absence of either photocatalyst or light, 3aa was not obtained at
all (entries 9 and 10).[18]
yields, respectively. Styrene derivatives also underwent the
present alkenylation and the reaction of 1i with 4-methoxystyrene
(2h) afforded 1,3-diarylpropene 3ih in 66% yield as a mixture of E
and Z isomers.[19] Under the standard conditions, less electron-
rich 4-acetoxystyrene (2i) gave the desired product 3bi in 20%
yield along with hydrated alcohol 3bi' in 20% yield. We
speculated that benzylic cation I in Scheme 1 would be trapped
by a trace amount of water in the reaction mixture to generate
3bi'.[14a,l] Therefore we ran the reaction in the presence of
tetrabutylammonium bromide (TBAB) in order to enhance the
deprotonation from I by increasing the solubility of the base. As a
result, the formation of 3bi' was suppressed and 3bi was obtained
in 37% yield. The alkenylation with simple styrene in the presence
of TBAB gave 3bj while the yield was lower than that of electron-
rich styrenes. The reaction with α-methylstyrene (2k) also
proceeded while a mixture of three isomers, 3ik', (E)-3ik, and (Z)-
3ik, was obtained.
With the optimized reaction conditions in hand, we then tried
this alkenylation with various benzylsulfoniums (Scheme 2). The
efficiency of the alkenylation depends on the electronic nature of
benzylsulfoniums: the yields of the products from electron-
deficient sulfoniums were higher than that from 1a. Functional
groups such as ester and cyano groups were well tolerated in this
reaction, giving 3ba, 3ca, and 3da, respectively. A methyl
substituent on the ortho position in 1e did not retard the reaction.
Electron-deficient sulfoniums 1f and 1g afforded 3fa and 3ga in
high yields, respectively. Bromo-substituted benzylsulfoniums 1h
and 1i were successfully converted to 3ha and 3ia without
touching their aryl–Br bonds. Substituents on the meta positions
did not hamper the reaction; products 3ja and 3ka were obtained
in good yields. π-Extended naphthylsulfoniums 1l and 1m also
gave alkenylation products 3la and 3ma.
R2
2 mol% fac-Ir(ppy)3
R2
2 equiv Na2CO3
R1
S
+
R
R
R1
NMP, blue LED, 16 h
Br
1 0.5 mmol
2 2 equiv
3
F
OMe
Br
F
MeO2C
OMe
3ib 77%
3bc 78%
SMe
Ph
2 mol% fac-Ir(ppy)3
2 equiv Na2CO3
Ph
S
Ph
+
R
R
NMP, blue LED, 16 h
Ph
Br
1 0.5 mmol
2a 2 equiv
3
MeO2C
MeO2C
MeO2C
SMe
OMe
Ph
Ph
Ph
Ph
3bd 74%
3be 77%
Ph
Ph
Ph
Ph
Ph
Ph
MeO2C
NC
Ph
3aa 64%
3ba 79%
3ca 73%
MeO2C
Br
F
3bg 84%
(isomer ratio = 60:40)
3bf 87%
(isomer ratio = 50:50)
CN
Ph
CF3
Ph
Ph
F
Ph
Ph
3da 77%
3fa 86%
3ea 79%
Br
3ga 80%
OMe
MeO2C
OAc
3ih 66% (E/Z = 59/41)
3bi 37%[a]
Ph
Ph
Ph
F
OH
Ph
Ph
Ph
Ph
Ph
Br
3ha 67%
3ia 75%
F
3ja 80%
MeO2C
OAc
MeO2C
3bj 31%[a]
3bi’
Ph
Ph
MeO
Ph
Ph
Ph
Br
Br
+
Ph
Ph
OMe
3ka 69%
3la 56%
3ma 56%
3ik’+3ik 58%
(3ik’:(E)-3ik:(Z)-3ik = 65:24:11)
Scheme 2. Scope of benzylsulfonium bromides.
The scope of alkenes was also investigated (Scheme 3).
1,1-Diphenylethylenes bearing electron-withdrawing fluoro or
electron-donating methoxy groups participated in the reaction,
giving 3ib or 3bc in good yields. The methylsulfanyl groups in 2e
left untouched in the reaction and 3be was obtained in 77% yield.
The alkenylation of 1b with unsymmetrical 1,1-diarylethylenes 2f
and 2g afforded E/Z mixtures of 3bf and 3bg in 87% and 84%
Scheme 3. Scope of alkenes. [a] With 1.0 equiv of nBu4NBr.
We then checked the effect of the counteranion of
benzylsulfoniums.
Instead of benzylsulfonium bromide 1i,
benzylsulfonium triflate 4i was involved in the reaction (Scheme
4a). As a result, not only the desired product 3ia but also tertiary
alcohol 3ia' was observed in the crude mixture. We inferred that
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10.1002/asia.201801732
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COMMUNICATION
the difference would be derived from the efficiency of the step D
in Scheme 1. When 1i is employed, counteranion of the benzylic
chlorobenzyl alcohol could be alkenylated to afford 3na in 69%
yield. Electron-rich benzylsulfonium 4o was also converted to
3oa in 54% yield although the yield was lower than those from
electron-deficient ones. It is noteworthy that this method allows
one to avoid the use of unstable electron-rich benzyl bromides.
Alkenylation of 2-arylbenzylsulfonium 4q proceeded in spite of the
steric hindrance, giving 3qa in 70% yield. This example shows
usefulness of this reaction (Scheme 6); while palladium-catalyzed
cross-coupling of 2-bromobenzyl bromide with arylboronic acids
was reported to take place at the benzylic C–Br bond (Scheme
6a),[21] that of 2-bromobenzyl alcohols took place at the aromatic
C–Br bonds without any protection of the hydroxy moiety.[22] After
the arylation, the hydroxy group was “activated” to the
corresponding sulfonium triflate and then sulfonium 4g was
alkenylated under the photoredox catalysis. This arylation–
activation–alkenylation sequence would provide a new synthetic
strategy for allylbenzene derivatives (Scheme 6b).
cation I should be Br– or NaCO3 (by generating NaBr) and the
–
anion would deprotonate I to afford 3ia. On the other hand, in the
case of 4i, far less basic TfO– would be reluctant to deprotonate I.
As a result, the reaction of I with water afforded 3ia'. An addition
of TBAB was again effective to accelerate the deprotonation and
suppress the formation of 3ia', as is the case with 3bi and 3bj,
affording 3ia in 72% yield (Scheme 4b, upper). Additionally, we
tried to obtain 3ia' selectively by running the reaction in the
presence of water. As expected, 3ia' was obtained in 65% yield
(Scheme 4b, lower). The reaction in the presence of MeOH gave
ether 3ia'' in 58% yield. Interestingly, formation of 3ia' is less
efficient when 2-bromobenzyl bromide was used instead of 4i: the
yield of 3ia' was only 27% and 3ia was the major product.
Therefore, it can be emphasized that the synthesis of 3ia' or 3ia''
is one feature of using benzylsulfoniums.
a) Arylation of 2-bromobenzyl bromide (ref. 21)
cat. Pd(PPh3)4
(a) Reaction of benzylsulfonium triflate 4i in the standard conditions
2 mol% fac-Ir(ppy)3
Br
Br
ArB(OH)2
Br
Br
Ph
Br
Ph
2 equiv 2a
Na2CO3 aq.
Ph
Br
Ar
2 equiv Na2CO3
S
Ph
OH
EtOH/toluene
+
NMP, blue LED, 16 h
X–
Reacts at the benzylic C–Br bond
3ia
3ia’
(4i)
(1i)
62% (NMR)
90% (NMR)
X = OTf
X = Br
23% (NMR)
Not detected
b) Arylation–activation–alkenylation sequence
OMe
2 mol% Pd(OAc)2
4 mol% JohnPhos
(b) Divergent synthesis from benzylsulfonium triflate 4i
2 mol% fac-Ir(ppy)3
Br
1.5 equiv 4-OMeC6H4B(OH)2
3 equiv KF
2 equiv 2a
Deprotonation
Br
Ph
Ph
OH
2 equiv Na2CO3
THF, 60 °C
80%
Arylation at the aromatic C–Br bond
OH
1 equiv TBAB
NMP, blue LED, 16 h
Br
3ia 72%
tetrahydrothiophene CH2Cl2, r.t.
TfOH 81%
S
–OTf
2 mol% fac-Ir(ppy)3
Br
Ph
Ph
2 equiv 2a
4i
2 equiv Na2CO3
OMe
OMe
OR
2 mol% fac-Ir(ppy)3
2 equiv 2a
2 equiv Na2CO3
1 equiv TBAB
NMP/ROH = 10/1
blue LED, 16 h
R = H 3ia’ 65%
R = Me 3ia’’ 58%
Trapping with nucleophiles
Ph
NMP, blue LED, 16 h
70%
Ph
S
Scheme 4. Reaction of benzylsulfonium triflate 4i.
–OTf
4q
2 mol% fac-Ir(ppy)3
2 equiv Na2CO3
1 equiv nBu4NBr
3qa
Ph
Ph
S
Ph
+
R
R
Scheme 6. Synthesis of 3qa starting from 2-bromobenzyl alcohol.
NMP, blue LED, 16 h
Ph
OTf
4 0.5 mmol
2a 2 equiv
3
2 mol% fac-Ir(ppy)3
2 equiv 2a
Ph
Br
Ph
Cl
Ph
OMe
Ph
2 equiv Na2CO3
S
Ph
Ph
Ph
Ph
NMP, blue LED, 16 h
Br
MeO2C
MeO2C
Br
1b 1.5 mmol
3ba 85%
3ia 72%
3na 69%
OMe
3oa 54%
2 mol% fac-Ir(ppy)3
2 equiv 2a
Br
Ph
Ph
2 equiv Na2CO3
1 equiv nBu4NBr
S
NMP, blue LED, 16 h
Ph
Ph
Ph
OTf
MeO
Ph
4i 1.5 mmol
3ia 86%
Scheme 7. Scale-up reactions.
3pa 61%
3qa 70%
Scheme 5. Scope of benzylsulfonium triflates.
Finally, we conducted the present alkenylation in a 1.5 mmol
scale; sulfonium bromide 1b and sulfonium triflate 4i reacted with
2a, respectively. In both cases, the corresponding alkenylation
Since 1-benzyltetrahydrothiophenium triflates 4 can be
synthesized from benzyl alcohols and tetrahydrothiophene,[20] the
scope of 1-benzyltetrahydrothiophenium triflates was also
examined (Scheme 5). The sulfonium salt prepared from 2-
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10.1002/asia.201801732
Chemistry - An Asian Journal
COMMUNICATION
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products 3ba and 3ia were obtained without loss of the yields
(Scheme 7).
In conclusion, we have developed alkenylation of
benzylsulfoniums by means of photoredox catalysis. The starting
benzylsulfonium triflates could be prepared from the
corresponding benzyl alcohols, allowing this transformation
[8]
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Further development on photoredox-mediated
transformations of organosulfur compounds is currently underway
in our laboratories.
Acknowledgements
This work was supported by JSPS KAKENHI Grant Numbers
JP16H04109, JP18H04254, JP18H04409, and JP18K14212.
S.O. acknowledges JSPS Predoctoral Fellowship and JSPS
Overseas Challenge Program for Young Researchers. T.R.
thanks NIGMS (GM125206).
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Keywords: photoredox catalysis, alkenylation, sulfonium, radical
reaction
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Malliaras, S. Bernhard, Chem. Mater. 2005, 17, 5712.
[18] A light on-off experiment was conducted to understand the reaction
mechanism. As shown in Figure S2 in the Supporting Information, the
alkenylation proceeded only when blue LED was irradiated. This result
suggests that the reaction would not proceed through radical chain
mechanism.
[5]
[6]
a) S.-M. Wang, H.-X. Song, X.-Y. Wang, N. Liu, H.-L. Qin, C.-P. Zhang,
Chem. Commun. 2016, 52, 11893; b) D. Uno, H. Minami, S. Otsuka, K.
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2353.
[19] The E/Z ratio of 3ih would depend on the reaction time. With a longer
reaction time, the ratio of the E isomer of 3ih was increased probably due
to E/Z isomerization.
[20] See the Supporting Information and Scheme 6.
[21] S. Langle, M. Abarbri, A. Duchêne, Tetrahedron Lett. 2003, 44, 9255.
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[7]
Reviews: a) Visible Light Photocatalysis in Organic Chemistry (Eds.: C.
R. J. Stephenson, T. P. Yoon, D. W. C. MacMillan), Wiley, Hoboken, 2018;
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10.1002/asia.201801732
Chemistry - An Asian Journal
COMMUNICATION
Entry for the Table of Contents
Layout 2:
(Please choose one layout)
COMMUNICATION
R2
S. Otsuka, K. Nogi, T. Rovis, H.
cat.fac-Ir(ppy)3
Na2CO3
R2
R1
Yorimitsu*
S
+
R
R
R1
NMP, blue LED
X
Page No. – Page No.
Visible light-mediated radical alkenylation of benzylsulfoniums was achieved by
means of fac-Ir(ppy)3 as a photocatalyst, giving allylbenzenes as products. A variety
of functional groups such as halogen, ester, and cyano groups were well tolerated in
this transformation. Starting benzylsulfoniums could be readily prepared from benzyl
alcohols via an acid-mediated substitution, increasing the synthetic utility of this
transformation.
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