C-H functionalization of enamides: Synthesis of β-amidovinyl sulfones via visible-light photoredox catalysis

Article abstract of DOI:10.1002/adsc.201200874

A mild, practical and environmentally friendly strategy to prepare β-amidovinyl sulfones has been developed based on the sulfonation of enamides or enecarbamates via visible-light photoredox catalysis. Direct C-H functionalizations of enamides or enecarbamates under the optimized conditions proceeded with a wide scope of substrates and remarkable selectivity to give functionalized vinyl sulfones with good to excellent yields. Copyright

Full text of DOI:10.1002/adsc.201200874

UPDATES
DOI: 10.1002/adsc.201200874
À
C H Functionalization of Enamides: Synthesis of b-Amidovinyl
Sulfones via Visible-Light Photoredox Catalysis
Heng Jiang,a Xuejiao Chen,a Yan Zhang,a,b,* and Shouyun Yua,*
a
State Key Laboratory of Analytical Chemistry for Life Science, Institute of Chemical Biology and Drug Innovation,
School of Chemistry and Chemical Engineering, Nanjing University, 22 Hankou Road, Nanjing 210093, Peopleꢀs Republic
of China
E-mail: njuzy@nju.edu.cn or yushouyun@nju.edu.cn
Laboratory of Chemical Genomics, School of Chemical Biology and Biochemistry, Pekin University, Shenzhen Graduate
b
School, Shenzhen 518055, Peopleꢀs Republic of China
Received: September 28, 2012; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200874.
Abstract: A mild, practical and environmentally
friendly strategy to prepare b-amidovinyl sulfones
has been developed based on the sulfonation of en-
amides or enecarbamates via visible-light photore-
Very recently, we have also described a mild and
practical method for alkylation, trifluoromethylation
and arylation of enamides and enecarbamates via visi-
ble-light photoredox catalysis using enamides as
SOMOphiles.[12] Stephenson and co-workers demon-
strated that 4-toluenesulfonyl chloride could be re-
duced to the sulfonyl radical via visible-light photore-
dox catalysis.[13] Based on these works, we are inter-
ested in the direct sulfonation of enamides with sulfo-
nyl chlorides to generate vinyl sulfones instead of the
addition of sulfonyl chlorides to C=C bonds to gener-
ate a-chlorosulfones. We envisaged that sulfonyl radi-
cal I, which could be generated from sulfonyl chloride
1 under photocatalysis conditions,[13] could add to en-
amide 2 to give the amido radical II. The resulting
À
dox catalysis. Direct C H functionalizations of en-
amides or enecarbamates under the optimized con-
ditions proceeded with a wide scope of substrates
and remarkable selectivity to give functionalized
vinyl sulfones with good to excellent yields.
À
Keywords: b-amidovinyl sulfones; C H activation;
iridium; photochemistry; redox chemistry
Vinyl sulfones are important building blocks in organ-
ic synthesis due to the versatility of the sulfone
moiety.[1] Moreover, structurally diverse vinyl sulfones
have been reported to be potent inhibitors of en-
zymes such as sortase,[2] cysteine proteases[3] and pro-
tein tyrosine phosphatases.[4] Vinyl sulfones containing
an amide functionality have been used as small mo-
lecular probes to investigate the mechanism of micro-
bial pathogenesis.[5] Due to the importance of vinyl
sulfones, it is of current research interest to develop
practical synthetic approaches to diverse compounds
containing the vinyl sulfone moiety.[6] However, to the
best of our knowledge, no general synthetic approach
to b-amidovinyl sulfones has been reported. Since en-
amides including enacarbamates are readily available
starting materials, the direct sulfonation of simple en-
amides represents a straightforward and flexible ap-
proach to this kind of motif.
Over the past five years, visible-light photoredox
catalysis has emerged as a powerful tool to promote
useful redox transformations[7] as demonstrated by
Figure 1. Rationale for the sulfonation of enamides and ene-
MacMillan,[8] Yoon,[9] Stephenson[10] and others.[11] carbamates.
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Table 1. Optimization or reaction conditions.[a]
Table 2. Scope of the sulfonyl chlorides.[a,b]
Entry Conditions
Yield [%][b]
1
2
standard conditions
no Ir(ppy)2A(dtbbpy)PF6
92
N
G
no reac-
tion[c]
0[d]
no reac-
tion[c]
20
3
4
no Na2HPO4
no light irradiation
5
6
open to air
Ru
N
G
10
ACHTUNGTRENNUNG
7
8
9
TEA instead of Na2HPO4
DMF as solvent
household bulb instead of blue LED
15
Trace
90
[a]
Reaction conditions: sulfonyl chloride 1a (0.75 mmol),
enamide 2a (0.5 mmol), base (1.5 mmol) and catalyst
(0.005 mmol) in acetonitrile (5 mL) was irradiated by
3 W blue LED for 4 h.
Isolated yield.
2a was fully recovered.
[b]
[c]
[d]
2a was decomposed.
[a]
Reaction conditions: N-vinylpyrrolidinone (0.5 mmol),
amido radical II could be easily oxidized to furnish
the N-acyliminium III. Upon deprotonation of III and
tautomerization, the amidovinyl sulfone 3 could be
formed as the final product (Figure 1).
sulfonyl chloride (0.75 mmol), Na2HPO4 (1.5 mmol) and
IrACHTNUTRGENN(UG ppy)2ACHTUNGTREN(NGUN dtbbpy)PF6 (0.005 mmol) in acetonitrile (5 mL)
were irradiated by 3 W blue LED for 4–24 h.
All the yields are isolated yields.
5 equiv. of methanesulfonyl chloride were used.
10 equiv. of sulfonyl chloride were used.
[b]
[c]
[d]
We have demonstrated that photoinduced reactions
are an efficient way to build structurally diversified
new compound libraries from which biologically
active molecules such as small molecular microRNA
modifiers could be identified.[14] As part of our con- tocatalyst Ir
ACTHUNRGTENNGU(ppy)2ACHUTNGTREN(NUGN dtbbpy)PF6 and Na2HPO4 for 4 h,
tinuing efforts to get new compound families based the configurationally pure E-vinyl sulfone 3a could be
on photo-initiated reactions,[12] we report here the isolated in a yield of 92% (Table 1, entry 1). Visible
visible-light photoredox catalyzed sulfonation of en- light either from 3 W blue LEDs or from a household
amides as a straightforward method to generate the fluorescent bulb could initiate the reaction smoothly
b-amidovinyl sulfone functionality.
Firstly we tested the feasibility of C H functionali-
zation of enamides with sulfonyl chlorides. When a so- establish the role of each component in the reaction
lution of 4-(trifluoromethyl)benzenesulfonyl chloride mixture. No desired product could be generated with-
(1a) and N-vinylpyrrolidinone (2a) in acetonitrile was out the photocatalyst, base or light source (entries 2–
irradiated by visible light in the presence of the pho- 4). When the reaction mixture was open to the air,
(entry 9).
À
A variety of control experiments was conducted to
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C H Functionalization of Enamides: Synthesis of b-Amidovinyl Sulfones
Table 3. Scope of the enamides and enecarbamates.[a,b]
[a]
Reaction conditions: 4-(trifluoromethyl)benzenesulfonyl chloride (0.75 mmol), enamide 2
(0.5 mmol), Na2HPO4 (1.5 mmol) and IrACHTNUGTRENN(UG ppy)2ACHTUNGTNER(NUGN dtbbpy)PF6 (0.005 mmol) in acetonitrile
(5 mL) were irradiated by 3 W blue LED for 5–18 h.
All the yields are isolated yields.
[b]
the yield of 3a dropped to 20% (entry 5). Ru
turned out to be much less efficient in this transfor- 2a was also achieved using alkanesulfonyl chlorides.
mation than Ir(ppy)2A(dtbbpy)PF6 (entry 6). When the The coupling between methanesulfonyl chloride with
ACHTUNGTREN(NUNG bpy)3Cl2 high yields. Remarkably, the successful sulfonation of
A
CHTUNGTRENNUNG
organic base triethylamine (TEA) was used instead of enamide 2a gave vinyl methyl sulfone 3o in 84% yield
Na2HPO4, most of the starting material 2a decom- although a large excess sulfonyl chloride was necessa-
posed and the desired product could only be obtained ry (10 equiv.). When 5 equiv. of methanesulfonyl chlo-
in 15% yield (entry 7). The solvent used in the reac- ride were used, the yield dropped to 53%. Other
tion also seemed to be important since the same reac- vinyl alkyl sulfones, such as 3p and 3q, could also be
tion in DMF could barely happen (entry 8).
prepared smoothly using this strategy.
This transformation was next applied to a variety of
We then explored the coupling between 4-
sulfonyl chlorides as shown in Table 2. To our delight, (trifluoroACTHNUTRGNEUmGN ethyl)benzenesulfonyl chloride (1a) with
the sulfonation of enamide 2a with all the sulfonyl different enamides or enecarbamates. The representa-
chlorides we tested so far worked quite well. Elec- tive examples are shown in Table 3. N-Vinyllactams
tron-poor arenesulfonyl chlorides coupled with enam- with different ring sizes could be sulfonated smoothly
ide 2a perfectly well to provide the corresponding to give sulfones 4a and 4b, respectively. Cyclic disub-
vinyl sulfones 3a–3e in up to 92% yields. Electron- stituted enamides were also good substrates in this re-
neutral arenesulfonyl chlorides also went through this action, the desired products 4c could be isolated in
transformation smoothly to give 3f and 3g in excellent 88% yield. Acyclic enamides were also suitable sub-
yields. Electron-rich arenesulfonyl chlorides worked strates for the sulfonation to give various b-amidovin-
slightly less efficiently. Sulfones 3h and 3i were ob- yl sulfones 4d–4g in good yields. It is noteworthy that
tained in 73% and 72% yields, respectively. Chloro- the resulting b-amidovinyl sulfones were in the E-con-
benzenesulfonyl chlorides with the chloride in the figuration exclusively. Enecarbamates could be also
ortho, meta or para position all served as suitable sub- sulfonated with this method to give sulfonyl enecarba-
strates in this reaction to give the corresponding vinyl mates 4h–4j in satisfactory yields.
sulfones 3j–3l. ortho-Chlorobenzenesulfonyl chloride
We next tested more challenge substrates
was slightly less effective because of steric hinderance. (Scheme 1). E-Enamide E-5, which is not an active
Heteroarenesulfonyl chlorides could also undergo this substrate with alkylating reagents under our standard
transformation to give vinyl sulfones 3m and 3n in conditions,[12] could be sulfonated with sulfonyl chlo-
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Scheme 1. Sulfonation of enamides E-5 and Z-5.
ride 1a fluently to give vinyl sulfone 6 in 69% yield. sulfones with good to excellent yields. The functional-
Interestingly, Z-enamide Z-5 could produce the exact- ized vinyl sulfones obtained in these reactions were in
ly same product 6 with comparable yield under the the E-configuration exclusively. This protocol could
same condition, which suggested that E- and Z-enam- thus serve as a facile approach to the new type of
ides shared the same intermediate.
vinyl sulfones which might find potential applications
A possible catalytic cycle was proposed as shown in as bioactive agents. Explorations on the biological ac-
Figure 2. The Ir3+ complex was excited by visible-light tivities of the resulting b-amidovinyl sulfones such as
to generate Ir3+*, which could be quenched by sulfo- inhibition on cancer cell growth and regulation on mi-
nyl chloride 1 to give Ir4+ and sulfonyl radical 7, re- croRNA are now underway.
spectively.[13] Then sulfonyl radical 7 added to enam-
ides, such as E- or Z-5, to provide the same amido
radical 8.[12,15] Amido radical 8 was oxidized by Ir4+ to Experimental Section
give N-acyliminium cation 9 and regenarate Ir3+.
After deprotonation and tautomerization of N-acyli- General Procedure
minium cation 9 assisted by base, the desired vinyl
sulfone 3 was finally produced.
A 10-mL round-bottom flask was equipped with a rubber
septum and magnetic stir bar and was charged with 1a
In summary, we have described a mild, practical
(0.5 mmol),
2a
(0.75 mmol),
IrACTHUNGTERNNU(G ppy)2CAHTUNGTRENN(UNG dtbbpy)PF6
and environmentally friendly procedure to prepare b-
amidovinyl sulfones. Starting from readily available
enamides and sulfonyl chlorides, sulfonation of enam-
ides could occur via a sulfonyl radical generated by
visible-light photoredox catalysis under the optimized
conditions at room temperature. This transformation
could tolerate different functional groups and resulted
in the formation of the corresponding b-amidovinyl
(0.005 mmol), Na2HPO4 (1.5 mmol,). The flask was evacuat-
ed and backfilled with argon for 3 times. CH3CN (5 mL)
was added with a syringe under argon. The mixture was
then irradiated by a 3 W blue LED strip at a distance of
5 cm. After the reaction was complete (as judged by TLC
analysis), the mixture was poured into a separatory funnel
containing 20 mL H2O and 30 mL EtOAc. The layers were
separated and the aqueous layer was extracted with EtOAc
(2ꢂ30 mL). The combined organic layers were dried over
Na2SO4 and concentrated under reduced pressure. The
crude product was purified by flash chromatography (hex-
anes/EtOAc) on silica gel to afford the desired product.
Acknowledgements
Financial support from the National Basic Research Program
of China (No. 2011CB935800), the National Natural Science
Foundation of China (2112109, 21102072), the Natural Sci-
ence Foundation of Jiangsu Province (BK2012012) and State
Key Laboratory of Bioorganic & Natural Products Chemis-
try, Shanghai Institute of Organic Chemistry, CAS is ac-
knowledged.
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6 C H Functionalization of Enamides: Synthesis of b-
Amidovinyl Sulfones via Visible-Light Photoredox Catalysis
Adv. Synth. Catal. 2013, 355, 1 – 6
Heng Jiang, Xuejiao Chen, Yan Zhang,* Shouyun Yu*
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