R. Chawla et al. / Tetrahedron Letters xxx (xxxx) xxx
3
reactions [10]. Very recently, Wei et al. have employed these com-
pounds as sulfonylating agents as well [11]. Consequently, herein
we describe the synthesis of (E)-vinyl sulfones from cinnamic acids
and arylazo sulfones mediated by visible light under photocata-
lyst- and external oxidant-free conditions (Scheme 1e).
(entries 15–18) and it was found that the ratio 2:1 of 1,4-dioxane/
H2O worked best for our reaction (entry 17). This observation can
be attributed to the fact that water as a solvent may promote the
homophotolysis of 2a but its addition in excess may lead to solu-
bility issues. Replacing the base Cs2CO3 with Na2CO3, K2CO3 or
1,8-diazabicyclo[5.4.0]- undec-7-ene (DBU)) did not show any bet-
ter results and Cs2CO3 remained the choice of base for the reaction
(entry 17 vs entries 19–21). The use of 1.5 equiv. of 2a led to a sub-
stantial decrease in the yield of 3a (entry 22). An increase in the
loading of 2a from 2.2 equiv. to 2.5 equiv. was conducive to the
efficiency of the reaction (entry 23) but further increase in the
amount of 2a and Cs2CO3 produced no significant increment in
the yield of 3a (entry 24). However, the yield of 3a decreased with
the decrease in the amount of Cs2CO3 (entry 25). Increasing the
reaction time to 24 h led to no further increase in the yield of
the product but lowering the reaction time was detrimental to
the efficieny of the reaction (entries 26 and 27). We next examined
the visible light source. In the presence of 3 W white LED and green
LED lamps, the product 3a was isolated in 32% yield and traces
respectively (entries 28 and 29). On dilution as well as on decreas-
ing the amount of solvent, the yield of product decreased (entries
30 and 31). As regards the use of arylazo sulfones, derivatives other
than 2a such as 4-methylphenylazo mesylate, 4-chlorophenylazo
mesylate or 4-fluorophenylazo mesylate were also tested but the
yield of the desired product 3a was observed to be lower than
the model reaction in all these cases (Scheme 2).
To probe the feasibility of our strategy, a model reaction was
performed using 4-methoxyphenylazo mesylate (2a) as the sul-
fonylating agent for the decarboxylative sulfonylation of cinnamic
acid 1a under 3 W blue LED lamps irradiation and photocatalyst-
free conditions (Table 1). Most satisfactorily, under these condi-
tions the desired product 3a was obtained in 32% yield (Table 1,
entry 1). In the absence of Cs2CO3, KI or light, no product could
be obtained similar to the observations of the previous report
[6l] (Table 1, entries 2–4). The use of a photocatalyst was found
to have no significant impact on the efficiency of the reaction
(Table 1, entries 5–7). In fact, the lower yields of the product can
be attributed to the partial absorption of light by the
photocatalysts leading to less dissociation of 2a into sulfonyl
radical.
Subsequently, a variety of solvents and solvent-systems were
screened to decide the best media for our reaction (Table 1, entries
8–14) and the best yield of the product was obtained in 1,4-diox-
ane/H2O (15:1) (entry 12). The ratio of solvents was also optimized
O
S
O
KI, Cs2CO3, open flask
Having identified the optimal reaction conditions, we next pro-
ceeded to examine the substrate scope of the reaction with respect
to cinnamic acids using 2a. A good variety of substituents on the
aromatic ring of cinnamic acids were amenable with our protocol
(Table 2, 3b-i). Electron-donating (Me, OMe) as well as electron-
withdrawing groups (halogens) on the aromatic ring of cinnamic
acids afforded the desired products in moderate yields. Cinnamic
COOH
X
N2SO2Me
Ph
Me
Ph
1,4-dioxane/H2O, blue LEDs, rt
3a
1a
2
63%, X = Me
41%, X = Cl
54%, X = F
Scheme 2. Reaction of cinnamic acid 1a with other arylazo sulfones 2.
Table 2
Substrate scope.a
a) Evidence in support of radical pathway
O
S
O
O
S
O
Standard reaction
KI, Cs2CO3, open flask
COOH
4-MeOC6H4N2SO2Me
COOH
Ar
Ar'N2SO2R
2
Ph
Ar
R
Ph
Me
N
conditions
1
3b,c
1,4-dioxane/H2O, blue LEDs, rt
TEMPO (2.5 equiv.)
traces
O
S
O
O
S
O
O
S
O
X= Me, 3b, 69%, 15h
X= OMe, 3c, 58%, 15h
X= F, 3d, 61%, 22h
X= Cl, 3e, 70%, 22h
X= Br, 3f, 64%, 22h
O
X
detected by
LC-MS
MeO
3g, 62%, 15h
3a, 77%, 18h
b) Role of air
COOH
O
S
O
Standard reaction
conditions
O
S
O
S
O
O
S
O
O
S
O
4-MeOC6H4N2SO2Me
Ph
Ph
Me
F
O
N2 atmosphere
traces
S
3k, 72%, 20h
3i, 53%, 24h
3h, 65%, 16h
3j, traces, 24h
Scheme 3. Control experiments.
O
S
O
O
S
O
Y= Me, 3m, 83%, 12h
Y= OMe, 3n, 73%, 12h
Y= F, 3o, 67%, 15h
Y= Cl, 3p, 72%, 15h
Y= Br, 3q, 70%, 15h
Y
3l, 79%, 12h
O
S
O
O
S
O
O
S
O
3s, 78%, 12h
3t, 65%, 12h
3r, 80%, 12h
O
S
O
S
O
O
3u, 0%, 24h
3v, 0%, 24h
aReaction conditions: 1a (0.5 mmol), 2a (2.5 equiv.), KI (1.0 equiv.), Cs2CO3
(3.0 equiv.), 1,4-dioxane/H2O 2:1 (1 mL), 3 W blue LEDs and open flask at rt,
12–24 h.
bIsolated yield.
cAll compounds are known in literature [3c,6d,12] and gave satisfactory
spectral (1H NMR, 13C NMR and HRMS) data.
Fig. 1. Result of irradiation ON/OFF experiment.
Please cite this article as: R. Chawla, S. Jaiswal, P. K. Dutta et al., Photocatalyst-free visible light driven synthesis of (E)-vinyl sulfones from cinnamic acids
Chawla, Ruchi
