Communication
1,4-diketones in reasonable yields (Table 2, entries 2–9). More-
over, the disubstituted substrates (Table 2, entry 10) and the
substrates bearing a sensitive free hydroxy group (Table 2,
entry 11) were also suitable. It is worthy to note that, the ben-
zene ring of the substrate can be replaced by heteroaryl ring
to provide furyl-substituted and thienyl-substituted 1,4-dike-
tones in 66 and 64% yields, respectively (Table 2, entries 12
and 13).
can be easily transferred to various biologically important
heterocycles, such as pyrrole 3a, furan 4a and thiophene 5a
in excellent isolated yields (Scheme 2).
To further expand the substrates scope, we next applied this
visible-light photocatalytic CÀC bond formation reaction to the
construction of unsymmetrical 1,4-diketones. It is noteworthy
that an excess of one b-ketosulfone is required under the best
reaction conditions in order to compete with undesired self-
coupling. As shown in Table 3, substituents with different elec-
tronic nature on the aryl ring, are well tolerated during the re-
action. Either the unsymmetrical 1,4-diketones bearing two dif-
Scheme 2. Two-step construction of heterocycles from b-ketosulfone 1a. Re-
action conditions: i). NH4OAc, AcOH, reflux; ii). Ac2O, HCl, 10–608C; iii). Law-
esson reagent, THF, 558C.
Table 3. . Construction of unsymmetrical 1,4-diketones.[a]
To obtain some mechanism insight for this process, we in-
vestigated the reaction of 1a in the presence of radical inhibi-
tor TEMPO under standard conditions. This visible light pro-
moted CÀC bond formation reaction is completely inhibited,
which indicated that a radical process is presumably involved
in the reaction pathway. Furthermore, the results of the reac-
tion of 1a with and without of light irradiation revealed that
a radical chain process is not the major reaction pathway,
although it cannot be fully ruled out (see the Supporting Infor-
mation for details).
Based on these results, a possible reaction pathway is pro-
posed in Scheme 3. The excited state of photoredox catalyst
RuII* was reductively quenched by Et3N to deliver the strong
reductant RuI (À1.33 V vs. SCE in CH3CN). Then single-electron
reduction of b-ketosulfone 1 (Ar=Ph, À0.79 V vs. SCE in
CH3CN)[19] with RuI afforded the key radical intermediate A
through the breakage of the a-CÀS bond (not PhÀSO2 bonds).
To better understand this selectivity, a theoretical calculation
was performed and the bond length of CH2ÀS bond (1.835 ꢁ)
was found longer than PhÀS bond (1.807 ꢁ), which indicated
the former more likely to undergo reductive cleavage pro-
cess.[11,13] On the other hand, another molecule of b-ketosul-
fone 1 would facilely isomerize to their enolate form B under
basic conditions. Subsequently, radical addition of A to the a-
position of B generated radical anion C, wherein the elimina-
tion of a phenylsulfonyl radical afforded enolate D.[20] Finally,
protonation of D gave the desired CÀC formation product 2,
whereas competitive hydrogen abstraction of A would provide
acetophenone as the byproduct.
[a] Reaction conditions: 1c, 1l–m (0.5 mmol); 1a, b, d, f, and
(0.75 mmol), [Ru(bpy)3Cl2.6H2O] (2.0 mol%), Et3N (5.0 equiv), K2CO3
(5.0 equiv), 3W white LED, CH3CN (10 mL), at room temperature.
g
a
[b] Yield of isolated product.
ferent phenyl rings (2ca) or the products containing with a
heteroaryl ring and a phenyl ring (2la–ma) can be successfully
prepared, albeit with moderate yields.
Perhaps more important, the 1,4-diketone 2a could be ob-
tained in 40% yield after a two-step sequential reaction from
commercially available phenylethyne without the purification
of b-ketosulfone intermediate [Eq. 1]. In addition, direct sun-
light irradiation of the reaction of 1a gave the desired 1,4-dike-
tone 2a with 53% yield after 8 h [Eq. 2].
According to reported methods,[17] the formed 1,4-diketone
from b-ketosulfone using a visible light photocatalytic strategy
In summary, we have developed an efficient visible light-in-
duced CÀS bond activation reaction of b-ketosulfones that op-
erates under mild reaction conditions. It renders this reaction
a useful method to various symmetrical and unsymmetrical
1,4-diketones from readily available starting materials. Further-
more, the desired products were well applied to the construc-
tion of synthetically significant pyrrole, furan, and thiophene
frameworks. The continuous discovery of other stable and
easily prepared organosulfur precursors is currently underway
in our laboratory.
Chem. Eur. J. 2014, 20, 3045 – 3049
3047
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