Angewandte
Chemie
DOI: 10.1002/anie.201208380
Visible-Light Photocatalysis
Tandem Cyclizations of 1,6-Enynes with Arylsulfonyl Chlorides by
Using Visible-Light Photoredox Catalysis**
Guo-Bo Deng, Zhi-Qiang Wang, Jia-Dong Xia, Peng-Cheng Qian, Ren-Jie Song, Ming Hu, Lu-
Bin Gong, and Jin-Heng Li*
The tandem cyclization strategy is of continuing interest in the
field of organic chemistry because it is a uniquely powerful
tool for the synthesis of polysubstituted polycyclic com-
pounds.[1–3] One typical transformation is the cyclization of
enynes, in particular the cyclization of 1,6-enynes because this
transformation offers unparalleled efficiency, atom economy,
and operational simplicity in the assembly of synthetically
versatile polycyclic compounds, and still remains an impor-
tant challenge. Generally, there are two basic methods for 1,6-
enyne cyclizations: through metal catalysis or a radical
reaction (thermal[2g] or photochemical cyclizations).[2,3] How-
ever, many enyne cyclizations are carried out at relatively
high reaction temperatures, and the photochemical cycliza-
tions often involve irradiation with ultraviolet light; more-
over, the substrate scope of these reactions is restricted to
endiynes and enyne allenes.[3] Common enynes can not absorb
either ultraviolet or visible light efficiently, thereby limiting
their applications in organic synthesis.
fonyl chlorides; this tandem cyclization is triggered by visible-
light photoredox catalysis and involves the use of 1,6-enynes
as radical acceptors, thus allowing the formation of a diverse
range of 10a,11-dihydro-10H-benzo[b]fluorene architec-
tures[8] in one step (Scheme 1). This work represents the
Scheme 1. Tandem cyclization of alkynes with alkenes and ArSO2Cl.
bpy=bipyridine.
first example of 1,6-enyne cyclization by using the visible-light
photoredox catalysis strategy. Most importantly, we show that
arylsulfonyl chlorides play two roles: they provide an ortho-
Recently, visible-light photoredox catalysis has attracted
great interest in organic synthesis community because visible
light is environmentally benign, mild, easy to handle, infin-
itely available, and has promising applications in industry.[4–6]
However, despite progress in this area, the in situ generation
À
C(aryl) H bond to participate in the tandem cyclization and
on cleavage of the SO2Cl group aryl radicals are formed.
We initially explored the tandem cyclization reaction of
methyl 2-(hydroxy(2-(phenylethynyl)phenyl)methyl)acrylate
(1a) with 4-nitrobenzene-1-sulfonyl chloride (2a), [Ru-
(bpy)3Cl2]·6H2O, Na2CO3, and 36 W compact fluorescent
light in MeCN at 408C: The desired 10a,11-dihydro-10H-
benzo[b]fluorene 3aa was formed, albeit in low yield, after
12 h (Table 1, entry 1).[9] Interestingly, the yield of 3aa was
dramatically enhanced to 79% when the reaction was
performed with 5 W blue LED light (entry 2). Extensive
screening of bases revealed that the use of an inorganic base
(Na2CO3 or NaOAc) facilitated the reaction, but the use an
organic base (Et3N) impeded it (entries 2–4). In addition, the
amount of base also affected the reaction: The yield of 3aa
was lowered to 64% when 2 equivalents of Na2CO3 were used
(entry 5). The reaction could proceed in the absence of base,
but the yield was decreased (entry 6). Next, both solvent and
reaction temperature were investigated (entries 2 and 7–11):
The reaction in MeCN at 408C gave the best results (entry 2).
The amount of Ru catalyst affected the reaction yield
(entries 12 and 13): The reaction in the presence of
10 mol% of Ru gave the identical results to those in the
presence of 5 mol% Ru, whereas the reaction with 2 mol% of
Ru led to a low yield of product 3aa. Two other visible-light
photoredox catalysts, [Ir(ppy)3] and Eosin Y, were also tested
(entries 14 and 15). With [Ir(ppy)3], a good yield was still
achieved (entry 14). Notably, the current reaction could be
successfully carried out under transition-metal-free condi-
tions: In the presence of Eosin Y, the desired 10a,11-dihydro-
À
of aryl radicals for C C(aryl) bond formation by visible light
photoredox catalysis is quite rare.[5] Efforts from the groups of
Deronzier,[5a–e] Sanford,[5f] and Kçnig[5g–i] have been devoted
to investigating aryldiazonium salts as a source of aryl radicals
À
for the C H arylation. Very recently, we found that arylsul-
fonyl chlorides could also be utilized as aryl radical precursors
for the sequential arylation of alkynes and carbocyclization
3
[5j,7]
À
with benzylic C(sp ) H bonds.
We hypothesized that
standard enynes could not absorb visible light efficiently, but
they may serve as a platform to trap radicals. Herein, we
report a new tandem cyclization of 1,6-enynes with arylsul-
[*] G.-B. Deng, Z.-Q. Wang, J.-D. Xia, P.-C. Qian, R.-J. Song, L.-B. Gong,
Prof. Dr. J.-H. Li
State Key Laboratory of Chemo/Biosensing and Chemometrics
College of Chemistry and Chemical Engineering, Hunan University
Changsha 410082 (China)
E-mail: jhli@hnu.edu.cn
Z.-Q. Wang, J.-D. Xia, M. Hu
Key Laboratory of Chemical Biology & Traditional Chinese Medicine
Research (Ministry of Education), Hunan Normal University
Changsha 410081 (China)
[**] We thank the Natural Science Foundation of China (No. 21172060)
and Fundamental Research Funds for the Central Universities
(Hunan University) for financial support.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2013, 52, 1535 –1538
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1535
Deng, Guo-Bo
