Visible-Light Photocatalytic Bicyclization of 1,7-Enynes toward Functionalized Sulfone-Containing Benzo[a]fluoren-5-ones

Article abstract of DOI:10.1002/adsc.201700124

A new visible-light photocatalytic arylsulfonylation and bicyclization of C(sp³)-tethered 1,7-enynes with sulfinic acids has been developed, delivering functionalized sulfone-containing benzo[a]fluoren-5-ones with generally good yields. This Eosin Y-catalyzed approach makes use of visible light as a safe and eco-friendly energy source to drive cascade cyclization reactions, resulting in continuous multiple bond-forming events including C–S and C–C bonds to efficiently construct polycyclic-linked alkyl aryl sulfones. (Figure presented.).

Full text of DOI:10.1002/adsc.201700124

Title: Visible-Light Photocatalytic Bicyclization of 1,7-Enynes toward
Functionalized Sulfone-Containing Benzo[a]fluoren-5-ones
Authors: Bo Jiang, Min-Hua Huang, Yi-Long Zhu, Wen-Juan Hao, Ai-
Fang Wang, De-Cai Wang, Feng Liu, Ping Wei, and Shu-
Jiang Tu
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Adv. Synth. Catal. 10.1002/adsc.201700124
Link to VoR: http://dx.doi.org/10.1002/adsc.201700124

10.1002/adsc.201700124
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Visible-Light Photocatalytic Bicyclization of 1,7-Enynes toward
Functionalized Sulfone-Containing Benzo[a]fluoren-5-ones
Min-Hua Huang,a,b,1 Yi-Long Zhu,a,b,1 Wen-Juan Hao,b Ai-Fang Wang,b De-Cai Wang,*,a Feng Liu,b
Ping Wei,a Shu-Jiang Tu,*,b and Bo Jiang*,b
a
Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing, 210009, Jiangsu, P. R. China, E-
mail: dcwang@njtech.edu.cn (DCW); Tel./fax: +86 25 58139397
School of Chemistry and Materials Science, Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional
b
Materials, Jiangsu Normal University, Xuzhou, 211116, P. R. China; E-mail: laotu@jsnu.edu.cn (SJT);
jiangchem@jsnu.edu.cn (BJ); Tel./fax: +86 516 83500065
These authors are equal to this work
1
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Accordingly, their synthesis has been actively
pursued in the organic community.[9] For instance,
Lei and Wang independently reported visible-light
photocatalytic sulfonyl radical additions to
unsaturated systems toward organosulfones.[10]
However, to the best of our knowledge, there is no
example of metal-free visible-light induced
bicyclization of 1,7-enynes for the construction of
sulfone-containing molecules using sulfinic acids as a
sulfonylating agent.
Abstract.
A
new
visible-light
photocatalytic
arylsulfonylation and bicyclizations of C(sp3)-tethered
1,7-enynes with sulfinic acids has been developed,
delivering
functionalized
sulfone-containing
benzo[a]fluoren-5-ones with generally good yields. This
Eosin Y-catalyzed approach makes use of visible light as
a safe and eco-friendly energy source to drive cascade
cyclization reactions, resulting in continuous multiple
bond-forming events including C−S and C−C bonds to
efficiently construct polycyclic-linked alkyl arylsulfone.
Keywords: Photocatalysis; Arylsulfonylation; 1,7-
Enynes; Bicyclizations; Benzo[a]fluorenes
Among the most attractive and appealing synthetic
strategies, visible light photoredox catalysis (VLPC)
has drawn special attention due to its intrinsic
characteristics of sustainability and green synthesis in
recent years.[1] Since the pioneering reports by
MacMillan, Yoon, and Stephenson groups,[2]
significant efforts have been directed to develop
visible light photoredox catalysis, which has made it
more powerful and applicable. Generally, the vast
majority of well-established VLPC strategies for
organic transformations include [2
cycloadditions,[3] α-functionalization of carbonyls,[4]
dehalogenation decarboxylative
reactions,[5]
couplings,[1e] radical C-H functionalization of
aromatic and oxidative
compounds,[6]
+
2]
Scheme 1. Photocatalytic Bicyclization of 1,7-Enynes for
the Synthesis of Benzo[a]fluoren-5-ones
Oxidative radical 1,7-enyne-cyclizations have
gradually become a powerful platform for rapid
collection of cyclic compounds with all-carbon
quaternary stereocenters via synergistic processes
across the C=C and CC bond systems in a single
step fashion.[11] Recently, we reported the addition of
sulfonyl radicals to N-tethered 1,7-enynes to access
sulfone-containing 3,4-dihydroquinolin-2(1H)-ones
(Scheme 1a) using TBHP as an oxidant.[12] During
this project, we planned the preparation of C(sp3)-
annulations.[1n,7] Among them, the last provides a
straightforward and atom economic protocol for the
construction of complex molecular structures with
high functional group compatibility as VLPC usually
behaves as an oxidation promoter to yield highly
reactive intermediates for further transformations. On
the other hand, organosulfones could serve as the
versatile precursors in numerous total syntheses and
display a range of significant biological activities.[8]
1
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10.1002/adsc.201700124
Advanced Synthesis & Catalysis
tethered 1,7-enyne-anchored starting materials by
taking advantage of a methodology in which visible-
light photocatalytic tandem bicyclizations across its
C=C and CC  system led to functionalized sulfone-
containing benzo[a]fluoren-5-ones (Scheme 1b). As
expected, this photoredox catalysis proceeded
smoothly, enabling sulfonyl radical-triggered 6-exo-
dig/5-endo-trig bicyclization to directly build up a
wide range of arylsulfonylated benzo[a]fluoren-5-
ones using Eosin Y as a photoredox catalyst. Herein,
we report this interesting visible-light photoredox
transformation. Notably, this is the first visible-light
photocatalytic bicyclizations of C(sp3)-tethered 1,7-
enynes for the metal-free synthesis these special
tetracyclic sulfone-containing benzo[a]fluoren-5-ones
with generally good yields using sulfinic acid as a
sulfonylation reagent.[13]
Our initial investigation commenced with the
treatment of the preformed C(sp3)-tethered 1,7-enyne
1a with p-tolyl sulfinic acid (2a) in a 1:2 mole ratio to
search the optimal conditions. The results are
summarized in Table 1. The reaction was carried out
in MeCN under Ar conditions at room temperature
using Eosin Y (1.0 mol%) as a photoredox catalyst
and K2CO3 (1.5 equiv) as a base promoter, and the
desired product 3a was generated in 58% yield.
Afterward, we attempted to utilize other photoredox
catalysts generally used in the radical reaction, such
using a variety of C(sp3)-tethered 1,7-enynes and aryl
sulfinic acids. We found that various substituents on
Table 1. Optimization of the reaction conditionsa
Entry Cat.
(1.0 mol %)
Eosin Y
Base
Solvent
Yield
(%)b
58
1
2
K2CO3
MeCN
MeCN
Ru(bpy)3Cl2· K2CO3
6H2O
30
3
4
fac-Ir(ppy)3 K2CO3
[Ru(bpy)3] K2CO3
(PF6)2
MeCN
MeCN
41
46
5
Eosin Y
Cs2CO3
MeCN
52
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
KHCO3
KOH
MeCN
MeCN
MeCN
MeCN
toluene
DCM
EA
acetone
THF
DCE
46
42
31
49
NR
63
65
62
73
45
30
K2HPO4
Na2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
as
Ru(bpy)3Cl2·6H2O,
fac-Ir(ppy)3
and
MeOH
1,4-dioxane 54
DMF
THF
[Ru(bpy)3](PF6)2, to improve this transformation,1 but
all of these catalysts showed inferior catalytic
performances under standard conditions, compared
with Eosin Y (entry 1 vs entries 2-4). As the next
optimization step, we performed the screening of a
variety of bases, including Cs2CO3, KHCO3, KOH,
K2HPO4, and Na2CO3, for this bicyclization by using
Eosin Y as the reaction catalyst (entries 5-9). All
these attempted bases did not improve the reaction
efficiency and the obtainable yield of 3a.
Subsequently, we investigated the solvent effect for
this transformation with use of various solvents such
as toluene, dichloromethane (DCM), ethyl acetate
(EA), acetone, tetrahydrofuran (THF), 1,2-
dichloroethane (DCE), methanol (MeOH), 1,4-
dioxane, and N,N-dimethylformamide (DMF). Use of
toluene as reaction media completely suppressed the
reaction process (entry 10) while in other four cases
of DCM, EA, acetone, and THF, the reaction could
work more efficiently, delivering higher yields of the
target product 3a than acetonitrile (entry 1 vs entries
11-14). Among them, THF gave the best outcome
(73% yield, entry 14). Other polar solvents, such as
DCE, MeOH, 1,4-dioxane, and DMF, all met with
little success in the present photocatalytic
bicyclization as lower conversions were observed
(entries 15-18). Without visible light, the reaction did
not proceed (entry 19). A significant drop in the yield
of 3a was obtained when the identical reaction was
conducted under air atmosphere (entry 20).
37
NRc
20
Eosin Y
K2CO3
THF
32d
a)Reaction conditions: C(sp3)-tethered 1,7-enyne 1a (1.0 equiv, 0.5 mmol),
p-tolyl sulfinic acid (2a, 2.0 equiv, 1.0 mmol), photocatalyst (1.0 mol%),
and base (1.5 equiv), solvent (2.0 mL), at room temperature under Ar
conditions, for 12 hours. b)Isolated yield based on substrate 1a. c)In the
darkness. d)Under air atmosphere. N.R. = no reaction.
the aromatic ring of both 1,7-enynes and sulfinic
acids were proven not to hamper this photoredox
catalysis, and a wide range of sulfone-containing
benzo[a]fluoren-5-ones 3a-3mm with structural
diversity can be generated in acceptable yields and a
functional-group-compatible fashion. As shown in
Scheme 2, p-tolyl (2a) and 4-chlorophenyl (2b)
sulfinic acids were first selected as representative
sulfonyl radial donators to evaluate the influence of
substituents (R) in the arylalkynyl moiety of 1. As
anticipated, all these transformations could occur
readily to access the expected benzo[a]fluoren-5-ones
3a-3l in 57%-88% yields. Various substituents
resided at different positions on the phenyl ring, such
as F (1b), Me (1c), Et (1d), MeO (1e), Cl (1f and 1g),
Br (1h), and t-Bu (1i), were compatible. The presence
of strong electron-donating substituents (t-butyl and
methoxy) at the para-position seemed to improve the
efficiency of the reaction, as the corresponding
products 3e and 3k-l could be isolated in slightly
higher yields. Unluckily, n-butyl counterpart 1j was
not an adaptable substrate for this reaction (Scheme 2,
3m), which may be caused by the relative instability
With the optimal reaction conditions in hand, (Table 1,
entry 14), we turned our attention to explore the
generality of this photocatalytic bicyclization by
2
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10.1002/adsc.201700124
Advanced Synthesis & Catalysis
Besides, heteroaryl linked 1,7-enynes 1z and 1aa
would be accommodated, confirming the reaction
efficiency, as indeno[1,2-h]quinolin-5-one 3dd and
fluoreno[2,1-b]thiophen-4-one 3ee were generated in
62% and 63% yields, respectively.
To further expand the scope of the reaction, 1,7-
enyne 1e was subjected with the reaction of aryl
sulfinic acids with different substituted patterns and
electronic properties. Sulfinic acids 2 bearing either
electronically rich (t-Bu, 2b and MeO, 2c ), neutral
(H, 2d) or poor (Cl, 2e; Br, 2f and CF3, 2g) groups at
different positions of the phenyl ring participated in
this photocatalysis, enabling sulfonyl radical-
triggered 6-exo-dig/5-endo-trig bicyclization to pave
the way for the collection of arylsulfonylated
benzo[a]fluoren-5-ones 3ff-3kk in 58%-85% yields.
Generally, electron-donating substituents on the
phenyl ring of 2 favor this transformation more than
their electron-deficient ones, as products 3ff and 3gg
could be provided in slightly higher yields (3ff and
3gg vs 3kk). A sterically demanding 2-chlorophenyl
analogue 2e proved to be high reactivity, in which
benzo[a]fluoren-5-one 3ii was furnished in 58% yield.
Additionally, substrate 2g carrying a strong electron-
withdrawing group (CF3) was also a suitable
sulfonylation reagent (3kk, 60%). Additional, alkyl
counterparts such as methyl (2h) and cyclopropyl (2i)
readily participated in the present photocatalytic
bicyclization, affording the corresponding sulfone-
containing benzo[a]fluoren-5-one products 3ll and
3mm in 50% and 62% yields, respectively. The
resultant densely functionalized benzo[a]fluoren-5-
one products were characterized by their NMR and
HRMS.
Scheme 2 Substrate Scope of Visible-Light Photoredox
Bicyclization
of the in-situ generated vinyl radical intermediate
during the reaction process. Next, we conducted
different functional groups (R1) like fluoro, chloro,
methyl and methoxy located at the 4- or 5-position of
the internal arene ring of 1,7-enynes 1 to explore its
synthetic utility. All those substituents (1k-1v) were
well tolerated under this system, giving access to the
corresponding benzo[a]fluoren-5-ones 3n-3y with
yields ranging from 49% to 89%. Subsequently,
electronic nature of substituents (R2) on the
arylalkenyl moiety was probed. Both electron-
donating (methyl, 1w) and electron-withdrawing
(chloro, 1x-1y) groups attached by different positions
were successfully engaged in the current
bicyclization reactions under the optimized
conditions (3z-3cc, Scheme 2). When chloro
functionality was installed at the meta-position of the
arylalkenyl moiety, two regioisomers 3bb and 3cc
were isolated in 27% and 30% yields, respectively.
Scheme 3. Control Experiments
In order to gain insights into the reaction mechanism
for accessing benzo[a]fluoren-5-ones 3, several
control experiments were performed, which are
shown in Scheme 3. The 1,7-enyne 1a was reacted
with 2a under standard conditions in the presence of
the radical scavenger TEMPO (2,2,6,6-tetramethyl-1-
piperidinyloxy)
or
BHT
(2,6-di-tert-butyl-4-
methylphenol) (1.5 equiv), but without observation of
the desired benzo[a]fluoren-5-one 3a (Scheme 3a),
indicating the possibility of a radical mechanism.
Next, without p-tolyl sulfinic acid, 1,7-enyne 1a did
not be converted into the expected benzo[a]fluoren-5-
one 4 (Scheme 3b), confirming that the arylsulfonyl
3
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10.1002/adsc.201700124
Advanced Synthesis & Catalysis
the methodology are currently underway in our
laboratory.
radical, generated in situ from aryl sulfinic acid,
triggered
addition-cyclization.
Accordingly,
sulfonylation occurred prior to the cyclization step.
Additionally, treatment with 1a and 2a in the absence
of K2CO3 only gave a trace amount of product 3a
under visible light irradiation (Scheme 3c), showing
that the base plays a pivotal role in the success of this
transformation.
On the basis of these results and previous report
survey,[10] a reasonable mechanism for the formation
of products 3 is proposed in Scheme 4. First, sulfinate
A is generated in situ from acid-base neutralization
Experimental Section
General Procedure for the Preparation of 3
Example for the synthesis of 3a: 11-phenyl-6a-
(tosylmethyl)-6,6a-dihydro-5H-benzo[a]fluoren-5-one
In dry Schlenk tube, 3-phenyl-1-(2-(phenylethynyl)phenyl)
but-3-en-1-one (1a, 0.5 mmol, 161 mg), Eosin Y (0.005
mol, 3.24 mg), K2CO3 (0.75 mmol, 104 mg) and THF (2.0
mL) were continuously added under argon atmosphere.
Then 4-methylbenzenesulfinic acid (2a, 1.0 mmol, 156
mg) was added into the above reaction system. The
solution was stirred at room temperature with the
irradiation of blue LEDs (12 W) for 12.0 h. After reaction
was completed monitored by TLC, the organic solvent was
removed under vacuum, and the residue was purified by
silica gel column with petrolem ether/ethyl acetate to
afford product 3a.
reaction between sulfinic acid
2
with base.
Subsequently, the excited state of Eosin Y* reacts
with free sulfinate A to generate Eosin Y radical
anion along with the formation of sulfonyl radical
B.[1b,10a] The radical addition of sulfonyl radical B to
alkenyl unit of 1,7-enynes 1 occurs, followed by
intramolecular 6-exo-dig/5-endo-trig bicyclization to
produce radical intermediate D.[11h] A single electron
transfer (SET) and proton transfer (PT) between
intermediate D and Eosin Y radical anion give the
final products 3 and Eosin Y–H hydride, which reacts
with proton to release H2 and regenerate Eosin Y.[10b]
Acknowledgements
We are grateful for financial support from the NSFC (No.
21232004, 21472071 and 21602087), PAPD of Jiangsu Higher
Education Institutions, the Outstanding Youth Fund of JSNU
(YQ2015003), NSF of Jiangsu Province (BK20151163 and
BK20160212), the Qing Lan Project and NSF of Jiangsu
Education Committee (15KJB150006).
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Scheme 4 Proposed Reaction Mechanism
In summary, we have established a new and visible-
light
photocatalytic
6-exo-dig/5-endo-trig
bicyclization of the preformed C(sp3)-tethered 1,7-
enynes and aryl sulfinic acids, by which a broad
range
of
functionalized
sulfone-containing
benzo[a]fluoren-5-ones with generally good yields
were synthesized using commercially available Eosin
Y. This metal-free protocol features broad scope of
substrates, high functional-groups tolerance and
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conditions, enabling radical-triggered multiple bond-
forming events including C−S and C−C bonds to
efficiently
construct
polycyclic-linked
alkyl
arylsulfones. Ongoing research including further
mechanistic details and the potential application of
4
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10.1002/adsc.201700124
Advanced Synthesis & Catalysis
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5
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10.1002/adsc.201700124
Advanced Synthesis & Catalysis
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