Visible-Light-Mediated Tandem Sulfonylation/Cyclization of Vinyl Azides with Sulfonyl Hydrazines for the Synthesis of 6-(Sulfonylmethyl)phenanthridines under Mild Conditions

Article abstract of DOI:10.1055/s-0037-1611758

A visible-light-mediated tandem sulfonylation/cyclization of vinyl azides with sulfonyl hydrazines was developed that provides an efficient and simple strategy for the synthesis of valuable 6-(sulfonylmethyl)phenanthridines with a broad substrate scope an

Full text of DOI:10.1055/s-0037-1611758

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© Georg Thieme Verlag Stuttgart · New York
2019, 30, A–F
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L.-L. Mao et al.
Letter
Synlett
Visible-Light-Mediated Tandem Sulfonylation/Cyclization of Vinyl
Azides with Sulfonyl Hydrazines for the Synthesis of 6-(Sulfonyl-
methyl)phenanthridines under Mild Conditions
Liu-Liang Mao*a
Li-Xia Quanb
Xian-Hong Zhua
Cong-Bin Jib
R1
R1
O
Ru(bpy)3Cl2 6H2O
N3
N
NHNH2
O
R3
S
+
R2
S
R3
TBPB, Na2CO3, MeCN
Blue LEDs, rt, Ar
O
An-Xi Zhou*a
Fayun Chenb
Da-Gui Zhenga
0
0
0
0-0
0
0
2-2
1
4
3-0
9
8
8
O
R2
28 examples, up to 84%
TBPB = tert-butyl peroxybenzoate
a Key Laboratory of Applied Organic Chemistry, Higher Institutions of
Jiangxi Province, Shangrao Normal University, Shangrao 334001,
P. R. of China
maoll14@lzu.edu.cn
zhouanxi@hotmail.com
b Shangrao Normal University, College of Chemistry and Environment
Science, Shangrao 334001, P. R. of China
Received: 31.01.2019
Accepted after revision: 27.02.2019
Published online: 10.04.2019
protocols is still highly desirable because of the extensive
uses of functionalized phenanthridines.
Visible-light-mediated organic photochemical synthesis
has recently attracted particular attention due to its envi-
ronmentally friendly and sustainable nature, as well as its
mild operating conditions; consequently, great progress has
been made in this area.11 In particular, the synthesis of the
phenanthridine skeleton through photoredox catalysis has
received a large amount of attention.12 However, the major-
ity of the resulting methodologies exhibit limited substrate
scopes, and only a few examples using vinyl azide sub-
strates have been explored.13
DOI: 10.1055/s-0037-1611758; Art ID: st-2019-w0052-l
Abstract A visible-light-mediated tandem sulfonylation/cyclization of
vinyl azides with sulfonyl hydrazines was developed that provides an ef-
ficient and simple strategy for the synthesis of valuable 6-(sulfonyl-
methyl)phenanthridines with a broad substrate scope and satisfactory
yields under mild conditions.
Key words photochemical reaction, tandem reaction, sulfonylation,
cyclization, sulfonyl hydrazines, phenanthridines
Phenanthridine and its derivatives represent privileged
subunits in materials chemistry and in medical chemistry
that have gained extensive attention due to their optoelec-
tronic properties and unique biological activities [Scheme
1(a)].1 Therefore, a considerable number of strategies have
been developed for the construction of functionalized
phenanthridines. Since its discovery in the 1980s, the
Bischler–Napieralski cyclization has been used extensively
to provide access to phenanthridines; however, this ap-
proach suffers from poor functional-group tolerance due to
the harsh conditions required.2 To overcome this limitation,
considerable efforts have been made to develop efficient
and simple strategies for the synthesis of phenanthridines.
Examples include the Morgan–Walls cyclization,3 the aza-
Wittig reactions,4 Pictet–Hubert condensation,5 Pd-cata-
lyzed tandem reactions,6 cyclization reactions employing
arynes,7 transition-metal-catalyzed annulations of imines,8
radical-mediated cyclizations,9 and microwave-promoted
cyclizations.10 Although these reports constitute significant
achievements, further exploration of mild and convenient
For instance, the Yu and Liu groups have reported the
preparation of functionalized phenanthridines through vis-
ible-light-mediated alkylations and trifluoromethylations
of vinyl azides [Scheme 1(b)].13a,b Visible-light-promoted
syntheses of phenanthridines through tandem sulfonyla-
tion and phosphorylation of vinyl azides have also been
demonstrated by the Yang and Wu groups, respectively
[Scheme 1(b)].13c,d Additionally, our group has successfully
used the same strategy to realize a cascade/tandem reac-
tion of vinyl azides for the construction of functionalized
phenanthridines by using arylsulfonyl chlorides as the sul-
fonylating reagent [Scheme 1(b)].13e Here, we report a visi-
ble-light-mediated tandem sulfonylation/cyclization of vi-
nyl azides with sulfonyl hydrazines to provide efficient and
simple access to 6-(sulfonylmethyl)phenanthridines
[Scheme 1(c)].
We first investigated the visible-light-mediated tandem
sulfonylation/cyclization of the vinylic azide 1a with the
hydrazine source 2 in MeCN. Because the structure of the
hydrazine source is crucial for the production of a N/C/S-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–F

B
L.-L. Mao et al.
Letter
Synlett
a) Examples of bioactive molecules containing a phenanthridine skeleton.
R
O
O
O
N
N
OMe
OMe
O
N
NH2
Chelerythrine
N
Antitumor agents
Trisphaeridine
OMe
OMe
N
O
N
MeO
HO
R
S
O
N
N
O
O
O
Fagaronine
H37Rv Inhibitor
Lycobetaine
b) Previous work
R-Br
PC = fac-Ir(ppy)3,
Ru(bpy)3Cl2 6H2O,
MesAcr+...
Umemoto's CF3 reagent
F3CSO2Na
R1
R1
R2
N3
N
+
RSO2Cl
Blue LEDs
ref.13
R/CF3/S/P
R2
R1HP(O)R2
Ar-N2BF4 + DABCO-(SO2)2
c) This work
R1
R1
O
N3
Rose Bengal, TBPB
N
O
R2
S
NHNH2
+
R2
Na2CO3, MeCN, Ar,
Blue LEDs
S
O
O
Scheme 1 Examples of bioactive molecules containing a phenanthridine skeleton (a) and examples of visible-light-mediated radical tandem cycliza-
tions of vinyl azides (b and c)
centered radical,14 a variety of hydrazine sources were pre-
pared and evaluated under photoredox conditions. When
azide 1a (1.3 equiv) was exposed to 5 W blue LED irradia-
tion in the presence of Ru(bpy)3Cl2·6H2O (2 mol%), tert-bu-
tyl peroxybenzoate (TBPB; 2.0 equiv), Na2CO3 (1.2 equiv),
and the sulfonyl hydrazine 2a (0.2 mmol) as a hydrazine
source, the corresponding sulfonylation/cyclization product
3aa was obtained in 74% yield. Other hydrazine sources
such as phenylhydrazine (2b), methyl hydrazinecarboxylate
(2c), and acetohydrazide (2d) were also examined, but none
of the desired products were detected (Scheme 2).
Ru(bpy)3Cl2·6H2O, 1.20 equivalents of Na2CO3, and 2.0
equivalents of TBPB in MeCN under 5 W blue LED irradia-
tion, which gave 3aa in 74% yield.
With the optimal reaction conditions in hand, we inves-
tigated the reactions of a variety of vinyl azides to define
the scope and limitations of our sulfonylation/cyclization
reaction (Scheme 3). Vinyl azides containing either elec-
tron-donating or electron-withdrawing groups at the C3
position (phenanthridine numbering) reacted smoothly
Following this, vinyl azide 1a and hydrazine 2a were
taken forward for further optimization of the reaction con-
ditions. The effects of varying the oxidant, solvent, and base
employed in the transformation were examined (see Sup-
porting Information, Tables S2–S4), but in all cases the
changes were found to be detrimental to the yield of 3aa.
The Ru(bpy)3Cl2·6H2O photocatalyst was found to be essen-
tial, as the reaction did not proceed when rhodamine B was
used instead (SI; Table S5, entry 1). Reactions conducted
with the photocatalyst under an air atmosphere or in the
absence of blue LED irradiation also failed to generate 3aa,
Ru(bpy)3Cl2 6H2O (2 mol%),
TBPB (2.0 equiv.)
N3
N
RNHNH2
+
R
Na2CO3 (1.2 equiv.),
MeCN, rt, Ar
Blue LEDs
1a
2
3
H
O
H
N
O
O
N
NH2
NH2
NH2
S
NH2
N
N
H
O
2c, 0%
O
H
2b, 0%
2d, 0%
2a, 74%
as did
Ru(bpy)3Cl2·6H2O (SI; Table S5, entries 2–4). The best result
was therefore obtained by using 2 mol% of
a reaction carried out in the absence of
Scheme 2 Screening of hydrazine sources for the tandem sulfonyla-
tion/cyclization of 1a
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–F

C
L.-L. Mao et al.
Letter
Synlett
taining a sterically hindered aromatic group (3ap), a naph-
thyl group (3au), or a thienyl group (3av) to provide the
corresponding products in good yields. Aliphatic sulfonyl
hydrazines, such as 1-phenylmethanesulfonohydrazide also
reacted with vinyl azide 1a to afford the expected products
(3aw, 3ax), albeit in reduced yields.
3
2
4
R1
Ru(bpy)3Cl2 6H2O, R1
1
TBPB, Na2CO3
N3
N
5
TsNHNH2
+
R2
Ts
MeCN, rt, Ar
Blue LEDs
6
R2
1
2a
3
To test the scalability of this tandem sulfonylation/cy-
clization reaction, we carried out a gram-scale experiment
under the standard conditions (Scheme 5). Remarkably, the
product was isolated in 72% yield, which demonstrates the
suitability of this reaction for scaling up.
To gain valuable insights into the visible-light-mediated
tandem sulfonylation/cyclization reaction, we conducted
some mechanistic experiments. When TEMPO was intro-
duced under the standard reaction conditions, the sulfo-
nylation/cyclization of vinyl azide 1a was suppressed com-
pletely [Scheme 5(b)]. Furthermore, the yield of 3aa de-
creased to 40% in the presence of 1,1-diphenylethene, and
the sulfonyl-radical-trapping product 4aa was isolated in
5% yield [Scheme 5(c)]. These results indicate that this visi-
ble-light-mediated sulfonylation/cyclization reaction prob-
ably proceeds via a sulfonyl radical intermediate.
R
3aa: R = Me, 74%
3ab: R = H, 72%
3ac: R = OMe, 74%
3ad: R = Cl, 71%
3ae: R = CF3, 68%
N
3af: 76%
Ts
Ph
N
N
Ts
Ts
Ts
3ag: 70%
Ts
N
MeO
N
N
Ts
3ah: 75%
3ai: 79%
3aj: 78%
S
N
N
Ts
Ts
N
Ts
R1
By combining the results of the above experiments with
previous reports in the literature,13e,15 we proposed a plau-
sible mechanism for the visible-light-mediated tandem sul-
fonylation/cyclization reaction, which is shown Scheme 6.
First, irradiation of the photocatalyst produces the excited
Ru(II)* complex, which reduces TBPB to generate a tert-bu-
toxy radical A and a Ru(III) complex. The tert-butoxy radical
A then oxidizes the sulfonyl hydrazine 2a to form sulfonyl
radical C after release of N2. Subsequently, the sulfonyl radi-
cal C reacts with the vinyl azide moiety of 1a to produce an
iminyl radical intermediate D after the loss of another mol-
ecule of N2. Finally, intramolecular radical addition to the
aromatic group occurs from intermediate D; this affords
radical E, which is then oxidized by Ru(III) and deprotonat-
ed by the base to generate the final product 3aa, with clo-
sure of the catalytic cycle.
To summarize, we have developed an efficient and sim-
ple method for the synthesis of functionalized phenanthri-
dines through visible-light-mediated tandem sulfonyla-
tion/annulation of vinyl azides and sulfonyl hydrazines un-
der mild reaction conditions. In this reaction, simple and
commercially available sulfonyl hydrazines emerge as effi-
cient sulfonylating reagents for the construction of new C–S
bonds with concomitant C–N bond formation. Further re-
search on the reactivity of vinyl azides is underway in our
laboratory.
3ak: R1 = Me, 80%
3al: R1 = Ph, trace
3am: trace
3am': trace
Scheme 3 Substrate scope for vinyl azides. Reaction conditions: 2a (0.2
mmol), 1 (1.3 equiv), Ru(bpy)3Cl2·6H2O (2 mol%), TBPB (2.0 equiv),
Na2CO3 (1.2 equiv), MeCN (2.0 mL), r.t., 5 W blue LED, under Ar. Isolat-
ed yields are reported.
with 2a to provide the products 3aa–ae in moderate to
good yields. Pleasingly, sterically hindered C1-substituted
biphenyl vinyl azides bearing methyl (1f), phenyl (1g), or
methoxy (1h) groups were also suitable substrates for this
transformation. Vinyl azide substrates containing two
methyl groups (1i) or a naphthalene moiety (1j) also react-
ed smoothly with 2a to give good yields of the correspond-
ing annulated phenanthridines. Methyl substitution at the
R1 position was tolerated in 1k, and phenanthridine 3ak
was isolated in 80% yield; however, a substrate containing a
phenyl group in this position (1l) proved intractable. An
isocyano group was found to be an unsuitable analogue for
the vinyl azide moiety used in this reaction (1m) and, like-
wise, a cyclization onto a heteroaromatic ring (1m′) failed
to generate a significant amount of the desired product.
Next, a series of sulfonyl hydrazines 2 were explored
under the optimized reaction conditions to examine this
area of the substrate scope (Scheme 4). To our delight, sul-
fonyl hydrazines with either electron-rich (3an and 3ao) or
electron-deficient (3aq–at) groups at the para-position of
the benzene ring were suitable for the reaction, and the tar-
get phenanthridines were obtained in yields of 70–84%. Ad-
ditionally, the reaction tolerated sulfonyl hydrazines con-
Funding Information
GJJ161059This work was supported by the Science and Technology
Research Projects of the Education Department of Jiangxi Province,
(Grant/Award Nos.: GJJ14716, GJJ151062, GJJ161059, GJJ161059)
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Ru(bpy)3Cl2 6H2O,
TBPB, Na2CO3
O
N3
N
NHNH2
O
R
S
+
MeCN, rt, Ar
Blue LEDs
S
R
O
O
1a
2
3
OMe
N
N
N
O
O
O
S
S
S
O
O
O
3an: 78%
3ao: 76%
3ap: 75%
R1
3aq: R1 = CN, 82%
3ar: R1 = NO2, 84%
3as: R1 = F, 77%
3at: R1 = Ph, 70%
N
N
O
O
S
S
O
O
3au: 74%
H
O
S
N
N
N
O
O
O
Bn
S
S
S
O
O
O
3av: 80%
3aw: 20%
3ax: 46%
Scheme 4 Substrate scope for sulfonyl hydrazines. Reaction conditions: 2 (0.2 mmol), 1a (1.3 equiv), Ru(bpy)3Cl2·6H2O (2 mol%), TBPB (2.0 equiv),
Na2CO3 (1.2 equiv), MeCN (2.0 mL), r.t., 5 W blue LED, under Ar. Isolated yields are reported.
standard conditions
N3
N
(a)
TsNHNH2
+
Ts
1a, 6.5 mol
3aa, 3.6 mol, 72%
2a, 5 mol
N3
standard conditions
N
TsNHNH2
+
(b)
Ts
TEMPO
1a
2a
3aa, 0%
Ph
Ts
N3
N
standard conditions
(c)
+
+
TsNHNH2
Ts
1,1-diphenylethene
4aa, 5%
1a
2a
3aa, 40%
Scheme 5 Gram-scale reaction and mechanistic experiments
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–F

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L.-L. Mao et al.
Letter
Synlett
Ru(II)
N2
N
N
Ts
Ts
3aa
1a
C
SET
NaHCO3
Na2CO3
N2
Ru(III)
O
Ph
O
OtBu
N
tBuO
N
Ts
A
Ts
TsNHNH2
Ts
C
2a
D
tBuOH + N2
E
Scheme 6 Proposed reaction mechanism
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Ed 2009, 48, 6713.
We are grateful for the financial support from the Science and Tech-
nology Research Projects of the Education Department of Jiangxi
Province (No. GJJ180880, GJJ161059, GJJ151062, and GJJ14716).
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Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1611758.
S
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p
p
orti
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gInformati
o
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