Further insight into the photochemical behavior of 3-aryl-N-(arylsulfonyl)propiolamides: tunable synthetic route to phenanthrenes

Article abstract of DOI:10.1039/c7ra00193b

Reported herein is further insight into the photochemical behaviour of 3-aryl-N-(arylsulfonyl)-propiolamides, which provides a straightforward way to access meaningful phenanthrenes. Mechanistic investigation indicated that aryl migration, C-C coupling, 1,3-hydrogen shift, desulfonylation and elimination were involved in the process. Moreover, this protocol allowed for scale-up using a flow reactor.

Full text of DOI:10.1039/c7ra00193b

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Further insight into the photochemical behavior of
3-aryl-N-(arylsulfonyl)propiolamides: tunable
synthetic route to phenanthrenes†
Cite this: RSC Adv., 2017, 7, 12022
*
Ming Chen, Xinxin Zhao, Chao Yang, Yanpei Wang and Wujiong Xia
Received 5th January 2017
Accepted 9th February 2017
Reported herein is further insight into the photochemical behaviour of 3-aryl-N-(arylsulfonyl)-
propiolamides, which provides a straightforward way to access meaningful phenanthrenes. Mechanistic
investigation indicated that aryl migration, C–C coupling, 1,3-hydrogen shift, desulfonylation and
elimination were involved in the process. Moreover, this protocol allowed for scale-up using a flow reactor.
DOI: 10.1039/c7ra00193b
rsc.li/rsc-advances
Phenanthrenes represent an important class of the simplest (phenylsulfonyl)propiolamide in CH3CN, we disclosed that
polycyclic aromatic hydrocarbons (PAHs) and they have received along with the main product 2-methylphenanthro[9,10-d]iso-
much interest in materials science and technology due to their thiazol-3(2H)-one
1,1-dioxide
through
the
Smiles
utility in functional materials, especially in photoelectronic rearrangement/Mallory reaction, a trace amount of phenan-
devices.1 Moreover, compounds containing this structural core threne was also observed aer removal of the sulfonylamide
are oen of great value in pharmaceutical chemistry because group (Scheme 1a).10 Such a result prompted us to explore
they generally display a broad spectrum of biological activities, suitable reaction conditions for the preferential preparation of
such as anti-microbial, anti-tumor, anti-inammatory, anti- phenanthrenes, and described in this paper are our continuous
malarial and others.2 Additionally, phenanthrene motifs can work on the photochemical behavior of 3-aryl-N-(arylsulfonyl)
be found in a large class of natural products.3 Therefore, propiolamides to provide a straightforward way to a series of
continuous effort has been devoted to the preparation of phenanthrenes tuned by base (Scheme 1b).
phenanthrene derivatives, and in the past decades, numerous
At the outset of further investigation, compound 1a was
relevant studies have been reported.4 Among all the available chosen as the model substrate and additives with different
strategies, the Lewis acid/base/metal mediated intramolecular property such as acid/base/salt were employed to modify the
annulation of alkynylated biaryls5 and various metal induced reaction, respectively. The results turned out to be that, unlike
intermolecular [4 + 2] cyclization of biaryls with alkynes6/bis(- the less effect of acid and salt, the addition of 1 equivalent of
pinacolatoboryl) alkenes7 represented the typical approaches to dimethylamine dramatically improved the yield of
A in
construct the phenanthrene skeleton. In addition, the prepa- comparison to the reaction with no additives (Table 1, entries 1–
ration of stilbene followed by photocyclization, radical or metal 4). Moreover, the increasing amount of dimethylamine to 3
catalyzed oxidative C–C coupling reactions has been extensively equivalents totally yielded the product A with the retard of
studied as well.8 Besides, the construction of a phenanthrene formation of 1b (Table 1, entry 5). Based on this observation,
ring could also be achieved via intramolecular ring-closing screening on the bases, such as triethylamine, methyl amine,
metathesis.9 Despite the above, most of these methods
suffered from certain limitations, such as the use of complex
and expensive metal catalysts which may be unfriendly to the
environment, not-easily accessible precursors, air- or moisture-
sensitive reagents, and harsh reaction conditions. Thus, devel-
opment of an efficient and convenient method for the
construction of phenanthrene scaffold from readily prepared
substrates is of important signicance.
During our recent exploration of the ultraviolet light induced
photochemical
behavior
of
N-methyl-3-phenyl-N-
State Key Lab of Urban Water Resource and Environment, Shenzhen Graduate School,
Harbin Institute of Technology, Harbin 150080, P. R. China. E-mail: xiawj@hit.edu.cn
† Electronic supplementary information (ESI) available: Experimental details,
product characterization and NMR spectral data. See DOI: 10.1039/c7ra00193b
Scheme 1 Previous work and this work.
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Table 1 Screening on the reaction conditionsa
R3 substituent on nitrogen atom (Table 2, A, B, C and Table 3,
R), it should be noted that the non-substituted phenanthrene A,
3-methylphenanthrene B, 3-methoxyphenanthrene C and the
heterocyclic aromatic compound R could be readily prepared in
moderate to excellent yields. Substrates with electrophilic
groups such as the halogen atom, triuoromethyl and cyano on
the para-position of aryl ring were perfectly tolerant with the
reaction conditions, generating the corresponding products in
30–72% yields (Table 2, D–G, J). Sterically bulkier group such as
butyl- or phenyl-substituted substrate 15a and 16a could furnish
the corresponding products as well in the yield of 68% and 56%,
respectively (Table 2, H and I). Substrate 18a with p-methyl on
both benzene rings afforded the single product K in 67% yield
(Table 2, K). The reaction of compound 19a bearing meta-methyl
on aryl ring delivered a mixture of regioisomers in a ratio of 4 : 3
(Table 2, L and L0), while the meta-uoro substituted substrate
20a gave two regioselective products M and M0 in a ratio of
8 : 1.4c As for the photoreaction of ortho-methyl substituted
substrates 21a, this protocol allowed access to both the desired
Yieldb (%)
A 1b
Entry Solvent
Additive (equiv.)
Light
t (h)
1
2
Toluene No additive
Toluene Acetic acid (1.0)
300 nm 1.5
300 nm 1.5
<5 71
<5 68
40 45
<5 65
3
4
5
Toluene Dimethylamine (1.0) 300 nm 1.0
Toluene NaCl (1.0) 300 nm 1.3
Toluene Dimethylamine (3.0) 300 nm 1.0
70
68
63
65
72
80
0
0
0
0
0
0
6
7
Toluene Et3N (3.0)
Toluene Methylamine (3.0)
300 nm 1.0
300 nm 1.2
8
Toluene Cyclohexamine (3.0) 300 nm 0.9
9
Toluene Morpholine (3.0)
Toluene Morpholine (1.0)
Toluene Morpholine (0.5)
Toluene Morpholine (1.0)
300 nm 0.7
300 nm 0.9
300 nm 1.2
300 nm 0.9
300 nm 1.2
300 nm 1.2
300 nm 1.0
300 nm 1.3
300 nm 1.0
300 nm 1.5
10
11
12c
13
14
15
16
17
18
19
20
21
22
1-methylphenanthrene
N and the unexpected 1-methyl-2-
55 20
80
(phenylethynyl)benzene N0 in a ratio of 3 : 5. In view of our
previous work, the arising of N0 was supposed possibly from the
Smiles rearrangement of 21a followed by the elimination of
sulfonylamide group. Replacing the methyl group with F, Cl
atom and triuoromethyl group equally afforded corresponding
structural isomers in different ratios (Table 2, N–Q). More
importantly, other polycyclic compounds, such as benzo[c]
phenanthrene and chrysene, could also be prepared via this
synthetic protocol (Table 3, T and U). In addition, the reaction
was also applicable for the synthesis of heteroaromatic
compounds and poly-substituted phenanthrene (Table 3, R, S
and V). Particularly, the yield of compound V reached to 50%
even the amount of 30a increased to 0.31 gram.
0
MeCN
MeOH
DMF
Morpholine (1.0)
Morpholine (1.0)
Morpholine (1.0)
Morpholine (1.0)
15 60
23 55
20 53
THF
72
70
0
0
Benzene Morpholine (1.0)
DCM Morpholine (1.0)
Toluene Morpholine (1.0)
Toluene Morpholine (1.0)
Toluene Morpholine (1.0)
Toluene Morpholine (1.0)
65 Trace
350 nm 12.0 58 Trace
HPML
MPML
4.0
6.0
60 Trace
62 Trace
300 nm 1.8
78
0
a
Reaction conditions: 1a (0.2 mmol), solvent (anhydrous 40 ml), under
b
air atmosphere, irradiation at room temperature. Isolated yield.
c
Under N2 atmosphere.
Notably, among all the prepared products, compounds A, B,
L0, N, K, U and V were identied as natural products and have
been reported in previous literatures,11–17 which indicated the
inherent synthetic utility of this novel protocol.
cyclohexamine and morpholine, was conducted which led to
comparable yields (Table 1, entries 6–9). Remarkably, the
decrease in the amount of morpholine to 1 equivalent gave the
best yield of 80% (Table 1, entry 10), whereas further decrease to
0.5 equivalent led to lower yield and poor selectivity (Table 1,
entry 11). Gladly, we found that reactions run equally efficiently
under air/inert gas conditions (Table 1, entries 10 and 12). Then
a set of solvents was screened and the reaction performed in
toluene provided the best result than in other medium (Table 1,
entries 13–18). It is noteworthy that the solvent plays a critical
role in the result in which most of the tested substrates have
been decomposed in strong polar solvents, leading to a low
yield of the desired product. The screening of light source
indicated that the suitable wavelength was proved to be 300 nm
(Table 1, entries 10, 19–21). Finally, we found there was a little
loss on yield (in comparison with entry 10) when the irradiation
time was increased to 1.8 hours (Table 1, entries 22).
Before the reaction mechanism was proposed, some control
experiments were conducted using 7a as representative
compound. First, 7a was irradiated under the standard condi-
tions for 0.2 h, fortunately, except for the product B, both the
rearrangement product 7c and a new compound 7d which was
quite labile and could be rapidly converted into B was isolated
(Scheme 2, eqn (1)). Further exploration disclosed that 7d was
derived from compound 7c, thus suggesting that product B was
probably resulted from a tandem three-step photo trans-
formation (Scheme 2, eqn (2)), and all these transformations
only took place under the irradiation of UV light.
Based upon the above results, a tentative mechanism for this
photoreaction was proposed as shown in Scheme 3. The alkyne
group of substrate a was rst excited to 1,2-biradicals which
initiated radical Smiles rearrangement/C–S bonding cascade
reaction to form the isolatable intermediate c,10 then subse-
quently oxidative cyclization delivered product b (Scheme 3,
path a).10 And cyclization reaction occurred to form the inter-
mediate II which underwent 1,3-H shi process with the mor-
pholine served as an assistant agent18 to form compound d that
Subsequently, a series of substrates were subjected to the
optimal conditions to investigate the reaction scope, and the
results were summarized in Tables 2 and 3. As expected,
substrate with the same aryl groups tethered to the sulfonyl or
the alkynyl resulted in the same adduct regardless of divergent
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Table 2 The photoinduced synthesis of phenanthrenesa
Table 3 The synthesis of PAHs and polysubstituted phenanthrenesa
Subs
R1
R2
R3
t (h)
Product
Yieldb (%)
1a
2a
3a
4a
5a
H
H
H
H
H
H
H
H
H
H
Me
Bn
Propargyl
Allyl
0.9
1.2
1.0
1.1
1.0
A
A
A
A
A
80
75
70
75
87
6a
7a
8a
9a
10a
11a
12a
13a
14a
15a
16a
17a
18a
Me
H
Me
OMe
H
F
CI
Br
CF3
Ph
H
Me
H
H
OMe
H
H
H
H
H
Me
Me
Allyl
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
1.0
1.2
1.2
2.5
2.5
1.2
1.2
2.5
1.5
2.5
1.2
2.0
1.2
B
B
B
C
C
D
E
F
G
H
I
85
90
80
55
65
55
72
30
70
56
84
55
67
a
All reactions are carried out under the standard conditions. b Isolated
yield. c Reaction is carried out on the amount of 0.31 gram.
t-Butyl
CN
Me
H
H
Me
J
K
Scheme 2 Control experiments.
Subs
R1
R2
R
t (h)
Product
Yieldb (%)
L/L0 (4/3)c
77d
74d
trapped by morpholine to produce morpholine-4-carboxamide f
(Scheme 3, path c). Furthermore, to add more credence to the
existence of compound f, N-benzylmorpholine-4-carboxamide
19a
20a
H
F
Me
H
Me
F
2.0
1.0
M/M0 (8/1)c
Subs
R1
t (h)
Productc
Yieldb (%)
21a
22a
23a
24a
Me
F
Cl
2.3
1.0
1.8
2.0
N/N0 (3/5)c
O/O0 (3/1)c
P/P0 (6/5)c
Q/Q0 (1/7)c
45d
93d
70d
50d
CF3
a
All reactions are carried out under the standard conditions. b Isolated
c
d
yield. The ratio was determinated by GC. The total yield of both
isomers.
was identied by the isolation of 7d as shown in Scheme 2. Then
a ring opening reaction of d occurred and afforded the 1,4-bir-
adical intermediate III aer exclusion of sulfur dioxide. The
nal phenanthrene product was formed from intermediate III
aer elimination of isocyanic intermediate e, which could be Scheme 3 The proposed reaction mechanism.
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Scheme 4 Gram scale study with a flow reactor.
(R3 ¼ Bn) was isolated in 40% yield during the reaction of
substrate 2a.19 It was reasonable to assume that some of the
intermediate c, which was arisen from the substrate containing
ortho-substituent on aryl ring, could not effectively undergo the
C–C coupling due to the steric hindrance, but be converted into
the diphenylethyne product instead aer removing the sulfo-
nylamide group with the aid of morpholine in the manner as
described above (Scheme 3, path b).
In order to demonstrate the synthetic value and utility of this
protocol, we turned our attention toward the preparation of
phenanthrenes on a large scale.20 With this aim in mind, we
carried out the photoreaction in a continuous ow reactor with
compound 1a, of which the loading was increased to 1.0 gram
and concentration to 10 mM, as the subject substrate. Then the
reaction was run for 4 hours to afford the target product A in
75% with little loss on yield (Scheme 4).
¨
S. Wittlin, B. Mordmuller and T. Kurz, J. Med. Chem., 2014,
57, 7971.
In conclusion, the photoreaction of easily accessible 3-aryl-N-
(arylsulfonyl)propiolamides to prepare phenanthrene deriva-
tives via radical Smiles rearrangement/elimination has been
developed. The addition of morpholine enables the 1,3-H shi
which is crucial to the regioselective formation of phenan-
threnes. Moreover, reasonable mechanism was elaborated in
detail based on the isolated intermediates. It's worthy to point
out that, using our established protocol, the desired substitu-
ents on phenanthrene rings could be simply introduced in
advance on the aromatic groups of the starting material.
Besides, the availability and efficiency of this approach makes it
appealing for the syntheses of certain natural products.
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Acknowledgements
We are grateful for the nancial support from China NSFC (No.
21372055, 21472030 and 21672047), SKLUWRE (No. 2015DX01),
the Fundamental Research Funds for the Central Universities
(Grant No. HIT.BRETIV.201310) and HLJNSF (B201406).
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