A radical cyclization cascade of 2-alkynylbenzonitriles with sodium arylsulfinates

Article abstract of DOI:10.1039/c8ob02288g

A convenient radical cyclization cascade procedure for the construction of sulfonated indenones from 2-alkynylbenzonitriles and sodium arylsulfinates has been explored under mild reaction conditions. The present methodology offers a low-cost and operationally straightforward approach to synthesizing various sulfonated indenones in moderate to good yields by simple use of cheap sodium persulfate as an oxidant and environmentally benign water as a co-solvent.

Full text of DOI:10.1039/c8ob02288g

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A radical cyclization cascade of
2-alkynylbenzonitriles with sodium arylsulfinates†
Cite this: Org. Biomol. Chem., 2018,
16, 7959
Bang Zhou, Wenqi Chen, Yuzhong Yang, Yuan Yang,
Yun Liang
Guobo Deng * and
*
A convenient radical cyclization cascade procedure for the construction of sulfonated indenones from
2-alkynylbenzonitriles and sodium arylsulfinates has been explored under mild reaction conditions. The
present methodology offers a low-cost and operationally straightforward approach to synthesizing
various sulfonated indenones in moderate to good yields by simple use of cheap sodium persulfate as an
oxidant and environmentally benign water as a co-solvent.
Received 15th September 2018,
Accepted 6th October 2018
DOI: 10.1039/c8ob02288g
rsc.li/obc
Functionalized indenones are important structural scaffolds in sulfonated indenones by these means are scarce. Therefore,
a variety of synthetic drugs,1 drug intermediates,2 and natural developing a simple and effective method of synthesizing
products,3 which display excellent biological and pharma- sulfonated indenones is an important and difficult
ceutical activities.4 There has been an ongoing interest in the work, due to their importance in biology and medicinal
development of simple, efficient methods for synthesizing chemistry.
indenone derivatives. Traditionally, methods for synthesizing
The radical cascade annulation reaction to construct
indenones include intramolecular Friedel–Crafts reactions,5 various heterocyclic frameworks is a well-established powerful
Grignard reactions,6 and Heck–Larock cyclization reactions.7 strategy in the synthesis processes,15 due to its simple oper-
Generally, palladium8 and rhodium9 are selected as metal cata- ation and cost reduction. For example, Lei and co-workers
lysts for the synthesis of indenones when the compounds such described an electro-oxidative direct arylsulfonylation of
as 2-halobenzaldehydes, 2-haloacetophenones, 2-iodo-benzo- ynones with sulfinic acids via a radical tandem cyclization
nitriles, and aroyl chlorides are used as substrates. Recently, strategy for the synthesis of sulfonated indenones.16 Recently,
the addition reactions to nitriles have been found to be radical addition–cyclizations of 2-alkynylbenzonitriles for the
efficient approaches for the construction of ketones.10 These construction of indenones have become one of the most
methods have made good progress in the synthesis of inde- popular methods for chemists. For example, Jiang, Tu and co-
none derivatives. However, the development of simple and workers reported the synthesis of 3-phosphinylated
low-cost approaches to synthesizing functional indenones is (Scheme 1a),17 3-alkylated (Scheme 1b)18 and 3-sulfonylated
still highly desirable.
indenones (Scheme 1c)19 by using 2-alkynylbenzonitriles as
Owing to its distinctive structure and electronic features, substrates. Despite great progress, employment of expensive
the sulfonyl group has been extensively exploited in pharma- metal catalysts and requirement of organometallics or per-
ceuticals, agrochemicals and bioactive compounds.11 The oxides in these methods still stimulate chemists to develop
common means for preparing sulfones including the oxidation more green and sustainable approaches. Besides, chemically
of sulfides,12 sulfonylation of arenes,13 and transition metal stable and readily available sodium arylsulfinates have been
catalyzed cross reactions are available.14 These strategies can widely utilized to obtain sulfone compounds through radical
serve as promising approaches for the synthesis of reactions.20 According to these above reactions, herein we
some sulfone derivatives. However, examples of synthesizing describe an efficient radical cascade annulation reaction for
the selective synthesis of various 3-sulfonyl indenones through
direct arylsulfonylation of 2-alkynylbenzonitriles with sodium
arylsulfinates.
National & Local Joint Engineering Laboratory for New Petro-chemical Materials and
Initially, we utilized 2-(phenylethynyl)benzonitrile 1a and
sodium 4-methylbenzenesulfinate 2a as the model substrates
to optimize the reaction conditions and these results are sum-
marized in Table 1. It was found that the desired product 3aa
was obtained in 36% yield when the reaction was performed in
the presence of a K2S2O8 oxidant (300 mol%) in CH3CN/H2O
Fine Utilization of Resources, Key Laboratory of Chemical Biology and Traditional
Chinese Medicine Research, Ministry of Education, Key Laboratory of the Assembly
and Application of Organic Functional Molecules of Hunan Province, China.
E-mail: gbdeng@hunnu.edu.cn, yliang@hunnu.edu.cn; Fax: +86(0731)88872533
†Electronic supplementary information (ESI) available. CCDC 1838957. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8ob02288g
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Na2S2O8 as an oxidant gave the best yield (67%), whereas other
oxidants such as (NH4)2S2O8, oxone, PhI(OAc)2, TBHP, DTBP,
and H2O2 did not or only sluggishly promoted this reaction
(Table 1, entries 3–8). Among a range of solvents tested,
CH3CN/H2O (9/1) was found to be the best one (Table 1,
entries 9–14). In contrast, the product 3aa was isolated in low
yield when the reaction was performed in the absence of H2O
or CH3CN (Table 1, entries 15 and 16). In addition, changing
the equivalents of Na2S2O8 did not further improve the reac-
tion either (Table 1, entries 17 and 18). It should be noted that
running the reaction at either 40 or 80 °C gave inferior results,
which demonstrates that temperature is important to the
product yield (Table 1, entries 19 and 20). The control experi-
ment showed that no reaction occurred in the absence of
Na2S2O8 (Table 1, entry 21).
With the optimized reaction conditions in hand, we evalu-
ated the scope and the generality of the reaction in the next
step (Scheme 2). First, we set out to explore the scope and the
limitations of the reactions of various 2-alkynylbenzonitriles
with sodium p-tolylsulfinate 2a toward the formation of sulfo-
nated indenones and the results are summarized in Scheme 2.
Under the standard conditions, a broad range of 1 bearing
electron-donating substituent groups including p-methyl,
Scheme 1 Radical cascade cyclization synthesis of indenones by using
2-alkynylbenzonitriles.
Table 1 Optimization of reaction conditionsa
Entry
Oxidant
Solvent (v : v)
Yieldb (%)
1
2
3
4
5
6
7
8
K2S2O8
Na2S2O8
(NH4)2S2O8
Oxone
PhI(OAc)2
TBHP
CH3CN/H2O (9 : 1)
CH3CN/H2O (9 : 1)
CH3CN/H2O (9 : 1)
CH3CN/H2O (9 : 1)
CH3CN/H2O (9 : 1)
CH3CN/H2O (9 : 1)
CH3CN/H2O (9 : 1)
CH3CN/H2O (9 : 1)
DMSO/H2O (9 : 1)
DCM/H2O (9 : 1)
1,4-Dioxane/H2O (9 : 1)
DCE/H2O (9 : 1)
36
67
21
Trace
n.r.
n.r.
n.r.
n.r.
Trace
15
DTBP
H2O2
9
Na2S2O8
Na2S2O8
Na2S2O8
Na2S2O8
Na2S2O8
Na2S2O8
Na2S2O8
Na2S2O8
Na2S2O8
Na2S2O8
Na2S2O8
Na2S2O8
—
10
11
12
13
14
15
16
17c
18d
19e
20f
21
7
Trace
24
48
36
Trace
43
32
29
55
n.r.
CH3CN/H2O (1 : 1)
CH3CN/H2O (3 : 1)
CH3CN
H2O
CH3CN/H2O (9 : 1)
CH3CN/H2O (9 : 1)
CH3CN/H2O (9 : 1)
CH3CN/H2O (9 : 1)
CH3CN/H2O (9 : 1)
a Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), oxidant
(0.6 mmol), solvent (2 mL), at 60 °C for 24 h under a N2 atmosphere.
n.r. = no reaction. b Isolated yield. c Na2S2O8 (0.40 mmol). d Na2S2O8
(0.80 mmol). e At 40 °C. f At 80 °C.
(9 : 1) at 60 °C for 24 h under a N2 atmosphere (entry 1). The
structure of 3aa was unambiguously established by X-ray crys-
tallographic analysis (CCDC 1838957†). Encouraged by this
result, we further optimized the reaction conditions by testing
Scheme 2 Variation of 2-alkynylbenzonitriles. Reaction conditions: 1
(0.2 mmol), 2a (0.4 mmol), Na2S2O8 (0.6 mol), v(CH3CN) : v(H2O) = 9 : 1
(total volume is 2 mL), 60 °C and 24 h under a N2 atmosphere. Isolated
various oxidants. The investigation results showed that using yields. a At 100 °C for 36 h.
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3,5-dimethyl, n-amyl, p-methoxy and m-methoxy groups on the
arylalkynyl moiety all worked well, efficiently converting into
the corresponding functionalized 3-sulfonylindenones 3ba–3fa
in good yields (66–71% yields). For example, the p-methoxy
substituted substrate 2e region-selectively formed 2-(4-methox-
yphenyl)-3-tosyl-1H-inden-1-one 3ea in 71% yield. Fluoro-
substituted and chloro-substituted 2-alkynylbenzonitriles were
tolerated well under the standard conditions, offering the
corresponding products 3ga and 3ha in 64% and 68% yields,
respectively. However, substrate 1i with an electron-withdraw-
ing NO2 group displayed lower reactivity, giving 3ia in 40%
yield. The results showed that the electron-donating group
substrates displayed higher reactivity than the electron-with-
drawing group substrates. It is noteworthy that substrates 1j
and 1k bearing diphenyl and naphthyl groups performed well
under the standard conditions, giving the corresponding poly-
cyclic aromatic hydrocarbon products 3ja and 3ka in 63% and
40% yields, respectively. Notably, heterocyclic and aliphatic
alkynes have not been reported in previous articles due to
their low reactivities. In our work, heterocyclic and aliphatic
alkynes (1l, 1m and 1n) successfully underwent radical
Scheme 3 Substrate scope of sodium sulfinates. Reaction conditions:
1a (0.2 mmol), 2 (0.4 mmol), Na2S2O8 (0.6 mol), v(CH3CN) : v(H2O) =
9 : 1 (total volume is 2 mL), 60 °C and 24 h under a N2 atmosphere.
cascade cyclization to give 3la, 3ma and 3na, albeit in low Isolated yields. a At 100 °C with 36 h.
yields. Subsequently, our study focused on the substitution
effect on the internal aryl ring of 2-alkynylbenzonitriles
(products 3oa–3ta). To our delight, the results showed that
both electron-donating substituents including methyl (1o) and
methoxy (1p) groups and electron-withdrawing substituents
fluorine (1q and 1r) and chlorine (1s and 1t) could be
smoothly transformed into the corresponding desired pro-
ducts in moderate to good yields under the optimized con-
ditions. For example, methyl substituted substrate 1o formed
the desired product 3oa in 63% yield. Fluoro-substituted
products 3qa and 3ra were obtained in 65% and 70% yields,
respectively. Finally, the symmetric substrate 1u was compati-
ble with the standard reaction conditions as well and gave the
corresponding product 3ua in 53% yield.
Next, the scope of sodium arylsulfinates was also explored
under the standard reaction conditions (Scheme 3). To our
delight, substrates 2b–2g, bearing electron-donating or elec-
tron-deficient groups on the aryl ring, were compatible under
the standard conditions. For example, substrate 2c with a
methoxy group was tolerated well under the standard con-
ditions, affording 3ac in 55% yield. Noticeably, the substrates
bearing an electron-deficient halo group, including Cl and I
on the aromatic ring, were well-tolerated under the reaction
conditions, thereby providing an opportunity for additional
modifications at the halogenated position (3ae and 3af).
Scheme 4 Control experiments.
Next, we investigated the reactivity of the substrate sodium piperidin-1-oxyl, butylated hydroxytoluene and 1,1-diphenyl-
naphthalene-2-sulfinate 2g, affording the corresponding ethylene resulted in the inhibition of the reaction (eqn (1) in
product 3ag in 52% yield. However, sodium trifluoromethane- Scheme 4). Obviously, when the reaction of 1a with 2a was
sulfinate 2h did not yield the corresponding product and carried out in the presence of radical scavenger 1,1-diphenyl-
formed compound 4, which was consistent with the results ethylene under standard reaction conditions, the formation of
reported by other chemists.21
the desired products was totally suppressed and adduct 5 was
To shed light on the possible mechanism of the reaction, obtained in 15% yield. The results suggest that the current
several control experiments were carried out, as shown in reaction is likely to involve a TS radical and involves a radical
Scheme 4. First, the addition of radical inhibitors tetramethyl- process. The result of an 18O-labeled experiment using H218O
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Open Research Fund Project of Key Laboratory of Hunan
Province (2018TP1017), and Hong Kong Scholars program
(XJ2017009).
Notes and references
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(eqn (2) in Scheme 4). Notably, compound 6 could not be con-
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not the intermediate of this reaction (eqn (3) in Scheme 4).
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Scheme 4). This result indicates that this radical tandem cycli-
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Based on the above control experiments and previous
mechanistic studies,20,22 a plausible mechanism is shown in
Scheme 5. First, sodium benzenesulfinate 2a is oxidized by
Na2S2O8 to generate benzenesulfonyl radical A.20a,c,d Then,
intermolecular addition of A to 1a offers the alkenyl radical
intermediate B.17–19,22 Intermediate B undergoes the 5-exo-dig
cyclization reaction to give intermediate C, which abstracts a
H-atom to produce unstable imine intermediate D. Finally,
imine intermediate D is hydrolyzed by water to yield the
desired product 3aa.17–19,22
In summary, we have demonstrated an efficient protocol for
the synthesis of sulfonated indenones via a radical cascade
cyclization of 2-alkynylbenzonitriles with sodium arylsulfi-
nates. This methodology allows one new C–C bond and C–S
bond formation through the cleavage of one C–N bond.
Furthermore, these synthetic 3-sulfonylindenones are
ubiquitous structural units in a number of biologically active
compounds; so the expansion of the synthetic application of
this skeleton is currently under investigation in our laboratory.
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