Sc(OTf)3-Catalyzed Synthesis of Symmetrical Dithioacetals and Bisarylmethanes Using Nitromethane as a Methylene Source

Article abstract of DOI:10.1021/acs.orglett.0c01831

Use of nitromethane as an electrophilic methylene source for the synthesis of symmetrical dithioacetals and bisarylmethanes has been showcased using Sc(OTf)3 as a catalyst. The procedure allows straightforward access to the densely functionalized dithioacetals and bisarylmethanes under mild conditions. Additionally, the method has been applied for the synthesis of antimalarial tetramethyl mellotojaponin C and anticancer dimeric phloroglucinol derivative.

Full text of DOI:10.1021/acs.orglett.0c01831

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Letter
Sc(OTf)3‑Catalyzed Synthesis of Symmetrical Dithioacetals and
Bisarylmethanes Using Nitromethane as a Methylene Source
Dattatraya H. Dethe,* Manmohan Shukla,† and Balu D. Dherange†
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ABSTRACT: Use of nitromethane as an electrophilic methylene source for the synthesis of symmetrical dithioacetals and
bisarylmethanes has been showcased using Sc(OTf)3 as a catalyst. The procedure allows straightforward access to the densely
functionalized dithioacetals and bisarylmethanes under mild conditions. Additionally, the method has been applied for the synthesis
of antimalarial tetramethyl mellotojaponin C and anticancer dimeric phloroglucinol derivative.
itromethane is the simplest nitro compound, often used
Nas a solvent, reagent, stabilizer, and fuel in motor sports
and rockets.1 In organic synthesis, owing to its strongly
electron-withdrawing nature, nitromethane has served as
pronucleophile to access a wide range of useful compounds
such as nitroalkenes, β-nitroalcohols (Henry reaction), amines,
and carbonyl compounds (Michael addition) (Scheme 1, B).2
The reactivity of nitromethane as a nitrogen donor was
recently explored by Jiao and co-workers in their Schmidt-type
synthesis of amides and nitriles (Scheme 1, B).3 In organic
synthesis, nitromethane is employed as one carbon building
block using a multistep reaction sequence.4 In a classical Nef
reaction, nitromethane on treatment with base forms nitronate
salts, which on subsequent treatment with aqueous acid
generate the corresponding ketone (Scheme 1, C).5 We
hypothesized that, analogous to the Nef process, nitromethane
can be used as an electrophilic methylene source. The aci form
of niromethane I on treatment with Lewis acid under mild
conditions would act as source of electrophilic carbon II,
which on nucleophilic attack of electron-rich substrates such as
thiol or arenes would lead to the intermediate III. A second
nucleophilic attack of thiol/arene on III along with the loss of
HNO species and water could result in the formation of
dithioacetal or bisarylmethanes (Scheme 1, C).
synthesis of dithioacetals by insertion of methylene into
disulfide using reduction of CO2 into methylene under the
NaBH4/I2 system (Scheme 1, A).12 Bisarylmethanes are
another class of compounds, structurally related to dithioace-
tals. Bisarylmethane-based/derived architectures are com-
monly found in supramolecules such as calixarenes,13
pillararene,14 and many other synthetic receptors.15 The
importance of bisarylmethanes in pharmaceuticals and natural
products has been well documented; for example, trimetho-
phrim is used as antibacterial,16 elvitegravir is used for HIV
treatment,17 PF-06827443 is used as an antitumor agent,18 and
papaverine isolated from Papaver somniferum is used for the
treatment of visceral spasm and vasospasm.19 In 2005,
Yonezawa and co-workers reported Lewis acid catalyzed
synthesis of symmetrical bisarylmethanes using α-methoxy
acetic acid as a methylene source (Scheme 1, A).20 In the
subsequent year, Yin and co-workers utilized InCl3·4H2O to
prepare bisarylmethanes using trioxane as a methylene
source.21a In 2012, Endo et al. disclosed one-pot palladium-
catalyzed Suzuki−Miyaura approach using diborylmethanes as
a methylene source to prepare bisarylmethanes.21b Recently,
Yan et al. reported CuCl2 catalyzed synthesis of methylene-
bridged bisindoles using N,N-dimethylaminoethanol as a
In the literature, numerous synthetic methods to access
dithioacetals have been documented and can be categorized in
two groups. The first one is homologation of disulfides using
diazomethane,6 sulfur ylide,7 zinc carbenoid,8 bromomethyl-
lithium,9 and acetone10 as a methylene source (Scheme 1, A).
The second method is condensation of aldehydes or halides
with thiols.11 Most recently, Xi and co-workers reported
Received: May 31, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01831
© XXXX American Chemical Society
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A

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Scheme 1. (A) Various Approaches for the Synthesis of
Dithioacetals and Bisarylmethanes. (B) Reactivity Patterns
of Nitromethane. (C) Nef Process vs Proposed Strategy.
(D) This Work
Table 1. Optimization of Reaction Conditions for the
a
Synthesis of Symmetrical Dithioacetals
b
entry
catalyst
additive
yield (%)
c
1
2
3
4
5
6
7
8
Cu(OTf)2
p-TsOH
Sc(OTf)3
Cu(OTf)2
0
c
0
c
0
LiClO4
LiClO4
LiClO4
LiClO4
LiClO4
LiClO4
LiClO4
LiClO4
LiClO4
LiClO4
LiClO4
NaClO4
NaOAc
LiCl
41
0
In(OTf)3
Yb(OTf)3
Sc(OTf)3
AgOTf
Fe(OTf)3
Bi(OTf)3
CF3SO3H
CSA
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
48
26
83
58
61
42
65
19
9
10
11
12
13
14
15
16
17
d
23
26
trace
trace
a
Reaction conditions: 1a (0.5 mmol), Sc(OTf)3 (20 mol %), LiOCl4
(0.5 mmol), nitromethane (1.0 mL), and 3 Å molecular sieves at 25
b
c
°C for 8 h, if otherwise stated. Isolated yields. No additives were
used. 20 mol % of additive was used. CSA: (1S)-(+)-10-
d
camphorsulfonic acid.
methylene source (Scheme 1, A).22 In addition, numerous
methods are reported for synthesis of unsymmetrical bisaryl-
methanes.23 Herein, we report the Sc(OTf)3-catalyzed efficient
method for the synthesis of symmetrical dithioacetals and
bisarylmethanes using abundant, inexpensive, and environ-
mentally benign nitromethane as the methylene source.
reduced to 20 mol %, the yield of reaction was dropped to
23%, indicating the necessity of stoichiometric amount of
LiClO4 (entry 14). Use of other lithium or sodium salts instead
of LiClO4 were not found to be fruitful (entries 15−17).
Once the optimal reaction conditions had been established,
reactions of a variety of thiols under identical conditions were
performed, and the results are summarized in Table 2. Higher
reactivity and yields were observed for aryl thiols containing
To evaluate the feasibility of the hypothesis, we began our
investigation with 4-methoxythiophenol (1a) as a test
substrate. The effect of various Lewis acids, additives, and
temperature were investigated, and the results are summarized
in Table 1. When 4-methoxythiophenol was treated with
Cu(OTf)2, p-TsOH, or Sc(OTf)3 at 25 °C or elevated
temperatures (25 → 100 °C), it did not generate the desired
dithioacetal 2a (Table 1, entry 1−3). It was thought that use of
only Lewis acid would not be enough to activate nitromethane
and there is a need for some additive. LiClO4 is well-known to
accelerate the rate of Diels−Alder reaction by coordinating
with dienophile.24 Kobayashi and co-workers demonstrated
that in Friedel−Crafts acylation, reactivity and turnover of
metal triflates were dramatically increased when combined
with LiClO4.25 Hence, it was decided to examine the effect of
addition of perchlorates along with Lewis acid. To our delight,
4-methoxythiophenol on treatment with 20 mol % of
Cu(OTf)2 and 1 equiv of LiClO4 yielded dithioacetal 2a in
41% yield (entry 4). When the same reaction was performed in
the absence of Cu(OTf)2, no product formation was observed
(entry 5), indicating that both Lewis acid and LiClO4 are
necessary in order to make NO2 as a good leaving group.
Among the other Lewis and Brønsted acids screened, namely,
In(OTf)3, Yb(OTf)3, Sc(OTf)3, AgOTf, Fe(OTf)3, Bi(OTf)3,
trifluoromethanesulfonic acid, and camphorsulfonic acid
(entries 6−13), best results were obtained with Sc(OTf)3
(83% yield, entry 8). When the equivalents of LiClO4 were
a
Table 2. Synthesis of Symmetrical Dithioacetals
a
Reaction conditions: 1 (0.5 mmol), Sc(OTf)3 (20 mol %), LiClO4
b
(0.5 mmol), and nitromethane (1.0 mL) at 25 °C. Reaction was
performed on 6 mmol scale.
B
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electron-donating groups (OMe, Me, Et, t-Bu) at the ortho or
para positions, giving dithioacetals with excellent yields (Table
2, entries 2a,b, 2d, 2f,g, 2k, 2n), while thiols bearing electron-
withdrawing substituents (F, Cl, Br, CO2Me) were found to be
comparatively less reactive and furnished the corresponding
dithioacetals in moderate to good yields (Table 2, entries 2e,
2h−j, 2l−m). It is worth mentioning that sterically demanding
dithioacetals were also synthesized in moderate yield from the
corresponding aryl thiols bearing substituents at both ortho
positions (Table 2, entries 2p−q). The method was also tested
for less reactive aliphatic thiols, generating corresponding
dithioacetals in moderate yield (2r−u). To demonstrate the
applicability of the present method for scale-up, the reaction of
1a was performed on 6 mmol scale under the identical
conditions, resulting in formation of 2a in 72% yield.
Unfortunately, arenes containing substituents with no
electron-donating effect (benzene or bromobenzene) did not
produce a detectable amount of corresponding bisaryl-
methanes. In addition, the reaction of mixed thiols or arenes
to yield unsymmetrical dithioacetals or bisarylmethanes
resulted in poor yields of the desired products owing to
formation of competing symmetrical products.
Plausible reaction mechanism for the synthesis of
dithioacetals is shown in Scheme 2. The mechanism is
Scheme 2. Plausible Reaction Mechanism for Synthesis of
Dithioacetals
After preparation of symmetrical dithioacetals, the method
was extended to synthesis of bisarylmethanes under identical
conditions. To our delight, when 2,4-dimethoxybenezene (3a)
was reacted under identical conditions, the desired bisaryl-
methane (4a) was furnished in 76% yield. Likewise, reactions
of various arenes were performed, and the results are
summarized in Table 3. Higher reactivity and yields were
a
Table 3. Synthesis of Symmetrical Bisarylmethanes
analogous to the classical Nef reaction.5 To begin with,
Sc(OTf)3 and Li from LiClO4 coordinates with the aci form of
nitromethane I to form intermediate II. Nucleophilic attack of
thiol on methylene carbon accounts for C−S bond formation
generating intermediate III. Nucleophilic attack of the second
thiol on III in SN2 fashion results in the formation of
dithioacetal along with the elimination of water and HNO. The
mechanism for the synthesis of bisarylmethanes would be
analogous to that of dithioacetal (not shown in Scheme 2).
Furthermore, to demonstrate the applicability of the
method, it was decided to expand the scope by synthesizing
the related biologically active molecules. In 2013, Kingston and
co-workers isolated mallotojaponins B (5) and C (6).26
Compounds 5−7 showed potent antimalarial activity against
chloroquine-resistant Plasmodium falciparum, with IC50 values
of 0.75, 0.14, and 2.2 μM, respectively.26 In 2016, the same
group reported the synthesis of mallotojaponin C (6),27 while
the synthesis of mallotojaponins B (5) and C (6) was reported
by Cariou and co-workers.28 In 2012, Singh and co-workers
reported synthesis of structurally related phloroglucinol
derivative 8 with potent anticancer and antidepressant
activity.29 Encouraged by the potent biological activity of
these molecules, we became interested in the synthesis of
tetramethyl mallotojaponin C (7) and phloroglucinol deriva-
tive 8 (Figure 1).
a
Reaction conditions: 3 (0.5 mmol), Sc(OTf)3 (20 mol %), LiClO4
(0.5 mmol), nitromethane (1.0 mL), and 3 Å molecular sieves at 25
b
c
°C for 8 h. Reactions were performed at 70 °C. Reaction was
d
performed on 6 mmol scale. 16% of ortho−para regioisomer was
formed.
observed for arenes bearing more electron-donating groups,
furnishing bisarylmethanes with good yields (Table 3, entries
4a, 4g−j). Reactions of arenes containing an amide group were
very slow at 25 °C, but elevation of temperature from 25 to 70
°C accelerated the reaction and resulted in increased yields
(entries 4d−f). Sterically demanding bisarylmethanes were
also synthesized in moderate to good yields from correspond-
ing arenes bearing substituents at both ortho positions (entry
4b, 4k−m). Reaction of 4-methoxybenzene (3c) led to the
formation of desired bisarylmethane 4c in 51% yield along with
16% of the ortho−para regioisomer.
Figure 1. Structures of mallotojaponins 5−7 and pholoriglucinol
derivative 8.
C
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Authors
To begin, compound 4b upon controlled acylation
generated acetophenone derivative 9, which on subsequent
alkylation with 1-bromo-3-methylbut-2-ene under basic
conditions furnished tetramethyl mallotojaponins C (7)
(Scheme 3).26 On the other hand, bisarylmethane 4b on
Manmohan Shukla − Department of Chemistry, Indian Institute
of Technology Kanpur, Kanpur 208016, India
Balu D. Dherange − Department of Chemistry, Indian Institute
of Technology Kanpur, Kanpur 208016, India; orcid.org/
0000-0002-4123-5012
Scheme 3. Synthesis of Tetramethyl Mallotojaponin C (7)
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01831
a
and Dimeric Phloroglucinol Derivative 8
Author Contributions
†M.S. and B.D.D. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.S. thanks UGC, New Delhi, and B.D.D. thanks CSIR, New
Delhi, for the award of a research fellowship. Financial support
from SERB Project No. EMR/2017/001455 is gratefully
acknowledged.
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* Supporting Information
The Supporting Information is available free of charge at
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Details of the experimental procedure and character-
ization data for all new compound (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Dattatraya H. Dethe − Department of Chemistry, Indian
Institute of Technology Kanpur, Kanpur 208016, India;
orcid.org/0000-0003-2734-210X; Email: ddethe@
iitk.ac.in
D
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