Selectfluor facilitated bridging of indoles to bis(indolyl)methanes using methyl: Tert -butyl ether as a new methylene precursor

Article abstract of DOI:10.1039/d1ob00120e

A novel, green and efficient method is developed for the synthesis of methylene bridged bis(indolyl)methanes in good to excellent yields. The reaction employs methyl tert-butyl ether (MTBE) as the methylene source and selectfluor as an oxidizing agent. The scope and versatility of the methods have been successfully demonstrated with 48 examples. The metal-free transformation process is suitable for scale-up production. A selectfluor-promoted oxidative reaction mechanism is proposed based on the results of the experimental studies. This journal is

Full text of DOI:10.1039/d1ob00120e

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Biomolecular Chemistry
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PAPER
Selectfluor facilitated bridging of indoles to
bis(indolyl)methanes using methyl tert-butyl
ether as a new methylene precursor†
Cite this: Org. Biomol. Chem., 2021,
19, 4076
Jiang Jin,a Yinghua Li,b Shiqun Xiang, b Weibin Fan,b Shiwei Guob and
a,b
Deguang Huang
*
A novel, green and efficient method is developed for the synthesis of methylene bridged bis(indolyl)
methanes in good to excellent yields. The reaction employs methyl tert-butyl ether (MTBE) as the methyl-
ene source and selectfluor as an oxidizing agent. The scope and versatility of the methods have been suc-
cessfully demonstrated with 48 examples. The metal-free transformation process is suitable for scale-up
production. A selectfluor-promoted oxidative reaction mechanism is proposed based on the results of
the experimental studies.
Received 22nd January 2021,
Accepted 7th April 2021
DOI: 10.1039/d1ob00120e
rsc.li/obc
available that detail the synthesis of BIMs with substituents in
the methylene group,9 BIMs that possess a methylene bridge
Introduction
Diarylmethanes and bis(heterocycle)methanes have attracted are difficult to synthesize using the usual method of coupling
significant attention from organic chemists in recent years indoles with formaldehyde. Therefore, a variety of methylene
owing to their broad spectrum of biological activities.1 For donor or precursor molecules have been explored in this
example, bis(indolyl)methanes (BIMs) are present in a large regard, such as the use of methanol as an alternative to for-
variety of natural products, and their derivatives exhibit maldehyde (Scheme 1a).10 Formic acid has also been used as a
diverse important biological and pharmacological activities source for the methylene group in the synthesis of BIMs.
(Fig. 1).2 Moreover, BIMs are used as dietary supplements to
However, an additional external hydrosilane is required to
help reduce the risk of developing breast and prostate cancer.3 provide the requisite hydrogen atoms (Scheme 1b).11 In par-
Furthermore, BIMs are used as active compounds against ticular, the use of amines and amides, acting as a carbon
recurrent respiratory papillomatosis.4 Recently, BIMs have source through C–N bond cleavage, has emerged as a useful
been studied as potential treatment options for a variety of strategy for the synthesis of BIMs (Scheme 1c).12 In addition,
viral and bacterial infections owing to their innate immune DMSO has previously been used in the synthesis of BIMs
modulating properties.5 In addition, the oxidized forms of using phosphoric acid as a promoter (Scheme 1d).13 Despite
BIMs and their derivatives have been utilized as dyes, as well these formidable achievements, certain problems are still
as colorimetric chemosensors.6 Furthermore, bis-1,3-dicarbo- unsolved, such as using metal catalysts, air sensitive reagents,
nyl compounds are valuable intermediates or precursors for harsh reaction conditions and non-environmentally friendly
organic synthesis and transition metal complexes.7 Owing to
the prevalence of methylene bridged compounds in natural
products and their versatile biological activity, there has been
significant interest in their synthesis.
Traditionally, BIMs can be prepared by reacting of indoles
with various aromatic or aliphatic aldehydes in the presence of
Lewis acids or Brønsted acids.8 Although many reports are
aCollege of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, P. R. China
bState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China.
E-mail: dhuang@fjirsm.ac.cn
†Electronic supplementary information (ESI) available: Characterization of com-
pounds. CCDC 2056479–2056483. For ESI and crystallographic data in CIF or
Fig. 1 Structures of some biologically active bis(indolyl)methane
other electronic format see DOI: 10.1039/d1ob00120e
alkaloids.
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Table 1 Optimization of the reaction conditionsa
Entry
Oxidant
Base
Solvent
Temp (°C)
Yieldb (%)
1
2
3
4
5
6
7
8
Selectfluor
Selectfluor
PhI(OAc)2
K2S2O8
O2
NFSI
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
CsF
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Toluene
DCE
100
100
100
100
100
100
100
100
100
100
100
100
100
80
91
Nonec
None
None
None
None
23
K2CO3
LiHMDS
DBU
57
21
13
71
79
85
38
90
9
10
11
12
13
14
15
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
THF
Dioxane
Dioxane
120
a Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), oxidant
(0.24 mmol), base (0.5 mmol), solvent (1.0 mL), air. b Isolated yield.
c Without MTBE. NFSI
= N-fluorobenzenesulfonimide, LiHMDS =
lithium bis(trimethylsilyl)amide, DBU = 1,8-diazabicyclo[5.4.0]undec-7-
ene, DCE = dichloroethane, THF = tetrahydrofuran.
Scheme 1 Different strategies used to synthesize substituted BIMs.
methods. Therefore, the development of a more sustainable
and mild protocol for the synthesis of BIMs is still a compel-
ling area of research.
MTBE, the desired product was not obtained (entry 2). When
different oxidants, for example PhI(OAc)2, K2S2O8 and oxygen,
were used in this coupling reaction the desired product was
not observed (entries 3–5). Moreover, the transformation did
not proceed when N-fluorobenzenesulphonimide (NFSI) was
used (entry 6). These results reveal that the selectfluor reagent
plays a paramount role in this transformation.
Other alkali salts and/or organic bases, such as CsF, K2CO3,
lithium bis(trimethylsilyl)amide (LiHMDS) and 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU), afforded the product 3a in lower
yields (entries 7–10). Subsequently, the effect of various sol-
vents, such as toluene, dichloroethane (DCE), tetrahydrofuran
(THF) and dioxane were studied, and dioxane was found to be
the optimal choice (entries 1 and 11–13). The effect of temp-
erature was also investigated. The percentage conversion of
indole was examined in a temperature range of 80–120 °C
(entries 14 and 15), and a temperature of 100 °C was selected
as the standard condition based on the transformation
efficiency of compound 3a. It was observed that a lower temp-
erature decreased the yield dramatically, down to 38% at 80 °C
The cleavage of ethers is one of the most fundamental
transformations in organic synthesis, mainly for the degra-
dation or transformation of polyfunctional molecules, particu-
larly in biologically active natural products.14 Based on this,
the utility of ethers as potential reactants in various organic
transformations is an attractive prospect. As a minimally toxic,
inexpensive and low-boiling point compound, methyl tert-
butyl ether (MTBE) is usually used as a solvent.15 However, to
the best of our knowledge, selective C–O bond cleavage in
MTBE and utilization of the released methyl groups as a
methylene precursor agent has not been explored to date.
Herein, we have developed a mild, simple, and environmen-
tally friendly method for synthesizing methylene bridged
compounds.
Results and discussion
All of the reactions were performed under air in 1 mL of (entry 14), and a higher temperature did not improve the
solvent. In order to elucidate the optimal reaction conditions, efficiency of production (entry 15).
1,2-dimethylindole (1a) and MTBE (2a) were selected as the
Using the optimal reaction conditions, we investigated the
initial substrates and the effects of different oxidants, bases, substrate scope and generality of the reaction (Table 2).
solvents and temperatures on the product yield were investi- Initially, we investigated N-methylindole with different substi-
gated (Table 1). Based on the experimental results obtained, it tuents at the C-2 position. The yield of the methyl-substituted
was concluded that the reaction of substrates 1a (0.2 mmol) substrates was higher than that of the phenyl-substituted sub-
and 2a (0.6 mmol) in dioxane at 100 °C for 20 h provided strates (3a, 3b); however, the yield was reduced for substrates
optimal results, with the production of compound 3a in a yield without substitutes (3c). We used a variety of N-substituted
of up to 91% (entry 1). However, without the presence of indoles and to our delight the resultant BIMs were obtained in
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Table 2 Scope of indoles with respect to the BIMs a
effect of steric hindrance on the reaction was studied by
employing a methyl group at different positions on the
benzene ring (3q, 3u–3w). The results demonstrated that the
positions of the substituents did not appear to exert a signifi-
cantly appreciable influence on the efficiency. Different sub-
strates were then evaluated to determine the source of methyl-
ene in the methylenation. As shown in Table 2, the methyl
ether, with a stronger electron-donating ability on the opposite
side, provides a better yield in the reaction (2a–2d). N,N,N,N-
Tetramethylethylenediamine
(TMEDA),
N,N-dimethyl-
formamide (DMF) and dimethyl sulfoxide (DMSO), acting as
methylene precursors, were also found to afford the methylene
bridged products (2e–2g).
The success of the methylenation of the indoles encouraged
us to further extend the substrate scope beyond the indoles.
1,3-dicarbonyl compounds are structurally unique and possess
a variety of interesting chemical properties.16 As shown in
Table 3, a total of 17 examples were selected in order to
analyse the reaction implications. The yield of 1,3-diphenylpro-
panedione, which possesses phenyl substituents on both
sides, was higher than those obtained for compounds with
substituents on only one side (5a–5c). Similarly, under the
Table 3 Scope of the 1,3-dicarbonyl compounds with respect to 5 a
a Reaction conditions:
(0.24 mmol), BuOK (0.5 mmol), dioxane (1.0 mL), 100 °C, 20 h, air,
isolated yield.
1 (0.2 mmol), 2a (0.6 mmol), selectfluor
t
good to excellent yields (3e–3k) (Fig. 2a).17 Interestingly, when
1-allyl-2-methyl-1H-indole weak electron-withdrawing groups
were present on the C-5 position of 1,2-dimethylindole (3l–3o).
Notably, the C–Cl and C–Br bonds remained intact during the
reaction, which provided was used as the reaction substrate,
the product 3i was obtained, which had undergone a double
bond rearrangement. High yields were also obtained if elec-
tron-donating groups and additional information for further
elucidation of the products obtained (3m, 3n). However, the
methoxy substituent in the same position had a slightly lower
yield (3p). We propose that this is because selectfluor could
also break the methoxy C–O bond on the benzene ring of
indole to reduce the contents of the substrate. Moreover, the
a Reaction conditions:
(0.2 mmol), BuOK (0.4 mmol), dioxane (1.0 mL), 80 °C, 20 h, air, iso-
lated yield.
4 (0.2 mmol), 2a (0.6 mmol), selectfluor
t
Fig. 2 Crystal structures of compounds 3j (a) and 5g (b).
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influence of the selectfluor, the reaction yield of the methoxy
substituent on the benzene ring was lower than expected (5d,
5e). High yields were also obtained when there were electron
donating groups and weak electron withdrawing groups on the
para position of the benzene ring of ethyl benzoylacetate (5e–
5i, 71–86%). The electron-withdrawing fluoro group furnishes
the product in a lower yield (5i, 71%) compared to the analo-
gous bromo and chloro groups (5g, 5h) (Fig. 2b).17 However, if
a strong electron withdrawing group, such as trifluoromethyl
substituted ethyl 3-(4-trifluoromethylphenyl)-3-oxopropionate,
is used for the reaction, the yield decreases significantly (5j).
The steric hindrance of the reaction was then studied by utilis-
ing a fluoro group (5i, 5k, 5l) in the para-, meta- and ortho-posi-
tions. The results revealed that the reaction yields decreased
from the substitution of the para-position (5i) to the ortho-
position (5l). Meanwhile, the reaction of MTBE with ethyl 3-
(furan-2-yl)-3-oxopropanoate and ethyl 3-oxo-3-(thiophen-2-yl)-
propanoate gave the target products in good yields of 85%
and 83%, respectively (5o, 5p). Notably, the target product
was also obtained when using ethyl acetoacetate as the reac-
tion substrate (5q). Arylamines have also been used to investi-
gate the availability of our protocol (Scheme 2). Very interest-
ingly, the desired products were successfully obtained (7a, 7b).
In addition, to further demonstrate the practicality and
efficiency of the developed methodology, a gram-scale reaction
was performed. As shown in Scheme 3, 3a (1.8 g, 75%) and 5a
(2.0 g, 83%) could be readily synthesized in a gram-scale
reaction.
Scheme 4 Schematic diagram of the controlled experiment.
Thus, we assume that the formation of neither the BIMs nor
bis-1,3-dicarbonyl compounds are not involved free radical
mediated processes. To examine the possibility of formal-
dehyde as an intermediate, the reaction of (c) and (d) was
implemented by adding polyformaldehyde. However, the
product 3a or 5a was not obtained. Considering the influence
of chloromethyl group within selectfluor, we used 8 instead of
MTBE to participate in the reaction under the best conditions.
However, the desired product was not observed in the reaction
((e) and (f)).
Several controlled experiments were conducted to gain
further understanding about the reaction mechanism
(Scheme 4). When a radical scavenger such as TEMPO was
added ((a) and (b)), the product 3a or 5a was obtained in the
nearly equal yield as that obtained without adding TEMPO.
On the basis of the above results, a possible mechanism
was proposed in Scheme 5. Initially, the intermediate A is gen-
erated by oxidation of MTBE in the presence of selectfluor and
tBuOK. Indoles 1 and 1,3-dicarbonyl compounds 4 then under-
Scheme 2 Reactions of different arylamines with MTBE.
Scheme 3 Gram-scale synthesis of 3a and 5a.
Scheme 5 Schematic diagram of the possible reaction mechanism.
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goes an electrophilic substitution with A to give unstable inter-
mediate B and D, respectively. Finally, in situ elimination of
tert-butanol from B and D produces reactive intermediate C
and E. Azafulvalene C was ready for accepting another 1 to
generate the final product 3. The final product 5 was formed
by reaction of intermediate E with the other portion of 4 via
Michael addition process.
Notes and references
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Conclusions
In summary, we have described a simple and efficient method
for the synthesis of BIMs and bis-1,3-dicarbonyl compounds
using MTBE as a methylene source. This work is superior to
the traditional method of synthesis owing to the simple reac-
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Experimental section
General experimental procedures
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A
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t
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research was supported by the National Natural Science
Foundation of China (Grant No. 21371171), and the Project
supported by the Science Foundation of the Fujian Province,
China (Grant No. 2020J01114).
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