N-Iodosuccinimide-Promoted Rapid Access to Indeno[1,2-c]pyrroles via [3+2] Annulation of Enamine-alkynes

Article abstract of DOI:10.1002/adsc.201600711

We have developed a metal-free, N-iodosuccinimide (NIS)-promoted, cascade strategy for the efficient synthesis of biologically important indeno[1,2-c]pyrroles via a [3+2] annulation process of enamine-alkynes. This methodology had shown a very broad scope

Full text of DOI:10.1002/adsc.201600711

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DOI: 10.1002/adsc.201600711
N-Iodosuccinimide-Promoted Rapid Access to Indeno[1,2-c]pyr-
roles via [3+2] Annulation of Enamine-alkynes
Jampani Santhia and Beeraiah Bairea,*
a
Department of Chemistry, Indian Institute of Technology Madras, Chennai – 600036, India
Fax : (+91)-044-2257-4202; phone: (+91)-044-2257-4206; e-mail: beeru@iitm.ac.in
Received: July 6, 2016; Revised: September 12, 2016; Published online: && &&, 0000
Dedicated to Prof. Thomas R. Hoye on the occasion of his 66th birthday.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600711.
Abstract: We have developed a metal-free, N-iodo- of indeno-pyrroles from diynones via enamine-al-
succinimide (NIS)-promoted, cascade strategy for kynes. Control experiments supported the involve-
the efficient synthesis of biologically important ment of NIS as an electrophilic activator via an ionic
indeno[1,2-c]pyrroles via a [3+2] annulation process mechanism rather than a radical pathway.
of enamine-alkynes. This methodology had shown
a very broad scope for diversely functionalized en-
amines and alkynes. We have also developed a one- Keywords: alkynes; cascade annulations; enamines;
pot, multicomponent strategy for the direct synthesis one pot reaction; pyrroles
Introduction
(NIS)-promoted, transition metal-free, cascade strat-
egy for the rapid generation of indeno[1,2-c]pyrroles
Cascade reactions are highly atom- and step-econom- via a [3+2] annulation of enamine-alkynes. According
ic and provide rapid access to complex frameworks.[1] to our design (Figure 1B), a tethered enaminone-
Recently there has been an increasing interest among alkyne substrate like 1 in the presence of an electro-
the synthetic community to design and develop novel philic promoter such as NIS or I2 can undergo a cas-
cascade strategies. Most of the molecules isolated cade [3+2] annulation process to give the indeno[1,2-
from nature contain either a carbocyclic or heterocy- c]pyrroles 2.
clic unit.[2] Cyclization reaction of alkynes with other
synthons is one of the most convenient strategies for
the synthesis of cyclic systems,[3] e.g., double insertion
of alkynes.[4,5]
Pyrrole is one of the most abundant heterocyclic
compounds in nature.[6] Pyrrole derivatives are em-
ployed as drugs in medicine,[7] and as optoelectronic
materials also.[8] They are also useful as synthons for
the synthesis of alkaloid natural products and drug
molecules. The indeno[1,2-c]pyrroles with a 6-5-5
framework are present in biologically important mol-
ecules[9] and alkaloid natural products (Figure 1A).
The interesting structural unit and important biologi-
cal properties associated with them attracted our at-
tention, to design and develop a mild and efficient
synthetic strategy.
To the best of our knowledge, there are only two
published reports on the synthesis of these structural
units both of which involve Pd-mediated cycliza-
Figure 1. Bioactive molecules with indeno[1,2-c]pyrrole unit
tion.[10] Herein we report an N-iodosuccinimide and our approach to this system.
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Results and Discussion
but the reaction was very slow (58 h) at room temper-
ature. After inncreasing the amount of NIS from 1.1
To test our hypothesis, we prepared a benzenoid to 1.5 equiv. (entry 5) the reaction was relatively
fused enaminone-alkyne 3a as a test substrate, from faster (36 h) and good yielding (71%) for 5a. When
2-iodobenzaldehyde employing an efficient, four-step 2 equiv. of NIS was used (entry 6), enamine-alkyne 3a
synthetic protocol via diynone 4a (see the Supporting was completely consumed within 4 h, at room temper-
Information). Initially, the enamine-alkyne 3a was ature and gave the tricyclic pyrrole 5a as the sole
treated with N-bromosuccinimide (NBS, 2 equiv.) and product in excellent yield (91%).
K2CO3 (2 equiv.) in acetonitrile at room temperature.
Subsequently, we studied this transformation with
Even after 12 h, there was no reaction observed different solvents for obtaining optimized conditions
(Table 1, entry 1).[11] But to our delight, replacement (entries 7–9), employing NIS (2 equiv.) as the promot-
of NBS with I2 (1.2 equiv.) resulted in the formation er. The reaction was unsuccessful in acetone. 1,4-Di-
of the expected tricyclic pyrrole unit 5a, but in only oxane and THF afforded 3a as sole product but in rel-
28% yield (entry 2).
atively lower yields, 76% and 68%, respectively
Encouraged by this result, we anticipated that in- (Table 1, entries 8 and 9). Hence MeCN was chosen
creasing the amount of I2 might give a better result. as the solvent for this transformation. As the temper-
Surprisingly, with 2 equivalents of I2 (entry 3) there ature increases, the reaction was quicker in acetoni-
was no noticeable improvement in the yield (36%) of trile but there was a drop in yield of 3a (Table 1, en-
the indeno-pyrrole 5a.[12] On the other hand, employ- tries 10 and 11). The reaction was successful in the ab-
ing 1.1 equiv. of NIS as the promoter (entry 4) result- sence of K2CO3 but slower and poor yielding
ed in the formation of pyrrole 5a in good yield (57%) (entry 12). With 1 equiv. of K2CO3 there was a slight
improvement in the yield (63%) of 3a.[13]
With an optimized process conditions in hand
[K2CO3 (2 equiv.), NIS (2 equiv.), in MeCN at room
tempeature], we turned our attention to assess the
scope of this new reaction. Initially a scope study for
various amines was performed. The enamines of elec-
tron-rich as well as electron-poor benzylamines (p-
OMe, p-F and m-CO2Me) were found to undergo this
cascade reaction very effectively under the optimized
reaction conditions to provide good yields of the cor-
responding pyrroles 6–8 (Scheme 1). The aniline-de-
rived enaminone was also very successful to generate
the tricyclic pyrrole 9.
Table 1. Reaction optimization study for the synthesis of
indeno[1,2-c]pyrroles.
We next explored the scope of the alkyne substitu-
ents (Scheme 2). Various tertiary alcohols and OTBS
ethers were found to be compatible with the reaction
conditions and gave the corresponding tricyclic pyr-
roles 5b–f in excellent yields (up to 89%) with varying
[a]
After 12 h, 40% of 5a and 18% of unreacted 3a were iso-
lated.
[b]
After 12 h, 66% of 5a and 10% of 3a were isolated.
After prolonged reaction times (24 h) there was no im-
[c]
provement in the formation of 5a.
Yields after chromatographic purification.
No K2CO3 was used.
1 equiv. of K2CO3 was used. THF=tetrahydrofuran,
[d]
[e]
[f]
NR=no reaction.
Scheme 1. Scope study for amines.
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Scheme 2. Scope study for alkyne substituents and enamine substituents.
reaction times (2–7 h). We also employed arylalkyne
substrates for the synthesis of 2,5-diarylpyrrole deriv-
atives 5n and 5o. In continuation, diversely substitut-
ed phenyl groups on the enamine part were tested
under our standard reaction conditions. Various
ortho-, meta- and para-substituted phenyl groups
smoothly underwent the cyclization to furnish the pyr-
roles 5g–l in excellent yields within 4–7 h. A long
chain OTBS ether was also successfully employed to
afford 5m.
Interestingly, when a TMS-acetylene was used as an
alkyne partner (Scheme 3A), e.g., the enaminone-
alkyne 10 underwent cyclization to give an insepara-
ble (1:1) mixture of two products in 83% yield. This
mixture was then treated with TBAF in THF to
remove the TMS groups present in order to achieve
any separation. Delightfully, both the compounds,
after TMS-removal, were separable and could be
characterized as tricyclic pyrrole 11-H (38%) and 2-
iodopyrrole 11-I (41%).
To understand the formation of iodo-pyrrole 11-I,
we designed two control experiments (Scheme 3B).
First, the compound 10 was treated with 4 equiv. each
Scheme 3. Direct synthesis of 2-iodoindeno[1,2-c]pyrrole 11-
of NIS and K2CO3 in MeCN at room temperature for I; and control experiments to reveal its mode of formation.
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4 h. To our surprise, this resulted in the formation of
iodo-pyrrole 11-I as the sole product in excellent
yield (78%). Secondly, the (1:1) mixture of 11-TMS
and 11-I obtained from the standard reaction condi-
tions (2 equiv. each of NIS and K2CO3) was further
subjected to same conditions. In this case all the 11-
TMS was consumed and converted into iodopyrrole
11-I without any loss in total yield. From these obser-
vations, it is clear that the 2-iodopyrrole 11-I was gen-
erated from the 11-TMS under the reaction condi-
tions.[14] The single crystal X-ray diffraction analysis
unambiguously confirmed the structures of both 11-H
and 11-I (Figure 2).[15]
Figure 2. ORTEP diagrams for 11-H and 11-I.
Next a scope study (Scheme 4A) for trialkylsilyl-
acetylenes was performed. The triethylsilyl (TES) and
triisopropylsilyl (TIPS) groups were used on the
alkyne moiety. In this case we have isolated only the
corresponding 2-trialkylsilylpyrrole derivatives 11-
TES and 11-TIPS. No traces of the 2-iodo-compound
11-I was observed. Furthermore, a scope study for en-
amine substituents with TMS-acetylene as the alkyne
partner was also carried out. p-Me, p-OMe, and p-F
substituted aryls underwent the cascade process
smoothly to afford the corresponding tricyclic pyr-
roles. The p-Me substrate gave only the corresponding
TMS product 12-TMS even with 4 equiv. of NIS. On
the other hand, the p-OMe analogue resulted in 2-
iodo-pyrrole 13-I as the only product with 2 equiv. of
NIS itself. This may be due to the fact that p-OMe is
Scheme 4. Scope study for trialkylsilyl groups, TMS-acety-
lene substrates and, synthetic utility of 13-I.
an electron-richer system than the p-Me one. In the the corresponding diynones 4a and 17 (Scheme 5).
case of the electron-poor p-F-phenyl substituent, the Both the ketones 4a and 17 upon treatment with
TMS group was lost under reaction conditions to give BnNH2 in the presence of MgSO4 at room tempera-
14-H.
ture for 12 h followed by the addition of NIS and
The synthesized tricyclic 2-iodopyrrole units can be K2CO3 (2 equiv. each for diyne 4a and 4 equiv. each
useful as building blocks for the synthesis of various for diyne 17) and acetonitrile, smoothly underwent
organic functional materials as well as for the bioac- formation of the respective enamines 3a and 10 fol-
tive molecules. In order to highlight the synthetic util- lowed by cyclization to afford the corresponding tricy-
ity of the indenopyrroles, one of the iodides, 13-I, was clic pyrroles 5a and 11-I in 73% and 54% yields, re-
converted into highly functionalized conjugated sys- spectively.
tems 15 and 16, by means of the Sonogashira coupling
with phenylacetylene and the Suzuki coupling reac- tails we have performed two control experiments
tion with Ph-B(OH)2, respectively (Scheme 4B). (Scheme 6A). In first experiment, to understand the
Finally, in order to understand the mechanistic de-
To further improve the synthetic potential of this role of the promoter, i.e., NIS, during the cascade
protocol, we have also performed this transformation process, we carried out the reaction in the absence of
as a one-pot, multicomponent process starting from NIS, both at room temperature and under reflux con-
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Scheme 5. One-pot, multicomponent strategy for the direct
synthesis of indeno[1,2-c]pyrroles from diynones.
ditions. Interestingly, there was no reaction observed
under both sets of conditions and all the starting ma-
terial 3a was recovered. This observation supports the
involvement of NIS during the designed [3+2] annu-
lation process.
Secondly, to distinguish between the radical and
ionic pathways, the reaction was performed in the
presence of a radical scavenger, TEMPO. In presence
of 2 equivalents of TEMPO, the reaction was success-
ful and gave the product 5a in 81% yield within 4 h at
room temperature. This experiment supports the ionic
mechanism over the radical pathway. Based on these
observations we propose
a possible mechanism
(Scheme 6B). Initially the iodonium ion (from NIS)
activates the alkyne in 18 towards intramolecular nu-
cleophilic addition of the enamine unit to give the
vinyl iodide 19. Upon proton abstraction with base,
19 undergoes an isomerization to an enamide ion 20.
This 20 after anionic electrocyclization followed by
iodide elimination results in the formation of the tri-
cyclic indeno[1,2-c]pyrrole 21.
Scheme 6. Control experiments and proposed mechanism.
The tricyclic 2-iodopyrroles generated via TMS-acety-
lene as alkyne partner, were well utilized for the syn-
thesis of highly conjugated functionalized molecules
which can be of potential use as organic materials. We
Conclusions
In conclusion we have developed a transition metal- have also shown that this transformation can be per-
free, NIS-promoted, cascade approach for the rapid formed as a one-pot multicomponent process to ach-
generation of indeno[1,2-c]pyrroles via a [3+2] annu- ieve the target indeno[1,2-c]pyrroles directly from the
lation of enamine-alkynes. This methodology has corresponding diynes. Control experiments support
shown broad scope for enamines (including amines the involvemnt of the NIS as an electrophilic activator
and alkyne substituents) as well as alkyne partners. via an ionic mechanism.
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Representative Synthesis of 2-Benzyl-3-(2-hydroxy-
propan-2-yl)-1-phenylindeno[1,2-c]pyrrol-8(2H)-one
(5a)
To a solution of the enamine 3a (40 mg, 0.101 mmol) in an-
hydrous acetonitrile (4 mL) at room temperature were
added N-iodosuccinimide (NIS) (45 mg, 0.202 mmol) and
K2CO3 (28 mg, 0.202 mmol) and the reaction mixture was
stirred at the same temperature for 6 h. The reaction mix-
ture was then diluted with water (10 mL) and extracted with
ethyl acetate (2ꢁ10 mL). The combined organic layer was
washed with brine (10 mL), and dried (MgSO4). Evapora-
tion of the solvent and purification of the crude material by
flash column chromatography (4:1 hexane:EtOAc) gave
indeno[1,2-c]pyrrole 5a as a dark yellow solid; yield: 35 mg
(0.09 mmol, 91%); mp 133–1358C. 1H NMR (400 MHz,
CDCl3): d=7.59 (2H, t, J=7.7 Hz), 7.40–7.47 (2H, m),
7.31–7.38 (4H, m), 7.24–7.30 (2H, m), 7.20 (1H, d, J=
7.4 Hz), 7.14 (1H, t, J=7.4 Hz), 6.94 (2H, d, J=7.1 Hz),
5.41 (2H, s) and 1.62 (6H, m); 13C NMR (100 MHz, CDCl3):
d=187.2, 141.1, 140.2, 139.4, 137.3, 135.7, 133.3, 129.6, 129.5,
129.1, 128.9, 128.5, 127.6, 127.3, 126.4, 125.4, 124.0, 123.2,
123.0, 71.5, 50.0 and 31.4; IR (neat): n=3426, 1601, 2925,
2853, 1674, 1491, 1376, 1376, 907, 770 cmÀ1; TLC: Rf =0.5
(4:1; hexand:EtOAc).
For full details of all experiments, and spectroscopic data
for all new compounds see the Supporting Information
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Acknowledgements
We thank Indian Institute of Technology Madras, Chennai
for the financial support through CHY/13-14/622/NFSC/
BEER grant. JS thanks IIT Madras for an HTRA fellowship.
We thank Mr. Ramkumar for the single crystal X-ray analy-
sis.
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[11] Use of increased reaction temperatures did not result
in any reaction.
[12] Use of 4 equivalents of I2 and elevated temperatures
failed to improve the yield of the 3a.
[13] There was no improvement observed in the yield of 3a
when employed at 808C in the presence of 1 equiv. of
K2CO3 and 2 equiv. of NIS in acetonitrile.
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paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. The ORTEP dia-
grams and further details are given in the Supporting
Information (Tables 1 & 2).
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8 N-Iodosuccinimide-Promoted Rapid Access to Indeno[1,2-
c]pyrroles via [3+2] Annulation of Enamine-alkynes
Adv. Synth. Catal. 2016, 358, 1 – 8
Jampani Santhi, Beeraiah Baire*
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