C3-Alkenylation between Pyrroles and Aldehydes Mediated by a Br?nsted Acid and a Br?nsted Base

Article abstract of DOI:10.1002/ejoc.201901228

A transition-metal-free, redox-neutral, organocatalytic C3-alkenylation of pyrroles is reported. Readily available aldehydes were employed as alkenylating agent and the reaction tolerates several key functional groups. The E-alkenylated products were isolated in moderate to exclusive selectivity. A one-pot two-fold alkenylation strategy is also developed for further downstream modifications. To show the applicability, synthetically challenging indolylpyrrole derivatives were synthesized using Cadogan cyclization.

Full text of DOI:10.1002/ejoc.201901228

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Accepted Article
Title: Transition-Metal-Free, Brønsted Acid/Brønsted Base Catalyzed
C3-Alkenylation of Pyrroles Using Aldehydes
Authors: Samrat Sahu, Avijit Roy, Mahadeb Gorai, Sudip Guria, and
Modhu Sudan Maji
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201901228
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10.1002/ejoc.201901228
European Journal of Organic Chemistry
COMMUNICATION
Transition-Metal-Free, Brønsted Acid/Brønsted Base Catalyzed
C3-Alkenylation of Pyrroles Using Aldehydes
Samrat Sahu,[a] Avijit Roy,[a] Mahadeb Gorai,[a] Sudip Guria[a] and Modhu Sudan Maji*[a]
Dedication ((optional))
Abstract: A transition-metal-free, redox-neutral, organocatalytic C3-
alkenylation of pyrroles is reported. Readily available aldehydes were
employed as alkenylating agent and the reaction allows tolerance of
several key functional groups in the products. The E-alkenylated
products were isolated in moderate to exclusive selectivity. A one-pot
two-fold alkenylation strategy is also developed for further
downstream modifications. To show the applicability, synthetically
challenging indolylpyrrole derivatives were synthesized using
Cadogen cyclization.
Introduction
Synthesis of functionalized pyrroles have drawn considerable
attention to the synthetic organic chemists over the course of
decades due to their widespread occurrence in a greater number
of natural products,[1] semiconductor materials for organic thin-
film transistors (OTFTs), organic photovoltaics (OPVs), light-
emitting diodes (LED), and nanocomposites.[2] Aside from
constituting the synthetic core of several essential biologically
active natural products (Figure 1),[1b] these compounds have
served as the precursors of other polypyrroles
Scheme 1. Literature reports on pyrrole C3-alkenylation and our hypothesis.
Wittig reaction, or via a one-pot three-component coupling type
reaction involving pyrrole ring formation.[4b,4m,5] Although these
classical methods have their own values, they are still plagued by
several limitations such as reaction requires high temperature,
installation of protecting group, synthesis of prefunctionalized
starting materials, and limited substrates scopes. With the
Figure 1. Pyrrole containing Natural Products.
advancement of transition-metal catalyzed cross-coupling
reactions, Gaunt et al. reported Pd(OAc)2 catalyzed regioselective
C3-alkenylation of N-TIPS pyrrole using O2 as an oxidant.
(e.g., prodigiosenes, porphyrins), indolizidine, indolizine, and
However, it is limited to activated alkenes and requires external
diketopiperazine alkaloids.[3,1d] As a synthetic intermediate,
oxidant (Scheme 1a).[5i] Wittig olefination of pyrrole-3-
pyrroles have well been utilized for the synthesis of several
carboxaldehyde is plagued by the usage of highly sensitive and
aromatic heterocycles, especially indole derivatives via classic
[4+2] annulation, 6π-electrocyclization as well as “back to front”
approach.[4] Traditionally, C-alkenylpyrroles have mainly been
made using Knoevenagel-type condensations, de-hydration,
along with the presence of few seminal reports on the synthesis
expensive triethylphosphine and scope of the reaction is limited
to only fumaronitrile as a coupling partner (Scheme 1b).[4m]
Considering the synthetic utilities of these powerful synthons
of 3-vinylpyrroles set the stage for us an in-depth investigation of
a new and alternative method from simple starting precursors via
[a]
Samrat Sahu, Avijit Roy, Mahadeb Gorai, Sudip Guria, Dr. Modhu
Sudan Maji
Department of Chemistry
Indian Institute of Technology Kharagpur
Kharagpur, West Bengal – 721302, India.
E-mail: msm@chem.iitkgp.ac.in
step-economic and mild protocol. We thereby report an
organocatalytic, redox-neutral, transition-metal-free strategy to
access a wide variety of 3-alkenylated pyrroles using aldehyde as
an alkenylating agent (Scheme 1c).
Homepage: www.msmlabiitkgp.com
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10.1002/ejoc.201901228
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Results and Discussion
regenerate the DBU back in the catalytic cycle, causing the
diminished yield. To overcome this, we thought of introducing a
second Brønsted base 1,1,3,3-tetramethylguanidine (TMG) in the
elimination step. To our joy, when 20 mol% of TMG was used in
addition to 20 mol% DBU, the yield of 5a improved significantly
(64%, entry 6). However, the E/Z selectivity of the product 5a
remained the same. To check the effect of the temperature on the
outcome, we next conducted the reaction at various temperatures
(entries 8-9). However, yields got diminished (43-57%) and E/Z
selectivity did not improve either. Changing the solvent to non-
polar and polar aprotic solvents did not improve the yield (43-51%,
entries 10-11). Increasing the amount of Cs2CO3 to 3.0 equiv was
also not effective (entry 12). The combination of DBU and TMG
bases was needed for the optimum yield, as a trace amount of 5a
was formed in the absence of DBU as evidenced by the controlled
experiment (entry 13).
An initial investigation suggested that, unlike our previous report
on C3-alkenylation of unprotected indoles,[6] a one-pot reaction
condition was found to be problematic on the pyrrole system
because of its known sensitivity towards the acidic environment
which leads to pyrrole decomposition. For the protection of
pyrrole's N-atom, as described earlier, the triisopropylsilyl (TIPS)
group was chosen because of its steric bulk which forces the
electrophilic attack through C3-position along with its easier
inclusion and subsequent removal under mild conditions. In this
context, sulfone starting precursors 3 and 4 were synthesized
according to the literature reported method.[7]
Table 1. Optimization of the reaction conditions for the synthesis of β-
alkyl-3-alkenyl pyrrolesa
Entry Base / Equiv
Catalyst (mol%)
Temp
( oC)
Yieldb
E/Zc
1
2
NaH/1.5
NaH/1.5
DBU/5
rt
<5%
<5%
<5%
8%
-
-
DBU/20
rt
3
NaH/1.5
DABCO/20
rt
-
4
Cs2CO3/1.5
Cs2CO3/2.5
Cs2CO3/2.5
NaH/2.5
DBU/20
70
70
70
70
80
40
70
70
70
70
3:1
3:1
3:1
3:1
2.7:1
3:1
3:1d
3:1e
3:1
3:1
5
DBU/20
16%
64%
53%
57%
43%
51%
43%
62%
<5%
6
DBU/20+TMG/20
DBU/20+TMG/20
DBU/20+TMG/20
DBU/20+TMG/20
DBU/20+TMG/20
DBU/20+TMG/20
DBU/20+TMG/20
TMG/20
7
8
Cs2CO3/2.5
Cs2CO3/2.5
Cs2CO3/2.5
Cs2CO3/2.5
Cs2CO3/3.0
Cs2CO3/2.5
9
10
11
12
13
[a] Reactions conditions: 4a (0.5 mmol), Cs2CO3 (1.25 mmol), DBU (0.1
mmol), TMG (0.1 mmol), THF (2.5 mL), 70 oC. [b] Isolated yield. [c] E/Z
ratio determined from the 1H NMR of the crude mixture. [d] Ether as
solvent. [e] CH2Cl2 as solvent. TMG = 1,1,3,3-tetramethylguanidine.
With these required sufones in hand, at first we concentrated
our attention towards finding an optimized condition. Taking 4a as
a model substrate, at first, we employed our standard conditions
obtained from the indole system (Table 1, entry 1).[6] Unfortunately,
it gave the desired product in trace amount. Increasing the
equivalency as well as changing the catalyst to DABCO did not
provide any fruitful results (entries 2-3). Switching the base from
NaH to Cs2CO3, and increasing the temperature to 70 oC provided
the desired alkenylated product 5a in only 8% yield (entry 4).
However, unlike the indole system where generally E-alkenylated
products were formed exclusively, a mixture of both the E- and Z-
isomers were isolated in moderate selectivity (E/Z 3:1). While
increasing the equivalency of Cs2CO3 did improve the yield to
16% (entry 5) we envisioned that Cs2CO3 might not be able to
Scheme 2. Scope of the aldehydes. [a] Reaction conditions: condition A: 4a-
4i (0.5 mmol), Cs2CO3 (1.25 mmol), DBU (0.1 mmol), TMG (0.1 mmol), THF
(2.5 mL), 70 oC, isolated yields; condition B: 4j-4q (0.5 mmol), NaH (0.75
mmol), DBU (0.1 mmol), THF (2.5 mL), rt for 0.5 h. [b] Crude 4 obtained from 3
was employed.
Having this optimized protocol in our hand, we intended to
investigate the scope of this method with respect to various
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COMMUNICATION
aldehydes (Scheme 2). Sulfonylpyrroles bearing linear as well as
branched alkyl chains such as ethyl, n-butyl, benzyl, and iso-
propyl underwent smooth sulfone elimination to provide
alkenylated products 5b-5d in 64-71% yields. We noticed that with
the increased branching at the β-position of the aldehydes
increases the E/Z selectivity of the alkenes (from 3:1 for 5a to 7:1
for 5d). Sulfone bearing ether and thioether functionalities were
also suitable reaction partners and provided alkenylated products
5e-5f in 54-71% yields. Albeit low yield, challenging 3-alkenyl
pyrroles such as 5g bearing two free N–H groups and 5h having
benzyl ester group were isolated successfully. It is important to
note that some of the β-alkyl-3-alkenylpyrroles 5a-5d are volatile
under high vacuum and require special care during isolation.
Tosyl protected indole substituted aldehyde was well tolerated
giving 5o in 65% yield. Sulfonylpyrroles having β,β-di-
substitutions were also compatible under the reaction conditions
furnishing 5p-5q in 57-61% yields.
Scheme 4. Scope of the one-pot bis-alkenylation of 1i. [a] Reaction Conditions:
Step 1: 1i (0.5 mmol), 2 (aldehyde 1, 0.6 mmol), PTSA·H2O (0.05 mmol),
PhSO2H (0.6 mmol), CHCl3 (2.5 mL), rt; Step 2: 2 (aldehyde 2, 0.55 mmol),
PhSO2H (0.6 mmol); Step 3: Cs2CO3 (2.0 mmol), DBU (0.125 mmol), TMG
(0.125 mmol), 70 oC, isolated yields.
To check the endurance of various functionalities on the
pyrrole ring, we next evaluated several pyrrole derivatives under
the reaction parameters (Scheme 3). However, unlike for the
aldehyde scope, where both the sulfone intermediates were
required to be isolated before proceeding to final reaction, we
opted for a one-pot sequential protocol which significantly
reduces the time, labor and cost. Pyrroles having aromatic as well
as aliphatic chains in the ring reacted elegantly providing
alkenylated products 6a-6c in 50-57% yields. It is important to
note that for compound 6b only (E)-product was isolated, which
proves the fact that steric bulk at the adjacent centers of the newly
formed double bond dictates the stereochemistry of the resulting
product. Pyrroles having ester groups at the C3- and C2-position
underwent smooth reaction to provide corresponding alkenes 6d-
6g in moderate yields (50-63%). Remarkably, vinylation was also
achieved using acetaldehyde, giving 6f in 63% yield. This, in turn,
is an extremely difficult challenge for transition metal catalyzed
cross-coupling reaction due to the difficulties in using ethylene
gas as a coupling partner. To check whether an active aldehyde
moiety in the pyrrole ring can be survived in the presence of
another external aldehyde, we took 2,5-dimethylpyrrole-3-
carboxaldehyde as a coupling partner. The mild nature of the
reaction allowed the survival of the ring aldehyde moiety and 6h
was isolated as a sole product in 61% yield. 2,5-Dimethylpyrrole
was found to be reactive enough and products 6i-6j were isolated
in 52-57% yields. Pyrroles having fused cyclohexyl group as well
as -CN and -SPh groups in the aromatic ring are also found to be
Scheme 3. Scope of the functionalized pyrroles. [a] Reaction Conditions: Step
1: 1 (0.5 mmol), 2 (0.6 mmol), PTSA·H2O (0.05 mmol), PhSO2H (0.6 mmol),
CHCl3 (2.5 mL) at rt; Step 2: Cs2CO3 (1.25 mmol), DBU (0.075 mmol) and TMG
(0.075 mmol), 70 oC, isolated yields.
While β-alkyl functionalities were readily incorporated under
this conditions, a major roadblock was observed when we tested
the compatibility of β-aryl group under the optimized conditions.
Fortunately, the combination of NaH and DBU was found to be
suitable for these transformations and exclusively (E)-isomer of
the products were isolated in comparison to their alkyl counterpart.
Aryl group bearing electron-withdrawing and donating groups in
the aromatic ring reacted elegantly giving 5j-5m in 57-80% yields.
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10.1002/ejoc.201901228
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suitable enough under the reaction conditions providing alkenes
6k-6l in 46 % and 39% yields.
to check whether indolyl pyrrole derivatives[8] can be made via
Cadogen cyclization (Scheme 5). To do so 6i, having one 2-
nitrophenyl moiety at the β-position of alkene were subjected
under the standard Cadogen cyclization conditions (Scheme 5).
Gratifyingly, indolyl pyrrole derivative 7 was isolated in 62% yield.
A similar two-fold protocol was also successfully applied on
substrate 6n to furnish 8 in 41% yield.
Encouraged by one-pot sequential protocol, we wanted to
take it one step further and simultaneously functionalized C3- and
C4-positions of pyrroles with electrophiles of our choice. This
flexibility and modularity not only opens up the further fine-tuning
of molecular complexity, but it also serves as a fundamental
approach to the modular synthesis of these important organic
backbones. To the best of our knowledge, despite having both
transition-metal-catalyzed and metal-free reports present in the
literature, till date only one report of –NH free one-pot bis-
alkenylation of pyrrole has been found in the literature.[5m]
Conclusions
In summary, we have developed a transition-metal-free, redox-
neutral C3-alkenylation of pyrrole using aldehyde as an
alkenylating agent. Tolerance of several key functional groups
such as ether, thioether, protected amine, benzyl and ethyl esters,
cyano, thiophenol in the aldehyde chain as well as pyrrole ring
opens up the possibilities of further post-synthetic modifications.
Remarkably, a one-pot sequential two fold alkenylation was also
achieved by using two different aldehydes, resulting the scope of
further fine tuning. To show the applicability of this protocol,
challenging indolyl pyrrole derivatives were synthesized using
Cadogen cyclization. Considering its wide applicability, we
believe this method will be a suitable alternative to the existing
ones.
Experimental Section
Scheme 5. Synthesis of indolyl-pyrrole compounds using Cadogen cyclization.
[a] Reaction Conditions: 6i, 6n (0.2 mmol), P(OEt)3 (0.5-1.0 mL), EtOAc (5 mL),
160 oC, isolated yields.
Condition A: Under an argon atmosphere in a flame-dried pressure tube,
the sulfonyl pyrrole 4 (1.0 equiv) was dissolved in dry THF and then
Cs2CO3 (2.5 equiv), DBU (20 mol%) and 1,1,3,3-tetramethylguanidine (20
mol%) were added. The mixture was stirred at 70 °C. The reaction mixture
was passed through celite pad and washed with DCM (3 × 5 mL). The
combined organic layers were dried over Na2SO4, concentrated and
purified immediately on a basic alumina column to furnish pure products
5a-5i.
To check the feasibility of this hypothesis, we took
commercially available 2,5-dimethylpyrrole as a pyrrole surrogate
and added phenylacetaldehyde and 2′-nitrophenylacetaldehyde
sequentially after the consumption of the first one (Scheme 4).
After complete consumption of both the starting materials all the
required bases and catalyst i.e., Cs2CO3 (4.0 equiv), DBU (0.25
equiv) were added in one-pot; and bis-alkenylated pyrrole 6m,
bearing two different alkenyl functionalities, was isolated in 30%
yield. This one-pot two-fold alkenylation strategy goes through
four different reactions with an average yield of 75% per step. It is
important to mention that adding 2.0 equivalent of aldehyde at the
beginning significantly deteriorate the reaction profile as well as
the yield of the reaction. Compatibility of phenylacetaldehyde and
phosphonate aldehyde was tested effortlessly and the
corresponding product 6o was isolated in 41% yield. To check
whether similar aliphatic aldehydes can be utilized for bis-
alkenylation we performed one reaction where butanal was added
as a second aldehyde. While both the sulfone as well as alkene
formation occurred as anticipated, unfortunately, we could not
isolate the final alkene 6p in sufficient purity due to the instability
of the alkene under air as well as column chromatographic
conditions.
Condition B: To a solution of sulfonyl pyrrole 4 (1.0 equiv) in dry THF
under an argon atmosphere in a flame dried pressure tube, NaH (60%
suspension in mineral oil, 1.5 equiv) and DBU (20 mol%) were added
successively and the mixture was stirred at room temperature. The
reaction was quenched carefully with distilled water. The organic layer was
separated, and the water layer was extracted two times with EtOAc (2 ×
10 mL). The combined organic layers were dried over Na2SO4,
concentrated and purified immediately on a basic alumina column to
furnish pure products 5j-5q.
Acknowledgments
M.S.M.
gratefully
acknowledge
DST/INSPIRE
Faculty
Award/2013/ DST/INSPIRE/04/2013/000681 for funding. We
thank DST (Sanction No. SR/FST/CSII-026/2013) for 500 MHz
NMR facility at IIT Kharagpur. S.S. sincerely thanks CSIR India
for fellowship.
Cadogen cyclization has been found to have profound
importance in the synthetic community and has been utilized over
the years for the synthesis of indoles skeleton. Tolerance of 2-
nitrophenylmoiety under the reaction conditions next prompted us
Keywords: Pyrrole • C-H Alkenylation • Redox-Neutral •
Transition-Metal-Free • Organocatalytic
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Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
Key Topic*
Samrat Sahu, Avijit Roy, Mahadeb
Gorai, Sudip Guria and Modhu Sudan
Maji*
Page No. – Page No.
Title: Transition-Metal-Free, Brønsted
Acid/Brønsted Base Catalyzed C3-
Alkenylation of Pyrroles Using
Aldehydes
*Pyrrole Alkenylation
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