Synthesis of star-shaped pyrrole-based C3-symmetric molecules via ring-closing metathesis, Buchwald–Hartwig cross-coupling and Clauson–Kaas pyrrole synthesis as key steps

Article abstract of DOI:10.1016/j.tetlet.2018.01.084

We have demonstrated three contrasting synthetic strategies to assemble pyrrole-based C₃-symmetric molecule. Here, ring-closing metathesis (RCM), Buchwald–Hartwig cross-coupling and Clauson–Kaas pyrrole synthesis have been used as key steps. In

Full text of DOI:10.1016/j.tetlet.2018.01.084

Accepted Manuscript
Synthesis of Star-Shaped Pyrrole-Based C -Symmetric Molecules via Ring-
3
Closing Metathesis, Buchwald–Hartwig Cross-Coupling and Clauson–Kaas
Pyrrole Synthesis as Key Steps
Sambasivarao Kotha, Saidulu Todeti, Tarasankar Das, Anindya Datta
PII:
S0040-4039(18)30129-1
https://doi.org/10.1016/j.tetlet.2018.01.084
TETL 49671
DOI:
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
26 December 2017
24 January 2018
28 January 2018
Please cite this article as: Kotha, S., Todeti, S., Das, T., Datta, A., Synthesis of Star-Shaped Pyrrole-Based C -
3
Symmetric Molecules via Ring-Closing Metathesis, Buchwald–Hartwig Cross-Coupling and Clauson–Kaas Pyrrole
Synthesis as Key Steps, Tetrahedron Letters (2018), doi: https://doi.org/10.1016/j.tetlet.2018.01.084
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Synthesis of Star-Shaped Pyrrole-Based C -Symmetric Molecules via Ring-
3
Closing Metathesis, Buchwald–Hartwig Cross-Coupling and Clauson–Kaas
Pyrrole Synthesis as Key Steps
a
a
a
a
Sambasivarao Kotha ,* Saidulu Todeti Tarasankar Das and Anindya Datta
,

1
Tetrahedron Letters
Synthesis of Star-Shaped Pyrrole-Based C -Symmetric Molecules via Ring-
3
Closing Metathesis, Buchwald–Hartwig Cross-Coupling and Clauson–Kaas
Pyrrole Synthesis as Key Steps
a
a
a
a
Sambasivarao Kotha ,* Saidulu Todeti
,
Tarasankar Das and Anindya Datta
a
Department of Chemistry, Indian Institute of Technology Bombay, Mumbai, 400 076, India
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
3
We have demonstrated three contrasting synthetic strategies to assemble pyrrole-based C -
symmetric molecule. Here, ring-closing metathesis (RCM), Buchwald–Hartwig cross-coupling
and Clauson–Kaas pyrrole synthesis have been used as key steps. Interestingly, readily available
starting materials such as 4-aminoacetophenone, 4-bromo acetophenone and 4-nitro acetophenone
have been used as starting materials. Grubbs’ first generation catalyst (G-I) is useful for one-pot
RCM and aromatization sequence without the involvement of additional reagents. We also report
photophysical properties of these star-shaped molecules.
Available online
Keywords:
trimerization
2
009 Elsevier Ltd. All rights reserved
ring-closing metathesis
Buchwald–Hartwig cross-coupling reaction
Clauson–Kaas pyrrole synthesis
Introduction
Trimerization of carbonyl compounds is instrumental
to assemble C -symmetric star-shaped molecules and they are
found to be useful core units in conjugated dendrimers and
3
1
1
-2
branched oligomers.
Moreover, conjugated star-shaped
3
4
molecules have been used in liquid crystals, photovoltaics,
organic field-effect transistors (OFETs),
1
b,5
organic light-
1b,6
2a
emitting diodes (OLEDs),
electroluminescent devices and
6
a,7
organic solar cells.
Several
C
3
-symmetric molecules
Figure 1. Bio-active molecules containing pyrrole moieties.
containing heterocycles (eg: carbazole, and N-heterocyclic
The metathesis strategy has emerged as one of the most
useful tool for the carbon-carbon bond formation in synthetic
organic chemistry. Ring-closing metathesis (RCM) is well
carbenes) have been synthesized and utilized in electro-optical
8
materials, and separation science. As part of our major program
1a,9
aimed at designing star-shaped C
here, we conceived new strategies to pyrrole-based C
symmetric molecules.
3
-symmetric molecules,
1
3
suited to prepare the cyclic olefins in high yield. With the
advent of structurally well-defined ruthenium based metathesis
precatalysts (Figure 2) during the last two decades, assembly of
heteroaromatics by intra and intermolecular C-C bond
3
-
Synthesis of pyrrole derivatives is of great interest due to their
applications in material science and pharmaceutical area. In
addition, a number of biologically important molecules contain
14
formation has gained momentum.
1
0
pyrrole as a core unit (Figure 1). For example, pyrrole
moieties have played an important role in haemoglobin function
11
to carry the oxygen in living organisms. Pyrrole derivative,
atorvastatin is used for reducing the cholesterol level and
ketorolac has been used as nonsteroidal anti-inflammatory
12
drug (NSAID).
Figure 2. Selected list of ruthenium-based metathesis catalysts.
In view of the importance of pyrroles, we conceived several
strategies to C
3
-symmetric molecule 4 containing three pyrrole
units. To the best of our knowledge there is no report available

2
in the literature to the synthesis of pyrrole-based C
star-shaped molecules. In our approach, pyrrole-based C
3
-symmetric
RCM require separate step for aromatization. Interestingly, our
approach involving G-I catalyst does not require aromatization
step which is better sustainable sequence.
3
-
symmetric star-shaped molecules are assembled by RCM,
15
Buchwald–Hartwig cross-coupling and Clauson–Kaas pyrrole
16
synthesis as key steps. We also studied photophysical
3
properties of these C -symmetric molecules.
Strategy
The retrosynthetic strategy to pyrrole-based star-shaped
compound 4 is shown in Figure 3. C -Symmetric star-shaped
3
compound 4 could be assembled from hexa-allyl derivative 3
via RCM followed by aromatization. The hexa-allyl derivative 3
could be assembled from di-allyl ketone 2 by trimerization
sequence which can be derived from readily available 4-amino
acetophenone (1) (path A). Alternatively, C
3
-symmetric product
-symmetric tri-bromide 5 (Figure
) via Buchwald–Hartwig cross-coupling reaction. However,
4
could be assembled from C
3
3
the tri-bromo derivative 5 can be derived from a readily
available 4-bromo acetophenone (6) by trimerization sequence.
These three distinctly different approaches provide several
3
opportunities to expand the chemical space of C -symmetric
Scheme 1. Synthesis of star-shaped compound 4 via RCM.
pyrrole derivatives. Finally, application of three fold Clauson–
Kaas pyrrole synthesis starting with 5 can generate the target
compound 4.
Later, alternate routes were developed to this star-shaped
pyrrole containing compound 4. In this regard, compound 9 was
reacted with 1,4-dimethoxy tetrahydrofuran (10) in acetic acid
under reflux condition for 6 h to deliver the pyrrole-based C
3
-
symmetric compound 4 in 63% yield (Scheme 2).
Scheme 2. Synthesis of star-shaped pyrrole derivative 4 by
using Clauson–Kaas pyrrole synthesis.
In another route, we prepared the tri-bromo derivative 5 by
Figure 3. Retrosynthetic routes to pyrrole-based C
symmetric star-shaped molecule 4.
3
-
using the literature procedure from commercially available 4-
20
bromo acetophenone (6).
Subsequently, the tri-bromo
compound 5 was subjected to Buchwald–Hartwig cross-
coupling reaction with pyrrole (11) in the presence of potassium
carbonate (K CO ) and, copper powder in dimethyl sulfoxide
(DMSO) at 130 ºC under inert atmosphere to obtain the star-
shaped molecule 4 in 68% yield (Scheme 3). The approaches
demonstrated here start with simple and readily available
starting materials. Moreover, these approaches are imbeded
with several diversity points in terms of strategies and coupling
partners. Furthermore, RCM and aromatization occur in one-pot
sequence unlike earlier reports.
Results and Discussion
To realize the strategy shown in Figure 3 (Path A),
commercially available 4-amino acetophenone (1) was treated
with allyl bromide in the presence of sodium hydride (NaH) in
dimethylformamide (DMF) at room temperature for 1 h which
delivered the N,N'-diallyl product 2 (68%). Then, the di-allyl
compound 2 was subjected to trimerization sequence with
2
3
2
thionylchloride (SOCl ) in EtOH under reflux condition to
generate the trimerized product 3 in 42% yield. To improve the
yield of the compound 3, we choose another route. In this
regard, we prepared tri-amine derivative 9 by using literature
17
procedure. Star-shaped tri-amine derivative 9 was subjected to
N-allylation by using allyl bromide in the presence of NaH in
DMF at room temperature for 2 h to give the hexa-allyl product
3
in 72% yield. Later, it was treated with Grubbs first
generation catalyst (G-I) in CH Cl at room temperature for 2 h
2
2
to obtain the ring-closing metathesis (RCM) product 4 (84%)
which underwent aromatization. The major driving force for
facile dehydrogenation sequence under the reaction conditions
is the gain in aromatization energy (Scheme 1). Such type of
non-metathetic behaviour of Grubbs catalyst is known in the
Scheme 3. Synthesis of star-shaped pyrrole derivative 4 via
Buchwald–Hartwig Cross-Coupling reaction.
1
8
19
literature. Notably, earlier approach to pyrroles involving

3
Time-resolved fluorescence measurements of star-shaped
derivatives 3 and 4
We also reported the photophysical properties of these star-
shaped molecules. Here, we have observed N,N’-diallyl C -
3
symmetric product 3 showing more lifetime than pyrrole-based
star-shaped derivative 4.
Steady-state and time-resolved fluorescence measurements
Figure 4) were performed in dilute solutions contained in
(
standard quartz cuvettes. The superimposability of excitation
and absorption spectra of the samples suggests a high degree of
purity of the compounds prepared. Emission spectrum for each
compound is obtained, with excitation at the wavelength of the
absorption maximum.
Steady-state and time resolved fluorescence
Absorption and fluorescence spectra were recorded on
a Jasco V530 spectrophotometer and a Varian Cary Eclipse
fluorometer, respectively. Band widths of 5 nm were used on
the excitation and emission sides for fluorescence
measurements. The samples were excited at 340 nm. TCSPC
measurements were performed on an IBH Fluorocube time
resolved fluorescence spectrophotometer. A Nanoled emitting at
340 nm was used to excite the samples. Instrument response
function was 800 ps. Further detail about the instrument is
1
0
0
0
.2
.9
.6
.3
4
IRF
4
Fit
3
Abs
Exc
Emi
3
2
1
0
1
1
0
0
Fit
0
.0
.9
0
3
Abs
Exc
Emi
21
0.6
0.3
0.0
10
10
available elsewhere. The lifetime values were obtained by
22
fitting the fluorescence decays to multiexponential functions
2
300
400
Wavelength (nm)
500
600
by an iterative reconvolution technique using reduced χ as the
0
1000
2000
3000
4000
5000
6000
2
Channel (ns)
parameter for goodness of fit (χ < 1.2 for a good fit). The fitting
function is as follows:
Figure 4: Lifetime decay of star-shaped pyrrole derivatives in
dichloromethane. Emission monitored at λem = 410 nm, λex
40 nm.
=
3
Corresponding lifetime values are given in Table 1. The decay
curves of compound 3 and 4 both show single exponential
nature (Table 1). These two compounds were shown
fluorescence studies in dichloromethane as a solvent. Here, we
Where I(t) and I(0) denote the fluorescence intensities
at time t and time zero after excitation, respectively. A and
i
th
denote the amplitude and lifetime, respectively, of the i
have observed N,N’-diallylated
C
3
-symmetric product
3
component of the decay.
showing longer lifetime than pyrrole-based star-shaped
derivative 4.
ACKNOWLEDGEMENTS
We thank the Department of Science and Technology
DST), New Delhi, India, for financial support and IIT-
Table 1: Time-correlated single-photon counting (TCSPC) data
analysis of C -symmetric compounds.
3
(
Bombay, for recording spectral data. S.K. thanks the
Department of Science and Technology for the award of a J. C.
Bose fellowship (SR/S2/JCB-33/2010), Praj industries, Pune for
Chair Professorship (Green Chemistry). S.T. and T.D. thanks
the IIT-Bombay for the award of a research fellowship.
χ2
compound
4
τ
1
(ns)
τ
2
(ns)
-
2.98
100%)
1.04
(
3
4.58
100%)
-
1.07
Supplementary Material
(
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/ …
These two
3
3 and 4 star-shaped C -symmetric
derivatives have similar spectroscopic properties. Structural
difference is in functionality at the end of star arm doesn’t have
any influence on these properties. These two symmetric
molecules can be modified further in versatile way of molecular
network structures.
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5
Highlights
Ring-closing metathesis product which underwent
aromatization.
Photophysical properties of star-shaped molecules
are studied.
N,N’-Diallyl C
3
-symmetric product showing more
lifetime than pyrrole-based star-shaped derivative.