Synthesis of energy transfer cassettes via click and Suzuki-Miyaura cross coupling reactions

Article abstract of DOI:10.1039/c6ra07861c

Novel cassettes capable of energy transfer involving simple synthetic methods viz., copper catalyzed azide-alkyne cycloaddition (click reaction) at the C-8 position and palladium catalyzed Suzuki-Miyaura cross coupling at the C-6 position have been represented. The resulting imidazo[1,2-a]pyrazine-triazole bridged coumarin cassettes are capable of energy transfer from a donor core to an acceptor moiety.

Full text of DOI:10.1039/c6ra07861c

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Synthesis of energy transfer cassettes via click and
Suzuki–Miyaura cross coupling reactions†
Cite this: RSC Adv., 2016, 6, 37664
*
*
Richa Goel, Vijay Luxami and Kamaldeep Paul
Received 26th March 2016
Accepted 3rd April 2016
DOI: 10.1039/c6ra07861c
www.rsc.org/advances
Novel cassettes capable of energy transfer involving simple synthetic
The major challenge currently encountered for efficient
methods viz., copper catalyzed azide–alkyne cycloaddition (click energy transfer cassettes either through space or through bonds
reaction) at the C-8 position and palladium catalyzed Suzuki–Miyaura is their syntheses, which are not straightforward4,5 and which
cross coupling at the C-6 position have been represented. The resulting require harsh conditions and tedious purication to isolate the
imidazo[1,2-a]pyrazine-triazole bridged coumarin cassettes are product with low to moderate yields.6
capable of energy transfer from a donor core to an acceptor moiety.
Click reactions7 and Suzuki–Miyaura cross coupling8 have been
proven to be powerful tools to construct heterocyclic molecular
frameworks. Herein, we have reported the rst sequential func-
tionalization at the C-8 and C-6 positions of imidazo[1,2-a]pyr-
azine by implementing “click” and “Suzuki–Miyaura cross
coupling” reactions, respectively. These have been leading to
energy transfer cassettes along with the determination of photo-
physical characterization of the synthesized compounds by
depicting the quantum yields. We have introduced various elec-
tron donating substituents viz., methyl, ethyl, hydroxy, and
methoxy as well as electron withdrawing substituents viz., formyl,
and acetyl to the donor part of the cassette, thereby determining
the overall quantum yield and energy transfer efficiency (ETE). The
effect of the spacer length between the donor and the acceptor has
also been studied to predict the energy transfer efficiency. To the
best of our knowledge, this is the rst report for the coupling of
imidazo[1,2-a]pyrazine based donors with coumarin acceptors in
a way that facilitate energy transfer through space.
To synthesize these energy transfer cassettes, our study was
initiated with the easily available starting material 2-bromoe-
thylamine hydrobromide 1 which was reuxed with sodium
azide in water for 12 h followed by stirring with sodium
hydroxide at room temperature to give 2-azidoethanamine 2.9
The reported tedious work up has been avoided in this case. The
azide has limited thermal stability but it could be stored in ether
or hexane for a longer period of time at a low temperature in
a pure state. To obtain the rst precursor, we have treated
6,8-dibromoimidazo[1,2-a]pyrazine 3 (obtained from bromina-
tion of 2-aminopyrazine followed by cyclization with 50%
aqueous chloroacetaldehyde)10 with 2-azidoethanamine 2 in the
presence of diisopropylethylamine (DIPEA) in acetonitrile at the
reux temperature for 24 h to obtain N-(2-azidoethyl)-6-bro-
moimidazo[1,2-a]pyrazin-8-amine 4 in 72% yield (Scheme 1).
Energy transfer (ET) cassettes involve the absorption of
a photon of light by one of the chromophores, resulting in
energy transfer. The donor molecule returned to the ground
state along with raising the acceptor to the excited state.1 The
energy transfer process can take place in two different ways; (i)
through bond energy transfer (TBET),2 and (ii) through space
energy transfer (uorescence resonance energy transfer, FRET).3
Energy transfer through both pathways involved the use of
a donor and an acceptor linked through a conjugated spacer in
the case of TBET and a non-conjugated spacer in the case of
FRET. The energy transfer through TBET does not require the
criterion of spectral overlap between the absorption spectrum of
the acceptor and the emission spectrum of the donor. On the
other hand, the FRET phenomenon requires the criterion of
spectral overlap between the absorption spectrum of the
acceptor and the emission spectrum of the donor. It can also
afford simultaneous recording ratio signals of two emission
intensities at different wavelengths, which could provide built-
in correction for the environmental effects and supply a facile
method for visualizing complex biological processes at the
molecular level, such as nucleic acid regulation, DNA detection,
protein labelling, and other biomarkers. Consequently,
studying the construction of energy donor–acceptor architec-
tures, or “energy transfer cassettes” is very interesting and
signicant to many chemists, biologists, environmentalists,
and medical scientists.
School of Chemistry and Biochemistry, Thapar University, Patiala-147004, India.
E-mail: kpaul@thapar.edu
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra07861c
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any catalyst, mixtures of 1,4-(7) and 1,5-(8) disubstituted triazole
derivatives in a ratio of 2 : 1 were obtained (Scheme 3). It has
been observed that thermal [2 + 3] dipolar cycloaddition of
alkyne to azide is not a regiospecic reaction.11
Strong Nuclear Overhauser Effect (NOE) was observed
between the H-50 triazole proton and the protons of the
substituted alkyl groups present at the N-10 and C-40 positions in
compound 7, suggesting that the triazole proton and the alkyl
groups are in close proximity, and conrmed the formation of
the 1,4-disubstituted triazole. The NOE spectrum of 1,5-disub-
stituted triazole 8 showed no correlation between the singlet of
the triazole at the C-40 position and the triplet of the substituted
alkyl group present at the N-10 position. The expected NOE
correlation of the triazole protons at the C-40 and C-50 alkyl
groups was observed (Scheme 3).
Scheme 1 Synthesis of N-(2-azidoethyl)-6-bromoimidazo[1,2-a]pyr-
azin-8-amine (4).
Scheme 2 Synthesis of 4-methyl-7-(prop-2-ynyloxy)-2H-chromen-
2-one (6).
1,4-Disubstituted triazole
7 was further used for the
synthesis of a library of compounds with Suzuki–Miyaura cross
coupling reactions at the C-6 position of imidazo[1,2-a]pyrazine.
To identify the optimal reaction conditions, various reported
Pd-catalysts, along with different combinations of bases and
organic solvents were examined in the reaction of imidazo[1,2-
a]pyrazine 7 with thiophen-2-yl boronic acid. A previous study12
showed that the base was a key reaction parameter, so, different
bases (Cs2CO3, K2CO3, and DIPEA) and solvents (CH3CN : H2O,
dioxane : H2O and DME : H2O) were reviewed rst. Both
affected the product yields as K2CO3 and dioxane : water gave
the best results. With the identication of the base and solvent,
several Pd catalysts were screened (Table S2†). Using [Pd(PPh3)4]
as the catalyst, 9 was obtained in 73% yield (Table S2,† entry 19),
while [Pd(PPh3)2Cl2], [Pd(dba)3] and [PdCl2(dppf)]$DCM had
lower yields of the desired products (Table S2,† entries 17, 18,
20). The best yield was obtained when the model reaction was
catalyzed with 5 mol% [Pd(PPh3)4] and using 1.0 equivalent of
thiophen-2-yl boronic acid, and 1.0 equivalent of K2CO3 in
dioxane : water (9 : 1) at the reux temperature for 6–8 h in
a sealed tube (Table S2,† entry 19).
The
second
precursor
4-methyl-7-(prop-2-ynyloxy)-2H-
chromen-2-one 6 was obtained in 81% yield via the reaction of
7-hydroxy-4-methyl-2H-chromen-2-one 5 with 2 equivalents of
a 80% solution of propargyl bromide in the presence of potassium
carbonate and acetone at room temperature for 12 h (Scheme 2).
With both precursors in hand, the copper-catalyzed azide–
alkyne cycloaddition (CuAAC) reaction of imidazo[1,2-a]pyr-
azine 4 and coumarin 6 was performed. To explore the feasi-
bility of the [2 + 3] cycloaddition of the azide–alkyne reaction,
various catalysts and solvents were screened (Table S1, ESI†).
Among the various catalysts (CuSO4$5H2O, CuBr, CuI and
CuCl2), CuSO4$5H2O showed the highest efficiency in the
presence of sodium ascorbate at room temperature (Table S1,†
entry 8). Several solvents were also tested; the ethanol and water
mixture was proven to be the most effective solvent (Table S1,†
entries 4, 8, 12). Increasing the temperature of the reaction did
not lead to any further improvement in the yield (Table S1,†
entries 13, 14). A slight decrease in the yield of the desired
product was observed upon increasing the loading of CuSO4
from 5 to 15 mol% (Table S1,† entries 15, 16).
With these optimized reaction conditions, the scope of the
reaction was further examined with various aryl boronic acids
(Table 1). The Suzuki reaction with imidazo[1,2-a]pyrazine 7
proceeded well to give the desired products irrespective of the
substitution on the phenyl ring. It has been observed that the
ve member rings were well compatible with the optimized
conditions, affording the corresponding products in moderate
yields (9–11, 58–75% yields). Both electron withdrawing
substituents viz., uoro (13), chloro (14), bromo (15), tri-
uoromethyl (16) and acetyl (24) as well as electron donating
substituents such as methyl (17), ethyl (18) and methoxy (21 and
22) were also compatible with the protocol, providing the ary-
lated imidazo[1,2-a]pyrazine in moderate to good yields. The
change in directing groups from ortho to para or vice versa did
not effect the reactivity. However, with the 2-hydroxyphenyl
substrate, the reactivity decreased signicantly, and the arylated
products 20 was obtained in only 53% yield (Table 1, entry 12),
perhaps because of the combination of both steric hindrance
and strong electron deciency.
With the optimized reaction conditions, compound 4 was
treated with 4-methyl-7-(prop-2-ynyloxy)-2H-chromen-2-one 6 in
the presence of 5 mol% CuSO4$5H2O and 10 mol% sodium
ascorbate in ethanol : water (8 : 2) at room temperature for 2 h
to give 1,4-disubstituted triazole 7 in 91% yield. However, when
the reaction was typically carried out with heating without using
Aer screening the 2-azidoethanamine linker, another alkyl
chain, 3-azidopropanamine, was investigated. 3-Bromopro-
Scheme 3 Thermal [2 + 3] cycloaddition reaction of 4 and 6.
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Table 1 Suzuki–Miyaura cross coupling reaction of ethyl triazole bridged imidazo[1,2-a]pyrazine–coumarin conjugates
Entry
1
Time (h)
ArB(OH)2
Product
Yield (%)
8
9 (62)
2
3
4
8
8
6
10 (75)
11 (58)
12 (71)
5
6
7
7
7
7
13 (80)
14 (82)
15 (64)
8
9
6
6
16 (71)
17 (65)
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Table 1 (Contd.)
Entry
10
Time (h)
ArB(OH)2
Product
Yield (%)
6
18 (66)
11
12
8
8
19 (74)
20 (53)
13
7
21 (90)
14
15
6
6
22 (91)
23 (92)
16
8
24 (90)
pylamine hydrobromide 25 was rst reacted with sodium azide to
give 26 followed by substitution at the C-8 position of 6,8-dibro-
moimidazo[1,2-a]pyrazine 3 to give 27 in 65% yield. Compound
27 was then coupled with coumarin 6 via a copper-catalyzed azi-
de–alkyne cycloaddition reaction to give 28 in 82% yield
(Scheme 4).
Compound 28 was further coupled with various aryl boronic
acids via Suzuki–Miyaura cross coupling reactions using 5 mol%
Scheme 4 Synthesis of propyl triazole bridged imidazo[1,2]-pyrazine–
coumarin conjugate (28).
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Table 2 Suzuki–Miyaura coupling reactions of propyl triazole bridged imidazo[1,2-a]pyrazine–coumarin conjugates
Entry
1
Time (h)
ArB(OH)2
Product
Yield (%)
8
29 (72)
2
3
4
8
8
6
30 (70)
31 (76)
32 (70)
5
6
7
7
33 (80)
34 (81)
7
8
7
6
35 (56)
36 (76)
9
6
37 (68)
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Table 2 (Contd.)
Entry
10
Time (h)
ArB(OH)2
Product
Yield (%)
6
38 (90)
Table 3 Photophysical characterization of the absorption and fluo- Table 4 Photophysical characterization of the absorption and fluo-
rescence properties of compounds 8–24 in acetonitrilea
rescence properties of compounds 28–38 in acetonitrilea
(nm)emi
(nm)max
abs
max
emi
l
Compd
l
(nm)
l (nm)
max
V
Ff
ETE (%)
abs
max
Compd
l
(nm)
(nm)
V
Ff
ETE (%)
28
29
30
31
32
33
34
35
36
37
38
315
316
314
315
317
316
315
312
315
317
380
376
375
380
380
390
378
377
375
380
500
26 250
29 450
27 900
28 050
29 700
30 850
31 300
27 400
27 550
33 250
31 400
0.11
0.11
0.13
0.14
0.27
0.35
0.13
0.15
0.16
0.17
0.47
76.0
76.0
71.7
69.5
41.3
15.2
71.7
67.3
65.2
0.5
7
9
315
315
316
316
315
312
315
317
316
314
315
312
319
314
316
380
375
375
377
377
378
385
415
385
378
378
415
378
375
375
495
475
39 600
49 700
44 600
25 350
22 400
19 850
39 050
49 250
33 400
26 950
49 800
26 150
28 000
20 350
30 900
31 400
40 800
0.12
0.22
0.24
0.20
0.15
0.18
0.30
0.34
0.32
0.15
0.13
0.36
0.016
0.20
0.16
0.48
0.40
73.9
52.1
47.8
56.5
67.3
60.8
34.7
17.3
15.2
67.3
71.7
N.D
96.5
56.5
65.2
N.D
N.D
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
275, 317
N.D
a
N.D: not determined.
275, 317
275, 317
22 containing the electron donating groups have lower
quantum yields (Ff ¼ 0.016 to 0.36) in comparison to
compounds 23 and 24 (Ff ¼ 0.40 to 0.48) with electron with-
drawing groups 4-formyl and 4-acetyl attached to the phenyl
ring at the C-6 position of imidazo[1,2-a]pyrazine. A similar
pattern of quantum yield was observed in the case of the propyl
chain in which the electron withdrawing group in 38 showed
the highest quantum yield (Ff ¼ 0.47). Overall, it was inferred
that compounds with electron donating groups showed lower
quantum yields in comparison to compounds with electron
withdrawing groups. An increase in the linker length also
resulted in similar or lower quantum yields. Energy transfer
efficiencies (ETE) based on the uorescence quantum yields have
a
N.D: not determined.
[Pd(PPh3)4], K2CO3 and dioxane : water at reux temperature for
6–8 h to obtain compounds 29–38 in 56–90% yields (Table 2).
Various spectroscopic measurements (Fig. S63–S64†) were
recorded for the molecules to test the cassettes, based on the
quantication of their efficiencies (Tables 3 and 4). Molar
absorptivities (V) and quantum yields (Ff) for all 28 compounds
were determined in acetonitrile. All of the derivatives have
similar absorption maxima (between 312 to 319 nm). However,
compounds 23, 24 and 38 containing electron withdrawing
groups such as formyl and acetyl attached to the phenyl ring
exhibited two absorption peaks at 275 nm and 317 nm. A similar
emission maximum was observed for all derivatives (between
375 to 390 nm). However, emission maxima to longer wave-
lengths (red shi) i.e. from 415 nm to 500 nm were observed for
compounds 15, 19, 23, 24 and 38. Compounds 17, 18, 20, 21 and
also been determined. The efficiency of the energy transfer (hETE
)
/
was evaluated using the equation13 hETE ¼ 1 ꢀ Ff(donor in dyad)
Ff(free donor). Here, Ff(donor in dyad) is the uorescence quantum
yield of the donor part in the dyad and Ff(free donor) is the uo-
rescence quantum yield of the donor when not connected to the
acceptor.
Compounds 7 and 28 with a bromo group at the C-6 position
of imidazo[1,2-a]pyrazine (at the donor part) acted as a good
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synthetic access to these conjugate cassettes from imidazo
[1,2-a]pyrazine and acceptor-based coumarins, which ensures
an excellent potential for developing future smarter hybrid
cassettes for valuable uorescence applications (bioimaging,
chemosensing, optoelectronics etc.). Thus, further experimental
and computational studies are now in progress, directed to
determine the key structural factors ruling the energy transfer
efficiency in these hybrid cassettes. We are convinced that we
herein communicated a new structural design to boost ETE,
which has great potential for inspiring the future development
of molecular cassettes on the basis of molecular hybridization.
Fig. 1 Spectral overlap of the emission spectrum of imidazo[1,2-a]
pyrazine (donor) and the absorption spectrum of coumarin (acceptor).
Acknowledgements
KP thanks DST, New Delhi (EMR/2014/000669) and UGC, New
Delhi (41-322/2012, SR) for providing funds. RG is indebted to
CSIR for SRF (09/677(0020)/2013.EMR-I). SAI labs, Thapar
University is also acknowledged for recording NMR spectra.
energy transfer cassette with ETE values of 73.9% and 76%,
respectively. The introduction of an electron donating groups
such as methyl (17 and 34) and ethyl (18 and 35) also showed
comparable ETE values (67.3% and 71.7%), while substitution
with halogen to a phenyl ring on the donor part of the cassettes
(13, 14, 15, 16, 31, 32 and 33) displayed low ETE values
(15.2–69.5%). The presence of electron donating groups viz.,
methoxy in compounds 21, 22 and 36, showed 56.5%, 65.2%
and 65.2% ETE, respectively. But the presence of 2-hydrox-
yphenyl (20) on the donor moiety showed signicant energy
transfer efficiency (96.5%). The presence of a naphthyl group
(19) and the electron withdrawing 4-formyl (23 and 38) and
4-acetyl (24) groups on the aromatic ring did not show any
energy transfer. Thus, compound 20 showed the maximum
energy transfer efficiency of 96.5% which indicated that this
compound behave as an energy transfer cassette to transfer
energy from the imidazo[1,2-a]pyrazine donor to the coumarin
acceptor. Thus, the emission spectra of imidazo[1,2-a]pyrazine
(donor) and the absorption spectra of coumarin (acceptor)
showed signicant spectral overlap for energy transfer (Fig. 1).
The energy transfer cassettes of imidazo[1,2-a]pyrazine–
coumarin conjugates were also compared with different known
molecular hybrids. It has been observed that compound 20
(ETE ¼ 96.5%) showed better or comparable results with uo-
rescein–coumarin cassettes for the detection of phosphodies-
terase I activity (ETE ¼ 94%),14 uorescein–coumarin cassettes
for ATP complexes (ETE ¼ 76% and 83%),15 Bodipy with phe-
nanthroline and terpyridine cassettes for light harvesting
system (ETE ¼ 82%),16 and Bodipy–cyanine based cassettes
(ETE ¼ 43–90%).17
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