Substituent effects on the kinetics for the chemiluminescence reaction of 6-arylimidazo[1,2-a]pyrazin-3(7H)-ones (Cypridina luciferin analogues): Support for the single electron transfer (SET)-oxygenation mechanism with triplet molecular oxygen

Article abstract of DOI:10.1016/j.tetlet.2005.09.014

Kinetics of chemiluminescence reactions of 2-methyl-6-phenylimidazo[1,2-a] pyrazin-3(7H)-one (1c, Cypridina luciferin analogue) and substituent effects of the 6-aryl group of derivatives 1 strongly suggest that the rate-determining step is a single electr

Full text of DOI:10.1016/j.tetlet.2005.09.014

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 7701–7704
Substituent effects on the kinetics for the chemiluminescence
reaction of 6-arylimidazo[1,2-a]pyrazin-3(7H)-ones (Cypridina
luciferin analogues): support for the single electron transfer
(
SET)–oxygenation mechanism with triplet molecular oxygen
a
a
a
a
Hiroyuki Kondo, Takayuki Igarashi, Shojiro Maki, Haruki Niwa,
b
a,
*
Hiroshi Ikeda and Takashi Hirano
a
Department of Applied Physics and Chemistry, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan
b
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Received 19 August 2005; revised 7 September 2005; accepted 7 September 2005
Available online 23 September 2005
Abstract—Kinetics of chemiluminescence reactions of 2-methyl-6-phenylimidazo[1,2-a]pyrazin-3(7H)-one (1c, Cypridina luciferin
analogue) and substituent effects of the 6-aryl group of derivatives 1 strongly suggest that the rate-determining step is a single elec-
tron transfer from an anion derived from 1 to a triplet molecular oxygen (O
2005 Elsevier Ltd. All rights reserved.
2
) in the oxygenation process.
ꢀ
ꢀ
1,5
Marine bioluminescent organisms, such as the crusta-
cean Cypridina (Vargula) and the jellyfish Aequorea,
peroxide intermediate (3 ) is suggested by Goto in
the chemiluminescence reaction of 2-methylimidazo-
[1,2-a]pyrazin-3(7H)-one in diglyme, containing t-BuOK
1
,2
3
have their own imidazo[1,2-a]pyrazin-3(7H)-one (imi-
dazopyrazinone) derivatives as a bioluminescent sub-
strate. Cypridina luciferin, for instance, is an
imidazopyrazinone having three appendages, (S)-2-
butyl, 3-indolyl, and 3-(1-guanidino)propyl groups, at
C2, C6, and C8, respectively. An imidazopyrazinone
ꢀ
as a base. The process from 1 to 3 is triggered by
deprotonation of 1 with a base. The successive single
electron transfer (SET) from the resulting 2 to O2
probably gives a radical 2 and the superoxide anion
O . Fast radical coupling of 2 and O followed by a
rearrangement affords a dioxetanone intermediate (4 )
via 3 . Finally, thermal decomposition of 4 with the
loss of carbon dioxide generates a singlet-excited state
of amidopyrazine anion (5 ), which emits light, yield-
ing 2-acetamide-5-arylpyrazine (6) via protonation of
5 . The SET from 2 to O is the most controversial
step and is suggested to be rate determining. However,
there has been no systematic study to prove that the
SET process exists.
ꢀ
Å
Å
ꢀ
Å
Å
ꢀ
2
2
ꢀ
substrate reacts with triplet molecular oxygen (O ) to
2
ꢀ
ꢀ
give an electronically excited amidopyrazine derivative
under regulation by a luciferin or apophotoprotein,
resulting in light emission with a high efficiency.
ꢀ
*
ꢀ
ꢀ
For several decades, we and other groups have investi-
gated chemiluminescence reactions of imidazopyrazi-
nones with O2 to establish the bioluminescence
2
1
,5
4
–7
mechanism. A plausible mechanism for the chemilu-
minescence reaction of 6-aryl-2-methylimidazopyrazi-
none (1), a Cypridina luciferin analogue, can be proposed
To verify the reaction mechanism, particularly the
involvement of SET, we investigated the kinetics of the
chemiluminescence reactions of 2-methyl-6-phenylimi-
dazopyrazinone (1c), its para-substituted phenyl deriva-
tives [1a, Ar = p-(CH ) NC H ; 1b, Ar = p-CH OC H ;
(
Scheme 1). Particularly, oxygenation of 1 to give a
3
2
6
4
3
6
4
1
d, Ar = p-ClC H ; 1e, Ar = p-NCC H ; Scheme 1], and
6 4 6 4
Keywords: Chemiluminescence; Imidazopyrazinone; Oxygenation;
the 3-indoyl derivative (1f), a prototype model of Cypri-
Substituent effects; Single electron transfer; Reaction rate.
8
*
Corresponding author. Tel./fax: +81 424 86 1966; e-mail: hirano@
pc.uec.ac.jp
dina luciferin. We report here that the oxygenation
ꢀ
ꢀ
reaction of 2 is controlled by SET from 2 to O2,
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.09.014

7702
H. Kondo et al. / Tetrahedron Letters 46 (2005) 7701–7704
rate determining SET step
O
O 3 2
O
O
fast
O
N
kox
O
radical
coupling
4
N1
N
N
base
N
+ O2
N
N
–
5
6
9
O2
N
8
Ar
N
H
Ar
N
Ar
N
7
Ar
N
1
2–
2·
3−
a: Ar = p-(CH ) NC H
6 4
3
2
b: Ar = p-CH OC H
3
6 4
c: Ar = C H5
6
O
O
d: Ar = p-ClC H4
O
O
6
*
hν
e: Ar = p-NCC H4
6
O
N
NH
H+
N
N
N
– CO2
f: Ar = 3-indolyl
+
–
N
N
5
Ar
N
Ar
N
H
Ar
N
–
–
6
5 *
4
Scheme 1.
and the SET process is the rate-determining step in the
whole chemiluminescence reaction of 1.
Next, we examined the chemiluminescence reactions of
1
0
1c in the presence of O at various concentrations.
2
The reactions of 1c were similarly traced in acetonitrile
ꢀ
1
15
First, we examined if 1,1,3,3-tetramethylguanidine
containing TMG (0.10 mol L ) at 25 ± 1 ꢁC. A linear
12
(
1
TMG) is an appropriate base for the kinetic study of
in acetonitrile. We considered TMG over t-BuOK
correlation between kobsd and [O ], k
0.00, gave the second-order rate constant at 25 ꢁC as a
slope (k = 22 mol L s , Fig. 2). These findings
= k [O ] +
obsd ox 2
2
9
ꢀ1
ꢀ1
16
because TMG, with a moderate basicity and a good
solubility in various organic solvents, is easier to handle.
A small portion (20 lL) of a stock solution of
ox
indicate that the rate constant is determined by the
ꢀ
oxygenation reaction of 2c and O . This conclusion
2
ꢀ3
ꢀ1
5
1
c (1.0 · 10 mol L ) in methanol was mixed with
matches that of Goto.
1
0
aerated acetonitrile
(2.0 mL) containing TMG at
2
5 ± 1 ꢁC. The reactions of 1c were traced by monitor-
The kox values for other substrates 1 are summarized
1
1
11–13,17
ing intensity (I) of the total emitted light, which
reached a maximum immediately after initiation of
the reaction and then decayed monotonically (Fig. 1).
The rate constant (kobsd) of the pseudo-first-order decay
was determined to be ca. 2.9 · 10
no significant dependency on the concentration of
in Table 1 together with kem and UCL
.
The
log[k (1)/k (1c)] values are correlated to the Hammett
ox
ox
1
8
rp constant of the p-substituent for 1a–e (Fig. 3): the
reaction was accelerated by an electron-donating group,
ꢀ
2
ꢀ1 12
ꢀ
s
.
Interestingly,
predicting a possible operation of SET from 2 to O in
2
the oxygenation process. To confirm this SET mecha-
ꢀ
4
ꢀ1
TMG ([TMG] = 1–100 · 10 mol L ) was observed
for (i) emission wavelengths from 5c (kem = 467 nm),
nism, we examined the correlation of the k values with
ox
ꢀ
*
p
ox
ꢀ
the anodic peak potentials ðE Þ of 2 measured with
(
ii) kobsd, (iii) chemiluminescence quantum yields
cyclic voltammetry in acetonitrile containing TMG
ꢀ
4
ꢀ1 19
(
UCL = 1.0–1.1 · 10 ), or (iv) chemical yields of the
(0.10 mol L ). As shown in Table 1, the k of 1
continuously increased with lowering E values of 2 .
ox
1
3
p
ox
ꢀ
product, 6c (78–91%). Therefore, TMG acts as a suit-
ꢀ
able base to generate 2c . It was also found that the
reaction of 2c with O proceeds at a constant rate.
This tendency matches the result of the Hammett corre-
ꢀ
lation of the log[k (1)/k (1c)] versus r . Judging from
2
ox
ox
p
0
0
0
0
0
.20
.15
.10
.05
.00
0
50 100 150 200 250
0
.0 0.2 0.4 0.6 0.8
time / s
−
3
−1
[
O ] / 10 mol L
2
Figure 1. Intensity (I) of the total emitted light over time observed in
ꢀ5
ꢀ1
the chemiluminescence of 1c (1.0 · 10 mol L ) in aerated aceto-
Figure 2. Correlation between the pseudo-first-order rate constants
(kobsd) for 1c and concentrations of O ([O ]).
ꢀ1
nitrile containing TMG (0.10 mol L ) at 25 ± 1 ꢁC.
2
2

H. Kondo et al. / Tetrahedron Letters 46 (2005) 7701–7704
7703
Table 1. Hammett constants (r
p
) and chemiluminescence data (kox, UCL, and kem) of 1 in acetonitrile containing TMG under air, oxidation potentials
p
ꢀ
ꢀ
ðE Þ of anions 2 in acetonitrile, and the calculated maximum rate constants (ket) of the SET from 2 to O
2
ox
a
ꢀ1
ꢀ1
b
ꢀ4
c
p
ox
ꢀ1
ꢀ1
Substrate [Ar]
r
p
k
ox (mol L s
)
U
CL · 10
k
em (nm)
E
k
et (mol L s )
V versus SCE
1
1
1
1
1
1
a [p-(CH
b [p-CH
c [C
d [p-ClC
e [p-NCC
3
)
2
NC
OC
6
H
4
]
ꢀ0.83
ꢀ0.27
+0.00
+0.23
+0.66
—
28
20
17
15
11
28
3.8
2.0
1.0
1.1
0.8
1.2
472
472
467
467
468
476
ꢀ0.23
ꢀ0.21
ꢀ0.19
ꢀ0.16
ꢀ0.13
ꢀ0.23
ꢀ3
4.8
2.2
1.0
0.32
0.10
4.8
3
6
H
4
]
6
H
5
]
6
H
4
]
6
H
4
]
f [3-indolyl]
a
ꢀ1
The kox values are observed oxygenation rate constants estimated to be kobsd/[O
determined with the method described in Note 16.
2
], where [O
2
] = 1.7 · 10 mol L . The kobsd values were
b
c
Chemiluminescence quantum yields.
Emission maxima of the chemiluminescence spectra.
1
ox
=2
ꢀ
derivative 1a. The E value of 2f is also similar to that
ꢀ
of 2a . These results indicate that the electronic effects
of the 3-indolyl group on the chemiluminescent reactiv-
ity of the imidazopyrazinone ring is comparable with
that of the 4-(N,N-dimethylamino)phenyl group. There-
fore, the 3-indolyl group in Cypridina luciferin acts as an
electron-donating group and plays an important role to
0
.2
.0
0
ꢀ
increase the efficiency of the oxygenation of 2 via the
SET mechanism.
-0.2
In conclusion, our experimental findings strongly sug-
gest that the chemiluminescence reaction of imidazopyr-
-
0.8 -0.4 0.0 0.4 0.8
azinone (1) proceeds in a mechanism including the
ꢀ
σp
rate-determining SET from 2 to O (Scheme 1). The
2
SET-oxygenation mechanism can explain the processes
Figure 3. Hammett plot of the log[kox(1)/kox(1c)] values for 1a–e
against the r constant.
ꢀ
ꢀ
from 2 to 3 , although direct evidence for the genera-
p
Å
Å
ꢀ
tion of 2 and O is not yet available. There is also a
2
strong suggestion that the Cypridina bioluminescence
1
ox
=2
p
ox
21
reaction is governed by the SET-oxygenation mecha-
the estimated oxidation potentials ðE ¼ E ꢀ 0:03Þ
ꢀ
1
nism, as predicted by Goto. Therefore, the SET-
of 2 and the reduction potential of O in acetonitrile
2
1
red
=2
20
oxygenation process may be regarded as the most likely
mechanism for bioluminescence and chemiluminescence
reactions, as has been reported for chemiluminescence
(
E
¼ ꢀ0:87 V vs SCE), the SET step is endergonic;
ꢀ
the free energy change of SET from 2 to O was cal-
2
0
1=2
1=2
culated to be positive ½DG ¼ E ꢀ E ¼ þ0:61 ꢁ
et
ox
red
2
8–30
reactions of several electron-rich compounds.
Fur-
þ0:71 eV > 0ꢂ. When it was assumed that the activation
0
et
ther study to reveal the complete chemiluminescence
reaction mechanism of imidazopyrazinones is now in
progress in our lab.
free energy for SET is nearly equal to DG , the maxi-
mum rate constant (k ) of the SET step for each 1
et
ꢀ
1
ꢀ1
was estimated to be 0.1–4.8 mol L s (Table 1) by a
2
2–24
theoretical treatment.
Therefore, the SET proceeds
far more slowly than the radical coupling of 2 and
Å
Å
ꢀ
8
O , whose rate constant is expected to be over
2
ꢀ1
ꢀ1
Å
0 mol L s , as is usual for alkyl radicals and O .
2
ꢀ
25
Acknowledgments
1
We thank Mr. T. Irie and Mr. H. Kubota at ATTO Co.
for their kind assistance in measuring chemilumines-
cence spectra. This work was supported by grants from
the Japanese Ministry of Education, Culture, Sports,
Science and Technology [No. 15404009 for H.N. and
No. 14050008 (Priority Area No. 417) for H.I.]. We also
thank Professor M. Ueda of Tohoku University for his
generous considerations.
While the k values for 1 are considerably smaller than
et
the corresponding kox values, the k values are linearly
et
correlated to the kox values, except for 1e with the
electron-deficient cyano group (k = 0.34k ꢀ 4.81,
et
ox
2
r = 0.999). This supports the assertion that the oxygen-
ation mechanism includes the rate-determining SET
step, as shown in Scheme 1. The large kox values com-
pared with the corresponding k values, with the excep-
et
tion of 1e in the kox ꢀ k correlation, may be ascribed to
et
the possible contribution of an intermolecular charge-
ꢀ
transfer interaction between 2 and O , which could
2
References and notes
2
6,27
accelerate the oxygenation reaction.
1
. Goto, T. Pure Appl. Chem. 1968, 17, 421–441.
The Cypridina luciferin model 1f showed a kobsd value
similar to that of the 4-(N,N-dimethylamino)phenyl
2. (a) Kishi, Y.; Goto, T.; Hirata, Y.; Shimomura, O.;
Johnson, F. H. Tetrahedron Lett. 1966, 3427–3436;

7704
H. Kondo et al. / Tetrahedron Letters 46 (2005) 7701–7704
(
b) Nakamura, H.; Aizawa, M.; Takeuchi, D.; Murai, A.;
17. Chemiluminescence reactions of imidazopyrazinones 1
gave the corresponding amidopyrazines 6 as main prod-
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3
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. McCapra, F.; Chang, Y. C. J. Chem. Soc., Chem.
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ꢀ
*
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.
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4
5
6
19. Cyclic voltammetry measurements were performed in
ꢀ
4
acetonitrile containing TMG (0.10 mol L ) and Et -
1
1
. Goto, T.; Inoue, S.; Sugiura, S. Tetrahedron Lett. 1968,
3
ꢀ
873–3876.
NClO4 (0.10 mol L ) as a supporting electrolyte at
. (a) Shimomura, O.; Johnson, F. H. Biochem. Biophys. Res.
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25 ± 1 ꢁC under Ar with an Ag/AgCl reference electrode
ꢀ
1
(scan rate: 100 mV s ). All reported potentials are
referenced to the oxidation potential of the ferrocene/
0
20
7
. (a) Hirano, T.; Gomi, Y.; Takahashi, T.; Kitahara, K.;
Chen, F. Q.; Mizoguchi, I.; Kyushin, S.; Ohashi, M.
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Hirano, T.; Niwa, H.; Ohashi, M. Chem. Lett. 1998, 1711–
ferricinium couple (E ¼ þ0:45 V versus SCE). Anion
ox
ꢀ
p
ox
2
showed irreversible waves to give E values with the
experimental errors within ± 0.01 V.
20. Sawyer, D. T.; Sobkowiak, A.; Roberts, J. L., Jr.
Electrochemistry for Chemists, 2nd ed.; John Wiley: New
York, 1995.
1712.
8
9
. (a) Goto, T.; Inoue, S.; Sugiura, S.; Nishikawa, K.; Isobe,
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Y.; Kayano, T.; Sato, K.; Goto, T. Bull. Chem. Soc. Jpn.
1
=2
ꢀ
21. The E
values of 2 were tentatively estimated to be
ox
p
E
ꢀ 0:03 Bird, A. J.; Faulkner, L. R. Electrochemical
ox
1992, 65, 2475–2479.
Methods: Fundamentals and Applications, 2nd ed.; John
. Kolthoff, I. M.; Chantooni, M. K., Jr.; Bhowmik, S. J.
Am. Chem. Soc. 1968, 90, 23–28.
Wiley: New York, 2001; p 239.
0
et
22. The ket values were estimated from the DG values by the
0
et
1
0. The O concentrations in aerated and oxygenated aceto-
equation: ket ¼ Z expðꢀDG =kBTÞ, where Z is the fre-
23
2
1
1
ꢀ1
ꢀ1
nitrile at 298 K were reported to be 1.7 and
quency factor taken as 1 · 10 mol L s , k
B
is the
ꢀ
3
ꢀ1
8
.1 · 10 mol L , respectively: Achord, J. M.; Hussey,
Boltzmann constant, and T is temperature in Kelvin.
Evaluation of thermal SET reactions using ket refers to the
C. L. Anal. Chem 1980, 52, 601–602. A solution with any
ꢀ
3
ꢀ1
24
concentration of O between 1.7 and 8.1 · 10 mol L
was prepared by bubbling a mixed gas of air and oxygen
through the solution. An accurate O concentration of the
studies by Fukuzumi et al.
2
23. Marcus, R. A. Annu. Rev. Phys. Chem. 1964, 15, 155–196.
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2
solution was determined by a comparison of the fluores-
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independently aerated and oxygenated solutions.
1
1. An intensity (I) of the total emitted light (400–700 nm) was
monitored using a Hamamatsu R5929 photomultiplier
tube powered by a Hamamatsu C4900 power supply. The
signal from the photomultiplier was collected on a PC
computer and the data were analyzed with the graphics
program Igor Pro, Version 4. 0. 8. 0 (Wave Metrics, Inc.).
2. The pseudo-first-order rate constant (kobsd) was obtained
1
by analysis using the equation: lnI
t
= ꢀkobsdt + lnI
0
,
where I and I are the intensities at t = 0 and t,
0
t
respectively. The experimental errors of kobsd were
within ± 10%.
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1
3. Chemiluminescence spectra were recorded on an ATTO
AB-1850 spectrometer. The UCL values were determined
as quantum yields relative to the UCL (0.013) of luminol in
1
4
DMSO containing t-BuOK/t-BuOH under air. The
experimental errors of UCL were within ± 10%. The yields
of 4 were determined by GC analyses.
28. Fletcher, A. N.; Heller, C. A. J. Phys. Chem. 1967, 71,
1507–1518.
1
1
4. Lee, J.; Seliger, H. H. Photochem. Photobiol. 1972, 15,
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227–237.
5. The emitted light was monitored at 470 ± 10 nm with a
JASCO FP-6500 spectrofluorometer in the experiments of
the concentration effects of O
2
on the chemiluminescence
reaction of 1c.
ꢀ
1
ꢀ1
1
6. The kox value of 22 mol L s for 1c was determined
with the same apparatus as in Note 15, while those shown
in Table 1 were determined with the apparatus described
in Note 11. The values for 1c from the two apparatuses
agreed within experimental error.