Base-induced chemiluminescence of 5-tert-butyl-1-(4-hydroxybenz[d]oxazol-6-yl)-4,4-dimethyl-2,6,7- trioxabicyclo[3.2.0]heptanes: Chemiluminescence-chemiexcitation profile in aqueous medium

Article abstract of DOI:10.1016/S0040-4039(01)01947-5

Bicyclic dioxetanes bearing a 2-aryl-4-hydroxybenz[d]oxazol-6-yl moiety (3a-3c) and their 2-alkyl-analogs (3d, 3e) were synthesized. All these dioxetanes (3a-3e) afforded light with high efficiency on treatment with tetrabutylammonium fluoride in acetonitrile. In the NaOH-H₂O system, chemiluminescence efficiency (Φ^CIEEL) for alkyl-analogs (3d, 3e) decreased markedly, while Φ^CIEEL for 3a-3c was still considerably high. Fluorescence study revealed that the marked decrease of Φ^CIEEL for alkyl-analogs would be attributed to a synergetic effect of decreased chemiexcitation yield and fluorescence yield of the emitter in NaOH-H₂O system.

Full text of DOI:10.1016/S0040-4039(01)01947-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 8869–8872
Base-induced chemiluminescence of
5-tert-butyl-1-(4-hydroxybenz[d]oxazol-6-yl)-4,4-dimethyl-2,6,7-
trioxabicyclo[3.2.0]heptanes: chemiluminescence–chemiexcitation
profile in aqueous medium
Masakatsu Matsumoto,* Yasuko Mizoguchi, Takuma Motoyama and Nobuko Watanabe
Department of Materials Science, Kanagawa University, Tsuchiya, Hiratsuka, Kanagawa 259-1205, Japan
Received 10 September 2001; revised 4 October 2001; accepted 12 October 2001
Abstract—Bicyclic dioxetanes bearing a 2-aryl-4-hydroxybenz[d]oxazol-6-yl moiety (3a–3c) and their 2-alkyl-analogs (3d, 3e) were
synthesized. All these dioxetanes (3a–3e) afforded light with high efficiency on treatment with tetrabutylammonium fluoride in
acetonitrile. In the NaOH–H2O system, chemiluminescence efficiency (FCIEEL) for alkyl-analogs (3d, 3e) decreased markedly, while
FCIEEL for 3a–3c was still considerably high. Fluorescence study revealed that the marked decrease of FCIEEL for alkyl-analogs
would be attributed to a synergetic effect of decreased chemiexcitation yield and fluorescence yield of the emitter in NaOH–H2O
system. © 2001 Elsevier Science Ltd. All rights reserved.
The intramolecular CIEEL (chemically initiated elec-
tron exchange luminescence) of a dioxetane bearing a
phenolic anion is a promising entry to highly efficient
chemiluminescent substrates.1–3 An adamantylidene-
dioxetane (1) is a typical example for such CIEEL-
active dioxetanes and its phosphate-protected form is
now used in modern chemiluminescence bioassays.3–5
The CIEEL from 1 occurs to emit light effectively in
aprotic solvents such as DMSO and acetonitrile while
its chemiluminescence efficiency (FCIEEL) decreases
markedly in aqueous medium (FCIEEL in H2O/FCIEEL in
nomenon observed for the CIEEL of 1 is that the
fluorescence maximum wavelength (umaxfl) of the emit-
ter does not agree with the maximum wavelength
(umaxCIEEL) of the CIEEL emission and is blue-shifted
by ca. 50 nm in aqueous medium, while the CIEEL
spectra and the fluorescence spectra of the emitter
coincide in aprotic solvents.7 Similar phenomena have
been observed for a bicyclic dioxetane (2).8,9 Thus,
there still remained an important question, namely how
is the marked decrease of chemiluminescence efficiency
of 1 and 2 in aqueous medium attributed to the singlet-
chemiexcitation yield (FS) and/or fluorescence yield
DMSO=1/39000).4–6
Another
noteworthy
phe-
*
O O
O O
-Y
O
O
Y-O
O
RO
RO
RO
OH
3a: R =
OMe
H
N
O
3b: R =
3c: R =
3d: R =
3e: R =
R
O O
O O
O O
t-Bu
t-Bu
F
HO
HO
MeO
O
O
2
Me
Bu
1
3
Scheme 1.
Keywords: singlet oxygenation; 1,2-dioxetane; CIEEL; aqueous medium.
* Corresponding author. E-mail: matsumo@chem.kanagawa-u.ac.jp
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01947-5

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M. Matsumoto et al. / Tetrahedron Letters 42 (2001) 8869–8872
(Ffl) of the emitter, though chemiexcitation yields have
been estimated for CIEEL-decay of various dioxetanes
in aprotic solvents (Scheme 1).
(1 mg) in CH2Cl2 (10 mL) was irradiated with a 940 W
Na lamp under an oxygen atmosphere at 0°C for 40
min to give a dioxetane (3a), which was obtained in
93.2% yield after chromatographic purification (SiO2/
hexane–AcOEt 3:1).10 All dioxetanes (3a–3e) synthe-
sized here were stable enough to permit handling at
room temperature.
In the course of our investigation on the design of new
chemiluminescent substrates with high efficiency, we
synthesized a variety of bicyclic dioxetanes, 5-tert-
butyl-4,4-dimethyl-2,6,7-trioxabicyclo[3.2.0]heptanes (3)
bearing a 2-substituted 4-hydroxybenz[d]oxazol-6-yl
moiety, and our findings can provide an answer to the
above question regarding the marked decrease of
FCIEEL in H2O.
When dioxetane (3a) was treated with a large excess of
tetrabutylammonium fluoride (TBAF) in acetonitrile,11
decomposition occurred rapidly to follow pseudo-first-
order kinetics independent of the TBAF concentration
and to emit yellow light (maximum wavelength:
umaxCIEEL=512 nm, chemiluminescence efficiency:
Benzoxazole-substituted dioxetanes (3a–3e) were syn-
thesized in>82% yield by singlet oxygenation of the
corresponding dihydrofurans (5a–5e), which were pre-
pared from 2-amino-3-methoxyphenol (4) by condensa-
tion with an orthoester and by successive demethylation
of the methoxy to a hydroxy group (Scheme 2). Typical
singlet oxygenation was as follows: a solution of dihy-
drofuran (5a) (100 mg) and tetraphenylporphin (TPP)
F
CIEEL=0.15,12 half-life of CIEEL-decay: t1/2=9.0 s).
Similar treatment of the other dioxetanes (3b–3e) with
TBAF/acetonitrile caused also the CIEEL-decay to
afford light whose chemiluminescent properties are
summarized in Table 1. It should be noted here that
CIEEL
fl
umax
for 3a–3e coincided with umax of the corre-
sponding spent reaction mixture. Table 1 shows that
a: R =
OMe
H
OH
OMe
H2N
HO
N
O
b: R =
c: R =
d: R =
e: R =
R
1) R-C(OMe)3
2) EtSNa/DMF
t-Bu
F
t-Bu
Me
Bu
O
O
5
4
OH
O
6
N
N
1O2
R
R
:B
O O
O
O
O
+
light
t-Bu
t-Bu
O
O
O
3
Scheme 2.
Table 1. Base-induced chemiluminescent decomposition of dioxetanes bearing a 2-substituted 4-hydroxybenz[d]oxazol-6-yl
group (3)
Dioxetane
TBAF–Acetonitrilea
NaOH–H2Ob
CIEEL
fl
CIEEL
fl
umax
(nm) umax (nm)
FCIEEL t1/2 (s)
umax
(nm) umax (nm)
FCIEEL
Relative FCIEEL t1/2 (s)
3a
3b
3c
3d
3e
1
512
543
533
466
463
466d
467g
512
545
534
466
463
466d
467g
0.15
0.08
0.12
0.12
0.19
0.12e
0.11
9.0
23
26
12
505
534
520
482
474
466d
467g
508
536
526
4.3×10−3
1.2×10−3
1.9×10−3
7.4×10−5
4.1×10−5
7.5×10−6f
1.1×10−5
58
16
26
1
0.55
0.10
0.15
29
66
73
85
75
408 (476)c
411 (470)c
415d
7.8
13e
25
180
810
2
416g
a A solution of a dioxetane in acetonitrile (1.0×10−6 mol dm−3, 1 mL) was added to a TBAF solution in acetonitrile (1.0×10−3 M, 2 mL) at 25°C.
b A solution of a dioxetane in acetonitrile (1.0×10−3 mol dm−3, 0.1 mL) was added to a NaOH solution in water (0.1 M, 2.9 mL) at 25°C.
c A figure in parenthesis means wavelength of a shoulder.
d Ref. 7.
e Ref. 5.
f Ref. 4.
g Ref. 9.

M. Matsumoto et al. / Tetrahedron Letters 42 (2001) 8869–8872
8871
Figure 1. Normalized spectra in NaOH–H2O: (A) (a) CIEEL of 2 and (b) fluorescence of 2,4,4-trimethyl-3-oxo-1-pentyl
3-hydroxybenzoate, (B) (a) CIEEL of 3d, (b) fluorescence of 6d excited with light of uex=320 nm and (c) fluorescence (not
normalized) of 6d excited with light of uex=370 nm, and (C) (a) CIEEL of 3a and (b) fluorescence of 6a.
both the chemiluminescence efficiency (FCIEEL) and
half-life of the CIEEL-decay (t1/2) were not substan-
tially different between 2-aryl- (3a–3c) and 2-alkyl-
analogs (3d, 3e) in acetonitrile, though considerable
difference in the color tone of luminescence was
observed: aryl-analogs (3a–3c) emit yellow light
(umaxCIEEL=512–543 nm), while alkyl-analogs (3d, 3e)
emit blue light (umaxCIEEL=463–466 nm).
as well as the fluorescence maximum of its spent reac-
tion mixture irrespective of the solvent system used:
umaxfl=512 nm, and Ffl=0.30 in TBAF–acetonitrile,
while umaxfl=508 nm, and Ffl=0.12 in NaOH–H2O
[Fig. 1, (C)].15 Using these fluorescence quantum yields
(Ffl) and chemiluminescence yields (FCIEEL) summa-
rized in Table 1, singlet-chemiexcitation yields (FS=
F
CIEEL/Ffl)
are
estimated
to
be
0.50
in
TBAF–acetonitrile and 0.036 in NaOH–H2O. Conse-
quently, decrease of FCIEEL for an aryl-analog (3a)
should be attributed mainly to low efficiency of the
singlet-chemiexcitation process in NaOH/H2O: (Ffl in
H2O)/(Ffl in acetonitrile)=0.4, while (FS in H2O)/(FS
in acetonitrile)=0.07.
Next, we carried out NaOH-induced decomposition of
the present dioxetanes (3a–3e). Treatment of 3a (1×10−3
M in acetonitrile, 0.1 mL) with NaOH (0.1 M in H2O,
2.9 mL) gave intense light with umaxCIEEL=505 nm,
F
CIEEL=4.3×10−3, and t1/2=29 s at 25°C. The chemilu-
minescence of 3a was intense and visible to the naked
eye even in NaOH–H2O.13 The other aryl-analogs (3b
and 3c) gave similarly intense light in NaOH–H2O (see
Table 1). On the other hand, the alkyl-analogs (3d, 3e)
gave light far less effectively than 3a–3c but a little
more than 1 and 2 in NaOH–H2O (see relative FCIEEL
in Table 1). The relation between the maximum wave-
length of the CIEEL emission (umaxCIEEL) and the
fluorescence maximum (umaxfl) of the spent reaction
mixture for aryl-analogs (3a–3c) was markedly different
from that for alkyl-analogs (3d, 3e) in NaOH–H2O.
As in the case of a pair (3a and 6a), the fluorescence
spectrum of the authentic emitter (6d) agreed both with
the CIEEL emission spectrum of 3d and with the
fluorescence spectrum of its spent reaction mixture in
TBAF–acetonitrile. Fluorescence yield (Ffl) was esti-
mated to be 0.20 for 6d in TBAF–acetonitrile, so that
singlet-chemiexcitation for an alkyl-analog (3d) should
be as effective as for an aryl-analog (3a) in TBAF–ace-
tonitrile. On the other hand, the fluorescence spectrum
of 6d in NaOH–H2O showed a peak at umaxfl=408 nm
with a shoulder at 470–480 nm (wavelength of excita-
tion: uex=320 nm) [Fig. 1, (B), b): compared with (A),
b)]. This spectrum coincided with that for the spent
reaction mixture after the CIEEL-decay of 3d in
NaOH–H2O. When light with wavelength at uex=370
nm was used for excitation, the fluorescence spectrum
of 3d in NaOH–H2O showed a peak at umaxfl=482 nm
but little peak at 408 nm [Fig. 1, (B), (c)]. Fluorescence
of the authentic emitter (6d) at umaxfl=482 nm, whose
Ffl was estimated to be 0.023, should correspond to the
CIEEL emission for 3d in NaOH–H2O, though its
intensity is far lower than that of fluorescence at
umaxfl=408 nm.
CIEEL
fl
Thus, umax
and umax of the spent reaction mixture
coincided for 3a–3c, while the spent reaction mixture of
alkyl-analogs (3d, 3e) showed a fluorescence peak at
umaxfl=408–411 nm accompanied with a shoulder peak
CIEEL
near the region where the CIEEL peak (umax
=
474–482 nm) appeared. The observed relations between
CIEEL
fl
umax
and umax for alkyl-analogs (3d, 3e) and for
aryl-analogs (3a–3c) were both different from that
fl
reported for 1 and 2, where umax of the emitter is
CIEEL
blue-shifted by ca. 50 nm from umax
in NaOH–
H2O.7,9
To confirm the results described above, we measured
the fluorescence spectra of ketoesters (6a) and (6d) as a
representative of an authentic emitter for 2-aryl- (3a–
3c) and 2-alkyl-analogs (3d, 3e), respectively. The
fluorescence maximum (umaxfl) of the authentic emitter
(6a) agreed with the CIEEL maximum (umaxCIEEL) of 3a
These results suggest that the authentic emitter (6d)
exists at least as two different species in NaOH–H2O, of
which one emitting fluorescence at 482 nm corresponds
to the CIEEL emitter produced from 3d, and the other

8872
M. Matsumoto et al. / Tetrahedron Letters 42 (2001) 8869–8872
species emitting fluorescence at 408 nm should be an
oxyanion of 6d effected by strong hydrogen bonding as
mentioned for the case of 1 and 2 by Adam.7,9,16
According to this suggestion, chemiexcitation yield (Fs)
is estimated to be 0.0032 for 3d in NaOH–H2O. There-
fore, the very low chemiluminescence efficiency of 3d in
NaOH–water would be attributed to a synergetic effect
of the low fluorescence yield of the emitter and low
chemiexcitation yield. Comparing the chemiluminescent
decomposition of an alkyl-analog (3d) with an aryl-
analog (3a), which emits light ca 60 times more effec-
tively than 3d in NaOH–H2O, one realizes that the
fluorescence efficiency (Ffl) of the emitter and the
chemiexcitation yield (Fs) are both higher for 3a than
for 3d.
5. Trofimov, A. V.; Mielke, K.; Vasil’ev, R. F.; Adam, W.
Photochem. Photobiol. 1996, 63, 463–467.
6. Matsumoto, M.; Arai, N.; Watanabe, N. Tetrahedron
Lett. 1996, 37, 8535–8538.
7. Adam, W.; Bronstein, I.; Trofimov, A. V. J. Phys. Chem.
A 1998, 102, 5406–5414.
8. Matsumoto, M.; Watanabe, N.; Kasuga, N. C.; Hamada,
F.; Tadokoro, K. Tetrahedron Lett. 1997, 38, 2863–2866.
9. Adam, W.; Matsumoto, M.; Trofimov, A. V. J. Org.
Chem. 2000, 65, 2078–2082.
10. Selected data for 3a: colorless granules (from hexane–
CH2Cl2), mp 150.5–151.5°C. 1H NMR (400 MHz,
CDCl3): lH 1.03 (s, 9H), 1.17 (s, 3H), 1.41 (s, 3H), 3.84
(d, J=8.2 Hz, 1H), 3.90 (s, 3H), 4.60 (d, J=8.2 Hz, 1H),
7.02 (d, J=8.9 Hz, 2H), 7.14 (d, J=1.0 Hz, 1H), 7.49 (d,
J=1.0 Hz, 1H), 8.15 (d, J=8.9 Hz, 2H); 13C NMR (100
MHz, CDCl3): lC 18.7, 25.2, 27.1, 36.9, 45.7, 55.5, 80.2,
103.3, 105.0, 111.5, 114.4, 116.5, 118.9, 129.5, 131.5,
134.0, 146.8, 151.0, 162.4, 163.2; mass (m/z, %) 425 (M+,
16), 369 (26), 285 (26), 268 (100).
The present results provide the first instance that
decreased chemiexcitation yield and decreased fluores-
cence yield of the emitter effect synergetically a marked
drop of chemiluminescence efficiency for the CIEEL-
decay of a dioxetane bearing a phenolic moiety in
NaOH–H2O. By an analogy, marked decrease of
chemiluminescence efficiency for 1 and 2 may be
attributed both to low chemiexcitation efficiency and
fluorescence efficiency of the emitter so low that a peak
could not be observed at a region where the CIEEL
peak appears in NaOH–H2O.
11. A solution of 3a in acetonitrile (1.0×10−6 mol dm−3, 1
mL) was added to a TBAF solution in acetonitrile (1.0×
10−3 M, 2 mL) at 25°C.
12. Chemiluminescence efficiency (FCIEEL) was based on the
reported value for tert-butyldimethylsilyl ether of 1:
FCL=0.29 in DMSO (Ref. 5).
13. Improved chemiluminescence yield as high as that of 3a
has been attained for the CIEEL of dioxetanes bearing a
4-acetyl-3-hydroxyphenyl or a 3-hydroxy-4-iminophenyl
moiety, though their light intensity is weak because of
long CIEEL-decay rate.6,14
References
14. Matsumoto, M.; Sakuma, T.; Watanabe, N. Lumines-
cence 2001, 16, 275–280.
1. Schuster, G. B. Acc. Chem. Res. 1979, 12, 366–373.
2. Catalani, L. H.; Wilson, T. J. J. Am. Chem. Soc. 1989,
111, 2633–2639.
3. Schaap, A. P.; Chen, T.-S.; Handley, R. S.; DeSilva, R.;
Giri, B. P. Tetrahedron Lett. 1987, 28, 1155–1158.
4. Adam, W.; Bronstein, I.; Edwards, B.; Engel, T.; Rein-
hardt, D.; Schneider, F. W.; Trofimov, A. V.; Vasil’ev, R.
F. J. Am. Chem. Soc. 1996, 118, 10400–10407 and refer-
ences cited therein.
15. Quinine bisulfate was used as the fluorescence standard.
16. Fluorescence of ketoester (6d) was observed at umax=341
nm in acetonitrile without base, while fluorescence of 6d
in aqueous acetonitrile (1:1) without base exhibited a
broad spectrum with two peaks at umax=358 and 470 nm.
Although further experiments should be required to dis-
cuss in detail, fluorescence at umax=341–358 nm is pre-
sumably attributed to undissociated phenolic form of 6d.