Calcium Peroxide Diperoxohydrate
J . Org. Chem., Vol. 67, No. 8, 2002 2419
preparation of a great amount of the endoperoxide and
the removing of the naphthalene derivative from the
reaction medium. However, water-soluble endoperoxides
have found valuable applications in biochemistry when
1
the releasing of a known amount of pure
2
O was
necessary.9
,20-26
Recently, Ca2 has been found to be the most environ-
+
1
mentally friendly catalyst to generate O
2
by dispropor-
1
tion of hydrogen peroxide. The precursor of O
identified as the isolable solid calcium peroxide diper-
oxohydrate CaO ‚2H which was described as unstable
2
has been
2
2 2
O
2
7
for temperature higher than 25 °C. This oxidizing agent
offers the advantage to generate singlet oxygen in pure
organic solvent without addition of aqueous hydrogen
peroxide. In this paper, we show that calcium peroxide
1
F igu r e 1. Luminescence electric signal of O
2
at 1270 nm
(4 mmol)
OH (20 mL) at 50 °C without (A) and with (B) addition
versus time during the thermolysis of CaO
in CH
2
2 2
‚2H O
3
of R-terpinene (0.02 mmol).
diperoxohydrate can be used as a simple heterogeneous
1
source of O
2
for the oxidation of organic compounds in
soluble cream-colored residue, which was shown to be
MeOH or in THF through the typical [4 + 2] and [2 + 2]
cycloadditions and the ene-reaction.
the anhydrous calcium peroxide, CaO
2
, as reported by
Gladisheva (eq 1).33
5
0 °C, 3 h
Resu lts a n d Discu ssion
CaO ‚2H O
2
8
2
2
1
3
Syn th esis a n d Ch a r a cter iza tion of Ca O
2
2 2
‚2H O .
CaO + 2H O + R O + (1 - R) O (1)
2
2
2
2
Although numerous calcium peroxides are reported in
the literature,2 Gladysheva has established that only
8,29
30
1
Formation of O
2
during the thermolysis of CaO
was proved unambiguously by detection of its monomol
emission at 1270 nm (eq 2).
2
2 2
‚2H O
two well-defined compounds may be obtained by reaction
of H
octahydrate peroxide CaO
diperoxohydrate peroxide CaO
pound can be prepared from Ca(OH)
2
O
2
with Ca(II) in aqueous solution: the stable
2
‚8H O and the thermolabile
2
kp
9
2
‚2H
2
,3
O
2
. This last com-
1
3
0-33
30,33
O2
8 O + hν (1270 nm)
(2)
CaO
2
‚8H
2
O,
2
2
2
9,34
29
CaO
peroxide, followed by separation of the precipitate on a
glass filter at -15 °C. In this work, CaO ‚2H was
synthesized from crystal of CaO ‚8H O with a 95% yield.
To avoid partial disproportionation of CaO ‚2H , the
2
,
or CaCl
2
by addition of concentrated hydrogen
1
Kinetics of O
2
formation during the thermolysis at 50
‚2H in methanol was
°
C of a suspension of CaO
2
2 2
O
2
2 2
O
studied by monitoring the luminescence signal at 1270
nm. Figure 1A shows a steep increase of the intensity
during the first 30 min, and then the signal slowly
2
2
2
2 2
O
wet peroxide obtained was rapidly and carefully deshy-
drated by filtration and used immediately or stored at
80 °C. Chemical titrations and thermogravimetric
analysis of the precipitate show the absence of free water
and the presence of three peroxide groups per atom of
calcium.
1
decreases during 2.5 h. Finally, all the available O
released within 3 h at 50 °C.
2
was
-
1
1
Yield of O
molysis of CaO
2 2
expression (3) where [ O ]cumul and [ O ]cumul are, respec-
2
. The yield (R) of O
2
produced by ther-
2
‚2H (eq 1) can be defined by the
2 2
O
1
3
tively, the cumulated concentrations of singlet and triplet
oxygen.
Heating CaO
2
‚2H
2 2
O in methanol at 50 °C leads to
the release of 1 molar equiv of oxygen and leaves an in-
1
1
[
O2]
[ O ]
2 cumul
cumul
1
(
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R )
)
(3)
1
3
[
CaO ‚2H O ]
[
O ]
+ [ O ]
2 cumul
2 2 2
(
2 cumul
(
1
2 cumul
Two different methods were used to determine [ O ] .
The first one (Figure 1) is based on the calibration of the
luminescence signal with an internal trap added to the
suspension.
(
(
26,35-37
R-Terpinene 3 was the chosen trap
1
because it reacts quickly and selectively with O
2
through
(
a [4 + 2] cycloaddition with negligible physical quench-
(
26) Di Mascio, P.; Sies, H. J . Am. Chem. Soc. 1989, 111, 2929-
38,39
ing.
The addition of R-terpinene 3 to a warmed
‚2H in MeOH leads to a decrease
2
914.
suspension of CaO
2
2 2
O
(27) Nardello, V.; Brivida, K.; Sies, H.; Aubry, J . M. Chem. Commun.
1
998, 599-600.
of the underlying area in Figure 1B). By comparing this
missing area with the total underlying area (Figure 1A),
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(
(
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3