6916 J . Org. Chem., Vol. 62, No. 20, 1997
Nojima et al.
methanol of spectrophotometric grade (Nacalai) were used
without further purification.
a) on the cycloreversion of dioxadiazole. Such a case
where the formation of 5 and N2O is predominant was
observed for the low-temperature reaction (e.g., -75 °C),
while that of 2 and N2 became comparable or faster at
ambient temperature as shown in Table 2. These ex-
perimental results indicates that the cycloreversion of 4
is governed by the activation enthalpy at low temperature
and by the entropy factor at ambient or higher temper-
atures. It is interesting to recall the contrasted result
that there was only a minor temperature effect on the
direction of cleavage of primary ozonides;20 this case may
be explained by no significant difference of reaction
enthalpies between the two paths of cleavage of primary
ozonide (cf. Scheme 2).
One possible scenario for the origin of the entropy
factor is a concerted nonsynchronous nature of the
cycloreversion of 4 to 5 (cf. path b in Scheme 3) involving
weak O-O bond fission, which would be more facile than
that of the N-O bond. Such nonsynchronous cyclorever-
sion tends to have a small activation entropy as reported
for 1,3-dipolar cycloreversions of tetrazoles.21 Therefore,
path b in Scheme 3 would be, although enthalpically
favored, entropically less favorable than path a. Thus,
the temperature dependence of the cycloreversion of 4
would be caused by such opposite directions between
enthalpy and entropy factors controlling pathways a and
b.
Deter m in a tion of Ga seou s P r od u cts Evolved d u r in g
th e P h otooxid a tion of 1. The relative sensitivity of N2/N2O
on the GC/MS spectroscopy at the ionizing voltage of 70 eV
was determined to be 0.91 ( 0.02 from a mixture of authentic
N2 and N2O gases of known ratio. A 1.65-mL solution of 0.15
mM MB or TPP (0.10 mM) and ∼3 mM of diazoalkane in a
1.7-mL Pyrex tube capped with a rubber septum (Aldrich) was
purged with N2-free O2 gas for 15 min. The solution was
irradiated with a 300 W medium pressure Hg lamp through a
5% KNO2 filter solution (i.e., >400 nm) for 30 min at 21 °C at
which diazoalkane was completely converted to ketones. In
the low-temperature experiment, the solution was purged with
N2-free O2 for 10 min at -30((3) °C or -75(( 2) °C to prevent
the contamination of air and was irradiated in the same way
but the irradiation time was 90 min.
After irradiation, the contents of N2 and N2O in solution
were analyzed by GC/MS spectroscopy from relative peak areas
at m/e 28 (N2) and 44 (N2O), respectively. Small amounts of
N2 and CO2 (m/e 44) leaked at the injection (less than 5% of
evolved gases) were corrected and the analyses were repeated
for 3-5 times.
La ser F la sh P h otolysis Stu d y. Laser flash photolysis
experiments were carried out on O2-saturated solution of diazo
compound (∼10-3 M) including sensitizer (∼10-4 M). All
experiments were carried out by flowing the solutions through
a 10 × 5 mm2 cell made of quartz in order to ensure that each
laser pulse irradiated a fresh volume of solution.
O2 (1∆g) phosphorescence (1270 nm) decay curves were
measured by the time-resolved single-photon counting method
as summarized in the followings. A Continuum Surelight I
Nd-YAG laser (532 nm, ∼10 ns, ∼3 mJ /pulse) was used as the
excitation source. The unfocussed 8-mm laser beam was
attenuated by being passed through a pinhole and neutral
density filters, and the total energy incident at the cuvette
was ca. 0.1 mJ /pulse. A sample cell was held in a specially
designed holder placed just ahead of a lens adapter, which was
used to focus the signal light onto the detector. A custom
interference filter (centered at 1270 nm, fwhm ) ca. 40 nm,
obtained from ASAHI SPECTRA Co. Ltd) and some cutoff
filters were placed between the lens adapter and the sample
holder to cut off unwanted scattered laser light. In this way,
the phosphorescence of O2 (1∆g) isolated from the signal light
was detected by a photomultiplier tube (R5509-41, HAMAMAT-
SU), which was located in the vidicon chamber (C6544,
HAMAMATSU) cooled at -80 °C utilizing liquid nitrogen as
the coolant medium. Signals from the photomultiplier were
amplified with a fast preamplifier (SR445, Stanford Research
Systems, Inc.) and stored in a multichannel scaler/averager
(SR430, Stanford Research Systems, Inc.) which counted
incoming pulses in successive time bins (5 ns). The counts of
O2 (1∆g) phosphorescence were prevented not to exceed 3% of
maximum counts (32 767) in order to maintain the single-
photon counting conditions.
The system to determine the rate constants of carbonyl oxide
formation is summarized as follows. A Spectron SL284G Nd-
YAG laser (266 nm, ∼6 ns, ∼25 mJ /pulse) was used as the
excitation source. The decay kinetics were observed by the
photomultiplier tube (PMT) monitoring system. This system
consisted of a 150 W Xe flash lamp (XF-80, Tokyo Instru-
ments), a SPEX 270M monochromator, and a HAMAMATSU
R-1221HA photomultiplier tube. The system was controlled
by a PC-9801 RA computer which was interfaced (GPIB) to a
GOULD 4072 digital oscilloscope for data acquisition. The
transient spectra were observed with a CCD monitoring
system. The CCD detector (ICCD-1024-MLDG-E, Princeton
Instruments) was controlled by a detector controller (ST-135,
Princeton Instruments) and a pulse generator (PG-200, Prin-
ceton Instruments). The system was controlled by a PC-9801
BA2 computer which was interfaced (GPIB) to the detector
controller.
Con clu sion s
The singlet oxygen oxidation of diazomethanes has
been clarified by substituent effects on the carbonyl oxide
yields as determined from the product ratios of N2/N2O,
and more directly by kinetics of the reaction of 1O2 with
1 and the formation of carbonyl oxides by the time-
resolved spectroscopy. Results are summarized as fol-
lows: (a) The predominant formation of dioxadiazole 4
in the 1O2 oxygenation of 1 was suggested, since the
product ratios of N2/N2O were not affected by protic
solvents. (b) Product selectivities in the cycloreversion
of 4 were controlled by the relative stability of resulting
carbonyl oxides, as supported by the effects of substitu-
ents on diazomethanes. (c) The lifetime of 1,2,3,4-
dioxadiazole 4 was too short (τ < 100 ns) to be observable
with the time-resolved spectroscopy under the present
conditions.
Exp er im en ta l Section
GC/MS analyses were carried out with a Shimadzu QP-5000
mass spectrometer using a 0.2 mm × 25 m capillary column
of CBP1-M50-025 (Shimadzu). GLC analyses were performed
with a Shimadzu GC-14A gas chromatograph, using a 2.5 mm
× 1 m column of Carbowax 300M, 2% on Chromosorb WAW
(GL Sciences).
Ma ter ia ls. Phenyldiazomethane,22 4-substituted phenyl-
diazomethanes,22 1-phenyldiazoethane,22 1-phenyldiazopro-
pane,22 diphenyldiazomethane,23 2-methyl-1-phenyldiazopro-
pane,23 2,2-dimethyl-1-phenyldiazopropane,23 and 1-phenyl-
2,2,2-trifluorodiazoethane24 were prepared by the reported
procedures.
Acetonitrile, benzene, and dichrolomethane (Tokyo Kasei)
were distilled over calcium hydride prior to use. Acetone and
(20) Flisza´r, S.; Renard, J . Can. J . Chem. 1970, 48, 3002-3018.
(21) Hong, S. Y.; Baldwin, J . E. Tetrahedron. 1968, 24, 3787-3794.
(22) Farnum, D. G. J . Org. Chem. 1963, 28, 870-872.
(23) Smith, L. I.; Howard, K. L. Organic Syntheses; Wiley: New
York, 1955; Collect. Vol. III, p 351.
(24) Shepard, R. A.; Wentworth, S. E. J . Org. Chem. 1967, 32, 3197-
3199.
All delay time of systems was controlled by a digital delay/
pulse generator (DG-535, Stanford Research system).
Ab In itio Ca lcu la tion s. Ab initio calculations were carried