1205
PHOTOCHEMICAL ISOMERIZATION OF a, N-DIPHENYLNITRONE
where a , the expansivity, is defined as -l/p(dp/dl), is very small. This is in agreement with the fact that the rate of change of density (or specific or equivalent volume) with temperature is small. When specific conductances for RfoOs and the molybdates of Li+, Na+, and K+ are compared a t 100' above the respective melting points, the decreasing order of conductance i s Lizi\/lo04 > NazMo042> KZMoo4> MoOa. This is also the order for the average activation energies, AH,, based on rgpecific conduct-
ances. This order for the activation energies is due perhaps to the fact that the polarizing powers of the cations and the coulombic forces between the ions of each substance are decreasing with increasing cation size. Quantitative interpretations of conductance data for molten molybdate systems must await the study of transport numbers for the conducting species. Acknowledgment. The authors are grateful to Mr. Daniel L. Akins for assistance in some of the experimental aspects of the research.
The Photochemical Isomerization of a,N-Diphenylnitrone
by Kinko Shinzawa and Ikuzo Tanaka Depurtment of Chemistry, Tokyo Institute of Technology, Ohokayama, Meguro-ku, Tokyo, Japan (Received September 68,1963)
The photocheinical isomerization of a,N-diphenylnitrone to a,N-diphenyloxazirane a t 3130 8. irradiahion was confirmed by iodometry and ultraviolet and infrared absorption spectroscopy. The oxazirane produced in diluted cyclohexane solution isomerized thermally to benzanilide a t high temperature (activation energy = 25.9 kcal./mole, frequency factor = 2.9 X l o l l set.-'). The oxazirane in concentrated solution decomposed to a substance having an aldehyde group even a t room temperature. The infrared spectrum of the oxazirane was taken a t '77'K. The quantum yield of the photoisomerization was independent of irradiation time, concentration of the nitrone, presence of oxygen, and temperature, but was affected by hydrogen bond formation. The quantum yield was 0.28 in the case of cyclohexane solution and 0.18 in ethanol solution in the temperature range from 30 to 75'. The trans + cis isomerization of the nitrone with irradiation could not be found. A mechanism of the reaction is proposed by consideration of the mobility of the oxygen attached to nitrogen and of trans + cis isomerization. At 2537 A. irradiation the oxazirane was produced primarily just as in the case of 3130 A. irradiation, but the oxazira,ne reacted futher by absorbing light of 2537 A.
Introduction Although there have appeared some on the photochemical, reaction of nitrones showing that their primary products were oxaziranes, the identity of the product in the case of a,N-diphenylnitrone has not been established and the mechanism of this reaction has not been discussed.
The chemical structure of a,N-diphenylnitrone is quite interesting. The features of its electronic (1) 1LI. Kamlet and L. Kaplan, J. Org. Chem., 22, 576 (1957). (2) F. Krohnke, Ann., 604, 203 (1957). (3) J. 5. Splitter and M.Calvin, J. Org. Chem., 23, 651 (1958). (4) H. Shindo and B. Umezawa, Chem. Pharm. BUZZ. (Tokyo), 10, 492 (1962).
Volume 68, Number 6
M a y , 1964
KINKOSHINZAWA AND IKUZO TANAKA
1206
structure are similar to those of pyridine K-oxide not only in the ground state but also in the excited state.6 Pyridine N-oxide and picoline Y-oxide were reported by Hata and Tanaka6 to liberate oxygen a t 2537 8. irradiation, and the latter to isomerize to 2-pyridinemethanol a t 3261 8. irradiation. In addition, the nitrone has a double bond as has stilbene,' azobenzene,s and benzylidenephenylimine, in which cases trans + cis photoisomerizations have been reported. This paper describes the identification of the product as oxazirane, reports measurements of the quantum yield under various conditions, and includes a discussion of the mechanism of the photochemical reaction on the basis of the mobility of the oxygen attached to nitrogen and of trans + cis isomerization.
gen, and its spectrum taken. Then the disk together with the cell was put out of, the spectrometer and irradiated a t 77°K. with 3130 A. light for 2.5 hr. through a KBr window of tjhe cell. Then the spectrum of the irradiated disk was taken again. Results and Discussion Formation of Oxazirane. The spectrum of the solution of a,K-diphenylnitrone changed markedly by irradiation with 3130 8. light as shown in Fig. 1.
I .o
I
Pro
Experimental
Materials. a,N-Diphenylnitronewas syn thesized and purified according to Foster, et aZ.10 Cyclohexane was Nichiri Co. reagent grade product and was purified by passing through silica gel and by distillation. Ethyl alcohol was Koso-Kagaku Co. reagent grade product. Light Sources. A high pressure mercury lamp was used for 3130 A. irradiation. The light was collimated and the stray radiation was prevented from entering the cell by a shielding box. The box was thermostated a t experimental temperatures. For 3130 A. irradiation a filter combination of cobalt sulfate-nickel sulfate solution and Hallio glass was used. The glass transmitted light of wave lengths longer than 3130 A. A low pressure mercury resonance lamp containing neon was used for the 2537 A. radiation source without filters, in which case the lamp was so separated from the cell that the 1849 8. light from the lamp was negligible by absorption of oxygen in the air between them. A Hallio glass cell and a quartz cell were used for 3130 and 2537 8. irradiation, respectively. Light Intensity Measurement. The intensity of the incident light was measured periodically by the uranyl oxalate actinometric method. l 1 The concentrations of the solutions were so adjusted that incident light was absorbed completely. Spectroscopy. Ultraviolet absorption spectra a t room temperature were taken with Shimazu QR-50 and Shimazu SV-5OA spectrophotometers and infrared absorption spectra with a Perkin-Elmer RIodel 112 equipped with a special cell designed for low temperature nieasurements.12 For reasons which will be apparent later, infrared spectra were taken both a t low temperature and room temperature. For low temperature measurements, a disk of the sample, which was formed by pressing a mixture of the sample and KRr powder, was set in the cell, cooled by liquid nitro7%
Journal
of
Physical Ch,e??vistrU
---___-.- --. __ __ - - -
,630 220 240 260 280 300 320 340 wavelength, m P I
I
I
I
Figure 1. The progressive decrease of the spectrum of the nitrone a t 3130 A. irradiation in cyclohexane solution. Numbers refer to time of irradiation in seconds, and dotted line denotes a spectrum of the product a t room temperature.
The first and second bands of the nitrone decreased and a new band with its maximum a t 223 mp appeared as the irradiation time increased, with an isosbestic point a t 232 mp. The reaction product has been considered to be a,K-diphenyloxazirane by the authors. The oxazirane was so unstable that it could not be obtained by usual organic methods. l 3 Consequently, ls4
( 5 ) T. Kubota and M.Yamakawa, Bull. Chem. Soe. Japan, 36, 1564 (1963). (6) N. Hata and 1. Tanaka, J . Chem. Phys., 36, 2072 (1962); Bull. Chem. SOC.Japan, 34, 1440, 1444 (1961).
(7) 2. Smakula, 2. physik. Chem., B25, 90 (1934). (8) G. S. Hartley, Nature, 140, 281 (1937); J . Chem. SOC..633 (1938). (9) E. Fisher and Y. Frei, J. Chem. Phys., 2 7 , 808 (1957). (10) (a) M . 0. Foster and H. Folmes, J . Chem. Soc., 242 (1908); (b) the authors are indebted to C. Shin for the preparation. (11) G. S. Forbes and L. J. Heidt, J. A m . Chem. SOC.,56, 2363 (1934). (12) The authors are indebted to the members of the laboratory of Prof. K . Kojima at Tokyo Institute of Technology for taking these spectra.
P H O T O C H E M ~ C A LISOMERIZATION O F
1207
a,N-DIPHENYLNITRONE
Table I“
H
H 322
20,000
D-(!!-NQ
-t7~-t.
295
16,700
0 H H 0 1
336
20,800
H -
I~
=
0 0
930
H H 0 1
a
BU
0
I 1 ~-Bu-X=C-(>=N-~-BU
~
245
H
H
~
14,000
‘d
0
~
223
y
-
t
- 252 ~ u 362
11,400, 15,800
t-Bu-X-C-
Y
‘c:
--N-t-Bu
None
0 ‘’
H 0 2 N - ~ - - C - - N I- t - B u
268
11,900
‘o/
The data except for those of a,N-diphenylnitrone and of its oxazirane are reprinted from l
