Solvent effect on photoisomerisation of 3-methyl-1-phenylbutane-1,2-dione 2-oxime

Article abstract of DOI:10.1248/cpb.50.969

Photoisomerisation of (2E)- and (2Z)-3-methyl-1-phenylbutane-1,2-dione 2-oxime (MPBDO) in several solvents was studied. With increasing dielectric constants of solvents, kinetic constants of forward reactions (E-form→Z-form) did not change appreciably but

Full text of DOI:10.1248/cpb.50.969

July 2002
Notes
Chem. Pharm. Bull. 50(7) 969—971 (2002)
969
Solvent Effect on Photoisomerisation of 3-Methyl-1-phenylbutane-
1,2-dione 2-Oxime
Hirohito IKEDA,* Takuya SUZUE, Miho YUKAWA, Tokihiro NIIYA, and Yoshinobu GOTO
Faculty of Pharmaceutical Sciences, Fukuoka University; Nanakuma, Jonan-ku, Fukuoka 814–0180, Japan.
Received December 13, 2001; accepted March 19, 2002
Photoisomerisation of (2E)- and (2Z)-3-methyl-1-phenylbutane-1,2-dione 2-oxime (MPBDO) in several sol-
vents was studied. With increasing dielectric constants of solvents, kinetic constants of forward reactions (E-
form→Z-form) did not change appreciably but those of reverse reactions (Z-form→E-form) decreased. The posi-
tive correlation was found between equilibrium constants of photoisomerisation and dielectric constants of sol-
vents.
Key words photoisomerisation; oxime; solvent effect; dielectric constant; equilibrium constant
determined with HF/6-31G. The electron correlation energy was corrected
by the second-order Moeller-Plesset (MP2) perturbation method.
It is well known that the compounds having CϭC, NϭN
or CϭN bond isomerise readily by light. Many reports on the
isomerisation of the oximes and their derivatives which have
CϭN bond are published,1—4) however, there are few papers
on the photochemical studies of a-oxo-oximes.5)
In the present paper, the kinetic study of photoisomerisation
of 3-methyl-1-phenylbutane-1,2-dione 2-oxime (MPBDO) in
several solvents is reported (Chart 1).
Results and Discussion
For the purpose of elucidation of the mechanism of pho-
toisomerisation of a-oxo-oximes, the investigation of solvent
effects on the isomerisation of E- and Z-MPBDO in several
solvents was first carried out until the photostationary state
(PSS) was attained. The irradiation was made repeatedly
three times in the same kind of solvent. The time of attain-
ment to the PSS was ca. 3—4 h.
According to the reaction procedure as described above,
the concentrations of E- and Z-MPBDO were determined at
regular intervals. The equilibrium constant (K) of the iso-
merisation shown in Chart 1 is expressed by Eq. 1 as follows,
Experimental
Materials 3-Methyl-1-phenylbutane-1-one (TOKYO KASEI KOGYO
Co. LTD.) and tert-butyl nitrite (ACROS ORGANICS) were purified by dis-
tillation prior to use. All solvents using photochemical reactions were HPLC
or spectroscopy grade obtained from KANTO CHEMICAL Co. INC.
Synthesis of MPBDO E- and Z-MPBDO were prepared by nitrosation
of 3-methyl-1-phenylbutane-1-one with tert-butyl nitrite in the presence of
hydrogen chloride by use of a similar procedure as described by Hartung and
Crossley for the synthesis of isonitrosopropiophenone.6) A mixture of E- and
Z-MPBDO (32.7 g) was obtained in 29% yield from 96.7 g of 3-methyl-1-
phenylbutane-1-one. The resulting mixture was separated by HPLC.
Kϭ[Z]PSS/[E]PSSϭk/kЈ
(1)
in which [E]PSS, [Z]PSS, k and kЈ are the concentration of E-
Separation of Resulting Mixture by HPLC HPLC was performed on and Z-MPBDO at PSS, the kinetic constant of forward reac-
JASCO PU-986/987 intelligent prep. pump and JASCO UVIDEC-100-III
UV spectrophotometer connected to a System Instruments Chromatocorder
11 integrator. Merck Lichrosorb Si60 (7 mm) (25 mmϫ250 mm, pre-packed)
column was used for HPLC analysis. As a mobile phase, ethyl acetate (25%
tion and that of reverse reaction, respectively. The summation
of kinetic constants (kϩkЈ) at regular intervals is calculated
by Eq. 2 as follows,
(v/v)) and hexane (75% (v/v)) were used. E- and Z-MPBDO were obtained
from the resulting mixture at the ratio of 6 to 4.
ln([A]Ϫ[A]PSS)ϭϪ(kϩkЈ)tϩln([A]0Ϫ[A]PSS
)
(2)
Photoisomerisation of MPBDO A solution of 1ϫ10Ϫ3 mol/l of E- or
Z-MPBDO was irradiated with a UVL-100HA high pressure (100 W) mer-
cury lamp equipped with water-cooling tube (Pyrex) or a UVL-32LB low
pressure (32 W) mercury lamp equipped with air-cooling tube (quartz)
(RIKO-KAGAKU SANGYO Co., LTD.) at a distance of 17 cm from a reac-
tion vessel (Pyrex). The temparature of the reaction vessel and the mercury
lamp was maintained at 300 K in the same thermostat during irradiation. The
reaction was followed by analysing the ratio Z/E using HPLC every 30 min.
HPLC was performed on Waters-600E-MSDS equipped with Waters 484
tunable absorbance detector connected to a System Instruments Chromato-
corder 11 integrator. Merck Lichrosphere Si60 (5 mm) (4 mmϫ250 mm, pre-
packed) column and ethyl acetate (25% (v/v)) and hexane (75% (v/v)) as a
mobile phase were used for HPLC analysis.
in which [A], [A]PSS and [A]0 are the concentration of start-
ing isomer (E- or Z-MPBDO) at the time t, that at PSS and
the initial concentration of starting isomer, respectively. K
was determined using [E]PSS and [Z]PSS. And the average of
(kϩkЈ) at regular intervals was calculated, and then, k and kЈ
were calculated using Eqs. 1 and 2. The dielectric constant
(e) of each solvent at 298 K from some literatures,9,10) k, kЈ
and K of photoisomerisation of MPBDO in each solvent are
shown in Table 1. With increasing dielectric constants of sol-
vent, k did not change appreciably but kЈ was liable to de-
crease. Using amides or alcohols as a reaction solvent, the
value of K is larger than that using the other solvent. As
shown in Fig. 1, the equilibrium constant K shows a linear re-
Computational Procedure The ab initio MO calculations were carried
out with the Gaussian 98 program.7) All of the geometry optimization and
energy calculation of an isomer in a solvent were carried out using an On-
sager reaction field model.8) The optimized geometries of each isomer were lation with the dielectric constant e, and the correlation coef-
ficient is 0.920.11) The value of K increases linearly with an
increase of that of e.
When the same experiments as described above were car-
ried out using the low pressure mercury lamp, the rate of
photoisomerisation was quite slower than that using high
Chart 1
pressure mercury lamp and the time for attainment of PSS
To whom correspondence should be addressed. e-mail: ikeda@fukuoka-u.ac.jp
© 2002 Pharmaceutical Society of Japan

970
Vol. 50, No. 7
Table 1. e of Solvent at 298 K, k, kЈ and K of Photoisomerisation of MPBDO in Each Solvent at 300 K
Solvent
e
k (hϪ1
)
kЈ (hϪ1
)
K
1,4-Dioxane
Benzene
Toluene
Diethyl ether
Diisopropyl ether
tert-Butyl methyl ether
Ethyl acetate
Tetrahydrofuran
2-Butanol
1-Butanol
2-Propanol
1-Propanol
Ethanol
Methanol
N,N-Dimethylformamide
N,N-Dimethylacetamide
2.210
2.270
2.380
4.200
4.490
4.500
6.020
7.580
16.56
17.51
19.92
20.45
24.55
32.66
36.71
37.78
0.549
0.476
0.379
0.469
0.469
0.495
0.390
0.486
0.386
0.558
0.502
0.453
0.523
0.400
0.459
0.462
1.090
0.966
0.829
1.100
1.130
1.120
0.852
1.070
0.669
0.975
0.815
0.781
0.880
0.627
0.597
0.509
0.504
0.493
0.458
0.432
0.414
0.440
0.458
0.457
0.576
0.572
0.617
0.580
0.593
0.639
0.771
0.908
Fig. 1. Relationship between K and e
Table 2. e of Solvent at 298 K, Etotal of Both Isomers and DE
Diethyl ether
1-Butanol
N,N-Dimethylformamide
e
4.20
17.51
36.71
Etotal of E-MPBDO (a.u.)
Etotal of Z-MPBDO (a.u.)
DE (kJ/mol)
Ϫ629.297313
Ϫ629.296699
1.61
Ϫ629.297621
Ϫ629.297066
1.46
Ϫ629.297688
Ϫ629.297140
1.44
was ca. 50—60 h. The low pressure mercury lamp emits
mainly a UV ray of 254 nm, while the high pressure mercury
lamp emits mainly a UV ray of 365 nm. According to some
reports,5,12) the wavelengths emitted from a low pressure
mercury lamp corresponds to p–p* absorption band of a-
oxo-oximes and those emitted from a high pressure mercury
lamp corresponds to n–p* absorption one. Thus, n–p* ab-
sorption of MPBDO may be closely related to this photoiso-
merisation.
both isomers and the difference (DE) in Etotal between both
isomers were calculated as shown in Table 2. With increasing
dielectric constants of solvents, DE tends to decrease. It is
considered that these calculation results support the experi-
mental results (the value of K in a solvent with high e is
larger than that in a solvent with low e).
The detailed investigation on the mechanisms of this reac-
tion using ab initio MO calculation will be reported in the
near future.
As a preliminary theoretical study on these experimental
results, the thermodynamic stabilities on ground state of E-
and Z-MPBDO in the three kinds of solvents (diethyl ether,
1-butanol and N,N-dimethylformamide) were investigated
using ab initio MO calculation. The MO calculation in the
presence of a solvent was performed using an Onsager reac-
tion field model.8) Based on the optimised geometries of E-
and Z-MPBDO in each solvent, the total energies (Etotal) of
Acknowledgements Thanks are due to the Information Technology
Center of Fukuoka University for use of the Fujitsu GP7000S1000 com-
puter, to the Computing and Communnications Center, Kyushu University
for use of the Fujitsu VPP5000/64 computer, and to the Reserch Center for
Computational Science, Okazaki National Research Institutes for use of the
NEC SX5 and Fujitsu VPP5000 computers.

July 2002
971
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