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Photoisomerization of p -nitrobenzaldehyde
TetrahedronLetters No.20, pp. 1689-1691,1970.
PergamonPrees.
Printed in (Seat &itain.
PHOTOISOMERIZATION OF E-NITROBENZALDEHYDE’ Gene W. Wubbels, Richard R. Hautala and Robert L. Letsinger Department of Chemistry, Northwestern University Evanston, Illinois 60201 (Receivedin USA 24 February1970; receivedin UK for publication31 March 1970)
A rule formulated by Sachs and Hilpe& states that aromatic nitro compounds which contain aliphatic C-H ortho to the nitro group are light sensitive.
The photoreactions,
which include the well-known isomerization of -o-nitrobenzaldehyde to -o-nitrosobenzoic acids3’ * and the photochromic transformations of substituted o-nitrotoluenes5, appear to proceed -via six membered cyclic transition states. m-Nitrobenzaldehyde and B-nitrobenzaldehyde are reported to be relatively stabile, failing to isomerize (though slowly yielding other products) when irradiated in ethanoF, methano16, ether6, toluene6, benzene7, or in the solid state7. We now wish to report that the photoinduced isomerization is more generalthan heretofore recognized and proceeds smoothly in an aqueous medium even when nitro and aldehyde groups are in m
positions.
Preliminary experiments showed that pnitrobenzaldehyde
(1.0 x 10m4M) reacted
readily when irradiated under nitrogen in distilled water at 25Owith light in the range of 290-400 nm.
The maximun in the uv spectrum at 267 nm disappeared and new maxima
developed at 287 and 308 nm with an isosbestic point at 282 nm. agreed well with that for g-nitrosobenzoic a noticeable
Neither oxygen nor iodide ion (10m2M)had
acid.
effect on the efficiency of the reaction.
Under the same conditions of irra-
diation m-nitrobenzaldehyde was essentially unchanged. benzaldehyde the solvent plays a critical role.
The final spectrum
In the isomerization of p-nitro-
No change was observed when g-nitroben-
zaldehyde was irradiated in hexane for periods longer than required for complete reaction in water.
p-Nitrobenzaldehyde
reacted slowly when irradiated in methanol or ethyl ether;
however, the spectral changes were complex( no isosbestic point) and indicated that the reaction followed a path different from that in water. For isolation of the product, 50 mg (0.33mmolbf e-nitrobenzaldehyde
in one liter of
doubly distilled water was irradiated through a Vycor filter sleeve with a 250 w Hanovia in vacua at 35-40° and the residue was lamp for two hours. The solvent was evaporated -dried by distillation of benzene and then taken up in methanol. Concentration of the
16s9
1630
rb.20
methanol extract afforded of its decomposition
40 mg (80%) of p-nitrosobenzoic
ment the irradiation
products
and, after evaporation solution and analysis
acid prepared
were extracted
of the organic
of a trace of hydrochloric
solvent,
The quantum yield for conversion
were heated with excess
was determined considerably
of p-nitrobenzaldehyde
molecules)
intermediate
to E-nitrosobenzoic
set at 270 run and the extent of at 270 nm).
than the photoisomerization
for which quantum yields of about 0.5 in a variety of solvents An attractive
for the isomerization
which could form by stepwise
transfer
This reaction
have been reported4.
is represented
(possibly
loss of the elements
in the presence of water.
of water,
The stability
assisted
afford p-nitrosobenzoic of m-nitrobenzaldehyde
water may stem from the inability of the meta isomer of this type.
f 0
III
is
of o-nitrobenzaldehyde, by III.
This
by water
of a hydrogen atom from the aldehyde group to the nitro group,
to the aldehyde or,
of the
the aqueous solution was irradia-
from the change in absorbance less efficient
Concentration
was added to the solution.
ted with light from a Bausch and Lomb monochromator reaction
methanol in the
showed a single product peak, and this peak
acid at 25O was found to be 0.037 (for this measurement
therefore
with
ln another experi-
with ethyl acetate from the aqueous solution
acid to effect esterification.
by gas chromatography
spectrum
independently.
was enhanced when authentic methyl p-nitrosobenzoate
intermediate,
identified by comparison
point (260°, turned brown; lit8 2509 and infrared
values for a sample of p-nitrosobenzoic
presence
acid,
could revert
acid by addition and to irradiation
in
to form a conjugated intermediate
No.20
REFERENCES 1. Photoinduced Substitution IX. & 03.
C_.,
in press.
Part VIII K. E. Steller and R. L. Letsmger,
This research was supported by the National Science
Foundation. 2.
F. Sachs and S. Hilpert, Cb.
3.
G. Ciamician and P. Silber, jb&l., a%0 (1901). J. G. Calvert and J. N. Pitts, Jr., “Photochemistry”,
4.
m.,
37, 3423 (1904). John Wiley and Sons, MC.,
New York, N. Y., 1966, p 479 and references therein. 5.
R. Exelby and R. Grinter, Ca.
6.
E. Ramberger and F. Elger, A>.
‘7. A. Kailan, Montash. CZ., 8.
F. J. Alway, Amer. CM.
3 4,
Rx.,
8% 247 (1965).
-4% Che_m., 1395 (1912).
32? 385 (1904).
288 (1929).
PergamonPrees.
Printed in (Seat &itain.
PHOTOISOMERIZATION OF E-NITROBENZALDEHYDE’ Gene W. Wubbels, Richard R. Hautala and Robert L. Letsinger Department of Chemistry, Northwestern University Evanston, Illinois 60201 (Receivedin USA 24 February1970; receivedin UK for publication31 March 1970)
A rule formulated by Sachs and Hilpe& states that aromatic nitro compounds which contain aliphatic C-H ortho to the nitro group are light sensitive.
The photoreactions,
which include the well-known isomerization of -o-nitrobenzaldehyde to -o-nitrosobenzoic acids3’ * and the photochromic transformations of substituted o-nitrotoluenes5, appear to proceed -via six membered cyclic transition states. m-Nitrobenzaldehyde and B-nitrobenzaldehyde are reported to be relatively stabile, failing to isomerize (though slowly yielding other products) when irradiated in ethanoF, methano16, ether6, toluene6, benzene7, or in the solid state7. We now wish to report that the photoinduced isomerization is more generalthan heretofore recognized and proceeds smoothly in an aqueous medium even when nitro and aldehyde groups are in m
positions.
Preliminary experiments showed that pnitrobenzaldehyde
(1.0 x 10m4M) reacted
readily when irradiated under nitrogen in distilled water at 25Owith light in the range of 290-400 nm.
The maximun in the uv spectrum at 267 nm disappeared and new maxima
developed at 287 and 308 nm with an isosbestic point at 282 nm. agreed well with that for g-nitrosobenzoic a noticeable
Neither oxygen nor iodide ion (10m2M)had
acid.
effect on the efficiency of the reaction.
Under the same conditions of irra-
diation m-nitrobenzaldehyde was essentially unchanged. benzaldehyde the solvent plays a critical role.
The final spectrum
In the isomerization of p-nitro-
No change was observed when g-nitroben-
zaldehyde was irradiated in hexane for periods longer than required for complete reaction in water.
p-Nitrobenzaldehyde
reacted slowly when irradiated in methanol or ethyl ether;
however, the spectral changes were complex( no isosbestic point) and indicated that the reaction followed a path different from that in water. For isolation of the product, 50 mg (0.33mmolbf e-nitrobenzaldehyde
in one liter of
doubly distilled water was irradiated through a Vycor filter sleeve with a 250 w Hanovia in vacua at 35-40° and the residue was lamp for two hours. The solvent was evaporated -dried by distillation of benzene and then taken up in methanol. Concentration of the
16s9
1630
rb.20
methanol extract afforded of its decomposition
40 mg (80%) of p-nitrosobenzoic
ment the irradiation
products
and, after evaporation solution and analysis
acid prepared
were extracted
of the organic
of a trace of hydrochloric
solvent,
The quantum yield for conversion
were heated with excess
was determined considerably
of p-nitrobenzaldehyde
molecules)
intermediate
to E-nitrosobenzoic
set at 270 run and the extent of at 270 nm).
than the photoisomerization
for which quantum yields of about 0.5 in a variety of solvents An attractive
for the isomerization
which could form by stepwise
transfer
This reaction
have been reported4.
is represented
(possibly
loss of the elements
in the presence of water.
of water,
The stability
assisted
afford p-nitrosobenzoic of m-nitrobenzaldehyde
water may stem from the inability of the meta isomer of this type.
f 0
III
is
of o-nitrobenzaldehyde, by III.
This
by water
of a hydrogen atom from the aldehyde group to the nitro group,
to the aldehyde or,
of the
the aqueous solution was irradia-
from the change in absorbance less efficient
Concentration
was added to the solution.
ted with light from a Bausch and Lomb monochromator reaction
methanol in the
showed a single product peak, and this peak
acid at 25O was found to be 0.037 (for this measurement
therefore
with
ln another experi-
with ethyl acetate from the aqueous solution
acid to effect esterification.
by gas chromatography
spectrum
independently.
was enhanced when authentic methyl p-nitrosobenzoate
intermediate,
identified by comparison
point (260°, turned brown; lit8 2509 and infrared
values for a sample of p-nitrosobenzoic
presence
acid,
could revert
acid by addition and to irradiation
in
to form a conjugated intermediate
No.20
REFERENCES 1. Photoinduced Substitution IX. & 03.
C_.,
in press.
Part VIII K. E. Steller and R. L. Letsmger,
This research was supported by the National Science
Foundation. 2.
F. Sachs and S. Hilpert, Cb.
3.
G. Ciamician and P. Silber, jb&l., a%0 (1901). J. G. Calvert and J. N. Pitts, Jr., “Photochemistry”,
4.
m.,
37, 3423 (1904). John Wiley and Sons, MC.,
New York, N. Y., 1966, p 479 and references therein. 5.
R. Exelby and R. Grinter, Ca.
6.
E. Ramberger and F. Elger, A>.
‘7. A. Kailan, Montash. CZ., 8.
F. J. Alway, Amer. CM.
3 4,
Rx.,
8% 247 (1965).
-4% Che_m., 1395 (1912).
32? 385 (1904).
288 (1929).