Photoinduced Aromatization of Unsymmetrically Substituted 1,4 ...

132 J. Chin. Chem. Soc., Vol. 54, No. 1, 2007 Memarian et al.
argon atmosphere.
RESULTS AND DISCUSSION
Irradiation (280 nm) of 15.10 -3 M solution of each
of 1a-1l in chloroform by bubbling oxygen or argon through
the solution was followed by thin layer chromatography
(TLC) until the total disappearance of 1a-1l. The results are
summarized in Table 1.
Since the aim of this study was (i). the investigation
of photochemical behavior of these compounds, dependent
on the type and nature of 4-substituent and (ii). to elucidate
the effect of oxygen atmosphere on the rate of reactions, we
have carried out simultaneously the photoreactions under
both oxygen and argon atmospheres. Because of the similarity
of the products obtained on irradiation under both atmospheres,
the products obtained under an argon atmosphere
have not been isolated, except for 1e, and the times
of total oxidation are determined by TLC monitoring. For
this purpose, we have taken the same volumes with equal
concentration of each of the compounds in two different
test tubes and irradiated them simultaneously and continuously
by purging with a stream of either oxygen or argon
until total disappearance of 1,4-dihydropyridines is observed
by following the reaction by TLC.
The observed fast reaction under oxygen atmosphere
is an indication of the involvement of the excited singlet
state of 1b, 1e, 1i, 1j, 1k and 1l in the reaction, since the excited
singlet state could not be quenched by triplet oxygen.
In the cases of 1a, 1c, 1d, 1f, 1g and 1h the excited triplet
state should be involved in the reaction, since a faster pho-
Scheme I
Table 1. Irradiation of dihydropyridines 1a-1l under O2 and Ar
O2 Ar
Product (%) I
Time (h) II
Product (%) III
Time (h) II
1a 3a (60) 0.66 3a 0.33
1b 3b (71) 5.5 3b 36
1c 3c (72) 9 3c 3.75
1d 3d (40) 6 3d 5.5
1e 2 (20)
3e (50)
9 2 (18)
3e (49)
22
1f 3f (84) 12.5 3f 8
1g 3g (73) 5.5 3g 3
1h 3h (67) 6 3h 4
1i 3i (65) 7.5 3i 8.5
1j 3j (80) 6 3j 15
1k 2 (63) 1.5 2 5
1l 2 (74) 2.25 2 5
I II
Isolated yield. Irradiation times refer to disappearance of the
starting material. III The products have not been isolated except
for 1e and identified only by TLC.
to-oxidation has been observed by bubbling argon through
the solution under irradiation.
We have also investigated the effect of the nature of
solvent on the rate of reaction. Whereas irradiation of only
compound 1a in ethanol solution after 15 minutes led to the
formation of compound 3a in 70% yield, other compounds
were obtained unchanged even after 5-13 hours of irradiation
in the same solvent. Therefore, we found that chloroform
is a suitable solvent for our reaction. Owing to the formation
of dichloromethane during the reaction, which has
been proved by GC-analysis of the reaction mixture and by
testing for acidity of the solution after irradiation (due to
formation of HCl), we will propose an electron transfer