Chen et al.
SCHEME 3. Photocyclization of 3a in Cyclohexane and Dichloromethane with UV Light
SCHEME 4. Preparation of 3,4-Diphenylpyrroles
from 3,4-Diphenyl-2,5-dihydropyrroles
disubstituted pyrrole system is probably the most difficult
to obtain since selective substitutions at one or more of
the â-positions have been a challenging goal in many
synthetic programs because of their tendency to react in
aromatic substitution reactions at the more electronically
favorable R-position of the heterocyclic ring. Many meth-
odologies for preparing 3,4-disubstituted pyrroles have
been reported in the literature, including (1) coupling of
imines and nitroalkanes,13 (2) using Friedel-Crafts
acylation with an electron-withdrawing group on the
pyrrole nitrogen,14 (3) use of 3,4-silylated precursors,15
(4) use of Michael acceptors with tosylmethyl isocyanide
(TOSMIC),16 (5) palladium-catalyzed cyclization of amino
allenes,17 (6) reduction of 3-and 4-pyrrolin-2-ones with
9-borabicyclo[3.3.1]nonane (9-BBN),18 and (7) multicom-
ponent coupling reactions.19 The preparation of 3,4-
disubstituted pyrroles from 3,4-disubstituted 2,5-dihy-
dropyrroles by photoreaction, as far as we know, has not
been reported. The photoconversion of 3,4-diaryl-2,5-
dihydropyrroles into 3,4-diarylpyrroles may present a
new methodology of preparation of 3,4-disubstituted
pyrroles. Similar results were obtained when other
solvents such as acetonitrile, chloroform, tetrahydrofu-
ran, and toluene were employed in reaction.
To explore the mechanism of photoconversion of 3,4-
diaryl-2,5-dihydropyrroles to 3,4-diarylpyrroles, the fol-
lowing investigations were carried out. First, DMPO (5,5-
dimethyl-1-pyrroline-1-oxide) was used as a spin trap to
capture the free radical hydrogen in solution, and the
result is presented in Figure 2. It is demonstrated by ESR
spectroscopy that the hydrogen free radical is produced
when 9a is irradiated with UV light in acetonitrile.
Second, it was also confirmed that hydrogen gas was
formed during the photoconversion of 3,4-diaryl-2,5-
dihydropyrroles to 3,4-diarylpyrroles by the appearance
) 434) was 100%, respectively. Moreover, the absorption
maximum band of 3c (λmax ) 270 nm, in cyclohexane)
was red shifted as much as 28 nm (<λmax ) 270-242 nm)
and blue shifted as much as 226 nm (<λmax ) 496-270
nm), respectively, via comparison with 3a and 3b. Similar
results were obtained by irradiation of other dia-
rylethenes with 2,5-dihydropyrrole in dichloromethane
and suggest that 3,4-diarylpyrroles are obtained from 3,4-
diaryl-2,5-dihydropyrroles by photoreaction in DCM.
We developed a preparative method suitable to obtain
nonphotochromic diarylethenes with a 2,5-dihydropyrrole
bridging unit. A class of N-substituted 3,4-diphenyl-2,5-
dihydropyrroles (Scheme 4) were prepared and employed
as templates. The results show that all compounds 9a-
13a can be converted into N-substituted 3,4-diphenyl
pyrroles 9c-13c in good yield (65-89%) in DCM by
photoreaction. The results also show that the photore-
action is readily formed, is very reliable, and appears to
be capable of tolerating different patterns. It is well-
known that the syntheses of pyrroles are an attractive
area in heterocyclic chemistry due to the fact that many
pyrroles are subunits of natural products, pharmaceutical
drugs, and agrochemicals.8 In particular, 3,4-disubsti-
tuted pyrroles have generated considerable interest ow-
ing to their remarkable diversity of biological activity. A
number of these compounds have been shown to possess
antidiabetic,9 fungicidal,10 herbicidal,11 or antibacterial12
properties. However, it is also noteworthy that the 3,4-
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(10) Nippon Soda Co., Ltd. Japanese Patent 81 79,672, 1981; Chem.
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(12) Umio, S., Kariyone, K. Japanese Patent 68 14,699, 1968; Chem.
Abstr. 1969, 70, 87560z.
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5004 J. Org. Chem., Vol. 70, No. 13, 2005