The Journal of Organic Chemistry
Article
(0.22 mmol) was dissolved in DCE (1.5 mL) and tributyltin
hydride (nBu3SnH) (1.2 equiv) was added in a 10 mL screw
caped quartz tube. Then the reaction mixture was irradiated
under ∼350 nm at room temperature for 16 h. However, no
product formation could be detected.
The heterocyclic core benzimidazole is well recognized in
supramolecular chemistry,43 organic synthesis,44 medicinal
chemistry,45 materials chemistry,46 etc. In Figure 5, we have
established the synthetic utility of this newly identified
photochemical reaction. The yield of 2aa was 98%; however,
unsubstituted benzimidazoles having different substitutions like
methyl, tert-butyl, fluoro, ethyl, methoxy, trifluoromethyl
groups, etc. at the aryl moiety were also efficiently converted
to the corresponding products (2ab−2ag). Notably, the
reactions were also highly successful with substitutions like
methyl, tert-butyl, fluoro, ethyl, methoxy, and chloro at the aryl
moiety of dimethyl-substituted benzimidazoles (2bb, 2bc, 2bd,
2be, 2bf, and 2bh). The compounds 2ba and 2bi were obtained
with 94 and 93% yields, respectively. The trifluoromethyl
(−CF3) substitution at aryl moiety of dimethyl substituted
benzimidazoles gave the desire product 2bg in 74% yield. To our
delight, the biphenyl group of dichloro substituted benzimida-
zoles was also efficiently converted to the desired product 2ca
with a 96% yield. The groups like methyl, tert-butyl, and ethyl at
the aryl ring of dichloro benzimidazole did not have any
significant impact on the synthesis of 2cb, 2cc, and 2ce.
Nevertheless, the fluoro, trifluoromethyl, and chloro group
containing aryl part gave the products 2cd, 2cg, and 2ch with 85,
79, and 87% yields. Similarly, the starting material with fluoro
substitution at the meta-position of the aryl group was
successfully converted to corresponding product 2ci with a
97% yield. Difluoro benzimidazole having unsubstituted and
ethyl group at the aryl skeleton also led to products 2da and 2de
with 90 and 92%, respectively.
Figure 7. Gram-scale synthesis.
Table 1. Photophysical Data of Benzimidazole-Fused
Phenanthridine Derivatives
a
b
c
entry
compound
λabs (nm)
λem (nm)
φF
1
2
3
4
5
6
7
8
2ac
2ad
2ae
2ba
2bb
2bc
2bd
2bf
2bi
346
331
346
354
330
347
338
359
354
351
341
344
351
357
354
357
374
369
373
401
383
381
394
384
386
374
401
379
373
378
399
379
0.61
0.94
0.90
0.67
0.73
0.66
0.96
0.84
0.94
0.18
0.58
0.64
0.72
0.39
0.37
0.31
9
10
11
12
13
14
15
16
2ca
2cb
2cc
2 cd
2ce
2cg
2ci
a
b
c
Absorption maxima in dichloromethane solution (3 × 10−6 M).
Emission maxima in dichloromethane solution (3 × 10−6 M).
Absolute fluorescence quantum yields were recorded using
corresponding absorption and emission parameter in dichloro-
methane solution (3 × 10−6 M).
In Figure 6, the substrate scope is shown for the
monosubstituted benzimidazole derivatives. Single regioisomers
were obtained for the aryl moiety of 3-bromo-benzimidazoles
with methyl, fluoro, and ethyl substitutions (4db, 4dd, and 4de).
Similarly, 4ed was obtained with an 84% yield as a major isomer.
A regioisomeric mixture of products (4eb, 4ee, 4ea, and 4fa)
was obtained for the unsymmetrical benzimidazole derivatives.
On the other hand, 3-chloro benzimidazoles bearing methyl,
tert-butyl, and fluoro group also reacted smoothly to deliver
corresponding products 4gb, 4gc, and 4gd. The structural
assignments for the other compounds were done with the help of
the X-ray crystal structure of 4gc, i.e., 11-chloro-2-fluorobenzo-
[4,5]imidazo[1,2-f ]phenanthridine (CCDC no. 2039807). The
position of the −Cl group is identified at the carbon-11. The −Cl
group shows the −I effect, and we assumed that the −Me group,
which shows the +I effect, is expected to be placed at the carbon-
12.
The synthetic utility of this photochemical conversion was
examined by scaling up the reaction up to 3.6 mmol of 2-([1,1′-
biphenyl]-2-yl)-1H-benzo[d]imidazole (1aa) and expected
product benzo[4,5]imidazo[1,2-f ]phenanthridine (2aa) was
isolated in 94% yield upon irradiation of ∼350 nm light for 24
h in the solvent DCE (Figure 7).
The absorption and emission spectra of benzimidazole-fused
phenanthridine derivatives were recorded in dichloromethane
of the compounds exhibited high fluorescence quantum yields
(φF,Table 1). For example, phenanthridine moiety with the
fluoro group in phenyl ring (2ad, 2bd, and 2bi) showed φF in the
range of 0.94−0.96. In contrast, the compound with dichloro
substituents in the benzimidazole-fused phenanthridine rings
(2ca) showed φF ∼ 0.18.
CONCLUSION
■
In summary, we have demonstrated an unprecedented photo-
chemical dehydrogenative C(sp2)−H and N(sp2)−H C−N
coupling of nonprefunctionalized systems under direct irradi-
ation (∼350 nm). Neither any added reagents nor prefunction-
alization of the substrates were needed for this transformation.
The weak or noncovalent interaction like N−H···π interaction
helped the molecules to absorb the light of lower energy than
required. The benzimidazole ring of 2-([1,1′-biphenyl]-2-yl)-
1H-benzo[d]imidazole led to the formation of imidazolyl-type
radical, which underwent ε-hydrogen abstraction to generate
1,6-diradical. Finally, the radical−radical coupling resulted in
benzo[4,5]imidazo[1,2-f ]phenanthridine. We anticipate that
the photochemical reaction described here can lead to the
initiation of a new research area under photochemistry.
EXPERIMENTAL SECTION
■
General Information. Commercially available reagents and
solvents were used as received. UV−visible spectra were recorded on
a JASCO V-730 UV−visible spectrometer, and emission spectral
studies were measured using a PerkinElmer LS-55 spectrophotometer
with an optical cell of a 1 cm path length. Absolute fluorescence
quantum yields in the solution state were performed by an integrating
sphere method using am Edinburgh FS30 spectrofluorometer. Column
chromatographic purifications of the compounds were performed using
9592
J. Org. Chem. 2021, 86, 9587−9602