Organic Letters
Letter
a
Scheme 3. Substrate Scope for 8-Aminoquinolines
potential for further transformation,19 we achieved the
nitration of the large sterically hindered 3a in moderate yield
under the condition of Ni(NO3)2·6H2O as the nitrating
reagent (eq 2). The structure of 5 was confirmed by single-
crystal X-ray analysis.17
With two series of triarylamine compounds (3a−o; 4a−l) in
hand, we proceeded to characterize their fluorescence
performance (Figure 2, Table 2). The UV absorption
a
t
1 (0.2 mmol), 2a (3.0 equiv), NiCl2 (10 mol %), BuONa (3.0
equiv), H2O (15.0 uL), DMF (2.0 mL), N2, sealed tube, isolated
b
yield. NaOH (3.0 equiv) was used instead of tBuONa and H2O; the
isolated yield is listed in parentheses.
Figure 2. Fluorescence spectra of (A) 3a−o and (B) 4a−l in toluene
compared with Al(q)3 in CH2Cl2 as reference. For the full-size panels
position of the 8-aminoquinoline ring, the reactions proceeded
well with good yields of the corresponding products (4a−l) by
simply changing the base from NaOH to tBuONa and water. A
possible reason might be that different substituents would have
various steric and electronic effects on the coordination of the
nickel catalyst and aminoquinolines. Thus the in situ formation
wavelengths of all compounds ranged from 363 to 412 nm.
(See the Supporting Information, Figure S3 and S4.) The
maximum fluorescence emission wavelengths of these com-
pounds were from 435 to 584 nm, ranging from blue to
yellowish orange. Among the compounds 3a−o, compound 3g
had the highest fluorescence quantum yield of 23%. The
quantum yields of triarylamine products with different
substituents on the quinoline ring (4a−l) were higher than
those of the products without substitution (3a). Because of the
steric effect of the C2 methyl group of the quinoline ring on
the inhibition of rotation, 4a gave the best quantum yields
(31%), and the heterocyclic groups C5-furan (4i) and C5-
pyridine (4j) also increased the quantum yields to 27 and 25%,
respectively.
Previously, triarylamine was demonstrated to have good AIE
performance.21 We therefore tested the AIE performance of
compound 3a and found that the fluorescence intensity
increased two-fold when the water fraction was increased from
0 to 95% (Figure 3A), indicating some potential in AIE, but its
background signal was a problem. This property of 3a could be
modified after the introduction of a less conjugated and rigid
TPE motif, which was demonstrated to play an important role
in AIE by Tang et al.22 As shown in Figure 3B, 3m with two
TPE motifs exhibited a 9.5 times better AIE effect when the
water fraction was increased from 0 to 95%. Furthermore,
compared with the single TPE and 3a (Figure 3A−D), 3m had
t
of the base by BuONa and H2O was required in some case.
When the quinoline ring had a methyl substitution at the C2
position, it could react with 9-(4-bromophenyl)-9H-carbazole
to form the corresponding product 4a in 68% yield. When a
phenyl group was attached to the C4 position, the desired
product 4b was obtained in 95% yield. As for the C5
substituents, when electron-withdrawing groups (5-Br, 4-
fluorophenyl, 4-trifluoromethylphenyl) or electron-donating
groups (phenyl, 4-methylphenyl, 1-naphthyl) were attached to
the C5 position, the corresponding products were generated in
62−92% yields(4c−h). Furthermore, when a heterocyclic
group, such as 3-furanyl or 4-pyridyl, was attached to the C5
position, the corresponding products 4i and 4j were generated
in 67 and 89% yields, respectively. Other positions, such as a
methoxy group at the C6 position (4k) or a methyl group at
the C7 position (4l), of the 8-aminoquinoline core were also
investigated, and the desired products were obtained in good
yields.
To show the potential use of this protocol, a gram-scale
synthesis was carried out. When using 2.0 mmol of 1a and 6.0
mmol of 2a under standard conditions, the corresponding
triarylamine product 3a could be obtained in 83% yield (1.04
g) (eq 1). Because the nitro-substituted quinolines have the
2516
Org. Lett. 2021, 23, 2514−2520