Organic Letters
Letter
during excitation. On the other hand, the presence of −OMe
and −Cl group at C-1 and C-2 does not significantly change
their quantum yields (see 3fa vs 3ia). Interestingly, fluoro- or
chloro-substituted at the C-3 position highly conjugated
compound 3la or 3ma show a dramatic increase in its
quantum yield versus that of nonsubstituted compound 3aa.
Such changes can be understood by evoking the enhanced
ICT. In addition, compounds having n-butyl, benzyl, and allyl
substituents on the indolyl N-atom give higher fluorescence
quantum yields as compared to the methyl substituent (see
3aa, 3ba, 3ca, and 3ea). The photophysical properties are not
affected by the presence of substituted phenyl, naphthyl,
pyrenyl, or pyridine groups on the N-atom (compounds 3ac,
3as, 3at, 3au, and 3av). Furthermore, a fluorosolvatochromic
study was also carried out on the compound 3aa (see SI).
Compound 3aa exhibits a moderate fluorescence quantum
yield from 0.12 in toluene to 0.49 in DCM.
Scheme 4. Plausible Reaction Mechanism
To examine the selectivity, a fluorescence screening of
various acids has been carried out (Figure 2). Hence, 3aa as
2a generated the coupling product 4a without CoCl2 in a yield
of 53% (Scheme 4c). The studies revealed that the Michael
addition process of the aniline to the quinone in the presence
of base at first. Under the standard conditions, the reaction of
compound 4a generated the product 3aa in a yield of 82%
(Scheme 4d). We surmised that the coupling product 4a
should be a key intermediate in this reaction. Without CoCl2,
the reaction of compound 4a generated the product 3aa in just
15% yield (Scheme 4e). And the reaction of compound 4a
under N2 delivered only a trace amount of the product
(Scheme 4f). The studies demonstrated the importance of
CoCl2 and O2.
Figure 2. Changes of the fluorescence spectra of the 3aa (3 × 10−5
M) with acid.
the model compound (3.0 × 10−5 M) in dichloromethane
(DCM) interacted with 1000 equiv of different acid such as
TsOH, HF, H2CO3, AcOH, HCl, HBr, HNO3, H2SO3, and
H3PO4. While strong emission enhancement was observed for
the majority of acids tested, surprisingly nearly no fluorescence
was observed due to fluorescence quenching with the
CH3SO3H added into the solution, confirming its suitability
for the selective sensing of CH3SO3H. Furthermore, using this,
one can detect CH3SO3H selectively as it changes to a
distinctly different color (red to blue). The quantitative
analysis was carried out by fluorescence spectral titration of
3aa with the varying concentration of CH3SO3H (0−1000
equiv) in DCM (Figure 2; see SI). After our investigation of
other compounds, each compound showed a colorimetric
response upon addition of CH3SO3H in contrast to other acids
(Figure 1; see SI). The colorimetric sensors are even better, as
the signaling event can be detected by the naked eye itself.
Control experiments were conducted to clarify the cyclo-
amination reaction pathway (Scheme 4). The studies revealed
that the radical scavenger TEMPO (2,2,6,6-tetramethyl-1-
piperidinyloxy) did not inhibit the reaction under standard
conditions, ruling out the radical mechanism (Scheme 4a). At
room temperature for 6 h, the reaction of model compounds
1a and 2a generated the coupling product 4a in a yield of 51%
(Scheme 4b). And the reaction of model compounds 1a and
On the basis of this and previous reports,18 a possible
reaction mechanism was proposed (Scheme 4). Initially, the
Michael addition of indolylnaphthoquinone (1a) and aniline
(2a) in the presence of base gave the intermediate A, which
was immediately oxidized to intermediate 4a by O2 or the
oxidative naphthoquinone.18a On the other hand, a cobalt salt
was oxidized to a Co(III) species in the presence of O2, which
then reacted with intermediate 4a giving the Co(III) species B.
Finally, the intermediate B underwent reductive elimination
producing the desired product 3aa and regenerating Co(III)
by the oxidant (O2). Although the above-mentioned
mechanism suggests a Co(III)/Co(I) cycle, the Co(IV)/
Co(II) cycle could not be excluded.18b,c,d
In conclusion, we have successfully demonstrated Co(II)-
catalyzed cycloamination reaction between indolylquinones
and various (hetero)aromatic amines to accomplish the
polycyclic N-heterocycle molecules. The significant aspects of
our work with tolerance of chloride, bromide, amino, thio, etc.
D
Org. Lett. XXXX, XXX, XXX−XXX