Synthesis of Carbazoles by a Merged Visible Light Photoredox and Palladium-Catalyzed Process

Article abstract of DOI:10.1021/acscatal.5b00817

Carbazoles have attracted great interest in recent years for a variety of applications in organic and medicinal chemistry as well as in materials science. In this work, an efficient method for the synthesis of carbazoles through the intramolecular C-H bon

Full text of DOI:10.1021/acscatal.5b00817

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Article
Synthesis of Carbazoles by a Merged Visible Light
Photoredox and Palladium Catalyzed Process
Sungkyu Choi, Tanmay Chatterjee, Won Joon Choi, Youngmin You, and Eun Jin Cho
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Synthesis of Carbazoles by a Merged Visible Light Photoredox and
Palladium Catalyzed Process
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Sungkyu Choi,a,b,‡ Tanmay Chatterjee,a,‡ Won Joon Choi,c Youngmin You,d,* and Eun Jin Choa,*
aDepartment of Chemistry, ChungꢀAng University, Seoul 156ꢀ756, Republic of Korea
bDepartment of Bionanotechnology, Hanyang University, Ansan, Kyeonggiꢀdo 426ꢀ791, Republic of Korea
cDepartment of Advanced Materials Science Engineering for Information and Electronics, Kyung Hee University, Yongin,
Gyeonggiꢀdo 446ꢀ701, Republic of Korea
dDivision of Chemical Engineering and Materials Science, Ewha Womans University, Seoul 120ꢀ750, Republic of Korea
ABSTRACT: Carbazoles have become of great interest in recent years for a variety of applications in organic and medicinal
chemistry as well as in materials science. In this work, an efficient method for the synthesis of carbazoles through the intraꢀ
molecular C−H bond amination of Nꢀsubstituted 2ꢀamidobiaryls has been developed. Under visible light and an aerobic atꢀ
mosphere, the transformation requires only catalytic amounts of Pd(OAc)2 and [Ir(dFppy)2phen]PF6 (dFppy = 2ꢀ(2,4ꢀ
difluorophenyl)pyridine; phen = 1,10ꢀphenanthroline), the latter of which is utilized in synthetic chemistry for the first time.
Spectroscopic and electrochemical studies revealed that the reaction is initiated by photoinduced electron transfer from a
palladacycle intermediate, formed from the 2ꢀamidobiaryl and PdII species, to the photoexcited Ir catalyst. This step triggers
reductive elimination in a PdIIIꢀcontaining palladacycle to produce the carbazole and a PdI species. The one electronꢀreduced
photocatalyst is reoxidized by O2 to generate the original form of the photocatalyst, and the PdI species can be oxidized to the
resting state through oxidative electron transfer to O2 or the excitedꢀstate photocatalyst.
KEYWORDS: carbazole • amination • CꢀH activation • visible light • photocatalysis
1. INTRODUCTION
compounds, transition metalꢀcatalyzed intramolecular C−H
bond amination of Nꢀsubstituted 2ꢀamidobiaryls are perꢀ
haps the most atomꢀeconomical route (Scheme 1).12, 13
Owing to their occurrence in a large number of bioactive
natural products and pharmaceutical agents, carbazoles
have garnered significant interest among synthetic organic
and medicinal chemists (Figure 1).1 The carbazole motif is
also found in a variety of electronic materials, including
photoconducting polymers and organic optoelectronic maꢀ
terials.2
Scheme 1. Intramolecular C−H Bond Amination Reactions of
N-Substituted Amidobiaryls.
Figure 1. Molecules containing the carbazole structural motif.
Consequently, intense efforts have been devoted towards
the development of methods that can be used to prepare
substances containing carbazole moieties.1f, 3ꢀ13 Among the
various routes devised thus far to synthesize these target
In 2005, Buchwald and coꢀworkers disclosed that the Pdꢀ
catalyzed intramolecular C−H bond amination of 2ꢀ
acetamidobiaryls resulted in the production of carbazole
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derivatives by using Cu(OAc)2 as the oxidant.12a Later,
Gaunt and coꢀworkers showed that Nꢀalkyl (Me, Bu, allyl,
desired carbazole 2a in a 52% yield (Table 1, entry 1). Noꢀ
t
1
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tably, strong groundꢀstate oxidants were not required for
this Pdꢀcatalyzed reaction. A variety of Pd catalysts were
employed in the reaction; Pd(OAc)2, the most widely used
catalyst in C−H bond functionalization reactions, was
found to be optimal (Table 1, entries 1−6). Control experiꢀ
ments revealed that visible light, the Pd catalyst, the photoꢀ
catalyst, and molecular oxygen were essential for transforꢀ
mation of 1a to 2a (Table 1, entries 1 vs 7−10). When the
reaction was performed at room temperature, a lower yield
of 2a was obtained (Table 1, entry 11).
and benzyl)ꢀsubstituted 2ꢀaminobiaryls could be transꢀ
formed to Nꢀsubstituted carbazoles upon treatment with
stoichiometric quantities of a hypervalent iodine complex
in the presence of a Pd catalyst.12b Hypervalent iodine comꢀ
plexes were also utilized as oxidants by Chang and coꢀ
workers to promote Cuꢀcatalyzed or metalꢀfree carbazole
formation from Nꢀsubstituted 2ꢀamidobiaryls.12c Youn also
reported that carbazoles could be produced by a Pdꢀ
catalyzed intramolecular C−H bond amination using oxone
as the oxidant in the presence of pꢀTsOH.12d Miura and his
coꢀworkers developed a Cuꢀcatalyzed synthesis using a
picolinamideꢀbased bidentate directing group and MnO2 as
terminal oxidant in the presence of AcOH in DMF.12e Reꢀ
cently, the same group reported an Irꢀcatalyzed process of
2ꢀaminobiaryls to 9Hꢀcarbazoles using Cu(OAc)2 and moꢀ
lecular oxygen as oxidants (Scheme 1).12g In addition, orꢀ
ganocatalytic intramolecular C−H bond amination of Nꢀ
acetyl amidobiaryls was done by the Antonchick group
using 2,2′ꢀdiiodoꢀ4,4′,6,6′ꢀtetramethylbiphenyl and AcOOH
as an organocatalyst and an oxidant, respectively.13
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Table 1. Palladium Catalyst Screening.a
Despite these advances, the processes described above
have limitations associated with the requirements of high
reaction temperatures and stoichiometric use of strong
groundꢀstate oxidants. This raises concerns regarding atom
economy and environmental issues. Therefore, a new apꢀ
proach for the highly efficient preparation of carbazoles,
with minimal reagent use, is highly desirable.
In another arena, visible light photoredox catalysis has
attracted substantial attention owing to its environmental
compatibility and versatility in a large number of synthetiꢀ
cally important reactions.14 Recently, visibleꢀlightꢀ
promoted, single electron transfer (SET) processes have
been devised for the oxidation or reduction of transitionꢀ
metal complex intermediates in Niꢀ,15 Cuꢀ,16 Rhꢀ,17 Auꢀ,18
and Pd19ꢀcatalyzed reactions.14f, 14h We envisioned that the
combined use of photoredox and transition metal catalysis
would be both environmentꢀfriendly and applicable to the
synthesis of industrially important substances. In such proꢀ
cesses, the use of photon energy would obviate the need to
use strong chemical additives.
aReaction scale: 1a (0.1 mmol). bThe yield was determined usꢀ
ing gas chromatography. cThe reaction mixture was deoxygenated
by repeating vacuumꢀfreezeꢀthaw cycles.
Table 2. Photocatalyst Screening.a,b
Experience gained in earlier studies of photoredox catalꢀ
ysis20 and C−H bond amination reactions12 enabled us to
design a protocol for the synthesis of Nꢀsubstituted carbaꢀ
zoles where an intramolecular C−H bond amination would
take place by visible light irradiation of a solution of a Nꢀ
substituted 2ꢀamidobiaryl in the presence of catalytic
amounts of PdII and a visibleꢀlightꢀabsorbing, electronꢀ
accepting photocatalyst under an O2 atmosphere (Scheme
1).
2. RESULTS AND DISCUSSION
To explore this proposal, a study was conducted using Nꢀ
benzenesulfonyl amidobiphenyl 1a as the model substrate
(Table 1). Visible light irradiation of a solution of 1a in airꢀ
saturated DMSO, containing 10 mol % of Pd(OAc)2 and 1
mol % of [Ru(bpy)3]Cl2, at 80 °C led to the production of
aReaction scale: 1a (0.1 mmol). bThe yield was determined usꢀ
ing gas chromatography.
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The new, merged catalytic, carbazoleꢀforming process,
Table 3. Substrate Scope of the Intramolecular C−H Bond
Amination Reaction of N-Benzenesulfonyl Amidobiaryls.a
1
was optimized in an exploration examining a variety of Ru,
Pt, and Ir based photocatalysts (1 mol %) in the presence of
10 mol % Pd(OAc)2 in oxygenated DMSO (Table 2). The
investigated photoredox catalysts included Pt(ppy)2acac
(4),20i [Ir(ppy)2(dtbbpy)]PF6 (5),21 facꢀIr(ppy)3 (6), and facꢀ
Ir(dFppy)3 (7) (Table 2; ppy = 2ꢀphenylpyridinato, acac =
2ꢀacetylacetonate, and dtbꢀbpy = 4,4′ꢀdiꢀtertꢀbutylꢀ2,2′ꢀ
bipyridine). Interestingly, IrIII complexes containing elecꢀ
tronꢀwithdrawing ligands, such as facꢀIr(dFppy)3 (7),
[Ir(dFppy)2phen]PF6 (8), and [Ir(dCF3ppy)2phen]PF6 (9;
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dCF3ppy
=
2ꢀ(2,4ꢀbis(trifluoromethyl)phenyl)pyridine),
promoted the highest yielding reactions (7, 83%; 8, 95%; 9,
95%). Based on these observations, [Ir(dFppy)2phen]PF6 (8)
was selected for use in further studies because it is more
readily available than [Ir(dCF3ppy)2phen]PF6 (9). It is notꢀ
ed that synthetic utility of the complexes, 8 and 9,22 has yet
to be fully evaluated to date.
The effects of several parameters including solvent, temꢀ
perature, and concentration, on the efficiency of the reacꢀ
tion were also examined. Among various solvents includꢀ
ing DCM, DMSO, DMF, and MeOH, DMSO showed the
best reactivity for the transformation. Although the reaction
proceeded at lower temperatures (25 oC, 40 C and 60 C),
o
o
o
80 C was chosen for the higher efficiency and reproduciꢀ
bility of the process. The highest yielding process occurred
when 10 mol
%
Pd(OAc)2 and
1
mol
%
[Ir(dFppy)2(phen)]PF6 (8) were utilized in 0.20 M DMSO
under an oxygen atmosphere and visibleꢀlight irradiation at
80 °C. Using these conditions, the substrate scope of the
carbazole forming process was investigated. A variety of
Nꢀbenzenesulfonyl amidobiaryls were converted to the corꢀ
responding Nꢀbenzenesulfonyl carbazoles in good to excelꢀ
lent yields (Table 3). Importantly, while the efficiencies of
previous reactions employed in other methods depended on
the substitution pattern of the amidobiaryls,12c the yields
obtained using this approach were not significantly altered
by the electronic properties or position of aryl ring substitꢀ
uents. Substrates 1d and 1i, which contain Cl groups at
different positions but both generate 3ꢀchloroꢀ9ꢀ
(phenylsulfonyl)ꢀ9Hꢀcarbazole (2d), reacted with nearly
equal efficiencies (Table 3, entries 4 and 9). Notably, the
reactions of 1h and 1i, which have the potential of producꢀ
ing two regioisomeric carbazoles, generated single carbaꢀ
zoles, 2h and 2i respectively, as a consequence of steric
effects (Table 3, entries 8 and 9).
Not only Nꢀbenzenesulfonyl amidobiaryls but also hetꢀ
eroatomꢀcontaining substrates worked for this process. Altꢀ
hough the reactivity was not as good as those of simple
biaryl systems, the reaction of Nꢀ(2ꢀ(thiophenꢀ3ꢀ
yl)phenyl)benzenesulfonamide (10) smoothly underwent
CꢀH amination to produce the tricyclic system 11 (Scheme
2). Notably, the process was highly regioselective, producꢀ
ing only single isomer 11 where heteroatoms N and S have
a syn orientation
aReaction scale: 1 (0.3 mmol). bIsolated yield based on an averꢀ
age of two runs. c15 mol % of Pd(OAc)2 was used.
Scheme 2. Intramolecular C−H Amination of N-(2-(thiophen-
3-yl)phenyl)benzenesulfonamide.
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The effects of Nꢀsubstituents on the efficiencies of the catalysis is applicable to production of pharmaceutically
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reaction were also explored (Table 4). Notably, Nꢀsulfonylꢀ
(pꢀtoluenesulfonyl (12) and methylsulfonyl (14)) and Nꢀ
acetyl (16a)ꢀcontaining substrates reacted to produce the
corresponding Nꢀsubstituted carbazoles (Table 4, entries
1−3). However, 2ꢀaminobiphenyl (18) and Nꢀalkylꢀ
substituted aminobiphenyls, such as benzyl (20) and ethyl
(22), did not undergo the intramolecular C−H amination
(Table 4, entries 4−6).23
and industrially relevant compounds.
Table 5. Substrate Scope of the Intramolecular C-H Bond
Amination Reaction of N-acetyl Amidobiaryls.a
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Table 4. NꢀSubstituent Effects.a
aReaction scale: 1 (0.3 mmol). bIsolated yield based on an averꢀ
age of two runs.
Scheme 3. Synthesis of an alkaloid, clausine C, through
deacetylation of 17d.
aReaction scale: Substrate (0.3 mmol). bIsolated yield based on an
average of two runs. c15 mol % of Pd(OAc)2 was used.
Specifically, Nꢀacetyl amidobiaryls containing both elecꢀ
tronꢀdonating and electronꢀwithdrawing aryl substituents
reacted smoothly to form the corresponding Nꢀ
acetylcarbazoles (Table 5). The reactivities of the Nꢀacetyl
substrates were lower than those of their Nꢀbenzenesulfonyl
analogs, as 15 mol % of Pd(OAc)2 was required to drive
these reactions to completion. However, the relatively facꢀ
ile removal of Nꢀacetyl as compared to Nꢀsulfonyl groups
makes the use of Nꢀacetyl amidobiaryls (16) more practical.
As the final phase of our study, spectroscopic and elecꢀ
trochemical investigations were performed in order to gain
insight into the reaction mechanism. Notably, the UV−vis
absorption spectrum of a DMSO solution containing
Pd(OAc)2 and 1a preincubated at 80 °C for 12 h under an
anaerobic atmosphere differed significantly from that of
DMSO solutions of 1a or Pd(OAc)2 (Supporting Inforꢀ
mation (SI), Figure S1). This observation may be indicative
of generation of a palladacycle. We obtained the ESI−MS
The results described above clearly demonstrate the feaꢀ
sibility and scope of the merger of Pd(OAc)2 and IrIII phoꢀ
toredox catalysts for transformation of amidobiaryls to the
corresponding carbazoles. To further explore utility of our
method, we attempted preparation of clausine C (an alkaꢀ
loid, Figure 1) using the merged catalysts. The intermediate,
an acetylated carbazole 17d (Table 5, entry 4), was preꢀ
pared according to our protocol. 17d was then converted to
clausine C by deacetylation under a basic condition
(Scheme 3). This demonstration illustrates that the merged
(positive)
spectra
that
revealed
presence
of
[KPdII(1a−H)(DMSO)3]+, supporting this notion (SI, Figure
S2). The formation of a trinuclear carbopalladacycle of
amidobiaryls under similar conditions was reported by
Gaunt and coꢀworkers.12b We hypothesized that the palꢀ
ladacycle might be the key species responsible for SET
with the photoexcited Ir catalyst. Comparisons of the oxiꢀ
dation potentials (Eox) of 1a (1.86 V vs SCE), 2a (1.83 V vs
SCE), and the palladacycle of 1a (1.21 V vs SCE) with that
of the excitedꢀstate reduction potential of 8 (E*red = 1.49 V
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vs SCE) revealed that exoergic SET to the excitedꢀstate of
PdI species (Path A). The PdI species is subsequently oxiꢀ
dized to generate the original PdII complex through SET to
the photoexcited Ir catalyst or molecular oxygen. However,
it is not possible to rule out a mechanism that follows path
B, in which one electron transfer to the photoexcited cataꢀ
lyst from PdIII intermediate occurs prior to reductive elimiꢀ
1
2
3
4
5
6
7
8
8 (8*) is allowed only from the palladacycle with a positive
driving force of 0.28 eV (SI, Figure S3). Actually, the
phosphorescence lifetime of 8* decreased in a concentraꢀ
tionꢀdependent manner from 1.48 ꢀs to 0.986 ꢀs upon the
addition of the palladacycle (0−0.800 mM; Figure 2(a)). A
pseudoꢀlinear fit of the data yielded an electronꢀtransfer
rate constant of 4.33±0.09 × 108 M−1 s−1, which correꢀ
sponded to a SET rate that was ca. 15 and 2 times faster
than those for the radiative (kr = 5.73 × 104 s−1) and nonꢀ
radiative (knr = 4.64 × 105 s−1) decay of 2.0 mM 8 (Figure
2(b)).22 The SET involved oxidation of PdII in the palꢀ
ladacycle, as similar photoinduced reductive quenching of
8* by Pd(OAc)2 occurred at a relative smaller the rate conꢀ
stant (3.98±0.12 × 108 M−1·s−1; SI, Figure S4). As expected,
phosphorescence quenching of 8* did not take place in the
presence of 1a and 2a (SI, Figure S5).
nation
(i.e.,
*[IrIV(dFppy)2phen•−]+
+
PdIII
→
[IrIII(dFppy)2phen•−]0 + PdIV).24 In either case, the one elecꢀ
tronꢀreduced Ir catalyst donates one electron to dioxygen
9
with
a
positive driving force of 0.65 eV
(e⋅[Ered([Ir(dFppy)2phen]+)
−
Ered(O2)];
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Ered([Ir(dFppy)2phen]+) = −1.30 V vs SCE and Ered(O2) =
−0.75 V vs SCE), being restored to its original state. Alterꢀ
natively, *[IrIV(dFppy)2phen•−]+ can be oxidatively
quenched to [IrIV(dFppy)2phen]2+ by O2, followed by SET
from the palladacycle (i.e., [IrIV(dFppy)2phen]2+ + PdII →
[IrIII(dFppy)2phen]+ + PdIII). Since oneꢀelectron oxidation
of 8 occurs at 1.58 V vs SCE, the driving force for the genꢀ
eration of the PdIII species is greater than the case by 8* by
0.09 eV. This may be indicative of preference for the oxiꢀ
dative quenching pathway, but it is likely that both (reducꢀ
tive and oxidative quenching of 8*) would be operative.
In the ensuing steps in the mechanistic pathway, the one
electronꢀreduced photocatalyst is oxidized to reform 8 upon
donation of the extra electron to O2. The PdIII palladacycle
undergoes reductive elimination to produce a C−N bond as
part of the carbazole ring system, as well as a PdI species.
Since the redox potential of PdII/I should be more cathodic
than that of PdIII/II, the PdI species should be capable of
reducing the photoexcited catalyst. However, direct moniꢀ
toring of this process was hampered presumably by the
short lifetime of this transient species.
O
Ph
S
N
O
H
1a
H
[PdII]
SO2Ph
N
[PdII]
hv
[IrIV
L
]+*
[IrIIIL]+
I
V
I
I
O2
O
(or by
)
2
O2
SO2Ph
0
[IrIII
L
]
N
H+
[PdIV
]
N
SO2Ph
SO2Ph
[PdI]
N
2a
[PdII]
0
[IrIII
L
]
O2
I
I
I
[IrIV
L
]+*
H+
I
O2
[IrIIIL]+
hv
SO2Ph
SO2Ph
N
N
[PdII]
[PdIII
]
+
*
[IrIV
L
]
0
[IrIII
L
]
Figure 2. (a) Phosphorescence decay traces of 1.00 mM
[Ir(dFppy)2(phen)]PF6 (8) (deaerated DMSO) with increasꢀ
O2
1A
[IrIIIL]+
= [Ir(dFppy)2phen]+
hv
ing concentrations of the palladacycle (0−0.800 mM) (λex
=
[IrIIIL]+
O2
377 nm, temporal resolution = 8 ns). (b) A pseudoꢀlinear fit
of the electron transfer rates, which were obtained from
Figure 3. Proposed mechanism for C−H bond amination reactions
of Nꢀsubstituted amidobiaryls.
electron transfer rate = 1/
τ −1/τ0, to the concentration of
palladacycle: and 0 correspond to the observed phosphoꢀ
τ
τ
3. CONCLUSION
rescence lifetimes in the presence and absence of palꢀ
ladacycle, respectively.
In conclusion, in the investigation described above we
have developed an efficient method for the synthesis of Nꢀ
substituted carbazoles, which involves intramolecular C−H
bond amination of Nꢀsubstituted 2ꢀamidobiaryls. The proꢀ
cess utilizes the merged visible light photoredox and Pd
catalysis and, as such, does not require strong oxidants for
regeneration of the Pd catalyst. In addition, observations
made in electrochemical and transient photoluminescence
spectroscopy studies showed that the catalysis is initiated
by SET from a palladacycle intermediate to the photoexcitꢀ
ed Ir complex. Moreover, the photocatalyst is regenerated
by molecular oxygenꢀpromoted oxidation of its reduced
These results enabled us to postulate the plausible mechꢀ
anism for the carbazoleꢀforming reaction (Figure 3). In the
initial step of the route, coordination of the amido group of
1a to Pd(OAc)2 facilitates cyclopalladation, leading to the
sixꢀmembered palladacycle 1A.12a The palladacycle, having
a PdII oxidation state, is oxidized to a PdIII complex by SET
to the photoexcited Ir catalyst (i.e., *[IrIV(dFppy)2phen•−]+),
with generation of the one electronꢀreduced catalyst
[IrIII(dFppy)2phen•−]0. Reductive elimination of the PdIIIꢀ
palladacycle leads to the formation of carbazole 2a and a
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form, and the catalytic PdII species is regenerated by SET
from a PdI species to molecular oxygen or the excited phoꢀ
tocatalyst. The strategy employed in this study, which
combines transition metal catalysis and photocatalysis and
negates the use of stoichiometric amounts of harsh or poꢀ
tentially toxic chemical additives, might be applicable in
the formation of other environmentally benign reactions.
(2) For some examples, see: (a) Grazuleviciusa, J. V.; Strohriegl, P.;
Pielichowskic, J.; Pielichowskic, K. Prog. Polym. Sci. 2003, 28,
1297ꢀ1353. (b) Brunner, K.; Dijken, A.; Börner, H.; Bastiaansen, J. J.
A. M.; Kiggen, N. M. M.; Langeveld, B. M. W. J. Am. Chem. Soc.
2004, 126, 6035ꢀ6042. (c) Liu, Z.; Guan, M.; Bian, Z.; Nie, D.; Gong,
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Wong, W.ꢀY.; Ho, C.ꢀL.; Gao, Z.ꢀQ.; Mi, B.ꢀX.; Chen, C.ꢀH.; Cheah,
K.ꢀW.; Lin, Z. Angew. Chem. Int. Ed. 2006, 45, 7800ꢀ7803. (e)
Hung,W.ꢀY.; Chi, L.ꢀC.; Chen, W.ꢀJ.; Chen, Y.ꢀM.; Choub, S.ꢀH.;
Wong, K.ꢀT. J. Mater. Chem. 2010, 20, 10113ꢀ10119. (f) Seo, H.ꢀJ.;
Song, M.; Jin, S.ꢀH.; Choi, J. H.; Yuna, S.; Kim, Y.ꢀI. RSC Adv. 2011,
1, 755ꢀ757.
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4. EXPERIMENTAL SECTION
9
Synthesis of N-Benzenesulfonyl Carbazole (2): An ovꢀ
enꢀdried resealable tube equipped with a magnetic stir bar
was charged with Nꢀbenzenesulfonyl aminobiaryl comꢀ
pound 1 (0.3 mmol), Pd(OAc)2 (10 mol %, 0.03 mmol) and
[Ir(dFppy)2(phen)]PF6, 8 (1 mol%, 0.003 mmol) in DMSO
(1.5 mL, 0.2 M). Then oxygen gas was bubbled through the
reaction mixture for 3 minutes and the tube was sealed with
a silicone septum screw cap. The test tube was then placed
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(3) For synthesis of carbazoles by Pdꢀcatalyzed domino NꢀH/CꢀH
bond activation, see: (a) Iwaki, T.; Yasuhara, A.; Sakamoto, T. J.
Chem. Soc. Perkin Trans. 1 1999, 1505ꢀ1510. (b) Bedford, R. B.;
Betham, M. J. Org. Chem. 2006, 71, 9403ꢀ9410. (c) Ackermann, L.;
Althammer, A. Angew. Chem. Int. Ed. 2007, 46, 1627ꢀ1629.
(4) For synthesis of carbazoles by dehydrogenative crossꢀcoupling of
substituted N,Nꢀdiarylamines, see: (a) Akermark, B.; Eberson, L.;
Jonsson, E.; Pettersson, E. J. Org. Chem. 1975, 40, 1365ꢀ1367. (b)
Gensch, T.; Roennefahrt, M.; Czerwonka, R.; Jaeger, A.; Kataeva, O.;
Bauer, I.; Knölker, H.ꢀJ. Chem. Eur. J. 2012, 18, 770ꢀ776. (c) Kumar,
V. P.; Gruner, K. K.; Kataeva, O.; Knölker, H.ꢀJ. Angew. Chem. Int.
Ed. 2013, 52, 11073ꢀ11077. (d) HernandezꢀPerez, A. C.; Collins, S.
K. Angew. Chem. Int. Ed. 2013, 52, 12696ꢀ12700.
(5) For synthesis of carbazoles by Pdꢀcatalyzed double amination of
2,2′ꢀdihaloꢀ1,1′ꢀbiaryls, see (a) Nozaki, K.; Takahashi, K.; Nakano,
K.; Hiyama, T.; Tang, H.ꢀZ.; Fujiki, M.; Yamaguchi, S.; Tamao, K.
Angew. Chem. Int. Ed. 2003, 42, 2051ꢀ2053. (b) Kuwahara, A.;
Nakano, K.; Nozaki, K. J. Org. Chem. 2005, 70, 413ꢀ419. (d) Kitaꢀ
waki, T.; Hayashi, Y.; Ueno, A.; Chida, N. Tetrahedron 2006, 62,
6792ꢀ6801.
o
under blue LEDs (7 W) at 80 C. The progress of the reacꢀ
tion was checked by TLC or gas chromatography. The reꢀ
action mixture was then diluted with dichloromethane
(DCM) and washed with brine. The organic layer was dried
over MgSO4, concentrated in vacuum, and purified by flash
column chromatography to furnish the pure Nꢀ
benzenesulfonyl carbazole, 2.
AUTHOR INFORMATION
(6) For synthesis of carbazoles by Cadogan cyclization of 2ꢀ
nitrobiaryls, see: (a) Ragaini, F.; Cenini, S.; Gallo, E.; Caselli, A.;
Fantauzzi, S. Curr. Org. Chem. 2006, 10, 1479ꢀ1510. (b) Sanz, R.;
Escribano, J.; Pedrosa, M. R.; Aguado, R.; Arnaiz, F. J. Adv. Synth.
Catal. 2007, 349, 713ꢀ718. (c) Kuethe, J. T.; Childers, K. G. Adv.
Synth. Catal. 2008, 350, 1577ꢀ1586. (d) MajgierꢀBaranowska, H.;
Williams, J. D.; Li, B.; Peet, N. P. Tetrahedron Lett. 2012, 53, 4785ꢀ
4788.
Corresponding Author
* odds2@ewha.ac.kr (Youngmin You)
* ejcho@cau.ac.kr (Eun Jin Cho)
Author Contributions
‡These authors contributed equally.
Notes
(7) For synthesis of carbazoles by the insertion of transition metal
(TM)ꢀnitrenoids into aromatic CꢀH bonds, see: Stokes, B. J.; Richert,
K. J.; Driver, T. G. J. Org. Chem. 2009, 74, 6442ꢀ6451.
The authors declare no competing financial interest.
(8) For synthesis of carbazoles by one pot RhIIIꢀcatalyzed CꢀH
borylation and CuIIꢀmediated ring closure of 2ꢀamino biaryls, see:
Jiang, Q.; DuanꢀMu, D.; Zhong, W.; Chen, H.; Yan, H. Chem. Eur. J.
2013, 19, 1903ꢀ1907.
ACKNOWLEDGMENT
This work was supported by the National Research Foundation of
Korea [NRFꢀ2014R1A1A1A05003274, NRFꢀ2014ꢀ011165, and
NRFꢀ2012M3A7B4049657] and the TJ Science Fellowship of the
POSCO TJ Park Foundation. Y.Y. acknowledges a grant from the
Ewha Womans University (1ꢀ2015ꢀ0447ꢀ001ꢀ1).
(9) For synthesis of carbazoles by benzyneꢀmediated cyclization of
Nꢀprotected and halogenated diarylamines, see: Noji, T.; Fujiwarw,
H.; Okano, K.; Tokuyama, H. Org. Lett. 2013, 15, 1946ꢀ1949.
(10) For synthesis of carbazoles by transition metalꢀfree cyclization
of 2ꢀnitrobiaryls with PhMgBr, see: Gao, H.; Xu, Q.ꢀL.; Yousufuddin,
M.; Ess, D. H.; Kürti, L. Angew. Chem. Int. Ed. 2014, 53, 2701ꢀ2705.
(11) For synthesis of carbazoles by photostimulated reactions of 2′ꢀ
Halo[1,1′ꢀbiphenyl]ꢀ2ꢀamines, see: Guerra, W. D.; Rossi, R. A.; Piꢀ
erini, A. B.; Barolo, S. M. J. Org. Chem. 2015, 80, 928ꢀ941.
(12) For synthesis of carbazoles by transition metalꢀcatalyzed intraꢀ
molecular CꢀH bond amination of Nꢀsubstituted 2ꢀamidobiaryls, see:
(a) Tsang, W. C. P.; Zheng, N.; Buchwald, S. L. J. Am. Chem. Soc.
2005, 127, 14560ꢀ14561. (b) JordanꢀHore, J. A.; Johansson, C. C. C.;
Gulias, M.; Beck, E. M.; Gaunt, M. J. J. Am. Chem. Soc. 2008, 130,
16184ꢀ16186. (c) Cho, S. H.; Yoon, J.; Chang, S. J. Am. Chem. Soc.
2011, 133, 5996ꢀ6005. (d) Youn, S. W.; Bihn, J. H.; Kim, B. S. Org.
Lett. 2011, 13, 3738ꢀ3741. (e) Takamatsu, K.; Hirano, K.; Satoh, T.;
Miura, M. Org. Lett. 2014, 16, 2892−2895. (f) Chng, L. L.; Yang, J.;
Wei, Y.; Ying, J. Y. Chem. Commun. 2014, 50, 9049ꢀ9052. (g) Suzuꢀ
ki, C.; Hirano, K.; Satoh, T.; Miura, M. Org. Lett. 2015, 17, 1597ꢀ
1600.
ASSOCIATED CONTENT
Supporting Information.
Experimental procedures, additional experimental data, analytical
1
data, and copies of H and 13C NMR spectra of compounds. This
information is available free of charge via the Internet at
http://pubs.acs.org/.
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(23) This lack of reactivity was due to reductive quenching of the
photoexcited catalysts by ‘free’ Nꢀalkyl aminobirayls. The rate conꢀ
stant for this photoinduced electron transfer from 20 approached to
2.0 × 109 M−1 s−1 (SI, Figure S6), being one order of magnitude faster
than that of the palladacycle of 1a.
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An efficient protocol for intramolecular CꢀH bond amination of Nꢀsubstituted amidobyaryls, leading to the formation of a library of Nꢀsubstituted carbazoles
using a merged palladium and visible light photoredox catalysis, has been developed. This method avoids the use of stoichiometric amounts of expensive
and/or toxic chemical additives, which merits special mention.
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