Rh(III)-Catalyzed Oxidative C-H Activation/Domino Annulation of Anilines with 1,3-Diynes: A Rapid Access to Blue-Emitting Tricyclic N,O-Heteroaromatics

Article abstract of DOI:10.1021/acs.orglett.0c01465

Disclosed herein is a rhodium-catalyzed oxidative C-H activation/domino annulation of N-Boc-anilines with 1,3-diynes for the construction of tricyclic N,O-heteroaromatics. This reaction features easily available substrates, mild reaction conditions, high regioselectivity, mono/diannulation selectivity, and intra/intermolecular annulation selectivity. Moreover, this synthetic protocol enables the rapid assembly of a library of blue-emitting molecules with high quantum yields, among of which two fluorophores with pure blue-emission in toluene are discovered.

Full text of DOI:10.1021/acs.orglett.0c01465

pubs.acs.org/OrgLett
Letter
Rh(III)-Catalyzed Oxidative C−H Activation/Domino Annulation of
Anilines with 1,3-Diynes: A Rapid Access to Blue-Emitting Tricyclic
N,O-Heteroaromatics
Shengyou Qian, Xingwen Pu, Guanjun Chang, Ying Huang, and Yudong Yang*
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ABSTRACT: Disclosed herein is a rhodium-catalyzed oxidative C−H
activation/domino annulation of N-Boc-anilines with 1,3-diynes for
the construction of tricyclic N,O-heteroaromatics. This reaction
features easily available substrates, mild reaction conditions, high
regioselectivity, mono/diannulation selectivity, and intra/intermolec-
ular annulation selectivity. Moreover, this synthetic protocol enables
the rapid assembly of a library of blue-emitting molecules with high
quantum yields, among of which two fluorophores with pure blue-
emission in toluene are discovered.
Scheme 1. Transition-Metal-Catalyzed Oxidative C−H
Activation/Domino Annulation of Anilines with 1,3-Diynes
olycyclic heteroaromatic molecules (PHAs) have received
Ptremendous attention, because of their wide applications
in bioimaging and optoelectronic devices such as organic light
emitting diodes (OLEDs), organic field effect transistors
(OEFTs), and organic solar cells.1 The luminescent and
electronic properties of PHAs could be tailored by changing
their degree of π-extension and doping heteroatoms into the π-
skeletons.2,3 As a result, the development of facile and efficient
synthetic strategies that enable the rapid construction and
screening of PHAs is highly desired.
In the past two decades, transition-metal-catalyzed oxidative
annulation of (hetero)aromatics with alkynes through
chelation-assisted C−H activation has emerged as one of the
most straightforward and efficient approaches to (hetero)-
arenes (Scheme 1a).4,5 In principle, the employment of
conjugated alkyne surrogates in these reactions would enable
the rapid assembly of two or more (hetero)aromatic rings and
replenishment of the structural diversity of PHAs. Despite a
large number of catalytic oxidative C−H annulations with
isolated alkynes have been developed, the elaboration of
analogous reactions using conjugated alkynes as the synthons is
still struggling. This is presumably attributed to the challenges
on selectivity control including regioselectivity in the alkyne
insertion and mono/diannulation selectivity.6 Recently, several
groups reported the transition-metal-catalyzed annulation of
(hetero)aromatics with 1,3-diynes for the synthesis of
bisheterocycles pioneered by Glorius.6a−c,g Herein, we disclose
a Rh(III)-catalyzed oxidative C−H activation/domino annu-
lation of N-Boc-anilines with 1,3-diynes for the synthesis of a
class of tricyclic N,O-heteroaromatics, in which the fused
heteroaromatic rather than bis-heteroaromatic skeletons are
delivered (Scheme 1b). Based on this protocol, a library of
Received: April 28, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01465
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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Letter
blue emitters with high quantum yields have been rapidly built
up.
Initially, a variety of reaction parameters, including catalysts,
additives, oxidants, solvents, and reaction temperature, have
been screened for the domino C−H annulation of N-Boc-
aniline (1a) and diphenylbuta-1,3-diyne (2a) (see Table 1). In
yield, toluene and 1,4-dioxane did not work (Table 1, entries
11−13). Increasing the temperature to 80 °C resulted in a
slightly decreased yield, whereas lower temperature (40 °C)
afforded 3a in a comparative yield (Table 1, entries 1, 14 and
15). However, diminished yields were obtained in the
following substrate scope examination when reactions were
conducted at 40 °C. Thus, 60 °C was selected as the optimal
reaction temperature. The attempts in the absence of
[Cp*RhCl2]2 or AgSbF6 were performed, demonstrating the
necessity of these two species (Table 1, entries 16 and 17). In
addition, no desired product was detected in the presence of
2.2 equiv of AgSbF6 without other oxidants (Table 1, entry
18).
a
Table 1. Optimization of Reaction Conditions
With the optimized condition in hand, we then explored the
substrate scope of anilines. As shown in Scheme 2, a broad
b
c
entry
catalyst
oxidant
Ag2O
solvent
yield (%)
d
1
[Cp*RhCl2]2
[Cp*IrCl2]2
[Cp*Co(CO)I2]
[RuCl2(p-cymene)]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
−
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
85
nd
nd
nd
trace
trace
trace
nd
69
72
a
Scheme 2. Scope of N-Boc-anilines
e
2
3
4
Ag2O
Ag2O
Ag2O
Ag2O
Ag2CO3
AgOAc
Cu(OAc)2
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
−
e
e
f
5
6
7
8
9
e
dg
,
10
11
12
13
14
15
16
17
18
CH2Cl2
toluene
dioxane
DCE
63
trace
trace
79
84
nd
dh
,
di
,
DCE
DCE
DCE
DCE
dj
,
e
dk
,
e
[Cp*RhCl2]2
[Cp*RhCl2]2
nd
nd
l
e
a
Reaction conditions: 1a (0.2 mmol, 1 equiv), 2a (0.24 mmol, 1.2
equiv), catalyst (5 mol %), AgSbF6 (20 mol %), PivOH (1 equiv),
b
oxidant (1.5 equiv), solvent (2 mL), 60 °C, N2, 24 h. DCE = 1,2-
c
d
e
dichloroethane. Isolated yield. Oxidant (2.0 equiv). nd = not
f
g
h
i
detected. Without PivOH. Catalyst (2.5 mol %). At 80 °C. At 40
°C. Without [Cp*RhCl2]2. Without AgSbF6. AgSbF6 (2.2 equiv).
j
k
l
the presence of [Cp*RhCl2]2 (5 mol %), AgSbF6 (20 mol %),
PivOH (1.0 equiv), and Ag2O (2.0 equiv), the desired product
3a could be provided in 85% isolated yield in DCE at 60 °C for
24 h (Table 1, entry 1). Changing the catalyst to [Cp*IrCl2]2,
[Cp*Co(CO)I2], and [RuCl2(p-cymene)]2 failed to promote
the reaction (Table 1, entries 2−4). The choice of additive and
oxidant play crucial roles in this reaction. In the absence of
PivOH, only a trace amount of the desired product was
detected and the starting material 1a could be recovered in
80% yield (Table 1, entry 5). These results indicated the
necessity of PivOH in the C−H activation/annulation process.
Ag2CO3 and AgOAc gave only trace amounts of product 3a,
whereas other oxidants such as MnO2, K2S2O8, CuO,
Cu(OAc)2, and O2 were found to be totally ineffective
(Table 1, entry 8; also see Table S1 in the Supporting
Information (entries 10−14)). In these attempts, N-Boc-
aniline 1a slightly decomposed and alkyne 2a remained intact.
Decreasing the amounts of oxidant led to a diminished yield of
69% (Table 1, entry 9). No reaction was observed in the
absence of an oxidant (Table S1, entry 28). The yield of 3a
was reduced to 72% when the catalyst loading was 2.5 mol %
(Table 1, entry 10). Subsequently, the effect of solvent was
examined. While CH2Cl2 afforded the targeted product in 63%
a
Reaction conditions: 1 (0.2 mmol, 1 equiv), 2 (0.24 mmol, 1.2
equiv), [Cp*RhCl2]2 (5 mol %), AgSbF6 (20 mol %), PivOH (0.2
mmol, 1 equiv), Ag2O (0.4 mmol, 2 equiv), DCE (2 mL), 60 °C, N2,
b
24 h. 5.2 mmol scale.
range of N-Boc-anilines bearing methyl, tert-butyl, methoxy,
phenyl, fluoro, chloro, bromo, trifluoromethyl, and ester on the
benzene ring could participate in this reaction to provide the
desired products in good to excellent yields (3b−3j). The
structure of compound 3b was unambiguously demonstrated
by single-crystal X-ray analysis (see Figure S1 in the
Supporting Information). The position of substituents on the
aryl ring of aniline 1 did not exhibit remarkable effect on the
B
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yields (3l−3n). Besides, N-Boc-2-naphthylamine could also
work well under the standard rhodium catalytic system (3k).
All of the examples showed that the diannulation could
proceed smoothly and no obvious monoannulation product
was detected.
orientations.7 The reaction exhibited no regioselectivity in the
case of unsymmetrical 1,4-diarylbuta-1,3-diyne (3z). No
desired product was detected in the attempts with buta-1,3-
diyn-1-ylbenzene and diethyl hexa-2,4-diynedioate (3aa and
3ab).
Next, the scope of 1,3-diynes was investigated (Scheme 3).
1,4-Diphenylbuta-1,3-diyne with various substituents under-
To get insight into the mechanism of this reaction, kinetic
isotope effect were performed. A kinetic isotope experiment
(KIE) value of 3.17 and 4.26 was observed in the intra-
molecular and intermolecular competitive reaction, respec-
tively (see Scheme 4). These results indicated that the C−H
bond cleavage might be involved in the rate-determining step.
a
Scheme 3. Scope of 1,3-Diynes
Scheme 4. Kinetic Isotope Experiments
On the basis of the above results and previous reports,5a,f,8,9
a plausible mechanism is proposed in Scheme 5. Initially, the
Scheme 5. Plausible Mechanistic Pathway
a
Reaction conditions: 1a (0.2 mmol, 1 equiv), 2 (0.24 mmol, 1.2
equiv), [Cp*RhCl2]2 (5 mol %), AgSbF6 (20 mol %), PivOH (0.2
mmol, 1 equiv), Ag2O (0.4 mmol, 2 equiv), DCE (2 mL), 60 °C, N2,
24 h.
went the domino annulation smoothly with 1a to give the
corresponding products in moderate to good yields (3o−3v).
Typically, electronic-donating substituents were more favor-
able than the electronic-withdrawing analogues in this
transformation (3o and 3p vs 3r−3v). Dialkyl diynes such as
dodeca-5,7-diyne exhibited a lower reactivity, delivering the
desired product in low yield (3y). Notably, this reaction
exhibited very high regioselectivity in the case of unsym-
metrical alkyl aryl diynes. As representative examples, the
annulation product 3w and 3x was exclusively afforded in 62%
and 57% yield when octa-1,3-diyn-1-ylbenzene and 1-methoxy-
4-(octa-1,3-diyn-1-yl)benzene were attempted. The structure
reaction of [Cp*RhCl2]2 with AgSbF6 produces a cationic
rhodium species A, which then coordinates to the carbonyl
group of aniline 1 to form a six-membered rhodacycle B via an
ortho C−H cleavage. Then, π-coordination and migratory
insertion of the 1,3-diyne 2 generates a cyclic organorhodium
complex D. Following reductive elimination delivers an indole
derivative E, which further undergoes an intramolecular
cyclization to provide the desired product 3. The released
[Cp*Rh(I)] is then oxidized to [Cp*Rh(III)] by AgI to
complete the catalytic cycle. Although no silver mirror
phenomenon was observed, the control experiment indicated
that an oxidant was required in this transformation (see Table
S1, entry 28).
1
1
of 3w was confirmed by H NMR, 13C NMR, and H−1H
NOESY analysis. These observations on the regioselectivity
were also consistent with the results of Glorius in the catalytic
C−H annulation for the synthesis of bis-heterocycle.6a This
regioselectivity might be attributed to the different electrostatic
interactions resulted from the two different alkyne insertion
C
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Among the organic electroluminescent materials that
provide primary color (red, green, and blue) emissions, the
development of blue emitters possessing high quantum
efficiency is a fascinating yet challenging task.10,11 Subse-
quently, the photophysical properties of 3 were measured (see
Table 2 and Figure 1, as well as Part X in the Supporting
Three new bonds (the C−C bond, C−N bond, and C−O
bond) and two fused heterocycles are formed in a single
operation. This reaction also provides a strategic approach to
rapidly assemble a series of blue-emitting fluorophores with
high quantum efficiencies, which may be potentially applied in
organic optoelectronic devices and bioprobes.
Table 2. Photophysical Properties of Representative
Compounds
ASSOCIATED CONTENT
■
sı
* Supporting Information
a
b
c
d
dye
λabs (nm)
λem (nm)
CIE
ΦF
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01465.
3a
3l
360
371
386
370
361
364
357
392
348, 363
352, 366
449
457
441, 467
462
453
456
(0.16, 0.14)
(0.16, 0.18)
(0.16, 0.20)
(0.18, 0.23)
(0.16, 0.16)
(0.17, 0.19)
(0.16, 0.14)
(0.17, 0.25)
(0.15, 0.08)
0.82
0.69
0.68
0.50
0.85
0.82
0.86
0.69
0.74
0.61
Detailed experimental procedures, characterization data,
photophysical data of related compounds, and crystallo-
graphic data (PDF)
3m
3n
3p
3q
3r
3v
3w
3x
Accession Codes
447
CCDC 1997767 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
452, 477
402, 427, 448
403, 428, 453
(0.16, 0.09)
a
b
Absorption maxima in toluene (1.0 × 10−5 M). Emission maxima in
c
toluene (1.0 × 10−5 M). CIE coordinates measured in toluene (1.0 ×
d
10−5 M). Absolute quantum yields in toluene (1.0 × 10−5 M)
determined with an integrating sphere system.
AUTHOR INFORMATION
■
Corresponding Author
Yudong Yang − Key Laboratory of Green Chemistry and
Technology of Ministry of Education, College of Chemistry,
Sichuan University, Chengdu 610064, People’s Republic of
China; orcid.org/0000-0002-7142-2249;
Email: yangyudong@scu.edu.cn
Authors
Shengyou Qian − Key Laboratory of Green Chemistry and
Technology of Ministry of Education, College of Chemistry,
Sichuan University, Chengdu 610064, People’s Republic of
China
Xingwen Pu − Key Laboratory of Green Chemistry and
Technology of Ministry of Education, College of Chemistry,
Sichuan University, Chengdu 610064, People’s Republic of
China
Guanjun Chang − State Key Laboratory of Environment-
friendly Energy Materials, School of Material Science and
Engineering, Southwest University of Science and Technology,
Mianyang 621010, People’s Republic of China; orcid.org/
0000-0002-9589-8030
Ying Huang − State Key Laboratory of Environment-friendly
Energy Materials, School of Material Science and Engineering,
Southwest University of Science and Technology, Mianyang
621010, People’s Republic of China; orcid.org/0000-0003-
4938-8870
Figure 1. Fluorescence spectra of representative compounds in
toluene (1.0 × 10−5 M).
Information). All the polycyclic N,O-heteroaromatics in tests
exhibited blue fluorescence in toluene solution with high
quantum yields (up to 0.86). While the position of substituents
on chromophoric core had a slight effect on the emissions
(Table 2, 3l−3n), altering the nature of substituents led to
significant hypochromatic or bathochromic shift in the
fluorescence spectra (Table 2, 3a, 3p−3r, 3v−3x). These
results indicated that the emission spectra could be easily
tuned by changing the electronic properties of substituents. It
is noteworthy that compound 3w (ΦF = 0.74) and 3x (ΦF =
0.61) prepared from alkyl aryl diynes exhibit pure blue
emission with Commission Internationale de I’Eclairage (CIE)
coordinates of (0.15, 0.08) and (0.16, 0.09) in solution,
respectively, which might be potential candidates for pure blue-
emitting organic optoelectronic devices and biological probe.
In conclusion, we have developed an unprecedented
rhodium-catalyzed oxidative C−H activation/domino annula-
tion of N-Boc-anilines with 1,3-diynes, which features easily
available substrates, excellent function group tolerance, mild
reaction conditions, high regioselectivity, mono/diannulation
selectivity, and intra/intermolecular annulation selectivity.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01465
Author Contributions
The manuscript was written through contributions of all
authors./All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
D
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Letter
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ACKNOWLEDGMENTS
■
This work was supported by grants from the National Key
R&D Program of China (No. 2019YFC1604804), National
Natural Science Foundation of China (No. 21502123), and the
Open Project of State Key Laboratory of Environment-friendly
Energy Materials (No. 19kfhg15). We appreciate the support
from the Comprehensive Training Platform of Specialized
Laboratory, College of Chemistry, Sichuan University.
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