Chiral Phosphoric Acid Catalyzed Atroposelective C?H Amination of Arenes

Article abstract of DOI:10.1002/anie.202000585

N-arylcarbazole structures are important because of their prevalence in natural products and functional OLED materials. C?H amination of arenes has been widely recognized as the most efficient approach to access these structures. Conventional strategies involving transition-metal catalysts suffer from confined substrate generality and the requirement of exogenous oxidants. Organocatalytic enantioselective C–N chiral axis construction remains elusive. Presented here is the first organocatalytic strategy for the synthesis of novel axially chiral N-arylcarbazole frameworks by the assembly of azonaphthalenes and carbazoles. This reaction accommodates broad substrate scope and gives atropisomeric N-arylcarbazoles in good yields with excellent enantiocontrol. This approach not only offers an alternative to metal-catalyzed C–N cross-coupling, but also brings about opportunities for the exploitation of structurally diverse N-aryl atropisomers and OLED materials.

Full text of DOI:10.1002/anie.202000585

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Accepted Article
Title: Chiral-Phosphoric-Acid-Catalyzed Atroposelective C-H Amination
of Arenes
Authors: Bin Tan, Wang Xia, Qian-Jin An, Shao-Hua Xiang, Shaoyu Li,
and Yong-Bin Wang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202000585
Angew. Chem. 10.1002/ange.202000585
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10.1002/anie.202000585
Angewandte Chemie International Edition
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Chiral-Phosphoric-Acid-Catalyzed Atroposelective C-H Amination
of Arenes
Wang Xia,[a] Qian-Jin An,[a] Shao-Hua Xiang,[a],[b] Shaoyu Li,*[a],[b] Yong-Bin Wang,[a] and Bin Tan*[a]
Dedication ((optional))
[a]
[b]
W. Xia, Q.-J. An, S.-H. Xiang, S. Li, Y.-B. Wang, Prof. Dr. B. Tan
Shenzhen Grubbs Institute and Department of Chemistry
Southern University of Science and Technology, Shenzhen, 518055, China
E-mail: tanb@sustech.edu.cn; lisy@sustech.edu.cn
S.-H. Xiang, S. Li
Academy for Advanced Interdisciplinary Studies
Southern University of Science and Technology, Shenzhen, 518055, China
Supporting information for this article is given via a link at the end of the document.
Abstract: N-arylcarbazole structures are of significant importance
due to their wide prevalence in natural products and functional
OLED materials. C-H amination of arene has been widely
organocatalytic enantioselective arene C-H amination with N-
nucleophiles remains hard to access due to the low activity of
aromatic ring and limited catalytic modes of asymmetric
recognized as the most efficient approach to access these structures. organocatalysis.
Conventional strategies involving transition metal catalyst always
suffered from confined substrate generality and the requirement of
exogenous oxidants. Organocatalytic enantioselective C-N chiral
axis construction remains elusive. Here we present the first
organocatalytic strategy for the synthesis of novel axially chiral N-
arylcarbazole frameworks via the assembly of azonaphthalenes and
carbazoles. This reaction accommodates broad substrate scope and
gives atropisomeric N-arylcarbazoles in good yields with excellent
enantiocontrol. This approach not only offers an alternative to metal-
catalyzed C-N cross-coupling, but also brings about opportunities for
the exploitation of structurally diverse N-aryl atropisomers and OLED
materials.
Organic molecules possessing N-aryl framework are ubiquitous
in natural products, pharmaceuticals, agrochemicals, and
functional materials.[1] Among these structures, N-arylcarbazoles
featuring high triplet energies and competitive hole transport
ability have been emerged as one of the most widely used host
materials for OLEDs in the recent years (Figure 1A).[2] Rapid
progress in such fields stimulates a continuous exploration of
structurally novel N-arylcarbazoles, particularly those bearing
distinct chiral elements. Therefore, the pursuit of catalytic and
enantioselective strategy to assemble such molecules has been
and continues to be highly enthralling. Conventional transition-
metal-catalyzed N-arylation protocols such as Ullmann,
Buchwald-Hartwig, and Chan-Lam-Evans coupling reactions
have become indispensable tools in C-N bond formation regime
(Figure 1B).[3] However, pre-functionalization of the arene
substrate and the superfluous waste production largely
hampered their efficiencies. To circumvent these drawbacks, a
series of strategies based on arene oxidative amination
Figure 1. Significance of N-arylcarbazole, conventional C-N bond formation
reactions were successively exploited using hypervalent iodine[4]
or transition metal[5] as the facilitator (Figure 1C). The amine-
arene coupling was further elegantly complemented by means of
novel photocatalysis[6] and electrocatalysis[7] established by
Nicewicz,[6a,b] Ackermann[7a] and their coworkers very recently.
Despite abovementioned contributions, confined substrate
scope significantly restricted their applications and harsh
conditions were adverse to enantiocontrol. On the other hand,
approaches to access N-aryl structures and our design for atroposelective
construction of novel N-arylcarbazoles.
We have formerly discovered that an azo group serves aptly
for aryl ring activation thus warrants organocatalytic C-H
arylation of arene.[8] This strategy realized the scarce aromatic
nucleophilic substitution with a couple of C-nucleophiles for the
production of privileged biaryl atropisomers with chiral
1
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10.1002/anie.202000585
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COMMUNICATION
phosphoric acid (CPA) catalysis,[9] while the employment of
typical N-nucleophiles remains absent. Given the importance of
the N-arylcarbazoles in chemical and material science, engaging
the carbazoles as competent N-nucleophiles for this
organocatalytic strategy to enable an alternative N-aryl
enantioselective cross-coupling became highly appealing
(Figure 1D). Notably, azo group could be transformed to an
amine readily and offer an effective functional handle for the
downstream diversity-oriented synthesis. Herein, we would like
to present our recent endeavors on the construction of
atropisomeric N-arylcarbazoles in line with our long-standing
research interest in exploiting axially chiral frameworks through
the first CPA-catalyzed atroposelective C-H amination of arenes.
Based on recent achievements in axial chirality domain[10] and
our understandings in organocatalytic construction of
atropisomeric frameworks,[11] azonaphthalene derivative 1a and
2-tert-butyl-9H-carbazole 2a were selected as model substrates.
We aimed to control the rotation of C-N axis by the installation of
a sterically hindered group on C2 position of carbazole substrate.
Delightfully, the desired axially chiral N-arylcarbazole 3a was
delivered in 64% isolated yield with 65% enantiomeric excess
(ee) when the reaction was facilitated by SPINOL-derived
phosphoric acid (S)-C1 (15 mol%) in CH2Cl2 at 40 °C (Table 1,
entry 1). Despite moderate enantioselectivity, this proof-of
principle result verified the feasibility of stereoinduction from
CPA to C-N axis via arene C-H amination. We then set out to
evaluate the substitution effect on the CPA and found that
(S)-C6 with phenanthryl on 6,6’-position was a superior facilitator,
boosting the enantioselectivity to 90% ee. Meanwhile, the
chemical yield was improved to 79% (Table 1, entry 6). Ensuring
optimization through the alternation of axially chiral backbones
of the catalyst met with failure (Table 1, entries 8-10).
Subsequent investigations revealed that chloroform was a
beneficial solvent to this reaction, producing the adduct 3a in
similar yield with 92% ee (Table 1, entries 11-14). Reduction of
catalyst loading to 10 mol% resulted in negligible effect on the
reaction outcome (Table 1, entry 15). Finally, the best results
were concluded with slight elevation of the temperature and the
introduction of 3Å MS (Table 1, entry 16).
Scheme 1. Generality of enantioselective arene C-H amination with carbazole
nucleophiles.[a]
Table 1. Optimization of the reaction conditions.[a]
T
(°C)
Time
(day)
Yield
(%)[b]
Ee
Entry
CPA
Solvent
(%)[c]
1
2
(S)-C1
(S)-C2
(S)-C3
(S)-C4
(S)-C5
(S)-C6
(R)-C7
(R)-C8
(R)-C9
(R)-C10
(S)-C6
(S)-C6
(S)-C6
(S)-C6
(S)-C6
(S)-C6
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
CCl4
40
40
40
40
40
40
40
40
40
40
40
40
40
40
50
50
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.5
2.5
2.5
2.5
3.0
3.0
64
78
78
9
65
74
87
62
72
90
-88
-25
-37
-24
92
67
90
77
91
93
3
4
5
32
79
71
30
23
5
6
7
8
9
10
11
12
13
14
15[d]
16[d],[e]
71
68
64
51
73
76
[a] Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), (S)-C6 (10 mol%) and 3Å
MS (64 mg) in CHCl3 (4 mL) at 50 °C. Isolated yield. Ee was determined by
chiral HPLC analysis.
DCE
toluene
CHCl3
CHCl3
Having established the optimal conditions, the substrate
generality of this reaction was then evaluated. First, various
azonaphthalene derivatives were tested and the reaction results
were summarized in Scheme 1A. Replacement of methyl ester
group by ethyl, isopropyl, tert-butyl or benzyl derivative could
give the expected product 3b-3e in similar yield with remarkable
[a] Reaction conditions: 1a (0.10 mmol), 2a (0.15 mmol), CPA (15 mol%) in 2
mL of solvent at 40 °C or 50 °C. [b] Isolated yield. [c] Determined by chiral
HPLC analysis. [d] 10 mol% of CPA was used. [e] 3Å molecular seive (MS)
was added. DCE, 1,2-dichlorethane.
2
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10.1002/anie.202000585
Angewandte Chemie International Edition
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enantiocontrol, respectively. The electronic nature of
substituents and substitution patterns resulted in limited effect
on enantioselectivity of 3f-3l. Notably, excellent yields were
obtained when substrates bearing a C3 substitution were
employed (3k and 3l). Subsequently, the substrate scope of 9H-
carbazole 2 was explored for this enantioselective CPA-
catalyzed arene C-H amination reaction (Scheme 1B). Aside
from the tert-butyl group in the model substrate, a wide range of
sterically hindered groups were substantiated to effect restricted
rotation of C-N axis, including isopropyl, cyclohexyl, bromine,
CF3 and aryl (3m-3t). C5 and C6 methyl substituted carbazoles
were also well tolerated for this reaction, furnishing axially chiral
N-arylcarbazoles 3u-3z in moderate to good yields with excellent
ee values (90-93%).
Noteworthy, indole substrate 4a with multiple reactive sites
commonly perceived as C-nucleophile in organocatalysis[12] was
validated to be applicable N-nucleophile for this transformation
by subtle modifications of standard reaction conditions (Table
S1), to generated the corresponding axially chiral N-arylindole
adducts 5a with remarkable enantiocontrol (93% ee), albeit with
low yield (Scheme 2). It should be noted that the bulky
substitution on C3 position of indole was crucial to allow the
formation of the desired N-aryl axis. Next, the substitution effect
on azonaphthalene was examined and most substrates gave N-
arylindole atropisomers in moderate yields with an enantiomeric
excess range from 92-96%, respectively (5d-g). Interestingly,
indicating the potential in large-scale chemical production. The
configurational stability of the generated products was
exemplified by heating a solution of product 3a in toluene at
110 °C for 48 h. HPLC analysis illustrated that no erosion of
enantiopurity in 3a was detected (Table S2). The high stability of
the chiral axis motivated us to extend their applications in
asymmetric catalysis as organocatalysts or ligands. Thus,
atropisomeric thiourea
7
and mono-phosphine 10 were
successively synthesized from 3a as shown in Figure 2B. The
two-step synthesis provided product 7 in 74% yield with no loss
of stereochemical purity. Meanwhile, mono-phosphine 10 was
afforded in 43% overall yield in 3 steps from 6 bearing a free
amine group on the naphthalene ring. The optical purities of both
products could be improved to >99% ee by means of semi-
preparative high-performance liquid chromatography purification.
Gratifyingly, thiourea 7 promoted the reaction of phosphorus
ylide 11 and N-Boc imine 12 smoothly to give the expected
product 13 in 71% yield with 80% ee after subjection to formalin
(Figure 2C).[13] N-arylcarbazole 10 could serve as an efficient
ligand for palladium catalyzed enantioselective allylic alkylation
of racemic 14 with malonate 15 nucleophiles. In the presence of
palladium catalyst (2 mol%) and 10 (4 mol%), these reactions
proceeded efficiently, to furnish the desired products 16a-c in
excellent yields with commendable stereoselectivity (85-95%
ee).[14] The absolute configuration of compound
8 was
determined as (aS) by X-ray crystallographic analysis (CCDC:
1963374) and those of other products were assigned by analogy.
A plausible stereochemical model featuring bifunctional CPA
facilitated H-bond activation and rearomatization-enabled central
to axial chirality transfer pathway were also provided in Figure
2D for current arene C-H enantioselective amination
reaction.[10j,k]
the introduction of
a methyl group on C3 position of
azonaphthalene significantly boosted the yield to over 90% with
enhanced enantioselectivity (5h, 99% ee). This positive
substitution effect was further substantiated by the reaction with
4b or 4c as the nucleophile (5i and 5j).
Scheme 2. Generality of the atroposelective arene C-H amination with indole
nucleophiles.[a]
[a] Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), (R)-C7 (10 mol%) in
CHCl3/CH2Cl2 (2/2 mL) at 25 °C for 1.5 days. Isolated yield. Ee was
determined by chiral HPLC analysis.
To verify the practicality of this protocol, a preparative-scale
synthesis of product 3a was carried out under the standard
conditions. As displayed in Figure 2A, negligible deterioration of
yield and identical stereochemical integrity were observed,
Figure 2. Synthetic transformations, applications and reaction pathway.
3
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Given the central role of di-carbazole substituted arenes in the
OLEDs materials, we then explored the synthesis of such
structural motifs possessing two chiral N-aryl axes[15] with the
developed strategy. Accordingly, 2,6-diazonaphthalene 17 was
prepared and subjected to the standard conditions with
carbazole as the nucleophile. Pleasingly, this double
atroposelective C-H amination reaction occurred to give the
desired 1,5-dicarbazole naphthalene derivative 18 with excellent
enantiocontrol (98% ee). However, only 27% yield was obtained
for this one-week reaction with the formation of mesomeric
byproduct 19 (16%) and about 10% of mono-amination product.
After evaluating a set of CPA catalysts, it was delightful to
identify 20 mol% of (S)-C2 with 1-pyrenyl group on 6,6’-position
of spiro-backbone greatly improved the yield to 51% without ee
erosion.[16] Meanwhile, a satisfactory diastereoselectivity (18:19
= 4:1) could be noted. To demonstrate the generality of this
transformation, other carbazoles with varied substituents on C2
position were then tested. As displayed in Scheme 3, all the
desired products (18b-d) were generated in moderate yields
with >90% ee values.
exploitation of their underlying applications are ongoing in our
laboratory.
Acknowledgements
We are grateful for financial support from the National Natural
Science Foundation of China (Nos. 21772081, 21825105,
21702092 and 21901105), Shenzhen Nobel Prize Scientists
Laboratory Project (C17213101), and Shenzhen Special Funds
for the Development of Biomedicine, Internet, New Energy, and
New Material Industries (JCYJ20170412151701379).
Conflict of interest
The authors declare no conflict of interest.
Keywords: N-arylcarbazole • C-N chiral axis • chiral phosphoric
acid • asymmetric catalysis • amination
Scheme 3. Generality evaluation of double enantioselective arene C-H
amination reaction.[a]
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In summary, we have developed the first chiral phosphoric
acid-catalyzed atroposelective arene C-H amination. This
nucleophilic aromatic substitution reaction represents as a
straightforward and alternative C-N bond formation method to
conventional metal-involved cross-couplings, delivering axially
chiral N-arylcarbazoles in good yields with remarkable
enantiocontrol through a rearomatization-enabled central to axial
chirality transfer pathway. Indoles as commonly known C-
nucleophiles in organocatalysis were also effective for this
transformation to give expected products possessing N-aryl
chiral axes with superior optical purities. Gram-scale synthesis,
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chiral organocatalyst and ligand substantiated the utility of the
generated constructs. Moreover, highly enantioenriched 1,5-
dicarbazole naphthalene derivatives with significant potential in
OLED materials were assembled via this protocol. Further
investigations about the synthesis of novel OLED materials and
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