Metal-free nitrogenation of 2-acetylbiphenyls: Expeditious synthesis of phenanthridines

Article abstract of DOI:10.1021/acs.orglett.5b00797

An intermolecular nitrogenation reaction toward the synthesis of phenanthridines has been developed. This metal-free protocol provides a novel nitrogen-incorporation transformation using azides as the nitrogen source. Phenanthridines, which are of great interest in pharmaceutical and medicinal chemistry, are synthesized efficiently in one step. Moreover, the byproducts derived from the Schmidt reaction are inhibited, which further demonstrated the high chemoselectivity of this transformation.

Full text of DOI:10.1021/acs.orglett.5b00797

Letter
pubs.acs.org/OrgLett
Metal-Free Nitrogenation of 2‑Acetylbiphenyls: Expeditious
Synthesis of Phenanthridines
Conghui Tang,† Yizhi Yuan,† and Ning Jiao*,†,‡
†State Key Laboratory of Natural and Biomimetic Drugs, School of Pharmaceutical Sciences, Peking University, Xue Yuan Road 38,
Beijing 100191, China
‡State Key Laboratory of Organometallic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: An intermolecular nitrogenation reaction toward
the synthesis of phenanthridines has been developed. This metal-
free protocol provides a novel nitrogen-incorporation trans-
formation using azides as the nitrogen source. Phenanthridines,
which are of great interest in pharmaceutical and medicinal
chemistry, are synthesized efficiently in one step. Moreover, the
byproducts derived from the Schmidt reaction are inhibited,
which further demonstrated the high chemoselectivity of this
transformation.
itrogen-containing compounds show great importance in
chemistry, biology, and material sciences.1 Many novel
and efficient methodologies on C−N bond formation for the
synthesis of nitrogen-containing compounds have been
developed over the past decades.2
Phenanthridines, which are representative core fragments in
a large number of natural products and alkaloids, have
generated great interest in the fields of medicinal chemistry,
biochemistry, and material sciences (Figure 1).3 Hence,
phenanthridine syntheses from iminyl radicals,12 photochemical
processes,13 and microwave-mediated cyclizations.14 Albeit
great achievements have been reached, further exploration of
convenient, efficient, and milder protocols are still significant
due to the broad applications of phenanthridine derivatives.
In the past decades, the direct functionalization of simple
substrates through C−H bond and/or C−C bond cleavage has
been significantly developed.15 Constructing nitrogen-contain-
ing compounds employing azides as a nitrogen source was a
classical as well as enduring strategy.16 The click chemistry has
been widely used in organic synthesis and in bioconjugation.17
Some well-known named reactions also realized nitrogen
incorporation using azides, such as the Schmidt reaction,18
Curtius rearrangement,19 and Staudinger reaction.20 Recently,
by using azide reagents, the direct functionalization of the C−H
bond to construct the C−N bond in the absence of an
additional oxidant has attracted considerable attention.21,22
Through this strategy, some acyclic nitrogen-containing
compounds have been easily prepared.
N
Figure 1. Examples of natural products and alkaloids containing a
However, by using azides as the nitrogen source, the
construction of N-heterocyclic compounds through two bond
cleavages and two bond formations in one step is still
challenging and has been rarely achieved. More recently,
Glorius and co-workers reported a Rh-catalyzed 1H-indazoles
synthesis with sulfonyl azide (Scheme 1, eq 1).23 The Ellman
group realized Rh-catalyzed acridines and phenazines synthesis
with aromatic azides (Scheme 1, eq 2).24 Jiao and co-workers
developed a Pd(II)-catalyzed pyrido[1,2-b]indazoles prepara-
tion with sodium azide (Scheme 1, eq 3).25 The transition
metal catalysts were required in these elegant works.
phenanthridine core.
considerable attention has been paid to the synthesis of
substituted phenanthridines for a long time. The Bischler−
Napieralski cyclization involving P4O10, POCl3, or PCl5 at
elevated temperature has been extensively used.4 To avoid the
harsh reaction conditions and limited functional group
tolerance, much effort has been focused on mild and efficient
synthetic routes to phenanthridines. Recent achievements
include palladium catalyzed cascade reactions,5 annulations
employing arynes,6 aza-Wittig reactions,7 oxidative cyclization
of 2-isocyanobiphenyls,8 biaryl-2-carbonitriles with organo-
metallic reagents,9 transition-metal-catalyzed cyclization of
imines,10 anionic ring closure reactions,11 UV promoted
Received: March 18, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b00797
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
gave a higher yield of 2a, but with a 32% yield of byproduct 3
(Table 1, entry 2). When the reaction atmosphere was changed
from air to dioxygen or argon, only a moderate yield of 2a was
obtained respectively (Table 1, entries 3, 4). It should be noted
that when TFA was changed by other acidic solvents, such as
MsOH, TCA, or Tf2NH, this reaction was completely inhibited
(Table 1, entries 5−7). Gratifyingly, when triflic acid (TfOH)
was added as an additive, 2a was highly selectively obtained
quantitatively (Table 1, entry 8). Decreasing the reaction
temperature from 80 to 60 °C gave a slightly lower yield of 94%
(Table 1, entry 9), while a reduced amount of the additive
TfOH (from 5 equiv to 2.5 equiv) led to a significant loss of 2a
(Table 1, entry 10). To our surprise, the target product 2a was
obtained in 86% yield even in the absence of FeCl2, indicating
that the FeCl2 was a promoter rather than a catalyst (Table 1,
entry 11). Consequently, we decided to use entry 11 as the
optimal conditions to investigate the application of this
transformation.
Scheme 1. Intermolecular Nitrogen Incorporation Using
Azides to Synthesize Heterocycles
Herein, we would like to report a novel metal-free
nitrogenation of 2-acetylbiphenyls for the efficient synthesis
of phenanthridine derivatives (Scheme 1, eq 4). To the best of
our knowledge, this is the first example of intermolecular
phenanthridine synthesis using azides as a N atom donor.
Multiple chemical bonds are transformed involving double C−
N bond formations. This reaction proceeds under mild
conditions with high efficiency and functional group tolerance.
In the beginning of this project, we employed 2-
acetylbiphenyl (1a) as the model substrate. When the reaction
of 1a and azidotrimethylsilane (TMSN3) was catalyzed by
FeCl2 at 60 °C in TFA as the solvent, to our delight, 6-
methylphenanthridine (2a) was obtained in 43% yield (Table 1,
entry 1). It is noteworthy that the Schmidt reaction product N-
acetyl-2-aminobiphenyl (3) was not detected in this case
(Table 1, entry 1). The reaction at higher temperature (80 °C)
Next, a variety of diaryl-ketones and aryl alkyl ketones were
tested as substrates under the standard conditions (Table 2).
Table 2. Substrate Scope of 6-Substituted Phenanthridines
a
Synthesis
b
entry
1, R1
2, yield
1
2
1a, Me
2a, 86%
2b, 83%
2c, 56%
2d, 95%
2e, 62%
2f, 95%
2g, 89%
2h, 94%
2i, 86%
2j, 83%
2k, 70%
2l, 86%
2m, 83%
2n, 96%
2o, 81%
2p, 81%
1b, 4-Me-C6H4
1c, 2-Me-C6H4
1d, 3,5-dimethyl-C6H3
1e, 4-t-Bu-C6H4
1f, 4-F-C6H4
c
3
a
4
Table 1. Optimization of the Reaction Conditions
5
6
7
1g, 4-Cl-C6H4
1h, 4-NO2-C6H4
1i, 4-CF3-C6H4
1j, 4-N,N-dimethyl-C6H4
1k, 2-naphthyl
1l, 3-pyridyl
8
9
10
11
12
13
14
15
16
b
yield (%)
catalyst
(mol %)
1m, CH2Ph
entry
additive
solvent t (°C) 2a
3
1n, t-Bu
1
2
FeCl2 (10)
FeCl2 (10)
FeCl2 (10)
FeCl2 (10)
FeCl2 (10)
FeCl2 (10)
FeCl2 (10)
FeCl2 (10)
FeCl2 (10)
FeCl2 (10)
−
−
−
−
−
−
−
TFA
60
80
80
80
80
80
80
80
60
60
43
67
55
46
0
N.D.
32
42
N.D.
trace
0
1o, c-hexyl
TFA
1p, n-pentyl
c
3
TFA
a
d
Reaction conditions: 1 (0.2−0.4 mmol), TMSN3 (2.0 equiv), TfOH
4
TFA
b
(5.0 equiv), TFA (1.0−2.0 mL), stirred at 60 °C under air. Isolated
yields. Reflux.
5
MsOH
TCA
Tf2NH
c
6
0
7
0
0
For diaryl-ketones, the position of the substituent on R1 (when
R1 = Ar) affected the reaction efficiency greatly. A methyl group
at the para position yielded 2b in 83% yield while the ortho
position yielded 2c in only 56% yield (Table 2, entries 2 and 3).
Two methyl substituents at the meta position showed excellent
reactivity with a 95% yield (2d) (Table 2, entry 4). A series of
para-substituted electron-donating and -withdrawing groups
were examined for details. All these substrates reacted smoothly
to afford the phenanthridines in good to excellent yields (Table
2, entries 5−10). When R1 was changed from substituted
phenyl rings to naphthalene, the corresponding phenanthridine
was generated in 70% yield (Table 2, entry 11). Notably,
aromatic heterocycles such as pyridine was also tolerated and
8
TfOH (5 equiv) TFA
99
94
75
0
9
TfOH (5 equiv) TFA
0
10
TfOH
(2.5 equiv)
TFA
0
11
none
TfOH (5 equiv) TFA
60
86
0
12
none
TfOH (5 equiv) TFA
80
81
0
a
Reaction conditions: 1a (0.4 mmol), TMSN3 (0.8 mmol, 2.0 equiv),
b
c
solvent (2.0 mL). Isolated yields. The reaction was carried out under
an O2 atmosphere. The reaction was carried out under an argon
d
atmosphere. TMS = trimethylsilyl, TFA = trifluoroacetic acid, TCA =
trichloroacetic acid, MsOH = methanesulfonic acid, TfOH =
trifluoromethanesulfonic acid, Tf2NH = trifluoromethanesulfonimide.
N.D. = not detected.
B
DOI: 10.1021/acs.orglett.5b00797
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
gave an 86% yield (Table 2, entry 12). For aryl alkyl ketones, all
of them performed well to give 6-alkyl substituted phenan-
thridines in 81%−96% yield (Table 2, entries 13−16).
Scheme 4. Proposed Reaction Mechanism
Next, a set of substrates bearing R2 substituents were tested
under the optimal conditions to figure out the regioselectivity
of this nitrogenation reaction (Table 3). In these cases, Me,
Table 3. Substrate Scope of Disubstituted Phenanthridines
a
Synthesis
bc
,
entry
R2
yield (5/6)
is easily generated from TMSN3 in the strong acidic media,
which attacks the substrate 4 to afford intermediate A. Then
intermediate A undergoes a dehydration process to afford
intermediate B, which executes a Friedel−Crafts reaction26
accompanied by loss of N2 to afford product 5 (Scheme 4, path
a). Alternatively, the aryl group migration of intermediate B
occurs to afford intermediate C. Product 6 is finally produced
through the electrophilic cyclization of intermediate C (Scheme
4, path b).
In summary, a metal-free intermolecular nitrogen incorpo-
ration approach to nitrogen-containing heterocycles has been
developed. Biologically important phenanthridine derivatives
are constructed concisely and efficiently with high chemo-
selectivity from simple 2-acetylbiphenyls and azides. Further
studies on the applications of this protocol are ongoing in our
laboratory.
1
2
3
Me (4a)
OMe (4b)
t-Bu (4c)
Cl (4d)
74% (5a/6a = 1:1)
71% (5b/6b = 1:2.6)
81% (5c/6c = 1:1.4)
47% (5d/6d = 1:3.3)
59% (6e)
d
4
5
OH (4e)
a
Reaction conditions: 4 (0.4 mmol), TMSN3 (0.8 mmol, 2.0 equiv),
TfOH (2.0 mmol, 5.0 equiv), TFA (2 mL), stirred at 60 °C under air.
b
c
d
Isolated yields. The product ratio was determined by 1H NMR. 10
mol % FeCl2 was added.
t
OMe, Bu, Cl substituents all showed good compatibility to
obtain products 5 and 6 (Table 3, entries 1−4). Interestingly,
the hydroxyl substituted substrate 4e highly regioselectively
delivered a single product (Table 3, entry 5), although the
reason is still unclear. The structure of 6e was determined by an
NOESY spectrum (see the Supporting Information).
To our delight, 2-biphenylcarboxaldehyde (7) was trans-
ferred into unsubstituted phenanthridine (8) successfully in
94% yield (Scheme 2). It is well-known that aldehyde usually
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures, full characterization of products, and
copies of NMR spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
S
Scheme 2. Nitrogenation of [1,1′-Biphenyl]-2-carbaldehyde
with TMSN3
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: jiaoning@pku.edu.cn.
Notes
The authors declare no competing financial interest.
underwent a Schmidt reaction18 under acidic conditions to give
nitriles, while the Schmidt reaction was completely inhibited in
the present system.
In order to clarify the detailed mechanism of this reaction, N-
acetyl-2-aminobiphenyl (3) was allowed to react under the
standard conditions, while 2a was not observed, which ruled
out the possibility of the relay of the Schmidt reaction with the
formation of 3 and then cyclization processes in this
nitrogenation reaction (Scheme 3).
ACKNOWLEDGMENTS
■
Financial support from the National Basic Research Program of
China (973 Program) (Grant No. 2015CB856600), the
National Natural Science Foundation of China (Nos.
21325206, 21172006), and the National Young Top-notch
Talent Support Program is greatly appreciated. We thank Kai
Wu in this group for reproducing the results of 2p and 5e.
REFERENCES
On the basis of the above-mentioned results, a proposed
reaction mechanism is represented in Scheme 4. Hydrazoic acid
■
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Scheme 3. Exclusion of the Reaction Intermediate
C
DOI: 10.1021/acs.orglett.5b00797
Org. Lett. XXXX, XXX, XXX−XXX

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