Facile synthesis of unsymmetrical acridines and phenazines by a Rh(III)-catalyzed amination/cyclization/aromatization cascade

Article abstract of DOI:10.1021/ja406131a

We report formal [3 + 3] annulations of aromatic azides with aromatic imines and azobenzenes to give acridines and phenazines, respectively. These transformations proceed through a cascade process of Rh(III)-catalyzed amination followed by intramolecular electrophilic aromatic substitution and aromatization. Acridines can be directly prepared from aromatic aldehydes by in situ imine formation using catalytic benzylamine.

Full text of DOI:10.1021/ja406131a

Communication
pubs.acs.org/JACS
Facile Synthesis of Unsymmetrical Acridines and Phenazines by a
Rh(III)-Catalyzed Amination/Cyclization/Aromatization Cascade
Yajing Lian,† Joshua R. Hummel,† Robert G. Bergman,‡ and Jonathan A. Ellman*,†
†Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States
‡Division of Chemical Sciences, Lawrence Berkeley National Laboratory, and Department of Chemistry, University of California,
Berkeley, California 94720, United States
S
* Supporting Information
with substitution on both rings can be challenging.16,17 In
contrast, the approach reported here provides very rapid access
to unsymmetrical derivatives with precise placement of diverse
functionality at almost any position.
We initiated our investigation by exploring the Rh(III)-
catalyzed addition of N-phenyl benzaldehyde imine (1a) to
phenyl azide (2a) (see the Supporting Information for the
optimization table). The use of a 10 mol % loading of the
convenient preformed cationic rhodium catalyst [Cp*Rh-
(CH3CN)3(SbF6)2] in dichloroethane (DCE) was found to be
optimal, providing 3a in 57% yield (eq 1). Because the aniline
ABSTRACT: We report formal [3 + 3] annulations of
aromatic azides with aromatic imines and azobenzenes to
give acridines and phenazines, respectively. These trans-
formations proceed through a cascade process of Rh(III)-
catalyzed amination followed by intramolecular electro-
philic aromatic substitution and aromatization. Acridines
can be directly prepared from aromatic aldehydes by in
situ imine formation using catalytic benzylamine.
h(III)-catalyzed C−H functionalization has proven to be a
Rversatile and highly functional-group-compatible approach
for the synthesis of important classes of heterocycles,1,2 with
additions to alkynes,3 alkenes,4 allenes,5 aldehydes,6 imines,6c
CO,7 isonitriles,8 isocyanates,9 and diazo compounds10 all having
been utilized. Capitalizing on recent reports of Rh(III)-catalyzed
C−H functionalization with aromatic and sulfonyl azide coupling
partners,11a−c,12,13 Glorius very recently described a novel Rh/
Cu-cocatalyzed synthesis of 1H-indazoles through C−H
amidation of benzimidates with sulfonyl azides followed by
oxidative N−N bond formation.11d Herein we report new formal
[3 + 3] annulations for the preparation of acridines and
phenazines by Rh(III)-catalyzed C−H amination with aromatic
azides followed by in situ intramolecular electrophilic aromatic
substitution and aromatization (Figure 1). Despite the
prevalence of acridines and phenazines in natural products,
pharmaceuticals, and materials (e.g., photosensitizers and
photocatalysts),14,15 regioselective preparation of derivatives
released upon cyclization and aromatization might interact with
the catalyst, CF3CO2CH2CF3 and acetic anhydride were used as
scavengers of this byproduct, resulting in significant increases in
the yield to 71% and 77%, respectively.
Having defined an effective catalyst and reaction conditions for
the synthesis of acridine 3a from preformed imine 1a, we next
explored the possibility of conducting the reaction directly from
aldehydes in the presence of a catalytic amount of an amine. This
approach would enhance the utility of the method because a vast
number of aldehydes are commercially available, thus providing
rapid access to a wide range of acridines. The proposed cascade
sequence would require in situ condensation of an aldehyde and
an amine to form the imine necessary to direct the C−H
amination, followed by cyclization to generate the acridine with
release of the amine for another catalytic cycle. Although we had
previously found that released aniline is detrimental to the
reaction, we reasoned that if catalytic amounts of the amine were
used, it might be sequestered as the imine until the reaction
neared completion.
The importance of the imine directing group was first
demonstrated by attempting the direct coupling of benzaldehyde
(4a), which resulted in only a trace amount of product (Table 1,
entry 1). Addition of 10 mol % aniline provided 3a in 15% yield
(entry 2), and including MgSO4 as drying agent along with 10
and 20 mol % of aniline further increased the yield to 26% and
Received: June 18, 2013
Figure 1. Heterocycles by tandem C−H amination and cyclization.
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
a
3o) groups. Moreover, thiophene could also be incorporated
(3j). While both electron-neutral and -rich aromatic aldehydes
were found to be suitable for this transformation, electron-poor
aldehydes afforded the products in higher yields (3d vs 3e, 3f,
and 3i). Aromatic aldehydes with ortho (3g), meta (3h) and para
(3b−f,k-o) substitution were compatible, with m-methyl-
substituted benzaldehyde exclusively providing product 3h
resulting from C−H activation at the less hindered site.
Interestingly, the yields obtained in this transformation were
not sensitive to the electronic or steric effects introduced by
substitution on the aromatic azide. Substitution with electron-
donating (3j, 3m−o), -neutral (3b−i), or -withdrawing (3k, 3l)
groups at the ortho (3n), meta (3o), or para (3k−m) position all
provided good to excellent yields. However, in contrast to the
complete regioselectivity observed for cyclization of a meta-
substituted aromatic aldehyde (see 3h), m-methyl substitution
on the phenyl azide proceeded with good but not absolute
regioselectivity favoring cyclization at the least hindered site
(3o). Heterocycles on both aldehyde (3i) and azide (3j) were
well-tolerated and provided the corresponding products in
moderate to good yields.
Table 1. In Situ Imine Formation with Catalytic Amine
a
Conditions: 4a (0.10 mmol) and 2a (0.15 mmol) in solvent (1.0 or
b
2.0 mL) for 20 h. 1H NMR yields using 2,6-dimethoxytoluene as an
c
external standard. Isolated yield at 0.20 mmol scale of 4a.
We next investigated the possibility of extending the substrate
scope to ketones, which would lead to acridines with substitution
at the 9-position. Ketones are much less efficiently converted to
imines than are aldehydes, so the protocol for in situ imine
formation using a catalytic amount of amine gave a yield of <10%,
not only with MgSO4 and molecular sieves but also with more
powerful water scavengers and Lewis acid additives such as Ti(i-
OPr)4. We therefore turned our attention to the use of
preformed imines under the conditions optimized for coupling
preformed aldimines with azides (eq 1). Under these conditions,
the ketimine-derived 9-substituted acridines were obtained in
good to excellent yields (Table 3). Both electron-deficient (6b)
40%, respectively (entries 3 and 4). While the use of anilines with
either electron-rich or -deficient substituents failed to improve
the yield (entries 5 and 6), benzylamine gave a slightly higher
yield (entry 7). Moreover, doubling the benzylamine loading
resulted in a further improvement of the yield to 65% (entry 8).
The branched and more sterically hindered cyclohexylamine was
not as effective (entry 9). However, diluting the reaction mixture
2-fold significantly enhanced the yield to 76% (entry 10), which
was comparable to the yield observed in the reaction with
preformed imine (eq 1). Although catalytic in situ imine
formation has been used in Rh(I) catalysis,18 to the best of our
knowledge this is the first example of its use in Rh(III)-catalyzed
C−H functionalization.
Table 3. Scope of Acridine Synthesis with Ketone-Derived
a b
,
Imines
The substrate scope was explored with a diverse set of aromatic
aldehydes and aromatic azides (Table 2). The reaction showed
excellent functional group compatibility, providing acridines 3 in
good yields for aldehydes substituted with chloro (3b), iodo
(3c), fluoro (3g), ester (3d, 3k−o), methoxy (3e), indole (3i),
and acetamide (3f) groups and aromatic azides substituted with
trifluoromethyl (3k), chloro (3l), methoxy (3m), and alkyl (3n,
a b
,
Table 2. Scope of Acridine Synthesis with Aldehydes
a
Conditions: imine (0.20 mmol), azide (0.30 mmol), and Ac2O (0.40
b
mmol) in DCE (2.0 mL) for 20 h. Isolated yields are shown.
and sterically hindered (6c) substituents could be installed at the
central position of the acridine ring. In analogy to the steric and
electronic effects observed for the reactions with aldehydes,
electron-donating groups (6e) resulted in lower yields than
electron-withdrawing (6d, 6j) and -neutral (6f) groups.
Moreover, meta substitution selectively generated the product
as a single isomer with amination at the least hindered site (6f).
In contrast to the high yields observed for the reaction with an
a
Conditions: aldehyde (0.20 mmol), azide (0.30 mmol), and MgSO4
b
(100 mg) in DCE (4.0 mL) for 20 h. Isolated yields are shown.
c
Formed in a 7:1 ratio with the 8-methyl-substituted isomer; the
combined yield is shown.
B
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Journal of the American Chemical Society
Communication
a b
,
ortho-substituted benzaldehyde (see 3g in Table 2), ortho
substitution on the ketimine resulted in a modest yield (6g),
although ortho substitution on the aromatic azide provided 6i in
high yield. An electron-deficient phenyl azide (6k) provided the
product in a moderate yield compared with the very good yields
observed for electron-rich (6j) and -neutral (6a−f) azides. A
heterocyclic azide (azidothiophene) afforded 6h in high yield.
Having established the broad scope of the synthesis of
acridines with or without substitution at the central 9-position,
we next considered the possibility of extending this formal [3 +
3] annulation approach to the synthesis of phenazines 8 from
azobenzenes 7 (Table 4). The conditions previously optimized
Table 5. Scope of Phenazine Synthesis
a
Table 4. Optimization of Phenazine Synthesis
a
Conditions: azobenzene (0.20 mmol) and azide (0.30 mmol) in
b
c
AcOH (4.0 mL) for 24 h. Isolated yields are shown. 8:1 ratio with
the separable 6-methyl-substitued minor isomer.
electron-withdrawing (8c, 8f, 8g−k) and -neutral (8a, 8b, 8e)
groups. Substitution at the ortho (8f), meta (8e), and para (8b−
d, 8g−k) positions was tolerated, and m-methyl substitution
produced a single isomer with amination at the least hindered site
(8e). Both electron-deficient (8a−h) and -neutral (8j, 8k) azides
were effective coupling partners, and ortho substitution was not
detrimental to the reaction yield (8k).
a
Conditions: 7 (0.10 mmol) and 2a (0.15 mmol) in solvent (2.0 mL)
b
for 24 h. 1H NMR yields using 2,6-dimethoxytoluene as an external
standard. Isolated yield at 0.20 mol scale of 7a.
The proposed mechanistic pathway for this cascade reaction is
shown in Scheme 1. Imine or azobenzene 9 undergoes ortho-
c
Scheme 1. Proposed Cascade Mechanism
for ketimines provided product 8a in low yield whether or not
acetic anhydride was used as an additive (entries 1 and 2). Glacial
acetic acid was evaluated as a solvent because we envisioned that
it might facilitate cyclization as well as sequester the released
aniline by hydrogen bonding or salt formation (entry 3).
Encouraged by the considerably improved yield, we next
explored alternative counterions. While the use of (Cp*RhCl2)2
and AgSbF6 resulted in a yield comparable to that observed with
the corresponding preformed catalyst (entry 4), use of the
completely noncoordinating counterion B(C6F5)4 resulted in a
significant improvement, providing 8a in 67% yield (entry 5). A
comparable yield was also obtained when unsymmetrical
azobenzene 7b was used (entry 6). With this substrate, the
reaction exclusively occurred on the unsubstituted ring,
consistent with the strong steric bias against Rh(III)-selective
C−H functionalization adjacent to a meta substituent.6a
We evaluated the reaction scope by preparing unsymmetrical
bis-substituted derivatives for which the regioselective placement
of functionality can be challenging using alternative methods
(Table 5).17 Unsymmetrical azobenzenes with 3,5-dimethylani-
line acting as a directing group were employed because this type
of azobenzene can readily be prepared by simple condensation
between commercially available anilines and 3,5-dimethylni-
trosobenzene. Consistent with the results for the acridine
synthesis, good functional group compatibility was observed, as
products with bromo (8b), chloro (8i), fluoro (8f), trifluor-
omethyl (8c), methoxy (8d), keto (8b−g), and ester (8a, 8h−k)
substitution were obtained. Azobenzenes with diverse electronic
properties proved to be effective substrates, although electron-
donating groups (8d) provided significantly lower yields than
directed C−H bond activation to give metallacycle 1019 followed
by coordination and migratory insertion of the azide to afford
metallacycle 12. This sequence of reactions corresponds to
mechanisms previously proposed for other Rh(III)-catalyzed
reactions with organic azides11a−c and is also consistent with the
lack of reactivity of the aromatic azide with the Rh(III) catalyst
unless the azobenzene or imine substrate is present. Protonation
of metallacycle 12 then releases diarylamine 13 and the Rh(III)
catalyst. Under the reaction conditions, 13 undergoes intra-
molecular electrophilic aromatic substitution to give 14, and
subsequent aromatization gives the desired acridine or phenazine
15. Under the standard conditions, 13 did not accumulate even
for the coupling of electron-deficient aryl azide 2b with
azobenzene 7b. However, when these coupling partners were
allowed to react at 90 °C for 7 h, a trace amount of 13 and ∼10%
phenazine 8a were detected by 1H NMR and LC-MS analyses.
C
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Journal of the American Chemical Society
Communication
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When the reaction was repeated on a larger scale, chromatog-
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material 7 along with a 9% yield of product 8a and ∼1%
uncyclized diarylamine 13.
In summary, formal [3 + 3] annulations of aromatic azides
with imines to give acridines and with azobenzenes to give
phenazines have been developed. These transformations proceed
by Rh(III)-catalyzed ortho C−H amination followed by
intramolecular electrophilic aromatic substitution and aromati-
zation. A broad range of acridines and phenazines can be
generated with precise placement of diverse functionality,
including unsymmetrical disubstituted derivatives. Moreover,
through the use of catalytic benzylamine to generate the requisite
imine in situ, aromatic aldehydes can be used to rapidly and
directly access acridines lacking substitution at the 9-position.
ASSOCIATED CONTENT
* Supporting Information
Procedures and spectral data. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
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(b) Bhanuchandra, M.; Yadav, M. R.; Rit, R. K.; Kuram, M. R.; Sahoo, A.
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AUTHOR INFORMATION
Corresponding Author
jonathan.ellman@yale.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by NIH (GM069559 to J.A.E.). R.G.B.
was supported by DOE (DE-AC02-05CH11231).
■
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