Rhodium(III)-Catalyzed Annulation of N-Methoxybenzamides with Heterobicyclic Alkenes by C–H Functionalization: Synthesis of Benzo[b]phenanthridinones

Article abstract of DOI:10.1002/ejoc.201700029

A protocol for the rhodium(III)-catalyzed annulation of N-methoxybenzamides with 7-oxa/azabenzonorbornadienes by C–H activation was developed. This method allowed the synthesis of polycyclic products containing an oxygen/nitrogen-bridged structure in excellent yields. The annulated products were successfully transformed into benzo[b]phenanthridinones by a ring-opening/aromatization sequence.

Full text of DOI:10.1002/ejoc.201700029

A Journal of
Accepted Article
Title: Rhodium(III)-Catalyzed Annulation of N-Methoxybenzamides with
Heterobicyclic Alkenes by C-H Functiona¬lization: Synthesis of
Benzo[b]phenanthridinones
Authors: Ying Cheng, Kanniyappan Parthasarathy, and Carsten Bolm
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201700029
Link to VoR: http://dx.doi.org/10.1002/ejoc.201700029
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10.1002/ejoc.201700029
European Journal of Organic Chemistry
COMMUNICATION
Rhodium(III)-Catalyzed Annulation of N-Methoxybenzamides with
Heterobicyclic Alkenes by C–H Functionalization: Synthesis of
Benzo[b]phenanthridinones
Ying Cheng, Kanniyappan Parthasarathy, and Carsten Bolm*
Abstract:
A
rhodium(III)-catalyzed annulation of N-methoxy-
benzamides with 7-oxa/azabenzonorbornadienes by C-H activation
is described. This method allows the synthesis of polycyclic products,
which contain oxygen/nitrogen bridged structure, in excellent yields.
The annulated products were successfully transformed into benzo-
[b]phenanthridinones by ring-opening/aromatization sequences.
a) Earlier methods
O
O
OMe
N
Y
H
Pd(II), Rh(III)
N
no further
functionalization
R
+
R''
R
R'
R'
R''
Y = H or OMe
b) This report
Transition metal-catalyzed C–H bond activation and annulations
with organic π-components provide an efficient method for the
construction of diverse heterocyclic compounds.[1] A variety of
organic π-components, such as alkynes,[2] alkenes,[3] and
allenes[4] have been used for such annulations. Aside from the
above organic π-components, a few examples involved ortho C-
H additions to heterobicyclic alkenes.[5] Thus, Qi and Li reported
O
H
X
O
X
N
OMe
+
Rh(III) cat
N
H
R
R
O
X = O, N-R
NH
MeSO3H, CHCl3
reflux, 2 h
then NEt3, 0 °C
R
a
rhodium(III)-catalyzed syntheses of ortho-naphthylated
products and cis-fused dihydrocarbazole derivatives by additions
of 2-arylpyridines onto heterobicyclic alkenes.[6] Later, we
developed rhodium-catalyzed hydroarylations of heterobicyclic
alkenes with NH-sulfoximines by ortho C–H functionalizations.[7]
Subsequently, we reported ruthenium(II)-catalyzed hydro-
arylations of strained oxa- and azabicyclic alkenes by C−H bond
activation.[8] Shortly after, Miura and Satoh have achieved
rhodium(III)-catalyzed direct coupling of arylphosphine oxides
with heterobicyclic alkenes.[9] Using cobalt-catalyzed C–H
activations, Cheng and co-workers recently developed a method
for the synthesis of dihydroepoxybenzofluorenone derivatives by
[3+2] annulations of aromatic and vinylic amides with bicyclic
alkenes,[10] and Li and co-workers reported couplings of N-
pyrimidinylindoles with 7-oxabenzonorbornadiene leading to 2-
naphthyl-N-pyrimidinylindoles.[11]
While annulations of N-methoxybenzamides with alkenes by
using Pd(II)[12] or Rh(III)[13] complexes have been reported
previously (Scheme 1 a), strained oxa- and azabicyclic carbon–
carbon double bonds have only scarcely been applied in such
ring formations. Combining both approaches, we have now
developed a rhodium(III)-catalyzed annulation of N-methoxy-
benzamides with 7-oxa/azabenzonorbornadienes by ortho C-H
activation leading to highly functionalized heterocycles that can
Scheme 1. Summary of previous and current work.
(1a) and 7-oxabenzonorbornadiene (2a) were used as
representative substrates, and the effect of rhodium complex,
oxidant/additive was examined (Table 1). A brief screening
revealed that the combination of [RhCp*Cl2]2/Cu(OAc)2 was
most effective forming annulation product 3aa in 97% yield
(Table
1,
entry
1).
Substituting
[RhCp*Cl2]2
by
[Cp*Rh(MeCN)3][SbF6]2 or [Cp*Rh(MeCN)3][BF4]2 gave 3aa in
lower yields (Table 1, entries 2 and 3). Rhodium(I) complexes
[RhCl(COD)]2, and Rh(PPh3)3Cl did not show any catalytic
activity for annulations (Table 1, entries 4 and 5). Next, the effect
of the oxidant/additive was examined. Changing from Cu(OAc)2
to Fe(OAc)2 afforded 3aa in only 44% yield (Table 1, entry 6).
Using CsOAc, KOAc, and NaOAc instead of Cu(OAc)2 proved
ineffective (Table 1, entries 7-9).
Table 1. Optimization studies for the annulation of N-methoxybenzamide (1a)
with 7-oxabenzonorbornadiene (2a).[a]
O
H
N
O
O
O
Rh-cat
OMe
+
N
oxidant/ additive
t-AmylOH, air, 60 °C
H
1a
2a
3aa
further
be
transformed
into
a
synthetically
straightforward manner
relevant
benzo[b]phenanthridinones in
Entry
Catalyst
[RhCp*Cl2]2
[Cp*Rh(MeCN)3][SbF6]2
Oxidant/Additive
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Fe(OAc)2
CsOAc
t (h)
Yield [%]
(Scheme 1 b). The first results are reported here.
1
2
3
4
5
6
7
3
97
62
58
0
To find suitable reaction conditions, N-methoxybenzamide
[b]
3
[b]
Dr. Y. Cheng, Dr. K. Parthasarathy,[#] Prof. Dr. C. Bolm
Institute of Organic Chemistry, RWTH Aachen University
Landoltweg 1, 52056 Aachen (Germany)
[Cp*Rh(MeCN)3][BF4]2
3
[*]
[b]
[RhCl(COD)]2
12
12
12
12
E-mail: carsten.bolm@oc.rwth-aachen.de
Rh(PPh3)3Cl[b]
[RhCp*Cl2]2
[RhCp*Cl2]2
0
[#]
New address: Department of Organic Chemistry, University of
Madras, Guindy Campus, Chennai-600025 (India)
44
trace
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/ejoc.201700029
European Journal of Organic Chemistry
COMMUNICATION
The results can be summarized as such: N-Methoxybenzamides
bearing both electron-donating and electron-withdrawing groups
at various positions of the arene (1a−1n) reacted smoothly with
7-oxabenzonorbornadiene (2a), affording the corresponding
annulated products (3aa-3na) in yields of 83-99%. Notably in
addition, meta-substituted N-methoxybenzamide (1l) gave a
single regioisomeric product (3la) in 98% yield. Also 1,3-
8
9
[RhCp*Cl2]2
[RhCp*Cl2]2
KOAc
12
12
trace
n.d.[c]
NaOAc
[a] Unless otherwise mentioned, all reactions were carried out using 0.30
mmol of 1a, 0.36 mmol of 2a, 2 mol % of catalyst, 2.0 equiv of oxidant/additive
and t-amyl alcohol (1.5 mL) at 60 °C. [b] Use of 5 mol % of catalyst. Cp* =
pentamethylcyclopentadiene. [c] Not detectable.
benzodioxol-5-yl-
and
2-naphthyl-substituted
N-
With this set of optimized reaction conditions, we further
investigated the substrate scope employing a variety of N-
methoxybenzamides (1a−1n) in their coupling with 2a. The
results are summarized in Scheme 2.
methoxybenzamides, 1m and 1n, reacted well providing only
one regioisomeric products (3ma and 3na) in yields of 90% and
99%, respectively.
In addition to 2a, heterobicyclic alkenes 2b-h were tested in
the annulation reaction. As depicted in Scheme 3, 7-oxabenzo-
norbornadienes 2b-d with electron-donating groups on the
arenes showed good reactivity with 1a, affording the
corresponding 3ab-3ac in high yields (93-94%). Alkenes bearing
naphthyl and phenanthyl substituent (2e and 2f) reacted
smoothly to give 3ae and 3af in 82% and 84% yield, respectively.
Finally, 7-azabenzonorbornadienes 3g and 3h afforded products
3ag and 3ah in 96% and 95% yield.
O
[Cp*RhCl2]2 (2 mol%)
Cu(OAc)2 (2.0 equiv)
O
O
NH
R1
O
OMe
+
N
R1
t-AmylOH, air, 60 °C
H
1
2a
3
O
O
O
NH
NH
O
NH
O
O
O
MeO
Me
H
X
O
X
R2
R3
[Cp*RhCl2]2 (2 mol%)
Cu/OAc)2 (2.0 equiv)
N
OMe
N
R3
+
H
R2
R3
t-AmylOH, air, 60 °C
3aa, 97%
3ba, 95%
3ca, 87%
R2
R2
X = O, NR
R3
1a
2
3
O
O
O
NH
NH
NH
O
O
O
O
O
O
O
tBu
I
Br
NH
O
NH
NH
NH
O
O
O
3da, 94%
3ea, 96%
3fa, 91%
OMe
Me
O
OMe
Me
3ac, 94%
O
O
O
O
3ab, 94%
3ad, 93%
3ae, 82%
NH
NH
NH
O
O
O
O
Cl
F
MeOOC
O
O
NH
O
NH
NH
N
N
COOMe
Boc
3ga, 90%
3ha, 88%
3ia, 91%
O
O
O
Me
3af, 84%
3ag, 96%
3ah, 95%
NH
NH
NH
O
O
O
O2N
Ph
Scheme 3. Results of rhodium-catalyzed C–H activation/annulation of N-
methoxybenzamide (1a) and various 7-oxa/azabenzonorbornadienes.
3ja, 83%
3ka, 94%
3la, 98%
O
O
A
plausible mechanism for the C–H bond activation/
O
O
annulation process providing product 3aa is shown in Scheme 4.
We assume that the catalysis proceeds via intermediate B,
which is formed from N-methoxybenzamide (1a) and rhodium
species A obtained by anion exchange between Cu(OAc)2 and
[Cp*RhCl2]2. Subsequent electrophilic C–H bond cleavage
affords five-membered rhodacycle C with release of AcOH.
Coordination of C to the double bond of the diene, here 7-
oxabenzonorbornadiene (2a), gives intermediate D. Insertion of
the coordinated diene into the rhodium–carbon bond of D
provides seven-membered rhodacycle E, which upon reductive
NH
NH
O
O
3ma, 90%
3na, 99%
Scheme 2. Results of rhodium-catalyzed C–H activation/annulation of various
N-methoxybenzamides and 7-oxabenzonorbornadiene (2a).
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10.1002/ejoc.201700029
European Journal of Organic Chemistry
COMMUNICATION
elimination and oxidative addition into the N–O bond affords
intermediate F.[14] Protonolysis with HOAc then leads to product
3aa and regenerates the initial Rh(III) species entering the next
catalytic cycle.[15]
sequences occurred providing benzo[b]phenanthridinones 5a-c
in 88-95% yield (Scheme 6). Deprotonation of 5a with sodium
hydride in DMF and reaction of the resulting anion with 1-
iodobutane then gave 81% of product 6 with known anti-hepatitis
C virus (HCV) activity (EC50: 3.2 µM).[17]
[Cp*RhCl2]2
Cu(OAc)2
MeOH, 3aa
O
1a
O
RhCp*(OAc)2
MeSO3H
CHCl3, 70 °C
NH
OMe
Rh
O
HOAc
O
O
A
NH
O
R1
2 HOAc
Cp*
R1
N
then Et3N
OMe
Cp*
N
Rh
AcO
F
3aa: R1 = H
3ca: R1 = Me
3ea: R1 = I
5a: R1 = H, 88%
5b: R1 = Me, 93%
5c: R1 = I, 95%
B
O
OMe
HOAc
O
N
N
Me
Rh Cp*
O
NaH
O
1-iodobutane
5a
N
Rh
Cp*
OMe
an anti-hepatitis
C virus (HCV)
active compound
O
DMF, 0 °C
E
C
6, 81%
N
Rh
OMe
O
2a
*Cp
Scheme 6. Syntheses of benzo[b]phenanthridinones 5a-c from products 3aa,
3ca and 3ea; preparation of anti-hepatitis C virus active compound 6.
D
Scheme 4. Proposed mechanistic pathway for the formation of 3aa.
In summary, we developed
a
rhodium(III)-catalyzed
annulation reaction with 7-oxa/azabenzonorbornadienes
involving C-H activations of N-methoxybenzamides. This method
provides polycyclic products containing oxygen/nitrogen-bridged
core structures in excellent yields. The annulated products were
successfully transformed into benzo[b]phenanthridinones by
ring-opening/aromatization sequences. Further, the synthetic
protocol was applied to an anti-HCV agent.
Finally, we briefly investigated the synthetic applicability of
products 3. With the intention to develop a one-pot synthesis of
benzo[b]phenanthridinones 5 via 3 starting from N-methoxy-
benzamides 1 and azabenzonorbornadienes 2, a mixture of 1a
and 2g was subjected to the standard catalyst/reagent
combination at 120 °C for 3 h (Scheme 5). However, instead of
the expected benzo[b]phenanthridinone 5 isoquinolin-1(2H)-one
(4) was obtained (in 92% yield). The same product was isolated
(in 90% yield) when 2h was applied instead of 2g. Thus both
reactions appeared to involve the expected annulated products
3ag and 3ah, but at the high temperature retro-Diels-Alder
reactions leading to 4 occurred faster than the intended ring
opening/deamidation sequence providing 5.[13a,16]
Experimental Section
General procedure: N-Methoxybenzamide 1 (0.30 mmol), heterobicyclic
alkene 2 (0.36 mmol), [Cp*RhCl2]2 (3.7 mg, 2 mol%), and Cu(OAc)2
(109.0 mg, 0.60 mmol) were placed in a Schlenk tube (25 mL) with a stir
bar. In air, tert-amyl alcohol (1.5 mL) was added by syringe, and the
reaction mixture was stirred at 60 °C for 3 h. Then, it was cooled to room
temperature and diluted with ethyl acetate (10 mL). The mixture was
filtered through a Celite pad and washed with ethyl acetate (3 x 20 mL).
The filtrate was concentrated, and the product was purified by column
chromatography using Et3N-deactivated silica gel.
O
O
+
[Cp*RhCl2]2 (2 mol%)
Cu(OAc)2 (2.0 equiv)
NH
OMe
N
H
1a
t-AmylOH, air, 120 °C
5 (not observed)
X
N
O
Acknowledgements
2g: X = COOMe
2h: X = Boc
NH
3ag or 3ah
from 2g: 92%
from 2h: 90%
Y.C. thanks the China Scholarship Council for pre-doctoral
stipends. K.P. greatly acknowledges support by the Alexander
von Humboldt foundation.
4
Scheme 5. Retro-Diels-Alder reactions of products.
Keywords: rhodium • N-methoxybenzamides • 7-
oxa/azabenzonorbornadienes • benzo[b]phenanthridinones
Benzo[b]phenanthridinones 5 could finally be prepared by
starting from 7-oxa cyclic products 3aa, 3ca and 3ea. Upon
treatment of these compounds with methyl sulfonic acid in
chloroform at 70 °C, smooth ring-opening/aromatization
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10.1002/ejoc.201700029
European Journal of Organic Chemistry
COMMUNICATION
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10.1002/ejoc.201700029
European Journal of Organic Chemistry
COMMUNICATION
COMMUNICATION
O
O
O
Ying Cheng, Kanniyappan
Parthasarathy, and Carsten Bolm*
O
OMe
N
Bu
NH
N
NH
H
X
Rh(III)
+
R
R
R
R
Page No. – Page No.
X
R'
R'
R'
Rhodium(III)-Catalyzed Annulation of
N-Methoxybenzamides with
Heterobicyclic Alkenes by C–H
Functionalization: Synthesis of
Benzo[b]phenanthridinones
R = H
HCV active compound
X = O, N-R
R'
Polycyclic products, active intermediates: Rhodium-catalyzed annulation
reactions provide polycyclic products in excellent yields starting from N-
methoxybenzamides with 7-oxa/azabenzonorbornadienes. The products retain the
oxygen/nitrogen-bridged structures, which show possible reactivity for further
transformations.
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