Substituent-guided switch between C-H activation and decarboxylative cross-coupling during palladium/copper-catalyzed cascade reactions of 2-aminobenzoates with 2-haloarylaldehydes

Article abstract of DOI:10.1002/chem.201301705

Cascade switch: Phenanthridines, pyrazole[4,3-c]quinolines and isocryptolepine were prepared in one step from the Pd/Cu-catalyzed reaction between potassium 2-aminobenzoates and 2-haloarylaldehydes (see scheme). Although the reactions of 2-aminobenzoates proceeded via a cascade imination/C-H functionalization, the reactions of 6-nitro-2-aminobenzoates ensued via a tandem imination/decarboxylative cross-coupling. Copyright

Full text of DOI:10.1002/chem.201301705

COMMUNICATION
Synthetic Methods
S. Bhowmik, G. Pandey,
S. Batra* ....................... &&&& — &&&&
Substituent-Guided Switch between
Cascade switch: Phenanthridines,
pyrazole[4,3-c]quinolines and isocryp-
tolepine were prepared in one step
from the Pd/Cu-catalyzed reaction
between potassium 2-aminobenzoates
and 2-haloarylaldehydes (see scheme).
Although the reactions of 2-aminoben-
zoates proceeded via a cascade imina-
À
C H Activation and Decarboxylative
ACHTUNGTRENNUNG
Cross-Coupling during Palladium/
Copper-Catalyzed Cascade Reactions
of 2-Aminobenzoates with 2-Halo-
À
tion/C H functionalization, the reac-
tions of 6-nitro-2-aminobenzoates
ensued via a tandem imination/decar-
boxylative cross-coupling.
AHCTUNGTREGUNaNN rylaldehydes
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DOI: 10.1002/chem.201301705
À
Substituent-Guided Switch between C H Activation and Decarboxylative
Cross-Coupling during Palladium/Copper-Catalyzed Cascade Reactions of
2-Aminobenzoates with 2-HaloACHTNUTRGNEaNUG rylaldehydes
Subhendu Bhowmik,[a] Garima Pandey,[a] and Sanjay Batra*[a, b]
The decarboxylative cross-coupling reactions between
haloarene and arene carboxylic acid have emerged as an ex-
ACHTUNGTRENNUNG
citing alternative to the traditional cross-couplings for the
construction of biaryls.[1,2] The strategy allows overcoming
the intrinsic disadvantages of cross-coupling reactions, such
as the necessity of employing an organometallic reagent
À
(Ar M) and the generation of stoichiometric amounts of
À
salts (M X). In addition, the use of carboxylic acids as nu-
cleophilic partner in the cross-coupling reaction confers sig-
nificant advantage in terms of cost, availability and ease of
use. Further, extrusion of CO2 as the waste product during
the reaction is considered to be environmentally benign.
The transformation typically employs bimetallic catalysis,
though for certain carboxylic acids the decarboxylation and
[2b,3]
Figure 1. Comparison of different intramolecular decarboxylative cross-
coupling protocols.
À
C C bond formation by same metal have been reported.
The bimetallic catalysis includes a conventional palladium
cycle to activate the electrophile and a copper- or silver-
mediated decarboxylative process. The synthetic potential of
the bimetallic approach has been realized by treating differ-
ent aryl halides,[4] tosylates[5] and triflates[6] with aromatic
and heterocyclic carboxylates in intermolecular fashion for
accessing biaryls, including drug intermediates.[7] In contrast,
only a few examples of intramolecular decarboxylative cou-
pling of benzoic acids are reported. The intramolecular de-
carboxylative arylation of benzoic acids with aryl halides
was successful with Pd-catalyst only while the intramolecu-
lar direct arylation of benzoic acids via tandem decarboxyla-
and an intramolecular decarboxylative cross-coupling reac-
tion still remains a target in organic synthesis. Recently,
Deng et al. reported a two-component tandem synthesis of
dibenzopyranone, but their approach involved initial inter-
molecular decarboxylative coupling followed by intramolec-
ular lactonization.[10] As part of our research program direct-
ed towards development of transition-metal based cascade
protocols for the synthesis of aza-heterocyles,[11] we sought
to investigate the reaction between potassium 2-aminoben-
zoates and 2-halobenzaldehydes in the presence Pd/Cu bi-
metallic catalyst system. We envisaged that in a cascade
event the aldimine formation may be followed by an intra-
molecular decarboxylative cross-coupling resulting in phe-
nanthridines. Working towards this objective, we have dis-
covered that absence of a nitro group ortho to the carboxy-
À
tion/C H activation was achieved with Pd/Ag bimetallic cat-
alyst.[3b,c,8,9] Noticeably, these intramolecular decarboxylative
cross-coupling protocols are limited to one-component reac-
tion only (Figure 1). Therefore, a two-component tandem
protocol that combines a functional group transformation
À
late promotes tandem imination/C H activation whereas
the presence of nitro group drives the reaction via imina-
tion/decarboxylative cross-coupling sequence to afford phe-
nanthridine derivatives. The approach can be extended to
efficient one-pot synthesis of pyrazoloACTHNUTRGNEUNG[4,3-c]quinoline and
isocryptolepine derivatives. Herein we present the details of
our preliminary results concerning this study.
Initially to test the feasibility of our strategy in a pilot ex-
periment, we treated potassium 2-aminobenzoate 1a with 2-
bromobenzaldehyde 2A in the presence of PdCl2 (5 mol%),
Cu2O (5 mol%), TBAB (10 mol%), 1,10-phenanthroline
(6 mol%) in DMF/NMP (9:1) as medium at 1508C. The re-
action was completed in 24 h and afforded a product in
[a] S. Bhowmik, G. Pandey, Dr. S. Batra
Medicinal and Process Chemistry Division
CSIR-Central Drug Research Institute
PO Box 173, Lucknow 226031 (India)
Fax: (+91)522-2771941
E-mail: batra_san@yahoo.co.uk
[b] Dr. S. Batra
Academy of Scientific and Innovative Research
New Delhi (India)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301705.
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Palladium/Copper-Catalyzed Cascade Reactions
COMMUNICATION
Although the preformed benzoimines have been used for
À
À
the synthesis of phenanthridines by C H activation/C C
cross-coupling,[14] to the best of our knowledge this consti-
À
tutes the first example of cascade aldimine formation/C H
À
activation/C C cross-coupling for the synthesis of phenan-
[15]
À
thridine. Given the significance of C H activation and the
importance of phenanthridine-based compounds in medici-
nal and materials chemistry for their biological activities[16]
and optoelectronic properties,[17] we were motivated to study
the scope of the reaction in detail.
After screening several combinations of palladium and
copper catalysts for the reaction of 1a with 2A, we found
that the best yields for 4aA (82%) were achieved when the
reaction was performed in the presence of Pd
ACHTUGNRTNE(NUNG PPh3)2Cl2
(1.5 mol%), Cu2O (1 mol%), 1,10-phenanthroline
Scheme 1. Synthesis of phenanthridine-4-carboxylic acid.
(2 mol%), TBAB (5 mol%), 4 ꢁ mol. sieves in dry DMF at
1608C under MW for 15 min (see the Supporting Informa-
tion). It is worth mentioning that the reaction was successful
only when the bimetallic catalyst system was used.
56% yield. The structure of the isolated product was estab-
lished as phenanthridine 4-carboxylic acid 4aA instead of
the expected phenanthridine 3 (Scheme 1). The presence of
the carboxylic acid group in 4aA was ascertained by trans-
forming it to the methyl ester 5. From this result it was infer-
À
To examine the scope of the tandem imination/C H acti-
vation reaction, various potassium 2-aminobenzoates (1a–e)
and 2-bromobenzaldehydes (2A–E) were treated under the
optimized conditions and the results are presented in
Scheme 3. All reactions of potassium 2-aminobenzoate (1a)
with differently substituted 2-bromobenzaldehydes (2B–E)
proceeded smoothly to afford the respective products (4aB,
À
red that the reaction proceeded via a tandem imination/C
H activation sequence rather than the expected tandem imi-
nation/decarboxylative cross-coupling sequence. Reports
demonstrating decarboxylative coupling being facilitated
under microwave (MW) conditions,[3d,5b,12] prompted us to
repeat the experiment under microwave irradiation at
1608C using molecular sieves as additive. The reaction was
expedited to be complete in 15 min, but the isolated product
was again 4aA, though in improved yield (66%). In the
light of these results, we sought to evaluate the potential of
the catalyst system for decarboxylative coupling. In this con-
text, we performed a reaction between potassium 2-nitro-
benzoate 6 and 2-bromobenzaldehyde 2A in the presence of
the bimetallic system under microwave since the nitro group
is known to enhance the copper-ligating quality of the car-
boxylate substrate.[4b] Pleasingly, the reaction was complete
in 10 min to afford the required 2’-nitrobiphenyl-2-carbalde-
hyde (7) in 90% yield (Scheme 2). Reductive cyclization of
7 with Fe-AcOH furnished the expected phenanthridine (3)
in 89% yield.[13] The result proved that though the bimetal-
lic catalyst system is suitable for the decarboxylative cou-
À
pling, the reaction proceeds via a tandem imination/C H ac-
tivation sequence even in the presence of a carboxylate
group.
Scheme 2. Two-step synthesis of phenanthridine.
Scheme 3. Scope of the synthesis of phenanthridine-4-carboxylic acids.
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S. Batra et al.
4aC, 4aD, 4aE) in good yields. Conversely, phenyl ring
bearing different substitutions in potassium 2-aminoben-
zoates produced varying results. The reactions of 2A and
2B with 1d carrying electron donating group proceeded in
identical fashion to afford the respective products 4dA and
4dB in good yields, but the reactions of 2A, 2B and 2D
with 1b and 1c bearing mild electron withdrawing groups
took longer (ca. 25 min) to complete and furnish the prod-
ucts (4bA, 4bB, 4bD, 4cA) in relatively lower isolated
yields. Notably, the newly formed phenanthridines 4bA,
4bB, 4bD and 4cA were sparingly soluble in organic sol-
vents. However, potassium 5-nitro-2-aminobenzoate (1e)
failed to react with 2A to give the expected product 4eA.
Perhaps the difficulty in the formation of aldimine with 5-
nitro-2-aminobenzoate could be attributed to the failure of
reaction with this substrate. Besides 2-bromobenzaldehyde,
experiments with 2-chlorobenzaldehyde and 2-iodobenzal-
dehyde revealed that although the former afforded the phe-
nanthridine-4-carboxylic acid in 20% yield, the latter failed
to react.
Scheme 5. Synthesis of isocryptolepine derivatives.
strategy, the reaction of 1a with N-methyl-2-bromo-3-indole-
carbaldehyde (10b) was performed, and smoothly furnished
the product 11b. In order to probe the feasibility of this re-
action with other halides, the chloro-derivatives 12a and
12b were prepared and treated with 1a. Unlike 2-chloro-
ACHUTNGERNbNUG enzHCATUNGTRENNaUGN ldehyde, these substrates smoothly gave the corre-
sponding products 11a and 11b, respectively, in good yields.
A plausible mechanism for the formation of phenanthri-
dine 4-carboxylic acid is illustrated in Scheme 6. Although
In our continuing interest for the synthesis of pyrazole-an-
nulated ring systems,[17] we next investigated several substi-
tuted 4-iodo-3-pyrazolecarbaldehydes as the starting materi-
als in the protocol. Accordingly, aldehydes 8A–D were
treated with potassium 2-aminobenzoate (1a) under the
standardized conditions. To our delight all reactions were
successful in furnishing the corresponding products 9aA,
9aB, 9aC and 9aD in good yields (Scheme 4).
Scheme 6. Plausible mechanism for the intramolecular cyclization via
À
tandem imination/C H activation.
potassium 2-aminobenzoate undergoes a cation exchange
with copper to afford intermediate I, Pd0 undergoes oxida-
tive addition to the 2-bromobenzaldehyde to form II. Subse-
quently, the imine intermediate III is formed via a dehydra-
tion step between I and II. It is speculated that the copper
carboxylate forms a six-member transition state with the
imine nitrogen, rendering it unavailable for decarboxylation.
Scheme 4. Scope of the synthesis of pyrazolo-[4,3-c]quinolines.
To further demonstrate the potential of the current two-
component cyclization reaction, we implemented this proto-
col for the synthesis of isocryptolepine analogues.[18] Treating
2-bromo-3-indolecarbaldehyde (10a) with 1a under the
standardized conditions resulted in the expected isocryptol-
epine derivative 11a in 65% yield (Scheme 5). It is worth
noting that the tandem sequence was successful with the un-
protected indole. However, to extend the usefulness of the
À
Simultaneously, the C H bond in the phenyl ring is activat-
ed and eliminates the bromine in the form of HBr, leading
to the formation of IV. Reductive elimination in IV results
in product V.
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COMMUNICATION
Finally, the mechanistic considerations generated interest
in investigating the fate of this tandem protocol in potassi-
um 2-amino-6-nitrobenzoate (1 f). Treating 1 f with 2-bromo-
benzaldehyde (2A) under the optimized conditions furnish-
ed a product (84%) within 15 min. Pleasingly, the structure
of the isolated product was established as 1-nitrophenanthri-
dine 4 fA, which implies that the reaction proceeds via a
tandem imination/decarboxylative sequence instead of the
À
imination/C H activation sequence. Buoyed with this suc-
cess, we tested the scope of the protocol by treating 1 f with
different 2-haloarylcarbaldehydes 2A,B,D,E and 7A,C
(Scheme 7). Gratifyingly, all the substrates underwent the
Scheme 8. Plausible mechanism for the formation of 1-nitrophenanthri-
dines via tandem imination/decarboxylative cross-coupling.
pot efficient synthesis of substituted pyrazoloACHTNUTRGNE[NUG 4,3-c]quino-
lines and isocryptolepines. The products generated herein
bear functional groups viable for further derivatization. The
simple starting materials, low catalyst loading, and short re-
action times are some of the promising features of this
methodology. In perspective, it would be interesting to
assess the scope of this method with 2-amino
ACHUTGTNREN(NUG het)ACHTUNGTRENNUNGaryl-
AHCTUNGTREGbNNNU enzoates and other strong coordinating functional groups
in place of the nitro group in potassium 2-aminobenzoates.
Work toward these objectives is underway and will be the
subject of future communication from our group.
Experimental Section
Scheme 7. Scope of the synthesis of 1-nitrophenanthridines and pyrazole-
ACHTUNGTRENNUNG[4,3-c]-quinolines.
Representative procedure: A microwave crimp top vial equipped with a
stir bar was charged with PdACTHNUTRGNEU(GN PPh3)2Cl2 (24.0 mg, 3 mol%), Cu2O (2.4 mg,
1.5 mol%), 1,10-phenanthroline (9.02 mg, 4 mol%), TBAB (37.0 mg,
10 mol%), potassium 2-aminobenzoate (1a; 0.2 g, 1.14 mmol) and 2-bro-
mobenzaldehyde (2A, 1.14 mmol, 0.13 mL), 4 ꢁ mol. sieves (0.2 g) in dry
DMF (2 mL). The resulting mixture was heated at 1608C for 15 min
under microwave irradiation. The DMF was then evaporated under re-
duced pressure and the residue was purified by silica gel column chroma-
tography (ethyl acetate/hexane, 30:70, v/v), yielding 4aA (0.21 g, 82%)
as a brown solid.
desired transformation to give the corresponding products
4 fB, 4 fD, 4 fE, 9 fA and 9 fC in 69–86% yields.
A plausible mechanistic pathway for the formation of 1-
nitrophenanthridine from potassium 6-amino-2-nitroben-
zoate is outlined in Scheme 8. It is assumed that the forma-
tion of imine III is followed by a decarboxylative process,
which is facilitated by the coordinating property of the nitro
group leading to the intermediate IV. This intermediate
then undergoes the usual transmetallation to give V fol-
lowed by reductive elimination resulting in product VI.
In conclusion, we have developed a Pd/Cu-catalyzed two-
component cyclization reaction, which can be tuned to pro-
Acknowledgements
S.B. and G.P. acknowledge the financial support from CSIR and UGC,
New Delhi in the form of fellowships. The authors gratefully acknowl-
edge the Sophisticated Analytical Instrumentation Facility of CSIR-
CDRI for providing the spectroscopic data (CDRI communication no.
8456). S.B. acknowledged funding from DST, New Delhi.
À
ceed either via tandem imination/C H activation or imina-
tion/decarboxylative cross-coupling sequence to afford sub-
stituted phenanthridines. The protocol is amenable to one-
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Keywords: C H activation · copper · decarboxylative cross-
coupling · nitrogen heterocycles · palladium
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Received: May 3, 2013
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