Ni(0)-Cu(i): a powerful combo catalyst for simultaneous coupling and cleavage of the C-N bond with cyclization to valuable amide-based pyrroles and 4-pyridones

Article abstract of DOI:10.1039/c5ra12115a

Unprecedented ligand-tuned Ni(0)-Cu(i) combo catalysis is demonstrated via sequential bond activated domino C-N/C-C coupled annulation with C-N bond cleavage to afford valuable polysubstituted amide-based pyrroles and 4-pyridones selectively from β-ketoan

Full text of DOI:10.1039/c5ra12115a

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Ni(0)-Cu(I): A powerful combo catalyst for simultaneous
coupling and cleavage of C-N bond with cyclization to
valuable amide-based pyrroles and 4-pyridones
Dipanwita Roy, Satinath Sarkar, Radha M. Laha, Nabyendu Pramanik and Dilip K. Maiti*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An unprecedented ligand-tuned Ni(0)-Cu(I) combo catalysis is
demonstrated via sequential bond activated domino C-N/C-C
coupled annulation with C-N bond cleavage to afford valuable
polysubstituted amide-based pyrroles and 4-pyridones selectively
from β-ketoanilides.
and 1,3-enynes with amines, cycloaddition, Fe(III), Ce(III), Ni(II) and
Rh(II)-catalyzed multicomponent reaction, C-H activation,
sequential C-H activation, multistep processes and other
strategies.7a,b,13 The 4-pyridone moiety is an important building
block of bioactive natural products and offers easy functionalization
to furnish key intermediates, chiral organocatalyst, bioactive
alkaloids, anti-HIV agent carbamoylpyridone (Figure 1),12e
anticancer, antiviral and antibacterial drugs, and material for
fabricating light-emitting devices, which led to development of few
synthetic approaches.12 Interestingly one direct synthetic approach
for pyridones from acetoacetanilides is achieved recently through
migration of substituents using one mole of sodium persulfate
(Na2S2O8) as an oxidant.12c However development of an catalytic
process is desirable for synthesis of the valuable heterocycles.
The C-N1 and C-C2 coupling are fundamental chemical processes
and have received considerable attention for easy construction of
N-bearing ubiquitous cyclic frameworks.3 The cleavage of C-N bond
have attracted increasing attention recently because of their varied
applications.4 The development of
a catalytic process for
simultaneous cleavage and building of C-N bonds is a significant
challenge in synthetic chemistry. It can be used for direct synthesis
of valuable heterocyclic compounds5 through designing of
a
suitable assembly of inexpensive precursors. In this regard, a
combination of two catalysts may be employed for executing such
two opposite chemical processes operating simultaneously.
Interestingly, Ni(0)-Cu(0)6a and Ni(II)-Cu(0)6e combo catalysts6 were
reported for C-C coupling reactions. Herein, an unprecedented
OHHO
HO
HO
OH
OH
O
NH
OMe
H
H
N
N
ligand-controlled Ni(0)-Cu(I) catalysis is
communicated for
N
N
F
O
O
executing diverse C-C/C-N coupled annulation with simultaneous
cleavage of C-N bond through sequential activation of several
bonds7 to furnish highly substituted valuable pyrroles8-11 and 4-
pyridones12 with outstanding selectivity.
O
O
O
HO
OH
N
H
OH
HO2C
CO2Me
F
N
R
OH
OH
Storniamide A
Carbamoylpyridone
Atorvastatin
Pyrrole is a characteristic structural motif of chlorophylls, vitamin
B-12, haemoglobin and numerous other valuable natural products.8
The substituted pyrroles-bearing amide functionality have found
important applications.9-11 For instance, pentasubstituted
Storniamide A10 is an antibiotic against Gram-positive bacteria and
Atorvastatin11 is the best-marketed cholesterol-lowering drug
(Figure 1). Recently few synthetic strategies for pentasubstituted
pyrroles are developed through cyclization of propargyl vinyl ethers
Figure 1. Useful pentasubstituted pyrrole and 4-pyridone
A general strategy for dual C-N/C-C coupled annulation cum
cleavage of C-N bond was aimed at (proposed path a, Scheme 1)
utilizing readily available inexpensive ‘two unit’ insertion synthons
acetoacetanilide1e,14 (1) and 1-phenylpropargyl alcohol (2), and ‘one
unit’ amine derivative (3) to afford directly the desired
pentasubstituted pyrroles (4, Scheme 1). During screening of a
suitable catalyst system acetanilide (3a, path a) or acetoacetanilide
(1a, path b) was our amine of choice instead of aniline (3b) to avoid
formation of undesired C-C triple bond-coupled byproduct(s). As a
a.Department of Chemistry, University of Calcutta, 92 A. P. C. Raod, Kolkata-
700009, India. Fax: +91-33-2351-9755; Tel: +91-33-2350-9937; e-mail:
dkmchem@caluniv.ac.in
†Electronic Supplementary Informaꢀon (ESI) available: Experimental procedures,
characterization data and NMR spectra. See DOI: 10.1039/c0xx00000x
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Scheme 1. Dual N-C/C-C coupled annulation via N-C bond cleavage
With the optimized conditions (entry 13, Table 1), we next
examined the scope of this domino process with various
acetoacetanilides 1b-g (Table 2). Notably, moderate to high yields
Table 2. Synthesized amide-functionalized pentasubstituted pyrroles (3)
Table 1. Development and optimization of the reactiona
Entry Catalystb
Reaction Conditionsd
3a, PhMe, reflux, 24 h
4,Yield(%)c
-
1
NiCl2(PPh3)2, Cu(0)
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
23
24
NiCl2.6H2O, CuI
NiCl2(PPh3)2, Cu(OTf)2
NiBr2, CuBr2
3a,PhMe, reflux, 22 h
3a,PhMe, reflux, 24 h
3a,PhMe, reflux, 28 h
3a,PhMe, reflux, 20 h
3a, PhMe, reflux, 24 h
3a, PhMe, reflux, 21 h
3a, PhMe, reflux, 21 h
3a, PhMe, reflux, 24 h
3a, PhMe, reflux, 23 h
3a, PhMe, reflux, 24 h
CpH, PhMe, reflux, 10 h
Ch, PhMe, reflux, 24 h
Ind, PhMe, reflux, 24 h
Cp, PhMe, reflux, 24 h
Cp, PhMe, reflux, 24 h
PCy3, PhMe, reflux, 24 h
PhMe, reflux, 24 h
-
-
-
-
-
-
-
-
Ni(OAc)2.4H2O, CuSO4.5H2O
NiCl2(PPh3)2, PhI(OAc)2
NiCl2(PPh3)2, Pd(OAc)2
NiCl2(PPh3)2, Ru(OAc)2
RhCODCl, Cu(OAc)2.H2O
Ni(COD)2, Cu(OTf)2
Ni(COD)2, CuI
-
4a, 32
4a,62
4a,55
4a,30
4a,25
4a,30
4a,49
-
Ni(COD)2,e CuI
Ni(COD)2, CuI
Ni(COD)2, CuI
Ni(COD)2, CuBr
Ni(COD)2,CuOTf
Ni(COD)2,CuOTf
Ni(COD)2,CuI
Ni(COD)2, CuI
Ni(COD)2,e CuI
3b,f CpH, PhMe, reflux, 24 h
3a, CpH, PhMe, reflux, 10 h
CpH, PhMe, reflux, 24 h
CpH, PhMe, reflux, 24 h
-
4b,30
-
-
Ni(COD)2,
CuI
aOne mmol of each precursor 1-3 was taken in 15 mL of toluene; bCatalyst
(10 mol% each); cProduct (4a) isolated after purification by column
d
chromatography; 30 mmol% of ligand; eNi(COD)2 (7 mol%) along with 1a and 2b
as substrates; f3b: Aniline.
part of our ongoing synthetic program for developing new combo
catalyst,6b we initially attempted using Ni(II)-Cu(0)6e and several
other potential combo catalysts. Unfortunately the reactions were
unsuccessful (entries 1-11, Table 1). We were delighted to have the
desired pentasubstituted pyrrole (4a, entry 1, Table 2) on use of
Ni(0)-Cu(I) combo catalyst, although the yield was very low (32%,
entry 12, Table 1). Surprisingly cleavage of C-N bond for generating
amine partner was observed using one of the versatile synthon
acetoacetanilide1e,14 (1a, path b, Scheme 1), but not with the
acetanilide (3a, path a). The optimization of the reaction was
explored employing cyclopentadiene (CpH), 1,3-cyclohexadiene
(Ch) and indene (Ind) ligands, variation of cuprous salts and solvent
(entries 13-20), and best result was obtained using Ni(COD)2-CuI
combo catalyst with CpH ligand to afford 4a in 62% yield after
refluxing the reaction mixture in toluene for 10 h. On treatment of
acetanilide (3a), 4-methyl acetoacetanilide (1b) and 2a under the
optimized conditions (entry 13) produced the product 4b (Table 2),
but not the proposed compound 4a (path a, Scheme 1). In the
catalyst controlled study desired product 4a was not found using
Ni(COD)2 and CuI in two separate reactions (entries 23,24, Table 1).
2 | J. Name., 2012, 00, 1-3
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(44–82%) were obtained utilizing acetoacetanilide and its
derivatives bearing alkyl and electron-donating groups to afford
pentasubstituted pyrroles (4b-k) within 8-10 h. However, the
reaction was unsuccessful when we replaced 1-phenylpropargyl
alcohol by the unsubstituted propargyl alcohol and the yield was
reduced (44%) on use of methyl substituted propargyl alcohol (2d,
entry 11). A cross coupling reaction was executed using 1a and 1b in
1:1 molar ratio (Scheme 2) to understand the electronic
requirement of this new cyclization process. Interestingly out of
four possible products (4a,b, 4l and 4m), 4b appeared as a major
product (55%) with coupling of two molecules of 1b-bearing
activated aromatic amine moiety. Cross coupling product 4l was
Scheme 3. Investigating the possible reaction pathway
O
OH
CH2
O
O
HN
CuI, CpH
Ni(COD)2,
+
(i)
NH CH3
Tolune, 110 oC, 10 h
H3C
N
5
1a
4n (Not found)
OH
O
O
+
CuI, CpH
,
Ni(COD)2
No product or
intermediate
(ii)
Tolune, 110 oC, 24 h
NH CH3
H
2a
1i
O2N
found as
a
minor product (15%) through coupling of
O
acetoacetanilide-bearing activated aromatic moiety (1b) and amine
from relatively less activated acetoacetanilide (1a). However, other
two possible isomers 4a and 4m were formed very negligible
amount (~ 1%), which were not isolated from the post reaction
mixture. The role of Cu(I) is expected towards selective activation of
≡C-H first and subsequently C≡C bond for annulaꢀon with the Ni-
chelated acetoacetanilide.
NH2
OH
O
O
CH3
HN
+
1. Ni(COD)2, CuI, CpH
Tolune, 110 oC,
2. Reaction quenched
with water and worked
up immediately
(iii)
NH CH3
+
H3C
H
2
h
N
O2
N
NO2
2a
1j
3c (10%)
NO2
4o (<5%)
(Scheme 4). The C-C/C-N coupled annulation with cleavage of C-N
bond proceeds through coupling between two molecules of
acetoacetanilide (1), and the corresponding pyrrole (4) was not
generated due to the steric and electronic forces exerted by the 2-
methyl furan during the combo catalysis. The multidimensional
application of 4-pyridone compounds led us to establish the
catalytic approach for direct synthesis of highly substituted
pyridones from acetoacetanilides in absence of any oxidant
(Scheme 4). The syntheses of functionalised 4-pyridines 6a-i were
Scheme 2. Cross coupling reaction.
Scheme 4. 2-Methyl furan tuned synthesis of 4-pyridones
R2
R2
O
O
O
N
CH3
CH3
+
OH
O
HN
O
O
H
N
Ni(COD)2, CuI,
H
+
NH CH3
Tolune, 110 oC, 3.5-5 h
N
H3
C
CH3
H3C
H
R4
R4
R2
1
2
R2
R2
6
4 (Not found)
H3CO
Our experiment using nonterminal alkyne (5) revealed terminal
alkyne is necessary for the unprecedented annulation process (eq. i,
Scheme 3). The reaction was completely blocked on use of aniline
(3b, entry 20, Table 1). Involvement of activated C3-H in the
H3
C
O
O
O
O
N
H
O
O
N
H
N
N
H
H
H
N
H3
C
N
CH3
H
H3C
CH3
H3C
CH3
annulation process was confirmed using
a
designed
acetoacetanilide (1i, eq. ii) bearing no such C-H bonds, which
resulted no adduct, intermediate or any product under the reaction
conditions. We have quenched most of the annulation reactions
(Table 2) to trap the C-N bond cleaved free-amine intermediate,
which were unsuccessful. However, treatment of acetoacetanilide-
bearing strongly deactivated aromatic moiety (1j, 4-nitrophenyyl,
eq. iii, Scheme 3) C-N bond breaking becomes easier and
corresponding 4-nitroaniline (3c) was trapped from the reaction
mixture, which confirms cleavage of C-N bond is an essential part of
the new catalysis process.
OCH3
6c: 3.5 h, 82%
CH3
6a: 4 h, 81%
6b: 4 h, 74%
Br
Cl
O
O
O
O
N
Cl
O
O
N
H
H
N
N
H
H
H
N
H
H3C
CH3
H3
C
N
CH3
H3
C
CH3
Cl
Cl
Br
6f: 5 h, 75%
6e: 5 h, 77%
6d: 4.5 h, 78%
EtO2C
OCH3
O
O
N
OCH3
O
O
O
O
N
H
N
H
H
Next we turned our attention to explore versatility of the combo
catalyst using various ligands under different reaction conditions.
After screening with several ligands, 2-methyl furan was found
effective for redirecting to the other annulation process to afford 4-
pyridone (6a) as the sole product under the reaction conditions
H
N
N
H
H
H3C
CH3
Cl
H3C
N
CH3
OCH3
H3C
CH3
OCH3
Cl
6g: 5 h, 74%
CO2Et
6h: 3 h, 81%
6i: 5 h, 79%
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References
accomplished in the fast reaction rate (3.5-5 h) and high yield (74-
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these studies in hand, the possible mechanism of the unusual
combo catalysis is depicted in Scheme 5. The oxidative insertion to
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combo catalyst to generate intermediate I, which subsequently
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