Pd-catalyzed carbonylative cycloamidation of ketoimines for the synthesis of pyrido[1,2-a]pyrimidin-4-ones

Article abstract of DOI:10.1039/c5cc02631h

The Pd(ii)-catalyzed pyridine-directed carbonylative cycloamidation of ketoimines has provided an efficient protocol for assembly of pyrido[1,2-a]pyrimidin-4-ones. This journal is

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ARTICLE TYPE
Pd-Catalyzed Carbonylative Cycloamidation of Ketoimines for
Synthesis of Pyrido [1, 2-a] Pyrimidin-4-Ones **
Ying Xie, Tengfei Chen, Shaomin Fu, Huanfeng Jiang, and Wei Zeng*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
and result in the occurrence of carbonylation reaction
(Scheme 1c). To identify this hypothesis, herein we report a
novel Pd-catalyzed pyridine-directed carbonylation to rapidly
assembly pyrido [1, 2-a] pyrimidin-4-ones (PPO) derived
from N-(2-pyridyl)-ketoimines, which was ever employed to
treat with isocyanates to furnish PPO,17 most of PPO
analogues are potent biological and drug molecules in medical
chemistry.18
Introduction
Transition metal-catalyzed carbonylation of organic molecules
such as aryl halides, 1 alkenes 2 and alkynes 3 etc, provides a
powerful tool to construct carbon-carbon and carbon-
heteroatom bonds. Among the various carbonylations,
chelation-assisted C-H functionalization is emerging as an
ideal site-selective carbonylation strategy due to its atom- and
step-economy. In this regards, various heteroatom-containing
functional groups such as amides, 4 nitrogen heterocycles, 4a, 4b,
a) Mo(0)-promoted carbonylation of ortho-haloarylimines
[Mo(CO)6] (1.0 equiv)
H2O (3.0 equiv)
R2
R2
N
N
CO (1 atm)
R2
5
8
hydroxy group,6 amines,7 and carboxylic acids etc., have
X
N
R1
nBu4NCl (1.0 equiv)
DMF, 160 oC, 1 h
(CO)
4
Mo
R1
R1
been widely employed as directing group to enhance Csp2-H
carbonylation. Moreover, several breakthrough in functional
group-directed Csp3-H carbonylation, has also been achieved
through Pd(II) or Ru(0)-catalyzed carbonylative Csp3-H
functionalization of aliphatic amides or amines by Yu,9 Gaunt
X = Br or I
R2 = alkyl or aryl
O
X
b) The coupling between aryl C(sp2)-beta C(sp3) of ketoimine
R2

R2
Pd(OAc)2 (10 mol %)
Bu4NBr (2 equiv)
O2 (1 atm), DMSO, 60 oC, 24 h
R3

N
N
R3
R1
R1
H
10
11
and Chatani, respectively. Consequently, developing new
c) This work: Pd(II)-catalyzed Csp3-H bond activation/carbonylation of ketoimines
type of ligand-directed carbonylations will continue to be
more desirable.
On the other hand, as the versatile synthon, imines are of
great importance in organic chemistry, transition metal-
catalyzed C-X (X= C, N etc.) coupling reactions derived from
O
R2
R2
R3

Pd
R2
R3
N
N
Pd(II)
N
C
O

N
R3
R1
N
N
R1
R1
A
E
B
Scheme 1. Ketoimines as a catalysis platform for N-heterocycle synthesis
imines are widely utilized in constructing kinds of nitrogen-
containing compounds,
12
but the carbonylation involving
imines was rarely reported. Up to now, only Iwasawa ever
developed a molybdenum-promoted carbonylative cyclization
of ortho-haloarylimines, in which carbonylation insertion
occurred through oxidative addition of Mo(0) to Csp2-halogen
bond (Scheme 1a).13 Recently, Yoshikai and co-workers
reported that Pd(II) could directly catalyze the intramolecular
oxidative cyclization of N-aryl ketoimine to construct indoles
via aryl C (sp2)-alkyl C (sp3) coupling (Scheme 1b).14 More
Results and Discussion
Initially, the carbonylation of N-(2-pyridyl)-ketoimine (1a)
was investigated as a model system. First attempts were
performed to screen palladium catalysts in the presence of
MnO2, K2CO3 and additive KI under an atmospheric pressure
o
of CO in DMF at 80 C (Table 1, entries 1-5), and we quickly
found that Pd(OAc)2 was an effective catalyst which could
provide the desired carbonylation product 2-phenyl-
pyrido[1,2-a]pyrimidin-4-one (2a) in 32% yield (entry 5). To
further improve the reaction outcome, we conducted this
reaction for optimizing oxidants including Na2S2O8, and
PhI(OAc)2 etc (entries 5-10). Among the tested oxidants,
Cu(OAc)2 was a suitable oxidant for this transformation, and
gave an improved yield of 36% (entry 10). Subsequently, we
continued to further evaluate the effect of various bases on
this carbonylation reaction for achieving satisfying yields
(entries 10-14), and found the yield of product 2a could be
increased to 50% with the use of 1.0 equiv of DABCO (1, 4-
diazabicyclo [2.2.2] octane) (entry 14). It is worth to note that
not employing KI as additive resulted in significantly
recently, we developed
a Pd-catalyzed pyridine-directed
oxidative [3+2] cycloaddition reaction for assembling
a
multisubstituted pyrrole skeleton from readily available 2-(N-
pyridyl)ketoimines and internal alkynes.15 Yoshikai’s and our
work indicated that β-C(sp3) of ketoimines could be directly
coupled with unsaturated carbon atoms. Considering that
carbon monoxide also belongs to an important unsaturated C1
source, and in connection with our continuing efforts to
explore novel reactions starting from imines to construct
nitrogen-containing compounds, 15, 16 we envisioned that a six-
membered cyclopalladium species (B) which derived from 2-
pyridine ketoimine A, will be possibly trapped by unsaturated
carbon atom-containing CO via carbonyl insertion process,
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reaction temperature is 100 oC.
decreased yield possibly due to that iodide anions could
enhance the carbonylation efficiency (compare entry 14 with
15).19 Finally, to our delight, increasing the reaction
With the optimized conditions in hand, we further examined
the scope and limitations of the pyridine-directed Csp3-H
activation/carbonylation reaction. As shown in Table 2,
common functional groups on the benzene rings attached to
the ketoimine carbon, were tolerated in this transformation to
furnish the corresponding carbonylation products in moderate
to excellent yield (entries 1-14), including electron-donating
groups, such as alkyl (2b-2d), alkyoxyl (2e), and acetal group
(2n), and electron-withdrawing groups including halogen (2f-
2j), nitrile (2k), ester (2l), and nitro groups (2m). It should be
noted that meso or ortho-substituted benzene ring led to a
lower yield possibly due to the increased steric hindrance
around the ketoimine (2b-2d; 2g-2i). Moreover, heteroaryl
(HetAr) substituted ketoimines (HetAr = 2-thiophene, 2-furan,
4-pyridyl, and 3-pyridyl) also allowed for this transformation
o
o
temperature from 80 C to 100 C significantly enhanced the
carbonylation reaction, and provided 2a in up to 88% isolated
yield (entry 17). Notably, lowering the reaction temperature to
60 oC led to poorer conversion (compare entries 14 and 17
with 16) [see Supporting Information (SI) for more details].
Table 1. Optimization of the reaction parameters a
N
Ph
Pd catalyst (10 mol %)
oxidant /CO (1 atm)
N
N
N
KI/base
DMF, 80 oC, 24 h
O
H3C
Ph
2a
1a
Entry
1
Catalyst
PdCl2
Oxidant
MnO2
Base
Yield (%) b
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
18
and
produced
the
desired
2-heterocycle-substituted
pyrido[1,2-a]pyrimidin-4-ones (2o - 2r) in 45-61% yields
(entries 15-18). Importantly, this reaction protocol could still
smoothly convert phenylpropone-derived ketoimine 1s to the
corresponding 3-methyl-2-phenyl-pyrido[1,2-a]pyrimidin-4-
one 2s (entry 19). Subsequently, we further investigated the
substitution effects from pyridine ring of ketoimines on this
transformation, and found electron–rich ketoimines offered
higher yield of products except N-(3-methoxyl pyridine)-
substituted ketoimine (1w) with stronger steric hindrance
around the reaction center; While electron-deficient substrates
inhibited cyclocarbonylation performance (entry 20-24,
compare 2t with 2u and 2w). Unfortunately, bromine-
containing substrates were sensitive to this transformation in
which bromine atom would be removed (1j and 1v).
2
Pd(TFA)2
MnO2
trace
trace
5
3
PdCl2(CH3CN)2 MnO2
4
PdCl2(PPh3)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
MnO2
5
MnO2
32
6
Na2S2O8
14
7
PhI(OAc)2 K2CO3
10
8
AgOAc
K2CO3
K2CO3
K2CO3
22
Table 2. Substrate scope for the Pd(II)-catalyzed carbonylative
Csp3-H bond cycloamidation of ketoimines a
9
BQ c
trace
36
3
1
N
O
R
R
Pd(OAc) (10 mol %)
2
1
Cu(OAc) /CO (1 atm)
R
N
N
2
N
2
R
KI/DABCO
10
11
12
13
14
15
16
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
3
o
R
DMF, 100 C, 24 h
2
2
1
R
b
Entry
Ketoimine (1)
Product (2)
Yield
NaHCO3
Cs2CO3
NaOAc
31
R
R
N
10
N
N
N
O
14
1
1a: R = H
2a: R = H
88
90
84
68
91
75
2
3
4
5
6
7
DABCO
DABCO
DABCO
DABCO
50
1b: R = p-Me
1c: R = m-Me
1d: R = o-Me
1e: R = p-MeO
1f: R = p-F
2b: R = p-Me
2c: R = m-Me
2d: R = o-Me
2e: R = p-MeO
2f: R = p-F
27 d
32 e
88 f
17
1g: R = p-Cl
2g: R = p-Cl
79
62
a
Unless otherwise noted, all the reactions were carried out using
ketoimine (1a) (0.10 mmol), KI (1.0 equiv) and CO (balloon
pressure) with Pd catalyst (10 mol %) in the presence of oxidant (1.0
equiv) and base (1.0 equiv) in DMF (1.0 mL) at 80 ˚C for 24 h in a
8
1h: R = m-Cl
2h: R = m-Cl
b
9
1i:R = o-Cl
1j: R = p-Br
2i: R = o-Cl
2a: R = p-H
54
82
sealed reaction tube, followed by flash chromatography on SiO2.
Isolated yield. c BQ stands for 1, 4-benzoquinone. d The reaction was
run in the absence of KI. e The reaction temperature is 60 C. f The
o
10
2
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effect experiments were conducted under the standard
conditions [Eq. (3)], and the corresponding KIE value (KH/KD
11
1k: R = p-CN
2k: R = p-CN
65
71
64
12
13
1l: R = p-CO2Me
1m: R = p-NO2
2l: R = p-CO2Me
=
1.59) indicated that imine-enamine equilibrium just
influence, but not solely dominate the reaction rate (see SI for
more details).
2m: R = p-NO2
O
Ph
O
N
N
Ph
O
Pd(OAc) (10 mol %)
2
N
Cu(OAc) /CO (1 atm)
N
N
O
N
2
94
(1)
(2)
+
Ph
N
N
KI/DABCO
O
o
Me
Ph
Ph
DMF, 100 C, 24 h
O
Ph
14
1n
2n
2z (50% yield)
1y
2y (not observed)
N
X
N
X
N
O
Pd(OAc) (10 mmol%)
2
N
N
N
N
CD OD/d6-DMF/Ar
3
D C
Ph
3
o
100 C, 24 h
15
16
1o: X = O
1p: X = S
2o: X = O
2p: X = S
52
61
81% D
d-1a
1a
N
O
Ph
Pd(OAc) (10 mol %)
2
N
Cu(OAc) /CO (1 atm)
N
N
2
N
N
N
(3)
or
N
H(D)
KI/DABCO
DMF, 100 C, 24 h
N
D C
3
Ph
o
H C
3
Ph
N
O
58
N
85% D
1a
KIE = 1.59
d-1a
2a or d-2a
1q
2q
17
Scheme 2. Preliminary mechanistic studies
N
O
N
N
N
N
45
17
N
1r
2r
18
19
Based on the above experimental results,
mechanism that involved a Pd(II)/Pd(0) redox process can be
a possible
N
Ph
proposed as shown in Scheme 3. Enamine A derived from
N
N
N
Me
14, 22
Me
imine/enamine-isomerization
can be electrophilically
Ph
O
1s
2s
attacked by Pd(II) to produce a palladacycle intermediate B
which can isomerize to form palladacycle intermediate C.
Subsequently, coordination and the migratory insertion of CO
into the N-Pd bond of intermediate C affords a seven-
membered palladacyle D, which can further lead to the
formation of carbonylative cycloamidation product 2 through
Pd(II) reductive elimination and generate a Pd(0) species
which can be oxidized by Cu(II) salts to generate the
catalytically active Pd(II) catalysts.
N
O
Ph
R
R
N
N
N
Ph
1t: R = 5-Me
2t: R = 5-Me
2u: R = 5-Cl
20
21
22
23
24
80
62
60
60
50
1u: R = 5-Cl
1v: R = 5-Br
2a: R = 5-H
1w: R = 3-MeO
1x: R = 5-CO2Me
2v: R = 3-MeO
2w: R = 5-CO2Me
N
NH
Ar
N
N
BABCO
Pd(II)
N
N
Ar
(II)Pd
A
Ar
1
B
a All the reactions were carried out using ketoimine (1a) (0.10 mmol),
KI (1.0 equiv) and CO (balloon pressure) with Pd(OAc)2 (10 mol %) in
the presence of Cu(OAc)2 (1.0 equiv) and DABCO (1.0 equiv) in DMF
(1.0 mL) at 100 ˚C for 24 h in a sealed reaction tube, followed by flash
chromatography on SiO2. b Isolated yield.
N
Cu(II)
N
(II)Pd
Ar
Pd(0)
C
N
Ar
N
N
CO
(II)
Pd
Ar
N
To gain preliminary insight into the reaction mechanism,
we conducted several controlled experiments. First, we tried
the carbonylative cycloamidation of N-phenyl ketoimine (1y)
under our standard conditions, no desired cyclocarbonylation
O
D
2
O
Scheme 3. Possible mechanism for the reaction
1
product 2y was detected by the H NMR and GC-MS methods.
On the contrary, only 50% yield of cascade cyclization
product 2z was produced [Scheme 2. Eq. (1)]. This result
Conclusions
20
In summary, we have developed an efficient method for the
synthesis of pyrido[1,2-a]pyrimidin-4-ones through Pd(II)-
catalyzed carbonylative cycloamidation of N-(2-pyridyl)
ketoimines under an atmospheric pressure of CO. This
transformation tolerates a variety of functionalities including
nitrile, chloride, alkyloxycarbonyl, nitro, and heteroaryl etc.,
which could be further employed as useful synthetic blocks
for constructing more complex biological molecules. Further
remarkably demonstrated that pyridyl nitrogen played a key
chelating site in this carbonylation process. Subsequently,
when N-(2-pyridyl) ketoimine 1a was subjected to
Pd(II)/CD3OD/Ar system for 24 h in the absence of carbon
monoxide, 81% deuterium incorporation was observed at the
imino methyl group of 1a [Eq. (2)] (see SI for more details),21
this experiment indicated that imine-enamine tautomerization
is facile under the reaction conditions. Finally, kinetic isotope
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DOI: 10.1039/C5CC02P63a1gHe 4 of 4
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mechanism about this reaction are under way in our laboratory.
Acknowledgments
The authors thank the NSFC (No. 21372085) and the GNSF
(No. 10351064101000000) for financial support.
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