Palladium-catalyzed oxidative carbonylation of N-aryl enamino esters with CO and alcohols: Synthesis of N-aryl aminomethylenemalonates

Article abstract of DOI:10.1039/c7cc02852k

A novel palladium-catalyzed regioselective oxidative carbonylation of tri-substituted alkenes with CO and alcohols for the synthesis of α,β-unsaturated esters has been developed. Experimental studies and DFT calculations suggested that the reaction proceeded through alkoxylation of the palladium(ii) catalyst, CO and CC double bond migratory insertion, β-(N)H elimination and tautomerization cascade steps. The reaction tolerates a wide range of groups and produces valuable aminomethylenemalonates in high yields.

Full text of DOI:10.1039/c7cc02852k

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Palladium-catalyzed oxidative carbonylation of N-aryl enamino
esters with CO and alcohols: synthesis of N-aryl
aminomethylenemalonates
Received 00th January 20xx,
Accepted 00th January 20xx
Ming Chen,† Le Yu,† Zhi-Hui Ren, Yao-Yu Wang and Zheng-Hui Guan*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
novel
palladium-catalyzed
regioselective
oxidative hindrance of tri-substituted alkenes prohibits the key β-H
carbonylation of tri-substituted alkenes with CO and alcohols for elimination of the palladium complex, thus making the
the synthesis of α,β-unsaturated esters has been developed. dicarbonylation products favored (Scheme 1b).14 Therefore,
Experimental studies and DFT calculations suggested that the palladium-catalyzed oxidative carbonylation of tri-substituted
reaction processed through alkoxylation of palladium(II) catalyst, alkenes for the regioselective synthesis of α,β-unsaturated
CO and C=C double bond migratory insertion, β-(N)H elimination carbonyl compounds remains a great challenge.
and tautomerization cascade steps. The reaction tolerates a wide
range
of
groups
and
produces
valuable
aminomethylenemalonates in high yields.
Carbonylation reaction is one of the most dominant themes in
modern synthetic chemistry.1 Transition-metal catalyzed
carbonylation of organic halides or pseudohalides,2 alkenes,3
alkynes,4 organometallic reagents,5 and C-H bonds of
hydrocarbons6 have been extensively studied and widely
applied in organic synthesis. Especially, transition-metal
catalyzed reductive carbonylation of alkenes, such as
hydroformylation,7 hydroesterification,8 and hydroamidation9
has been well-established for the synthesis of various alkyl
carbonyl compounds. For example, hydroformylation of
alkenes for the synthesis of aldehydes has been one of the
largest processes in chemical industry.9,10 However, since the
reductive property of carbon monoxide and steric sensitive of
the carbonylations,6c transition-metal catalyzed oxidative
carbonylation of alkenes for the synthesis of valuable α,β-
unsaturated carbonyl compounds has remained largely
undeveloped.11 Recently, a few elegant palladium-catalyzed
oxidative carbonylation of alkenes for atom-economic
synthesis of α,β-unsaturated esters have emerged.12 However,
due to the steric sensitive cis-β-H elimination was involved,
this class of reaction was generally restricted to using terminal
alkenes, and less steric-hindered primary alcohols as the
substrates (Scheme 1a). In comparison with the terminal
alkenes, the low binding affinity of tri-substituted alkenes to
the palladium center and slow migratory insertion make this
process challenging.13 Furthermore, the intrinsic steric
Scheme 1 Palladium-catalyzed oxidative alkoxycarbonylation of alkenes.
Methylenemalonates are a class of versatile building
blocks
in
organic
synthesis.
Especially,
N-aryl
aminomethylenemalonates are the key synthetic precursors
for the preparation of quinolones, 1,3,4-oxadiazoles, 3H-indole
alkaloids, 4-phenylthioquinolines, and chiral amino acid
derivatives. The laters are prevalent structural motifs in drugs,
such as norfloxacin, flumequine, ciprofloxacin, fleroxacin,
moxifloxacin, gemifloxacin, grepafloxacin and antibacterial
reagents.15 However, N-aryl aminomethylenemalonates were
not easily accessible by traditional protocols.16 In connection
with our interest in carbonylations,17 we hypothesized that
palladium-catalyzed oxidative carbonylation of readily
available N-aryl enamino esters with CO and alcohols would be
an ideal and straightforward method for the synthesis of N-
aryl
aminomethylenemalonates.
However,
additional
Key Laboratory of Synthetic and Nature Molecule Chemistry of Ministry of
Education, Department of Chemistry & Materials Science, Northwest University,
Xi’an 710127, P.R. China. E-mail: guanzhh@nwu.edu.cn
† These authors contributed equally (M.C. and L.Y.)
Electronic Supplementary Information (ESI) available:
challenges should be overcome: (a) palladium-catalyzed
intramolecular oxidative cyclization of N-aryl enamino esters
towards indoles must be suppressed,18 (b) palladium-catalyzed
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carbonylation of N-H bond of enamino esters to form ureas or reaction (Table 1, entries 9-11). And KBr was lessVeiefwfeAcrtticl(eeOnntlirnye
carbamates should also be inhibited.17d In this paper, we 8 vs 12). Finally, the reaction temperaturDeOwI:a1s0.a10ls3o9/vCa7CriCe0d2,8a5n2dK
describe the development of a novel palladium-catalyzed 60 °C gave the best yield (80%) of 3aa (Table 1, entries 13-15).
oxidative carbonylation of N-aryl enamino esters with CO and
Table 2 Palladium-catalyzed carbonylation of various N-aryl enamino esters 1 with CO
and methanola
alcohols for the synthesis of N-aryl aminomethylenemalonates
under mild conditions (Scheme 1c).
Table 1 Optimization of the reaction conditionsa
Entry
Oxidant
Additive
Solvent
Yield of 3aa (%)
1b
2
Cu(OAc)2
Oxone
DMF
DMF
5
0
3
4
5
6
7
8
9
10
11
12
13c
14d
15e
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
DMF
18
9
1,4-dioxane
Toluene
DMSO
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
11
28
51
72
71
69
56
42
36
80
n.r.
KI
NaI
TBAI
I2
KBr
KI
KI
KI
a
Reaction conditions: 1a (0.2 mol), MeOH (5.0 equiv), Pd(OAc)2 (5 mol%),
Cu(OAc)2 (1.2 equiv), KI (0.2 equiv), solvent (2 mL), CO (balloon pressure), 80
b
c
d
e
oC; isolated yield. Cu(OAc)2 (3.0 equiv). 100 oC. 60 oC. 40 oC, n.r.= no
reaction.
We began our study with the palladium-catalyzed
oxidative carbonylation of the readily available N-phenyl
enamino ester 1a with CO and methanol. The carbonylation
product N-phenyl aminomethylenemalonate 3aa was formed
in only 5% yield along with indole and acetanilide as the
a
Reaction conditions: 1 (0.2 mmol), MeOH (5.0 equiv), Pd(OAc)2 (5 mol%),
Cu(OAc)2 (1.2 equiv), KI (0.2 equiv), CH3CN (2 mL) at 60 oC under CO (balloon
pressure); isolated yield.
With the optimized reaction conditions established, the
scope of the reaction was investigated (Table 2). This new
carbonylation reaction displayed high functional group
tolerance and proved to be a quite general methodology. N-
Aryl enamino ester with p-fluoro on aryl ring 1b gave the
desired product 3ba in 92% yield. All of the o-, m- and p-chloro
o
byproducts in the presence of Pd(OAc)2 and Cu(OAc)2 at 80 C
in DMF (Table 1, entry 1). Importantly, the carbonylation of
enamino ester was observed. Encouraged by this preliminary
result, we have tried to optimize the reaction conditions for
this palladium-catalyzed oxidative carbonylation (Table 1).
Firstly, different oxidants, such as CuCl2, Oxone and K2S2O8
were screened (Table 1, entry 2). No 3aa was observed under
these conditions, but N,N’-diphenylurea, which generated
from palladium-catalyzed N-H bond carbonylation of aniline
(hydrolyzed from N-phenyl enamine ester 1a), was obtained as
the main product in the presence of CuCl2.17d Therefore,
Cu(OAc)2 was retried as the oxidant. Fortunately, the
acetanilide and indole byproduct were significant suppressed
by verification of the loading of Cu(OAc)2 oxidant (Table 1,
entry 3). Then, different solvent were optimized for further
improving the reaction efficiency (Table 1, entries 4-7). To our
delight, the desired N-phenyl aminomethylenemalonate 3aa
was obtained in 51% yield in CH3CN (Table 1, entry 7). Next, KI
which has been shown to improve the efficiency of palladium-
catalyzed carbonylations in our previous reports,17 was added
as an additive to the current reaction. The yield of 3aa was
further improved to 72% (Table 1, entry 8). Other iodide
compounds such as NaI, TBAI, and I2 are inferior to KI in this
substituted N-Aryl enamino esters 1c-1e were tolerated in the
reaction, and o-chloro substituted N-Aryl enamino ester 1e
gave the highest yield (93%). Notably, sensitive functional
groups in the palladium-catalyzed cross-coupling reactions,
such as bromo and iodo, were tolerated as well, thus resulting
the corresponding bromo or iodo substituted products 3fa
in 84% and 60% yields, respectively. N-Aryl enamino esters
with various electron-withdrawing groups on aryl ring 1h 1l
such as acetyl, ester, cyano and nitro, proceeded smoothly to
produce the desired carbonylation products 3ha 3la in 81-95%
yields. However, the substrates bearing electron-donating
group, such as methyl, afforded the desired carbonylation
product 3ma in slightly lower yield.
N-Naphthyl enamino ester 1n was also allowed to
produce the N-naphthyl aminomethylenemalonate 3na in 66%
yield under the reaction conditions. Furthermore, different
groups on α-position of N-phenyl enamino esters were
-3ga
-
,
-
2 | J. Name., 2012, 00, 1-3
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investigated under the standard conditions. The α-ethyl
substituted N-phenyl enamino esters 1o-1p produce the
corresponding carbonylation product 3oa-3pa in good to high
yields. α-Aryl (or α-hetero-aryl) substituted N-phenyl amino-
methylenemalonates 3qa-3sa were achieved by the
carbonylation reaction albeit in slightly low yields. Notably, (Z)-
Methyl-3-(phenylamino)-acrylate was found compatible with
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DOI: 10.1039/C7CC02852K
the
reaction
and
afforded
the
corresponding
Scheme 2 Possible pathways for the carbonylation reaction.
aminomethylenemalonate 3ta in 84% yield. In addition, the
expected carbonylation product 3ua was only obtained in 24%
yield when (Z)-4-(phenylamino)pent-3-en-2-one 1u was used
as the substrate.
optimized geometries (SDD basis was used for Pd)). The
reaction process starts with a stepwise addition of methanol to
the Pd(OAc)2 via hydrogen bond mediated TS-1 and 8-
membered ring TS-2 transition structures. Then an
intramolecular proton transfer occurs, the H from 1st added
methanol migrates to the acetate group via transmission of
the 2nd methanol. The second step is carbonyl insertion, the
carbon monoxide addition and methanol-acetic acid dimer
dissociation to the Pd center occurs simultaneously. The
Table 3 Palladium-catalyzed carbonylation of N-phenyl enamino ester 1a with CO and
various alcohols 2a
intermediate
B isomerizes to C via a three-membered ring
transition structure TS-5 (ΔG = 9.0 Kcal/mol) and then
coordinates to the C=C double bond in enamino ester 1a to
give rise to intermediate D. In 1a, the lone pair electrons in N
atom and double bond would form a delocalized π bonds with
four electrons distributed in three atoms. The N atom and
CEster atom in double bond are electronically negative and
CAmino atom in double bond is electronically positive. Therefore,
the Pd would insert to the CEster atom first, while in the four-
membered ring form transition state for the ester migration,
the coordination center for Pd transfer to CAmino atom, and this
process has been verified by the intrinsic reaction coordinate
a
Reaction conditions: 1a (0.2 mmol), alcohol (5.0 equiv), Pd(OAc)2 (5 mol%),
Cu(OAc)2 (1.2 equiv), KI (0.2 equiv), CH3CN (2.0 mL) at 60 oC under CO (balloon
pressure), isolated yield (for all of products E/Z ≈ 1). b alcohol (2.0 equiv).
(IRC) path calculation. Upon intermediate E, there are three
Different alcohols were also studied to determine their
reactivity in the carbonylation reaction (Table 3). It was
noteworthy that all of the primary alcohols, phenol, secondary
alcohol and even tertiary alcohol were tolerated in the
reaction. Primary alcohols, such as ethanol, n-propanol, n-
butanol, and benzyl alcohol, produced the corresponding
carbonylation products 3ab-3ae in 65-82% yields. In particular,
phenol converted to the desired product 3af in 80% yield
under oxidative reaction conditions. Due to the transition-
metal catalyzed carbonylation was generally sensitive in steric
effects, secondary and tertiary alcohol are generally inactive in
carbonylation reactions.19 However, 2-propanol and tert-butyl
alcohol reacted smoothly to produce the corresponding N-
phenyl aminomethylenemalonates 3ag and 3ah in 85% and
84% yields, respectively, under the reaction conditions.
possible elimination pathways β-Ha, β-Hb, and β-Hc, as
discussed above and illustrated in Scheme 2. As β-Hb
elimination path is experimentally excluded, we will focus on
the competition between the remained two pathways. As
shown in Fig. S2 (see supplementary information), the Pd atom
and β-Ha are in cis position, while the β-Hc is in trans-position.
Thus, only in case the -CH(CO2Me)2 group rotates for 180º, the
β-Hc elimination pathway is conformationally accessible.
Moreover, the oxygen atom of esters group coordinate with
the Pd central, the rotation of the -CH(CO2Me)2 group is
prohibited. Hence, only the β-Ha elimination pathway is
conformational and energetically available. The β-(N)H is
extracted by Pd-acetate group via TS-7 and departed with
them to form intermediate
isomerization from imines
F
F
. The last step in this route is the
to the carbonylation product N-
Palladium-catalyzed oxidative carbonylation of alkenes
always proceeds via migratory insertion of C=C double bond
into acylpalladium complex sequential with β-H elimination
steps.12 Thus, β-Ha, β-Hb, and β-Hc elimination were possible
for this carbonylation in principle (Scheme 2). However, the
phenyl aminomethylenemalonate
3. In addition, we also
explored the palladium-catalyzed C-H activation and
carbonylation pathway,17b and which has been energetically
ruled out as the high barrier for the formation of octahedron
Pd(IV) intermediates (see supplementary information, Fig. S3).
To further confirm the reaction mechanism, the
carbonylation of N-methyl enamino ester 1u, which has no
hydrogen on nitrogen atom, was conducted under the
standard conditions. Conformable, no reaction was observed
(Eq. 1). This result also suggested that the β-(N)H elimination
pathway is more likely.
smooth reaction of 1q-
1t indicates that the β-Hb elimination is
prohibited in current reaction situation. In order to clarify
which route in β-Ha or β-Hc elimination was the real reaction
pathway, DFT calculations has been performed.
The free energy profile for the carbonylation reaction
pathway is show in supplementary information, Fig. S1
(obtained at M06/6-311++G** level with B3LYP/6-31G*
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Notes and references
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On the basis of aforementioned results, a proposed
catalytic cycle was depicted in Scheme 3. Initially, an
alkoxylpalladium species was generated from ligand
A
exchange by loss of HOAc. Then, alkoxylpalladium
underwent coordination of CO to form intermediate
A
B
.
Wang, Z. Wang and K. Ding, J. Am. Chem. Soc., 2015, 137
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Migratory insertion of CO into the MeO-Pd bond of the
intermediate produces intermediate Next, the
coordination and 1,2-migratory insertion of C=C double bond
into intermediate generates intermediate β-(N)H
Elimination of delivers the imine intermediate and
palladium hydride species. Finally, isomerization of imine
intermediate gives the carbonylation product The
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Scheme 3 Proposed mechanism for palladium-catalyzed oxidative carbonylation of N-
aryl enamino esters with CO and alcohols.
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In conclusion, we have developed a novel palladium-
catalyzed oxidative carbonylation of tri-substituted alkenes
with CO and alcohols for the synthesis of α,β-unsaturated
esters. The use of N-H enamino esters as the substrates was
significant for the oxidative carbonylation reaction, which was
making the challenging β-H elimination step proceeded
smoothly. The resulted N-aryl aminomethylenemalonates
products are valuable for pharmaceutical chemistry. The
experiment results and DFT calculations suggested that the
oxidative carbonylation proceeded through alkoxylation of
palladium catalyst, CO and alkenyl C=C double bond migratory
insertion, β-(N)H elimination and tautomerization cascade
steps. The readily available N-aryl enamino esters, valuable N-
aryl aminomethylenemalonates, good functional groups
tolerance, mild conditions and high yields make this
unprecedented oxidative carbonylation reaction attractive for
organic synthesis. Further scope and mechanistic studies of
the reaction are underway in our laboratory.
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This work was supported by generous grants from the
National Natural Science Foundation of China (NSFC-21622203,
21472147, and 21272183) and Fund of Northwest University
(334100036).
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