Palladium-catalyzed cross-coupling of enamides with sterically hindered α-bromocarbonyls

Article abstract of DOI:10.1039/c5cc10653b

Palladium-catalyzed intermolecular alkylation of enamides with α-bromo carbonyls was developed. Under mild reaction conditions, various cyclic and acyclic enamides reacted well with α-bromo carbonyls to afford the corresponding multi-substituted alkene pr

Full text of DOI:10.1039/c5cc10653b

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ARTICLE TYPE
Palladium-Catalyzed Cross-Coupling of Enamides with Sterically Hindered α-
Bromocarbonyls
a
a
a
a
a
a,b
Ran Ding, Zhi-Dao Huang, Zheng-Li Liu, Tian-Xiang Wang, Yun-He Xu * and Teck-Peng Loh *
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
ABSTRACT: Palladium-catalyzed intermolecular alkylation of
enamides with α-bromo carbonyls was developed. Under mild
reaction conditions, various cyclic and acyclic enamides reacted
well with α-bromo carbonyls to afford the corresponding multi-
substituted alkene products in good yields. The coupling
products could be converted into very useful γ-amino acid, δ-
amino alcohol, 1,4-dicarbonyl and γ-lactam derivatives.
would like to communicate a palladium-catalyzed alkylation
reaction of enamides with α-bromo carbonyls, via which various
tri- and tetra-substituted functional alkene compounds could be
obtained (Scheme 1).
45
1
0
Enamines are used widely in organic synthesis as synthetic
[
1]
equivalents to metal enolates. They have been shown to
function as versatile synthetic intermediates, reacting with a
1
2
2
3
3
4
5
0
5
0
5
0
[
2]
wide variety of electrophiles under mild reaction conditions.
However, one of the major limitations in this reaction is the
[
3]
inability to react with sterically hindered alkyl halides. Based on
the previous work from our group on the C-H functionalization
[4]
of β-H enamides, we envisage that carrying out the reaction
with bulky halides via metal-catalyzed coupling reaction using
enamides, a derivative of enamine may solve this problem. If
successful, this method will provide an easy entry to the bulky
alkyl-substituted alkenes as the versatile precursor of γ-amino
Scheme 1. Simple Access for Synthesis of Diverse γ-Amino Acids, δ-
Amino Alcohols, 1,4-Dicarbonyls and γ-Lactams.
Initially, the reaction between enamide 1a and ethyl 2-
bromo-2-methylpropanoate 2a was established as the model
reaction to optimize the reaction conditions. The results are
[
5]
acids, δ-amino alcohols, 1,4-dicarbonyls and γ-lactams. It is
important to note that elegant works on the coupling of α-halo
carbonyl compounds with alkenes have been recently reported,
which could sufficiently avoid the β-hydride elimination side-
summarized in Table 1. First, the combined system of Pd(TFA)2
with PPh in the presence of AgOAc could afford the desired
3
[18]
[6]
product 3a in 23% yield (entry 1).
Further screening of
reaction in traditional Mizoroki-Heck coupling reaction. The
pioneering work reported by Oshima developed a cobalt(II)
catalyzed Mizoroki-Heck type reaction between alkylhalides and
palladium catalysts showed that the Pd(PPh ) is more efficient
3 4
to afford the desired product in a 70% yield (entry 4).
However, other silver compounds were found to be inefficient
to give the product. Only moderate yield of the product 3a
could be obtained when other solvent such as dichloroethane,
acetonitrile or toluene was applied in this reaction (entry 9,
[7]
styrene derivatives. Subsequently, different transition-metal
catalyzed alkenylation reactions of alkylhalides via radical
[
8]
[9]
[10]
process have been discovered by Fu, Lei, Nishikata,
[
11]
[12]
[13]
Alexanian,
Zhou,
and Wei, respectively. Recently, the
1
0).
Several phosphine ligands were screened, and the P(4-MeO-
light-induced and transition-metal catalyzed intermolecular
addition of alkyl radicals to alkenyl double bonds to form the
Ph)3 could increase the product’s yield. Further studies
showed that the yield of the product would be decreased in
the absence of 20 mol% PPh3 as ligand, and the control
experiment showed that no desired product could be detected
in the absence of the silver acetate. Finally, the catalyst
loading (Figure 1, entries 14-18), reaction temperature (Figure
1
Lei,
,4- or 1,5-dicarbonyl compounds also have been reported by
[14]
[15]
[16]
[17]
[18]
[19]
Zhang,
Wan,
Li,
Tambar,
Zhu,
and
[
20]
Melchiorre, etc. However, it was noted that only di- and tri-
substituted olefin products could be synthesized. Herein, we
a
Hefei National Laboratory for Physical Sciences at the Microscale
1
, entry 18) and the ratio of palladium catalyst with the
and Department of Chemistry, University of Science and Technology of
China, Hefei, 230026, China
phosphine ligand (Figure 1, entry 15) were also examined, a
76% yield of the product 3a could be obtained under the
following reaction conditions: 5 mol% Pd(PPh ) as catalyst, 1.5
b
Division of Chemistry and Biological Chemistry, School of Physical
3
4
and Mathematical Sciences, Nanyang Technological University,
Singapore 637371
equiv. AgOAc as additive and 20 mol% P(4-MeO-Ph) as ligand
3
in dichloromethane solution with stirring for 12 hours at 80 °C
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DOI: 10.1039/C5CC10653B
under argon atmosphere (Figure 1, entry 17).
Figure 2. Palladium-Catalyzed Alkylation of Different
a, b, c
Enamides.
Pd(PPh3)4 (5 mol%)
P(4-MeO-Ph)3 (20 mol%)
AgOAc (1.5 equiv)
Figure 1. Optimization of Reaction Conditions of Alkylation of
Enamide 1a with Ethyl 2-Bromo-2-methylpropanoate.
NHAc
AcHN
a
OEt
OEt
R
+
Br
R
DCM
O
X n
O
2a
X n
NHAc
1
3
NHAc
NHAc
OEt
OEt
O
OEt
OEt
Me
O
O
Cl
3
a (12 h)
6%
NHAc
3b (12 h)
3c (12 h)
7
71%
54%
NHAc
NHAc
OEt
OEt
F
MeO
O
O
O
O
Br
3
d (12 h)
3e (12 h)
3f (12 h)
77%
49%
70%
NHAc
NHAc
NHAc
OEt
OEt
OEt
OEt
OEt
O
O
O
OAc
g (12 h)
5%
NHAc
3
3h (12 h)
68%
3i (12 h)
6
35%
NHAc
NHAc
OEt Me
O
O
O
Me
k (21 h)
5%
Z:E = 51:49)
3
j (21 h)
3
3l (21 h)
92%
7
86%
(
Z:E > 99:1)
(
(Z:E > 99:1)
NHAc
NHAc
NHAc
OEt
OEt
OEt
O
O
O
MeO
F
OMe
3o (21 h)
7%
3
n (21 h)
a
3m (21 h)
4%
(Z:E = 82:18)
NHAc
8
Reaction conditions: To a mixture of 1a (0.3 mmol), 2a (0.6 mmol), catalyst,
80%
8
(Z:E > 99:1)
(
Z:E = 96:4)
ligand, additive and solvent (1ml) was allowed to stir for 15 h at 80 °C under
argon atmosphere. NMR yields (mesitylene was used as internal standard).
Isolated yields. No desired product. The reaction was conducted at 60°C.
NHAc
b
c
NHAc
d
e
OEt
OEt
OEt
O
O
O
O
Cl
Br
3
p (21 h)
2%
Z:E > 99:1)
NHAc
3q (21 h)
82%
3r (21 h)
With the optimized reactions conditions in hand, the
following substrate scope of different enamide was tested.
First, various substituted cyclic enamides were subjected into
this reaction. It was found that the substrate with electron-
withdrawing group would slightly decrease the product yield
69%
9
(
(Z:E > 99:1)
(Z:E = 93:7)
NHAc
OEt
OEt
S
O
O
3
s (21 h)
5%
Z:E > 99:1)
3t (21 h)
92%
6
(
Figure 2, 3c, 3e). While higher yields of the corresponding
(
(Z:E > 99:1)
a
products could be obtained for the substrates with electron-
donating groups (Figure 2, 3b and 3f). Other cyclic enamide
such as N-(2H-chromen-4-yl)acetamide was also tolerated to
afford the coupling product in 68% yield (Figure 2, 3h).
However, only 35% yield of the product was found when the
N-(acenaphthylen-1-yl)acetamide was used as substrate
Reaction conditions: To a mixture of 1 (0.3 mmol), 2a (0.6 mmol,), Pd(PPh
mol%), ligand (20 mol%), AgOAc (1.5 equiv) and solvent (1 ml) at 80 °C under argon
atmosphere. Isolated yields. Z/E ratio was determined from H NMR spectra of
3 4
) (5
b
c
1
the product.
5
Figure 3. Palladium-Catalyzed Alkylation of Enamides with
a, b, c
Different α-Bromo Carbonyls.
(
Figure 2, 3i). Next, different acyclic enamides as substrates
AcHN
R1
NHAc
X n
R1
R2
O
Pd(PPh3)4 (5 mol%)
P(4-MeO-Ph)3 (20 mol%)
AgOAc (1.5 equiv)
were screened in this reaction (Figure 2, 3j-3t). The
substituents such as different halides, methoxyl group on the
phenyl ring could be tolerated in coupling reaction, and the
corresponding products could be obtained in good yields. It
was worth noting that the substituents at the ortho-position of
phenyl ring would significantly affect the stereoselectivity of
the products (Figure 2, 3k and 3m). When the heteroaryl
substituted enamides were used, the coupling products 3r and
R2
R3
R3
R
+
Br
R
O
X n
NHAc
DCM
1
2
4
NHAc
NHAc
CO Et
2
OMe
OBn
2
CO Et
O
O
4
a (12 h)
4b (24 h)
50%
4c (24 h)
39%
50%
NHAc
NHAc
NHAc
t
O Bu
OBn
O
4e (11 h)
80%
3
s could be obtained only in moderate yields respectively.
In sequence, other α-bromo-substituted carbonyl
O
O
4
d (15 h)
5%
4f (15 h)
90%
(Z: E > 99:1)
5
compounds as coupling partners were examined in this
coupling reaction. The product 4a could be obtained in 50%
yield when the diethyl 2-bromomalonate was coupled with 1a.
In addition, the α-bromo-substituted ketone also could react
well with the enamides (Figure 3, products 4b and 4g
respectively). It was observed that less substituted α-bromo
ester would lead to a decreased product yield (Figure 3,
product 4c). Also the tert-butyl
NHAc
O
4
g (24 h)
7
7%
(
Z: E > 99:1)
a
Reaction conditions: To a mixture of 1 (0.3 mmol), 2 (0.6 mmol), Pd(PPh
3 4
) (5
1
0 mol%), ligand (20 mol%), AgOAc (1.5 equiv) and solvent (1 mL at 80 °C under argon
b
c
1
atmosphere. Isolated yields. Z/E ratio was determined from H NMR spectra of
the product
2
|
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substituted ester (Figure 3, product 4d) was not as efficient as
and 5j could be achieved in high yields via the cyclization
ethyl 2-bromo-2- methylpropanoate or benzyl 2-bromo-2- 30 reaction of 5e and 5f (Scheme 2, eq 9 and and eq 10).
methylpropanoate (Figure 3, product 4f) in coupling reaction.
Moreover, acyclic enamides were found to be more reactive
than cyclic enamides to couple with α-bromo-substituted ester
or ketone partners (Figure 3, products 4f and 4g respectively).
Scheme 3. Experiments for elucidating the possible mechanistic
pathway.
5
Scheme 2. The Application of Coupling Products for Synthesis of
γ-Amino Acid, δ-Amino Alcohol, 1,4-Dicarbonyl and γ-Lactam
a, b
1
0
Derivatives.
3
4
4
5
0
5
To better understand this palladium-catalyzed coupling
reaction between enamides and α-bromo-substituted carbonyls,
a series of control experiments were performed to elucidate the
[21]
possible mechanistic pathway.
When TEMPO (2,2,6,6-
tetramethyl-1-piperidinyloxy) was introduced to this reaction
under standard reaction conditions, the trapped byproduct
hydroxylamine A could be obtained with trace amount of
desired product. Another trapping experiment using
diphenylethylene was performed, it was found that a mixture
products of 3a and 3a’ could be obtained which was consistent
[17a]
with the previously reported result.
Moreover, there was
only ring-opening product E was observed.
Figure 4. Proposed Catalytic Cycle.
a
b
The detailed reaction conditions please see supporting information. Isolated
yields.
1
2
2
5
0
5
To demonstrate the synthetic applications, further
transformations of the coupled products have been performed.
We treated 3a and 3j with aqueous HBr solution (48%) to give
the corresponding 1,4-dicarbonyl compounds 5a and 5b
5
5
6
0
5
0
On the basis of these results, a plausible mechanism was
(
Scheme 2, eq 1 and eq 2) in 84 and 87% yields respectively, but
[11b, 12b, 18]
proposed as shown in Scheme 3.
At the beginning, an
higher temperature was required for the synthesis of 5a.
Hydrogenation of 4e and 4f would afford the corresponding γ-
amino acids 5c and 5d (Scheme 2, eq 3 and eq 4) in good yields
enolate radical II could be generated via palladium-catalyzed C–
Br bond cleavage following which could add to the double bond
[22]
of enamide. Subsequently, the new radical intermediate II
could react with the Pd(I) species and lead to the
(
84 and 81% respectively), similar good results also could be
I
observed for the ethyl-substituted 3a and 3j (Scheme 2, eq 5 and
eq 6). Further reduction of the saturated esters 5e and 5f with
DIBAL in DCM afforded the alcohols 5g and 5h both in moderate
yields (Scheme 2, eq 7 and eq 8). Under the basic condition, with
the use of aqueous LiOH in THF solution, the lactam products 5i
carbonpalladium intermediate IV, then the reductive elimiantion
can take place to afford the desired product and release the Pd(0)
catalyst for next catalytic cycle in the presence of AgOAc.
Conclusions
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In conclusion, a palladium-catalyzed alkylation reaction of
enamides with α-bromo-substituted carbonyls has been
developed, and the corresponding products could be obtained in
good yields. Importantly, the β-hydride elimination side-reaction
could be well suppressed under the optimized reaction
conditions. The coupling products could be easily transformed
into very useful γ-amino acids, δ-amino alcohols, 1,4-dicarbonyls
and γ-lactams with high efficiency. Further applications of
current methodology in the synthesis of pharmaceutical
molecules and natural products are in progress in our laboratory.
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