Palladium-catalyzed cascade metallo-ene/Suzuki coupling reaction of allenamides

Article abstract of DOI:10.1039/c7cc00191f

A new type of cascade metallo-ene/Suzuki coupling reaction of allenamides catalyzed by palladium is described. A variety of polyfunctionalized 2,3-dihydropyrrole derivatives, which are important structural motifs for bioactive molecules, were furnished wi

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Palladium-Catalyzed Cascade Metallo-Ene/Suzuki Coupling
Reaction of Allenamides
Hanbing Liang,a Fachao Yan,a Xu Dong,a Qing Liu,a Xiaobing Wei, a Sheng Liu,c Yunhui Dong*,a and
Hui Liu*,a,b,c
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A new type of cascade metallo-ene/Suzuki coupling reaction of
allenamides catalyzed by palladium has been described. A variety
of polyfunctionalized 2,3-dihydropyrrole derivates which are
important structural motifs for bioactive molecules, were
furnished with excellent yields. Two new Csp3-Csp2 bonds were
constructed in one pot efficiently. The reductive elimination from
π-allyl palladium complex presented excellent regioselectivity to
terminal C1 position. The unique terminal alkene was one of the
most easily functional group, providing these molecules potential
transformation to much more complicated molecules.
O
O
N
N
F
O
F3C
HO
N
N
CN
N
O
H
O
OPh
Vildagliptin (Novartis)
Cytotoxic Activity
Serotonin Reuptake Inhibitor
Eli Lilly and Company
Patent: US6353008 B1, 2002
HO
O
Me
Cl
N
O
Me
N
N
H2N
Me
OH
CN
O
O
Anti-Juvenile
Pirprofen
Hormone Activity
Saxagliptin (BMS)
The synthesis of polyfunctionalized pyrroles remains an
attractive objective due to the versatile existing in biological,
pharmaceutical and organic chemistry.1 A lot of drugs and
bioactivated compounds have been reported, such as
serotonin reuptake inhibitor from Eli Lilly,2 Vildagliptin from
Novartis,3 Saxagliptin from BMS,4 Pirprofen and so on (Figure
1).5 Therefore, it is quite important to develop efficient
methodologies for the construction of pyrrole scaffold.
Figure 1. Representative bioactive molecules bearing 2,3-
dihydropyrrole scaffolds.
reaction of allenamides to construct 1,2,3,4-tetrahydro-
pyridine derivatives.12 Considering our interest on ene
reactions, we attempt to introduce the allenamines to ene
reaction, developing novel methodologies in the construction
of functionalized pyrroles.
In the past few decades, numerous elegant methodologies
have been developed to construct the pyrrole motif. In
particularly, transition metal-catalyzed cyclization reactions
represent a powerful and atom economic strategy,6 such as
Ni,7 Pd,8 Pt,9 Rh10 and Ru11. During our research on the
cyclization reactions, the allenamines as a unique building
block cause our attention, which is potential to construct
nitrogen-containing heterocyclic derivatives easily. Very
recently, our group realized a pd-catalyzed cyclization-Heck
The ene reaction is mechanistically related to the much more
famous Diels-Alder reaction, both of which proceeds through cyclic
transition states involving six electrons. However, the higher
activation energy for an ene reaction limits the application of ene
reaction in organic synthesis.13 In the past few decades, a series of
mild ene reactions have been developed by chemists, during which
metallo-ene reaction shows highly efficiency and promising
application.14 Recently, some catalytic, enantioselective metallo-
ene processes are realized, arousing more interests to metallo-ene
reaction.15 It is well known intramolecular metallo-ene reaction,
working via the interception of allylmetal intermediates with
adjoining π-unsaturation, is an important subdivision of ene
reaction,16 which has made significant contributions to build
densely functionalized cyclic molecules.14h, 17
Although the metallo-ene reaction could be seen as
mechanistically analogous to traditional ene reaction, significant
differences between these two processes are still apparent
(Scheme 1). According to the traditional ene reaction, [1,5]-hydride
shift is underwent accompanying the formation of new Csp3-Csp3
bond. In the metallo-ene reaction of diene, the generated transient
alkylmetal intermediate could produce a new intermediate I, which
could be further functionalized via coupling reactions (Scheme 1a).
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providing 3ab in 70% yield without the detection of 3aa (Entry 11).
The results from bidentate ligands indicated the unsuccessful of
transmetalation with boronic acid 2a, which might due to the big
steric-hindrance of bidentate ligands. We are excited to achieve an
excellent yield of 95% when PdCl2(PPh3)2 was applied (Entry 16).
Two rhodium catalysts were examined, while no positive results
were detected (Entries 17 and 18). Basing on these results, the
optimized conditions were determined to include PdCl2(PPh3)2
combined with K3PO4 in dioxane at 50oC under N2 atmosphere
(Entry 16).
With the optimized conditions in hand, we then aimed to explore
the substrate scope of this cascade metallo-ene/Suzuki coupling
reaction. A variety of boronic acid were examined under the
optimal conditions. The substrates with electron-donating group
methoxyl or methyl at ortho/para-position afforded the
corresponding products in excellent yields (3aa-3ad). The (4-(tert-
butyl)phenyl)boronic acid could also form 2,3-dihydropyrrole
derivates 3ae in 91% yield. Remarkably, the halides substitutions at
boronic acid showed not only excellent compatibility but also
excellent convertion (3af-3aj). Notably, bromo could survive very
well under these conditions without bromo corrosion (3ah and 3ai).
The electron-withdrawing substituents, such as CF3, CN and NO2,
could efficiently produce the desired molecules (3ak-3am).
Furthermore, condensed rings such as naphthalen-1-ylboronic acid
and benzofuran-2-ylboronic acid could also deliver the coupling
Scheme 1. The Traditional ene Reaction and Metallo-ene Reaction.
In the allene system, the insertion of π-allylic metal-complex to
the internal or external double bond of allene could generate
intermediates II or III, which could be transformed into more
functionalized molecules (Scheme 1b). Bäckvall group made a great
contribution to the carbocyclization of allene recently,18 including
that a stereoselective pd-catalyzed carbocyclization of allenic allylic
Table 1. Palladium-Catalyzed Metallo-Ene/Suzuki Coupling
Reaction of Allenamides a
carboxylates.18x In 2008, Tsukamoto group reportd a pd-catalyzed
19
arylative cyclization of allenyl enones.
However, there is few
report on the allenamides metallo-ene reaction.
Allenamides, functionally derived from allenamines, involving the
amido group inducing delocalization of the lone-pair electrons on
nitrogen and consequently exhibiting improved stability, have
Entry
Base
[Cat]
ligand
Solvent
Yield of 3aa (%)b
1
2
K2CO3
K2CO3
Pd(PPh3)4
Pd(PPh3)4
\
\
toluene
dioxane
45
54
become
a versatile and powerful building block in synthetic
chemistry.20 Pursuing our interests in developing new
methodologies for the synthesis of nitrogen-containing heterocyclic
derivatives, we hypothesize that the allenamines would be
competent at performing an intramolecular metallo-ene reaction,
furnishing a π-allylic intermediate IV, which could be further
functionalized via coupling reaction (Scheme 1c).
3
4
5
6
K2CO3
K2CO3
K2CO3
Na2CO3
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
\
\
\
\
DMF
50
33
53
45
DCM
MeCN
dioxane
7
KOAc
K3PO4
Et3N
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
\
\
dioxane
dioxane
dioxane
dioxane
36
75
32
45
8
In this work, we realized
a palladium-catalyzed metallo-
9
\
ene/Suzuki coupling reaction of allenamides to construct
polyfunctionalized 2,3-dihydropyrrole derivates, accompanying two
new Csp3-Csp2 bonds formation. We initially investigated the
10
Cy2NMe
\
9
K3PO4
Pd(OAc)2
PPh3
dioxane
50
cascade
reaction
with
allenamines
1a
with
(4-
10
11
12
K3PO4
K3PO4
K3PO4
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
dppm
dppe
dppp
dioxane
dioxane
dioxane
ND
70c
ND
methoxyphenyl)boronic acid 2a in the presence of 10 mol%
Pd(PPh3)4 and 2.0 equiv. of K2CO3 at 50 °C in toluene under N2. To
our delight, the desired 2,3-dihydropyrrole derivates 3aa was
isolated in 45% yield (Entry 1). Then different solvents were
screened, such as dioxane, DMF, DCM and MeCN, among which
dioxane could increase the yield slightly to 54% (Entries 2-5).
Subsequently, both inorganic and organic bases were investigated
(Entries 2-5). Gratifyingly, the yield was increased dramatically to 75%
when K3PO4 was applied (Entry 8). When Pd(OAc)2 combined with
PPh3 was used as catalyst, product 3aa was obtained in 50% yield.
However, the employment of bidentate ligands, such as dppm,
dppp, dppb, dppf and Binap, could not promote this transformation,
accompanying the formation of 4,4'-dimethoxy-1,1'-biphenyl and
decomposition of 1a (Entries 10,12-15). To our surprise, when dppe
was used, the metallo-ene/reduction reaction was happened,
13
14
15
K3PO4
K3PO4
K3PO4
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
dppb
dppf
dioxane
dioxane
dioxane
ND
ND
ND
Binap
16
K3PO4
PdCl2(PPh3)2
\
dioxane
96
17
K3PO4
[Rh(OH)cod]2
PPh3
dioxane
ND
18
K3PO4
[RhClcod]2
PPh3
dioxane
ND
aReaction conditions: 1a (0.2 mmol), 2a (0.22 mmol), [Pd] (0.02
mmol), base (0.4 mmol), solvent (2.0 mL). bIsolated yields. c3ab was
obtained.
2 | J. Name., 2012, 00, 1-3
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Table 2. Scope of Cascade Metallo-Ene/Suzuki Coupling Reactiona,b
Ar
AcO
PdCl2(PPh3)2 (10 mol%)
ArB(OH)2
+
K3PO4 (2.0 equiv)
Dioxane, 50℃, 8 h
N
N
PG
3
PG
1
2
OMe
Me
OMe
N
N
N
Ts
Ts
Ts
3aa: 96%
3ab: 93%
3ac: 95%
tBu
F
Me
N
N
N
Ts
Ts
Ts
Scheme 2. Proposed Mechanism for the Cascade Metallo-
Ene/Suzuki Coupling Reaction.
3af: 98%
3ae: 91%
3ad: 92%
-
III
.
Subsequently, transmetalation with boronic acid 2a
, which then formed the
Cl
Br
produced palladium complex 2a
-
I
Br
N
N
N
second Csp3-Csp3 bond and gave final product 3aa. It was
worth mentioning that highly reductive elimnation selectivity
with terminal C1 but not internal C3 was observed.
Ts
Ts
Ts
3ag: 97%
3ah: 97%
3ai: 96%
In conclusion, we have developed
a highly efficient
F
CF3
palladium-catalyzed cascade metallo-ene/Suzuki coupling
reaction of allenamides, delivering polyfunctionalized 2,3-
dihydropyrrole derivates in excellent yields. In this
transformation, two new Csp3-Csp2 bonds were constructed in
one pot. The reductive elimination from π-allyl palladium
F
CN
N
N
N
Ts
3aj: 98%
Ts
Ts
3ak: 95%
3al: 88%
O
complex 2a-I presented excellent regioselectivity, providing
NO2
the terminal C1 position coupling product without detection of
internal C3 coupling product. The unique terminal alkene as
one of the most easily functional group endowed these
molecules versatile transformation to much more complicated
molecules. Further applications are in progress in our
laboratory.
N
N
N
Ts
Ts
Ts
3ao: 80%
3am: 85%
3an: 83%
OMe
F
NO2
N
N
N
We thank the National Natural Science Foundation of China
(21302057 and 21405095), the Young Talents Joint Fund of
Shandong Province (ZR2015JL005), CPSF (2016M590736),
Special Funding for Postdoctoral Innovation Project of
Shandong Province (201501002), and The Technology Project
of General Administration of Quality Supervision, Inspection
and Quarantine (2016IK202). We are grateful for the support
from Zibo Positive Additive Co., Ltd.
Ms
Ms
Ms
3bm: 93%
3ba: 84%
3bf: 89%
aReaction Conditions: 1a (0.2 mmol), 2 (0.22 mmol), PdCl2(dppf)
(0.02 mmol), K3PO4 (0.4 mmol), dioxane (2.0 mL), 50oC, 8h. bIsolated
yields.
products in good yields (3an-3ao). In addition, the corresponding
products from methyl sulfonyl substrates were obtained in satisfied
yields as well (3ba, 3bf, and 3bm). Although p-nitrobenzenesulfonyl
protected allenamides could deliver the corresponding products,
the easy decomposition made the products be stored for only few
hours (See SI).
Notes and references
1
(a) G. Cirrincione, A. M. Almerico, E. Aiello and G. Dattolo:
Aminopyrroles, in: R. A. Jones (Ed.), Pyrroles Part Two: The
Synthesis Reactivity and Physical Properties of Substituted
Pyrroles, John Wiley & Sons, New York, 1992. 299; (b) R. J.
Sundberg, in: A. R. Katritzky, C. W. Rees (Eds.),
Combined with the proposed mechanism by Bäckvall group,18
a
possible pathway for this palladium-catalyzed cascade metallo-
ene/Suzuki coupling reaction of allenamides was shown in Scheme
2. In the beginning, oxidative addition of an allyl acetate 1a to Pd(0)
gives intermediate 1a-I (η1 form), which also existed as π-allyl
palladium intermediate 1a-II (η3 form). The following insertion to
the allene generates the first Csp3-Csp3 bond formation and metallo
-ene reaction delivered another π-allyl palladium intermediate 1a
Comprehensive Heterocyclic Chemistry, Pergamon Press,
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2
L. W. Hertel and Y. C. Xu, Eli Lilly and Company (US), Patent:
US6353008 B1, 2002, 14.
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