Palladium-Catalyzed Hydrocarbonylative Cyclization of 1,5-Dienes

Article abstract of DOI:10.1021/acs.orglett.9b02230

A novel and atom-economic palladium-catalyzed isomerization-hydrocarbonylative cyclization reaction of 1,5-dienes to 2-alkylidenecyclopentanones has been developed, which provides a rapid and straightforward approach to 2-alkylidenecyclopentanones with high stereoselectivity. The reaction was found to proceed via alkene isomerization and selective hydrocarbonylative cyclization to generate 2-alkylidenecyclopentanones with high selectivity.

Full text of DOI:10.1021/acs.orglett.9b02230

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Palladium-Catalyzed Hydrocarbonylative Cyclization of 1,5-Dienes
Suchen Zou,† Bao Gao,† Yao Huang,† Tianze Zhang,† and Hanmin Huang*,†,‡
†Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemistry, Center for Excellence in
Molecular Synthesis, University of Science and Technology of China, Chinese Academy of Sciences, Hefei, 230026, People’s
Republic of China
‡State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou, 730000, People’s Republic of China
S
* Supporting Information
ABSTRACT: A novel and atom-economic palladium-catalyzed
isomerization-hydrocarbonylative cyclization reaction of 1,5-dienes
to 2-alkylidenecyclopentanones has been developed, which provides
a rapid and straightforward approach to 2-alkylidenecyclopenta-
nones with high stereoselectivity. The reaction was found to
proceed via alkene isomerization and selective hydrocarbonylative
cyclization to generate 2-alkylidenecyclopentanones with high selectivity.
Scheme 1. Background of the Development Method
he cyclopentanones are valuable core structures of a
plethora of nature products and bioactive compounds, as
T
well as versatile intermediates in the synthesis of complex
molecules.1 In particular, 2-alkylidenecyclopentanones have
been identified as versatile building blocks for the synthesis of
various pharmaceuticals and bioactive molecules.2,3 Conse-
quently, general strategies for the efficient and selective synthesis
of cyclopentanones are highly desirable but remain under-
developed. During the past few decades, extensive efforts have
been devoted to the ring expansion reactions. Among them,
transition-metal-catalyzed ring expansion of functionalized
cyclobutanol is particularly attractive for the synthesis of 2-
alkylidenecyclopentanones. However, the tedious procedure for
the synthesis of cyclobutanol limited the large-scale synthesis
(Scheme 1a).4 Alternatively, ring-closing reactions of alkynals
and thioester olefins under the catalysis of transition metals have
also been established for the generation of 2-alkylidenecyclo-
pentanones, but the relative higher catalyst loading and
stoichiometric amount of additive required significant reduce
the synthetic practice (Scheme 1b).5,6 For all of these
approaches, the step and atom economies are imperfect, because
of the requirement for prefunctionalization and/or the need for
additional reagents in the coupling step.
Although intramolecular carbonylative cyclization reactions,
such as the Pauson−Khand reaction, have been studied
extensively,7 the preparation of 2-alkylidenecyclopentanones
through analogous cyclization reactions finds considerable less
precedent. In this context, Alexanian and co-workers developed
a Pd-catalyzed carbonylative Heck-type reaction of alkenyl
iodides in the presence of a base. In that reaction, the key
acylpalladium species was produced via sequential oxidative
addition and CO insertion with alkyl iodides as starting
materials, resulting in lower atom economy (see Scheme 1c).8
Transition-metal-catalyzed hydrocarbonylation reactions that
allow the direct use of simple alkenes as starting materials have
emerged as atom-economic methods to install carbonyl groups.9
In this context, one could establish a cyclization reaction with
1,5-dienes initiated via hydrocarbonylation to form 2-
alkylidenecyclopentanones in the presence of CO that possesses
complete atom economy (Scheme 1c). A conventional
mechanism would include hydropalladation of alkene, CO
insertion, and intramolecular alkene insertion.
However, the realization of such a process is challenging, since
the site-selective hydropalladation of one alkene moiety among
the two reactive alkene moieties is difficult. Moreover, the
intramolecular Heck reaction initiated via the hydropalladation
may occur to compete with the desired carbonylation reaction.
Received: June 28, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02230
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
In continuation of our efforts on the hydrocarbonylation
reactions,10 herein we report a palladium-catalyzed selective
hydrocarbonylative cyclization of 1,5-dienes, leading to the
efficient synthesis of 2-alkylidenecyclopentanones.
Initially, we commenced our studies by attempting the
proposed carbonylative cyclization of 2-phenyl-1,5-diene 1a in
anisole at 80 °C under CO (20 atm). With Pd(COD)Br2 as a
catalyst precursor, CH3SO3H as an additive, a variety of
commercially available ligands, including diphosphine and
monophosphine, were evaluated, and the results revealed that
some commonly used ligands, such as DPPB, DPPP, and DPPF
(Table 1, entries 1−3), were ineffective for this carbonylation.
were assessed as well, yet variation of these parameters could not
deliver better results (see the Supporting Information (SI)).
Finally, we were pleased to find that the combined yield could be
increased to 81% in the presence of MsOH and 1 mol % of
catalyst under 30 atm of CO at 90 °C (Table 1, entry 16).
With the optimized reaction conditions established, we
examined the scope of this new reaction with a variety of
substituted dienes. As summarized in Table 2, in all cases, 2-
a
Table 2. Substrate Scope
b
Table 1. Optimization of Reaction Conditions
yield,
2a+2a′
a
entry
[Pd]
ligand
DPPB
DPPP
DPPF
DPEPhos
PPh3
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
acid
(%)
2a:2a′
−
−
1
2
3
4
Pd(COD)Br2
Pd(COD)Br2
Pd(COD)Br2
Pd(COD)Br2
Pd(COD)Br2
Pd(COD)Br2
PdCl2
Pd(PhCN)2Cl2
PdBr2
Pd2(dba)3
Pd(COD)Br2
Pd(COD)Br2
Pd(COD)Br2
Pd(COD)Br2
Pd(COD)Br2
Pd(COD)Br2
MsOH
MsOH
MsOH
MsOH
MsOH
MsOH
MsOH
MsOH
MsOH
MsOH
−
0
0
0
−
15
21
78
58
65
62
trace
0
trace
73
22
54
81
88:12
88:12
91:9
90:10
88:12
89:11
−
−
−
91:9
93:7
91:9
92:8
c
5
6
7
8
9
d
10
11
12
13
14
15
NMP·HCl
PhSO3H
CF3CO2H
TsOH
e
16
MsOH
a
MsOH = methanesulfonic acid; TsOH = p-toluenesulfonic acid.
Reaction conditions: 1a (0.4 mmol), [Pd] (0.02 mmol, 5 mol %),
b
ligand (0.024 mmol, 6 mol %), acid (0.04 mmol, 10 mol %), anisole
(1.0 mL), CO (20 atm), 80 °C, 12 h, the combined yield based on the
diene and the ratio (2a:2a′) of the crude reaction mixture was
determined by GC and GC-MS analysis using n-dodecane as the
c
d
internal standard. Ligand (0.048 mmol, 12 mol %). [Pd] (0.01
e
mmol, 2.5 mol %). 1a (1.0 mmol), Pd(COD)Br2 (0.01 mmol, 1
mol %), Xantphos (0.012 mmol, 1.2 mol %), acid (0.1 mmol, 10
mol %), anisole (2.0 mL), CO (30 atm), 90 °C, 16 h.
a
Reaction conditions: 1 (1.0 mmol), Pd(COD)Br2 (1.0 mol %),
To our delight, the desired 2-alkylidenecyclopentanone (2a +
2a′) was obtained in a combined yield of 15% with good
stereoselectivity when DPEPhos (Table 1, entry 4) was
introduced into the catalytic system. Further optimization of
the ligands disclosed that Xantphos (Table 1, entry 6) could
deliver the carbonylative adducts in a combined yield of 78%
with higher stereoselectivity. Examination of palladium
precursors revealed that Pd(COD)Br2/Xantphos created the
most reactive catalyst for this transformation to give the
carbonylation product with high stereoselectivity, and other
palladium precursors showed lower activity (Table 1, entries 7−
10). No desired product formed in the absence of acid (Table 1,
entry 11), and we also found that sulfonic acids such as
methanesulfonic acid (MsOH) were superior to other acids.
Moreover, the reaction temperature, solvent, and CO pressure
Xantphos (1.2 mol %), MsOH (0.1 mmol), CO (30 atm), anisole (2.0
mL), 90 °C, 16 h. The isolated yield based on the diene and the ratio
(E/Z) of the crude reaction mixture was determined by GC and GC-
MS. bPd(COD)Br2 (5.0 mol %), Xantphos (6 mol %). c120 °C.
dPhSO3H (0.02 mmol). PhSO3H (0.1 mmol). MsOH (0.3 mmol).
e
f
g24 h.
phenyl-1,5-dienes bearing both electron-donating and electron-
withdrawing groups on the aromatic rings were all smoothly
converted to the corresponding (E)-2-alkylidenecyclopenta-
nones in moderate to good yields (33%−76%) with good to
excellent stereoselectivities. Obviously, the 2-phenyl-1,5-dienes
with electron-donating groups provided the desired products
with good yields (50%−76%, 2a−2g) and higher selectivities.
B
DOI: 10.1021/acs.orglett.9b02230
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
The structure of the desired product 2g was unambiguously
confirmed by single-crystal X-ray diffraction (XRD) analysis. In
contrast, 2-phenyl-1,5-dienes substituted by electron-withdraw-
ing groups such as −CF3 (2l), −CN (2m), and −CO2Me (2n),
led to relative lower yields (33%−57%) and selectivities. The
steric effect was observed in the transformation. The carbon-
ylation reaction of ortho-substituted dienes, such as 1d and 1p,
gave relative lower yields and selectivities. Pleasingly, the
reaction performed well for the diene substituted with 2-
naphthyl (2o, 66% yield). Substituents could be incorporated
into the tether-backbone of the dienes to give the corresponding
adduct (2q). In addition, heteroaromatic-ring-substituted 1,5-
dienes provided corresponding product in moderate yields with
excellent stereoselectivities (2r and 2s). Encouraged by our
success with aromatic diene, aliphatic diene was then explored.
Under the optimized reaction conditions, the large steric 2-
adamantyl substituted diene (1t) can be applied to this protocol,
delivering the desired product (2t) in 28% yield with trans-
configuration. The analogue 1,6-dienes were also tested, but no
desired carbonylative cyclization reactions were observed. To
our great delight, the target reaction proceeded smoothly at a
lower catalyst loading (0.1 mol %), affording the desired 2-
alkylidenecyclopentanone 2a in 49% yield with excellent
stereoselectivity (see the SI).
alkylidenecyclopentanone 2a via hydrocarbonylation and Heck
reaction.
On the basis of precedent results10 and the above
experimental data, a tentative catalytic cycle is proposed in
Scheme 2. The reaction begins with the generation of palladium
Scheme 2. Plausible Reaction Mechanism
To gain some insight into the mechanism of the present
carbonylation reaction, some control experiments were
conducted. Treatment of diene 1a with MsOH at 90 °C for
16 h, the 1,4-diene (3a) was obtained in 93.3% yield with E:Z =
87:13. The 1,4-dienes can be converted to the corresponding
product in 60% yield under the standard reaction conditions
(Figure 1). The reaction was monitored using substrate 1a to
hydride species from the reaction of Pd(0) and MsOH. Next,
intermolecular hydropalladation of the 1,4-diene A, which is
produced from the isomerization of 1,5-diene under the acidic
condition,11 generated the intermediate B. Subsequent insertion
of CO into the alkyl−palladium bond affords the acyl palladium
complex C, followed by the intramolecular CC double bond
insertion to afford the intermediate D. Finally, β-hydride
elimination leads to the desired product and regenerates the
palladium hydride species for the next catalytic cycle.
In summary, we have developed an atom-economic protocol
for the synthesis of 2-alkylidenecyclopentanones via Pd-
catalyzed isomerization−cyclocarbonylation of 1,5-dienes. The
reaction proceeded well to furnish the desired products with
good functional-group compatibility and high stereoselectivity,
which provided a rapid and reliable approach to 2-
alkylidenecyclopentanones. Mechanistic studies suggested that
the selective isomerization of 1,5-dienes occurred first to form
1,4-dienes, which subsequently were converted to the desired
products through hydrocarbonylation and intramolecular Heck
reaction. Further studies to apply this strategy are in progress in
our laboratory.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b02230.
Figure 1. Reaction profile of Pd-catalyzed hydrocarbonylative
cyclization of 1a.
Experimental details and full spectroscopic data for all
new compounds (PDF)
Accession Codes
detect whether the diene isomerization occurred. It illustrated
that the 1,5-diene 1a was transformed to 1,4-dienes A and A′
from the beginning of the process and then the 1,4-diene was
slowly transformed to the desired product 2a (Figure 1). Based
on these studies, it can be reasoned that selective isomerization
of 1,5-diene occurred first to form thermodynamic stable (E)-
1,4-diene, which was subsequently converted to the desired 2-
CCDC 1937070 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
C
DOI: 10.1021/acs.orglett.9b02230
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: hanmin@ustc.edu.cn.
ORCID
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Hanmin Huang: 0000-0002-0108-6542
Notes
The authors declare no competing financial interest.
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