Accessing 1,3-Dienes via Palladium-Catalyzed Allylic Alkylation of Pronucleophiles with Skipped Enynes

Article abstract of DOI:10.1021/acs.orglett.7b01960

An unprecedented palladium-catalyzed allylic alkylation of pronucleophiles with unactivated skipped enynes has been developed. This method provides a straightforward access to a wide array of 1,3-dienes without the need to preinstall leaving groups or employ extra oxidants. The reaction exhibited high atom economy, good functional group tolerance, excellent regioselectivities, and scalability. With D₂O as cosolvent, deuterium could be incorporated in high efficiency.

Full text of DOI:10.1021/acs.orglett.7b01960

Letter
pubs.acs.org/OrgLett
Accessing 1,3-Dienes via Palladium-Catalyzed Allylic Alkylation of
Pronucleophiles with Skipped Enynes
Shang Gao,† Hao Liu,† Chi Yang, Zhiyuan Fu, Hequan Yao,* and Aijun Lin*
State Key Laboratory of Natural Medicines (SKLNM) and Department of Medicinal Chemistry, School of Pharmacy, China
Pharmaceutical University, Nanjing 210009, P. R. China
S
* Supporting Information
ABSTRACT: An unprecedented palladium-catalyzed allylic
alkylation of pronucleophiles with unactivated skipped enynes
has been developed. This method provides a straightforward
access to a wide array of 1,3-dienes without the need to
preinstall leaving groups or employ extra oxidants. The
reaction exhibited high atom economy, good functional
group tolerance, excellent regioselectivities, and scalability. With D2O as cosolvent, deuterium could be incorporated in high
efficiency.
1,3-Dienes are crucial skeletons found in numerous natural
products and bioactive molecules,1 such as antiproliferative
agent dolatrienoic acid,2 (−)-zampanolide,3 antitumor anti-
biotic anthramycin,4 and bacteriostatic antibiotic (−)-macro-
lactin A.5 Moreover, they also serve as privileged building
blocks in organic synthesis.6 Thus, the development of efficient
methods to construct 1,3-dienes has been actively pursued by
synthetic chemists. Traditional methods to build 1,3-dienes
include the Horner−Wadsworth−Emmons reaction,7 Wittig
reaction,8 transition-metal-catalyzed (dehydrogenative) cross-
coupling,9 olefin metathesis,10 and others.11 As an alternative,
transition-metal-catalyzed allylic substitution provides a new
method to construct 1,3-dienes (Scheme 1a).12 Furthermore,
the allylic C−H oxidative functionalization of 1,4-dienes has
attracted chemists’ research interests over the last few decades
(Scheme 1b).13 However, some unavoidable limitations of
previous methods still exist with respect to the formation of
stoichiometric valueless byproducts, prefunctionalization of
substrates, and employment of stoichiometric oxidants. An
undeveloped route to 1,3-dienes that is ideal from the
perspective of atom economy and sustainable chemistry is
redox-neutral allylic alkylation with unactivated skipped enynes
because of its inherent advantages of bypassing prefunctional-
ization of the substrates.
Since transition-metal-catalyzed redox-neutral allylic alkyla-
tion of internal or terminal alkynes has been described,14 we
were drawn to the possibility that skipped enynes15 could
provide a new avenue to develop a general catalyst system for
building 1,3-dienes with a range of pronucleophiles (Scheme
1c). Mechanistically, the 1,3-dienes could be produced through
the following process: (1) skipped enynes transform to vinyl
allene intermediates Int I through M−H insertion and
subsequent β-hydrogen elimination, which was followed by
the addition of M−H species again to deliver vinyl (π-allyl)
metal Int II; (2) Int II converts into Int III via π−σ−π
isomerization; (3) pronucleophiles capture Int III to afford the
corresponding 1,3-dienes. With this strategy selected, the
regioselectivities and substrate scope are potential issues to be
determined. Moreover, the success of this idea depends largely
on the isomerization process and the choice of transition-metal
catalysts. As a continuation of our previous work on the
palladium-catalyzed allylic alkylation of alkynes,16 herein we
describe an unprecedented palladium-catalyzed redox-neutral
allylic alkylation of pronucleophiles with skipped enynes to
construct 1,3-dienes with high atom economy.
Scheme 1. Transition-Metal Catalyzed Allylic Alkylation To
Construct 1,3-Dienes
We commenced our study with 1-methyl-3-phenylindolin-2-
one 1a and pent-4-en-1-yn-1-ylbenzene 2a as substrates, and
the results are summarized in Table 1. A series of acids typically
employed for palladium-catalyzed redox-neutral allylic alkyla-
tion were examined (Table 1, entries 1−5), and a good result
was observed with PhCO2H as additive, giving the desired 1,3-
Received: June 27, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01960
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Organic Letters
Letter
a
a−c
Table 1. Optimization of Reaction Conditions
Scheme 2. Substrate Scope of Skipped Enynes
b
T
(°C)
yield
ratio (3aa/
c
entry
catalyst
additive
(%)
3aa′)
1
2
3
4
5
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd2(dba)3
100
100
100
100
100
100
100
100
TsOH·H2O
(PhO)2POOH
Ph3CCO2H
CH3CO2H
PhCO2H
trace
38
6.7:1
5.7:1
6.3:1
8.2:1
8.0:1
86
92
93
d
6
PhCO2H
84
d
7
d
PhCO2H
<10
<10
8
Pd2(dba)3·
CHCl3
PhCO2H
9
Pd/C
100
100
100
100
80
PhCO2H
PhCO2H
PhCO2H
nd
nd
nr
10
11
12
13
14
15
Pd(OAc)2
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
39
74
50
nd
4.3:1
5.2:1
3.9:1
PhCO2H
PhCO2H
PhCO2H
60
40
a
Reaction conditions: 1a (0.25 mmol), 2a (0.50 mmol), [Pd] (5.0 mol
%), additive (5.0 mol %) in 1.0 mL of toluene at 100 °C under argon
a
b
Reaction conditions: 1a (0.25 mmol, 1.0 equiv), 2 (0.50 mmol, 2.0
for 12 h. Isolated yields and the yields are reported as a mixture of E
c
d
and Z isomers. The ratios were determined by 1H NMR. 2.5 mol %
of catalyst and 2.5 mol % of PhCO2H were used. nr = no reaction. nd
= not detected.
equiv), Pd(PPh3)4 (5.0 mol %), and PhCO2H (5.0 mol %) in toluene
(1.0 mL) at 100 °C under argon for 12 h. Isolated yields and the
b
c
yields are reported as a mixture of E and Z isomers. The ratios of 2E/
d
e
f
g
2Z were determined by 1H NMR. 24 h. 85 °C. 120 °C. 3.0 mol %
of Pd(PPh3)4 and 3.0 mol % of PhCO2H were used.
dienes 3aa and 3aa′ in 93% total yield. Other acids such as
TsOH·H2O, (PhO)2POOH, Ph3CCO2H, and CH3CO2H
showed inferior performance with respect to both product
yields and ratios of 3aa/3aa′. The structure of 3aa was assigned
by X-ray crystallographic analysis (see the Supporting
Information for details).17 The role of each reactant was
further tested. The 1,3-dienes could still be obtained in 84%
yield without obvious loss of stereoselectivity when the catalyst
loading was reduced to 2.5 mol % (Table 1, entry 6).
Pd(PPh3)4 was critical to this reaction because no product was
detected with Pd/C or Pd(OAc)2 as catalysts, and poor yields
were obtained with Pd2(dba)3 or Pd2(dba)3·CHCl3 as catalysts
(Table 1, entries 7−10). The reaction did not occur under
catalyst-free conditions (Table 1, entry 11). Control experi-
ments revealed that PhCO2H played a vital role in the reaction
(Table 1, entry 12). The yields and ratios of 3aa/3aa′ were
reduced at lower temperatures (Table 1, entries 13 and 14),
and no reaction was observed when the reaction temperature
was below 40 °C (Table 1, entry 15).
With the optimized conditions in hand, we then investigated
the scope and generality of this reaction with a series of skipped
enynes, and the results are summarized in Scheme 2. Skipped
enynes bearing both electron-donating groups (−Me, −OMe)
and electron-withdrawing groups (−CF3, −F, −Cl) on the
phenyl group gave the desired products 3ab−af in good yields
from 88% to 96%. Ortho-substituted skipped enyne 2g
exhibited lower reactivity due to steric hindrance, while meta-
substituted skipped enyne 2h afforded 3ah in 89% yield. When
the phenyl group was changed to biphenyl and 1-naphthalenyl
groups, the reaction proceeded smoothly to deliver 3ai in 95%
yield and 3aj in 62% yield, respectively. Moreover, heterocycles
and functional groups such as thiophene-3-yl and ferrocenyl
groups could also be tolerated to give 3ak in 94% yield and 3al
in 96% yield. For 2m with an alkyl substituent, the allylic
alkylation proceeded regioselectively at the terminal position to
produce 3am in good yield. To further test the practicability of
this reaction, a gram-scale reaction was conducted with 3.0 mol
% of Pd(PPh3)4 as catalyst, and 3aa was generated in 86% yield
(1.576 g, 2E/2Z = 10.6:1).
Next, we turned our attention to explore the generality of the
pronucleophiles under the standard conditions and the results
are summarized in Scheme 3. Indolinones with electron-
donating groups (−OMe, −Me) and an electron-withdrawing
group (−F) afforded 3ba−da in 74−83% yields. The
nonsteroidal anti-inflammatory drug phenylbutazone was
efficiently allylic alkylated at the C3-position to provide 3ea
in 78% yield. Cyano, nitro, and acetyl groups were also
compatible with this catalytic system and delivered 3fa−ha in
53−97% yields. Moreover, difluoride product 3ia was achieved
in 72% yield, and the incorporated phosphonate group
provided a handle to further derivation. The diketones 1j and
1k could be elaborated efficiently to give 3ja in 67% and 3ka in
93% yield. Moreover, 3ka (2.33 g) could be synthesized in 92%
yield on a 10.0 mmol scale with the reaction time prolonged to
24 h. To our delight, 1,3-dienoic moiety could also be
introduced to the unactivated cyclic ketone with L-proline and
TsOH·H2O as co-catalyst,16b delivering 3la in 60% yield. In
addition, heteronucleophiles such as indoline and tetrahydro-
quinoline could also generate 3ma and 3na in 97% and 96%
yields through C−N bond formation. It is worth noting that the
complex natural product rutecarpine, a COX-2 inhibitor, was
modified successfully to provide 3oa in 57% yield. When
B
DOI: 10.1021/acs.orglett.7b01960
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a−c
Scheme 3. Substrate Scope of Pronucleophiles
Figure 1. X-ray structure of 6a.
Scheme 5. Proposed Mechanism
a
Reaction conditions: 1 (0.25 mmol, 1.0 equiv), 2a (0.50 mmol, 2.0
equiv), Pd(PPh3)4 (5.0 mol %), and PhCO2H (5.0 mol %) in toluene
b
(1.0 mL) at 100 °C under argon for 12 h. Isolated yields and the
c
yields are reported as a mixture of E and Z isomers. The ratios of 2E/
d
e
2Z were determined by 1H NMR. CH3CN was used as solvent. 10.0
f
mmol scale, 24 h. Pd(PPh3)4 (10.0 mol %), (4−F-C6H4)3P (20.0 mol
%), TsOH·H2O (15.0 mol %), and L-proline (30.0 mol %) were used.
obtained in 89% yield. The deuterated ratios at the α,β,γ-
positions were >99%, >99%, and 21%, respectively, which
indicated that the β-hydrogen elimination of intermediate B to
vinyl allene C was reversible as shown in Cycle I of Scheme 5b
g
h
PhCO2H (0.25 mmol, 1.0 equiv) was used. CH3CO2H (0.25 mmol,
1.0 equiv) was used.
(see the SI for details).18 On the basis of previous reports,19
a
benzoic acid and acetic acid were used as pronucleophiles, 3pa
and 3qa were obtained in 67% and 61% yields through C−O
bond construction, while PhOH or BnOH could not offer the
corresponding product.
To take advantage of the 1,3-dienes, 3aa was further
transformed to various attractive structures (Scheme 4).
plausible catalytic cycle is proposed as shown in Scheme 5b.
First, oxidation of Pd(PPh3)4 with benzoic acid initiates the
catalytic cycles and affords the hydridopalladium species A.
Hydropalladation of 2 with A affords the vinyl palladium
intermediate B. β-Hydrogen elimination of B produces the
phenyl allene C and intermediate A (catalytic cycle I). Next,
hydropalladation of A with phenyl allene C delivers the vinyl
(π-allyl)palladium species D (catalytic cycle II), which delivers
intermediate E through π−σ−π isomerization. Capture of
intermediate E with pronucleophiles 1 affords the allylic
alkylated products 3 in addition to Pd(0), which can reenter the
next catalytic cycle.
Scheme 4. Derivation of the Allylic Alkylated Product 3aa
In summary, we have developed a novel palladium-catalyzed
allylic alkylation of various pronucleophiles with unactivated
skipped enynes to construct 1,3-dienes through C−C, C−N,
and C−O bond formation. Various functionalized 1,3-dienes
were obtained in good yields with high atom economy in
excellent regioselectivities, which also provided a platform to
streamlining the synthesis of complex molecules. The in situ
formed vinyl allene intermediate bypassed preinstallation of
leaving groups to the nucleophilic partner bearing the
dienophile in allylic substitution and employment of extra
oxidants in allylic C−H oxidation functionalization.
Through catalytic hydrogenation, indolinone 4 containing a
long-chain alkyl group could be obtained in 93% yield.
Moreover, through a tandem Diels−Alder/oxidation process,
indolinone 5 with a polysubstituted phenyl group was achieved
in 71% yield. When N-benzylmaleimide was selected as
dienophile, polycyclic compounds 6a and 6b were isolated in
72% total yield (6a/6b = 1.5:1). The structure of 6a was
assigned by X-ray crystallographic analysis (Figure 1; see the SI
for details).17
ASSOCIATED CONTENT
■
S
* Supporting Information
To gain insight into the mechanism of this reaction, a
deuteration reaction was conducted (Scheme 5a). When D2O
was used as a cosolvent, deuterated product 3aa-dn could be
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01960.
C
DOI: 10.1021/acs.orglett.7b01960
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
1H and 13C NMR spectra for all new compounds (PDF)
Letter
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: hyao@cpu.edu.cn, cpuhyao@126.com.
*E-mail: ajlin@cpu.edu.cn.
ORCID
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Chi Yang: 0000-0002-3436-8554
Hequan Yao: 0000-0003-4865-820X
Aijun Lin: 0000-0001-5786-4537
Author Contributions
†S.G. and H.L. contributed equally to this work.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Generous financial support from the National Natural Science
Foundation of China (NSFC21502232 and NSFC21572272),
the Natural Science Foundation of Jiangsu Province
(BK20140655), and the Foundation of State Key Laboratory
of Natural Medicines (ZZYQ201605) is gratefully acknowl-
edged.
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DOI: 10.1021/acs.orglett.7b01960
Org. Lett. XXXX, XXX, XXX−XXX