Palladium-Catalyzed Tandem Dehydrogenative [4 + 2] Annulation of Terminal Olefins with N-Sulfonyl Amides via C-H Activations

Article abstract of DOI:10.1021/acs.orglett.0c01011

A palladium-catalyzed tandem dehydrogenative [4 + 2] annulation of terminal olefins with N-sulfonyl amides via C(sp²)-H activation, allylic C(sp³)-H activation, and homoallylic C(sp³)-H elimination processes has been developed. Promoted by the DMSO ligand, various benzamides, heterocyclic arylamides, alkenyl carboxamides, and commercial olefins are found to be efficient substrates to construct important heterocyclic compounds bearing a vinyl substituent with high E stereoselectivity. Using air as the terminal oxidant also provides a great advantage regarding environmental friendliness.

Full text of DOI:10.1021/acs.orglett.0c01011

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Letter
Palladium-Catalyzed Tandem Dehydrogenative [4 + 2] Annulation
of Terminal Olefins with N‑Sulfonyl Amides via C−H Activations
Manman Sun, Weida Chen, Xiangyu Xia, Guodong Shen, Yongmin Ma, Jianguo Yang, Hanfeng Ding,
and Zhiming Wang*
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ABSTRACT: A palladium-catalyzed tandem dehydrogenative [4 +
2] annulation of terminal olefins with N-sulfonyl amides via
C(sp2)−H activation, allylic C(sp3)−H activation, and homoallylic
C(sp3)−H elimination processes has been developed. Promoted by
the DMSO ligand, various benzamides, heterocyclic arylamides,
alkenyl carboxamides, and commercial olefins are found to be
efficient substrates to construct important heterocyclic compounds
bearing a vinyl substituent with high E stereoselectivity. Using air as the terminal oxidant also provides a great advantage regarding
environmental friendliness.
he Tsuji−Trost reaction is well established as a powerful
ylation,4h,5a fluorination,4c and others4g have also been
developed (Scheme 1b).6 However, palladium-catalyzed
tandem reactions involving allylic C(sp3)−H functionalization
are rarely reported.7 In 2008, the Shi group reported a
palladium-catalyzed tandem dehydrogenative diamination of
terminal olefins (Scheme 1c).8 In this report, the used N,N-di-
tert-butylthiadiaziridine 1,1-dioxide was a dual nitrogen source.
After allyl sulfamide was formed first, the retained N−H bond
and carbon−carbon double bond subsequently underwent
palladation and β-hydride elimination pathways, affording the
cyclic diamine product. This tandem reaction shows great
advantage in enabling the rapid construction of complexly
cyclic compounds from simple starting materials. Hence,
determining how to utilize a suitable substrate with dual
nucleophilic centers to perform the dehydrogenative annula-
tion with terminal olefins involving allylic C(sp3)−H
functionalization is challenging and rewarding.
T
tool in organic synthesis, for the retained degree of
unsaturation in the allylic products is easily manipulated to
access various versatile functional groups (Scheme 1a).1
Scheme 1. Pd-Catalyzed Allylic Substitution Reactions
Palladium-catalyzed directed C(sp2)−H functionalization of
amides is an appealing area,9 especially the cascade cyclization
reactions,9b,10 since the CONHR group can serve not only as a
directing group for C(sp2)−H functionalization but also as a
precursor to the C−N bond. Hence, we hypothesized that
benzamides could be chosen as the dual nucleophilic sources
to perform the dehydrogenative [4 + 2] annulation with
terminal olefins. However, palladium-catalyzed [4 + 2]
annulation of benzamides with isolated alkenes is still a
Compared with using the preactivated alkenes with an allylic
leaving group (LG), such as ester, halide, carbonate,
carbamate, and phosphate, directly oxidative allylic C(sp3)−
H cleavage of simple alkenes to generate the π-allyl palladium
intermediates is more challenging, owing to the weak
coordination ability of the C(sp3)−H bond with palladium2
and the competitive addition reaction of the carbon−carbon
double bond with palladium.3 Nevertheless, with the assistance
of ligands4 and/or by modification of reaction conditions,5
numerous reactions including palladium-catalyzed allylic C-
(sp3)−H oxygenation,4a,5c amination,4b,d alkylation,4e,i−m ar-
Received: March 19, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01011
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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a
Table 1. Optimization of Reaction Conditions
b
entry
catalyst
ligand
oxidant
yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Pd(TFA)2
Pd(OAc)2
PdCl2
−
−
−
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
Cu(OAc)2
Cu(OAc)2
−
56
44
47
93
trace
70
57
53
64
46
44
0
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
DMSO
−
tetramethylene sulfoxide
diphenyl sulfoxide
methyl phenyl sulfone
PPh3
2,5-dimethyl-1,4-benzoquinone
[1,1′-binaphthalene]-2,2′-diol
pyridine
phenanthroline
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
c
8
d
72
56
10
8
62
trace
27
51
23
e
BQ
Ag2CO3
PhI(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
f
g
h
22
DMSO
a
Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), Pd-catalyst (0.02 mmol), ligand (0.04 mmol), oxidant (0.04 mmol), and solvent (2 mL) at
b
c
d
e
f
g
100 °C under air for 18 h. Isolated yield. The solvent is DMSO. At 90 °C for 24 h. Pd(TFA)2 (0.01 mmol). The solvent is 1,4-dioxane. The
solvent is PhCl. The solvent is MeCN under refluxing.
h
challenge,11 mostly tending to perform the C(sp2)−H
olefination/annulation processes.12 Toward this goal, 1,3-
dienes have been developed to successful substances to
conduct the [4 + 2] annulation with amides through π-allyl
palladium intermediate.12a,13 However, the 1,3-dienes should
be synthesized from corresponding aldehydes. Given the
abundance of commercial terminal olefins (>1600) versus the
relative scarcity of 1,3-dienes (120), we anticipated that if a
carefully chosen Pd-catalytic system could promote the
dehydrogenative [4 + 2] annulation of terminal olefins, a
significant synthetic advantage in forming bioactively and
medicinally important 3,4-dihydroisoquinolones would be
provided.14 Herein, we describe the palladium-catalyzed
tandem dehydrogenative [4 + 2] annulation of terminal olefins
with N-sulfonyl amides via C(sp2)−H activation, allylic
C(sp3)−H activation, and homoallylic C(sp3)−H elimination
processes, in which the DMSO ligand plays a significant role
(Scheme 1d). Moreover, the reaction proceeds by using air as
the terminal oxidant, which avoids the waste of the oxidation
and decreases the environmentally hazardous byproducts
obtained with excessive oxidant.15
Substrates 1a and 2a were carried out to determine the
optimal reaction conditions (Table 1). Initially, with 10 mol %
Pd(TFA)2 and 20 mol % Cu(OAc)2, the two substrates
afforded the desired dehydrogenative [4 + 2] annulation
product 3aa in 56% yield with E stereoselectivity in p-xylene at
100 °C for 18 h under air (entry 1). Examination of other Pd-
catalysts showed that more electrophilic Pd(TFA)2 had a
higher yield than Pd(OAc)2 and PdCl2 (entries 2 and 3).
Considering that sulfoxide ligands such as DMSO can promote
the allylic C(sp3)−H functionalization,4a,6a we added 20 mol %
DMSO to the reaction. To our delight, an excellent 93% yield
was obtained (entry 4). However, when the reaction was
performed in DMSO, only trace product was detected (entry
5), which excluded the effect of the polarity of DMSO.
Unfortunately, other sulfoxide ligands as well as sulfone, PPh3,
benzoquinone, and binaphthol ligands did not give a higher
yield than DMSO (entries 6−11). Nitrogen-based ligands such
as pyridine and phenanthroline were not suitable ligands for
this reaction (entries 12 and 13). These results demonstrate
the beneficial effect of the DMSO ligand. We consider that this
effect may be due to the DMSO ligation to palladium.16 The in
situ formed Pd(DMSO)2(TFA)2 catalyst exhibits both hard O-
and soft S-bound DMSO ligands,16a which may work
cooperatively in the interconversion between the relatively
hard Pd(II) and soft Pd(0) redox states.16b The O-DMSO
ligand is kinetically labile, contrasting with the behavior of a
nitrogen-based ligand, which might be important to enable
weakly coordinating substrates to coordinate to Pd(II), such as
the C(sp2)−H bond and the allylic C(sp3)−H bond,
facilitating the electrophilic C−H cleavages.6a The kinetically
labile nature also may be important to activate the π-allyl
palladium intermediates, facilitating the nucleophilic π-allyl
palladium functionalization.4a The S-DMSO ligand can
stabilize the Pd(0) complex, which inhibits the formation of
palladium black and facilitates the oxidation of the Pd(0)
catalyst.16c The yield decreased to 72% when the temperature
reduced to 90 °C for 24 h (entry 14). The yield decreased to
56% when the amount of Pd(TFA)2 decreased to 5 mol %
(entry 15). Omitting Cu(OAc)2 led to a sharp decrease of the
B
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yield (entry 16). Changing the oxidant to BQ, Ag2CO3,
PhI(OAc)2 or solvent to 1,4-dioxane, PhCl, MeCN could not
improve the reaction (entries 17−22). Eventually, satisfactory
reaction conditions were obtained as shown in entry 4.
As shown in Scheme 2, a gram-scale reaction of substrates 1a
and 2a was carried out under optimized reaction conditions.
Scheme 3. Dehydrogenative [4 + 2] Annulation of Terminal
Olefins with 1a
Scheme 2. Dehydrogenative [4 + 2] Annulation of Amides
with 2a
(3-Buten-1-yl)-naphthalene 2i also worked smoothly, giving
3ai in 85% yield. γ,δ-Unsaturated esters, ketones, nitrile, and
amide were also applicable for the dehydrogenative [4 + 2]
annulation. Products 3aj−3ap were generated in 35−88%
yields. The yield of 3an was relatively low, which may be due
to the relatively low boiling point of hex-5-en-2-one 2n. When
4-phenoxy-1-butene 2q was conducted in this reaction, the
desired product 3aq was obtained in 60% yield. However, the
C−H olefination/annulation product 4d was also obtained in
15% yield. We suspected that the homoallylic unsaturated
bond benefited the dehydrogenative [4 + 2] annulation.
Excitingly, the methyl group at the 2-position of olefin did not
retard the dehydrogenative [4 + 2] annulation, and 3ar was
obtained in moderate yield.
According to all above-mentioned results and previous
reports,8,12b,16,17 a plausible mechanism was proposed as
shown in Scheme 4. First, the catalyst Pd(TFA)2 and ligand
DMSO form a new Pd(DMSO)2(TFA)2 catalyst in situ,16
which then complexes with the N−H bond and the C(sp2)−H
bond in 1a to form a five-membered palladacycle I.12b After
coordination of olefin to palladium and removal of an allylic
hydrogen of 2a, the π-allyl palladium complex II forms.8
Gratifyingly, the reaction performed smoothly, affording
desired product 3aa in 87% yield. Then the scope was
expanded to a variety of amides. Benzamides with both
electron-donating and electron-withdrawing substituents func-
tioned well, achieving desired products 3ba−3ka in 62−93%
yields. Electron-rich benzamides exhibited slightly better
reactivities than electron-deficient benzamides (cf. 3ba, 3ca,
3ea vs 3fa−3ka). meta-Substituted benzamide 1c afforded one
regioisomer 3ca in 86% yield. ortho-Substituted benzamide 1d
formed 3da in 62% yield, slightly lower than the yields of 3ba
and 3ca. The two reactions suggested that the steric factor
could influence the Pd-catalyzed ortho C(sp2)−H activation
process. Heterocyclic arylamides were also compatible with
this reaction. Products 3la−3na were achieved in good yields.
Furthermore, alkenyl carboxamides also worked well under
those optimized conditions to synthesize another important
type of heterocyclic compounds, 5,6-dihydropyridinones.
Bicyclic product 3oa was obtained in 73% yield. α,β-
Disubstituted products 3pa and 3qa were obtained respectively
in 67% and 80% yields.
Scheme 4. Proposed Plausible Mechanism
The influence of a benzamide directing group was also
explored. When N-tosylbenzamide 1r was reacted with 2a,
dehydrogenative [4 + 2] annulation product 3ra was obtained
in 69% yield; however, the C−H olefination/annulation
product 4a was also obtained in relatively high ratio.12b
Replacing the −CH3 group with the −CF3 or −CN group
decreased the ratio of C−H olefination/annulation product 4b
or 4c. These reactions demonstrated that the stronger acidity
of the N−H bond benefited the dehydrogenative [4 + 2]
annulation.
Subsequently, the scope was expanded to terminal olefins
(Scheme 3). Both electron-donating and electron-withdrawing
substituents on the phenyl ring of 4-phenyl-1-butenes were
well tolerated, giving products 3ab−3ah in 68−86% yields. 1-
C
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Authors
Reductive elimination of II gives the allyl intermediate III and
Pd(0) species. The intermediate III has been detected by high
resolution mass spectra analysis. Subsequently, the coordina-
tion of Pd(II) catalyst with the N−H bond and retained
carbon−carbon double bond in allyl intermediate III forms the
complex VI which then undergos the nitrogen palladation of
alkene to afford V.16c Finally, β-H elimination affords the
dehydrogenative [4 + 2] annulation product 3aa with
regeneration of Pd(0) species. Cu(OAc)2 and O2 oxidize the
Pd(0) species to a Pd(II) catalyst to participate in the next
cycle.16c
Manman Sun − Advanced Research Institute and Department of
Chemistry, Taizhou University, Taizhou 318000, P. R. China
Weida Chen − Advanced Research Institute and Department of
Chemistry, Taizhou University, Taizhou 318000, P. R. China
Xiangyu Xia − Advanced Research Institute and Department of
Chemistry, Taizhou University, Taizhou 318000, P. R. China
Guodong Shen − School of Chemistry and Chemical
Engineering, School of Pharmacy, Liaocheng University,
Liaocheng 252000, P. R. China; orcid.org/0000-0002-
0631-1913
Yongmin Ma − Advanced Research Institute and Department of
Chemistry, Taizhou University, Taizhou 318000, P. R. China
Jianguo Yang − Advanced Research Institute and Department of
Chemistry, Taizhou University, Taizhou 318000, P. R. China;
orcid.org/0000-0003-4268-7384
Because the intermediate III was present at a trace level, 2-
allyl benzamide 5 (a mixture of Z/E diastereomers) was
prepared and subjected to the standard reaction conditions to
gain more insight into the possible mechanism (Scheme 5).
Scheme 5. Cyclization of 2-Allyl Benzamide
Hanfeng Ding − Department of Chemistry, Zhejiang University,
Hangzhou 310058, P. R. China; orcid.org/0000-0002-
1781-4604
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01011
Notes
The authors declare no competing financial interest.
Quantitative product 3aa could be obtained in just 1 h,16b,c
which was in agreement with the proposed mechanism. The
faster cyclization rate of 2-allyl benzamide compared to 2-
alkenyl benzamide12b indicated that the stronger acidity of the
N−H bond in the directing group and the homoallylic
unsaturated bond in the terminal olefin benefited the
competition of dehydrogenative [4 + 2] annulation from C−
H olefination/annulation.
In summary, with the assistance of the DMSO ligand, we
have developed the palladium-catalyzed tandem dehydrogen-
ative [4 + 2] annulation of terminal olefins with N-sulfonyl
amides via C(sp2)−H activation, allylic C(sp3)−H activation,
and homoallylic C(sp3)−H elimination processes. The mostly
used commercial olefins represent an atom-efficient and step-
efficient approach to synthesize bioactively important and
medicinally important heterocyclic compounds bearing a vinyl
substituent with high E stereoselectivity. Various substituted
benzamides, heterocyclic arylamides, and alkenyl carboxamides
are found to be efficient substrates. Using air as the terminal
oxidant also provides a great advantage regarding environ-
mental friendliness.
ACKNOWLEDGMENTS
This work was supported by Natural Science foundation of
Zhejiang Province (Nos. LQ20B020007, LY18B020002).
■
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01011.
■
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Experimental details, NMR (1H, 13C) spectra for all new
compounds and high resolution mass spectra of
intermediate III (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Zhiming Wang − Advanced Research Institute and Department
of Chemistry, Taizhou University, Taizhou 318000, P. R.
China; orcid.org/0000-0002-6583-3826;
Email: zhiming@tzc.edu.cn, wzmmol@hotmail.com
D
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