Tuning Vinylethylene Carbonates into [4 + 2] Cycloaddition via Silylation and Vinylogous Peterson Elimination

Article abstract of DOI:10.1021/acs.orglett.0c01690

Vinylethylene carbonates have been extensively used to trigger [3 + n] or [5 + n] cycloaddition via the formation of η3-allylic intermediates, while the important [4 + n] cycloaddition has not been explored yet. Here, we report a new strategy to convert vinylethylene carbonates into 4-(trimethylsilyl)but-2-en-1-ols, which can readily undergo [4 + 2] cycloaddition by in situ formation of 1,3-dienes. This novel reaction involves [PdII]-catalyzed decarboxylative silylation, [FeIII]-catalyzed vinylogous Peterson elimination, and subsequent [4 + 2] cycloaddition to afford a multisubstituted cyclohexa-1,4-diene skeleton.

Full text of DOI:10.1021/acs.orglett.0c01690

pubs.acs.org/OrgLett
Letter
Tuning Vinylethylene Carbonates into [4 + 2] Cycloaddition via
Silylation and Vinylogous Peterson Elimination
Wei Feng,† Li Xu,† Deng-Yuan Li,* and Pei-Nian Liu*
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ABSTRACT: Vinylethylene carbonates have been extensively
used to trigger [3 + n] or [5 + n] cycloaddition via the formation
of η3-allylic intermediates, while the important [4 + n] cyclo-
addition has not been explored yet. Here, we report a new strategy
to convert vinylethylene carbonates into 4-(trimethylsilyl)but-2-en-
1-ols, which can readily undergo [4 + 2] cycloaddition by in situ formation of 1,3-dienes. This novel reaction involves [PdII]-
catalyzed decarboxylative silylation, [FeIII]-catalyzed vinylogous Peterson elimination, and subsequent [4 + 2] cycloaddition to afford
a multisubstituted cyclohexa-1,4-diene skeleton.
inylethylene carbonates have been widely used to
frameworks, which are prevalent in drugs, agrochemicals, and
bioactive molecules.8 For example, Diels−Alder provides
access to 6-membered cyclic motifs in an efficient and
convergent manner.9 Because the vinylethylene carbonate-
derived η3-allylic intermediates cannot be used directly in [4 +
n] cycloaddition due to their reactivity properties, the
development of [4 + n] cycloaddition of vinylethylene
carbonates still remains a great challenge.
V
generate η3- allylic intermediates upon decarboxylation,1
which react with various nucleophiles such as phenols,
thiophenols, and anilines to construct various carbon−carbon
and carbon−heteroatom bonds.2,3 Recently, cycloadditions of
vinylethylene carbonates such as [3 + n] or [5 + n]
cycloaddition have attracted great interest, which allows
construction of various carbocyclic and heterocyclic skeletons
(Scheme 1a).4−7 For instance, Zhang et al. achieved [3 + 2]
4-(Trimethylsilyl)but-2-en-1-ols are important compounds
and have been widely applied to construct various cyclic
skeletons because they can undergo [4 + n] cycloaddition via
1,3-diene generated by vinylogous Peterson elimination.10−12
However, only a few 4-(trimethylsilyl)but-2-en-1-ols are
reported from propargyl alcohols by a multistep operation.10,11
In addition, the vinylogous Peterson elimination of 4-
(trimethylsilyl)but-2-en-1-ols is relatively underexploited.11
Silylation, by introducing silicon functional groups,13 can
make reactions much more selective and flexible. For example,
a cross-aldol reaction between two α-H-carbonyl compounds
proceeds with poor selectivity because the two reactants can
serve as both electrophiles and nucleophiles. In contrast, after
the conversion of one carbonyl compound into an enol silane
via silylation, the so-called Mukaiyama aldol reaction between
the enol silanes (nucleophile only) and the other carbonyl
compounds is much more selective.14 Taking advantage of
silylation, we here report a new strategy to tune the reactivity
of vinylethylene carbonates to generate 4-(trimethylsilyl)but-2-
en-1-ols, which can readily undergo [4 + 2] cycloaddition by in
Scheme 1. Tunable Cycloadditions of Vinylethylene
Carbonates
cycloadditions with various electrophilic partners,5 and Zhao et
al. and Glorius et al. independently reported [5 + 4]
cycloadditions to synthesize highly functionalized rings of
different sizes.6a,b Glorius et al. subsequently achieved the first
enantioselective [5 + 2] annulation of vinylethylene carbonates
with enals via cooperative N-heterocyclic carbene organo-
catalysis and palladium catalysis,6c while Liang et al. reported
ligand-controlled [3 + 2] and [3 + 3] cycloadditions between
vinylethylene carbonates and naphthols.7 Despite these
extensive studies, we are unaware of reports of [4 + n]
cycloadditions of vinylethylene carbonates.
Received: May 18, 2020
The [4 + n] cycloadditions, especially [4 + 2] cycloaddition,
play key roles in synthetic organic chemistry because they can
rapidly generate challenging but synthetically valuable cyclic
https://dx.doi.org/10.1021/acs.orglett.0c01690
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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Letter
situ formation of 1,3-dienes (Scheme 1b). The cascade
silylation/elimination/[4 + 2] cycloaddition reaction affords
multisubstituted cyclohexa-1,4-dienes in good yields. The
reaction involves [PdII]-catalyzed decarboxylative silylation
and [FeIII]-catalyzed vinylogous Peterson elimination. To the
best of our knowledge, this is the first example of tuning
vinylethylene carbonates into [4 + n] cycloaddition.
Initially, we carried out the reaction between vinylethylene
carbonate 1a and 1,1,1,2,2,2-hexamethyldisilane (HMDS) in
the presence of Pd(OAc)2 (10 mol %) at 60 °C in THF (Table
1). The silylated product (E)-2-phenyl-4-(trimethylsilyl)but-2-
We screened ligands in order to improve the yield of 2a. Of
various common ligands, the tridentate ligand terpyridine
(Tpy) is the best, increasing the yield of 2a to 77% (entry 12,
Table 1; see Supporting Information, Table S2). The Tpy
ligand may work well because the Tpy-chelated [PdII] catalysts
is quite stable and possesses greater electrophilicity to activate
alkene of 1a.15 We also tried to optimize catalyst and ligand
loading. Decreasing the amounts of catalyst and ligand
simultaneously to 5 mol % or even 1 mol % afforded the
best result to generate 2a in 77% yield (entries 13 and 14,
Table 1).
Using the optimal conditions to form 2a, we proceeded to
investigate cascade silylation/elimination/[4 + 2] cyclo-
addition of vinylethylene carbonates with alkynes. Of various
common Fe catalysts, Fe2(SO4)3 gave the best results,
furnishing cyclohexa-1,4-diene 4a in 68% isolated yield (entries
15−18, Table 1). Other common Lewis acids such as Ag2SO4,
CeSO4, CoSO4·H2O, Bi2(SO4)3, and In2(SO4)3 did not
improve the results (see Supporting Information, Table S3).
We were able to reduce catalytic loading to 15 mol % while
maintaining 68% isolated yield (entries 19 and 20, Table 1). It
is noteworthy that the structure of 4a was confirmed using
single-crystal X-ray diffraction analysis, NMR, and high-
resolution mass spectrometry (see the Supporting Informa-
tion).
a
Table 1. Optimization of Reaction Conditions
b
c
H2O
(equiv)
yield
(2a, %)
yield
entry
catalyst
LA
(4a, %)
1
2
3
4
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OPiv)2
Pd2(dba)3
Pd(TFA)2
PdCl2
Pd(PPh3)4
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
6
23
48
55
39
17
53
38
4
12
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
d
5
6
e
Next we explored the scope of the silylation of vinylethylene
carbonates (Scheme 2). First, vinylethylene carbonates 1
containing electron-donating groups such as Me, MeO, or MeS
at the para position of the benzene ring reacted with HMDS,
providing the products 2b−2d in yields of 73−84%. The
7
8
9
10
11
12
13
14
15
16
17
18
19
22
ND
ND
77
f
a
g
Scheme 2. Substrate Scope of Silylation
77
h
i
77 (75)
h
FePO4
Fe(OTf)3
FeCl3
Fe2(SO4)3
Fe2(SO4)3
Fe2(SO4)3
ND
47
62
h
h
h
68
hj
,
i
68 (66)
hk
,
20
63
a
Reaction conditions: 1a (0.5 mmol), HMDS (0.75 mmol),
Pd(OAc)2 (0.05 mmol), THF (1.5 mL), 60 °C, under Ar for 12 h,
then 3a (1 mmol), LA (0.125 mmol), THF (1.5 mL), 80 °C, under
b
Ar for 12 h. The yield was determined by 1H NMR using
c
trimethyl(phenyl)silane as the internal standard. Isolated yields are
d
e
f
g
indicated. 30 °C. 90 °C. Tpy (0.05 mmol) was added. Pd(OAc)2
h
(0.025 mmol) and Tpy (0.025 mmol) were used. Pd(OAc)2 (0.005
mmol) and Tpy (0.005 mmol) were used, 24 h. Isolated yield at 1.0
mmol scale. Fe2(SO4)3 (0.075 mmol) was used. Fe2(SO4)3 (0.05
i
j
k
mmol) was used.
en-1-ol (2a) was isolated in 6% yield, and no Z-2a was
observed (entry 1, Table 1). Addition of H2O slightly
increased the yield of 2a (entry 2, Table 1), and the best
yield was obtained with 20 equiv of H2O (entries 3 and 4,
Table 1). Increasing or decreasing the reaction temperature
reduced the yield of 2a (entries 5 and 6, Table 1). Screening
many common solvents, including THF, MeCN, DMF,
toluene, DMSO, MeOH, DCM, and dioxane identified THF
as the best (see Supporting Information, Table S1). Of various
palladium catalysts, Pd(OAc)2 was found to be the most
suitable (entries 7−11, Table 1). Notably, neither Pd(PPh3)4
nor PdCl2 led to formation of 2a.
a
Reaction conditions: 1 (0.5 mmol), HMDS (0.75 mmol), Pd(OAc)2
(0.005 mmol), Tpy (0.005 mmol), H2O (20 equiv), THF (1.5 mL),
under Ar for 24 h. Isolated yields are indicated. The E/Z ratio was
determined by H NMR.
b
1
B
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Letter
substitution of electron-withdrawing F, Cl, Br, or CF3 groups at
the same position also gave products 2e−2h in moderate
yields. NOESY experiments with 2b showed that H1 was close
to H2 and H3, confirming the E-configuration of the silylated
products (see the Supporting Information). It is noteworthy
that the similar reactions catalyzed by [Pd0] catalysts via
formation of η3-allylic intermediates produce the Z-stereo-
isomer.3a,b The result of obtaining E-stereoisomer of 2b, as well
as the ineffectiveness of Pd(PPh3)4 catalyst (entry 11, Table 1),
suggest that the catalyst in this reaction may be [PdII] instead
of [Pd0].
Scheme 3. Substrate Scope of Cascade Silylation/
Elimination/[4 + 2] Cycloaddition
a
Vinylethylene carbonates substituted with electron-donating
MeO or electron-withdrawing Cl at the meta-position reacted
well with HMDS, selectively producing 2i and 2j in respective
yields of 74% and 65%. Vinylethylene carbonates substituted
with o-MeO also gave the desired product 2k in 61% yield.
Biphenyl and naphthyl vinylethylene carbonates reacted to give
the corresponding products 2l and 2m in good yield. The
substrate 3-thienyl vinylethylene carbonate gave 2n as a single
stereoisomer in 52% yield. The substrate 2-furanyl vinyl-
ethylene carbonate gave 2o in 63% yield as a mixture of two
stereoisomers, reflecting the oxygen atom in the furan ring
might affect the catalytic process. Alkyl vinylethylene carbonate
also gave the desired product 2p in 49% yield as a mixture of
two stereoisomers. Interestingly, 4-phenyl-4-(prop-1-en-2-yl)-
1,3-dioxolan-2-one generated 2q as a single stereoisomer with
only 36% yield, suggesting that substitution on the C = C bond
impeded silylation.
Next, we investigated the scope of organosilicon sources.
(Me2PhSi)2 and (Me2ThSi)2 underwent silylation with 1a to
produce the two stereoisomers 2r and 2s in respective yields of
72% and 52%. (Ph3Si)2, in contrast, gave no desired product,
presumably because of steric effect.
Using the optimized conditions to form 4a, we analyzed the
scope of vinylethylene carbonates 1 (Scheme 3). Regardless of
whether R1 on its phenyl group was electron-donating or
-withdrawing, the vinylethylene carbonates readily underwent
the cascade reaction with alkyne 3a to give products 4a−4i in
yields of 43−76%. Changing the benzene ring of the
vinylethylene carbonates to a naphthalene or thiophene ring
and then reacting it with alkyne 3a afforded products 4j and 4k
in respective yields of 66% and 42%.
When we analyzed the scope of alkynes 3, we found that
vinylethylene carbonate 1a reacted with diethyl but-2-
ynedioate to give 4l in 61% yield. Di-tert-butyl but-2-ynedioate
also reacted with 1a to afford 4m, albeit in slightly lower yield.
Interestingly, the reaction of ethyl 4,4,4-trifluorobut-2-ynoate
with 1a gave the mixture of cyclohexa-1,4-dienes 4n and 4n′ in
respective yields of 30% and 15%, revealing the indistinctive
regioselectivity of the cycloaddition. However, diaryl- or
alkylacetylene did not afford the desired cyclohexa-1,4-dienes
under optimized reaction conditions.
To further explore the flexibility of this cascade reaction, we
tried to react vinylethylene carbonate 1a with other
dienophiles. Various N-substituted maleimides reacted with
1a to afford the desired cis-products 4o−4q in yields of 55−
62%, and dimethyl fumarate reacted with 1a to form the
corresponding trans-product 4r in 59% yield. Interestingly,
diethyl diazene-1,2-dicarboxylate also reacted with 1a, forming
4s in slightly lower yield. Notably, the moderate yields for the
cascade reaction might be caused by self-cycloaddition of in
situ generated dienes to form dimers, which were detected by
GC−MS.
a
Reaction conditions: 1 (0.5 mmol), HMDS (0.75 mmol), Pd(OAc)2
(0.005 mmol), Tpy (0.005 mmol), H2O (20 equiv), THF (1.5 mL),
60 °C, under Ar for 24 h, then 3 (1 mmol), Fe2(SO4)3 (0.075 mmol),
THF (2 mL), 80 °C, under Ar for 12 h. Isolated yields are indicated.
To demonstrate the usefulness of the cascade silylation/
elimination/[4 + 2] cycloaddition, further transformation of
products 4 was investigated. DDQ efficiently oxidized 4a−4d
to dimethyl [1,1′-biphenyl]-3,4-dicarboxylate 5a−5d in 86−
89% yields. Treating 4a with LiAlH4 in Et2O generated 2,5-
dihydro-[1,1′-biphenyl]-3,4-diyl)dimethanol 6 in 64% yield
(see the Supporting Information).
To clarify the promoting effect of H2O, the silylation of 1a
was carried out under the standard conditions without H2O.
GC−MS analysis of the reaction mixture indicated that most of
1a was converted to (E)-trimethyl(3-phenyl-4-
((trimethylsilyl)oxy)but-2-en-1-yl)silane (D), which was then
converted to 2a after quenching with H2O. The results indicate
that H2O promotes the hydrolysis of D to form 2a.
Furthermore, when Pd(OAc)2, Tpy ligand and HMDS were
combined in THF, TMS-OAc could be detected by GC−MS,
suggesting that Tpy-chelated [PdII]-TMS complex might be
generated as the active catalyst.
On the basis of the above studies as well as literature
reports,2d a tentative mechanism involving [PdII]-catalyzed
decarboxylative silylation, [FeIII]-catalyzed vinylogous Peterson
elimination, and [4 + 2] cycloaddition has been proposed
(Scheme 4). First, Pd(OAc)2 reacts with Tpy and HMDS to
generate the active Tpy-[PdII]-TMS catalyst, which coor-
dinates with the alkene 1 to generate A. Regioselective addition
produces alkyl-[PdII] intermediate B. After C−O bond
cleavage and elimination of CO2, intermediate C forms and
then reacts with HMDS to afford intermediate D, and the
regenerated Tpy-[PdII]-TMS catalyst. D undergoes hydrolysis
to give product 2. Finally, [FeIII]-catalyzed vinylogous Peterson
elimination of 2 forms 1,3-diene E,10,11 which undergoes [4 +
C
https://dx.doi.org/10.1021/acs.orglett.0c01690
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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Letter
Pei-Nian Liu − Shanghai Key Laboratory of Functional
Materials Chemistry, Frontiers Science Center for
Scheme 4. Proposed Mechanism
Materiobiology and Dynamic Chemistry, Key Laboratory for
Advanced Materials and School of Chemistry and Molecular
Engineering, East China University of Science and Technology,
Shanghai 200237, China; orcid.org/0000-0003-2014-
2244; Email: liupn@ecust.edu.cn
Authors
Wei Feng − Shanghai Key Laboratory of Functional Materials
Chemistry, Frontiers Science Center for Materiobiology and
Dynamic Chemistry, Key Laboratory for Advanced Materials
and School of Chemistry and Molecular Engineering, East
China University of Science and Technology, Shanghai 200237,
China
Li Xu − Shanghai Key Laboratory of Functional Materials
Chemistry, Frontiers Science Center for Materiobiology and
Dynamic Chemistry, Key Laboratory for Advanced Materials
and School of Chemistry and Molecular Engineering, East
China University of Science and Technology, Shanghai 200237,
China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01690
2] cycloaddition with dienophiles 3 to give the cascade
product 4.
Author Contributions
†W.F. and L.X. contributed equally.
Notes
In summary, we have developed a cascade silylation/
elimination/[4 + 2] cycloaddition reaction of vinylethylene
carbonates, which efficiently constructs multisubstituted cyclo-
hexa-1,4-diene skeleton in good yields. The reaction proceeds
via the [PdII]-catalyzed decarboxylative silylation of vinyl-
ethylene carbonates to form 4-(trimethylsilyl)but-2-en-1-ols,
which undergo the sequential [FeIII]-catalyzed vinylogous
Peterson elimination and [4 + 2] cycloaddition with
dienophiles. This new protocol tuning vinylethylene carbo-
nates into [4 + 2] cycloaddition may be applied to the
generation of 6-membered cyclic frameworks, which are
prevalent in drugs, agrochemicals, and bioactive molecules.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Project Nos. 21672059, 91845110, and
21925201), the Program of the Shanghai Committee of
Science and Technology (Project No. 18520760700), the
Program for Eastern Scholar Distinguished Professor, and the
Fundamental Research Funds for the Central Universities.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01690.
■
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