Silver(I) catalyzed intermolecular alkene carbo-alkoxylation through cleavage of allylic ether C–O bond

Article abstract of DOI:10.1016/j.tetlet.2020.151895

All rights reserved. First example of allylic ether's dimerization reaction was developed through olefin's C–O bond insertion via Ag(I) catalysis. Other aromatic olefins, such as styrene and its electro-poor analogues could participate to provide the ally

Full text of DOI:10.1016/j.tetlet.2020.151895

Tetrahedron Letters xxx (xxxx) xxx
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Tetrahedron Letters
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Silver(I) catalyzed intermolecular alkene carbo-alkoxylation through
cleavage of allylic ether C–O bond
⇑
Bai-Ling Chen, Yu-Jiang Wang, Ming-Zhe Shao, Zili Chen
Department of Chemistry, Renmin University of China, Beijing 100872, China
a r t i c l e i n f o
a b s t r a c t
Article history:
All rights reserved. First example of allylic ether’s dimerization reaction was developed through olefin’s
C–O bond insertion via Ag(I) catalysis. Other aromatic olefins, such as styrene and its electro-poor ana-
logues could participate to provide the allylic ether’s C–O bond insertion products.
Ó 2020 Elsevier Ltd. All rights reserved.
Received 21 January 2020
Revised 2 March 2020
Accepted 31 March 2020
Available online xxxx
Keywords:
Silver catalysis
Allylic ether
Carbo-alkoxylation
Alkene insertion
Introduction
We recently explored the gold catalyzed enyne cycloisomeriza-
tion reaction of 1,6-enyne 1a by using IprAuCl (5 mol%)/AgSbF
6
Insertion of the alkene or alkyne groups into the ether’s C–O
bond is an appealing and useful synthetic tool for the construction
of a new ether or vinyl ether structural moieties. In this regard,
some transition metals, such as: Pd(0), Au(I) and Pt(II), could be
employed to mediate the intra- or intermolecular alkyne’s inser-
tion into ether’s C–O bond (Scheme 1, Eq. 1) [1–3]. Nevertheless,
the similar approach involved the alkene groups seems to be very
challenging. To date, only a few examples have ever been reported
to realize C–O bond’s olefin-insertion reaction [4]. For example, in
(10 mol%) as the catalyst combination [8]. Except for the desired
major cycloisomerization product 2, an unexpected dimerization
product 3a could be separated from the reaction mixture in less
than 5% yield (Table 1, entry1). According to 3a’s structure [9], it
seems that a double bond rather than a triple bond was activated
to insert into the allylic ether bond, wherein, two 1,6-enyne sub-
strates were dimerized to provide 3a diastereoselectively, with
two propargylic units kept intact.
3a’s novel structure prompted us to further explore this dimer-
ization reaction. After a series of experiments, we found that silver
2
012, Hu and his colleagues reported the gold catalyzed inter-
molecular carboalkoxylation of alkenes by using acetals as the
ether-type substrates (Scheme 1, Eq. 2) [5]. Moreover, intermolec-
ular [3+2] cycloaddition of expoxide with olefin could be carried
out through epoxide C–O bond’s activation by using Fe(II) or Sc
salt AgSbF
6
could mediate 3a’s formation. As shown in Table 1,
in CH Cl
2
when 1,6-enyne 1a was treated with 10 mol% of AgSbF
at rt, dimerization product 3a could be obtained in 41% yield,
6
2
0
together with the formation of its inseparable diastereomer 3a
0
(
[
III) catalysts or under photochemical condition (Scheme 1, Eq. 3)
6]. Despite these achievements, however, acetals and epoxides
(3a/3a = 5/1, Table 1, entry 2). Other silver salts, such as: AgNTf,
AgOTf, AgBF
are inferior to that of AgSbF
such as: CHCl , DCE and CH
4 4
and AgClO , were also tested. But their performances
are not typical ether substrates. Examples of direct intra- or inter-
molecular olefin-insertion into C–O bond, which involved the other
classical types of ethers, are very scarce.
We herein want to report the first example of allylic ether’s
dimerization reaction through silver catalysis [7], in which, an
insertion of the alkene group into allylic ether’s C–O bond
occurred. In addition, the intermolecular insertion of aromatic ole-
fin into allylic ether C–O bond could also be achieved.
6
(entry 3–6). Then, various solvents,
3
CN, were scrutinized (entry 7–9). It
3
was found that DCE (DCE) was the best reaction media (entry 8),
0
in which, total yields of 3a and 3a was equal to 46%
0
(3a/3a = 5/1). Reducing AgSbF
6
’s loading to 5 mol% would lead
to a relatively lower reaction yield (entry 10). If 10 mol% of AgSbF
was added in two batches, the reaction yield could be improved to
54% (entry 11).
6
With the optimized conditions in hand, we turned to extending
⇑
Corresponding author.
E-mail address: zilichen@ruc.edu.cn (Z. Chen).
the substrate scope by using 10 mol% of AgSbF
Dimerization reactions of a series of allylic esters with different
6
as the catalyst.
https://doi.org/10.1016/j.tetlet.2020.151895
040-4039/Ó 2020 Elsevier Ltd. All rights reserved.
0
Please cite this article as: B.-L. Chen, Y. J. Wang, M. Z. Shao et al., Silver(I) catalyzed intermolecular alkene carbo-alkoxylation through cleavage of allylic
ether C–O bond, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.151895

2
B.-L. Chen et al. / Tetrahedron Letters xxx (xxxx) xxx
Table 2
a
Ag(I) mediated dimerization reactions of various allylic ethers.
Scheme 1. Transition metal catalyzed insertion of alkene or alkyne group into C–O
bond.
substitution patterns were explored (Table 2). At first, various
ether O-substituents, such as: 1,1-diemthyl propargylic group,
methyl and ethyl groups, were tested, which readily provided the
desired olefin’s C–O insertion products. The relatively low yields
aUnless noted, all reactions were carried out on 0.1 mmol scale in 2 mL dry DCE at rt
0
of 3b and 3b might be owing to the enhanced steric hindrance
for 4 h with the addition of 10 mol% of AgSbF
second batch of AgSbF6 was added 0).
bIsolated yields.
6
added in two batches (2 h later, the
0
of the 1,1-diemthyl propargylic unit (compounds 3b and 3b are
inseparable, Table 2). However, to our surprise, the phenol group
(Scheme 2, 1i) were not tolerated in this reaction. It seems that
only the cinnamyl type ether could participate in the dimerization
reaction. As shown in Scheme 2, the allylic methyl ether 1j gave no
desired C–O insertion product. Various substituted cinnamyl
groups were then examined, in which, the electro deficient groups,
such as bromo and chloro substituted cinnamyl ethers (Table 2,
0
0
3
f/3f and 3g/3g ), performed better than that of their electro-rich
0
0
counterparts (Table 2, 3e/3e and 3h/3h ). Furthermore, when the
electro-rich 3-(furan-2-yl) allylic ether 1 k (Scheme 2) was treated
6
with AgSbF , only a mixture of inseparable products was obtained.
We were wondering if this olefin’s insertion into C–O bond
reaction could be extending to other aromatic olefins. Thus, a series
Scheme 2. Unsuccessful substrates for allylic ether’s dimerization reaction.
Table 1
a,b,c
Silver mediated dimerization of 1,6-enyne 1a.
0
c,d
Cata (mol%)
Solvent
2’s Yield
3a&3a Yield (dr)
1
2
3
4
5
6
7
8
9
1
1
IPrAuCl/AgSbF
6
(5/10)
CH
CH
CH
CH
CH
CH
CHCl
DCE
3
CH CN
2
2
2
2
2
2
Cl
Cl
Cl
Cl
Cl
Cl
2
2
2
2
2
2
81%
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
>5%
41% (5/1)
Trace
14% (~3/1)
Trace
19% (~5/1)
20%
46% (5/1)
Trace
33% (5/1)
54% (5/1)
AgSbF
6
(10)
AgNTf (10)
AgOTf (10)
AgBF (10)
4
AgClO
AgSbF
AgSbF
AgSbF
AgSbF
AgSbF
4
(10)
(10)
(10)
(10)
(5)
6
6
6
6
6
3
0
1
DCE
DCE
e
(10)
a
b
c
Unless noted, all reactions were carried out on 0.1 mmol scale in 2 mL anhydrous solvent at rt for 4 h.
0
3
a, 3a ’s structure only shows relative configuration.
Isolated yields.
d
e
a/3a ratio was determined by 1H NMR spectral data.
0 mol% of AgSbF were added in two batches.
0
3
1
6
Please cite this article as: B.-L. Chen, Y. J. Wang, M. Z. Shao et al., Silver(I) catalyzed intermolecular alkene carbo-alkoxylation through cleavage of allylic
ether C–O bond, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.151895

B.-L. Chen et al. / Tetrahedron Letters xxx (xxxx) xxx
3
Table 3
a
Ag(I) mediated carbo-oxylation of styrene 4a with various allylic ethers.
Scheme 4. Control experiments of 1c’s reaction in moisture DCE and allylic alcohol
7’s reaction.
aUnless noted, all reactions were carried out on 0.1 mmol scale (equivalent ratio:
4
a/1 = 4/1) in 2 mL dry DCE at rt for 4 h with the addition of 10 mol% of AgSbF
6
added in two batches (2 h later, the second batch of AgSbF6 was added).
b
Isolated yields.
of allylic ethers were evaluated by using styrene as the reaction
partner. As shown in Table 3, excess amount of 4a was added to
compete with allylic ether’s dimerization reaction. As shown in
Table 3, it was found that electro deficient substituents, such as
bromo and chloro groups substituted cinnamyl ethers (Table 3,
5
c and 5d) worked better than their electro-rich analogues (5b
and 5e) did in styrene’s C–O insertion reactions. A series of ether
O-substituents, such as: methyl (Table 3, 5a-e), ethyl (5f), propar-
gylic group (5g) and benzyl (5h) groups, were tolerated (Table 3).
Next, except for styrene 4a, other aromatic olefins were exam-
ined by using methyl p-chloro cinnamyl ether 1g as the reaction
partner. As shown in Scheme 3, the reactions of electro deficient
aromatic alkenes, such as: p-ClPh (Scheme 3, 4b), p-FPh (4c), p-
BrPh (4d) and o-ClPh (4e) substituted olefins, went smoothly to
provide the desired olefin insertion products in moderate yields
Fig. 1. X-ray chromatograph for compound 8.
(Scheme 3, 6b-e), together with the formation of small amount
of 1g’s dimerization products. However, the electro rich aromatic
alkene 4f could not give the insertion product, in which, no dimer-
ization product 3g and 3g’ were obtained either. This result indi-
cated that 1g’s dimerization might be inhibited in the presence
of 4f.
(
Scheme 4, Eq. 1), while its syn-diastereomer could not be detected.
Compound 8’s structure was determined by its single-crystal X-ray
diffraction, in which, a (1S*, 2S*) type trans- configuration has been
confirmed (Fig. 1). When water’s amount was improved to 10
equivalence, no desired reaction was observed (Scheme 4, Eq. 1).
Trapping the allylic ether 1c with other nucleophile, such as phenol
In order to elucidate the reaction mechanism, three control
experiments were performed. When 1c’s dimerization reaction
9
, could provide para-allylic substituted phenol compound 10 in
70% yield (Scheme 4, Eq. 2) [11]. Treating allylic alcohol 7 with
AgSbF in DCE gave only trace amount of dimerization products
Scheme 4, Eq. 3).
A plausible simple mechanism was then proposed. As shown in
was performed in DCE with the addition of 2 equivalent of H
dimerization product 3c and 3c could be synthesized in 40% yield.
In addition, a water trapping product 8 was separated in 5% yield
2
O,
0
6
(
Scheme 5, 1c’s dimerization might be started from ether activation
+
by Ag cation, which provided the allylic cation intermediate I with
the removal of AgOMe. Intermediate I reacted with another allylic
ether molecule to give intermediate II, which was then trapped by
0
AgOMe to afford the desired dimerization products 3c and 3c . The
anti-configuration major product 3c would be thermodynamically
favored in this process. Similarly, intermediate II could be trapped
by H O in the presence of 2 equivalence of water, providing com-
2
pound 8 in low yield (Scheme 5). Based on Eq. 1’s result (Scheme 3),
trapping intermediate II with AgOMe should be preferred in the
presence of H
cage effect [10].
2
O, possibly because of a certain degree of solvent
Scheme 3. Ag(I) mediated carbo-oxylation of various aromatic olefins with allylic
ether 1g.
Please cite this article as: B.-L. Chen, Y. J. Wang, M. Z. Shao et al., Silver(I) catalyzed intermolecular alkene carbo-alkoxylation through cleavage of allylic
ether C–O bond, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.151895

4
B.-L. Chen et al. / Tetrahedron Letters xxx (xxxx) xxx
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(
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(
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4
f’s (scheme 3) unreactivity.
In summary, we have reported Ag(I) catalyzed allylic ether’s
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reaction: C. Bruneau Angew. Chem. Int. Ed. 44 (2005) 2328–2334;
[
dimerization reaction through the C–O bond scission and olefin
insertion [12]. The insertion reaction could also be extended to
styrene and its electro poor analogues. Although there are some
limitation upon ether substrates and alkene groups, this is the first
example of intermolecular olefin-insertion into the C–O bond,
which involved various types of allylic ethers.
(
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Declaration of Competing Interest
(j) R. Dorel, A.M. Echavarren, J. Org. Chem. 80 (2015) 7321–7332.
9] a’s relative chemistry could be deduced from compound 8, of which, the
crystal structure have been obtained.
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S.M. da Silva, W.J. Baader J. Org. Chem. 78 (2013) 4432–4439;
[
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
[
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[
5
311.
Acknowledgments
12] General procedure for silver mediated dimerization of allylic ether 1c: To a
suspension of 5 mol% of AgSbF6 (3.4 mg) in DCE (2 mL) at rt, was added allylic
ether 1c (30 mg, 0.2 mmol). After the reaction mixture was stirred at rt for 2h,
the second batch of AgSbF6 (3.4 mg, 5 mol%) was added. The reaction was then
kept at rt with TLC monitoring until complete consumption of the starting
material. Concentration of the reaction mixture, followed by purification
product through flash chromatography (petroleum/EtOAc = 100/1 to 10/1 as
Financial Support for this work with the grant from the National
Sciences Foundation of China (Nos. 21871294) is gratefully
acknowledged.
0
the eluent) afforded 3c (15.5 mg) and 3c (3 mg) as two colorless oil products
(
(
Appendix A. Supplementary data
63% total yield, 3c/3c, = 5/1). The spectral data of ((4S*,5S*,E)-5-methoxy-4-
methoxymethyl)pent-1-ene-1,5-diyl)dibenzene 3c: 1H NMR (400 MHz,
CDCl3): d 7.43–7.33 (m, 2H), 7.27 (m, 7H), 7.18 (m, 1H), 6.29 (d, J = 15.8 Hz,
H), 6.15–6.06 (m, 1H), 4.26 (d, J = 6.6 Hz, 1H), 3.59–3.48 (m, 1H), 3.36–3.29
m, 3H), 3.22 (s, 3H), 2.30–2.19 (m, 1H), 2.19–2.00 (m, 2H). 13C NMR (101
MHz, CDCl3) d 140.22, 137.90, 131.68, 128.91, 128.60, 128.36, 127.65, 127.02,
26.08, 83.39, 71.45, 58.96, 57.12, 45.53, 31.16.; HRMS (APCI) Calcd for
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2020.151895.
1
(
1
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0
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Please cite this article as: B.-L. Chen, Y. J. Wang, M. Z. Shao et al., Silver(I) catalyzed intermolecular alkene carbo-alkoxylation through cleavage of allylic
ether C–O bond, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.151895