Atom- and Step-Efficient Construction of Five-Membered Carbocycles with Alkenes and Alkynes Catalyzed by AgSbF6

Article abstract of DOI:10.1021/acscatal.8b02448

We report a cationic intermolecular cyclization between simple alkenes and alkynes catalyzed by AgSbF₆ with great selectivity, involving alkyl C-H bond cleavage. This methodology could supply important multisubstituted cyclopentene scaffolds. The remarkable tolerance of functional groups in the reaction allows great possibility for further transformations.

Full text of DOI:10.1021/acscatal.8b02448

Letter
pubs.acs.org/acscatalysis
Cite This: ACS Catal. 2018, 8, 7760−7765
Atom- and Step-Efficient Construction of Five-Membered
Carbocycles with Alkenes and Alkynes Catalyzed by AgSbF6
Xiang Su,† Bifeng Chen,† Shaohong Wang,‡ Hui Chen,‡ and Chao Chen*,†
†Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education, MOE), Department of
Chemistry, Tsinghua University, Beijing, 100084, China
‡Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Photochemistry, Institute of Chemistry,
Chinese Academy of Sciences. Beijing, 100190, China
S
* Supporting Information
ABSTRACT: We report a cationic intermolecular cyclization
between simple alkenes and alkynes catalyzed by AgSbF6 with
great selectivity, involving alkyl C−H bond cleavage. This
methodology could supply important multisubstituted cyclo-
pentene scaffolds. The remarkable tolerance of functional groups
in the reaction allows great possibility for further transformations.
KEYWORDS: intermolecular cyclization, alkenes, alkynes, cyclopentene derivatives, AgSbF6 catalysis
Scheme 1. Silver-Catalyzed Efficient Cyclization of Alkenes
and Alkynes to Five-Membered Carbocycles
he cationic cyclization reactions of alkenes and alkynes
1
Tare powerful methods to prepare various carbocycles.
These types of reactions are normally initiated by proton
(involving Lewis or Brønsted acids) or carbocation species
(generated in situ) and terminated at π-bonds as electrophilic
substitution. In contrast, it was relatively rare for the alkyl C−
H bond to trap the carbocation, let alone with great selectivity.
When enynes, polyolefins, or polyacetylenes are used as the
substrates, polycyclic products could be generated via intra-
molecular processes, and these types of reactions are applied to
synthesize many complex natural products.2−4 Although these
methods are very effective, there is a critical challenge for the
intermolecular cyclization between the alkenes and alkynes,
because the intrinsic electrophilic properties of these substrates
would result in self-dimerization or oligomerization. Another
inevitable problem is the control of regioselectivity, in
particular, when unsymmetric alkenes or alkynes are involved.
Although many efforts have been made in this part, most
successful cases are accomplished via the combination of
special substrates and a cationic center, such as using allylic
alcohols, propargyl alcohols, or acetals as the substrates.5 In
2010, Echavarren and co-workers reported a gold(I)-catalyzed
intermolecular [2 + 2] cycloaddition of terminal alkynes with
alkenes leading to cyclobutenes.6 Inspired by this elegant work,
as well as our recent success in electrophilic activation of
simple alkynes,7 we hypothesized that initiation of this process
by other catalysts might alter the reaction outcome and/or
barrier. Remarkably, a suitable Lewis acid AgSbF6 could
catalyze the cationic intermolecular cyclization between simple
alkenes and alkynes with great selectivity, involving alkyl C−H
bond cleavage and yielding entirely different products and
multisubstituted cyclopentene derivatives (see Scheme 1).
Recently, several other groups and our group reported
intramolecular cationic cyclization reactions of alkenes and
alkynes promoted by super acid through the formation of a C−
C bond converted at the alkyl C−H bond.7a,8 These reactions
could supply even polycyclic carbocyles in one step from rather
simple starting materials. Here, we envisioned an economic
and straightforward synthesis of cyclopentene derivatives with
one alkene and one alkyne as the substrates. Treating the
mixture of alkyne 1a and alkene 2a with the super acid TfOH
could generate the desired product 3aa in 40% yield (see
Scheme 2, entry 1). However, the contaminant of generated
alkene dimer made the mixture inseparable, and the conversion
ratio of alkyne 1a was merely 60%. The use of Tf2NH led to
lower efficiency (Scheme 2, entry 2), but HSbF6 catalyst could
produce the desired product 3aa in 70% yield, despite some
isomers in ∼15% yield (Scheme 2, entry 3). The Lewis acid
AgSbF6 then was tested, surprisingly giving desired product
3aa in 90% yield, meanwhile the dimerization of 2a was
suppressed efficiently (Scheme 2, entry 4). The success of the
catalyst AgSbF6 was assumably attributed to the balance
between the affinity of the Ag ion with alkynes versus
alkenes.9,10 The utilization of other silver salts or other solvents
instead of CHCl3 was not successful to give the desired
product (Scheme 2, entries 5−16). And the co-catalyst
PPh3AuCl/AgSbF6 was capable of catalyzing the reaction but
resulted in slightly diminished yield (Scheme 2, entry 17).
Received: June 24, 2018
Revised: July 22, 2018
Published: July 24, 2018
© XXXX American Chemical Society
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Scheme 2. Condition Optimization of Construction of Five-
Membered Carbocycles with 1a and 2a
Scheme 3. Reaction Scope of Alkynes with 2a
a
b
Determined by 1H NMR versus an internal stardard. Isolated yield.
c
Reaction conditions: PPh3AuCl/AgSbF6 (5 mol %/6 mol %), 60 °C.
However, alone PPh3AuCl catalyst failed to initiate the process
(Scheme 2, entry 18). Lower catalyst loading led to decreased
yields and conversion rate (Scheme 2, entries 19 and 20).
Because of the importance of cyclopentenes,11 the reactions
were extended with more substrates (Scheme 3). First, a range
of alkynes with various substitutents were tested in the
reactions. As shown in Scheme 3, diaryl acetylenes 1b−1e with
the F-, Br-, Me-, and MeO-groups on the phenyl rings all
proceeded smoothly under standard conditions to give
products 3ba−3ea in good yields. The use of unsymmetric
diaryl acetylenes 1f−1h afforded the corresponding products
3fa−3ha with great regioselectivity (>50:1), probably because
the generated carbocation center near the electron-rich phenyl
is more stable. Remarkably, aryl-alkyl acetylenes 1i−1l also
worked well in the cyclization reactions with 2a to give
products 3ia−3la. To our surprise, halogen atoms (including
Cl, Br, and I) on the alkyl groups (1i, 1k, and 1l) all tolerated
this AgSbF6-catalyzed reaction. Finally, bromo-phenyl acety-
lene 1m also fit this AgSbF6-catalyzed cyclization reaction to
form 3ma, albeit in slightly low yield. The residence of the
halogen atoms in the products left the possibility for further
useful transformations.
lesser steric hindrence in the product 3ah. And, interestingly,
the two neighboring H atoms on the five-membered ring were
transconfigured. The shift of the double bond was also found
in the formation of 3ai. When cyclic substrates 2j and 2k were
used, spirocycles were easily produced. When phenyl-cyclo-
hexene 2l reacted with 1a, indene product was generated via
C−C bonds formed on the phenyl ring. Interestingly, when
alkene 2i reacted with terminal alkynes 1n, 1o, and 1p, all the
reactions produced cyclopentenes with shifted double bonds.
This may be due to the higher stability of the formed tertiary
carbocation, in comparison to a secondary carbocation. The
reaction of alkyne 1e and alkene 2m with long alkyl chain
could proceed smoothly with good regioselectivity, leading to
the five-membered ring product 3em, instead of any larger-ring
derivatives. When using proton initiator (TfOH or Tf2NH) for
the reaction of 1e and 2m, there was no desired cyclopentene
product; instead, isomers of alkene 2m with the double bond
shifted to internal were produced. Therefore, AgSbF6
possessed unique advantages, compared to proton catalysts.
Excitingly, this skeleton widely occurs in many biologically
active compounds, natural products, and natural productlike
molecules.12 Thus, we examined the possibility of modification
of these new products. To our delight, the bromoethyl product
3ka could be easily transformed via substitution by an azido
group (to compound 4), thiocyano group (to compound 5), or
elimination by KOH (to compound 6). Moreover, the bromo
product 3ma could be performed with the Sonogashira or
Next, a variety of alkenes were evaluated with symmetric or
unsymmetric acetylenes for this AgSbF6-catalyzed cyclization
(Scheme 4). Many commercially available alkenes 2b−2i with
2, 3, and 4-substituents were tested with tolane 1a. The use of
different alkenes 2c and 2a gave the same product 3aa,
indicating the carbocationic process of this reaction, as well as
2e and 2f. When 2h was employed, the double bond in the
product was shifted to the cyclohexyl ring, probably due to the
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Scheme 4. Reaction Scope of Various Alkenes with 1
Figure 1. Products derivatization and structure determination of 8
from 3ca.
Scheme 5. Controlled Experiments for the Catalyst Role
Suzuki coupling reaction to give products 7a and 7b.13 Finally,
the product 3ca could be irradiated to give fused phenanthrene
8. The structure of 8 was unequivocally confirmed by XRD
analysis as well (see Figure 1, Cambridge Crystallographic
Data Centre No. CCDC-1584141).
Mechanistically, it is noteworthy to discover the great
selectivity between simple alkenes and alkynes in the presence
of AgSbF6 catalyst. We were curious about the trigger mode of
AgSbF6 catalyst. Thus, monitoring the reaction of 1a with 2a
under 5 mol % and 10 mol % AgSbF6 by GC analysis showed
that the formation of cyclopentene 3aa under 10 mol %
AgSbF6 loading is obviously faster than the reaction with 5
mol AgSbF6 (see Figure S1 in the Supporting Information). So
the catalyst loading is very crucial for this transformation. At
the same time, there was a dramatic rate increase after 2 h
under both conditions. The slow conversion ratio (<20%)
within the first 2 h suggests that this reaction requires a long
initiation process. Controlled experiments then were con-
ducted [see Scheme 5, eqs 1−4]. Surprisingly, even within 0.5
h, the reaction can reach beyond 60% yield (>70% conversion)
[Scheme 5, eqs 1 and 3], and is in sharp contrast to the
standard protocol (0.5 h, ∼15% conversion). For the
pretreatment of AgSbF6 with alkene 2a, the reaction rate
declined slightly, compared with the standard condition
[Scheme 5, eq 2]. Whereas for the pretreatment of AgSbF6
in CHCl3, the catalyst could be deactivated and not be capable
of catalyzing the cyclization. Through these series of
experimental results, we proposed that this reaction was
induced by the action of AgSbF6 with alkynes, and the in-situ-
generated reactive intermediates could trigger the subsequent
cyclization with alkenes.
In addition, we made some attempts to shed more light on
the catalyst activation step. Chemical shift had changed
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obviously via 13C NMR spectra, compared with the alone
alkyne 1d in CDCl3, which revealed the dynamic coordination
between 1d and AgSbF6 and the formed complex was not
stable (see Figure S2 in the Supporting Information).14
Monitoring the reactions of 1a with 2a under different
substrates concentration via gas chromatography (GC)
analysis showed the relative rate of cyclopentene 3aa formation
with higher alkyne concentration was faster (see Figures S4
and S5 in the Supporting Information); but no obvious
acceleration was observed with higher alkene concentration.
These results further proved the coordination between alkyne
and AgSbF6 was more favorable.
Furthermore, for a deep insight into the alkyl C−H cleavage
mechanism, (S)-2m was prepared and subjected to react with
1e, thus enantioenriched product (R)-3em was synthesized
with some erosion of ee [Scheme 5, eq 5]. The comparation of
its derivative 9 with simulating result suggested that the
enantiomer ratio was 90:10. This result implied that most of
the C-figuration was retained in the cyclization process and
suggested the alkyl C−H cleavage was very likely a concerted
process.7b
intermediate is quenched by the acid to give the product
and regenerates the catalyst AgSbF6.16
In summary, we reported a novel cationic intermolecular
cyclization between the alkenes and alkynes catalyzed by
AgSbF6. The remarkable tolerance of functional groups in the
reaction allowed a great possibility for further transformations.
These findings encourage us finding more Ag-catalyzed new
reactions based on C−C bond formation on the inert C−H
bond.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.8b02448.
Experimental procedures, the data, and the copies of 1H
and 13C NMR (PDF)
(CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: chenchao01@mails.tsinghua.edu.cn.
■
Based on the above controlled experiments, plausible
catalytic cycles are proposed in Scheme 6. In this reaction,
ORCID
Hui Chen: 0000-0003-0483-8786
Chao Chen: 0000-0002-7644-0884
Scheme 6. Proposed Mechanism
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Thank Prof. Dr. Lei Jiao of Tsinghua University for guidance
on this work. We are thankful for the financial support from
National Natural Science Foundation of China (No.
21672120), The National Key Research and Development
Program of China (No. 2016YFB0401400), the Fok Ying
Tong Education Foundation of China (Grant No. 151014),
and the Postdoctoral Foundation of the Peking−Tsinghua
Joint Center for Life Sciences (for B.C.).
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(16) Although we have received some tips for the reaction
mechanism, it is difficult to fully understand the reaction pathway
at this stage. No long-life intermediate was observed via IR in situ. To
shed more light on the reaction mechanism at the molecular level,
density functional theory (DFT) calculations were then conducted.
The energies of intermediate II, IV, IV′, and VIII were estimated, but
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state, was not successful; see page S33 in the Supporting Information.
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