Triazene-Activated Donor-Acceptor Cyclopropanes: Ring-Opening and (3 + 2) Annulation Reactions

Article abstract of DOI:10.1021/acs.orglett.0c01527

Donor-acceptor cyclopropanes substituted with 3,3-dialkyltriazenyl groups are described herein. The strong electron-donating character of the triazene renders the cyclopropanes highly reactive, allowing for catalyst-free ring-opening reactions with methanol and tetracyanoethylene under mild conditions. The triazene-substituted cyclopropanes could also be used as substrates in Lewis acid catalyzed (3 + 2) annulations with silyl enol ethers.

Full text of DOI:10.1021/acs.orglett.0c01527

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Letter
Triazene-Activated Donor−Acceptor Cyclopropanes: Ring-Opening
and (3 + 2) Annulation Reactions
Abdusalom A. Suleymanov, Eliott Le Du, Zhaowen Dong, Bastian Muriel, Rosario Scopelliti,
́
̂
Farzaneh Fadaei-Tirani, Jerome Waser,* and Kay Severin*
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ABSTRACT: Donor−acceptor cyclopropanes substituted with 3,3-
dialkyltriazenyl groups are described herein. The strong electron-donating
character of the triazene renders the cyclopropanes highly reactive,
allowing for catalyst-free ring-opening reactions with methanol and
tetracyanoethylene under mild conditions. The triazene-substituted cyclopropanes could also be used as substrates in Lewis acid
catalyzed (3 + 2) annulations with silyl enol ethers.
yclopropanes substituted with both electron-withdrawing
and electron-donating groups in vicinal position display a
which provide the right amount of activation, while ensuring
sufficient stability of the three-membered ring. Indeed, too
much activation can result in spontaneous rearrangement of
cyclopropanes into larger 5-membered heterocycles.6
A good balance between stability and reactivity has
traditionally been realized with carbon- and oxygen-based
donor groups, whereas nitrogen-based substituents were less
investigated, despite their synthetic relevance.2 To address this
limitation, the Waser group has introduced phthalimide-,
succinimide-, and nucleobase-substituted cyclopropanes
(Scheme 1b).2e,7 In this work, we introduce 3,3-dialkyltria-
zenes as new nitrogen-based donors. We show that triazenes
provide an exceptional activation of the cyclopropane ring,
allowing for ring-opening and -expansion reactions under mild
conditions.
C
high reactivity, which can be accentuated by using catalysts.1,2
Typically, catalytic activation is achieved by coordination of a
Lewis or Brønsted acid to the acceptor group.2 The acid
facilitates heterolytic C−C bond cleavage of the strained ring,
allowing for reactions with electrophiles or nucleophiles, as
well as cycloaddition and rearrangement reactions (Scheme
1a). Alternatively, formyl-acceptor groups can be activated by
Scheme 1. Reactivity of D−A Cyclopropanes (A) and
Nitrogen-Based Donor Groups (B)2−7
Over the last five years, we have studied the chemistry of 1-
alkynyltriazenes.8 These investigations have revealed that the
triazene group is a very good electron donor, enabling an
electrophilic attack to the triple bond. In view of these results,
we were interested in exploring if triazenes could be used as
donors in D−A cyclopropanes to expand the range of useful
nitrogen substituents.
A common procedure for the synthesis of diester-substituted
cyclopropanes is the Rh-catalyzed cyclopropanation of olefins
with diazomalonates.9 This approach could be implemented
for the synthesis of cyclopropanes bearing triazene groups. The
required vinyltriazenes starting materials 1a−1e were accessed
by coupling of vinyl Grignard reagents with nitrous oxide and
lithium amides8g,10 or by hydrogenation of alkynyltriazenes.11
amine catalysts via iminium formation.3 The catalytic
activation of donor−acceptor (D−A) cyclopropanes via
interaction of the donor group with Lewis bases is less
common.4 Examples of isolable D−A cyclopropanes, which are
able to undergo ring-opening reactions without external
catalysts, remain scarce.5 The paucity of such cyclopropanes
is related to the difficulty of finding donor/acceptor pairs,
Received: May 4, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01527
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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Cyclopropane 2a could then be prepared by reaction of (E)-
3,3-dicyclohexyl-1-vinyltriaz-1-ene (1a) with dimethyl diazo-
malonate (1.2 equiv) in the presence of commercially available
Rh2(esp)2 (0.2 mol %) as catalyst (Scheme 2). The desired
Scheme 2. Synthesis of D−A Cyclopropanes with Triazene
a
Groups
Figure 1. 13C NMR chemical shifts of different D−A cyclopropanes
(a) and (b) relative stability of carbocations (ΔG298 (kcal·mol−1)).
The reference values were taken from the literature (ref 13). All
spectra were measured in CDCl3. The calculations for the
carbocations were performed at the M06-2X/6-311+G(d,p) level.
kinetic15 point of view. The calculations on M06-2X/6-
311+G(d,p) level showed that carbocation A is 19 kcal·mol−1
more stable than B. Accordingly, the triazene group was
expected to provide a very strong activation of the cyclo-
propane ring.
Diverse reactivity studies with these new cyclopropanes 2
were then carried out. When 2a was dissolved in methanol, the
ring-opened adduct 3a (Scheme 3) could be isolated in
quantitative yield after heating to 60 °C for 2 h, followed by
evaporation of the solvent. The structure of 3a could be
confirmed by X-ray crystallography. Analogously, reaction of
a
Reaction conditions: vinyltriazene (1 equiv), diazomalonate (1.2
equiv), Rh2(esp)2 (0.2−0.5 mol %), DCM (0.1 M), 0 °C to rt.
product was obtained in good yield, and the procedure was
further optimized to gram scale with an isolated yield of 87%.
Compound 2a is a colorless crystalline solid, and its structure
could be confirmed by single-crystal X-ray diffraction. Using
this optimized procedure, we could prepare cyclopropanes
2b−2e having additional phenyl, 4-anisyl, 2-thienyl, or
cyclopropyl groups in C3 position from the corresponding
cis-1,2-disubstituted vinyltriazenes 1b−1e. The isolated yields
of 2b−2e varied from 52% to 65%. According to the 1H NMR
spectra, the cis stereochemistry is conserved in the products.
The 13C NMR spectrum of cyclopropane 2a in CDCl3
shows a chemical shift for the C2 atom next to the triazene
group at 55.6 ppm. Calculation of 13C NMR chemical shifts for
DFT-optimized structures of 2a gave a similar value.12 For
comparison, the corresponding values for “standard” D−A
cyclopropanes having aryl, cyclopropyl, or phthalimidyl donor
groups lie in the range of 31−35 ppm, and cyclopropanes with
poor donor groups such as n-butyl and hydrogen show a C2
signal at 24 and 17 ppm, respectively (Figure 1a).13 The
strongly shifted C2 signal of 2a was a first indication for the
pronounced electronic effect of the triazene group.
Scheme 3. Methanol Addition Reactions
Further evidence for the strong electron-donating properties
of the triazene group were obtained by calculating the relative
stability of carbocations A and B (Figure 1b). This approach,
which was developed by Werz,14 allows comparison of the
donor capabilities of different substituents. As a reference, we
used the p-methoxyphenyl group, which is known to be one of
the strongest donors among the reported stable D−A
cyclopropanes, both from a thermodynamic14 as well as a
B
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methanol with cyclopropanes 2b−2e gave the addition
products 3b−3e in quantitative yields as a mixture of two
diastereoisomers (ratio: ∼1:1). In all cases, the bond between
the triazene and the diester was broken, showing again that it
was a better donor than a (hetero)arene or a cyclopropyl
group. Preliminary mechanistic investigations indicate that
methanol addition proceeds with participation of the ester
group: more details are given in the Supporting Information.
For comparison, we have attempted reactions with methanol
using three literature-known D−A cyclopropanes (donor = p-
methoxyphenyl, N-phthalimide, or O-acetate). No reaction was
observed, even after prolonged (24 h) heating at 60 °C. These
results underline the unique donor properties of the triazene
group.
Next, we have explored the reactivity of 2a toward
electrophiles. Ring-opening of D−A cyclopropanes with
electrophiles is generally more challenging.2 Cyclopropane 2a
reacted with tetracyanoethylene (TCNE) in a formal (3 + 2)
cycloaddition reaction, in the absence of any external catalyst,
to give the highly functionalized cyclopentane 4a in 65% yield
(Scheme 4).16 Similarly, we were able to prepare the addition
bearing triazene groups could be activated by Lewis or
Brønsted acids because the triazene groups are known to be
acid sensitive.17
As a benchmark reaction, we have examined the addition of
silyl enol ethers to cyclopropane 2a. For D−A cyclopropanes
with N-donor groups, this reaction is known to occur with a
Lewis acid catalyst such as SnCl4.7 Screening of different LA
catalysts and reaction conditions revealed that hafnium
triflate18 can catalyze the desired reaction between 2a and
silyl enol ethers at −40 °C (see the SI for details).
Using the optimized reaction conditions, we have tested
different acetophenone-based silyl enol ethers (5a−5c, Scheme
5). Bulky TBDPS and TIPS groups allowed isolation of the
a
Scheme 5. (3 + 2) Annulation with Silyl Enol Ethers
a
Scheme 4. (3 + 2) Annulation with TCNE
a
Reaction conditions: 2a (1 equiv), enol ether (1.2 equiv), Hf(OTf)4
*
(10 mol %), DCM (0.1 M), −40 °C, 20 h. After recrystallization
from MeOH.
cyclopentanes 6a and 6b in good yields with a dr of 4:1.
Recrystallization of 6a from methanol improved the dr to 16:1.
Analysis of 6a by NOESY NMR spectroscopy revealed that the
major diastereoisomer has a trans configuration of the triazene
and the OSiR3 group. The same selectivity had been observed
for reactions with N-phthalimide D−A cyclopropanes.7a
Utilization of a silyl enol ether with a less bulky TMS group
(5c) gave cyclopentane 6c in lower yield (57%) and a dr of
2.6:1. Next, we tested silyl enol ethers with different aryl
groups. Electron-withdrawing 4-fluorophenyl and electron-
donating 4-anisyl led to the formation of 6d and 6e in 54% and
88% yield, respectively. Interestingly, the dr of the products
was only slightly influenced by the electronic properties of the
aryl groups. On the other hand, using a substrate with a bulky
1-naphthyl substituent gave 6f almost exclusively as one
a
Reaction conditions: 2a (1 equiv), TCNE (1.2 equiv), acetone (0.1
M), 40 °C, 12 h.
products 4d and 4e, albeit with reduced yields of 40% and
46%, respectively. The cyclopentanes 4d and 4e were isolated
as mixtures of diastereoisomers, with prevalence of the trans
isomer (dr = 2:1). As for the reactions with methanol, we
found that D−A cyclopropanes with p-methoxyphenyl or N-
phthalimide donor groups did not react with TCNE under
these conditions.
After having established catalyst-free reactions with meth-
anol and TCNE, we turned our attention to less reactive
reaction partners, which require activation of the acceptor
group by a catalyst. A priori, it was not clear if cyclopropanes
C
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́
Zhaowen Dong − Institut des Sciences et Ingenierie Chimiques,
diastereoisomer (dr >20:1) in 49% yield. Cyclopentane 6f was
additionally characterized by single-crystal X-ray diffraction.
In conclusion, we report the first synthesis of D−A
cyclopropanes substituted with dialkyltriazene groups by a
simple Rh-catalyzed reaction of vinyltriazenes with dimethyl
diazomalonate. The triazene group was found to provide an
exceptionally strong activation of the cyclopropane ring,
allowing for catalyst-free reactions with methanol and
TCNE. Combined with the good stability of the new
cyclopropanes, this type of reactivity is unique, as evidenced
by the fact that “standard” D−A cyclopropanes with 4-
methoxyphenyl or N-phthalimide donor groups do not react
under these conditions. The usefulness of the new triazenyl
cyclopropanes was further highlighted by their compatibility
with Lewis acid catalysts in a (3 + 2) cycloaddition with silyl
enol ethers, which delivered highly functionalized cyclo-
pentanes. Taken together, our findings highlight the potential
of the triazene group in activating cyclopropane ring systems.
The results are also relevant from the more general perspective
of triazene chemistry. So far, most investigations about
triazenes have focused on 1-aryltriazenes,19 with studies of 1-
vinyl-11,20 and 1-alkynyltriazenes8 as recent additions. There
are very few reports about 1-alkyltriazenes and no examples of
alkyltriazenes with additional functional groups.21 The D−A
cyclopropane chemistry described above offers a means to
prepare structurally and functionally complex alkyltriazenes.
Further reactivity studies are ongoing in our laboratories.
́
́
Ecole Polytechnique Federale de Lausanne (EPFL), 1015
Lausanne, Switzerland
́
Bastian Muriel − Institut des Sciences et Ingenierie Chimiques,
́
́
Ecole Polytechnique Federale de Lausanne (EPFL), 1015
Lausanne, Switzerland
́
Rosario Scopelliti − Institut des Sciences et Ingenierie
́
́
Chimiques, Ecole Polytechnique Federale de Lausanne (EPFL),
1015 Lausanne, Switzerland; orcid.org/0000-0001-8161-
8715
́
Farzaneh Fadaei-Tirani − Institut des Sciences et Ingenierie
́
́
Chimiques, Ecole Polytechnique Federale de Lausanne (EPFL),
1015 Lausanne, Switzerland; orcid.org/0000-0002-7515-
7593
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01527
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by funding from the Ecole
■
́
́
Polytechnique Federale de Lausanne (EPFL) and the Swiss
National Science Foundation (Grant No. 200021_165788).
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01527.
Experimental details and analytical data of the new
compounds (PDF)
Accession Codes
CCDC 1995157 and 1995416−1995418 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
́
̂
́
Jerome Waser − Institut des Sciences et Ingenierie Chimiques,
́
́
Ecole Polytechnique Federale de Lausanne (EPFL), 1015
Lausanne, Switzerland; orcid.org/0000-0002-4570-914X;
Email: jerome.waser@epfl.ch
́
Kay Severin − Institut des Sciences et Ingenierie Chimiques, Ecole
́
́
Polytechnique Federale de Lausanne (EPFL), 1015 Lausanne,
Switzerland; orcid.org/0000-0003-2224-7234;
Email: kay.severin@epfl.ch
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Abdusalom A. Suleymanov − Institut des Sciences et Ingenierie
́
́
Chimiques, Ecole Polytechnique Federale de Lausanne (EPFL),
1015 Lausanne, Switzerland
́
Eliott Le Du − Institut des Sciences et Ingenierie Chimiques,
́
́
Ecole Polytechnique Federale de Lausanne (EPFL), 1015
Lausanne, Switzerland
D
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