(3 + 2)-Cycloaddition of Donor-Acceptor Cyclopropanes with Selenocyanate: Synthesis of Dihydroselenophenes and Selenophenes

Article abstract of DOI:10.1021/acs.orglett.0c03329

We present a Lewis-acid-catalyzed (3 + 2)-cycloaddition of donor-acceptor cyclopropanes and selenocyanate (as its tetramethylammonium salt) for the synthesis of dihydroselenophenes. The transformation proceeded with moderate to excellent yields and showed a high functional group tolerance. Further oxidation using DDQ delivered selenophenes.

Full text of DOI:10.1021/acs.orglett.0c03329

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Letter
(3 + 2)-Cycloaddition of Donor−Acceptor Cyclopropanes with
Selenocyanate: Synthesis of Dihydroselenophenes and
Selenophenes
Anu Jacob, Peter G. Jones, and Daniel B. Werz*
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ABSTRACT: We present a Lewis-acid-catalyzed (3 + 2)-cycloaddition of donor−acceptor cyclopropanes and selenocyanate (as its
tetramethylammonium salt) for the synthesis of dihydroselenophenes. The transformation proceeded with moderate to excellent
yields and showed a high functional group tolerance. Further oxidation using DDQ delivered selenophenes.
Scheme 1. Previous Work on the Formation of Sulfur/
Selenium-Containing Heterocycles via D-A Cyclopropane
Chemistry and Our Novel Route to Dihydroselenophenes
onor−acceptor (D−A) cyclopropanes, the smallest
D
carbocyclic ring systems, have attracted the attention of
synthetic chemists because of their versatility as masked 1,3-
zwitterionic building blocks.1 Pioneering reports by Wenkert
and Reissig2 on these highly strained molecules3 date back to the
late 1970s. Though the chemistry of D−A cyclopropanes had
remained dormant for some decades, the area has seen a revival
in the recent past, unlocking the synthetic utility of these
interesting molecules. The “push effect” by the donor and the
“pull effect” by the acceptor groups promote the lability of the
bond that connects them, thus facilitating a wide variety of
transformations including cycloadditions, rearrangements,4 and
ring-opening reactions.5 Of these, the types most investigated
are cycloadditions with various 1,2-, 1,3- and 1,4-dipoles
including dienes,6 carbonyls,7 imines,8 nitrones,9 alkynes,10
heterocumulenes,11 and other dipolarophiles.12 Although
syntheses of oxygen- and nitrogen-containing heterocycles
employing D−A cyclopropanes have been extensively studied,
there are few routes to sulfur or selenium analogues. In 2012, the
Stolz group demonstrated that a stoichiometric amount of
Sn(OTf)2 enabled the conversion of D−A cyclopropanes to
thioimidates in the presence of isothiocyanate,13 and tetrasub-
stituted thiophenes have been synthesized in two steps utilizing
trans-2-aroyl-3-arylcyclopropane-1,1-dicarboxylates and in situ-
generated mercaptoaldehyde.14 In 2017, our group reported the
synthesis of tetrahydrothio- and tetrahydroselenophenes in a
(3 + 2)-cycloaddition of D−A cyclopropane with thio- and
selenoketones, respectively.15 Shortly after this, Yazaki and
Ohshima were able to conduct a similar transformation with
thionoesters.16 Recently, Guo and co-workers reported the
Yb(OTf)3-catalyzed cycloaddition of D−A cyclopropanes with
thiourea to provide dihydrothiophenes (Scheme 1a).17
leading to dihydroselenophenes (Scheme 1b). These com-
pounds might also be precursors for selenophenes, which have
found prominent roles not only in optoelectronics,18 lumines-
cent materials,19 and semiconductors20 but also as antiox-
idants,21 antinociceptives,22 and anti-inflammatory agents.23
Selenophenes are usually synthesized from alkynes, dienes,
and β-diketo compounds on treatment with elemental selenium
Received: October 5, 2020
Because the selenocyanate anion can be regarded as
deaminated and deprotonated selenourea, we conjectured that
selenocyanate might show a reaction with D−A cyclopropanes,
© XXXX The Authors. Published by
American Chemical Society
https://dx.doi.org/10.1021/acs.orglett.0c03329
Org. Lett. XXXX, XXX, XXX−XXX
A

Organic Letters
pubs.acs.org/OrgLett
Letter
or other selenium sources.24 However, routes to partially
saturated dihydroselenophenes are scarce in the literature.25
To test our notion that such a (3 + 2)-cycloaddition reaction
of selenocyanate and a D−A cyclopropane might be feasible, we
started by using cyclopropane 1a as a model compound. As
suitable selenocyanate sources, NH4SeCN, KSeCN,
NBu4SeCN, and NMe4SeCN were chosen. The initial experi-
ments were performed by mixing cyclopropane 1a and
tetramethylammonium selenocyanate in THF for 12 h at
25 °C with various Lewis acids such as AlCl3, MgI2, TiCl4, and
Sc(OTf)3; however, no reaction was observed (Table 1, entries
Scheme 2. (3 + 2)-Cycloaddition Reaction with Respect to
Different D−A Cyclopropanes
a
a
Table 1. Optimization of the Reaction Conditions
b
entry
Lewis acid
solvent
THF
THF
THF
THF
THF
THF
MeCN
MeCN
THF/MeCN
THF/MeCN
T (°C)
yield of 3a (%)
1
2
3
4
5
6
7
8
9
AlCl3
MgI2
TiCl4
25
25
25
25
25
65
65
65
65
65
0
0
0
0
21
35
0
Sc(OTf)3
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
c
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
22
52
91
d
e
e
f
g
f
10
a
Reaction conditions: 1a (100 μmol), 2 (250 μmol), Lewis acid (20
b
mol %), solvent (1 mL), under Ar, 12 h. Yields refer to purified and
isolated products. 30 mol % of Yb(OTf)3. 30 mol % of Lewis acid
was used. The ratio of the solvent system is 1:1. The reaction was
stirred for 6 h. 60 mol % of Lewis acid was used.
a
c
d
Reaction conditions: 1 (100 μmol), 2 (250 μmol), Yb(OTf)3 (60
e
f
mol %), THF/MeCN (1:1) (2 mL), at 65 °C under Ar, 6−20 h;
b
g
yields refer to purified and isolated products. Large scale: 71% yield
c
for 1.5 mmol of starting material. The reaction was run for 4 h at 25
°C.
1−4). With Yb(OTf)3 (20 mol %), the desired product was
formed in 21% yield (entry 5). The investigations with
cyclopropane 1a and NH4SeCN were unsuccessful because of
the tendency of NH4SeCN to decompose to elemental
selenium. Thereafter, KSeCN was chosen as the selenocyanate
source (entry not shown in Table 1). KSeCN together with 18-
crown-6 was envisioned as providing a more “naked” and thus
more nucleophilic selenocyanate, facilitating the desired
reaction, yet in fact, it exhibited poor reactivity with D−A
cyclopropanes. Under the same reaction conditions, NBu4SeCN
gave the desired product in 9% yield (entry not shown in Table
1). An increase in temperature to 65 °C using NMe4SeCN
increased the product yield from 21 to 35% (entry 6). A change
of the solvent to acetonitrile, in which the salt is much better
soluble than in THF, proved to be unsuccessful when 20 mol %
of the Lewis acid was used (entry 7). Increasing the amount of
Lewis acid to 30 mol % afforded the product in 22% yield (entry
8). Additionally, we observed a sharp increase in the yield with
increasing catalyst loading. An exploration of various solvents
showed that a 1:1 mixture of THF and acetonitrile proved to be
the best choice and furnished 3a in 6 h in 91% yield (entries 9
and 10).
compounds 3a and 3c unambiguously confirmed the structure
of the product. Cyclopropane 1f was completely consumed in
4 h at 25 °C to deliver the corresponding product in 58% yield,
consistent with its high reactivity. Notably, cyclopropane 1k
underwent smooth transformation under the standard con-
ditions to give product 3k in 74% yield. More steric bulk, with
the o-methyl substituent on the phenyl donor, afforded product
3l in 69% yield. However, the mesityl variant 3m showed a
strong decrease in the yield to 34%. The naphthyl group with an
extended π system furnished the corresponding dihydroseleno-
phene 3n in 71% yield. Different ester groups, such as benzyl,
ethyl, and tert-butyl esters, as acceptor moieties were found to be
tolerant under reaction conditions and provided very good
yields (3p−3r). Heterocyclic donors such as thienyl and
succinimide moieties delivered the corresponding product in
54 and 30% yield, respectively (3s, 3t).
To demonstrate the utility of the dihydroselenophene moiety,
products 3 were oxidized to selenophenes. DDQ proved to be
the agent of choice. We tried various solvents such as DCM,
DCE, and toluene; however, the best results were obtained in
benzene (Scheme 3). All selenophenes 4 were obtained in good
to excellent yields. The structure of 4a was confirmed by X-ray
crystallography. Unfortunately, a combination of the two
reactions (cycloaddition and oxidation) in one pot did not
provide the selenophenes.
With the optimized reaction conditions in hand, we probed
the generality of this method using tetramethylammonium
selenocyanate (2) and variously substituted D−A cyclo-
propanes. The reaction proceeded smoothly with several
p‑substituted aryl cyclopropanes under the optimized reaction
conditions, providing the desired products 3a−3j in good to
excellent yields (Scheme 2). Single-crystal X-ray analysis of
To shed light on the reaction mechanism for the
dihydroselenophene formation, we performed several experi-
B
https://dx.doi.org/10.1021/acs.orglett.0c03329
Org. Lett. XXXX, XXX, XXX−XXX

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pubs.acs.org/OrgLett
Letter
only 10% yield. Full conversion up to a yield of 91% was only
found with more than 2 equiv (Scheme 4b). The intermediate of
the potential mechanism, imino dicarboxylate 3aa, was isolated
in 46% yield when the reaction was carried out for 5 min at room
temperature. When we performed the same reaction with D−A
cyclopropane 1p, bearing benzyl ester groups, we were able to
isolate 40% of BnSeCN 5 as side product. Thus, we assume that
the selenocyanate has a dual role: it serves as a reagent to
incorporate the selenium, but its second crucial role is to cleave
one of the ester moieties by acting as a nucleophile; therefore, 2
equiv is required. Taking these considerations into account, we
propose the following mechanism (Scheme 5). Yb(OTf)3
Scheme 3. Oxidation of Dihydroselenophenes to
Selenophenes
a
Scheme 5. Proposed Mechanism
a
Reaction conditions: 3 (100 μmol), DDQ (100 μmol), C6H6 (2
mL), under Ar, 8 h; yields refer to purified and isolated products.
b
Large scale: 73% for 1 mmol of 3a.
ments. First, we wished to establish whether the initial attack of
the selenocyanate takes place via an SN2-like mechanism.
Therefore, we used enantiomerically (94% ee) enriched D−A
cyclopropane (Scheme 4a). As shown by HPLC measurements,
coordinates to the acceptor moieties in the D−A cyclopropane.
The activated ring allows a nucleophilic attack of the
selenocyanate in an SN2-like fashion, opening the highly strained
cyclic system. The emerging malonate attacks the carbon of the
nitrile, leading to a kinetically favored five-membered ring
system. Excess selenocyanate in the reaction medium attacks the
alkyl moiety of the ester, setting the stage for decarboxylation,
followed by formation of dihydroselenophene.
Scheme 4. Control Experiments with Respect to the
Mechanism
In conclusion, we have demonstrated an attractive and
efficient strategy for the construction of dihydroselenophenes by
Lewis-acid-catalyzed (3 + 2)-cycloaddition reaction of donor−
acceptor cyclopropanes and selenocyanate (as its tetramethyl-
ammonium salt). Numerous functional groups are tolerated.
The emerging dihydroselenophenes were easily oxidized by
DDQ to selenophenes. The special substitution pattern might
be interesting for applications in materials chemistry. Current
studies using thiocyanates are underway and will be reported in
due course.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03329.
Detailed experimental procedures and analytical data, 1H
and 13C spectra for all new compounds, and details on
mechanistic experiments (PDF)
a slight loss of enantiomeric purity was found for the product
(89% ee). However, no racemic mixture was obtained. Thus, we
assume that the initial attack proceeds via an SN2-like
mechanism, but because of the large amount of Lewis acid, a
significant background reaction converting one enantiomer of
the cyclopropane to the other takes place. Second, we wished to
understand why more than 2 equiv of selenocyanate is required.
As shown in Scheme 4b, 1 equiv delivered the desired product in
Accession Codes
CCDC 2034505−2034507 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
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C. G.; Studer, A. Org. Lett. 2016, 18, 5576−5579. (i) Garve, L. K. B.;
Jones, P. G.; Werz, D. B. Angew. Chem., Int. Ed. 2017, 56, 9226−9230.
̈
(j) Lucht, A.; Patalag, L. J.; Augustin, A. U.; Jones, P. G.; Werz, D. B.
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
Angew. Chem., Int. Ed. 2017, 56, 10587−10591. (k) Wallbaum, J.;
Garve, L. K. B.; Jones, P. G.; Werz, D. B. Org. Lett. 2017, 19, 98−101.
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AUTHOR INFORMATION
Corresponding Author
■
̈
Daniel B. Werz − Technische Universitat Braunschweig, Institute
of Organic Chemistry, 38106 Braunschweig, Germany;
orcid.org/0000-0002-3973-2212; Email: d.werz@tu-
braunschweig.de
́
(o) Richmond, E.; Vukovic, V. D.; Moran, J. Org. Lett. 2018, 20, 574−
577. (p) Augustin, A. U.; Jones, P. G.; Werz, D. B. Chem. - Eur. J. 2019,
25, 11620−11624. (q) Lu
G.; Reissig, H.-U.; Sarkar, B.; Werz, D. B. Chem. - Eur. J. 2019, 25,
10359−10365. (r) Kreft, A.; Lucht, A.; Grunenberg, J.; Jones, P. G.;
̈
cht, A.; Sobottka, S.; Patalag, L. J.; Jones, P.
Authors
̈
Anu Jacob − Technische Universitat Braunschweig, Institute of
Organic Chemistry, 38106 Braunschweig, Germany
Peter G. Jones − Institute of Inorganic and Analytical Chemistry,
Technische Universitat Braunschweig, 38106 Braunschweig,
̈
Werz, D. B. Angew. Chem., Int. Ed. 2019, 58, 1955−1959. (s) Guin, A.;
Rathod, T.; Gaykar, R. N.; Roy, T.; Biju, A. T. Org. Lett. 2020, 22,
2276−2280. (t) Mondal, B.; Das, D.; Saha, J. Org. Lett. 2020, 22, 5115−
5120.
̈
Germany
(6) (a) Zhu, W.; Fang, J.; Liu, Y.; Ren, J.; Wang, Z. Angew. Chem., Int.
Ed. 2013, 52, 2032−2037. (b) Ma, C.; Huang, Y.; Zhao, Y. ACS Catal.
2016, 6, 6408−6412. (c) Dey, R.; Banerjee, P. Org. Lett. 2017, 19, 304−
307. (d) Blom, J.; Vidal-Albalat, A.; Jørgensen, J.; Barløse, C. L.; Jessen,
K. S.; Iversen, M. V.; Jørgensen, K. A. Angew. Chem., Int. Ed. 2017, 56,
11831−11835.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03329
Notes
The authors declare no competing financial interest.
(7) (a) Pohlhaus, P. D.; Sanders, S. D.; Parsons, A. T.; Li, W.; Johnson,
J. S. J. Am. Chem. Soc. 2008, 130, 8642−8650. (b) Benfatti, F.; de
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(c) Sabbatani, J.; Maulide, N. Angew. Chem., Int. Ed. 2016, 55, 6780−
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2062.
ACKNOWLEDGMENTS
A DAAD fellowship to A.J. is gratefully acknowledged.
■
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