The Cyclopropyl Group as a Neglected Donor in Donor-Acceptor Cyclopropane Chemistry

Article abstract of DOI:10.1021/acs.orglett.8b00603

D-A cyclopropanes bearing a simple cyclopropyl group as donor are shown to undergo a variety of [3+n]-cycloaddition reactions (n = 2-4). This behavior contrasts sharply with that of common D-A cyclopropanes with aliphatic donors. Kinetic experiments demonstrate that, in terms of donor ability, the cyclopropyl substituent lies between electron-rich and electron-neutral aryl donors.

Full text of DOI:10.1021/acs.orglett.8b00603

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
The Cyclopropyl Group as a Neglected Donor in Donor−Acceptor
Cyclopropane Chemistry
Alexander Kreft, Peter G. Jones, and Daniel B. Werz*
†
‡
,†
†
‡
Institut fu
̈
r Organische Chemie and Institut fu
̈
r Anorganische und Analytische Chemie, TU Braunschweig, Hagenring 30, 38106
Braunschweig, Germany
*
S Supporting Information
ABSTRACT: D−A cyclopropanes bearing a simple cyclopropyl
group as donor are shown to undergo a variety of [3+n]-
cycloaddition reactions (n = 2−4). This behavior contrasts
sharply with that of common D−A cyclopropanes with aliphatic
donors. Kinetic experiments demonstrate that, in terms of donor
ability, the cyclopropyl substituent lies between electron-rich and electron-neutral aryl donors.
onor−acceptor (D−A) cyclopropanes are one of the most
Scheme 1. Cyclopropyl Group in D−A Cyclopropane
Chemistry
D
versatile three-carbon-atom building blocks in organic
1
synthesis. These highly strained and, thus, “spring-loaded”
molecules are substituted with an electron-donating and an
electron-withdrawing group in vicinal positions, a substitution
pattern that allows facile heterolytic cleavage of the correspond-
ing bond in the cyclopropane by stabilizing a 1,3-zwitterionic
structure. Consequently, these systems are easily able to undergo
2
3
nucleophilic or electrophilic ring-opening, rearrangement and
formal cycloaddition reactions.
4
Whereas ring-opening reactions produce 1,3-bifunctionalized
carbon chains, rearrangements and especially formal cyclo-
addition reactions allow fast and efficient access to complex
(hetero)cyclic motifs. Nevertheless, established donor moieties
generally involve aryl groups or heteroatoms (such as O and N);
there are very few examples with aliphatic substituents. However,
the cyclopropyl group itself is known to stabilize a positive charge
in α-position because of an orbital overlap of the cyclopropane
5
Walsh orbital with an emerging empty orbital. Thus, we
considered whether simple cyclopropyl moieties would allow a
similar chemistry to that observed with common aryl-substituted
D−A cyclopropanes. A scrutiny of the literature revealed that, as
early as 1991, Herndon and co-workers had reported the
synthesis and a formal cycloaddition reaction of a D−A
cyclopropane bearing both a methoxy and a cyclopropyl group
the cyclopropyl residue would stay intact under our selected
reaction conditions. Finally, we wanted to compare the reactivity
of this special D−A cyclopropane with that of aryl-substituted
systems, which are widely used in this field of chemistry by our
group and many others.
The desired starting material was readily available in a simple
two-step synthesis beginning from commercially available
cyclopropane carbaldehyde 1. Knoevenagel condensation with
dimethyl malonate and subsequent Johnson−Corey−Chaykov-
sky reaction with trimethylsulfoxonium iodide yielded the
product 2 in an overall yield of 66% (Scheme 2).
6
,7
as donor on the same carbon (Scheme 1, eq 1). However, in
such experiments the effect of the cyclopropyl moiety alone
could not be examined. In 2016, Tomilov et al. reported that a
D−A cyclopropane possessing a phenyl-substituted cyclopropyl
group as donor is able to undergo ring-opening reactions of both
three-membered rings in the presence of strong Lewis acids,
forming, e.g., either cyclohexene derivatives or open-chain
products in an elegant way (Scheme 1, eq 2). In these
transformations, the system acted as a formal six carbon
At the beginning of our studies, we chose various aromatic
aldehydes as substrates for formal [3 + 2]-cycloadditions,
yielding the corresponding tetrahydrofuran derivatives (Scheme
8
synthon.
3
). The reactions were carried out using conditions established
Because of these observations, we were interested in
establishing whether cycloaddition reactions could be triggered
by the use of a simple biscyclopropyl dicarboxylate without
incorporation of another donor (Scheme 1, eq 3) and whether
Received: February 19, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00603
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Synthesis of Cyclopropyl Cyclopropane
Dicarboxylate 2
π-system by using 1-naphthaldehyde as starting material yielded
the product 4j in 71% yield. Single crystals of both diastereomers
of the ferrocenyl-substituted tetrahydrofuran derivative 4k were
obtained, and X-ray crystallographic analysis confirmed the
integrity of the intact cyclopropyl donor and the formation of the
desired 5-membered heterocycle.
As the next step, further [3 + 2]-cycloaddition reactions were
studied. The formal cycloaddition with thioketone 5 yielded the
tetrahydrothiophene 6 in an excellent yield of 99% following a
by Johnson and co-workers; a catalytic amount of Sn(OTf) as
2
9
10
Lewis acid in CH Cl at room temperature was employed.
2
2
procedure recently established by our group (Scheme 4, eq 1).
a
a
Scheme 3. Synthesis of Tetrahydrofurans
Scheme 4. Various Formal [3 + 2]-Cycloadditions
a
All yields represent isolated products. E = CO Me, PMP = p-
2
methoxyphenyl.
The transformation with triazinane 7 led to the pyrrolidine 8 in
8
9% (Scheme 4, eq 2; the structure was also elucidated by X-ray
11
crystallography), and the insertion of phenyl isothiocyanate 9
using a protocol of Stoltz and co-workers gave the corresponding
12
thioimidate 10 in 69% yield (Scheme 4, eq 3).
To examine potential syntheses of larger heterocycles, both a
formal [3 + 3]- and a formal [3 + 4]-cycloaddition were studied.
The reactions with various phenyl-substituted nitrones of type 11
yielded the tetrahydro-1,2-oxazines in very good yields of up to
1
3
9
0% (12a−c) following a procedure by Kerr (Scheme 5, eq 1).
The conversion of 2 with benzodithioloimine 13 promoted by
TiF4 as Lewis acid was more challenging and gave the
Scheme 5. Formal [3 + 3]- and [3 + 4]-Cycloaddition
Reactions
a
Reaction conditions: 2 (0.2 mmol), 3 (1.2−3.0 equiv), Sn(OTf) (0.1
2
equiv), CH Cl (2 mL), rt. All yields represent isolated products.
2
2
Fortunately, a first reaction with benzaldehyde as substrate
yielded the desired product 4a in 98% yield. Consequently, we
subjected other aromatic aldehydes to these conditions. All of
these transformations proceeded smoothly with yields up to
92%. Both electron-poor and electron-rich benzaldehydes were
tolerated, but the diastereomeric ratios of the products were only
moderate, with a preference for the cis-isomer (4b−g). Sterically
more crowded substrates with two tert-butyl groups or three
methoxy groups gave the products in reasonable yields of 63% or
80%, but with a somewhat better dr (4h, 4j). An extension of the
B
DOI: 10.1021/acs.orglett.8b00603
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
corresponding seven-membered ring 14 in a moderate yield of
14
51% (Scheme 5, eq 2).
In order to quantify the donor ability of the cyclopropyl moiety
with respect to phenyl-derived donors, the reaction rates of
various D−A cyclopropanes for the formal cycloaddition with 4-
1
9
fluorobenzaldehyde were measured by F NMR spectroscopy.
Previous studies of this cycloaddition showed a stepwise
mechanism with an initial nucleophilic attack of the aldehyde
at the positively polarized atom of the D−A cyclopropane ring
9
together with simultaneous bond-breaking. Taking into account
these results, a more electron-rich donor should lower the energy
of the transition state because of its better ability to polarize the
bond that is broken during the reaction. If this step is the rate-
determining step, a more electron-rich donor will lead to a faster
transformation. As expected, the reaction of the electron-rich p-
methoxyphenyl-substituted cyclopropane (Figure 1, pink dots)
Figure 2. Energy difference of the hypothetical isomeric cations C1 and
C2 for different residues R. A negative value indicates a better
stabilization of the cation next to the three-membered ring; a positive
value shows a better stabilization next to the residue R. All calculations
were performed on the DFT level of theory using the PW6B95
functional and the triple-ζ basis def2-TZVP together with the dispersion
1
5
correction D3BJ.
can give an indication as to which of the two compared groups is
the better donor in D−A cyclopropane chemistry with respect to
the formal zwitterionic structure of these substrates. As
estimated, a positive charge is much better stabilized by a
cyclopropyl group than by an isopropyl residue (ΔG = −14.4
−1
kcal·mol , Figure 2a) or an alkyne residue (ΔG = −13.7 kcal·
−1
mol , Figure 2b). Furthermore, a better stabilization of the
cationic structure C1 is observed in comparison to electron-poor
aromatic residues and surprisingly even to an alkene residue
(
Figure 2c−f). If the residue is changed to a simple phenyl- or
halide-substituted phenyl group, the cation is slightly favored at
the benzylic position (Figure 2g−j). In the case of the very
electron-rich p-methoxyphenyl, the cationic structure C2 is the
−1
most stable (ΔG = 10.0 kcal·mol , Figure 2k).
In summary, we have shown that the cyclopropyl group is able
to act as an excellent donor in D−A cyclopropane chemistry.
Various [3+n]-cycloaddition reactions catalyzed by Lewis acids
were accomplished, thereby generating a variety of heterocycles
bearing an intact cyclopropyl group. Moreover, kinetic studies
revealed that the donor ability of the cyclopropyl group ranges
between that of a phenyl residue and a p-methoxyphenyl group.
Simple computational studies comparing the stabilization of
positive charge either adjacent to cyclopropyl or adjacent to
various aryls confirm this view qualitatively.
Figure 1. Measurement of the relative reaction rate of 4-
fluorobenzaldehyde with different D−A cyclopropanes. The reaction
1
9
progress was monitored by F NMR spectroscopy. Reaction
conditions: D−A cyclopropane (1.00 equiv), 4-fluorobenzaldehyde
(
1.00 equiv), Sn(OTf) (8 mol %), CD Cl (0.10 M).
2 2 2
was significantly faster than the same reaction with a phenyl-
substituted derivative (Figure 1, black dots), which showed after
4
h an NMR yield of the desired product of only about 20%. The
D−A cyclopropane with a cyclopropyl group as donor showed
after 4 h an NMR yield of ∼69%, about 10 times slower than the
p-methoxyphenyl-substituted cyclopropane (Figure 1, blue
dots). While the reaction rate with an o-methoxyphenyl-
substituted cyclopropane lay in the same range as that of the
unsubstituted starting material (Figure 1, red dots), the reaction
with the p-methylphenyl cyclopropane was about twice as fast as
the phenyl cyclopropane and thus nearly half as fast as the
cyclopropyl-substituted D−A cyclopropane (Figure 1, green
dots).
Finally, we sought to compare the cyclopropyl donor to π-
system-derived donors by computational means (Figure 2).
However, instead of looking into transition-state structures, we
simply calculated the energy difference of the hypothetical
isomeric cations C1 and C2 on the basis of density functional
theory for various residues R. Although these results represent
thermodynamic data, the trend of the kinetic data is roughly
mirrored. Thus, we were able to rank the ability to stabilize a
positive charge in α-position. In a first-order approximation, this
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b00603.
Detailed experimental procedures and analytical data for
all new compounds (PDF)
Accession Codes
CCDC 1823935−1823937 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 emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1
EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: d.werz@tu-braunschweig.de.
C
DOI: 10.1021/acs.orglett.8b00603
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
ORCID
Letter
2017, 359, 3848. (h) Wang, Z.-H.; Zhang, H.-H.; Wang, D.-M.; Xu, P.-
F.; Luo, Y.-C. Chem. Commun. 2017, 53, 8521. (i) Borisov, D. D.;
Novikov, R. A.; Tomilov, Y. V. Angew. Chem., Int. Ed. 2016, 55, 12233.
(j) Chidley, T.; Vemula, N.; Carson, C. A.; Kerr, M. A.; Pagenkopf, B. L.
Org. Lett. 2016, 18, 2922. (k) Curiel Tejeda, J. E.; Irwin, L. C.; Kerr, M.
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Daniel B. Werz: 0000-0002-3973-2212
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
A.K. thanks the State of Lower Saxony for a Lichtenberg
Fellowship in the CaSuS (Catalysis for Sustainable Synthesis)
program. We thank Philip M. Rutz, Nico Weichbrodt, and Stefan
Brenner (all TU Braunschweig) for their kind support of this
work.
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