Nucleophilic Ring Opening of Donor-Acceptor Cyclopropanes Catalyzed by a Br?nsted Acid in Hexafluoroisopropanol

Article abstract of DOI:10.1021/acs.orglett.7b03688

A general, Br?nsted acid catalyzed method for the room temperature, nucleophilic ring opening of donor-acceptor cyclopropanes in fluorinated alcohol solvent, HFIP, is described. Salient features of this method include an expanded cyclopropane scope, including those bearing single keto-acceptor groups and those bearing electron-deficient aryl groups. Notably, the catalytic system proved amenable to a wide range of nucleophiles including arenes, indoles, azides, diketones, and alcohols.

Full text of DOI:10.1021/acs.orglett.7b03688

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Nucleophilic Ring Opening of Donor−Acceptor Cyclopropanes
Catalyzed by a Brønsted Acid in Hexafluoroisopropanol
Edward Richmond, Vuk D. Vukovic, and Joseph Moran*
́
University of Strasbourg, CNRS, ISIS UMR 7006, 67000 Strasbourg, France
S
* Supporting Information
ABSTRACT: A general, Brønsted acid catalyzed method for
the room temperature, nucleophilic ring opening of donor−
acceptor cyclopropanes in fluorinated alcohol solvent, HFIP, is
described. Salient features of this method include an expanded
cyclopropane scope, including those bearing single keto-
acceptor groups and those bearing electron-deficient aryl
groups. Notably, the catalytic system proved amenable to a
wide range of nucleophiles including arenes, indoles, azides,
diketones, and alcohols.
onor−acceptor (DA) cyclopropanes serve as “spring-
D
loaded” synthetic intermediates owing to the inherent C−
C bond polarization afforded by the synergistic combination of
donor and acceptor substituents.1 Consequently, they have
found widespread application in modern synthetic organic
chemistry, and a range of catalytic methodologies have been
developed that engage DA-cyclopropanes in cycloaddition,2 1,3-
difunctionalization,3 and homoconjugate addition-type reac-
tions4 with a variety of nucleophiles including amines, alcohols,
thiols, carboxylic acids, and azides.5 Outside of reactions with
heteroatomic nucleophiles, a handful of catalytic arylation
reactions of DA-cyclopropanes have been developed; however,
currently developed systems are typically limited to a single class
of nucleophiles such as indole derivatives,6 anilines,7 electroni-
cally activated anisole derivatives,8 and 2-naphthols.9 Typically,
all of the aforementioned catalytic systems employ high (10−20
mol %) loadings of Lewis acidic catalysts,10 namely rare-earth
triflates, with the reactions typically operating at elevated
temperatures. As these Lewis acidic catalyst systems seek to
take advantage of dual coordination to geminal diester-bearing
cyclopropanes, a striking limitation in terms of cyclopropane
substitution becomes apparent. Likewise, the activation of such
DA-cyclopropanes relies on polarization of the C−C bond by the
synergistic interaction of the D and A groupings. Consequently,
in almost all conjugate addition-type ring openings, the tolerated
donor substitution is limited to electron-donating or -neutral aryl
substituents (Figure 1, top).
Figure 1. Existing Lewis acid catalyzed methodologies for the
nucleophilic ring opening of diester-bearing DA-cyclopropanes. This
work: Brønsted acid catalyzed nucleophilic ring opening of a wide range
of DA-cyclopropanes.
isopropanol (HFIP)12,13 acts as a highly active and general
catalyst system for the nucleophilic ring opening of DA-
cyclopropanes. A wide array of nucleophiles (C-, O-, N-) can
be employed in the protocol, and typical limitations of the DA-
cyclopropane architecture have also been surpassed, allowing
chalcone derived cyclopropanes bearing a single keto-acceptor
motif to engage in catalytic nucleophilic ring opening (Figure 1,
This work). The developed method is also highly user-friendly
providing numerous practical advantages; the reactions operate
at ambient temperature, under open-flask conditions with most
reactions typically complete within 3 h.
Given our recent investigations into aggregation phenomena
in acid catalysis11 we recognized an opportunity to develop an
alternative Brønsted acid catalyzed protocol for the activation of
DA-cyclopropanes. We hypothesized that a strong Brønsted acid
catalyst system would provide a dual advantage in the activation
of DA-cyclopropanes, allowing both a wider nucleophile scope
and the employment of a wider range of cyclopropane scaffolds
beyond the typical structural limitations.
Herein, we report the successful realization of such a scenario,
whereby trifluoromethanesulfonic acid (TfOH) in hexafluoro-
Received: November 28, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03688
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Organic Letters
Letter
Synthetic investigations began with a benchmark reaction
the arylative ring opening of DA-cyclopropane 1a14 in
combination with 1,3,5-trimethoxybenzenea reaction known
to proceed at 80 °C in the presence of 20 mol % of Yb(OTf)3.8c It
was rapidly established that, by slight modification of catalytic
conditions already established within the group,11d the arylative
ring opening of such cyclopropanes could be initiated at rt by
TfOH when employing HFIP as solvent. Further slight tweaking
established 10 mol % of TfOH in HFIP as a reliable system for
this transformation leading to complete conversion in just 3 h
and a 95% yield of the desired arylated material 2a after simple
column chromatography (Scheme 1).
nucleophile in this system generating arylated product 2i in
97% yield. A DA-cyclopropane bearing a 2-fluorophenyl
substituent was also well tolerated in this catalytic system,
delivering arylated products 2j, 2k, and 2l with 1,3,5-
trimethoxybenzene, mesitylene, and 2,6-dimethylphenol, respec-
tively. As a further demonstration of the generality of the
developed catalytic system, DA-cyclopropanes bearing a 4-
nitrophenyl group and a 4-benzonitrile aryl substituent were also
smoothly engaged in arylation with 1,3,5-trimethoxybenzene and
mesitylene (2m−2o) in good to excellent yields.
Attention next turned to DA-cyclopropanes bearing a single
ketone substituent as the “acceptor” motif. To the best of our
knowledge, such scaffolds have yet to find widespread uptake as
substrates in catalytic cyclopropane methodology development
owing to the prior focus on Lewis acid catalyzed methods
believed to require a geminal diester cyclopropane substitution
pattern. Nevertheless, upon reaction with 1,3,5-trimethoxyben-
zene, chalcone derived cyclopropane 1k delivered the desired
arylated product 2p in 98% yield (Scheme 2). Furthermore, this
Scheme 1. Scope of Brønsted Acid Catalyzed Arylative Ring
Opening of DA-Cyclopropanes Bearing a Geminal Diester
a
Motif
Scheme 2. Scope of Brønsted Acid catalyzed Arylative Ring
Opening of DA-Cyclopropanes Bearing a Keto-Acceptor
a
Group
a
b
Isolated yields after column chromatography. Isolated as a mixture
of regioisomers; see Supporting Information (SI) for further details.
c
d
Reaction performed at 40 °C for 4 h. Reaction performed at 50 °C
a
b
Isolated yields after column chromatography. Reaction performed
c
for 24 h.
with 5 mol % TfOH. Isolated as a mixture of regioisomers; see the SI
for further details. Reaction performed at 80 °C.
d
Application of these catalytic reaction conditions across a
range of DA-cyclopropane substrates was then undertaken. 1,2,4-
Trimethoxybenzene also proved to be an excellent nucleophile
furnishing the desired product 2b in 85% yield. Reaction of
various cyclopropane substrates bearing electron-donating or
-neutral aryl substituents with a range of di- and trimethox-
ybenzene derivatives provided the desired arylated products in
moderate to good yields (2c−2h) including arylated diethylester
2f. 2,6-Dimethylphenol also proved to be an excellent
reaction was successfully scaled up to 2.5 mmol, catalyzed by only
5 mol % TfOH, and duly delivered 0.971 g of the arylated
product in 99% yield. Variation of the nucleophilic component in
this reaction system proved unproblematic (2q−2s), as did
variation of the chalcone-derived cyclopropane scaffold to
include electron-rich aryl groups (2t and 2u) and halogenated
aryls (2v and 2w). Notably, mesitylene (2r), naphthalene
derivatives (2s and 2w), 2,6-dimethylphenol (2t), and anisole
B
DOI: 10.1021/acs.orglett.7b03688
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Organic Letters
Letter
(2u) were all tolerated in the reaction system in excellent yield.
Once again, DA-cyclopropanes bearing electron-deficient aryl
substituents (4-NO2 and 4-CF3) were successfully engaged in
arylative ring opening (2x, 2y, and 2z) with only moderate
heating required in the case of the reactions employing the 4-
nitro substituted starting material. Additionally, cyclopropanes
bearing heterocyclic motifs were also successfully engaged in
arylative ring opening with 1,3,5-trimethoxybenzene under
slightly modified conditions;15 thiophene-bearing products 3a
and 3b were isolated in 60% and 93% yield respectively, yet
methyl-furan bearing product 3c proved sensitive to decom-
position during purification and was only isolated in trace
amounts (Scheme 3).
Scheme 4. Scope of Brønsted Acid Catalyzed Nucleophilic
Ring Opening of DA-Cyclopropanes
a
Scheme 3. Brønsted Acid Catalyzed Arylative Ring Opening
a
of DA-Cyclopropanes Bearing Heterocyclic Donor Groups
a
b
Isolated yields after column chromatography. Reaction performed
c
with 5 mol % B(C6F5)3·H2O in MeNO2 at 80 °C for 24 h. Reaction
performed with 10 mol % B(C6F5)3·H2O at 80 °C for 24 h. Reaction
d
performed with 5 mol % B(C6F5)3·H2O.
a
Isolated yields after column chromatography.
Scheme 5. A Plausible Mechanistic Scenario
Further investigation of this reaction system revealed that the
same catalytic conditions were able to promote cyclopropane
ring opening with nucleophiles other than activated arenes.
Reaction with diketones proceeded smoothly to yield highly
functionalized products 4a and 4b in 78% and 50% yield,
respectively. Investigation of the reactions of DA-cyclopropanes
with indoles revealed HFIP to be an inefficient solvent for such
reactions, most likely owing to its ability to hydrogen bond to
basic groups, thus attenuating the nucleophilicity of these
species. Rather, N-methylindole proved to be a competent
reaction partner with DA-cyclopropanes when employing
B(C6F5)3·H2O as a catalyst in nitromethane, a catalytic duo
previously investigated in our research group.11a Under these
conditions, products 4c and 4d were delivered in 85% and 68%
yield, respectively. Ring opening with TMS-N3 also proved facile
with the developed catalytic protocol, with products 4e−4h
isolated in good to excellent yield. Additionally, primary alcohols
could be employed as nucleophiles with no change in reaction
conditions required and products 4i−4k were delivered as
expected in up to 90% yield (Scheme 4).
In a system designed to shed light on the mechanistic course of
such cyclopropane opening reactions, treatment of enantiopure
cyclopropane (S)-1a with 1,3,5-trimethoxybenzene under stand-
ard conditions delivered (R)-2a in high yield.16 This observation
is in line with prior Lewis acid catalyzed protocols8c and suggests
that, under Brønsted acid catalyzed conditions, cyclopropane
ring opening occurs via a predominantly SN2-like mechanistic
pathway. This allows the following mechanistic rationale to be
proposed (Scheme 5). Initial protonation of the cyclopropane
“acceptor-motif” by a Brønsted acid catalyst17 leads to an increase
in cyclopropane C−C bond polarization, thus activating the
benzylic carbon to nucleophilic attack in a mechanism analogous
to an SN2-like displacement. Subsequent enol-tautomerization or
protonation, together with proton abstraction, regenerates the
Brønsted acid to enable turnover of the cycle and in the process
delivers the desired ring-opened product.
In conclusion, a unified Brønsted acid catalyzed nucelophilic
ring opening of DA-cyclopropanes has been developed. The
salient features of this system are the operator-friendly reaction
procedure and the wide scope with respect to both the
cyclopropane scaffold and nucleophile. The combination of
TfOH and HFIP provides a simple yet highly active Brønsted
acid system allowing novel classes of cyclopropane, including
those bearing electron-deficient aryl groups and those derived
from chalcones, to engage in nucleophilic ring-opening reactions
for the first time. Initial observations suggest that ring opening
occurs via an SN2-like mechanistic pathway; however,
comprehensive mechanistic studies into the Brønsted acid
catalyzed activation of small-ring cycloalkyl species are currently
ongoing within our laboratory and will be reported in due course.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03688.
Optimization data, experimental procedures, character-
ization of new compounds, and spectral data (PDF)
C
DOI: 10.1021/acs.orglett.7b03688
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
2016, 81, 3917. For an enantioselective multinucleophile (thiols,
alcohols, carboxylic acids) ring opening of diketo-bearing DA-
cyclopropanes, see: (o) Xia, Y.; Lin, L.; Chang, F.; Fu, X.; Liu, X.;
Feng, X. Angew. Chem., Int. Ed. 2015, 54, 13748. For ring-opening
reactions with silyl enol ethers, see: (p) Fang, J.; Ren, J.; Wang, Z.
Tetrahedron Lett. 2008, 49, 6659. (q) Qu, J. P.; Liang, Y.; Xu, H.; Sun, X.-
L.; Yu, Z. X.; Tang, Y. Chem. - Eur. J. 2012, 18, 2196. (r) Xu, H.; Qu, J.-P.;
Liao, S.; Xiong, H.; Tang, Y. Angew. Chem., Int. Ed. 2013, 52, 4004. For a
redox/acid catalyzed, C−C bond forming reactions with naphthoqui-
nones, see: (s) Luecht, A.; Patalag, L. J.; Augustin, A. U.; Jones, P. G.;
Werz, D. B. Angew. Chem., Int. Ed. 2017, 56, 10587.
(6) (a) Harrington, P.; Kerr, M. A. Tetrahedron Lett. 1997, 38, 5949.
(b) Kerr, M. A.; Keddy, R. G. Tetrahedron Lett. 1999, 40, 5671.
(c) Grover, H. K.; Lebold, T. P.; Kerr, M. A. Org. Lett. 2011, 13, 220.
(d) Wales, S. M.; Walker, M. M.; Johnson, J. S. Org. Lett. 2013, 15, 2558.
(e) de Nanteuil, F.; Loup, J.; Waser, J. Org. Lett. 2013, 15, 3738.
(7) Kim, A.; Kim, S.-G. Eur. J. Org. Chem. 2015, 2015, 6419.
(8) (a) Ivanova, O. A.; Budynina, E. M.; Grishin, Y. K.; Trushkov, I. V.;
Verteletskii, P. V. Eur. J. Org. Chem. 2008, 2008, 5329. (b) Jiang, X.; Lim,
Z.; Yeung, Y.-Y. Tetrahedron Lett. 2013, 54, 1798. (c) Talukdar, R.; Saha,
A.; Tiwari, D. P.; Ghorai, M. K. Tetrahedron 2016, 72, 613.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: moran@unistra.fr.
ORCID
Joseph Moran: 0000-0002-7851-6133
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This project has received funding from the European Research
Council (ERC) under the European Union’s Horizon 2020
research and innovation programme (Grant Agreement No.
639170) and by grants from LabEx “Chemistry of Complex
Systems”. V.D.V. thanks the French government for an MRT
fellowship. We thank Dr. Helmut Muenster of MasCom and Dr.
Jean-Louis Schmitt (ISIS, CNRS, UMR7006) for HRMS
analysis.
(9) Kaicharla, T.; Roy, T.; Thangaraj, M.; Gonnade, R. G.; Biju, A. T.
Angew. Chem., Int. Ed. 2016, 55, 10061.
REFERENCES
■
(10) The Brønsted acid catalyzed ring-opening of 1-alkenyl cyclo-
propanes has been reported, including an example of arylative reactivity
with benzene. (a) Tsuge, O.; Kanemasa, S.; Otsuka, T.; Suzuki, T. Bull.
Chem. Soc. Jpn. 1988, 61, 2897. A single example of TfOH-catalyzed
arylative ring opening of a siloxy/keto DA-cycloproane has also been
reported. (b) Wilsdorf, M.; Leichnitz, D.; Reissig, H.-U. Org. Lett. 2013,
15, 2494.
(11) (a) Dryzhakov, M.; Hellal, M.; Wolf, E.; Falk, F. C.; Moran, J. J.
Am. Chem. Soc. 2015, 137, 9555. (b) Dryzhakov, M.; Moran, J. ACS
Catal. 2016, 6, 3670. (c) Dryzhakov, M.; Richmond, E.; Li, G.; Moran, J.
J. Fluorine Chem. 2017, 193, 45. (d) Vukovic, V. D.; Richmond, E.; Wolf,
E.; Moran, J. Angew. Chem., Int. Ed. 2017, 56, 3085.
(1) For reviews on DA-cyclopropanes, see: (a) Reissig, H.-U.; Zimmer,
R. Chem. Rev. 2003, 103, 1151. (b) Yu, M.; Pagenkopf, B. L. Tetrahedron
2005, 61, 321. (c) Schneider, T. F.; Kaschel, J.; Werz, D. B. Angew.
Chem., Int. Ed. 2014, 53, 5504. (d) Cavitt, M. A.; Phun, L. H.; France, S.
Chem. Soc. Rev. 2014, 43, 804. (e) Grover, H. K.; Emmett, M. R.; Kerr,
M. A. Org. Biomol. Chem. 2015, 13, 655. (f) Budynina, E. M.; Ivanov, K.
L.; Sorokin, I. D.; Melnikov, M. Y. Synthesis 2017, 49, 3035.
(2) For selected recent examples of DA-cyclopropanes engaging in
catalytic [3 + 2]-cycloadditions, see: (a) Parsons, A. T.; Smith, A. G.;
Neel, A. J.; Johnson, J. S. J. Am. Chem. Soc. 2010, 132, 9688. (b) de
Nanteuil, F.; Waser, J. Angew. Chem., Int. Ed. 2011, 50, 12075. (c) Cui,
B.; Ren, J.; Wang, Z. J. Org. Chem. 2014, 79, 790. (d) Garve, L. K. B.;
Kreft, A.; Jones, P. G.; Werz, D. B. J. Org. Chem. 2017, 82, 9235.
(e) Augustin, A. U.; Sensse, M.; Jones, P. G.; Werz, D. B. Angew. Chem.,
Int. Ed. 2017, 56, 14293. For selected recent examples of DA-
cyclopropanes engaging in catalytic [3 + 3]-cycloadditions, see:
(f) Zhou, Y.; Li, J.; Ling, L.; Liao, S.; Sun, X.; Li, Y.; Wang, L.; Tang,
Y. Angew. Chem., Int. Ed. 2013, 52, 1452. (g) Zhang, H.; Luo, Y.; Wang,
H.; Chen, W.; Xu, P. Org. Lett. 2014, 16, 4896. (h) Chidley, T.; Vemula,
N.; Carson, C. A.; Kerr, M. A.; Pagenkopf, B. L. Org. Lett. 2016, 18, 2922.
(3) For recent examples, see: (a) Garve, L. K. B.; Barkawitz, P.; Jones,
P. G.; Werz, D. B. Org. Lett. 2014, 16, 5804. (b) Banik, S. M.; Mennie, K.
M.; Jacobsen, E. N. J. Am. Chem. Soc. 2017, 139, 9152. (c) Wallbaum, J.;
Garve, L. K. B.; Jones, P. G.; Werz, D. B. Org. Lett. 2017, 19, 98.
(4) For a comprehensive account and mechanistic study into the
addition of H−X to cyclopropanes, see: Lambert, J. B.; Napoli, J. J.;
Johnson, K. K.; Taba, K. N.; Packard, B. S. J. Org. Chem. 1985, 50, 1291.
(5) For the use of thiols as nucleophiles, see: (a) Braun, C. M.; Shema,
A. M.; Dulin, C. C.; Nolin, K. A. Tetrahedron Lett. 2013, 54, 5889.
(b) Wang, H.-P.; Zhang, H.-H.; Hu, X.-Q.; Xu, P.-F.; Luo, Y.-C. Eur. J.
Org. Chem. 2015, 2015, 3486. For ring opening with bromide, see:
(c) Xu, W.; Dolbier, W. R., Jr; Salazar, J. J. Org. Chem. 2008, 73, 3535.
For ring opening with azides, see: (d) Emmett, M. R.; Grover, H. K.;
Kerr, M. A. J. Org. Chem. 2012, 77, 6634. (e) Ivanov, K. L.; Villemson, E.
V.; Budynina, E. M.; Ivanova, O. A.; Trushkov, I. V.; Melnikov, M. Y.
Chem. - Eur. J. 2015, 21, 4975. For ring opening with phenolates, see:
(f) Lifchits, O.; Alberico, D.; Zakharian, I.; Charette, A. B. J. Org. Chem.
2008, 73, 6838. For reaction with amines, see: (g) Blanchard, L. A.;
Schneider, J. A. J. Org. Chem. 1986, 51, 1372. (h) Wurz, R. P.; Charette,
A. B. Org. Lett. 2005, 7, 2313. (i) Lifchits, O.; Charette, A. B. Org. Lett.
2008, 10, 2809. (j) Martin, M. C.; Patil, D. V.; France, S. J. Org. Chem.
2014, 79, 3030. (k) Zhou, Y.-Y.; Wang, L.-J.; Li, J.; Sun, X.-L.; Tang, Y. J.
Am. Chem. Soc. 2012, 134, 9066. (l) So, S. S.; Auvil, T. J.; Garza, V. J.;
Mattson, A. E. Org. Lett. 2012, 14, 444. For ring opening with
organoboron reagents, see: (m) Nguyen, T. N.; Nguyen, T. S.; May, J. A.
Org. Lett. 2016, 18, 3786. (n) Ortega, V.; Csaky, A. G. J. Org. Chem.
(12) For examples of other HFIP-enabled strong Brønsted acid
catalyzed reactions, see: (a) Saito, A.; Takayama, M.; Yamazaki, A.;
Numaguchi, J.; Hanzawa, Y. Tetrahedron 2007, 63, 4039. (b) Motiwala,
́
H. F.; Charaschanya, M.; Day, V. W.; Aube, J. J. Org. Chem. 2016, 81,
1593. (c) Zeng, X.; Liu, S.; Xu, B. Org. Lett. 2016, 18, 4770. (d) Liu, W.;
Wang, H.; Li, C.-J. Org. Lett. 2016, 18, 2184. (e) Kamitanaka, T.;
Morimoto, K.; Tsuboshima, K.; Koseki, D.; Takamuro, H.; Dohi, T.;
Kita, Y. Angew. Chem., Int. Ed. 2016, 55, 15535.
(13) For studies pertaining to the properties of HFIP in the context of
organic reactivity, see: (a) Berkessel, A.; Adrio, J. A. J. Am. Chem. Soc.
2006, 128, 13412. (b) Vuluga, D.; Legros, J.; Crousse, B.; Slawin, A. M.
Z.; Laurence, C.; Bonner-Delpon, D. J. Org. Chem. 2011, 76, 1126.
(c) Holloczki, O.; Berkessel, A.; Mars, J.; Mezger, M.; Wiebe, A.;
Waldvogel, S. R.; Kirchner, B. ACS Catal. 2017, 7, 1846. For select,
recent transformations mediated by HFIP in the absence of additional
acid, see: (d) Champagne, P. A.; Benhassine, Y.; Desroches, J.; Paquin, J.
F. Angew. Chem., Int. Ed. 2014, 53, 13835. (e) Khaksar, S. J. Fluorine
Chem. 2015, 172, 51. (f) Motiwala, H. F.; Vekariya, R. H.; Aube, J. Org.
Lett. 2015, 17, 5484. (g) Wen, H.; Wang, L.; Xu, L.; Hao, Z.; Shao, C.-L.;
Wang, C. Y. Adv. Synth. Catal. 2015, 357, 4023. (h) Singh, P.; Peddinti,
R. K. Tetrahedron Lett. 2017, 32, 4753. (i) Tang, R.-J.; Milcent, T.;
Crousse, B. Eur. J. Org. Chem. 2017, 2017, 4753.
(14) See the Supporting Information for full structural details of the
cyclopropanes and for details of their preparation.
(15) Using TfOH as a catalyst with heterocyclic substrates led to
significant decomposition and lower yields of products. Consequently,
B(C6F5)3·H2O was employed as a milder Brønsted acidic catalyst.
(16) See the Supporting Information for further details.
(17) NMR titration of TfOH into a solution of cyclopropane/HFIP
indeed reveals a downfield shift of cyclopropane methylene protons by
1H NMR, and a downfield shift of keto-carbonyl by 13C NMR. Taken
together, this is highly suggestive of ‘acceptor’-protonation occurring
facilely under the reaction conditions.
D
DOI: 10.1021/acs.orglett.7b03688
Org. Lett. XXXX, XXX, XXX−XXX