Synthesis of Cyclopropanes via 1,3-migration of acyloxy groups triggered by formation of α-Imino Rhodium Carbenes

Article abstract of DOI:10.1021/acs.orglett.0c01764

A novel and highly efficient synthetic approach to cyclopropanes was realized via 1,3-migration of acyloxy groups triggered by α-imino rhodium carbenes. Excellent chemoselectivity ensured broad compatibility of common functional groups. Merits such as readily available substrates, mild reaction conditions, and time-saving processes qualified this transformation as an attractive alternative strategy to synthesize multifunctionalized cyclopropanes. Primary investigations and discussion on the mechanism are presented.

Full text of DOI:10.1021/acs.orglett.0c01764

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Letter
Synthesis of Cyclopropanes via 1,3-Migration of Acyloxy Groups
Triggered by Formation of α‑Imino Rhodium Carbenes
Ze-Feng Xu,* Jincheng Liu, Xin Chang, Tao Chen, Huaping Xu, Shengguo Duan, and Chuan-Ying Li*
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ABSTRACT: A novel and highly efficient synthetic approach to cyclopropanes was realized via 1,3-migration of acyloxy groups
triggered by α-imino rhodium carbenes. Excellent chemoselectivity ensured broad compatibility of common functional groups.
Merits such as readily available substrates, mild reaction conditions, and time-saving processes qualified this transformation as an
attractive alternative strategy to synthesize multifunctionalized cyclopropanes. Primary investigations and discussion on the
mechanism are presented.
etallocarbene is one of the most important intermediates
in modern organic chemistry, which shows versatile
tricyclic ketone in 53% yield via a 1,3-methyl migration mediated
by a large excess of copper bronze in refluxing benzene under a
nitrogen atmosphere (Scheme 1A). Due to the rigid con-
formation of the bicyclic skeleton, only 1,3-migration was
achievable in the reaction.13a In 2005, Wang and co-workers
reported the second 1,3-migration, in which the β-tosyl-α-diazo
ester was converted to cyclopropane via rhodium-catalyzed 1,3-
H migration (Scheme 1B). The presence of the β-tosyl group
induced a predominant rigid conformation in which the
migrating C−H bond paralleled the RhC bond, and the
conformation favored the 1,3-migration over 1,2- or 1,5-
migration.13b Accordingly, a conformational requirement was
essential in both cases, whereas such a particular conformational
requirement, on the other hand, limited the application of 1,3-
migration in organic synthesis.
M
reactivity in the synthesis of complex chemicals.1 In 2008,
Gevorgyan, Fokin, andco-workersrevealedthatrhodium(II)salt
could efficiently catalyze the transformation of 1-sulfonyl-1,2,3-
triazole to α-imino rhodium carbene,2 which enriches carbene
chemistry remarkably.3,4
It is well-known that intramolecular 1,n-migration (or named
as 1,n-insertion) is a common reaction mode in carbene
chemistry.5 The popular 1,2-migration of hydride, alkyl, as well
as a heteroatom could deliver various substituted alkenes;6 1,4-
and 1,5-H migration are convenient ways to prepare four- and
five-membered cycles, respectively.7,8 Specifically, we reported a
1,4-halo migration of α-imino rhodium carbene producing a
dihydroquinoline skeleton.9 The 1,6-, 1,7-, and even 1,14-
migration involving carbene were reported, as well.10,11
Cyclopropaneisuniquenotonlyduetoitsspecialstructurebut
also due to its widespread presence in diverse bioactive
molecules. The synthesis of cyclopropane is a well-investigated
topic, in which Simmons−Smith−Furukawa cyclopropanation,
Corey−Chaykovsky cyclopropanation, and transformations
derived from them are the most popular classical methods.12
However, the chemoselectivity of the reaction may be an issue
when substrates bear carbene-sensitive or base-sensitive groups.
It is well-known that 1,3-migration triggered by carbene,
predictably affording cyclopropane, is difficult to occur because a
high barrier was required for the four-membered ring transition
state. To the best of our knowledge, only two cases were reported
before.13 In 1962, Yates and Danishefsky reported that bicyclic
diazoisofenchone could be converted to cyclopropane-fused
Transformation of carbene is a long-term topic in our group.14
Except for the above-mentioned 1,4-halo migration,9 formal 1,2-
migration of acyloxy in a special α-imino rhodium carbene 2 was
realized recently in our lab (Scheme 1C), and the in situ
generated azadiene 4 was utilized as an effective 1-aza-[4C]
synthon in the synthesis of piperidine derivatives.15 Benefiting
from the stability of the five-membered 1,3-dioxolane
Received: May 25, 2020
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A

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Scheme 1. Background and Reaction Design
Scheme 2. Synthesis of Triazole 5
the substrate, nothing recognizable was obtained (entry 1, Table
1). In order to improve the stability of the carbocation in
a
Table 1. Optimization of Reaction Conditions
b
entry
[Rh]
solvent
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
temp (°C)
yield (%)
c
1
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(esp)2
Rh2(oct)4
Rh2(piv)4
Rh2(tpa)4
Rh2(s-ntv)4
Rh2(s-ptv)4
Rh2(s-nttl)4
Rh2(piv)4
Rh2(piv)4
Rh2(piv)4
Rh2(piv)4
Rh2(piv)4
Rh2(piv)4
Rh2(piv)4
Rh2(piv)4
Rh2(piv)4
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
60
reflux
60
60
60
60
0
0
d
2
3
4
5
6
7
8
9
90
74
92
86
14
87
75
16
91
0
85
83
73
80
0
DCE
DCE
DCE
10
11
12
13
14
15
16
17
DCM
chloroform
toluene
ethyl acetate
DMF
DMSO
DCE
DCE
60
60
60
60
e
18
f
98
89
0
19
20
DCE
a
intermediate 3, the 1,2-acyloxy migration had excellent chemo-
selectivity. It is well-known that the six-membered ring was also
stable enough and easily accessible. Accordingly, we envisioned
whether, if a two-carbon linker was introduced between triazole
and acyloxy, a similar six-membered 1,3-dioxane intermediate 7
would be generated, which may overcome the unachievable
barrier in forming a four-membered transition state in the direct
1,3-migration reaction. The following ring opening by C−O
bond cleavage along with the departure of rhodium catalyst
produces 1,3-dipole 8, which undergoes convenient ring closure
to give cyclopropane 9 as the formal 1,3-acyloxy migration
product (Scheme 1D). Based on the above proposal, we report
herein the successful 1,3-migration of the acyloxy group induced
by α-imino rhodium carbene and the application of such a
method in cyclopropane synthesis.
Reagents and conditions: 5c (0.2 mmol, 1.0 equiv), [Rh] (5 mol %),
b
c
solvent (1 mL), N2 atmosphere. Isolated yield. 5a was used, and the
reaction time was 40 min. 5b was used, and the reaction time was 40
min. 3 mol % of Rh2(piv)4 was used. 2 mol % of Rh2(piv)4 was used.
Ts = tosyl, DCE = 1,2-dichloroethane, DCM = dichloromethane,
DMF = dimethylformamide, DMSO = dimethyl sulfoxide.
d
e
f
Initially, as depicted in Scheme 2, 1-sulfonyl-4-(2-acetox-
ylethyl)-1,2,3-triazole 5a−c were prepared conveniently within
three simple steps starting from commercially available materials
(see Supporting Information for details).16,17 gem-Dimethyl-
substituted triazole 5a was first submitted to Rh2(OAc)4 (5 mol
%) in refluxing 1,2-dichloroethane (DCE) under a nitrogen
atmosphere. Unfortunately, alongwithgradualdecompositionof
intermediate 8, 5b was employed but produced no desired
product either (entry 2). Gratifyingly, when gem-diphenyl-
substituted triazole 5c was exposed to the same conditions, the
desired 1,3-acyloxy migration product 9c was obtained in 90%
yield (entry 3). No predicted side products, as shown in Scheme
1E, were detected, illustrating excellent chemoselectivity of the
1,3-migration.
B
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Consequently, 5c was selected as the model substrate to
establish the optimal reaction conditions. Various rhodium(II)
saltswereevaluated(entries4−10). Generally, most ofthe tested
rhodium salts exhibited high efficiency in this transformation.
When 5 mol % of Rh2(esp)2 was used, the desired 9c was
generated in 74% yield (entry 4); Rh2(oct)4 performed well, and
9c was isolated in 92% yield (entry 5); bulky catalyst Rh2(piv)4
worked well, too, giving 9c in 84% yield (entry 6), whereas
bulkier Rh2(tpa)4 was slower in the reaction, and 9c was isolated
in only 14% yield (entry 7); bowl-shaped catalysts exhibited
diverse activities in the reaction; for instance, Rh2(s-NTV)4 and
Rh2(s-PTV)4 were suitable catalysts for the transformation and
produced 9c in 87 and 75% yields, respectively (entries 8 and 9);
however, when Rh2(s-NTTL)4 was employed, only 16% yield
was achieved (entry 10). Rh2(piv)4 was proven to be more
suitable for further screening because, at a lower temperature (60
°C), 91% yield of 9c was obtained (entry 11). Further survey of
solvents was conducted. Reaction in refluxing DCM could not
provide any product, which attributed to the low boiling point
(entry 12); chloroform and toluene were also suitable for the
reaction (85 and 83% yield, respectively, entries 13 and 14).
Remarkably, polar solvents such as ethyl acetate and DMF could
also be employed (73 and 80% yields, respectively, entries 15 and
16), and no side reaction between triazole and DMF18a occurred,
indicating high chemoselectivity of this reaction. DMSO could
coordinate to the vacant site of the rhodium catalyst,18b thus
giving no product (entry 17). The dosage of Rh2(piv)4 could be
reduced to 3 mol %, producing 9c in nearly quantitative yield
(98%, entry 18). No 9c was generated without rhodium catalyst
(entry 20).
Scheme 3. Reaction Scope
As depicted in Scheme 3, the scope of the 1,3-migration was
evaluated starting with various sulfonyl groups in 1,2,3-triazole 5.
Electron-donating-group-substituted aryl sulfonyls were per-
fectly compatible, and the corresponding cyclopropanes 9c−e
were obtained in excellent yields (95−98%); however, pure p-
bromophenyl-sulfonyl-substituted product 9f was not obtained
directly because of partial hydrolysis (aldehyde 12 was
generated; see eq 1); it could be reduced in situ to 13f, which
Condition a: 5 (0.2 mmol), Rh2(piv)4 (3 mol %), DCE (2.0 mL), 60
°C, 15 min, N2. Condition b: crude 9 from 0.2 mmol 5, NaBH4 (1.0
a
equiv), MeOH (4.0 mL), rt, 15 min, N2. 13 was obtained from a
one-pot reaction from triazole 5, and the yield was calculated based
on the corresponding triazole.
effectively, and 9h was generated in 77% yield. Different kinds of
migrating acyloxy groups were examined next. Propionyloxy
migrated favorably, delivering 9i in 94% yield, whereas benzoxy
groups slightly retarded the reaction, and 9j and 9k were
generatedin77−85%yields. Gratifyingly, theBocgroupsurvived
during the reaction, affording 9l in excellent yield (94%). Various
R1 and R2 groups were compatible, as well. When R1 and R2 were
the same group, 9m-q, 9aa, and 13ab could be isolated in 80−
99% yields. If R1 was different than R2, 9r-z and 9ac-ag were
obtained in 70−98% yields, yet the diastereoselectivity was low
for most examples except for 9ag (dr 4.5:1). Common
functionalities, including strong electron-withdrawing and
electron-donating groups (such as sulfonyls and −OMe), were
compatible (9z, 13ae, and 9ag), and the position of the
functionalities was not limited (9aa, 13ab, 9ac, 13ae, and 9ag).
was isolated in 84% yield in two steps. The steric effect of sulfonyl
influenced the reaction marginally as 2,4,6-tri-isopropylphenyl-
and2-naphthyl-substituted9dand9gwereproducedinexcellent
yield, as well (96 and 92%). Alkyl sulfonyl performed less
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Heteroaromatics were also tolerated (9af and 9ag). Remarkably,
carbene-sensitive groups, such as a carbon−carbon double bond
and triple bond, survived in 9q and 9x. Gratifyingly, spiro[2.4]-
heptane 9ah could be generated in satisfactory yield, elucidating
the power of this protocol in organic synthesis.
trimethoxybenzene as the internal standard (Figure 1).
Obviously, the initial reaction rate of electron-rich triazole 5m
The reaction could be enlarged to a preparative scale with the
yields maintained (Scheme 4). For instance, 9c, 9n, and 9q could
Scheme 4. Reactions in Preparative Scale
Figure 1. Competitive reaction of 5m and 5o. Conditions: 5m (0.2
mmol), 5o (0.2 mmol), Rh2(piv)4 (6 mol %), DCE (4.0 mL), 60 °C,
time, N2. The yields were obtained by 1H NMR with 1,3,5-
trimethoxybenzene (0.4 mmol) as the internal standard.
was faster than that of electron-poor triazole 5o, illustrating that
the electron-donating group accelerated the reaction, whereas
the electron-withdrawing group decelerated the reaction. This
result is consistent with the fact that the electron-donating group
increases the stability of intermediate 8. After 7 min, the yield of
9o was higher than the yield of 9m, and we believe this fact is
attributed to the relative stability of 9o and 9m. 9m, which bears
an electron-rich aromatic ring, is easier to participate in a ring-
opening reaction possibly because of the Lewis acidity of the
rhodium catalyst (see eq 4). Accordingly, based on the
experimental facts reported herein as well as in the literature,
the mechanism proposed in Scheme 1D was reasonable.
be obtained in even higher yields via 1.0 mmol scale reactions
under standard conditions (Scheme 4A). Gratifyingly, the
dosage of the catalyst could be reduced to 1 mol % when the
reactions were conducted in 1.5 mmol scale, and the yields of 9d,
9l, and 9p were almost the same as that obtained under standard
conditions (Scheme 4B).
The potential of the protocol in organic synthesis was further
illustrated by derivatization of the target cyclopropanes. As
revealed, the imino group of 9f could be hydrolyzed easily.
Accordingly, a one-pot synthesis of aldehyde 12 from triazole 5f
was conducted, and 12 was obtained in 85% yield (eq 1). As
performed for 9f, the imine group in 9c could also be reduced
with 1.0 equiv of NaBH4, giving sulfamide 13c in 97% yield (eq
2). Interestingly, when 1.0 equiv of LiAlH4 was used as the
reductant,α-aminoketone14cwasproducedin98%yield(eq3).
The cyclopropane ringin 9m could be opened easily under acidic
conditions, as well, and diene 15m could be afforded in 80% yield
after treatming 9m with wet silica gel in methanol at rt (eq 4).
To confirm whether the migration is intramolecular or not, a
mixture of 5e and 5j was submitted to the standard conditions.
Only intramolecular migrating products 9e and 9j were isolated
in 84 and 75% yields, respectively, and no cross-migrating
products were detected (eq 5). The acyl group was essential for
the 1,3-migration reaction because when hydroxyl-substituted
triazole 5ai was submitted to the standard conditions, no 1,3-
migration product 9ai was detected, whereas cyclobutanone 16
was generated in 65% yield via O−H insertion and rearrange-
ment (eq 6). Subsequently, competitive reaction between same
amount of gem-di-p-tolyl triazole 5m and gem-di-p-Cl-phenyl
triazole5owasconductedunderthestandardconditions, andthe
In conclusion, a novel and practical synthetic method of
functionalizedcyclopropanewasachievedvia1,3-migrationofan
1
reaction progress was monitored by H NMR with 1,3,5-
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acyloxy group triggered by α-imino rhodium carbene. Acyloxy
reacted with rhodium carbene chemospecifically, and the
excellent chemoselectivity ensured compatibility of common
functional groups, especially carbene-sensitive groups. Readily
available substrates, mild reaction conditions, and time-saving
processes enhanced the synthetic potential of this protocol. The
reaction could be scaled up without decreasing the yields with
only 1 mol % of Rh catalyst. A three-membered ring, ester, and
imine could act as powerful functional groups for further
transformation, and several useful derivatizations have been
realized, illustrating the power of this protocol. Investigations of
the 1,3-migration of other groups induced by diverse types of
carbenes, as well as the detailed mechanism, are ongoing in our
laboratory.
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ASSOCIATED CONTENT
* Supporting Information
■
sı
TheSupportingInformation isavailablefreeofchargeat https://
pubs.acs.org/doi/10.1021/acs.orglett.0c01764.
Details of experimental procedures (PDF)
1H and 13C NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Ze-Feng Xu − Department of Chemistry, Zhejiang Sci-Tech
University, Hangzhou 310018, China; Key Laboratory of
Organofluorine Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032,
China; orcid.org/0000-0002-4531-6266;
(2)Horneff, T.;Chuprakov, S.;Chernyak, N.;Gevorgyan, V.;Fokin, V.
V. Rhodium-catalyzed transannulation of 1,2,3-triazoles with nitriles. J.
Am. Chem. Soc. 2008, 130, 14972−14974.
Email: xuzefeng@zstu.edu.cn
Chuan-Ying Li − Department of Chemistry, Zhejiang Sci-Tech
University, Hangzhou 310018, China; orcid.org/0000-
0001-9400-6830; Email: licy@zstu.edu.cn
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Authors
Jincheng Liu − Department of Chemistry, Zhejiang Sci-Tech
University, Hangzhou 310018, China
Xin Chang − Department of Chemistry, Zhejiang Sci-Tech
University, Hangzhou 310018, China
Tao Chen − Department of Chemistry, Zhejiang Sci-Tech
University, Hangzhou 310018, China
Huaping Xu − Department of Chemistry, Zhejiang Sci-Tech
University, Hangzhou 310018, China
Shengguo Duan − Department of Chemistry, Zhejiang Sci-Tech
University, Hangzhou 310018, China; orcid.org/0000-
0002-8004-2053
Complete contact information is available at:
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triazole as a carbene precursor, see: (a) Miura, T.; Nakamuro, T.; Liang,
C. J.; Murakami, M. Synthesis of trans-cycloalkenes via enantioselective
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https://pubs.acs.org/10.1021/acs.orglett.0c01764
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (21871235, 21801224, and 21901230), Natural Science
Foundation of Zhejiang Province (LQ18B020009), Fundamen-
talResearchFundsofZhejiangSci-TechUniversity(2019Y003),
and Science Foundation of Zhejiang Sci-Tech University
(ZSTU) under Grant Nos. 16062193-Y and 18062301-Y for
supporting this work.
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