Dirhodium(II)-Catalyzed (3 + 2) Cycloaddition of the N-Arylaminocyclopropane with Alkene Derivatives

Article abstract of DOI:10.1021/acs.orglett.8b00904

Several (3 + 2) cycloaddition reactions catalyzed by dirhodium(II) complexes between N-arylaminocyclopropane and alkenes derivative have been developed. Preliminary mechanism studies suggest that dirhodium(II) complexes may decrease the bond-dissociation

Full text of DOI:10.1021/acs.orglett.8b00904

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Dirhodium(II)-Catalyzed (3 + 2) Cycloaddition of the
N‑Arylaminocyclopropane with Alkene Derivatives
Yi Kuang,†,§ Yangbo Ning,‡ Jin Zhu,† and Yuanhua Wang*,‡
†Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610046, China
‡College of Chemistry, Sichuan University, Chengdu 610046, China
§University of Chinese Academy of Sciences, Beijing 100049, China
S
* Supporting Information
ABSTRACT: Several (3 + 2) cycloaddition reactions catalyzed by dirhodium(II) complexes between N-arylaminocyclopropane
and alkenes derivative have been developed. Preliminary mechanism studies suggest that dirhodium(II) complexes may decrease
the bond-dissociation energy (BDE) of the N−H bond of N-arylaminocyclopropanes for N−H bond activation, thus facilitating
the formation of N-centered radicals by loss of a hydrogen radical.
he formation of compounds possessing a five- or six-
membered ring by the ring-opening reaction of a smaller
Initially, the α, β-unsaturated ester 2a was selected to trap the
generated distonic radical cation from N-phenylaminocyclo-
propane 1a (Table 1). The desired cycloaddition product 3aa
was obtained in moderate yield when 1 mol % of Rh2(esp)2 and
T-HYDRO was used in the reaction with dichloromethane
(DCM) as the solvent under an argon atmosphere (Table 1,
entry 1). Of particular interest was the yield of 3aa increasing to
68% when the T-HYDRO was left out (Table 1, entry 2). No
conversion to the desired product occurred without a metal
catalyst, but 1a spontaneously decomposed in DCM (Table 1,
entry 3). Further solvent screening disclosed nonpolar n-hexane
as the best solvent (Table 1, entries 4−7). In n-hexane, the
yield of product 3aa increased to 84% catalyzed by Rh2(esp)2
(Table 1, entry 5). It was reasoned that the decomposition rate
of 1a slowed in n-hexane. Coordinating solvents, such as
dimethoxymethane (DME), inhibited the reaction (Table 1,
entry 6). No products were formed when the strong
coordinating solvent dimethylformamide (DMF) was used
(Table 1, entry 7). This indicated that the axial sites of the
Rh2(II,II) catalyst play an important role in the reaction.
Decreasing the Rh2(esp)2 catalyst loading to 0.1 mol % gave the
lowest yield of 3aa (Table 1, entry 8). Other Rh2(II,II)
compounds were found to still be feasible in the reactions.
Doyle’s Rh2(cap)4 catalyst generated a high yield of 88%,
though the catalyst was insoluble in n-hexane (Table 1, entry
9). Another Doyle’s catalyst, Rh2(5S,R-MenPY)4,8 was found to
exhibit excellent solubility in n-hexane and even generate a 94%
product yield (Table 1, entry 10). When the reaction was
T
ring, particularly a cyclopropane or cyclobutane, is widely used
in synthetic chemistry.1 The N-arylaminocyclopropane 1 is
prone to single-electron oxidation of the nitrogen, and
subsequent ring opening, to generate a distonic radical cation,
which can be trapped by an alkene or alkyne to form a
cycloamine2 or triplet oxygen to yield a dioxolane (Scheme 1).3
These characteristics make the N-arylaminocyclopropane motif
a useful building block for entry into a variety of biologically
active compounds.4 To advance synthetic methodology, several
research studies have focused on this ring-opening reaction and
have developed ways of accelerating the process of radical
cation generation. These methods include using the stoichio-
metric oxidant ceric ammonium nitrate (CAN),5 the catalytic
single-electron oxidizing FeIII(phen)3(PF6)3,3a the hydrogen-
abstracting agents benzoyl peroxide or tert-butyl peroxide/UV
light,3a and visible-light photocatalysis with ruthenium catalysts
(Scheme 1).2 Fortunately, recent studies have reported an
efficient synthesis of compounds of structure 1, which has
provided a great opportunity to develop a good synthetic
methodology for the above reaction.6 In continuation of our
study of reactions of dirhodium(II) complexes (Rh2(II,II)) with
tert-butyl hydroperoxide (TBHP),7 we envisaged Rh2(esp)2/
70% TBHP in water (T-HYDRO) generating tert-butylperoxy
radicals that could induce the ring-opening reaction of
aminocyclopropane 1. During the investigation, it was
discovered that the Rh2(II,II) complex could catalyze the
ring-opening reaction of 1 efficiently by itself under an argon
atmosphere (Scheme 1). Therefore, the efforts of this
investigation have focused on this ring-opening reaction
catalyzed by Rh2(II,II) complexes.
Received: March 20, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00904
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Distonic Radical Cation Generation and
Cycloaddition Reactions
Table 1. Optimization of Reaction Conditions
b
c
entry
conditions
solvent
yield (%) (cis/trans)
d
1
Rh2(esp)2, TBHP
Rh2(esp)2
DCM
45
2
3
4
5
6
7
DCM
68
No catalyst
DCM
NR
Rh2(esp)2
toluene
hexane
DME
69
Rh2(esp)2
84
Rh2(esp)2
50
Rh2(esp)2
DMF
NR
e
8
Rh2(esp)2
hexane
hexane
hexane
hexane
hexane
hexane
hexane
hexane
hexane
hexane
DCM
60
9
Rh2(cap)4
88 (40/60)
94 (41/59)
92
e
10
Rh2(5S,R-MenPY)4
Rh2(5S,R-MenPY)4
Rh2(5S,R-MenPY)4
Rh2(OAc)4
ef
,
11
12
13
14
15
16
17
18
g
60
44 (40/60)
70 (39/61)
41 (41/59)
80 (44/56)
trace
Rh2(oct)4
[Rh(CH2CH2)2Cl]2
[Rh(Cp*)Cl2]2
Fe(phen)3(PF6)3
Fe(phen)3(PF6)3
53 (64/36)
a
Reaction conditions: 1a (0.5 mmol, 0.2 M in degassed solvent), 2a
(2.5 mmol), catalyst (1 mol %) under argon at rt for 18 h, unless
b
c
1
otherwise noted. Isolated yield. Determined by H NMR spectros-
d
e
copy of crude product. 2 equiv of TBHP in water. 0.1 mol % of
f
g
catalyst. Reaction was conducted in the dark. 0.01 mol % catalyst, 48
h. esp = α,α,α’,α’-tetramethyl-1,3-benzenedipropionate, cap =
caprolactamate, 5S,R-MenPY = (S)-(1R,2S,5R)-2-isopropyl-5-methyl-
cyclohexyl 2-oxopyrrolidine-5-carboxylate, oct = n-octyl, NR = no
reaction, phen = 1,10-phenanthroline monohydrate. For more
information, see the SI.
conducted in the dark with Rh2(5S,R-MenPY)4, the yield of 3aa
reached 92%, which indicated the reaction was not photo-
sensitive (Table 1, entry 11). Even 0.01 mol % of Rh2(5S,R-
MenPY)4 gave the desired product in 60% yield after 48 h
(Table 1, entry 12). Regrettably, the obtained products were
optically inactive, though the Rh2(5S,R-MenPY)4 was a chiral
catalyst. The Rh2(II,II) compounds bearing carboxylate ligands
(i.e., Rh2(OAc)4, Rh2(oct)4) were not as efficient as the amide
Rh2(II,II) catalysts (Table 1, entries 13 and 14). For
comparison, rhodium metal complexes in other oxidation
states and additional metal complexes were also evaluated in
the reactions (Table 1, entries 15−18). The Rh(I) species
generated a product in not so high yield, while the Rh(III)
species provided decent product yield but was still inferior to
the Rh2(5S,R-MenPY)4 catalyst (Table 1, entries 15 and 16).
Except for the cobalt species, other metal complexes each
generated product 3aa in poor yield (see Supporting
Information (SI)). Fe(phen)3(PF6)3, which has been reported
as an efficient catalyst to oxidize 1a and generate the radical,
catalyzed the reaction in DCM with 53% yield (Table 1, entry
18). Nevertheless, due to solubility issues, no reaction in n-
hexane was observed with Fe(phen)3(PF6)3 (Table 1, entry
17). Despite this, the measured diastereomeric ratios of the
selected reactions in Table 1 gave moderate values, which
suggested that a free radical mechanism was operative.
a
Scheme 2. Substrate Scope of N-Arylaminocyclopropanes
a
Reaction conditions: 1b−q (1 mmol, 0.2 M in degassed solvent), 2a
(5 mmol), catalyst (0.1 mol %) at rt for 18 h, unless otherwise noted.
Isolated yields of two isomers. cis/trans ratio was determined by H
NMR spectroscopy of crude products. NR = no reaction. 36 h.
1
b
or electron-withdrawing groups, such as CF3, Cl, and p-phenyl,
all gave high yields of products 3ba−ja. Multiple substituted
cyclopropylanilines with the 3,5-dimethyl group 1l and 3,5-
disubstitued trifluoromethyl 1m generated products 3la and
3ma in 76% and 80% yields, respectively. However, steric
interactions played a major role in the reaction. o-Methyl-
substituted cyclopylamines 1j gave the least yields of product
3ja. A poor yield of product 3ka was obtained when the bulky
o-phenyl 1k was applied. Nonetheless, the 1- or 2-naphthalene-
substituted cyclopylamines 1n and 1o both gave good yields of
the products 3na and 3oa. Substrate 1p bearing the pyridine
Next, with the optimized reaction conditions, Rh2(5S,R-
MenPY)4 was selected to investigate the scope of the reaction
with respect to different N-arylaminocyclopropanes 1. As
shown in Scheme 2, various substituted versions of 1 were
synthesized according to the reported procedure6 and reacted
with ester 2a to yield the desired products 3 with moderate
diastereomeric ratios. Compounds of structure 1 bearing
electron-donating groups, such as methoxy, methyl, and ethyl,
B
DOI: 10.1021/acs.orglett.8b00904
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 3. Substrate Scope of Alkene and Alkyne Derivatives
a
Reaction conditions: 1a (1 mmol, 0.2 M in degassed solvent), 2b−x (5 mmol) or 4a−d (5 mmol), Rh2(5S,R-MenPY)4 (0.1 mol %) at rt for 18 h,
b
c
unless otherwise noted. Isolated yields of two isomers. The cis/trans ratio was determined by 1H NMR spectroscopy of crude products. 36 h. DCM
as solvent.
group gave only a trace product due to the pyridine group to
which Rh2(II,II) was coordinated, which inhibited coordination
of amine to Rh2(II,II) complex. It is worth mentioning that no
reaction occurred when the intrinsically reactive molecule 1-
methyl-2-phenyl-2-azabicyclo[3.1.0]hexane (1q) was used (see
the SI), suggesting that the hydrogen was important in the
reaction. Subsequently, a variety of α,β-unsaturated esters 2b−q
were catalyzed by Rh2(5S,R-MenPY)4 and reacted with 1a for
18 h at room temperature under an argon atmosphere (Scheme
3). The various substituted unsaturated esters 2b−n produced
cycloaddition products 3ab−an with good yields, but with
moderate diastereomeric ratios. A diastereomeric ratio of 83/17
for 3ah with the trans configuration prevailing was obtained
when trisubstituted esters 2h were used. Additionally, the cis
configuration of 3ai predominated (71/29) when the esters 2i
contained a hydroxyl group. The β-substituted methyl ester 2j
gave the β-amino ester 3aj, while the β-substituted phenyl ester
2k gave the γ-amino ester 3ak. The crystal structure of
(1RS,2RS,3SR)-3ak was obtained by crystallizing the insepa-
rable product mixture, and the γ-amino ester motif of 3ak was
confirmed by X-ray crystallography. It was reasoned that this
different regioselectivity derived from the radical stability of the
intermediate.2e In contrast to the pyridine substituent, when the
amide group was present in the substrate 2l, the reactions
proceeded smoothly, providing 3al in 71% yield. In particular,
the reaction of the cis- or trans-dimethyl fumarate 2m and 2n
with 1a produced the products 3am and 3an in yields of 84%
and 76% respectively, with a similar diastereomeric ratio. In
addition, after treatment with the cis product 3ba with CAN,
the desired β-amino ester 6a was obtained in 79% yield (see the
SI). It was reported the 6a can undergo facile transformation to
the chiral β-amino acid by chiral resolution.9
Furthermore, except for the α,β-unsaturated esters, the
reaction of 1a was probed using different alkenes. Reactions
with the aromatic alkenes 2q−x, bearing either electron-rich or
electron-poor substituents on the aromatic ring, proceeded
smoothly with 1a to yield the desired products 3aq−ax with
moderate diastereomeric ratios. Modest yields were obtained
when alkyne derivatives 4a−d were used in the reactions after
an extended period of time. Though no reaction occurred with
cyclohexenone 2o (Scheme 3), 1a reacted with 2H-chromen-2-
one 4d to generate a product mixture in 53% yield containing
the cycloaddition compound 5ad and amide compound 5ae. It
was supposed that 5ae formed spontaneously through the
intramolecular amide reaction with 5ad.
Next, we investigated the plausible mechanism for the
reactions. The key issue is how the Rh2(II,II) catalyst initiates
the radical process. Unsubstituted aminocyclopropanes are
reported to undergo facile oxidation to a radical cation species,
in which the oxidation process is essentially barrier free.10
However, compound 1 generated radicals slowly due to
stereoelectronic and resonance effects, which is in accordance
with the report by Zheng et al.2a In Zheng’s report, no (3 + 2)
cycloaddition product was obtained without light or Ru catalyst.
Despite this, the conversion of 1a reached 14% after 12 h
without light and a Ru catalyst, indicating that 1a decomposed
and generated a radical or active species, thus leading to a
reaction pathway different from the (3 + 2) cycloaddition. UV/
vis spectra demonstrate that the Rh2(II,II) catalyst coordinated
with 1a to form a metal complex. The characteristic band of
C
DOI: 10.1021/acs.orglett.8b00904
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Rh2 π* to Rh2 σ* moved from 665 to 592 nm when the
Rh2(esp)2 coordinated to 1a in DCM. In regard to the UV/vis
spectra analysis, no change in the oxidative state of Rh2(esp)2
was observed (Figure S1). Chirik et al. recently reported a
study addressing the weakness of the N−H, and they found
that the bond-dissociation energy (BDE) of an N−H decreased
when the metal was coordinated to an amine.11 On the basis of
this work, it is inferred that the N−H bond of 1 may be
activated by the Rh2(II,II) by axially coordinating to 1, which
facilitates proton transfer (PT) and electron transfer (ET) to
the BDEN−H beyond the BDEO−H of MEHQ. On the basis of
the investigation, the proposed mechanism is described in
Scheme 4. Since 1a decomposes spontaneously to generate
Scheme 4. Proposed Mechanism
lose the hydrogen radical as a result of the decreased BDEN−H
.
Experiments were designed to demonstrate how the Rh2(II,II)
complex tunes the BDE of the N−H bond by comparing
various efficiencies catalyzed by Rh2(esp)2 and Rh2(5S,R-
MenPY)4 in the reaction. Table 2 displays the radical
Table 2. Rh2(II,II) Complex Tunes the BDE of the N−H
a
Bond of 1a
b
entry
substrate
catalyst
additive
yield (%)
1
2
1a
1a
1a
1a
1a
1a
1m
1m
none
none
MEHQ
BHT
trace
trace
65
unidentified radicals, these trace radicals abstract H atom from
the Rh2(II,II)-1a complex A and form the N-centered radical B.
This is followed by cyclopropane ring-opening to generate
radical C, which is trapped by 2a to form intermediate D. The
formation of D occurs by intramolecular radical addition to
yield E. After hydrogen atom transfer (HAT) from complex A,
the desired product 3aa is obtained and regenerates the N-
centered radical B. During the catalytic cycle, it was proposed
that the Rh2(II,II) complex “tames” the radicals to complete the
cycloaddition reaction efficiently.
This study detailed the development of a (3 + 2)
cycloaddition reaction catalyzed by Rh2(II,II) complexes
between N-arylaminocyclopropane and alkenes derivatives.
The reaction exhibited a wide tolerance for various functional
groups and could be used to produce β-amino acids. It appears
that the Rh2(II,II) complex coordinates axially to the N-
arylaminocyclopropane to activate the N−H bond and
decreases the BDE of N−H bond in the aminocyclopropane.
This mechanism facilitates formation of an N-centered radical
by loss of a hydrogen radical. Future experiments will focus on
elucidating the structure of Rh2(II,II) complexes related to the
N−H bond activation and broadening the scope of this
method.
c
3
Rh2(esp)2
MEHQ
MEHQ
BHT
4
Rh2(5S,R-MenPY)4
Rh2(esp)2
76
c
5
<5
6
7
8
Rh2(5S,R-MenPY)4
Rh2(5S,R-MenPY)4
Rh2(5S,R-MenPY)4
BHT
64
none
80
MEHQ
trace
a
Reaction conditions: 1a (0.5 mmol, 0.2 M in degassed solvent), 2a
(2.5 mmol), catalyst (0.1 mol %), and additive (20 mol %) at rt for 18
h, unless otherwise noted. Isolated yields of two isomers. 1 mol % of
catalyst was used.
b
c
terminating reagents 4-methoxyphenol (MEHQ) or butylated
hydroxytoluene (BHT) with different BDEO−H that were added
to the reactions. The experimental results showed that MEHQ
(BDEO−H = 81.7 kcal/mol)12 was unable to inhibit the reaction
catalyzed by Rh2(esp)2 or Rh2(5S,R-MenPY)4, indicating that
these two Rh2(II,II) compounds lowered the BDEN−H of 1a to
be less than the BDEO−H of MEHQ. However, when BHT
(BDEO−H = 79.9 kcal/mol)12 was added, the reaction catalyzed
by Rh2(esp)2 was efficiently inhibited, but not for the reaction
catalyzed by Rh2(5S,R-MenPY)4 These results suggest that the
BDEN−H of 1a, tuned by Rh2(5S,R-MenPY)4, was lower than
the BDEO−H of BHT. This can help clarify why Rh2(5S,R-
MenPY)4 was sought out. Further experimentation demon
strated that the BDEN−H plays an important role in the reaction.
As literatures reported, the electron-withdrawing groups on the
arenes increase the BDEN−H of the aromatic amines.12 The 1m
bearing two CF3 groups substituted phenyl group of 1a
catalyzed by Rh2(5S,R-MenPY)4 to yield 3ma in 80% yield.
However, the addition of MEHQ inhibited this reaction
effectively (Table 2, entry 8). It is reasoned that 1m has such
a high BDEN−H value that Rh2(5S,R-MenPY)4 cannot reduce
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00904.
Additional experimental procedures and spectroscopic
data for all new compounds (PDF)
D
DOI: 10.1021/acs.orglett.8b00904
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Accession Codes
Letter
(11) Bezdek, M. J.; Guo, S.; Chirik, P. J. Science 2016, 354, 730.
(12) Lucarini, M.; Pedulli, G. F. Chem. Soc. Rev. 2010, 39, 2106.
CCDC 1581725, 1581760, and 1581762 contain the
supplementary 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 Author
*E-mail: yhwang@scu.edu.cn.
ORCID
Yuanhua Wang: 0000-0003-0514-6976
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful for financial support from National Science
Foundation of China (Grant No. 21272162).
■
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DOI: 10.1021/acs.orglett.8b00904
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