Copper(I)-Catalyzed Intramolecular Trifluoromethylation of Methylenecyclopropanes

Article abstract of DOI:10.1021/acs.orglett.5b02940

Copper(I)-catalyzed intramolecular trifluoromethylation of methylenecyclopropanes has been developed to produce a variety of CF₃-substituted dihydronaphthalenes in moderate to good yields, relying on the construction of C(sp²)-CF

Full text of DOI:10.1021/acs.orglett.5b02940

Letter
pubs.acs.org/OrgLett
Copper(I)-Catalyzed Intramolecular Trifluoromethylation of
Methylenecyclopropanes
Zi-Zhong Zhu,† Kai Chen,† Liu-Zhu Yu,† Xiang-Ying Tang,‡ and Min Shi*,†,‡
†Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Mei
Long Road, Shanghai 200237, China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Road, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: Copper(I)-catalyzed intramolecular trifluoromethyla-
tion of methylenecyclopropanes has been developed to produce a
variety of CF3-substituted dihydronaphthalenes in moderate to good
yields, relying on the construction of C(sp2)−CF3 bonds under mild
conditions. The reactions proceed through a radical process under
copper(I) catalysis with a good compatibility for the functional
group.
he trifluoromethyl group is a very important structural
motif in many agrochemicals and pharmaceuticals because
CF3-containing molecules usually have remarkable biological
activity, high hydrophobicity, and metabolic stability (Figure
1).1 Over the past decades, research to efficiently introduce a
Scheme 1. Synthesis of CF3-Substituted
Dihydronaphthalenes
T
Figure 1. CF3-containing drugs.
several years, much attention has been paid to the transition
metal or Lewis acid catalyzed transformations of MCPs to
rapidly construct complex and interesting organic com-
pounds.14 At the same time, the radical initiated ring-opening
processes of MCPs have been also explored extensively.15
On the basis of the above successful examples of
trifluoromethylation and our ongoing interest in the exploration
of new reactivity of MCPs, we hypothesized that the direct
trifluoromethylation of MCPs to obtain a variety of CF3-
substituted dihydronaphthalenes would be possible through a
Cu(I)-catalyzed free radical process (Scheme 1).16 Herein, we
report the successful construction of C(sp2)−CF3 bonds relying
on the radical initiated ring opening of MCPs and the
subsequent cyclization to afford CF3-substituted dihydronaph-
thalenes.
trifluoromethyl group into organic molecules has been a very
important part of organic chemistry.2 Trifluoromethylations,
especially transition-metal-mediated or -catalyzed trifluoro-
methylation reactions, efficiently offer a large number of CF3-
containing compounds. Thus far, the construction of C(sp3)−
CF3 bonds has been reported widely through a transition-metal
catalyzed process.3 Moreover, trifluoromethylation of aromatic
compounds has also been achieved through a transition-metal-
catalyzed cross-coupling process, resulting in the construction
of C(sp2)−CF3 bonds.4 More recently, difunctionalization of
alkynes has become a useful method to access CF3-containing
molecules, also having a C(sp2)−CF3 bond. For example, the
groups of Szabo,5 Sodeoka,6 and Cho7 have developed the
́
difunctionalization of terminal alkynes to construct C(sp2)−
CF3 bonds, respectively. In addition, the groups of Liu,8 Liang,9
Hou,10 Ding,11 and Fu12 achieved trifluoromethylation of
internal alkynes (Scheme 1).
We first investigated the reaction between methylenecyclo-
propane (1a) and the Togni reagent (II) to identify the optimal
Methylenecyclopropanes (MCPs) are highly strained but
readily accessible molecules and have been used very often as
important building blocks in organic synthesis.13 In the past
Received: October 11, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b02940
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Letter
reaction conditions, and the results are summarized in Table 1.
Using CuI (10 mol %) as the catalyst in DCE (1,2-
Scheme 2. Substrate Scope of 1
a
Table 1. Optimization of the Reaction Conditions
b
entry
catalyst
solvent
temp/°C
yield [%]
1
2
3
CuI
DCE
80
80
80
80
48
52
70
72
CuTc
CuTc
CuTc
DCE
MeCN
MeCN
c
4
a
Reaction conditions: 1a (0.2 mmol), Togni reagent (II) (0.3 mmol),
b
catalyst (10 mol %), and solvent (1.5 mL) were used. Determined by
c
1H NMR spectroscopy. Togni reagent (II) (0.4 mmol) was used.
dichloroethane) at 80 °C, CF3-substituted dihydronaphthalene
2a was obtained in 48% yield (Table 1, entry 1). We also
utilized other catalysts instead of CuI, and found that CuTc was
the best one, affording the desire product 2a in 52% yield
(Table 1, entry 2). Next, we utilized different solvents in this
reaction to examine the solvent effects and found that MeCN
was the solvent of choice, giving 2a in 70% yield (Table 1, entry
3). The examination of reaction temperature revealed that this
reaction should be carried out at 80 °C (for more details about
the optimization of this reaction, see Table S1 in the
Supporting Information). To our delight, the yield of 2a
could be improved to 72% when 2.0 equiv of the Togni reagent
(II) was used in the presence of CuTc (10 mol %) in MeCN
(1.5 mL) at 80 °C (Table 1, entry 4).
a
b
Using DCE as the solvent. A second portion of CuTc (10 mol %)
c
and the Togni reagent (II) (2.0 equiv) was added after 6 h. The ratio
of regioisomers. Isolated yields are provided.
Having the optimized conditions in hand, we turned our
attention to evaluating the generality of the reaction using a
variety of MCPs 1b−1k, bearing either electron-donating or
-withdrawing groups on the aromatic ring. The results are
shown in Scheme 2. When a substituent at the ortho-position of
the aromatic ring was an electron-donating substituent such as
an alkyl or Ts-protected amino group, the reaction proceeded
smoothly to give the desired product 2b or 2c in 83% or 65%
yield, respectively. The ORTEP drawing of 2b is shown in
Figure 2, and its CIF data have been indicated in the
Supporting Information. While the substituent was a strongly
electron-withdrawing group such as NO2, the product 2d was
obtained in a slightly lower yield. By replacing NO2 by CN, the
reaction proceeded efficiently in DCE to afford the desired
product 2e in 63% yield. Subsequently, we examined the
influence of the substituents at the meta-position of the
aromatic ring. For substrate 1f, the desired product 2f could be
obtained as the sole product in 70% yield. While, in the case of
1g, the reaction proceeded smoothly to give the desired
product 2g along with its regioisomer 2g′ in 61% total yield as
the ratio of 3:1. As for substrate 1h, in which the meta-position
of the aromatic ring was substituted by NO2, the corresponding
product 2h and its regioisomer 2h′ could be also obtained in
62% total yield along with the ratio of 2:1. When substituents at
the para-position of the aromatic ring were OMe and CN, the
desired products 2i and 2j were afforded in 76% and 63%
yields, respectively. However, as for MCP 1k, in which the
aromatic ring was p-ClC6H4, the product 2k could be obtained
in 50% yield when a second portion of CuTc and the Togni
reagent (II) was added after 6 h. In the case of substrate 1l, in
which R2 = H was replaced by a phenyl group, the reaction
Figure 2. X-ray crystal structure of product 2b.
should be carried out in DCE to give the desired product 2l in
56% yield. As for substrates 1m−1o, in which the aromatic ring
had two substituents, the corresponding products 2m−2o were
formed in 70−52% yields. The trisubstituted substrate 1p
produced the corresponding product 2p in 80% yield. A
substrate containing an indole group (1q) instead of a phenyl
counterpart has been also utilized for this reaction. However,
we found that the reaction system became complex and no
desired product was formed under the standard conditions. As
for substrates 1s and 1t, both of them are unstable because they
could easily decompose even at low temperature under an
argon atmosphere. As for methylenecyclobutane 1u, the
reaction did not take place under the standard conditions.
To demonstrate the synthetic utility of this method, further
transformations of 2b were performed (Scheme 3). Product 2b
could be readily oxidized by 3.0 equiv of NBS (N-bromo-
succinimide) to give the corresponding CF3-substituted
naphthalene 3b in 69% yield (Scheme 3a). In the presence of
B
DOI: 10.1021/acs.orglett.5b02940
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Letter
from the Togni reagent (II) in the presence of copper(I). The
CF3 radical adds to the CC bond of MCP to give
intermediate A, which undergoes a ring-opening process to
give the alkyl radical intermediate B. The key intermediate B
undergoes direct radical cyclization with an aromatic ring to
afford intermediate C, which is oxidized by Cu(II) to give the
desired product 2 and release a proton.
Scheme 3. Transformation of the Product 2b
In summary, we have disclosed a novel synthetic protocol for
the construction of CF3-substituted dihydronaphthalenes
through an efficient copper(I)-catalyzed trifluoromethylation
of methylenecyclopropanes. The mechanistic studies indicate
that the reaction proceeded through a CF3 radical addition to
the CC bond of MCP, followed by sequential ring opening
and oxidative cyclization to afford the desired product. The
product 2b can be easily transformed to other useful
trifluoromethylated compounds under mild conditions. Efforts
are in progress in the application of this new methodology to
synthesize interesting biologically active compounds in our
laboratory.
NBS (6.0 equiv), 2b could be further oxidized to the CF3-
substituted naphthaldehyde 4b in 61% yield under identical
conditions (Scheme 3b). Furthermore, product 2b could also
be transformed to the CF3-substituted epoxide 5b in the
presence of m-CPBA (2.0 equiv) (Scheme 3c). On the basis of
the above-mentioned results, it is obvious that more function-
alized CF3-containing compounds can be easily obtained from
product 2.
To gain mechanistic insight into this reaction, control
experiments were conducted. As shown in Scheme 4, when the
ASSOCIATED CONTENT
* Supporting Information
■
S
Scheme 4. Mechanistic Investigations
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b02940.
X-ray structural data (CIF) of compound 2b (CCDC
1407656) (CIF)
General experimental procedure and characterization
data of the products; copies of NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: mshi@mail.sioc.ac.cn.
Notes
reaction mixture was stirred at 80 °C for 30 min in the presence
of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) (2.0 equiv),
the CF3 radical was captured by TEMPO to give the
corresponding TEMPO adduct in 22% 19F NMR yield along
with 2a in only 28% 19F NMR yield (Scheme 4a). If the
reaction was performed with the addition of BHT (butylated
hydroxytoluene) (2.0 equiv) under the standard conditions, 2a
could not be detected (Scheme 4b), suggesting that the
reaction may undergo a radical process.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the National
Basic Research Program of China (973)-2015CB856603 and
the National Natural Science Foundation of China (20472096,
21372241, 21361140350, 20672127, 21421091, 21372250,
21121062, 21302203, 20732008, and 21572052).
A plausible mechanism is depicted in Scheme 5 on the basis
of the aforementioned control experiments and previously
reported literature.3c,k,11,12 The CF3 radical can be generated
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