Copper-Catalyzed Domino Cyclization/Trifluoromethylthiolation of Unactivated Alkenes: Access to SCF3-Containing Pyrrolines

Article abstract of DOI:10.1021/acs.orglett.8b00614

A novel and efficient copper-catalyzed cascade cyclization/trifluoromethylthiolation of unactivated olefins has been achieved with the stable and readily available AgSCF₃ as the SCF₃ source. A range of SCF₃-substituted pyr

Full text of DOI:10.1021/acs.orglett.8b00614

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Copper-Catalyzed Domino Cyclization/Trifluoromethylthiolation of
Unactivated Alkenes: Access to SCF ‑Containing Pyrrolines
3
Kang Guo, Honglin Zhang, Shujun Cao, Chen Gu, Huating Zhou, Jie Li, and Yingguang Zhu*
Jiangsu Key Laboratory of Pesticide Science and Department of Chemistry, College of Sciences, Nanjing Agricultural University,
Nanjing 210095, China
*
S Supporting Information
ABSTRACT: A novel and efficient copper-catalyzed cascade cycliza-
tion/trifluoromethylthiolation of unactivated olefins has been achieved
with the stable and readily available AgSCF as the SCF source. A range
3
3
of SCF -substituted pyrrolines have been easily obtained under mild
3
conditions in good yields via the present process. This method
represents a facile and rapid access to valuable pyrrolines with
fluorine-containing groups, and it is amenable to gram-scale synthesis.
ompounds bearing fluorine-containing groups have been
widely used as pharmaceuticals, agrochemicals, and
that involve the N−O bond cleavage of acyl oximes having an
olefin moiety and the subsequent alkene difunctionalization
C
11f,g,14b,16a,e,h,18b
materials, because of their unique physical, chemical, and
process are rare.
Herein, we report a novel
1
biological properties. Because of its strong electron-with-
copper-catalyzed domino cyclization and trifluoromethylthiola-
tion of acyl oxime-tethered unactivated olefins to deliver a
drawing capability and high lipophilicity, the incorporation of
the trifluoromethylthio (SCF ) group into an organic molecule
range of SCF -featured pyrrolines in good yields, where the
3
3
often significantly improves the metabolic stability, bioavail-
ability, and bioactivity. Therefore, the development of effective
stable and easily available AgSCF is employed as a SCF
source.
3
3
2
strategies for the introduction of the SCF group has attracted
much attention from chemists. Traditional indirect methods
Our studies commenced with the examination of the effect of
acyloxy leaving groups on the reactivity of olefinic O-acyl
oximes and the product yield. As shown in Scheme 1, when O-
3
3
for the formation of the SCF group include the trifluor-
3
4
omethylation of thiols and their derivatives and the halogen-
fluorine exchange reactions, in which additional synthetic steps
5
a
Scheme 1. Screening of O-Acyl Oximes
are needed. Recently, various direct strategies for the
6
construction of C−SCF bond, including nucleophilic, electro-
3
7
8
9
philic, radical, and oxidative trifluoromethylthiolations have
been developed. Because of its stability and ready availability,
AgSCF3 has recently been reported as an effective and
straightforward trifluoromethylthiolating reagent in the C−
6a,d−f,h−j,7f−h,8,9c,d
SCF bond-forming reactions.
Despite signifi-
3
cant progress, the difunctionalization-type trifluoromethylth-
iolation of alkenes with AgSCF as the SCF source has been
3
3
7
h,8a,d,g
3
less explored.
Thus, new and efficient C(sp )−SCF
3
a
bond-forming processes involving the difunctionalization of
alkenes, especially unactivated alkenes, remain highly desirable.
O-Acyl oximes are exceptionally versatile building blocks for
the construction of structurally diverse and valuable nitrogen-
All reactions were carried out with acyl oxime 1 (0.30 mmol),
AgSCF (0.45 mmol), and Cu(acac) (0.060 mmol) in DCE (3.0 mL)
at 80 °C under N
3
2
for 12 h. Isolated yield based on 1.
2
10
containing heterocycles via N−O bond cleavage. To date, a
variety of methods for the synthesis of N-heterocycles through
the N−O bond cleavage of O-acyl oximes have been developed,
benzoyl oxime 1a was treated with AgSCF in the presence of
Cu(acac) (20 mol %) in 1,2-dichloroethane (DCE) at 80 °C
for 12 h, the desired SCF -containing pyrroline 2a was obtained
in 72% isolated yield. Other O-aroyl oximes with electron-
donating or electron-withdrawing groups on the phenyl ring
3
2
11
12
mainly including transition-metal catalysis, such as Pd, Rh,
3
1
3
14
15
16
17
Ru, Fe, Co, Cu, etc., microwave or UV irradiation, and
1
8
visible-light photoredox catalysis. Difunctionalization of
alkenes has become a powerful tool for the incorporation of
two functional groups across a double bond in one step, and
thus for a rapid increase of molecular complexity and diversity.
Although much progress has been made in the construction of
N-heterocycles with acyl oximes as the precursors, examples
(
1ac, 1ad, and 1ae) also gave product 2a, albeit with slightly
lower yields. However, less activated and more stable O-acetyl
Received: February 20, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00614
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
,
a b
oxime 1ab afforded a much lower yield of 29%. Thus,
benzoyloxy group was chosen as a suitable leaving group of
the acyl oxime substrates.
Scheme 2. Substrate Scope
We next set out to optimize the reaction conditions with O-
benzoyl oxime 1a as a model substrate (Table 1). Various metal
a
Table 1. Optimization of the Reaction Conditions
b
entry
catalyst
Cu(acac)2
solvent
t (°C)
yield (%)
1
2
3
4
5
6
7
8
9
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
80
80
80
80
80
80
80
80
80
80
80
80
80
80
50
100
80
80
72
0
NiCl (dme)
2
Fe(acac)2
Pd(OAc)2
Cu(OAc)2
Cu(OTf)2
CuCl
0
0
81
67
65
60
68
61
36
46
26
63
trace
51
67
trace
CuBr
Cu(MeCN) PF
4
6
10
11
12
13
14
15
16
17
18
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
CH CN
3
DMF
DMA
dioxane
toluene
DCE
DCE
c
DCE
d
DCE
a
All reactions were performed with 1a (0.30 mmol), AgSCF (0.45
3
mmol), and catalyst (0.060 mmol) in solvent (3.0 mL) at 80 °C for 12
b
c
h unless otherwise noted. Isolated yields. PPh (0.060 mmol) was
3
d
added. 2,2′-Bipyridine (0.060 mmol) was added.
a
salt catalysts were first tested. No product was observed when
All reactions were performed with acyl oxime 1 (0.30 mmol),
(0.45 mmol), and Cu(OAc) (0.060 mmol) in DCE (3.0 mL)
at 80 °C under N for 12 h unless otherwise noted. Isolated yields.
NiCl (dme), Fe(acac) , or Pd(OAc) was employed as the
AgSCF
3
2
2
2
2
b
catalyst (Table 1, entries 2−4). Other Cu(II) and Cu(I) salts,
2
c
d
Run for 24 h. Cu(CH CN) PF (20 mol %) was used.
including Cu(OAc) , Cu(OTf) , CuCl, CuBr, and Cu-
3
4
6
2
2
(
MeCN) PF were also found to be effective catalysts for this
4 6
Naphthyl- and 2-thienyl-substituted acyl oximes were also
effective substrates, affording the corresponding products 2i
and 2j in 65% and 60% yields, respectively. The aryl-substituted
substrates bearing substitutions at the alkyl chain moieties were
also compatible with the reaction, leading to the desired
products (2k−2n) in satisfactory yields. The alkyl-substituted
O-acyl oxime could also be transformed to the product (2o) in
reaction (Table 1, entries 5−9). Among the copper salts
surveyed, Cu(OAc) proved to be the best catalyst, and gave
product 2a in 81% yield (Table 1, entry 5). Screening of
solvents showed that DCE was superior to other solvents, such
2
as CH CN, DMF, DMA, 1,4-dioxane, and toluene (Table 1,
3
entries 5 and 10−14). The reaction temperature had a
remarkable effect on this transformation, and a lower (50 °C)
or a higher temperature (100 °C) resulted in inferior yields
6
1% yield.
To further demonstrate the synthetic utility of this method,
(
Table 1, entries 15 and 16). The addition of ligands, such as
reduction of the product 2a was achieved by the use of
diisobutylaluminum hydride (DIBAL-H), thus affording cis-
pyrrolidine 3 in 81% yield and with good diastereoselectivity
PPh and 2,2′-bipyridine, led to a lower yield and trace amounts
3
of product, respectively (Table 1, entries 17 and 18).
Having established the optimized reaction conditions, we
subsequently investigated the generality of this domino
cyclization/trifluoromethylthiolation process. As exemplified
in Scheme 2, the present reaction can be extended to a variety
(
dr >15:1) (see Scheme 3).
To understand the reaction mechanism, some control
experiments were conducted using substrate 1a. No reaction
of olefinic O-acyl oximes to give the SCF -containing pyrroline
3
products in 60%−89% yields. For the aryl-substituted O-acyl
oximes with either electron-donating or electron-withdrawing
groups at the para, meta, and ortho positions of the phenyl ring,
the reaction proceeded smoothly and gave the desired products
Scheme 3. Further Transformation of Product 2a
(
1
2a−2h) in good yields. As illustrated in the case of acyl oxime
f, pyrroline 2f can be obtained on gram scale in 89% yield. 2-
B
DOI: 10.1021/acs.orglett.8b00614
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
occurred in the absence of a copper catalyst, suggesting that the
copper salt plays an important role in this transformation
Scheme 5. Proposed Mechanism
(Scheme 4, eq 1). When the mixture of 2,2′-biquinoline and
Scheme 4. Mechanistic Investigations
16g,h
to give the Cu(I) and carbocation D,
which is then attacked
by SCF anion to produce the final product 2a (path II).
3
In conclusion, we have developed a novel copper-catalyzed
domino cyclization/trifluoromethylthiolation of unactivated
alkenes bearing an O-acyl oxime moiety. A variety of SCF3-
containing pyrrolines can be effectively constructed in good
yields from olefinic O-acyl oximes and the stable, easily
handled, and readily available AgSCF . The reaction enables the
3
3
simultaneous formation of a C−N bond and a C(sp )−SCF
bond, and is amenable to gram-scale synthesis. Preliminary
3
Cu(OAc) in DCE was heated at 80 °C for 5 h, a purple
solution was observed, indicating that a Cu(I)/biquinoline
complex was formed in situ via reduction or disproportionation
2
mechanistic investigations indicated that the SCF radical might
not be involved in the process.
3
of Cu(OAc) (Scheme 4, eq 2; see the Supporting Information
2
1
6d
ASSOCIATED CONTENT
Supporting Information
(
SI) for details). These results suggest that a Cu(I) species
might be involved in the domino reaction. When isolated
CuSCF was used instead of AgSCF , product 2a can be
■
*
S
3
3
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00614.
obtained in a good yield of 76% in the absence of a copper
catalyst (Scheme 4, eq 3). The results indicate that CuSCF3,
which could be generated in situ from the copper salt and
Experimental procedures and characterization data
6
e,f
AgSCF3, is likely to be an intermediate in the reaction. The
reaction was almost completely suppressed upon the addition
of the radical scavenger 2,2,6,6-tetramethyl-1-piperidinyloxy
(PDF)
AUTHOR INFORMATION
(
TEMPO) under standard conditions, and only trace amounts
■
Corresponding Author
*E-mail: ygzhu@njau.edu.cn.
ORCID
of 2a were obtained, indicating that this reaction might proceed
through a radical pathway (Scheme 4, eq 4). In order to
investigate whether or not the SCF radical is involved in this
3
process, F CSSCF , which was reported to generate the SCF
3
3
3
8a
Yingguang Zhu: 0000-0002-0429-6369
Notes
radical in the presence of Ag(I), was prepared in situ and
subjected to the reaction conditions (Scheme 4, eq 5).
However, no reaction was observed, implying that the SCF3
radical is not very likely to be involved in the domino
cyclization and trifluoromethylthiolation process.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Based on the above results and the literature report-
We are grateful for the financial support from the National
Natural Science Foundation of China (No. 21502096), the
Natural Science Foundation of Jiangsu Province (No.
BK20150652), the Fundamental Research Funds for the
Central Universities (No. KJQN201629), and “333 High
Level Talent Project” of Jiangsu Province.
6
e,f,16d,f,h,18b
s,
a plausible catalytic pathway is proposed in
Scheme 5. Initially, the metathesis between Cu(I) and AgSCF
3
6e,f
generates CuSCF3,
which reductively cleaves the N−O bond
1
6d,f,18b
of acyl oxime 1a to form iminyl radical
A and
Cu(II)SCF species. Subsequently, an intramolecular 5-exo-
3
trig cyclization of iminyl radical A gives C-centered radical
1
6d,h,18b
B,
which is trapped by the Cu(II)SCF species to
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Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.8b00614
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