Copper-catalyzed intramolecular oxytrifluoromethylthiolation of unactivated alkenes

Article abstract of DOI:10.1021/ol502624z

A mild, versatile, and convenient method for efficient intramolecular oxytrifluoromethylthiolation of unactivated alkenes catalyzed by Cu(OAc)₂ has been developed. The reactions were carried out under aerobic conditions and formed a variety of isoxazolines bearing a -SCF₃ substituent. (Chemical Equation Presented).

Full text of DOI:10.1021/ol502624z

Letter
pubs.acs.org/OrgLett
Terms of Use
Copper-Catalyzed Intramolecular Oxytrifluoromethylthiolation of
Unactivated Alkenes
Liping Zhu, Guoqiang Wang, Quanping Guo, Zhaoqing Xu,* Di Zhang, and Rui Wang*
Key Laboratory of Preclinical Study for New Drugs of Gansu Province, School of Basic Medical Sciences, Lanzhou University,
Lanzhou 730000, China
*
S Supporting Information
ABSTRACT: A mild, versatile, and convenient method for efficient intramolecular oxytrifluoromethylthiolation of unactivated
alkenes catalyzed by Cu(OAc) has been developed. The reactions were carried out under aerobic conditions and formed a
2
variety of isoxazolines bearing a −SCF substituent.
3
8
he modification of drug candidates by introduction of
phthalimide (III) and its analogues, and trifluoromethanesul-
9
T
fluorine or fluorine-containing moieties has become an
fenmide (IV) and its analogues, were developed and employed
1
important strategy in drug discovery. Among the fluorine-
in electrophilic trifluoromethylthiolation under transition-metal-
catalytic or transition-metal-free conditions. However, the
substrate scopes are mainly restricted in strong nucleophilic
compounds, such as terminal alkynes, indoles, and oxindoles, or
the α-position of carbonyl compounds (eq 2). Thus, developing
a methodology for trifluoromethylthiolation of unactivated
substrates, especially for construction of C −SCF is highly
containing groups, the trifluoromethylthio group (−SCF ) has
3
attracted increasing interest for its strong electron-withdrawing
2
effect and high lipophilicity property. Accordingly, in the past
several decades, numerous methods to introduce −SCF onto
3
organic compounds have been reported. Typical methods
include a halogen−fluorine exchange reaction (often under
sp3
3
3
harsh conditions) or trifluoromethylation of sulfur-containing
desirable.
4
compounds.
Transition-metal-catalyzed difunctionalization of unactivated
alkenes attracts much attention, which represents a versatile and
step economical strategy to furnish multiple C−C bond/C−
heteroatom bonds in a single step. In this area, one of the most
synthetically important reactions is transition-metal-catalyzed
radical addition to alkenes. Despite the successful employment of
nucleophilic or electrophilic SCF reagents in trifluoromethylth-
iolation, the radical-type reaction by using SCF has been very
seldom studied. Recently, Buchwald and Qing reported
oxytrifluoromethylation of unactivated alkenes, respectively.
Recently, a few new synthetic approaches for construction of
C−SCF have been developed. For instance, transition-metal-
3
10
mediated or -catalyzed trifluoromethylthiolation uses nucleo-
philic SCF sources (CuSCF , AgSCF , or NMe SCF , etc.;
3
3
3
4
3
5
Scheme 1, eq 1). More recently, several new electrophilic SCF
reagents, such as hypervalent iodonium ylide (I), trifluorome-
thylthiolated thioperoxy reagent (II), N-(trifluoromethylthio)-
3
6
3
7
•
3
11
12
Scheme 1. Trifluoromethylthiolation Using Nucleophilic or
Electrophilic SCF Reagents
However, the oxytrifluoromethylthiolation for difunctionaliza-
3
tion of unactivated alkenes is still unknown.
Isoxazoline motifs exhibit interesting biological activities and
serve as versatile organic synthesis intermediates. The
1
3
1
4
modification of isoxazoline by introduction of −SCF is of
3
great interest both for biology and organic synthesis research. We
15
envisioned that by using alkene-substituted oxime as substrate
isoxazoline featuring an SCF group can be easily synthesized. As
3
part of our research interests on copper-catalyzed synthesis of
16
heterocycles, we disclose here the first example of Cu(II)-
catalyzed intramolecular oxytrifluoromethylthiolation of unac-
tivated alkenes for the synthesis of C −SCF . The mechanistic
sp3
3
study indicated that a radical pathway was involved in this
transformation. Notably, in the reaction, AgSCF was directly
3
Received: September 4, 2014
Published: October 2, 2014
©
2014 American Chemical Society
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dx.doi.org/10.1021/ol502624z | Org. Lett. 2014, 16, 5390−5393

Organic Letters
Letter
used as SCF source without other oxidant’s assistance or the
With the optimal conditions (Table 1, entry 3) in hand, we
then studied the substrate scope for the oxytrifluoromethylth-
iolation reaction (Scheme 2). The aromatic oximes with
3
preparation of trifluoromethylthiolated hypervalent iodine.
Moreover, the radical addition product can be easily transferred
to a synthetic useful building block, α-trifluoromethylthio alcohol
derivatives in one step.
a
Scheme 2. Investigation of Substrate Scope
The investigation was initiated by using oxime 1a as substrate,
1
.5 equiv of AgSCF (2) as trifluoromethylthio reagent, 20 mol %
3
of Cu(OTf) as catalyst, and DMF as solvent. The reaction was
2
stirred at 80 °C for 12 h under aerobic conditions and gave the
desired product 3a in 38% yield (the same yield was obtained
under argon atmosphere, Table 1, entry 1). We then tested other
Table 1. Optimization of Reaction Conditions
a
b
entry
catalyst
solvent
DMF
yield (%)
1
2
3
4
5
6
7
8
9
Cu(OTf)2
CuCl2
38
64
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Cu(OAc)2
CuCl
93 (90)
59
CuBr
67
CuI
65
FeCl3
<5
In(OTf)3
La(OTf)3
Sc(OTf)3
Zn(OTf)2
Al(OTf)3
NiCl .6H O
trace
trace
<5
10
11
12
13
14
15
16
17
18
19
20
21
22
trace
trace
trace
50
2
2
a
All reactions were carried out by using 0.2 mmol of 1, 1.5 equiv of 2,
c
Cu(OAc)2
0
.04 mmol of Cu(OAc) , and 2 mL of DMF. Yields refer to isolated
2
b
d
Cu(OAc)2
trace
52
yields. Relative configuration was not assigned.
Cu(OAc)2
CH CN
3
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
toluene
1,4-dioxane
DCE
66
electron-donating or electron-withdrawing group at the o-, m-,
or p- position were all worked well under standard conditions
(3b−i). Other substituted aryl, naphthyl, and benzyl oximes also
reacted smoothly (3j−m,p). The oxytrifluoromethylthiolation of
oximes with heteroarene substituents gave the bihetero products
49
89
e
f
DMF
85
DMF
67
DMF
trace
3
n and 3o with acceptable yields (65% and 52%). We were
a
All reactions were carried out by using 1 (0.2 mmol), 2 (1.5 equiv),
pleased to find that the catalytic system worked well with
aliphatic substrate, providing the corresponding product 3q in
catalyst (20 mol %), and solvent (2 mL), stirred at 80 °C for 12 h,
except as noted. Yields were determined by GC with internal
standard; isolated yield was indicated in parentheses. 22 mol % of
b
7
6% yield. The α-phenyl oxime was cyclized with excellent
c
17
diastereselectivity (dr = 25:1, 3r). Then, the current method-
d
1
,10-phenanthroline was used. 22 mol % of 2,2′-bipyridine was used.
e
f
ology was successfully employed to build a quaternary carbon
center with −SCF (3s). To further investigate the substrate
At 40 °C. At 120 °C.
3
scope, the oxytrifluoromethylthiolation reaction was employed
for the synthesis of a six-membered ring. To our satisfaction,
under the stated conditions, the desired product was obtained in
good yield (3t). Interestingly, when 1u was used, the alkene
migration product 3u was formed instead of the trifluorome-
thylthiolation.
To further illustrate the synthetic utility of oxytrifluorome-
thylthiolation product, isoxazoline 3a was applied to a reductive
ring-opening reaction (Scheme 3). The reaction proceeded
copper catalysts, such as CuCl , Cu(OAc) , CuCl, CuBr, and CuI
2
2
(
entries 2−6). The copper salts with different valencies were all
effective for this reaction, and Cu(OAc) gave the highest yield
2
(
93%, entry 3). Other transition-metal catalysts were also
examined in the reaction: FeCl and Sc(OTf) only afforded less
3
3
than 5% desired product; In(OTf) , La(OTf) , Zn(OTf) ,
3
3
2
Al(OTf) , and NiCl ·6H O were completely ineffective for this
3
2
2
reaction (entries 7−13). Using of ligands, namely 1,10-
phenanthroline and 2,2′-bipyridine, led to poor yields (entries
smoothly and gave the highly functional SCF -containing
3
14 and 15). The solvent screening indicated that DMF is the best
building block, 3-hydroxy-1-phenyl-4-trifluoromethylthio-1-bu-
tanone 4 in 87% yield.
To understand the mechanism, some reactions were carried
out (Scheme 4, see the Supporting Information for details).
When 2,6-di-tert-butyl-4-methylphenol (BHT, 1.5 equiv) was
added to the reaction under standard conditions, no desired
choice for the reaction (entries 16−19). The attempts to increase
the yields by variation of the reaction temperature were also
failed (entries 20 and 21). It should be noted that the reaction did
not occur in the absence of catalyst, which revealed that copper
was crucial to this transformation (entry 22).
5
391
dx.doi.org/10.1021/ol502624z | Org. Lett. 2014, 16, 5390−5393

Organic Letters
Letter
•
Scheme 3. Synthetic Utility of Current Method
Scheme 5. Mechanistic Study for SCF Formation
3
Scheme 4. Mechanistic Study for Oxime Radical Formation
•
SCF , and it may only act as the promoter for the oxime radical
3
•
18
generation. CuSCF is able to decompose to release SCF .
3
3
When it was applied in the reaction instead of AgSCF , 3a was
3
observed in 58% yield (eq 7).
Although the mechanism of oxytrifluoromethylthiolation is
not completely clear yet, on the basis of previous reports and the
above experimental results, the catalytic cycle could including the
following components (see the Supporting Information for
1
9
details): (i) Cu(OAc) catalyzes the formation of oxime
2
•
radical; (ii) AgSCF serves as the precursor to generate SCF
3
3
0
20
and gives Ag as by product; (iii) the oxime radical adds to C
C bond via an intramolecular way and the result alkyl radical is
•
trapped by SCF , by which the difunctionalization of alkene is
3
realized through an oxytrifluoromethylthiolation fashion.
In conclusion, we have developed a novel and convenient
method for the intramolecular oxytrifluoromethylthiolation of
alkenes for the first time. All of the reactions were carried out
under aerobic conditions, directly utilizing the air stable and
a
b
c
Standard conditions. GC yield with internal standard. Isolated
easily prepared AgSCF as SCF source, and without assistance of
yield.
3
3
other oxidants. This methodology allows rapid access to a variety
of isoxazolines bearing a −SCF substituent. Moreover, the
3
product formed. However, the ESI-HRMS analysis of reaction
mixture indicated the formation of BHT-oxime adduct (eq 4).
Using 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO, 1.5 equiv)
synthetic utility of product was demonstrated by one step
synthesis of highly functional C −SCF -containing building
sp3
3
block 4 in 87% yield.
as additive, the TEMPO addition product 5 was obtained in 83%
•
yield (eq 5, entry 1). Although no SCF radical trapping product
3
ASSOCIATED CONTENT
Supporting Information
■
was detected in the experiments, these results indicated that
oxime radical might generate in the system. We then set up a
series of reactions to clarify the most active species which
catalyzed the generation of oxime radical in the reaction mixture.
Without any metal additives, 5 formed very slowly (after 2 h, 27%
yield; after 12 h, 63% yield. entry 2). However, when 20 mol % of
*
S
Experimental details, characterization data, and NMR spectra.
This material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
Cu(OAc) was used, after only 0.5 h the radical addition product
■
2
was obtained in 98% yield (entry 3). In the case of 1.5 equiv of
AgSCF , a moderate reaction rate was observed (entry 4). It is
* E-mail: zqxu@lzu.edu.cn.
* E-mail: wangrui@lzu.edu.cn.
3
interesting that CuSCF showed similar catalytic activity to
3
Cu(OAc) (entry 5). These results indicated that, compared with
2
Notes
AgSCF , a catalytic amount of copper additive was more efficient
3
The authors declare no competing financial interest.
to promote this rapid TEMPO-trapping reaction and possibly
also serve as the catalyst/promoter for the oxime radical
formation in oxytrifluoromethylthiolation reaction.
ACKNOWLEDGMENTS
■
We are grateful for the grants from the NSFC (Nos. 21202072,
21102141, and 21432003) and the Fundamental Research Funds
for the Central Universities (Nos. 860976 and 861188).
Recently, Rueping and co-workers reported an example in
•
8c
which AgSCF decomposes to give SCF . In fact, we noticed a
3
3
silver mirror in each case of Scheme 2, which might form in the
•
•
SCF -producing process. To further investigate the SCF
3
3
REFERENCES
■
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3
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Organic Letters
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5
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dx.doi.org/10.1021/ol502624z | Org. Lett. 2014, 16, 5390−5393