Photoredox-catalysed chloro-, bromo- and trifluoromethylthio-trifluoromethylation of unactivated alkenes with sodium triflinate

Article abstract of DOI:10.1039/c7cc01903c

A mild and transition-metal-free protocol is herein presented for chloro-, bromo- and trifluoromethylthiotrifluoromethylation of unactivated alkenes. The easy-handling Langlois reagent, as well as N-halophthalimide and N-trifluoromethylthiosaccharin, is used in this method. In the presence of an organic photoredox catalyst N-methyl-9-mesityl acridinium, a broad range of desired products were afforded in satisfactory yields upon visible-light irradiation via a radical process.

Full text of DOI:10.1039/c7cc01903c

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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DOI: 10.1039/C7CC01903C
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Photoredox‐catalysed chloro‐, bromo‐ and trifluoromethylthio‐
trifluoromethylation of unactivated alkenes with sodium triflinate
Jing Fang,a,† Zhong‐Kui Wang,a,† Shu‐Wei Wu,a Wei‐Guo Shen,a Gui‐Zhen Aoa and Feng Liu*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
species, N‐Methyl‐9‐mesityl acridinium (Mes‐Acr+) with
excited‐state reduction potential (E1/2red*) > 1.8 V (vs. SCE),12,13
is sufficient for single electron oxidation of the Langlois
reagent (Eox = 1.05 V vs. SCE) to afford a CF3 radical.11e,14
A mild and transition‐metal‐free protocol is herein presented for
chloro‐, bromo‐ and trifluoromethylthiotrifluoromethylation of
unactivated alkenes. The easy‐handling Langlois reagent, as well
as N‐halophthalimide and N‐trifluoromethylthiosaccharin, are
used in the method. In the presence of organic photoredox
catalyst N‐methyl‐9‐mesityl acridinium, a broad range of desired
products were afforded in satisfied yields upon visible‐light
irradiation via a radical process.
Previous work:
Ir-photoredox
organic photoredox
Cat.
H
or
H
+ CF3SO2Na
R
CF3
(1)
R
ref . 11c-e
Introduction of a trifluoromethyl group (CF3) into a potentially
useful organic compound could lead to profound changes of its
biological, chemical and/or physical properties.1 Consequently,
it has been of great interest to develop practical synthetic
methods for preparation of trifluoromethylated compounds.2
Radical trifluoromethylation, especially, via photoredox
catalysis, has received great attention. In recent years, a
variety of CF3 radical precursors, such as CF3I,3 CF3SO2Cl,4 Togni
reagent,5 Umemoto reagent,6 Langlois/Baran reagents
(CF3SO2Na,7 (CF3SO2)2Zn8), and Ruppert‐Prakash reagent9
(TMSCF3) have been extensively used for incorporation of a CF3
group into diverse skeletons. As a stable, inexpensive, easily
stored and handled reagent, sodium triflinate was developed
by Langlois and co‐workers in 1991.10 However, there are
currently few operational methods aiming to use a photoredox
catalyst in combination with Langlois reagent toward
generating CF3 radical for organic synthesis (Scheme 1).11 To
access trifluoromethylated compounds, we recently became
interested in utilizing the combination of organic photoredox
catalyst and Langlois reagent owing to the cost and
environmental benefits.11a As one of the Fukuzumi acridinium
O
OH
or
MeO-H or
F3C
F3C
H
CF3
Ar
N
CF3SO2Na
Ru-photoredox
Cat.
N
+
(2)
(3)
R
CF3
ref . 11b
R
ArN2 BF4
O
N3
organic photoredox
Cat.
CF3
CF3SO2Na
+
R
R
H2O
ref . 11a
This work:
X
organic photoredox
Cat.
CF3
CF3SO2Na
R
R
R
or
SCF3
O
O
CF3
N
X
or
N SCF3
S
O
O
O
X = Cl, Br
Scheme 1 Visible‐light‐induced trifluoromethylation with CF3SO2Na.
Although various methods have been developed for vicinal halo
trifluoromethylation of alkenes,15 most of these protocols required
corrosive or gaseous reagents such as CF3SO2Cl,15b‐d SOX2,15a or
ICF3.15e Therefore, more convenient and practical strategies for
halotrifluoromethylation are highly desirable. As part of our interest
in synthesis of trifluoromethylated organic molecules,5a,11a we
a. Jiangsu Key Laboratory of Translational Research and Therapy for Neuro‐Psycho‐
Diseases and Department of Medicinal Chemistry, College of Pharmaceutical
Sciences, Soochow University, 199 Ren‐Ai Road, Suzhou, Jiangsu 215123, People’s
Republic of China. E‐mail: fliu2@suda.edu.cn
b. Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032,
People’s Republic of China.
report
herein
a
novel
photoredox‐catalysed
vicinal
† These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: Experimental details,
characterization data, and copies of 1H and 13C NMR spectra for all compounds. See
DOI: 10.1039/x0xx00000x
difunctionalization of unactivated alkenes with CF3SO2Na and N‐
halophthalimide/N‐trifluoromethylthiosaccharin, incorporating not
This journal is © The Royal Society of Chemistry 20xx
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only the CF3 group but also the Cl, Br or SCF3 in one pot (Scheme 1). both the photocatalyst and light were essenVtiieawl Arftioclre Ontlhinies
DOI: 10.1039/C7CC01903C
chlorotrifluoromethylation (entries 9 and 10).
The introduction of a halogen atom allows further transformations.
Table 1 Optimization of reaction conditions a
Mes-Acr (1 mol%)
Cl
CF3SO2Na (2 equiv)
R
CF3
O
O
Mes-Acr (1 mol%)
CF3SO2Na (2 equiv)
R
Cl
2f (1.2 equiv), DCE, white LEDs
1
TFA (2 equiv), rt, 16-24 h
3
CF3
N
N
[Cl] (1.2 equiv), DCE
O
O
O
O
Cl
Cl
Cl
TFA (2 equiv), rt, LEDs, 24 h
O
1a
O
CF3
CF3
CF3
N
7
N
3
3a
O
O
Cl
3c (2-Cl), 74%
3d (3-Cl), 65%
3e (4-Cl), 64%
entry
LEDs
chlorine source
yield (%)b
3b
3a
, 84%
, 73%
1
2
white
white
white
white
white
white
white
blue
2a
2b
2c
2d
2e
2f
62
55
44
35
61
73
45
71
0
Cl
Cl
Cl
O
O
Cl
H
N
H
N
S
CF3
CF3
CF3
O
Ph
O
O
3
3g
3h
, 69%
, 74%
O
3f
, 86%
Br
4
O
Cl
Cl
O
Cl
CF3
CF3
5
CF3
Br
F
OHC
6
O
3k
, 62%
3j
, 53%
3i
, 54%
7
2g
2f
Cl
O
Cl
Cl
Cl
Ts
CF3
8
O
CF3
N
CF3
CF3
CF3
Br
14
7
9c
10d
–
2f
O
3o, 85%
3m
, 72%
3n
, 60%
Cl
3l, 65%
white
2f
0
CF3
O
Cl
Cl
Cl
a1a (0.4 mmol), Mes‐Acr+ (1 mol%), CF3SO2Na (0.8 mmol), chlorine source (0.48
mmol), TFA (0.8 mmol), DCE (4 mL), Argon balloon, 5 W LEDs. bIsolated yield. cIn
the dark. dNo photoredox catalyst.
CF3
O
3
3
N
CO2Et
CF3
Cl
Ts
EtO2C
3q
3p
, 63%
, 88%
3s
3r
, 41%
, 41%
Scheme 2 Substrate scope for chlorotrifluoromethylation of unactivated
alkenes.
O
O
O
O
O
O
O
O
S
S
S
S
Cl
Cl
Cl
Cl
O2N
MeO
With the optimal chlorotrifluoromethylation protocol in hand, we
aimed to demonstrate the scope of substrates. As shown in Scheme
2, a variety of unactivated olefins were effectively converted into
the corresponding desired products in moderate to good yields. The
reaction was found to tolerate a series of different functional
groups, including imides, esters, amides and ketones. Additionally,
the substrate bearing both –CHO and electron‐rich arene, which are
sensitive groups, was smoothly transformed into the desired
product in 62% yield (3k). This protocol could be extended to
geminally disubstituted alkenes, furnishing the trifluoromethylated
products in good yields (3n−p). In the case of internal alkenes, the
desired products (3q and 3r)16 were obtained in satisfied yields.
Moreover, the diene (1s, diethyl 2,2‐diallylmalonate) was subjected
to the standard reaction conditions, as might be expected, giving
the cyclized product syn‐3s in 41% yield,16 which indicates that a
radical cascade happened.
Encouraged by success of the chlorotrifluoromethylation, the
bromotrifluoromethylation of unactivated alkenes was also
achieved in a similar manner using N‐bromophthalimide (NBP) as
the bromine source (Scheme 3). For this reaction, the terminal
monosubstituted and geminally disubstituted alkenes were both
employed in good yields (4a−f). Notably, a range of functional
groups were tolerated.
2c
2d
2b
2a
Mes
O
O
O
N
Cl
N Cl
N
O
Cl
S
N
O
O
O
Me ClO4
Mes-Acr
2e
2f
2g
The initial investigation was focused on the reaction of N‐(but‐3‐
enyl)phthalimide 1a with Langlois reagent and various chlorine
sources (Table 1). Solvents and organic acid additives were first
screened upon irradiation by white LEDs (light emitting diodes)
using 4‐toluenesulfonyl chloride 2a as the chlorine source (see ESI†).
We were delighted to identify DCE (1,2‐dichloroethane) as the best
solvent and TFA (trifluoroacetic acid) as the best acid additive in
terms of chemical yield. Next, an evaluation of different sulfonyl
chlorides (2a−d) and N‐chloroimides (2e−g) showed that N‐
chlorophthalimide (NCP, 2f) was the most effective one (Table 1,
entries 1−7), affording the desired product 3a in good yield (entry 6,
73% yield). It is worth mention that irradiation with blue LEDs also
gave satisfied yield (entry 8). Control experiments revealed that
2 | J. Name., 2012, 00, 1‐3
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DOI: 10.1039/C7CC01903C
O
Mes
Mes-Acr (1 mol%)
CF3SO2Na (2 equiv)
CF3SO2Na
Br
*
Mes-Acr
N
Br
R
CF3
R
NBP (1.2 equiv), DCE, white LEDs
R
N
1
TFA (2 equiv), rt, 16-24 h
O
hv
1
4
NBP
Me
Mes-Acr

SO2
SET
O
Br
O
Br
Br
CF3
CF3
CF3
OHC
O
CF3
CF3
R
N
O
B
3
Mes-Acr
Mes-Acr
Cl
O
CF3
C
R
O
4c, 60%
4b, 71%
4a, 72%
A
SET
N
X
O
O
Br
Br
O
O
O
O
O
N
H
Br
CF3
S
CF3
O
O
N
CF3
NH
X
Cl
O
4f, 75%
4d, 76%
4e, 82%
CF3
O
O
R
D
Scheme 3 Bromotrifluoromethylation of unactivated alkenes.
3 or 4
Scheme 5 Proposed mechanism for the halotrifluoromethylation of alkenes.
Control experiments were performed to gain more insights on
the reaction mechanism (Scheme 4). In the presence of radical
scavenger TEMPO, no desired product was found from the reaction
mixture, along with the formation of the adduct TEMPO‐CF3 5
(Scheme 4, eq 1).17 We further conducted the competitive reaction
using tetrabutylammonium chloride as the chlorine anion source
under the bromotrifluoromethylation conditions, only affording the
brominated product 4c (Scheme 4, eq 2). The result could rule out
the formation of carbocation intermediate B which might be
engaged in Cu‐15a or Ru‐catalyzed15d reactions (Scheme 5).
Inspired by the halotrifluoromethylation of alkenes, we next
explored trifluoromethylthiotrifluoromethylation of unactivated
alkenes using N‐trifluoromethylthiosaccharin 6 as the SCF3 transfer
reagent (Scheme 6).19 For this reaction, p‐toluenesulfonic acid
(PTSA) was determined as the best acid additive. Under the catalytic
reaction conditions, the desired products bearing various functional
groups were achieved from the corresponding alkenes in satisfied
yields (7a−g).
O
O
N
Mes-Acr (5 mol%)
CF3SO2Na (2 equiv)
S
SCF3
O
standard conditions
SCF3
R
CF3
2f
with
R
6 (1.5 equiv), DCE, white LEDs
N
O
N
3a
(1)
+
1
PTSA (2 equiv), rt, 4 h
O
7
TEMPO (2 equiv)
6
not detected
CF3
O
5
O
SCF3
Cl
SCF3
1a
SCF3
H
CF3
Br
N
CF3
N
O
CF3
7
Ph
O
CF3
O
O
O
7c, 64%
7a
, 78%
OHC
OHC
7b, 72%
4c
+
, 34%
standard conditions
(2)
with NBP
O
Ph
Cl
SCF3
SCF3
SCF3
O
CF3
O
CF3
SCF3
n-Bu4NCl (1.2 equiv)
CF3
OHC
3
3
OHC
CF3
TsO
CF3
, 58%
O
1k
3k (not detected)
7g
7f
7d
, 69%
, 64%
7e
, 74%
Scheme 4 Control experiments.
Scheme 6 Trifluoromethylthiotrifluoromethylation of unactivated alkenes.
Reaction conditions: a mixture of 1 (0.4 mmol), CF3SO2Na (0.8 mmol), 6 (0.6
mmol), PTSA (0.8 mmol) and Mes‐Acr+ (5 mol%) in DCE (4 mL) was
irradiated by white LEDs at rt for 4 h. Isolated yields.
Based on the experimental results18 and related reports,11a,15
a
plausible catalytic mechanism is proposed in Scheme 5. The initial
step involves single electron oxidation of Langlois reagent by the
photoexcited Mes‐Acr+ (Mes‐Acr+*), followed by generating a CF3
radical and an acridine radical C. Addition of the CF3 radical to the
alkene 1 leads to the β‐trifluoromethyl carbon‐centered radical A.
The subsequent step is a halogen atom transfer from N‐
halophthalimide to give the halotrifluoromethylation products 3 or
4, as well as the nitrogen‐centered radical D which could oxidize the
acridine radical C by a single electron transfer (SET) process to
restart the catalytic cycle.
In conclusion, we have demonstrated a mild and efficient method
for the preparation of β‐chloro, bromo and trifluoromethylthio
trifluoromethylated compounds from unactivated alkenes via
organic photoredox catalysis upon visible‐light irradiation. The solid
and easy‐handling Langlois reagent, as well as N‐halophthalimide
and N‐trifluoromethylthiosaccharin, are used in the system. This
catalytic protocol is tolerant of a range of functional groups,
displaying potential for further applications in medicinal and
agrochemical research.
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Acknowledgements
Grateful for the financial support from the National Natural
Science Foundation of China (Grant No. 21302134).
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6
7
1
16 The diastereomeric ratio was determined by H NMR or LC‐
MS of the product: 3q (1.2 : 1, dr), 3r (10 : 1, dr), 3s (cis :
trans = 11 : 1), 7g (1.2 : 1, dr).
17 Only trace amount of TEMPO‐CF3 was detected by 19F NMR.
This could be rationalized regarding the potential (vs. SCE):
Eox (CF3SO2Na) = 1.05 V, Eox (TEMPO) = 0.76 V.
18 The fluorescence quenching experiments could also support
the proposed mechanism (see ESI†).
19 X. Shao, C. Xu, L. Lu and Q. Shen, Acc. Chem. Res., 2015, 48
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1227.
4 | J. Name., 2012, 00, 1‐3
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