Metal-free radical trifluoromethylation of β-nitroalkenes through visible-light photoredox catalysis

Article abstract of DOI:10.1039/c7cc02589k

A catalytic method for functional group interconversion is immensely important in modern sciences. Here, we report an efficient catalytic conversion of nitroalkenes to highly stereoselective 1-trifluoromethylalkenes at room temperature. This unprecedented metal-free photocatalytic strategy is simple and operates under visible-light irradiation using the commercially available CF₃ source.

Full text of DOI:10.1039/c7cc02589k

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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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Metal-Free Radical Trifluoromethylation of β-Nitroalkenes
through Visible-Light Photoredox Catalysis‡
Siba P. Midya,1,2† Jagannath Rana,1,2† Thomas Abraham,1 Bhaskaran Aswin,1 and Ekambaram
Balaraman*,1,2
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Catalytic method for functional group interconversion is possessing broad substrate scope under mild reaction
immensely important in modern science. Here, we report an conditions is highly desired and synthetically very demanding.
efficient catalytic conversion of nitroalkenes to highly
O
Cl
stereoselective 1-trifluoromethylalkenes at room temperature.
This unprecedented metal-free photocatalytic strategy is simple
and operates under visible-light irradiation using the commercially
available CF3 source.
CF3
HN
O
O
N
Cl
O
S
CF3
HN
Me
CF3
N
O
H2N
N
OH
Viroptic (Antiviral agent)
HO
Anti-inflammatory agent
Anticancer agent
The trifluoromethyl group (-CF3) constitutes a very important
chemical entity found in pharmaceuticals, agrochemicals, and
material sciences mainly because of its excellent metabolic
stability and excessive lipophilicity.1-2 Undoubtedly, the
introduction of the CF3 group into organic scaffolds can change
the physical properties of molecular substances, such as
solubility, membrane permeability.3 Consequently, immense
efforts have been devoted to incorporating the trifluoromethyl
F3C
H2N
N
F5
N
CN
Cl
H
CO2H
F
N
CN
Aminotransferase inhibitor
CF3
CF3
O
CF3
Cl
Cl
Herbicide
Insecticide analogs
F3C
CN
O
O
CF3
group into organic molecules. Despite
a
significant
O
OH
breakthrough on trifluoromethylation of arenes,4 examples
that describe the synthesis of 1-trifluoromethylalkenes is very
limited in the contemporary science.4o,5 Transition-metal
catalyzed trifluoromethylation of vinylboronic acids have
independently been developed by Liu,6 Shen,7 and Buchwald.8
However, considering the difficulties in the preparation of
vinylboronic acids and their derivatives, Hu9 and Liu10 et al.,
reported the transition-metal catalyzed decarboxylative
trifluoromethylation of α,β-unsaturated carboxylic acids using
the Togni reagent and Langlois reagents, respectively.
Recently, trifluoromethylation of β-nitroalkenes promoted by
a stoichiometric amount of iron(III)salts at 120 oC using the
Togni reagent as CF3-source was reported.11 In this context, a
new efficient method for constructing Cvinyl-CF3 bonds,
Cl
Me
Cyhalothrin Pesticide
Me
NH
O
Panomifene
Antiestrogenic agent
Scheme 1. The importance of 1-trifluoromethylalkenes.
In recent times, a visible-light mediated photoredox catalysis is
emerging has a powerful synthetic tool in chemical sciences.
MacMillan and co-workers developed an elegant approach to
trifluoromethylation of arenes under visible light irradiation at
room temperature using the Ru-based polypyridyl complex as
a
photoredox catalyst and CF3SO2Cl as a source of
trifluoromethyl group.12 Subsequently, fewer methods were
developed for trifluoromethylation under visible-light
photoredox catalysis.13 However, the use of expensive metal
catalysts (Ru and Ir), and the problems involved in removing
potential toxic metal impurities from the final product are
some common concerns. Hence, development of alternative
metal-free trifluoromethylation performed under mild and
practical conditions is highly desirable and very demanding.
Here, we report an operationally simple, efficient transition-
metal-free stereoselective trifluoromethylation of β-
1Catalysis Division, Dr.Homi Bhabha Road, CSIR-National Chemical Laboratory
(CSIR-NCL), Pune - 411008, India.
2Academy of Scientific and Innovative Research (AcSIR), New Delhi - 110025, India.
E-mail: eb.raman@ncl.res.in; balaramane2002@yahoo.com
†Equal contribution.
‡Electronic Supplementary Information (ESI) available: [details of experimental
procedure, characterization of new compounds and copy of NMR data]. See
DOI:10.1039/x0xx00000x
nitroalkenes
using
the
commercially
available
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trifluoromethylating source. To the best of our knowledge, no at increased temperature (60 °C) didn’t yield the desired
example of the room temperature metal-free catalyzed direct product, which excludes homolytic bond cleavage of the
formation of the Cvinyl-CF3 bond has been reported under starting material to trifluoromethyl radical under these
photoredox conditions.
conditions. Notably, no Z-isomer was observed under our
standard conditions (19FNMR spectroscopy).
Table 1. Optimization of the reaction conditions.a
Scheme 2. Various s strategies to 1-trifluoromethylalkenes.
We began our direct stereoselective trifluoromethylation of β-
nitroalkenes with an evaluation of a range of photoredox
catalysts, solvents, light source, and additives using (E)-1-
methoxy-4-(2-nitrovinyl)benzene 1a as a benchmark substrate.
aUnless otherwise noted, the reaction performed under an inert atmosphere of argon.
Reaction conditions: 1a (0.125mmol), CF3SO2Cl (0.3mmol), photocatalyst (5 mol%),
The commercially available and relatively economical CF3SO2Cl additive (0.3 mmol) and solvent (0.5 mL) under visible-light irradiation at room
temperature for 32 h. bYield determined by GC using m-xylene as an internal standard.
cReaction under direct sun-light. dReaction performed under dark condition. NR = No
(2) was used as a CF3-source (Table 1). Upon visible-light
irradiation in the presence of catalytic amount of photoredox
catalyst and K2HPO4 as additive under argon atm at room
temperature enabled (E)-selective 3a in 74% yield (Table 1,
entry 2). A series of photoredox catalysts was examined, and
among them, eosin-Y was found to be an optimal
photocatalyst for this transformation (Table 1, entries 1-6).
The necessity of each of the key reaction components
(photoredox catalyst, additive and light source) was
demonstrated (Table 1, entries 7-9). Significantly, the choice of
additive was found to have a dramatic effect on the yield of 3a
and with K2HPO4 proving to be optimal for the present strategy
(Table 1, entries 1, and 10-12). We believe that a weakly Lewis
basic nature of K2HPO4 would facilitate the formation of CF3
radical.12-13 We have observed that the use of CH3CN as the
reaction.
Having identified optimal conditions for a metal-Free, room-
temperature, radical trifluoromethylation of β-nitroalkenes,
we sought to define the scope of β-nitroalkenes 1. As shown in
Table 2, the present metal-free catalyzed the direct formation
of Cvinyl-CF3 bond strategy displayed high functional-group
tolerance and proved to be an efficient method for the
preparation of E-selective 1-trifluoromethylalkene in good to
excellent yields (up to 81%) under very mild conditions.
Notably, the addition of the trifluoromethyl radical is viable
when aromatic substituents are present, due to the easy
formation of benzyl radicals. Thus, the present strategy is
highly applicable to β-nitrostyrene derivatives. The β-
nitrostyrene bearing electron-donating groups at the meta and
para position of the phenyl ring, such as methyl, methoxy and
thiomethyl proceeded smoothly and offered the
corresponding 1-trifluoromethylalkenes in good yields (Table
reaction medium was found to provide an excellent yield of 3a
.
Further screening using common solvents, including apolar
arenes (toluene and o-xylene), or polar Et2O and DMSO proved
ineffective. Notably, other commonly employed solvents in
photoredox catalysis, such as DMF, MeOH, THF and DCE
afforded the product 3a in 0%, 31%, 39%and 27% respectively
(Table 1, entries 13-15). While trace amounts of the product
were formed in the absence of additive; no reaction was
observed in the absence of photoredox catalyst as well as
upon exclusion of light (Table 1, entries 7-9). Other
commercially available trifluoromethylating reagents (Langlois
reagent, Togni reagent, CF3CO2Na and (CF3CO)2O; see Scheme
2b) were examined and among them, CF3SO2Cl was found to
be effective for this transformation. Reactions in the dark and
2, products 3a-3c, 3i-3j, and 3m). Various halides substitutes
are also well tolerated and gave the desired product in good
yields (products 3e in 70%, 3f in 63%, 3k in 67%, and 3l in
66%). Indeed, electron-withdrawing group substituted β-
nitroalkene showed better reactivity and to afford the
expected (E)-1-trifluoromethylalkenes (3h) in 81% yield. These
results indicate that the electronic nature of the nitroalkenes
has considerable influence on our room-temperature metal-
free trifluoromethylation strategy. However, a substituent at
the ortho position of the phenyl ring (1-methoxy-2-(2-
2 | J. Name., 2012, 00, 1-3
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nitrovinyl)benzene) gave desired CF3-product in very poor yield be involved in the catalytic cycle. Furthermore, the yield of 3a
6% yield) with 87% recovery of starting material, which could was completely dropped when no photoredox catalyst was
(∼
be explained by steric reasons. The reaction of 2-nitrovinyl present in the reaction and/or under dark conditions (Table 1,
naphthalene also proceeded well, yielding the desired product entries 8-9). A competition experiment with a different
up to 64% yield. Thus, the reaction of 1n-1p under optimal electronic substituent (para position of the phenyl ring) on β-
conditions gave the corresponding (E)-1-trifluorometylated nitroalkenes revealed that electron-withdrawing groups
products 3n (60%),3o (59%), and 3p (64%), respectively. We facilitate the reaction. This finding can be rationalized
further applied the optimized reaction conditions to different regarding stabilization of the benzylic radical by strong
heterocyclic, substituted nitroalkenes and are amenable to this electron-withdrawing group (Scheme 4c). Notably, under an
trifluoromethylation protocol. Thus, treatment of 1q and 1r optimal condition, other alkenes such as styrene, β-bromo,
under
optimal
conditions
gave
the
expected and β-carboxy-styrene failed to yield the product 3a. This
trifluoromethylated product 3q in 66% and 3r in 65% yields. result clearly indicates that denitration is very crucial in the
Notably, under optimal condition aliphatic and arylaromatic catalytic cycle and the nitro radical does not facilitate the
nitroalkenes
(1s-1t)
failed to yield the corresponding radical chain process.15
trifluoromethyled products, due to less stability of the carbon-
centred radical (see Scheme 5).
Table 2.Stereoselectivetrifluoromethylationof β-nitroalkenes under metal-free
(a) Radical trapping experiment
Eosin-Y (5 mol%)
CF3
NO2
photoredox catalysis.a,b
K2HPO4 (2.5 equiv)
CF3SO2Cl
+
visible light
MeCN / Ar atm / 24 h
MeO
MeO
(b) TEMPO:CF3 adduct
2
1a
3a (0%)
Eosin-Y (5 mol%)
TEMPO
K2HPO4 (2.5 equiv)
NO2
CF3
CF3SO2Cl
+
R
R
CH3
CN, Ar at
m
Eosin-Y (5 mol%)
3
1
2
+
CF3SO2Cl
(by GC-MS)
RT
visible light
MeCN / Ar atm / 8 h
N
O
N
O
2
CF3
CF3
CF3
CF3
CF3
4
(c) Intermolecular competition experiment
NO2
Me
MeS
MeO
3b
3c
3d (68%)c
3a (74%)
3e (70%)
(71%)
(73%)
MeO
std. condition
12 hr
CF3
1a
CF3
+
CF3
CF3
CF3
CF3
O2N
CF3SO2Cl
2 (1 equiv)
O2
N
MeO
NO2
3a
(6%)
3h (14%)
F
Cl
Ph
3a 3h
:
(
= 1:2.3)
CF3
O2
N
3h
3g
(81%)
(69%)
3f
(63%)
1h
(d) via denitration
CF3
CF3
CF3
CF3
X
std.condition
+
CF3SO2Cl
F
MeO
MeO
2
3a
Br
Cl
3k
Me
OMe
yield of 3a
X
3l
(66%)
3j
(67%)
CF3
3i (60%)
(63%)
H
0%
0%
Br
CF3
trace
CO2H
NO2
CF3
CF3
74%
OMe
MeO
MeO
Scheme 4. Mechanistic studies.
OMe
3m (69%)
3p (64%)
3n (60%)
3o
(59%)
Ph
In the cyclic voltammetric study, the observed reduction
potential of β-nitroalkene (1d) (-0.968 V and -0.848 V vs. SCE)14
is quite lower than the reduction potential of triflyl chloride (-
0.81 V vs. SCE).12 Furthermore, the Stern-Volmer fluorescence
quenching experiment illustrates that the excited state of
CF3
CF3
CF3
S
3q (66%)
S
CF3
3r (65%)
3s (trace)d
3t (n.d)
aUnless otherwise noted, the reaction performed under an inert atmosphere of argon.
Reaction conditions: 1a-t (0.25mmol), (0.625mmol), eosin-Y (5 mol%), K2HPO4
2
eosin-Y* is quenched in the presence of triflyl chloride (
2
) The
Stern-Volmer constant of
2
and β-nitroalkene (3d) are 71 x 10-3
(0.625mmol), and CH3CN (1 mL)under visible-light irradiation at room temperature.
bAverage yields of two independent reactions. cGC yield. dBased on 19F NMR. n. d: not
detected.
and 3.4 x 10-3, respectively (Figure 2).14 These results clearly
support the oxidative quenching pathway in the catalytic cycle.
To our delight, we have successfully shown the scalability and
practical viability of this catalytic protocol under standard
conditions.14 To gain an insight into the course of the reaction,
we conducted a series of control experiments (Scheme 4).
When we performed the reaction in the presence of the
radical
scavenger
2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO), the reaction was completely inhibited. To our
delight, performing the reaction in the presence of eosin-Y,
CF3SO2Cl, and TEMPO under standard conditions yielded the
trifluoromethyl-TEMPO product, which was detected on GC-
MS. These results indicate that radical reaction pathway could
Figure 2. Eosin-Y emission quenching with CF3SO2Cl and 4-fluoro-nitroalkene.
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Based on preliminary results and known literature,12-13,15
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plausible mechanism for the trifluoromethylation of β-
nitroalkenes is shown in Scheme 5. The photocatalyst eosin-Y
absorbs light in the visible region (λmax = 539 nm) to give a
photoexcited singlet state, 1eosin-Y*, which undergoes
intersystem crossing to form the long-lived triplet state 3eosin-
2. (a) P. Kirsch, Modern Fluoroorganic Chemistry: Synthesis Reactivity,
Applications; Wiley-VCH: Weinheim, 2005; (b) I. Ojima (Ed.), Fluorine in
Medicinal Chemistry and Chemical Biology; Wiley-Blackwell:
Chichester, 2009.
Y*. The reaction proceeds first via single-electron reduction
red
(SET) of triflyl chloride (2, E1/2 = −0.81 V vs. SCE) by the
3. (a) M. Shimizu and T. Hiyama, Angew. Chem. Int. Ed., 2005, 44, 214-
231; (b) O’Hagan, Chem. Soc. Rev., 2008, 37, 308-319; (c) T. Liang, C.
N. Neumann and T. Ritter, Angew. Chem. Int. Ed., 2013, 52, 8214-
8264.
3eosin-Y*. The radical anion of
2 immediately collapsed and
•
generates a more stable CF3 radical with the release of SO2
and chloride, which is an entropically driven process.12 The
radical addition of trifluoromethyl radical to β-nitroalkene
4
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Subsequently, oxidation of the radical intermediate
Eosin-Y radical cation to afford the expected 1-
1
produces carbon-centred radical
(
D
).
D
by the
(C)
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14. See the ESI.†
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