Sunlight-driven trifluoromethylation of olefinic substrates by photoredox catalysis: A green organic process

Article abstract of DOI:10.1016/j.crci.2015.01.013

The principles and utility of photoredox catalysis in organic synthesis are described. After a brief description of the features of the two types of catalytic photoredox processes following the reductive quenching cycle (RQC) and the oxidative quenching cycle (OQC), the discussion is focused on organic transformations based on OQC, in particular the trifluoromethylation of olefinic substrates with electrophilic trifluoromethylating reagents furnishing solvolytic addition products and substitution products. It is concluded that catalytic photoredox systems are green from the point of view of harmfulness, safety, and energy source (visible light, including sunlight). Future prospects of photoredox catalysis will be also discussed.

Full text of DOI:10.1016/j.crci.2015.01.013

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Contents lists available at ScienceDirect
Comptes Rendus Chimie
www.sciencedirect.com
South Europe-Japan Joint Forum on inorganic chemistry and its interfaces
(Strasbourg, France, 17 & 18 October 2014)
Sunlight-driven trifluoromethylation of olefinic substrates
by photoredox catalysis: A green organic process
*
Munetaka Akita , Takashi Koike
Chemical Resources Laboratory, Tokyo Institute of Technology, R1-27 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
The principles and utility of photoredox catalysis in organic synthesis are described. After a
brief description of the features of the two types of catalytic photoredox processes
following the reductive quenching cycle (RQC) and the oxidative quenching cycle (OQC),
the discussion is focused on organic transformations based on OQC, in particular the
trifluoromethylation of olefinic substrates with electrophilic trifluoromethylating
reagents furnishing solvolytic addition products and substitution products. It is concluded
that catalytic photoredox systems are green from the point of view of harmfulness, safety,
and energy source (visible light, including sunlight). Future prospects of photoredox
catalysis will be also discussed.
Received 26 December 2014
Accepted after revision 29 January 2015
Available online xxx
Keywords:
Photoredox catalysis
Trifluoromethylation
Visible light
Olefinic substrates
´
ß 2015 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
respectively, has been studied to a considerable extent
[3]. Such studies are invaluable from the viewpoint of
The sun provides huge and inexhaustible energy to the
Earth, andthemajorcomponentisvisiblelight. Itisneedless
to say that plants utilize the energy for their growth as well
as for oxygen evolution [1]. Many scientists in various fields
have made challenges to analyze and understand the
mechanisms of the complicated biological and chemical
systems and to develop chemical systems that mimic
functions and reactions occurring therein, occasionally
called‘‘artificialphotosynthesis’’[2]. Inthefield ofinorganic
chemistry, electron transfer and energy transfer have been
the major concerns and many excellent chemical systems
have been developed so far [3]. In contrast, little attention
has been paid to applications to catalytic transformations
and attempts have been limited. For example, the
reduction of small inorganic molecules such as proton and
carbon dioxide giving hydrogen and carbon monoxide,
energy storage, but much less attention has been paid to
catalytic organictransformations. Althoughattemptsatthem
have been made sporadically and the obtained results have
been reported [4], they have not been studied systemati-
cally until recently. Taking into account this situation, we
have made challenges toward developing catalytic organic
transformations with two different strategies. One is
photoredox catalysis (For selected reviews on photoredox
catalysis see [5]), which is the subject of the present
account, and the other is bimetallic photocatalysis, which
was reviewed previously [6].
On the other hand, introduction of fluorine functional
groups to organic skeletons causes dramatic changes of
their properties such as liposolubility, water repellency,
stability, and heat resistance and, in particular, various
medical and agricultural fluorinated chemicals have been
developed and launched by making use of these favorable
properties[7]. Forthesepurposes, many kindsoftechniques
for fluorination of organic compounds have been developed
and even now new fluorination methods are demanded.
*
Corresponding author.
E-mail address: matika@res.titech.ac.jp (M. Akita).
http://dx.doi.org/10.1016/j.crci.2015.01.013
1631-0748/ß 2015 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: Akita M, Koike T. Sunlight-driven trifluoromethylation of olefinic substrates by
photoredox catalysis: A green organic process. C. R. Chimie (2015), http://dx.doi.org/10.1016/j.crci.2015.01.013

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We combined these two problems and set up develop-
ment of novel trifluoromethylation methods of organic
compounds involving carbon–carbon bond formation by
the sunlight- or visible light-promoted photoredox cataly-
sis as one of our research programs several years ago.
2. Photoredox catalysis
It has been recognized that photoreactions follow
pathways different from those of thermal reactions so as
to accelerate the reaction or even furnish different
products [8]. Most of organic compounds, however, are
colorless and, as a result, they cannot absorb sunlight (i.e.
visible light). If one wants to utilize the sunlight energy
(visible light), some photosensitizer should be added to
absorb visible light. There are many kinds of photosensi-
tizers ranging from organic dyes to metal complexes, and
we have employed metal photosensitizers such as
[Ru(bipy)3]2+ salts (1a in Fig. 1) and the isoelectronic
cyclometalated pyridylphenyliridium complexes (e.g.,
Ir(ppy)3 (1b)) [9]. Because their photophysical properties
have been studied for many years, their features, including
those of the photo-excited states, are now understood in
detail. They have intense absorption bands in the region of
visible light and the lifetime of the photo-excited states is
of the order of the microsecond, which is long enough to
interact with and activate organic molecules.
Scheme 1. (Color online.) Photoredox cycles: oxidative and reductive
quenching cycles.
designed, a donor D and an acceptor A in the reaction
system can undergo 1e-oxidation and 1e-reduction to
form D+ and Aꢀduring the catalytic cycle, respectively. This
cycle is called reductive quenching cycle (RQC), because the
photoexcited species M* is reduced in the first step. But
readers should note that the external substrate is oxidized
in the first step of RQC. The catalytic cycle can also be
initiated by transfer of the higher energy electron of the
photoexcited species to an acceptor followed by oxidation
of an external substrate, and this cycle is called oxidative
quenching cycle (OQC; the lower cycle in Scheme 1).
This photoredox system has been utilized for many
transformations, including the reduction of small inorgan-
ic molecules as mentioned above. In those systems,
however, sacrificial reducing/oxidizing reagents (e.g.,
trialkylamine) were added to supply/remove electrons
to/from the reaction substrate. In the present photoredox
systems, however, if the unique redox system can be
combined with chemical reactions demanding these two
redox functions, no sacrificial redox reagents needs to be
added, in other words, the system is redox-neutral, as it will
be described in the following sections.
Catalytic photoredox reactions developed in our
laboratory during the last several years are summarized
in Scheme 2 [10] (For a review, see also [5l-o]) and are
divided into two categories, where the reactions follow
either RQC or OQC. All reactions involve C-centered organic
radicals as key intermediates, and we have found that the
OQC system is particularly useful for trifluoromethylation of
olefinic substrates (reactions 2b–g), which is the main
subject of the present account. Before discussion of
trifluoromethylation, one typical example following RQC
will be briefly described in the next section.
The key concept of photoredox catalysis is summarized
in Scheme 1 [5]. The first event occurring upon visible light
irradiation is metal-to-ligand charge transfer (MLCT) to
form the singlet photo-excited species, which is converted
into the triplet excited species (M*) via intersystem
crossing in an almost quantitative yield. The resulting
triplet species with a
ms-order lifetime has a hole in the
lower SOMO (the solid oval line) and a higher energy
electron in the higher SOMO (in the dotted oval line),
which can serve as an oxidant and a reductant, respective-
ly. If an electron donor is present nearby (the upper cycle in
Scheme 1), electron transfer from the donor to the hole
generates the anionic metal species (Mꢀ) together with the
oxidized cationic organic species (D+). The resultant
anionic metal species may be oxidized by an acceptor in
the reaction system to generate the reduced acceptor (Aꢀ)
in addition to the original ground-state neutral metal
species M. Thus if an appropriate reaction system can be
3. Reactions mediated by reductive quenching cycle
(RQC) [10a–d]
Generation of organic radicals under mild reaction
conditions has remained to be a problem to be solved even
now. During the course of our study, we revealed that
hydrocarbyl radicals can be readily generated from the
corresponding organoborates by the action of photoredox
Fig. 1. Structures and photophysical properties of representative
photoredox catalysts.
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Scheme 2. (Color online.) Photoredox-catalyzed reactions developed in our laboratory.
catalysis [10d] When a mixture of potassium organotri-
fluoroborate 2 (K[R–BF3]) and enone derivatives 3 dis-
solved in an acetone–MeOH mixed solvent containing
1 mol % of the iridium catalyst 1c was irradiated by blue
LED lamps, Giese-type reaction proceeded to give the C–C
coupling products 4 in good yields (Scheme 3).
The reaction cannot be driven by [Ru(bipy)3](PF6)2
1aꢁPF6, but they can be by the iridium catalyst 1c. The
difference can be ascribed to the different reduction
potentials of the photo-excited species (0.43 V (1a) vs.
0.91 V (1c); cf. oxidation potential for K[PhCH2–BF3]:
0.67 V); the reduction potential of the photo-excited
Scheme 3. Giese-type reaction of organoborates.
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iridium catalyst 1c is enough to oxidize the organoborate 2,
but that of the ruthenium catalyst 1aꢁPF6 is not. As for the
scope for the enone derivatives 3, various kinds of
substrates including cyclic ones can be applied. On the
other hand, as for the scope for the organoborates 2,
tertiary alkyl and aryl derivatives gave the products in
good yields, while secondary and primary alkyl derivatives
turned out to be sluggish, presumably because of the
instability of the corresponding alkyl radical intermedi-
ates. In these cases, however, the replacement of the
fluoride substituents attached to the boron atom by less
electronegative oxygen groups (B(OCH2)3CCH3; triolbo-
Scheme 5. Generation of organic radicals via RQC and OQC.
rate) improved the yields to
a considerable extent.
Heteroaryltriolborates and -heterosubstituted methyl
derivatives (K[X–CH2–BF3]; X= OR [10][10l], NR2 [10m])
also afforded the desired products in good yields.
a
by blue LED lamps in the presence of the iridium
photosensitizer 1b for 2 h, 1-aryl-3,3,3-trifluoropropanol
7 was obtained in an almost quantitative yield as a result of
regiospecific difunctionalization (oxytrifluoromethylation)
of styrene, i.e. the CF3 and OH groups are introduced to the
The reaction mechanism can be interpreted as in
Scheme 4. The photoexcited iridium species Ir* undergoes
1e-transfer from borate 2 to generate the organic radical
Rꢁand the reduced metal species Irꢀ. The radical adds to the
b
- and a-carbon atoms of styrene, respectively, in a
specific manner (Scheme 6). In this case, all examined Ru
and Ir photosensitizers work well, but we prefer to use the
neutral species Ir(ppy)3 1b because of its solubility in
organic solvents. As will be described below, a significant
solvent effect is observed for the present transformation,
and such an effect can be studied only with such a soluble
catalyst as 1b.
b-carbon atom of enone 3 to generate the radical
intermediate, which is reduced by Irꢀto give the enolate
intermediate and the original ground-state metal catalyst.
Final protonation of the enolate furnishes the product 4. As
is evident from the mechanism, no sacrificial redox reagent
is needed and thus it turns out that the present catalytic
photoredox system is redox-neutral.
Scheme
oxytrifluoromethylation, which can be applied to many
kinds of styrene derivatives including naphthyl, - and
6 also shows the scope of the present
a
b-
substituted styrenes, and even vinyl ether. Simple olefin
such as 1-octene, however, turned out to be sluggish. In
other words, olefin substituents, which can stabilize the
radical at the
a-position, such as aryl groups and hetero-
functional groups are essential for the present transforma-
tion.
When the reaction was carried out in the presence of
oxygen nucleophiles in place of H2O, various oxytrifluor-
omethylated products 7 such as ether (from alcohol) and
ester (from carboxylic acid) were obtained in good yields
(Scheme 6).
Scheme 4. (Color online.)
A plausible reaction mechanism for the
Giese-type reaction.
As described in the introductory part, we planned to
develop catalytic organic transformations promoted by
sunlight. Then we carried out the reaction under daylight
and obtained the products in yields comparable to those
obtained by irradiation with blue LED lamps (Scheme 7).
Although we did not examine all reactions under daylight,
all reactions we examined under daylight successfully
afforded the desired products in good yields.
In order to obtain information on the reaction mecha-
nisms, a couple of experiments were conducted. First, CV
analysis revealed that Umemoto’s reagent 5 (Eox = –0.75 V)
could be readily oxidized by the photoexcited species of
the metal catalyst (e.g., Ered(1b) = –2.14 V). Second,
4. Reactions mediated by oxidative quenching cycle
(OQC) [10e–k]
The catalytic reaction following RQC indicates that
organic radicals can be generated by 1e-oxidation of
electron-rich organic substrates. Taking into consideration
this result, it is readily expected that organic radicals may
also be generated by 1e-reduction of electron-deficient
substrates via OQC (Scheme 5). Then we examined the
generation of the trifluoromethyl radical from electrophilic
trifluoromethylating reagents such as Umemoto’s reagent 5
(trifluoromethylsulfonium salt of dibenzothiophene) [11]
and Togni’s reagents 6 (hypervalent iodine species) [12].
b
-pinene with the bicyclic structure afforded a ring-
opened product via a ring-opening process of the strained
tertiary radical intermediate. Finally, Stern–Volmer plots
revealed that luminescence from the photoexcited iridium
species Ir* was not quenched by styrene but by Umemoto’s
reagent 5. These experimental results suggest the plausible
mechanism based on OQC summarized in Scheme 8. The
photoexcited metal species Ir* transfers one electron to
4.1. Oxytrifluoromethylation of olefins giving 1-aryl-3,3,3-
trifluoropropanols [10f]
When a mixture of styrene and Umemoto’s reagent 5
dissolved in an acetone–H2O mixture (9 = 1) was irradiated
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Scheme 6. Oxytrifluoromethylation of olefins.
5 to form the oxidized metal species Ir+ together with
trifluromethyl radical (after elimination of dibenzothio-
Scheme 4, the initial redox events are opposite but the
sequential oxidation/reduction processes make both cata-
lytic transformations redox-neutral.
phene), which adds to the
b-carbon atom of the styrene
derivative to give the stable benzyl radical intermediate
8. Subsequent 1e-transfer from the radical to Ir+ regener-
ates the ground state Ir catalyst 1b and the benzyl cation
intermediate 9, to which oxygen nucleophiles add to
furnish the oxytrifluoromethylated product 7 after depro-
tonation. When compared with the mechanism shown in
An intramolecular version of the present catalytic
transformation provides synthetic methods for trifluor-
omethylated lactone derivatives from alkenoic acids
(reaction 2c in Scheme 2) [10g]. In addition, combination
of the oxytrifluoromethylation with acidic dehydration
provides a new substitution method of a vinylic hydrogen
atom by a CF3 group (10), and this process was applied to
the synthesis of a precursor of panomifene, an anti-
estrogen drug (Scheme 9) [10f].
4.2. Aminotrifluoromethylation of olefins giving amides of
1-aryl-3,3,3-trifluoropropylamine [10h]
When the oxytrifluoromethylation of olefins was
carried out in the presence of nitrile and H2O, aminotri-
fluoromethylation occurred to give amides of 1-aryl-3,3,3-
trifluoropropylamines 11 (Scheme 10). The reaction
Scheme 7. (Color online.) Sunlight-promoted oxytrifluoromethylation of
styrene.
Scheme
8. (Color
online.)
A
plausible
mechanism
for
oxytrifluoromethylation.
Scheme 9. Formal substitution of a vinylic hydrogen atom by a CF3 group.
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reagent 5 was used in place of 6a, DMSO-adduct 13 (35%)
was formed in addition to 12 (45%) (entry 2). Further
irradiation of the mixture for 20 min caused complete
conversion of 13 into 12 (86%) (entry 2’). Furthermore,
when [Ru(bipy)3](PF6)2 1aꢁPF6 was used in place of 1b, the
DMSO-adduct 13 was formed exclusively in 92% yield, but
the reaction was slow. It took 2 h for the complete
conversion of styrene (entry 3). Further irradiation of
the reaction mixture caused conversion of only a part of 13
into 12 (entry 3’). However, when NEt3, a base, was added
to the reaction mixture of entry 3 containing 13 as the
dominant component, 13 was converted to 12 spontane-
ously and quantitatively (entry 3’’). Quantitative conver-
sion of 13 to 12 was also observed, when a catalytic
amount of Ir(ppy)3 1b was added to the reaction mixture of
entry 3 and then the mixture was further irradiated for 1 h
(entry 3’’’). The reaction with 1a and 6a was slow but
selective (entry 4).
Scheme 10. (Color online.) Aminotrifluoromethylation of olefins.
mechanism can be interpreted in terms of a Ritter-type
mechanism, where the carbocationic intermediate 9 is first
trapped by nitrile and the resultant adduct is hydrolyzed
by H2O.
The present aminotrifluoromethylation turns out to be
tolerable with many functional groups such as halogens,
carbonyl and amino groups, and even Bpin group and,
therefore, can serve as a late-stage trifluoromethylation
method for bioactive compounds with many functional
groups and a vinyl group, i.e. a trifluoromethyl group can
be introduced to the vinyl part at a final stage of a sequence
of transformations.
The following features are noted for the present
reaction. (i) For the trifluoromethylating reagents, Togni’s
reagent 6a is superior to Umemoto’s reagent 5. (ii) For the
photosensitizers, Ir(ppy)3 catalyst 1b is superior to
[Ru(bipy)3]2+ catalyst 1aꢁPF6. (iii) The DMSO-adduct 13
is an intermediate for 12. (iv) Two mechanisms are
operating for the conversion of 13 into 12; one is a
base-promoted Kornblum oxidation-type mechanism and
the other is photoredox catalysis.
4.3. Ketotrifluoromethylation of olefins giving 1-aryl-3,3,3-
trifluoropropan-1-one [10i]
b
-Alkyl-substituted styrenes were also converted into
the corresponding 2-alkyl-3,3,3-trifluoropropiophenone
120. To our surprise,
-alkyl-substituted derivatives were
a
During the course of our study, we found an intriguing
solvent effect for the trifluoromethylation of olefins, i.e.
ketone derivatives are obtained when the reactions were
carried out in DMSO. It turned out that the reaction
depended not only on the photocatalysts, but also on the
trifluoromethylating reagents.
When a DMSO solution of styrene and Togni’s reagent
6a was irradiated in the presence of Ir(ppy)3 1b (5 mol%) by
blue LED lamps for 10 min (in an NMR tube), 3,3,3-
trifluoropropiophenone 12 was formed in 75% yield as a
sole product (entry 1; Scheme 11). But when Umemoto’s
converted to 12 via a C–C bond cleavage process, and the
eliminated alkyl groups were detected as sulfonium salts
14 (Scheme 12).
The above-mentioned features are explained by the
reaction sequence shown in Scheme 13. The photoredox
catalysis first generates the benzyl cation-type intermedi-
ate 9 (Scheme 8), which is captured by DMSO to form the
adduct 15.
In the presence of a base, 15 is converted to 12 via a
Kornblum oxidation type-mechanism (path i), i.e. deproto-
nation from the methyl group in the SMe2 part, followed by
Scheme 11. Ketotrifluoromethylation of styrene.
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For the alternative photoredox mechanism—path (ii)—,
we have no experimental evidence. Our proposed mecha-
nism involves a 1e-reduction of 15 by the photo-excited Ir*
species followed by a sequence of S–O bond cleavage, C-to-
O migration of the R group, and oxidation by Ir+, giving the
cationic intermediate 16, which is converted into the
product 12 via O–H/Me bond heterolysis induced by SMe2
(generated in the step of the S–O bond cleavage). It is
noteworthy that, in the case of
a-alkylstyrene, some
photoredox catalysis operates for the C–C bond cleavage
process.
Thus we observed remarkable solvent effects for the
present solvolytic trifluoromethylation of olefins. It should
be noted that, by simply changing the solvent, olefins
undergo oxytrifluoromethylation (reactions 2b,c in
Scheme 2), aminotrifluoromethylation (reaction 2d), and
ketotrifluoromethylation (reaction 2e) to give the
functionalized trifluoromethylated derivatives.
4.4. Trifluoromethylation of olefinic substrates via
substitution reactions [10j,k]
Scheme 12. Ketotrifluoromethylation of olefins.
The above-mentioned mechanisms of the solvolytic
trifluoromethylation reactions involving the cationic
intermediates prompted us to examine substitution
reactions of olefinic substrates via deprotonation from
the cationic intermediate 9.
We first examined direct trifluoromethylation of olefins
with various CF3 sources and photoredox catalysts, but it
turned out to be unsuccessful. Then we designed a
coupling reaction between the electron-deficientꢁCF3
radical and the electron-rich alkenylborates 17, K[R–
CH=CH–BF3], which proceeded efficiently to give the
substituted products 18 (Scheme 14) [10j]. We also found
that 1,1-diarylethene derivatives underwent direct tri-
fluoromethylation without activation by the borate
auxiliary (Scheme 15) [10k]. The cationic intermediate is
formed by following the processes explained above, and
subsequent deboronation or deprotonation from the
intramoleculardeprotonationof thebenzylichydrogenatom
associated with the elimination of SMe2, leads to 12. This
reaction pathway is supported by theobservationthat, when
the reaction was carried out in DMSO-d6, CD3SCD2H was
formed as confirmed by the characteristic quintet 1H NMR
signal for the CD2H group. Because the reaction proceeds in
the absence of a base and in the presence of the iridium
catalyst (entry 3’’’), some photocatalytic mechanism—path
(ii)—should also work (see below).
The extremely fast reaction observed when Togni’s
reagent 6a is employed as the CF3-source (entry 1) can be
ascribed to o-iodobenzoate, the byproduct (Y) of the initial
OQC cycle, which works as a base to accelerate path (i). In
the case of the reaction of a-alkylstyrenes, the alkyl group
at the benzylic position in 15 cannot be removed by the
action of a base—path (i)—and, therefore, some other
mechanism should operate.
b
-position of 9 should furnish the substituted products
18 and 19 (Scheme 16). The reaction of borates 17 should
be promoted by the attractive electrostatic interaction
between the two reagents as designed above, and the
driving force for the 1,1-diarylethene should be formation
of the very stable diarylmethyl radical.
Scheme
13. (Color
online.)
Plausible
mechanisms
for
ketotrifluoromethylation.
Scheme 14. Trifluoromethylation of alkenylborates.
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Scheme 16. A mechanism for the formation of the substitution products.
the cationic +CF3 species to unsaturated bonds [13] (See
also [11c]). Instead, in the present system, the -CF3-
b
substituted cationic species 9 is generated via a sequence
promoted by photoredox catalysis, i.e. the CF3 radical
generated by the action of OQC—step (i)—adds to olefin to
generate the benzylic radical intermediate 8—step (ii)—,
which is oxidized by the action of OQC—step (iii)—to give
the cation 9. As mentioned in the introductory part,
photoredox catalysis can be a powerful synthetic tool
when it is combined with organic transformations
demanding such a pair of redox processes. Recently, an
increasing number of photoredox-catalyzed trifluoro-
methylation reactions have been reported [5], and this
technique can be used complimentarily together with, for
example, transition metal catalysis [14] and electrocata-
lysis.
Scheme 15. Trifluoromethylation of 1,1-diarylethenes.
4.5. Summary of reaction mechanisms
The mechanisms of the solvolytic and substitution
reactions discussed above are summarized in Scheme
17. Trifluoromethyl radicalꢁCF3, the key intermediate of
these transformations, is generated by 1e-reduction of the
electrophilic trifluoromethylating reagents 5 or 6 via OQC
and adds to olefins to form the benzyl radical intermediate
8, which is converted into the benzyl cation intermediate 9
by the action of the latter part of OQC. Subsequent
processes are dependent on the reaction conditions. In
the presence of a nucleophile, it adds to the cationic center
to afford the solvolysis products 7 or 11, which may
undergo further transformation (e.g., an oxidation giving
the ketone product 12). In the other cases, the nucleophile
does not attack the cationic center, but the boron-
5. Photoredox catalysis, a green method for radical
generation: future prospects
Organic radicals are usually generated by thermolysis,
photolysis, and redox reactions of appropriate precursors
having weak bonds, which usually contain heavy elements
(mostly harmful; e.g., Sn) or are potentially explosive (e.g.
O-O bond). Additionally, all processes require fossil fuel as
an energy source to drive the reactions.
In sharp contrast to the classical radical chemistry,
photoredox catalysis turns out to be superior to the
classical methods and green from the point of view of the
above-mentioned aspects. All you need are reagents,
catalyst, flask, and sunlight.
functional group or the proton at the
eliminated to furnish the substituted products 18 and 19.
The -CF3-substituted cationic species may be
b-position is
b
9
generated by the interaction of olefin with trifluoromethyl
cation +CF3 (Scheme 17), but such a reaction has not been
reported so far, presumably because of the low reactivity of
Photoredox catalysis is, of course, not almighty. It
becomes powerful only when the target reaction meets
the above-mentioned requirements. But the scope of
Scheme 17. (Color online.) Trifluoromethylation of olefins.
Please cite this article in press as: Akita M, Koike T. Sunlight-driven trifluoromethylation of olefinic substrates by
photoredox catalysis: A green organic process. C. R. Chimie (2015), http://dx.doi.org/10.1016/j.crci.2015.01.013

G Model
CRAS2C-4041; No. of Pages 10
M. Akita, T. Koike / C. R. Chimie xxx (2015) xxx–xxx
9
Fig. 2. The growing numbers of published items and citations relevant to ‘‘photoredox catalysis’’ as of 24 January 2015.
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Please cite this article in press as: Akita M, Koike T. Sunlight-driven trifluoromethylation of olefinic substrates by
photoredox catalysis: A green organic process. C. R. Chimie (2015), http://dx.doi.org/10.1016/j.crci.2015.01.013