Principles and Applications of Photoredox Catalysis: Tri

1． Photoredox Catalysis

Photoredox catalysis 1 has attracted increasing attention as a method for useful and green chemical processes in the �eld of synthetic organic chemistry, because these catalytic reac-tions are promoted by simply irradiating reaction mixtures with visible light including sunlight. Because inexhaustible solar energy is abundant on the earth as supplied from the sun, this catalytic system is sometimes called solar synthesis. 1j

Photoredox catalysis is characterized as catalytic organic reactions involving radical intermediates generated by SET (single electron transfer) processes, i.e. 1e─ redox processes, which are induced by the action of a photoredox catalyst excited by visible light.

The concept of photoredox catalysis was established in 1980’s by chemists including Japanese pioneers. 2 In its infancy it was applied to rather simple organic transformations and did not attract much attention from synthetic organic chemists. Later on, however, photoredox catalysis came to arouse the interest of many organic chemists through its applications to more complicated organic processes including C─ C bond for-mation and recently the �eld has developed explosively. This renaissance in organic photoredox catalysis began in the late 2000’s as can be seen from the publication dates of various review articles, 1 and our �rst communication was published in early 2009. 3a

In this review article, discussion of the principles and fea-tures of photoredox catalysis will be followed by illustration using typical examples with emphasis on reactions developed in the authors’ laboratory. 3,4 We published a review article in Japanese in this journal in 2014 4c but the present review article includes more recent progresses.1.1　 Principles of Photoredox Catalysis

Most organic compounds cannot absorb visible light, because they are colorless. Some sensitizer, therefore, must be added to the reaction system so that it can absorb solar energy.

Typical colored catalysts include coordination and organome-tallic compounds (e.g. ruthenium polypyridine complexes, cyclometalated iridium complexes, and relevant copper, plati-num and gold complexes), while recently organic dyes have also become popular. 3,5

Photoredox catalysis will be explained here taking [Ru(bipy) 3]

2+ (Ru) 6 as an example (Scheme 1). This ruthenium complex exhibits an intense absorption band around 450 nm, which is in the higher energy region of sunlight. Irradiation of the photocatalyst (Ru II) with visible light leads, via MLCT transition (metal─ to─ ligand charge transfer; from the frontier metal d orbital to the ligand π ＊ orbital), to the excited singlet

Principles and Applications of Photoredox Catalysis: Tri�uoromethylation and Beyond

Munetaka Akita ＊ and Takashi Koike ＊

Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology

R1─ 27 4259 Nagatsuta, Midori─ ku, Yokohama 226─ 8503, Japan

(Received June 13, 2016; E─ mail: [email protected])

Abstract: Photoredox catalysis has attracted the increasing attention of chemists in various �elds from the viewpoints of green chemistry, solar chemistry, clean redox processes, and radical chemistry. Discussion of the principles and features of photoredox catalysis will be followed by illustration using typical examples with emphasis on the reactions developed in the authors’ laboratory which proceed via an oxidative quenching cycle accompanying reduction of external substrates in the �rst stage. Photoredox catalysis turns out to be quite effective for trifluoromethylation of olefinic substrates with electrophilic tri�uoromethylating reagents such as Umemoto’s and Togni’s reagents, 1e─ reduction of which generates the key intermediate, tri�uoromethyl radi-cal. Key features of photoredox catalysis will be discussed in terms of visible light─ driven reactions, radical reactions, redox─ neutral system and atom economy. The concept can be extended to other radical systems such as N─ and C─ centered radicals. This review article will be closed with an overview of future prospects for pho-toredox catalysis.

Scheme 1.　Principles of photoredox catalysis.

（ 8 ） J. Synth. Org. Chem., Jpn.1036

state, which is subsequently converted to the excited triplet species ( ＊Ru II) via intersystem crossing. The resultant excited triplet species has a lifetime of μs order, which is long enough for most chemical processes on a timescale of ns─ ps order.

The excited triplet species can function in two ways via SET (single electron transfer) processes. The electron in the higher SOMO (indicated by the ellipse) and the hole in the lower SOMO (indicated by the dashed ellipse) may serve as a 1e─ reductant and a 1e─ oxidant, respectively. When the present photochemical system is combined with chemical reactions, which require the redox properties of the photo─ excited cata-lyst, those reactions will proceed upon simply irradiating the mixture with visible light (sunlight).

The two types of the photocatalytic reaction mechanisms are called the oxidative quenching cycle (abbreviated as OQC in this article) and the reductive quenching cycle (abbreviated as RQC), which are initiated by oxidation and reduction processes of the photo─ excited species, respectively (Scheme 1). In the oxidative quenching cycle, the electron in the higher SOMO is transferred to an external acceptor (A) nearby to form the anionic radical of the acceptor (A -) together with the oxidized Ru species (Ru III), which then may receive an electron from a donor (D) to afford the cationic radical species (D +) together with the ground state metal catalyst (Ru II). Thus simple irradia-tion of the reaction system containing the photoredox catalyst drives the reaction cycle to perform sequential 1e─ reduction and 1e─ oxidation of external substrates. The other photoredox cycle RQC is initiated by an oxidative process with the hole in the lower SOMO, and the photoexcited species interacts with the external reagents in the order of donor ( ＊Ru II+D→Ru I+D +) and then acceptor (Ru I+A→Ru II+A -). It should be noted that, in the first SET process of OQC, oxidation of the excited cata-lyst causes reduction of the external substrate, while in that of RQC the external substrate is oxidized.

Photoredox catalysis has been frequently utilized in the �eld of inorganic photochemistry, in particular, for reduction of small inorganic molecules such as H + (to H 2) and CO 2 (to CO). 7 In these cases, sacri�cial redox reagents (e.g. trialkyl-amine for reduction) are used for the �rst SET process just to inject an electron or a hole into the reaction system and thus their organic skeletons are not incorporated into the product, hence such systems are not atom ─ economic. As exempli�ed in Scheme 1 (the lower scheme) for the oxidative quenching cycle, if the radical species formed by the �rst SET process can be functionalized and then incorporated into the product, the system can be made more atom ─ economic. Furthermore, the electron provided by the photo─ excited metal species returns to the metal catalyst through the functionalization process ( ＊Ru II→S -─ (functionalization)→P -→Ru III) to render the sys-tem redox ─ neutral. A scheme for RQC similar to the one shown for OQC is possible but omitted for lack of space. The electron circulates as indicated by the arrows in the scheme, and thus the photoredox catalyst serves as an electron media-tor in the catalytic system. 8 To be noted is that the reaction sequence from S - to P - involves radical intermediates. 9

The authors’ group regards this advanced system, where the initial reactant S, whole or its part, is incorporated into the product, as photoredox catalysis, but the non─ atom─ economic systems using sacri�cial redox reagents are often called pho-toredox catalysis by other researchers.

It is remarkable that the redox power of the catalyst is

enhanced by the photo─ excitation. For reduction via OQC, for example, the energy level of the highest occupied orbital is a rough measure of the reducing power of the species. When the HOMO level of the ground state and the higher SOMO level of the excited state are compared, it can be seen that the reduc-ing power of the photo─ excited species is enhanced by the energy gap between the HOMO and the higher SOMO, i.e. the HOMO─ LUMO gap. Similar consideration of the oxidation process via RQC reveals that the oxidizing power of the photo─ excited species is also enhanced by the energy gap between the LUMO of the ground state catalyst and the lower SOMO of the photo─ excited species. Thus light energy is converted to enhanced redox power of the catalyst.

Practical examples of the two processes, which were deve-loped at an early stage of our study, are shown in Scheme 2 referring to oxy─ tri�uoromethylation of styrene derivatives 3d and Giese─ type reaction 10 of organyltri�uoroborate with elec-tron─ de�cient ole�ns. 3e In these reactions, tri�uoromethyl and organyl radicals are generated by the SET processes to and from the photo─ excited catalyst with elimination of the auxi-liary groups. Subsequent capture of the radical intermediates by the ole�nic substrates (I’) followed by the reverse SET pro-cesses from and to I’ affords the ground state catalyst and the carbo─ cationic (I +) and carbo─ anionic intermediates (I -) respectively, which are further functionalized by the action of the nucleophile or proton to furnish the respective products. It should be pointed out that, although the orders of the two redox processes are reversed, (i) these two processes are redox ─ neutral and (ii) a part of the �rst redox reagent (S in Scheme 1) is incorporated into the product (P), i.e. they are more atom ─ economic than the system using a sacri�cial reagent. Thus for photoredox catalysis, proper design of the catalytic system is essential.

1.2　 Physical Properties of Photoredox CatalystsThe redox potentials of the photo─ excited species 5 are the

most important parameters of these catalysts, and those of the representative photo─ excited catalysts are shown in Figure 1. 1d,h The oxidation and reduction potentials of the photo─ excited species ＊ PC n +, PC ( n + 1 )/ ＊PC n + and ＊ PC n +/PC ( n - 1 ), are compara-ble respectively to the reducing and oxidizing power of ＊ PC n +. When the absolute values of the potentials, PC ( n + 1 )/ ＊PC n + or

Scheme 2.　 Two typical photoredox catalytic reactions following OQC and RQC.

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＊ PC n +/PC ( n - 1 ), exceed respectively those of the reduction or oxidation potentials of the reactants, the SET processes may be viable.

The redox potentials of the metal photoredox catalysts are dependent on several factors including the kind of central metal (Ru vs. Ir), the charge (neutral vs. cationic), and the electron─ donating property and the charge of the chelating ligand(s). A catalyst appropriate for the purpose of each par-ticular reaction should be chosen. For example, for an oxida-tive transformation via RQC, it is better to choose a catalyst having a ＊ PC n +/PC ( n - 1 ) value larger than that of the reactant, and vice versa for OQC. Recently, organic dyes have drawn attention from the viewpoint of element strategy (abundance and cost). Among these, the acridinium salt F shows extraordi-narily strong oxidizing power, 12,13 while, for reductive transfor-mations, metal catalysts are generally still superior to organic ones. The catalytic performance, however, is not determined solely by the redox potentials. Other factors such as the multi-plicity and the lifetime of the excited species should also in�u-ence it.

The enhancement in redox power due to photo─ excitation (see above) can also be demonstrated by analysis of electro-

chemical data. For example, for [Ru(bipy) 3] 2+ (A), when the

reduction potentials of the ground state, Ru II/Ru I, and the excited state, ＊ Ru II/Ru I, which are rough measures of their oxi-dizing power, are compared, the oxidizing power is signi�cantly enhanced by 2.17 V (-1.74 V→0.43 V; vs. FeCp 2) upon photo─ excitation. Similarly, the reducing power is enhanced by 2.12 V (0.88 V→-1.24 V). The differences are indicated with the arrows in Figure 1 and roughly comparable to the HOMO─ LUMO gap of [Ru(bipy) 3]

2+ estimated from the onset of its visible absorption (2.2 V). 14

1.3　 Generation of Organic Radicals by Photoredox CatalysisSelective generation of organic radicals under mild reac-

tion conditions still remains a key issue to be solved in the �eld of synthetic organic chemistry. 9

The key concept for the radical generation steps shown in Scheme 2 can be generalized according to Scheme 3. Organic radicals may be generated either by 1e─ reduction of electron─ de�cient precursors following OQC or by 1e─ oxidation of electron─ rich precursors following RQC with elimination of the auxiliary groups (X or Y respectively) which facilitate the SET processes.

Generation of radicals is the �rst step of any radical reac-

Figure 1.　 Redox potentials of photo─ excited photoredox catalysts and selected organic substrates. The potentials are reported in V with reference to FeCp 2. The numbers in parentheses are absorption maxima of the catalysts (in nm).

Scheme 3.　Key concept for generation of organic radicals induced by photoredox catalysis.


tion process, and organic radicals are usually generated either by homolysis of a weak bond in a precursor or redox processes of a precursor (Scheme 4).

The homolysis of precursors is usually induced by ther-molysis, photolysis, radiolysis, electrolysis etc. Azobis-(isobutyronitrile) (AIBN) and benzoyl peroxide (BPO) are typical initiators of radical polymerization and both of them decompose upon heating or UV irradiation to generate radi-cals. Bonds associated with a heavy element are, in general, so weak that compounds containing them are frequently utilized for radical reactions, as typically exempli�ed by organotin compounds. These precursors are convenient but have some drawbacks. For example, AIBN and BPO are intrinsically explosive and therefore must be kept in a refrigerator in the dark and handled with care. Furthermore, compounds con-taining heavy elements are frequently so harmful that their use must be limited as much as possible. Organic radicals can also be generated by redox processes either by the action of a stoi-chiometric amount of a reductant or an oxidant or by electrol-ysis. The stoichiometric use of redox reagents, however, causes formation of waste, while electrolysis requires special equip-ment.

Furthermore any methods that consume fossil fuel will generate CO 2.

Radical generation by the action of photoredox catalysis turns out to be superior to the conventional methods with respect to each of the aspects mentioned above, i.e. it is safe and has no need for harmful reagents, special equipment, or use of fossil fuel (instead it can directly utilize sunlight), mak-ing this catalytic system green.

2． Photoredox Catalyzed Reactions

The photoredox catalyzed reactions developed in our labo-ratory are summarized in Table 1, 3 and reaction conditions and yields for selected examples of the catalytic reactions via OQC are summarized in Table 2. The two reactions OQC─ b ─ (i) and RQC─ c have been already discussed above (Scheme 2), and our study demonstrated that OQC is quite an ef�cient mechanism for generating the �uorinated methyl radicals, ·CF 3 and ·CF 2H. The present review focuses on those reactions occurring via OQC, and the following section deals with tri�uoromethylation reactions of ole�nic substrates (shaded in Table 1).2.1　 Tri─ and Di─ �uoromethylative Difunctionalization of

Unsaturated Hydrocarbons via Oxidative Quenching Cycle (OQC ─ b ─ g)

Oxy─ tri�uoromethylation giving 1 (OQC─ b ─ (i)) has already been shown in Scheme 2. Umemoto’s reagent (5─ tri-

�uoromethyldibenzothiophenium salt) 15 and Togni’s reagent containing a hypervalent iodine atom, 16 which are electrophilic tri�uoromethylating reagents, turn out to be ef�cient CF 3─ radical sources upon combination with OQC.

Introduction of �uorine─ containing functional groups into organic skeletons is a matter of urgency for the development of new drugs and agrochemicals. The present photocatalytic method has the potential to contribute to this development. 17,18

2.1.1　 Regiospeci�c Fluoromethylative Difunctionalization of Ole�ns

The �uoromethylative reactions OQC─ c ─ g can be inter-preted in terms of the key cationic intermediate I + appearing in Scheme 2. CF 3 radical is generated by 1e─ reduction (the �rst SET process) of the electrophilic tri�uoromethylating reagent followed by elimination of dibenzothiophene (from Umemoto’s reagent) or o ─ iodobenzoate (from Togni’s reagent). Subsequent addition of the resultant electron─ de�cient CF 3 radical to the β ─ carbon atom of the C─ C double bond in a styrene derivative gives the benzyl radical intermediate I’, which is further con-verted to the benzyl cation intermediate I + via SET to the oxi-dized metal center. In the presence of oxygen nuclephiles such as water, alcohol, and carboxylic acid, these can add to the cationic center in I + to give the oxy─ tri�uoromethylated products 1. The regiochemistry is determined at the stage of the addition of CF 3 radical, which occurs at the β ─ carbon atom so as to form the more stable benzyl radical intermediate I’.

The proposed mechanism is based on the following experi-mental results.

(1) The oxidation potential of the photo─ excited iridium catalyst B (Ir IV/ ＊Ir III=-2.14 V) is enough to undergo SET to Umemoto’s reagent (E red=-0.75 V) and Togni’s reagent (E red=-1.34 V), and Umemoto’s reagent can also be reduced by [Ru(bipy) 3]

2+ (Ru III/ ＊Ru II=-1.24 V).(2) Intermediacy of a radical species is con�rmed by the

formation of a ring─ opened, rearranged product, trans ─ 1─ (2,2,2─ tri�uoroethyl)─ 4─ (1─ hydroxy─ 1─ methyl)ethyl-cyclohexene from the strained bicyclic substrate, β ─ pinene.

(3) Stern─ Volmer plots 5 reveal that the phosphorescence from the photo─ excited iridium catalyst ＊ Ir is not quenched by styrene but by Umemoto’s reagent, indi-cating that ＊ Ir interacts with Umemoto’s reagent.

ATRA (atom transfer radical addition) via a radical chain reaction is frequently proposed and actually observed as an alternative mechanism for the addition reactions of radical intermediates to ole�ns. 19 But for the oxy─ tri�uoromethylation (OQC─ b ─ (i)) the time─ conversion plots (Figure 2) clearly indi-cate that the photoredox mechanism rather than the ATRA mechanism operates, because the reaction proceeded only while the light source was turned on.

Intramolecular oxy─ tri�uoromethylation of alkenoic acid affords the tri�uoromethylated lactones (4 and 5; OQC─ c ─ (i)), 3i and spiro ─ compounds (6; OQC─ c ─ (ii)) 3p,v are obtained from cyclic ole�ns with nucleophilic pendant groups (Tables 1 and 2).

The corresponding oxy─ di�uoromethylation following a similar mechanism should be feasible (OQC─ b ─ (ii)). 3s Because, however, the CF 2H reagents corresponding to Umemoto’s and Togni’s reagents are not available, we had to look for an alter-native and �nally found that Hu’s reagent, S ─ di�uoromethyl─

Scheme 4.　Conventional radical generation methods.

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S ─ 2─ pyridyl─ N ─ p ─ tolylsulfonylsulfoxyimene, 20 served as an excellent CF 2H source to afford the desired products 2.

Furthermore the oxy─ tri�uoromethylation was extended to acetylenes to afford tetra─ substituted trans ─ ole�ns bearing a CF 3 group 3 in a stereo─ and regio─ selective manner (OQC─ b ─ (iii); Scheme 5). 3r Here Yagupolskii’s reagent was used instead of Umemoto’s reagent, which gave a substantial amount of tri�uoromethylated dibenzothiophene as byprod-uct. The reaction may follow a mechanism similar to that pro-

posed for ole�ns (Scheme 2) and the nucleophilic tri�ate anion, the counteranion of Yagupolskii’s reagent, adds to the cationic intermediate I + to afford trans ─ alkenyl tri�ate 3. The trans ─ stereochemistry should result from electrostatic repulsion between the highly negatively charged CF 3 group and the tri-�ate anion. The resultant alkenyl tri�ate 3 is of synthetic importance, because it is susceptible to Pd─ catalyzed cross─ coupling reactions with organometallic nucleophiles with retention of the ole�n con�guration. Thus by combining pho-

Table 1.　 Photoredox─ catalyzed reactions developed in the authors’ laboratory since 2009. The reactions are divided into those following OQC and those following RQC.


toredox catalysis and cross─ coupling catalysis in a sequential manner, acetylenes can be converted to tetra─ substituted CF 3─ ole�ns with high regio─ and stereo─ selectivity.2.1.2　 Regiospeci�c Difunctionalization of Ole�ns via

Solvolytic Tri�uoromethylationThe carbo─ cationic intermediate I + (Scheme 2) undergoes

solvolysis in nucleophilic solvents to give different products

depending on the solvent used (Scheme 6).In the presence of a nitrile, nucleophilic addition to I + gives

the amino─ tri�uoromethylated products (OQC─ d; Tables 1 and 2) via a Ritter─ type reaction mechanism (Scheme 6). The adduct is hydrated to afford the amide product 7.

When the reaction was carried out in DMSO, another sol-volytic reaction occurred to afford the keto─ tri�uoromethy-

Table 2.　 Reaction conditions and yields of selected examples of the phooredox reactions via OQC. Light source: LED lamps. The reactions were carried out at r.t. For the symbols U, B etc., see Figure 1 and Table 1.

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lated products, 3,3,3─ tri�uoropropiophenone derivatives 8 (Scheme 6). 3k The addition of DMSO to I + forms the alkoxy-sulfonium species G, which is an intermediate in the DMSO─ oxidation reactions of alkyl halides leading to carbonyl com-pounds upon interaction with a base and can be detected and characterized by NMR spectroscopy. 21 The experimental results obtained have been interpreted in terms of a combina-tion of Kornblum oxidation─ type mechanism and photoredox mechanism. The residue from Togni’s reagent, o ─ iodobenzo-ate, can serve as a base to promote the deprotonation of G. The reactions of β ─ substituted styrene derivatives afforded the corresponding β ─ substituted products 8 (Table 2). To our sur-

prise, α ─ substituted derivatives, for which the Kornblum─ type reaction is not viable because the α ─ alkyl group in G cannot be removed by the action of a base, also gave the dealkylated par-ent 3,3,3─ tri�uoropropiophenone 8 along with alkyldimethyl-sulfonium salt ([R─ SMe 2]

+) (Table 2). Although no positive evidence has been obtained for a photoredox mechanism for the conversion from G to 8, the results for the α ─ substituted derivatives support its viability. 1e─ Reduction of G would give the alkoxy radical intermediate H, which may undergo H─ migration and 1e─ oxidation to afford 8.2.1.3　 Preparation of Tri�uoromethylated Ole�ns

As described in textbooks of organic chemistry, a carboca-tionic species is an intermediate of E1 elimination to give ole-�ns via deprotonation, and ole�nic products may be obtained from I + bearing an appropriate leaving group.

A simple ole�n is not reactive enough toward the electron─ de�cient CF 3 radical for the addition process preceding the formation of I +. But by increasing the electron─ density of the C─ C double bond by introduction of a BF 3 functional group, alkenyltri�uoroborate becomes reactive enough toward CF 3 radical to give the substituted product 10 via deboronation from I + (OQC─ f; Scheme 7). Furthermore, 1,1─ diarylethenes, though lacking activation by the BF 3 group, can be tri�uoro-methylated to give the substituted products 9 (OQC─ g), pre-sumably because the addition of the CF 3 radical is facilitated by stabilization of the resultant radical adduct by the two aryl groups directly bonded to the radical center.

2.1.4　 Tri�uoromethyl Cation EquivalentIt would appear that the carbocationic species I +,

which are the key intermediates for the reactions described in this section, should be readily formed by interaction of ole�ns with the CF 3─ cation but in fact such a reaction has never been reported. Instead, in the present system, the cationic species I + is formed from an ole�n by addition to it of the CF 3 radical followed by 1e─ oxidation of the resultant radical intermediate I’, and photoredox catalysis plays the key roles at the

Figure 2.　 Lighting on─ off experiment for oxy─ tri�uoromethylation of styrene (OQC─ b ─ (i)) (observed by 1 H NMR).

Scheme 5.　 Regio─ and stereo─ selective conversion of acetylene to tetra─ substituted ole�ns bearing a CF 3 substituent.

Scheme 6.　Solvolytic tri�uoromethylative ole�n difunctionalization.


stages of (i) the radical generation from an electrophilic tri�uo-romethylating reagent via 1e─ reduction and (ii) the conversion of the radical intermediate I’ to the carbocationic intermediate I + via 1e─ oxidation, making the reaction sequence redox─ neu-tral (Scheme 8). Thus the action of the combination of an electrophilic tri�uoromethylating reagent and OQC on an ole-�n can be considered to be equivalent to addition of trifluoro-methyl cation to the olefin.

Nowadays tri�uoromethylation has been achieved not only by this combination of electrophilic CF 3 reagents with OQC but also by the combination of nucleophilic CF 3 reagents with RQC, and this has led to date to the development of many new reaction systems. 22

2.2　 Generation of Heteroatom Radicals via OQCThe success of tri�uoromethylation mediated by photore-

dox catalysis prompted us to develop methods of generating other types of radical species.2.2.1　 Generation of N─ centered Radicals:

Amino─ hydroxylation of Ole�nsOrganic nitrogen radicals are versatile synthons comple-

menting functionalized nucleophilic and electrophilic aminat-ing reagents. We examined generation of a nitrogen─ centered radical by the combination of electrophilic reagents and OQC (Scheme 3) and chose originally aminopyridinium precursors. Of the aminopyridinium salts with various protective groups examined, the tosylated precursor worked well. When photo-catalytic reactions were conducted under conditions similar to those of the oxy─ tri�uoromethylation, amino─ hydroxylated products 11 were obtained regiospeci�cally in good yields (OQC─ h ─ (i); Tables 1 and 2). 3o The proposed mechanism is similar to that for oxy─ tri�uoromethylation (Scheme 9). SET from the photo─ excited catalyst to the amino precursor gives the amino radical with elimination of pyridine. The generated amino radical adds to the β ─ carbon atom of a styrene deriva-tive to give the benzyl radical intermediate I’, which is con-verted to the amino─ hydroxylated product 11 by way of I +.

Although amino─ hydroxylation was achieved with the tosyl reagent, it turned out to be dif�cult to remove the tosyl protecting group from the product 11 under mild conditions. We next went on to explore the reaction with iminopyridinium ylides, which can be activated by Sc(OTf) 3 in place of a proton as in the case of the aminopyridinium salt (see the structures in brackets in Scheme 9). 3u As a result, the tri�uoroacetyl deriva-tive underwent the aminohydroxylation to give 11’ in satisfac-tory yield (OQC─ h ─ (ii); Table 1) and that the tri�uroacetyl protecting group was readily removed by simple acid or base treatment.

Amino─ hydroxylation of ole�ns is a valuable and unique synthetic method to introduce simultaneously two different groups, one amino and the other hydroxyl, onto the vicinal positions in a carbon skeleton. The Os─ catalyzed reaction 23 has been used for this purpose but is associated with problems of regioselectivity and toxicity of the Os reagent. The present method is superior to the Os system with respect to these issues.2.2.2　 Penta�uorosulfanylphenylation

The penta�uorosulfanyl group (SF 5) is an extraordinarily electron─ withdrawing group with a Hammett σ p value of 0.68 and an electronegativity χ of 3.65, both of which are even

larger than those of the CF 3 group (σ p=0.54, χ=3.36). In particular, the χ value is close to that of the �uorine atom (4.0). In addition to these electronic effects, the enhanced thermal and chemical stabilities as well as lipophilicity of SF 5 derivatives is attracting interest in the �elds of agrochemicals, pharmaceuticals, and materials. In addition to the �uorination of organosulfur precursors, cross─ coupling with penta�uorosulfanylaryl compounds is seen as a versatile synthetic route for the preparation of aryl derivatives containing this group. We explored a synthetic route via radical interme-diates and found a penta�uorosulfanylphenyl-ation of ole�ns (OQC─ i; Table 1). 3t Penta�uo-

Scheme 7.　Preparation of ole�ns with a CF 3 substituent.

Scheme 9.　Photoredox─ catalyzed amino─ hydroxylation of ole�n.

Scheme 8.　 Formal addition of a CF 3 cation to ole�n by the action of photoredox catalysis.

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rosulfanylphenyl radical is generated by SET─ reduction of the hypervalent diaryliodonium precursor, [F 5S─ C 6H 4─ I─ Mes] + with elimination of mesityl iodide and the resultant radical reacts with ole�ns in a manner similar to the solvolytic tri�uo-romethylative difunctionalization (section 2.1.1) to afford the corresponding adducts 13.

Thus photoredox catalysis turns out to be quite effective for regiospeci�c difunctionalization of ole�ns and acetylenes (Scheme 3).

3． Features of Photoredox Catalysis

The features of photoredox catalysis discussed thus far may be summarized as follows:

1) driven by visible light: Photoredox catalysis is driven by irradiation with sources of visible light including sun-light and LED lamps. Although we did not examine all these reactions under sunlight, those for which we did so were all successful, and the reaction time and yields were comparable to those when carried out with LED and xenon lamps. It should be noted that, in principle, no fossil fuel is needed.

2) radical reactions: The catalytic reaction sequence involves SET processes from/to the photo─ excited cata-lyst so as to generate radical species as the key interme-diates of the transformations.

3) redox ─ neutral: The electron transferred from or to the catalyst in the �rst SET process returns respectively to or from the catalyst in the second SET process to form the product, i.e. the electron formally serves as a catalyst 8 and the photoredox catalyst serves as a mediator of the SET processes. The photoexcited electron goes down through a cascade of the SET processes (Scheme 1, lower Scheme).

4) atom ─ economic: The reactant (or part of it) incorpo-rated in the �rst SET process is included in the skeleton of the product. In addition, no sacri�cial redox reagent is needed.

We have demonstrated that photoredox catalysis is a pow-erful synthetic tool, in particular, for incorporation of a CF 3 group into unsaturated organic compounds via radical inter-mediates. The key concept is facile interconversion between non─ radical and radical species via SET processes made possi-ble by virtue of features 2) and 3) of photoredox catalysis. In well─ designed systems, the combination of radical generation from a non─ radical precursor and subsequent 1e─ redox pro-cess of the functionalized radical gives rise to ionic intermedi-ates, which are further susceptible to electrophilic or nucleo-phile functionalizations. Thus, the precursors can be converted into the reactive functionalized ionic species via SET processes, and the origin of the enhanced reactivity should be ascribed to the light energy initially adsorbed by the photoredox catalysts as discussed in 1.1. and 1.2.

In conclusion, photoredox catalysis is a green chemical sys-tem in terms of the points discussed above.

4． Future Prospects: “and beyond”

Photoredox catalysis has now become popular among syn-thetic organic chemists, because the principles are simple, the catalysts are commercially available, and solar energy (sun-light) is available on the earth when it is �ne.

In principle, there remains much room for the generation

of many kinds of new radical species following Scheme 3. In addition, the possibilities for functionalization of the resultant radical species are unlimited.

Another way to expand photoredox catalysis is merger with other catalytic systems such as organic and organometal-lic catalysis, and now many chemists are making their own way forward on the basis of this concept. For example, MacMillan’s group reported asymmetric alkylation of aldehydes at a very early stage of the history of photoredox catalysis, where the electron─ de�cient aldehyde is activated as an enamine upon condensation with a chiral amine so that the resultant elec-tron─ rich chiral enamine reacts with the benzyl radical enanti-oselectively (Scheme 10(a)). 24 In the case of our �rst contribu-tion to this �eld (RQC─ a), too, an aldehyde was converted to an enamine so as to undergo 1e─ oxidation by the photo─ excited catalyst. 3a

Recently, photoredox catalysis is being combined with organometallic cross coupling. The �rst attempt was made by Osawa and us for copper─ free Sonogashira coupling, the cata-lyst system of which consisted of [Ru(bipy) 3]

2+ (A)─ Pd(CH 3CN) 2Cl 2─ PBu t 3─ NEt 3, although the reaction mecha-nism was not elucidated (Scheme 10(b)). 25 Later on, Sanford reported a more sophisticated example of cross coupling via the higher valent Pd(II)─ Pd(IV) redox cycle (via OQC)

Scheme 10.　Merger with other catalytic systems.


(Scheme 10(c)). 26 Pd(II) species are not electron─ rich enough to undergo 2e─ oxidative addition of aryl halides, and thus an organyl radical generated by the reductive photoredox catalysis was introduced onto the Pd center via 1e─ oxidative addition. Subsequent 1e─ oxidation followed by reductive elimination successfully afforded the coupling product. Recently, Macmillan reported asymmetric cross─ coupling between an aryl halide and carboxylic acid (Scheme 10(d)). 27 In the latter two examples, arenediazonium salts and carboxylic acids totally different from the conventional organometallic nucleo-philes (e.g. Grignard reagents and organoboronic acids) are used as equivalents of the latter so as to expand the scope not only of the cross coupling catalysis but also of photoredox catalysis. In this case, too, there are many other types of cataly-sis to be merged with photoredox catalysis.

Another aspect is the development of reactors. Although most of the catalytic reactions can be driven by sunlight, for steady production of organic compounds some arti�cial light source (e.g. LED lamps) needs to be used and in such cases one must �nd a way to improve the ef�ciency of the catalysis. In addition, for large scale production special reactors should be designed, because light may not pass through a thick reactor. The problems should be solved by, for example, a micro�ow system. 28

Such combinations of the science and technology of pho-toredox catalysis are promising ways to open up a new world of organic synthesis.

Just leave your flask under sunlight, and you will obtain the desired product in a good yield!

AcknowledgementsWe are grateful to the coworkers listed in refs. 3 for their

invaluable contributions to advancing the science of photore-dox catalysis. Financial support from our government (KAKENHI Nos. 26288045, 23750174, 15K13689, and 15J12072) is also gratefully acknowledged.

References and Notes 1） (a) Yoon, T. P.; Ischay, M. A. Nat. Chem. 2010, 2, 527. (b) Narayanam,

J. M. R.; Stephenson, C. R. J. Chem. Soc. Rev. 2011, 40, 102. (c) Xuan, J.; Xiao, W.─ J. Angew. Chem. Int. Ed. 2012, 51, 6828. (d) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem. Rev. 2013, 113, 5322. (e) Hari, D. P.; König, B. Angew. Chem. Int. Ed. 2013, 52, 4734. (f) Ravelli, D.; Fagnoni, M.; Albini, A. Chem. Soc. Rev. 2013, 42, 97. (g) Fukuzumi, S.; Ohkubo, K. Chem. Sci. 2013, 4, 561. (h) Reckenthälera, M.; Griesbeck, A. G. Adv. Synth. Catal. 2013, 355, 2727. (i) Hopkinson, M. N.; Sahoo, B.; Li, J.─ L.; Glorius, F. Chem. Eur. J. 2014, 20, 3874. (j) Schultz, D. M.; Yoon, T. P. Science 2014, 343, 985. (k) Nicewicz, D. A.; Nguyen, T. M. ACS Catal. 2014, 4, 355. (l) Duret, G.; Quinlan, R.; Bisseret, P.; Blanchard, N. Chem. Sci. 2015, 6, 5366. (m) Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. J. Org. Chem. 2016, 81, 6898. (n) thematic issue: Photochemistry in Organic Synthesis, Chem. Rev. 2016, 116, 9629. See also refs. 4.

2） (a) Hedstrand, D. M.; Kruizinga, W. H.; Kellog, R. M. Tetrahedron Lett. 1978, 19, 1255. (b) Pac, C.; Ihama, M.; Yasuda, M.; Miyauchi, Y.; Sakurai, H. J. Am. Chem. Soc. 1981, 103, 6495. (c) Ishitani, O.; Pac, C.; Sakurai, H. J. Org. Chem. 1984, 49, 26. (d) Goren, Z.; Willner, I. J. Am. Chem. Soc. 1983, 105, 7764. (e) Ishitani, O.; Pac, C.; Sakurai, H. J. Org. Chem. 1983, 48, 2941. (f) Cano─ Yelo, H.; Deronzier, A. J. Chem. Soc., Perkin Trans. 2 1984, 1093. (g) Hironaka, K.; Fukuzumi, S.; Tanaka, T. J. Chem. Soc., Perkin Trans. 2 1984, 2705. (h) Mashraqui, S. H.; Kellog, R. M. Tetrahedron Lett. 1985, 26, 1453. (i) O. Ishitani, O.; Ihama, M.; Miyauchi, Y.; Pac, C. J. Chem. Soc., Perkin Trans. 1 1985, 1527. (j) Tomioka, H.; Ueda, K.; Ohi, H.; Izawa, Y. Chem. Lett. 1986, 1359. (k) Fukuzumi, S.; Mochizuki, S.; Tanaka,

T. J. Phys. Chem. 1990, 94, 722. (l) Okada, K.; Okamoto, K.; Morita, N.; Ohkubo, K.; Oda, M. J. Am. Chem. Soc. 1991, 113, 9401.

3） (a) Koike, T.; Akita, M. Chem. Lett. 2009, 166. (b) Yasu, Y.; Koike, T.; Akita, M. Chem. Commun. 2012, 48, 5355. (c) Koike, T.; Yasu, Y.; Akita, M. Chem. Lett. 2012, 41, 999. (d) Yasu, Y.; Koike, T.; Akita, M. Angew. Chem. Int. Ed. 2012, 51, 9567. (e) Yasu, Y.; Koike, T.; Akita, M. Adv. Synth. Cat. 2012, 354, 3414. (f) Yasu, Y.; Koike, T.; Akita, M. Chem. Commun. 2013, 49, 2037. (g) Yasu, Y.; Koike, T.; Akita, M. Org. Lett. 2013, 15, 2136. (h) Miyazawa, K.; Yasu, Y.; Koike, T.; Akita, M. Chem. Commun. 2013, 49, 7249. (i) Yasu, Y.; Arai, Y.; Tomita, R.; Koike, T.; Akita, M. Org. Lett. 2014, 16, 780. (j) Tomita, R.; Yasu, Y.; Koike, T.; Akita, M. Beilstein J. Org. Chem. 2014, 10, 1099. (k) Tomita, R.; Yasu, Y.; Koike, T.; Akita, M. Angew. Chem. Int. Ed. 2014, 53, 7144. (l) Miyazawa, K.; Koike, T.; Akita, M. Adv. Synth. Catal. 2014, 356, 2749. (m) Li, Y.; Miyazawa, K.; Koike, T.; Akita, M. Org. Chem. Frontiers 2015, 2, 319. (n) Chinzei, T.; Miyazawa, K.; Yasu, Y.; Koike, T.; Akita, M. RSC Advances 2015, 5, 21297. (o) Miyazawa, K.; Koike, T.; Akita, M. Chem. Eur. J. 2015, 21, 11677. (p) Noto, N.; Miyazawa, K.; Koike, T.; Akita, M. Org. Lett. 2015, 17, 3710. (q) Asano, M.; Tomita, R.; Koike, T.; Akita, M. J. Fluorine Chem. 2015, 179, 83. (r) Tomita, R.; Koike, T.; Akita, M. Angew. Chem. Int. Ed. 2015, 54, 12923. (s) Arai, Y.; Tomita, R.; Ando, G.; Koike, T.; Akita, M. Chem. Eur. J. 2016, 22, 1262. (t) Li, Y.; Koike, T.; Akita, M. Synlett 2016, 27, 736. (u) Miyazawa, K.; Koike, T.; Akita, M. Tetrahedron 2016, in press (DOI: 10.1016/j.tet.2016.05.045). (v) Noto, N.; Koike, T.; Akita, M. J. Org. Chem. 2016, 81, 7064.

4） For review articles, see (a) Inagaki, A.; Akita, M. Coord. Chem. Rev. 2010, 254, 1220. (b) Koike, T.; Akita, M. Synlett 2013, 24, 2492. (c) Akita, M.; Inagaki, A.; Koike, T. J. Synth. Org. Chem., Jpn. 2014, 72, 538. (d) Koike, T.; Akita, M. Inorg. Chem. Front. 2014, 1, 562. (e) Koike, T.; Akita, M. J. Fluorine Chem. 2014, 167, 30. (f) Koike, T.; Akita, M. Top. Catal. 2014, 57, 967. (g) Koike, T.; Akita, M. in “Ruthenium in Catalysis 2014” ed. Bruneau, C.; Dixneuf, P.; Springer, 2014, 48, 371. (h) Akita, M.; Koike, T. Compt. Rend. Chim. 2015, 18, 742. (i) Koike, T.; Akita, M. Org. Biomol. Chem. 2016, 14, 6886. (j) Koike, T.; Akita, M. Org. Chem. Front. 2016, in press (DOI: 10.1039/C6QO00139D). (k) Koike, T.; Akita, M. Acc. Chem. Res. 2016, in press (DOI: 10.1021/acs.accounts.6b00268).

5） (a) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Principles of Molecular Photochemistry, University Science Books, 2008. (b) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Modern Molecular Photochemistry of Organic Molecules, University Science Books, 2010.

6） Meyer, T. J. Acc. Chem. Res. 1989, 22, 163, and references cited therein.

7） See, for example, Takeda, H.; Ishitani, O. Coord. Chem. Rev. 2010, 254, 346. See also, ref. 6.

8） Studer, A.; Curran, D. P. Nature Chem. 2014, 6, 765. 9） (a) Encyclopedia of Radicals in Chemistry, Biology and Materials, Vols.

1─ 4, edited by Chatgilialoglu, C.; Studer, A. John Wiley & Sons, Chichester, 2012. (b) Zard, S. Z. Radical reactions in organic synthesis, Oxford University Press, 2003.

10） (a) Burke, S. D.; Fobare, W. F.; Armistead, D. M. J. Org. Chem. 1982, 47, 3348. (b) Giese, B.; Dupuis, J. Angew. Chem., Int. Ed. Engl. 1983, 22, 622. (c) Giese, B.; González─ Gómez, J. A.; Witzel, T. Angew. Chem., Int. Ed. Engl. 1984, 23, 69.

11） Tucker, J. W.; Corey, C. R. J. J. Org. Chem. 2012, 77, 1617.12） (a) Fukuzumi, S.; Kotani, H.; Ohkubo, K.; Ogo, S.; Tkachenko, N. V.;

Lemmetyinen, H. J. Am. Chem. Soc. 2004, 126, 1600. (b) Ohkubo, K.; Mizushima, K.; Iwata, R.; Fukuzumi, S. Chem. Sci. 2011, 2, 715. (c) Hoshino, M.; Uekusa, H.; Tomita, A.; Koshihara, S.; Sato, T.; Nozawa, S.; Adachi, S.; Ohkubo, K.; Kotani, H.; Fukuzumi, S. J. Am. Chem. Soc. 2012, 134, 4569.

13） A good example of the superiority of an organic dye can be seen in the reaction via RQC shown below. Only the organic dye F gave the product in a satisfactory yield in accord with the reduction potentials of the photo─ excited catalysts shown in parentheses. 3n)

14） Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater. 2011, 23, 2367.

15） (a) Umemoto, T.; Ishihara, S. Tetrahedron Lett. 1990, 31, 3579. (b)

Vol.74 No.11 2016 （ 17 ） 1045

Umemoto, T.; Ishihara, S. J. Am. Chem. Soc. 1993, 115, 2156. (c) Umemoto, T. Chem. Rev. 1996, 96, 1757.

16） (a) Eisenberger, P.; Gischig, S.; Togni, A. Chem. Eur. J. 2006, 12, 2579. (b) Charpentier, J.; Früh, N.; Togni, A. Chem. Rev. 2015, 115, 650.

17） (a) Hiyama, T. Organofluorine Compounds: Chemistry and Applica-tions; Springer, Berlin, 2000. (b) Ojima, I. Fluorine in Medicinal Chemistry and Chemical Biology; Wiley─ Blackwell, Chichester, 2009. (c) P. Kirsch, Modern Fluoroorganic Chemistry; Wiley─ VCH, Weinheim, 2013. (d) Kirsch, P. Modern Fluoroorganic Chemistry; Wiley─ VCH, Weinheim, 2013. (e) Meanwell, N. A. J. Med. Chem. 2011, 54, 2529.

18） (a) Haszeldine, R. N. J. Chem. Soc. 1949, 2856. (b) Langlois, B. R.; Laurent, E.; Roidot, N. Tetrahedron Lett. 1991, 32, 7525. (c) Dolbier, Jr., W. R. Chem. Rev. 1996, 96, 1557. (d) Kamigata, N.; Fukushima, T.; Terakawa, Y.; Yoshida, M.; Sawada, H. J. Chem. Soc. Perkin Trans. 1 1991, 627. (e) Shibata, N.; Matsnev, A.; Cahard, D. Beilstein J. Org. Chem. 2010, 6, No. 65. (f) Studer, A. A. Angew. Chem. Int. Ed. 2012, 51, 8950. (g) Macé, Y.; Magnier, E. Eur. J. Org. Chem. 2012, 2479. (h) Barata─ Vallejo, S.; Lantaño, B.; Postigo, A. Chem. Eur. J. 2014, 20, 16806. (i) Zhang, C. Org. Biomol. Chem. 2014, 12, 6580. (j) Alonso, C.; de Marigorta, E. M.; Rubiales, G.; Palacios, F. Chem. Rev. 2015, 115, 1847. (k) Ravelli, D.; Protti, S.; Fagnoni, M. Chem. Rev. 2016, 116, 9850. See also ref. 2.

19） In the ATRA mecha-nism, the adduct radi-cal may not be oxi-dized by the second SET process (a) but instead abstract Y· from X─ Y to generate the product and X·, which adds to the ole-�n to propagate the radical chain reaction without the action of the photo─ excited spe-cies (b). Thus photore-dox catalysis only plays the role of initia-tor. The two mechanisms can be discriminated by a couple of experi-ments. One is the “lighting on─ off” experiment. If the reaction follows the ATRA mechanism (b), the photo─ initiated reaction proceeds even in the dark (while the light is turned off), once the reaction has been initiated by irradiation. On the other hand, in the case of the photore-dox mechanism (a), the reaction proceeds only while the light is still on. Determination of quantum yield is another method of discrimina-tion. The ATRA mechanism may be viable when Y is a halogen atom due to C─ halogen bonds being rather weak. See, (a) Curran, D. P. Synthesis, 1988, 489. (b) Wallentin, C.─ J.; Nguyen, J. D.; Finkbeiner, P.; Stephenson, C. R. J. J. Am. Chem. Soc. 2012, 134, 8875. (c) Arceo, E.; Montroni, E.; Melchiorre, P. Angew. Chem. Int. Ed. 2014, 53, 12064.

20） (a) Zhang, W.; Wang, F.; Hu, J. Org. Lett. 2009, 11, 2109. (b) Bizet, V.; Kowalczyk, R.; Bolm, C. Chem. Soc. Rev. 2014, 43, 2426.

21） (a) Kornblum, N.; Powers, J. W.; Anderson, G. J.; Jones, W. J.; Larson, H. O.; Levand, O.; Weaver, W. M. J. Am. Chem. Soc. 1957, 79, 6562. (b) Kornblum, N.; Jones, W. J.; Anderson, G. J. J. Am. Chem. Soc. 1959, 81, 4113.

22） (a) Nagib, D. A.; MacMillan, D. W. C. Nature 2011, 480, 224. (b) Merino, E.; Nevado, C. Chem. Soc. Rev. 2014, 43, 6598. (c) Wang, S.─ M.; Han, J.─ B.; Zhang, C.─ P.; Qin, H.─ L.; Xiao, J.─ C. Tetrahedron 2015, 71, 7949. (d) Lefebvre, Q.; Hoffmann, N.; Rueping, M. Chem. Commun. 2016, 52, 2493. See also ref. 4.

23） (a) Bodkin, J. A.; McLeod, M. D. J. Chem. Soc., Perkin Trans. 1 2002, 2733. (b) Reiser, O. Angew. Chem., Int. Ed. Engl. 1996, 35, 1308. (c) O’Brien, P. Angew. Chem. Int. Ed. 1999, 38, 326. (d) Nilov, D.; Reiser, O. Adv. Synth. Catal. 2002, 344, 1169. (e) MuÇiz, K. Chem. Soc. Rev. 2004, 33, 166. (f) Christie, S. D. R.; Warrington, A. D. Synthesis 2008, 1325.

24） (a) Nicewicz, D. A.; MacMillan, D. W. C. Science 2008, 322, 77. See

also, (b) Huo, H.; Shen, X.; Wang, C.; Zhang, L.; Röse1, P.; Chen, L.─ A.; Harms, K.; Marsch, M.; Hilt, G.; Meggers, E. Nature 2014, 515, 100. (c) Wang, C.; Qin, J.; Shen, X.; Riedel, R.; Harms, K.; Meggers, E. Angew. Chem. Int. Ed. 2016, 55, 685.

25） (a) Osawa, M.; Nagai, H.; Akita, M. Dalton Trans. 2007, 827. For the bimetallc photocatalysis, see ref. 4(a) and references cited therein. See also, (b) Murata, K.; Ito, M.; Inagaki, A.; Akita, M. Chem. Lett. 2010, 39, 915. (c) Nitadori, H.; Takahashi, T.; Inagaki, A.; Akita, M. Inorg. Chem. 2012, 51, 51. (d) Murata, K.; Inagaki, A.; Akita, M. Dalton Trans. 2013, 42, 6989. (e) Murata, K.; Inagaki, A.; Akita, M.; Halet, J.─ F.; Costuas, K. Inorg. Chem. 2013, 52, 8030. (f) Yasu, Y.; Inagaki, A.; Akita, M. J. Organomet. Chem. 2014, 753, 48. (g) Kozawa, K.; Inagaki, A.; Akita, M. Chem. Lett. 2014, 43, 290. (h) Mukuta, T.; Fukazawa, N.; Murata, K.; Inagaki, A.; Akita, M.; Tanaka, S.; Koshihara, S.; Onda, K. Inorg. Chem. 2014, 53, 2481. (i) Murata, K.; Saitoh, K.; Kikuchi, S.; Akita, M.; Inagaki, A. Chem. Commun. 2015, 57, 5717. (j) Shiba, Y.; Inagaki, A.; Akita, M. Organo-metallics 2015, 34, 4844. (k) Phungsripheng, S.; Kozawa, K.; Akita, M.; Inagaki, A. Inorg. Chem. 2016, 55, 3750.

26） (a) Kalyani, D.; McMurtrey, K. B.; Neufeldt, S. R.; Sanford, M. S. J. Am. Chem. Soc. 2011, 133, 18566. See also, (b) Skubi, K. L.; Blum, T. R.; Yoon, T. P. Chem. Rev. 2016, 116, 10035.

27） (a) Zuo, Z.; Cong, H.; Li, W.; Choi, J.; Fu, G. C.; MacMillan, D. W. C. J. Am. Chem. Soc. 2016, 138, 1832. See also, (b) Gui, Y.─ Y.; Sun, L.; Lu, Z.─ P.; Yu, D.─ G. Org. Chem. Front. 2016, 3, 522.

28） (a) Neumann, M.; Zeitler, K. Org. Lett. 2012, 14, 2658. See also, (b) Oelgemöller, M. Chem. Rev. 2016, 116, 9664.

PROFILE

Munetaka Akita is Professor and Director of Laboratory for Chemistry and Life Science at Tokyo Institute of Technology. He received his master and PhD degrees from Kyoto University (with Prof. Makoto Kumada in 1981) and Osaka University (with Prof. Akira Nakamura in 1984), respectively. In 1984 he moved to Tokyo Institute of Technology as a research associate and was appointed as a professor in 2002. Recently, his research interests involve application of carbon─ rich organometallics to molecular devices, pho-toredox catalysis, and supramolecular sys-tems based on aromatic systems.

Takashi Koike was born in 1977 in Toyama, Japan. He received his doctoral degree from Tokyo Institute of Technology in 2005 under the guidance of Prof. Takao Ikariya. After his graduate career, he joined the Professor Robert H. Grubbs’ group in California Insti-tute of Technology, USA, as a postdoctoral research scholar. In 2007 he returned to Tokyo Institute of Technology as an assistant professor of Chemical Resources Laboratory. In 2016 he was posted to Institute of Innova-tive Research. His research interests are in synthetic organic chemistry and organome-tallic chemistry. Recently, his efforts have been directed to the development of visible─ light─ driven photocatalysis.


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