1． Introduction

Because of the unique properties of the �uorine atom, the signi�cant emergence of �uorine chemistry has been seen as an integral part of synthetic chemistry over the past few decades. Driven by the striking success of �uorine chemistry, it is not an exaggeration to say that selective mono�uorination is being established via various �uorination techniques, such as electro-philic, nucleophilic, and radical �uorination techniques. Indeed, various functionalized �uorinated organic compounds have been synthesized and currently improve our daily lives. 1 The next challenge, therefore, is to ef�ciently prepare highly �uorinated compounds. One of the most de�nitive approaches to their synthesis is to employ the corresponding synthons with a de�ned partial �uorination pattern. This approach, however, strongly depends on the availability of suitable �uorine─ con-taining synthons. Another approach is the derivatization of readily available per�uorinated compounds.

Based on this concept, our efforts have been focused on the development of a new transformation of tetra�uoroethylene (TFE, 1; CF 2=CF 2) into a variety of poly�uorinated molecules for the following reasons. First, TFE is the simplest per�uoro-alkene and it is an industrially economical feedstock for the preparation of PTFE and co─ polymers with other alkenes. 2 Second, TFE is an environmentally benign feedstock with near─ zero global warming potential (GWP), 3 while most �uo-rocarbons and hydro�uorocarbons are extremely potent green-house gases. In addition, even though homogeneous catalytic reactions involving C─ F bond activation have received an increased amount of attention, 4 no catalytic reactions involv-ing 1 had been reported until we reported the �rst catalytic transformation reaction. 5

Against such a background, we envisioned that the scope of TFE as synthetic reagents should have been broadened, even though most transition─ metal species bearing a highly �uorinated ligand have been regarded as the deactivated spe-cies in catalytic reactions due to their excessive electron─ with-drawing characteristics. This account is a full overview of our studies on the development of the transition─ metal─ catalyzed, or ─ mediated, transformation reactions of tetra�uoroethylene,

which include (1) palladium─ catalyzed cross─ coupling reac-tions via a C─ F bond cleavage of TFE, (2) copper─ mediated 1,2─ diarylation of tetra�uoroethylene, and (3) nickel─ cata-lyzed co─ and cross─ trimerization of TFE, ethylene, and/or aldehydes (Scheme 1).

2． Pd(0)─ Catalyzed Coss─ coupling Reactions of Tetra�uoroethylene via C─ F Bond Cleavage

Tri�uorovinyl compounds, such as α, β,β ─ tri�uorostyrene and their derivatives, have attracted increased attention, because they are regarded as potential monomers for the prep-aration of polymers with a per�uorinated main chain. 6 Never-theless, conventional routes for their preparation have thus far not been fully established. For instance, most of the initial preparation routes for tri�uorostyrenes required multistep reactions. 7 A few reactions substituting the �uorine atom on TFE with a strong carbon nucleophile, which involve an addition─ elimination mechanism, are considered classic proce-dures. 8 They often, however, suffer from undesired side─ reactions, such as a multi─ substitution, even at low reaction temperatures. 8a,c Pd(0)─ catalyzed cross─ coupling reactions of

Scheme 1

Transition─ Metal Mediated Transformations of Tetra�uoroethylene into Various Poly�uorinated Organic Compounds

Masato Ohashi and Sensuke Ogoshi ＊

Faculty of Engineering, Osaka University Yamada─ oka, Suita, Osaka 565─ 0871, Japan

(Received July 13, 2016; E─ mail: ogoshi@chem.eng.osaka-u.ac.jp)

Abstract: This account describes our studies on the transition─ metal mediated transformation of tetra�uoro-ethylene into a variety of poly�uorinated organic compounds. We have divided our account into three main parts. First, we describe a series of palladium─ catalyzed cross─ coupling reactions that involve a C─ F bond cleavage of tetra�uoroethylene as a key elementary reaction step. Also described are reactions that enable the construction of tetra�uoroethylene─ bridging structures, such as R─ CF 2CF 2─ R’ and R─ CF 2CF 2─ H, from tetra-�uoroethylene using either copper(I) or nickel(0) species. This account emphasizes the merits of a derivatiza-tion of readily available per�uorinated compounds to valuable highly �uorinated organic compounds.

Vol.74 No.11 2016 （ 19 ） 1047

tri�uorovinylzinc, stannane, or borate reagents emerged in the 1980s as more direct synthetic methods. 9

2.1　 Negishi─ type Cross─ coupling with Diarylzinc ReagentsAt the outset of our exploration of palladium─ catalyzed

cross─ coupling reactions of TFE, our primary interest was whether a C─ F bond cleavage of TFE could occur on palla-dium. Inspired by a groundbreaking study of C─ F bond acti-vation of TFE on platinum, 10 we started investigating the reactivity of (η 2─ CF 2=CF 2)Pd(PPh 3) 2 (2a) toward lithium iodide. As a result, the C─ F bond cleavage on palladium took place at room temperature within a few minutes, affording a tri�uorovinyl palladium(II) iodide (3; Scheme 2, Figure 1). 11 Complex 3 was the �rst example of a mononuclear tri�uoro-vinyl complex generated by the carbon─ �uorine bond cleavage of TFE with a well─ de�ned structure. 12 The formation of a strong Li─ F bond might have been a driving force behind the reaction. An attempt at thermal cleavage of the C─ F bond in 2a in the absence of lithium iodide resulted in its decomposi-tion along with the liberation of a TFE molecule and the pre-cipitation of Pd black.

We then examined a stoichiometric reaction of 3 with diphenylzinc (4a) at room temperature for the following two reasons: (a) the expected product, α, β,β ─ tri�uorostyrene (5a), is known to undergo [2+2] cyclodimerization in a head─ to─ head manner at 80 ℃, 9d,13 and (b) TFE itself is known to react with organometallic reagents with strong nucleophilicity, such as organolithium and Grignard reagents, via an addition─ elimination mechanism. 8 As a result, the reaction with 4a pro-ceeded smoothly at room temperature in the presence of LiI and dibenzylideneacetone (DBA) to give 5a in 87% yield (Scheme 3). In contrast, the corresponding reaction conducted in the absence of LiI and DBA resulted in the formation of a complicated mixture that contained only a small amount of 5a. The role of lithium iodide in this reaction was to the forma-tion of reactive zincates such as Li[ArZnXI] (X=Ar or I, vide infra). 14

In line with these stoichiometric reactions, the cross─

coupling reaction of TFE with 4a, which was prepared by treating ZnCl 2 with 2 equiv of PhMgCl in situ, proceeded ef�-ciently at 40 ℃ in the presence of 0.01 mol% of Pd 2(dba) 3 and lithium iodide, affording 5a in 81% yield (TON=8,100, Scheme 4). 11 It should be mentioned that this reaction gave neither bisaryl─ substituted nor higher─ substituted products, which indicated that the monoarylated product would be smoothly released from the catalyst in the presence of an excess amount of TFE. We also found that the rate of the catalytic reaction was accelerated by the omission of a PPh 3 ligand. The reactions with �uoro─ substituted aryl zinc reagents took place without the occurrence of C(aryl)─ F bond cleavage to afford the corresponding products in moderate yields. Relatively lower isolated yields were caused either by their high volatility or by the undesired cyclodimerization at a higher concentra-tion.

On the basis of the aforementioned stoichiometric reac-tions, we proposed the catalytic cycle depicted in Scheme 5. Coordination of a TFE molecule to Pd(0) would take place to generate an η 2─ TFE species. The oxidative addition of a C─ F bond to Pd(0) would then be promoted by lithium iodide to generate a tri�uorovinyl palladium(II) intermediate. Trans-metalation with Li[ArZnXI] would yield a transient aryl palladium(II) intermediate, which would then undergo reduc-tive elimination to yield a (α, β,β ─ tri�uoro)styrene derivative along with regeneration of the Pd(0) species. The addition of lithium iodide is essential not only for accelerating cleavage of the carbon─ �uorine bond, but also for enhancing the reactivity of arylzinc reagents via the formation of zincates, such as Li[ArZnXI].

Figure 1.　 ORTEP Drawing of 3 with thermal ellipsoids at the 30% probability level. H atoms have been omitted for clarity.

Scheme 2

Scheme 4 a

Scheme 3

（ 20 ） J. Synth. Org. Chem., Jpn.1048

2.2　 Suzuki─ Miyaura─ type Cross─ coupling with ArylboronatesOur next concern was to apply the C(sp 2)─ F bond activa-

tion methodology to a Suzuki─ Miyaura type of C─ C bond formation reaction, which generally offers the advantages of tolerance across a broad range of functional groups. 15 Most of the reported Suzuki─ Miyaura types of cross─ coupling reac-tions that involve a C─ F bond cleavage of either highly elec-tron─ de�cient organo�uorine compounds or those bearing a directing group have traditionally been conducted in the presence of a base. 16 The role of a base in a Suzuki─ Miyaura coupling reaction is generally considered to follow one of two patterns: either converting a neutral organoboron compound into a nucleophilic boronate, or converting a palladium halide intermediate into an active palladium species via a ligand exchange reaction by the base. 17 Some coupling reactions with organoboron reagents are known to proceed under neutral conditions, in which a palladium alkoxy or acyl complex is generated as an active species in situ via the oxidative addition of a C─ O bond. 16a,18 We speculated that the base─ free Suzuki─ Miyaura cross─ coupling reaction of organo�uorine com-pounds would be developed if a transition─ metal �uoride generated via C─ F bond cleavage would be suf�ciently reactive to activate an organoboron compound.

We started exploring an active palladium species that could cleave the C─ F bond of TFE without additives, because our original protocol using lithium iodide eliminated the chance to generate a transition─ metal �uoride intermediate in return for ef�cient C─ F bond cleavage. As a result, tri�uorovinyl-palladium(II) �uoride (6) was generated by the thermolysis of (η 2─ CF 2=CF 2)Pd(PCy 3) 2 (2b) in THF at 100 ℃ under a TFE atmosphere (Scheme 6). 19 In the 19 F NMR spectrum of 6, characteristic up�eld─ shifted resonance assignable to a �uo-rine adjacent to palladium appeared at -317.9 ppm. As far as we could ascertain, complex 6 is the �rst example of a structur-

ally well─ de�ned oxidative addition product of TFE on a transition metal without the use of Lewis acid additives. In addition, �uoropalladium complexes generated via the oxida-tive addition of a C─ F bond are very rare. 16i,20

We next examined the reactivity of 6 toward 5,5─ dimethyl─ 2─ phenyl─ 1,3,2─ dioxaborinane (8a). Treatment of 6 with 4 equiv of 8a in the presence of DBA at 100 ℃ for 2 h afforded 5a in 75% yield (Scheme 7). 19 In contrast, no C─ C bond forma-tion occurred, even for a prolonged reaction time, when 8a was treated with the other palladium halides (9). These observa-tions clearly demonstrated that the C─ C bond formation with organoboron reagents is unique to palladium �uoride 6.

Based on these stoichiometric reactions, we successfully established the base─ free Pd(0)─ catalyzed cross─ coupling of TFE with arylboronates employing 10 mol % Pd(dba) 2 and 20 mol % P iPr 3 as a catalytic precursor (Scheme 8). 19 Although this catalytic reaction leaves much to be desired regarding the catalyst loading and the product yield, it is of great signi�cance in preparing substituted tri�uorostyrenes bearing nitro, alde-hyde, ester, and cyano groups. These functional groups can easily react with Grignard reagents that are required for the in situ preparation of organozinc reagents, and therefore, such products are dif�cult to synthesize from a coupling reac-tion with organozinc reagents.

We also extended this base─ free cross─ coupling reaction to other organo�uorine molecules. Although a Pd(0) catalyst was ineffective in a base─ free coupling reaction of �uoroarenes, the reaction of either vinylidene �uoride (10) or hexa�uoropropy-lene (11) took place to give the corresponding products (12 and

Scheme 7

Scheme 8 a

Scheme 5

Scheme 6

Vol.74 No.11 2016 （ 21 ） 1049

13; Scheme 9). In contrast, our attempt at extending this reac-tion to chlorotri�uoroethylene gave no coupling products, probably due to the generation of the unreactive tri�uorovinyl-palladium chloride intermediate. Concurrent with our studies, Yamakawa and Shen independently developed Pd─ catalyzed coupling reactions of chlorotri�uoroethylene with arylboronic acids in the presence of a base. 12,21

We proposed the catalytic reaction mechanism as follows (Scheme 10). Coordination of a TFE molecule to palladium would occur to generate an η 2─ TFE species. The combination of Pd(0) and trialkylphosphines with a strong σ ─ donor ability would then facilitate the oxidative addition of a C─ F bond of 1 to Pd(0) with no additives, yielding a tri�uorovinylpalladium(II) �uoride intermediate. The transmetalation with arylboronates followed by reductive elimination would afford the coupling product 5 along with a regeneration of the Pd(0) species and �uoroboranes. Another possible mechanism involving con-certed bimolecular elimination via a four─ membered transient intermediate could afford the product. 22 It should be re-emphasized that no extraneous base is required in this reac-tion, although extraneous base is generally requisite for the Suzuki─ Miyaura cross─ coupling reaction to promote a trans-metalation step with organoboron reagents.

2.3　 Hiyama─ type Cross─ coupling with ArylsilanesIn addition to a base─ free Suzuki─ Miyaura type cross─

coupling reaction of TFE, we have also developed a related Hiyama─ type cross─ coupling reaction with aryltrialkoxysi-

lanes (Scheme 11). 23 Unlike the base─ free Suzuki─ Miyaura type reaction, the optimal ligand in a base─ free Hiyama─ type cross─ coupling reaction was PCyp 3 (Cyp=c ─ C 5H 9). In addi-tion, the time─ product yield curve of the catalytic reaction indicated the existence of an induction period. On the basis of the hypothesis that a sharp increase in the generation rate of the product was caused by the �uorosilanes that were gene-rated in situ as the coupling reaction proceeded, we demon-strated how the catalytic reaction was obviously accelerated by the addition of 10 mol % of FSi(OEt) 3. Under the optimal reaction conditions, the coupling reaction of TFE with PhSi(OMe) 3 took place at 80 ℃ to afford 5a in 94% yield. In addition to a variety of trimethoxyarylsilanes, an alkenyl-siloxane was also applicable to this coupling reaction.

Although the mechanistic details of the base─ free Hiyama coupling reaction have not been fully revealed at this time, there is no doubt that metal �uoride complexes play a crucial role. Fluorosilanes accelerated the Pd(0)─ catalyzed Hiyama coupling reaction of TFE, whereas it was not necessary with a base─ free mechanism. Based on the fact that the Ni(0)─ catalyzed Hiyama─ type coupling reaction of �uoroarenes, in which [Ni 2(

iPr 2Im) 4(COD)] is highly ef�cient in the oxidative addition of a C─ F bond on �uoroarenes, 16h,m,24 required no �uorosilane additives, we proposed that �uorosilanes would facilitate the oxidative addition of a C─ F bond of TFE toward Pd(0). We excluded the mechanism involving a free �uoride anion, because the coupling reaction of TFE did not proceed when TBAF was an additive.2.4　 Lithium Salt Promoted the Selective Monoalkylation of

TFEWe have also developed related monoalkylation reactions

of TFE, which involve a classic addition─ elimination mecha-nism without the use of a palladium catalyst (Scheme 12). 25 In the presence of lithium iodide, the reaction of TFE with dieth-ylzinc (15) in t ─ BuCN at 60 ℃ for 24 h resulted in the forma-tion of 1,1,2─ tri�uoro─ 1─ butene (16) in 51% yield. The yield of 16 at approximately 50% was rationalized by the lack of reactivity of ethylzinc iodide toward TFE under the same reac-

Scheme 9

Scheme 10

Scheme 11 a,b,c

（ 22 ） J. Synth. Org. Chem., Jpn.1050

tion conditions. Monitoring the reaction in the absence of lithium iodide by NMR spectroscopy demonstrated the forma-tion of a carbozincation product, EtCF 2CF 2ZnEt (18). This observation indicated that the use of lithium iodide would facilitate β ─ �uoride elimination. 26 On the other hand, tri�uo-rovinylzinc iodide (19) was obtained in 20% yield and 16 was not generated at all when 15, instead of diphenylzinc 4a, was applied to the Negishi─ type cross─ coupling reaction with TFE. 11

Furthermore, we also demonstrated that the addition of lithium chloride improved the rate and selectivity of the reac-tion of benzylmagnesium chloride (20a) with TFE at room temperature for 0.5 h to give 21a in a quantitative yield (Scheme 13). 25 Lithium chloride might enhance the reactivity of 20a as a result of the formation of the ate species. 14 Although simple alkylmagnesium reagents such as ethyl-magnesium chloride retarded the reaction, both benzyl and allyl magnesium chloride derivatives participated in the reac-tion.

3． Construction of a Tetra�uoroethylene─ bridging Structure

We next attempted to construct a tetra�uoroethylene─ bridging motif (R─ CF 2CF 2─ R’), since TFE had the potential to be an ideal starting material. Tetra�uoroethylene─ bridging

skeletons are typically prepared either via the �uorination of a C≡C triple bond with F 2 gas or via the deoxo�uorination of 1,2─ diketones with �uorination reagents. 27 However, both syn-thetic routes face the issue of low functional group tolerance and require the use of toxic or costly reagents. Thus, the deve-lopment of an ef�cient and straightforward alternative has been in high demand. Against such a background, our atten-tion was initially drawn to the 1,2─ addition of organocopper reagents to TFE, the so─ called carbocupration of TFE. While alkenes are not considered to be applicable to carbocup ration with the exception of highly strained cyclopropenes, 28 per�uo-roalkenes with electrophilicity should be able to undergo car-bocupration. In addition, �uoroalkylcopper(I) species are assumed to be a key intermediate in copper mediated �uoroal-kylation reactions. 29

3.1　 Cu(I)─ Mediated Diarylation of Tetra�uoroethyleneThe exposure of (phen)CuPh (22), 30 which was prepared by

the reaction of 5,5─ dimethyl─ 2─ phenyl─ 1,3,2─ dioxaborinane 8a, CuO tBu, and 1,10─ phenanthroline (phen), to TFE in THF at 40 ℃ for 24 h resulted in the clean formation of (phen)CuCF 2CF 2Ph (23) in 88% yield as a brown solid (Scheme 14). 31 Although organoboron compounds are less common in the preparation of organocopper reagents, 32 their use confers great advantages with regard to ease of handling and functional group tolerance. Furthermore, the very weak Lewis acidic nature of organoboronate compounds is quite important for the preparation of 23 because Lewis acidic species can cause decomposition of the complex through α ─ or β ─ �uorine elimi-nation. In fact, the reaction using PhMgBr instead of 8a did not produce 23, and tri�uorostyrene 5a was obtained in 70% yield as a result of β ─ �uorine elimination.

This is the �rst example of the preparation of a �uoroal-kylcopper complex via carbocupration of a per�uoroalkene, 33 whereas known �uoroalkylcopper complexes are generally synthesized via transfer of a �uoroalkyl group from a �uoroal-kyl halide, �uoroalkyl silane, or �uoroalkane. It is noteworthy that 23 is bench─ stable under N 2 at room temperature and can be stored for at least a few months without decomposition. We were successful in preparing a p ─ �uorophenyl analogue, (phen)-CuCF 2CF 2(p ─ C 6H 4F) (23─F), and were able to unambiguously determine its molecular structure by X─ ray diffraction study (Figure 2). As with the dynamic behavior in solution of the known �uoroalkylcopper complexes, 34 23─F existed as a mix-ture of a neutral form and an ionic form, [(phen) 2Cu]-[Cu{CF 2CF 2(p ─ C 6H 4F)} 2] in THF─ d 8 solution.

Fluoroalkylcopper 23 could be transformed into 1,2─ diaryl─ 1,1,2,2─ tetra�uoroethanes via the coupling reaction with iodoarenes: the reaction of iodobenzene (24a) with 23 (1.2 equiv) in THF at 60 ℃ gave 1,1,2,2─ tetra�uoro─ 1,2─ diphenylethane (25a) in 98% yield (Scheme 15). 31 Both elec-tron─ de�cient and electron─ rich iodoarenes (p ─ X─ C 6H 4─ I; X=CF 3 or OMe) afforded the corresponding products in high

Scheme 12

Scheme 13 a

Scheme 14

Vol.74 No.11 2016 （ 23 ） 1051

yields, while the latter required a longer reaction time. A variety of functionalized iodoarenes bearing a formyl, ester, cyano, boronate, phosphate, amino, bromo, or chloro group were tolerant of the reaction conditions. It should be men-tioned that 2─ iodopyridine and alkenyl iodide also reacted with 23 to give the corresponding products in excellent yields. Although the coupling reaction with bromobenzene did not take place, electron─ de�cient 4─ bromophthalic anhydride par-ticipated in the coupling reaction to give the corresponding product. We successfully applied the present method to the expeditious synthesis of a liquid─ crystalline compound bear-ing a tetra�uoroethylene─ bridging structure (26), which was synthesized in six steps from commercially available 4─ pentyl-

phenylboronic acid (Scheme 16). This procedure was superior to the previously reported preparation route 35 in terms of not only the number of steps but also the ready availability of the starting material that constructs a tetra�uoroethylene─ bridg-ing key structure.

In addition to the above─ mentioned stepwise reaction using 23, a one─ pot synthesis of 1,2─ diaryl─ 1,1,2,2─ tetra�uo-roethanes could be utilized without isolation of the corre-sponding �uoroalkylcopper intermediates (Scheme 17). 31 Vari-ous halogen─ substituted arylboronates, such as �uorine, chlorine, and bromine substituents, were converted into 1,2─ diaryl─ 1,1,2,2─ tetra�uoroethanes 25. The use of electron─ rich arylboronates yielded the products in high yields, although the carbocupration step with electron─ de�cient arylboronates required longer reaction times to generate the �uoroalkyl-copper intermediates. It should be mentioned that no signi�-cant side products were obtained in the aforementioned one─ pot sequential reaction, while a complicated mixture was obtained when all of the starting materials, including arylboro-nate, were mixed before heating.

Furthermore, treatment of 23 with MgBr 2 immediately

Figure 2.　 ORTEP Drawing of 23─F with thermal ellipsoids at the 30% probability level. H atoms have been omitted for clarity.

Scheme 16 a

Scheme 15 a

Scheme 17 a,b

（ 24 ） J. Synth. Org. Chem., Jpn.1052

gave α, β,β ─ tri�uorostyrene 5a via β ─ �uorine elimination. In this reaction, the Lewis acidic metal species would accelerate the β ─ �uorine elimination via the transient 6─ membered struc-ture as depicted in Scheme 12. Based on this �nding, we have also developed a copper─ mediated one─ pot synthesis of tri�u-orostyrene derivatives (Scheme18). 36 Exposure of a mixture of arylboronates 8, CuO tBu, and phen to TFE (3.5 atm) followed by the addition of sodium iodide gave tri�uorostyrene deriva-tives 5 in moderate yields.

3.2　 Ni(0)─ Catalyzed Linear Selective co─ Trimerization of TFE and Ethylene

During the course of our continuous studies, we reaf�rmed that oxidative cyclization between TFE and an unsaturated compound with Ni(0) might be a key reaction step in C─ C bond formation reactions. 37 A limited number of reports have focused on �ve─ membered nickelacycles generated via the oxi-dative cyclization of TFE with other unsaturated compounds 38 and on their applications to catalytic transformations. 38f Based on the early study employing ethylene and styrene with chro-mium, 39 it is possible that the �ve─ membered nickelacycle generated via the oxidative cyclization of TFE and ethylene with Ni(0) may serve as an intermediate for the co─ trimeriza-tion of TFE and ethylene.

A �ve─ membered nickelacycle, (CF 2CF 2CH 2CH 2)- Ni(PPh 3) 2 (28), was isolated quantitatively by successive treat-ment of a mixture of Ni(cod) 2 and PPh 3 with ethylene (27) followed by TFE, whereas the known octa�uoronickela-cyclopentane, (CF 2CF 2CF 2CF 2)Ni(PPh 3) 2,

37g,i was generated when the Ni(0)/PPh 3 mixture was exposed to TFE prior to the ethylene treatment (Scheme 19). 40 The X─ ray diffraction study of 28 clearly demonstrated that the nickelacyclopentane framework was derived from one TFE and one ethylene unit (Figure 3). Further treatment of 28 with ethylene led to the formation of 5,5,6,6─ tetra�uoro─ 1─ hexene (29) in 80% yield, and (η 2─ CH 2=CH 2)Ni(PPh 3) 2 was generated concomitantly in the crude reaction mixture. The co─ trimerization product was obtained as the sole product, and neither 3,3,4,4─ tetra�uoro─ 1─ hexene nor any C 8, or higher, products were detected in the crude product. This observation led to the suggestions that the migratory insertion of ethylene to the Ni─ CH 2 bond rather than to the Ni─ CF 2 bond in 27 took place selectively to gene-rate a tentative seven─ membered nickelacycle intermediate (30) and that β ─ hydride elimination from 30 would be considerably faster than further ethylene insertion to the Ni─ C bonds in 30.

We successfully applied this procedure to the catalytic co─

trimerization of TFE and ethylene, which led to 29 under the optimized reaction conditions (TFE: ethylene=5 atm: 25 atm, Ni(cod) 2/PCy 3 (10 and 20 mol%, respectively)), albeit with a low turnover number of 13 (Scheme 20). 40 Attempts at expand-ing the substrate scope with respect to other alkenes failed; neither the use of vinylidene �uoride, or chlorotri�uoroethy-lene in place of TFE nor the use of 1─ hexene or styrene instead of ethylene gave the corresponding co─ trimerization products. In contrast, the catalytic reaction of 2─ allylstyrene (31) with TFE in the presence of Ni(cod) 2 and PPh 3 (1 and 2 mol %, respectively) resulted in smooth progress, and afforded a 2─ methylene─ 2,3─ dihydroindene derivative (32) in 58% yield. Monitoring the catalytic reaction using 31 at -30 ℃ by means of NMR spectroscopy, we detected a transient intermediate (33) on the basis of the observation of the characteristic high─ �eld shifted resonances assignable to the allyl group coordi-nated to nickel (δ H 3.64, 4.21, 4.43; δ C 87.5 and 96.4) as well as the four inequivalent 19 F nuclei. This intermediate as well as

Scheme 18 a,b,c

Scheme 19

Figure 3.　 ORTEP Drawing of 28 with thermal ellipsoids at the 30% probability level. H atoms have been omitted for clarity.

Scheme 20

Vol.74 No.11 2016 （ 25 ） 1053

unreacted 33 was fully converted into 32 at room temperature.On the basis of these �ndings, we proposed the catalytic

cycle for the reactions of TFE with either ethylene 27 or 31, which was a widely accepted mechanism observed in ethylene trimerizations that involves metallacycle intermediates (Scheme 21). 41 Based on the observation of the transient inter-mediate 33, we can stipulate that the oxidative cyclization of TFE and ethylene occurred prior to that of the two units of ethylene.

We also demonstrated that, although stoichiometric, nickela cycle 28 is very useful for the preparation of cross─ trimerization products of TFE, ethylene, and α, β ─ unsaturated carbonyl compounds. For instance, both ethyl acrylate and chalcone reacted with 28 to afford the corresponding β ─ disubstituted ketones (34 and 35) in 27 and 60% isolated yields, respectively (Scheme 22a). 40 On the other hand, the use of phenyl─ 1─ propenyl ketone resulted in the formation of an E/Z mixture of 6,6,7,7─ tetra�uoro─ 3─ methyl─ 1─ phenyl─ 2─ hepten─

1─ one (36) in 61% isolated yield, while the formation of another η 3─ oxallyl nickelacycle intermediate (37) was isolated when the reaction was conducted at room temperature (Scheme 22b).3.3　 Ni(0)─ Catalyzed Linear Selective Cress─ trimerization of

TFE, Ethylene, and AldehydesOn the basis of the aforementioned unique reactivity of

the α ─ CH 2 moiety in 28, we formulated another hypothetical catalytic cycle that involves the nucleophilic addition of a �ve─ membered nickelacycle to aldehyde (Scheme 23). As in the case of the co─ trimerization reaction of TFE and ethylene, the oxi-dative cyclization of TFE and ethylene with Ni(0) �rst occurs to generate a 2,2,3,3─ tetra─ �uoronickelacyclopentane species. A nucleophilic addition of the Ni─ CH 2 moiety to the carbonyl group in an aldehyde then proceeds to afford a seven─ mem-bered oxanickelacycle intermediate. Subsequent β ─ hydride elimination is followed by reductive elimination, yielding a 4,4,5,5─ tetra�uoro─ 1─ pentanone derivative along with a regeneration of the Ni(0) species. Indeed, NBO charge distri-bution analysis demonstrated that the α ─ carbon in 28 is the most nucleophilic among the related �ve─ membered nickela-cycles (Figure 4). In addition, treating 28 with an excess amount of benzaldehyde (38a) in C 6D 6 at 40 ℃ gave 39a in 94% yield. 42

In line with the foregoing hypothesis, we developed a Ni(0)─ catalyzed cross─ trimerization reaction of TFE, ethylene, and aldehydes leading to �uorinated ketones 39 (Scheme 24). 42 Employing 5 mol% of Ni(cod) 2 and IPr as a catalyst in toluene at 150 ℃ promoted the cross─ trimerization of TFE, ethylene 27, and benzaldehyde 38a in a regioselective manner. Although 5,5,6,6─ tetra�uoro─ 1─ hexene 29 was one of the potential side─ reaction products, it was not detected in this crude reaction mixture. In addition, elevating the reaction temperature to 150 ℃ was key to the successful development of selective and ef�cient cross─ trimerization. While the catalytic reaction con-ducted at 40 ℃ gave a signi�cant amount of another envisaged side─ reaction pro duct, benzyl benzoate, the reaction conducted at 150 ℃ suppressed the progress of the undesired homo─ Tishchenko reaction. 43 We then examined the scope and limita-tions with respect to various arylaldehydes, and the present

Scheme 21

Scheme 23

Scheme 22

（ 26 ） J. Synth. Org. Chem., Jpn.1054

system showed high functional group tolerance. However, nei-ther p ─ chloro─ nor p ─ bromo─ benzaldehyde afforded the desired products, which might have been due to the occurrence of an undesired oxidative addition of a C─ X bond (X=Cl or Br).

4． Conclusion

We have presented an overview of our work on transforma-tions of tetra�uoroethylene (TFE) that is readily available to industries into a variety of valuable poly�uorinated organic compounds by using appropriate transition─ metal species. One of the critical factors that enables the transformation of poly-�uoroalkenes such as TFE involves overcoming the long─ standing dif�culty of employing a proper combination of transition─ metals and ligands, because most transition─ metal species bearing a highly─ �uorinated ligand are considered to be deactivated species in catalytic reactions due to an excessive electron─ withdrawing nature. In addition, we isolated some key intermediate complexes and demonstrated their molecular structures, and this information will prove to be a guide for the future development of further transformation reactions. Indeed, by taking full advantage of our accumulated knowl-edge, we have developed other types of transformations of poly�uorinated compounds such as per�uoroarenes as well as tri�uoromethylacetophenones. 44 Our future work will focus continuously on a further expansion of the scope of readily

available poly�uorinated compounds as synthetic reagents. To achieve this goal, it will also be important to gain deeper insights into other key elementary reaction mechanisms of poly�uorinated compounds in the presence of transition─ metal species.

AcknowledgmentThe authors wish to thank their co─ authors. Financial

support by Grants─ in─ Aid for Young Scientists (A) form JSPS and from The Noguchi Institute to M. O. and by Grants─ in─ Aid for Scienti�c Research (A) and Scienti�c Research on Innovative Areas “Molecular Activation Directed toward Straightforward Synthesis” from MEXT, and A─ STEP from JST to S. O. is greatly appreciated. We also express our thanks to Daikin Industries, Ltd., for supplying TFE.

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Figure 4.　 Natural population analysis of (CF 2CF 2CF 2CF 2)Ni(PPh 3) 2 (I), (CF 2CF 2CH 2CH 2)Ni(PPh 3) 2 (II), and (CH 2CH 2CH 2CH 2)Ni(PPh 3) 2 (III). In these diagrams, the nickel, phosphorus, carbon, �uorine, and hydrogen atoms are in dark green, yellow, black, pale purple, and sky blue, respectively. Red and blue values represent positive and negative charge, respectively. All phenyl groups on the phos-phorus atoms have been omitted for clarity.

Scheme 24 a,b

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PROFILE

Masato Ohashi received his Ph.D. from Tokyo Institute of Technology under the supervision of Professor Hiroharu Suzuki in 2003. After joining the research group of Kazushi Mashima at Osaka University as a JST post─ doctoral fellow, he moved to Aachen in 2006, where he was a Humboldt research follow with Jun Okuda (RWTH─ Aachen). In 2007, he joined the group of S. Ogoshi at Osaka University as an assistant professor. In 2012, he was appointed to Associate Professor of Osaka University. His current research interests include transition metal─ catalyzed transformation reactions of organo�uorine compounds as well as their mechanistic investigation.

Sensuke Ogoshi studied at Osaka University and received his Ph.D. under the supervision of Professor Shinji Murai in 1993. In that year he joined the faculty at Osaka University as an assistant professor. He was promoted to an associate professor in 1999, and is a full professor since 2007. In the meantime, he pursued his academic career also at University of Alberta during 1996─ 1997, where he joined the research group of Professor Jeffrey M. Stryker (JSPS Fellow-ships for Research Abroad). His research has been directed toward the discovery of new transition metal complexes that can act as a key reaction intermediate in new transforma-tion reactions of unsaturated compounds. In addition, he has been focusing on the correla-tion between structure and reactivity of organo─ transition─ metal complexes.

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