Olefin Polymerization and Copolymerization Catalyzed By

Ole n Polymerization and Copolymerization Catalyzed by Dinuclear Catalysts Having Macrocyclic Ligands

Daisuke Takeuchi ＊

＊ Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University 3 Bunkyo ─ cho, Hirosaki 036 ─ 8561, Japan

(Received June 30, 2019; E ─ mail: [email protected])

Abstract: Recently dinuclear complexes have attracted attention as catalysts for ole n polymerization. This account describes the synthesis and catalytic behavior of dinuclear Pd, Ni, Co, and Fe complexes with cyclic ligands. The two metal centers of the dinuclear complexes are located in close proximity owing to the rigid, cyclic ligands. These dinuclear complexes show higher catalytic activity, and/or produce polymers with higher molecular weight than the corresponding mononuclear complexes. The dinuclear catalysts were shown to have higher thermal stability during polymerization. They also enable the synthesis of branched ethylene/acrylate copolymer with acrylate units incorporated into the main chain, and the selective incorporation of non ─ conjugated dienes in the copolymerization with ethylene, reactions which could not be achieved using their mononuclear analogues.

lene with 1,1 ─ disubstituted ole ns, which are hardly incorpo- 1． Introduction rated in the copolymerization catalyzed by the corresponding 11 Metal complexes have been extensively used as catalysts for mononuclear complex. Ef cient trapping of the 1,1 ─ disubsti- organic synthesis and enable reactions that can hardly be tuted ole ns via agostic interaction of the monomer with the achieved otherwise. Most of the metal complexes used in second metal center is proposed to be of key importance for organic synthesis are mononuclear catalysts, which contain the success of this process. one metal center in the complex. Recently, dinuclear and multi- There have been many examples of dinuclear catalysts with nuclear complexes, which have two or more metal centers in structures containing two mononuclear centers tethered with a the complex, are also attracting attention as catalysts for exible spacer. Such complexes can adopt various conforma- organic reactions, because they can sometimes achieve particu- tions, some of which are not suitable for ef cient cooperation larly stereoselective and/or accelerated reactions owing to between the two metal centers. Recently, dinuclear complexes cooperation between the metal centers. 1 ─ 3 with rigid frameworks have been designed. In these complexes, Metal complexes also catalyze polymerization reactions the two metal centers are positioned in close proximity with ef ciently. Some polymerization reaction mechanisms involve each other, and conformational change of the complexes is cooperation of two metal complexes. For example, metal ─ restricted. Such dinuclear complexes with a rigid framework mediated polymerizations of heterocyclic monomers and can be categorized into four groups (Figure 1). (meth)acrylates are sometimes accelerated by the addition of Lewis acidic metal complexes. 4,5 In these cases the Lewis acids coordinate and activate the monomer toward nucleophilic attack by the growing species. Dinuclear complex catalysts have been also used in the polymerization of these monomers, and ef cient and stereoselective polymerizations have been achieved. 6,7 Transition metal complexes have been extensively used as 8 catalysts for the polymerization of ethylene and α ─ ole ns. Recently designed metal complex catalysts have enabled living ─ Figure 1. Four kinds of dinuclear complexes with rigid framework polymerization of ethylene and α ─ ole ns, stereospeci c used as the catalyst for ethylene/ole n polymerization. polymerization of α ─ ole ns, and copolymerization of ethylene and α ─ ole ns with various comonomers including polar, Type I dinuclear complexes have two metal centers on a vinylic monomers. Mononuclear complexes are used as the rigid planar framework. Marks designed dititanium, dizirco- catalysts in most cases, where the polymerization proceeds via nium, and dinickel ─ salicylaldimine complexes having a naph- coordination and insertion of the monomer on the metal cen- thalene backbone (Figure 2). 12 The dinickel complex has a ter. short Ni ─ Ni distance (Ni ─ Ni=5.8024 Å), and allows more Dinuclear and multinuclear complexes have also attracted incorporation of norbornene unit in the copolymerization attention as catalysts for ethylene and ole n polymerization. 9 with ethylene than the corresponding mononickel catalyst. 12d A The dinuclear complexes often show higher catalytic activity similar dinickel complex, but with a different salicylaldimine and/or produce a polymer with higher molecular weight than site, affords linear branched polyethylene in ethylene polymeri- 10 13 the related mononuclear catalyst. For example, a dinuclear Ti zation via a tandem ─ type mechanism. catalyst reported by Marks enables copolymerization of ethy- Type II dinuclear complexes have an oligoarylene frame-

1136 （ 84 ） J. Synth. Org. Chem., Jpn. work with restricted rotation about the aryl ─ aryl linkage. Agapie reported terphenylene ─ based dinickel and dizirco- nium complex catalysts of this type. 14 The dinickel complex 14a has a relatively short Ni ─ Ni distance (Ni ─ Ni=7.1 Å), and enables copolymerization of ethylene with amino ─ ole ns, a process which has been dif cult to be achieve using mononickel catalysts. 14c Type III di ─ and trinuclear complexes have two and three metal centers respectively, on a rigid, cyclic ligand. Lee reported such a dinickel complex with cyclic ligands, which has a Ni ─ Ni distance of 8.869 Å, and is active for ethylene polymerization. 15 A triiron complex with a cyclic tris{bis(imino)pyridine} ligand shows higher activity for ethylene polymerization than the monoiron complex, 16 while a trinickel complex with a cyclic tris{salicylaldimine} ligand brings about isotactic polymerization of propylene, which cannot be achieved by the mononickel complex. 17 Type IV dinuclear complexes have two mononuclear centers connected by a rigid bridge. Chen and Ma reported several types of dititanium and dinickel complexes categorized in this 18 group. Although their metal ─ metal distances are rather long, they show higher catalytic activity, and/or produce polymer with larger molecular weight than the corresponding mononuclear complexes. We designed a series of dinuclear complexes with cyclic 19 ligands categorized as type III. They have double ─ decker type structures, with two mononuclear centers connected by two rigid bridges. One of the apical positions of the metal center is covered by the other metal complex moiety. The metal ─ metal distance is short, and limited conformational change occurs during the catalysis. In this account, we describe our work on synthesis of dinuclear Ni, Pd, Co, and Fe complexes with cyclic ligands, and their application as catalysts for ole n polymerization and copolymerization. 2． Syntheses of Cyclic Ligands and their Transition Metal Complexes 2.1 Synthesis of Cyclic Ligands It has been reported that the reaction of 2,2’ ─ ethylene bisaniline with 2,6 ─ diacetylpyridine affords the mono ─ imine (one ─ to ─ one reaction product) rather than the cyclic bis{bis(imino)pyridine} ligand. 20 However, our attempt to carry out the same reaction in reuxing EtOH in the presence of a few drops of acetic acid for 36 h lead to successful synthesis of the ligand L1a in 52% yield. 21 Other bisanilines with ortho ─ and/or para ─ substituents, which were synthesized according to the procedure in Scheme 1, also reacted with 2,6 ─ diacetylpyridine to give the corresponding cyclic bis{bis(imino) pyridine} ligands (L1b d) in 40 ─ 95% yield (Scheme 2). Bisani- lines with a rigid xanthene─ backbone were synthesized either by the reported procedure, 22 or by the procedure shown in Scheme 2. The reaction of the rigid bisanilines with 2,6 ─ diacetylpyridine took place in reuxing 1 ─ butanol in the presence of a catalytic amount of p ─ TsOH to give the corre- 1 13 1 sponding ligands (L2a,b, 66 ─ 74% yield). H and C{ H} NMR analyses of the ligand L2 showed that the signals due to the two methyl groups at the 9 ─ position of the xanthene group appear at different positions, which indicates that the two Figure 2. Representative examples of di ─ or trinuclear complexes bis(imino)pyridine groups of the ligand are orientated in the with a rigid framework used as the catalyst for ethylene/ ole n polymerization. same direction. X ─ ray crystal structure analysis of the ligand L1b, on the

Vol.77 No.11 2019 （ 85 ） 1137 Scheme 1. Reagents: (a) (Boc) 2O; (b) (CH 2=CH)BF 3K; (c) Pd(OAc) 2, Et 3N; (d) Pd ─ C, H 2; (e) TFA; (f) MeSO 3H; (g) Br 2; (h) NaN 3, CuI, N,N ─ dimethylethylenediamine.

23 Scheme 2 ligands (L3a,b) in 77% yield (Scheme 3). X ─ ray crystal structure analysis of the ligand L3a showed the distance between two naphthalene groups is 3.494 Å owing to π ─ π stacking. The cyclic ligand L3d could also be synthesized by a similar reaction of the bisaniline with 2,3 ─ butanedione (69% yield).

Scheme 3

other hand, indicated that the two bis(imino)pyridine groups of the ligand are orientated in different directions, as well as the presence of π ─ π stacking between the two pyridine groups of the ligand (Figure 3).

Figure 3. ORTEP view of ethylene ─ bridged L1b. In contrast, the reaction of 2,2’ ─ ethylene bisaniline with acenaphthenequinone afforded a mixture of various products, Bisanilines of xanthene structure reacted smoothly with and isolation of the cyclic ligand L3c was not successful. The acenaphthenequinone to afford the corresponding bis(diimine) reaction of 2,2’ ─ ethylene bisaniline with acenaphthenequinone

1138 （ 86 ） J. Synth. Org. Chem., Jpn. Scheme 4. Reagents: (a) Pd(PPh 3) 4, Na 2CO 3; (b) acetyl chloride; (c) 4,5 ─ diaminoxanthenes, p ─ TsOH.

in the presence of ZnCl 2 did give the corresponding dizinc complex in 81% yield, but demetallation to remove zinc using dipotassium oxalate did not proceed. The cyclic bis(salicylaldimine) ligands were synthesized 24 according to Scheme 4. Suzuki ─ coupling reaction of the 25 bis(boronic acid) shown with OH ─ protected 3 ─ bromo ─ salicylaldehyde, 26 followed by deprotection afforded the bis(salicylaldehyde), which was subjected to condensation with the bisaniline to give the bis(phenoxyimine) ligands L4 in 69 ─ 74% yield. 2.2 Synthesis of Transition Metal Complexes Containing Cyclic Ligands The reaction of the cyclic bis{bis(imino)pyridine} ligands

(L1, L2) with FeCl 2 or CoCl 2 afforded the corresponding diiron and dicobalt complexes (Figure 4, Fe 2(L1), Co 2(L1),

Fe 2(L2), Co 2(L2)). X ─ ray crystal structure analysis of the dicobalt complex Co 2(L1b) showed that the two bis(imino)pyridine Co moieties stack in an antiparallel manner at a distance of 7.74 Å. The Co and Fe complexes are paramagnetic and show 1 H NMR signals in the range -30 ─ 120 and -30 ─ 90 ppm, respectively. The Fe and Co complexes Fe 2(L1a) and Co 2(L1a) 2＋ 2－ readily undergo isomerization to [Fe(L1a)] FeCl 4 and 2＋ 2－ 2＋ 2－ [Co(L1a)] CoCl 4 , respectively, in methanol. [Fe(L1a)] FeCl 4 1 is diamagnetic and shows H NMR signals in the range 1 ─ 9 ppm (Figure 5). The reaction of the cyclic bis(diimine) ligands L3 with two equivalents of PdMeCl(cod) gave the dipalladium complexes

Pd 2(L3). The reaction of the ligand L3b with PdMeCl(cod) in one ─ to ─ one molar ratio also afforded the dipalladium complex

Pd 2(L3b), rather than the monopalladium complex Pd(L3b). Thus, the formation of the second palladium center is much faster than the formation of the rst palladium center owing to the allosteric effect. Attempted synthesis of the Pd complex

Pd 2(L3d) using the cyclic ligand L3d was not successful. This could be due to the more exible structure of L3d compared to L3a and L3b. NMR analysis of the dipalladium complex

Pd 2(L3b) showed two pairs of Pd ─ Me signals at 0.85, 0.74, -0.50, and -0.56 ppm. The intensity ratio of the signals at 0.85 and 0.74 ppm is 3.9:1, and the intensity ratio of the signals at 0.85 and 0.74 ppm to those at -0.50 and -0.56 ppm is 2.7:1. The signals at 0.85 and 0.74 ppm are assigned to the Pd ─

Me of Pd 2(L3b) (X=Me, Y=Cl and X=Cl, Y=Me). The signals at -0.50 and -0.56 ppm are due to Pd 2(L3b)(anti) (X=Me, Y=Cl and X=Cl, Y=Me), and the up eld shift of these Pd ─ Me signals is ascribed to the shielding effect of the acenaph- thene group. In the presence of 2.2 molar equivalent of Figure 4. Dinuclear metal complexes containing the cyclic ligands, - NaBARF (BARF=B{C 6H 3(CF 3) 2─ 3,5} 4 ), the dipalladium and the corresponding mononuclear complexes. complex did not show the signals due to the anti isomers at -0.50 and -0.56 ppm. Thus, the two Pd centers of the cationic The cyclic bis(salicylaldimine) ligands L4 were rst treated form of the complex are orientated to the same direction. with NaH, then (Me 3P) 2NiMeCl was added to give the corre-

Vol.77 No.11 2019 （ 87 ） 1139 3.2 Copolymerization of Ethylene with Polar Vinyl Monomers and Non conjugated Dienes by the Double decker Type Dinickel ─Complexes ─ Although the late transition ─ metal catalyzed copolymerization of ethylene with α ─ ole ns often suffers from a signi - cant decrease in polymerization rate and polymer yields compared to ethylene homopolymerization, the polymerization of

ethylene by the dinickel catalyst Ni 2(L4b)/Ni(cod) 2 was not retarded by 1 ─ hexene. The absence of butyl branches in the copolymer indicates negligible incorporation of 1 ─ hexene. In contrast, ethylene polymerization in the presence of 1,6 ─ hepta-

diene catalyzed by Ni 2(L4b)/Ni(cod) 2 afforded the copolymer having the 1,2 ─ trans ─ cyclopentylene structure formed by the incorporation of 1,6 ─ heptadiene and accompanying cyclization (3.6 mol %) (Scheme 5). 24,31 The copolymer produced had a much higher molecular weight, and a less branched structure than the polyethylene obtained under similar reaction conditions but without added diene. Copolymerization of ethylene with 1,7 ─ octadiene also afforded the corresponding copolymer 1 2＋ 2－ Figure 5. H NMR spectra of (i) [Fe(L1a)] FeCl 4 (CD 3OD) and containing the 1,2 ─ trans ─ cyclopentylene group (Scheme 5). 2＋ 2－ [Co(L1a)] CoCl 4 (D 2O). Scheme 5 sponding dinickel complexes Ni 2(L4). X ─ ray crystal structure analysis of the dinickel complexes Ni 2(L4a) and Ni 2(L4b) showed that the distance between two nickel centers is 4.73 ─ 4.87 Å, which is the shortest of any known dinickel complexes with bis(phenoxyimine) ligands. 3． Ethylene Polymerization and Copolymerization by the Double ─ decker Type Dinickel Complex Catalysts Nickel complexes with bulky salicylaldimine ligands catalyze ethylene polymerization to yield high ─ molecular ─ weight polyethylenes. 27 They also enable ethylene polymerization in polar solvents, polymerization in the absence of cocatalyst, and random copolymerization of ethylene with functionalized norbornenes. 28 These nickel catalysts are not stable at elevated temperature because of the formation of bis ─ ligated nickel species. 29 3.1 Ethylene Polymerization by Dinickel Complexes

The dinickel complex Ni 2(L4b) brought about ethylene polymerization in the presence of Ni(cod) 2 as a phosphine 30 scavenger. The catalytic activity of Ni 2(L4b) was higher than that of the mononickel complex Ni(L7b). The molecular weight of the produced polyethylene was much higher than that obtained using Ni(L7b), and had a wider molecular weight The formation of the 1,2 ─ trans ─ cyclopentylene group in distribution. Ni 2(L4b) was able to promote ethylene polymeri- this case is ascribed to a chain ─ walking reaction (β ─ hydrogen zation at 60 ℃. In contrast, Ni(L7b) only showed very low elimination/re ─ insertion) occurring prior to cyclization activity at this temperature. The dinickel complex without (Figure 6 (i)). The repeating unit with pendant vinyl groups, bulky ortho ─ substituents, Ni 2(L4a), was also effective for ethy- which is formed by the incorporation of the diene without lene polymerization. It showed higher activity at 60 ℃ than at cyclization, was also present in the copolymer. GPC analysis r.t. The polyethylenes obtained using the dinickel catalysts of the ethylene ─ 1,7 ─ octadiene copolymer showed a bimodal

Ni 2(L4a) and Ni 2(L4b) contained a crystalline polymer frac- molecular weight distribution, while that of the homopolyethy- tion with a less branched structure, whereas the mononickel lene obtained by Ni 2(L4b)/Ni(cod) 2 under similar reaction complex Ni(L7b) did not produce polymer containing this less conditions showed unimodal distribution. The molecular branched fraction. The cooperation between the two nickel weight of the high ─ molecular ─ weight fraction was mostly centers in the dinuclear complexes is responsible for their high double that of the low ─ molecular ─ weight fraction. These thermal stability and the formation of the less ─ branched, results may indicate that the produced copolymer adopts a lad- crystalline polyethylene with a higher molecular weight. der structure formed by selective cross ─ linking between the two polymer chains by the 1,7 ─ octadiene. The copolymerization of 2,2 ─ diallyl ─ 5,5 ─ dimethyl ─ 1,3 ─ dioxane by the dinickel catalyst yielded the polymer with pendant allyl groups without

1140 （ 88 ） J. Synth. Org. Chem., Jpn. Figure 6. Mechanism of reaction of (i) 1,7 ─ octadiene and (ii) butenoate with the dinickel catalyst. cyclized units (incorp.=1.8 mol %) (Scheme 5). In contrast, 4． Ole n Polymerization and Copolymerization by the Double reaction of ethylene and 1,7 ─ octadiene using mononickel cata- decker Type Dinuclear Dipalladium Complex Catalysts ─ lyst Ni(L7b)/Ni(cod) 2 or ethylene and 1,5 ─ hexadiene using dinickel catalyst Ni 2(L4b)/Ni(cod) 2 afforded the corresponding Palladium complexes with diimine ligands catalyze product in much smaller amount, and the diene unit was polymerization of ethylene, propylene, various α ─ ole ns, and 32,33 absent from the copolymer (Scheme 5). cyclopentenes. One of the features of Pd ─ catalyzed ole n

The dinickel complex Ni 2(L4b) also brought about copoly- polymerization is isomerization of the growing terminal dur- merization of ethylene with various comonomers. The copoly- ing chain growth via β ─ hydrogen elimination/re ─ insertion merization of ethylene with tert ─ butyl butenoate and ethyl (chain walking). Due to this chain walking reaction, highly pentenoate proceeded with the dinickel catalyst to give the branched polyethylene forms during ethylene polymerization, copolymer containing respectively 1.4 and 0.4 mol % of the while polymers with less branched structure form in the repeating unit due to the polar comonomer (Scheme 6). In polymerization of α ─ ole ns higher than C6. The copolyme- both cases the ester group was linked with the polymer chain rization of ethylene with polar monomers, such as acrylates, via a ─ CH 2─ CH 2─ group. This is ascribed to the formation of also proceeds under Pd catalyst to give branched copolymer 34 the stable, six ─ membered chelate structure via a chain walking with the acrylate units located at the termini of the branches. reaction after the insertion of butenoate to the polymer ─ nickel 4.1 Polymerization of Ethylene and α ─ Ole ns by the Double ─ bond (Figure 6 (ii)). The dinickel complex Ni 2(L4b) was also decker Type Dinuclear Dipalladium Complexes effective for the copolymerization of ethylene with 5 ─ norbor- The dipalladium complex Pd 2(L3b) catalyzed polymeri- nene ─ 2 ─ carboxylic acid methyl ester (incorp.=4.1 mol %), but zation of ethylene and various α ─ ole ns (such as 1 ─ butene, 1 ─ was not active for ethylene/methyl acrylate copolymerization. hexene, 1 ─ octene, and 4 ─ methyl ─ 1 ─ pentene) in the presence of NaBARF. 35 At room temperature, activity of the dipalladium Scheme 6 complex toward ethylene polymerization was slightly lower than that of the monopalladium complex Pd(L6b). The poly-

ethylene obtained using the dipalladium complex Pd 2(L3b) had higher molecular weight, and much fewer branches than that obtained with the mononuclear complex Pd(L6b). The

dipalladium catalyst Pd 2(L3b) favors ethylene insertion into

the Pd ─ CH 2─ bond more signi cantly than into the Pd ─ CHR ─ bond, and/or the equilibrium is shifted to the Pd ─ CH 2─ bond more signi cantly in the dinuclear catalysts.

The dinuclear Pd complex Pd 2(L3b) showed higher activity toward ethylene polymerization at 60 ─ 100 ℃ than the mononuclear complex Pd(L6b), and was still active after a reaction time of 24 h at 100 ℃, while the mononuclear complex lost its activity almost completely within 5 h. The molecular weight of Thus, these dinickel catalysts are effective for the copoly- the polyethylene obtained at 100 ℃ was lower than that merization of ethylene with ole n comonomers containing obtained at 60 ℃, but was still higher than that produced by vinyl or ester groups connected by a spacer at an appropriate mononuclear complex Pd(L6b). C ─ H activation of the diimine distance. Ef cient interaction between the two nickel centers ligand by the Pd center, which is known to take place at ele- and the vinyl groups or ester group in the comonomer vated temperature during polymerization by the monopalla- (Figure 6) plays an important role in the ef cient incorpora- dium complex, 36 is retarded in the dipalladium complex tion of the latter. because of the restricted rotation of the N ─ aryl groups within

Vol.77 No.11 2019 （ 89 ） 1141 the rigid cyclic structure. value almost halfway between that of polyethylene (99

The dipalladium complex Pd 2(L3b) showed higher activity branches/1,000 C), and of polyhexene (72 branches/1,000 C) toward polymerization of α ─ ole ns (such as 1 ─ hexene and 1 ─ obtained under similar conditions, and that did not change octene) than the mononuclear complex Pd(L6b), and afforded signi cantly during the polymerization. Thus, ethylene and 1 ─ a polymer of higher molecular weight. It is worth noting that hexene are introduced into the copolymer almost equally in the the polymers produced by the dipalladium catalyst have much copolymerization catalyzed by the monopalladium complex. lower degree of branching (22 ─ 26 branches/1,000 C) and are 4.2 Copolymerization of Ethylene with Acrylates and Acrylic crystalline with T m=92 ─ 98 ℃. The formation of the relatively Anhydride by the Double ─ decker Type Dinuclear Dipalla- linear polymer is ascribed to predominant 2,1 ─ insertion of the dium Complexes monomer into the Pd ─ carbon bond followed by a selective Copolymerization of ethylene with methyl acrylate by the chain walking reaction before the insertion of the new mono- monopalladium complex Pd(L6b) (ethylene=1 MPa, [methyl mer. This is in contrast to the result with the mononuclear acrylate]/[Pd]=1,670) afforded a copolymer containing complex, where both 1,2 ─ and 2,1 ─ insertion of the monomer 1.1 mol % of the repeating unit from acrylate (M n=3,400) take place to almost similar extent and afford a polymer with (Scheme 7). 23 The acrylate units located mainly on the termini higher degree of branching than that given by the dinuclear of the branches (terminal units=79%). The dipalladium com- complex (48 ─ 72 branches/1,000 C, T m=3 ─ 18 ℃). The polyme- plex Pd 2(L3b) yielded a copolymer with increased incorpora- rization of 4 ─ methyl ─ 1 ─ pentene by the dipalladium complex tion of methyl acrylate (5.2 mol %), and with higher molecular Pd 2(L3b) proceeded in living manner at -20 ℃ and afforded a weight (M n=63,700). In contrast to the mononuclear complex ─ polymer with narrow molecular weight distribution. In this catalyzed case, the acrylate unit was introduced predominantly case, the selectivity for the 2,1 ─ insertion was very high (97%). into the main chain (main chain units=77%). The catalytic

The dipalladium complex Pd 2(L3b) was active for the activity of Pd 2(L3b) was higher than Pd(L6b), and the degree copolymerization of ethylene with 1 ─ hexene. As the polymeri- of branching of the copolymer formed by the dipalladium zation time became longer, the elution peak of the produced complex Pd 2(L3b) (43 branches/1,000 C) was lower than with copolymer in GPC shifted toward the high molecular weight the monopalladium complex Pd(L6b) (93 branches/1,000 C). region. A DSC chart of the polymer formed at 0.5 h showed an endothermic peak at 101 ℃, which is due to the linear polyhe- Scheme 7 xene block. In contrast, the polymer formed at 2 h showed a broad endothermic peak at 50 ─ 80 ℃, due to the more branched, repeating unit formed by the reaction of ethylene, in addition to a weak peak at 101 ℃. This result indicates that 1 ─ hexene is preferentially consumed in the early stage of the polymerization to give the polymer having a relatively linear block. As 1 ─ hexene is consumed, ethylene also reacts with the catalyst, and the polymer having linear and branched blocks forms. Such block copolyole ns having blocks with both high and low crystallinity have attracted attention due to their potential use as thermoplastic elastomers. 37 The preferential A plausible mechanism for the introduction of the acrylate reaction of 1 ─ hexene over ethylene in the polymerization by unit into the polymer main chain is shown in Figure 7. Initially the dipalladium complex Pd 2(L3b) is noteworthy because ethy- acrylate undergoes 2,1 ─ insertion into the Pd ─ carbon bond. In lene generally shows higher polymerizability than 1 ─ hexene the case of a monopalladium complex, fast chain walking then toward copolymerization by conventional catalysts. Ethylene ─ takes place to give the stable, six ─ membered chelate complex. 1 ─ hexene copolymerization by the monopalladium complex Insertion of ethylene after the chain walking causes the acry- Pd(L6b) afforded a polymer having 78 ─ 83 branches/1,000 C, a late unit to locate at the termini of branches. In the case of the

Figure 7. Mechanism for the introduction of the acrylate unit into the main chain, and at a branch terminus.

1142 （ 90 ） J. Synth. Org. Chem., Jpn. dipalladium complex, a similar 2,1 ─ insertion of acrylate takes and carboxylic acid groups by methanolysis of the copolymer place into one of the Pd ─ carbon bonds. (Scheme 9). However, the ester group at the end of the polymer chain 5． Ethylene Polymerization and Oligomerization by the Double interacts with the other Pd center, which retards fast isomeri- decker Type Dinuclear Diiron and Dicobalt Complex ─ zation of the chain end via β ─ hydrogen elimination. The inser- Catalysts tion of ethylene into the intermediate before chain walking then causes acrylate unit incorporation into the polymer main Iron and cobalt complexes with bis(imino)pyridine ligands chain. catalyze polymerization of ethylene to afford linear polyethy- 39,40 Copolymerization of ethylene with tert ─ butyl acrylate by lene. Bulky substituents on the ortho position of the N ─ the dipalladium complex Pd 2(L3b) afforded the copolymer aryl group are essential for the formation of high molecular having the acrylate unit exclusively incorporated into the main weight polymer, and catalysts without ortho ─ substituents chain. DSC analysis of the polymer showed a broad endother- afford oligomer rather than polymer. 41 mic peak at 8 ─ 29 ℃, whereas that of the polymer obtained The iron catalyst is also active for isospeci c polymeri- using the monopalladium complex showed only a glass transi- zation of propylene. 42 tion temperature at -84 to -87 ℃. Treatment of the ethylene ─ Diiron and dicobalt complexes with xanthene linkers, tert ─ butyl acrylate copolymer with Me 3SiI afforded its hydro- Fe 2(L2) and Co 2(L2), promoted ethylene polymerization or lyzed copolymer (Scheme 8). The hydrolyzed copolymer had a oligomerization in the presence of organoaluminum cocata- 21,43 higher Young’s modulus (4.6 MPa) than the original copoly- lysts (Figure 8). The diiron and dicobalt complexes Fe 2(L2b) mer (1.6 MPa), probably owing to hydrogen bonding between and Co 2(L2b) afforded polyethylene with linear structure. On the carboxylic acid groups. the other hand, less bulky diiron complex Fe 2(L2a) produced

oligoethylene. Fe 2(L2a)/MMAO gave the oligomer with ethyl

Scheme 8 branches selectively. The oligomer produced by Fe 2(L2a)/

Me 3Al, however, had both ethyl and propyl branches (Scheme 10). The former oligomer had both vinyl and vinylene terminal groups (35 and 65%, respectively), whereas the latter

The dipalladium complex Pd 2(L3b) was also effective for the copolymerization of ethylene with acrylic anhydride (ethylene=1 MPa, [methyl acrylate]/[Pd]=420) (Scheme 9). 38 In this copolymerization the dipalladium complex Pd 2(L3b) again shows higher activity, and affords higher molecular weight copolymer (M n=26,400) than the monopalladium complex

Pd(L6b) (M n=3,290). The degree of branching of the copolymer formed by the dipalladium complex Pd 2(L3b) (58 branches/1,000 C) was lower than for the monopalladium complex Pd(L6b) (103 branches/1,000 C), and it contained 2.6 mol % of the repeating unit from acrylic anhydride. Acrylic anhydride was introduced mainly as a ve ─ membered cyclic anhydride group (69%, trans:cis=86:14) formed by the cyclization of the acrylic anhydride, in addition to a minor unit containing the uncyclized, monoacrylic anhydride group. The monopalladium complex Pd(L6b) afforded the copolymer with 0.8 mol % of the repeating unit from acrylic anhydride. In this case the acrylic anhydride was predominantly introduced as the uncyclized, monoacrylic anhydride unit located at the end of chain branches (83%). The acid anhydride groups of the Figure 8. Ethylene polymerization or oligomerization by iron and copolymer could be transformed to a mixture of methyl ester cobalt catalysts.

Scheme 9

Vol.77 No.11 2019 （ 91 ） 1143 Figure 9. Mechanism for the formation of ethyl and propyl branches in ethylene oligomerization using Fe 2(L2a). was selectively terminated by vinyl (99%). This is in contrast to 30 min at this temperature. The polymer produced had a nar- most of the monoiron and monocobalt complexes with rower molecular weight distribution (M w/M n=1.75 ─ 2.77) than bis(imino)pyridine ligands which afford linear vinyl ─ termi- that obtained at r.t.-60 ℃ (M w/M n=48 ─ 67). In contrast to the nated poly ─ or oligoethylenes. A monoiron complex with an diiron catalyst, the catalytic activity of the mononuclear cata- N ─ naphthyl group afforded branched oligoethylene, which lyst Fe(L5b)/MMAO decreased as the polymerization tempera- contained methyl, ethyl, and propyl branches. Dicobalt com- ture increased, and it lost its activity within 7 min at 100 ℃. plex Co 2(L2a) also produced oligoethylene, but its structure The diiron catalyst is also active for propylene polymeri- was linear. zation to give isotactic polymer. Furthermore, the molecular The selective formation of ethyl ─ and/or propyl ─ branched weight of the isotactic polypropylene produced is higher than oligoethylene is ascribed to the mechanism shown in Figure 9. that given by the mononuclear complex. Due to its less sterically hindered structure, diiron catalysis 6． Conclusion easily results in chain transfer to the coordinated ethylene. Usually such reaction leads to the formation of vinyl ─ termi- We have developed dinuclear complexes with cyclic ligands nated oligomer. In the case of this diiron catalyst however, the having double ─ decker type structure. These show higher cata- vinyl ─ terminated oligomer formed after the chain transfer lytic activity, and/or produce polymers with higher molecular reaction is trapped by the two iron centers, and re ─ insertion of weight than the corresponding mononuclear complexes. They the oligomer into the ethyl ─ iron bond takes place. 1,2 ─ inser- also enabled some copolymerizations which have not been tion of the vinyl ─ terminated oligomer forms an ethyl branch, achieved by their mononuclear analogues. High thermal stabi- whereas 2,1 ─ insertion leads to a propyl branch. In the case of lity is another common feature of these double ─ decker type the Fe 2(L2a)/MMAO system, the bulky chain end formed after dinuclear catalysts. The further development of new di ─ or the 2,1 ─ insertion undergoes β ─ hydrogen elimination rather multinuclear metal complexes containing macrocyclic ligands than the insertion of ethylene, leading to the vinylene group. is expected to make possible the synthesis of new polymers Diiron and dicobalt complexes with ethylene linkers which previously have been dif cult to prepare.

Fe 2(L1) and Co 2(L1) were effective for ethylene polymerization to give linear polyethylene. The molecular weight of the poly- Acknowledgements ethylene obtained with these catalysts at room temperature The author would like to express his sincere gratitude to under 1 atom ethylene (M w=1,190,000 (Co 2(L1b)/MMAO), Professor Kohtaro Osakada of Tokyo Institute of Technology

M w=1,000,000 (Fe 2(L1c)/MMAO)) was much higher than that for helpful suggestions and discussions. The author would also produced by the corresponding mononuclear catalysts like to express his appreciation to all of the past and present

(M w＜40,000). It is worth noting that the dicobalt complex members of his research group at Tokyo Institute of Technol- without a substituent at the 6 ─ position Co 2(L1a) produced ogy and at Hirosaki University, and his coworkers for their polyethylene with M w=211,000. This is in contrast to the cor- great contributions to these studies. This work was supported responding monocobalt complex Co(L5a) that did not afford by JSPS KAKENHI Grant JP15H03814. polyethylene. The dinuclear structure of the complex is responsible for the formation of high molecular weight polymer, as chain transfer reactions are suppressed as a result of coopera- References tion between the two metal centers. 1） (a) Haak, R. H.; Wezenberg, S. J.; Kleij, A. W. Chem. Commun. 2010, 46, 2713. (b) Buchwalter, P.; Rosé, J.; Braunstein, P. Chem. Rev. 2015, The diiron catalyst Fe 2(L1b)/MMAO catalyzed ethylene 115, 28. (c) Paull, D. H.; Abraham, C. J.; Scerba, M. T.; Alden ─ polymerization at 80 ─ 100 ℃. The highest polymerization Danforth, E.; Lectka, T. Acc. Chem. Res. 2008, 41, 655. (d) Shibasaki, activity was achieved at 100 ℃, and it kept its activity over

1144 （ 92 ） J. Synth. Org. Chem., Jpn. M.; Kanai, M.; Matsunaga, S.; Kumagai, N. Acc. Chem. Res. 2009, 27, 2166. (b) Salata, M.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 12. 42, 1117. (e) Matsunaga, S.; Shibasaki, M. Bull. Chem. Soc. Jpn. 2008, (c) Salata, M. R.; Marks, T. J. Macromolecules 2009, 42, 1920. (d) 81, 60. (f) Jacobsen, E. N. Acc. Chem. Res. 2000, 33, 421. (g) Ma, Weberski, Jr., M. P.; Chen, C.; Delferro, M.; Marks, T. J. Chem. Eur. J. J. ─ A.; Cahard, D. Angew. Chem. Int. Ed. 2004, 43, 4566. 2012, 18, 10715. 2） (a) Chen, Z.; Furutachi, M.; Kato, Y.; Matsunaga, S.; Shibasaki, M. 13） Shu, D.; Mouat, A. R.; Stephenson, C. J.; Invergo, A. M.; Delferro, Angew. Chem. Int. Ed. 2009, 48, 2218. (b) Yamatsugu, K.; Yin, L.; M.; Marks, T. J. ACS Macro Lett. 2015, 4, 1297. Kamijo, S.; Kimura, Y.; Kanai, M.; Shibasaki, M. Angew. Chem. Int. 14） (a) Radlauer, M. R.; Day, M. W.; Agapie, T. J. Am. Chem. Soc. 2012, Ed. 2009, 48, 1070. (c) Kato, N.; Mita, T.; Kanai, M.; Therrien, B.; 134, 1478. (b) Radlauer, M. R.; Day, M. W.; Agapie, T. Organometal- Kawano, M.; Yamaguchi, K.; Danjo, H.; Sei, Y.; Sato, A.; Furusho, lics 2012, 31, 2231. (c) Radlauer, M. R.; Buckley, A. K.; Henling, L. S.; Shibasaki, M. J. Am. Chem. Soc. 2006, 128, 6768. (d) Sone, T.; M.; Agapie, T. J. Am. Chem. Soc. 2013, 135, 3784. (d) Radlauer, M. Yamaguchi, A.; Matsunaga, S.; Shibasaki, M. J. Am. Chem. Soc. R.; Agapie, T. Organometallics 2014, 33, 3247. (e) Sampson, J.; Choi, 2008, 130, 10078. G.; Akhtar, M. N.; Jaseer, E. A.; Theravalappil, R.; Al ─ Muallem, H. 3） Active sites having two metal centers can be found in enzymes, and A.; Agapie, T. Organometallics 2017, 36, 1915. promote reactions ef ciently. For example, urease has two nickel cen- 15） Na, S. J.; Joe, D. J.; S, S.; Han, W. ─ S.; Kang, S. O.; Lee, B. Y. J. ters in close proximity, and activation of urea and water on the two Organomet. Chem. 2006, 691, 611. nickel centers enables the ef cient hydrolysis of urea. (a) Karlin, K. 16） Liu, J.; Li, Y.; Liu, J.; Li, Z. Macromolecules 2005, 38, 2559. D.; Science 1993, 261, 701. (b) Jabri, E.; Carr, M. B.; Hausinger, R. P.; 17） Chen, Z.; Zhao, X.; Gong, X.; Xu, D.; Ma, Y. Macromolecules 2017, Karplus, P. A. Science 1995, 268, 998. (c) Göbel, M. W. Angew. Chem. 50, 6561. Int. Ed. 1994, 33, 1141. 18） (a) Han, S.; Yao, E.; Qin, W. Zhang, S.; Ma, Y. Macromolecules 2012, 4） (a) Aida, T.; Inoue, S. Acc. Chem. Res. 1996, 29, 39. (b) Kuran, W. 45, 4054. (b) Wang, R.; Sui, X.; Pang, W.; Chen, C. ChemCatChem Prog. Polym. Sci. 1998, 23, 919. 2016, 8, 434. (c) Na, Y.; Wang, X.; Lian, K.; Zhu, Y.; Li, W.; Luo, Y.; 5） (a) Kuroki, M.; Aida, T.; Inoue, S. J. Am. Chem. Soc. 1987, 109, 4737. Chen, C. ChemCatChem 2017, 9, 1062. (d) Chen, Z.; Yao, E.; Wang, (b) Kuroki, M.; Watanabe, T.; Aida, T.; Inoue, S. J. Am. Chem. Soc. J.; Gong, X.; Ma, Y. Macromolecules 2016, 49, 8848. (e) Rong, C.; 1991, 113, 5903. (c) Akatsuka, M.; Aida, T.; Inoue, S. Macromolecules Wang, F.; Li, W.; Chen, M. Organometallics 2017, 36, 4458. 1994, 27, 2820. (d) Takeuchi, D.; Watanabe, Y.; Aida, T.; Inoue, S. 19） (a) Takeuchi, D. Kobunshi 2019, 68, 57. (b) Takeuchi, D.; Shigenaga, Macromolecules 1995, 28, 651. (e) Isoda, M.; Sugimoto, H.; Aida, T.; T.; Chiba, Y.; Iwasawa, T.; Osakada, K. Kobunshi Ronbunshu 2018, 75, Inoue, S. Macromolecules 1997, 30, 57. (f) Takeuchi, D.; Aida, T. 507. Macromolecules 1996, 29, 8096. (g) Collins, S.; Ward, D. G.; Suddaby, 20） Gibson, V. C.; McTavish, S.; Redshaw, C.; Solan, G. A.; White, A. J. K. H. Macromolecules 1994, 27, 7222. (h) Stojcevic, G.; Kim, H.; P.; Williams, D. J. Dalton Trans. 2003, 221. Taylor, N. J.; Marder, T. B.; Collins, S. Angew. Chem. Int. Ed. 2004, 43, 21） Takeuchi, D.; Takano, S.; Takeuchi, Y.; Osakada, K. Organometallics 5523. 2014, 33, 5316. 6） (a) Hirahata, W.; Thomas, R. M.; Lobkovsky, E. B.; Coates, G. W. J. 22） (a) Nowick, J. S.; Ballester, P.; Ebmeyer, F.; Rebek, Jr., J. J. Am. Chem. Am. Chem. Soc. 2008, 130, 17658. (b) Widger, P. C. B.; Ahmed, S. M.; Soc. 1990, 112, 8902. (b) Bühlmann, P.; Nishizawa, S.; Xiao, K. P.; Hirahata, W.; Thomas, R. M.; Lobkovsky, E. B.; Coates, G. W. Chem. Umezawa, Y. Tetrahedron 1997, 53, 1647. Commun. 2010, 46, 2935. (c) Widger, P. C. B.; Ahmed, S. M.; Coates, 23） Takano, S.; Takeuchi, D.; Osakada, K.; Akamatsu, N.; Shishido, A. G. W. Macromolecules 2011, 44, 5666. (d) Ahmed, S. M.; Poater, A.; Angew. Chem. Int. Ed. 2014, 53, 9246. Childers, M. I.; Widger, P. C. B.; LaPointe, A. M.; Lobkovsky, E. B.; 24） Takeuchi, D.; Chiba, Y.; Takano, S.; Osakada, K. Angew. Chem. Int. Coates, G. W.; Cavallo, L. J. Am. Chem. Soc. 2013, 50, 18901. (e) Ed. 2013, 52, 12536. Nakano, K.; Hashimoto, S.; Nozaki, K. Chem. Sci. 2010, 1, 369. (f) 25） (a) Verner, E.; Katz, B. A.; Spencer, J. R.; Allen, D.; Hataye, J.; Hiranoi, Y.; Hatanaka, M.; Nakano, K. J. Polym. Sci. Part A: Polym. Hruzewicz, W.; Hui, H. C.; Kolesnikov, A.; Li, Y.; Luong, C.; Martelli, Chem. 2017, 55, 2150. A.; Radika, K.; Rai, R.; She, M.; Shrader, W.; Sprengeler, P. A.; 7） Examples of polymerization by dinuclear complexes having photosen- Trapp, S.; Wang, J.; Young, W. B.; Mackman, R. L. J. Med. Chem. sitizing metal complex moiety. (a) Inagaki, A.; Edure, S.; Yatsuda, S.; 2001, 44, 2753. (b) Hayashi, K.; Yamazoe, A.; Ishibashi, Y.; Kusaka, Akita, M. Chem. Commun. 2005, 5468. (b) Son, C.; Inagaki, A. Dalton N.; Oono, Y.; Nozaki, H. Bioorg. Med. Chem. 2008, 16, 5331. (c) Trans. 2016, 45, 1331. (c) Kikuchi, S.; Saito, K.; Akita, M.; Inagaki, Nicolaou, K. C.; Snyder, S. A.; Huang, X.; Simonsen, K. B.; Koumbis, A. Organometallics 2018, 37, 359. A. E.; Bigot, A. J. Am. Chem. Soc. 2004, 126, 10162. 8） (a) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem. Int. 26） Hirotsu, M.; Ohno, N.; Nakajima, T.; Kushibe, C.; Ueno, K.; Ed. 1999, 38, 428. (b) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Kinoshita, I. Dalton Trans. 2010, 39, 139. Chem. Rev. 2000, 100, 1169. (c) Imanishi, Y.; Naga, N. Prog. Polym. 27） Wang, C.; Friedrich, S.; Younkin, T. R.; Li, R. T.; Grubbs, R. H.; Sci. 2001, 26, 1147. (d) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. Bansleben, D. A.; Day, M. W. Organometallics 1998, 17, 3149. 2003, 103, 283. (e) Camacho, D. H.; Guan, Z. Chem. Commun. 2010, 28） (a) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.; 46, 7879. (f) Nakamura, A.; Ito, S.; Nozaki, K. Chem. Rev. 2009, 109, Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460. (b) Connor, 5215. (g) Takeuchi, D. Dalton Trans. 2010, 39, 311. (h) Carrow, B. P.; E. F.; Younkin, T. R.; Henderson, J. I.; Hwang, S.; Grubbs, R. H.; Nozaki, K. Macromolecules 2014, 47, 2541. Roberts, W. P.; Litzau, J. J. J. Polym. Sci. Part A. Polym. Chem. 2002, 9） (a) Li, H.; Marks, T. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15295. 40, 2842. (c) Göttker ─ Schnetmann, I.; Korthals, B.; Mecking, S. J. (b) Delferro, M.; Marks, T. J. Chem. Rev. 2011, 111, 2450. (c) McInnis, Am. Chem. Soc. 2006, 128, 7708. J. P.; Delferro, M.; Marks, T. J. Acc. Chem. Res. 2014, 47, 2545. (d) 29） (a) Connor, E. F.; Younkin, T. R.; Henderson, J. I.; Waltman, A. W.; Stürzel, M.; Mihan, S.; Mülhaupt, R. Chem. Rev. 2016, 116, 1398. Grubbs, R. H. Chem. Commun. 2003, 2272. (b) Waltman, A. W.; 10） (a) Jüngling, S.; Mülhaupt, R.; Plenio, H. J. Organomet. Chem. 1993, Younkin, T. R.; Grubbs, R. H. Organometallics 2004, 23, 5121. 460, 191. (b) Noh, S. K.; Kim, S.; Kim, J.; Lee, D. ─ H.; Yoon, K. B.; 30） Takeuchi, D.; Chiba, Y.; Takano, S.; Kurihara, H.; Kobayashi, M.; Lee, H. ─ B.; Lee, S. W.; Huh, W. S. J. Polym. Sci., Polym. Chem. 1997, Osakada, K. Polym. Chem. 2017, 8, 5112. 35, 3717. (c) Noh, S. K.; Kim, J.; Jung, J.; Ra, C. S.; Lee, D. ─ H.; Lee, 31） Dario, P.; Takeuchi, D. Chem. Rev. 2018, 118, 8983. H. B.; Lee, S. W.; Huh, W. S. J. Organomet. Chem. 1999, 580, 90. (d) 32） (a) Camacho, D. H.; Guan, Z. Chem. Commun. 2010, 46, 7879. (b) Mitani, M.; Oouchi, K.; Hayakawa, M.; Yamada, T.; Mukaiyama, T. Dong, Z.; Ye, Z. Polym. Chem. 2012, 3, 286. (c) Ye, Z.; Xu, L.; Dong, Polym. Bull. 1995, 35, 677. (e) Soga, K.; Ban, H. T.; Uozumi, T. J Mol. Z.; Xiang, P. Chem. Commun. 2013, 49, 6235. (d) Guo, L.; Dai, S.; Sui, Cat. A: Chem. 1998, 128, 273. (f) Spaleck, W.; Küber, F.; Bachmann, X.; Chen, C. ACS Catal. 2016, 6, 428. (e) Wang, F.; Chen, C. Polym. B.; Fritze, C.; Winter, A. J Mol. Cat. A: Chem. 1998, 128, 279. (g) Alt, Chem. 2019, 10, 2354. (f) Takeuchi, D.; Osakada, K. Polymer 2008, H.; Ernst, R. Inorg. Chim. Acta 2003, 350, 1. (h) Lee, M. H.; Kim, S. 49, 4911. (g) Takeuchi, D.; Osakada, K. Polymer 2016, 82, 392. (h) K.; Do, Y. Organometallics 2005, 24, 3618. (i) Kuwabara, J.; Takeuchi, Takeuchi, D. Macromol. Chem. Phys. 2011, 212, 1545. (i) Takeuchi, D. D.; Osakada, K. Organometallics 2005, 24, 2705. (j) Kuwabara, J.; Polym. J. 2012, 44, 919. (j) Takeuchi, D. Polym. J. 2018, 50, 572. (k) Takeuchi, D.; Osakada, K. Chem. Commun. 2006, 3815. (k) Lee, S. H.; Takeuchi, D.; Park, S.; Osakada, K. J. Synth. Org. Chem., Jpn. 2008, Wu, C. J.; Joung, U. G.; Lee, B. Y.; Park, J. Dalton Trans. 2007, 4608. 66, 1049. 11） (a) Li, H.; Li, L.; Marks, T. J. Angew. Chem. Int. Ed. 2004, 43, 4937. 33） (a) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. (b) Li, H.; Li, L.; Marks, T. J.; Liable ─ Sands, L.; Rheingold, A. L. J. 1995, 117, 6414. (b) McLain, S. J.; Feldman, J.; McCord, E. F.; Am. Chem. Soc. 2003, 125, 10788. (c) Wang, J.; Li, H.; Guo, N.; Li, L.; Gardner, K. H.; Teasley, M. F.; Coughlin, E. B.; Sweetman, K. J.; Stern, C. L.; Marks, T. J. Organometallics 2004, 23, 5112. Johnson, L. K.; Brookhart, M. Macromolecules 1998, 31, 6705. (c) 12） (a) Rodriguez, B. A.; Delferro, M.; Marks, T. J. Organometallics 2008, Gottfried, A. C.; Brookhart, M. Macromolecules 2001, 34, 1140. (d)

Vol.77 No.11 2019 （ 93 ） 1145 Gottfried, A. C.; Brookhart, M. Macromolecules 2003, 36, 3085. (e) PROFILE McCord, E. F.; McLain, S. J.; Nelson, L. T. J.; Ittel, S. D.; Tempel, D.; Killian, C. M.; Johnson, L. K.; Brookhart, M. Macromolecules 2007, Daisuke Takeuchi is a Professor of Hirosaki 40, 410. (f) Park, S.; Takeuchi, D.; Osakada, K. J. Am. Chem. Soc. University. He received his B. Eng. (1994) 2006, 128, 3510. (g) Okada, T.; Park, S.; Takeuchi, D.; Osakada, K. and M. Eng. (1996) from the University of Angew. Chem. Int. Ed. 2007, 46, 6141. (h) Okada, T.; Takeuchi, D.; Tokyo under the direction of Prof. Shohei Shishido, A.; Ikeda, T.; Osakada, K. J. Am. Chem. Soc. 2009, 131, Inoue and Prof. Takuzo Aida. In 1998, he 10852. (i) Park, S.; Okada, T.; Takeuchi, D.; Osakada, K. Chem. Eur. joined the Chemical Resources Laboratory J. 2010, 16, 8662. (j) Takeuchi, D. J. Am. Chem. Soc. 2011, 133, 11106. (the former name of the Laboratory for 34） (a) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. Chemistry and Life Science) of Tokyo Insti- 1996, 118, 267. (a) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, tute of Technology as a Research Associate M. J. Am. Chem. Soc. 1998, 120, 888. in the Group of Prof. Takeshi Endo, where he 35） Takano, S.; Takeuchi, D.; Osakada, K. Chem. Eur. J. 2015, 21, 16209. received his Ph.D. (2000). He was promoted 36） Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.; Brookhart, M. to assistant professor (2000) and to associate J. Am. Chem. Soc. 2000, 122, 6686. professor (2006) in the group of Prof. Kohtaro 37） (a) Crawford, K. E.; Sita, L. R. ACS Macro Lett. 2015, 4, 921. (b) Osakada at the Tokyo Institute of Technology. Leone, G.; Mauri, M.; Bertini, F.; Canetti, M.; Piovani, D.; Ricci, G. He was appointed as a professor of Hirosaki Macromolecules 2015, 48, 1304. (c) Ohtaki, H.; Deplace, F.; Vo, G. D.; University in 2018. His research interests in- LaPointe, A. M.; Shimizu, F.; Sugano, T.; Kramer, E. J.; Fredrickson, clude the design of metal complex catalysts G. H.; Coates, G. W. Macromolecules 2015, 48, 7489. (d) O’Connor, K. and development of polymerization reactions S.; Watts, A.; Vaidya, T.; LaPointe, A. M.; Hillmyer, M. A.; Coates, G. for synthesis of polymers and oligomers with W. Macromolecules 2016, 49, 6743. (e) Lian, K.; Zhu, Y.; Li, W.; Dai, new structures. S.; Chen, C. Macromolecules 2017, 50, 6074. 38） Takeuchi, D.; Iwasawa, T.; Osakada, K. Macromolecules 2018, 51, 5048. 39） (a) Bianchini, C.; Giambastiani, G.; Rios, I. G.; Mantovani, G.; Meli, A.; Segarra, A. M. Coord. Chem. Rev. 2006, 250, 1391. (b) Gibson, V. C.; Redshaw, C.; Solan, G. A. Chem. Rev. 2007, 107, 1745. (c) Flisak, Z.; Sun, W. ─ H. ACS Catal. 2015, 5, 4713. (d) Small, B. L. Acc. Chem. Res. 2015, 48, 2599. 40） (a) Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.; McTavish, S. J.; Solan, G. A.; White, A. J. P.; Williams, D. J. Chem. Commun. 1998, 849. (b) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc. 1998, 120, 4049. (c) Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan, G. A.; Strömberg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1999, 121, 8728. 41） (a) Small, B. L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 7143. (b) Britovsek, G. J. P.; Mastroianni, S.; Solan, G. A.; Baugh, S. P. D.; Redshaw, C.; Gibson, V. C.; White, A. J. P.; Williams, D. J.; Elsegood, M. R. J. Chem. Eur. J. 2000, 6, 2221. (c) Zhang, W.; Sun, W. ─ H.; Redshaw, C. Dalton Trans. 2013, 42, 8988. 42） (a) Pellecchia, C.; Mazzeo, M.; Pappalardo, D. Macromol. Rapid Commun. 1998, 19, 651. (b) Small, B. L.; Brookhart, M. Macromolecules 1999, 32, 2120. 43） Takano, S.; Takeuchi, Y.; Takeuchi, D.; Osakada, K. Chem. Lett. 2014, 43, 465.

1146 （ 94 ） J. Synth. Org. Chem., Jpn.