Controlled Cyclopolymerization of Dienes by Late Transition Metal Complexes

Daisuke Takeuchi, Sehoon Park, and Kohtaro Osakada*

Chemical Resources Laboratory, Tokyo Institute o f Technology 4259, Nagatsuta, Yokohama 226-8503, Japan

(Received July 22, 2008; E-mail: kosakada@res.titech.ac.jp)

Abstract: This article reviews our recent studies on cyclopolymerization of 1,6-heptadiene derivatives cat-alyzed by late transition metal complexes. Pd-diimine complexes initiate cyclopolymerization of the 1,6-heptadienes with functional groups to afford the polymers containing functionalized trans-l,2-disubsti-tuted cyclopentane rings along the polymer chain. Copolymerization of the dienes with ethylene and a-olefins also causes quantitative cyclization of the diene during the polymer growth. 1,6-Octadiene and higher mono-terminal dienes undergo the cyclopolymerization accompanied by chain walking of the grow-ing polymer, leading to the polymers with trans-l,2-disubstituted cyclopentane groups located regularly along the linear polymer chain. The Fe and Co complexes with a bis(imino)pyridine ligand promote the polymerization of unsubstituted 1,6-heptadiene with quantitative cyclization to yield the polymers contain-ing five-membered rings. The Fe complex-catalyzed reaction affords the polymers composed of cis-fused five-membered rings, whereas the polymer obtained by using Co catalyst contains trans-fused rings selec-tively.

1. Introduction

Cationic monoorganopalladium complexes are regarded as the stable intermediate of synthetic organic reactions such as carbonylation and Mizoroki-Heck type reactions. The coordination of olefin to the Pd center, followed by its migra-tory insertion and /3-hydrogen elimination of the resulting alkylpalladium species, complete the Mizoroki-Heck reac-tion, which is widely used in the synthesis of various organic compounds and materials.2'3 The complexes formulated as

[PdR(L)2(solv)]+ (L = phosphine, amine, etc.) are often employed as the catalysts of these reactions, and their cation-ic metal center and a labile solvent ligand facilitate coordina-tion of olefin and CO, giving a [PdR(L)2(olefin(or CO))]+ type intermediate in the catalytic cycles.

The choice of bulky diimine ligands and very weakly coordinating BARF (B[C6H3(CF3)2-3,5]4-) counter anion enabled Pd-complex-catalyzed high-mass polymerization of ethylene and a-olefins as well as copolymerization of ethylene with acrylic esters.4 6 Successive insertion of the olefin into the Pd-polymer bond takes place under mild conditions. Although the reaction accompanies /3-hydrogen elimination of the grow-ing polymer, it does not lead to chain transfer and causes branching of resulted polyethylene and chain-straightening of

poly(a-olefin). Cationic Pd complexes with bispyrazolyl hg-ands such as [(N"N)PdMe(ethylene)][B(C6F5)4] (N"N =

(3,5-Met-pyrazolyl)2CHSi(p-tolyl)3) were recently reported to catalyze oligomerization of ethylene via frequent chain transfer.? Thus, the complexes catalyze single and multiple insertion of olefins into the Pd-C bond to produce aryl-alkenes, or oligo- and poly(alkene)s, as shown in Scheme 1. The kind of the olefins, the steric and electronic properties of supporting ligands, and the coordination ability of counter anions, influence rates of insertion as well as /3-hydrogen elimination of alkylpalladium species to govern the reaction

pathways.We have been engaged in the study of the polymerization

Scheme 1. Reactions of olefins by Pd complexes,

of new monomers using late transition metal complexes, and found that the Brookhart-type cationic palladium complexes, with a slight modification. Pd complexes-catalyzed ring-opening polymerization of methylenecyclopropanes as well as alternating copolymerization of the monomers with CO to form the polymers with unique structures and proper-ties (Scheme 2).$ Among intense research trends to develop transition-metal-catalyzed cyclization of enynes and related doubly unsaturated compounds, such as Pauson-Khand reac-tions,9 Widenhoefer et al. reported that the cationic Pd-phenanthroline complex with BARF counter anion cat-alyzes cycloisomerization and cyclizative hydrosilylation of 1,6-heptadiene derivatives with polar functional groups

(Scheme 3(i)).10 The former reaction involves insertion of an olefinic

group of the diene into the Pd-H bond, cyclization via intramolecular insertion of the remaining C=C bond into the Pd-C bond, and /3-hydrogen elimination of the alkylpalladium species. They clarified the mechanism by detailed kinetic stud-

Vo1.66 No.l1 2008 (13) 1049

Scheme 2. Polymerization and copolymerization of 2-aryl-l-methylenecyclopropanes by Pd complexes.

Scheme 3. Cyclization reactions and cyclopolymerization of 1,6-heptadienes.

ies of the intermediate compounds. Successive inter- and intramolecular insertion of C=C bonds of the diene molecules is included also in the cyclopolymerization of a, w-dienes cat-alyzed by early transition metal (Ti, Zr) complexes (Scheme 3(ii).11,12 The cyclopolymerization lacks in selectivity on the ring-formation, and often produces the polymer having an alkenyl pendant due to incomplete cyclization of the diene and low cis-trans selectivity and tacticity of the cyclic substituents. Coates and Waymouth reported cyclopolymerization of 1,5-hexadiene by optically active zirconocenes to afford an optically active polymer in spite of a moderate degree of trans-cis control.l1d,1

The cyclopolymerization of the dienes catalyzed by Pd

complexes would enable synthesis of the polymers containing

functionalized five- or six-membered rings because the most

Pd catalysts are tolerant to polar functional groups both in the

synthetic organic reactions and in polymer synthesis. In this

account, we describe our recent work on cyclopolymerization

of dienes.

2. Cyclopolymerization of Functionalized Deenes' 3

Isopropylidene diallylmalonate 1 undergoes cyclopolymeri-zation in the presence of a cationic palladium complex formed by mixing A and NaBARF (BARF = B[C6H3(CF3)z-3,5]4) to afford the polymer with five-membered rings (Scheme 4).

Scheme 5 summarizes the monomers that can be used in the cyclopolymerization. Ittel et al. suggested that unsubstituted a,w-dienes may be unsuited for the polymerization using Pd complexes due to the formation of stable it-allyl palladium species that do not cause further polymer growth.4`' Sub-stituents at proper positions in the monomers in Scheme 5 enhance the cyclization in the polymerization because of the Thorpe-Ingold effect.14 These monomers actually afford the

polymers with five-membered rings with various substituents by the reactions catalyzed by the Pd complexes (Table 1, runs 1-7). The polymerization of monomer 3 having an indane-dione structure, affords the polymer with narrow molecular weight distribution, and the molecular weight is close to the calculated value based on monomer-to-Pd molar ratio (Mn = 9200, MW/Mn = 1.12, M,, ca1ca = 9100) (Table 1, run 3). 13C{'H} NMR spectrum of poly(3) is shown in Figure 1

, which indicates selective formation of trans-l,2-disubstituted cyclopentane ring.

Scheme 6 shows two typical sequences of the polymer by stereoselective polymerization, threo-diisotactic and threo-disyndiotactic tetrads of the repeating units. Complex A has a Czv symmetrical structure and polymerization using it as the catalyst precursor forms an atactic polymer of 1. An X-ray crystallographic study of C revealed Cz symmetrical coordination of the diimine ligand to Pd, as shown in Scheme 7. Pd catalysts, formed from C-F and NaBARF,

promote the cyclopolymerization of 1 effectively to afford poly-1 with Mn = 1200-9000 (Table 1, runs 8-11). The 13C{'H} NMR spectrum of poly-1 obtained by D/NaBARF

catalyst exhibits large and small peaks due to CH carbons at S 46.5 and 46.6, which are assigned to rrr and mrr teterads respectively, and much weaker peaks at b 46.6-47.2. Relative intensity of the signals indicates that the rr triad is 83%

Scheme 4. Cyclopolymerization of diene by Pd complex.

Scheme 5. Monomers that undergo Pd-promoted cyclopolymerization.

1050 (14) J. Synth . Org . Chem ., Jpn.

Table 1. Polymerization of dienes by Pd complexes.a

a [Pd cat.] = 0 .01 mmol, [NaBARF] = 0.012 mmol, [diene] = 0.7 mmol, r.t.

b Determined by 1H NMR. c Determined by GPC using CHC13 as eluent

(based on polystyrene standard). d Determined by 13C{1H} NMR. a [diene]

= 0 .4 mmol, at -10 •Ž. f Isolated yield. g Determined by GPC using DMF as

eluent (based on polystyrene standard).

Figure 1. 3C {' H} NMR spectrum of poly(3).

Scheme 6. Threo-diisotactic and threo-disyndiotactic repeating units.

(threo-diisotactic stereochemistry) (run 9). The polymer obtained by C/NaBARF shows lower rr selectivity (66%)

(run 8).

Scheme 7. Pd Complexes with C2 symmetrically coordinated diimine ligand.

3. Polymerization Mechanism

The polymers obtained by the above catalysts contain trans-fused five-membered rings in high selectivity. Tacticity of the cyclic substituents along the polymer chain is influ-enced significantly by the structure of the diimine ligand. Scheme 8 depicts the proposed mechanism of the ring forma-tion during the polymer growth. 2,1-Insertion of an olefinic

group of the diene forms 5-hexenylpalladium intermediates (i) and (ii), which differ in relative stereochemical relationship between the secondary carbon attached to Pd and the coordi-nated olefinic group. In fact, the intermediate (i) is formed selectively, or undergoes preferential cyclization to afford the trans-five-membered rings.

Scheme 8. Proposed mechanism for ring-formation.

Initial 1,2-insertion of a C=C bond of the monomer into the Pd-C bond and subsequent inteamolecular 1,2-insertion of the remaining alkenyl group (cyclization) would produce the repeating units with a six-membered ring, but the obtained polymers do not contain these structures at all. Preferential 1,2-insertion of a-olefin is common in the poly-merization catalyzed by early transition metal complexes, whereas Ni and Pd-diimine complexes were reported to pro-mote not only 1,2-insertion but also 2,1-insertion in the olefin polymerization.l s

The tacticity of the polymer (the relative stereochemistry of the trans-fused five-membered rings) is determined at the stage of intermolecular insertion of a C=C bond of the diene into the Pd-cyclopentylmethyl bond of the growing polymer end. Scheme 9 depicts a possible pathway for formation of

Vo1.66 No.11 2008 (15) 1051

the isotactic-type polymer. Coordination of the si-face, for example, of a vinyl group of the monomer, followed by inser-tion of the C=C bond into the Pd-C bond and trans-cycliza-tion, would generate a new five-membered ring with

(R)-configuration at the polymer end. Repetition of stereo-selective coordination of the diene to Pd and migratory inser-tion of the C=C bond, followed by cyclization, produces the

polymer containing the five-membered rings with the same stereochemistry. The auxiliary ligand with substituents ori-ented to form C2 symmetrical space around the Pd center serves to regulate stereochemistry of the monomer insertion to form the isotactic type polymer, similarly to the polymeri-zation of a-olefins catalyzed by the metallocenes with a C2-symmetrical structure.16 Ni complexes, having the diimine ligand contained in E and F, were reported to cat-alyze isotactic polymerization of propylene, which can be ascribed to the C2 coordination of the ligand with bulky sub-stituents of the 2-tent-butylphenyl or 2-isopropyl-6-methylphenyl groups at opposite sides of the coordination

plane

Scheme 9. Possible pathway for formation of the isotactic-type polymer.

4. Copolymerization of Diene with Ethylene and

a-Olecns

Palladium complexes promote random copolymerization

of diallylmalonate with ethylene accompanied by quantitative cyclization of the diene monomer to give the polymer with

cyclopentane groups. The molar ratio of the repeating unil from diene in the copolymer ranges from 3 to 42%. The

copolymer with 3% of the repeating unit from 1 contains a highly branched polymer chain formed via insertion of ethy-lene, accompanied by chain-walking, whereas the copolymer

with 42% of the repeating unit from 1 has linear ethylene linkage between the five-membered rings (Scheme 10).

Heating the copolymers of diallylmalonate and dial-

lyldimethyldioxane with ethylene in the presence of Bronsted

Scheme 10. Copolymerization of diene with ethylene.

acid leads to hydrolysis of the ester group to give the copoly-mer with dicarboxylic acid and diol groups, respectively. These polymer reactions proceed smoothly in almost quanti-tative ring-opening (Scheme 11).

5. Cyclization-Chain-Walking Polymerization of Dienes

by Palladium Complexes' 8

Diimine-palladium complexes promote polymerization of allyl(crotyl)malonate to attain quantitative conversion in 24 h to give the polymer with alternating repeating units of trans-l,2-disubstituted cyclopentane ring and trimethylene

group (Scheme 12). The polymer does not show H NMR signal due to methyl branch. The polymer structure indicates that the polymerization proceeds via cyclization of the diene followed by chain-walking of the Pd center to form Pd-pri-mary alkyl intermediate, which allows insertion of another diene monomer (Scheme 13). The diene monomers with longer alkyl group on 7-position also undergo a smooth cyclization-chain-walking reaction to end up with the poly-

Scheme 12. Cyclization-chain-walking polymerization

of dienes by Pd complexes.

1052 (16) J . Synth . Org . Chem . , Jpn.

Scheme 13. Mechanism of cyclization-chain-

walking polymerization.

mer containing trans-l,2-cyclopentane group and

oligomethylene groups. The polymer does not have braches

as a result of random chain-walking reaction, which is

ascribed to the selective insertion of a C=C bond of the

monomer to the Pd-primary alkyl intermediate due to steric

reason. Similar selective insertion of olefin to the Pd-primary

alkyl bond rather than Pd-secondary alkyl bond is also

observed in a-olefin polymerization by the Pd-diimine com-

plexes.s Polymer growth that involves insertion of a C=C

double bond and a chain-walking reaction was reported in

Ni-catalyzed polymerization of a-olefinsl7e19 and trans-2-

butenel7c° 20 and classified as their isomerization polymerization.

In addition to the diene with malonate group, monomers

with cyclic amide groups, the tosylamide group, and the

cyclic acetal group also undergo similar cyclopolymerization

accompanying a chain-walking reaction. The polymerization

of the diene with acetal group proceeds in living fashion at

-20 •Ž to give the polymer with narrow molecular weight

distribution (MwIMn = 1.20).

6. Cyclopolymerization of 1,6-Heptadiene by Iron and

Cobalt Complexes21

Fe and Co complexes with bis(imino)pyridine ligands in conjunction with MMAO (MMAO = modified methylalumi-noxane) show high catalytic activity toward ethylene poly-merization.22 In contrast to the Pd complexes, the Fe and Co complexes produce linear polyethylene, because of the poly-mer growth without isomerization of the polymer end via the chain-walking reaction. The Fe-catalyzed polymerization of ethylene was reported to involve chain transfer via transmeta-lation of the polymer chain to organoaluminum co-catalyst and via a-hydrogen elimination, although the Co-catalyzed ethylene polymerization is free from the former chain transfer reaction. The polymerization of ethylene using the Fe cata-lyst in the presence of ZnEt2 causes rapid and reversible transfer of the growing polymer between Fe and Zn, and forms the polymer or oligomer with desired molecular weights. The Co catalyst does not promote the polymeriza-

tion of a-olefins due to slow insertion of the substituted

olefins into the Co-C bond, while the Fe complexes polymer-

ize propylene via 2,1-insertion of the monomer.23

The polymerization of 1,6-heptadiene 8 by a Fe-bis-

(imino)pyridine complex proceeds to produce the polymer with cyclopentanediyl groups. Table 2 summarizes results of the polymerization. 13C NMR spectrum of the polymer showed quantitative cyclization of diene monomer during the

polymerization. The five-membered ring is controlled to produce a cis-structure in high selectivity (> 95%) (Scheme 14). 'H NMR spectrum of the polymer showed the signals assigned to terminal 2- and/or 3-cyclohexenyl group at ca. 5.4. The relative intensity ratio of these signals of the termi-nal groups and those of main chain indicates that the molec-ular weights of the polymer are 3000-14000.

Cobalt complexes with a bis(imino)pyridine ligand also

promote cyclopolymerization of 8 to afford the polymer with a cyclopentane ring controlled in trans-configuration, exclu-sively (Scheme 15). The polymer is soluble in CHC13, although the polymer obtained from the Fe-catalyzed reac-tion is insoluble in CHCl3 and THF. A catalyst composed of

Table 2. Polymerization of diene catalyzed by Fe and Co complexes.

Scheme 14. Polymerization of diene by Fe complex.

Vol.66 No.11 2008 (17) 1053

I and MMAO copolymerizes 8 with ethylene (1 atm) in

toluene to produce the polymer with five-membered rings

(Scheme 16). Poly(8-co-ethylene) is insoluble in THE and

CHCI3 at room temperature, but dissolves in 1,1,2,2-tetra-

chloroethane at 130 •Ž. 'H NMR spectrum of the polymer

indicates incorporation of the diene unit to the copolymer in

By varying the ethylene pressure and the concentration of 8, the content of the diene unit in the copolymer can be increased by up to 50%. Small broad 'H NMR signals due to the terminal CH=CH2 group are observed at S 4.9 and 5.9, but the copolymer obtained from 8 and C2D4 does not show 'H NMR signals at the positions, indicating that the

growing polymer undergoes chain transfer via /3-hydrogen elimination of the -CH2-CH2-Co (or -CD2-CD2-Co) end

group.

Scheme 15. Polymerization of diene by Co complex.

Scheme 16. Copolymerization of ethylene with 1,6-heptadiene by Co complex.

Figure 2 summarizes DSC results of the copolymers.

Polyethylene obtained by I/MMAO shows the endothermic

peak due to Tn, at 131 •Ž. Poly(8-co-ethylene) with low

amounts of the monomer unit from 8 (3% and 35%) also

shows T,n at lower temperature (116 •Ž and 109 •Ž, respec-

Figure 2. DSC profiles of poly( 1 ,6-heptadiene-co-ethylene).

tively). The latter polymer exhibits glass transition at -25 •Ž,

which is lower than Tg of poly(1,6-heptadiene) obtained by

I/MMAO. Melting point becomes negligible in the polymer

containing 50% of the repeating unit from 8. Thus, thermal

properties of the polymers vary depending on the content or

the diene monomer unit.

The cyclopolymerization involves insertion of a C=C

bond of the monomer and subsequent isomerization of the

polymer end via the cyclization. The classical isomerization

polymerization using cation initiator tends to form the poly-mers with thermodynamically favored structures. The Fe

complex-catalyzed cyclopolymerization formed the polymers

with a kinetically favored structure, owing to the stereochem-

istry of the cyclization promoted by the complexes.

7. Conclusion

We have developed novel cyclopolymerization of dienes

by late transition metal complexes, which shows high stereos-

electivity. It is also possible to locate various functional

groups in the prearranged location along the polymer chain.

Some of the polymerization reactions show a living character.

The formed polymers have novel structures and are expected

to show characteristic physical properties.

Acknowledgements

We thank our coworkers for their great contributions to

these studies. This work was supported by Grant-in-Aid for

Young Scientists for Scientific Research from the Ministry of

Education, Science, Sports and Culture, Japan.

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PROFILE

Daisuke Takeuchi is Associate Professor of Tokyo Institute of Technology. He was horn in 1972 in Ishikawa, Japan. He received his B.Eng. (1994) and M. Eng. (1996) degrees from the University of Tokyo under the direction of Prof. Shohei lnoue and Prof. Takuzo Aida. In 1998, he joined Tokyo Institute of Technology as a Research Associate in the Group of Prof. Takeshi Endo, where he received Ph.D. degree (2000). He was promoted to an assistant professor in 2000 and to an associate professor in 2006. His research interests include development of novel polymerization and oligomerization and their control by using metal complexes.

Sehoon Park is post-doctoral fellow at Tokyo Institute of Technology. He was born in Seoul, Korea in 1977 and received his B.Sc. (2002) from Chungang

University (Korea) and Ph.D. degree (2007) from Tokyo Institute of Technol-ogy. He received financial support from the Japanese government as a Monbuk-agakusho scholarship student. His research interests include the develop-

ment of new polymerization methods by using late-transition metal complexes and supramolecular polymer chemistry based on hydrogen bonds.

Kohtaro Osakada is Professor of Tokyo Institute of Technology. He was horn in Okayama in 1955. He received his Bachelors degree in 1977 and his Doctor of Engineering degree in 1982 from Tokyo University under the direction of Prof. Sadao Yoshikawa. He joined the Research Laboratory of Resources Uti-lization (the former name of Chemical Resources Laboratory) of the Tokyo Institute of Technology as an assistant professor in the group of Prof. Takakazu Yamamoto and Prof. Akio Yamamoto in 1982. He was appointed as a professor of Chemical Research Laboratory in 1999. His current research covers the structure and chemi-cal properties of new organotransition-metal complexes, the synthesis of poly-mers using metal-catalyzed C-C bond-forming reactions, and interlocked molecular assembly composed of organometallic molecules.

1056 (20) J. Synth. Org. Chem., Jpn.