ARTICLE pubs.acs.org/Organometallics
Rhodium/Chiral Diene-Catalyzed Asymmetric Cyclopolymerization of Achiral 1,8-Diynes Yoshitaka Ichikawa, Takahiro Nishimura,* and Tamio Hayashi* Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
bS Supporting Information ABSTRACT: Cyclopolymerization of nitrogen-bridged 1,8diynes containing one terminal and one internal alkyne in the presence of a rhodium/chiral diene catalyst gave enantiomerically enriched polymers with a helix chirality that keep their chiral structures in solution. This polymerization proceeded through alternating reaction of the terminal and the internal alkynes, forming polyacetylenes with a 1,2-dialkylidene heterocyclic unit.
’ INTRODUCTION A rhodium complex coordinated with a diene ligand has been recognized as an effective catalyst for polymerization of terminal alkynes, giving π-conjugated polymers with unique properties as potential functional materials.1,2 For example, living polymerization of phenylacetylenes has been achieved with rhodium complexes coordinated with nbd (2,5-norbornadiene)3 or tfb (tetrafluorobenzobicyclo[2.2.2]octatriene)4 (Scheme 1). Asymmetric polymerization of achiral terminal alkynes has been reported where the helical conformation of polymers formed from achiral monomers is maintained with assistance of hydrogen bonding interactions of the functional groups incorporated into the monomer and steric repulsions of the bulky substituents.5,6 For example, Aoki reported that the chiral information of helical polymers, produced from achiral arylacetylenes bearing two hydroxymethyl groups on the benzene ring, was maintained for a long period in chloroform solution, whereas their chirality was lost in polar solvents (Scheme 2).5a,hj The synthesis of helical polyacetylenes that keep their chirality in solution without difficulty is one of the challenging objectives in the field of polymer chemistry.7 In this context, we focused on cyclopolymerization810 of achiral nonconjugated diynes containing both terminal and internal alkynes, which is catalyzed by a rhodium complex (Scheme 3). This cyclopolymerization consists of two reaction steps: a chain-growth intermolecular insertion of the terminal alkyne into a rhodiumcarbon bond (A) and an intramolecular insertion of the internal alkyne (B). Because the terminal alkyne has higher reactivity than the internal one toward insertion,11 the intermolecular reaction of the diyne forms alkenylrhodium species depicted as I. The lower energy r 2011 American Chemical Society
Scheme 1
Scheme 2
of cyclization will promote the intramolecular reaction on I, leading to alkenylrhodium II, which then reacts with the terminal alkyne on another molecule of the diyne. It follows that this polymerization proceeds through the alternating reaction of A and B. This type of ring-forming polymerization introduces the cyclic structure onto the polymer main chain, and these rings are expected to stabilize the helical conformation of polymers by Received: January 31, 2011 Published: March 24, 2011 2342
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their rigid structure. RajanBabu reported the helical chirality in (Z,Z)-1,2-dialkylidenecycloalkanes, which are formed by palladiumcatalyzed cyclization of 1,n-diynes with R3SiSnR0 3.12 Although this type of cyclopolymerization of nonconjugated diynes has been studied by use of a rhodium catalyst,1315 helixsense-selective asymmetric cyclopolymerization of achiral diynes has not been reported, to the best of our knowledge.1618 Here we report that the asymmetric cyclopolymerization is realized by use of a rhodium/chiral diene catalyst for the reaction of 1, 8-diynes where the two alkynes are terminal and internal ones.
Scheme 3. Cyclopolymerization of 1,n-Diynes
Table 1. Rhodium-Catalyzed Asymmetric Cyclopolymerization of Diynes 1ada
polymer entry
a
X (monomer)
b
Rh cat. yield (%)
1
CH2 (1a)
i
0d
2
OCH2 (1b)
i
0d
Mn
Mw/Mn [R]Dc
8
3
CH2OCH2 (1c)
i
10
4300
1.45
4
CH2OCH2 (1c)
ii
99
8100
1.66
5
5e
CH2OCH2 (1c)
ii
40
5800
1.63
þ35
6e
CH2N(Ph)CH2 (1d)
ii
44
3300
1.19
þ115
Polymerization conditions: [monomer] 50 mM; [catalyst] 2.5 mM in CH2Cl2, 20 °C for 24 h. b Isolated yield. c In CHCl3 (c 0.10). d [2 þ 2 þ 2] cycloadducts were formed. e In toluene.
’ RESULTS AND DISCUSSION In the first set of experiments, the polymerization of 1,n-diynes 1ad, which are composed of an aromatic terminal alkyne and an internal alkyne tethered at the ortho-position on the benzene ring, was examined using a rhodium complex coordinated with a chiral diene ligand (2a; Me-tfb*)6,19,20 (Table 1). The reaction of 1,6-diyne 1a in the presence of a cationic rhodium complex Rh((R,R)-2a)[(η6-C6H5)BPh3], which is an active catalyst for helix-sense-selective polymerization of achiral arylacetylenes,6 in dichloromethane at 20 °C for 24 h gave no polymers, where the formation of [2 þ 2 þ 2] cycloadducts21 was observed instead (entry 1). Oxygen-bridged 1,7-diyne 1b did not give polymers either (entry 2). On the other hand, 1,8-diyne 1c, which is expected to form a seven-membered ring at the intramolecular insertion, gave polymer (poly(1c)) in 10% yield with 4300 molecular weight (entry 3). The yield of the polymer was greatly improved by use of an alkenylrhodium complex,22 [(Me-tfb*)Rh{C(Ph)dCPh2}(PPh3)], in situ generated from [RhCl((R,R)-2a)]2, triphenylvinyllithium, and triphenylphosphine, to give poly(1c) with a molecular weight of 8100 in 99% yield (entry 4). Unfortunately, its optical rotation value was low ([R]D 5, c 0.10 in CHCl3). Although a higher optical rotation ([R]D þ35, c 0.10 in CHCl3) was observed for poly(1c) produced by the reaction in toluene (entry 5), poly(1c) was found to rapidly lose its optical rotation in chloroform solution (Figure 1, line a).23 In contrast to the oxygen-bridged 1,8-diyne 1c, nitrogenbridged 1,8-diyne 1d gave polymer (poly(1d)) with a high optical rotation value, [R]D þ115 (entry 6), and the value in chloroform solution was maintained constant for one month (Figure 1, line b).
Figure 1. Time dependence of [R]25D (c 0.10, CHCl3) of polymers in CHCl3 solution at 25 °C. (a) Poly(1c) (entry 5 of Table 1); (b) poly(1d) (entry 6). 2343
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The reaction of nitrogen-bridged 1,8-diyne 1d was examined under several reaction conditions, the results being summarized Table 2. Rhodium-Catalyzed Asymmetric Cyclopolymerization of Nitrogen-Bridged 1,8-Diyne 1da
in Table 2. The reaction solvents had a significant influence on the reactivity and the helix-sense enantioselectivity. The reaction in dichloromethane gave poly(1d) in 49% yield, but its [R]D value was low (þ65) (entry 2). The reaction in a polar solvent, THF or DMF, gave little or no polymers (entries 3 and 4). Aromatic solvents showed a good performance in the polymerization of 1d Table 3. Rhodium-Catalyzed Asymmetric Cyclopolymerization of Nitrogen-Bridged 1,8-Diynes 1doa
polymer b
yield (%)
Mn
Mw/Mn
polymer [R]Dc
entry
2
Ar
Ar
b
yield (%)
Mn Mw/Mn [R]Dc
entry
ligand
solvent
1
(R,R)-2a
toluene
44
3300
1.19
þ115
1
Ph
Ph (1d)
46
3700
1.23
þ236
2
(R,R)-2a
CH2Cl2
49
3700
1.24
þ65
2
4-MeOC6H4 Ph (1e)
38
3200
1.11
þ159
3
(R,R)-2a
THF
3
3500
1.17
þ17
3
4-FC6H4
Ph (1f)
21
3400
1.13
þ230d
4 5
(R,R)-2a (R,R)-2a
DMF benzene
0 37
4
4-ClC6H4
Ph (1g)
16
3200
1.14
þ130d
3500
1.19
þ167
5
Ph
2-MeC6H4 (1h)
36
3300
1.12
þ318
6
(R,R)-2a
p-xylene
49
4100
1.31
þ165
7
(R,R)-2a
C6H5Cl
56
3800
1.26
þ193
6 7
Ph Ph
Mes (1i) 4-MeC6H4 (1j)
32 67
3400 3900
1.17 1.28
772 þ83
8
(R,R)-2a
C6H5F
46
3700
1.23
þ236d
8
Ph
4-C12H25C6H4 (1k)
67
5100
1.34
þ68
9
(S,S)-2a
C6H5F
46
3600
1.21
231d
9
Ph
4-i-PrC6H4 (1l)
62
3900
1.24
þ235
10
(R,R)-2b
C6H5F
77
5800
1.65
þ116
10 Ph
4-PhC6H4 (1m)
53
3900
1.21
þ222
11
(R,R)-2c
C6H5F
89
6500
1.75
119
11 Ph
4-BrC6H4 (1n)
23
3400
1.14
þ96d
12
(R)-2d
C6H5F
0
12 Ph
4-IC6H4 (1o)
63
3400
1.18
þ150
Polymerization conditions: 20 °C; 24 h; [monomer] 50 mM; [catalyst] 2.5 mM. b Isolated yield. c In CHCl3 (c 0.10). d c 0.30. a
1
Polymerization conditions: 20 °C; 24 h; [monomer] 50 mM; [catalyst] 2.5 mM. b Isolated yield. c In CHCl3 (c 0.30). d c 0.10. a
Figure 2. CD and UVvis spectra of poly(1d) measured in CHCl3 (0.10 mM) at 20 °C. Ligand: (a) (R,R)-2a (entry 8 of Table 2); (b) (S,S)-2a (entry 9 of Table 2). 2344
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Figure 3. Time dependence of [R]25D (c 0.10, CHCl3) of polymers in CHCl3 solution at 25 °C: (a) poly(1e) (entry 3 of Table 3), (b) poly(1k) (entry 8), (c) poly(1m) (entry 10), (d) poly(1n) (entry 11). (e) Time dependence of [R]25D (c 0.10, DMSO) of poly(1d) (entry 1) in DMSO solution at 25 °C.
Scheme 4
Scheme 5 Figure 4. 13C NMR spectra of (a) poly(1d) (entry 8 of Table 2) and (b) poly(1d-13C) (Scheme 5) in CDCl3. Each spectral datum was accumulated 8192 times.
(entries 1, 58), the best enantioselectivity ([R]D þ236) being observed in fluorobenzene (entry 8).23 The CD spectrum of poly(1d) obtained in fluorobenzene had clear positive Cotton effects in the region from 300 to 450 nm (Figure 2). Use of the S, S isomer of ligand 2a gave optically active poly(1d), displaying opposite Cotton effects (entry 9 and Figure 2), indicating that the helical chirality of poly(1d) is kinetically controlled by the chirality of the diene ligand. Chiral tfb ligands 2b20a,b and 2c6,20a displayed a high catalytic activity, giving the polymer in 77% and 89% yields, respectively (entries 10 and 11). The formation of poly(1d) was not observed24 with diene ligand (R)-2d, which is readily prepared from a natural product25 (entry 12). The results obtained for the asymmetric polymerization of nitrogen-bridged 1,8-diynes bearing several substituents on the benzene rings Ar1 and Ar2 are summarized in Table 3. The reaction of diynes 1eg substituted with electron-donating
and -withdrawing aryl groups (Ar1) on the alkyne terminus gave helical polyacetylenes, poly(1e)(1g) in 1638% yields (entries 24). The present polymerization can also be applied to monomers with a variety of aryl groups (Ar2) on the nitrogen atom. Bulky aromatic rings, such as o-tolyl (1h) and mesityl (1i) groups, gave the corresponding polymers with relatively high specific rotations (þ318 and 772, respectively) (entries 5 and 6). 1,8-Diynes bearing alkyl (1j1l), phenyl (1m), and halo (1n and 1o) substituents at the para-position on the benzene rings (Ar2) gave poly(1j)(1o) in 2367% yields (entries 712). It is noteworthy that no rapid racemization of these polymers was observed in solution.23 For example, poly(1m), whose optical rotation had been [R]D þ222 at the beginning, displayed the rotation of [R]D þ199 after one month in chloroform solution at 25 °C (Figure 3, line c). The racemization of poly(1d) was also very slow in DMSO at 25 °C (Figure 3, line e), while a partial decomposition of poly(1d) was observed at 50 °C. The cyclic structure generated upon the cyclopolymerization of diyne is important for constructing the helix-sense chirality. Thus, the polymerization reaction of monoacetylene 3, which is similar to diyne 1d in its basic skeleton but lacks the internal 2345
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Organometallics Scheme 6
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chiral structures in solution for a long time without serious racemization. This polymerization proceeds through alternating insertion of the terminal and the internal alkynes into a rhodium carbon intermediate.
’ EXPERIMENTAL SECTION
Scheme 7
alkyne, gave poly(3) in 21% yield (Scheme 4). The [R]D of poly(3) was zero, and no Cotton effect was observed in its CD spectrum, implying that the cyclic structure is required for induction of the helical chirality in the polymer main chain. The polymerization of selectively 13C-labeled 1,8-diyne 1d-13C26 provided significant information about the structure of poly(1d) (Scheme 5). The 13C NMR spectrum of poly(1d-13C) showed the existence of major resonance peaks (δ 135148) that are assigned to sp2 carbons, and only small peaks of acetylenic sp carbons were observed (Figure 4), indicating that almost all the internal alkynes were converted into the alkene moieties in the polymerization of monomer 1d. In other words, the present rhodium-catalyzed asymmetric polymerization of 1,8-diynes was demonstrated to proceed through the “cyclopolymerization” mechanism, and the mechanism of exclusive reaction of the terminal alkynes leaving the internal alkynes intact is ruled out. Treatment of poly(1c), obtained from oxygen-bridged 1,8-diyne 1c, with a catalytic amount of RuCl3 3 3H2O and NaIO4 at 0 °C for 20 h brought about partial oxidation to give hemiacetal 4a. Its structure was fully characterized by spectroscopic studies of ethyl acetal 4b,27 which was obtained in 1% yield (from poly(1c)) by treatment of 4a with ethanol in chloroform. A plausible mechanism for the formation of 4a is illustrated in Scheme 6. Oxidative cleavage of the trisubstituted carboncarbon double bond of the polymer main chain forms the keto carboxylic acid intermediate 4c. The spontaneous intramolecular cyclization of 4c would give hemiacetal 4a.28 Thus, these results indicate that poly(1c) is composed of a seven-membered ring and rhodium/diene-catalyzed polymerization proceeds through an insertion of internal alkynes into organorhodium species via 7-exo cyclization (Scheme 7).
’ CONCLUSIONS In summary, a rhodium complex coordinated with a chiral tfb ligand catalyzed the asymmetric cyclopolymerization of nitrogen-bridged 1,8-diynes containing both terminal and internal alkynes to give helix-sense-selective polymers, which keep their
General Procedures. All anaerobic and moisture-sensitive manipulations were carried out with standard Schlenk techniques under predried nitrogen. NMR spectra were recorded on a JEOL JNM LA500 spectrometer (500 MHz for 1H, 125 MHz for 13C). Chemical shifts are reported in δ ppm referenced to an internal SiMe4 standard for 1H NMR and chloroform-d (δ 77.16) for 13C NMR: the following abbreviations are used: s: singlet, d: doublet, t: triplet, q: quartet, quint: quintet, sept: septet, m: multiplet, br: broad. Infrared (IR) spectra were recorded on a Shimadzu IRPrestige-21 spectrometer. High-resolution mass spectra were obtained with a Bruker micrOTOF spectrometer. Number-average molecular weights (Mn) and molecular weight distributions (Mw/Mn) of polymers were estimated by GPC (Shodex KF805L and KF803L) eluted with THF by polystyrene calibration at 40 °C. CD and UVvis spectra were recorded on a JASCO J-820 spectropolarimeter. Optical rotations were measured on a JASCO P-2200 polarimeter. Unless otherwise specified, UVvis and CD spectra of polymers were measured at a concentration of 0.10 mM based on the monomer unit at 20 °C. Materials. CH2Cl2, CH3CN, DMF, p-xylene, and C6H5F were distilled over CaH2 under nitrogen. NEt3 was distilled over KOH under nitrogen. Benzene was distilled over benzophenone ketyl under nitrogen. C6H5Cl was distilled over P2O5 under nitrogen. THF, 1,4-dioxane, and toluene were purified by passing through a neutral alumina column under nitrogen. General Procedure for Table 1-i. Rh((R,R)-2a)[(η6-C6H5)BPh3] (10 μmol, 5 mol %) was dissolved in CH2Cl2 (0.5 mL) at room temperature. To the solution of the catalyst was added a CH2Cl2 (3.5 mL) solution of monomer 1 (0.20 mmol). After the reaction solution was stirred at 20 °C for 24 h, the mixture was quenched with a large amount of MeOH to precipitate a solid, which was filtered and dried under vacuum. General Procedure for Tables 1-ii, 2, and 3. To a solution of [RhCl(diene*)]2 (10 μmol, 5 mol % Rh) and PPh3 (14.3 mg, 50 μmol) in solvent (0.50 mL) was added triphenylvinyllithium (0.10 M toluene solution, 0.25 mL, 25 μmol), and the mixture was stirred at room temperature for 5 min. To the solution of the catalyst was added monomer 1 (0.20 mmol) in solvent (3.25 mL). After the reaction solution was stirred at 20 °C for 24 h, the mixture was quenched with a large amount of MeOH to precipitate a yellow solid, which was filtered and dried under vacuum.
’ ASSOCIATED CONTENT Supporting Information. Text and figures giving experimental procedures and spectral characterization data. This material is available free of charge via the Internet at http:// pubs.acs.org.
bS
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected].
’ ACKNOWLEDGMENT We acknowledge Dr. Masashi Shiotsuki (Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University) for fruitful discussions and his kind assistance in CD and UVvis spectroscopy. This work was supported by a 2346
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Organometallics Grant-in-Aid for Scientific Research (19105002 and 22750090) from the MEXT, Japan. Y.I. thanks the JSPS for a Research Fellowship for Young Scientists.
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