Stereoselective Synthesis of (R, R)-, (S, S)-, and (R, S) - Nature

Polymer Journal, Vol. 35, No. 7, pp 592—597 (2003)

Stereoselective Synthesis of (R, R)-, (S, S)-, and (R, S)-Poly(2,3dihydroxy-1,4-naphthylene) Derivatives by Asymmetric Oxidative Coupling Polymerization Shigeki H ABAUE, Tomoaki S EKO, Masaya I SONAGA, Hiroharu A JIRO, and Yoshio O KAMOTO† Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464–8603, Japan (Received March 31, 2003; Accepted May 12, 2003) ABSTRACT: The asymmetric oxidative coupling polymerization of (R)- and (S )-3,3 -dihydroxy-2,2 -dimethoxy1,1 -binaphthalene 1 and (S , S )-, (R, R)-, and (S, R)-3,3 -dihydroxy-2,2 ,3 ,2 -tetramethoxy-1,1 : 4 ,1 -ternaphthyl with the CuCl-2,2 -isopropylidenebis(4-phenyl-2-oxazoline) (Phbox) catalyst was performed to stereoselectively obtain poly(2,3-dihydroxy-1,4-naphthylene) derivatives. The polymerization with the bisoxazoline proceeded in a ligand controlled manner regardless of the monomer stereostructure in contrast to that using diamines, such as N, N, N , N tetramethylethylenediamine, which afforded polymers under substrate control. For example, the polymerization of (R)and (S )-1 with (S )Phbox produced the polymers with preferential · · ·SSSS· · · and · · ·RSRS· · · chain structures, respectively. KEY WORDS Asymmetric Oxidative Coupling Polymerization / Poly(1,4-naphthylene) / Bisoxazoline / 1,1 -Bi-2-naphthol / Ternaphthyl /

Poly(2,3-dihydroxy-1,4-naphthylene), in which the 1,1 -bi-2-naphthol unit is connected at its 4,4 positions, has a rigid main chain with continuous axial dissymmetry. The structures of the · · ·RRRR· · ·/· · ·SSSS· · · and · · ·RSRS· · · isomers are quite different from each other. The former has a stable one-handed helical main chain conformation, whereas, in the latter, all the hydroxyl groups exist on one side of a plane through the main chain. Therefore, each polymer should show characteristic properties based on the main chain structure with helically arranged functional groups or on a single side. The stereoselective synthesis of poly(2,3-dihydroxy-1,4-naphthylene) is very interesting from the standpoint of developing novel func-

tional materials.1 We reported the first synthesis of poly(2,3-dihydroxy-1,4-naphthylene) derivatives by the asymmetric oxidative coupling polymerization (AOCP) using optically active 3,3 -dihydroxy-2,2 -dimethoxy1,1 -binaphthalene 1 as a monomer,2 while a stepwise synthetic approach to the stereochemically pure oligomer, such as the octinaphthalene derivatives, has been known.3 Furthermore, we recently found that the AOCP of the commercially available 2,3dihydroxynaphthalene 2 using the copper(I) complexes of the C2 -symmetric bisoxazoline derivatives as a catalyst under an O2 atmosphere quantitatively produced a polymer,4 although the AOCP of 2 with the CuCl2 -(−)-sparteine [(−)Sp] and CuCl-(S )-(+)-1-(2pyrrolidinylmethyl)pyrrolidine [(+)PMP] complexes, which have been used as effective reagents for the cou-

Scheme 1. †

To whom correspondence should be addressed.

592

Scheme 2.

Stereoselective Synthesis of Poly(1,4-naphthylene) Derivatives

Scheme 4.

size exclusion chromatographic (SEC) analyses were carried out on a Shodex GPC-System-21 equipped with Shodex UV-41 and Shodex RI-71S detectors using Shodex GPC KF-806L and KF-803 columns connected in series (eluent: THF). Calibration was performed using standard polystyrenes. Optical rotation and circular dichroism (CD) spectra were measured on a JASCO P1030 polarimeter at 25 ◦ C and a JASCO J-720L apparatus, respectively.

Scheme 3.

pling reaction of 2-naphthol derivatives,5–7 was unsuccessful in producing a low molecular weight oligomer in poor yield.2, 4 However, the enantioselectivity of the polymers obtained by the polymerization of 2 with the Cu(I)-bisoxazoline systems was estimated to be low. In this study, the AOCP of (R)-, (S )-1, and three stereoisomers of 3,3 -dihydroxy-2,2 ,3 ,2 tetramethoxy-1,1 : 4 ,1 -ternaphthyl [(S , R)-, (S , S )-, and (R, R)-3] using the CuCl-2,2 -isopropylidenebis(4phenyl-2-oxazoline) (Phbox) catalyst was carried out and the poly(2,3-dihydroxy-1,4-naphthylene) derivatives having (R, R)-, (S , S )-, and (R, S )-structures were selectively synthesized. EXPERIMENTAL Measurements 1 H and 13 C NMR spectra were measured on a Varian Gemini-2000 (400 MHz for 1 H) spectrometer in CDCl3 . Infrared (IR) spectra were recorded on a JASCO FT/IR-620 spectrometer. Mass spectra were taken on a JEOL LMS-AX505HA or a Voyager Elite MALDI-TOF mass spectrometer (PE Biosystems). The Polym. J., Vol. 35, No. 7, 2003

Materials Compound 3 was synthesized by the coupling reaction between the mono-benzylated 1 and 3-benzyloxy2-naphthol (3 equiv.) using the CuCl-(+)PMP catalyst (10 mol%) in CH2 Cl2 at room temperature under an O2 atmosphere,3, 8 followed by methylation and debenzylation of the hydroxyl groups (total yield: 40%). Compound 3 was separated by silica gel column chromatography (hexane/AcOEt/CHCl3 = 3/1/1) into meso38 and rac-3 (38 : 62). The prepared rac-3 was further resolved into enantiomers (> 99%ee) by high performance liquid chromatography (HPLC) using a chiral column [Chiralpak AD (2 cm (i.d.) × 25 cm), Daicel] (eluent: hexane/2-propanol = 9/1). The absolute configuration of the optically active 3 was confirmed by conversion of the hydroxyl groups into methyl groups.9 rac-3: mp = 244–245 ◦ C. 1 H NMR (400 MHz, CDCl3 ): δ 3.60 (s, 6 H, –OCH3 ), 3.63 (s, 6 H, –OCH3 ), 6.19 (s, 2 H, –OH), 7.10–7.42 (m, 10 H, aromatic), 7.51 (s, 2 H, aromatic), and 7.81 (d, J = 8.4 Hz, 2 H, aromatic). IR (KBr, cm−1 ): 3377 (br), 2939, 1508, 1468, 1428, 1385, 1247, and 1012. Mass (FAB) m/z 533 ([M + H]+ ). (R, R)-3: [α]25 D = + 94 ◦ (c = 1.56, THF), mp = 248– 249 ◦ C. (S , S )-3: [α]25 D = − 92 ◦ (c = 0.40, THF), mp = 248–249 ◦ C. The monomer 1, reagents, and solvents used in the polymerization were synthesized or purchased as previously reported.2, 4 Polymerization A monomer was added to a mixture of copper chlo593

S. H ABAUE et al.

Table I. Oxidative coupling polymerization of 1 at room temperature for 24 ha Entry

1

Catalyst

1g 2 3 4 5

R S R S R

CuCl-TMEDA CuCl-(S )Phbox CuCl-(S )Phbox CuCl-(R)Phbox CuCl-(R)Phbox

Yieldb (%) 71 63 50(20)h 41(21)h 58

Mn (×103 ) (Mw /Mn )c 5.2 (1.6) 5.6 (1.5) 3.3 (1.3) 3.0 (1.4) 4.8 (1.5)

S : Rd (S : R)e 39 : 61 (19 : 81) 76 : 24 (89 : 11) 80 : 20 (37 : 63) 23 : 77 (64 : 36) 23 : 77 (11 : 89)

[α]25 D f (deg.) +145 −137 +31 −55 +151

Solvent: CH2 Cl2 (entry 1) and THF (entries 2–5). b Methanol-insoluble part of poly1 . Determined by SEC in THF (polystyrene standard). d Determined by 13 C NMR analysis. e Estimated from the S : R and Mn values.10 f In CHCl3 . g Ref 2. h THF-soluble part. a

c

Scheme 5.

ride and a diamine in a solvent ([monomer] = 0.35 M, [Cu(I)]/[ligand]/[monomer] = 0.2/0.25/1). After stirring the mixture at room temperature for 24 h under an O2 atmosphere, the solvent was evaporated and acetylation was carried out using an excess amount of acetyl chloride and pyridine in CH2 Cl2 . Poly1 or poly3 was isolated as the methanol-insoluble fraction by centrifugation, followed by repeated washing with methanol, and drying in vacuo. RESULTS AND DISCUSSION The AOCP results of the optically active 1 with the CuCl complexes of (S )- and (R)-Phbox in tetrahydrofuran (THF) at room temperature for 24 h under an O2 atmosphere are summarized in Table I, together with that of the polymerization using N, N, N , N tetramethylethylenediamine (TMEDA) as a ligand.2 The polymers obtained from (R)-1 with (S )Phbox and from (S )-1 with (R)Phbox after acetylation of the hydroxyl groups were partially insoluble in THF (entries 3 and 4), whereas the polymerizations of the (S )-isomer with (S )Phbox and vice versa afforded the polymers fully soluble in CHCl3 and THF (entries 2 and 5). After the polymerization of (S )-1 with the (R)Phbox ligand, a part of the obtained polymer was further alky594

Scheme 6.

lated with an excess amount of 1-bromohexane to give a methanol-insoluble polymer (poly1 ) in 34% yield (total yield, Mn = 3.7 × 103 , Mw /Mn = 1.3), which was fully soluble in CHCl3 and THF in contrast to the corresponding poly1 . On the other hand, poly[(R)-1 ] prepared in the same way using (R)Phbox [58% total yield (methanol-insoluble part), Mn = 5.0 × 103 , Mw /Mn = 1.7] was also soluble in these solvents. The estimated number average molecular weight of the former was quite smaller than that of the latter. These results suggest that the stereochemistry of poly1 should significantly affect the polymer solubility as will be described later. Figure 1 shows the MALDI-MS spectrum of poly1 (THF-soluble part, Mn = 1.8 × 103 , Mw /Mn = 1.6) obtained with (R)Phbox from (S )-1 (matrix = trans-3indoleacrylic acid (IAA) with NaCl). The peaks appeared for approximately every molecular weight of the monomer unit (344) and a small peak with the largest m/z value of around 3900, which corresponds to about 11 repeat units, was observed. In a previous study, the stereochemistry of the newly formed carbon–carbon bonds (S : R) during the polymerization was estimated from the methyl carbon abPolym. J., Vol. 35, No. 7, 2003

θ


Figure 1. MALDI-TOFMS spectrum of poly[(S )-1] (THFsoluble part) obtained with the CuCl-(R)Phbox catalyst (Table I, entry 4) (matrix = IAA with NaCl). Figure 3. CD spectra of poly1 s (a) obtained using (S )Phbox from (S )-1 (Table I, entry 2), (b) from (R)-1 (entry 3), (c) obtained using (R)Phbox from (S )-1 (entry 4), and (d) from (R)-1 (entry 5) (naphthalene unit, in CHCl3 ).

Figure 2. 13 C NMR spectra of carbonyl and methyl carbons in acetyl groups of poly1 ’s obtained with the CuCl-(S )Phbox catalyst (a) from (S )-1 (Table I, entry 2) and (b) from (R)-1 (entry 3) (CDCl3 , 60 ◦ C).

sorption of the acetyl groups in the 13 C NMR spectrum of poly1 .2 Figure 2 shows the 13 C NMR spectra of the polymers obtained from (S )- and (R)-1 with the CuCl-(S )Phbox catalyst (entries 2 and 3). The spectral patterns were quite different from each other, indicating that the polymers mainly consist of the SSS structure for the former and RSR for the latter, based on the stereostructure of the used monomer. The evaluated stereoselectivity, S : R, is listed in Table I. Poly1 obtained using an achiral ligand, TMEDA, showed the stereoselectivity, S : R = 39 : 61. The clear match/mismatch effect between the monomer 1 and chiral diamine ligands, such as (+)PMP and (−)Sp, was observed during the AOCP of 1 as previously reported.2 Accordingly, the monomer structure signifiPolym. J., Vol. 35, No. 7, 2003

cantly affected the stereoselectivity during these polymerizations. On the other hand, the polymerization with (S )- or (R)-Phbox gave a polymer with an S or R-selectivity (76–80%), respectively, regardless of the monomer chirality. Therefore, the polymerization proceeded under a almost complete ligand control, in marked contrast to the polymerization with other diamines. Poly(2,3-dihydroxy-1,4-naphthylene), preferentially with the · · ·RS· · · structure, in addition to those having 89% of an S - or R-configuration, were selectively synthesized by this system. The polymers prepared from (S )- or (R)-1 showed negative or positive specific rotations, whose values were quite different between those obtained using (S )Phbox (Table I, entries 2 and 3) and (R)Phbox (entries 4 and 5). Almost mirror image CD spectral patterns were observed for the polymers obtained from (S )- and (R)-1 and the absorption intensities around 230 nm were in good agreement with the [α]D values (Figure 3). These results support the stereochemistry estimated by the 13 C NMR analysis for the poly1 ’s. The oxidative coupling polymerization of 3 with TMEDA and (S )Phbox was carried out (Table II). The polymerizations of the optically active 3, the (R, R)- and (S , S )-isomers, resulted in lower yields than those of (S , R)-3, probably due to the lower solubility of these isomers toward polymerization solvents. The stereochemistry of the bonds between the monomer units (S : R) for the polymers obtained from the optically active 3 was determined by the 13 C NMR analysis in CDCl3 at 60 ◦ C (Figure 4a and 4b). The polymerization of (R, R)-3 with the CuCl-TMEDA catalyst showed a slightly higher selectivity (30 : 70) than 595

S. H ABAUE et al.

Table II. Oxidative coupling polymerization of 3 at room temperature for 24 ha Entry

3

1 2 3 4 5

RR SS RR SR SR

Catalyst CuCl-TMEDA CuCl-(S )Phbox CuCl-(S )Phbox CuCl-TMEDA CuCl-(S )Phboxg

Yieldb (%) 50 54 35 86 63

Mn (×103 ) (Mw /Mn )c 2.6 (2.8) 4.0 (1.6) 2.7 (1.4) 4.5 (2.4) 4.1 (1.3)

S : Rd (S : R)e 30 : 70 (8 : 92) 73 : 27 (92 : 8) 68 : 32 (19 : 81) – –

[α]25 D f (deg.) +176 −192 +123 – −49

Solvent: CH2 Cl2 (entries 1 and 4) and THF (entries 2, 3, and 5). b Methanolinsoluble part of poly3 . c Determined by SEC in THF (polystyrene standard). d Determined by 13 C NMR analysis. e Estimated from the S : R and Mn values.11 f In CHCl3 . g Polymerization time = 0.33 h.

θ

a

Figure 5. CD spectra of poly3 s (a) obtained using TMEDA from (R, R)-3 (Table II, entry 1), (b) obtained using (S )Phbox from (S, S )-3 (entry 2), (c) from (R, R)-3 (entry 3), and (d) from (S, R)-3 (entry 5) (naphthalene unit, in CHCl3 ).

Figure 4. 13 C NMR spectra of methyl carbon in acetyl groups of poly3 ’s (a) obtained with the CuCl-(S )Phbox catalyst from (S , S )-3 (Table II, entry 2), (b) from (R, R)-3 (entry 3), (c) from (S , R)-3 (entry 5) (CDCl3 , 60 ◦ C).

that of 1, again indicating that the stereochemistry of the polymerization was significantly affected by the monomer structure. In contrast, the polymers produced from (S , S )- and (R, R)-3 using the (S )Phbox ligand preferentially possessed an S -configuration (approximately 70%), demonstrating the ligand controlled stereochemistry. For example, the S -content in poly[(S , S )-3 ] prepared with (S )Phbox was estimated to be 92%, which showed a higher [α]D value and CD intensity around 230 nm than those of the other poly3 ’s, as well as that of poly1 (Figure 5). The model coupling reaction of 3-benzyloxy-2naphthol with (S )Phbox in THF gave the coupling product in 43%ee (S ), although the actual enantioselectivity in the polymerization of 2 is not clear at present.4 596

The coupling reaction using the CuCl-(S )Phbox catalyst in THF at room temperature should proceed with an enantioselectivity of about 36–60%ee, slightly depending on the monomer structure, such as a unimer, dimer, and trimer, but almost independent of the monomer chirality. Because of the meso-structure of the monomer, the stereoselectivity of poly[(S , R)-3 ] cannot be directly determined from the 13 C NMR analysis and estimated to be (SSS , RRR, RRS , and SSR) : (SRS , RSR, RRS , and SSR) = 50 : 50 for the polymer obtained using TMEDA and 53 : 47 for that prepared with (S )Phbox (Figure 4c). Judging from the CD spectral pattern (Figure 5d), the latter polymer should be slightly rich in the S -configuration, which is based on the · · ·RSSSR· · · structure, because the 1 : 1 absorption of the methyl carbon in acetyl groups is observed for the · · ·RSSRS · · · chain in the 13 C NMR spectrum. Based on the assumption that the polymerization of (S , R)-3 with the CuCl-(S )Phbox catalyst proceeds Polym. J., Vol. 35, No. 7, 2003


in an S -selective manner as observed for those of 1, 2, and the optically active 3, the structures such as · · ·RSSRSSRSSRSSRS · · · and · · ·RSSS RSRSSS RSRS · · · are likely stereoregular chains and these ideal poly[(S , R)-3 ]’s show a 50 : 50 peak area ratio in the 13 C NMR spectrum. This suggests that the observed ratio in the 13 C NMR spectrum, 53 : 47, for the polymer obtained using (S )Phbox does not necessarily have a poor selectivity. Poly[(S , S )-3 ] obtained with the (S )Phbox ligand was evaluated to have a 84%ee (S ) main-chain structure and showed the specific rotation of −192◦ . Assuming that the stereoselectivity of the main chain directly contributes to the specific rotation of the polymer, the selectivity of poly[(S , R)-3 ] having a specific rotation of −49 ◦ was evaluated to be 21%ee (S ) in the whole chain and the selectivity of the polymerization, S : R = 85 : 15 considering the mesostructure of the monomer and the estimated molecular weight.12 In conclusion, the poly(2,3-dihydroxy-1,4naphthylene) derivatives having various stereostructures, such as the (S , S )-, (R, R)-, and (S , R)-isomers, were selectively synthesized by the AOCP of the optically active tetrahydroxybinaphthyl and hexahydroxyternaphthyl derivatives using the Cu(I)-bisoxazoline catalyst, which proceeded under ligand control.

Polym. J., Vol. 35, No. 7, 2003

REFERENCES AND NOTES 1. For reviews: a) L. Pu, Chem. Rev., 98, 2405 (1998). b) M. Putala, Enantiomer, 4, 243 (1999). c) L. Pu, Macromol. Rapid Commun., 21, 795 (2000). 2. S. Habaue, T. Seko, and Y. Okamoto, Macromolecules, 35, 2437 (2002). 3. K. Fuji, T. Furuta, and K. Tanaka, Org. Lett., 3, 169 (2001). 4. S. Habaue, T. Seko, and Y. Okamoto, Macromolecules, 36, 2604 (2003). 5. M. Smrcina, J. Pol´akov´a, S. Vyskocil, and P. Kocovsky, J. Org. Chem., 58, 4534 (1993). 6. M. Nakajima, I. Miyoshi, K. Kanayama, S. Hashimoto, M. Noji, and K. Koga, J. Org. Chem., 64, 2264 (1999). 7. X. Li, J. Yang, and M. C. Kozlowski, Org. Lett., 3, 1137 (2001). 8. K. Tsubaki, H. Tanaka, T. Furuta, K. Tanaka, T. Kinoshita, and K. Fuji, Tetrahedron, 58, 5611 (2002). 9. K. Tanaka, T. Furuta, K. Fuji, Y. Miwa, and T. Taga, Tetrahedron: Asymmetry, 7, 2199 (1996). 10. For poly[(R)-1 ]: S /100 = [(DP − 1)S /100]/(2DP − 1), R/100 = [DP + (DP − 1)R/100]/(2DP − 1), DP = degree of polymerization. 11. For poly[(RR)-3 ]: S /100 = [(DP − 1)S /100]/(3DP − 1), R/100 = [2DP + (DP − 1)R/100]/(3DP − 1). 12. S /100 = [0.605(3DP − 1) − DP]/(DP − 1).

597