[Rh(L-alaninate)(1,5-Cyclooctadiene)] Catalyzed Helix-Sense-Selective Polymerizations of Achiral Phenylacetylenes

The [Rh(L-alaninate)(cod)] (cod = 1,5-Cyclooctadiene) complex was synthesized and characterized. Asymmetric polymerizations of achiral phenylacetylene with two hydroxyl groups and a dodecyl group (DoDHPA) were performed by using the rhodium complex as the catalyst to provide polymers with a higher molecular weight (>105) than the polymers obtained using the [Rh(cod)Cl]2 initiator systems. The resulting polymers showed circular dichroism (CD) signals at approximately 310 and 470 nm, indicating that they have a preferential one-handed helical structure. The helix sense in the polymer main chain was controlled by the sign of the catalyst chirality. These findings suggest that the rhodium complex with a chiral amine is the true active species for the helix-sense-selective polymerization of DoDHPA. The [Rh(L-alaninate)(cod)] complex also exhibits high catalytic activity in the polymerization of phenylacetylene (PA) to give a high yield and molecular weight. All these results demonstrate that this Rh complex is an excellent catalyst for the polymerization of phenylacetylene monomers.


Introduction
Asymmetric catalysis is one of the best methods for the synthesis of optically active organic compounds [1,2]. The design and development of novel catalytic systems for asymmetric synthesis and polymerizations have been active research areas in recent years [3][4][5][6][7][8][9][10]. In particular, in the field of phenylacetylene (PA) asymmetric polymerizations, Rh catalysts have been found to play a vital role and cannot be replaced by other metal catalysts [11].
T. Aoki et al. [12] polymerized p-dodecyloxy-m,m-dihydroxyphenylacetylene (DoDHPA) by [Rh(nbd)Cl] 2 /(S)-PEA (here, nbd is 2,5-norbornadiene and PEA is 1-phenylethylamine) to produce the corresponding polymer that exhibited large cotton effects in the circular dichroism (CD) spectra. The polymerization was named helix-sense-selective polymerization (HSSP). In this HSSP, the chirality polymerization (SHSSP) was also carried out for phenylacetylenes with an L-amino ether residue and two hydroxy groups (ORDHPAs: OADHPA, OVDHPA, and OPDHPA) [13] (Chart S1, Supplementary Materials) by using the achiral [Rh(nbd)Cl]2 catalytic system. The chiral amino ether residues in the monomer and the resulting polymers were thought to act as the ligand and cocatalyst of HSSP instead of (R)-and (S)-PEA. However, the mechanism of SHSSP was not proven directly. Therefore, isolation of the real active species of HSSP or SHSSP was desired but was difficult due to the low stability of the Rh-amine complex [14,15].
In this paper, we report for the first time the crystal chiral [Rh(L-alaninate)(cod)] (cod = 1,5-Cyclooctadiene) catalyst (see Scheme 1) that is very easily obtained and is suitable for the asymmetric polymerization of DoDHPA. The catalytic efficiency for DoDHPA and PA was found to be very high.

Experimental Section
All solvents were dried using standard methods and were distilled over CaH2 under reduced pressure prior to use. Commercially available compounds were used without further purification. RhCl3·3H2O and all other chemicals products including the L-amino acids and PA were purchased from Shanghai Darui Finechem Co., Ltd (Darui, Shanghai, China).
According to Scheme 1, the [Rh(L-alaninate)(cod)] complex was synthesized and isolated; the following reaction procedures were conducted under dry nitrogen. [Rh(cod)Cl]2: The compound was synthesized and purified according to the procedure reported in the literature (see Figure 1A; Figure S1 and S2, Supplementary Materials) [16].
[Rh(L-alaninate)(cod)]: A solution of L-alanine (13.20 mg, 0.130 mmol) and NaOH (5.50 mg, 0.130 mmol) in H2O (0.60 mL) was added to a yellow suspension of [Rh(cod)Cl]2 (30.00 mg, 0.06 mmol) in methanol (2.00 mL). The obtained clear yellow solution was stirred for 1 h at room temperature. The mixture was cooled at 5 °C until precipitation out from methanol, and then the precipitate was isolated by centrifugation. The product was obtained as a yellow powder (30.00 mg, 0.100 mmol, 77.9%). 1

Experimental Section
All solvents were dried using standard methods and were distilled over CaH 2 under reduced pressure prior to use. Commercially available compounds were used without further purification. RhCl 3 ·3H 2 O and all other chemicals products including the L-amino acids and PA were purchased from Shanghai Darui Finechem Co., Ltd. (Darui, Shanghai, China).
According to Scheme 1, the [Rh(L-alaninate)(cod)] complex was synthesized and isolated; the following reaction procedures were conducted under dry nitrogen.
[Rh(cod)Cl] 2 : The compound was synthesized and purified according to the procedure reported in the literature (see Figure 1A; Figures S1 and S2, Supplementary Materials) [16].
[Rh(L-alaninate)(cod)]: A solution of L-alanine (13.20 mg, 0.130 mmol) and NaOH (5.50 mg, 0.130 mmol) in H 2 O (0.60 mL) was added to a yellow suspension of [Rh(cod)Cl] 2 (30.00 mg, 0.06 mmol) in methanol (2.00 mL). The obtained clear yellow solution was stirred for 1 h at room temperature. The mixture was cooled at 5 • C until precipitation out from methanol, and then the precipitate was isolated by centrifugation. The product was obtained as a yellow powder (30.00 mg, 0.100 mmol, 77.9%). 1  (see Figure 1B). 13   In this work, the rhodium catalyst [Rh(L-alaninate)(cod)] was purified using the recrystallization method [17][18][19][20]. The isolated [Rh(L-alaninate)(cod)] complex was a yellow powder, that was crystallized from dichloromethane. The catalyst has a polygonal shape, and the crystal surface is very smooth ( Figure S11 Figure S14 and S15, Supplementary Materials). We concluded that the crystals of the catalysts were obtained by the recrystallization.
The 1 H-NMR spectrum ( Figure 1) of the [Rh(L-alaninate)(cod)] dissolved in DMSO-d6 was obtained. The peaks of (-CH3), amino group (-NH) and methyne (-CH) in L-alaninate appeared separately at 1.16-1.17 ppm, 3.66 ppm and 3.07-3.08 ppm. The coordination of L-alanine to rhodium is supported. When L-alanine was coordinated to rhodium, the chemical shift of the olefinic and methylene protons in cod changed from 4.45, 2.43, and 1.99 ppm ( Figure 1A) to 3.89, 2.29, and 1.69 ppm ( Figure 1B), respectively [6]. The coordination of cod to rhodium is also supported. This chemical shift change arises because L-alanine provides an easily accessible coordination vacancy through a potential effect on the metal center [21]. On the basis of these 1 H-NMR spectra as well as the results of the 13 C NMR, IR, and elemental analyses, it was concluded that the novel chiral rhodium catalyst containing cod and L-alanine ([Rh(L-alaninate)(cod)]) was synthesized successfully.

Instruments
The number and weight-averaged molecular weights (M n and M w , respectively) and dispersity (M w /M n ) of the polymers were measured by gel permeation chromatography (GPC) (Polymer Laboratories, United Kingdom) at 40 • C. A nuclear magnetic resonance (Mode, Bruker, Germany) instrument ( 1 H NMR and 13 C NMR) was used to determine the molecular structure of the monomer and the catalysts. Infrared spectra were recorded using an IR-spectrum One B (PE, Austin, TX, USA) instrument. CD spectra were recorded using a MOS-450 (Bio-Logic, Seyssinet-Pariset, France) instrument. SEM images were recorded using an S-4300 (Hitachi, Tokyo, Japan) apparatus. XRD spectra were recorded using a D8-FOCUS (Bruker, Karlsruhe, Germany) instrument. Mass spectra were recorded using an UltiMate 3000 UPLC/Q-Exactive Orbitrap MS (Thermo, Hampton, NH, USA) instrument.

Results and Discussion
In this work, the rhodium catalyst [Rh(L-alaninate)(cod)] was purified using the recrystallization method [17][18][19][20]. The isolated [Rh(L-alaninate)(cod)] complex was a yellow powder, that was crystallized from dichloromethane. The catalyst has a polygonal shape, and the crystal surface is very smooth ( Figure S11 The 1 H-NMR spectrum (Figure 1) of the [Rh(L-alaninate)(cod)] dissolved in DMSO-d 6 was obtained. The peaks of (-CH 3 ), amino group (-NH) and methyne (-CH) in L-alaninate appeared separately at 1.16-1.17 ppm, 3.66 ppm and 3.07-3.08 ppm. The coordination of L-alanine to rhodium is supported. When L-alanine was coordinated to rhodium, the chemical shift of the olefinic and methylene protons in cod changed from 4.45, 2.43, and 1.99 ppm ( Figure 1A) to 3.89, 2.29, and 1.69 ppm ( Figure 1B), respectively [6]. The coordination of cod to rhodium is also supported. This chemical shift change arises because L-alanine provides an easily accessible coordination vacancy through a potential effect on the metal center [21]. On the basis of these 1 H-NMR spectra as well as the results of the 13 C NMR, IR, and elemental analyses, it was concluded that the novel chiral rhodium catalyst containing cod and L-alanine ([Rh(L-alaninate)(cod)]) was synthesized successfully.
Here, we report the asymmetric polymerization of achiral phenylacetylene monomer (DoDHPA) by using the crystalline chiral Rh catalyst, as shown in Scheme 2. The achiral monomer (DoDHPA) was polymerized by using the binary [Rh(cod)Cl]2/(chiral amine) catalytic system, with [Rh(cod)Cl]2 as the main catalyst and L-prolinol, L-valinol, L-alaninol, or (S)-PEA as cocatalysts, to produce polymers in low yield (8%-31%) with moderate Mw 2.93 × 10 4 -28.50 × 10 4 (Table 1, entries 1-4). Similar results were also reported by Trhlikova and Mawatari [22,23]. To obtain polymers with higher yield and Mw values, we attempted to prepare a novel high-efficiency catalyst [Rh(L-alaninate)(cod)]. We polymerized DoDHPA using the isolated [Rh(L-alaninate)(cod)] catalyst without a cocatalyst to lead a high yield (72%) and large molecular  (Table 1, entries 1-4). Similar results were also reported by Trhlikova and Mawatari [22,23]. To obtain polymers with higher yield and M w values, we attempted to prepare a novel high-efficiency catalyst [Rh(L-alaninate)(cod)]. We polymerized DoDHPA using the isolated [Rh(L-alaninate)(cod)] catalyst without a cocatalyst to lead a high yield (72%) and large molecular weight (55.43 × 10 4 ), as presented in Table 1  The CD spectra of the polymers are shown in Figure 2. The polymers exhibited Cotton effects with molar ellipticity (θ) (deg cm 2 /dmol) at 310 and 430 nm. The peak at 310 nm may arise from a chiral position between the adjacent pendant groups [11,12]. The absorption band at 430 nm was assigned to the conjugated main chain. Therefore, the polymers had chirality in the main chain, that is, a one-handed helical backbone. The (θ) value of poly (DoDHPA) obtained by using the isolated [Rh(L-alaninate)(cod)] was higher ( When [Rh(L-alaninate)(cod)] and [Rh(L-alaninate)(nbd)] were used as catalysts, even though the Rh-N bonds dissociate, the Rh-O bonds are preserved (Chart 1). This differences between the bonds are due to the stronger donating and acceptor abilities of the O-atom compared to the N-atom. The Rh-N bond length is greater than the Rh-O bond length, enabling proton transfer from the NH group to the coordinated monomer [17,24]. The synthesis and characterization of [Rh(L-alaninate)(cod)] and [Rh(L-alaninate)(nbd)] can confirm that the Rh complex containing the amine group may be the real active species of HSSP or SHSSP (Chart 1), providing direct evidence for our previous results [13]. Therefore, the isolated [Rh(L-alaninate)(cod)] and [Rh(L-alaninate)(nbd)] also maintains a higher  When [Rh(L-alaninate)(cod)] and [Rh(L-alaninate)(nbd)] were used as catalysts, even though the Rh-N bonds dissociate, the Rh-O bonds are preserved (Chart 1). This differences between the bonds are due to the stronger donating and acceptor abilities of the O-atom compared to the N-atom. The Rh-N bond length is greater than the Rh-O bond length, enabling proton transfer from the NH group to the coordinated monomer [17,24]. The synthesis and characterization of [Rh(L-alaninate)(cod)] and [Rh(L-alaninate)(nbd)] can confirm that the Rh complex containing the amine group may be the real active species of HSSP or SHSSP (Chart 1), providing direct evidence for our previous results [13]. Therefore, the isolated [Rh(L-alaninate)(cod)] and [Rh(L-alaninate)(nbd)] also maintains a higher helical selective ability than the binary [Rh(cod)Cl]2/L-alaninol and [Rh(nbd)Cl]2/L-alaninol catalytic system in HSSP of DoDHPA.   When [Rh(L-alaninate)(cod)] and [Rh(L-alaninate)(nbd)] were used as catalysts, even though the Rh-N bonds dissociate, the Rh-O bonds are preserved (Chart 1). This differences between the bonds are due to the stronger donating and acceptor abilities of the O-atom compared to the N-atom. The Rh-N bond length is greater than the Rh-O bond length, enabling proton transfer from the NH group to the coordinated monomer [17,24]. The synthesis and characterization of [Rh(L-alaninate)(cod)] and [Rh(L-alaninate)(nbd)] can confirm that the Rh complex containing the amine group may be the real active species of HSSP or SHSSP (Chart 1), providing direct evidence for our previous results [13]. Therefore, the isolated [Rh(L-alaninate)(cod)] and [Rh(L-alaninate)(nbd)] also maintains a higher helical selective ability than the binary On the other hand, we also investigated role of co-catalysts on HSSP of DoDHPA. As shown in Figure 3, when the binary [Rh(cod)Cl] 2 /chiral amine catalytic system was used to polymerize DoDHPA, the type of chiral cocatalysts affected the (θ) value of the polymers. The (θ) value of the polymer obtained by [Rh(cod)Cl] 2 /(S)-PEA was higher (Figure 3, No. 1) than those obtained by using L-prolinol, L-valinol and L-alaninol (Figure 3, Nos. 2-4). This behavior may be due to the larger steric hindrance of (S)-PEA compared to those of L-prolinol, L-valinol, and L-alaninol. These findings indicated that the (S)-PEA is a good cocatalyst in the HSSP of DoDHPA.
Polymers 2018, 10, 1223 7 of 10 in Figure 3, when the binary [Rh(cod)Cl]2/chiral amine catalytic system was used to polymerize DoDHPA, the type of chiral cocatalysts affected the (θ) value of the polymers. The (θ) value of the polymer obtained by [Rh(cod)Cl]2/(S)-PEA was higher (Figure 3, No. 1) than those obtained by using L-prolinol, L-valinol and L-alaninol (Figure 3, Nos. 2-4). This behavior may be due to the larger steric hindrance of (S)-PEA compared to those of L-prolinol, L-valinol, and L-alaninol. These findings indicated that the (S)-PEA is a good cocatalyst in the HSSP of DoDHPA.  We also studied the effect of the solution on the HSSP of DoDHPA. When toluene was used as the polymerization solvent, [Rh(L-alaninate)(cod)] improved the catalytic performance to provide polymers with higher Mw, yields and (θ) values ( Table 1, No. 5 and Figure 4) compared to those obtained using CH2Cl2, benzene, and acetone as the solvents. The polarity of the solvent strongly  We also studied the effect of the solution on the HSSP of DoDHPA. When toluene was used as the polymerization solvent, [Rh(L-alaninate)(cod)] improved the catalytic performance to provide polymers with higher M w , yields and (θ) values ( Table 1, No. 5 and Figure 4) compared to those obtained using CH 2 Cl 2 , benzene, and acetone as the solvents. The polarity of the solvent strongly affected the polymerization of DoDHPA. The polarity values of toluene and benzene are lower than those of the other solvents. DoDHPA and poly(DoDHPA) have better solubilities in toluene. It is important to mention that the solvents affect the catalytic activity. These differences may be due to the different solvents affecting the stability of the Rh complex acting as the active species of the asymmetric polymerization. These findings indicate that the chiral rhodium complex with L-alaninate as the ligand is effective for HSSP. The [Rh(L-alaninate)(cod)] and [Rh(L-alaninate)(nbd)] catalyst can induce HSSP of DoDHPA.
On the other hand, we also investigated role of co-catalysts on HSSP of DoDHPA. As shown in Figure 3, when the binary [Rh(cod)Cl]2/chiral amine catalytic system was used to polymerize DoDHPA, the type of chiral cocatalysts affected the (θ) value of the polymers. The (θ) value of the polymer obtained by [Rh(cod)Cl]2/(S)-PEA was higher (Figure 3, No. 1) than those obtained by using L-prolinol, L-valinol and L-alaninol (Figure 3, Nos. 2-4). This behavior may be due to the larger steric hindrance of (S)-PEA compared to those of L-prolinol, L-valinol, and L-alaninol. These findings indicated that the (S)-PEA is a good cocatalyst in the HSSP of DoDHPA. Figure 3. CD spectra of chiral poly(DoDHPA) obtained using the [Rh(cod)Cl]2/(chiral amine) catalytic system (CD spectra were determined in a THF solution with 1.000 mmol/L poly(DoDHPA)) ( Table 1,   We also studied the effect of the solution on the HSSP of DoDHPA. When toluene was used as the polymerization solvent, [Rh(L-alaninate)(cod)] improved the catalytic performance to provide polymers with higher Mw, yields and (θ) values ( Table 1, No. 5 and Figure 4) compared to those obtained using CH2Cl2, benzene, and acetone as the solvents. The polarity of the solvent strongly Polymerization of phenylacetylene (PA) [25] was also performed using a [Rh(L-alaninate)(cod)] catalyst in different solvents at 30 • C for 24 h. The obtained results are summarized in Table 2. The yield and M w of polymers were very high, with values of 78.6-92.5% and 10 5 -3 × 10 5 , respectively. When methanol is used as the solvent, the highest yield and M w values were obtained. This is because the methanol hydroxyl and the amino group of the active species (Chart 1) can form a hydrogen bond, improving the activity of the polymerization species and increasing the polymerization yield. Additionally the surface tension and the boiling point of methanol are low, and the poly(PA) solubility in methanol is strong, which can effectively reduce the viscosity of the system and promote the polymerization. Therefore, [Rh(L-alaninate)(cod)] used in a CH 3 OH solution was a high-efficiency catalyst for the polymerization of PA.