Enantiomorphic Site Control Mechanism Research Articles

Asymmetric Kinetic Resolution Polymerization of Racemic Lactide Mediated by Axial-Chiral Thiourea/Phosphazene Binary Organocatalyst.

Asymmetric kinetic resolution polymerization (AKRP) provides an ideal way to obtain highly isotactic polylactide (PLA) with superior thermal-mechanical properties from racemic lactide (rac-LA). However, the development of a new catalytic system with concurrent high activity and selectivity at ambient temperature remains a great callenge. Here, a series of simple and effective binary organocatalytic pairs containing axial-chiral thioureas and commercially available phosphazene bases were designed. These chiral binary organocatalytic pairs allow for both high polymerization activity and moderate enantioselectivity for AKRP of rac-LA at room temperature, yielding semi-crystalline and metal-free stereoblock PLA with a melting temperature as high as 186 °C. The highest kinetic resolution coefficient (krel) of 8.5 at 47 % conversion was obtained, and D-LA was preferentially polymerized via kinetic resolution with a maximum selectivity factor (kD/kL) of 18.1, indicating that an enantiomorphic site control mechanism (ESC) was involved.

Highly Isoselective Ring‐Opening Polymerization of rac‐Lactide Using Chiral Binuclear Aluminum Catalyst

The microstructure can significantly affect the physical and mechanical properties of polymers. Five chiral binuclear aluminum methyl complexes (rac1 to rac5) are synthesized by using chiral binaphthalene diamine ligand and characterized by both 1 H and 13 C NMR spectroscopy. The crystal structures of rac3 and rac5 are also identified by single crystal X-ray diffraction. In the presence of i PrOH initiator, these aluminum complexes catalyze the ring-opening polymerization (ROP) of rac-lactide (rac-LA) in a controlled manner and produce polymers with high to excellent isoselectivity (Pm up to 0.93). Even with a catalyst loading as low as 0.1%, polylactide with a Pm of 0.83 can still be obtained. Kinetic studies reveal the first-order dependence on monomer concentration whereas kinetic resolution polymerization provides the evidence to support that these binuclear aluminum catalysts catalyzed ROP of rac-LA adopts enantiomorphic site control mechanism. Furthermore, this strategy can also be applied to the ROP of both ε-caprolactone and δ-valeroactone.

Highly Active Chiral Oxazolinyl Aminophenolate Magnesium Initiators for Isoselective Ring-Opening Polymerization of rac-Lactide: Dinuclearity Induced Enantiomorphic Site Control

The most important challenge in ring-opening polymerization of rac-lactide today is to obtain isotactic PLAs with high molecular weights. The goal of our work is to address this important challenge by developing novel catalysts that enable precise control over stereoselectivity and produce high molecular weight PLAs at the same time. Here we report some rare examples of isoselective magnesium complexes toward the ROP of rac-LA, which are supported by chiral oxazolinyl aminophenolate ligands. Preliminary kinetic investigations confirmed that isotactic PLAs could be achieved via an enantiomorphic site control mechanism by using these chiral magnesium complexes. A pair of racemic magnesium initiators proved to integrate hyperactivity (TOF up to 54 000 h–1 at 25 °C) with sufficient isoselectivity (Pm = 0.80) toward the ROP of rac-LA, leading to the formation of isotactic stereoblock PLA with high molecular weight (Mn = 461 kg mol–1) and a semicrystalline property (Tm = 164 °C). Detailed structural investigati...

Macrocyclic Trinuclear Nickel Phenoxyimine Catalysts for High-Temperature Polymerization of Ethylene and Isospecific Polymerization of Propylene

A series of macrocyclic multinucleating phenoxyimine ligands and the corresponding neutral binuclear (2-Ni2) and trinuclear (3-Ni3) nickel catalysts have been efficiently synthesized. The trinuclear nickel complex 3-Ni3 showed high activity, high thermal stability, and slow chain transfer in ethylene polymerization, thus producing polyethylene with high molecular weight and low branch density. Highly regiospecific and isospecific polymerization of propylene was also achieved with 3-Ni3, generating regioregular and highly isotactic propylene with high Tm and crystallinity. This is the first example of regio- and stereocontrolled propylene polymerization promoted by nickel phenoxyimine catalysts. Statistical analysis suggested selective 1,2-insertion and enantiomorphic site control mechanism in the chain propagation step, likely caused by the unique steric effect of macrocyclic ligands and the potential cooperative effect.

2-Methoxyethylamino-bis(phenolate)yttrium Catalysts for the Synthesis of Highly Isotactic Poly(2-vinylpyridine) by Rare-Earth Metal-Mediated Group Transfer Polymerization

Highly isotactic poly(2-vinylpyridine) (P2VP) was synthesized by the group transfer polymerization of the prochiral 2-vinylpyridine (2VP) with 2-methoxyethylaminobis(phenolate)yttrium complexes. Isotacticities of up to Pm = 0.92, narrow molecular weight distributions, and high molecular weights were achieved by steric modifications of the variable bisphenolate ligand structure. The resulting polymer samples were characterized by thermoanalysis (DSC, TGA), GPC, and 13C NMR. The origin of the isotactic microstructure was attributed to an enantiomorphic site control mechanism based on 13C NMR mechanistic studies and allowed new insights into 13C pentad assignments.

Stereospecific catalytic precision polymerization of 2-vinylpyridine via rare earth metal-mediated group transfer polymerization with 2-methoxyethylamino-bis(phenolate)-yttrium complexes

C 1-Symmetric 2-methoxyethylamino-bis(phenolate)-yttrium complexes for the stereospecific polymerization of 2-vinylpyridine.

Stereoselective ROP of rac-Lactide Mediated by Enantiopure NNO-Scorpionate Zinc Initiators

The reaction of the enantiopure NNO-scorpionate ligand (R,R)-bpzmmH with [ZnR2] (R = Me, Et, CH2SiMe3) in a 1:2 molar ratio afforded the dinuclear tris-alkyls [Zn(R)(κ2-NNμ-O)Zn(R)2] (R = Me (1), Et (2), CH2SiMe3 (3)). The reaction of the bimetallic tris-alkyls 1–3 with an aromatic alcohol and thioalcohol yielded the dinuclear zinc complexes containing aryloxide/arylthioxide ligands [(ZnR)2(κN:κN-μ-O)(μ-EAr)] (Ar = 2,6-C6H3Me2; E = O, R = Me (4), Et (5), CH2SiMe3 (6); E = S, R = Me (7), Et (8), CH2SiMe3 (9)) by an alcoholysis or thioalcoholysis reaction. The structures of the different compounds were determined by spectroscopic methods and, in addition, the X-ray crystal structures of 2 and 5 were also unambiguously established. Alkyl- and aryloxide/thioaryloxide-containing enantiopure zinc complexes 3–6 and 9 can act as single-component living initiators for the ring-opening polymerization of lactides, affording materials with low molecular weights under mild conditions in only a few hours. Interestingly, microstructural analysis of poly(rac-lactide) by 1H NMR spectroscopy revealed that the most sterically hindered initiator, [(ZnCH2SiMe3)2(R,R-bpzmm)(μ-OAr)] (6), is one of the first examples of a zinc complex capable of exerting a significant influence on the degree of enantioselectivity, with enriched-isotactic PLAs (Pi = 0.74) produced through an enantiomorphic site control mechanism.

Ring-Opening Polymerization of Cyclic Esters and Trimethylene Carbonate Catalyzed by Aluminum Half-Salen Complexes

A series of ONO-tridentate Schiff base ligands derived from chiral and achiral amino alcohols and amino acids were synthesized and reacted with AlEt(3) to provide dimeric aluminum complexes. These complexes were tested for the ring-opening polymerization (ROP) of rac-lactide at 70 °C in toluene, producing poly(lactide) with up to 82% isotacticity. The most active of these aluminum complexes was chosen to perform ring-opening homopolymerizations of rac-lactide, trimethylene carbonate (TMC), rac-β-butyrolactone (rac-β-BL), δ-valerolactone (δ-VL), and ε-caprolactone (ε-CL). Kinetic parameters were investigated, and each polymerization was found to be first order with respect to monomer concentration. Fractional orders were observed with respect to catalyst concentration, indicating catalyst aggregation during the polymerization processes. Activation parameters were determined for all monomers, with their ΔG(‡) values at 90 °C being in the order rac-lactide ≈ rac-β-BL > δ-VL > TMC ≈ ε-CL. Fineman-Ross and kinetic studies of the copolymerization of rac-lactide and δ-VL both indicate that the rate of rac-lactide enchainment is higher than that of δ-VL, resulting in a tapered copolymer. In addition, single crystals of one of these aluminum complexes were grown in the presence of rac-lactide and characterized using X-ray crystallography. The unit cell contains two lactide monomers, one D- and one L-lactide, adding further proof that polymerization takes place via an enantiomorphic site control mechanism.

Latent Bimodal Polymerization of 1-Hexene by a Titanium-Based Diastereomeric Catalyst Containing a rac/meso-Aminodiol Ligand

A New titanium catalyst bearing an aminodiol ligand was synthesized as racemic (C2), as meso, and as a diastereomeric mixture. The racemic catalyst (rac) produced isotactic polymer by enantiomorphic-site control mechanism whereas meso catalyst (meso) produced atactic oligomers. The rac and the diastereomeric catalyst (mix) produced polymers with single-site catalyst behavior at ambient temperatures. When the polymerization temperature was lowered, rac gave high molecular weight polyhexene with polydispersity value approaching close to a living system whereas the mix exhibited a latent bimodal behavior. meso produced just oligomers and did not show any change in behavior with polymerization temperature. Deliberate mixing of rac and meso produced two polyhexene fractions: high molecular weight polymer and oligomers indicating an independent behavior of the mixed catalyst.

Synthesis, structure, and catalytic properties of ansa-zirconocenes, Me 2Si(RInd) 2ZrCl 2 (R=2- p- or 3- p-tolyl)

ansa-Ligands, Me 2Si(RInd) 2 [R=2- p ( 1) and 3- p-tolyl ( 2)], have been prepared as a mixture of rac- and meso-isomers in a ratio of 1:1 from the reactions of Me 2SiCl 2 with cyanocuprates of 1- p-tolylindene and 2- p-tolylindene in diethyl ether, respectively. Pure rac-isomers, 1r and 2r, have been isolated by fractional recrystallization of 1 and 2. Thermal diastereomerization of 1 and 2 between rac- and meso-isomers has been studied by 1H-NMR spectroscopy. These results imply that 1 and 2 undergo the diastereomerization process by migration of silicon moiety and hydrogen atom, respectively. ansa-Zirconocenes, Me 2Si(RInd) 2ZrCl 2 [R=2- p ( 3) and 3- p-tolyl ( 4)], have been prepared from the reactions of the corresponding dilithiated ligands with ZrCl 4 in diethyl ether and hexane at −78°C. The molecular structure of rac- 3 has been determined by a single crystal X-ray diffraction study. Propylene polymerization has been studied using rac- 3 and meso- 4 in the presence of methylaluminoxane (MAO). Catalyst rac- 3 produces a highly isotactic polypropylene (PP) at 0°C by the enantiomorphic site control mechanism and meso- 4 affords a syndiotactic dominant PP having a rr triad of 49% at −30°C by the chain end control mechanism.

Enantioselective cyclopolymerization of 1,5-hexadiene catalyzed by chiral zirconocenes: a novel strategy for the synthesis of optically active polymers with chirality in the main chain

Enantioselective cyclopolymerization represents a novel strategy for the synthesis of optically active main-chain chiral polymers. Cyclopolymerization of 1,5-hexadiene using the optically active catalyst precursor, (R,R)-(EBTHI)ZrBINOL ((R,R)-1) [EBTHI = ethylene-1,2-bis([eta][sup 5]-4,5,6,7-tetrahydro-1-indenyl); BINOL = 1,1[prime]-bi-2-naphtholate], yields optically active poly(methylene-1,3-cyclopentane) (PMCP) with a molar optical rotation of [[Phi]][sup 28][sub 405] +51.0[degrees] (c = 0.80, CHCl[sub 3]). The molar optical rotation for the polymer derived from (R,R)-1 is higher than that of the model compound trans-(1R,3R)-1,3-dimethylcyclopentane and is also temperature dependent, suggesting that the polymer adopts chiral conformations which contribute to the observed optical rotation. The microstructure of the polymer was interpreted by [sup 13]C NMR at tetrad resolution. A statistical model for the microstructure based on an enantiomorphic site control mechanism provided good agreement with the experimental data. On the basis of this model, the enantioface selectivity for the cyclopolymerization of 1,5-hexadiene in the presence of catalysts derived from (R,R)-1 is 91% at 20[degrees]C, indicative of a highly isotactic microstructure. The molar optical rotation of poly(methylene-1,3-cyclopentane) is independent of molecular weight, which provides experimental support for an enantiomorphic site control mechanism. The absolute configuration of PMCP was tentatively assigned on the basis of the sign of the optical rotation of the model compoundmore » trans-(1R,3R)-1,3-dimethylcyclopentane and the known enantioface selectivity of 1 with similar [alpha]-olefins. 46 refs., 12 figs., 5 tabs.« less