Exploring Electronic versus Steric Effects in Stereoselective Ring‐Opening Polymerization of Lactide and β‐Butyrolactone with Amino‐alkoxy‐bis(phenolate)–Yttrium Complexes

Abstract

A series of methoxy-amino-bis(phenol)s (ONOO(R(1),R(2)))H(2) possessing on the phenol rings R(1) ortho substituents with variable steric and electronic properties (R(1)=CMe(2)Ph, 1; CMe(2)tBu, 3; CMe(2)(4-CF(3)C(6)H(4)), 5; CPh(3), 9; Cl, 10) has been synthesized and further reacted with [Y{N(SiHMe(2))(2)}(3)](THF)(2) to give cleanly the corresponding yttrium compounds [Y(ONOO(R(1),R(2))){N(SiHMe(2))(2)}(thf)(n)] (Y-x); the solid-state structures of Y-3 and Y-10 have been determined. These amido complexes have been used as initiators for the ring-opening polymerization (ROP) of rac-lactide (LA) and rac-β-butyrolactone (BBL) to provide heterotactically enriched poly(lactic acid)s (PLAs) and syndiotactically enriched poly(3-hydroxybutyrate)s (PHBs), respectively, by means of a chain-end control mechanism. Most of these polymerizations proceeded in a controlled fashion, giving polymers with narrow polydispersities and experimental molecular weights in good agreement with calculated values. The nature of the R(1) ortho substituents has a profound impact on the rates and, more spectacularly, on the stereocontrol of the polymerizations. The heterotactic stereocontrol in the ROP of rac-LA appears to be governed essentially by steric considerations; the larger the substituent, the higher the heterotacticity: R(1)=Cl (P(r)=0.56)≪CMe(3) (P(r)=0.80)≪CMe(2)Ph (P(r)=0.90)<CMe(2)(4 CF(3)-Ph) (P(r)=0.93-0.94)≤CMe(2)tBu (P(r)=0.94-0.95)≤CPh(3) (P(r)=0.95-0.96). On the other hand, the syndiotactic stereocontrol in the polymerization of rac-BBL follows a quite different trend: R(1)=Cl (P(r)=0.42-0.45)≪CMe(2)tBu (P(r)=0.62-0.70)<CMe(3) (P(r)=0.80)≤CMe(2) (4 CF(3)-Ph) (P(r)=0.82-0.84)<CMe(2)Ph (P(r)=0.89)<CPh(3) (P(r)=0.94), which suggests the involvement of electronic interactions. DFT computations on model intermediates confirmed a stabilizing C-H···π interaction between a methylene C-H of the ring-opened BBL unit and the π system of one of the ortho-aryl substituents of the ONOO(R(1)) ligand; by contrast, for model intermediates in the ROP of LA, no such C-H···π interaction involving the methyl group of lactate was observed.

Ring-opening polymerization of lactides by (pyrazol-1-ylmethyl)pyridine Zn(II) and Cu(II) complexes: Kinetics, mechanism and tacticity studies

“…This observation is consistent with those of Carpentier and co-workers. 21 Greater activities of Zn(II) initiators 1 and 3 in comparison to the Cu(II) catalysts 2 and 4 agree with literature reports. 22 We also observed some influence of monomer stereochemistry on the kinetics of the polymerization reactions.…”

Rare-Earth Complexes Supported by Tripodal Tetradentate Bis(phenolate) Ligands: A Privileged Class of Catalysts for Ring-Opening Polymerization of Cyclic Esters

“…It is noteworthy that, in sharp contrast, this [Y{ON(OMe)-O Cl,Cl }(OiPr)] catalyst is essentially nonstereoselective in the ROP of rac-LA (P r = 0.56) and rac-BL (P r = 0.42−0.45). 23 Obviously, considerations different from the steric and electronic ones mentioned above for the ROP of rac-LA and rac-BL are at work in the ROP of of β-malolactonates. However, their exact nature still remains unknown; it is unclear if this might be related to the dinuclear nature of this peculiar catalyst precursor.…”

“…DFT computations performed on putative intermediates indeed indicated that such interactions may stabilize the active species by 5−10 kcal mol −1 and freeze the growing macromolecular chain in a specific conformation, enhancing stereocontrol ( Figure 2); 23 23 (b) In the NOESY 1 H− 1 H NMR spectrum of the corresponding isopropoxide species Y{ON(OMe)O CMe2Ph,CMe2Ph }(OiPr)-(THF) (generated upon alcoholysis of the former amido species), CH(OiPr)···aryl(cumyl) close contacts were also evidenced. 23 (c) Despite the fact that species of the type Y{ON(OMe)O CMe2Ph,R }(OiPr)(THF) (i.e., similar to those computed in Figure 2) could never be generated cleanly, close analogues based on tridentate ONO pyridine-bisphenolate ligands were prepared and structurally characterized; hence, in {ONO CMe2Ph,Me }Al((R)-OCH(Me)CH 2 CO 2 Me), the two cumyl substituents point in the direction of the β-alkoxybutyrate moiety, with short contacts (∼2.6−2.8 Å) between the phenyl π system and one methylene C−H of the butyrate moiety. 68 Recently, Rieger and co-workers studied the kinetics, determining the enthalpies and entropies of the ratedetermining steps through temperature-dependent in situ IR measurements and performed DFT computations on the ROP of BL initiated by the mononuclear 55 Ln{ON(OMe)O tBu2 }(N-(SiHMe 2 ) 2 )(THF) (Ln = Sm, Tb, Y, Lu).…”

Structurally diverse dysprosium and yttrium complexes containing an amine-bis(benzotriazole phenolate) ligand: Synthesis, characterization and catalysis for lactide polymerization

“…Based on the excellent catalytic properties for lactide polymerization catalyzed by wellcharacterized rare-earth metal complexes [12][13][14][15][16], amine-bis(benzotriazole phenolate) dysprosium and yttrium complexes 1-6 are expected to behave as air-stable catalysts for ROP of lactide (LA).…”

“…For instance, a series of aminoalkoxybis(phenolate) yttrium complexes were developed by Carpentier and co-workers; such a yttrium amide with the bulkier ortho substituents installed on the amine-phenolate ligands was shown to be a high active initiator with great stereocontrol for producing highly heterotactic PLAs [12,13]. Recently, Yao and Shen et al…”