Stereo-electronic contributions in yttrium-mediated stereoselective ring-opening polymerization of functionalracemicβ-lactones: ROP of 4-alkoxymethylene-β-propiolactones with bulky exocyclic chains

Abstract

Stereoselective ring-opening polymerization (ROP) of cyclic esters is the privileged strategy to access to stereoregular polyesters that are widely applied in various domains, such as in particular the biomedical and...

Metal-catalyzed stereoselective ring-opening polymerization of functional β-lactones: methylene-alkoxy-fluorinated polyhydroxyalkanoates unveil the role of non-covalent interactions

“…In rac-BPL CH2OCF2CHF2 , the outer methylene hydrogens are replaced by fluorine in order to inhibit the outer OC̲ R*H̲ 2 ⋯Cl(ligand) NCIs, and ultimately to evaluate the impact of such an interaction on the resulting PHA stereo-Scheme 1 Stereoselective ROP of various chiral racemic 4-substituted-β-propiolactones, rac-BPL FG s (FG = Me, CH 2 CH 2 OCH 2 Ph, CH 2 OCH 2 R* with R* = Me, nBu, allyl, Bn, CH 2 OZ with Z = iPr, tBu, Ph, SitBuMe 2 , CO 2 R* with R* = Me, Allyl, Bn) performed with Y{ONXO Cl2 }/iPrOH catalyst systems, illustrating the importance of the -CH 2 OCH 2 R* exocyclic side chain to impart stereoregularity on these dichloro-substituted bisphenolate yttrium catalysts. 9,[11][12][13][14][15][16][17] regularity. Hence, the ROP of rac-BPL CH2OCF2CHF2 was studied: (i) to determine and rationalize the activity of catalysts, especially in relation to the stereoelectronic effects of the phenolate R substituents, (ii) to fully characterize the resulting PBPL CH2OCF2CHF2 s at the molecular and microstructural levels by NMR spectroscopy, SEC, mass spectrometry, DSC and TGA thermal analyses, and (iii) finally to assess the catalyst ability in affording stereoregular PHAs.…”

“…Typical rac-BPL CH2OCF2CHF2 polymerization procedure [10][11][12][13][14][15][16][17] In a typical experiment (Table 1, entry 4), in a glovebox, a Schlenk flask was charged with [Y(N(SiHMe 2 ) 2 ) 3 ](THF) 2 (8.8 mg, 14.0 µmol) and {ONNO tBu2 }H 2 (1c, 6.2 mg, 9.8 µmol), and toluene (0.4 mL) was next added. To this solution, iPrOH (75.7 μL of a 1% (v/v) solution in toluene, 1 equiv.…”

“…Thus, alkyl (FG = Me), (di)methylene (thio)alkoxide (FG = CH 2 OMe, CH 2 OiPr, CH 2 OtBu, CH 2 OAll, CH 2 OBn, CH 2 OSitBuMe 2 , CH 2 OPh, CH 2 SPh, and CH 2 CH 2 OBn), and ester (FG = CO 2 Me, CO 2 All, and CO 2 Bn) substituted β-propiolactones enabled the estimation of the influence of short/long, polar/non-polar, alkyl/functional lactone pending groups on their ROP mediated by the Y{ONXO R2 }/iPrOH catalyst platform (Scheme 1). [7][8][9][10][11][12][13][14][15][16][17] Extended insights into the stereoselectivity of the resulting PHAs by 13 C NMR analyses first suggested that, by itself, neither the length, steric bulkiness, electronic environment nor chemical functionality (alkyl vs. ether vs. ester) provided by the FG exocyclic substituent, may enable the preparation of PHAs other than atactic or syndiotactic PHAs. While the chemical synthesis of syndiotactic PHAs − that arise from a regular "chain-end stereocontrol mechanism" thanks to the sterically bulky substituents installed on the ligand of the catalyst, 5,[7][8][9][10][11][12][13][14][15][16][17] and that otherwise cannot be found as natural polymers since the PHAs produced naturally or biosynthetically from microorganisms or enzymes are invariably isotactic polymers − was thereby rewardingly established, isotactic polymers were only exceptionally obtained from these Y{ONXO R2 }/ iPrOH catalyst systems.…”

“…[7][8][9][10][11][12][13][14][15][16][17] Extended insights into the stereoselectivity of the resulting PHAs by 13 C NMR analyses first suggested that, by itself, neither the length, steric bulkiness, electronic environment nor chemical functionality (alkyl vs. ether vs. ester) provided by the FG exocyclic substituent, may enable the preparation of PHAs other than atactic or syndiotactic PHAs. While the chemical synthesis of syndiotactic PHAs − that arise from a regular "chain-end stereocontrol mechanism" thanks to the sterically bulky substituents installed on the ligand of the catalyst, 5,[7][8][9][10][11][12][13][14][15][16][17] and that otherwise cannot be found as natural polymers since the PHAs produced naturally or biosynthetically from microorganisms or enzymes are invariably isotactic polymers − was thereby rewardingly established, isotactic polymers were only exceptionally obtained from these Y{ONXO R2 }/ iPrOH catalyst systems. 18 Indeed, only the 4-alkoxymethylenesubstituted β-propiolactones, simultaneously featuring inner and outer methylene groups apart from the side-chain oxygen atom, namely rac-BPL C̲ H̲ 2 ̲ OC̲ H̲ 2̲ R* (R* = H, CHvCH 2 , Ph), returned the isotactic corresponding PBPL C̲ H̲ 2̲OC̲H̲ 2̲R* s, provided the yttrium bisphenolate ligand was bearing ortho-halogen substituents (R = F, Cl, Br).…”

Crystal Structure of Poly(7-heptalactone)

“…In fact, few studies of the polymerization of η-heptalactone to PHL have appeared in the literature, , one of which concerns the enzymatic polymerization catalyzed by lipase, and one paper reports the ROP of η-heptalactone employing metal-catalyzed coordinative-insertion polymerization . In particular, although the ROP of lactones of different ring sizes promoted by metal-based catalysts has been extensively studied, − the ROP of the eight-membered lactone, η-heptalactone, has been reported only recently employing a discrete lanthanum compound, La­[N­(SiMe 3 ) 2 ] 3 , and salen-ligated yttrium and lanthanum complexes as catalysts to produce high molecular mass PHL . In the same report, the yttrium and lanthanum catalysts have been employed for the stereoselective copolymerization of η-heptalactone with eight-membered cyclic diester (diolide) to produce block copolymers of PHL with poly­(3-hydroxybutyrate) .…”