Yttrium Catalysts Research Articles

Phosphorus-containing stereocontrolled polyhydroxyalkanoates by yttrium-mediated ring-opening copolymerization of β-lactones

Poly(3-hydroxybutyrate) (PHB) polymers functionalized with pendent diphenyl-phosphinate moieties were prepared by the simultaneous ring-opening copolymerization (ROCOP) of racemic or enantiopure β-butyrolactone (rac- or (S)-BPLMe, respectively) with racemic-(4-oxooxetan-2-yl)methyl diphenylphosphinate (rac-BPLOP). The reactions were mediated by yttrium catalysts which tunable ancillary structure enabled preparing copolymers containing syndio-enriched (Pr up to 0.84; Pr is the probability of racemic linkage), atactic (Pr ∼ 0.50) and highly isotactic (Pr < 0.06) poly(BPLMe) (P(BPLMe), aka PHB for polyhydroxybutyrate) segments, starting from rac-BPLMe or (S)-BPLMe, respectively, as assessed by detailed NMR analyses of the copolymer samples. The amount of BPLOP incorporated within the P[(BPLMe)x-co-(BPLOP)y] samples were in line with the initial comonomer loadings (1.8–8.1 mol% vs. 2–10 mol%). The ROCOP was rather well-controlled affording linear α-isopropoxy, ω-crotonyl telechelic and cyclic random copolymers with Mn,NMR up to 15,000 g.mol−1 and ÐM = 1.34–1.95. Detailed MALDI and high-resolution ESI mass spectrometric analyses evidenced the formation of three major series of macromolecules, namely the cyclic one and two linear ones end-capped with an α-isoproxy and either an ω-hydroxy or a ω-crotonate groups; for each of these series, the populations containing y = 0, 1 or 2 BPLOP repeating units were unambiguously distinguished. The thermal characteristics of the P[(BPLMe)x-co-(BPLOP)y] copolymers were investigated by DSC and TGA, revealing in comparison to those of the parent PHB homopolymers, tunable features as a function of their molar mass, tacticity and BPLOP content.

Divergent Synthesis of Multi-Substituted Aminotetralins via [4+2] Annulation of Aldimines with Alkenes by Rare-Earth-Catalyzed Benzylic C(sp3 )-H Activation.

The search for efficient and selective methods for the divergent synthesis of multi-substituted aminotetralins is of much interest and importance. We report herein for the first time the diastereoselective [4+2] annulation of 2-methyl aromatic aldimines with alkenes via benzylic C(sp3 )-H activation by half-sandwich rare-earth catalysts, which constitutes an efficient route for the divergent synthesis of both trans and cis diastereoisomers of multi-substituted 1-aminotetralin derivatives from readily accessible aldimines and alkenes. The use of a scandium catalyst bearing a sterically demanding cyclopentadienyl ligand such as C5 Me4 SiMe3 or C5 Me5 exclusively afforded the trans-selective annulation products in the reaction of aldimines with styrenes and aliphatic alkenes. In contrast, the analogous yttrium catalyst, whose metal ion size is larger than that of scandium, yielded the cis-selective annulation products. This protocol features 100 % atom-efficiency, excellent diastereoselectivity, broad substrate scope, and good functional group compatibility. The reaction mechanisms have been elucidated by kinetic isotope effect (KIE) experiments and the isolation and transformations of some key reaction intermediates.

Divergent Synthesis of Multi‐Substituted Aminotetralins via [4+2] Annulation of Aldimines with Alkenes by Rare‐Earth‐Catalyzed Benzylic C(sp3)−H Activation

AbstractThe search for efficient and selective methods for the divergent synthesis of multi‐substituted aminotetralins is of much interest and importance. We report herein for the first time the diastereoselective [4+2] annulation of 2‐methyl aromatic aldimines with alkenes via benzylic C(sp3)−H activation by half‐sandwich rare‐earth catalysts, which constitutes an efficient route for the divergent synthesis of both trans and cis diastereoisomers of multi‐substituted 1‐aminotetralin derivatives from readily accessible aldimines and alkenes. The use of a scandium catalyst bearing a sterically demanding cyclopentadienyl ligand such as C5Me4SiMe3 or C5Me5 exclusively afforded the trans‐selective annulation products in the reaction of aldimines with styrenes and aliphatic alkenes. In contrast, the analogous yttrium catalyst, whose metal ion size is larger than that of scandium, yielded the cis‐selective annulation products. This protocol features 100 % atom‐efficiency, excellent diastereoselectivity, broad substrate scope, and good functional group compatibility. The reaction mechanisms have been elucidated by kinetic isotope effect (KIE) experiments and the isolation and transformations of some key reaction intermediates.

Ring-opening (co)polymerization of chiral seven-membered lactones mediated by achiral yttrium catalysts: insights into the catalyst stereocontrol by mass spectrometry

The ROCOP of equimolar mixtures of (R)-CLnBu/(S)-CLMe enabled the assessment of the ROP stereocontrol ability of yttrium catalysts through the degree of alternation via detailed mass spectrometric analyses.

Effect of Additives in the Hydroamination/Cyclization of Aminoalkenes Catalyzed by a Binaphtholate Yttrium Catalyst

AbstractA series of various solvents and additives were tested in enantioselective hydroamination/cyclization reactions of aminoalkenes catalyzed by a binaphtholate yttrium catalyst. The functional group tolerance of the catalyst and the influence on the reaction rate and enantioselectivity was studied. Some weakly coordinating polar solvents, such as Et2O, MTBE, and chlorobenzene led to slightly increased reaction rates compared to the less polar solvent benzene, presumably due to a better stabilization of the polar transition state. Stronger binding solvents and additives, such as THF, DMAP, pyrrolidine, n‐propylamine, and 1‐phenylethylamine, decrease the reaction rate and diminish the enantioselectivity of the hydroamination product. Some additives, such as THF, Et2O, MTBE, chloro‐ and bromobenzene, as well as (+)‐sparteine resulted in slightly higher enantioselectivities in the cyclization of the model substrate C‐(1‐allylcyclohexyl)methylamine, although this observation was not generally true for other aminoalkene substrates. The reaction rates and enantioselectivities were depressed in the presence of (−)‐sparteine using the (R)‐binaphtholate‐ligated catalyst. In case of C‐(1‐allylcyclohexyl)methylamine, the enantioselectivity was switched from 76% ee favoring the (S)‐enantiomer of the hydroamination product when using (+)‐sparteine to 22% ee in favor of the (R)‐enantiomer when (−)‐sparteine was used. The rates of cyclization of aminoalkenes and the resulting enantioselectivities significantly depend on substrate concentration with the highest rate (13.6 h−1) and enantioselectivity (68% ee) observed in dilute conditions (0.05 M) compared to a concentrated solution (0.5 M, 5.0 h−1, 35% ee) for 2,2‐dimethylpent‐4‐enylamine. These observations indicate that the reaction mechanism is shifted in favor of a slower, less enantioselective catalytic cycle involving a higher coordinate species when higher substrate concentrations or stronger binding additives are present.magnified image

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

ROP of bulky functional β-lactones mediated by an achiral diamino-bis(phenolate) yttrium catalyst affords syndio-enriched polyhydroxyalkanoates.

Regioselective Benzylic C–H Alumination and Further Functionalization of 2-Alkylpyridines by Yttrium Catalyst

The regioselective benzylic C–H alumination of various symmetrical and nonsymmetrical 2,6-alkylpyridines catalyzed by a half-sandwich yttrium complex has been developed. The resulting alkylaluminum species bearing a pyridyl ring moiety can be transformed to the corresponding alcohols by oxidation. Moreover, the Cu-catalyzed allylation and benzylation and Pd-catalyzed arylation reactions can be applied to the resulting alkylaluminum species, giving the corresponding α-mono-allylated, α-mono-benzylated, and α-mono-arylated 2-alkylpyridine derivatives, respectively, in good to high yields. Unsymmetrically substituted 2,6-dialkylpyridines can be synthesized from simple symmetrical substrates by repeating such C–H alumination/C–C bond formation processes. Because of the high regioselectivity of the present C–H alumination and the subsequent C–C bond formation, this protocol may serve as a desirable method for the synthesis of nonsymmetrically-2,6-disubstituted pyridine derivatives.

Expanding the Chiral Monoterpene Pool: Enantioselective Diels-Alder Reactions of α-Acyloxy Enones.

An enantioselective Diels-Alder (DA) reaction of α-acyloxy enones has been developed to synthesize chiral oxidized cyclohexenes. Yttrium(III) triflate, in conjunction with a chiral pyridinebisimidazoline (PyBim) ligand, was found to catalyze the asymmetric [4 + 2] cycloaddition with a variety of dienes and α-acyloxy enone dienophiles. Using this method, terpinene-4-ol, a key intermediate in the synthesis of commercial herbicide cinmethylin, can be prepared in four steps from isoprene. A combination of kinetic data and NMR studies support a mechanism involving reversible binding of a dienophile to a yttrium catalyst followed by cycloaddition with a diene as the rate-determining step.

Synthesis of Thermoplastic Elastomers by Yttrium-Catalyzed Isospecific Trans-1,4-Polymerization of (E)-1,3-Pentadiene

Abstract Regio- and stereoselectivity control in the polymerization of 1,3-conjugated dienes is of much interest and importance. We report herein for the first time the isospecific trans-1,4-selective living polymerization of (E)-1,3-pentadiene (EPD) by a C5H5-ligated yttrium catalyst (C5H5)Y(CH2C6H4-NMe2-o)2/[Ph3C][B(C6F5)4], which afforded crystalline isotactic-trans-1,4-polypentadiene showing good elastic properties without vulcanization (tensile strength up to 7.1 MPa with elongation at break of 2600%). The DFT calculations revealed that the stereoselectivity was mainly due to steric repulsion between the C5H5 ligand in the catalyst and the methyl group in EPD.

Yttrium-catalyzed cis-1,4-Selective Polymerization of 2-(4-Halophenyl)-1,3-Butadienes and Their Copolymerization with Isoprene

Polymerization of 2-(4-halophenyl)-1,3-butadiene (2-XPB) and their copolymerization with isoprene using a yttrium catalyst have been examined. The β-diketiminato yttrium bis(alkyl) complex (1) activated by [Ph3C][B(C6F5)4] and AliBu3 shows high cis-1,4-selectivity (>98%) for the polymerization of 2-XPB (2-XPB = 2-FPB, 2-ClPB and 2-BrPB) to afford halogenated plastic poly(dienes) with glass transition temperatures of 30–55 °C. Moreover, the copolymerization of 2-XPB with isoprene (IP) has also been achieved by this catalyst, and the insertion ratios of 2-XPB can be facilely tuned in a full range of 0%–100% simply by changing the 2-XPB-to-IP ratio. Quantitative hydrogenation of cis-1,4-poly(2-XPB) results in perfect alternating ethylene-halostyrene copolymers, and an alternating copolymer of 4-vinylbenzoic acid with ethylene is obtained by a consecutive reaction of ethylene-4-bromostyrene copolymer with nBuLi, CO2 and HCl.

Regiodivergent C-H Alkylation of Quinolines with Alkenes by Half-Sandwich Rare-Earth Catalysts.

The regiodivergent catalysis of C-H alkylation with alkenes is of great interest and importance but has remained hardly explored to date. We report herein the first regiodivergent C-H alkylation of quinolines with alkenes by half-sandwich rare-earth catalysts. The regiodivergence was achieved by fine-tuning the metal/ligand combination or steric and electronic properties of the catalysts. The use of the C5Me5-ligated scandium catalyst Sc-3 for the reaction of quinolines with styrenes and that of the C5Me4H-ligated yttrium catalyst Y-2 for the reaction with aliphatic olefins exclusively afforded the corresponding C8-H alkylation products, thus constituting the first example of direct C8-H alkylation of neutral quinolines. In contrast, the Sc-3-catalyzed reaction of 2-arylquinolines with aliphatic olefins and the Y-2-catalyzed reaction with styrenes selectively gave the 2-aryl o-C-H alkylation products. On the basis of the catalyst/substrate-controlled regiodivergence, the sequential regiospecific dialkylation of quinolines with two different alkenes has also been achieved. DFT studies revealed that the C-H activation of 2-phenylquinoline at both the C8 position and an ortho position of the 2-phenyl substituent was possible, and these two types of initially formed C-H activation products were interconvertible through the coordination and C-H activation of another molecule of quinoline. The regioselectivity for the C-H alkylation reactions was governed not only by the ease of the initial formation of the C-H activation products but also by the energy barriers for their interconversions, as well as by the energy barriers or steric and electronic influences in the subsequent alkene insertion processes. This work has not only constituted an efficient protocol for the selective synthesis of diversified quinoline derivatives but also offered unprecedented insights into the C-H activation and transformation of quinolines and may help in the design of more efficient, selective, or complementary catalysts.

C–H Bond Activation of Silyl-Substituted Pyridines with Bis(Phenolate)Yttrium Catalysts as a Facile Tool towards Hydroxyl-Terminated Michael-Type Polymers

Herein, silicon-protected, ortho-methylated hydroxy-pyridines were reported as initiators in 2-aminoalkoxy-bis(phenolate)yttrium complexes for rare earth metal-mediated group-transfer polymerization (REM-GTP) of Michael-type monomers. To introduce these initiators, C−H bond activation was performed by reacting [(ONOO)tBuY(X)(thf)] (X = CH2TMS, thf = tetrahydrofuran) with tert-butyl-dimethyl-silyl-functionalized α-methylpyridine to obtain the complex [(ONOOtBuY(X)(thf)] (X = 4-(4′-(((tert-butyldimethylsilyl)oxy)methyl)phenyl)-2,6-di-methylpyridine). These initiators served as functional end-groups in polymers produced via REM-GTP. In this contribution, homopolymers of 2-vinylpyridine (2VP) and diethyl vinyl phosphonate (DEVP) were produced. Activity studies and end-group analysis via mass spectrometry, size-exclusion chromatography (SEC) and NMR spectroscopy were performed to reveal the initiator efficiency, the catalyst activity towards both monomers as well as the initiation mechanism of this initiator in contrast to commonly used alkyl initiators. In addition, 2D NMR studies were used to further confirm the end-group integrity of the polymers. For all polymers, different deprotection routes were evaluated to obtain hydroxyl-terminated poly(2-vinylpyridine) (P2VP) and poly(diethyl vinyl phosphonate) (PDEVP). Such hydroxyl groups bear the potential to act as anchoring points for small bioactive molecules, for post-polymerization functionalization or as macroinitiators for further polymerizations.

Isospecific Group-Transfer Polymerization of Diethyl Vinylphosphonate and Multidimensional NMR Analysis of the Polymer Microstructure

Control of stereoregularity is an integral part of a precision polymerization method and for the development of functional materials. Yttrium- and aluminum-based catalysts are known for converting diethyl vinylphosphonate (DEVP) into its stimuli-responsive polymer in a precise but stereoirregular way. Herein, we present Y- and Al-based constrained geometry complexes (CGCs) to induce isotacticity without losing control over other macromolecular parameters. After having established convenient synthesis routes and detailed structural analyses, these CGCs showed exceedingly high turn-over frequencies (up to 45 000 h–1) in the group-transfer polymerization of DEVP. The initiator efficiencies (≤99%) and dispersities (≤1.02) strongly depended on the substitution pattern of the applied ligands. An analysis of the microstructure using multidimensional NMR (1H–1H and 1H–13C(−31P)) correlation experiments demonstrated significant disparities for the stereospecificity of the CGCs and enabled a reliable signal assignment. The yttrium catalysts produced highly isotactic poly(diethyl vinylphosphonate), likely following a chain-end control mechanism, whereas the aluminum complexes produced less defined polymers.

Rare-Earth-Metal-Catalyzed Kinetic Resolution of Chiral Aminoalkenes via Hydroamination: The Effect of the Silyl Substituent of the Binaphtholate Ligand on Resolution Efficiency.

The kinetic resolution of α‐substituted aminopentenes via intramolecular hydroamination was investigated using various 3,3′‐silyl‐substituted binaphtholate yttrium catalysts. High efficiencies in the kinetic resolution were observed for methyl‐, benzyl‐, and phenyl‐substituted substrates utilizing the cyclohexyldiphenylsilyl‐substituted catalyst 2c with resolution factors reaching as high as 90(5) for hex‐5‐en‐2‐amine (3a). Kinetic analysis of the enantioenriched substrates with the matching and mismatching catalyst revealed that the efficiency of catalyst 2c benefits significantly from a favorable Curtin–Hammett pre‐equilibrium and by a large kfast/kslow ratio. Other binaphtholate catalysts were less efficient due to a less favorable Curtin–Hammett pre‐equilibrium, which often favored the mismatching substrate‐catalyst combination. Cyclization of the matched substrate proceeds generally with large trans‐selectivity, whereas the trans/cis‐ratio for mismatched substrates is significantly diminished, favoring the cis‐cyclization product isomer in some instances.

Yttrium Catalyzed Dialkyl Vinyl Phosphonate Polymerization: Mechanistic Insights on the Precision Polymerization from DFT

Single-site yttrium catalysts have the ability to produce poly(dialkyl vinyl phosphonate) with a very narrow molecular weight distribution. Following the report of Rieger and co-workers ( J. Am. Ch...

Air-stable di-nuclear yttrium complexes as versatile catalysts for lactide polymerization and copolymerization of epoxides with carbon dioxide or phthalic anhydride

Four novel di-nuclear yttrium catalysts coordinated by bis(benzotriazole iminophenolate) (BiIBTP) ligands have been synthesized and structurally characterized. The one-pot procedure of BiIBTP pro-ligand with [Y(NO3)3.6H2O] (1.0 equiv.) and 2-nitrophenol derivative (1.0 equiv.) in the presence of triethylamine (5.0 equiv.) under refluxing MeOH solution gave bimetallic yttrium nitrophenolate complexes 1–4, [(μ-BiIBTP)Y(L)]2 (BiIBTP = C13CBiIBTP or C83CBiIBTP; L = 2-nitrophenolate or 2,4-dinitrophenolate) with good to high yields. The solid-state structure of 1 and 3–4 exhibits a di-yttrium(III) di-nitrophenolate species with the pentadentate BiIBTP ligand chelating two metal atoms. Versatile catalysis towards ring-opening polymerization of cyclic ester and copolymerization of internal epoxides with carbon dioxide (CO2) or phthalic anhydride (PA) catalyzed by these di-yttrium nitrophenolate complexes was systematically studied. Experimental results revealed that air-stable complexes 3 and 4 were shown to effectively catalyze not only lactide polymerization but also copolymerization of cyclohexene oxide (CHO) with CO2 or PA. Among these compounds, di-Y complex 4 was the most active catalyst for the ring-opening polymerization of l-lactide in combination with 9-anthracenemethanol, providing poly(l-lactide)s having the expected molecular weight and narrow molecular weight distribution. Furthermore, catalyst 3 was capable of copolymerizing CO2 and CHO in the presence of co-catalysts under mild conditions, producing perfectly alternating poly(cyclohexene carbonate)s with the controllable character. In addition, the molecular weight controllable poly(PA-co-CHO)s with high ester linkage contents could be afforded on copolymerizing PA and CHO utilizing di-yttrium complex 3 as the catalyst. This work provides the facilely synthesized di-nuclear yttrium complexes as versatile catalysts for three different types of polymerization catalysis.

Catalyst‐Sidearm‐Induced Stereoselectivity Switching in Polymerization of a Racemic Lactone for Stereocomplexed Crystalline Polymer with a Circular Life Cycle

AbstractConstruction of robust, stereocomplexed (sc) crystalline material, based on a recently discovered infinitely recyclable polymer system, requires blending of enantiomeric polymer chains produced from respective enantiopure, fused six‐five bicyclic lactones. Herein, the stereoselective polymerization of the racemic monomer by yttrium catalysts bearing tetradentate ligands is reported, where the tethered donor sidearm switches the heteroselectivity of the catalyst to isoselectivity when it is changed from the β‐OMe to β‐NMe2 sidearm. The latter catalyst produces an isotactic stereoblock polymer (Pm up to 0.95) that forms the crystalline sc‐material with a Tm of up to 171 °C. This sc‐material can be fully depolymerized back to rac‐monomer in a quantitative yield and purity, thus establishing its circular life cycle.

Catalyst-Sidearm-Induced Stereoselectivity Switching in Polymerization of a Racemic Lactone for Stereocomplexed Crystalline Polymer with a Circular Life Cycle.

Construction of robust, stereocomplexed (sc) crystalline material, based on a recently discovered infinitely recyclable polymer system, requires blending of enantiomeric polymer chains produced from respective enantiopure, fused six-five bicyclic lactones. Herein, the stereoselective polymerization of the racemic monomer by yttrium catalysts bearing tetradentate ligands is reported, where the tethered donor sidearm switches the heteroselectivity of the catalyst to isoselectivity when it is changed from the β-OMe to β-NMe2 sidearm. The latter catalyst produces an isotactic stereoblock polymer (Pm up to 0.95) that forms the crystalline sc-material with a Tm of up to 171 °C. This sc-material can be fully depolymerized back to rac-monomer in a quantitative yield and purity, thus establishing its circular life cycle.

Development of Group 3 Catalysts for Alternating Copolymerization of Ethylene and Styrene Derivatives

Alternating copolymers have the clearest and most defined microstructures among manmade polymers, having been promising building blocks to access synthetic polymers able to mimic biomaterials. The most successful approaches employ donor–acceptor monomer couples, enantiomers with different substituents, as well as specially designed cyclic monomers containing various units through ionic and living radical polymerizations. Herein we report the catalytic behaviors of rare-earth metal-based catalyst systems toward the direct copolymerization of ethylene with a series of unmasked polar styrenes and nonpolar styrenes. For the copolymerization of ethylene with para-methoxystyrene, the pyridyl side-armed fluorenyl-supported yttrium catalyst was inert, while its scandium analogue displayed moderate activity to give a random copolymer; the half-sandwich fluorenyl scandium catalyst provided a gel product. In contrast, the methyl-substituted N-heterocyclic carbene (NHC) side-armed fluorenyl scandium catalyst showed t...

Stereoselective Copolymerization of Unprotected Polar and Nonpolar Styrenes by an Yttrium Precursor: Control of Polar-Group Distribution and Mechanism.

Styrene underwent unprecedented coordination-insertion copolymerization with naked polar monomers (ortho-/meta-/para-methoxystyrene) in the presence of a pyridyl methylene fluorenyl yttrium catalyst. High activity (1.26×106 g molY-1 h-1 ) and excellent syndioselectivity were observed, and high-molecular-weight copolymers (24.6×104 g mol-1 ) were obtained. The insertion rate of the polar monomers could be adjusted in the full range of 0-100 % simply by changing the loading of the polar styrene monomer. Strikingly, the copolymers had tapered, gradient, and even random sequence distributions, depending on the position of the polar methoxy group on the phenyl ring and thus on its mode of coordination to the active metal center, as shown by tracking the polymerization process and DFT calculations.