Concepts for Stereoselective Acrylate Insertion

Various phosphinesulfonato ligands and the corresponding palladium complexes [{((P^∧O)­PdMeCl)-μ-M}<i>_n</i>] ([{(^<b>X</b><b>1-Cl</b>)-μ-M}_<i>n</i>], (P^∧O) = κ²-<i>P</i>,<i>O</i>-Ar₂<i>P</i>C₆H₄SO₂<i>O</i>) with symmetric (Ar = 2-MeOC₆H₄, 2-CF₃C₆H₄, 2,6-(MeO)₂C₆H₃, 2,6-(<i>i</i>PrO)₂C₆H₃, 2-(2′,6′-(MeO)₂C₆H₃)­C₆H₄) and asymmetric substituted phosphorus atoms (Ar¹ = 2,6-(MeO)₂C₆H₃, Ar² = 2′-(2,6-(MeO)₂C₆H₃)­C₆H₄; Ar¹ = 2,6-(MeO)₂C₆H₃, Ar² = 2-<i>c</i>HexOC₆H₄) were synthesized. Analyses of molecular motions and dynamics by variable temperature NMR studies and line shape analysis were performed for the free ligands and the complexes. The highest barriers of Δ<i>G</i>^⧧ = 44–64 kJ/mol were assigned to an aryl rotation process, and the flexibility of the ligand framework was found to be a key obstacle to a more effective stereocontrol. An increase of steric bulk at the aryl substituents raises the motional barriers but diminishes insertion rates and regioselectivity. The stereoselectivity of the first and the second methyl acrylate (MA) insertion into the Pd–Me bond of in situ generated complexes ^<b>X</b><b>1</b> was investigated by NMR and DFT methods. The substitution pattern of the ligand clearly affects the first MA insertion, resulting in a stereoselectivity of up to 6:1 for complexes with an asymmetric substituted phosphorus. In the consecutive insertion, the stereoselectivity is diminished in all cases. DFT analysis of the corresponding insertion transition states revealed that a selectivity for the first insertion with asymmetric (P^∧O) complexes is diminished in the consecutive insertions due to uncooperatively working enantiomorphic and chain end stereocontrol. From these observations, further concepts are developed

Concepts for Stereoselective Acrylate Insertion

Various phosphinesulfonato ligands and the corresponding palladium complexes [{((P^∧O)­PdMeCl)-μ-M}<i>_n</i>] ([{(^<b>X</b><b>1-Cl</b>)-μ-M}_<i>n</i>], (P^∧O) = κ²-<i>P</i>,<i>O</i>-Ar₂<i>P</i>C₆H₄SO₂<i>O</i>) with symmetric (Ar = 2-MeOC₆H₄, 2-CF₃C₆H₄, 2,6-(MeO)₂C₆H₃, 2,6-(<i>i</i>PrO)₂C₆H₃, 2-(2′,6′-(MeO)₂C₆H₃)­C₆H₄) and asymmetric substituted phosphorus atoms (Ar¹ = 2,6-(MeO)₂C₆H₃, Ar² = 2′-(2,6-(MeO)₂C₆H₃)­C₆H₄; Ar¹ = 2,6-(MeO)₂C₆H₃, Ar² = 2-<i>c</i>HexOC₆H₄) were synthesized. Analyses of molecular motions and dynamics by variable temperature NMR studies and line shape analysis were performed for the free ligands and the complexes. The highest barriers of Δ<i>G</i>^⧧ = 44–64 kJ/mol were assigned to an aryl rotation process, and the flexibility of the ligand framework was found to be a key obstacle to a more effective stereocontrol. An increase of steric bulk at the aryl substituents raises the motional barriers but diminishes insertion rates and regioselectivity. The stereoselectivity of the first and the second methyl acrylate (MA) insertion into the Pd–Me bond of in situ generated complexes ^<b>X</b><b>1</b> was investigated by NMR and DFT methods. The substitution pattern of the ligand clearly affects the first MA insertion, resulting in a stereoselectivity of up to 6:1 for complexes with an asymmetric substituted phosphorus. In the consecutive insertion, the stereoselectivity is diminished in all cases. DFT analysis of the corresponding insertion transition states revealed that a selectivity for the first insertion with asymmetric (P^∧O) complexes is diminished in the consecutive insertions due to uncooperatively working enantiomorphic and chain end stereocontrol. From these observations, further concepts are developed

Concepts for Stereoselective Acrylate Insertion

Various phosphinesulfonato ligands and the corresponding palladium complexes [{((P^∧O)­PdMeCl)-μ-M}<i>_n</i>] ([{(^<b>X</b><b>1-Cl</b>)-μ-M}_<i>n</i>], (P^∧O) = κ²-<i>P</i>,<i>O</i>-Ar₂<i>P</i>C₆H₄SO₂<i>O</i>) with symmetric (Ar = 2-MeOC₆H₄, 2-CF₃C₆H₄, 2,6-(MeO)₂C₆H₃, 2,6-(<i>i</i>PrO)₂C₆H₃, 2-(2′,6′-(MeO)₂C₆H₃)­C₆H₄) and asymmetric substituted phosphorus atoms (Ar¹ = 2,6-(MeO)₂C₆H₃, Ar² = 2′-(2,6-(MeO)₂C₆H₃)­C₆H₄; Ar¹ = 2,6-(MeO)₂C₆H₃, Ar² = 2-<i>c</i>HexOC₆H₄) were synthesized. Analyses of molecular motions and dynamics by variable temperature NMR studies and line shape analysis were performed for the free ligands and the complexes. The highest barriers of Δ<i>G</i>^⧧ = 44–64 kJ/mol were assigned to an aryl rotation process, and the flexibility of the ligand framework was found to be a key obstacle to a more effective stereocontrol. An increase of steric bulk at the aryl substituents raises the motional barriers but diminishes insertion rates and regioselectivity. The stereoselectivity of the first and the second methyl acrylate (MA) insertion into the Pd–Me bond of in situ generated complexes ^<b>X</b><b>1</b> was investigated by NMR and DFT methods. The substitution pattern of the ligand clearly affects the first MA insertion, resulting in a stereoselectivity of up to 6:1 for complexes with an asymmetric substituted phosphorus. In the consecutive insertion, the stereoselectivity is diminished in all cases. DFT analysis of the corresponding insertion transition states revealed that a selectivity for the first insertion with asymmetric (P^∧O) complexes is diminished in the consecutive insertions due to uncooperatively working enantiomorphic and chain end stereocontrol. From these observations, further concepts are developed

Stereoselectivity in Metallocene-Catalyzed Coordination Polymerization of Renewable Methylene Butyrolactones: From Stereo-random to Stereo-perfect Polymers

Coordination polymerization of renewable α-methylene-γ-(methyl)­butyrolactones by chiral <i>C</i>₂-symmetric zirconocene catalysts produces stereo-random, highly stereo-regular, or perfectly stereo-regular polymers, depending on the monomer and catalyst structures. Computational studies yield a fundamental understanding of the stereocontrol mechanism governing these new polymerization reactions mediated by chiral metallocenium catalysts

Selective Reduction of CO₂ to CH₄ by Tandem Hydrosilylation with Mixed Al/B Catalysts

This contribution reports the first example of highly selective reduction of CO₂ into CH₄ via tandem hydrosilylation with mixed main-group organo-Lewis acid (LA) catalysts [Al­(C₆F₅)₃ + B­(C₆F₅)₃] {[Al] + [B]}. As shown by this comprehensive experimental and computational study, in this unique tandem catalytic process, [Al] effectively mediates the first step of the overall reduction cycle, namely the fixation of CO₂ into HCOOSiEt₃ (<b>1</b>) via the LA-mediated CO activation, while [B] is incapable of promoting the same transformation. On the other hand, [B] is shown to be an excellent catalyst for the subsequent reduction steps 2–4, namely the hydrosilylation of the more basic intermediates [<b>1</b> to H₂C­(OSiEt₃)₂ (<b>2</b>) to H₃COSiEt₃ (<b>3</b>) and finally to CH₄] through the frustrated Lewis pair (FLP)-type Si–H activation. Hence, with the required combination of [Al] and [B], a highly selective hydrosilylative reduction of CO₂ system has been developed, achieving high CH₄ production yield up to 94%. The remarkably different catalytic behaviors between [Al] and [B] are attributed to the higher overall Lewis acidity of [Al] derived from two conflicting factors (electronic and steric effects), which renders the higher tendency of [Al] to form stable [Al]–substrate (intermediate) adducts with CO₂ as well as subsequent intermediates <b>1</b>, <b>2</b>, and <b>3</b>. Overall, the roles of [Al] and [B] are not only complementary but also synergistic in the total reduction of CO₂, which render both [Al]-mediated first reduction step and [B]-mediated subsequent steps catalytic

Mechanism of Insertion Polymerization of Allyl Ethers

The copolymerization of ethylene (E) with allyl ethyl ether (AEE) by [di­(2-dianisyl)­phosphine-2-yl]­benzene­sulfonato Pd­(II) as a catalyst is investigated by DFT calculations and compared with the copolymerization of E with diallyl ether (DAE). For AEE, both 1,2- and 2,1-monomer insertions lead to a very stable O-Chelate product (a five-membered and a four-membered ring, respectively) that hinders any further ethylene insertion. As for DAE, a first 2,1-insertion (favored by 1.8 kcal mol^–1 vs the 1,2-insertion) leads to the four-membered O-Chelate product that easily evolves to the most stable intermediate with the second DAE CC bond coordinated to the metal promoting the following 1,2-insertion. The 2,1 + 1,2 DAE insertion product, bearing a five-membered cyclic unit, is stabilized by a β-agostic interaction that easily opens in favor of E coordination and insertion. Based on the proposed copolymerization mechanism, the stereochemistry of the E/DAE copolymer is studied and the experimental microstructure explained. Finally, [di­(2-anisyl)­phosphine-2-yl]­benzenesulfon­(methyl)­amido Pd­(II) species showing a greater regioselectivity toward a first DAE 2,1-insertion (ΔΔ<i>G</i> of −3.6 kcal mol^–1) are suggested to be a promising catalyst

A Comprehensive Mechanistic Picture of the Isomerizing Alkoxycarbonylation of Plant Oils

Theoretical studies on the overall catalytic cycle of isomerizing alkoxycarbonylation reveal the steric congestion around the diphosphine coordinated Pd-center as decisive for selectivity and productivity. The energy profile of isomerization is flat with diphosphines of variable steric bulk, but the preference for the formation of the linear Pd-alkyl species is more pronounced with sterically demanding diphosphines. CO insertion is feasible and reversible for all Pd-alkyl species studied and only little affected by the diphosphine. The overall rate-limiting step associated with the highest energetic barrier is methanolysis of the Pd-acyl species. Considering methanolysis of the linear Pd-acyl species, whose energetic barrier is lowest within all the Pd-acyl species studied, the barrier is calculated to be lower for more congesting diphosphines. Calculations indicate that energy differences of methanolysis of the linear versus branched Pd-acyls are more pronounced for more bulky diphosphines, due to involvement of different numbers of methanol molecules in the transition state. Experimental studies under pressure reactor conditions showed a faster conversion of shorter chain olefin substrates, but virtually no effect of the double bond position within the substrate. Compared to higher olefins, ethylene carbonylation under identical conditions is much faster, likely due not just to the occurrence of reactive linear acyls exclusively but also to an intrinsically favorable insertion reactivity of the olefin. The alcoholysis reaction is slowed down for higher alcohols, evidenced by pressure reactor and NMR studies. Multiple unsaturated fatty acids were observed to form a terminal Pd-allyl species upon reaction with the catalytically active Pd-hydride species. This process and further carbonylation are slow compared to isomerizing methoxycarbonylation of monounsaturated fatty acids, but selective

A Comprehensive Mechanistic Picture of the Isomerizing Alkoxycarbonylation of Plant Oils

Theoretical studies on the overall catalytic cycle of isomerizing alkoxycarbonylation reveal the steric congestion around the diphosphine coordinated Pd-center as decisive for selectivity and productivity. The energy profile of isomerization is flat with diphosphines of variable steric bulk, but the preference for the formation of the linear Pd-alkyl species is more pronounced with sterically demanding diphosphines. CO insertion is feasible and reversible for all Pd-alkyl species studied and only little affected by the diphosphine. The overall rate-limiting step associated with the highest energetic barrier is methanolysis of the Pd-acyl species. Considering methanolysis of the linear Pd-acyl species, whose energetic barrier is lowest within all the Pd-acyl species studied, the barrier is calculated to be lower for more congesting diphosphines. Calculations indicate that energy differences of methanolysis of the linear versus branched Pd-acyls are more pronounced for more bulky diphosphines, due to involvement of different numbers of methanol molecules in the transition state. Experimental studies under pressure reactor conditions showed a faster conversion of shorter chain olefin substrates, but virtually no effect of the double bond position within the substrate. Compared to higher olefins, ethylene carbonylation under identical conditions is much faster, likely due not just to the occurrence of reactive linear acyls exclusively but also to an intrinsically favorable insertion reactivity of the olefin. The alcoholysis reaction is slowed down for higher alcohols, evidenced by pressure reactor and NMR studies. Multiple unsaturated fatty acids were observed to form a terminal Pd-allyl species upon reaction with the catalytically active Pd-hydride species. This process and further carbonylation are slow compared to isomerizing methoxycarbonylation of monounsaturated fatty acids, but selective

Rare-Earth Half-Sandwich Dialkyl and Homoleptic Trialkyl Complexes for Rapid and Stereoselective Polymerization of a Conjugated Polar Olefin

Under ambient conditions, discrete half-sandwich rare-earth (RE) dialkyls, [η⁵-(1,3-(SiMe₃)₂C₉H₅)]]­RE­(CH₂SiMe₃)₂(THF) (RE = Sc, Y, Dy, Lu), catalyze rapid <i>and</i> stereoselective coordination polymerization of β-methyl-α-methylene-γ-butyrolactone (_βMMBL), a conjugated polar olefin and a member of the naturally occurring or biomass-derived methylene butyrolactone family. Within the present RE series, the complex of the largest ion (Dy³⁺) exhibits the highest activity, achieving a high turnover frequency of 390 min^–1, and also produces the highly isotactic polymer P_βMMBL (<i>mm</i> = 91.0%). This stereoregular polymer is thermally robust, with a high glass-transition temperature of 280 °C, and is resistant to all common organic solvents. Other half-sandwich RE catalysts of the series are also highly active and produce polymers with a similarly high isotacticity. Intriguingly, even simple homoleptic hydrocarbyl RE complexes, RE­(CH₂SiMe₃)₃(THF)₂ (RE = Sc, Y, Dy, Lu), also afford highly isotactic polymer P_βMMBL, despite their much lower polymerization activity, except for the Lu complex, which maintains its high activity for both types of complexes. Computational studies of both half-sandwich and simple hydrocarbyl yttrium complexes have revealed a stereocontrol mechanism that well explains the observed high stereoselectivity of _βMMBL polymerization by both types of catalysts. Specifically, the experimental stereoselectivity can be well rationalized with a monometallic propagation mechanism through predominantly chain-end stereocontrol in the coordination–addition polymerization. In this mechanism, formation of an isotactic polymer chiefly originates from interactions between the methyl groups on the chiral β-C atom of the five-membered ring of both the coordinated monomer and the last inserted _βMMBL unit of the chain, and the auxiliary ligand on the metal makes a negligible contribution to the stereocontrol of the polymerization

Chain Propagation and Termination Mechanisms for Polymerization of Conjugated Polar Alkenes by [Al]-Based Frustrated Lewis Pairs

A combined experimental and theoretical study on mechanistic aspects of polymerization of conjugated polar alkenes by frustrated Lewis pairs (FLPs) based on <i>N</i>-heterocyclic carbene (NHC) and Al­(C₆F₅)₃ pairs is reported. This study consists of three key parts: structural characterization of active propagating intermediates, propagation kinetics, and chain-termination pathways. Zwitterionic intermediates that simulate the active propagating species in such polymerization have been generated or isolated from the FLP activation of monomers such as 2-vinylpyridine and 2-isopropenyl-2-oxazolineone of which, IMes⁺-CH₂C­(Me)(C₃H₂NO)­Al­(C₆F₅)₃^– (<b>2</b>), has been structurally characterized. Kinetics performed on the polymerization of 2-vinylpyridine by I^<i>t</i>Bu/Al­(C₆F₅)₃ revealed that the polymerization follows a zero-order dependence on monomer concentration and a first-order dependence on initiator (I^<i>t</i>Bu) and activator [Al­(C₆F₅)₃] concentrations, indicating a bimolecular, activated monomer propagation mechanism. The Lewis pair polymerization of conjugate polar alkenes such as methacrylates is accompanied by competing chain-termination side reactions; between the two possible chain-termination pathways, the one that proceeds via intramolecular backbiting cyclization involving nucleophilic attack of the activated ester group of the growing polymer chain by the <i>O</i>-ester enolate active chain end to generate a six-membered lactone (δ-valerolactone)-terminated polymer chain is kinetically favored, but thermodynamically disfavored, over the pathway leading to the β-ketoester-terminated chain, as revealed by computational studies