QM/MM Studies on Scandium-Catalyzed Syndiospecific Copolymerization of Styrene and Ethylene

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.

Ancillary ligand effects on α-olefin polymerization catalyzed by zirconium metallocene: a computational study

The polymerization of α-olefins catalyzed by zirconium metallocene catalyst was systematically studied through experiments and density functional theory (DFT) calculations. Having achieved an agreement between theory and experiment, it was found that the effect of the catalyst ligand on the C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C insertion reaction was significantly greater than that on the β-H elimination reaction. Therefore, the molecular weight of polymers can be increased by improving the activity of the CC insertion. In addition, in comparison with propylene, the chain length of α-olefins can directly affect the stereotacticity of polymerization products, owing to steric hindrance between the polymer chain and monomer.

The polymerization of α-olefins catalyzed by zirconium metallocene catalyst was systematically studied through experiments and density functional theory (DFT) calculations.

Synthesis of allylanilines via scandium-catalysed benzylic C(sp3)-H alkenylation with alkynes

The ortho-selective benzylic C(sp³)-H alkenylation of 2-methyl tertiary anilines with internal alkynes has been achieved for the first time by using a half-sandwich scandium catalyst. This protocol provides a straightforward route for the synthesis of a new family of 2-allylaniline derivatives, featuring broad substrate scope, 100% atom-efficiency, high yields, and high chemo-, regio-, and stereoselectivity.

Recent progress in rare-earth metal-catalyzed sp2 and sp3 C–H functionalization to construct C–C and C–heteroelement bonds

The review presents rare-earth metal-catalyzed C(sp2/sp3)–H functionalization accessing C–C/C–heteroatom bonds and olefin (co)polymerization, highlighting substrate scope, mechanistic realization, and origin of site-, enantio-/diastereo-selectivity.

Mechanistic Studies for Palladium Catalyzed Copolymerization of Ethylene with Vinyl Ethers

The mechanism of ethylene with vinyl ether (VE, CH2=CHOEt) copolymerization catalyzed by phosphine-sulfonate palladium complex (A) was investigated by density functional theory (DFT) calculation. On achieving an agreement between theory and experiment, it is found that the favorable 1,2-selective insertion of VE into the complex A originates from stronger hydrogen interaction between the oxygen atom of VE and the ancillary ligand of catalyst A. Additionally, VE insertion is easier into the ethylene pre-inserted intermediate than that into the catalyst to form the resultant copolymers with the major units of OEt in chain and minor units of OEt at the chain end. The effect of β-OEt and β-H elimination was explored to elucidate chain termination and the molecular weight of copolymers. Furthermore, a family of cationic catalysts has been demonstrated to copolymerize ethylene with VE along with our modified cationic complex B with higher incorporation of VE and reactivity in comparison with complex A, which was modelled computationally by increasing the strong interactions between the catalyst and monomer moiety. Other than VE, the activity of cationic complex B for copolymerization of vinyl chloride and methacrylate is also computed successfully.

Stereoselective copolymerization of 4-(N,N-diphenylamino)styrene and isoprene by a C5H5-ligated scandium catalyst: synthesis of amino-functionalized crystalline styrenic thermoplastic elastomers

Amino-functionalized crystalline styrenic thermoplastic elastomers were prepared by carrying out stereoselective copolymerizations of 4-(N,N-diphenylamino)styrene and isoprene with the use of a C₅H₅-ligated scandium catalyst.

Theoretical insight into the redox-switchable activity of group 4 metal complexes for the ring-opening polymerization of ε-caprolactone

DFT calculations indicate that the Lewis acidity of the catalytic metal center M (Ti, Zr, Hf) explains the redox-switchable activity observed for the ring-opening polymerization of ε-caprolactone by ferrocene derived metal complexes.

A computational study of the reactivity of rare-earth/phosphorus Lewis pairs toward polymerization of conjugated polar alkenes

The polymerization mechanism of methyl methacrylate (MMA) catalyzed by rare-earth/phosphorus (RE/P) Lewis pairs has been systematically studied through density functional theory (DFT) calculations.

“C–H⋯π Interaction” regulates the stereoselectivity in olefin polymerization

DFT calculations reveal that a coordinating THF plays an essential role in regulating the stereoselectivity via C–H⋯π noncovalent interaction.

Sequence-controlled ethylene/styrene copolymerization catalyzed by scandium complexes

Sequence controlled E/St copolymerization was achieved by tuning the electrophilicity of scandium center and the steric around it, isolating multi-block, random, “pseudo random” and alternating E/St copolymers.

Origin of stereoselectivity and multidimensional quantitative analysis of ligand effects on yttrium-catalysed polymerization of 2-vinylpyridine

Through a combination of density functional theory (DFT) calculations and multivariate regression analysis, the origin of the stereoselectivity of yttrium-catalysed polymerization of 2-vinylpyridine (2VP) has been investigated.

Engineering of Syndiotactic and Isotactic Polystyrene-Based Copolymers via Stereoselective Catalytic Polymerization

This contribution presents an updated overview of the different copolymers containing stereoregular polystyrene blocks. Special emphasis is placed on syndiospecific and isospecific copolymerization of styrene with co-monomers (ethylene and α-olefins, conjugated and non-conjugated dienes, styrene derivatives, etc.). The catalytic systems involved are described and the polymerization mechanisms are discussed. Alternative approaches (simultaneous, living, chain-transfer and graft copolymerization) and the resulting detailed structures and characteristics of the copolymers are also reported.

DFT Studies on cis-1,4-Polymerization of Dienes Catalyzed by a Cationic Rare-Earth Metal Complex Bearing an Ancillary PNP Ligand

Dnsity functional theory (DFT) calculations have been carried out for the highly selective cis-1,4-polymerization of butadiene catalyzed by a cationic rare-earth metal complex bearing an ancillary PNP ligand. It has been found that the chain initiation and propagation of butadiene polymerization occurs via the favorable cis-1,4-insertion route. The trans-1,4 and 1,2-insertion are unfavorable both kinetically and thermodynamically. The chain growth follows the π-allyl-insertion mechanism. The analyses of energy decomposition of transition states indicate that the likelihood of rival insertion pathways is predominantly controlled by the interaction energy of butadiene with a metal center and the deformation energy of butadiene moiety. The electronic factor of the central metal has a decisive influence on the cis- vs. trans-insertion and the regioselectivity (cis-1,4- vs. cis-1,2-insertion) is mainly determined by steric hindrance. Tetrahydrofuran (THF) coordination made monomer insertion less favorable compared with THF-free case and had more noticeable impact on the trans-monomer insertion compared with the cis case. During the chain propagation, cis-insertion of monomer facilitates THF de-coordination and the THF molecule could therefore dissociate from the central metal.

Copolymerization of propylene with styrene and ethylene by a THF-containing half-sandwich scandium catalyst: efficient synthesis of polyolefins with a controllable styrene content

A novel family of multi-block propylene–styrene copolymers with syndiotactic styrene–styrene blocks and random propylene–ethylene–styrene terpolymers with a broad composition range were obtained by using (C₅Me₄SiMe₃)Sc(CH₂SiMe₃)₂(THF).

Effects of nucleophilic ligands on the chain initiation efficiency of polar monomer polymerizations catalyzed by 2-methoxyethylaminobis(phenolate)yttrium complexes: a DFT study

DFT calculations suggest that less steric hindrance and a good orbital match induced by nucleophilic ligands are beneficial to the chain initiation of ROP and GTP, respectively.

DFT study on 1,7-octadiene polymerization catalyzed by a non-bridged half-titanocene system

For (η⁵-C₅Me₅)TiCl₂(O-2,6-ⁱPr₂C₆H₃)/MAO system, the selectivity of repeated insertion and intramolecular cyclization of 1,7-octadiene were explored in detail by DFT method.

Half-sandwich rare-earth-catalyzed olefin polymerization, carbometalation, and hydroarylation

The search for new catalysts for more efficient, selective chemical transformations and for the synthesis of new functional materials has been a long-standing research subject in both academia and industry. To develop new generations of catalysts that are superior or complementary to the existing ones, exploring the potential of untapped elements is an important strategy. Rare-earth elements, including scandium, yttrium, and the lanthanides (La-Lu), constitute one important frontier in the periodic table. Rare-earth elements possess unique chemical and physical properties that are different from those of main-group and late-transition metals. The development of rare-earth-based catalysts by taking the advantage of these unique properties is of great interest and importance. The most stable oxidation state of rare-earth metals is 3+, which is difficult to change under many reaction conditions. The oxidative addition and reductive elimination processes often observed in catalytic cycles involving late transition metals are generally difficult in the case of rare-earth complexes. The 18-electron rule that is applicable to late-transition-metal complexes does not fit rare-earth complexes, whose structures are mainly governed by the sterics (rather than the electron numbers) of the ligands. In the lanthanide series (La-Lu), the ionic radius gradually decreases with increasing atomic number because of the influence of the 4f electrons, which show poor shielding of nuclear charge. Rare-earth metal ions generally show strong Lewis acidity and oxophilicity. Rare-earth metal alkyl and hydride species are highly reactive, showing both nucleophilicity and basicity. The combination of these features, such as the strong nucleophilicity and moderate basicity of the alkyl and hydride species and the high stability, strong Lewis acidity, and unsaturated C-C bond affinity of the 3+ metal ions, can make rare-earth metals unique candidates for the formation of excellent single-site catalysts. This Account is intended to give an overview of our recent studies on organo rare-earth catalysis, in particular the synthesis and application of half-sandwich rare-earth alkyl complexes bearing monocyclopentadienyl ligands for olefin polymerization, carbometalation, and hydroarylation. Treatment of half-sandwich rare-earth dialkyl complexes having the general formula CpMR2 with an equimolar amount of an appropriate borate compound such as [Ph3C][B(C6F5)4] can generate the corresponding cationic monoalkyl species, which serve as excellent single-site catalysts for the polymerization and copolymerization of a wide range of olefin monomers such as ethylene, 1-hexene, styrene, conjugated and nonconjugated dienes, and cyclic olefins. The cationic half-sandwich rare-earth alkyl complexes can also catalyze the regio- and stereoselective alkylative alumination of alkenes and alkynes through insertion of the unsaturated C-C bond into the metal-alkyl bond followed by transmetalation between the resulting new alkyl or alkenyl species and an alkylaluminum compound. Moreover, a combination of deprotonative C-H bond activation of appropriate organic compounds such as anisoles and pyridines by the rare-earth alkyl species and insertion of alkenes into the resulting new metal-carbon bond can lead to catalytic C-H bond alkylation of the organic substrates. Most of these transformations are unique to the rare-earth catalysts with selectivity and functional group tolerance different from those of late-transition-metal catalysts.

Syndioselective coordination polymerization of unmasked polar methoxystyrenes using a pyridenylmethylene fluorenyl yttrium precursor

A new family of syndiotactic polystyrene derivatives containing a methoxyl group was established successfully by employing a yttrium catalyst.

Insights into the epimerization activities of RaCE and pAGE: the quantum mechanics/molecular mechanics simulations

Ruminococcus albus cellobiose 2-epimerase (RaCE) and N-acetyl-d-glucosamine 2-epimerase from porcine kidney (pAGE) belong to the AGE superfamily and have a detectable AGE activity.

Mechanistic Studies of Ethylene and α-Olefin Co-oligomerization Catalyzed by Chromium-PNP Complexes

To explore the possibility of producing a narrow distribution of mid- to long-chain hydrocarbons from ethylene as a chemical feedstock, co-oligomerization of ethylene and linear α-olefins (LAOs) was investigated, using a previously reported chromium complex, [CrCl(3)(PNP(OMe))] (1, where PNP(OMe) = N,N-bis(bis(o-methoxyphenyl)phosphino)methylamine). Activation of 1 by treatment with modified methylaluminoxane (MMAO) in the presence of ethylene and 1-hexene afforded mostly C(6) and C(10) alkene products. The identities of the C(10) isomers, assigned by detailed gas chromatographic and mass spectrometric analyses, strongly support a mechanism that involves five- and seven-membered metallacyclic intermediates comprising of ethylene and LAO units. Using 1-heptene as a mechanistic probe, it was established that 1-hexene formation from ethylene is competitive with formation of ethylene/LAO co-trimers, and that co-trimers derived from one ethylene and two LAO molecules are also generated. Complex 1/MMAO is also capable of converting 1-hexene to C(12) dimers and C(18) trimers, albeit with poor efficiency. The mechanistic implications of these studies are discussed and compared to previous reports of olefin co-trimerization.