Scandium Catalyst Research Articles

Dehydrocoupling of boranes with amines using a scandium catalyst

The scandocene alkyl complex (C5Me5)2ScCH2SiMe3 was found to be an efficient catalyst for the dehydrocoupling of the non-cyclic boranes, dicyclohexylborane and thexylborane, with amines under mild conditions. The reactions afforded the corresponding aminoboranes in high yields with good functional group tolerance. The stoichiometric reaction of scandium alkyl with amine led to the isolation of a scandium amide complex, which was shown to be an active species during the catalysis. Although a borane-coordinated scandium hydride was also obtained from the stoichiometric experiment, it was not involved in the catalytic cycle. In addition, kinetic studies provided insight into this intermolecular dehydrogenation reaction.

Enantioselective C-H Alkenylation of Ferrocenes with Alkynes by Half-Sandwich Scandium Catalyst.

The enantioselective C-H alkenylation of ferrocenes with alkynes is, in principle, a straightforward and atom-efficient route for the construction of planar-chiral ferrocene scaffolds bearing alkene functionality but has remained scarcely explored to date. Here we report for the first time the highly enantioselective C-H alkenylation of quinoline- and pyridine-substituted ferrocenes with alkynes by a half-sandwich scandium catalyst. This protocol features broad substrate scope, high enantioselectivity, and 100% atom efficiency, selectively affording a new family of planar-chiral ferrocenes bearing N/alkene functionalities. The mechanistic details have been clarified by DFT analyses. The use of a quinoline/alkene-functionalized ferrocene product as a chiral ligand for asymmetric catalysis is also demonstrated.

Benzylic C-H addition of aromatic amines to alkenes using a scandium catalyst.

An efficient and selective benzylic C(sp3)-H addition of o-CH3-substituted tertiary aromatic amines to alkenes has been achieved using an anilido-oxazoline ligand supported scandium catalyst, which provides an atom-economic method for the synthesis of a new family of alkylated tertiary anilines. A wide range of amine and alkene substrates are compatible with the catalyst system.

Syndiospecific coordination (Co)polymerization of carbazole-substituted styrene derivatives using the scandium catalyst system

Perfect syndiospecific polymerization of carbazole-substituted styrene derivatives was achieved using a rare-earth metal catalyst. Also copolymerization of FSt with styrene afforded copolymers with gradient sequence distributions and easily tunable incorporation rate.

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.

Copolymerization of ethylene and halogenated styrenes using scandium catalysts

Copolymerization of ethylene (E) and halogenated styrenes is screened by half-sandwich scandium catalyst 1 and constrained geometry scandium catalysts 2–4. The methyl-substituted N-heterocyclic carbene fluorenyl catalyst 3 initiates the E copolymerization with para-fluorostyrene (pFSt), para-chlorostyrene (pClSt) and para-bromostyrene (pBrSt) to pseudo-random copolymers with an incorporation level of up to 46.3 mol%, 45.1 mol% and 37.5 mol%, respectively. Increasing reaction temperatures from 20 to 60 °C results in an increment of E/pFSt copolymerization activity from 5.64 × 105 to 8.10 × 105 g molSc−1h−1. In contrast, a higher temperature dramatically depresses the E/pClSt copolymerization (4.86 × 105 g molSc−1h−1 at 20 °C vs 0.90 × 105 g molSc−1h−1 at 60 °C). At 20 °C, the E/pBrSt copolymerization only affords a little pure polyethylene, and the E-pBrSt copolymer with 37.5 mol% pBrSt is obtained at 0 °C. The pseudo-random copolymers of E and halogenated styrenes give one glass transition temperature and decomposition temperature on DSC and TGA curves.

Construction of All-Carbon Quaternary Stereocenters by Scandium-Catalyzed Intramolecular C-H Alkylation of Imidazoles with 1,1-Disubstituted Alkenes.

The exo-selective C-H cycloaddition of imidazoles to 1,1-disubstituted alkenes has been achieved for the first time by using half-sandwich scandium catalysts. A wide range of imidazole compounds bearing various 1,1-disubstituted aliphatic alkenes, styrenes, dienes, and enynes have been selectively converted in high yields to the corresponding bicyclic imidazole derivatives bearing β-all-carbon-substituted quaternary stereocenters. By using a chiral half-sandwich scandium catalyst, the asymmetric exo-selective cyclization has also been achieved with a high level of enantioselectivity.

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.

Scandium-Catalyzed para-Selective Alkylation of Aromatic Amines with Alkenes.

An efficient para-alkylation of primary and secondary anilines with a variety of sterically encumbered alkenes using a simple β-diketiminato scandium catalyst is reported. This protocol features 100% atom economy, excellent chemo- and regioselectivity, broad substrate scope, and good functional group tolerance. Mechanistic studies disclosed that the reaction probably proceeded via a tandem hydroamination/Hofmann-Martius rearrangement.

Scandium-Catalyzed Regio- and Stereoselective Cyclopolymerization of Functionalized α,ω-Dienes and Copolymerization with Ethylene.

The precise control of regio- and stereochemistry in the cyclopolymerization of heteroatom-functionalized α,ω-dienes is of much interest and importance, but has remained a challenge to date. We report herein the regio-, diastereoselective and stereoregular cyclopolymerization of ether- and thioether-functionalized 1,6-heptadienes by a half-sandwich scandium catalyst. The polymerization of 4-benzyloxy-1,6-heptadiene selectively afforded the corresponding benzyloxy-functionalized cyclic polymer composed of 1,2,4-cis-substituted-ethylenecyclopentane (ECP) microstructures in a isospecific fashion (95% mmm). In contrast, the polymerization of 4-phenylthio-1,6-heptadiene exclusively yielded 1,2-trans-1,4-cis-ECP units with high isotacticity (95% rrr). The DFT calculations revealed that an interaction between the scandium atom in the catalyst and the heteroatom in a diene monomer played an important role in controlling the regio- and stereochemistry of the diene cyclopolymerization. The copolymerization of functionalized 1,6-heptadienes with ethylene has also been achieved in a controlled fashion.

Computational Investigation of Scandium-Based Catalysts for Olefin Hydroaminoalkylation and C–H Addition

Great progress has been achieved in the olefin hydroaminoalkylation by using amines, which is an atom-efficient route for the synthesis of alkylated amine derivatives. However, success in the catalytic olefin hydroaminoalkylation with a simple tertiary amine is hitherto very limited. In this study, density functional theory was applied to investigate the hydroaminoalkylation of olefins with tertiary amines, catalyzed by a series of homoleptic tris(benzyl) scandium complexes. It is found that the catalytic performance can be improved via substitution of electron-withdrawing groups and modifying ligand frameworks to reduce their steric hindrance. In addition, the potential applications of scandium catalysts in the α-C(sp3)–H alkylation of various heteroatom-containing (P, As, O, S, and Se) substrates were explored. The results suggest that alkyl sulfides and selenides are promising substrates to undergo α-C(sp3)–H addition to olefins. Importantly, the effects of ligand backbone and substituent on catalytic ...

Synthesis of Self-Healing Polymers by Scandium-Catalyzed Copolymerization of Ethylene and Anisylpropylenes.

Self-healing materials are of fundamental interest and practical importance. Herein we report the synthesis of a new class of self-healing materials, formed by the copolymerization of ethylene and anisyl-substituted propylenes using a sterically demanding half-sandwich scandium catalyst. The copolymerization proceeded in a controlled fashion, affording unique multi-block copolymers composed of relatively long alternating ethylene- alt-anisylpropylene sequences and short ethylene-ethylene units. By controlling the molecular weight and varying the anisyl substituents, a series of copolymers that show a wide range of glass-transition temperatures ( Tg) and mechanical properties have been obtained. The copolymers with Tg below room temperature showed high elastic modulus, high toughness, and remarkable self-healability, being able to autonomously self-heal upon mechanical damage not only in a dry environment but also in water and aqueous acid and alkaline solutions, while those with Tg around or above room temperature exhibited excellent shape-memory property. The unique mechanical properties may be ascribed to the phase separation of the crystalline ethylene-ethylene nanodomains from the ethylene- alt-anisylpropylene matrix.

Hydroaminoalkylation of sterically hindered alkenes with N,N-dimethyl anilines using a scandium catalyst.

An atom- and step-economical C(sp3)-H addition of N,N-dimethyl anilines to various sterically demanding 1,1- and 1,2-disubstituted alkenes has been achieved by using a simple β-diketiminato ligand supported scandium dialkyl complex in combination with a borate compound. The corresponding C(sp3)-C(sp3) bond forming reaction occurs with excellent regioselectivities to give a variety of tertiary aromatic amines.

Mechanism and Effect of Polar Styrenes on Scandium-Catalyzed Copolymerization with Ethylene.

Copolymerization of ethylene (E) and polar vinyl monomers remains a problem because E propagation is hindered. Herein, for the first time, we report the copolymerization of E and polar styrenes (SR ) by using an oxophilic scandium catalyst that exhibits higher turnover frequencies than both E and SR homopolymerizations when R is an electron-withdrawing group. This positive comonomer effect was elucidated through computing reaction profiles of E/SF copolymerization at the DFT (B3PW91) level of theory. It reveals that the secondary interaction between Sc3+ and phenyl of the last and penultimate inserted SF units leads to a decrease of the E insertion barrier, because the electron-withdrawing substituent enhances the electrophilicity of Sc3+ by an inductive effect mediated by the secondary interaction. After three consecutive insertions of the E units, the secondary interaction is lost and the SF insertion is kinetically preferred over the E insertion. This process is in line with the NMR spectrum analyses which show that the resultant copolymers mainly contain SR (E)x SR sequences where x≤3.

Enantioselective Construction of Silicon‐Stereogenic Silanes by Scandium‐Catalyzed Intermolecular Alkene Hydrosilylation

AbstractThe catalytic asymmetric construction of silicon‐stereogenic silanes is of great interest and significance, but has met with only limited success to date. We herein report the enantioselective hydrosilylation of alkenes with dihydrosilanes by a chiral half‐sandwich scandium catalyst, which constitutes an efficient and general route for the synthesis of a wide range of enantioenriched silicon‐stereogenic silanes from easily accessible starting materials. This reaction features a broad substrate scope, high yields, and high enantioselectivity. Some of the chiral tertiary silane products were also converted into valuable derivatives, such as chiral silanol, quaternary silane, and benzosilole compounds.

Enantioselective Construction of Silicon-Stereogenic Silanes by Scandium-Catalyzed Intermolecular Alkene Hydrosilylation.

The catalytic asymmetric construction of silicon-stereogenic silanes is of great interest and significance, but has met with only limited success to date. We herein report the enantioselective hydrosilylation of alkenes with dihydrosilanes by a chiral half-sandwich scandium catalyst, which constitutes an efficient and general route for the synthesis of a wide range of enantioenriched silicon-stereogenic silanes from easily accessible starting materials. This reaction features a broad substrate scope, high yields, and high enantioselectivity. Some of the chiral tertiary silane products were also converted into valuable derivatives, such as chiral silanol, quaternary silane, and benzosilole compounds.

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...

Convenient synthesis of versatile syndiotactic polystyrene materials containing pendant alkenyl groups with a scandium catalyst system

The syndioselective copolymerization of styrene with five alkenyl functionalized styrenic monomers, with the use of a scandium catalyst, produced a new family of versatile syndiotactic polystyrene materials with pendant alkenyl groups.

Copolymerization of ethylene with styrene catalyzed by a scandium catalyst

The pseudo-random copolymers of ethylene and styrene can also be generated by a rare-earth metal based catalyst system.

α-C-H Alkylation of Methyl Sulfides with Alkenes by a Scandium Catalyst.

The C-H addition of sulfides to alkenes is an atom-efficient route for the functionalization and modification of sulfide compounds through C-C bond formation, but this transformation is highly challenging. We report here the regioselective α-C(sp3)-H addition of a wide range of methyl sulfides to a variety of olefins and dienes by a half-sandwich scandium catalyst. This protocol provides a unique route for the synthesis of diverse sulfide derivatives through C-C bond formation at a sulfur-adjacent carbon atom in a 100% atom efficient fashion.