Shaowen Zhang

Regio- and Stereochemical Control in Ocimene Polymerization by Half-Sandwich Rare-Earth Metal Dialkyl Complexes

The polymerization of ocimene has been first achieved by half-sandwich rare-earth metal dialkyl complexes in combination with activator and Al(i) Bu3 . The regio- and stereoselectivity in the ocimene polymerization can be controlled by tuning the cyclopentadienyl ligand and the central metal of the complex. The chiral cyclopentadienyl-ligated Sc complex 1 prepares syndiotactic cis-1,4-polyocimene (cis-1,4-selectivity up to 100%, rrrr = 100%), while the corresponding Lu, Y, and Dy complexes 2-4 and the achiral pentamethylcyclopentadienyl Sc, Lu, and Y complexes 5-7 afford isotactic trans-1,2-polyocimenes (trans-1,2-selectivity up to 100%, mm = 100%).

A highly active chiral (S,S)-bis(oxazoline) Pd(II) alkyl complex/activator catalytic system for vinyl polymerization of norbornene in air and water

A series of air/water-tolerant chiral Pd alkyl complexes (R2-(S,S)-BOZ)PdR′ (1–3, R = CH(CH3)2, Ph; R′ = Me, Ph) were synthesized and structurally characterized. In the presence of an excess of activator, such as [Ph3C][B(C6F5)4], [PhMe2NH][B(C6F5)4], or B(C6F5)3, the air/water-tolerant chiral Pd(II) alkyl complexes 1–3 could promote the norbornene polymerization in air and water using unpretreated technical grade solvent and monomer with extremely high activity up to 1.7 × 109 g PNB per molPd per h.

Cationic Half-Sandwich Rare-Earth Metal Alkyl Species Catalyzed Polymerization and Copolymerization of Aryl Isocyanides Possessing Polar, Bulky, or Chiral Substituents

Two types of half-sandwich rare-earth metal dialkyl complexes (Cp)Ln(CH2SiMe3)2(THF) 1–6 (1: Cp = (3aR,4R,8R,8aR)-cyclopentadienyl ligand (Cpx*′), Ln = Sc; 2: Cp = Cpx*′, Ln = Y; 3: Cp = Cpx*′, Ln = Lu; 4: Cp = (3aR,8aS)-cyclopentadienyl ligand (Cpx*′′), Ln = Sc; 5: Cp = Cpx*′′, Ln = Y; 6: Cp = Cpx*′′, Ln = Lu) have been synthesized and structurally characterized by X-ray diffraction. The reaction of these half-sandwich rare-earth metal dialkyl complexes with almost one equivalent of activator (borate or borane) quantitatively afforded the cationic half-sandwich rare-earth metal alkyl species, which are the first example of rare-earth metal catalysts for the polymerization of five functional aryl isocyanides containing polar ester [4-ethoxycarbonyl phenyl isocyanide (EPI)], bulky naphthyl [2-naphthyl isocyanide (NI)] or tetraphenylethylene [4-isocyano-4′-(1,2,2-triphenylvinyl)-1,1′-biphenyl (ITPB)], or chiral ester [(1S,2R,5S)-2-isopropyl-5-methylcyclohexyl 4-isocyanobenzoate (D-IMCI) and (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl 4-isocyanobenzoate (L-IMCI)] substituents with extremely high activities (up to 107 g of polymer molLn−1 h−1) under mild conditions. The resulting functional polyisocyanides have high molecular weights (Mn = 4.4–21.4 × 104 g mol−1) as well as broad molecular weight distributions (Mw/Mn = 3.20–6.82) and exhibit good solubility, aggregation-caused quenching (ACQ) nature, or single-handed helical conformation. Moreover, the complex 1/activator binary system can also promote the helix-sense-selective copolymerization of chiral D-IMCI or L-IMCI with achiral ITPB at high activities (up to 106 g of polymer molLn−1 h−1), affording new random poly(D/L-IMCI-co-ITPB)s with D/L-IMCI contents in the range of 9–76 mol% and different degrees of single-handed helical conformations. In contrast with the AIE nature of the ITPB monomer, these copolymers containing ITPB units exhibit ACQ nature similar to poly(ITPB). A plausible coordination–insertion mechanism is proposed based on ESI-MS spectroscopy, providing insight into the initiation and termination polymerization process of aryl isocyanides catalyzed by rare-earth metal catalysts.

Syndiotactic polymerization of styrene and copolymerization with ethylene catalyzed by chiral half-sandwich rare-earth metal dialkyl complexes

The syndiotactic polymerization of styrene (St) and the copolymerization of St with ethylene (E) were carried out by using a series of chiral half-sandwich rare-earth metal dialkyl complexes (Cpx*) as the catalysts. The complexes are Ln(CH2SiMe3)2(THF) (1−4: Ln = Sc (1), Ln = Lu (2), Ln = Y (3), Ln = Dy (4)) bearing chiral cyclopentadienyl ligand containing bulky cylcohexane derivatives in the presence of activator and AliBu3. For the St polymerization, a high activity up to 3.1 × 106 g of polymer molLn−1·h−1 and a high syndiotactic selectivity more than 99% were achieved. The resulting syndiotactic polystyrenes (sPSs) have the molecular weights (Mn) ranging from 3700 g·mol−1 to 6400 g·mol−1 and the molecular weight distributions (Mw/Mn) from 1.40 to 5.03. As for the copolymerization of St and E, the activity was up to 2.4 × 106 g of copolymer molSc−1·h−1·MPa−1, giving random St-E copolymers containing syndiotactic polystyrene sequences with different St content in the range of 15 mol%−58 mol%. These results demonstrate that the bulky cyclopentadienyl ligands of the chiral half-sandwich rare-earth metal complexes effectively inhibit the continued insertion of St monomers into the (co)polymer chain to some extent in comparison with the known half-sandwich rare-earth metal complexes.

Pd-Catalyzed C(sp3)–C(sp2) cross-coupling of Y(CH2SiMe3)3(THF)2 with vinyl bromides and triflates

Pd-Catalyzed C(sp3)-C(sp2) cross-coupling of Y(CH2SiMe3)3(THF)2 with vinyl bromides and triflates has been developed for efficient synthesis of various allyltrimethylsilanes. The cross-coupling reaction was conducted at room temperature with low catalyst loading of either Pd(PPh3)4 or Pd(PPh3)2Cl2, and exhibited high efficiency and a broad substrate scope. In combination with the cross-coupling by the Lewis-acid catalyzed Hosomi-Sakurai reaction, a novel three-component one-pot cascade reaction was then accomplished to deliver homoallylic alcohols and ethers with high regioselectivity and diastereoselectivity. The three-component reaction defined the yttrium complex as a novel one-carbon synthon, which could either trigger bifunctionalization of alkenes or link two electrophiles and would find applications in organic synthesis.

A displacement-type fluorescent probe reveals active species in the coordinative polymerization of olefins

The correct identification of active species is an important prerequisite to study the mechanism of coordinative polymerization of olefins, which can afford important theoretical guidance for the design and synthesis of new organometallic catalysts and high-performance polyolefin materials. However, it is very difficult to isolate and identify the catalytically active species generated from organometallic complex/activator/AlR3 ternary systems during the coordinative polymerization of olefins. Herein, we present a new strategy to detect the catalytically active species in the coordination polymerization of olefins through the utilization of a displacement-type fluorescent probe containing an organometallic complex and an aluminum alkyl complex bearing the same chelating ligand. Using fluorescence spectroscopy and in situ1H NMR spectroscopy, the truly active species is identified and a plausible mechanism is proposed, which provides new insights into the cis-1,4-polymerization of isoprene catalyzed by rare earth metal monoalkyl complex/activator/AlR3 ternary systems.

Controlled Polymerization of Isoprene with Chromium-Based Metal-Organic Framework Catalysts: Switching from Cyclic to cis-1,4-Selectivity Depending on Activator

Chromium-based metal-organic framework (MOF) Cr-MIL-100/101 activated by activator and aluminum trialkyl compound serve as unique, highly efficient heterogeneous single-site catalysts for the controlled polymerization of isoprene, which not only exhibit quasi-living nature in isoprene polymerization but also unprecedentedly switch from cyclic to cis-1,4-selectivity depending on the activator used to yield low molecular weight cyclic PIPs or extremely high molecular weight cis-1,4-PIPs. Such heterogeneous Cr-MOF catalysts can be recycled approximately five times. Based on nitrogen sorption isotherm tests and powder X-ray diffraction, a cationic mechanism is suggested, in which the polymerization takes place inside the open nanochannels of MOF catalysts and the space confinement effect of narrow open nanochannels originated from the coordination of PhNMe2 from activator [PhNHMe2 ][B(C6 F5 )4 ] with the multiple metal centers of MOF catalysts might give a rational explanation for such controlled adjustment on the PIPs structure and properties.

[Ph3C][B(C6F5)4]: A Highly Efficient Metal‐Free Single‐Component Initiator for the Helical‐Sense‐Selective Cationic Copolymerization of Chiral Aryl Isocyanides and Achiral Aryl Isocyanides

Commercially available [Ph3 C][B(C6 F5 )4 ] served as a highly efficient metal-free and single-component initiator not only for the carbocationic polymerization of polar and bulky aryl isocyanides with extremely high activity up to 1.2×107  g of polymer/(molcat.  h), but also for the helical-sense-selective polymerization of chiral aryl isocyanides and copolymerization with achiral aryl isocyanides to afford high-molecular-weight functional poly(aryl isocyanide)s with good solubility as well as AIE characteristics and/or a single-handed helical conformation.

Cis-1,4-Polymerization of Isoprene by 1,3-Bis(oxazolinymethylidene)isoindoline-Ligated Rare-Earth Metal Dialkyl Complexes

A series of novel chiral nonmetallocene pincer-type rare-earth metal dialkyl complexes bearing the chiral monoanionic tridentate C2-symmetric 1,3-bis(oxazolinymethylidene)isoindoline (BOXMI-H) ligand (BOXMI)Ln(CH2SiMe3)2 1–3 (1: Ln = Sc, yield = 57%; 2: Ln = Lu, yield = 55%; 3: Ln = Y, yield = 62%) have been prepared in moderate yields via the acid-base reaction between the BOXMI ligand and rare-earth metal tri(trimethylsilylmethyl) complexes. The X-ray diffractions show that both of the complexes 1 and 2 contain one BOXMI ligand and two trimethylsilylmethyl ligands, adopting a distorted trigonal bipyramidal configuration. In the presence of a cocatalyst such as borate and AlR3, these complexes 1–3 exhibit high activities of up to 6.8 × 104 (g of polymer)/(molLn h) and high cis-1,4 selectivities of up to 97% in the polymerization of isoprene in toluene, yielding the cis-1,4-polyisoprenes with heavy molecular weights (Mn of up to 710,000 g/mol) and bimodal molecular weight distributions (Mw/Mn = 2.0–4.5).