Polystyrene attached titanocene species. Preparation and reactions

Sheltering Mono-P-Ligated Metal Complexes in Porous Polystyrene Monolith: Effect of Aryl Pendant Stabilizers on Catalytic Durability

Metal centers that can generate coordinatively unsaturated metals in accessible and stable states have been developed using synthetic polymers with sophisticated ligand and scaffold designs, which required synthetic efforts. Herein, we report a simple and direct strategy for producing polymer-supported phosphine-metal complexes, which stabilizes mono-P-ligated metals by modulating the electronic properties of the aryl pendant groups in the polymer platform. A three-fold vinylated PPh3 was copolymerized with a styrene derivative and a cross-linker to produce a porous polystyrene-phosphine hybrid monolith. Based on the Hammett substituent constants, the electronic properties of styrene derivatives were modulated and incorporated into the polystyrene backbone to stabilize the mono-P-ligated Pd complex via Pd-arene interactions. Through NMR, TEM, and comparative catalytic studies, the polystyrene-phosphine hybrid, which induces selective mono-P-ligation and moderate Pd-arene interactions, demonstrated high catalytic durability for the cross-coupling of chloroarenes under continuous-flow conditions.

Click Chemistry with Cyclopentadiene

Cyclopentadiene is one of the most reactive dienes in normal electron-demand Diels-Alder reactions. The high reactivities and yields of cyclopentadiene cycloadditions make them ideal as click reactions. In this review, we discuss the history of the cyclopentadiene cycloaddition as well as applications of cyclopentadiene click reactions. Our emphasis is on experimental and theoretical studies on the reactivity and stability of cyclopentadiene and cyclopentadiene derivatives.

C(sp3 )-H Oxidative Addition and Transfer Hydrogenation Chemistry of a Titanium(II) Synthon: Mimicry of Late-Metal Type Reactivity

Two-electron reduction of the Ti^IV compound (^ket guan)(Im^Dipp N)Ti(OTf)₂ (3) gives the arene-masked complex (^ket guan)(η⁶ -Im^Dipp N)Ti (1) in excellent yield. Upon standing in solution, 1 converts to a Ti^IV metallacycle (4) through dehydrogenation of a pendant isopropyl group. Spectroscopic evidence shows this transformation initially proceeds via the oxidative addition of a C(sp³ )-H bond and can be reversed upon exposure of 4 to H₂ . Interestingly, treatment of 1 with cyclohexene gives cyclohexane and 4 via a titanium-mediated transfer hydrogenation reaction, a process that can be extended to catalytically hydrogenate other unsaturated hydrocarbons under mild conditions. These results, rare for the early-metals, suggest 1 possesses chemical characteristics reminiscent of noble, late-metals.

Enantioselective hydrogenations with chiral titanocenes

In this review article chiral titanocenes and their application for the enantioselective hydrogenations of different unsaturated compounds are discussed, with a special emphasis on the kinetics and the practicality of the developed systems. The nature of enantioselectivity and the hydrogenation mechanisms are reviewed as well. Catalyst immobilization and the different immobilization techniques are examined.