Iron-Catalyzed Oxidative C–H/C–H Cross-Coupling between Electron-Rich Arenes and Alkenes

A novel oxidative C–H/C–H cross-coupling reaction between electron-rich arenes and alkenes is established utilizing FeCl₃ as the catalyst and DDQ as the oxidant. Interestingly, direct arylation products are obtained with diaryl-ethylenes and double arylation products are obtained with styrene derivatives, which show high chemoselectivity and good substrate scope. A radical trapping experiment and EPR (electron paramagnetic resonance) experiments indicate that this reaction proceeds through a radical pathway in which DDQ plays a key role in the aryl radical formation. XAFS (X-ray absorption fine structure) experiments reveal that the oxidation state of the iron catalyst does not change during the reaction, suggesting that FeCl₃ might be used as a Lewis acid. Finally, a detailed mechanism is proposed for this transformation

Direct Observation of Reduction of Cu(II) to Cu(I) by Terminal Alkynes

X-ray absorption spectroscopy and <i>in situ</i> electron paramagnetic resonance evidence were provided for the reduction of Cu­(II) to Cu­(I) species by alkynes in the presence of tetramethylethylenediamine (TMEDA), in which TMEDA plays dual roles as both ligand and base. The structures of the starting Cu­(II) species and the obtained Cu­(I) species were determined as (TMEDA)­CuCl₂ and [(TMEDA)­CuCl]₂ dimer, respectively

Capping Ligands as Selectivity Switchers in Hydrogenation Reactions

We systematically investigated the role of surface modification of nanoparticles catalyst in alkyne hydrogenation reactions and proposed the general explanation of effect of surface ligands on the selectivity and activity of Pt and Co/Pt nanoparticles (NPs) using experimental and computational approaches. We show that the proper balance between adsorption energetics of alkenes at the surface of NPs as compared to that of capping ligands defines the selectivity of the nanocatalyst for alkene in alkyne hydrogenation reaction. We report that addition of primary alkylamines to Pt and CoPt₃ NPs can drastically increase selectivity for alkene from 0 to more than 90% with ∼99.9% conversion. Increasing the primary alkylamine coverage on the NP surface leads to the decrease in the binding energy of octenes and eventual competition between octene and primary alkylamines for adsorption sites. At sufficiently high coverage of catalysts with primary alkylamine, the alkylamines win, which prevents further hydrogenation of alkenes into alkanes. Primary amines with different lengths of carbon chains have similar adsorption energies at the surface of catalysts and, consequently, the same effect on selectivity. When the adsorption energy of capping ligands at the catalytic surface is lower than adsorption energy of alkenes, the ligands do not affect the selectivity of hydrogenation of alkyne to alkene. On the other hand, capping ligands with adsorption energies at the catalytic surface higher than that of alkyne reduce its activity resulting in low conversion of alkynes

Cu(II)–Cu(I) Synergistic Cooperation to Lead the Alkyne C–H Activation

An efficient alkyne C–H activation and homocoupling procedure has been studied which indicates that a Cu­(II)/Cu­(I) synergistic cooperation might be involved. <i>In situ</i> Raman spectroscopy was employed to study kinetic behavior, drawing the conclusion that Cu­(I) rather than Cu­(II) participates in the rate-determining step. IR, EPR, and X-ray absorption spectroscopy evidence were provided for structural information, indicating that Cu­(I) has a stronger interaction with alkyne than Cu­(II) in the C–H activation step. Kinetics study showed Cu­(II) plays a role as oxidant in C–C bond construction step, which was a fast step in the reaction. X-band EPR spectroscopy showed that the coordination environment of CuCl₂(TMEDA) was affected by Cu­(I). A putative mechanism with Cu­(I)–Cu­(II) synergistic cooperation procedure is proposed for the reaction

Supported Single-Site Ti(IV) on a Metal–Organic Framework for the Hydroboration of Carbonyl Compounds

A stable and structurally well-defined titanium alkoxide catalyst supported on a metal–organic-framework (MOF) of UiO-67 topology (<b>ANL1-Ti­(O</b>^{<b><i>i</i></b>}<b>Pr)</b>_<b>2</b>) was synthesized and fully characterized by a variety of analytical and spectroscopic techniques, including BET, TGA, PXRD, XAS, DRIFT, SEM, and DFT computations. The Ti-functionalized MOF was demonstrated active for the catalytic hydroboration of a wide range of aldehydes and ketones with HBpin as the boron source. Compared to traditional homogeneous and supported hydroboration catalysts, <b>ANL1-Ti­(O</b>^{<b><i>i</i></b>}<b>Pr)</b>_<b>2</b> is completely recyclable and reusable, making it a promising hydroboration catalyst alternative for green and sustainable chemical synthesis. In addition, <b>ANL1-Ti­(O</b>^{<b><i>i</i></b>}<b>Pr)</b>_<b>2</b> catalyst exhibits remarkable hydroboration selectivity toward aldehydes vs ketone in competitive study. DFT calculations suggest that the catalytic hydroboration proceeds via a (1) hydride transfer between the active Ti-hydride species and a carbonyl moiety (rate-determining step) and (2) alkoxide transfer (intramolecular σ-bond metathesis) to generate the borate ester product