Mechanistic Investigations of the Rhodium Catalyzed Propargylic CH Activation

Previously we reported the redox-neutral atom economic rhodium catalyzed coupling of terminal alkynes with carboxylic acids using the DPEphos ligand. We herein present a thorough mechanistic investigation applying various spectroscopic and spectrometric methods (NMR, <i>in situ</i>-IR, ESI-MS) in combination with DFT calculations. Our findings show that in contrast to the originally proposed mechanism, the catalytic cycle involves an intramolecular protonation and not an oxidative insertion of rhodium in the OH bond of the carboxylic acid. A σ-allyl complex was identified as the resting state of the catalytic transformation and characterized by X-ray crystallographic analysis. By means of ESI-MS investigations we were able to detect a reactive intermediate of the catalytic cycle

Univalent Gallium Salts of Weakly Coordinating Anions: Effective Initiators/Catalysts for the Synthesis of Highly Reactive Polyisobutylene

The scope of the univalent gallium salts [Ga­(C₆H₅F)₂]⁺[Al­(OR^F)₄]⁻ and the new completely characterized [Ga­(1,3,5-Me₃C₆H₃)₂]⁺[Al­(OR^F)₄]⁻ (R^F = C­(CF₃)₃) was investigated in terms of initiating or catalyzing the synthesis of highly reactive poly­(2-methylpropylene)highly reactive polyisobutylene (HR-PIB)in several solvents. A series of polymerization reactions proved the high efficiency and quality of the univalent gallium salts for the polymerization of isobutylene. The best results were obtained using very low concentrations of [Ga­(C₆H₅F)₂]⁺[Al­(OR^F)₄]⁻ (down to 0.007 mol%) while working at reaction temperatures of up to ±0 °C and in the noncarcinogenic and non-water hazardous solvent toluene. Under these conditions, HR-PIB with an α-content of terminal olefinic double bonds up to 91 mol% and a molecular weight of 1000–2000 was obtained in good yields. Upon changing [Ga­(C₆H₅F)₂]⁺[Al­(OR^F)₄]⁻ for the electron richer [Ga­(1,3,5-Me₃C₆H₃)₂]⁺[Al­(OR^F)₄]⁻, polymerization temperatures could be increased to +10 °C. The reactivity of the gallium­(I) cations therefore seems to be tunable through ligand exchange reactions. Experimental results, density functional theory calculations, and mass spectrometric investigations point toward a coordinative polymerization mechanism

Univalent Gallium Salts of Weakly Coordinating Anions: Effective Initiators/Catalysts for the Synthesis of Highly Reactive Polyisobutylene

The scope of the univalent gallium salts [Ga­(C₆H₅F)₂]⁺[Al­(OR^F)₄]⁻ and the new completely characterized [Ga­(1,3,5-Me₃C₆H₃)₂]⁺[Al­(OR^F)₄]⁻ (R^F = C­(CF₃)₃) was investigated in terms of initiating or catalyzing the synthesis of highly reactive poly­(2-methylpropylene)highly reactive polyisobutylene (HR-PIB)in several solvents. A series of polymerization reactions proved the high efficiency and quality of the univalent gallium salts for the polymerization of isobutylene. The best results were obtained using very low concentrations of [Ga­(C₆H₅F)₂]⁺[Al­(OR^F)₄]⁻ (down to 0.007 mol%) while working at reaction temperatures of up to ±0 °C and in the noncarcinogenic and non-water hazardous solvent toluene. Under these conditions, HR-PIB with an α-content of terminal olefinic double bonds up to 91 mol% and a molecular weight of 1000–2000 was obtained in good yields. Upon changing [Ga­(C₆H₅F)₂]⁺[Al­(OR^F)₄]⁻ for the electron richer [Ga­(1,3,5-Me₃C₆H₃)₂]⁺[Al­(OR^F)₄]⁻, polymerization temperatures could be increased to +10 °C. The reactivity of the gallium­(I) cations therefore seems to be tunable through ligand exchange reactions. Experimental results, density functional theory calculations, and mass spectrometric investigations point toward a coordinative polymerization mechanism

Mechanism of the Ti^III-Catalyzed Acyloin-Type Umpolung: A Catalyst-Controlled Radical Reaction

The titanium­(III)-catalyzed cross-coupling between ketones and nitriles provides an efficient stereoselective synthesis of α-hydroxyketones. A detailed mechanistic investigation of this reaction is presented, which involves a combination of several methods such as EPR, ESI-MS, X-ray, in situ IR kinetics, and DFT calculations. Our findings reveal that C–C bond formation is turnover-limiting and occurs by a catalyst-controlled radical combination involving two titanium­(III) species. The resting state is identified as a cationic titanocene-nitrile complex and the beneficial effect of added Et₃N·HCl on yield and enantioselectivity is elucidated: chloride coordination initiates the radical coupling. The results are fundamental for the understanding of titanium­(III)-catalysis and of relevance for other metal-catalyzed radical reactions. Our conclusions might apply to a number of reductive coupling reactions for which conventional mechanisms were proposed before