- Cyclopentadienone Iridium Bipyridyl Complexes: Acid-Stable Transfer Hydrogenation Catalysts ORGANOMETALLICS 2023
Mechanism-Guided Design of Robust Palladium Catalysts for Selective Aerobic Oxidation of Polyols.
Journal of the American Chemical Society
The palladium complex [(L1)Pd(mu-OAc)]2[OTf]2 (L1 = neocuproine) is a selective catalyst for the aerobic oxidation of vicinal polyols to alpha-hydroxyketones, but competitive oxidation of the ligand methyl groups limits the turnover number and necessitates high Pd loadings. Replacement of the neocuproine ligand with 2,2'-biquinoline ligands was investigated as a strategy to improve catalyst performance and explore the relationship between ligand structure and reactivity. Evaluation of [(L2)Pd(mu-OAc)]2[OTf]2 (L2 = 2,2'-biquinoline) as a catalyst for aerobic alcohol oxidation revealed a threefold enhancement in turnover number relative to the neocuproine congener, but a much slower rate. Mechanistic studies indicated that the slow rates observed with L2 were a consequence of precipitation of an insoluble trinuclear palladium species─(L2Pd)3(mu-O)22+─formed during catalysis and characterized by high-resolution electrospray ionization mass spectrometry. Density functional theory was used to predict that a sterically modified biquinoline ligand, L3 = 7,7'-di-tert-butyl-2,2'-biquinoline, would disfavor the formation of the trinuclear (LPd)3(mu-O)22+ species. This design strategy was validated as catalytic aerobic oxidation with [(L3)Pd(mu-OAc)]2[OTf]2 is both robust and rapid, marrying the kinetics of the parent L1-supported system with the high aerobic turnover numbers of the L2-supported system. Changes in ligand structure were also found to modulate regioselectivity in the oxidation of complex glycoside substrates, providing new insights into structure-selectivity relationships with this class of catalysts.
View details for DOI 10.1021/jacs.2c10667
View details for PubMedID 36657018
Potassium Trimethylsilanolate-Promoted, Anhydrous Suzuki-Miyaura Cross-Coupling Reaction Proceeds via the "Boronate Mechanism": Evidence for the Alternative Fork in the Trail.
Journal of the American Chemical Society
Previous studies have shown that the critical transmetalation step in the Suzuki-Miyaura cross-coupling proceeds through a mechanism wherein an arylpalladium hydroxide complex reacts with an aryl boronic acid, termed the oxo-palladium pathway. Moreover, these same studies have established that the reaction between an aryl boronate and an arylpalladium halide complex (the boronate pathway) is prohibitively slow. Herein, studies on isolated intermediates, along with kinetic analysis, have demonstrated that the Suzuki-Miyaura reaction promoted by potassium trimethylsilanolate (TMSOK) proceeds through the boronate pathway, in contrast with other, established systems. Furthermore, an unprecedented, binuclear palladium(I) complex containing a mu-phenyl bridging ligand was characterized by NMR spectroscopy, mass spectrometry, and computational methods. Density functional theory (DFT) calculations suggest that the binuclear complex exhibits an open-shell ground electronic state, and reaction kinetics implicate the complex in the catalytic cycle. These results expand the breadth of potential mechanisms by which the Suzuki-Miyaura reaction can occur, and the novel binuclear palladium complex discovered has broad implications for palladium-mediated cross-coupling reactions of aryl halides.
View details for DOI 10.1021/jacs.1c08283
View details for PubMedID 35230833
Condensing water vapor to droplets generates hydrogen peroxide.
Proceedings of the National Academy of Sciences of the United States of America
It was previously shown [J. K. Lee et al., Proc. Natl. Acad. Sci. U.S.A, 116, 19294-19298 (2019)] that hydrogen peroxide (H2O2) is spontaneously produced in micrometer-sized water droplets (microdroplets), which are generated by atomizing bulk water using nebulization without the application of an external electric field. Here we report that H2O2 is spontaneously produced in water microdroplets formed by dropwise condensation of water vapor on low-temperature substrates. Because peroxide formation is induced by a strong electric field formed at the water-air interface of microdroplets, no catalysts or external electrical bias, as well as precursor chemicals, are necessary. Time-course observations of the H2O2 production in condensate microdroplets showed that H2O2 was generated from microdroplets with sizes typically less than 10 m. The spontaneous production of H2O2 was commonly observed on various different substrates, including silicon, plastic, glass, and metal. Studies with substrates with different surface conditions showed that the nucleation and the growth processes of condensate water microdroplets govern H2O2 generation. We also found that the H2O2 production yield strongly depends on environmental conditions, including relative humidity and substrate temperature. These results show that the production of H2O2 occurs in water microdroplets formed by not only atomizing bulk water but also condensing water vapor, suggesting that spontaneous water oxidation to form H2O2 from water microdroplets is a general phenomenon. These findings provide innovative opportunities for green chemistry at heterogeneous interfaces, self-cleaning of surfaces, and safe and effective disinfection. They also may have important implications for prebiotic chemistry.
View details for DOI 10.1073/pnas.2020158117
View details for PubMedID 33229543