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1,4-naphthoquinones (NQs) catalytically oxidize H2S to per- and polysufides and sulfoxides, reduce oxygen to superoxide and hydrogen peroxide, and can form NQ-SH adducts through Michael addition. Here, we measured oxygen consumption and used sulfur-specific fluorophores, liquid chromatography tandem mass spectrometry (LC-MS/MS), and UV-Vis spectrometry to examine H2S oxidation by NQs with various substituent groups. In general, the order of H2S oxidization was DCNQ ~ juglone > 1,4-NQ > plumbagin >DMNQ ~ 2-MNQ > menadione, although this order varied somewhat depending on the experimental conditions. DMNQ does not form adducts with GSH or cysteine (Cys), yet it readily oxidizes H2S to polysulfides and sulfoxides. This suggests that H2S oxidation occurs at the carbonyl moiety and not at the quinoid 2 or 3 carbons, although the latter cannot be ruled out. We found little evidence from oxygen consumption studies or LC-MS/MS that NQs directly oxidize H2S2–4, and we propose that apparent reactions of NQs with inorganic polysulfides are due to H2S impurities in the polysulfides or an equilibrium between H2S and H2Sn. Collectively, NQ oxidation of H2S forms a variety of products that include hydropersulfides, hydropolysulfides, sulfenylpolysulfides, sulfite, and thiosulfate, and some of these reactions may proceed until an insoluble S8 colloid is formed. 

We have developed an efficient method to generate highly active nickel–palladium bimetallic nanoparticles supported on multi-walled carbon nanotubes (Ni–Pd/MWCNTs) by dry mixing of the nickel and palladium salts utilizing the mechanical energy of a ball-mill. These nanoparticles were successfully employed in Sonogashira cross-coupling reactions with a wide array of functionalized aryl halides and terminal alkynes under ligand and copper free conditions using a Monowave 50 heating reactor. Notably, the concentration of palladium can be lowered to a minimum amount of 0.81% and replaced by more abundant and less expensive nickel nanoparticles while effectively catalyzing the reaction. The remarkable reactivity of the Ni–Pd/MWCNTs catalyst toward Sonogashira cross-coupling reactions is attributed to the high degree of the dispersion of Ni–Pd nanoparticles with small particle size of 5–10 nm due to an efficient grinding method. The catalyst was easily removed from the reaction mixture by centrifugation and reused several times with minimal loss of catalytic activity. Furthermore, the concentration of catalyst in Sonogashira reactions can be reduced to a minimum amount of 0.01 mol% while still providing a high conversion of the Sonogashira product with a remarkable turnover number (TON) of 7200 and turnover frequency (TOF) of 21 600 h −1 . The catalyst was fully characterized by a variety of spectroscopic techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). 

Naphthoquinone (1,4-NQ) and its derivatives (NQs, juglone, plumbagin, 2-methoxy-1,4-NQ, and menadione) have a variety of therapeutic applications, many of which are attributed to redox cycling and the production of reactive oxygen species (ROS). We previously demonstrated that NQs also oxidize hydrogen sulfide (H2S) to reactive sulfur species (RSS), potentially conveying identical benefits. Here we use RSS-specific fluorophores, mass spectroscopy, EPR and UV-Vis spectrometry, and oxygen-sensitive optodes to examine the effects of thiols and thiol-NQ adducts on H2S-NQ reactions. In the presence of glutathione (GSH) and cysteine (Cys), 1,4-NQ oxidizes H2S to both inorganic and organic hydroper-/hydropolysulfides (R2Sn, R=H, Cys, GSH; n = 2–4) and organic sulfoxides (GSnOH, n = 1, 2). These reactions reduce NQs and consume oxygen via a semiquinone intermediate. NQs are also reduced as they form adducts with GSH, Cys, protein thiols, and amines. Thiol, but not amine, adducts may increase or decrease H2S oxidation in reactions that are both NQ- and thiol-specific. Amine adducts also inhibit the formation of thiol adducts. These results suggest that NQs may react with endogenous thiols, including GSH, Cys, and protein Cys, and that these adducts may affect both thiol reactions as well as RSS production from H2S.