853 pubs · 25,329 cites · h-index 82
The influence of specific adsorption of the sulfo group in the perfluorosulfonic acid ionomer, Nafion, on the oxygen reduction reaction (ORR) of a carbon-supported Pt/C catalyst using a thin-film rotating disk electrode was investigated. The relationship between the catalyst activity and coating of the Pt/C catalyst with Nafion was quantitatively evaluated through electrochemical measurements, operando X-ray absorption spectroscopy (XAS), and CO stripping voltammetry. Activity of the Pt/C catalyst decreased with increasing ionomer-to-carbon weight ratios. To date, the effect of specific adsorption on a catalyst has been investigated using CO stripping voltammetry. However, quantitative evaluation of specific adsorption at the ORR potential (0.50–1.0 V) has not been performed yet. We quantitatively evaluated the specific adsorption during the ORR by measuring the 5d orbital vacancy of the Pt/C catalyst using operando XAS. The difference in the electronic structures of Pt in the high potential range, with and without the ionomer, was successfully established.
The oxygen reduction reaction (ORR) activity and the coverage of oxide species over a Pd core–Pt shell catalyst on carbon support (Pt/Pd/C) with various Pd core sizes (2.3, 4.3, and 8.0 nm) were investigated in the temperature range from 25 to 60 °C and compared with a Pt/C catalyst (TEC10V30E, Tanaka Kikinzoku Kogyo). The apparent rate constant ( k app ) of Pt/Pd/C increased with increasing core size at 25 °C. However, k app of Pt/Pd/C started to decrease at 50–60 °C, while that of Pt/C behaved according to the general Arrhenius equation. Eventually, the 2.3 nm core showed the highest k app at 60 °C, and the 8.0 nm core was almost the same as that of Pt/C. According to the electrochemical measurements, the coverage of oxide species on Pt/Pd/C was quite smaller than that of Pt/C. However, it increased dramatically with increasing temperature from 25 to 60 °C. Among the Pt/Pd/C, the 8.0 nm core showed the most obvious oxide coverage increase at 60 °C, which was almost identical to that of Pt/C. In contrast, the 2.3 nm core showed the lowest oxide coverage at 60 °C, which was expected to be the cause of the largest k app . Operando X-ray absorption spectroscopy indicated that the Pt–Pt bond length in Pt/Pd/C was shorter than that in the Pt/C at 25 °C due to compressive surface strain from the Pd core, which is the reason why Pt/Pd/C has higher activity than Pt/C. On the other hand, as Pd has a thermal expansion coefficient higher than that of Pt, Pt/Pd/C showed a Pt–Pt bond length larger than that of Pt/C at 60 °C. A longer Pt–Pt bond length extension was observed at 60 °C in the 8.0 nm core compared to that in the other catalysts.
This study provides evidence of the enhancement of the oxygen reduction reaction (ORR) activity of Pt/C and Pd-core Pt-shell catalysts upon ionic liquid modification. Under high potential at 1.15 V (vs. reversible hydrogen electrode), the lower coverage of the oxide species on the Pt surface modified with [7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD)][bis(trifluoromethylsulfonyl)imide (NTf2)] ionic liquid (IL) was confirmed by electrochemical analysis. Operando X-ray absorption spectra (XAS) of the ionic liquid-modified Pd-core Pt-shell catalyst displayed smaller 5d orbital vacancies than the naked catalyst. Shrinkage of Pt-Pt bond length was also displayed, indicating the suppression of oxidation. Thus, we conclude that the IL-modified systems are effective in avoiding the adsorption of oxide species on the Pt surface and improving the oxygen reduction activity.
The first examples of metal-catalyzed extended Pummerer reactions through the activation of sulfoxides are described. The copper-catalyzed reactions of ketene dithioacetal monoxides with alkynyl sulfides and ynamides provided a wide variety of γ,γ-disulfanyl-β,γ-unsaturated carbonyl compounds with an accompanying oxygen rearrangement. The products can be easily converted into 1,4-dicarbonyl compounds and substituted heteroaromatics. DFT calculations and mechanistic experiments revealed a new interesting stepwise addition/oxygen rearrangement mechanism.
The reaction of terminal alkynes with sulfide (I) proceeds with complete regio‐ and stereoselectivities.
Regio- and stereoselective addition of a silyl-substituted alkynyl sulfide to various terminal alkynes proceeds in the presence of a palladium catalyst to give (Z)-1-thio-1,3-enynes, which are useful building blocks for the synthesis of polysubstituted 1,3-enynes. Addition to internal alkynes also takes place under solvent-free conditions (see scheme).
Bis(cyclopentadienyldicarbonyliron) ([CpFe(CO)2]2) serves as a carbon monoxide source in carbonylative coupling reactions. Treatment of aryl iodides with primary amines in the presence of DBU and [CpFe(CO)2]2 under palladium catalysis provides the corresponding benzamides in good yields. Similar reactions with phenols and thiols provide the corresponding benzoate esters and thioesters, respectively. A catalytic amount of DMAP as an additive promoted the carbonylative coupling reactions with primary alcohol and secondary amine.
Strike while the iron is hot! A BF3-mediated formal [2+2] cycloaddition between (1-alkynyl)iron complexes and aromatic aldehyde imines occurs, which is followed by ring-opening of the initially formed azacyclobutenes to yield (2-alkenyl-1-imino)iron complexes (see scheme). The resulting iron complexes undergo deferric substitution reactions with water under oxidative conditions to afford the corresponding cinnamamides with high E selectivity.
Palladium-catalyzed arylative cyclization of propargyl-substituted malonate esters with aryl halides offers a stereoselective approach to alkylidenecyclopropanes. The reaction proceeds by an anti-carbopalladation pathway, which guarantees the exclusive stereocontrol of the resulting double bond. The highly strained as well as densely substituted skeletons of the products facilitate further versatile transformations, which underscores the importance of the products as synthetic intermediates.
Cat. in the bag: The pictured copper complex can promote CC bond cleavage through retro-allylation of homoallyl alcohols to form allylcopper species. This process is applicable to catalytic allylation of aldehydes and imines with homoallyl alcohols. The method has also been extended to regioselective allenylation and propargylation of imines.
Treatment of tetrathiafulvalene (TTF) with aryl bromides in the presence of cesium carbonate and a palladium catalyst results in direct arylation of TTF. The direct arylation is inherently straightforward and efficient, hence easily creating a large library of aryl TTFs for systematic investigations of structure–property relationships. The present protocol will thus represent an efficient method for exploiting TTF-based interesting functional materials.
About 40 years have passed since methylene dizinc reagent was discovered as a substitute for Wittig reagent. Density functional theory (DFT) calculations have been performed to understand the reaction pathways of methylenation of carbonyl compounds with bis(iodozincio)methane. The present computational/theoretical study concluded that the methylenation reaction with gem-dizinc reagent proceeds as a two-step reaction, that is, methylene addition (RDS) and olefination. In the first step, the nucleophilic attack of the CH2 group enhanced by two Zn proceeds under the assistance of the electrophilic activation of the carbonyl group with the Zn atom. In the second step, the olefination is facilitated by both Zn atoms of the gem-dizinc reagent without an electron transfer process.
A concise and diversity-oriented route to trifluoromethylbenzo[b]furans has been devised. A variety of phenols are directly converted to the corresponding 2-methylthio-3-trifluoromethylbenzo[b]furans by new triflic-anhydride-mediated extended Pummerer annulation reactions with trifluoromethylketene dithioacetal monoxide. The methylthio group of the products undergoes further transformations, which increase the diversity of available trifluoromethylbenzo[b]furans.
Useful intermediates: Addition of silyl-substituted chloroalkynes to terminal alkynes, namely chloroalkynylation, proceeds under palladium catalysis to afford (Z)-1-chloro-1,3-enynes (see scheme). The carbon–chlorine bond of the adducts is easily convertible to a carbon–carbon bond by using palladium-catalyzed cross-coupling reactions to provide a variety of 1,3-enynes.
The title reaction affords the 2‐phosphorylated indoles in moderate to excellent yields.
Rhodium-catalyzed regioselective arylzincation of terminal allenes affords synthetically useful functionalized allylzinc reagents. The allylzinc reagents react with a variety of electrophiles such as acetonitrile, offering a more versatile three-component coupling reaction.
A facile route to homoallyl alcohols bearing a trisubstituted double bond has been devised. The palladium-catalyzed reactions of aryl halides with the alcohols thus synthesized result in regiospecific allyl transfer from the alcohols to aryl halides via retro-allylation, providing allylarenes having two substituents at the 1 and 2 positions of the allyl moiety. Optically active homoallyl alcohols transfer their chirality at the hydroxylated carbon to the benzylic carbon of the product.
Carbometalation: Cobalt salts catalyze benzylzincation of alkynes to afford benzylated multisubstituted alkenes with high regio- and stereoselectivity. The scope of the reaction is wide enough to apply unfunctionalized alkynes as well as arylacetylenes. The reaction offers a new route to the regio- and stereoselective synthesis of an estrogen receptor antagonist (see scheme).
The motorcyclization diaries: Palladium-catalyzed intramolecular carboacetoxylation of 4-pentenyl-substituted malonates proceeds with the aid of titanium tetraisopropoxide and iodobenzene diacetate. The transformation represents the first PdII/PdIV catalytic oxidative process that forms an Csp3Csp3 bond as well as a useful method for the synthesis of acetoxymethyl-substituted cyclopentanes or bicyclic lactones.