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1. Unconventional mechanism and selectivity of the Pd-catalyzed C–H bond lactonization in aromatic carboxylic acid

   https://doi.org/10.1038/s41467-022-27986-6  

   Xu, Li-Ping ; Qian, Shaoqun ; Zhuang, Zhe ; Yu, Jin-Quan ; Musaev, Djamaladdin G. ( January 2022 , Nature Communications)

   The search for more effective and highly selective C–H bond oxidation of accessible hydrocarbons and biomolecules is a greatly attractive research mission. The elucidating of mechanism and controlling factors will, undoubtedly, help to broaden scope of these synthetic protocols, and enable discovery of more efficient, environmentally benign, and highly practical new C–H oxidation reactions. Here, we reveal the stepwise intramolecular S_N2 nucleophilic substitution mechanism with the rate-limiting C–O bond formation step for the Pd(II)-catalyzed C(sp³)–H lactonization in aromatic 2,6-dimethylbenzoic acid. We show that for this reaction, the direct C–O reductive elimination from both Pd(II) and Pd(IV) (oxidized by O₂oxidant) intermediates is unfavorable. Critical factors controlling the outcome of this reaction are the presence of the η³-(π-benzylic)–Pd and K⁺–O(carboxylic) interactions. The controlling factors of the benzylic vs ortho site-selectivity of this reaction are the: (a) difference in the strains of the generated lactone rings; (b) difference in the strengths of the η³-(π-benzylic)–Pd and η²-(π-phenyl)–Pd interactions, and (c) more pronounced electrostatic interaction between the nucleophilic oxygen and K⁺cation in the ortho-C–H activation transition state. The presented data indicate the utmost importance of base, substrate, and ligand in the selective C(sp³)–H bond lactonization in the presence of C(sp²)–H.

    

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2. Heterogeneous catalysis by ultra-small bimetallic nanoparticles surpassing homogeneous catalysis for carbon–carbon bond forming reactions

   https://doi.org/10.1039/d0nr04105j  

   Norouzi, Nazgol ; Das, Mrinmoy K. ; Richard, Alexander J. ; Ibrahim, Amr A. ; El-Kaderi, Hani M. ; El-Shall, M. Samy ( October 2020 , Nanoscale)

   null (Ed.)

   Palladium catalyzed cross-coupling reactions represent a significant advancement in contemporary organic synthesis as these reactions are of strategic importance in the area of pharmaceutical drug discovery and development. Supported palladium-based catalysts are highly sought-after in carbon–carbon bond forming catalytic processes to ensure catalyst recovery and reuse while preventing product contamination. This paper reports the development of heterogeneous Pd-based bimetallic catalysts supported on fumed silica that have high activity and selectivity matching those of homogeneous catalysts, eliminating the catalyst's leaching and sintering and allowing efficient recycling of the catalysts. Palladium and base metal (Cu, Ni or Co) contents of less than 1.0 wt% loading are deposited on a mesoporous fumed silica support (surface area SA BET = 350 m 2 g −1 ) using strong electrostatic adsorption (SEA) yielding homogeneously alloyed nanoparticles with an average size of 1.3 nm. All bimetallic catalysts were found to be highly active toward Suzuki cross-coupling (SCC) reactions with superior activity and stability for the CuPd/SiO 2 catalyst. A low CuPd/SiO 2 loading (Pd: 0.3 mol%) completes the conversion of bromobenzene and phenylboronic acid to biphenyl in 30 minutes under ambient conditions in water/ethanol solvent. In contrast, monometallic Pd/SiO 2 (Pd: 0.3 mol%) completes the same reaction in three hours under the same conditions. The combination of Pd with the base metals helps in retaining the Pd 0 status by charge donation from the base metals to Pd, thus lowering the activation energy of the aryl halide oxidative addition step. Along with its exceptional activity, CuPd/SiO 2 exhibits excellent recycling performance with a turnover frequency (TOF) of 280 000 h −1 under microwave reaction conditions at 60 °C. Our study demonstrates that SEA is an excellent synthetic strategy for depositing ultra-small Pd-based bimetallic nanoparticles on porous silica for SCC. This avenue not only provides highly active and sintering-resistant catalysts but also significantly lowers Pd contents in the catalysts without compromising catalytic activity, making the catalysts very practical for large-scale applications. 

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3. Mechanistic Details of the Pd‐catalyzed and MPAA Ligand‐Enabled β‐C(sp 3 )‐H Acetoxylation of Free Carboxylic Acid

   https://doi.org/10.1002/asia.202201145  

   Xu, Li‐Ping ; Li, Na ; Musaev, Djamaladdin G. ( December 2022 , Chemistry – An Asian Journal)

   Transition metal‐catalyzed C−H bond oxidation of free carboxylic acid stands as an economic, selective, and efficient strategy to generate lactones, hydroxylated products, and acetoxylated products and attracts much of the chemists’ attention. Herein, we performed a density functional theory study on the mechanism and selectivity in Pd‐catalyzed and MPAA ligand‐enabled C−H bond acetoxylation reaction. It was found that the ligand, base, and substrate are important in determining the reaction mechanism and the selectivity. The acetic anhydride additive is critical in leading the reaction to be acetoxylation, instead of the lactonization, through a facile σ‐bond metathesis mechanism that leads to the Pd‐OAc in‐termediate. Our study sheds light on the further development of transition metal‐catalyzed C−H bond oxidation reactions.

    

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4. Selectivity for ethanol partial oxidation: the unique chemistry of single-atom alloy catalysts on Au, Ag, and Cu(111)

   https://doi.org/10.1039/C9TA04572D  

   Li, Hao ; Chai, Wenrui ; Henkelman, Graeme ( October 2019 , Journal of Materials Chemistry A)

   Recently, we found that the atomic ensemble effect is the dominant effect influencing catalysis on surfaces alloyed with strong- and weak-binding elements, determining the activity and selectivity of many reactions on the alloy surface. In this study we design single-atom alloys that possess unique dehydrogenation selectivity towards ethanol (EtOH) partial oxidation, using knowledge of the alloying effects from density functional theory calculations. We found that doping of a strong-binding single-atom element ( e.g. , Ir, Pd, Pt, and Rh) into weak-binding inert close-packed substrates ( e.g. , Au, Ag, and Cu) leads to a highly active and selective initial dehydrogenation at the α-C–H site of adsorbed EtOH. We show that many of these stable single-atom alloy surfaces not only have tunable hydrogen binding, which allows for facile hydrogen desorption, but are also resistant to carbon coking. More importantly, we show that a rational design of the ensemble geometry can tune the selectivity of a catalytic reaction. 

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5. The Synergy of Dilute Pd and Surface Oxygen Species for Methane Upgrading on Au 3 Pd(111)

   https://doi.org/10.1002/ente.201900732  

   Do, Quan K. ; Tran, Hung-Vu ; Wang, Shengguang ; Grabow, Lars C. ( September 2019 , Energy Technology)

   The grandest challenge in the direct upgrade of methane to useful chemicals is finding a catalyst that is active for the difficult C—H bond scission yet selective such that not all of the C—H bonds break to form coke or CO₂. The Au₃Pd(111) surface is studied for methane activation, particularly due to its surface geometry containing a single Pd atom fully surrounded by Au atoms. Through density functional theory (DFT) calculations, the nonoxidative and oxidative mechanisms for methane activation are examined. In the nonoxidative mechanism, the initial C—H bond scission on Au₃Pd(111) resembles that of Pd(111), but the subsequent dehydrogenation steps are more difficult than the first, mimicking pure Au(111). In the oxidative mechanism, the dilute Pd atom in Au₃Pd(111) draws oxygenated adsorbates to the weakly binding Au surface to assist in the activation of methane. In both cases, Au₃Pd(111) merges the activity of pure Pd(111) and the selectivity of pure Au(111), indicating the promise of isolated reactive metals as a potential catalyst for the upgrade of methane and other dehydrogenation processes.

    

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