Electrophilic Arene Hydroxylation and Phenol O?H Oxidations Performed by an Unsymmetric μ‐η1:η1‐O2‐Peroxo Dicopper(II) Complex

Reactions of the unsymmetric dicopper(II) peroxide complex [Cu^II₂(μ‐η¹:η¹‐O₂)(m‐XYL^N3N4)]²⁺ ( 1 O₂ , where m‐XYL is a heptadentate N‐based ligand), with phenolates and phenols are described. Complex 1 O₂ reacts with p‐X‐PhONa (X=MeO, Cl, H, or Me) at ?90 °C performing tyrosinase‐like ortho‐hydroxylation of the aromatic ring to afford the corresponding catechol products. Mechanistic studies demonstrate that reactions occur through initial reversible formation of metastable association complexes [Cu^II₂(μ‐η¹:η¹‐O₂)(p‐X‐PhO)(m‐XYL^N3N4)]⁺ ( 1 O₂ ?X‐PhO) that then undergo ortho‐hydroxylation of the aromatic ring by the peroxide moiety. Complex 1 O₂ also reacts with 4‐X‐substituted phenols p‐X‐PhOH (X=MeO, Me, F, H, or Cl) and with 2,4‐di‐tert‐butylphenol at ?90 °C causing rapid decay of 1 O₂ and affording biphenol coupling products, which is indicative that reactions occur through formation of phenoxyl radicals that then undergo radical C? C coupling. Spectroscopic UV/Vis monitoring and kinetic analysis show that reactions take place through reversible formation of ground‐state association complexes [Cu^II₂(μ‐η¹:η¹‐O₂)(X‐PhOH)(m‐XYL^N3N4)]²⁺ ( 1 O₂ ?X‐PhOH) that then evolve through an irreversible rate‐determining step. Mechanistic studies indicate that 1 O₂ reacts with phenols through initial phenol binding to the Cu₂O₂ core, followed by a proton‐coupled electron transfer (PCET) at the rate‐determining step. Results disclosed in this work provide experimental evidence that the unsymmetric 1 O₂ complex can mediate electrophilic arene hydroxylation and PCET reactions commonly associated with electrophilic Cu₂O₂ cores, and strongly suggest that the ability to form substrate?Cu₂O₂ association complexes may provide paths to overcome the inherent reactivity of the O₂‐binding mode. This work provides experimental evidence that the presence of a H⁺ completely determines the fate of the association complex [Cu^II₂(μ‐η¹:η¹‐O₂)(X‐PhO(H))(m‐XYL^N3N4)]ⁿ⁺ between a PCET and an arene hydroxylation reaction, and may provide clues to help understand enzymatic reactions at dicopper sites.     

Enantioselective Palladium‐Catalyzed Insertion of α‐Aryl‐α‐diazoacetates into the O?H Bonds of Phenols

A palladium‐catalyzed asymmetric O H insertion reaction was developed. Palladium complexes with chiral spiro bisoxazoline ligands promoted the insertion of α‐aryl‐α‐diazoacetates into the O H bond of phenols with high yield and excellent enantioselectivity under mild reaction conditions. This palladium‐catalyzed asymmetric O H insertion reaction provided an efficient and highly enantioselective method for the preparation of synthetically useful optically active α‐aryl‐α‐aryloxyacetates.     

α,β,γ,δ,ε,η-Bi2O3电子结构和光学性质的第一性原理研究

基于密度泛函理论的CASTEP模块研究了α, β, γ, δ, ε和η-Bi₂O₃晶型, 计算分析了其几何结构、 能带结构、 电子态密度和光学性质. 结果表明, α, ε和η相均为层状结构, 其中, α和ε相为单层—Bi—O—结构, 而η相为双层—Bi—O—结构; β, γ和δ相为—Bi_m—O_n—交错结构, 其中δ相交错尤为密集, 呈现导体特性. 各晶相的导带均由Bi 6p态构成, 价带由O_2p态起主导作用. 电势电位分析结果表明, 6种晶相价带电位均在H₂O/O₂之下, 具有强氧化能力, 与实验报道的光催化氧化能力大小顺序γ-Bi₂O₃>β-Bi₂O₃>α-Bi₂O₃>δ-Bi₂O₃一致, 而导带还原电位低于H₂/H₂O, 预测纯Bi₂O₃很难具备催化产氢能力. 光学性质分析发现, γ和δ相的起始响应波长较大, 说明其应具备红外激发的性质. 这些结果可为获得偏红外激发和较宽光谱响应的Bi₂O₃材料研究提供理论基础, 为研发和应用Bi₂O₃及其复合物提供重要的指导.     

Regioselective C?F Bond Activation of Hexafluoropropylene on Palladium(0): Formation of a Cationic η2‐Perfluoroallylpalladium Complex

A chemoselective C(sp²) F or C(sp³) F bond activation of hexafluoropropylene (HFP) was achieved by adopting the proper combination of a Lewis acid co‐additive with a ligand which coordinates Pd⁰. The treatment of [(η²‐HFP)Pd(PCy₃)₂] with B(C₆F₅)₃ allowed a chemoselective C(sp³) F bond cleavage of HFP to give a unique cationic perfluoroallypalladium complex. In this complex, the coordination mode of the perfluoroallyl ligand was considered to be of the unique η²‐fashion.     

Rhodium‐catalyzed coupling reactions of β‐ or α,β‐substituted vinylpyridines with alkenes via C?H bond activation

β‐ or α,β‐Substituted vinylpyridines react with 3,3‐dimethylbut‐1‐ene in the presence of Wilkinson catalyst [RhCl(PPh₃)₃] to give the corresponding alkylated products along with unusually isomerized products. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:346–350, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10045     

Regioselective Insertion of o‐Carborynes into the α‐C?H Bond of Tertiary Amines: Synthesis of α‐Carboranylated Amines

o‐Carboryne can undergo α‐C H bond insertion with tertiary amines, thus affording α‐carboranylated amines in very good regioselectivity and isolated yields. In this process, the nucleophilic addition of tertiary amines to the multiple bond of o‐carboryne generates a zwitterionic intermediate. An intramolecular proton transfer, followed by a nucleophilic attack leads to the formation of the final product. Thus, regioselectivity is highly dependent upon the acidity of α‐C H proton of tertiary amines. This approach serves as an efficient methodology for the preparation of a series of 1‐aminoalkyl‐o‐carboranes.     

Redox‐Triggered C?C Coupling of Diols and Alkynes: Synthesis of β,γ‐Unsaturated α‐Hydroxyketones and Furans by Ruthenium‐Catalyzed Hydrohydroxyalkylation

Direct ruthenium‐catalyzed C C coupling of alkynes and vicinal diols to form β,γ‐unsaturated ketones occurs with complete levels of regioselectivity and good to complete control over the alkene geometry. Exposure of the reaction products to substoichiometric quantities of p‐toluenesulfonic acid induces cyclodehydration to form tetrasubstituted furans. These alkyne‐diol hydrohydroxyalkylations contribute to a growing body of merged redox‐construction events that bypass the use of premetalated reagents and, hence, stoichiometric quantities of metallic by‐products.     

Isomerization of Olefins Triggered by Rhodium‐Catalyzed C?H Bond Activation: Control of Endocyclic β‐Hydrogen Elimination

Five‐membered metallacycles are typically reluctant to undergo endocyclic β‐hydrogen elimination. The rhodium‐catalyzed isomerization of 4‐pentenals into 3‐pentenals occurs through this elementary step and cleavage of two C H bonds, as supported by deuterium‐labeling studies. The reaction proceeds without decarbonylation, leads to trans olefins exclusively, and tolerates other olefins normally prone to isomerization. Endocyclic β‐hydrogen elimination can also be controlled in an enantiodivergent reaction on a racemic mixture.     

Unusual Folding Propensity of an Unsubstituted β,γ‐Hybrid Model Peptide: Importance of the C?H⋅⋅⋅O Intramolecular Hydrogen Bond

The single‐crystal X‐ray diffraction analysis of a β,γ‐hybrid model peptide Boc‐β‐Ala‐γ‐Abu‐NH₂ revealed the existence of four crystallographically independent molecules ( A , B , C and D conformers) in the asymmetric unit. The analysis revealed that unusual β‐turn‐like folded structures predominate, wherein the conformational space of non‐proteinogenic β‐Ala and γ‐Abu residues are restricted to gauche‐gauche‐skew and skew‐gauche‐trans‐skew orientations, respectively. Interestingly, the U‐shaped conformers are seemingly stabilised by an effective unconventional C? H ??? O intramolecular hydrogen bond, encompassing a non‐covalent 14‐membered ring‐motif. Taking into account the signs of torsion angles, these conformers could be grouped into two distinct categories, A / B and C / D , establishing the incidence of non‐superimposable stereogeometrical features across a non‐chiral one‐component peptide model system, that is, “mirror‐image‐like” relationships. The natural occurrence of β‐Ala and γ‐Abu entities in various pharmacologically important molecules, coupled with their biocompatibilities, highlight how the non‐functionalised β,γ‐hybrid segment may offer unique advantages for introducing and/or manipulating a wide spectrum of biologically relevant hydrogen bonded secondary structural mimics in short synthetic peptides.