Coupling Titanium and Chromium Catalysis in a Reaction Network for the Reprogramming of [BH4]− as Electron Transfer and Hydrogen Atom Transfer Reagent for Radical Chemistry

We describe a unique catalytic system with an efficient coupling of Ti- and Cr-catalysis in a reaction network that allows the use of [BH₄]⁻ as stoichiometric hydrogen atom and electron donor in catalytic radical chemistry. The key feature is a relay hydrogen atom transfer from [BH₄]⁻ to Cr generating the active catalysts under mild conditions. This enables epoxide reductions, regiodivergent epoxide opening and radical cyclizations that are not possible with cooperative catalysis with radicals or by epoxide reductions via Meinwald rearrangement and ensuing carbonyl reduction. No typical S_N2-type reactivity of [BH₄]⁻ salts is observed.     

Hydrolysis of epoxides and aziridines catalyzed by polymer-supported quarternary ammonium bisulfate

Macroporous resin （D201）-supported quartemary ammonium bisulfate （D201-HSO4） was prepared and effectively used in catalyzing the hydrolysis of epoxides or aziridines under mild and non-metal conditions to give the corresponding 1,2-diols and β-amino alcohols in high yields. The catalyst was facilely prepared and recyclable.     

苯环上氯取代位对四苯基金属卟啉催化烯烃环氧化性能的影响

以苯乙烯、环己烯和反式二苯乙烯为烯烃底物,以双氧水、叔丁基过氧化氢和异丙苯过氧化氢为氧化剂,以苯环上对位和邻位氯取代的四苯基金属卟啉为仿生催化剂,对烯烃的催化环氧化反应进行了对比研究.讨论了不同氯取代位的四苯基金属卟啉对烯烃环氧化性能的影响.实验结果表明,在没有助催化剂存在下,邻位氯代的四(2,6-二氯苯基)铁(锰)卟啉对烯烃的环氧化具有优异的催化性能,烯烃底物的转化率和环氧选择性都比对位氯代的四苯基铁(锰)卟啉高,且反应条件温和.其中FeⅢ(TDCPP)Cl的催化性能最好,环氧化选择性最高,催化氧化苯乙烯时,环氧苯乙烷的选择性达到了90.4%.相同金属离子不同配体的金属卟啉传递氧原子的能力为TDCPP>T(p-Cl)PP>TPP.氧化剂的结构对环氧化物的选择性有较大影响.过氧键连有吸电子基团的异丙苯过氧化氢对环氧化物的选择性最高.根据实验结果,对金属卟啉催化环氧化机理进行了分析.     

Potassium Acetate/18-Crown-6 Pair: Robust and Versatile Catalyst for Synthesis of Polyols from Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides†

Efficient synthesis of polyester polyols with tunable molecular weight and microstructures from cyclic anhydride/epoxide mixtures by taking advantage of chain transfer reaction remains a great challenge, because most of the catalysts exhibit poor tolerance to chain transfer agent (CTA). In this contribution, we demonstrated that potassium acetate (KOAc) and 18-crown-6 (18-C-6) combination has great potential in the synthesis of diverse polyester polyols with controllable molecular weight and high-end group fidelity. Compared with KOAc, KOAc/18-C-6 pair could induce a much faster chain transfer between the active and dormant chains, and thus produce polyester polyols with narrow and monomodal distribution. In addition, polyester polyols could be efficiently prepared in laboratory by using commercially available cyclic anhydride without further purification (containing about 2% diacid residual as CTA) with an extremely low catalyst loading ([catalyst pair] : [anhydride] : [epoxide] = 1 : 50000 : 250000, [catalyst pair] = 0.0004 mol%). KOAc/18-C-6 could also promote the self-switchable copolymerization of cyclic anhydride/epoxide/cyclic ester mixtures. Ring- opening copolymerization of cyclic ester was initiated automatically after the full conversion of cyclic anhydride, finally producing polyester polyols with ABA-type block structure.      

One-Carbon Homologation of α,β-Unsaturated Aldehydes: Access to α-Tertiary β,γ-Unsaturated Aldehydes

An efficient protocol for the synthesis of enolizable α-substituted β,γ-unsaturated aldehydes is reported. The developed strategy involves two steps, epoxidation and Meinwald rearrangement, to result in a one-carbon homologation of α,β-unsaturated aldehydes enabling the insertion of a CHR unit.     

A Nuclearity-Dependent Enantiodivergent Epoxide Opening via Enthalpy-Controlled Mononuclear and Entropy-Controlled Dinuclear (Salen)Titanium Catalysis

A nuclearity-dependent enantiodivergent epoxide opening reaction has been developed, in which both antipodes of chiral alcohol products are selectively accessed by mononuclear (salen)Ti^III complex and its self-assembled oxygen-bridged dinuclear counterparts within the same stereogenic ligand scaffold. Kinetic studies based on the Eyring equation revealed an enthalpy-controlled enantio-differentiation mode in mononuclear catalysis, whereas an entropy-controlled one in dinuclear catalysis. DFT calculations outline the origin of the enantiocontrol of the mononuclear catalysis and indicate the actual catalyst species in the dinuclear catalytic system. The mechanistic insights may shed a light on a strategy for stereoswichable asymmetric catalysis utilizing nuclearity-distinct transition-metal complexes.     

Exposed Zinc Sites on Hybrid ZnIn2S4@CdS Nanocages for Efficient Regioselective Photocatalytic Epoxide Alcoholysis

Photocatalytic epoxide alcoholysis through C−O bond cleavage and formation has emerged as an alternative to synthesizing anti-tumoral pharmaceuticals and fine chemicals. However, the lack of crucial evidence to interpret the interaction between reactants and photocatalyst surface makes it challenging for photocatalytic epoxide alcoholysis with both high activity and regioselectivity. In this work, we report the hierarchical ZnIn₂S₄@CdS photocatalyst for epoxide alcoholysis with high regioselectivity nearly 100 %. Mechanistic studies unveil that the precise activation switch on exposed Zn acid sites for C−O bond polarization and cleavage has a critical significance for achieving efficient photocatalytic performance. Furthermore, the establishment of Z-scheme heterojunction facilitates the interface charge separation and transfer. Remarkably, the underlying regioselective photocatalytic reaction pathway has been distinctly revealed.