TEMPO催化醇氧化反应进展

选择性催化氧化醇类化合物为相应的醛或酮是一类重要的官能团转化反应.四甲基哌啶氧化物(TEMPO)是一种含有稳定的氮氧自由基(NO·)的有机小分子催化剂,NO·可通过自身的强选择性,在加快醛或酮转化的同时不会过氧化成为羧酸.本文阐述了TEMPO催化体系催化醇选择性氧化反应的机理,在此基础上详述了过渡金属/TEMPO、非过...     

化学氧化预处理对MFC性能的影响

以漂白硫酸盐竹浆为原料,结合PFI磨的前期磨浆顶处理和高压均质机的后期高压均质化处理制备微纤化纤维素(MFC).探讨了由2,2,6,6-四甲基哌啶氧化物自由基(TEMPO)、NaCIO和NaBr组成的氧化体系的主要工艺参数(TEMPO、NaBr及NaOH用量)对竹浆MFC主要性能(如相对保水值)的影响,确定了最佳的化学预处理工艺.基于氧化后浆料的黏度与最终所得MFC的相对保水值之间的关系以及在优化氧化条件下NaOH用量与氧化后浆料黏度的关系,建立了预测MFC相对保水值的数学方程.     

Practical TEMPO‐Mediated Oxidation of Alcohols using Different Polymer‐Bound Co‐Oxidants

Hypochlorite and chlorite exchange resins are evaluated as co‐oxidants or oxidants, respectively, for the oxidation of alcohols to the corresponding aldehydes, ketones or carboxylic acids. Polymer‐bound hypochlorite can act as a co‐oxidant in TEMPO‐mediated oxidations of alcohols. The formation of aldehydes only works under weakly acidic conditions. However, the cheap hypochlorite exchange resin is less efficient as co‐oxidant compared to the use of ionically immobilised bisacetoxybromate(I) anions. In contrast, the chlorite exchange resin is a highly potent co‐oxidant for the preparation of carboxylic acids from the corresponding primary alcohols in the presence of TEMPO. It is demonstrated that in this case also the resin acts as a polymer‐bound co‐oxidant for both oxidation steps. Yields are commonly excellent as is also demonstrated for examples taken from natural product synthesis which include acid labile glycosides. In most cases, work‐up of this heavy metal‐free oxidation is kept to a minimum. It often includes filtration followed by removal of the solvent.     

Sol‐Gel Ormosils Doped with TEMPO as Recyclable Catalysts for the Selective Oxidation of Alcohols

Organically modified silicas doped with TEMPO prepared via the sol‐gel method are highly recyclable catalysts of the selective Montanari‐Anelli oxidation of 1‐nonanol; They show a notable ”positive feedback” effect of matrix alkylation on the catalyst activity which is typical of doped sol‐gel materials and markedly differentiates the behaviour of these materials from that of analogous silica‐supported TEMPO.     

Aryl Ketone-Catalyzed Light-Driven Radical Oxy-Functionalization of Non-Acidic C(sp3)−H Bonds Enabled by Triple Role of TEMPO

A light-driven radical oxy-functionalization of non-acidic aliphatic and benzylic C(sp³)−H bonds was achieved with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in a single-step by employing an aryl ketone as a sole catalyst. The transformation was initiated by homolysis of a C(sp³)−H bond in the starting substance by the photo-excited aryl ketone. The derived carbon radical was then promptly trapped by TEMPO leading to the formation of the TEMPO-adduct. The derived TEMPO-adduct is synthetically versatile as an alcohol equivalent since it can be readily converted to the corresponding ketone by oxidation and to the corresponding alcohol by reduction. The key to realizing the present catalytic reaction relies on the distinctive triple role of TEMPO, which acts as the precursor of the oxygen functionality, the oxidant to regenerate the aryl ketone, and the protective group for the derived TEMPO-adduct from over-oxidation.     

纳米银在TEMPO催化氧化黄麻纤维上的微波原位合成

金属纳米粒子一纤维素复合材料是一种具有生物相容性和抗菌性的可再生纳米材料,由于其应用功能广泛而备受关注。本文首次研究了不使用还原剂条件下’采用微波原位合成法在丁EMPO（2,2,6,6一四甲基哌啶氧化物自由基）选择性氧化黄麻纤维上简便、快速制备银纳米粒子。微波加热5min,纤维上的银纳米粒子平均尺寸为50．0±2．0hm,而加热10min则为90．0±4．7nm。该多功能黄麻一银纳米粒子复合物具有优异的热稳定性和高结晶度。     

Post‐polymerization modification by nitroxide radical coupling

Post‐polymerization modification is an attractive approach to extend applications and convert commodity plastics into products with new, desirable and tunable properties. Among the post‐polymerization modification methods, the nitroxide radical coupling (NRC) reaction has been shown to be a convenient and versatile way to graft specific functionalities onto polymer chains and to control the onset and yield of polymer crosslinking during peroxide‐initiated processes. The use of 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) and its derivatives as controllers of scorch in crosslinking and as functionalizers in functionalization reactions is thoroughly described. Examples are also given of graft polymerization from macroalkoxyamines generated by NRC and grafting of nitroxides by irradiation processes. In addition, in this review we attempt to demonstrate the broad applications of the NRC reaction in the preparation of polymers with a multitude of functionalities and elaborate architectures. The examples discussed here concern the use of atom transfer and single electron transfer NRC reactions to design a variety of polymers with asymmetrical structure and the use of the radical crossover reaction, based on the alkoxyamine dynamic covalent bond, to generate reversible polymer structures and switchable functional polymers. © 2018 Society of Chemical Industry     

有机单体空间构型对共轭微孔聚合物催化性能的影响

以TEMPO自由基功能化2,5-二溴-N-(2,2,6,6-四甲基哌啶)苯甲酰胺为有机单体，与平面三角构型(D3h对称性)的1,3,5-三乙炔苯和四面体构型(Td对称性)的四(4-乙炔基苯)甲烷分别通过Sonogashira-Hagihara偶联反应获得CMP-3-TEMPO和CMP-4-TEMPO。扫描电子显微镜照片(SEM)显示两种TEMPO功能化CMPs形貌上有显著差异，CMP-3-TEMPO为不规则的棒状和块状微米聚合物，CMP-4-TEMPO为尺寸较均一的微米球聚合物。固态电子顺磁共振谱(EPR)证明了CMP-3-TEMPO和CMP-4-TEMPO均含有TEMPO自由基。两种CMPs催化剂均可将5-羟甲基糠醛(5-HMF)选择性催化氧化成2,5-二甲酰基呋喃(2,5-DFF)，但在催化效率和循环利用率方面存在显著差异。有机配体的空间立体构型对CMPs的形貌和催化性能有重要影响，选择具有空间立体构型的有机单体，所合成的CMP-4-TEMPO具有更稳定的结构，催化循环性能更优异。     

Diverse nanocelluloses prepared from TEMPO-oxidized wood cellulose fibers: Nanonetworks,nanofibers, and nanocrystals

When wood cellulose fibers are oxidized with NaClO and catalytic amounts of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and NaBr in water at pH 10, significant amounts of sodium carboxylate groups (≤1.7 mmol/g) are introduced into the oxidized celluloses. The original fibrous morphologies and cellulose I crystal structures are unchanged by oxidation. The TEMPO-oxidized cellulose fibers can be converted to partially fibrillated nanonetworks, completely individualized cellulose nanofibers with high aspect ratios, and needle-like cellulose nanocrystals with low aspect ratios by controlling the conditions of mechanical disintegration in water. It is therefore possible to prepare diverse nanocelluloses with different morphologies and properties from the same TEMPO-oxidized cellulose fibers, for various end uses and applications. All TEMPO-oxidized nanocelluloses contain large amounts of carboxylate groups. These provide scaffolds for versatile surface modification of nanocelluloses by simple ion exchange of sodium for other metal ions and alkylammonium ions.     

Improved preparative electrochemical oxidation of d-glucose to d-glucaric acid

The 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO) mediated electrochemical oxidation of d-glucose to d-glucaric acid on a synthetically useful scale is reported. Using TEMPO and a graphite felt anode combined with a stainless steel cathode, d-glucose was oxidized under different conditions (pH, temperature, co-oxidant), and the reaction outcomes were analyzed. Optimized conditions for such oxidation are provided along with few new interesting results unique to this reaction, such as the appearance of a novel triacid.