Palladacycles in catalysis - a critical survey

The application of palladacycles as catalysts for cross-coupling and similar reactions is reviewed. In the majority of cases palladacycles are likely to serve as a source of highly active but unstable zero-valent palladium species. In this respect the palladacycles resemble the so-called phosphine-free catalysts. The advantages and limitations of palladacycle catalysts are discussed.     

Cyclizations via Palladium-Catalyzed Allylic Alkylations [New Synthetic Methods (79)]

The history of ring systems in organic chemistry parallels their synthetic accessibility. Transition-metal-catalyzed cyclizations offer a new opportunity to create carbo- and heterocyclic compounds with great facility. Among these methods, allylic alkylations catalyzed by palladium have proven unusually productive because of the extraordinary chemo-, regio-, and diastereoselectivity and the continuing possibility for the development of enantioselectivity. The rules for ring closure differ from those for non-transition-metal-catalyzed reactions. A major benefit is the ability to generate medium (eight-, nine-, ten-, and eleven-membered) and large rings in preference to normal (five-, six- and seven-membered) rings. With the appropriate substrate, efficient macrocyclizations are possible under conditions of normal concentrations. A second major benefit derives from the complementary stereochemistry of the metalcatalyzed substitution (net retention of configuration) compared to non-metal-catalyzed reactions (inversion of configuration). Further, the requirement for the substrate to conform to the transition-metal template may impose a stereochemical preference in the intermediate that ultimately translates into the thermodynamically less stable organic product regardless of the stereochemistry of the starting material. While more work has focused on carbocyclic synthesis, the possibilities for heterocyclic synthesis are just beginning to be tapped. In addition to forming heterocycles by C? C bond formation, use of a heteroatom as a nucleophile has already proven effective for oxygen and nitrogen, with other nucleophiles awaiting investigation. New dimensions for cyclization via allylic alkylation arise by generating the requisite π-allylpalladium intermediates by methods other than palladium(0)-initiated allylic ionizations. In addition, metals other than palladium will clearly expand the possibilities, but as yet remain untapped.     

Intramolecular,Stoichiometric (Li,Mg, Zn) and Catalytic (Ni,Pd, Pt) Metallo-Ene Reactions in Organic Synthesis [New Synthetic Methods (75)]

Metallo-ene reactions, hardly recognized until very recently, have experienced a breathtaking development when applied in an intramolecular sense. Efficient regio- and stereoselective magnesium-ene cyclizations have served as a cornerstone for numerous syntheses of structurally diverse natural products (e.g., sesquiterpenes of marine or plant origin, alkaloids, fragrances, insect defense compounds, and a fungitoxin). A brilliant example is the synthesis of the elusive odorant (+)-khusimone which outshines 20 years of work in the field of tricyclovetivane synthesis. Palladium-, platinum-, and nickel-catalyzed versions of the metallo-ene reaction are in a comparatively early stage of exploration, but, nevertheless, reveal intriguing potential. Hence an almost 100% stereospecific C? O→C? ;Pd-→ C? C chirality transfer permits simple and selective, cis- or trans-annelation processes. The mild cyclization conditions are compatible with various functional groups, such as nitrogen moieties, which offer interesting perspectives for the preparation of heterocycles (e.g., alkaloids) difficult to obtain by other methods. Carbon monoxide insertion reactions of the cyclized σ-metal intermediates were shown to afford annelated cyclopentanones and cyclopentenones with concomitant stereocontrolled formation of four carbon–carbon bonds. These and other observations, highlighted in this article, provide a platform for further extensions and applications of this powerful method in organic synthesis.     

Polymeric Catalysts

The present-day position in the field of polymeric catalysts is outlined. The following selected groups of polymeric catalysts are discussed: synthetic hydrolases, immobilized enzymes, phase-transfer catalysts, nucleophilically active bases, polymers with conjugated π-systems, photosensitizers, polymers as carriers for catalytically active metals or ions, and immobilized homogeneous catalysts. Polymeric catalysts have the following valuable properties: insoluble polymeric catalysts are readily separable from reaction solutions and can often be re-used without loss of activity; a hydrophobic matrix protects the organometallic active center from deactivation by oxygen and water; by fixation of finely divided metals on an ion exchanger, multistage reactions may be effected successively in one reactor. Polymeric carriers may influence the catalytic properties; for example, in the case of immobilized enzymes on polyionic carriers the pH of the activity maximum may be shifted.     

Properties of Ce–Zr–La–O nano-system with ruthenium modified surface

Structural peculiarities of Ce–Zr–La–O and Ce–Zr–La–O/Ru samples in mean of catalytic properties are compared. The samples (Ce:Zr = 1:1, La = 10÷30 mol.%, Ru = 1.5 wt.%) were obtained by sol–gel method (X-samples) and co-precipitation (P-samples). It is shown that Ce_0.45Zr_0.45La_0.1O_2−δ/Ru X-samples are characterized by high thermal stability and the highest catalytic activity in partial methane oxidation reaction. According to XRD, BET, FTIR, EPR and XPS data it is concluded that the difference in the samples catalytic activity is caused by various disposition of Ru-containing phase on the support surface. The distinction in the dimension of Ru-containing particles (3D or 2D) is conditioned by structural peculiarities of Ce_0.45Zr_0.45La_0.1O_2−δ and Ce_0.35Zr_0.35La_0.3O_2−δ P- and X-samples.     

N-Allyl-1,3-oxazines via a facile keto-ene/cyclization tandem reaction

The intramolecular keto-ene/cyclization tandem reaction of γ-N-allylamino ketones is an effective means of producing 1,3-oxazines. The reaction usually requires high temperatures and/or pressures. We discovered that N,N-diallyl amines undergo the reaction at lower temperatures than their monoallyl analogs. An extreme example, 1-N,N-diallylamino-9,10-anthraquinone, undergoes the keto-ene reaction near ambient temperature. In the case of 1-N,N′-dialkylaminoanthraquinones, electron deficient ene components can even be used, allowing the preparation of a broad spectrum of oxazines. Furthermore, the N-allyl-1,3-oxazine can be easily deallylated to produce a 1,3-oxazine that contains a secondary amine.     

基于新型电荷生成层的P-i-N结构白色有机叠层电致发光器件

采用2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline(BCP): 5 wt.% cesium carbonate(Cs₂CO₃)和N, N'-Bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine(NPB): 20 wt.% molybdenum oxide(MoO₃)分别作为器件的电子注入层和空穴注入层,研究了N型和P-i-N结构有机电致发光器件的载流子传输特性.载流子传输层中BCP: Cs₂CO₃和NPB:MoO₃的引入有效增强了载流子注入能力,从而降低了器件的驱动电压.基于新型电荷生成层BCP: 5 wt.% Cs₂CO₃/ NPB: 20 wt.% MoO₃制备了色稳定、高效率P-i-N结构有机叠层器件.与单元器件相比,引入新电荷生成层有机叠层器件的最大电流效率增大了2.5倍,表明该电荷生成层可以有效地将电子和空穴分别注入到相邻发光单元中.采用该电荷生成层制备了P-i-N结构白色有机叠层器件,器件的上下发光单元分别为橙光和蓝光发射.当发光亮度从500增加到5 000 cd/m²时,器件的色坐标稳定在(0.33, 0.29)附近,接近白光等能点.利用单色发光单元堆叠制备白色有机叠层器件的方法为实现色稳定、高效率的白色有机电致发光器件提供了一种有效的途径.     

基于B3PyMPM∶Cs高效叠层OLED器件的制备

以B 3 PyMPM∶Cs/Al/HAT-CN作为电荷生成单元制备高效叠层绿色磷光有机电致发光器件,叠层器件的最大电流效率和最大流明效率分别为172.2 cd/A和111.0 lm/W,在5 mA/cm^2电流密度下,叠层器件的电压和亮度分别为传统器件的2.04倍和2.84倍.为了探究叠层器件性能优于传统器件的原因,研究了电荷生成单元内的电荷产生和注入过程,以及薄层铝对电子注入特性和电荷生成单元稳定性的影响.实验结果表明,电荷能够有效地在电荷生成单元内产生并顺利注入电子传输层中,B3PyMPM∶Cs和HAT-CN间Al薄层的插入能够进一步提高电子注入效率及器件结构的稳定性.