Conformational and electronic consequences in crafting extended, π-conjugated, light-harvesting macrocycles

The synthesis of a new series of free‐base, Ni^II and Zn^II 2,3,12,13‐tetra(ethynyl)‐5,10,15,20‐tetraphenyl porphyrins is described. Upon heating, two of the four ethynyl moieties undergo Bergman cyclization to afford the monocyclized 2,3‐diethynyl‐5,20‐diphenylpiceno[10,11,12,13,14,15‐jklmn]porphyrin in 30 %, 10 %, and trace yields, respectively. The structures of all products were investigated by using quantum chemical calculations and the free‐base analogue was isolated and crystallized; all compounds show significant deviation from the idealized planar structure. No fully‐cyclized bispiceno[20,1,2,3,4,5,10,11,12,13,14,15‐fghij]porphyrin was isolated from the reaction mixture. To understand why only two of the four enthynyl groups undergo Bergman cyclization, the reaction coordinates were examined by using DFT at the PWPW91/cc‐pVTZ(‐f) level coupled to a continuum solvation model. The barrier to cyclization of the second pair of ethynyl groups was found to be 5.5 kcal mol^?1 higher than the first, suggesting a negative cooperative effect and significantly slower rate for the second cyclization. Cyclization reactions for model porphyrin–enediynes with ethene‐ and H‐functionality substitutions at the meso‐phenyl rings were also examined, and found to have a similar barrier to diradical formation for the second cyclization event as for the first in these highly planar molecules. By enforcing an artificial 30° cant in two of the pyrrole rings of the porphyrin, the second barrier was increased by 2 kcal mol^?1 in the ethene model system; this suggests that the disruption of the π conjugation of the extended porphyrin structure is the cause of the increased barrier to the second cyclization event.     

Experimental and theoretical characterization of the aromatization, epimerization, and fragmentation reactions of bi-2H-azirin-2-yls prepared from 1,4-diazidobuta-1,3-dienes

1,4‐Diazidobuta‐1,3‐dienes (Z,Z)‐ 10 , 17 , and 21 were photolyzed and thermolyzed to yield the pyridazines 13 , 20 , and 23 , respectively. To explain these aromatic final products, the generation of highly strained bi‐2H‐azirin‐2‐yls 12 , 19 , and 22 and their valence isomerization were postulated. In the case of meso‐ and rac‐ 22 , nearly quantitative formation from diazide 21 , isolation as stable solids, and complete characterization were possible. On the thermolysis of 22 , aromatization to 23 was only a side reaction, whereas equilibration of meso‐ and rac‐ 22 and fragmentation, which led to alkyne 24 and acetonitrile, dominated. Prolonged irradiation of 22 gave mainly the pyrimidine 25 . The change of the configuration at C‐2 of the 2H‐azirine unit was observed not only in the case of bi‐2H‐azirin‐2‐yls 22 but also for simple spirocyclic 2H‐azirines 29 at a relatively low temperature (75 °C). The fragmentation of rac‐ 22 to give alkyne 24 and two molecules of acetonitrile was also studied by high‐level quantum chemical calculations. For a related model system 30 (methyl instead of phenyl groups), two transition states TS‐ 30 – 31 of comparable energy with multiconfigurational electronic states could be localized on the energy hypersurface for this one‐step conversion. The symmetrical transition state complies with the definition of a coarctate mechanism.     

An efficient, overall [4+1] cycloaddition of 1,3-dienes and nitrene precursors

Intermolecular cycloadditions of conjugated dienes and nitrene precursors usually produce aziridines. A generally useful method was lacking to directly provide the [4+1] cycloadducts, 3-pyrrolines. We have realized this transformation by using an uniquely active catalyst, copper(II) 1,1,1,5,5,5-hexafluoroacetylacetonate ([Cu(hfacac)(2)]). The method is applicable to a wide array of dienes with good yields. When 1,4-disubstituted dienes are used as substrates, good-to-excellent cis or trans selectivity can be obtained. Interestingly, the cis or trans preference depends on the nature of the substituents, rather than diene geometry. Mechanistic studies reveal that the [4+1] cycloaddition proceeds through diene aziridination and subsequent ring expansion. Among common copper catalysts, only [Cu(hfacac)(2)] can efficiently catalyze both steps, which explains the unique efficiency of the catalyst.     

The reactivity of a silacyclopentadienylidene towards aldehydes: silole ring expansion and the formation of base-stabilized silacyclohexadienones

Tossing aldehydes into the ring: The reaction of a silacyclopentadienylidene with aldehydes leads to C=O bond cleavage with the formation of base-stabilized silanones and cyclopropanation of the adjacent C=C bond, followed by silole ring expansion to give silicon analogues of cyclohexadienones.     

The route to a feasible hydrogen-storage material: MOFs versus ammonia borane

The replacement of fossil fuels is one of the greatest challenges that chemistry and material sciences will have to face in the near future. While hydrogen seems to be the most likely candidate for this, a material able to store the hydrogen itself is sorely needed. Intense research in the past decade has narrowed down the field of possible concepts to two materials: ammonia borane with chemically bound hydrogen atoms and metal-organic frameworks with physisorbed hydrogen molecules. Herein we want to give an overview of the strengths and weaknesses of each concept, discuss the challenges that need to be overcome, and try to compare the future capabilities of these two materials.     

Lewis acid/base catalyzed [2+2]-cycloaddition of sulfenes and aldehydes: a versatile entry to chiral sulfonyl and sulfinyl derivatives

The first catalytic asymmetric synthesis of β-sultones is reported. This development has enabled a rapid access to a number of highly enantioenriched biologically interesting sulfonyl and sulfinyl compound classes, which makes use of the inherent ring strain of the four-membered heterocycles. The products possess either two vicinal stereocenters, such as in β-hydroxy-sulfonamides, -sulfonates, -sulfones, -sulfonic acids, -sulfinic acids, γ-sultines, and γ-sultones or a single stereocenter, such as in α-branched alkyl or allyl sulfonic acids. This work also represents the first application of sulfene intermediates in asymmetric catalysis. The reactivity of a sulfene normally acting as an electrophile could be reverted by the formation of a nucleophilic zwitterionic sulfene-amine adduct. To achieve a combination of high enantioselectivity and reactivity, cooperative catalytic action of a chiral nucleophilic tertiary amine (the cinchona alkaloid derivative diydroquinine 2,5-diphenyl-4,6-pyrimidinediyl diether ((DHQ)(2)PYR)) and Bi(OTf)(3) or In(OTf)(3) was of primary importance.     

Novel and efficient copper-catalysed synthesis of nitrogen-linked medium-ring biaryls

Herein, a new copper-catalysed strategy for the synthesis of rare nitrogen-linked seven-, eight- and nine-membered biaryl ring systems is described. It is proposed that the reaction proceeds through a highly activated intramolecularly co-ordinated copper catalyst. The process is technically simple, proceeds under relatively mild conditions, displays a broad substrate scope and forms biologically valuable products that are difficult to synthesise by other methods. We envisage that this methodology will prove useful in a wide synthetic context, with possible applications in both target-oriented and diversity-oriented synthesis.     

热处理对La0.63Gd0.2Mg0.17Ni3.1Co0.3Al0.1储氢合金微观组织和电化学性能的影响

在氩气气氛和1173 K保温条件下对La0.63 Gd0.2 Mg0.17Ni3.1 Co0.3 Al0.1储氢合金进行不同时间(t=8 ～168 h)的热处理,采用电感耦合等离子发射光谱(ICP)、X射线衍射(XRD)、电子探针显微分析方法(EPMA)和电化学测试分析方法对比研究了退火时间对合金显微组织演化和电化学性能的影响.研究结果表明,铸态合金组织由Ce2 Ni7型、Gd2Co7型、Pr5 Co19型、PuNi3型和CaCu5型相组成,其Ce2 Ni7型相的丰度为78.9％,随退火时间的延长,退火合金中Ce2 Ni7型相的丰度逐渐增加,当退火时间t=168 h时其相丰度达到94.5％,Ce2 Ni7型相结构的晶胞参数和晶胞体积随退火时间增加而减小.电化学测试分析表明,退火合金电极的电化学性能与Ce2 Ni7型相的丰度有密切关系,退火时间对合金电极的活化性能影响不大,但合金电极放电容量随退火时间的延长逐渐提高,当t=168 h时,合金电极放电容量达到最大值386.8mAh·g-1;退火时间对合金电极循环稳定性的提高和改善有不同程度的影响,当退火时间t=16～168 h时,经100次充放电循环后,其电极容量保持率S100=90.3％～91.5％.热处理能有效改善合金电极电化学反应的动力学性能,但不同退火时间对合金电极的高倍率放电性能影响不明显.     

热处理对La4MgNi19储氢电极合金结构和性能的影响

以感应熔炼和不同的热处理工艺制备了La₄MgNi₁₉合金, 用X射线衍射(XRD)和电化学测试系统研究了该合金的相结构和电化学性能. 结构分析表明: 当热处理工艺为900 °C+水淬时, 合金主要由CaCu₅结构的LaNi₅相和少量未知相组成; 当热处理工艺为900 °C退火时, 合金主要由Pr₅Co₁₉、Ce₅Co₉结构的(La, Mg)₅Ni₁₉相及少量CaCu₅结构的LaNi₅相组成. 淬火和退火后合金的电化学循环稳定性(S₁₀₀)分别为49.7%及76.0%, 合金电极的电化学性能和相结构密切相关. 退火热处理有利于生成Pr₅Co₁₉、Ce₅Co₉型相. 在La-Mg-Ni 系储氢合金中, La₄MgNi₁₉合金电化学循环稳定性不及La₃MgNi₁₄合金.