EPR studies of peroxide decomposition, radical formation and reactions relevant to cross-linking and grafting in polyolefins

EPR spectroscopy has been employed to detect directly radicals formed from a variety of polyolefins (PE, PP and EPM) during reaction with peroxide-derived alkoxyl radicals generated by thermolysis. Conditions have been chosen to reflect those employed in polyolefin grafting, degradation and cross-linking. Radical assignment is assisted, in particular, by the recognition of the effects of chirality on the β-proton hyperfine splittings. Quantitative analysis provides information on the selectivity of the initial attack (e.g. methine protons>methylene protons for PP); notable differences in selectivity between alternating EPM and other EPM samples are discussed. The detection of longer-lived allyl radicals detected for PE is explained in terms of alkyl radical disproportionation with subsequent reaction of the product alkene.     

Enhanced Reaction Efficiency in Continuous Flow

Continuous flow reactors are enabling tools that can significantly benefit chemical reactions, especially those that are path length dependent (e.g., photochemical), mixing or transport dependent (e.g., gas-liquid), exothermic, or utilize hazardous or unstable intermediates. In this review, it is demonstrated how the nearly instantaneous mixing, exceptionally fast mass transfer, safe access to high temperatures and pressures, and high surface area to volume ratio can be leveraged to improve product yield, reaction rates and/or selectivity. By showcasing five synthetic methodologies examined by our group, it is hoped that the reader will gain an appreciation of the accessible and transformative nature of flow chemistry for improving existing transformations, enabling rapid optimization, and for developing new methodologies that depend on precise parameter controls.     

Controlling site selectivity in palladium-catalyzed C-H bond functionalization

Effective methodology to functionalize C-H bonds requires overcoming the key challenge of differentiating among the multitude of C-H bonds that are present in complex organic molecules. This Account focuses on our work over the past decade toward the development of site-selective Pd-catalyzed C-H functionalization reactions using the following approaches: substrate-based control over selectivity through the use of directing groups (approach 1), substrate control through the use of electronically activated substrates (approach 2), or catalyst-based control (approach 3). In our extensive exploration of the first approach, a number of selectivity trends have emerged for both sp(2) and sp(3) C-H functionalization reactions that hold true for a variety of transformations involving diverse directing groups. Functionalizations tend to occur at the less-hindered sp(2) C-H bond ortho to a directing group, at primary sp(3) C-H bonds that are β to a directing group, and, when multiple directing groups are present, at C-H sites proximal to the most basic directing group. Using approach 2, which exploits electronic biases within a substrate, our group has achieved C-2-selective arylation of indoles and pyrroles using diaryliodonium oxidants. The selectivity of these transformations is altered when the C-2 site of the heterocycle is blocked, leading to C-C bond formation at the C-3 position. While approach 3 (catalyst-based control) is still in its early stages of exploration, we have obtained exciting results demonstrating that site selectivity can be tuned by modifying the structure of the supporting ligands on the Pd catalyst. For example, by modulating the structure of N-N bidentate ligands, we have achieved exquisite levels of selectivity for arylation at the α site of naphthalene. Similarly, we have demonstrated that both the rate and site selectivity of arene acetoxylation depend on the ratio of pyridine (ligand) to Pd. Lastly, by switching the ligand on Pd from an acetate to a carbonate, we have reversed the site selectivity of a 1,3-dimethoxybenzene/benzo[h]quinoline coupling. In combination with a growing number of reports in the literature, these studies highlight a frontier of catalyst-based control of site-selectivity in the development of new C-H bond functionalization methodology.     

Supramolecular Self-Assembled Ligands in Asymmetric Transition Metal Catalysis

The design of novel chiral ligands is at the core of asymmetric catalysis. The catalytic characteristics of a transition metal catalyst such as activity, selectivity and stability can be fine-tuned by optimization of the steric and electronic properties of the coordinating ligands. In asymmetric transformations, catalyst optimization still relies to a large extent on trial-and-error and educated guesses. New strategies based on combinatorial screening and high-throughput experimentation have been introduced for the design and optimization of new ligands and catalytic systems. Supramolecular bidentate ligands that form by self-assembly of building blocks are particularly suited for this combinatorial approach as the potential number of catalysts grows exponentially with the number of building blocks synthesized. Catalytic systems based on supramolecular interactions have proven to be highly advantageous in creating large ligand libraries for high-throughput screening, which allows optimization of activity and selectivity for a variety of reactions. In this review we describe the progress in this field.     

光化学微反应技术的基础及研究进展

光化学转化是有效利用光能实现化学反应的重要途径,微反应技术为提高其过程效率提供了一个强有力的平台。本文首先指出微反应器相比于传统釜式光反应器,在光强分布、过程放大、光能利用效率等诸多方面存在明显的优势,能够实现光化学反应过程的高效强化。简要地介绍了光化学转化及光化学微反应技术的基本特征,然后系统地综述了光化学微反应器的设计构建及其在有机合成、聚合等方面的应用,并详细介绍了自动化控制的光化学微反应系统及应用。重点介绍了微反应技术在紫外光、可见光辐照下的光化学合成进展及其过程放大。最后,对光化学微反应技术的研究进展进行总结,并对其发展趋势进行了展望。     

Chiral monodentate phosphine ligand MOP for transition-metal-catalyzed asymmetric reactions

Chiral monophosphines, whose chirality is due to biaryl axial chirality, have been prepared from enantiomerically pure 2, 2'-dihydroxy-1,1'-binaphthyl and demonstrated to be highly efficient chiral ligands for transition-metal-catalyzed organic transformations, especially for reactions where chelating bisphosphine ligands cannot be used. The high efficiency is observed in palladium-catalyzed asymmetric hydrosilylation of a wide variety of olefins such as alkyl-substituted terminal olefins and in asymmetric reactions via pi-allylpalladium intermediates represented by asymmetric reduction of allylic esters with formic acid.     

Surface Chemistry of Hydrocarbon Fragments on Transition Metals: Towards Understanding Catalytic Processes

The central issue of this review is the importance of selectivity in heterogeneous catalysis. The implications of our mechanistic studies on model systems to the interpretation of catalytic processes are discussed. In particular, it is our thesis that the optimization of catalytic selectivity may require a different approach than that needed to increase overall catalytic activity. This is so because a given intermediate in the reaction mechanism may be slow to form, but able to react by following more than one relatively fast pathway. In the case of hydrocarbon reforming, while activity is often controlled by alkane activation, selectivity is likely to depend on the regioselectivity of the subsequent dehydrogenation of the resulting alkyl surface intermediates. Similar arguments can be used to explain the selectivity between dehydrogenation and dehydration reactions on alcohols. Additional complications arise from the build up of carbonaceous deposits on the surface of the catalyst under reaction conditions. Surface-science studies have led to the conclusion that these deposits modify the properties of the exposed catalytic metal, opening up new channels for mild reactions but still allowing for the occurrence of the more demanding reactions involved in hydrocarbon reforming and oxidation processes. Lastly, catalyst modification can be introduced in a controlled fashion, as in the case of the bestowing of chirality to normal hydrogenation catalysts by the addition of small amounts of chiral cocatalysts to the reaction mixture. A brief survey of our recent investigation of these systems is provided.     

Synthesis and Vibrational Circular Dichroism Analysis of N-Heterocyclic Carbene Precursors Containing Remote Chirality Centers

VCD analysis of 16 diastereomeric pairs of NHC precursors containing two isolated chirality centers and different substitution patterns identified VCD transitions characteristic of the chirality center in the imidazolium ring or in the side chain, which, in contrast to ECD and OR, could be utilized to assign the two chirality centers separately by simple comparison, regardless of the type and position of achiral aromatic substituents. While the ECD and OR data showed great dependence on the position of an achiral substituent such as a methoxy group, characteristic experimental VCD transitions remained consistent and they could be used to determine the absolute configuration of all the regio- and stereoisomers and substituted analogues. VCD, ECD and OR approaches were evaluated, and several carbene precursors were found, for which only the VCD method could distinguish the four stereoisomers. With t-butyl, phenyl or 2-naphthyl substituents at the C-1′ chirality center, the ECD spectra of the C-1′ epimers were near-identical, and hence it was only the VCD approach that showed distinct differences suitable for the configurational assignment. The chiroptical characterization of our diastereomeric pairs of NHC precursors enables the future application of related derivatives having different substitution patterns in stereoselective transformations.     

Surface-enhanced Raman study of electrochemical and photocatalytic degradation of the azo dye Janus Green B

The photocatalytic degradation of Janus Green B azo dye over silver modified titanium dioxide films was investigated by surface-enhanced Raman spectroscopy (SERS). An optimized SERS-active substrate was employed to study the photodegradation reaction of Janus Green B. Considering that photocatalytic degradation processes of organic molecules adsorbed on TiO₂ might involve either their oxidation or reduction reaction, the vibrational spectroelectrochemical study of the dye was also performed, in order to clarify the transformations involved in initial steps of its photochemical decomposition. In order to understand the changes in Raman spectra of Janus Green B after photodegradation and/or electrochemical processes, a vibrational assignment of the main Raman active modes of the dye was carried out, based on a detailed resonance Raman profile. Products formed by electrochemical and photochemical degradation processes were compared. The obtained results revealed that the first steps of the degradation process of Janus Green B involve a reductive mechanism.     

Photochemical Production of Vitamin D2, Scale‐up and Optimization

An effective low cost method for the photochemical production of vitamin D₂ has been studied, including a 200 L batch photoreactor and a continuous reactor. Following optimization of the irradiation time (550 min), initial concentration of ergosterol (1.4 mg/mL) and the number of lamps employed (16) for the batch reactor, the conversion of ergosterol is ca. 50 %, and the selectivity yield of pre‐vitamin D₂ is ca. 84 %. The corresponding data for the continuous reactor are 73 % and 62 %, respectively.     

Selective growth of single-walled carbon nanotubes over Co–MgO catalyst by chemical vapor deposition of methane

The selective synthesis of SWCNTs with narrow chirality and diameter distribution by methane decomposition over a Co–MgO catalyst is reported. Raman spectroscopy, temperature programmed oxidation (TPO), UV–Vis–NIR absorption spectroscopy, and nitrogen physisorption were used to probe SWCNTs morphology, reaction selectivity, SWCNTs chirality and diameter distribution, and carbon yield. The catalyst was examined by nitrogen physisorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), and UV–Vis-diffuse reflectance spectroscopy to elucidate the structure and chemical state of the species responsible for SWCNT growth. The results established a clear link between the degree of dispersion of Co species inside the MgO lattice and the catalyst activity and selectivity for SWCNT growth. High dispersion and stabilization of Co species influenced catalytic activity for methane decomposition and the high SWCNT selectivity. The yield of carbon and SWCNT selectivity increased with an increase in temperature, however, SWCNTs diameter distribution shifts to larger diameter tubes as synthesis temperature was increased.     

Nitrogen Nucleophiles in Au-Catalyzed Dehydrative Cyclization Reactions

Mild conditions for the gold-catalyzed dehydrative cyclization of carbamate-protected azaallylic alcohols to form saturated nitrogen heterocycles are reported. The cyclization reactions are high-yielding, operationally easy to perform, and provide heterocycles with a synthetically useful vinyl group, strategically located on the ring system, which can facilitate further transformations for target oriented synthesis. It is also demonstrated through chirality transfer experiments that the mechanism can be either cationic in nature or a Au-catalyzed addition/elimination sequence. The diverging mechanistic scenario is dependent on the nature of the substituents on the allylic alcohol and necessitates judicious substrate design.     

Selective synthesis of single-walled carbon nanotubes on Fe–MgO catalyst by chemical vapor deposition of methane

The selective synthesis of single-walled carbon nanotubes (SWCNTs) with narrow chirality and diameter distribution by methane decomposition over Fe–MgO catalyst is reported. The catalyst was examined by nitrogen physisorption, X-ray diffraction, temperature programmed reduction, X-ray photoelectron spectroscopy, and UV–Vis diffuse reflectance spectroscopy to elucidate the structure and chemical state of the species responsible for SWCNT growth. High resolution electron microscopy, Raman and optical absorption spectroscopy, temperature programmed oxidation, energy dispersive X-ray spectroscopy and nitrogen physisorption were used to probe reaction selectivity, SWCNT chirality and diameter distribution, carbon yield and effectiveness of purification protocols. The yield of carbon increased with an increase in temperature, although SWCNTs selectivity decreased above the optimum synthesis temperature. Results established a clear link between the degree of dispersion of iron oxide species inside the MgO lattice and the catalyst selectivity for SWCNT growth.     

Deliberately Losing Control of C−H Activation Processes in the Design of Small-Molecule-Fragment Arrays Targeting Peroxisomal Metabolism

Combined photochemical arylation, “nuisance effect” (S_NAr) reaction sequences have been employed in the design of small arrays for immediate deployment in medium-throughput X-ray protein–ligand structure determination. Reactions were deliberately allowed to run “out of control” in terms of selectivity; for example the ortho-arylation of 2-phenylpyridine gave five products resulting from mono- and bisarylations combined with S_NAr processes. As a result, a number of crystallographic hits against NUDT7, a key peroxisomal CoA ester hydrolase, have been identified.     

光化学反应器

光化学反应过程由于具有选择性好且可在常温常压下进行等特点而在施工领域有着良好的应用前景，光化学反应器作为光化学生产过程的核心设备，在光化学工艺的应用中具有十分重要的地位。本文从工程应用的角度出发，介绍了常见的光化学反应器的类型，总结了描述光化学反应器行为的模型，并讨论了有关光化学反应器的设计计算及放大等问题。     

Biocatalysis Fueled by Light: On the Versatile Combination of Photocatalysis and Enzymes

Enzymes catalyze a plethora of highly specific transformations under mild and environmentally benign reaction conditions. Their fascinating performances attest to high synthetic potential that is often hampered by operational obstacles such as in vitro cofactor supply and regeneration. Exploiting light and combining it with biocatalysis not only helps in overcoming these drawbacks, but the fruitful liaison of these two fields of “green chemistry” also offers opportunities to unlock new synthetic reactivities. In this review we provide an overview of the wide variety of photo-biocatalysis, ranging from the photochemical delivery of electrons required in redox biocatalysis and photochemical cofactor and reagent (re)generation to direct photoactivation of enzymes enabling reactions unknown in nature. We highlight synthetically relevant transformations such as asymmetric reactions facilitated by the combination of light as energy source and enzymes’ catalytic power.     

A New Type of Stereoselectivity in Baeyer–Villiger Reactions: Access to E‐ and Z‐Olefins

A new concept for accessing configurationally defined trisubstituted olefins has been developed. Starting from a common ketone precursor of the type 4‐ethylidenecyclohexanone, Baeyer–Villiger monooxygenases are employed as catalysts in diastereoselective Baeyer–Villiger reactions leading to the corresponding E‐ or Z‐configurated lactones. Wild‐type cyclohexanone monooxygenase (CHMO) as catalyst delivers the E‐isomers and a directed evolution mutant the opposite Z‐isomers. Subsequent transition metal‐catalyzed chemical transformations of a key product containing a vinyl bromide moiety provide a variety of different trisubstituted E‐ or Z‐olefins. A model based on QM/MM sheds light on the origin of this unusual type of diastereoselectivity. In contrast to this biocatalytic approach, traditional Baeyer–Villiger reagents such as m‐CPBA fail to show any selectivity, 1:1 mixtures of E‐ and Z‐olefins being formed.     

分子印迹在化学传感器及阵列领域的研究进展

分子印迹聚合物因其具有高选择性、预定识别性、制备简单、价格低廉等优点,成为当前研究的热点之一。本文对分子印迹聚合物在化学传感器及传感器阵列领域的应用进展进行了综述。作为选择性化学传感器,重点探讨了分子印迹聚合物在电化学传感器、光化学传感器和质量敏感型传感器中的应用。相对于分子印迹传感器,基于分子印迹聚合物交叉敏感性原理的分子印迹聚合物传感器阵列的应用较少,仅对有限例子的光化学传感器阵列进行了探讨。受益于印迹机理的深入理解,分子印迹化学传感器及其阵列将在食品分析、药物控制、环境监测等领域表现出较好的应用前景。     

On the Centenary of Emanuel Gil-Av,Former Professor of the Weizmann Institute of Science and Pioneer of Enantioselective Chromatography

Emanuel Gil-Av (Zimkin) was born in 1916 and passed away in 1996. The introduction of chemical selectivity into the chromatographic separation process constitutes his main scientific contribution. This approach culminated in the first enantiomeric separation of racemic amino acids by gas chromatography on an optically active chiral stationary phase in 1966, together with Binyamin Feibush and Rosita Charles-Sigler. Thus the year 2016 marks an important date to remember and to appreciate the invaluable achievements of Gil-Av in the realm of chirality carried out at the Weizmann Institute of Science, Israel.     

The Chirality of Aggregated Yariv Reagents Correlates with Their AGP-Binding Ability**

Yariv reagents are glycosylated triphenylazo dyes that bind to arabinogalactan proteins (AGPs), proteoglycans found in plant cell walls that are integral for plant growth and development. Yariv reagents are widely utilized as imaging, purification, and quantification tools for AGPs and represent the only small molecule probe for interrogating AGP function. The ability of Yariv reagents to bind to AGPs is dependent on the structure of the terminal glycoside on the dye. The reason for this selectivity has not been understood until the present work. Using circular dichroism spectroscopy, we show that the Yariv reagents form supramolecular aggregates with helical chirality. More significantly, the ability of the Yariv reagent to bind AGPs is correlated with this helical chirality. This finding paves the way towards developing a more detailed understanding of the nature of the Yariv-AGP complex, and the design of AGP-binding reagents with higher affinities and selectivities.