Unconventional mechanism and selectivity of the Pd-catalyzed C–H bond lactonization in aromatic carboxylic acid

The search for more effective and highly selective C–H bond oxidation of accessible hydrocarbons and biomolecules is a greatly attractive research mission. The elucidating of mechanism and controlling factors will, undoubtedly, help to broaden scope of these synthetic protocols, and enable discovery of more efficient, environmentally benign, and highly practical new C–H oxidation reactions. Here, we reveal the stepwise intramolecular SN2 nucleophilic substitution mechanism with the rate-limiting C–O bond formation step for the Pd(II)-catalyzed C(sp3)–H lactonization in aromatic 2,6-dimethylbenzoic acid. We show that for this reaction, the direct C–O reductive elimination from both Pd(II) and Pd(IV) (oxidized by O2 oxidant) intermediates is unfavorable. Critical factors controlling the outcome of this reaction are the presence of the η3-(π-benzylic)–Pd and K+–O(carboxylic) interactions. The controlling factors of the benzylic vs ortho site-selectivity of this reaction are the: (a) difference in the strains of the generated lactone rings; (b) difference in the strengths of the η3-(π-benzylic)–Pd and η2-(π-phenyl)–Pd interactions, and (c) more pronounced electrostatic interaction between the nucleophilic oxygen and K+ cation in the ortho-C–H activation transition state. The presented data indicate the utmost importance of base, substrate, and ligand in the selective C(sp3)–H bond lactonization in the presence of C(sp2)–H.

Advances in [1,2]-Sigmatropic Rearrangements of Onium Ylides via Carbene Transfer Reactions

This review article summarizes progress made in [1,2]-sigmatropic rearrangements using carbenes in the ylide formation step. While other rearrangements, such as the [2,3]-sigmatropic, Doyle-Kirmse, or Sommelet-Hauser rearrangements, have been studied in detail over the past decades, studies on [1,2]-sigmatropic rearrangements are still limited. Based on the application of diazoalkanes as carbene precursors, research on diazoalkanes in ylide formation reactions started flourishing in the 1990s. This review covers milestones from the beginning of [1,2]-sigmatropic rearrangements using carbenes to generate ammonium, oxonium and other ylide species and should serve as an overview to further promote research in this area.

Influence of Chitosan-Based Carbon Dots on Astaxanthin Production of Green Alga Tetraselmis sp.

CDs are carbon fluorescent nanomaterials that have gained significant attention in recent years owing to their unique properties. In this work, we utilized a simple solution to produce CDs with func-tionalized amino groups via a one-step carbonization from a chitosan precursor. This simultaneous approach does not use special reagent for either the formation step or the amino-functionalization step of CDs. The as-prepared amino-functionalized CDs that possesses expected characteristics, such as nano-size distribution, monodispersible, high blue light emission, high absolute quantum yield of 5.52%, and functionalized amino groups on the surface. Furthermore, this work demonstrated the low cytotoxicity and high biocompatibility of the CDs, through the improvements in the astaxanthin production of alga Tetraselmis sp. (more than doubled (up to 0.044 mg/L), relative to the control). Thus, as-prepared CDs have promising properties not only for applications in bioimaging, drug delivery or sensors, but also as promoter in algal biorefinery

Catalytic enantioselective C(sp3)–H functionalization involving radical intermediates

Recently, with the boosted development of radical chemistry, enantioselective functionalization of C(sp3)–H bonds via a radical pathway has witnessed a renaissance. In principle, two distinct catalytic modes, distinguished by the steps in which the stereochemistry is determined (the radical formation step or the radical functionalization step), can be devised. This Perspective discusses the state-of-the-art in the area of catalytic enantioselective C(sp3)–H functionalization involving radical intermediates as well as future challenges and opportunities.

Topography of the Free Energy Landscape on the Claisen-Schmidt Condensation: Solvent and Temperature Effect in the Rate-Controlling Step

Recent studies have assigned hydroxide elimination and C=C bond formation step in base-promoted aldol condensation the role of having a strong influence in the overall rate reaction, in contrast to...

Periodic Nanowire Arrays with Alternating Compositions and Structures Fabricated using a Simultaneous Nanowire Formation Step

Arrays of alternating metallic nanostructures present hybrid properties, which are useful for applications such as photonics and catalysis. Block copolymer films provide versatile templates for fabricating periodic arrays of nanowires....

Chiral phosphoric acid-catalyzed stereodivergent synthesis of trisubstituted allenes and computational mechanistic studies

Chiral molecules with multiple stereocenters are widely present in natural products and pharmaceuticals, whose absolute and relative configurations are both critically important for their physiological activities. In spite of the fact that a series of ingenious strategies have been developed for asymmetric diastereodivergent catalysis, most of these methods are limited to the divergent construction of point chirality. Here we report an enantioselective and diastereodivergent synthesis of trisubstituted allenes by asymmetric additions of oxazolones to activated 1,3-enynes enabled by chiral phosphoric acid (CPA) catalysis, where the divergence of the allenic axial stereogenicity is realized by modifications of CPA catalysts. Density functional theory (DFT) calculations are performed to elucidate the origin of diastereodivergence by the stacking- and stagger-form in the transition state (TS) of allene formation step, as well as to disclose a Münchnone-type activation mode of oxazolones under Brønsted acid catalysis.

Tracking Hydroplasticization by DSC: Movement of Water Domains Bound to Poly(Meth)Acrylates during Latex Film Formation

The film formation step of latexes constitutes one of the challenges of these environmentally friendly waterborne polymers, as the high glass transition (TG) polymers needed to produce hard films to be used as coatings will not produce coherent films at low temperature. This issue has been dealt by the use of temporary plasticizers added with the objective to reduce the TG of the polymers during film formation, while being released to the atmosphere afterwards. The main problem of these temporary plasticizers is their volatile organic nature, which is not recommended for the environment. Therefore, different strategies have been proposed to overcome their massive use. One of them is the use of hydroplasticization, as water, abundant in latexes, can effectively act as plasticizer for certain types of polymers. In this work, the effect of three different grafted hydroplasticizers has been checked in a (meth)acrylate copolymer, concluding that itaconic acid showed the best performance as seen by its low minimum film-formation temperature, just slightly modified water resistance and better mechanical properties of the films containing itaconic acid. Furthermore, film formation monitoring has been carried out by Differential Scanning Calorimety (DSC), showing that itaconic acid is able to retain more strongly the water molecules during the water losing process, improving its hydroplasticization capacity.

Experimental study of sediment supply control on step formation, evolution, and stability

We present results from an experimental campaign conducted in a steep flume subject to longitudinal width variations and different sediment feed rates. The experiments were designed to study how sediment supply influences step formation, step location, and step stability. Our results show that steps are more likely to form in segments of the channel where the width narrows because of particle jamming, and these steps are also more stable. Sediment feed increases particle activity which generates a dynamic channel morphology with steps forming and collapsing. A comparison with experiments without sediment feed shows that sediment supply does not inhibit step formation. Time series of step formation, evolution, and destruction show that the maximum number of steps is achieved when the sediment feed is larger than zero but smaller than the transport capacity. We summarize this outcome in a conceptual model where the dependence of step frequency on sediment supply is expressed by a bell curve. Sediment yield measured at the channel outlet followed the sediment feed at the inlet closely, even when we fed 50 % more and 50 % less than the calculated transport capacity. This outcome challenges the applicability of the concept of transport capacity to steep channels and highlights the key role played by sediment feed in dictating sediment yield and channel response. Finally, we detected a positive correlation between sediment concentration and step destruction, which stresses the importance of particle interactions for step formation and stability.

THE STUDY OF LEVEL FORMATION AND DISCLOSURE OF NORMAL CRACKS IN REINFORCED CONCRETE ELEMENTS

This article is devoted to the study of the main patterns of normal cracks multilevel formation in reinforced concrete elements and structures. A critical analysis of the latest publications related to the theme of these studies is carried out. The classification is carried out and the main shortcomings of the known works on the four identified areas of research are identified. The most important elements of the method of reinforced concrete beams experimental research are developed and briefly described. The main characteristics of the prototypes, the conditions for their manufacture and storage are given. The most important results of testing reinforced concrete beams, concerning the main parameters and characteristics of level formation and crack disclosure are highlighted. It was confirmed that the process of normal cracks formation is indeed multilevel and is accompanied by a gradual disruption of the bond between reinforcement and concrete. According to the results of experimental studies, it has been established that in the real design of bending reinforced concrete elements, when calculating their crack resistance, it is possible to restrict oneself to only 2 levels of normal cracking. The calculation of the step and normal cracks opening width in beams research was carried out according to the real multilevel and traditional "one-level" schemes of their formation. These calculations were carried out according to the author's general and simplified methods, according to a method that uses a linear function of the average adhesion stresses of reinforcement with concrete, according to the methods of current domestic and European standards and building rules. Their statistical comparison with the experimental studies results confirmed the effectiveness of those calculation methods in which the crack formation step is directly related to the basic laws and parameters of reinforcement to concrete adhesion. The results of this comparison showed that the author's general method, based on the use of a nonlinear function of the reinforcement with concrete average adhesion stresses, turned out to be the most accurate. Keywords: reinforced concrete, elements, adhesion, reinforcement, cracks, levels of formation, step, width.