In Silico Investigation of Palladium-Catalyzed Chemoselective Monoalkoxycarbonylation of 1,3-diynes for Conjugated Enynes Synthesis.

The palladium-catalyzed monoalkoxycarbonylation of 1,3-diynes provides a chemoselective method for the construction of synthetically useful conjugated enynes. Here, in silico unraveling the detailed mechanism of this reaction and the origin of chemoselectivity were conducted. It is shown that the alkoxycarbonylation reaction preferably proceeds by a NH-Pd pathway, which including three substeps: hydropalladation, CO migratory insertion and methanolysis. The effectiveness of the NH-Pd catalytic system is attributed to the alkynyl-palladium π-back-bonding interaction, C-H⋅⋅⋅π interaction in reactant moiety and d-pπ conjugation between the Pd center and alkenyl group. The hydropalladation step was identified as the rate- and chemoselectivity-determining step, and the first alkoxycarbonylation requires a much lower energy barrier in comparison with the second alkoxycarbonylation, in line with the experimental outcomes that the monoalkoxycarbonylation product was obtained in high yield. Distortion-interaction analysis indicates the more favorable monoalkoxycarbonylation (compared to double alkoxycarbonylation) is caused by steric effect.

Bifunctional Ligand Enables Gold-Catalyzed Propargyl C-H Functionalization via Reactive Gold Allenylidene Intermediate.

Gold allenylidene species have been seldom exploited as reactive intermediates in synthetically versatile catalytic reactions. By employing alkynylbenziodoxoles as the substrates and bifunctional WangPhos as the metal ligand, this work demonstrated ready catalytic access to these intermediates of general substitution patterns and their electrophilic reactivities at the γ-carbon center with a diverse range of nucleophiles. The reaction is driven by the reductive decomposition of the benziodoxole moiety and achieves the replacement of a propargylic proton with an N/O/C-based nucleophile, hence realizing reactivity umpolung. Corroborated by Density Functional Theory (DFT) calculations, the reaction mechanism involves a mild propargylic deprotonation. In contrast to prior works employing a tertiary amine functionality, a weakly BrØnsted-basic amide group in WangPhos is surprisingly effective in deprotonation at the propargylic position under a gold-ligand cooperation regime.

Palladium-Catalyzed Cascade Cyclization for the Synthesis of Fused Benzo-Aza-Oxa-[5-6-5] Tetracycles.

A novel and expedient cascade strategy has been demonstrated for the synthesis of fused benzo-aza-oxa-[5-6-5] tetracycles in high yields and diastereoselectivities (up to 20 : 1 dr). The strategy was fulfilled through palladium-catalyzed oxidative convergent assembly of functionally divergent anilines and 3-butenoic acid with five chemical bonds constructed. Coupled with control experiments and deuterium labelled studies, DFT calculations were performed for the proposed mechanism. The utility of the illustrated strategy is emphasized by gram-scale syntheses, late-stage functionalization, and the transformation to a key core of natural products such as martinellic acid and seneciobipyrrolidine.

Impacts of water-sediment regulation on spatial-temporal variations of heavy metals in riparian sediments along the middle and lower reaches of the Yellow River.

The water-sediment regulation scheme (WSRS) of dams influences the desorption, resuspension, and deposition processes of riparian sediments, which in turn affect the spatial-temporal variations of heavy metals (HMs) in riparian sediments and leads to severe degradation of soil and water quality. However, the difference between the trapping effect of dams and the redistribution effects of the WSRS on HMs in riparian sediments, as well as the consecutively seasonal change of HMs after the WSRS, are rarely reported. To fill this gap, the concentrations of six HMs including Cd, Cr, Cu, Ni, Pb, and Zn in riparian sediments along the Xiaolangdi Dam (XLD) Reservoir and its downstream reach were investigated, and the contamination level and potential ecological risk of HMs were assessed, to differentiate the effects of the XLD and its WSRS on the concentration, contamination level, and potential ecological risks of HMs. The results indicated that the mean HM concentrations in riparian sediments were higher than the background values in the study area and showed significant spatial and temporal variations. However, the regional differences of HM concentrations caused by the trapping effect of the XLD were less than the seasonal differences caused by the redistribution effects of the WSRS. The contamination and ecological risk assessment indicated that riparian sediments in the study area were contaminated by the six HMs, particularly by Cd and Pb, which overall exhibited a high and moderate ecological risk, respectively. The sources for Pb were likely agricultural inputs, while the sources for Cd should be attributed to both industrial and agricultural inputs. Overall, the trapping effect of the XLD led to the accumulation of HMs in riparian sediments along the reservoir area, while the regulation effects of the WSRS resulted in the redistribution of HMs in riparian sediments from the reservoir area to the downstream reach.

Theoretical Elucidation of the Mechanism and Kinetic Experimental Phenomena on the Esterification of α-Tocopherol with Succinic Anhydride: Catalysis of a Histidine Derivative vs an Imidazolium-Based Ionic Liquid.

DFT calculations have been conducted to gain insight into the mechanism and kinetics of the esterification of α-tocopherol with succinic anhydride catalyzed by a histidine derivative or an imidazolium-based ionic liquid (IL). The two catalytic reactions involve an intrinsically consistent molecular mechanism: a rate-determining, concerted nucleophilic substitution followed by a facile proton-transfer process. The histidine derivative or the IL anion is shown to play a decisive role, acting as a Brönsted base by abstracting the hydroxyl proton of α-tocopherol to favor the nucleophilic substitution of the hydroxyl oxygen of α-tocopherol on succinic anhydride. The calculated free energy barriers of two reactions (15.8 kcal/mol for the histamine-catalyzed reaction and 22.9 kcal/mol for the IL-catalyzed reaction) together with their respective characteristic features, the catalytic reaction with a catalytic amount of histamine vs the catalytic reaction with an excessed amount of the IL, rationalize well the experimentally observed kinetics that the former has faster initial rate but longer reaction time while the latter is initiated slowly but completed in a much shorter time.

Effects of road and river networks on sediment connectivity in mountainous watersheds.

Road and river networks in mountainous watersheds play an important role in transporting eroded sediments. However, the underlying transport mechanisms remain poorly understood, particularly in terms of how alterations to flow paths caused by road and river networks influence sediment connectivity. Therefore, using data from the Dongshuanghe (DSH) watershed in the Dabie Mountain in Central China, this study investigated the spatial relationships between road and river networks, analyzed the effects of road and river networks on the spatial distribution of sediment connectivity, and determined the primary factors influencing sediment connectivity. The primary factors were identified using the index of connectivity (IC), buffer analysis, a random forest (RF) model, and a geographical detector model (GDM). The results indicated that road and river networks were spatially closely associated: closer to rivers, the density of high-grade roads increased, while farther from road-river crossings, the length and density of rivers and high-grade roads decreased. Both road and river networks affected the IC. In particular, for permanent drainage lines or local sinks, the average value of ICR (i.e., IC affected by road networks, -0.97) was higher than that of IC (-2.17). Thus, values of ICR decreased substantially with increasing distance to the closest roads (R2 = 0.73). However, beyond a threshold of 150 m, the effect of roads on ICR gradually diminished. In addition, the structural characteristics of road networks, particularly slope (Road_S), had greater explanatory power for spatial variation in the ICR. In conclusion, compared to the river networks, the effect of road networks on ICR was more important, which was not only reflected in the spatial distribution of ICR, but also in the factors influencing ICR.

Palladium-catalysed selective oxidative amination of olefins with Lewis basic amines.

Amines are prominent in natural products, pharmaceutical agents and agrochemicals. Moreover, they are synthetically valuable building blocks for the construction of complex organic molecules and functional materials. However, amines, especially aliphatic and aromatic amines with free N-H bonds, tend to coordinate with transition metals and deactivate the catalyst, posing a tremendous challenge to applying Lewis basic amines in the amination of olefins. Here we present an example of oxidative amination of simple olefins with various Lewis basic amines. The combination of a palladium catalyst, 2,6-dimethyl-1,4-benzoquinone and a phosphorous ligand leads to the efficient synthesis of alkyl and aryl allylamines. A series of allylamines were obtained with good yields and excellent regio- and stereoselectivities. Intramolecular amination to synthesize tetrahydropyrrole and piperidine derivatives was also realized. Mechanistic investigations reveal that the reaction undergoes allylic C(sp3)-H activation and subsequent functionalization.

Hypervalent Iodine-Mediated Diastereoselective α-Acetoxylation of Cyclic Ketones.

A binary hybrid system comprising a hypervalent iodine(III) reagent and BF3•OEt2 Lewis acid was found to be effective for the diastereoselective α-acetoxylation of cyclic ketones. In this hybrid system, BF3•OEt2 Lewis acid allowed the activation of the hypervalent iodine(III) reagent and cyclic ketones for smooth α-acetoxylation reaction, achieving high diastereoselectivity. This hypervalent iodine-mediated α-acetoxylation of the cyclic ketone reaction plausibly undergoes an SN2 substitution mechanism via an α-C-bound hypervalent iodine intermediate. The diastereoselectivity of the reaction mainly originates from thermodynamic control.

Theoretical Insight into the Conversion Mechanism of Glucose to Fructose Catalyzed by CrCl2 in Imidazolium Chlorine Ionic Liquids.

To better understand the efficient transformation of glucose to fructose catalyzed by chromium chlorides in imidazolium-based ionic liquids (ILs), density functional theory calculations have been carried out on a model system which describes the catalytic reaction by CrCl2 in 1,3-dimethylimidazolium chlorine (MMImCl) ionic liquid (IL). The reaction is shown to involve three fundamental processes: ring opening, 1,2-H migration, and ring closure. The reaction is calculated to exergonic by 3.8 kcal/mol with an overall barrier of 37.1 kcal/mol. Throughout all elementary steps, both CrCl2 and MMImCl are found to play substantial roles. The Cr center, as a Lewis acid, coordinates to two hydroxyl group oxygen atoms of glucose to bidentally rivet the substrate, and the imidazolium cation plays a dual role of proton shuttle and H-bond donor due to its intrinsic acidic property, while the Cl- anion is identified as a Bronsted/Lewis base and also a H-bond acceptor. Our present calculations emphasize that in the rate-determining step the 1,2-H migration concertedly occurs with the deprotonation of O2-H hydroxyl group, which is in nature different from the stepwise mechanism proposed in the early literature. The present results provide a molecule-level understanding for the isomerization mechanism of glucose to fructose catalyzed by chromium chlorides in imidazolium chlorine ILs.