Amino acids and peptides with bulky side chains are of significant importance in organic synthesis and modern medicinal chemistry. The efficient synthesis of these molecules with full enantiocontrol ...and high diversity remains challenging. Herein we report a Pd-catalyzed ligand-enabled γ-C(sp
)-H arylation of
-leucine and its derived peptides without using an external directing group (DG)
a less favored six-membered palladacycle. Structurally diverse bulky side chain amino acids and peptides were accessed in a step-economic fashion and the reaction could be conducted on a gram scale with retention of chirality. The resulting amino acids can be used as chiral ligands in Co(iii)-catalyzed enantioselective C(sp
)-H amidation. It is worth noting that the weakly coordinating carboxylate DG outcompetes the strongly coordinating bidentate DG of the peptide backbone, providing the products of γ-C(sp
)-H arylation of Tle residue exclusively. This protocol represents the first example of late stage C(sp
)-H functionalization of peptides using a weakly coordinating directing group.
An unprecedented enantioselective synthesis of spiro‐γ‐lactams via a sequential C−H olefination/asymmetric 4+1 spirocyclization under a simple CoII/chiral spiro phosphoric acid (SPA) binary system is ...reported. A range of biologically important spiro‐γ‐lactams are obtained with high levels of enantioselectivity (up to 98 % ee). The concise, asymmetric synthesis of an aldose reductase inhibitor was successfully achieved. Notably, contrast to previous reports that relied on the use of cyclopentadienyl or its derivatives (achiral Cp*, CptBu, or chiral Cpx) ligated CoIII complexes requiring tedious steps to prepare, cheap and commercially available cobalt(II) acetate tetrahydrate was used as an efficient precatalyst.
An unprecedented sequential C−H olefination/asymmetric 4+1 spirocyclization under a simple CoII/chiral spiro phosphoric acid (SPA) binary system is reported, providing a straightforward route for the asymmetric synthesis of spiro‐γ‐lactams with high enantioselectivities (up to 98 % ee).
Rational utilization of soil resources and remediation of contaminated soils are imperative due to the rapidly growing demand for clean soils. Currently, many in-situ remediation technologies are ...less suitable at low-permeability sites due to the limitations of soil permeability. This work defines a low-permeability site as a site with hydraulic conductivity less than 10−4 cm/s, and summarizes the migration characteristics of representative contaminants at low-permeability sites, and discusses the principles and practical applications of different technologies suitable for the remediation of low-permeability sites, including electrokinetic remediation technology, polymer flushing technology, fracturing technology, and in-situ thermal remediation technology. Enhanced and combined remediation technologies are further described because one remediation technology cannot remediate all contaminants. The prospects for the application of remediation technologies to low-permeability sites are also proposed. This work highlights the necessity of low-permeability sites remediation and the urgent need for new remediation technologies, with the hope to inspire future research on low-permeability sites.
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•Low-permeability sties are defined as contaminated sites with a permeability coefficient less than 10−4 cm/s.•Many remediation techniques are less suitable at low-permeability sites.•Macro- and micro-factors jointly influence the migration of contaminants at low-permeability sites.•New remediation techniques are urgently needed for the remediation of low-permeability sites.
Nanogold catalysis in fine chemical synthesis is examined. Achievements in nanogold catalysis includes the development of a gold-palladium catalytic system.
The site‐selective functionalization of unactivated C(sp3)−H bonds remains one of the greatest challenges in organic synthesis. Herein, we report on the site‐selective δ‐C(sp3)−H alkylation of amino ...acids and peptides with maleimides via a kinetically less favored six‐membered palladacycle in the presence of more accessible γ‐C(sp3)−H bonds. Experimental studies revealed that C−H bond cleavage occurs reversibly and preferentially at γ‐methyl over δ‐methyl C−H bonds while the subsequent alkylation proceeds exclusively at the six‐membered palladacycle that is generated by δ‐C−H activation. The selectivity can be explained by the Curtin–Hammett principle. The exceptional compatibility of this alkylation with various oligopeptides renders this procedure valuable for late‐stage peptide modifications. Notably, this process is also the first palladium(II)‐catalyzed Michael‐type alkylation reaction that proceeds through C(sp3)−H activation.
The site‐selective δ‐C(sp3)−H alkylation of amino acids and peptides with maleimides is enabled by a palladium catalyst. C−H bond cleavage occurs reversibly and preferentially at γ‐methyl over δ‐methyl C−H bonds while the subsequent alkylation proceeds exclusively at the six‐membered palladacycle generated by δ‐C−H activation. The selectivity can be explained by the Curtin–Hammett principle. PA=picolinamide.
Biaryl atropisomers are of great importance in natural products, pharmaceuticals, and asymmteric synthesis. The efficient synthesis of these chiral scaffolds with full enantiocontrol and high ...diversity remains challenging. Reported herein is a Pd‐catalyzed atroposelective C−H allylation with tert‐leucine as an efficient catalytic chiral transient auxiliary. A wide range of enantioenriched biaryl aldehydes were prepared in synthetically useful yields with excellent enantioselectivity (up to >99 % ee) through β‐O elimination. The reaction could be carried out on a gram scale without erosion of the ee value. A variety of axially chiral carboxylic acids could be obtained with high enantiopurity. The resulting axially chiral biaryl aldehydes and carboxylic acids might be used in asymmetric synthesis as chiral ligands and/or organocatalysts.
A Pd‐catalyzed atroposelective C−H allylation with allylic surrogates is reported. tert‐Leucine was identified as an efficient catalytic transient chiral auxiliary. A range of enantioenriched biaryls were prepared in synthetically useful yields with enantioselectivities up to >99 % ee through β‐O elimination. The reaction could be scaled up and the products could be further converted into enantiomerically pure axially chiral carboxylic acids.
Dibenzocyclooctadiene lignans are an interesting class of molecules because of their unique structure based on an axially chiral biaryl moiety as well as their significant biological activity. ...Herein, we describe the development of a palladium‐catalyzed atroposelective C−H alkynylation and its application in gram‐scale, stereocontrolled formal syntheses of (+)‐isoschizandrin and (+)‐steganone. tert‐Leucine was identified as an efficient, catalytic transient chiral auxiliary. A wide range of enantiomerically enriched biaryl compounds were prepared by this approach in good yields (up to 99 %) with excellent enantioselectivity (up to >99 % ee).
Locked into position with a Pd key: A palladium‐catalyzed atroposelective C−H alkynylation was developed and applied to gram‐scale, stereocontrolled formal syntheses of (+)‐isoschizandrin (see scheme) and (+)‐steganone. tert‐Leucine was identified as an efficient catalytic transient chiral auxiliary for this transformation, which enabled the preparation of a wide range of enantiomerically enriched biaryl compounds in good yields.
Esters are valuable commodity chemicals widely found in agrochemicals, pharmaceuticals, and advanced materials. They are also important synthetic building blocks for chemical transformation. Recent ...advances of ester synthesis via transition-metal-catalyzed C–H activation have provided highly efficient and atom-economical alternatives to the traditional methods. Herein, we summarize recent advances on ester synthesis via transition-metal-catalyzed C–H activation. On the basis of the modes of reactivity and the types of C–H bonds, transition-metal-catalyzed alkoxycarbonylation of C–H bonds with various esterification reagents will be discussed in sections and . Finally, hydroesterification/hydroacylation of C–C or C–O double bonds via formate C–H activation will be discussed in sections and .
•Three kinds of nano-particles/polymer systems were proposed to modify SK-70 asphalt.•Influences of nano-particles/polymer on the properties of SK-70 asphalt were tested with TFOT, DSR and BBR ...tests.•Morphologies of modified asphalts and SK-70 asphalt were measured with SEM.•The chemical bondings of modified asphalts and SK-70 asphalt were analyzed through FTIR.
At present, most asphalts and modified asphalts cannot meet requirements of both high temperature stability and low temperature cracking resistance simultaneously. Hence, the objective of this paper is to find new modification system to improve the high and low temperature properties of SK-70 base asphalt. In this paper, nano-zinc oxide (nano-ZnO) particles, nano-titanium dioxide (nano-TiO2) particles, nano-calcium carbonate (nano-CaCO3) particles, styrene-butadiene-styrene (SBS) and styrene-butadiene-rubber (SBR) were selected as modifiers. The modified asphalt samples with three or two kinds of modifiers were prepared, and the softening point, ductility and penetration of modified asphalt samples were measured and compared. Eventually, three optimal nano-particles/polymer modification systems for SK-70 base asphalt including 3%nano-ZnO/0.5%nano-TiO2/3.7%SBS, 5%nano-ZnO/4.2%SBS, 5%nano-CaCO3/4%SBR were proposed. To study the effects of these three optimal modification formulations on high and low temperature properties of base asphalt, dynamic shear rheometer (DSR), thin film oven test (TFOT), and beam bending rheometer (BBR) tests were conducted on the asphalt of SK-70 and the nano-particles/polymer modified asphalts. The micro-morphology of SK-70 and the modified asphalts was examined using scanning electron microscope (SEM) and the reactions between modifiers and SK-70 base asphalt were studied by infrared spectrum instrument. From the test results, it was observed that the nano-materials were well dispersed in base asphalt, and could increase the dispersibility of polymer in base asphalt and improve the compatibility between polymer and base asphalt. Furthermore, the nano-materials could improve the high and low temperature properties of SK-70 base asphalt. Additionally, the results also revealed that for polymer modified asphalts the physical reaction between polymers and asphalt played the dominant role in the modification process, while for nano-particles/polymer modified asphalts, both physical and chemical reaction occurred in the modification process.
Modular steel construction (MSC) comprises off-site manufactured volumetric modules and on-site assembly, leading to reduced construction periods, improved quality, and reduced waste of resources. ...This environmentally friendly solution has been extensively used for low-rise buildings as an alternative to traditional on-site construction. The popularity of MSC has now spread to mid-to-high rise applications in seismic regions to meet the increasing urban construction demand, owing to its significant technical advantages. The influence of earthquake becomes critical as the height of the building increases. Hence, this paper presents a state-of-the-art review of the seismic performance of mid-to-high MSC and articulates the key technical issues. The module classification is presented as a brief introduction of MSC, followed by discussion of the structural system. Afterwards, the seismic performance of the lateral force resisting system and recent innovations on the connection system are reviewed in detail, on which the seismic performance of MSC highly depend. The global seismic response analysis methodology, characteristics, failure mode as well as the current design criteria are evaluated, providing a more comprehensive understanding of the seismic performance of MSC. Finally, the recently developed isolation systems for MSC are introduced. As a currently developing area, there is great potential for innovation in mid-to-high rise MSC. Despite progressively increasing research exploring the seismic performance of MSC, a comprehensive understanding of this topic has not been achieved, hindering the further prevalence of mid-to-high rise MSC in areas with potential seismic hazards. Given this situation, several key research areas are suggested thereupon at last, aiming to promote the further extended application of MSC in seismic regions.
•Module classification and structural form of modular steel construction (MSC).•State-of-the-art review on the seismic performance of mid-to-high rise MSC.•Research progress on the lateral force resisting system and connection system.•Currently available seismic design criteria are summarized and evaluated.•Key research areas are identified for extended seismic application of MSC.
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