Complete Conversion of Racemic Enol Ester Epoxides into Optically Active α-Acyloxy Ketones

Complete Conversion of Racemic Enol Ester Epoxides into Optically Active α-Acyloxy Ketone

Enantioselective Allylation of Ketones Catalyzed by <i>N</i>,<i>N</i>‘-Dioxide and Indium(III) Complex

Complexes of (S)-pipecolic acid-, l-proline-, and other amino acid-derived N,N‘-dioxides coordinated with different metal ions have been investigated in the enantioselective allylation of ketones. A variety of aromatic ketones were found to be suitable substrates in the presence of the L1-In(III) complex, and afforded the corresponding homoallylic alcohols with good enantioselectivites (up to 83% ee) and moderate to high yields (up to 94%). On the basis of the experimental results, a possible catalytic cycle including a transition state has been proposed to explain the origin of the reactivity and asymmetric inductivity, and a bifunctional catalyst was described with Lewis base N-oxide activating tetraallyltin and Lewis acid indium activating ketone

Highly Enantio- and Diastereoselective Brassard Type Hetero-Diels−Alder Approach to 5-Methyl-Containing α,β-Unsaturated δ-Lactones

Two efficient new chiral copper (II) Schiff base complexes were developed for the highly enantio- and diastereoselective HDA reaction of Brassard type diene 1b with aldehydes, to afford the corresponding 5-methyl-containing α,β-unsaturated δ-lactone derivatives in moderate yields, high enantioselectivities (up to 99% ee) and excellent diastereoselectivities (up to 99:1 anti/syn). On the basis of the absolute configuration of 4a−4j disclosed by X-ray diffraction and CD analysis, a possible transition-state model for the enantio- and diastereoselective catalytic reaction has been proposed

Asymmetric Carbonyl-Ene Reaction Catalyzed by Chiral <i>N,N′</i>-Dioxide-Nickel(II) Complex: Remarkably Broad Substrate Scope

Asymmetric Carbonyl-Ene Reaction Catalyzed by Chiral N,N′-Dioxide-Nickel(II) Complex: Remarkably Broad Substrate Scop

Highly Enantioselective Allylation of α-Ketoesters Catalyzed by <i>N</i>,<i>N</i>‘-Dioxide−In(III) Complexes

An efficient asymmetric allylation of α-ketoesters was catalyzed by the N,N‘-dioxide−In(III) complex in excellent yields (up to 99%) and high enantioselectivities (up to 94% ee) for a variety of substrates under mild reaction conditions. On the basis of experimental results, a possible catalytic cycle including a transition state has been proposed to explain the origin of the reactivity and asymmetric inductivity, and a bifunctional catalysis was described with Lewis base N-oxide activating tetraallylstannane and Lewis acid indium activating α-ketoester

Asymmetric Three-Component Radical Alkene Carboazidation by Direct Activation of Aliphatic C–H Bonds

Azide compounds are widely present in natural products and drug molecules, and their easy-to-transform characteristics make them widely used in the field of organic synthesis. The merging of transition-metal catalysis with radical chemistry offers a versatile platform for radical carboazidation of alkenes, allowing the rapid assembly of highly functionalized organic azides. However, the direct use of readily available hydrocarbon feedstocks as sp3-hybridized carbon radical precursors to participate in catalytic enantioselective carboazidation of alkenes remains a significant challenge that has yet to be addressed. Herein, we describe an iron-catalyzed asymmetric three-component radical carboazidation of electron-deficient alkenes by direct activation of aliphatic C–H bonds. This approach involves intermolecular hydrogen atom transfer between a hydrocarbon and an alkoxy/aryl carboxyl radical, leading to the formation of a carbon-centered radical. The resulting radical then reacts with electron-deficient alkenes to generate a new radical species that undergoes chiral iron-complex-mediated C–N3 bond coupling. An array of valuable chiral azides bearing a quaternary stereocenter were directly accessed from widely available chemical feedstocks, and their synthetic potential is further demonstrated through more facile transformations to give other valuable enantioenriched building blocks

Indium(III)-Catalyzed Asymmetric Hetero-Diels–Alder Reaction of Brassard-Type Diene with Aliphatic Aldehydes

A highly diastereo- and enantioselective hetero-Diels–Alder (HDA) reaction of a Brassard-type diene with aliphatic aldehydes has been developed. The chiral N,N′-dioxide L2/In(OTf)3 complex was efficient toward the obtention of the corresponding β-methoxy-γ-methyl α,β-unsaturated δ-lactones in good yields (up to 86%) as well as dr and ee values (up to 97:3 cis/trans and 94% ee). In addition, the product 4a could be easily transformed into the methyl-protected epi-prelactone B by hydrogenation

Experimental Investigations of the Turbulent Boundary Layer for Biomimetic Protrusive Surfaces Inspired by Pufferfish Skin: Effects of Spinal Density and Diameter

Pufferfish is known for its extension of tiny spine-covered skin that appears to increase skin drag and may act as turbulisors, reducing overall drag while serving a protective function. Therefore, the present study addresses a neglected aspect of how spines affect the turbulent boundary layer (TBL) for drag reduction in the pufferfish skin. Particle image velocimetry (PIV) was utilized to investigate the TBL structure on the biomimetic spine-covered protrusion samples inspired by the back skin of the pufferfish. The comparison samples of two sparse “k-type” arrangements (hexagon and staggered) for three types of rough element sizes with roughness heights k+ = 5.5–6.5 (nearly hydraulically smooth) and smooth case in bulk Reynolds numbers (Reb = 37,129 and 44,554) were tested. The results of turbulence statistics of these samples indicate that both the sample (type hexagon) for large rough density (λ = 0.0215) with small roughness elements and the sample (type staggered) for small rough density (λ = 0.0148) with large roughness elements have a drag reduction rate of 5–11%. These two kinds of bionic surfaces have a similar morphology to that seen in the distribution of pufferfish spines and probably serve a similar hydrodynamic function. Vortex identification shows that the spines in the front section for large density with small rough elements stabilize the TBL and generate many small-scale vortices and the dense spines with large rough elements at the back section have the effect of separating the vortices. The retrograde vortex generated by them is beneficial to increasing the driving force of the pufferfish. In addition, these two rough surfaces may effectively delay the separation of the TBL. These results will provide a preliminary research foundation for the development of a more practical prototype of the bionic drag-reducing surfaces and strengthen the theoretical investigation concerning drag reduction exploration

Coupled Bionic Drag-Reducing Surface Covered by Conical Protrusions and Elastic Layer Inspired from Pufferfish Skin

Inspired by the drag-reducing properties of the cone-like spines and elastic layer covering the pufferfish skin, important efforts are underway to establish rational multiple drag-reducing strategies for the development of new marine engineering materials. In the present work, a new drag-reducing surface (CPES) covered by conical protrusions (sparse “k-type” with rough height k+ = 13–15) and an elastic layer are constructed on copper substrate via a hybrid method, combining the sintering and coating processes. The drag-reducing feature of the prepared CPES biomimetic surface is achieved by rheometer and particle image velocimetry (PIV) experiments. To comprehensively investigate its drag reduction mechanism, the porous copper substrate (PCS), copper substrate (CS), conical protrusion resin substrate (CPRS), and conical protrusion porous copper substrate (CPPCS) were used for a comparative analysis. In laminar flow, we discovered that the conical protrusion structure and wettability of the elastic surface coupling affect the CPES sample’s drag-reducing performance (7–8%) and that the interface produced slip to reduce the viscous drag. In turbulent flow, the CPES biomimetic surface exhibits an 11.5–17.5% drag-reducing performance. Such behavior was enabled by two concurrent mechanisms: (i) The conical protrusions as vortex generators enhance the number of vortices and the wake effect, enabling faster movement of downstream strips, reducing viscous drag; (ii) The conical protrusion elements break and lift large-scale vortices to produce numerous small-scale vortices with low energy, effectively weakening perturbations and momentum exchange. Additionally, the elastic layer shows high adhesion and stability on copper substrate after sandpaper abrasion and water-flow erosion tests. The copper substrate surface formed by the sintering method is also covered with dense porous structures, which gives the elastic layer and conical protrusions excellent combined robustness. Our findings not only shed new light on the design of robust drag-reducing surfaces but also provide new avenues for underwater drag reduction in the field of marine applications

Indium(III)-Catalyzed Asymmetric Hetero-Diels–Alder Reaction of Brassard-Type Diene with Aliphatic Aldehydes

A highly diastereo- and enantioselective hetero-Diels–Alder (HDA) reaction of a Brassard-type diene with aliphatic aldehydes has been developed. The chiral N,N′-dioxide L2/In(OTf)3 complex was efficient toward the obtention of the corresponding β-methoxy-γ-methyl α,β-unsaturated δ-lactones in good yields (up to 86%) as well as dr and ee values (up to 97:3 cis/trans and 94% ee). In addition, the product 4a could be easily transformed into the methyl-protected epi-prelactone B by hydrogenation