Polymer-Supported Hantzsch 1,4-Dihydropyridine Ester: An Efficient Biomimetic Hydrogen Source

Organocatalytic Reduction of Aromatic Nitro Compounds: The Use of Solid-Supported Phenyl(2-quinolyl)methanol

The reduction of aromatic nitro compounds has been performed employing a catalytic amount of Wang resin-supported phenyl(2-quinolyl)methanol (Wang-PQM) in the presence of an excess of NaBH₄ to regenerate the reactive reducing species at the end of the process. The reduction products are easily isolated through a simple filtration/extraction protocol, and the catalyst can be efficiently recovered and recycled. The condensation route is generally preferred, and azo- and/or hydrazo-arenes can be easily prepared in high yields.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

Facile fabrication of highly controllable gating systems based on the combination of inverse opal structure and dynamic covalent chemistry

A three-dimensional (3D) inverse opal with periodic and porous structures has shown great potential for applications not only in optics and optoelectronics, but also in functional membranes. In this work, the benzaldehyde group was initially introduced into a 3D nanoporous inverse opal, serving as a platform for fabricating functional membranes. By employing the dynamic covalent approach, a highly controllable gating system was facilely fabricated to achieve modulable and reversible transport features. It was found that the physical/chemical properties and pore size of the gating system could easily be regulated through post-modification with amines. As a demonstration, the gated nanopores were modified with three kinds of amines to control the wettability, surface charge and nanopore size which in turn was exploited to achieve selective mass transport, including hydrophobic molecules, cations and anions, and the transport with respect to the physical steric hindrance. In particular, the gating system showed extraordinary reversibility and could recover to its pristine state by simply changing pH values. Due to the unlimited variety provided by the Schiff base reaction, the inverse opal described here exhibits a significant extendibility and could be easily post-modified with stimuli-responsive molecules for special purposes. Furthermore, this work can be extended to employ other dynamic covalent routes, for example Diels-Alder, ester exchange and disulfide exchange-based routes.

Aza-Wittig Rearrangements of N-Benzyl and N-Allyl Glycine Methyl Esters. Discovery of a Surprising Cascade Aza-Wittig Rearrangement/Hydroboration Reaction

Treatment of N-(arylmethyl)-N-aryl or N-allyl-N-aryl glycine methyl ester derivatives with (n)Bu2BOTf and (i)Pr2NEt effects an aza-Wittig rearrangement that provides N-aryl phenylalanine methyl ester derivatives and N-aryl allylglycine methyl ester derivatives, respectively, in good yield with moderate to good diastereoselectivity. Under similar conditions, analogous substrates bearing N-carbonyl groups are converted to 1,4,2-oxazaborole derivatives. Additionally, N-allyl-N-aryl glycine methyl ester derivatives subjected to similar conditions at elevated temperatures undergo an aza-[2,3]-Wittig rearrangement, followed by a subsequent hydroboration oxidation reaction, to afford substituted amino alcohol products.

Domino Hydrogenation-Reductive Amination of Phenols, a Simple Process To Access Substituted Cyclohexylamines

Phenols can be efficiently reduced by sodium formate and Pd/C as the catalyst in water and in the presence of amines to give the corresponding cyclohexylamines. This reaction works at rt for 12 h or at 60 °C under microwave dielectric heating for 20 min. With the exception of aniline, primary, secondary amines, amino alcohols, and even amino acids can be used as nucleophiles. The reductive process is based on a sustainable hydrogen source and a catalyst that can be efficiently recovered and reused. The protocol was developed into a continuous-flow production of cyclohexylamines in gram scale achieving very efficient preliminary results (TON 32.7 and TOF 5.45 h(-1)).

One-pot pseudo three-component reaction of nitroketene-N,S-acetals and aldehydes for synthesis of highly functionalized hexa-substituted 1,4-dihydropyridines

Pseudo three-component condensation of aliphatic/aromatic/α,β-unsaturated aldehydes and nitroketene-N,S-acetals to afford diversely functionalized hexa-substituted 1,4-dihydropyridines under 2-aminopyridine catalysis was achieved.

Multifunctional organic polymeric catalysts and reagents

A series of polystyrenes bearing multiple different functional groups has been synthesized, and these materials have been used as catalysts and reagents in a variety of organic reactions. Polymers functionalized with various combinations of amine, phenol, phosphine, and thiourea groups have been prepared in both non-cross-linked (soluble) and cross-linked (insoluble) formats. Reactions catalyzed by these polymers include Morita–Baylis–Hillman (MBH), alkyne to 1,3-diene isomerization, and decarboxylative Doebner–Knoevenagel reactions. Furthermore, Wittig and tandem Wittig/reduction reactions have been performed using heterogeneous polymeric reagents possessing a combination of amine and phosphine groups.

Magnetic nano-Fe3O4-supported 1-benzyl-1,4-dihydronicotinamide (BNAH): synthesis and application in the catalytic reduction of α,β-epoxy ketones

A novel magnetically recoverable organic hydride compound was successfully constructed by using silica-coated magnetic nanoparticles as a support. An as-prepared magnetic organic hydride compound, BNAH (1-benzyl-1,4-dihydronicotinamide), showed efficient activity in the catalytic reduction of α,β-epoxy ketones. After reaction, the magnetic nanoparticle-supported BNAH can be separated by simple magnetic separation which made the separation of the product easier.

Transfer hydrogenation with Hantzsch esters and related organic hydride donors

In recent years, Hantzsch esters and their related organic hydride donors have been widely utilized in biomimetic approaches of asymmetric transfer hydrogenation (ATH) reactions. Various compounds containing C=C, C=N and C=O unsaturated functionalities could be reduced in the presence of organocatalysts or transition metal complexes, affording versatile chiral building blocks in high yields and excellent enantioselectivities under mild conditions. In this critical review, recent advances in this area are summarized and classified according to unsaturated functional groups being reduced and catalytic systems employed (91 references).