Enantioselective Organocopper-Catalyzed Hetero Diels-Alder Reaction through in Situ Oxidation of Ethers into Enol Ethers

Recycling Polyester and Polycarbonate Plastics with Carbocation Lewis Acidic Organocatalysts

The effective management of plastic waste is critical for environmental sustainability. This work explores the use of carbocation catalysts for the recycling of common polyesters and polycarbonates through alcoholysis. We demonstrate complete depolymerization of end-of-life materials and investigate the relationship between the catalytic reactivity and the structural features of the carbocation compounds, including the cations and their counteranions. Carbocations function as Lewis acids, facilitating the interaction with carbonyls in polymer chains. Moreover, our approach enables the hierarchical degradation of the polyester blends. This research not only elucidates the catalytic role of carbocations in the alcoholysis of these polymers, but also establishes a metal-free process for the efficient recycling of waste plastics.

Double Hydride Transfer Enabled Substitution of All Hydrogens at α,β-Positions of Cyclic Amines: Access to α,β-Unsaturated Lactams

A one-shot substitution of all hydrogens at the α,β-positions of saturated cyclic amines was achieved. The key feature of this reaction is the sequential involvement of intra- and intermolecular redox processes. When N,O-acetals obtained through an internal redox process were treated with a catalytic amount of Zn(OTf)2 and an excess amount of benzylidene barbiturates, three key transformations involving intermolecular redox process occurred successively to afford α,β-unsaturated lactams in moderate to good chemical yields.

Knoevenagel-IMHDA and -IMSDA sequences for the synthesis of chiral condensed O,N-, S,N- and N-heterocycles

Domino Knoevenagel-cyclization reactions of styrene substrates, containing an N-(ortho-formyl)aryl subunit, were carried out with N-substituted 2-cyanoacetamides to prepare tetrahydro-4H-pyrano[3,4-c]quinolone and hexahydrobenzo[j]phenanthridine derivatives by competing IMHDA and IMSDA cyclization, respectively. The diastereoselective IMHDA step with α,β-unsaturated amide, thioamide, ester and ketone subunits as a heterodiene produced condensed chiral tetrahydropyran or thiopyran derivatives, which in the case of Meldrum's acid were reacted further with amine nucleophiles in a multistep domino sequence. In order to simplify the benzene-condensed tricyclic core of the targets and get access to hexahydro-1H-pyrano[3,4-c]pyridine derivatives, a truncated substrate was reacted with cyclic and acyclic active methylene reagents in diastereoselective Knoevenagel-IMHDA reactions to prepare novel condensed heterocyclic scaffolds. The chemo-, regio- and diastereoselectivity of the cyclization step were investigated and structural elucidation was aided by single crystal X-ray analysis.

Oxoammonium-Catalyzed Ether Oxidation via Hydride Abstraction: Methodology Development and Mechanistic Investigation Using Paramagnetic Relaxation Enhancement NMR

Hydride abstraction represents a promising yet underexplored approach in the functionalization of C-H bonds. In this work, we report the oxidation of α-C-H bonds of ethers via oxoammonium catalysis using 3-chloroperbenzoic acid (mCPBA) as the terminal chemical oxidant or by means of electrochemistry. Mechanistic studies revealed intricate equilibria and interconversion events between various catalytic intermediates in the presence of mCPBA, which alone however was incompetent to drive catalytic turnover. The addition of a small amount of strong acid HNTf2 or weakly coordinating salt NaSbF6 turned on catalytic turnover and promoted ether oxidation with excellent efficiency. NMR experiments leveraging paramagnetic relaxation enhancement effect allowed for quantification of open-shell catalytic intermediates in real time during the reaction course, which aided the identification of catalyst resting states and elucidation of reaction mechanisms.

Asymmetric Double Oxidative [3 + 2] Cycloaddition for the Synthesis of CF3-Containing Spiro[pyrrolidin-3,2'-oxindole]

An asymmetric double oxidative [3 + 2] cycloaddition is reported. Oxidation of 3-((2,2,2-trifluoroethyl)amino)indolin-2-ones and β-aryl-substituted aldehydes simultaneously and subsequent asymmetric cycloaddition in the presence of the chiral amino catalyst generated highly functionalized chiral CF₃-containing spiro[pyrrolidin-3,2'-oxindole] with four contiguous stereocenters stereoselectively, which is characterized by directly constructing two C-C bonds from four C(sp³)-H bonds. This new method features mild conditions, broad substrate scope, and excellent functional group compatibility.

Synthetic applications of hydride abstraction reactions by organic oxidants

Carbon-hydrogen bond functionalizations provide an attractive method for streamlining organic synthesis, and many strategies have been developed for conducting these transformations. Hydride-abstracting reactions have emerged as extremely effective methods for oxidative bond-forming processes due to their mild reaction conditions and high chemoselectivity. This review will predominantly focus on the mechanism, reaction development, natural product synthesis applications, approaches to catalysis, and use in enantioselective processes for hydride abstractions by quinone, oxoammonium ion, and carbocation oxidants. These are the most commonly employed hydride-abstracting agents, but recent efforts illustrate the potential for weaker ketone and triaryl borane oxidants, which will be covered at the end of the review.