A transition metal free expedient approach for the C C bond cleavage of arylidene Meldrum's acid and malononitrile derivatives

Donor-Acceptor Covalent Organic Frameworks as a Heterogeneous Photoredox Catalyst for Scissoring Alkenes to Carbonyl Constituents

The conversion of alkenes to carbonyl constituents via the cleavage of the C═C bond is unique due to its biological and pharmacological significance. Though a number of oxidative C═C cleavage protocols have been demonstrated for terminal and electron-rich alkene systems, none of them were optimized for electron-deficient and conjugated alkenes. In this work, a covalent organic framework containing triphenylamine and triazine units was revealed to cleave the C═C bond of alkenes under very mild conditions involving visible light irradiation due to its photoredox property. The alkenes can be conveniently broken across the double bond to their constituent carbonyl derivatives on light irradiation in the presence of air and the covalent organic framework photocatalyst. This protocol is applicable for a wide range of alkenes in an aqueous acetonitrile medium with high functional group tolerance and regioselectivity. Though the electron-deficient alkenes required tetramethylethylene diamine as a sacrificial donor, the electron-rich alkenes do not demand any additives.

Donor-Acceptor Cyclopropanes: Activation Enabled by a Single, Vinylogous Acceptor

A novel class of highly activated donor-acceptor cyclopropanes bearing only a single, vinylogous acceptor is presented. These strained moieties readily undergo cycloadditions with aldehydes, ketones, thioketones, nitriles, naphth-2-ols and various other substrates to yield the corresponding carbo- and heterocycles. Diastereocontrol can be achieved through the choice of catalyst (Brønsted or Lewis acid). The formation of tetrahydrofurans was shown to be highly enantiospecific when chiral cyclopropanes are employed. A series of mechanistic and kinetic experiments was conducted to elucidate a plausible catalytic cycle and to rationalize the stereochemical outcome.

Selected applications of Meldrum's acid - a tutorial

Due to its unique structure and the vast array of substituents that can be attached to its core, Meldrum's acid is a molecule with exceptional chemical properties. In water, it has a remarkably low pKa value of about 4.9. Its C5 position is readily involved in electrophilic substitution reactions whereas the C4 and C6 positions are easily attacked by nucleophiles. At elevated temperatures Meldrum's acid undergoes distinctive decomposition pathways, which can be used in cycloaddition and acylation reactions. In this Tutorial Review, the authors intend to introduce the principles of the synthetic chemistry of Meldrum's acid and provide the essential knowledge for the design and preparation of compounds with desired properties. As there are many reviews focusing on a specific detail of Meldrum's acid chemistry, we would like to give a broader picture of this diverse molecule for undergraduate and graduate students as well as experienced lab leaders. For achieving this goal, some recent advances in using Meldrum's acid derivatives in synthetic scenarios are presented with the hope to further stimulate and promote research leading to additional innovative applications of this synthetically highly relevant molecule.

Electro-organic synthesis of tetrahydroimidazo[1,2-a]pyridin-5(1H)-one via a multicomponent reaction

Electro-synthesis through a one-pot three-component condensation of corresponding aldehydes, Meldrum's acid, and 2-(nitromethylene)imidazolidine resulted in a series of novel tetrahydroimidazo[1,2-a]pyridine-5(1H)-one derivatives containing an electronegative pharmacophore (=CNO₂). The process was carried out in propanol medium with sodium bromide presented as electrolyte, inside an undivided cell with good to excellent yields. As a powerful entry into fused polycyclic structures related to bioactive heterocycles, this green protocol shows great potential.