Innovative green oxidation of amines to imines under atmospheric oxygen

Dioxygen compatible electron donor-acceptor catalytic system and its enabled aerobic oxygenation

The photochemical properties of Electron Donor-Acceptor (EDA) complexes present exciting opportunities for synthetic chemistry. However, these strategies often require an inert atmosphere to maintain high efficiency. Herein, we develop an EDA complex photocatalytic system through rational design, which overcomes the oxygen-sensitive limitation of traditional EDA photocatalytic systems and enables aerobic oxygenation reactions through dioxygen activation. The mild oxidation system transfers electrons from the donor to the effective catalytic acceptor upon visible light irradiation, which are subsequently captured by molecular oxygen to form the superoxide radical ion, as demonstrated by the specific fluorescent probe, dihydroethidine (DHE). Furthermore, this visible-light mediated oxidative EDA protocol is successfully applied in the aerobic oxygenation of boronic acids. We believe that this photochemical dioxygen activation strategy enabled by EDA complex not only provides a practical approach to aerobic oxygenation but also promotes the design and application of EDA photocatalysis under ambient conditions.

Green imine synthesis from amines using transition metal and micellar catalysis

Imines are a versatile class of chemicals with applications in pharmaceuticals and as synthetic intermediates. While imines are conventionally synthesized via aldehyde-amine condensation, their direct preparation from amines can avoid the need for the independent preparation of the aldehyde coupling partner and associated constraints with regard to aldehyde storage and purification. The direct preparation of imines from amines typically utilizes transition metal catalysis and is often well-aligned with green chemistry principles. This review provides a comprehensive overview of transition metal catalysed imine synthesis, with a particular focus on the copper-catalyzed oxidative coupling of amines. The emerging application of micellar catalysis for imine synthesis is also surveyed due to its potential to avoid the use of hazardous solvents and intensify these reactions through reduced catalyst loadings and locally increased reactant concentrations. Future directions relating to the confluence of these two areas are proposed towards the more sustainable preparation of imines.

Two distinct protocols for the synthesis of unsymmetrical 3,4-disubstituted maleimides based on transition-metal catalysts

Two tandem catalytic systems are described for the synthesis of novel 3,4-disubstituted maleimides using the same Ugi adducts as starting materials. 4-Aryl-3-pyrrolyl- and 4-aryl-3-indolyl-maleimides were successfully obtained via a Pd(OAc)2/PPh3 based protocol. In contrast, maleimide-fused pyrrolo and indolo[1,2-a]quinolines were obtained in a complementary methodology using CuI/L-proline. These strategies involved a combination of benzylic amine oxidation, trans-amidation, intramolecular Knoevenagel condensation, and N-arylation reactions. Computational investigations provide further insights into this reaction sequence.

Tailoring flavin-based photosensitizers for efficient photooxidative coupling of benzylic amines

Photooxidative coupling of benzylic amines using naturally abundant O2 as an oxidant under visible light irradiation is an alternative green approach to synthesis imines and is of both fundamental and practical significance. We investigated the photophysical properties of flavin (FL) that is a naturally available sensitizer and its derivatives, i.e. 9-bromoflavin (MB-FL), 7,8-dibromoflavin (DB-FL) and 10-phenylflavin (Ph-FL), as well as the performance of these FL-based sensitizers (FLPSs) in the photooxidative coupling of benzylic amines to imines combining experimental and theoretical efforts. We showed that chemical functionalization with Br and phenyl effectively improves the photophysical properties of these FLPSs, in terms of absorption in the visible light range, singlet oxygen quantum yields, triplet lifetime, etc. Apart from nearly quantitative selectivity for the production of imines, the performance of DB-FL is superior to those of other FLPSs, and it is among the best photocatalysts for imine synthesis. Specifically, 0.5 mol% DB-FL is capable of converting 91% of 0.2 mmol benzylamine and more than 80% of 0.2 mmol fluorobenzylic amine derivatives into their corresponding imines in 5 h batch runs. Mechanistic investigation finely explained the observed photophysical properties of FLPSs and highlighted the dominant role of electron transfer in FLPS sensitized coupling of benzylic amines to imines. This work not only helps to understand the pathways for photocatalysis with FLPSs but also paves the way for the design of novel and efficient PSs to promote organic synthesis.

Photoinduced Nitroarenes as Versatile Anaerobic Oxidants for Accessing Carbonyl and Imine Derivatives

Herein, we report a protocol for the anaerobic oxidation of alcohols, amines, aldehydes, and imines promoted by photoexcited nitroarenes. Mechanistic studies support the idea that photoexcited nitroarenes undergo double hydrogen atom transfer (HAT) steps with alcohols and amines to provide the respective ketone and imine products. In the presence of aldehydes and imines, successive HAT and oxygen atom transfer (OAT) events occur to yield carboxylic acids and amides, respectively. This transformation is amenable to a continuous-photoflow setup, which led to reduced reaction times.