Electrogenerated N-Heterocyclic Carbene in Ionic Liquid: An Insight into the Mechanism of the Oxidative Esterification of Aromatic Aldehydes

First example of organocatalysis by cathodic N-heterocyclic carbene generation and accumulation using a divided electrochemical flow cell

In this paper we present the first electrochemical generation of NHC carried out in a divided flow cell. The flow cell operated in the recycle mode. The need for a divided cell derived from the anodic electroactivity of the electrogenerated carbene. In order to have NHC accumulation in the catholyte, the Nafion membrane (cell separator) was pretreated with an alkaline solution. The formation of NHC was quantified as its reaction product with elemental sulfur. The NHC was successfully used as organocatalyst in two classical umpolung reactions of cinnamaldehyde: its cyclodimerization and its oxidative esterification.

A convenient green protocol for oxidative esterification of arylaldehydes over Pd NPs decorated polyplex encapsulated Fe3O4 microspheres

A mild, competent and eco-friendly protocol has been developed for oxidative esterification of various aldehydes over a post-synthetically modified and Pd fabricated chitosan-starch dual functionalized Fe3O4 microspheres as a magnetically isolable heterogeneous and biocompatible nanocatalyst. Molecular O2 was used as an oxidant in the reaction. A series of aldehydes was directly esterified with MeOH in excellent yields without any pre-activation and involvement of harsh chemicals/conditions. Structural features of the catalyst were assessed through FT-IR, FE-SEM, TEM, EDX, molecular mapping, XRD, VSM and ICP-OES techniques. Due to magnetic core, the catalyst was easily isolated using an external magnet and reused for 8 times in succession, retaining its morphology and catalytic activity.

Chitosan-starch biopolymer modified kaolin supported Pd nanoparticles for the oxidative esterification of aryl aldehydes

A mild and efficient green protocol has been disclosed for selective oxidative esterification of various aldehydes over a novel Pd fabricated chitosan-starch polyplex encapsulated Kaolin (Kaolin@CS-starch-Pd) as a heterogeneous and reusable biocompatible nanocatalyst. Molecular oxygen was used as an oxidizing agent to generate water as the sole by-product. A wide variety of aldehydes was converted to their methyl esters in high yields. The process involved gentle reaction conditions to avoid any type of pre-activation. Structural features of the catalyst were determined through FT-IR, FE-SEM, TEM, EDX, elemental mapping, XRD and ICP-OES analyses. The material was found to be stable enough toward Pd leaching. Durability of Kaolin@CS-starch-Pd was further justified by retaining its catalytic activity through successful reusability for several times.

Electrogenerated NHCs in Organic Synthesis: Ionic Liquids vs Organic Solvents Effects

In the last twenty years, N-heterocyclic carbenes (NHCs) have been extensively studied for their application as organocatalysts in stereoselective synthesis as well as ligands for transition metals-promoted synthetic methodologies. Derived mainly from azolium salts, NHCs have demonstrated exceptional versatility in their generation usually performed by deprotonation or reduction (chemical or electrochemical). In particular, the generation of NHC under electrochemical conditions, starting from azolium-based ionic liquids, has proven to be a successful green approach and demonstrated wide applicability in organic synthesis. In this Personal Account, the application of electrogenerated NHCs in organic synthesis will be discussed, with a particular attention to the different reactivity in ionic liquids compared to classical organic solvents.

Formation and Properties of Poly(Ionic Liquid)-Carbene Nanogels Containing Individually Stabilized Silver Species

Imidazolium-based ionic liquids have the ability to undergo a variety of chemical reactions through an N-heterocyclic carbene (NHC) intermediate, which has expanded the chemical toolbox for new applications. Despite their uses and exploration, the carbene-forming properties and applications of their polymeric congeners, poly(ionic liquid)s (PILs), is still underdeveloped. Herein, we explore the NHC-forming properties of a theophylline-derived PIL for nanogel synthesis. Using silver oxide as both the carbene-forming reagent and cross-linker, nanogels containing individually stabilized ions can be created with different sizes and morphology, including large "galaxy-like" superstructures. Using high-resolution TEM techniques, we directly observed the sub-nanometer structure of these constructs. These features combined exemplify the unique chemistry of poly-NHCs for single-metal-ion-stabilization nanogel design.

Ionic liquids as an electrolyte for the electro synthesis of organic compounds

The use of ionic liquids (ILs) as a solvent and an electrolyte for electro organic synthesis has been reviewed.

Ionic liquids as precatalysts in the highly stereoselective conjugate addition of α,β-unsaturated aldehydes to chalcones

Imidazolium-based ionic liquids (ILs) serve both as recyclable reaction media and as precatalysts for the N-heterocyclic carbene-catalyzed conjugate addition of α,β-unsaturated aldehydes to chalcones. The reaction produces a broad scope of 1,6-ketoesters incorporating an anti-diphenyl moiety in high yields and with high stereoselectivity. In recycling experiments, the IL can be reused up to five times with retained reactivity and selectivity. Moreover, the 1,6-ketoesters form self-assembled organogels in aliphatic hydrocarbons. The reaction protocol is robust, easily operated, scalable and highly functionalized compounds can be obtained from inexpensive and readily accessible starting materials.