Synthetically versatile imidazolium chlorides, especially 1,3‐bis‐(2,6‐diisopropylphenyl)‐imidazolium chloride (IPrHCl) along with copper (I) iodide and cesium carbonate, were found to be useful to form C−O bonds via coupling of phenols and aryl/heteroaryl bromides. In this catalysis, the metal:ligand (Cu:IPr) loading was optimised to 1.0 mol%, making it an inexpensive and a simple one pot catalytic route for synthesising a variety of diaryl ethers. The catalytic cycle was probed by UV‐Vis, FT‐IR and HRMS techniques confirming involvement of a copper‐imidazolium‐carbonate complex.
Studies of environmentally benign catalytic methods are of great value in modern chemical synthesis, especially the chemo‐selective construction of chemical bonds under green conditions. This work elucidates such preferential synthesis of C−N bond over C−O bond via selective N‐arylation of 3‐aminophenols using 1,3‐bis‐[2‐hydroxyphenyl] imidazolium chloride (IHPHCl) and copper iodide as catalyst (1 mol %) in aqueous medium. Presence of chelating group (−OH) on IHPHCl enhances N‐selectivity. Overall this is a simple and green method for selective N‐arylation of 3‐aminophenols with good substrate scope and yields (60–88 %). GC‐MS, HRMS and other spectroscopic techniques were utilised in detailing the kinetics and mechanistic aspects.
A single step approach for the synthesis of non-symmetrical tetraaryl-α-diimines via the transimination route at room temperature was developed. The reported procedure was found to be an elegant route for making non-symmetrical tetraaryl-α-diimines, which upon cyclization yields a series of new non-symmetrical 1,3,4,5-tetraarylimidazolium chlorides, as stable N-heterocyclic carbene (NHC) precursors.
The journey of “carbenes” is more than a century old. It began with a curiosity to understand a then less familiar carbon moiety in its divalent state. It reached an important milestone in the form of 1,3-imidazolium-based N-heterocyclic carbenes (NHCs), where the quest for bottleable carbenes was achieved through simple and elegant synthetic routes. The properties of these carbenes were finely tunable through the steric and electronic factors via chemical modifications. Thus, it became one of the unique and extensively studied ligands for its properties and applications. This chapter first briefs about structural details of NHCs and different synthetic routes for the preparation of imidazolium-based NHC precursors. The later section focuses on various methods for characterizing the steric and electronic properties of these ligands and their metal intermediates, which are crucial for developing efficient catalytic processes. Finally, the chapter concludes with NHC-metal-mediated catalytic applications and its immediate challenges.
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