Current Organic Chemistry - Volume 20, Issue 5, 2016

Transition-metal-catalyzed oxidative and decarboxylative acylations of sp2 C-H bonds bearing conventional directing groups with various acyl equivalents, such as aldehydes, alcohols, alkylbezenes, α-keto acids, etc. are described. These protocols present an effective and promising route to obtain aryl ketones, which are found to be important precursors for the formation of bioactive molecules and functional materials. Thus, oxidative and decarboxylative C-H acylations potentially provide a catalytic alternative to overcome the limitations of classical Friedel-Crafts acylation. This review is intended to give an overview on recent advances on catalytic acylation reactions through C-H bond activation.

Development of cross-coupling chemistry by direct functionalization of C–H or heteroatom–H bonds under oxidative conditions has recently emerged as powerful and ideal methods for the formation of carbon-carbon and carbon-heteroatom bonds. Several transition metal catalysts were successfully utilized for these kinds of transformations over the past several years. Among them, copper salts are of special importance because of their easy availability, bio-compatibility and economical advantages. This review summarizes recent advances over the past 3 years in copper-catalyzed inter- and intramolecular reactions involving C–H and X–H (X= carbon or heteroatom) under oxidative dehydrogenation process in achieving C–C, C–O, C–N, C–S and C–P bond formations.

In green chemistry, the most ideal synthetic route to biaryl compounds, which are important in manufacturing fine chemicals, from non-functionalized starting aromatic materials appears to be the oxidative couplings that directly use two molecules of C–H groups for a new bond-forming event to reduce the synthetic steps and byproduct generation. However, these methods for reacting non-activated aromatic substrates are known to have difficulty in achieving selective cross-biaryl-couplings because of the over-oxidation of the products and competitive homodimer formations by undesired coupling between the same aryl substrates. In order to realize an efficient cross-coupling, therefore, unique chemoselective nature of the oxidant for the aromatic rings that discriminate aromatic substrates is required together with the sufficient oxidation ability. Using hypervalent iodine reagent, we have recently pioneered a number of new metal-free oxidative aromatic coupling methods for providing several types of mixed biaryls in high yields with perfect cross-coupling selectivities. They are classified into three types of the aromatic ring activation modes based on the pattern of the reaction mechanisms, which involve i) the aromatic cation radicals, ii) diaryliodonium salts, and iii) C-reactive phenoxenium ions and their N-analogues. As a result, the strategies now cover a wide range of the aromatic substrates. Hypervalent iodine reagents having low toxicities, mild reactivities, ready availability, high stabilities, easy handling, and ease of recovery and recyclability are useful alternatives to metal-based oxidants and catalysts for performing metal-free synthesis. We fully summarize the outcomes and clarify the synthetic values of our hypervalent iodine methods among the oxidative crosscoupling area.

Supported ionic liquids refer to ionic liquids that are immobilized on supports (e.g. silica-based materials, magnetic nanoparticles, carbon nanotube, etc.). The synergistic effect of ILs and supports can derive a series of novel performances and properties while retain the advantages of both species. As green media in organic catalytic reactions and asymmetric reactions, they have many advantages over free ionic liquids, including avoiding the leaching of ionic liquids and catalysts, reducing their amount, and improving the recoverability and reusability of both themselves and catalysts. Moreover, supported ionic liquids themselves possess catalytic activity and selectivity via mediating the process of some reactions. Therefore, much research effort has been devoted to studying the supported ionic liquids. Over the last decade, the dramatic advantages of SILs inspire drastic reformation in ionic liquid area. In this article, we overview mainly the application and development of solid-supported ionic liquid using covalent bonds in catalytic reactions, while their stability and the interaction among support, ionic liquid and catalyst are discussed.