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      Oxidative coupling between C(sp2)–H and C(sp3)–H bonds of indoles and cyclic ethers/cycloalkanes

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          Abstract

          Cross-dehydrogenative-coupling (CDC) between C–H/C–H bonds of indoles and cyclic ethers/cycloalkanes is made viable through a simple transition-metal-free pathway.

          Cross-dehydrogenative-coupling (CDC) between C–H/C–H bonds of indoles and cyclic ethers/cycloalkanes is made viable through a simple transition-metal-free pathway. With the aid of only di- tert-butyl peroxide, a number of inactive cyclic ethers and cycloalkanes can be directly coupled with indole derivatives in satisfactory yields.

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          Cross-dehydrogenative coupling (CDC): exploring C-C bond formations beyond functional group transformations.

          Synthetic chemists aspire both to develop novel chemical reactions and to improve reaction conditions to maximize resource efficiency, energy efficiency, product selectivity, operational simplicity, and environmental health and safety. Carbon-carbon bond formation is a central part of many chemical syntheses, and innovations in these types of reactions will profoundly improve overall synthetic efficiency. This Account describes our work over the past several years to form carbon-carbon bonds directly from two different C-H bonds under oxidative conditions, cross-dehydrogenative coupling (CDC). We have focused most of our efforts on carbon-carbon bonds formed via the functionalization of sp(3) C-H bonds with other C-H bonds. In the presence of simple and cheap catalysts such as copper and iron salts and oxidants such as hydrogen peroxide, dioxygen, tert-butylhydroperoxide, and 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ), we can directly functionalize various sp(3) C-H bonds by other C-H bonds without requiring preactivation. We demonstrate (1) reaction of alpha-C-H bonds of nitrogen in amines, (2) reaction of alpha-C-H bonds of oxygen in ethers, (3) reaction of allylic and benzylic C-H bonds, and (4) reaction of alkane C-H bonds. These CDC reactions can tolerate a variety of functional groups, and some can occur under aqueous conditions. Depending on the specific transformation, we propose the in situ generation of different intermediates. These methods provide an alternative to the separate steps of prefunctionalization and defunctionalization that have traditionally been part of synthetic design. As a result, these methods will increase synthetic efficiencies at the most fundamental level. On an intellectual level, the development of C-C bond formations based on the reaction of only C-H bonds (possibly in water) challenges us to rethink some of the most fundamental concepts and theories regarding chemical reactivities. A successful reaction requires the conventionally and theoretically less reactive C-H bonds to react selectively in the presence of a variety of functional groups. With further investigation, we expect that C-C bond formations based on cross-dehydrogenative coupling will have a positive economic and ecological impact on the next generation of chemical syntheses.
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            The atom economy--a search for synthetic efficiency.

            B. Trost (1991)
            Efficient synthetic methods required to assemble complex molecular arrays include reactions that are both selective (chemo-, regio-, diastereo-, and enantio-) and economical in atom count (maximum number of atoms of reactants appearing in the products). Methods that involve simply combining two or more building blocks with any other reactant needed only catalytically constitute the highest degree of atom economy. Transition metal-catalyzed methods that are both selective and economical for formation of cyclic structures, of great interest for biological purposes, represent an important starting point for this long-term goal. The limited availability of raw materials, combined with environmental concerns, require the highlighting of these goals.
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              Direct C-H transformation via iron catalysis.

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                Author and article information

                Journal
                OBCRAK
                Organic & Biomolecular Chemistry
                Org. Biomol. Chem.
                Royal Society of Chemistry (RSC)
                1477-0520
                1477-0539
                2016
                2016
                : 14
                : 9
                : 2608-2612
                Article
                10.1039/C6OB00076B
                26842403
                86ff9cc4-42f8-4950-a982-9c3e80c528a0
                © 2016
                History

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