Cross-coupling reaction between two C–H bonds has become a fundamental strategy in synthetic organic chemistry. With its increasing importance in green chemistry, atom economy, and step economy, it...

En Route to Intermolecular Cross-Dehydrogenative Coupling Reactions.

Alkylation of arenes is one of the most fundamental transformations in chemical synthesis and leads to privileged scaffolds in many areas of science. Classical methods for the introduction of alkyl groups to arenes are mostly based on the Friedel-Crafts reaction, radical additions, metalation, or prefunctionalization of the arene: these methods, however, suffer from limitations in scope, efficiency, and selectivity. Moreover, they are based on the innate reactivity of the starting arene, favoring the alkylation at a certain position and rendering the introduction of alkyl chains at other positions much more challenging. This can be addressed by the use of a directing group that facilitates, in the presence of a metal catalyst, the regioselective alkylation of a C-H bond. These directed alkylations of C-H bonds in arenes will be comprehensively summarized in this Review.

Beyond Friedel and Crafts: Directed Alkylation of C-H Bonds in Arenes.

The highly regioselective electrophotocatalytic C–H functionalization of ethers is described. These reactions are catalyzed by a trisaminocyclopropenium (TAC) ion at mild electrochemical potential ...

Electrophotocatalytic C-H Functionalization of Ethers with High Regioselectivity.

Cross-dehydrogenative coupling (CDC) is a process in which, typically, a C–C bond is formed at the expense of two C–H bonds, either catalyzed by metals or other organic compounds, or via uncatalyzed processes. In this perspective, we present various modes of C–H bond-activation at sp3 centers adjacent to ether oxygen atoms, followed by C–C bond formation with aromatic systems as well as with heteroaromatic systems. C–N bond-formation with NH-containing heteroaromatics, leading to hemiaminal ethers, is also an event that can occur analogously to C–C bond formation, but at the expense of C–H and N–H bonds. A large variety of hemiaminal ether-forming reactions have recently appeared in the literature and this perspective also includes this complementary chemistry. In addition, the participation of C–H bonds in alcohols in such processes is also described. Facile access to a wide range of compounds can be attained through these processes, rendering such reactions useful for synthetic applications via Csp3 bond activations.

/pdf/cross-dehydrogenative-coupling-and-oxidative-amination-54mfmm4bil.pdf

Cross-dehydrogenative coupling and oxidative-amination reactions of ethers and alcohols with aromatics and heteroaromatics.

Minisci alkylation is of prime importance for its applicability in functionalizing diverse heteroarenes, which are core structures in many bioactive compounds. In alkyl radical generation processes, precious metal catalysts, high temperatures and excessive oxidants are generally involved, which lead to sustainability and safety concerns. Herein we report a new strategy using diacetyl (2,3-butanedione) as an abundant, visible light-sensitive and “traceless” hydrogen atom abstractor to achieve metal-free cross-dehydrogenative Minisci alkylation under mild conditions. Mechanistic studies supported hydrogen atom transfer (HAT) between an activated C(sp3)–H substrate and diacetyl. Moreover, with the assistance of di-tert-butyl peroxide (DTBP), the scope of the reaction could be extended to strong aliphatic C–H bonds via diacetyl-mediated energy transfer. The robustness of this strategy was demonstrated by functionalizing complex molecules such as quinine, fasudil, nicotine, menthol and alanine derivatives.

/pdf/diacetyl-as-a-traceless-visible-light-photosensitizer-in-2axjav1tud.pdf

Diacetyl as a “traceless” visible light photosensitizer in metal-free cross-dehydrogenative coupling reactions

An important challenge in the Cα-heteroarylation of ethers is the requirement of a large excess amount of ethers (that are used as solvents in many cases) to achieve effective transformations. This drawback has significantly restricted the Cα-heteroarylation of ethers to the use of simple and easily accessible ether substrates. To overcome this limitation, a new, efficient, N-hydroxysuccinimide (NHS) mediated, mild and metal-free CDC strategy for the direct Cα-heteroarylation of diverse ethers has been developed. Different to our previous benzaldehyde mediated photoredox Cα-heteroarylation, we have identified NHS as a new and efficient mediator without using light. A distinct non-photoredox engaged hydrogen-atom-transfer (HAT) mechanism that used a nitrogen-centered radical cation produced from NHS is initially revealed. Notably, only 5–10 equivalents of ethers as coupling partners are used, which allows for structurally diverse and complex ethers to engage in this process, to create highly medicinally relevant Cα-heteroarylated ethers. Furthermore, more structurally diverse heterocyclics can serve as reactants for this process.

/pdf/direct-ca-heteroarylation-of-structurally-diverse-ethers-via-4cygagki1m.pdf

Aoxia Liu

Papers

Direct Cα-heteroarylation of structurally diverse ethers via a mild N-hydroxysuccinimide mediated cross-dehydrogenative coupling reaction.