Acceptorless cross-dehydrogenative coupling for C(sp3)-H heteroarylation mediated by a heterogeneous GaN/ketone photocatalyst/photosensitizer system.

Alkanes are naturally abundant chemical building blocks that contain plentiful C(sp3)-H bonds. While inert, the activation of C(sp3)-H via hydrogen atom abstraction (HAT) stages an appealing approach to generate alkyl radicals. However, prevailing shortcomings include the excessive use of oxidants and alkanes that impede scope. We herein show the use of gallium nitride (GaN) as a non-toxic, recyclable, heterogeneous photocatalyst to enable alkyl C(sp3)-H in conjunction with the catalytic use of simple photosensitizer, benzophenone, to promote the desired alkyl radical generation. The dual photocatalytic cycle enables cross-dehydrogenative Minisci alkylation under mild and chemical oxidant-free conditions.

Photocatalytic Decarboxylative Minisci Reaction Catalyzed by Palladium-Loaded Gallium Nitride.

The decarboxylative Minisci reaction is a versatile tool for the direct C-H alkylation of heteroarenes, where stoichiometric amounts of oxidants or expensive, precious metal reagents are commonly used. Herein, we reported a photodriven decarboxylative Minisci reaction enabled by a gallium nitride-based heterogeneous photocatalyst under mild conditions. This method can be effectively applied to a broad substrate scope of acids, including primary, secondary, and tertiary carboxylic acids and N-heteroarenes effectively. The practicability and robustness of the approach are demonstrated for the functionalization of biologically active compounds.

Ruthenium(ii)-catalyzed regioselective 1,6-conjugate addition of umpolung aldehydes as carbanion equivalents.

One of the most efficient and reliable approaches to construct C-C bonds involves the conjugate addition of carbon nucleophiles to electron-deficient ketones. Yet, 1,6-conjugate additions of extended conjugated systems largely remain underexplored due to difficulties in controlling the regioselectivity. Herein, we report umpolung aldehydes as carbanion equivalents for highly regioselective 1,6-conjugate addition reactions to unsaturated ketones, with preliminary studies of the enantioselective variant. The synergy of ruthenium(ii) catalyst and electron-rich, bidentate phosphine ligand is essential for the reactivity and selectivity under mild reaction conditions.

En Route to Intermolecular Cross-Dehydrogenative Coupling Reactions.

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, its development has sky-rocketed within the last 20 years, with the term "cross-dehydrogenative coupling (CDC)" popularized and progressed by the group of Li and others to describe direct Y-Z bond formations from Y-H and Z-H bonds under oxidative conditions. Among all types of CDC reactions, the C-C bond formations are of prime importance in building up the molecular complexity but their categorization currently remains disarray due to a wide diversity, resulting in frequent display in separate topics. In this Perspective, a contemporary categorization via C-H activation strategies is presented herein, which could be vital for future CDC designs. With this mechanism-based categorization and discussion, we wish that this minireview will help more synthetic chemists gain insight into the design of CDC reactions and inspires more ideas on this topic.