Growing Utilization of Radical Chemistry in the Synthesis of Pharmaceuticals

Programmable C-N Bond Formation through Radical-Mediated Chemistry in Plasma-Microdroplet Fusion

Non-thermal plasma discharge produced in the wake of charged microdroplets is found to facilitate catalyst-free radical mediated hydrazine cross-coupling reactions without the use of external light source, heat, precious metal complex, or trapping agents. A plasma-microdroplet fusion platform is utilized for introduction of hydrazine reagent that undergoes homolytic cleavage forming radical intermediate species. The non-thermal plasma discharge that causes the cleavage originates from a chemically etched silica capillary. The coupling of the radical intermediates gives various products. Plasma-microdroplet fusion occurs online in a programmable reaction platform allowing direct process optimization and product validation via mass spectrometry. The platform is applied herein with a variety of hydrazine substrates, enabling i) self-coupling to form secondary amines with identical N-substitutions, ii) cross-coupling to afford secondary amine with different N-substituents, iii) cross-coupling followed by in situ dehydrogenation to give the corresponding aryl-aldimines with two unique N-substitutions, and iv) cascade heterocyclic carbazole derivatives formation. These unique reactions were made possible in the charged microdroplet environment through our ability to program conditions such as reagent concentration (i. e., flow rate), microdroplet reactivity (i. e., presence or absence of plasma), and reaction timescale (i. e., operational mode of the source). The selected program is implemented in a co-axial spray format, which is found to be advantageous over the conventional one-pot single emitter electrospray-based microdroplet reactions.

Mechanochemical Radical Transformations in Organic Synthesis

Organic synthesis has historically relied on solution-phase, polar transformations to forge new bonds. However, this paradigm is evolving, propelled by the rapid evolution of radical chemistry. Additionally, organic synthesis is witnessing a simultaneous resurgence in mechanochemistry, the formation of new bonds in the solid-state, further contributing to this shift in the status quo. The aforementioned advances in radical chemistry have predominantly occurred in the solution phase, while the majority of mechanochemical synthesis advances feature polar transformations. Herein, we discuss a rapidly advancing area of organic synthesis: mechanochemical radical reactions. Solid-state radical reactions offer improved green chemistry metrics, better reaction outcomes, and access to intermediates and products that are difficult or impossible to reach in solution. This review explores these reactions in the context of small molecule synthesis, from early findings to the current state-of-the-art, underscoring the pivotal role solid-state radical reactions are likely to play in advancing sustainable chemical synthesis.

Controllable Chemoselectivity Cascade Reactions for the Synthesis of Phenanthrenols via Palladium-Catalyzed-Suzuki/Heck Reaction and Michael Addition

Palladium serves as a multi-functional catalyst which is controllable by tuning reaction conditions. This work demonstrated the utilization of a palladium catalyst for the synthesis of phenanthrenols by cascade palladium-catalyzed Suzuki/Heck reaction between chalcone and 2-bromophenylboronic acid, followed by Michael addition. The sequential reaction could be controlled by reactivity of the palladium catalyst in different solvents and concentrations of reagents. This protocol could be applied to a broad range of substrates to give products in low to good yields.