Catalytic, Green-Synthetic, and Rearrangement Strategies for Biologically Active Piperazine Derivatives

The piperazine scaffold is central to contemporary medicinal chemistry, serving as the structural foundation of numerous FDA-approved therapeutics with diverse pharmacological profiles, including anticancer, antiviral, antibacterial, antifungal, and neuroprotective activities. Conventional synthetic routes to piperazine derivatives are often burdened by hazardous reagents, stoichiometric waste, poor atom economy, and limited scalability, necessitating more efficient, selective, and environmentally responsible alternatives. This review, covering the period 2014-2026, provides a comprehensive and critically evaluated account of recent advances in catalytic, green synthetic, and rearrangement-based strategies for the construction of piperazine derivatives. Among catalytic approaches, homogeneous systems including palladium- and copper-mediated Narylation, ruthenium-catalyzed borrowing-hydrogen annulation, and photoredox C-H functionalization afford structurally diverse scaffolds with excellent regio- and chemoselectivity. Complementary heterogeneous methodologies employing Pd/C, Ni/SiO2, and zeolite-supported catalysts offer operational simplicity, recyclability, and compatibility with sustainable solvent systems. Green synthetic methods, including microwave-assisted synthesis, solvent-free mechanochemistry, biocatalytic amination, and continuous-flow chemistry, are discussed with respect to their contributions to reduced reaction times, higher yields, improved energy efficiency, and diminished environmental burden, as quantified through green chemistry metrics such as atom economy (AE), Efactor, process mass intensity (PMI), and reaction mass efficiency (RME). Rearrangement strategies, including Mumm rearrangement and Ugi-Smiles cyclization, are highlighted as powerful, convergent approaches enabling access to complex piperazine architectures in fewer synthetic steps. A dedicated structure-activity relationship (SAR) analysis connecting synthetic modifications with biological activity is also presented. Collectively, the convergence of catalytic innovation with sustainable synthesis principles is transforming piperazine chemistry, enabling greener, faster, and more economically viable routes to bioactive compounds of pharmaceutical relevance.