Abstract
The palladium-catalyzed Heck coupling represents a powerful method for constructing complex molecular structures, including biologically active ingredients. Implementing alternative reaction media in this industrially important transformation is crucial for developing environmentally safer processes and advancing sustainability. Herein, we report the first successful phosphine-free Heck protocol conducted in the biomass-derived solvent 1,4-pentanediol (1,4-PDO). The effects of key reaction parameters, i.e., Pd source, base type, and solvent water content, were systematically evaluated using iodobenzene and styrene as model substrates. Using the Pd(OAc)(2)/Et(3)N catalyst system, 4-substituted iodobenzene and styrene derivatives were subsequently employed to investigate the effects of their substituents' electronic parameters on reaction efficiency and functional-group tolerance. Excellent linear correlations (R(2) > 0.9) were observed between the substituents' Hammett σ constants and performance. Taking advantage of the products' significant temperature-dependent solubility in 1,4-PDO, facile product separation via simple filtration can be achieved, avoiding high-solvent-consuming column chromatography and facilitating solvent recovery. Using this approach, 24 products were synthesized and isolated with moderate to high yields (53-83%) and high purities (>90%). Solvent and catalyst recycling were demonstrated over three cycles. This protocol reduces the E-factor by more than 90% relative to conventional Heck conditions, highlighting its potential for sustainable synthetic applications.