Zeolite-supported palladium nanoparticles as universal heterogeneous Tsuji–Trost allylic alkylation catalysts

Abstract

The Tsuji–Trost allylic alkylation is mainly performed in homogeneous systems; however, developing efficient alternative, heterogeneous catalysts remains crucial for sustainable synthesis. This study introduces zeolite-supported palladium catalysts as viable heterogeneous catalysts in this reaction. The active catalysts, Pd@USY and Pd@deAl-USY, were synthesized via controlled impregnation and reduction to achieve ultrasmall Pd nanoparticles (2.2 nm and 2.5 nm in diameter, respectively). Pd@lay-MFI, serving as a reference material containing large nanoparticles (>20 nm), proved inactive in the reaction. Among the prepared materials, Pd@USY exhibited optimal performance in the model Tsuji–Trost reaction between diethyl malonate and allyl acetate, achieving complete conversion within 2 hours under mild conditions (room temperature, dichloromethane, K₂CO₃), with activity directly correlated to nanoparticle size: inactive Pd@lay-MFI featured substantially bigger particles, while sub-3 nm particles in active catalysts enabled efficient substrate activation. The reaction scope demonstrated broad substrate compatibility, though nucleophilicity and α-substitution heavily influenced reactivity, and bulky substituents reduced conversion due to zeolite pore diffusion constraints and steric hindrance during nucleophilic attack. Catalyst reuse was feasible for at least two cycles before the catalyst became deactivated. The analysis of the deactivation mechanism is ongoing. Notably, the loss of activity was reversible, as regeneration successfully restored catalytic performance. The catalyst was stable against sintering, leaching, or poisoning. In conclusion, Pd@USY represents a promising heterogeneous alternative for Tsuji–Trost allylic alkylation, combining high activity, selectivity, and operational simplicity.