Hydrophobic species-enabled acid–base multi-catalysis for stereoselective access to renewable trans-anethole

trans-Anethole (trans-AN) is widely applied in food, daily necessities, and pharmaceuticals and is typically available from inefficient natural oil extraction or complex organic transformations over mineral acid or noble metals. Here, a green and sustainable route was developed to stereoselectively produce trans-AN (ca. 90% selectivity) over an organic polymeric phosphonate-hafnium catalyst (PAS-Hf) through the cascade transfer hydrogenation and dehydration of biomass-based 4'-methoxypropiophenone (4-MOPP), with an environmental impact factor (E-factor) of 47.73. The porous structure and the enhanced hydrophobicity of the spherical catalyst PAS-Hf ensured the formation of more accessible and stable Lewis (Hf4+) and Brønsted (SO3H) acid active sites, which could be used for rapid conversion of biomass-based 4-MOPP to AN (100% conversion, 97.2% yield) in 0.5-2 h (TOF: 9.3 h-1). Density functional theory (DFT) calculations elucidated that the addition of PAS-Hf could remarkably facilitate the overall conversion process by decreasing the reaction energy barrier (151.33 to 48.27 kJ mol-1) of the rate-determining step. The good thermal stability and heterogeneity of the bifunctional catalyst were responsible for its constant activity during at least five consecutive cycles. The synergistic/relay catalysis of Lewis acid-base and Brønsted acid species could be extended to more than ten kinds of aldehydes and ketones. This acid-base multi-catalytic protocol has considerable potential in cascade biomass conversion via heterogeneous catalysis without any base additive.