Solvent‐Controlled Chemoselectivity in Brønsted Acidic Deep Eutectic Mixtures: A Metal‐Free One‐Pot Cascade Meyer‐Schuster/C–P Bond Formation

One-pot tandem methodologies offer powerful opportunities to enhance step economy while reducing waste, energy input, and operational complexity, thereby directly aligning with the Principles of Green Chemistry. Thus, herein, we report a one-pot cascade, metal-free Meyer-Schuster/C-P (and C-Cl) bond-forming protocol enabled by Brønsted Acidic Deep Eutectic Solvents (BADESs), which act simultaneously as solvent, promoter and chemoselectivity controller. This eutectic medium enables the in situ generation of α,β-unsaturated aldehydes from propargylic alcohols and their interception by secondary phosphine oxides, allowing both orthogonal transformations to operate: i) simultaneously from the beginning of the reaction in a single and operationally simple step; and ii) under mild, air- and moisture-compatible conditions. Remarkably, the modulation of the HBA/HBD ratio within the same BADES system allows predictable switching between the selective formation of α-hydroxy or α-chloro allyl phosphine oxides, without changing reagents or reaction conditions. Additionally, the protocol features: i) broad substrate scope; ii) high chemoselectivity and functional-group tolerance; and iii) enables product isolation by simple precipitation/filtration, thereby avoiding VOC-intensive chromatographic purification. Moreover, the eutectic medium can be recycled, the reaction is readily scalable to gram scale, and the process displays a favorable environmental footprint, with an E-factor as low as 11.