Rapid Dual Proton–Hydride Donor Transfer Hydrogenation of α, β-Unsaturated Ketones with Built-In Visual Endpoint Detection

Decoupling proton and hydride delivery in transfer hydrogenation (TH) enables improved control over reactivity and selectivity, allowing mild reductions and site-specific deuterium incorporation beyond conventional single-donor systems. Herein, we report a rapid and highly chemoselective TH of α,β-unsaturated ketones using bench-stable, proton-responsive imidazole–phenol Ru(II)-p-cymene catalysts that operate via a dual-donor strategy, with methanol as the proton source and hydrosilanes as the hydride donor, enabling fast and selective C═C reduction at room temperature within 2–5 min. Reversible ligand switching in the bifunctional Ru(II) complex facilitates efficient metal–ligand cooperation, while a built-in chromatic response offers real-time visual end point detection and prevents over-hydrogenation. Detailed kinetic and spectroscopic analyses identify a silane-derived Ru–H active intermediate and demonstrate the crucial involvement of polar protic solvents in the rate-determining step. Control experiments indicate that selectivity arises from π-conjugation and carbonyl polarity, which facilitate substrate activation at Ru(II) and efficient proton-hydride transfer; the method also enables site-specific deuterium incorporation in saturated chalcones across diverse substrates. This dual-reagent strategy expands the reactivity of Ru(II)–arene complexes toward selective olefin reduction, while offering a practical, operationally simple protocol with built-in reaction control.