Near-instantaneous chemoselective transfer hydrogenation of aldehydes with visual endpoint reporting and ultrahigh TOF

A bench-stable Ru(II)-<i>para</i>-cymene catalyst bearing proton-responsive ligands enables rapid and chemoselective transfer hydrogenation (TH) of aldehydes under mild, base-free conditions, achieving turnover frequencies (TOFs) up to 166 s^-1 with dual proton and hydride donors. A distinct, reversible green-to-yellow color change allows real-time visual endpoint detection and helps prevent over-reduction. The catalyst remains recyclable over multiple cycles with sustained activity and selectivity, providing a practical, self-indicating platform for efficient transfer hydrogenation in complex synthetic workflows.