pH-Dependent Catalytic Activity and Chemoselectivity in Transfer Hydrogenation Catalyzed by Iridium Complex with 4,4′-Dihydroxy-2,2′-bipyridine

Transfer hydrogenation catalyzed by an iridium catalyst with 4,4′‐dihydroxy‐2,2′‐bipyridine (DHBP) in an aqueous formate solution exhibits highly pH‐dependent catalytic activity and chemoselectivity. The substantial change in the activity is due to the electronic effect based on the acid–base equilibrium of the phenolic hydroxyl group of DHBP. Under basic conditions, high turnover frequency values of the DHBP complex, which can be more than 1000 times the value of the unsubstituted analogue, are obtained (up to 81 000 h−1 at 80 °C). In addition, the DHBP catalyst exhibits pH‐dependent chemoselectivity for α,β‐unsaturated carbonyl compounds. Selective reduction of the CC bond of enone with high activity are observed under basic conditions. The ketone moieties can be reduced with satisfactory activity under acidic conditions. In particular, pH‐selective chemoselectivity of the CO versus CC bond reduction was observed in the transfer hydrogenation of cinnamaldehyde. pH‐Dependent substituent effects: Transfer hydrogenation catalyzed by an iridium catalyst with the 4,4′‐dihydroxy‐2,2′‐bipyridine (DHBP) ligand in a formate solution shows highly pH‐dependent catalytic activity. High turnover frequency values of the DHBP complex, which can be more than 1000 times the value of the unsubstituted analogue under basic conditions, were obtained under mild conditions (see figure).