Development of an Efficient and Durable Photocatalytic System for Hydride Reduction of an NAD(P)+ Model Compound Using a Ruthenium(II) Complex Based on Mechanistic Studies

The mechanism of photocatalytic reduction of 1-benzylnicotinamidium cation (BNA+) to the 1,4-dihydro form (1,4-BNAH) using [Ru(tpy)(bpy)(L)]2+ (Ru-L2+, where tpy = 2,2′:6′,2′′-terpyridine, bpy = 2,2′-bipyridine, and L = pyridine and MeCN) as a photocatalyst and NEt3 as a reductant has been clarified. On the basis of this mechanistic study, an efficient and durable photocatalytic system for selective hydride reduction of an NAD(P)+ model compound has been developed. The photocatalytic reaction is initiated by the formation of [Ru(tpy)(bpy)(NEt3)]2+ (Ru-NEt32+) via the photochemical ligand substitution of Ru-L2+. For this reason, the production rate of 1,4-BNAH using [Ru(tpy)(bpy)(MeCN)]2+ (Ru-MeCN2+) as a photocatalyst, from which the quantum yield of photoelimination of the MeCN ligand is greater than that of the pyridine ligand from [Ru(tpy)(bpy)(pyridine)]2+ (Ru-py2+), was faster than that using Ru-py2+, especially in the first stage of the photocatalytic reduction. The photoexcitation of Ru-NEt32+ yields [Ru(tpy)(bpy)H]+ (Ru-H+), which reacts with BNA+ to give 1:1 adduct [Ru(tpy)(bpy)(1,4-BNAH)]2+ (Ru-BNAH2+). In the presence of excess NEt3 in the reaction solution, a deprotonation of the carbamoyl group in Ru-BNAH2+ proceeds rapidly, mainly forming [Ru(tpy)(bpy)(1,4-BNAH-H+)]+ (Ru-(BNAH-H+)+). Although photocleavage of the adduct yields 1,4-BNAH and the cycle is completed by the re-coordination of a NEt3 molecule to the Ru(II) center, this process competes with hydride abstraction from Ru-(BNAH-H+)+ by BNA+ giving 1,4-BNAH and [Ru(tpy)(bpy)(BNA+-H+)]2+. This adduct was observed as the major complex in the reaction solution after the photocatalysis was depressed and is a dead-end product because of its stability. Based on the information about the reaction mechanism and the deactivation process, we have successfully developed a new photocatalytic system using Ru-MeCN2+ with 2 M of NEt3 as a reductant, which could reduce more than 59 equivalent amounts of an NAD(P)+ model, 1-benzyl-N,N-diethylnicotinamidium cation, selectively to the corresponding 1,4-dihydro form in a 6 × 10−4 quantum yield using 436-nm light.