title: 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.
abstract: 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-L(2+), where tpy = 2,2':6',2''-terpyridine, bpy = 2,2'-bipyridine, and L = pyridine and MeCN) as a photocatalyst and NEt(3) 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)(NEt(3))](2+) (Ru-NEt(3)(2+)) via the photochemical ligand substitution of Ru-L(2+). For this reason, the production rate of 1,4-BNAH using [Ru(tpy)(bpy)(MeCN)](2+) (Ru-MeCN(2+)) 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-py(2+)), was faster than that using Ru-py(2+), especially in the first stage of the photocatalytic reduction. The photoexcitation of Ru-NEt(3)(2+) yields [Ru(tpy)(bpy)H](+) (Ru-H(+)), which reacts with BNA(+) to give 1:1 adduct [Ru(tpy)(bpy)(1,4-BNAH)](2+) (Ru-BNAH(2+)). In the presence of excess NEt(3) in the reaction solution, a deprotonation of the carbamoyl group in Ru-BNAH(2+) 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 NEt(3) 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-MeCN(2+) with 2 M of NEt(3) 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 x 10(-4) quantum yield using 436-nm light.
authors.affiliation: Department of Chemistry, Tokyo Institute of Technology, O-okayama 2-12-1, E1-9, Meguro-ku, Tokyo 152-8551, Japan.
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Matsubara
authors.forename: Kichitaro
authors.initials: K
authors.lastname: Koga
authors.forename: Atsuo
authors.initials: A
authors.lastname: Kobayashi
authors.forename: Hideo
authors.initials: H
authors.lastname: Konno
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Sakamoto
authors.forename: Tatsuki
authors.initials: T
authors.lastname: Morimoto
authors.forename: Osamu
authors.initials: O
authors.lastname: Ishitani
mesh_terms: D002384:Catalysis; D009249:NADP / Q000737:chemistry*; D010777:Photochemistry; D011725:Pyridines / Q000737:chemistry*; D017975:Ruthenium Compounds / Q000737:chemistry*
chemical_list: D011725:Pyridines; D017975:Ruthenium Compounds; D009249:NADP