Published on Web 07/12/2010
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
Yasuo Matsubara,† Kichitaro Koga,† Atsuo Kobayashi,‡,§ Hideo Konno,‡,#
Kazuhiko Sakamoto,‡ Tatsuki Morimoto,† and Osamu Ishitani*,†
Department of Chemistry, Tokyo Institute of Technology, O-okayama 2-12-1,
E1-9, Meguro-ku, Tokyo 152-8551, Japan, and Graduate School of Science and Engineering,
Saitama UniVersity, 255 Shimo-Okubo, Saitama 338-8570, Japan
Received May 11, 2010; E-mail: ishitani@chem.titech.ac.jp
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-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.
Introduction
NAD(P)H models with a chiral center or chiral centers, and
some of their reactions yielded high enantiomeric excesses.1f,2
Reductions of various kinds of organic compounds using the
coenzyme NAD(P)H and model compounds have been well
studied for several decades.1 For example, asymmetric reduc-
tions of unsaturated substrates have been reported using
From the standpoint of organic synthesis, the stoichiometric
requirement of the NAD(P)H model is a problem; therefore, a
catalytic system is needed to reduce the oxidized form.3,4 The
system has to provide two types of selectivities: (1) chemose-
lectivity, where only hydride reduction should occur without
producing the corresponding radical coupling products, and (2)
† Tokyo Institute of Technology.
‡ Saitama University.
§ Present address: DIC Corp., Oaza Komuro, Ina-machi, Kitaadachi-gun,
Saitama 362-8577, Japan.
# Present addresses: Research Institute for Innovation in Sustainable
Chemistry, National Institute of Advanced Industrial Science and Technol-
ogy, 1-1-1 Higashi, Tsukuba 305-8565, Japan.
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10.1021/ja1040122 2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 10547–10552 10547