title: pH-Dependent catalytic activity and chemoselectivity in transfer hydrogenation catalyzed by iridium complex with 4,4'-dihydroxy-2,2'-bipyridine.
abstract: 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 degrees C). In addition, the DHBP catalyst exhibits pH-dependent chemoselectivity for alpha,beta-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.
authors.affiliation: Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5-2, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan. himeda.y@aist.go.jp
authors.forename: Yuichiro
authors.initials: Y
authors.lastname: Himeda
authors.forename: Nobuko
authors.initials: N
authors.lastname: Onozawa-Komatsuzaki
authors.forename: Satoru
authors.initials: S
authors.lastname: Miyazawa
authors.forename: Hideki
authors.initials: H
authors.lastname: Sugihara
authors.forename: Takuji
authors.initials: T
authors.lastname: Hirose
authors.forename: Kazuyuki
authors.initials: K
authors.lastname: Kasuga