Atom-transfer radical reactions catalyzed by a coordinatively unsaturated diruthenium amidinate, [(η5-C5Me5)Ru(μ2-i-PrNdouble bond; length as m-dashC(Me)Ni-Pr)Ru(η5-C5Me5)]+

Atom-transfer radical cyclization (ATRC) and addition (ATRA) catalyzed by a coordinatively unsaturated diruthenium amidinate complex , [(η5-C5Me5)Ru(μ2-i-PrNdouble bond; length as m-dashC(Me)Ni-Pr)Ru(η5-C5Me5)]+, are investigated, and their features are compared with those of atom-transfer radical polymerization (ATRP). As an example of ATRC, a cationic diruthenium amidinate is found to exhibit excellent catalytic reactivity for the cyclization of N-allyl α-halogenated acetamides including an alkaloid skeleton at ambient temperature. A catalytic species generated in situ from a halide complex, (η5-C5Me5)Ru(μ2-i-PrNdouble bond; length as m-dashC(Me)Ni-Pr)Ru(η5-C5Me5)(X) [X=Cl, Br] and sodium salts of weakly coordinating anions such as NaPF6 and NaBPh4 also shows high catalytic activity; this actually provides a solution for a problematic instability of as the practical catalyst. The in situ-generated catalyst species is also active towards the intermolecular ATRA of α,α,γ-trichlorinated γ-lactam with alkenes at rt to afford the corresponding α-alkylated γ-lactams in moderate yields. Examination of ATRP of methyl methacrylate (MMA) showed that both the isolated [Y=PF6] and in situ-generated [Y=PF6] are effective for the polymerization of MMA in the presence of 2-bromoisobutylate as the initiator. Use of the isolated catalyst results in controlled polymerization at initial stage of the reaction; in contrast, the polymerization with in situ-generated catalyst produces poly(MMA) with wide molecular weight distribution. The isolated catalyst is powerful for the activation of a C–Br bond of macromolecule initiators; BrCMe2CO2[O(CH2)4]n-n-Bu (Mn=3800; Mw/Mn=1.2) initiated ATRP of MMA even at 25 °C to afford the poly(THF)–poly(MMA) block copolymer of Mn=26,000 and Mw/Mn=1.2 with the aid of . The roles of the coordinatively unsaturated ruthenium species for these reactions are discussed.