Mononuclear copper complexes, and coordinatively unsaturated ruthenium complexes, [Cp*Ru(i-Pr-amidinate)RuCp*]+ and [Cp*Ru(OMe)]2, for the Atom-Transfer Radical Reactions

Copper complexes as catalysts

Cyclization of N-tosyl or N-benzyl-N-allyltrichloroacetamide to the corresponding γ-lactam proceeds smoothly using copper complex catalysts.

Using copper complexes with N-containing ligands such as TMEDA and bipyridine dramatically increases the reactivity. We have applied this reaction system to successfully prepare complex organic compounds.

Ruthenium complexes bearing amidinate ligands as catalysts

We have recently reported the synthesis, characterization, and dynamic behavior of ruthenium amidinate complexes. In addition, we described their unique properties, which afford the reactive coordinatively unsaturated species. Thus, these amidinate complexes should have a high activity toward catalytic reactions.

As an example of atom transfer radical cyclization, a cationic diruthenium amidinate exhibits an excellent catalytic reactivity for the cyclization of N-allyl α-halogenated acetamides, including an alkaloid skeleton at ambient temperature. This indicates that our coordinatively unsaturated ruthenium amidinates are very active catalysts toward atom transfer radical cyclization reactions.

Ruthenium complexes bearing alkoxide ligands as the catalyst

Next, we examined dinuclear ruthenium complexes with alkoxide ligands as a cyclization reaction catalyst. A diruthenium alkoxide can be easily synthesized to generate highly coordinatively unsaturated species.

As expected, ruthenium alkoxides are very active catalyst toward atom transfer cyclization reactions and catalyze a wide range of reactions of starting substrates. In this reaction system, ruthenium alkoxide is very active, but has a short lifetime. However, the addition of two-electron donor ligands such as pyridine and triphenylphosphine improves the catalyst efficiency and affords the corresponding γ-lactams in high yields. Thus, a ruthenium alkoxide/pyridine system is a more practical reaction system.