Enantio- and Regioselective Epoxidation of Olefinic Double Bonds in Quinolones, Pyridones, and Amides Catalyzed by a Ruthenium Porphyrin Catalyst with a Hydrogen Bonding Site

Abstract

An array of differently substituted 3-alkenylquinolones was synthesized, and the enantio- and regioselectivity of their Ru-catalyzed epoxidation were studied. A precursor ruthenium(II) complex with a chiral tricyclic γ-lactam skeleton (octahydro-1H-4,7-methanoisoindol-1-one) was available by Sonogashira cross-coupling with a monobromo-substituted ruthenium(II) porphyrin. Enantioselective epoxidation reactions (60-83% yield, 85-98% ee) were achieved with this catalyst, and it was shown that the enantioselectivity depends critically on the presence of a two-point hydrogen bond interaction between the γ-lactam site of the catalyst and the δ-lactam (quinolone) site of the substrate. DFT calculations support the hypothesis that the reaction occurs via a hydrogen-bound transition state, in which the 3-alkenylquinolone adopts an s-trans conformation. The calculations further revealed that this transition state is preferred over a competing s-cis transition state because it exerts less strain in the rigid backbone and because the hydrogen bond interaction is more stable. The catalyst loading required for complete conversion was low (<0.2 mol %), and turnover numbers exceeding 4000 were recorded. It was shown that there is little, if any, inhibition of the catalytic process by other quinolones, which could potentially compete with the binding site. A mechanistic model for the catalytic reaction is presented. In accordance with this model 3-alkenylpyridones reacted with similar enantioselectivities as the respective quinolones. The epoxidation products were unstable, however, and the enantiomeric purity (77-87% ee) of the products could be established only after derivatization. Primary alkenoic acid amides also underwent the epoxidation but gave the respective products in lower enantioselectivities (70% and 45% ee), presumably because the enantioface differentiation is hampered by the increased flexibility of the substrates, which exhibit two or three rotatable single bonds between the binding site and the reactive olefinic double bond.