Synthesis of some [1,2-bis(alkoxycarbonyl)ethyl]ruthenium(II) complexes and their catalytic activities for cis-trans isomerization of dialkyl maleates

Abstract

The hydridoruthenium(II) complex [RuCl(CO)H(PPh₃)₃](1) reacted smoothly with dimethyl maleate and diethyl maleate at room temperature to give insertion products, [[graphic omitted]-OR}Cl(CO)(PPh₃)₂](2; R = Me) and (3; R = Et), respectively. Complex (2) was also formed by reaction of (1) with dimethyl fumarate. Complexes (2) and (3) were converted into binuclear complexes [{[graphic omitted])OR](µ-Cl)(CO)(PPh₃)}₂](4; R = Me) and (5; R = Et), respectively, by heating above 100 °C. Complex (4) reacted with Na[BH(pz)₃](pz = 1-pyrazolyl) at ambient temperature to afford [Ru{CH(CO₂Me)CH₂CO₂Me}(CO){BH(pz)₃}(PPh₃)]. Complexes (1), (2), and (4) were found to catalyze cis-trans isomerization of dialkyl maleates above 100 °C. A kinetic study of the isomerization of dimethyl maleate revealed that the reaction rates were proportional to the concentrations of both the substrate and the catalyst, and that the ratio of the catalytic constants per ruthenium atom is about 1.6:1.0:0.36 for (1), (2), and (4), respectively, at 120 °C. Addition of PPh₃ to the catalyst accelerated the reaction rates. It is deduced that the isomerization proceeds through an insertion–β-elimination mechanism.