Ruthenium- and Rhodium-catalyzed Carbenoid Reactions of Diazoesters in Hexaalkylguanidinium-based Ionic Liquids

Hexaalkylguanidinium-based room-temperature ionic liquids were investigated as solvents for the cyclopropanation of styrene with diazoacetates catalyzed by Rh2(OAc)4 or [Ru2(μ-OAc)2(CO)4]n. While the yields of the formed cyclopropanes are much lower compared to the reactions performed in dichloromethane, the diastereomeric ratio is not significantly affected by the change of the reaction medium. Immobilization of the catalysts is only partially successful. In contrast to this intermolecular reaction, the Ru-catalyzed formation of a β -lactam by an intramolecular carbenoid C–H insertion of an α-methoxycarbonyl-α-diazoacetamide occurs in high yield, similar to the Rh2(OAc)4-catalyzed reaction. The cis → trans isomerization of the resulting 1-tert-butyl-3-methoxycarbonyl-4-phenylazetidin-2-one is accelerated in the ionic liquid N,N-dibutyl-N′,N′-diethyl-N′′,N′′-dihexylguanidinium triflate.