Ethylene carbonate splitting and Claisen-type self-addition of γ-butyrolactone promoted by an oxygen-bridged Ga/P FLP

Abstract

The versatile reactivity of the geminal oxygen-bridged frustrated Lewis pair (FLP) Bis₂Ga–O–P^tBu₂ (Bis = CH(SiMe₃)₂; GaOP) is presented towards a series of CO double bonds, strained oxygen-containing rings, alkynes plus alkenes and a C–Cl bond. By adding benzaldehyde, cyclopentanone (CP), γ-butyrolactone (GBL) or ethylene carbonate (EC), classical 1,2-adducts are formed. The reversibility of the GaOP·EC adduct stands out, enabling a splitting of EC after several days or when heated to 70 °C, resulting in FLP adducts of CO₂ and ethylene oxide (EO). The latter consists of a six-membered ring and is obtained via a ring-opening reaction, as verified by a further reaction involving the similar propylene oxide (PO). The GaOP·GBL adduct is further converted, with two adduct molecules combining into a 2 : 1 FLP adduct in a Claisen-type addition and ring-opening. Tests towards various acetylene containing species revealed that the ratio of deprotonation to ring-closure product can be adjusted. While PhCCH is almost exclusively bound as the deprotonation product, for Me₃SiCCH, the ring-closure product also forms. In the reaction with benzyl chloride, Bis₂(Cl)Ga–O–P(Bn)^tBu₂ is formed by a C–Cl bond activation.