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-PtBu2 (Bis = CH(SiMe3)2; GaOP) is presented towards a series of C=O 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 to a 2:1 FLP adduct in a Claisen-type addition. Tests towards various acetylene containing species revealed that the ratio of deprotonation to ring-closure product can be adjusted. While PhC≡CH is almost exclusively bound as the deprotonation product, for Me3SiC≡CH also the ring-closure product forms. In the reaction with benzyl chloride, Bis2(Cl)Ga-O-P(Bn)tBu2 is formed by a C-Cl bond activation.