Thermodynamic and kinetic studies of hydride transfer from Hantzsch ester under the promotion of organic bases

Base-promoted hydride transfer (BPHyT), a strategy for the upconversion of hydride donors, was studied here in a three-component system, composed of Hantzsch ester (HEH), acridinium derivatives and organic bases. Based on the thermodynamic parameters of hydricity and pKa, we proposed a thermodynamic cycle to evaluate the apparent hydricity of HEH/base combinations, as well as the overall driving force of BPHyT. Brønsted-type linear analysis indicated that the base used in BPHyT is much more effective to regulate the reaction kinetics, compared to conventional Brønsted acid or base catalysis. Structure-reactivity relationships showed that the hydride acceptor and the base contribute equally to regulate the kinetics of BPHyT. Kinetic isotope effects suggested that the hydride transfer is involved in the rate-determining step. Reductions of the polar CC bonds by HEH/base combinations were performed to confirm the feasibility of applying BPHyT in organic synthesis.