Thermodynamics of the elementary steps of organic hydride chemistry determined in acetonitrile and their applications

Abstract

Hydride transfer extensively exists in nature and is widely studied by chemists and biologists. Chemists have designed and synthesized a series of organic hydride compounds as models to mimic the NADH coenzyme and typical unsaturated compounds as hydride acceptors and systematically investigated the thermodynamics of the elementary steps and mechanisms of hydride transfer. This review focuses on ∼5600 thermodynamic driving forces of 421 organic hydrides and unsaturated compounds releasing or accepting hydride or H₂ experimentally determined in acetonitrile, which is a systematic database on the thermodynamics of organic hydride chemistry for chemists to query and use, greatly enriching the presented bond energy database. Based on the depth of research on thermodynamics, the understanding on hydride transfer has developed, experiencing 3 periods, i.e., stage 1: hydride transfer thermodynamics in 6 possible elementary steps, stage 2: H₂ (H⁻–H⁺ and 2H˙) transfer thermodynamics in 8 possible elementary steps, and stage 3: H₂ (H⁻–H⁺ and 2H˙) transfer thermodynamics in 20 possible elementary steps. The considerable amount of thermodynamic data that has been reviewed and compared provides us a golden opportunity to comprehensively discuss and reveal the redox abilities of NADH models and unsaturated compounds. To enable chemists to intuitively judge the redox properties of organic hydride donors and acceptors, such as hydride-donating and hydride-accepting ability, hydrogen-atom-donating and hydrogen-atom-accepting ability, proton-donating and proton-accepting ability, and electron-donating and electron-accepting ability, respectively, based on the 5600 thermodynamic data, we try to classify them into 5 classes according to the mass of the thermodynamic driving forces values, i.e., very strong, strong, medium, weak, and very weak. Most importantly, the applications of the determined thermodynamic database on the identification of mechanisms, designing hydrogenation and reduction reactions, understanding mechanisms based on thermodynamics, and evaluation of H₂ generation, the H-transfer kinetics and bond energy calculation are well demonstrated in this review.