Thermodynamics regulated organic hydride/acid pairs as novel organic hydrogen reductants

Abstract

Organic hydride/acid pairs (XH/HB pairs) have been used as novel hydrogen reductants in hydrogenation reactions. In this work, the thermodynamic possibilities and abilities of XH/HB pairs as potential hydrogen reductants for 216 organic hydrides are well evaluated and discussed. A vital thermodynamic parameter, defined as the thermodynamic driving force of XH/HB pairs releasing two hydrogen ions and denoted as ΔG_PH⁻(XH/HB), is employed to quantitatively measure the thermodynamic abilities of XH/HB pairs as potential organic hydrogen reductants. The thermodynamic abilities of XH/HB pairs as organic hydrogen reductants could be conveniently regulated by choosing suitable organic hydrides and acids with various acidities. For an XH, the regulable thermodynamic range for the reduction abilities of XH/HB pairs is estimated as 41 kcal mol⁻¹ from ΔG_PH⁻(XH/HClO₄) to ΔG_PH⁻(XH/PhOH). Moreover, another thermodynamic parameter, pK_a(HB_S), is proposed and how to choose specific acids (HB_S) to construct XH/HB_S pairs as thermodynamically equivalent organic hydrogen reductants to Hantzsch ester (HEH) is shown. It is indisputable that the XH/HB pair strategy could realize the transformation of N-substituted organic hydrides from simple hydride reductants to thermodynamics regulated hydrogen reductants, which would greatly enrich the molecule library of novel hydrogen reductants.