Thermodynamic Evaluations of Amines as Hydrides or Two Hydrogen Ions Reductants and Imines as Protons or Two Hydrogen Ions Acceptors, as Well as Their Application in Hydrogenation Reactions

Abstract

Since the hydrogenation of imines (X) and the dehydrogenation of amines (XH 2 ) generally involve the two hydrogen ions (H − + H + ) transfer, the thermodynamic abilities of various amines releasing hydrides or two hydrogen ions as well as various imines accepting protons or two hydrogen ions are important and characteristic physical parameters. In this work, the pK a values of 84 protonated imines (XH + ) in acetonitrile were predicted. Combining Gibbs free energy changes of amines releasing hydrides in acetonitrile from our previous work with the pK a (XH + ) values, the Gibbs free energy changes of amines releasing two hydrogen ions and imines accepting two hydrogen ions were derived using Hess's law by constructing thermochemical cycles, and the thermodynamic evaluations of amines as hydrides or two hydrogen ions reductants and imines as protons or two hydrogen ions acceptors are well compared and discussed. Eventually, the practical application of thermodynamic data for amines and imines on hydrogenation feasibility, mechanism, and possible elementary steps was shown and discussed in this paper from the point of thermodynamics.

Thermodynamic evaluations of the acceptorless dehydrogenation and hydrogenation of pre-aromatic and aromatic N-heterocycles in acetonitrile

“…aΔ G H − PR ( YH 2 ) and Δ G H − PA ( Y ) values are from ref. 20.bΔ G H 2 R ( YH 2 ) = Δ G H − PR ( YH 2 ) + Δ G PA (H − ).cΔ G H 2 A ( Y ) = −Δ G H 2 R ( YH 2 ).…”

“…For better comparing the relationship between the thermodynamic abilities and chemical structures, all 84 amines ( YH 2 ) were divided into 4 groups, 20 comprising 1,4-dihydropyridine compounds ( Y I H 2 , 1H 2 –62H 2 ), 1,2-dihydropyridines ( Y II H 2 , 63H 2 –73H 2 ), 1,2-dihydro-3-substituted-indoline analogs ( Y III H , 74H 2 –78H 2 ), and general amines ( Y IV H 2 , 79H 2 –84H 2 ) (Scheme 4). Among the 4 groups, Y I H 2 , Y II H 2 , and Y III H 2 were 1,4-dihydro nitric six-member heterocycles, 1,2-dihydro nitric six-member heterocycles, and 1,2-dihydro nitric five-member heterocycles, respectively, which all belong to pre-aromatic N-heterocycles.…”

“…Step 2 is the chemical process of YH 2 releasing two hydrogen ions, YH 2 → Y + H − + H + , and the corresponding thermodynamic driving force was defined as the Gibbs free energy of YH 2 releasing two hydrogen ions, Δ G H − PR (YH 2 ). 20 Step 3 is the chemical process of H − reacting with H + to form H 2 , H − + H + → H 2 , and the related thermodynamic driving force was defined as the Gibbs free energy of one molar H − reacting with one molar H + to generate one molar H 2 , Δ G PA (H − ). The Δ G PA (H − ) value was reported as −76.0 kcal mol −1 in acetonitrile.…”

“…3–8 This work follows on from our previous study into the thermodynamics of hydrogen transfer for amines. 20 This present work focuses on thermodynamic evaluations of the acceptorless dehydrogenation and hydrogenation of a special category of N-heterocycles, namely, pre-aromatic and aromatic N-heterocycles, in acetonitrile.…”

Thermodynamic H-Abstraction Abilities of Nitrogen Centered Radical Cations as Potential Hydrogen Atom Transfer Catalysts in Y–H Bond Functionalization

“…Considering the stability, possible side effects, and resulting concentration of X •+ , the H-affinities of X •+ could not be directly measured by isothermal titration calorimetry or other bond energy measurement methods. In this work, the p K a values of 120 XH + (protonated N-containing compounds) in AN are predicted, and the H-affinities of 120 X •+ are derived from the reduction potentials of X •+ and p K a of XH + employing Hess’ law by constructing a thermodynamic cycle. − Moreover, the thermodynamic H-abstraction properties of 120 X •+ (Scheme ) in AN have been discussed in detail.…”