Quantum-Chemical Study of “Hydride” Mobility in the Molecules of Chalcogenopyrans

Abstract

An approach is proposed for the quantum-chemical investigation of "hydride ion" transfer based on analysis of the similarity of the order of variation in the ionization potentials, enthalpies, and free energies of affinity to the hydride ion, the hydrogen atom, and the proton in the substrate molecules and also the derivatives of their cations, radicals, and ions to the experimentally established "hydride" series. It was established that the experimental "hydride" mobility series of six chalcogenopyrans based on "semicyclic" 1,5-diketones agrees with the quantum-chemically calculated ionization potentials of the molecules and with the affinity of the respective radicals to the hydrogen atom participating in the transfer. It was found that direct removal of a hydride ion and initial deprotonation of the substrates followed by the removal of two electrons are unlikely. "Hydride" shift mechanisms, in which the first stage is transfer of an electron or hydrogen atom from the chalcogenopyran molecules, are feasible.Reactions including stages that can formally be represented as hydride-ion transfer are of great significance in chemistry [1]. Examples include the transformations of 1,5-dicarbonyl compounds and chalcogenopyrans in a mixture of chalcogenopyrylium salts and chalcogenocyclohexanes or chalcogenopyrylium salts and other reduced products [2]. The following mechanisms of "hydride" transfer have been discussed for a wide range of chemical compounds [1]:1. Direct abstraction of a hydride ion (-H -). 2. Successive ejection of an electron, a proton, and another electron (-e -, -H + , -e -). 3. Removal of an electron and a hydrogen atom (-e -, -H · ). 4. Transfer of a hydrogen atom and an electron (-H · , -e -). 5. Deprotonation followed by the transfer of two electrons (-H + , -e -, -e -). The mechanism of the "minus electron, minus electron, minus proton" type (-e -, -e -, -H + ) should not be realized since the formation of the dication-radical is unlikely.However, in our opinion, there also exists a mechanism not examined in the present paper, involving initial protonation of the heterocycle and a final stage with release of H 2 [3] or another electrically neutral molecule [4]. The possible protonation of a heterocycle at a chalcogen atom was demonstrated conclusively in the papers of Lambert and others [5]. __________________________________________________________________________________________ 1 N.

A comparative theoretical study of the hydride transfer mechanisms during LiAlH 4 and LiBH 4 reductions

“…Hydride transfer is a key process in hydride reduction and so, to elucidate the associated reaction mechanism, it is important to understand the hydride reduction mechanism. In previous works, the hydride transfer mechanisms in many biological and organic reduction systems were investigated using either experimental or theoretical approaches [13][14][15][16][17][18][19][20][21][22][23][24]. Although these mechanisms have not yet been fully explained due to the variety of possible pathways involving the overall transfer of two electrons and one proton, the following hydride transfer mechanisms have been proposed: (1) direct transfer of a hydride ion (H -) in a single step, (2) two-step transfer of an electron before or after the transfer of a hydrogen atom and (3) transfer of two electrons and one proton in three separate steps.…”

“…Theoretical studies aimed at understanding the hydride transfer mechanism have also been conducted, focusing on the effects of the charge density, ionization potential and proton affinity of the reactant and transition state [13][14][15][16][17][18]21,23,24]. However, as far as we know, there are no reports of detailed charge density analysis along the reaction coordinates during hydride transfer, although it requires complete information regarding the dynamics of the mechanism.…”