Theoretical Study of Oxidation of Cyclohexane Diol to Adipic Anhydride by [Ru^IV(O)(tpa)(H₂O)]²⁺ Complex (tpa ═ Tris(2-pyridylmethyl)amine)

Abstract

The catalytic conversion of 1,2-cyclohexanediol to adipic anhydride by Ru(IV)O(tpa) (tpa ═ tris(2-pyridylmethyl)amine) is discussed using density functional theory calculations. The whole reaction is divided into three steps: (1) formation of α-hydroxy cyclohexanone by dehydrogenation of cyclohexanediol, (2) formation of 1,2-cyclohexanedione by dehydrogenation of α-hydroxy cyclohexanone, and (3) formation of adipic anhydride by oxygenation of cyclohexanedione. In each step the two-electron oxidation is performed by Ru(IV)O(tpa) active species, which is reduced to bis-aqua Ru(II)(tpa) complex. The Ru(II) complex is reactivated using Ce(IV) and water as an oxygen source. There are two different pathways of the first two steps of the conversion depending on whether the direct H-atom abstraction occurs on a C-H bond or on its adjacent oxygen O-H. In the first step, the C-H (O-H) bond dissociation occurs in TS1 (TS2-1) with an activation barrier of 21.4 (21.6) kcal/mol, which is followed by abstraction of another hydrogen with the spin transition in both pathways. The second process also bifurcates into two reaction pathways. TS3 (TS4-1) is leading to dissociation of the C-H (O-H) bond, and the activation barrier of TS3 (TS4-1) is 20.2 (20.7) kcal/mol. In the third step, oxo ligand attack on the carbonyl carbon and hydrogen migration from the water ligand occur via TS5 with an activation barrier of 17.4 kcal/mol leading to a stable tetrahedral intermediate in a triplet state. However, the slightly higher energy singlet state of this tetrahedral intermediate is unstable; therefore, a spin crossover spontaneously transforms the tetrahedral intermediate into a dione complex by a hydrogen rebound and a C-C bond cleavage. Kinetic isotope effects (k(H)/k(D)) for the electronic processes of the C-H bond dissociations calculated to be 4.9-7.4 at 300 K are in good agreement with experiment values of 2.8-9.0.

Efficient Vanadium‐Catalyzed Aerobic C−C Bond Oxidative Cleavage of Vicinal Diols

“…Beside the intrinsic problem related to the use of stoichiometric amounts of oxidants, most of these reagents suffer from severe drawbacks in terms of selectivity, toxicity, storage, handling, cost and solubility. More recently, catalytic procedures have been reported using homogeneous iron, molybdenum, copper, manganese, ruthenium, cobalt, palladium, or polyoxometalate complexes. Sacrificial stoichiometric organic or inorganic oxidants are however still required ,,.…”

“…Sacrificial stoichiometric organic or inorganic oxidants are however still required ,,. When O 2 is used as primary oxidant, high pressures, or activating sacrificial substrates,, are required or the scope of the substrate is limited to the more reactive tertiary or benzylic glycols . Therefore, the design of more efficient and environmentally benign catalytic processes for the C−C bond cleavage of diols remains a topic of key importance.…”

Oxidation of Organic Substrates with RuIV=O Complexes Formed by Proton-Coupled Electron Transfer

“…Figure 5 DFT-optimized structure of the hydrogen-bonded adduct between 6 and hfp Determinations of KIEs for the oxidations with 6-8 have been also conducted with propan- 43b or with methanol, [1,1,1-2 H 3 ]methanol, and [1-2 H]methanol. 48,56 C-H/C-D isotopic substitution in the substrates affords significant KIE values, in the range 1.8-3.3 for all the oxidants and for both substrates, whereas the KIE values for the O-H/O-D isotopic substitution are 1.0-1.1 for all oxidants and substrates, indicating there is no KIE for the O-H bond. Consequently, the rate-determining step for substrate oxidation with 6-8 involves the abstraction of a hydrogen atom from the α-C-H bond of the alcohol, rather than from the O-H group.…”

Iron, ruthenium and osmium

“…[Ru IV (O)-(tpa)(H 2 O)] 2+ catalyses the conversion of cyclohexane diol to adipic anhydride, ending up as a bis-aqua Ru II (tpa) complex, from which the catalyst can be regenerated by Ce(IV). 96 Ce(IV) oxidises [Ru(TPA)(bpy)] 2+ to a Ru(IV)-oxo complex, [Ru(O)(H + TPA)(bpy)] 3+ , in which one of the pyridinyl arms is uncoordinated. In MeCN solution, the Ru(IV) complex oxidises and oxygenates compounds with saturated C-H bonds.…”