Multielectron Atom Transfer Reactions of Perchlorate and Other Substrates Catalyzed by Rhenium Oxazoline and Thiazoline Complexes:  Reaction Kinetics, Mechanisms, and Density Functional Theory Calculations

Abstract

The title complexes, the Re(O)L(2)(Solv)(+) complexes (L = hoz, 2-(2'-hydroxyphenyl)-2-oxazoline(-) or thoz, 2-(2'-hydroxyphenyl)-2-thiazoline(-); Solv = H(2)O or CH(3)CN), are effective catalysts for the following fundamental oxo transfer reaction between closed shell molecules: XO + Y --> X + YO. Among suitable oxygen acceptors (Y's) are organic thioethers and phosphines, and among suitable oxo donors (XO's) are pyridine N-oxide (PyO), t-BuOOH, and inorganic oxyanions. One of the remarkable features of these catalysts is their high kinetic competency in effecting perchlorate reduction by pure atom transfer. Oxo transfer to rhenium(V) proceeds cleanly to afford the cationic dioxorhenium(VII) complex Re(O)(2)L(2)(+) in a two-step mechanism, rapid substrate (XO) coordination to give the precursor adduct cis-Re(V)(O)(OX)L(2)(+) followed by oxygen atom transfer (OAT) as the rate determining step. Electronic variations with PyO derivatives demonstrated that electron-withdrawing substituents accelerate the rate of Re(VII)(O)(2)L(2)(+) formation from the precursor adduct cis-Re(V)(O)(OX)L(2)(+). The activation parameters for OAT with picoline N-oxide and chlorate have been measured; the entropic barrier to oxo transfer is essentially zero. The potential energy surface for the reaction of Re(O)(hoz)(2)(OH(2))(+) with PyO was defined, and all pertinent intermediates and transition states along the reaction pathway were located by density functional theory (DFT) calculations (B3LYP/6-31G). In the second half of the catalytic cycle, Re(O)(2)L(2)(+) reacts with oxygen acceptors (Y's) in second-order reactions with associative transition states. The rate of OAT to substrates spans a remarkable range of 0.1-10(6) L mol(-)(1) s(-)(1), and the substrate reactivity order is Ph(3)P > dialkyl sulfides > alkyl aryl sulfides > Ph(2)S approximately DMSO, which demonstrates electrophilic oxo transfer. Competing deactivation and inhibitory pathways as well as their relevant kinetics are also reported.

Oxidorhenium(V) Complexes with Tridentate and Tetradentate Phenol‐Based Ligands

“…As the active species, an oxidized form of our compounds with a dioxidorhenium(VII) core is conceivable since the Re VII complex of the type [ReO 2 (hoz) 2 ] + with the (hydroxyphenyl)oxazoline ligand (hoz) has been crystallographically characterized. [46] However, the identification of the active catalyst has so far remained elusive. Attempts to use H 2 O 2 instead of TBHB as the oxidant gave no conversion to the epoxide.…”

“…[1][2][3][4][5][6][7][8][9] Mechanistic aspects were thoroughly investigated by Espenson and co-workers to reveal the two most prominent requirements for a given rhenium(V) compound to be an active catalyst: the active site must allow coordination of the oxygen atom donor such as pyridine Noxide, dimethyl sulfoxide, or tert-butyl hydroperoxide (TBHP), [10] and the breaking of the O-X bond within the oxygen donor (e.g., in Py-O) is facilitated by nucleophiles. [3] Rhenium(V) compounds were particularly well studied, because they are easily prepared and are usually moistureand air-stable.…”

Oxygen Atom Transfer

“…It should be noted that the first-order rate constants measured for ClO 4 À and ClO 3 À at substrate saturation are comparable, and this rate constant corresponds to the OAT step following anion coordination to rhenium(V) (Figure 3.27). 74 Hence, chlorate is a superior oxidant in this instant not because it undergoes faster OAT but because it is a better ligand than perchlorate (see K values in Figure 3.27). The oxazoline catalyst shows product inhibition at high chloride concentrations, which is not surprising.…”

Oxidorhenium(V) Complexes with Oxazolinylmethoxido Ligands – Structure and Catalytic Epoxidation

“…[3][4][5][6][7] Abu-Omar et al demonstrated that oxidorhenium(V) complexes containing 2-(2Ј-hydroxyphenyl)-2-oxazoline exhibit interesting catalytic behaviour for oxygen atom transfer reactions. [4,8,9] Most interestingly, they were found to successfully catalyze the reduction of perchlorate (ClO 4 -) to chloride ions, [4,8] a reaction that is environmentally interesting (waste water cleaning) but kinetically hindered. [10] The success of oxazoline ligands as stabilizing agents in catalysis as well as our ongoing interest in rhenium-catalyzed epoxidations [11][12][13][14][15] prompted us to investigate a largely unexplored oxazoline ligand in oxidorhenium(V) chemistry.…”