Chemoselective Oxidation of 3,5-DI-tert-butylcatechol by Molecular Oxygen. Catalysis by an Iridium(III) Catecholate through its Dioxygen Adduct

The selective oxidation of 3,5-di-tert-butylcatechol( DTBCH,) to 3,5-di-tert-butyl-o-benzoquinon(eD TBQ) by molecular oxygen is catalyzed by the Ir(II1) catecholate complex [(triphos)Ir(DTBC)]+ through its dioxygen adduct [(triphos)Ir(OO)(DTBSQ)]+ [triphos = MeC(CH2PPh2),;D TBC = 3,5-di-tert-butylcatecholateD; TBSQ = 3,5-di-tert-butylsemiquinonate],T he following overall stoichiometric equation is suggested by analysis of the oxidation products and H20z: DTBCHz + 0, - DTBQ + H202. The rates of reaction of the substrate as well as the formation of products are shown to be first order with respect to catalyst and substrate concentrations and zero order with respect to the partial pressure of O2 in the range 15-725 psi. The kinetic parameters for the oxidation reaction are estimated in the temperature range 38-60 OC. Kinetic and thermodynamic data are consistent with a rate-determining step involving the attack of free catechol to the O2 adduct. For partial pressures of 0, higher than 725 psi, the oxygenation of DTBCHz to 3,5-di-rert-butyl-l -oxacyclohepta-3,5-diene-2,7-dionceo mpetes with DTBQ formation.