A NOVEL OXYGEN-CARRYING AND ACTIVATING SYSTEM OF RHODIUM(III) - OXIDATION AND OXYGENATION REACTIONS OF 3,5-DI-TERT-BUTYLCATECHOL CATALYZED BY A RHODIUM(III) CATHECOLATE THROUGH ITS (ETA-1-SUPEROXO)(ETA-2-SEMIQUINONATO)RHODIUM(III) COMPLEX

The coordinatively unsaturated Rh(III) catecholate complex [(triphos)Rh(3,5-DBCat)]+ (2+) has been synthesized by oxidative addition of 3,5-DBQ to the 16-electron fragment [(triphos)RhClj [triphos = MeC(CH2PPh2)3; 3,5-DBCat = 3,5-di-tert-butylcatecholate; 3,5-DBQ = 3,5-di-tert-butyl-o-benzoquinone]. Complex 2+ undergoes electron-transfer reactions that encompass the Rh(III), Rh(II), and Rh(I) oxidation states of the metal and the catecholate, semiquinone, and quinone oxidation levels of the quinoid ligand. The Rh(II) derivative [(triphos)Rh(3,5-DBCat)] (3) and the Rh(III) semiquinone complex [(triphos)Rh(3,5-DBSQ)]2+ (42+) have been characterized by X-band ESR spectroscopy [3,5-DBSQ = 3,5-di-tret-butylsemiquinonate]. Below 10 °C, complex 2+ in CH2Cl2 or MeCN picks up dioxygen in a reversible manner, forming a diamagnetic dioxygen adduct, which has been identified by NMR and electrochemical techniques as [(triphos)Rh(η1-O2)(η2-3,5-DBSQ)]+ (5+). The η1-superoxo complex 5+ is able to oxygenate inorganic substrates such as SO2 and PPh3 to sulfate and phosphine oxide, respectively. While the reaction with SO2 is stoichiometric, leading to the stable η2-sulfate-O,O complex [(triphos)Rh(SO4)]+, that with PPh3 is catalytic under an oxygen atmosphere. The starting catecholate compound 2+ is recovered intact after the catalysis cycle. 3,5-Di-tert-butylcatechol is both oxidized and oxygenated by 5+ in a catalytic way producing o-quinone, muconic acid anhydride, and 2H-pyran-2-one. The oxidation to quinone, which is the prevailing reaction, is accompanied by production of H2O2. The characteristic features of the present nonenzymatic oxygenation of 3,5-di-tert-butylcatechol with molecular oxygen are as follows: (i) the process takes place in an intermolecular fashion, (ii) the catechol is oxygenated into the Hamilton intermediates of either the intra-diol or extra-diol type, and (iii) the oxygenation does not proceed through a quinone intermediate. © 1990, American Chemical Society. All rights reserved.