Specific Oxidation of 2 by O2 via the Coordination of in Situ Generated HOOH. Implications for the Rh(III)/Cu(II)-Catalyzed O2 Oxidation of 1-Alkenes to 2-Ketones

Article abstract of DOI:10.1021/ja00349a028

The oxidation of 1-hexene to 2-hexanone catalyzed by Rh(III)/Cu(II) mixtures is investigated.In order to study the reactions that rhodium undergoes to form an active catalyst, 2 (A) is used as a catalyst precursor.A number of results are obtained that indicate that this species must be converted to a rhodium(III) complex before catalysis occurs.With A as a catalyst precursor in the absence of Cu(II), long induction are observed for catalytic oxidations.Rhodium(I) is oxidized to rhodium(III) chloride during the induction period.Furthermore, at higher chloride/rhodium ratios (up to a 10:1 mole ratio), greater initial rates and catalyst stabilities are found.These observations are used as partial justification for characterizing rhodium(III) as an active catalyst in the oxidation of 1-hexene to 2-hexanone.The oxidation of 2 to rhodium(III) chloride is investigated in detail.An unusual mechanism for this reaction is proposed.Hydrogen peroxide, produced in situ from the reduction of O2 by alcohol solvent, oxidizes 2.An intermediate hydroperoxide complex is formed in the course of the oxidation to rhodium(III) which contains a coordinated carbonyl ligand (B).This intermediate is studied in dilute solution and is found to decompose immediately when attempts are made to isolate it.Very few stable hydroxyperoxide and alkylperoxide complexes of the platinum metals have been reported; some are capable of oxidizing terminal olefins to 2-ketones.In contrast, the oxidation of 2 to rhodium(III) chloride under identical conditions is much faster and proceeds by a mechanism avoiding detectable quantities of this hydroperoxo intermediate, while 2 is not oxidized even after 48 h.The oxidation of A to B occurs only in solvents capable of reducing O2.