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A study of catalytic autoxidation of organic substrates using H2/O2 mixtures in the presence of rhodium complexes containing dimethylsulfoxide ligands

University of British Columbia

Dimethylacetamide (DMA) solvent is oxidized catalytically to CH₃CON(CH₃)CH₂OOH and CH₃CON(CH₃)CHO under H₂/0₂ mixtures at 50°C in the presence of the dimethylsulfoxide complex RhCl₃(DMSO)₃ (I) at a rate which is much faster than peroxide-initiated autoxidation of DMA under O₂ alone. The hydroperoxide is thought to be the initial product, and the N-formyl derivative its decomposition product. An accompanying metal-catalyzed hydrogenolysis of 0₂ leads to H₂0₂ and H₂0. Hydrogen peroxide and CH₃CON(CH₃)CH₂OOH are the only products formed in the early stages of the catalytic reaction. The maximum rate of gas uptake in this initial region is independent of the partial pressure of 0₂, but shows linear dependences on Rh and H₂. Stoichiometry, rate and spectral data are consistent with an initiation reaction between complex I and H₂, and then 0₂ to give a catalytically active RhIII (0₂=) (DMA) species (II) (eq. 1) [formula omitted] The autoxidation of DMA and the hydrogenolysis of 0₂ are postulated to occur via independent pathways involving II (eqs. 2 and 3). [formula omitted] In the absence of H2, II degenerates to catalytically inactive species. The role of H₂ in the DMA autoxidation is thought to be the regeneration of Rh I species and hence II, from deactivated forms of II. Eventual slow, irreversible deactivation of the catalyst and the probable participation of the H₂0₂ product in peroxide initiated free-radical autoxidations complicate the interpretation of later stages of reaction. Diphenylsulfide (DPS) is catalytically oxidized to the sulfoxide by complex I under H₂/O₂ in DMA at 50°C, but accompanying oxidation of the solvent persists even in the presence of a 100-fold excess of DPS over Rh. Oxidation of the sulfide is thought to involve H₂0₂ liberated in the catalytic hydrogenolysis of 0₂. Complex I in CH₂C1₂ or C₂H₄CL₂ reacts with CO to give the dimethylsulfide complex RhCL₃(DMS)₃ via a facile reduction of DMSO ligands. Dimethylsulfoxide is reduced also by RhI species in CH₂CL₂ in the presence of two equivalents of acid to yield DMS, RhIII and H₂0. However, Rh I /2H⁺/DMS0 systems are relatively stable in DMA, because of the proton affinity of the solvent. Complex I reacts also with the strongly basic tertiary amine NEt₃ via a redox process in which the RhIII is reduced to Rh I with an accompanying dehydrogenation of the amine (eq. 4). RhCl₃ + 3NEt₃ → RhCl + 2NEt₃ HCl + CH₂=CHNEt₂ (4) The resulting ethenamine then reacts with I to give the ƞ¹-ylidic complex, RhCl₃(DMS̠O)₂(⁻CH₂CH=⁺NEt₂). Data from an earlier thesis, on a reaction between complex I and 1,8-bis(dimethylamino)naphthalene (or Proton Sponge), are reinterpreted in terms of a similar redox reaction that gives an N-carbene fragment (eq. 5),which is stabilized within the RhIII complex, RhCl₃(DMS̠O)₂(=CH-N(Me)-C₁₀H₆NMe₂•HCl). [formula omitted]

Text

2010-06-16

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http://hdl.handle.net/2429/25791

Chemistry

University of British Columbia

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