Mechanistic aspects of the oxidative functionalization of ethane and ethanol by platinum(II) salts in aqueous medium. Role of platinum(II) -olefin and platinum(IV) -alkyl intermediates

The relative rate of CH bond activation by the Pt(II) ion decreased in the order HCH₂CH₃ > HCH₂CH₂OH > HCH(OH)CH₃. The platinum(II)-ethylene complex, [PtCl₃(C₂H₄)]^-1, 1, was the key intermediate in the oxidation of ethane, ethanol, and diethyl ether to 1,2-ethanediol by platinum(II) in aqueous medium. In particular, the intermediary of 1 in the oxidation of ethanol to 1,2-ethanediol and 2-chloroethanol was verified through labelling studies. In D₂O, 1, upon oxidation with one of a number of oxidants, converted to [PtCl₅(CH₂CH₂OD)]^2-, 2. 2 in turn decomposed to a mixture of 1,2-ethanediol and 2-chloroethanol on heating. The rate conversion of 1 was a function of pH, the anions present, and the oxidant used. While the conversion of 1 to 2 involved a nucleophilic attack by water for hydroxide ion), such a step was not observed in the absence of an oxidant. On basic D₂O, the sequential replacement of Cl^- by OD^- in 1 occurred to successively form [PtCl₂(OD)(C₂H₄)]^-, [PtCl(OD)₂(C₂H₄)]^- and [Pt(OD)₃(C₂H₄)]^-. The process was reversed upon acidification. The species [PtCl₅(CH₂CHO)]^2-, 3, appeared to be the source for the small quantities of hydroxy- and/ or chloroacetaldehyde formed during the oxidation of 1. 3 was synthesized independently by the reaction of acetaldehyde with a mixture of PtCl₄^2-, and PtCl₆^2- in aqueous medium. When 1 was oxidized by Cl₂ in CD₃OD solution, the principal product was [PtCl₅(CH₂CO₂D)]^2- 4, when a small amount of water was present, and CD₃OCH₂CH₂OCD₃ in the absence of water.