Formation of 1,2-Dioxanes Using Tris(2,4-pentanedionato)manganese(III)

Tris(2,4-pentanedionato)manganese(III) was easily prepared by the direct oxidation of 2,4-pentanedione with potassium permanganate [76]. The synthesis of 1,2-dioxane was normally carried out as follows: To a solution of 1,1-diphenylethene in glacial acetic acid was added freshly prepared tris(2,4-

R1 R2


AcOH air, rt




Scheme 4 Reaction of alkenes with tris(2,4-pentanedionato)manganese(IH) in acetic acid at room temperature in air pentanedionato)manganese(III) at a molar ratio of 1: 1, and the solution stirred at ambient temperature in a round-bottomed flask equipped with a calcium chloride drying tube until the brown color of the Mn(III) species turned a transparent pale yellow. The solvent was removed in vacuo and the residue triturated with water followed by extraction. After separation by silica gel TLC or column chromatography, crystalline 1,2-dioxan-3-ol was obtained in excellent yield (Scheme 4) [77]. Other 1,1-disubstituted ethenes also gave similar results. However, the yields from the reaction of aliphatic terminal alkenes and cyclic alkenes were poor. In addition, under an oxygen atmosphere, formally produced ligand radicals were preferentially oxidized before reacting with the alkenes, and the formation of the 1,2-dioxan-3-oles was consequently prevented. It was found that the best results were obtained by the reaction in air at room temperature (23 °C). The formation of 1,2-dioxane is characteristic for manganese(III). Other transition metal complexes, such as Cr(III), Fe(III), Co(III), Ni(II), and Cu(II), do not produce the reaction [77].

For the synthesis using tris(2,4-pentanedionato)manganese(III), the

1.2-dioxan-3-ol scaffold was limited to bear only an acetyl and methyl functionality derived from 2,4-pentanedione without alkenes. Therefore, in order to apply the synthesis of various endoperoxide analogs, the behavior of man-ganese(III) acetate in acetic acid at ambient temperature in the presence of

1.3-dicarbonyl compounds needed attention.

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