Mechanism for the Formation of 1,2-Dioxan-3-ols

1.2-Dioxan-3-ol derivatives are formed at ambient temperature during the aerobic oxidation catalyzed by the Mn(II)-Mn(III) redox system. The manganese(III)-enolate complex A would be formed by the reaction of

1.3-dicarbonyl compounds with manganese(III) acetate during the first stage (Scheme 31). In fact, the corresponding enolate complex could be isolated by the reaction of manganese(III) acetate with 4,4,4-trifluoro-1-phenylbutane-1,3-dione in acetic acid at 23 °C for 1 h [106]. The enolate complex formation is the rate-determining step [44,124] and the key to the catalytic reaction. When alkenes are present in the reaction system, complexes between the alkenes (electron donor) and the manganese(III)-enolate complex (electron acceptor) are immediately formed in situ and a one-electron-transfer oxidation easily occurs to produce the corresponding carbon radicals B and release manganese(II) acetate [124]. The carbon radicals B take up dissolved molecular oxygen in the solvent to form the peroxy radicals C. The per-oxy radicals C could be reduced by the manganese(II)-enolate complex D, which would be formed by the ligand-exchange reaction of the released man-ganese(II) acetate with 1,3-dicarbonyl compounds, followed by cyclization to finally produce the thermodynamically stable OH-axial 1,2-dioxanes [168].

The manganese(III) species A should be reproduced in the reaction system, and the catalytic cycle must be continued until the added alkenes and 1,3-dicarbonyl compounds are completely consumed [46,77,79,80,83,106, 125-128,134,135,142,143]. The mechanism for the formation of peroxides is summarized in Scheme 31.

The formation of manganese(II)-enolate complex D is essential for the catalytic reaction since the complex D is lightly oxidized by the peroxy radicals C, dissolved oxygen, or other added metal oxidants (such as man-ganese(III) acetate, cobalt(III) acetate, potassium permanganate, lead(IV) acetate, chromium(IV) trioxide, thallium(III) acetate, ammonium cerium(IV) nitrate, copper(II) acetate, and iron(III) perchlorate) though the oxidation of manganese(II) acetate itself is very slow in air at ambient temperature [80]. Therefore, it is possible to use manganese(II) acetate instead of

Scheme 31 Mechanism of the manganese(III)-catalyzed aerobic oxidation

manganese(III) species for the aerobic oxidation [79-82,137]. The hydroperoxidation of cyclic amides could also be explained by a similar mechanism for the manganese(III)-catalyzed aerobic oxidation. The cyclization between the hydroperoxy group and amido carbonyl group might not occur because the electrophilicity of the amido carbonyl carbon is poor.

In contrast, when the reaction is carried out at elevated temperatures using a stoichiometric amount of manganese(III) acetate, the reaction is dramatically changed. The formed carbon radicals B would be quickly oxidized by manganese(III) species in the reaction at elevated temperature due to the absence of dissolved molecular oxygen and the presence of sufficient man-ganese(III) acetate or cooxidant such as copper(II) acetate [44]. As a result, carbocations E would be produced and would cyclize at the carbonyl oxygen to give furan or lactone derivatives [135,169-173].

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