Glycol cleavage mechanism

The solvent isotope effect suggests that no O-H cleavage is involved in the slow step and the effect of O-methylation indicates that a cyclic complex is involved. The induction factor is probably obscured by the reaction of Mn(III) and Mn02 with pinacol itself. The typical glycol-cleavage mechanism advocated for oxidations by Pb(IV) and I(VII) (p. 349) may well operate, viz. 

II. Mechanism of Glycol-cleavage Oxidation by Lead Tetraacetate. 11... 

Fig. 17.23. Mechanism of the glycol cleavage with NaI04 or H5I06, respectively. A diester of iodo(VII) acid (periodic acid) is formed initially. The ester decomposes in a one-step reaction in which three valence electron pairs are shifted simultaneously.

Fig. 17.24. Standard mechanism of the glycol cleavage with Pb(0Ac)4. The reaction proceeds preferentially via a cyclic diester of Pb(IV) acid, which decomposes in a one-step reaction to Pb(0Ac)2 and two equivalents of the carbonyl compound.

Fig. 14.19. The standard mechanism (transition state C) and the alternative mechanism (transition state B) of glycol cleavages with Pb(OAc)4. The trans-glycol A reacts slowly via the monoester B of Pb(IV) acid, while the isomeric di-glycol reacts fast via the cyclic diester C of lead(IV) acid.

Lead tetraacetate is an alternative to periodate for glycol cleavage. It is particularly useful for glycols that have low solubility in the aqueous media used for periodate reactions. A cyclic mechanism is indicated by the same kind of stereochemistry-reactivity relationships discussed for periodate. Unlike periodate, however, glycols that cannot form cyclic intermediates are eventually oxidized. For... 

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