Mechanism of glycol cleavage

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

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.

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. 

Another is the oxidation of a secondary alcohol to a ketone (9-3), where A and B are alkyl or aryl groups and Z is also CrOjH. In the lead tetraacetate oxidation of glycols (9-7) the mechanism also follows this pattern, but the positive leaving group is carbon instead of hydrogen. It should be noted that the cleavage shown is an example of an E2 elimination. 

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. 17.58. The mechanism of the McMurry reaction. A mixture of diastereomers of the dititanium(III) glycolate C is either directly generated via the initially formed Ti(in) ketyl B (variant 1) or in multiple steps (variant 2). At sufficiently high temperatures, C is then reduced to a mixture of diastereomers of the corresponding dititanium(III) glycolate G. The latter decomposes via homolytic cleavage of one of its C—0 bonds to furnish the radical intermediate H. If the only C—0 bond left in this radical also breaks homolytically—which partly occurs without ( ) prior rotation around the C-C(OTiCl) bond—the alkene is formed as an F,Z-mixture. Its composition may (somewhat) depend on the configuration of the dititanium(II) glycolate precursor G (cf. Figure 17.56).

Fig. 14.49. Mechanism of the McMurry reaction. The heterocyclic monotitanium glycolates A and B or analogous dititanium glycolates decompose at higher temperatures via heterolytic cleavage of one of their C—O bonds and form the radical intermediate C. The alkene is formed by cleavage of the second C—O bond. This alkene is not obtained as a single stereoisomer because of the free rotation about the C—C(O) bond in the radical intermediate C. Note that the alkene is formed with a cis, fraraj-selectivity that is independent of the configuration of the titanium glycolate precursor(s).

On p. 590 we discuss briefly the mechanism of the Os04-alkene-L reaction. The reaction in which cis diols R(OH)2 react with 0s03py2 is thought to proceed via nucleophilic attack by the diol on osmium pyridine species. This is consistent with kinetic data and also with l80-labelling experiments on thymine glycols which demonstrated that there was no C—O bond cleavage.530... 

Oxidative cleavage of 1,2-glycols. Trahanovsky et al.1 have studied the relative rates of oxidative cleavage of 1,2-glycols with CAN and with lead tetraacetate and have concluded that the mechanism in the case of CAN involves formation of a monodentate complex followed by a one-electron cleavage to give an intermediate radical that is oxidized further (scheme I). The cleavage with lead tetraacetate (1, 554-... 

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