Glycol cleavage by lead tetraacetate

Whereas GeCl2 and SnCl2 are strong reducing agents, many divalent lead compounds, such as the halides and the diacetate, are perfectly stable, thanks to the inert pair effect. Like 

The mechanism involves the formation of a cyclic tetravalent plumbate ester intermediate  

The five-membered ring then disintegrates via a pericyclic process to generate two carbonyl fragments and lead diacetate. 

Interestingly, a cyclic mechanism is not an absolute requirement for explaining the formation of the observed products. An E2-like elimination from a plumbate monoester can also account for the observed products. 

Support for this alternative mechanism comes from LTA-mediated cleavage of a conforma-tionally restricted cyclohexane-1,2-diol (see below), where the OH groups are locked in a trans orientation that does not permit the formation of a cyclic lead intermediate  

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This lack of an entirely consistent pattern has not prevented the successful application of glycol-cleavage by lead tetraacetate to a wide variety of structural problems. In relating oxidation data to structure, it has been necessary only to recognize that a single approach to a chemical problem is never fully adequate and that other lines of evidence must be sought. 

In the first of three comprehensive studies of the relative rates of glycol cleavage by lead tetraacetate in acetic acid, Criegee noted that with the six pairs of cis-trans isomers or near isomers listed in Table 1 the cis isomer reacts much faster than the trans. On the assumption that the relationship is general, and that the... 

Grundmann s aldehyde synthesis also starts from acid chlorides. The latter are converted by way of the diazo ketones or acetoxy ketones into the glycols, which are subjected to cleavage by lead tetraacetate. <%,/ -Unsaturated aldehydes cannot be obtained by this method.457... 

GRUNDMANN ALDEHYDE SYNTHESIS. Transformation of an acid into an aldehyde of the same chain length by conversion of the acid chloride via the diazo ketone to the acetoxy ketone, reduction with aluminum isopropoxide and hydrolysis to the glycol, and cleavage with lead tetraacetate. 

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

Although carbinols were found not to be oxidized by lead tetraacetate in acetic acid, they were attacked in less polar solvents, and glycol-cleavage was also more rapid in these solvents23 for example, the rate of oxidation of trans-l, 2-cyclohexanediol in tetrachloroethane was 5,000 times that in acetic acid. The kinetics was no longer true second-order in the media of low polarity, but approached this state as the concentration of acetic acid... 

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-... 

Criegee reaction. Oxidative cleavage of vicinal glycols by lead tetraacetate. 

The tetrahydrochromane (2) is oxidized by /M-chloroperbenzoic acid to 6-ketonon-anolide (5) under carefully controlled conditions (2.8 equiv. of peracid, methylene chloride, room temperature). The reaction is postulated to involve the hydroxy peracid (4) as an intermediate which undergoes fragmentation. Use of perphthalic acid gives the glycol (3). which on cleavage with lead tetraacetate gives (5). 

Glycol cleavage by oxidation with sodium periodate, lead tetraacetate, or sodium bismuthate, is a useful method for determination of the ring size of glycosides as well as for the assignment of anomeric configuration and position of substituents on the ring. (See Chapters VI and I.)... 

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