Periodate ester

The cleavage of 1,2-diols 1 by periodic acid is associated with the name of the French chemist Malaprade. The reaction mechanism is related to that outlined above, and is likely to involve a five-membered ring periodate ester intermediate 7 ... 

There are ample evidences which suggest that a cyclic periodate ester is the intermediate 148). Cyclobutane-1,2-diols can also be cleaved oxidatively and this aspect has been used in organic synthesis. Thus, photocycloaddition of l,2-bis(tri-methylsiloxy)cyclobutene to (—)piperitone (381) gave the photoadduct (438). Desilyla-... 

An alternative method of cleaving C=C bonds is to use OSO4 in conjunction with NaI04. The diol product forms a periodate ester, which decomposes to give two molecules of aldehyde. These are themselves oxidized by the periodate to carboxylic acids. 

The oxidation with periodic acid also occurs via cyclic intermediate periodate ester. Periodate ester undergoes rearrangement of the electrons, which leads to the formation of a C-C bond and two C=0 bonds (Scheme 7.30). 

The mechanism of cleavage by periodate is consistent with a cyclic, five-membened ring intermediate (10) shown in Scheme 6. Support comes from the fact that the cis isomers of cyclic diols are more reactive than the trans isomers, threo-, 2-Aio i undergo oxidation faster than the erythro isomers and the inert behavior of diaxial rrans-1,2-diols which cannot form a cyclic periodate ester." ... 

Evidence for the formation of diol-periodate esters Diol-periodate esters are recognized as intermediates in the oxidation of 1,2-... 

The formation of diol-periodate esters is supported by physical evidence. The addition of ethane-1,2-diol to periodate solutions causes an initial rapid change in the uv absorption spectrum, followed by a slower change as the oxidation proceeds and lOJ is formed. Similar results are observed for other 1,2-diols except for highly substituted diols such as pinacol (Buist et al ). Buist and Bunton have shown that the cyclic periodate esters formed in alkaline solution from 1,2-diols and periodate can be detected by nmr. The initial fall in pH which occurs in the oxidations of ethane-1,2-diol and lightly substituted diols is also attributed to ester formation (Malaprade, Buist and Bunton ). Cyclic triesters, similar to the cyclic diesters formed from 1,2-diols, are formed from cyclic compounds containing the cis-l,2,3-triol system (Barker and Shaw , Dijkstra and from 1,2-0-isopropylidene-a-D-glucofuranose. In the latter case the presence of the triester has been demonstrated by nmr (Berlin and van Rudloff ). Monoesters of periodic acid have not been detected in any system, but they are postulated as intermediates in the formation of cyclic diesters from 1,2-diols (section 1.3.5). 

Taylor pointed out that the kinetics observed for the oxidation of ethanediol are equally consistent with a mechanism involving the direct formation of the products, and an inactive diol-periodate ester, C, in equilibrium with the reactants, viz. 

Angyal and Young report second-order kinetics for the oxidation of the camphane-2,3-diols. The cis- isomers are oxidised much more rapidly than the trans- isomers, and a temperature of 80 °C had to be used for kinetic measurements on the latter. It seems likely that the rigidity of the camphane skeleton prevents the formation of a cyclic diol-periodate ester from the trans- isomers. Possibly the reaction at 80° is completely different in nature from the normal oxidation of 1,2-diols by periodate. The same workers report that cholestane-3j8,6j8,7a-triol, in which the and 7a hydroxyl groups are axial-axial, is inert towards periodate. 

It is evident that the order of reactivity of cis and trans isomers is variable. Their reactivities may well be related to the ease of formation of a cyclic periodate ester, but again any interpretation is limited by uncertainty concerning the rate limiting stage. Barlow and Guthrie later reported their work in more detail, and included kinetic data for the pH range 4-7, at 0 as well as at 25°C. They suggest that the amino-altrosides are oxidised by an unknown mechanism, different from the mechanism of oxidation of the other six isomers. 

Dahlgren and Reed ). Shiner and Wasmuth suggest that the oxidation of 1,2-diketones proceeds via a cyclic periodate ester, similar to that postulated for the oxidation of 1,2-diols, but formed by nucleophilic attack of periodate on carbon, viz. 

The actual presence of a cyclic periodate ester or other intermediate has not been demonstrated experimentally. Dahlgren and Reed suggest that the general base catalysis could be due either to an interaction of the catalyst with periodate in a prior equilibrium, or to proton abstraction prior to formation of the cyclic ester. 

At25 °C,k() = 0.24sec SfcH = 26l.mole sec , and oH = 2.6x 10 Lmole sec". The latter coefficient is about 1/40 of the rate coefficient for a diffusion controlled reaction in water. Kaiser and Weidman suggest that the intermediate is similar to the cyclic periodate ester postulated for the 1,2-diol oxidations, viz. 

H5IO6 or lOJ on the hydroxyl group to form a periodate ester whose rate of decomposition is fast. The same workers also studied the oxidation of p-hy-droxyphenyl phosphate by periodate (aqueous solutions, pH 0-10, 25 C). The kinetics are second-order, and the rate maxima are at pH s 0.7 and 7.5. Bunton and Hellyer accounted for the pH dependence by assuming (a) that both the ester and its monoanion react with H5IO6, and (b) that the ester monoanion and dianion react with a periodate monoanion. The decrease in rate beyond pH 7.5 shows that reactions involving the phenoxide ion or the periodate dianion are unimportant. 

Periodate ions IO4 react with 1,2-diols to form periodate esters 64 that are unstable and release iodate ions IO3". During that process, the carbon bond is oxidatively cleaved and two aldehydes result. Here, the desired aldehyde 66 and formaldehyde (65) are formed. 

Periodic acid and periodates have found wide application in organic chemistry as powerfiil oxidizing reagents [729-731]. The well-known periodate glycol oxidation involves the cyclic periodate esters as key intermediates however, none of the organic periodate esters have been isolated as stable compounds. 

The cyclic periodate ester breaks down spontaneously, as shown below, creating a pair of carbonyl groups and a pentavalent, pentacoordinate iodine intermediate ... 

Enzyme-catalyzed oxidation of the primary hydro)yl group at carbon 6 of a hexose yields a uronic acid, examples of which are common in both the plant and animal worlds. Periodic acid cleaves the carbon-carbon bond of a glycol in a reaction that proceeds through a cyclic periodic ester. This reaction was once useful in carbohydrate stmcture determination. 

The mechanism of this transformation proceeds through a cyclic periodate ester, which decomposes to give two carbonyl groups. 

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