Vicinal Diols to Carbonyl Compounds

The carbon-carbon bond between two vicinal free hydroxyl groups can be cleaved to give two carbonyl compounds. This reaction is invaluable in the structure determination and transformation of sugars. In connection with hydroxylation of double bonds, it can be used as an alternative to the cleavage of double bonds by ozone [95i] (see equation 305). 

Two oxidants essentially dominate these oxidations lead tetraacetate in organic solvents and periodic acid in aqueous media. On occasion, other oxidation reagents cause the cleavage of vicinal diols ceric ammonium nitrate [424], sodium bismuthate [482, 483], chromium trioxide [482, 555], potassium dichromate with perchloric acid [949], manganese dioxide [817], and trivalent [779, 789] or pentavalent [798] iodine compounds. 

The mechanism of the cleavage of vicinal diols can be represented by the reaction of diols with periodic acid, HIO4 or H5IO6 (equation 296). 

The cleavage of secondary straight-chain diols is a good source of aldehydes [433, 448, 775, 789, 795] (equations 297-299). 

In saccharides, cleavage by periodic acid or sodium periodate occurs between any carbons possessing/ree hydroxy lie groups. Thus glucosazone treated with periodic acid in aqueous ethanol at room temperature is degraded to the l,2-bis(phenylhydrazone) of mesoxalaldehyde (propanone-dial) in 85% yield [759]. 

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Red lead, Pb304, in acetic acid oxidizes a-hydroxy acids to carbonyl compounds [143, 433] and cleaves vicinal diols to carbonyl compounds... 

Sodium periodate (sodium metaperiodate), NaI04 (mp 300 °C dec), which is commercially available, is applied mainly in aqueous or aqueous-alcoholic solutions. Like the free periodic acid, sodium periodate cleaves vicinal diols to carbonyl compounds [762], This reaction is especially useful in connection with potassium permanganate [763, 764] or osmium tetroxide [765], Such mixed oxidants oxidize alkenes to carbonyl compounds or carboxylic acids, evidently by way of vicinal diols as intermediates. Sulfides are transformed by sodium periodate into sulfoxides [322, 323, 766, 767, 768, 769, 770, 771, 772], and selenides are converted into selenoxides [773]. Sodium periodate is also a reoxidant of lower valency ruthenium in oxidations with ruthenium tetroxide [567, 774],... 

Cleavage of vicinal diols to carbonyl compounds can be achieved by using periodic acid (HIO4) (Section 15.12). 

Periodic Acid Cleavage of Carbohydrates Another method used to determine the size of carbohydrate rings is cleavage by periodic acid. Recall that periodic acid cleaves vicinal diols to give two carbonyl compounds, either ketones or aldehydes, depending on the substitution of the reactant (Section 11-1 IB). 

N03)j, a newcomer to the arena of oxidants, is useful for the acetoxylation of aromatic side chains in benzylic positions [415, 416] and for the oxidation of methylene or methyl groups that are adjacent to aromatic rings to carbonyl groups [238, 415, 417]. The reagent also oxidizes alcohols to aldehydes [418, 419, 420, 421] and phenols to quinones [422, 423], cleaves vicinal diols to ketones and a-hydroxy ketones to acids [424, 425], and converts diaryl sulfides into sulfoxides [426]. A specialty of ammonium cerium nitrate is the oxidative recovery of carbonyl compounds from their oximes and semicarbazones [422, 427] and of carboxylic acids from their hydrazides [428] under mild conditions. 

The results of some oxidations with potassium permanganate differ depending on the pH of the reaction. For example, stearolic acid gives 9,10-diketostearic acid at pH 7-7.5 (achieved with carbon dioxide) and azelaic acid on treatment at pH 12 [864]. In some reactions, potassium permanganate is used as a catalyst for oxidation with other oxidants, such as sodium periodate. Thus alkenes are cleaved to carbonyl compounds or acids via vicinal diols obtained by hydroxylation with potassium permanganate, followed by cleavage by sodium periodate [763, 552]. 

While anodic oxidation of simple alcohols is of limited utility, direct oxidation of vicinal diols and related compounds is an excellent method for achieving oxidative cleavage to the corresponding carbonyl or carbonyl-acetal products, as in Eqs. (53) and (54). Unlike certain chemical glycol cleavage reactions, the anodic method is not limited by the stereochemistry of the substrate oxygens [132]. 

NaBiO3 has been used in the oxidation of alcohols, phenols, and olefins. Allylic and benzylic alcohols are oxidized by NaBiOs to f/,/furisalurated and aromatic aldehydes, respectively [260]. In aqueous H3PO4 or AcOH, NaBiO3 cleaves the C-C bond of vicinal diols to give two molecules of carbonyl compounds (Scheme 14.121) [261]. When treated with NaBiO3 in aqueous AcOH, 2-hydroxycyclohexanone is converted to 6-oxohexanoic acid (Scheme 14.122) [262]. 

Vicinal diols can be transformed to carbonyl compounds in the presence of electrophilic catalysts. This reaction, called the pinacol rearrangement, is discussed in detail in Section 5.5. 

Acid-catalyzed rearrangement of vicinal diols (pinacols) to carbonyl compounds. 

The methods for oxidation of sugars discussed so far leave the basic skeleton intact. A reagent that leads to C-C bond rupture is periodic acid, HIO4. This compound oxidatively degrades vicinal diols to give carbonyl compounds. 

Periodic acid oxidation (Section 15 12) finds extensive use as an analytical method m carbohydrate chemistry Structural information is obtained by measuring the number of equivalents of periodic acid that react with a given compound and by identifying the reaction products A vicinal diol consumes one equivalent of penodate and is cleaved to two carbonyl compounds... 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

Vicinal diol is oxidized to the two corresponding carbonyl compounds using Pb(OAc)4, lead tetraacetate (LTA). 

With sodium borohydride and catalytic amounts of titanyl acetoacetonate, a,fi-unsaturated carbonyl compounds give allyl alcohols regioselectively, whereas a-diketones and acyloins are reduced to vicinal diols.325 Enantioselectivities in the reduction of acetophenone, catalysed by 1,3,2-oxazaborolidones, have been examined using the AM1-SCF MO method. The optimized geometries, thermal enthalpies, and entropies of R and S transition states in the stereo-controlling steps of the reduction have been obtained.326... 

Osmium tetroxide reacts with double bonds to form cyclic osmate(VI) diesters (10), which can then be hydrolyzed to provide vicinal diols in good yields. > If, however, sodium perio te is also present, the diol is cleaved, as in Scheme 2, and carbonyl compounds are the final products. Periodate serves the additional purpose of regenerating osmium tetroxide, thus permitting the use of this expensive and toxic reagent in minimum amounts. 

The oxidative cleavage of carbon-carbon bonds in vicinal diols [756, 759] is a reaction widely used in saccharide chemistry. Besides its application in this reaction, periodic acid achieves the oxidative coupling [757] or oxidation to quinones [758] of polynuclear aromatic hydrocarbons, the oxidation of methyl groups in aromatic compounds to carbonyl groups [760], the conversion of epoxides into dicarbonyl compounds [761], and the oxidative cleavage of trimethylsilyl ethers of acyloins to carboxylic acids [755]. 

In addition, manganese oxide oxidizes benzylic methyl or methylene to carbonyl [814] and cleaves the carbon bonds of vicinal diols [577, 522]. It converts amines into imines [577, 525] tertiary amines into secondary amines [572], formamides [526, 527, 525], or ketones aromatic primary amines [575, 525] and hydrazo compounds [525] into azo compounds hy-droxylamines into nitroso [576] or nitro compounds [525] hydrazones into diazo compounds [520] phosphines into phosphine oxides [575] thiols into disulfides [576] and sulfides into sulfoxides [541]. 

The spectrum of applications of potassium permanganate is very broad. This reagent is used for dehydrogenative coupling [570], hydrox-ylates tertiary carbons to form hydroxy compounds [550,831], hydroxylates double bonds to form vicinal diols [707, 296, 555, 577], oxidizes alkenes to a-diketones [560, 567], cleaves double bonds to form carbonyl compounds [840, 842, 552] or carboxylic acids [765, 841, 843, 845, 852, 869, 872, 873, 874], and converts acetylenes into dicarbonyl compounds [848, 856, 864] or carboxylic acids [843, 864], Aromatic rings are degraded to carboxylic acids [575, 576], and side chains in aromatic compounds are oxidized to ketones [566, 577] or carboxylic acids [503, 878, 879, 880, 881, 882, 555]. Primary alcohols [884] and aldehydes [749, 868, 555] are converted into carboxylic acids, secondary alcohols into ketones [749, 839, 844, 863, 865, 886, 887], ketones into keto acids [555, 559, 590] or acids [559, 597], ethers into esters [555], and amines into amides [854, 555] or imines [557], Aromatic amines are oxidized to nitro compounds [755, 559, 592], aliphatic nitro compounds to ketones [562, 567], sulfides to sulfones [846], selenides to selenones [525], and iodo compounds to iodoso compounds [595]. 

In the previous chapter, one of the first reactions that we studied was the pinacol-pinacolone rearrangement, in which a symmetrical diol gave a carbonyl compound. There are many ways to make a vicinal diol, e g. the addition of H202 to a carbon/carbon double bond. Such reactions were... 

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