Regeneration from hydrazones

Carbonyl compounds are regenerated from hydrazones by Pd-catalyzed reaction with tin(ll) chloride. ... 

Oxidative cleavage of hydrazones. Carbonyl compounds can be regenerated from 2,4-dinitrophenylhydrazones, tosylhydrazones, or N-methyl-N-tosylhydra-zones in 80-95% yield by oxidation with sodium nitrite in TFA or HOAc (0-5°, 1-2 hours). In the case of tosylhydrazones, the other product of cleavage has been identified as tosyl azide, the product of reaction of tosylhydrazine with nitrous acid. 

It is usually unnecessary to isolate such hydrazones during the working up the mixture is partitioned between water and a water-insoluble solvent such as ether. The hydrazones then remain in the aqueous phase, and the other organic substances are removed in the organic phase. The carbonyl components are finally regenerated from the aqueous hydrazone phase by acid hydrolysis. Considerable fractionation of mixed carbonyl compounds can be achieved by varying the reaction conditions, notably the temperature or pH, of either the condensation or the hydrolysis. Acid-sensitive carbonyl compounds, e.g., citral, may be isolated by exchange from the hydrazone by an excess of formaldehyde.932... 

Regeneration of carbonyl compounds from hydrazones (75-98%) [60] and semicarbazones (55-90%) [61] has also been achieved with bismuth trichloride. 

Benzeneseleninic anhydride (PhSe0)20 is highly recommended for the regeneration of ketones from hydrazones, oximes, and semicarbazones. Ketones have been regenerated from the same range of derivatives using a stoicheiometric amount of titanous chloride in buffered acetic acid and dioxan. ... 

Some of the oil boiling at 168 to 171 (45 at 4 to 5 mm. pressui e) was treated with bisulphite. The aldehyde regenerated from the bisulphite compound distilled at 174 to 179, and was found to be heavier than water. The body was identified as benzaldehyde by the phenyl-hydrazone melting at 156, and the semi-carbazone of the melting-point 213 to 214. ... 

Ketoximes are converted into the corresponding ketones using two molar equivalents of pyridinium chlorochromate. " Parent aldehydes (including aliphatic) and ketones can be regenerated from aryl hydrazones using ben-zeneseleninic anhydride or hydrogen peroxide and potassium carbonate. ... 

For the regeneration of sensitive, optically active aldehydes and ketones from their SAMP- or RAMP-hydrazones, two mild cleavage methods are used routinely. 

Regeneration of carbonyl compounds from certain derivatives. Ketones can be recovered in satisfactory yield from the phenylhydrazones, p-nitrophenyl-hydrazones, tosylhydrazoncs, oximes, and semicarhazones by reaction with 1 equivalent of (C6H,Se0)20. 2,4-Dinilrophenylhydrazones and N,N-dimethyl-hydrazones arc inert under even rather vigorous conditions. The reagent can also be used to regenerate aldehydes from oximes or tosylhydrazones. 

Reduction of saturated carboxylic acids with the borane derivative, thexylchloro-borane, provides a direct route to aldehydes without their prior conversion to carboxylic acid derivatives. The aldehydes initially formed were isolated either as their bisulfite adducts or as their hydrazones, from which the aldehydes were regenerated. 

Of special importance to the applications of hydrazones and oximes to carbon-carbon bond formation is the observation that these derivatives are considerably more resistant to hydrolysis than are either en-amines or imines. Thus, methods for mild regeneration of the carbonyl group from these derivatives are especially significant. Among the reagents/systems developed are oxidation with bromine reaction... 

Cleavage of 2,4-dinitrophenylhydrazone and oximes. Keeney devised a semimicro colorimetric procedure for estimation of 2,4-dinitrophenylhydrazones which involved use of levulinic acid as acceptor molecule for regeneration of ketones from the derivatives by exchange. He found that hydrazones of nonconjugated ketones are split much more rapidly than are those of conjugated or aromatic ketones. 

Unsymmetrically substituted irondiene carbonyl complexes are chiral, and some are easily accessible in an enantiomerically pure form by resolution Complexes 1.152 (R = H, Me, rt-Bu, Y = CHO) or 1.153 (Y = CHO), bearing an aldehyde functional group, can be resolved via chiral hydrazones [526], chiral aminals [527], or derivatives of ephedrine 1.61 [528], The diastereoisomers thus formed are separated by chromatography, and the aldehydes are easily regenerated. The resolution of trimethyienemethane complexes 1.154 can be accomplished similarly [529]. Complexes bearing an ester functionality 1.152 (R = COOMe) are resolved through lactates, which are subsequently treated with KOH/MeOH [530], From these enantiomerically pure complexes, classical reactions lead to other systems such as 1.152 (R = CH2OH, Y = CHO) [531], 1.152 (R = w-Bu,... 

Table 1 Reagents for Regeneration of Carbonyl Compounds from Ketone and Aldehyde Hydrazones...

A new method for cleavage was recently reported by Enders. Treatment of the hydrazone with three equivalents of sodium perborate regenerate the carbonyl compound in 70-95% yield according to equation (36). This oxidative cleavage was chemoselective and left carbon-carbon double bonds intact. This chemoselectivity plus the neutral hydrolytic conditions and the low cost of the oxidant make this method quite attractive, liie main disadvantage of this method is the relatively long reaction time required, ranging from 4-24 h for aliphatic ketone hydrazones to 2-3 d for aromatic ketone hydrazones. 

Many other reagents have been used to cleave hydrazones and to regenerate carbonyl groups. Table 1 lists some of these other examples of procedures to regenerate carbonyl compounds from their N,N-d -alkylhydrazones. 

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