J. Chem. Soc. Perkin. Trans. 2563-2565 (1988)
Amberlyst 15, a superior, mild and selective catalyst for carbonyl regeneration from nitrogenous derivatives.
Regeneration of the carbonyl group from its derivatives under mild conditions is an i portant process in synthetic organic chemistry. Hydrazones and oximes are employed as ketone and aldehyde functional group equivalents in synthesis, both because they provide convinient protection for the carbonyl groupand because they are important synthetic intermediates. There has therefore been considerable interest in the development of mild techniques for the conversion of oximes, hydrazones and tosylhydrazones back into carbonyl compounds. Most of the reported methods require strongly oxidative or reducing, acidic or basic media, or tedious procedures and/or expensive reagents. Regeneration of carbonyl compounds via equilibrium exchange, by use of acetone with boron trifluoride-ether complex as catalyst has been reported to be efficient only with tosylhydrazones.
In connection with our studies on reactions carried out with heterogenous catalysts, we have found that Amberlyst 15, a macroreticular ion-exchange resin which contains strongly acidic sulphonic groups, is an excellent and far superior catalyst for regeneraton of carbonyl compounds fron nitrogenous derivatives when acetone is used as exchange reagent.
Our method is carried out at room tempreature or at 80°C by simply dissolving the appropriate carbonyl derivative in acetone-water, with a certain amount of Amberlyst 15 as catalyst, and leaving the mixture for 5-24 hours. However, for aldehyde regeneration, it was found convinient to add 10 eq. paraformaldehyde also.
Yields are high and work-up is exceedingly simple, only involving solvent removal, extracton with pentane, filtration and evaporation, to obtain the product in a high state of purity (at least 97%); no further preparation steps are required.
Our procedure is applicable to various nitrogenous derivatives such as tosylhydrazones, oximes, 2,4-dinitrophenylhydrazones and semicarbazones. Limited reaction times are necessary; groups which are acid-sensitive such as furyl derivatives, esters or ethers survive and, above all, excellent results areobtained in aldehyde regeneration. We consider that the combination of Amberlyst 15 with acetone is one of the most convinient of the existing methods for carbonyl regeneration from nitrogenous derivatives, on the basis of its simplicity and cheapness, and the high yields obtained with a very large variety derivatives, under mild conditions.
General procedure
A 250 ml flask, equipped with reflux condenser and magnetic stirrer was charged with the appropriate nitrogenous derivative (10 mmol) in acetone-water (10:1; 66 ml)(100 mmol paraformaldehyde was added for aldehyde regeneration) and Amberlyst 15 (2 g). Stirring was continued at room temperature or at 80°C for the appropriate time. The progress of the reaction can be conviniently monitored by TLC or GLC. The acetone was removed under reduced pressure. Extraction with pentane (3x20 ml), filtration and evaporation gave the pure carbonyl compound at least 97% pure.
Phenylacetone tosylhydrazone gave 91% phenylacetone in 24 hours at room temp.
4-Methylbenzaldehyde tosylhydrazone gave 86% p-methylbenzaldehyde in 24 hours at 80°C.
2-Methoxybenzaldehyde tosylhydrazone gave 88% 2-methoxybenzaldehyde in 4 hours at 80°C.
Cyclohexanone oxime gave 92% cyclohexanone in 21 hours at room temp.
