Oxidation of olefin-type substrates with KMnO₄ yields mostly dihydroxylated or hydroxy oxo products in alkaline or neutral media, and the double bond is cleaved to the corresponding acids in acidic solutions^1-9. A series of studies on the mechanism and intermediates of such oxidations in aqueous solution have revealed the transient formation of aldehydes, which, however, undergo further oxidation by manganese(II) and/or manganese(IV) intermediates⁸. No attempt was made at utilizing the cleavage reaction on a preparative scale, although the possibility of quenching the manganese intermediates for saving the aldehydes had been pointed out^8b-e. Under phase-transfer conditions, isolated examples of olefin to aldehyde conversion have been reported^2,7. There is, however, no convenient procedure for double bond cleavage to aldehydes in aqueous organic mixtures which would not require special additivies (phase-transfer agent) or techniques (chemical quenching). We now report a simple method in THF/water, in which the solvent plays the role of a quenching reagent, permitting good yields of the aldehydes.

In a mechanistic study on 1,5-benzodiazepines, we needed compound R⁴CHO for identifying a reaction intermediate. Attempts at converting the 4-methyl of R⁴CH₃ to a formyl group via oxidation or dihalogenation/hydrolysis were unsuccessful. We have found however that treatment of a dilute THF solution of 1 (with R¹=R⁴, R²=H, and R³=Ph) by a concentrated aqueous solution of KMnO₄ afforded the desired aldehyde R⁴CHO and benzaldehyde in 80% yield. The lack of extensive overoxidation is surprising since in the overwhelming majority of reported cases cleavage leads to the corresponding acids. The testing of this procedure on some other olefinic compounds indicates that it may be useful in the synthesis of a variety of aldehydes. Examples are listed in Table I. Apparently, activation of the double bond by conjugation increases, whereas adjacent bulky groups decrease the yield.