Acid-Catalyzed Hydrolysis of Vinyl Acetals. Reaction through the Acetal Rather Than the Vinyl Ether Functional Group

Article abstract of DOI:10.1021/ja00200a043

The hydrolysis of three vinyl acetals, 9-methoxy-1-oxacyclonon-2-ene and the cis- and trans-1-propenyl methyl acetals of acetaldehyde, was monitored by 1H NMR spectroscopy in dilute minaral acid solution, and rates of the reactions were also measured in dilute mineral acids and semicarbazide, acetic acid, and cacodylic acid buffers.The NMR spectra showed biphasic changes, which could be assigned to the formation and decay of enol intermediates generated by initial acetal group hydrolysis.Biphasic absorbance changes also appeared in the ultraviolet spectra of thesesubstrates reacting in dilute mineral acid solution, and rate constants for two successive reactions could be evaluated.These rate constants were assigned to the individual reaction steps, and further support for the assigned reaction path was obtained, by comparison with rate constants measured in the buffer solutions, where acetal hydrolysis was the only reaction observed, and also by comparison with rates of ketonization of the enols generated independently from the corresponding trimethylsilyl enol ethers.Solvent isotope effects provided yet more evidence for the assigned reaction path.Estimates of rates of hydrolysis of the vinyl ether groups of these substrates show that these vinyl ether functions are too unreactive to compete with acetal hydrolysis.Simultaneous vinyl ether and acetal hydrolysis, however, does occur in the case of 6-membered cyclic vinyl acetals, such as 2-methoxy-2,3-dihydropyran, and this is probably because intramolecular recapture of the alkoxycarbocationic intermediate slows acetal hydrolysis in the case of these smaller ring compounds.