Concerted Bimolecular Substitution Reactions of Acetal Derivatives of Propionaldehyde and Benzaldehyde

Article abstract of DOI:10.1021/ja00202a034

The propionaldehyde acetal derivatives 1-acetoxy-1-methoxy-3-(4-methoxyphenyl)propane, 1-methoxy-3-(4-methoxyphenyl)-1-(4-nitrophenoxy)propane, and 1-azido-1-ethoxypropane undergo concerted bimolecular substitution reactions with anionic nucleophiles.The products are unstable adducts that break down to the corresponding aldehydes.The observed rate constants follow Swain-Scott correlations with s values of 0.1-0.2, which indicate a low sensitivity to the attacking nucleophile and are consistent with an "exploded" transition state.Water shows a small negative deviation from these plots but reacts through a stepwise mechanism with a highly unstable oxocarbenium ion intermediate.Similarly, an acetal derivative of benzaldehyde, azidomethoxyphenylmethane, undergoes rapid concerted bimolecular substitution with good nucleophiles.The unexpectedly large s value of 0.4 is similar to that observed for a 4-nitrobenzyl system but, remarkably, the reactivity toward thiocyanate is >1E5-fold greater than that of benzyl azide and ca. 100-fold greater than that of 1-azido-1-ethoxypropane, which provides evidence for a benzylic effect.Water exhibits a positive deviation of ca. 30-fold from the Swain-Scott correlation for anionic nucleophiles, and hydrolysis of this compound occurs by a concurrent stepwise mechanism with a diffusionally equilibrated oxocarbenium ion intermediate.The consequences of these results for reactions of glycosyl derivatives are discussed.