Acid-catalyzed hydrolysis of 4-diazo-3-isochromanone: the effect of coplanarity on the carbon protonation of α-phenyl-α-carbonyldiazo compounds

Hydrolysis of the cyclic α-phenyl-α-carbonyl-diazo compound, 4-diazo-3-isochromanone, in dilute aqueous perchloric acid solutions was found to give the hydronium ion isotope effect [Formula: see text] which shows that this reaction occurs by rate-determining hydronation of the substrate on the carbon atom α to its diazo group. Comparison of the rate constant obtained, [Formula: see text] with that for the corresponding acyclic analog, methyl phenyldiazoacetate, indicates that the cyclic compound is 57 times less reactive. Semi-empirical AM1 molecular orbital calculations suggest that this difference in reactivity is caused by enforced near-coplanarity of the diazo and phenyl groups in the cyclic substrate, as opposed to a staggered arrangement of these groups in the acyclic analog; this coplanarity then enhances delocalization of negative charge from the diazo α-carbon atom into the phenyl group, which reduces the negative charge density on the α-carbon atom and slows the rate of reaction. Key words: hydrolysis, diazoalkanes, charge delocalization, AM1 calculations.