Heteroaromatic hydrogen exchange reactions. Part VIII. The ionisation of 1,3-dimethylindolin-2-one

Abstract

The kinetics of iodination at the 3-position of 1,3-dimethyl- and [3-²H₁]-1,3-dimethyl-indolin-2-one are reported for buffered mildly alkaline 50%(w/w) aqueous methanol solutions at 25°. Unlike the iodination of many ketones these reactions (even in the presence of excess of substrate and with large amounts of added iodide ion) do not follow a zero-order rate expression, but show behaviour intermediate between a zero-and first-order dependence on [I₃^–]. This implies that the reactive enolate ion intermediate undergoes protonation as rapidly as reaction with iodine to give 3-iodo-1,3-dimethylindolin-2-one. Analysis of the kinetic data shows that 1,3-dimethylindolin-2-one ionises nearly 100-fold faster than acetone and at least 10⁶ times faster than a simple amide such as acetamide. This makes the substrate one of the most acidic amides known. The deuterium isotope effect associated with the ionisation is 6.3 ± 0.6.