1822 J . C . S . Perkin I 1 Heteroaromatic Hydrogen Exchange Reactions . Part VIII . l The lonisa - tion of 1 . 3 - Dimethylindolin - Zone

The kinetics of iodination at the 3-position of 1.3-dirnethyland [3-2HI]-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 zeroand first-order dependence on [I3-]. This implies that the reactive enolate ion intermediate undergoes protonation as rapidly as reaction wi th iodine to give 3-iodo-1.3-dimethylindolin-2-one. Analysis of the kinetic data shows that 1.3-dirnethylindolin-2one ionises nearly 100-fold faster than acetone and at least 1 O8 times faster than a simple arnide such as acetamide. This makes the substrate one of the most acidic arnides known. The deuterium isotope effect associated with the ionisation is 6.3 f 0.6.