Primary 2H kinetic isotope effects in the base-catalysed 1,3-prototropic rearrangement of 1-methylindene and 1,3-dimethylindene

Abstract

Rate constants and primary deuterium kinetic isotope effects (p.d.k.i.e.s.) have been determined for the base-catalysed rearrangement of 1-methylindene (1) and 1,3-dimethylindene (3) in the solvent o-dichlorobenzene at 30°C. The structurally similar tertiary amines 1-aza-adamantane, quinuclidine, 1,3-diaza-adamantane, DABCO, and 1,3,5-triaza-adamantane were used as catalysts. Substrate (3) gave a lower isotope effect than (1). The response in p.d.k.i.e. due to a change in basicity of the amine catalyst is very similar for (1) and (3). Despite its intermediate basicity, DABCO showed a significantly lower p.d.k.i.e. for both substrates. The p.d.k.i.e.s. obtained for the other bases, with a pK at or around 2.6, were almost constant. The consequences of a possible influence of internal ion-pair return on the rearrangement of the substrates are discussed and related to the measured p.d.k.i.e.s. The similarity between the substrates and the symmetry in the rearrangement of (3) permits some rationalizations to be made.