How does organic structure determine organic reactivity? The effect of ortho-dimethyl groups on the nucleophilic substitution and alkene-forming elimination reactions of ring-substituted cumyl derivatives

The addition of a pair of ortho-methyl groups to ring-substituted cumyl derivatives to give the corresponding 2,6-dimethylcumyl derivatives X-1-Y leads to modest (<5-fold) changes in the observed rate constant for reaction in 50:50 (v:v) trifluoroethanol-water (I = 0.50, NaClO₄). The reactions of X-1-Y proceed by a stepwise mechanism through the liberated 2,6-dimethylcumyl carbocations X-2 that partition between nucleophilic addition of solvent and deprotonation to give good yields (≥ 67%) of the corresponding 2-(2,6-dimethylaryl)propenes X-3. The carbocations X-2 are also trapped by nucleophilic addition of azide ion to give good yields (≥ 68% at [N₃^-] = 0.50 M) of the corresponding 2,6-dimethylcumyl azides X-1-N₃. In the presence of high concentrations of azide ion there are constant limiting yields of the alkenes X-3, which shows that X-2 also undergo significant reactions with azide ion as a Bronsted base. The product rate constant ratios for partitioning of the 2,4,6-trimethylcumyl carbocation Me-2 between reaction with azide ion as a Lewis and a Bronsted base, k(az)/k(B), the nucleophilic addition of azide ion and solvent, k(az)/k(s) (M^-1), and deprotonation by solvent, k(az)/k(e) (M^-1), were combined with (k(az) + k(B)) = 5 x 10⁹ M^-1 s^-1 for the diffusion-limited reaction of azide ion to give absolute rate constants for the reactions of Me-2. The data show that the addition of a pair of ortho-methyl groups to the 4-methylcumyl carbocation to give the sterically hindered Me-2 results in a 70-fold decrease in the rate constant for nucleophilic addition of solvent to the benzylic carbon, but a 60-fold increase in the rate constant for deprotonation of the carbocation by solvent.