Theoretical studies on the base-catalysed rearrangement of 4,4′-disubstituted benzils in the gas phase and aqueous solution

Abstract

The base-catalysed rearrangements of disubstituted aryl benzils in the gas phase and in solution (water) have been investigated theoretically by the AM1 method. The solvent effects were accounted for using the Cramer–Truhlar SM2.1 solvation model. The calculated Gibbs free energy changes (ΔG^‡) for the overall reactions (k_obs) are dissected into component parts, i.e. for the equilibrium step(K) of reactants ⇌ intermediate (ΔG_I^o) and rearrangement step (k₂) of intermediate → transition state (ΔG_I^‡); ΔG^‡=ΔG_I^o+ΔG_I^‡. The Hammett ρ values for the migrating (ρ_X) and non-migrating rings (ρ_Y) are then calculated for each step. Thus, the overall ρ(k_obs) value can be dissected into two components, ρ(K)+ρ(k₂). It has been shown that (i) the substituent effects of the 4,4′-disubstituted aryl benzils are not in general additive, ρ(k_obs)≠ρ_X+ρ_Y, due to the cross-interaction term, ρ_XY, and (ii) in the gas phase, the equilibrium step is considerably more important than the rearrangement step, ρ(K)ρ(k₂), whereas in solution (water) the two steps contribute comparably, ρ(K)≃ρ(k₂). It is also notable that in the rearrangement step (k₂) the carbonyl carbon of the non-migrating ring actually becomes more electron-deficient [ρ_Y(k₂) < 0] on going from the intermediate to the transition state. This is in contrast to the migrating ring which becomes more electron-rich [ρ_X(k₂) > 0]. Fair agreement of the ρ(k_obs) values are obtained between those for the theoretical solution phase (6.7 in water) and for the experimentally observed [5.7 in 7%(v/v) aqueous Me₂SO] for the symmetrically disubstituted aryl benzils (σ_X=σ_Y).