THEORETICAL-STUDIES ON THE BASE-CATALYZED REARRANGEMENT OF 4,4'-DISUBSTITUTED BENZILS IN THE GAS-PHASE AND AQUEOUS-SOLUTION

The base-catalysed rearrangements of disubstituted aryl benzils in the gas phase and in solution (water) have been investigated theoreticall y by the AM1 method. The solvent effects were accounted for using the Cramer-Truhlar SM2.1 solvation model. The calculated Gibbs free energy changes (Delta G(double dagger) for the overall reactions (k(obs)) ar e dissected into component parts, i.e. for the equilibrium step(K) of reactants reversible arrow intermediate (Delta G(1) degrees) and rearr angement step (k(2)) of intermediate --> transition state (Delta G(1)( double dagger)); Delta G(double dagger) = Delta G(1) degrees + Delta G (1)(double dagger). The Hammett rho values for the migrating (rho(X)) and non-migrating rings (rho(Y)) are then calculated for each step. Th us, the overall rho(k(obs)) value can be dissected into two components , rho(K) + rho(k(2)). It has been shown that (i) the substituent effec ts of the 4,4'-disubstituted aryl benzils are not in general additive, rho(k(obs)) not equal rho(X) + rho(Y), due to the cross-interaction t erm, rho(XY), and (ii) in the gas phase, the equilibrium step is consi derably more important than the rearrangement step, rho(K) much greate r than rho(k(2)), whereas in solution (water) the two steps contribute comparably, rho(K) similar or equal to rho(k(2)). It is also notable that in the rearrangement step (k(2)) the carbonyl carbon of the non-m igrating ring actually becomes more electron-deficient [rho(Y)(k(2)) < 0] on going from the intermediate to the transition state. This is in contrast to the migrating ring which becomes more electron-rich [rho( X)(k(2)) > 0]. Fair agreement of the rho(k(obs)) values are obtained b etween those for the theoretical solution phase (6.7 in water) and for the experimentally observed [5.7 in 7%(v/v) aqueous Me(2)SO] for the symmetrically disubstituted aryl benzils (sigma(X) = sigma(Y)).