Application of the Structural Correlation Method to Ring-Flip Processes in Benzophenones

Thirty-eight crystallographically independent structures of thirty-two benzophenones were retrieved from the Cambridge Structural Data Base. All except one show an helical propeller conformation. The torsional angles of the two rings 0, and 02 were plotted one against the other in order to identify the threshold enantiomerization mechanism by applying the structural correlation method to the potential ring-flip processes. Molecular mechanics (MM) calculations on benzophenone gave the corresponding calculated potential energy surface. An excellent agreement with the calculated route for a one-ring flip was obtained from the conformational map of the crystallographic data, especially in benzophenones where the C=0 bond is involved in intramolecular hydrogen bonding. The strong preference for this route is rationalized by the tendency to maximize the Ar-C=0 conjugation interaction during the rotation. The structural correlation method can be used also to evaluate trends in the changes of the structural parameters, such as bond lengths and angles in approaching the rotational transition state. Similar trends are obtained from analysis of the X-ray data and from MM calculations on the one-ring flip of benzophenone.