NOVEL CONCEPTS IN DIRECTED BIARYL SYNTHESIS .39. THE INFLUENCE OF ALUMINUM TRICHLORIDE ON A CONFIGURATIVELY LABILE LACTONE-BRIDGED BIARYL - QUANTUM-CHEMICAL CALCULATIONS AND OPTICAL SPECTROSCOPY

Semiempirical and ab initio calculations on the binding properties of the AlCl3 complex of 1,3-dimethylbenzo[b]naphtho[1,2-d]pyran-6-one are reported. Compounds of this type are key intermediates in the stereoselective synthesis of axially chiral biaryl systems. Semiempirical (MNDO, PM3) and ab initio (STO-3G, 3-21G) calculations give important information about conformational changes that occur by complexing the free lactone with AlCl3: the calculations predict a significant influence on the lengths of the (C=O)exo and (C-O)endo bonds, and a distinct planarization of the heterocyclic lactone part of the complexed benzonaphthopyranone. IR and Raman spectroscopy was applied in order to confirm the theoretical predictions. The good agreement of results obtained by semiempirical frequency calculations with the experimental data supports the assignment of vibrational bands affected on coordination. The highest shift caused by coordination (- 152 cm-1) is observed for nu(C=O).