Synthesis, Chiroptical Properties, and Their Theoretical Simulation of Some Highly Rotating Benzotricamphor Derivatives

The large molecules 1–3 (69, 90, and 102 atoms, respectively), prepared by cyclotrimerization of enantiomerically pure derivatives of (2)-bornyl acetate, show intense ECD spectra, high optical rotation (OR) values (200–1300, in absolute value) dominated in sign and order of magnitude by the lowest-energy Cotton effects, that is, they are the ideal candidates to test the reliability of our ‘‘approximate’’ (TDDFT/B3LYP/6-31G* or smaller basis set) approach to the calculation of chiroptical properties. As a matter of fact, a correct simulation of the OR values and ECD spectra of 1 and 2 can be obtained even using STO-3G basis set and semiempirical or molecular mechanics input geometries: for 1, at the TDDFT/B3LYP/STO-3G level, the OR values are of the order of 500–550, versus an experimental value ranging between 660 and 690, depending on the solvent. On the contrary, the case of 3 (exp. OR between 21330 and 21500) is really complex (for instance, the OR values range between 23216 and 2729 (TDDFT/B3LYP/6-31G* calculations) or 21824 and 2444 (TDDFT/B3LYP/STO-3G calculations)), making the comparison between calculated and experimental values more dif.cult. The behavior of 3 is due to its molecular .exibility, whereas 1 is a really rigid molecules and 2 behaves (vide infra) as it were a rigid system. These observations strongly indicate that the conformational freedom constitutes one of the major dif.culties for a correct but simple simulation of the chiroptical properties.