Synthesis, chiroptical properties, and their theoretical simulation of some highly rotating benzotricamphor derivatives

Abstract

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 difficult. The behavior of 3 is due to its molecular flexibility, 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 difficulties for a correct but simple simulation of the chiroptical properties.

Synthesis, Molecular Structure, and Chiroptical Properties of Dibenzylidene Derivatives of Bicyclo[3.3.1]nonane and Brexane

“…The latter apparently originates from the exciton coupling 46 of the two electrically allowed π→π* transitions, which are polarized along the long axis of the unsaturated chromophore. 47 In concordance with the exciton interaction of the two nonconjugated chromophores, the strong absorption maximum at the same wavelength is observed in the UV spectrum of 2. This was further corroborated by a comparison with the monofunctional congener (+)-4, which exhibited positive CE of moderate intensity attributable to a π→π* transition of the unsaturated chromophore at 295 nm (Fig.…”

Highly diastereoselective synthesis of a novel functionalized benzocyclotrimer

“…The mixture of diastereomers was dehydrobrominated with potassium tert-butoxide in THF, 26 to afford the alkenyldibromide 8 in good yield (93%) (Scheme 1). The key reagent 9 for the cyclotrimerization was obtained either by proton abstraction of halide 6 with lithium diisopropylamide (LDA) 11,13,[27][28][29][30][31][32][33] or by removal of one of the bromine atoms of the dibromide 8 with nbutyl lithium 30,31,34,35 followed by a trans-metalation reaction with trimethyltin chloride in 96% yield in both cases. The high chemoselectivity of removal of the bromine atom in 8 was surprisingly: indeed, the bromine atoms attached to the aromatic ring remained completely unaffected.…”

Synthesis, Enzymatic Resolution, and Stereochemical Characterization of Isoparaconic Acid Derivatives: A Combined Experimental and Theoretical Investigation

“…Therefore, thanks to all these powerful tools, the problem of AC determination of organic molecules looks now near to its ultimate solution. However, in spite of this significant progress, some relevant problems still remain: in particular, the treatment of large conformationally flexible molecules . In this case, many conformers showing different (even opposite) behavior are simultaneously present, making the correct chiroptical data prediction very difficult.…”

“…However, in spite of this significant progress, some relevant problems still remain: in particular, the treatment of large conformationally flexible molecules. [35][36][37] In this case, many conformers showing different (even opposite) behavior are simultaneously present, making the correct chiroptical data prediction very difficult. Clearly, this case presents many formidable obstacles: a really accurate determination of the conformers population and a rigorous knowledge of geometry is required.…”