Circular Dichroisms of Mono- and Dibromo[2.2]paracyclophanes: A Combined Experimental and Theoretical Study

摘要

Circular dichroisms (CDs) of planar chiral 4-bromo[2.2]paracyclophane (1 ) and three isomeric dibromo[2.2]paracyclophanes (p -2 , m ′-2 , and o ′-2 ) were investigated experimentally and theoretically. They all exhibited strong multisignate Cotton effects (CEs) at the ~(1)L_(b), ~(1)L_(a), and ~(1)B transitions of the component (bromo)benzene chromophore and were comparable to each other. For all of the cyclophanes examined, the enantiomer that eluted earlier from a chiral high-performance liquid chromatography column (Chiralcel IA or IB) exhibited negative and positive CEs at the ~(1)L_(b) and ~(1)L_(a) bands, respectively, which were followed by a more complicated pattern of CDs at the higher-energy bands. These CD features were well reproduced by quantum chemical calculations, allowing us to unambiguously assign the absolute configurations of the first-eluted enantiomers as R _(p) in all of the cases examined. Interestingly, the CDs of 1 and 2 , although largely comparable in shape, were still sensitive to the number and pattern of bromine substitution, showing closer resemblance between m ′-2 and o ′-2 and between p -2 and 1 . The theoretical calculations also reproduced successfully these spectral resemblance between them. The anisotropy (g ) factors for the ~(1)L_(b) bands of these cyclophanes were considerably large (～10~(–2)), whereas those for the ~(1)L_(a) band were conventional in the order of 10~(–3). In addition, a weak CE was observed in the low-energy region at around 320 nm, which turned out to originate from the interplanar interaction and is hence assigned to the “cyclophane band”. The experimental g factors of this band were fairly large in the order of 10~(–2), but the computation turned out to be quite challenging and were less well reproduced theoretically, ascribable to the forbidden nature of the transition.