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

Abstract

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.

Nanosized Carbon Macrocycles Based on a Planar Chiral Pseudo Meta‐[2.2]Paracyclophane

“…Accordingly, the first and second eluted fraction can be assigned as R P ‐ and S P ‐ enantiomer, respectively. These results are consistent with the observations of pseudo meta dibromo[2.2]PCP [15b] …”

Fluorescence detected circular dichroism (FDCD) for supramolecular host–guest complexes

“…The enantiomers of the paracyclophane-derived dye MPCP, ( S P )-MPCP (( S p )- 3a ) and ( R P )-MPCP (( R p )- 3b ) were prepared through a stepwise coupling procedure following the synthetic route shown in Scheme 1a . Starting from unsubstituted [2.2]paracyclophane, an aromatic substitution with bromine under iron catalysis resulted in ( rac )-4-bromo[2.2]paracyclophane ( 1 ) in excellent yield, 53 followed by a preparative HPLC with a chiral stationary phase 54 to separate the two enantiomers of racemic 1 , ( S p )- 1a and ( R p )- 1b . Subsequently a Pd-catalysed Suzuki cross-coupling of ( S p )- 1a and ( R p )- 1b with 4-pyridyl boronic acid was carried out to obtain ( S p )- 2a and ( R p )- 2b with 93% ee and 99% ee respectively and 79% yield.…”

Transannular Hydrogen Bonding in Planar‐Chiral [2.2]Paracyclophane‐Bisamides

“…In addition, Mori and coworkers recently attributed detailed Cotton effects for [2.2]pCp-bromides to the substitution pattern. [70] It is envisioned that different Cotton effects would be observed for the planar-chiral compounds (R p )-1b and (R p )-3 resulting from their transannular H-bonding properties. Trans-annular H-bond capable ps-o-bisamide (R p )-1b and monoamide (R p )-3 have similar Cotton effects in the higher energy region, with (R p )-3 exhibiting a more intense absorbance in the cyclophane region (250-285 nm) compared to (R p )-1b ( Figure 6).…”