strong chiroptical responses
The unique combination of geometric and electronic properties explains the remarkable magnitude of the Cotton effects in the circular dichroism spectra of new enantiomerically pure alleno-acetylenic macrocycles (see picture). The macrocycles (P,P,P,P)-(−)-1 (red) and (M,M,M,M)-(+)-1 (blue) were prepared in three steps starting from optically pure 1,3-di-tert-butyl-1,3-diethynylallenes.
New enantiomerically pure alleno–acetylenic macrocycles were prepared by oxidative homocoupling of optically active 1,3-diethynylallenes. Enantiomer separation resulted from a combined strategy of synthesis and chiral HPLC techniques. Two other achiral stereoisomers were also isolated and fully characterized. In addition, the X-ray structures of the chiral D4- and C2-symmetric macrocycles are reported. The chiroptical properties of these macrocycles are discussed on the basis of quantum chemical calculations, by using the CAM-B3LYP functional. Studies were carried out to investigate the vibronic fine structure observed experimentally in the UV/Vis and CD spectra. The origin of the intense chiroptical response of the chiral alleno–acetylenic macrocycles is explained by considering the topology of the molecular orbitals involved, thus relating electronic properties to structural features. Further analysis of the canonical molecular orbitals and the electron localization function (ELF) shows that these macrocycles belong to a relatively rare class of highly stable and formally anti-aromatic Hückel compounds.
Enantiomerically pure alleno-acetylenic oligomers of defined lengths were synthesized by the palladium-mediated oxidative homocoupling of optically pure 1,3-diethynylallenes. The large amplification of their chiroptical properties strongly suggests the formation of helical secondary structures. This assignment is supported by time-dependent quantum chemical calculations.