Trimethylsumanene: enantioselective synthesis, substituent effect on bowl structure, inversion energy and electron conductivity

Abstract

C₃ symmetric chiral trimethylsumanene was enantioselectively synthesized through Pd-catalyzed syn-selective cyclotrimerization of an enantiomerically pure iodonorbornenone, ring-opening/closing olefin metathesis, and oxidative aromatization where the sp³ stereogenic center was transmitted to the bowl chirality. Chiral HPLC analysis/resolution of the derivatives were also achieved. Based on theoretical calculations, the columnar crystal packing structure of sumanene and trimethylsumanene was interpreted as due to attractive electrostatic or CH–π interaction. According to the experimental and theoretical studies, the bowl depth and inversion energy were found to increase on methylation for sumanene in contrast to corannulene. Dissimilarities of the effect of methylation on the bowl structure and inversion energy of sumanene and corannulene were ascribed to differences in steric repulsion. A double-well potential model was fitted to the bowl structure–inversion energy correlation of substituted sumanenes, with a small deviation. The effects of various substituents on the sumanene structure and bowl-inversion energy were analyzed by density functional theory calculations, and it was shown that the bowl rigidity is controlled by a combination of electronic and steric effects of the substituents. The electron conductivity of trimethylsumanene was investigated by time-resolved microwave conductivity method, compared with that of sumanene.