Electron Transfer-Induced Conformational Changes of Highly Hindered Aromatic Compounds. The Case of Hexakis(alkylsulfonyl)benzenes

Abstract

The monoelectronic reduction of hexakis(alkylsulfonyl)benzenes (alkyl = methyl 1a, ethyl 1b, butyl 1c, iso-propyl 1d, and iso-butyl 1e) to the corresponding radical anion in dimethylformamide involves two widely separated redox systems, except in the case of 1e that shows a single perfectly reversible system. Electrochemical data supported by calculations of molecular modeling (DFT and PM3 methods) show the existence of a four-member square scheme for which the neutral and radical anion species can both exist under chair and boat conformations. The relative stability of the conformers was found to be strongly dependent on the nature of the alkyl substituents. Generally, the most stable neutral forms adopt a chairlike geometry, and the radical anions adopt a boatlike geometry. For the most hindered compound 1e, the steric contribution of the (iso-butylsulfonyl) substituents becomes so strong that the conformational changes are considerably slowed, resulting in a frozen chair conformation