Solvent effects on isomer distributions in acylation of 1,2,3,4-tetramethyldibenzofuran: regiospecific solvation of the intermediate σ-complex

A large solvent effect is observed in Friedel–Crafts acylation of 1,2,3,4-tetramethyldibenzofuran (TMD). 8-Acyl-TMD is by far the major product in nitrohydrocarbons while 7-acyl-TMD is favoured in chloro-hydrocarbons and is the major product in many cases. The influence of temperature, and the nature of the reaction medium, acylating agents, and added nitro-compounds on the isomer distribution, are examined in detail. It is demonstrated that neither rearrangement of the products, nor thermodynamic differences between the 7-and 8-position of TMD, nor reduction in electrophilicity of the acylium ion by solvation are responsible for the solvent effect. Specific association of a nitrohydrocarbon solvent with the intermediate σ-complex leading to 8-acyl-TMD is presented as the pathway of the reaction.