Solvent effects on isomer distributions and relative rates in Friedel–Crafts benzylation and benzoylation of dibenzofuran derivatives

Abstract

The positional reactivity order in Friedel–Crafts benzoylation and benzylation of dibenzofuran (DBF) is found to be 2 > 3 > 1 4. Both the partial rate factors and the positional selectivity for the benzylation of DBF are very low compared with those of benzoylation. In competitive benzoylation of 1,2,3,4-tetramethyldibenzofuran (TMD)versus DBF, a large solvent effect has been observed on the relative rate (TMD versus DBF) as well as on the isomer ratio of 8- to 7-benzoyl-TMD, which appears to be due to the difference in solvation of the intermediate σ-complex leading to each of the products. In contrast, in the case of competitive benzylation of TMD versus DBF, neither solvent effect on the relative rate nor any isomer ratio was observed. The nature of the transition state determining the relative rate and the positional reactivity for benzylation has been deduced from these results.