POSITIONAL REACTIVITY OF DIBENZOFURAN IN ELECTROPHILIC SUBSTITUTIONS

Isomer distributions of dibenzofuran (DBF) in Friedel-Crafts acylations, Friedel-Crafts alkylations, and nitrations have been determined. The 2- and 3-positions of DBF represents most of the total reactivity. However, the ratio of 2- to 3-isomers greatly varied, depending on the nature of the electrophile. The positional reactivities have been found to be in the following sequence: 2- > 3- > 1- > 4-positions for Friedel-Crafts acylations, Friedel-Crafts benzylations, and nitrations with alkyl nitrate/Lewis acid or nitronium tetrafluoroborate. The ratios for acylations varied over a range from 13.1 to 2.9, while for benzylations and nitrations from 2.0 to 1.0. In contrast, for nitrations of DBF with nitric acid a different reactivity order was found: 3- > 2- > 1- > 4-, with the ratio varying from 0.8 to 0.03 depending on the nature of solvents used. The selectivity for the 3-substitution increased with increase in nitronium ion-like character of nitrating reagents. In particular, nitration with nitric acid in dichloromethane gave mostly 3-nitro-DBF (95% of the four possible isomeric mixture). The charge-transfer nitration with tetranitromethane under the UV irradiation has shown a similar isomer distribution to that in nitration with nitric acid. The MNDO calculations predicts that the late transition-state model (by sigma-complex) favors reactions at the 2-position while the early transition-state model (by HOMO electron density) leads to the 3-substitution.