Positional Reactivity of Dibenzofuran in Electrophilic Substitutions

Article abstract of DOI:10.1021/jo00015a020

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 (95percent 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 ?-complex) favors reactions at the 2-position while the early transition-state model (by HOMO electron density) leads to the 3-substitution.