Effect of Crown Ethers on the Selectivity of Electrophilic Aromatic Nitration

Article abstract of DOI:10.1021/jo00221a023

Relative rates, isomer distributions, and partial rate factors have been determined for the nitration reactions of benzene, toluene, m-xylene, anisole, and mesitylene with tetrabutylammonium nitrate and trifluoroacetic anhydride in homogeneous CH2Cl2 solutions.The observed selectivity values have been compared with those obtained when 12-crown-4, 15-crown-5, 18-crown-6, 21-crown-7, or 24-crown-8 was added to the nitrating mixture.Small effects, if any, were observed with the smaller ligands, but marked variations in both substrate and positional selectivity appeared in the presence of 18-crown-6, 21-crown-7, and 24-crown-8.Such effects were found to depend on crown ether concentration and to vanish when KClO4 was added.A reaction scheme is proposed where both uncomplexed and complexed nitronium ion contribute to variable extents to the overall reaction.The selectivity of the crown ether associated electrophile relative to the unassociated one seems to result from the balance of two different effects: an increase in the sensitivity to the electronic effects of the substituents and a larger crowding in the transition state.The largest effects on selectivity were observed with 21-crown-7 which causes ortho-para-directing groups to act as essentially para-directing groups.Data for toluene nitration in the presence of 21-crown-7 fit very well the Brown selectivity relationship; the selectivity factor spanning a remarkably wide range on changing the crown ether concentration.