Regioselective Carbon-Oxygen Bond Cleavage Reactions of Aromatic Ethers and Esters with Potassium Metal/18-Crown-6/THF as the Electron-Transfer Reagent

Article abstract of DOI:10.1021/jo00257a007

The facile carbon-oxygen bond cleavage reactions of a variety of aromatic ethers and esters were studied with an electron-transfer reagent that consisted of potassium metal and 18-crown-6 in tetrahydrofuran (THF).The relative rates of carbon-oxygen bond cleavage, with these aromatic ethers and esters under electron-transfer conditions, was studied in competition with the standard, diphenyl ether.The order of relative reactivities was as follows: phenyl 2-hydroxybenzoate (10) (<*>2.0); phenyl 2-methoxybenzoate (11) (<*>2.0); phenyl 4-methoxybenzoate (12) (<*>2.0); 2-methoxyphenyl benzoate (13) (<*>2.0); 4-methoxyphenyl benzoate (14) (<*>2.0); 4-hydroxyphenyl benzoate (15) (1.6); benzyl benzoate (9) (1.3); 4-methoxyphenyl phenyl ether (5) (1.15); phenyl benzoate (8) (1.04); diphenyl ether (1) (1.0); dibenzyl ether (2) (0.98); 2-methoxyphenyl phenyl ether (6) (0.97); benzyl phenyl ether (3) (0.96); phenethyl phenyl ether (7) (0.77); and 4-hydroxyphenyl phenyl ether (4) (<*>0.1).From these relative rates, it is clear that electron-donating groups increase the relative rates of C-O bond cleavage in aromatic esters.Conversely, a methoxyl group has no apparent effect on the relative rates of aromatic ether C-O bond cleavage, while a hydroxyl group dramatically decreases the rate.Proton donors, tert-butyl alcohol and hydroquinone, decreased the rate of diphenyl ether cleavage.The regioselectivity for C-O bond cleavage of esters 8-15, i.e., carbonyl-oxygen versus carboxyl-carbon cleavage, showed selective carbonyl-oxygen bond cleavage.The regiochemistry for C-O bond cleavage reactions for the ethers 3-6 showed a preference for the benzyl-oxygen bond, whereas substitution of either a hydroxyl and methoxyl group in the 4 position of diphenyl ether provided data that showed selective cleavage in the substituted aryl groups.Substitution of a 2-methoxyl group in diphenyl ether changed the rgiochemistry to cleavage in the unsubstituted aryl group.The overall results also imply that the theory of regioconservation of spin density may be operative in the cleavage of carbon-oxygen bonds in aromatic ethers and esters under electron-transfer conditions.