Facile synthesis of 2-aryl or β,γ-unsaturated esters via 1,2-Migration from aryl or α,β-unsaturated ketones using thallium(III) p-tosylate

The experiment reports that 2-aryl esters can be efficiently synthesized via 1,2-aryl migration from aryl ketones using thallium(III) p-tosylate in high yields. To determine optimum conditions for conversion of aryl ketones to 2-aryl esters, the effects of solvents were examined. An initial reaction of 4'-methoxypropiophenone and perchloric acid using thallium(III) p-tosylate in ethanol afforded ethyl 2-(4-methoxyphenyl)propanoate in only 10% yield after 24 h at room temperature. However, the corresponding reaction in ethanol/triethyl orthoformate (4/1) was completed in 1 h between 0 °C and room temperature to give ethyl 2-(4-methoxyphenyl)propanoate in 94% yield. The presence of triethyl orthoformate induced rapid ketalization of enol intermediate and facilitated 1,2-migration of the 4-methoxyphenyl group. The relative effectiveness of several metal salts was also examined for conversion of 2',4'-dimethoxypropiophenone to ethyl 2-(2,4-dimethoxyphenyl)propanoate. The solvents were evaporated off under reduced pressure, and the residue was dissolved in methylene chloride. The white precipitate was filtered off, and the resulting yellow solution was poured into saturated NaHCO3 solution and extracted with methylene chloride. The combined organic layers were dried over anhydrous MgSO4, filtered, and concentrated in vacuo. The residue was purified by vacuum distillation using a Kugelrohr apparatus to give 4g as a colorless liquid.