Epoxidation of Alpha-Methylstyrene and Rearrangement to 2-Phenylpropanal
Epoxidation of Alpha-Methylstyrene
A mixture of 1.2 g (10 mmol) of alpha-methylstyrene, 25 mL of methylene chloride, and 4 g of anhydrous sodium carbonate is placed in a 100mL flask fitted with a magnetic stirring bar and in a water bath. Stir the suspension and add 3.0 mL of 32% peracetic acid solution (14 mmol; Aldrich Chemical Co.; d=1.13 g/mL) over a period of 5-10 min. maintaining the temperature at or slightly below room temperature. Frothing will occur. Allow the mixture to stir for 2 h at or below 25°C. Filter the reaction mixture and wash the filter cake with 4 mL of methylene chloride. To remove the excess peracid, prepare a small column of alumina using a disposable glass Pasteur pipet. Wedge a small piece of filter paper into the bowl of the pipet using a wooden applicator stick and fill with two inches of alumina (80-200 mesh). Using another pipet, transfer the reaction solution through the alumina column. Test this filtrate for peracid with starch iodide test paper. If necessary, refilter through another alumina column. Remove the solvent at ambient temperature.
Rearrangement of Epoxide with Boron Trifluoride Etherate
One half of the epoxide product mixture is dissolved in 10 mL of methylene chloride in a small sample vial fitted with a magnetic stirring bar. Add 4 drops of boron trifluoride solution (1:2 in CH2Cl2) to the vial while stirring. Cap the vial and let the reaction proceed for 15 min. Carefully transfer the reaction mixture through a small alumina column, prepared as before, to remove the BF3. Remove the solvent and analyze the product via IR or NMR (the aldehyde proton gives a doublet at 9.8 ppm, CDCl3. Unreacted styrene will be polymerized by the BF3 and can be removed as described below. The purchased alpha-methylstyrene reagent contains some polymeric material.
Reference: David L. Garin, et. al., J Chem Ed, Volume 73 (6), 555 (1996)
2-phenylpropanal and P2P from Alpha-Methylstyrene
118g alpha-Methylstyrene was added to a mixture of 160g of bromine and 1200ml 15% H2SO4, and the mixture was heated at 80°C for 8h. The mixture was cooled to room temperature and the organic layer separated, and the aqueous layer extracted with benzene. The extracts and organic layer was mixed and dried over Na2SO4. The benzene was evaporated under vacuo, and the residue was vacuum distilled, and the fraction boiling at 75-85°C/8mmHg weighed 75g and was purified via its bisulfite adduct to give a product mixture boiling at 78-81°C/4 mmHg in 57% yield, consisting of 48.5% 2-phenylpropanal and 51.5% phenyl-2-propanone.
Reference: CA 71, 61016x (1969)
2-Phenylpropanal can also be synthesized from phenylmethylglycidic ester, which in turn can be made by Darzens condensation of acetophenone and ethyl chloroacetate.