Ionic liquids can effectively accelerate slow N-benzylation reactions utilizing dibenzyl carbonate as an alkylating reagent. Dibenzyl carbonate (DBzlC) has been used to benzylate phenylacetonitrile, benzyl phenylacetate and phenol. Selective N, N-dibenzylation of primary aliphatic amines was carried with dibenzyl carbonate in the presence of phosphonium salts.
To a stirred solution of benzyl alcohol (0.216 g, 2.00 mol), tributylphosphine (0.303 g, 1.50 mmol), and CyTMG (0.394 g, 2.00 mmol) in DMF (2.00 mL), CO2 was added at room temperature. After 15 min, tetrabromomethane (0.663 g, 2.00 mmol) was added, the reaction vessel was sealed, and the contents were stirred for 2 h. Thereafter, the reaction mixture was diluted with ethyl acetate, washed successively with 0.5 m aqueous HCl and saturated aq. NaHCO3 solution, and dried over Na2SO4. Diphenylmetha- nol was added to the organic solution as an internal standard and the yield of dibenzyl carbonate was determined from the relative integrals of a methylene peak of the carbonate and a methine peak of diphenylmethanol in the 1H NMR spectrum of the ethyl acetate solution. Dibenzyl carbonate, prepared by a larger scale reaction of CO2 with benzyl alcohol (1.08 g, 10.0 mmol), was isolated by column chromatography (cyclohexane/ethyl acetate, 50:1) in 67.3% yield (0.816 g).
Dibenzyl carbonate (DBC) is commonly used as a benzylating agent. Dimethyl carbonate and excess of benzyl alcohol undergoes transesterification in the presence of CsF/α-Al2O3 (cesium fluoride/aluminum oxide) to form DBC. The formation of N,N-dibenzyl derivatives by the reaction of primary aliphatic amines with DBC in the presence of phosphonium salts has been investigated.
