Manganese Acetate-Mediated Phosphorylation of Indoles

Mn(OAc)(3)-mediated selective radical phosphoryiation of indoles with dialkylphosphites or diphenylphosphine oxide in moderate to good yields is described. The reaction proceeds under mild conditions in air, no need to add extra additives through a radical process involving phosphorus-centered radical generated from dialkylphosphites or diphenylphosphine oxide mediated by Mn(OAc)(3), selectively added to 3-position of 2-substituted indoles or 2-position of 3-substituted indoles to form carbon radical intermediate, followed by oxidation with Mn(OAc)(3) and deprotonation to afford products 2- or 3-phosphonyl indoles. This protocol provides an efficient and general method for the preparation of 2- and 3-phosphoryl indoles. The detailed investigation revealed that I) the yields of N-methyl indoles were higher than NH unprotected analogues; 2) 2-Phenyl or 2-methyl indoles were unsuitable for the reaction with dialkylphosphites, but were suitable for the reaction with diphenyl phosphine oxide; 3) Indoles bearing electron-withdrawing groups such as F, Cl, Br on the phenyl ring were favored to the reaction. A representative procedure for this reaction is as follows: To a solution of substituted indoles 1 or 4 (I mmol) and dialkylphosphites 2 (2 mmol) in HOAc (3 mL) heated with oil bath (50 degrees C), Mn(OAc)(3) (3 mmol) was added in portions within 0.5 h, then, the mixture was allowed to stir further for 0.5 k After removal of the most solvent under reduced pressure, 50 mL water was added to the residue and the mixture was extracted with Et0Ac (20 mL 3). The combined organic layers were dried over anhydrous Na2SO4, filtered, concentrated under reduced pressure, and the residue was purified by flash silica-gel column chromatography (using mixture of petroleum ether/ethyl acetate las eluent from ratio of 20/1 to 1/2) to afford the desired 2- or 3-phosphonyl indoles 3 or 5.