PHOSPHORYL TRANSFER FROM PHENYL AND 4-NITROPHENYL PHOSPHATES IN APROTIC AND PROTIC SOLVENTS. AMINE CATALYSIS AND FORMATION OF OXYPHOSPHORANE AND METAPHOSPHATE INTERMEDIATES

Article abstract of DOI:10.1016/0040-4020(80)80159-1

The behavior of 4-nitrophenyl dihydrogen phosphate, ArOPO3H2, and of its tetra-n-butylammonium and tetramethyl ammonium salts, ArOPO3H^-R4N⁺, ArOPO3^2-2(R4N⁺), was studied in aprotic solvents, in the absence and in the presence of increasing amounts of alcohols or water.The reactions were investigated in the absence of amines, and in the presence of hindered and unhindered amines, diisopropylethylamine and quinuclidine.The course of the reactions was followed at 35 deg C or at 70 deg C by (31)P and (1)H NMR spectrometry.Values for the approximate half-times of the reactions were estimated (+/-25percent) from the times at which reactantsignal intensity becomes equal to product signal intensity.The mononitrophenyl ester transfers its phosphoryl group to alcohols and water from the diprotonated acid by the addition-elimination mechanism via oxyphosphorane intermediates, and from the monoanion and dianion by the elimination-addition mechanism via the monomeric metaphosphate intermediate, PO3^-.Formation of PO3^- is faster from dianion than from monoanion in acetonitrile and in alcohol solutions.Conversely, PO3^- is generated at a faster rate from monoanion than from dianion in aqueous solution.This effect results from a decrease in the rate of formation of PO3^- in the solvent series: acetonitrile > alcohols > water.The rate depression as a function of the medium is greater for the dianion than for the monoanion, and is attributed to greater solvation of the more polar phosphate ground state than of the less polar transition state in the more polar protic solvents.Unhindered amines add to 4-nitrophenyl phosphate monoanion, but not to the dianion.The oxyphosphorane intermediate thus formed collapses to aroxide ion and a protonated dipolar phosphoramide which is rapidly deprotonated by the relatively basic 4-nitrophenoxide: ArOPO3H^- + CH(CH2CH2)3N(acetonitrile<*>CH(CH2CH2)3N⁺P(O)(OH)O^- + ArO^-<*>CH(CH2CH2)3N⁺PO3^2- + ArOH<*>CH(CH2CH2)3N + PO3^-.The postulated formation of PO3^- by this route, explains why the addition of quinuclidine to an acetonitrile solution containing the monoanion salt, ArOPO3H^-R4N⁺, and t-BuOH produces t-butyl phosphate at a faster rate than the addition of diisopropylethylamine to the same solution. 2,4-Dinitrophenyl phosphate, which was previously studied by the same techniques, reacts via oxyphosphorane intermediates from the diprotonated and the monoanion forms, and via monomeric metaphosphate, from the dianion form.