Evidence That Metaphosphate Monoanion Is Not an Intermediate in Solvolysis Reactions in Aqueous Solution'

The slope of plots of log k2 against the pKa of the leaving group for reactions of uncharged, monoanionic, and dianionic oxygen nucleophiles with three phosphorylated pyridine monoanions changes from PI, = -0.98 to -0.79 as the pK, of the oxygen nucleophile increases from 3.6 to 15.7 (aqueous solution; 25 "C; ionic strength, 1.5). This coupling between the strength of the nucleophile and the measure of bond cleavage is described by an interaction coefficient, pxy = dp,,/dpK,,, = dp,,,/dpK,, = 0.013, and provides evidence for concerted phosphoryl transfer between pyridine and oxygen bases. The solvolysis of phosphorylated pyridines and of acetyl phosphate in aqueous solution also shows the behavior expected for a bimolecular substitution reaction with no metaphosphate intermediate: (1 ) The values of log k2 for reactions of oxygen nucleophiles, including water, with phosphorylated y-picoline monoanion follow a single line of slope 0.51 in a plot against log k2 for the corresponding bimolecular reactions with methyl 2,4-dinitrophenyl phosphate monoanion (Kirby, A. J.; Younas, M. J . Chem. SOC. B 1970, 1165). (2) Water behaves as expected for a nucleophile of its pKa in a Br~nsted-type plot of log k2 for reaction with phosphorylated y-picoline against the pK, of oxygen nucleophiles. (3) The value of PI, = -1.02 for the hydrolysis of phosphorylated pyridines is less negative than the value of Ow' = -1.25 for complete breaking of the P-N bond. (4) The value of Pi, = -1.02 for the reaction of water with phosphorylated pyridines fits the correlation of PI, against the pKa of oxygen nucleophiles for concerted, bimolecular reactions, with a slope ofp, = a~l,/dpKn,, = 0.013. (5) The reactions of acetyl phosphate monoanion and dianion with aqueous alcohols show selectivity that depends on the pKa of the alcohol, with a larger selectivity for the less reactive dianion [50% aqueous alcohol (v/v), 55 "C]. Metaphosphate monoanion has been discussed as a possible reaction intermediate since 1955 when eq 1 was proposed in order to explain the rapid hydrolysis of phosphate monoester monoanScheme I