Phosphoryl transfer between pyridines

Hydrolysis of Phosphates. The conclusion that phosphoryl transfer between amines is concerted does not exclude the formation of a metaphosphate intermediate in reactions with weaker nucleophiles, including water. Pyridine is a much stronger nucleophile than water and it is possible that phosphoryl transfer between pyridines is concerted because the metaphosphate ion does not have a significant lifetime in the presence of pyridine the metaphosphate ion might exist for a short time in water. However, it is much more difficult to determine whether or not the mechanism is concerted for a reaction with water than for a reaction with pyridine. 

A series of kinetic experiments reported by Herschlag and Jencks (7) addressed the influence of metal ions on phosphoryl transfer reactions for non-enzymic phosphoryl transfer between pyridine and carboxylate ions (and other nucleophiles). The reaction of actetate ion with phosphorylated -y-picoline monoanion and the reverse reaction of y-picoline with acetyl phosphate dianion are... 

Scheme 4 Transfer of the phosphoryl group between pyridine nucleophiles - an open concerted mechanism shown also is the charge distribution in the transition structure and that expected for two putative intermediates...

The role of zinc ions as catalysts for phosphoryl transfer between phosphorimidazole and pyridine-2-carboxaldoxime is attributed to a lowering of the electrostatic barrier for attack of the anion of (26) on phosphorus it may also act as a template for the proper alignment of the entering and leaving groups. With amine nucleophiles it was found that zinc, in common with several bivalent cations, inhibited attack on phosphorimidazole. 

These results are consistent with the observation that increasing basicity leads to a more favorable solvation energy for transfer of pyridines from the gas phase to water. The available data are consistent with a value of d for desolvation of approximately —0.2 (10, 13-16). The observed P value of — 0.1 for the reactions of quinuclidines with 2,4-dinitrophenyl phosphate and phosphorylated pyridine would then reflect the difference between the value of Pd = —0.2 and a value of Pnuc = 0.1 for the bond-forming step (equation 2). 

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