Solvolysis of α-Arylvinyl Ethyl Phosphates in Aqueous and Methanolic Solutions: Intramolecular Participation by the oxy Group

Article abstract of DOI:10.1021/jo00198a003

The α-arylvinyl ethyl phosphates 2a-c were synthesized and their solvolysis reactions were studied in acidic aqueous solution and methanol.In aqueous solution both the anionic and neutral diesters are subject to acid-catalyzed hydrolysis.Solvent deuterium isotope effects, the observation of general acid catalysis by formic acid, and comparison with the reaction characteristics of the acid-catalized hydrolysis of other vinyl esters and ethers indicate that the anionic diesters hydrolyze via an A_SE2 mechanism.The ratio of the rate constants for hydronium ion catalyzed hydrolysis of anionic and protonated 2c, k_-/k₀, is 14.1 +/- 0.3.This modest acceleration is consistent with weak electrostatic stabilization of the developping cation by the anionic site, although an inductive effect must also contribute.A previously reported correlation of log k_H+ for a series of alkenes of the type R₁R₂C=CH2 with the sum of the ?_P⁺ parameters for R₁ and R₂ and k_- for 2a at 25 deg C were used to calculate a value of -0.52 for the ?_P⁺ parameter of the anionic <(ethyloxy)phosphoryl>oxy group.The estimated ?_P⁺ of the protonated group is -0.43.These parameters and direct comparisons of hydrolysis rates of 2a-c with the hydrolysis rates of the corresponding triesters 1a-c and the α-arylvinyl methyl ethers confirm that the <(ethyloxy)phosphoryl>oxy group, in both of its ionization states, is intermediate between a oxy group and an alkoxy group in its ability to stabilize a cation.The solvolysis of 2a-c in methanol/HCl proceeds via two routes.An HCl-catalyzed pathway has characteristics very similar to the acid-catalyzed hydrolysis reactions.Another path, the rate of which is independent of HCl concentration, apparently involves intramolecular general acid catalysis of methanolysis by the prot onated <(ethyloxy)phosphoryl>oxy group.Differences in the ionization caonstants of the diesters in the two solvents apparently account for the divergent solvolysis behavior.