14535-12-7Relevant articles and documents
Reactions of Charged Substrates. 5. The Solvolysis and Sodium Azide Substitution Reactions of Benzylpyridinium Ions in Deuterium Oxide
Buckley, Neil,Oppenheimer, Norman J.
, p. 7360 - 7372 (1996)
Second-order rate constants and activation values were measured for the reactions with NaN3 of a series of 4-Y-substituted (Y = MeO, Me, H, Cl, and NO2) benzyl 3'-Z-substituted (Z = CN, CONH2, H, F, Ac) pyridinium chlorides in deuterium oxide. 3'-Cyanopyridine substrates reacted much faster than nicotinamide and pyridine substrates; in the pyridine series the 4-me, 4-H, and 4-Cl benzyl analogs did not react for up to 6 months at 96 deg C in 1.7 M NaN3.The 3'-cyanopyridine substrates do not exhibit borderline kinetic behavior, but the nicotinamide substrates do.The Hammett plot is flat for the NaN3 reaction of 3'-cyanopyridine substrates and increasingly V-shaped for the nicotinamide and pyridine substrates. the values of βLG (four-point plot) for the NaN3 reaction of the 4-MeO benzyl substrates is -1.45, which is usually interpreted as being a very "late" activated complex.Two-pint Broensted "plots" for the other benzyl derivatives and for two N-methylpyridinium ions give values of βLG in the same range.The second-order rate constant and activation values for N-methyl-3'-cyanopyridinium iodide are within the same range as those for the benzyl substrates.For the hydrolysis reaction, the Hammett plot is linear for 3'-cyanopyridine substrates (ρ' = -1.24) and flat for the nicotinamide substrates.The extent of hydrolysis of 0.005-0.05 M solutions of the 3'-cyanopyridine substrates depended on the initial concentration of substrate, and hydrolysis was slowed significantly or stopped completely in the presence of exogenous 3-cyanopyridine.These results show that an equilibrium is established among the products for the 4-MeO, 4-Me, 4-H, and 4-Cl substrates; the 4-NO2 substrate reacted too slowly to discern any difference.Data for the extent of hydrolysis were fitted by an equation derived assuming the equilibrium.Despite this limitation on a classic test of mechanism, the rates and ρ values are consistent with direct displacement by solvent and not with a unimolecular process.These results, which are rationalized in terms of the Pross-Snaik model, suggest that there are no ion-dipole complex intermediates in the benzyl series and show that borderline kinetic behavior is a function of leaving group ability and is not necessarily related to a change in mechanism.A computational approach was used to evaluate anomalous βLG values for the hydrolysis and nucleophilic substitution reactions of the methylpyridinium ion substrates.It was found that neither the Nu-substrate bond lengths nor the difference in charge matched the βLG values.The value if ΔΔSactiv. of -15 gibbs/mol between (4-methoxybenzyl)-3'-cyanopyridinium chloride and the corresponding dimethylsulfonium chloride in the NaN3 reaction, which is the result of the solvation of the pyridine at the transition state and the lack of solvation of SMe2, is used to argue that the source of NAD+ glycohydrolase "catalysis" of NAD+ bond cleavage is the result of desolvation of the leaving group upon binding.
