779285-57-3Relevant academic research and scientific papers
A Single Transition State in the Transfer of the Methoxycarbonyl Group between Isoquinoline and Substituted Pyridines in Aqueous Solution
Chrystiuk, Edwin,Williams, Andrew
, p. 3040 - 3046 (2007/10/02)
N-Methoxycarbonylisoquinolinium ion reacts with nucleophiles in aqueous solution according to the equation .The rate constants, k2, for attack of substituted pyridines on the isoquinolinium species exhibit a linear Broensted relationship (βnuc=0.58) over a range of nucleophile pKa greater than and less than the pKa of isoquinoline.The Broensted data indicate a smaller change in effective charge on nucleophilic nitrogen for formation of a putative tetrahedral intermediate than for its decomposition to product.This is opposite to what is expected for the stepwise process where the largest bonding change to attacking nitrogen is in the addition step.The results are consistent with a single transition state in the transfer of the methoxycarbonyl group between pyridines in aqueous solution; they contrast with those for reaction of pyridines and tertiary amines with neutral acyl derivatives where relatively stable zwitterionic tetrahedral intermediates have been demonstrated.The transition state for transfer between pyridines is symmetrical, and the effective charge on its pyridine nitrogen is consistent with about 40percent of a single bond between nitrogen and acyl carbon.An imbalance of effective charge indicates that the MeO-CO component of the transition state has considerable acylium ion character pointing to an almost square-planar structure.
Solvent Effect on the Relative Nucleofugalities of Pyridines and Phenoxide Ions: Broensted-Type Plots in the Pyridinolysis of 2,4-dinitrophenyl Methyl Carbonate and 2,4-Dinitrophenyl Acetate in Aqueous Ethanol
Castro, Enrique A.,Borquez, Maria T.,Parada, Paulina M.
, p. 5072 - 5077 (2007/10/02)
Curved Broensted-type plots (log kN vs. pKa) are obtained for the nucleophilic reactions of the title in 44 wt percent aqueous ethanol, at 25 deg C, ionic strength 0.2 M (KCl).The curves are described by a semiempirical equation based on the existence of a tetrahedral intermediate (T+/-) in the reaction path and a change of the rate-determining step from breakdown to formation of T+/- as the pyridine is more basic.The pKa value at the center of curvature (pKa0) increases with increasing inductive electron withdrawal from the nonleaving group of the substrate (pKa0 7.6 and 8.0 for the acetate and carbonate esters, respectively), favoring amine expulsion, relative to that of 2,4-dinitrophenoxide ion, from T+/-.The pKa0 values for the present reactions are larger than those for the same reactions in water, which shows that amine expulsion from T+/- is fovored over 2,4-dinitrophenoxide expulsion by the change to a less polar solvent.Specific acid catalysis of aminolysis is found in the reaction of both substrates with 4-cyanopyridine at low pH. The most likely mechanism involves amine attack to the substrate to form T+/-, followed by protonation equilibrium with T+ and breakdown to products of T+/- and T+.
