A Single Transition State in the Transfer of the Methoxycarbonyl Group between Isoquinoline and Substituted Pyridines in Aqueous Solution

Article abstract of DOI:10.1021/ja00244a028

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 (β_nuc=0.58) over a range of nucleophile pK_a greater than and less than the pK_a 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.