Evidence for a Single Transition State in the Transfer of the Phosphoryl Group (-PO32-) to Nitrogen Nucleophiles from Pyridino-N-phosphonates

Article abstract of DOI:10.1021/ja00336a046

The reaction of pyridinio-N-phosphonates with pyridines in aqueous buffers has been demonstrated to involve nucleophilic attack at phosphorus.The second-order rate constants obey the equation log k_xpy = 0.15pK_xpy - 0.86 for attack on isoquinolino-N-phosphonate over a wide range of pyridine basicity indicating a single transition state; this is consistent with a concerted transfer of the phosphoryl group rather than with a stepwise mechanism involving metaphosphate ion in a ternary encounter complex with donor and acceptor.Transfer of the phosphoryl group to pyridine from substituted pyridinio-N-phosphonates obeys the equation log k = -0.92pK_xpy + 5.24 and leads to a β_eq of 1.07 for substituent effect on the equilibrium constant for transfer.The effective charge at nitrogen, in the transition state, indicated by these values favors weak P-N bonding.An imbalance of -0.77 effective charge units between entering and leaving nitrogen in the transition state is proposed to derive from the charge on the PO3 atoms, which therefore do not bear a full negative charge in the transition state.Transfer of the phosphoryl group from isoquinolinio-N-phosphonate to amines has been investigated kinetically, and the results are also consistent with weak bonding between phosphorus and nitrogen in the transition state.