Kinetic isotope effects on ethyl metaphosphate transfer from a phosphoramidate to ethanol

Article abstract of DOI:10.1021/ja00105a008

Kinetic isotope effects of nitrogen and hydrogen on thermolysis of O-ethyl N-mesityl- (1) and O-ethyl N-phenylphosphoramidates (2) in anhydrous ethanol at 80 °C are reported. The solvent hydrogen effect k_EtOH/k_EtOD is equal to 0.62 ± 0.03 for 1 and 0.67 ± 0.05 for 2. The kinetic nitrogen isotope effect k₁₄/k₁₅ was found to be equal to 1.0056 ± 0.0011 for 1 and 0.9925 ± 0.0024 for 2. These results are consistent with a mechanism which involves proton transfer from the OH group to the nitrogen moiety. The recalculated nitrogen kinetic effect for the second step (amine departure) is equal to 1.021 for 1 and 1.008 for 2. On this basis a metaphosphate-like transition state or "exploded" S_N2(P) transition state is proposed for solvolysis of 1. With its less bulky substituent, an earlier transition state in the unimolecular process or a change into the addition-elimination mechanism is considered for phosphoramidate 2.