O-Ethyl Phosphoramidic Acids with Sterically Demanding N-Substituents: Useful Precursors of Ethyl Metaphosphate on Thermolysis

Article abstract of DOI:10.1021/jo00095a013

Kinetics of the thermal fragmentation of four N-substituted derivatives of O-ethyl phosphoramidic acids, (EtO-P(O)(NRR')(OH), were examined.When N contained either of the sterically demanding mesityl or 1-adamantyl groups, the reaction followed first-order kinetics, both in the absence and prasence of an alcohol trapping reagent.In the former case, the product was a pyrophosphate(RR'N(EtO)(O)P-O-P(O)(OEt)OH).In the latter case, phosphorus was trapped as a dialkyl phosphate.Both reactions are therefore indicated to follow an elimination-addition mechanism, with ethyl metaphosphate as transient intermediate.The pyrophosphate is derived from reaction of the metaphosphate with unreacted phosphoramidic acid.With less bulky substituents (N-phenyl or N,N-diethyl), mixed first- and second-order kinetics were followed in the absence of a trapping agent; some bimolecular interaction of the substrate to form the pyrophosphate product is indicated by the second-order kinetics.Product analyses and quantitative measurements were made with 31P NMR spectroscopy.From all phosphoramidic acids, the intermediate metaphosphate was effectively trapped by reaction with the OH group on the surface of solid silica gel.The presence of covalently bonded phosphate on the surface was shown by 31P and 29Si CP/MAS NMR spectroscopy.