NMR study of iminophosphine oligomers by 31P, 15N and 31P, 13C heteronuclear shift correlation

2D 31P, 15N heteronuclear shift correlation, based on a non-refocused INEPT pulse sequence with 15N detection, was applied to a 15N NMR study of the azadiphosphiridines 1-5 which are formed by [2 + 1] cycloaddition of iminophosphines, RPNR′. The pulse sequence used allowed consistent assignment of both the 31P and 15N NMR spectra in one experiment; by a slight modification, also the signs of numerous 31P-15N coupling constants across one or two bonds could be determined. Comparison of the data with those of the azatriphosphetidine 6 (obtained from ID 1H,15N and 31P,15N INEPT spectra) showed the influence of the ring size and conformational effects on the NMR parameters. 31P,13C shift-correlated 2D spectra were further used to measure (including sign determination) and assign 31P-13C coupling constants across one to three bonds in 1-5. Based on the analysis of the coupling information and the known conformation of 4a, the relative stereochemistry of the diastereomers 4a and b could be derived; further, the comparison of the 31P-13C coupling data of 1-3 with those of the 4a and b allowed the conformations of these derivatives, which were previously unknown, to be determined. The derived information of the constitution and stereochemistry of 1-5 gives insight into mechanistic details of the iminophosphine [2 + 1] cycloaddition.