C-13, N-15, AND P-31 NMR SPECTROSCOPIC STUDIES OF N-AROYL-P,P,P-TRIPHENYLPHOSPHA-LAMBDA-5-AZENES

The 13C, 15N, and 31P NMR spectra of a series of N-aroyl-P,P,P-triphenylphospha-X5-azenes 3 are reported. The syntheses of several heretofore unknown members of this class of compounds are also reported along with the syntheses of 15N-labeled compounds. The parent substituted benzamides 4, including 15N-labeled derivatives, have also been examined by 13C and 15N NMR spectroscopy. Correlation of the 31P, 15N, and various 13C chemical shifts with Hammett substituent constants suggests that 3 can be viewed as being made up of three interacting dipolar subunits as shown in Figure 1. This is based on the slope of the plots of the chemical shifts for various atoms vs the Hammett constants. The response of the one-bond coupling constants h/PN and le/PC to changes in substituent is consistent with pπ-dπ and pπ-o* (using a P-C a* orbital) between the P and N atoms. Alternatively, one can explain the changes in 1Jpn, 1JPC and also VNCon the basis of electrostatic interactions based on the structure in Figure 1, by assuming that adjacent atoms with opposite charge will attract, shorten the bond, increase the s character of the orbitals making up the bond, and thus increase the one-bond coupling constant. The opposite will be true for atoms that repel one another. The 13C and 15N chemical shifts in 3 correlate with those in 4. From the sign of the slopes of these correlations, one can show that the positively charged phosphorus atom of the phosphazenyl group polarizes the rest of the molecule (N, CO, and aryl group) relative to the benzamides. The ipso carbon atoms in N-aryl-P,P,P-triphenylphospha-X5-azenes 1, N-(arylsulfonyl)-P,P,P-triphenylphospha-X5-azenes 2, 3, 4, the anilines 5, and arenesulfonamides 6 have all been compared by treatment with the Taft DSP and the DSP-NLR equations. The conclusions are that the Ph3P=N group is a strongly electron donating group and is even more electron donating than NH2. The Ph3P=NSO2 group is moderately electron withdrawing while the H2NS02 group is much more strongly electron withdrawing. Ph3P=NC(O) and H2NC(0) both appear to be about equally moderately electron withdrawing with the former group (in 3) being a little less electron withdrawing. This small difference between these groups may be the result of the ability of the nitrogen to interact more strongly with the aromatic ring by back donation. © 1990, American Chemical Society. All rights reserved.