N-PHENYL-P,P,P-TRIARYLPHOSPHA-LAMBDA-5-AZENES, TRIARYLPHOSPHINES, AND TRIARYLPHOSPHINE OXIDES - SUBSTITUENT EFFECTS ON N-15, P-13, AND C-13 NMR-SPECTRA

The syntheses and N-15, P-31, and C-13 NMR spectra of a series of N-phenyl-P,P,P-triarylphospha-lambda-5-azenes 4 and the P-31 and C-13 NMR spectra of the corresponding series of triarylphosphines 5 and triarylphosphine oxides 6 are reported. The substituent effects on the chemical shifts can be best accommodated and rationalized by use of a model for system 4 whereby the dipole of the aryl group and its pendant R group polarizes the rest of the molecule. This includes the P and N atoms and phenyl ring, where an electron-withdrawing R group increases the electron density on the P, N, and ipso C-1 while decreasing the electron density on C-3 and C-4 of the N-phenyl ring (Figure 3). A similar polarization pattern for the phosphine oxide series 6 is suggested. In the phosphine series 5, the chemical shift data is consistent with the lone electron pair on the phosphorus atom delocalizing into the aryl rings. The coupling constant data, in particular 1J(PN) for series 4 and 1J(PC) for series 4-6, were examined with use of the Hammett monosubstituent parameter (MSP) and the Taft dual-substituent parameter (DSP) approaches. For systems 4 and 6, without a lone electron pair on the phosphorus atom, a better electron-donating substituent increases the one-bond P-C(Ar) coupling constant. On the contrary, in the phosphine series 5, where there is a lone electron pair on the phosphorus, a better electron-withdrawing substituent increases the one-bond P-C(Ar) coupling constant. DSP treatment of 1J(PC), and comparing to the few related systems in the literature, shows three types of systems. One, which includes 4 and 6, has an atom, phosphorus in these cases, that does not have a lone pair of electrons attached to the ring to which is attached an atom with a lone pair of electrons. Here, the resonance effect on 1J(PC) predominates. A second series, which includes phosphines 5, has a lone pair on the atom attached to the aryl ring. In these cases, the resonance effect is approximately 50% greater than the inductive effect. Finally, the third series, exemplified by two examples from the literature, has a tetrahedral atom (without a lone pair) attached to the aryl ring and this in turn is attached to tetrahedral atoms without lone electron pairs. In these case, the resonance and inductive effects are fairly comparable.