NMR spectroscopic and computational study of static and dynamic E/Z isomerism in aminoiminophosphines R2N-P = NMes

The P-31 NMR spectra of the iminophosphines R2N-P=NMes* [R = Me (2a), Et (2b), i-Pr (2c); Mes* = 2,4,6-tri-tert-butylphenyl] were studied in the solid state under magic angle spinning conditions and in solution. Evaluation of the solid-state data in connection with CIAO quantum chemical model calculations allows the characterization of the P-31 nuclear magnetic shielding tensors and leads to assignment of Z-configuration for the double bond in 2a and of the E-configuration for the double bonds in 2b and c. Comparison of the shifts delta(iso) with the chemical shift in solution (delta(soln)) indicates that this configuration is retained in solution only for 2c, whereas solutions of 2a and b contain rapidly exchanging equilibrium mixtures of both E- and Z-isomers. The analysis of the temperature dependence of delta(soln) allows for the first rime the evaluation of Delta H and Delta S for this isomerization. The results reveal that in the case of 2b the configuration of the more stable stereoisomer is opposite to the conformation observed in the solid state. Copyright (C) 2000 John Wiley & Sons, Ltd.