Computational Studies on Azaphosphiridines, or How to Effect Ring-Opening Processes through Selective Bond Activation

The relative energies of azaphosphiridine and its isomers, the ring stability towards valence isomerization, and the ring strain, as well as the kinetics and thermodynamics of possible ring-opening reactions of P-III derivatives (1-5) and P-V chalcogenides (6-9; O to Te), were studied at high levels of theory (up to CCSD(T)). The barrier to inversion at the nitrogen atom in the trimethyl-substituted P-III derivative 5 increases from 12.11 to 15.25 kcal mol(-1) in the P-oxide derivative 6 (P-V); the relatively high barrier to inversion at the phosphorus in 5 (75.38 kcal mol(-1)) points to a configurationally stable center (MP2/def2-TZVPP//BP86/def2-TZVP). The ring strain for azaphosphiridine 5 (av. 22.6 kcal mol(-1)) was found to increase upon P-oxidation (6) (30.8 kcal mol(-1); same level of theory). Various ring-bond-activation processes were studied: N-protonation of PIII (5) and P-V (6, 7) derivatives leads to highly activated species that readily undergo P-N bond cleavage. In contrast, metal chlorides such as LiCl, CuCl, CuCl2, BeCl2, BCl3, AlCl3, TiCl3, and TiCl4 show little P-N bond activation in 5 and 7. Remarkably, TiCl3 selectively activates the C-N bond, and induces stronger bond activation for P-V (6, 7) than for P-III azaphosphiridines (5). The ring-expanding rearrangement of PV azaphosphiridines 6-9 to yield PIII 1,3,2-chalcogena-azaphosphetidines 32a-d is predicted to be preferred for the heavier chalcogenides 7-9, but not for the P-oxide 6. The first comparative analysis of three bond strength parameters is presented: 1) the electron density at bond critical points, 2) Wiberg's bond index, and 3) the relaxed force constant. This reveals the usefulness of these parameters in assessing the degree of ring bond activation.