Nucleophilic substitution at phosphorus centers (SN2@P)

We have studied the characteristics of archetypal model systems for bimolecular nucleophilic substitution at phosphorus (S(N)2@P) and, for comparison, at carbon (S(N)2@C and silicon (S(N)2@Si) centers. In our studies, we applied the generalized gradient approximation (GGA) of density functional theory DFT) at the OLYP/ TZ2P level. Our model systems cover nucleophilic substitution at carbon in X- + CH3Y (S(N)2@C), at silicon in X- + SiH3Y (S(N)2@Si), at tricoordinate phosphorus in X- + PH2Y (S(N)2@P3), and at tetracoordinate phosphorus in X- + POH2Y (S(N)2@P4). The main feature of going from S(N)2@C to S(N)2@P is the loss of the characteristic double-well potential energy surface (PES) involving a transition state [X-CH3-Y](-) and the occurrence of a single-well PES with a stable transition complex, namely, [X-PH2-Y](-) or [X-POH2-Y](-). The differences between S(N)2@P3 and S(N)2@P4 are relatively small. We explored both the symmetric and asymmetric (i.e. X, Y = Cl, OH) S(N)2 reactions in our model systems, the competition between backside and frontside pathways, and the dependence of the reactions on the conformation of the reactants. Furthermore, we studied the effect, on the symmetric and asymmetric S(N)2@P3 and S(N)2@P4 reactions, of replacing hydrogen substituents at the phosphorus centers by chlorine and fluorine in the model systems X- + PR2Y and X- + POR2Y, with R=Cl, F. An interesting phenomenon is the occurrence of a triple-well PES not only in the symmetric, but also in the asymmetric S(N)2@P4 reactions of X-+ POCl2-Y.