A linear three-center four electron bonding identity nucleophilic substitution at carbon, boron, and phosphorus. A theoretical study in combination with Van't Hoff modeling

摘要

We studied various identity nucleophilic substitution reactions based on an S_N2 reaction profile. With calculations and experimental geometries concerning the nature of the various complexes indicated as stable, intermediate and transition state we were able to show the additional value of van't Hoff 's tetrahedron by changing its geometry via a trigonal pyramid into a trigonal bipyramid. The ratio of the apical and the corresponding tetrahedral bond distance is then 1.333. This value has been used in general as a calibration point for the understanding of the (in)stabilities of the complex formation on the S_N2 reaction coordinate. The relevance of this approach has been also proved for enzymatic reactions focused on carbon and phosphorus substrates. Furthermore, it could be established that identity proton-in-line displacements are fully comparable with the relocation of carbon in a nucleophilic substitution reaction as Cl~- + CH_3Cl. The significance of this information will afford new insight in the dynamics of a linear three-center four-electron complex.