Trigonal pyramidal carbon geometry as model for electrophilic addition–substitution and elimination reactions and its significance in enzymatic processes

Abstract

ABSTRACT:Various examples are given in which compounds are characterized as products or intermediates in a (distorted) trigonal pyramidal (TP) geometry. These observations have taken place mainly in the field of carbocation chemistry. Special attention is given to carbenium ions formed by halogen addition to 1,1-diarylsubstituted ethylenes focused on the electronic effects of the C-halogen bond as axial bond in a TP geometry with regard to the -distribution in the rest of the molecular system. The experimental verification is accompanied by quantum chemical calculations. We also used the TP structure as a reactive model for specific enzymatic reactions. The relevance of this geometry is shown for the dehalogenation reaction of the nucleophilic displacement in dichloroethane catalyzed by haloalkane dehalogenase and for the decarboxylation of l-ornithine with ornithine decarboxylase under loss of carbon dioxide.

Small‐scale and automatable high‐throughput compositional analysis of biomass

“…We focused our attention on a model derived from the dynamics of a regular tetrahedron as originated by van't Hoff (vtH) in which the interstitial carbon changes its position from tetrahedral into a trigonal (bi)pyramidal configuration as proposed by Hughes and Ingold for S N 2 reactions1. This model based on the geometry of the corresponding transition state has been used for the introduction of dimensionless ratio numbers derived from the ab initio values in comparison with the vtH model as given by Buck2–5. As an illustration of this reaction type, we demonstrate the reaction progress for the identity reaction as shown in Figure 1.…”

Molecular model studies based on ab initio calculations of nucleophilic and electrophilic addition‐substitution reactions focused on carbon–halogen compounds

“…With the exception of F and apparently CN in the axis of the trigonal pyramid, the other elements Cl, Br, and I are good candidates for these partly bonded structures with I > Br > Cl ) F, CN as has been described by Buck [2]. Other nobond structures are high energetic because of the unfavorable charge polarization in the principal axis.…”

“…We gave a comprehensive description under Refs. [2] (p. 210) and [3] (p. 1612). Also, in this case, the CAF bond is quite different from the other halogens.…”

“…There is sin h for the equatorial bond length, besides the parameter 1 À cos h, in the trigonal bipyramidal to complete our model description [2][3][4][5]. With the ab initio values of h as shown in Table VI, we calculated the CACN bond length in the corresponding trigonal bipyramid.…”

“…This model based on the geometry of the corresponding transition state has been used for the introduction of dimensionless ratio numbers derived from the ab initio values in comparison with the vtH model as given by Buck [2][3][4][5]. As an illustration of this reaction type, we demonstrate the reaction progress for the identity reaction as shown in Figure 1.…”