Soft dissymmetry

The HSAB theory of Pearson has been one of the key organizing concepts in the study of nucleophiles. This theory is applied and examined in Chapters 15 and 16. In Chapter 15, Fuji applies the HSAB principles to design nucleophilic reagents for cleaving C-X bonds. Fuji notes that all bonds are made of a combination of Lewis acid and Lewis base and have hard-soft dissymmetry for the typical C-X bond, the carbon is a soft acid and the X is a hard base. Thus, in accord with the HSAB principles, a soft base (the nucleophile) and a hard acid are required to cleave this bond selectively. Applying these ideas, Fuji then shows the utility of several soft base-hard acid reagents for cleaving various C-X bonds in complex molecules. 

All chemical bonds, except for symmetrically substituted ones, are inherently dissymmetric in two respects charge dissymmetry and hard-soft dissymmetry. The reversal of charge dissymmetry by modifying the structural unit (umpolung) has been well documented and has become one of the important principles in synthetic organic chemistry (16). On the other hand,... 

The HSAB concept is helpful here, too each soft or hard fiagment strives for stabilization on a corresponding center (symbiotic effect). Complex A exhibits a hard/ soft dissymmetry (NH3 is hard, 1 soft), whereas in complex B the hard Co " center is stabilized exclusively by hard ligands. 

In the intermediate of higher oxidation state, the metal is harder, and a hard-soft dissymmetry of the hgands favors the irreversible elimination of the product. In this way the key role of oxidative addition and its reverse reaction in the homogeneous catalysis of C-C and C-H coupling reactions can be understood in the simplest case, the interplay of oxidative addition and reduetive elimination ean be represented by a redox reaction (Eq. 2-83). 

The soft Rh complex anion A readily imdergoes oxidative addition of methyl iodide. Insertion of CO into the Rh-C bond of the resulting complex B then gives the acetyl rhodimn complex C. Owing to a hard-soft dissymmetry, rapid elimination of acetyl iodide occurs. This initial product of the reaction is immediately solvolyzed by methanol to give acetic acid. The rate-determining step is believed to be the oxidative addition of methyl iodide to the Rh complex. 

Bases containing the elements P, As, Sb, Se, and Te are soft compounds according to the HSAB concept and therefore form stable complexes with the soft transition metals and thus deactivate the catalysts. The hard oxygen and nitrogen bases hardly react with die transition metals due to the hard/soft dissymmetry. 

Optimal conditions will be achieved when the substrate bond R-X possesses maximal hard-soft dissymmetry. 

AUcoxy complexes of transition metals are generally less stable because of the presence of a hard(OR)/soft(M) dissymmetry. The elimination of aUcene from the more stable alkyl metal complexes generally requires drastic conditions (Eq. 2-70). 

---