Lewis base addition reactions with

P NMR spectra indicate the T-shape geometry is retained in solution at -30°C but that the molecule is fluxional at room temperature. Rh(PPh3)3 undergoes a range of addition reactions with Lewis bases (CO, PF3, NH3) to afford various 16- and 18-electron species (Figure 2.14). 

As a result of having unshared pairs of electrons, S4N4 undergoes addition reactions in which it functions as a Lewis base. For example, with BF3 the reaction can be shown as... 

Considerable effort has been devoted to the development of enantiocatalytic MBH reactions, either with purely organic catalysts, or with metal complexes. Paradoxically, metal complex-mediated reactions were usually found to be more efficient in terms of enantioselectivity, reaction rates and scope of the substrates, than their organocatalytic counterparts [36, 56]. However, this picture is actually changing, and during the past few years the considerable advances made in organocatalytic MBH reactions have allowed the use of viable alternatives to the metal complex-mediated reactions. Today, most of the organocatalysts developed are bifunctional catalysts in which the chiral N- and P-based Lewis base is tethered with a Bronsted acid, such as (thio)urea and phenol derivatives. Alternatively, these acid co-catalysts can be used as additives with the nucleophile base. 

The next step involves the addition of nonanoyl chloride and is also understood using the Lewis definition. The carbonyl (C = 0) group on nonanoyl chloride is very polar, yielding an electron-deficient carbon, which is then a Lewis acid. The negative cholesterol ion is an electron donor, or Lewis base, and reacts with the positive carbon center to form a new bond and release a chloride ion. This chloride ion is then neutralized by the pyridinium ion in another Lewis acid-base reaction to form pyridine hydrochloride. Pyridine hydrochloride is soluble in water, while the newly synthesized cholesteryl nonanoate is not. Addition of the reaction mixture to aqueous sulfuric acid causes only the desired product, cholesteryl nonanoate. 

On the contrary, the addition of an appropriate Lewis base may coirpete with the reaction intermediate(s) for the adsorption, favouring the complete hydrogenation of Q to DHQ. This Lewis base has to be stable in reaction conditions... 

The solubility of metal salts is also affected by tiie presence of certain Lewis bases tiiat react with metal ions to form stable complex ions. Complex-ion formation in aqueous solution involves the displacement by Lewis bases (such as NH3 and CN ) of water molecules attached to the metal ion. The extent to which such complex formation occurs is expressed quantitatively by the formation conr stant for tiie complex ion. Amphoteric metal hydroxides are those sligjitly soluble metal hydroxides that dissolve on addition of eitiier acid or base. Acid-base reactions involving the OH" or H2O groups bound to the metal ions give rise to tiie amphoterism. 

Thus in the aldol addition, just as in the case of epoxide-opening reactions, the chloride ion, formed as a necessary consequence of the mechanism of Lewis base catalysis with chlorosilanes, is not innocuous. In fact, it is a competent nucleophile that can attack an aldehyde or an epoxide activated by the Lewis base-coordinated silicenium cation in an intermolecular fashion. The desire to understand these two seemingly inconsistent results obtained in our study of the Lewis base-catalyzed reactions of trichlorosilanes presented an opportunity for the development of novel catalytic processes. For example, if a chloride ion can capture these activated electrophiles, could other exogenous nucleophiles be employed to intercept these reactive intermediates If so, a wide variety of bond-forming processes mediated by the phosphoramide-bound chiral Lewis acid [LB SiCls]" would be feasible. At this point it remained unclear if (1) an exogenous nucleophile could compete with the ion-paired chloride and (2) what kinds of nucleophiles could be compatible with the reaction conditions. 

---