Hydrogen bonds cation solvation

In the course of our investigations to develop new chiral catalysts and catalytic asymmetric reactions in water, we focused on several elements whose salts are stable and behave as Lewis acids in water. In addition to the findings of the stability and activity of Lewis adds in water related to hydration constants and exchange rate constants for substitution of inner-sphere water ligands of elements (cations) (see above), it was expected that undesired achiral side reactions would be suppressed in aqueous media and that desired enanti-oselective reactions would be accelerated in the presence of water. Moreover, besides metal chelations, other factors such as hydrogen bonds, specific solvation, and hydrophobic interactions are anticipated to increase enantioselectivities in such media. 

Polar protic solvents also possess a pronounced ability to separate ion pairs but are less favorable as solvents for enolate alkylation reactions because they coordinate to both the metal cation and the enolate ion. Solvation of the enolate anion occurs through hydrogen bonding. The solvated enolate is relatively less reactive because the hydrogen-bonded enolate must be disrupted during alkylation. Enolates generated in polar protic solvents such as water, alcohols, or ammonia are therefore less reactive than the same enolate in a polar aprotic solvent such as DMSO. 

Water owes its superiority as a solvent for ionic substances not only to its polarity and its high dielectric constant, but to another factor as well it contains the —OH group and thus can form hydrogen bonds. Water solvates both cations... 

Polar solvents solvate ionic species and thus favor ion formation and separation in S l reactions. Part of the effect is due to dielectric constant, which facilitates charge separation (Table 9.6), and part to the ability of the solvent to solvate both anion and cation. Dipolar protic solvents are chosen since they are able to solvate both anions and cations (Figure 9.20). Solvation of cations involves interaction between lone pairs and the positive center and solvation of anions the formation of hydrogen bonds. The solvation of any type of ion increases with its charge and decreases with its size. Typically, S l reactions are carried out in polar solvents such as water and alcohols. 

Cement-forming liquids are strongly hydrogen-bonded and viscous. According to Wilson (1968), they must (1) have sufficient addity to decompose the basic powder and liberate cement-forming cations, (2) contain an acid anion which forms stable complexes with these cations and (3) act as a medium for the reaction and (4) solvate the reaction products. 

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