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Introduction to Resolution

As illustrated earlier, that the chiral compounds synthesized from achiral starting materials and reagents are usually racemic (that is a 50:50 mixture of enantiomers). A process of Separation of racemates into their component enantiomers is called resolution. Because enantiomers have similar physical properties, like melting point and solubility, resolution is very difficult. Diastereomers, in contrast, have distinct physical properties and this fact is employed to achieve resolution of racemates. A race mate's reaction of with an enantiomerically pure chiral reagent gives a mixture of diastereomers that can be separated. By reversing the first reaction then leads to the separated enantiomers plus the recovered reagent.

Several types of chemical and physical reactions, that is including salt formation, can be used to achieve the diastereomeric intermediates needed for separation. The picture given below demonstrate this usual principle, by showing how a nut having a right-handed thread (R) could serve as a "reagent" to discriminate and separate a mixture of right- and left-handed bolts of similar weight and size. Only the two right-handed partners may be interact to give a fully-threaded intermediate, so separation is quite simple. The resolving moiety, that is the nut, is then removed, leaving the bolts separated into their right and left-handed forms. Chemical reactions of enantiomers are generally not so considerably distinct, but a practical distinction is yet possible.

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Crystallization

Since the enantiomers have similar physical and chemical properties in achiral environments, separation of the stereoisomeric components of a racemic mixture or racemate is usually not possible by the conventional methods of distillation and crystallization.

Diastereomer Separation

As described earlier, the distinct physical properties of diastereomers renders them more easily separated than enantiomers. So, reaction of a racemate with an enantiomerically pure chiral reagent gives a mixture of diastereomers that can then be separated. By Reversing the first reaction then leads to the separated enantiomers plus the recovered reagent. 

Kinetic Resolution

If a racemic acyl chloride is reacted with an enantiomerically pure alcohol, as displayed in the equation, a mixture of diastereomeric esters will be produced. These are created through diastereomeric transition states, thus the rate of reaction for each of the enantiomeric acyl chlorides will be distinct. One enantiomer may be completely transformed before the other begins to react, If the rates are sufficiently different, So achieving a kinetic resolution.

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Parallel Kinetic Resolution

Not all possible kinetic resolution reaction systems demonstrate the high selectivity factors (s > 120) feature of the above illustrations. With this approach when s is smaller a serious problem, is that the relative concentration of the less reactive enantiomer get increased as the conversion proceeds, resulting in an increase in effective rate for the not wanted transformation. Subsequently, conversions have to be kept well below 50% if product enrichment is desired or above 50$ for reactant enrichment. A way to solution this problem is to conduct a simultaneous reaction that removes the unfavoured enantiomer at similar time that the desired resolution is occurring.

Dynamic Kinetic Resolution

If a racemic substrate goes through racemization at a rate higher than the enantioselective reaction that discriminates the enantiomeric components, the result may in theory be isolated in up to 100% yield and, depending on s, 100% ee. The resulting dynamic resolution is a unique case of the Curtin-Hammett law. The S and R enantiomers of a racemate have identical enthalpy or free energy, and will go through chemical reaction with achiral reagents, solvents or catalysts by way of enantiomeric transition states, so that ΔGR‡ = ΔGS‡.

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