Stereoheterotopic groups

A stereochemical condition that describes a stereoheterotopic group C in the compound XABC2 containing a prochiral tetrahedral atom X having substituents A, B, C, and C. 

Asymmetric synthesis starts with a prochiral compound. This is a compound which is not chiral, but can be converted into a chiral compound by a chiral (bio) catalyst. Subsequently, two types of prochiral compounds can be distinguished The first one has a stereoheterotopic face (which usually is a double bond) to which an addition reaction takes place. An example is the conversion of the prochiral compound propene into 1,2-epoxypropane (which has two enantiomers, of which one may be preferentially formed using an enantioselective catalyst). The second type of prochiral compound has two so-called enantiotopic atoms or groups. If one of these is converted, the compound becomes chiral. Meso-compounds belong to this class. Figure 10.5 and 10.6 show some examples of the different types of asymmetric catalysis with prochiral compounds. 

Compounds with asymmetric centers can be obtained from prochiral starting molecules by either face-selective reactions [1] (stereoheterotopic facial addition) or group-selective reactions (stereoheterotopic ligand substitution). The transition states of these selective stereodivergent reactions must be diastereomeric, and the kinetics are the same as those of parallel reactions with different products (enantiomers or diastereo-mers). The selectivity in the stereoselective event leading to the different transition states can never be exceeded by the final yield of the major stereoisomer. 

Stereoheterotopic A term that includes both enantiotopic and diastereotopic atoms, groups and faces. Equivalent atoms, groups and faces would be homotopic. 

Consider the CH2 group of 2-butanol. There are no symmetry operations in 2-butanol, and as such the two hydrogens of the CH2 cannot be interconverted by a symmetry operation. Therefore, these two hydrogens are expected to be different from one another in all meaningful ways, such as NMR shift, acidity, C-H bond length, bond dissociation energy, reactivity, etc. They have the same connectivity, but there is no symmetry operation that interconverts them in any conformation. They are stereoheterotopic, and defined specifically as diastereotopic. 

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