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Stereospecific and Stereoselective Reactions

The stereochemical outcome of a reaction is described by two terms. A stereospecific reaction leads to a stereoselective reaction.
The stereochemical outcome of a reaction is described by two terms. A stereospecific reaction leads to a stereoselective reaction. The similarity of the terms makes it difficult for many students to remember which is which or even know the difference. But let us examine each of the terms in more detail.

Stereospecific Reactions

Definition - When the stereochemistry of the reactant completely determines the stereochemistry of the product in a stereospecific reaction, there is no other alternative.

Numbers of products - Specific products are generated from stereospecific reactions.

Effects - It is the stereochemistry of the reactants that determines the final product of a stereospecific reaction.

Stereospecific reactions give a stereoisomer of the product from one reactant when they are carried out with stereoisomeric starting materials.

You should think of synergistic and antiferromagnetic addition reactions when you hear the word stereospecific reactions. An anti-addition reaction, for example, is the halogenation of alkanes.

Here we have a situation where a particular stereoisomer of a starting material becomes a particular stereoisomer in the end product. A meso compound is all the product is. In a similar way, cis-stilbene is enantiomerically identical to its trans counterpart. It is important to note that because of the stereo specificity of the halogenation reaction mechanism (anti addition), you will never get a pair of enantiomers when you use trans-stilbene, while the cis-stilbene will never produce the meso compound. Hence, if the starting materials of a reaction are diastereomeric and the end product is also diastereomeric, the reaction will be stereospecific.

Stereoselective Reactions

Definition - Stereoselective reactions take place when the reaction path of choice is favorable to the formation of the stereoisomer, but other pathways are not favorable to the formation of the stereoisomer.

A stereoselective reaction can produce multiple products.

Effects - Differences in the steric and electronic effects determine selectivity of reaction pathways (bumper groups cause steric hindrance).

There are two or more stereoisomers that can be formed in stereoselective reactions, but only one of them will predominate over the other.

A stereoselective reaction occurs when more than one reaction occurs between a set of reactants under the same conditions, resulting in stereoisomers that result in greater amounts of one product than the others.

Enantioselective reactions produce only enantiomers, and are therefore stereoselective.

Diastereoselective stereochemistry refers to stereoselective reactions in which products may be diastereomers.

This is a reaction that can produce two different stereoisomers, but only one of them is measured, while the other is minor. An example would be a different epoxidation reaction.

Two stereoisomers result from the initial reaction. That is an enantiomer. Initial reactions do not show a preference between the two stereoisomers. This does not show stereoselectivity. Enantiospecific reactions can also be described as stereochemical reactions, since the products are enantiomers. Sharpless epoxidation is an important step in organic synthesis because it's a stereoselective reaction. Stereoselective methods and the mechanisms behind these reactions, however, are typically quite complicated.
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