Stereospecific reactions, definition examples

However, if both maleic and fumaric acid gave the dl pair or a mixture in which the dl pair predominated, the reaction would be stereoselective but not stereospecific. If more or less equal amounts of dl and meso forms were produced in each case, the reaction would be nonstereoselective. A consequence of these definitions is that if a reaction is carried out on a compound that has no stereoisomers, it cannot be stereospecific, but at most stereoselective. For example, addition of bromine to methylacetylene could (and does) result in preferential formation of trans-1,2-dibromopropene, but this can be only a stereoselective, not a stereospecific reaction. 

Eliel (1962) proposed the following definition Stereospecific as applied to a reaction means that stereoisomerically different starting materials give rise to stereoisomerically different products. The reactions of bromine with traws-butene-2 and cis butene-2 are examples of stereospecific reactions. The trans isomer yields the meso-dibromide (V) whereas the cis isomer affords the di-dibromide VI). 

The term stereoselective is often confused with the term stereospecific, and the literature abounds with views as to the most satisfactory definition. To offer some clarification, it is perhaps timely to recall a frequently used term, introduced a decade or so ago, namely the stereoelectronic requirements of a reaction. All concerted reactions (i.e. those taking place in a synchronised process of bond breaking and bond forming) are considered to have precise spatial requirements with regard to the orientation of the reactant and reagent. Common examples are SN2 displacement reactions (e.g. Section 5.10.4, p. 659), E2 anti) elimination reactions of alkyl halides (e.g. Section 5.2.1, p.488), syn (pyrolytic) elimination reactions (Section 5.2.1, p.489), trans and cis additions to alkenes (e.g. Section 5.4.5, p. 547), and many rearrangement reactions. In the case of chiral or geometric reactants, the stereoisomeric nature of the product is entirely dependent on the unique stereoelectronic requirement of the reaction such reactions are stereospecific. 

Complications of reaction mechanism are avoided by an operational definition of a reaction. A reaction converts a compound of specifiable structure into a second of specifiable structure in a stereospecific process. The new compound possesses one new ligand not present in the starting material. For example,... 

According to the second school of thought - first proposed by Zimmerman and coworkers and strongly advocated by Eliel - a reaction is stereospecific if stereomerically-distinct reactants yield stereomerically-distinct products. The wording of this definition is troublesome - with respect to enantiomers, as well as diastereomers. The following examples demonstrate the inadequacy of this definition. 

Following our definition of stereospecificity given above, it follows that both stereoselective reactions (transformations (155-163a/b) and nonstereoselective ones (transformations 157a/b-163a/b) can be stereospecific. Therefore, the first part of the statement "all stereospecific processes are stereoselective, but all stereoselective processes are stereospecific " is not true. By our definition, all stereospecific processes are not stereoselective for example, stereospecific transformations 157a/b-163a/b can be nonstereoselective. 

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