Stereospecific reactions, definition

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. 

A stereospecific chemical reaction is one in which starting substrates or reactants, differing only in their configuration, are converted into stereoisomeric products. Note, with this definition a stereospecific reaction has to be stereoselective whereas the inverse statement (that is, with respect to a stereoselective reaction or process) is not necessarily true. 2. Referring to reactions that act on only one stereoisomer (or, have a preference for one stereoisomer). Thus, many enzyme-catalyzed reactions are stereospecific, and characterization of that stereospecificity is always an issue to be addressed for a particular enzyme. 

The cheletropic elimination of nitrogen from diazenes 8.1 and 8.2 is a stereospecific reaction (for definition of a stereospecific reaction, see section 1.5). 

Remember the lesson from Problem 8-17 anti addition of a symmetric reagent to a symmetric ci5-alkene gives racemic product, while anti addition to a rrani-alkene gives meso product. This fits the definition of a stereospecific reaction, where different stereoisomers of the starting material cis and trans) are converted into different stereoisomers of product (a dl-pair and meso form). 

For the purposes of this treatise, the definition of asymmetric synthesis is a modification of that proposed by Morrison and Mosher [1] and as such will be applied to stereospecific reactions in which a prochiral unit in either an achiral or a chiral molecule is converted, by utility of other reagents and/or a catalytic antibody, into a chiral unit in such a manner that the stereoisomeric products are produced in an unequal manner. As such, the considerable body of work devoted to antibody-catalysis of stereoselective reactions including chiral resolutions, isomerizations and rearrangements are considered to be beyond the scope of this discussion. For information regarding these specific topics and more general information regarding the catalytic antibody field the following papers... 

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). 

Since the two reactants cis and trans butenes are stereoisomers, being diastereomers, the product from cis is a meso compound and that from the trans give a pair of enantiomers, by definition both the reactions are stereospecific. 

However, the foregoing observations cannot definitely be ascribed to triplet CH2 for the following reasons. Dilution of the reaction mixture with argon while maintaining constant total pressure leads to decreased deactivation efficiency, as pointed out by Frey, and therefore the loss of stereospecificity may result from geometric isomerization of the initial excited adducts rather than from a different mechanism of reaction of triplet methylene. 

Let us now define three terms that refer to reactions stereoselective, stereospecific and stereoelectronic. There has been a difference of opinion about the use of the first two we shall use the definitions suggested by Zimmerman2 and now adopted by most authors. 

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. 

Although such a definition is seemingly quite clear and unique, the practical exploitation of the above criterion is complicated by the fact that the scission and formation of bonds is a microscopic process, inaccessible to direct experimental observation. This, of course, suggests the necessity of searching other, more easily exploitable, criteria of concert. One such criterion is the remarkable stereospecificity accompanying the formation of products in allowed pericyclic reactions [60,61]. The fact that the origin of the synchronisation in the process of scission and the formation of the bonds was always intuitively related to a certain energetic stabilisation led to another widespread opinion that all allowed reactions are automatically concerted. On the other hand nonconcertedness, advocated by frequently observed stereo-randomization [60] was practically always expected in forbidden reactions. 

Our overall plan in this paper is this. Initially, we shall set down a few definitions, and a point of view. We shall then describe a number of principles or causes of stereoselection associated with bonding, steric, thermodynamic, electrical, mechanical, etc., factors, inherent in the stereoprocess. The relation between the properties of intermediates, e.g. carbanion or syn-anti isomers, etc., rate and equilibrium conditions and stereoselectivity will be dealt with. Throughout, we shall examine representative systems and reactions to see how relevant the principles of stereoselection are. No attempt will be made to venture into several important areas, e.g. enzyme stereospecificity, stereoregular polymers, etc. 

---