Stereospecific process, definition

It will be appreciated that, with these definitions, any stereospecific process is also stereoselective however, not all stereoselective reactions are stereospecific. In some papers, the terms are modified with the prefixes, enantio- or diastereo-. 

The most definitive stereochemical studies have concerned configurational changes at the a carbon. Alexander et al. (2, 138) showed that (-I-)540-CpFe(CO)2CH(Me)Ph reacts with neat SOg at —60° or — 10°C, or with SO2 in saturated pentane at 27°C, to give the corresponding iS-sulfinate. The specific rotation of the product, [a]546, varied somewhat with the method of synthesis (—186°, —176°, and —158°, respectively). It was concluded that the insertion is a substantially stereospecific process, but whether it involves retention or inversion could not be determined. 

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,... 

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. 

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 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. 

Although scheme (138) is the standard mechanism for the radical-catalyzed isomerization of isomeric alkenes, kinetic data for both substitution and isomerization are sparse. Using cis- or frcms-diiodo-ethene and labeled iodine atoms, Noyes et al. (1945) demonstrated that iodine atoms exchanged with predominant retention isomerization was the slower process, the barrier being <4 kcal/mole. Corresponding studies with dibromoethene and bromine atoms indicate a barrier of ca. 3 kcal/mole (Steinmetz and Noyes, 1952) in which bromine-atom departure from and isomerization of the intermediate were competitive. Qualitative selective or stereospecific radical-initiated additions to alkenes have since indicated that radical intermediates probably have stereostability, but the studies cited are definitive. The kinetic analysis provided the essential model for SS in mechanistic schemes such as (138), whether for SE, SH or SN processes. 

Generally speaking, these distinctions have not been observed by biochemists. Stereoselective has been little used, and stereospecific has been used to cover almost all aspects of the impact of stereochemical influences on reactions in living tissues or enzyme systems. Consider, for instance, the enzymatic hydration of fumarate by the enzyme, fumarase. Since there is a relationship between the structure of the substrate and product, the process could be described as stereospecific. Yet the definition of stereospecific requires that it be shown that the isomer of fumaric acid gives rise to a product which is stereochemically different from L-malate. Since the enzyme, however, does not catalyze any reaction with the (Z)-isomer (maleic acid) it is not clear whether stereospecific actually applies. 

These metal-alkynyl complexes can be protonated to afford the free alkynes and parent cobalt hydroxo complex (comparable reactivity to their alkyl and aryl congeners), but have proven inert toward oxygenation and carbonylation. They are also thermally stable up to 100 °C. Attempts to explore the reactions of these compounds with unsaturated hydrocarbons were typically fruitless. The one exception is the reaction between 53 and its parent alkyne (HC = C02Me, Scheme 6), which under benzene reflux effects catalytic, stereospecific, linear trimerisation of the alkyne to afford ( , )-buta-l,3-dien-5-yne. The reaction was, however, slow (4.5 turnovers in 20 h) and suffered from catalytic deactivation due to hydrolysis of 53, which subsequently reacted with adventitious CO2 to irreversibly form an inert /x-carbonato complex. The catalytic cycle was concluded to involve initially a double coordination-insertion of the C = C bond of methylpropiolate into the Co-Caikyne linkage. Subsequent hydrolysis of the Co-C bond by a third equivalent of HC = CC02Me would then afford the observed product and regenerate 53. However, a definitive explanation for the stereospecificity of the process was not established. 

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