Inchemistry,stereoselectivity[1] is the property of achemical reaction in which a singlereactant forms an unequal mixture ofstereoisomers during a non-stereospecific creation of a newstereocenter or during a non-stereospecific transformation of a pre-existing one.[2] The selectivity arises from differences insteric andelectronic effects in themechanistic pathways leading to the different products. Stereoselectivity can vary in degree but it can never be total since theactivation energy difference between the two pathways is finite: both products are at least possible and merely differ in amount. However, in favorable cases, the minor stereoisomer may not be detectable by the analytic methods used.
Anenantioselective reaction is one in which oneenantiomer is formed in preference to the other, in a reaction that creates an optically active product from an achiral starting material, using either a chiral catalyst, anenzyme or a chiral reagent. The degree of selectivity is measured by theenantiomeric excess. An important variant iskinetic resolution, in which a pre-existing chiral center undergoes reaction with a chiral catalyst, an enzyme or a chiral reagent such that one enantiomer reacts faster than the other and leaves behind the less reactive enantiomer, or in which a pre-existing chiral center influences the reactivity of a reaction center elsewhere in the same molecule.
Adiastereoselective reaction is one in which onediastereomer is formed in preference to another (or in which a subset of all possible diastereomers dominates the product mixture), establishing a preferred relative stereochemistry. In this case, either two or more chiral centers are formed at once such that one relative stereochemistry is favored,[3] or a pre-existing chiral center (which needs not be optically pure) biases the stereochemical outcome during the creation of another. The degree of relative selectivity is measured by thediastereomeric excess.
Stereoconvergence can be considered an opposite of stereospecificity, when the reaction of two different stereoisomers yield a single product stereoisomer.
The quality of stereoselectivity is concerned solely with the products, and their stereochemistry. Of a number of possible stereoisomeric products, the reaction selects one or two to be formed.
Stereomutation is a general term for the conversion of one stereoisomer into another. For example, racemization (as in SN1 reactions), epimerization (as in interconversion of D-glucose and D-mannose inLobry de Bruyn–Van Ekenstein transformation), or asymmetric transformation (conversion of a racemate into a pure enantiomer or into a mixture in which one enantiomer is present in excess, or of a diastereoisomeric mixture into a single diastereoisomer or into a mixture in which one diastereoisomer predominates).[4]
An example of modest stereoselectivity is thedehydrohalogenation of 2-iodobutane which yields 60%trans-2-butene and 20%cis-2-butene.[5] Since alkenegeometric isomers are also classified as diastereomers, this reaction would also be called diastereoselective.
Cram's rule predicts the major diastereomer resulting from the diastereoselective nucleophilic addition to a carbonyl group next to a chiral center. The chiral center need not be optically pure, as the relative stereochemistry will be the same for both enantiomers. In the example below the (S)-aldehyde reacts with a thiazole to form the (S,S) diastereomer but only a small amount of the (S,R) diastereomer:[6]
TheSharpless epoxidation is an example of an enantioselective process, in which anachiralallylic alcohol substrate is transformed into an optically active epoxyalcohol. In the case of chiral allylic alcohols, kinetic resolution results. Another example isSharpless asymmetric dihydroxylation. In the example below the achiral alkene yields only one of the possible 4 stereoisomers.[7]
With astereogenic center next to the carbocation the substitution can be stereoselective in inter-[8] and intramolecular[9][10] reactions. In the reaction depicted below the nucleophile (furan) can approach the carbocation formed from the least shielded side away from the bulkyt-butyl group resulting in high facial diastereoselectivity:
Pinoresinol biosynthesis involved a protein called adirigent protein. The first dirigent protein was discovered inForsythia intermedia. This protein has been found to direct the stereoselective biosynthesis of (+)-pinoresinol fromconiferyl alcohol monomers.[11] Recently, a second, enantiocomplementarydirigent protein was identified inArabidopsis thaliana, which directs enantioselective synthesis of (−)-pinoresinol.[12]
-Pinoresinol_Biosynthesis.svg)
