Hi Aamir,

An example of stereoselectivity is the dehydrohalogenation of 2-iodo-butane which yields 60% trans-2-butene and 20% cis-2-butene. Since alkene geometric isomers are also classified as diastereomers, this reaction would also be called diastereoselective. So called Cram's rule predicts the majordiastereomer 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. Also, the addition of formic acid to norbornene is also stereospecific because the exo isomer is formed exclusively without any of the endo isomer.

Before giving examples of regioselectivity, let me, Aamir, justify that in chemistry that is the preference of one direction of chemical bond making or breaking of over all other possible directions. It can often apply to which of many possible positions a reagent will affect, such as which proton a strong base will abstract from an organic molecule, or where on a substituted benzene ring a further substituent will add. A specific example is a halohydrin formation reaction with 2-propenylbenzene. Because of the preference for the formation of one product over another, the reaction is selective. This reaction is regioselective because it selectively generates one constitutional isomer rather than the other. Certain examples of regioselectivity have been formulated as rules for certain classes of compounds under certain conditions, many of which are named. Among the first introduced to chemistry students are Markovnikov's rule for the addition of protic acids to alkenes, and the Fürst-Plattner rule for the addition of nucleophiles to derivatives of cyclohexene, especially epoxidederivatives.

Regioselectivity in ring-closure reactions is subject to Baldwin's rules. If there are two or more orientations that can be generated during a reaction, one of them is dominant (e.g., Markovnikov/anti-Markovnikov addition across a double bond). Regioselectivity can also be applied to specific reactions such as addition to pi ligands. Selectivity also occurs in carbene insertions, for example in the Baeyer-Villiger reaction. In this reaction an oxygen is regioselectively inserted near an adjacent carbonyl group. In ketones this insertion is directed toward the carbon which is more highly substituted (i.e. according Markovnikov's rule). For example, in a study involving acetophenones this oxygen was preferentially inserted in-between the carbonyl and the aromatic ring to give acetyl aromatic esters, not methyl benzoates.

Best wishes,

Ilya