Asymmetric Induction: Chelation, Non-Chelation, Cram-Reetz & Evans Models

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

What dictates the formation of one product over another in a reaction? In this lesson, we will study the stereochemistry concept of asymmetric induction by examining chelation, non-chelation, the Cram-Reetz, and the Evans models to see how one stereoisomer can be preferentially formed over another.

How do You Know What Will Happen?

If someone were to ask you what the weather forecast for the week was and how the weather person knew what would you say? Aside from weather being a complex science you might say to whom you were talking to that the forecasters have predicted what the weather will be based on the current patterns they observe and then computer models that help them accurately predict what will happen weather-wise.

What if that same person then asked you how one can know what the stereochemical outcome of a chemical reaction would be? It turns out that also can be a complex question to answer, but by using some simple models and patterns it's possible to fairly accurately predict what the product of a reaction will look like in terms of stereochemistry. In our lesson today we are going to be looking at four distinct models and theories that can help chemists predict beforehand what will occur before they actually run a chemical reaction. Let's see how we can do this!

Asymmetric Induction

Stereochemistry is the branch of chemistry that primarily deals with the three-dimensional shape of organic compounds. The concept of asymmetric induction is this idea that in a chemical reaction, one specific stereoisomer is formed preferentially over others because of the asymmetry of one of the reagents that's used in the reaction. Stereoisomers are compounds that have the same chemical formula, the same atom connectivity, but differ in the way they occupy three-dimensional space.

In order to rationalize the observed stereochemistry of a reaction product, many models and theories have been formulated. Let's break down four of the more popular models next to see how they work.

The Chelation Model

When metal atoms are used in a chemical reaction they can vastly affect the stereochemical outcome of the reaction due to chelation, which is when the metal atom coordinates to other atoms in the molecule, especially atoms like oxygen. For example, a ketone can be selectively reduced to an alcohol using titanium chloride in the reaction, which causes chelation to occur, and only one stereoisomer to be formed.

Titanium metal can chelate in between both oxygen atoms to allow for the formation of a single stereoisomer

In this reaction the titanium chelates between the two oxygen atoms so that the reducing agent has to come from the bottom face of the molecule and gives only the observed stereoisomer.

The Non-Chelation Model

What if we had an atom in our reactant that wasn't able to chelate very well with a metal, such as chlorine? This model is known as the non-chelation model and has been used to explain the observed products in chemical reactions. As an example consider a ketone being reduced to an alcohol and the substrate has a chlorine as well.

Example of a non-chelation effect reaction

In this case, the reaction produces the product in which the alcohol group and chlorine are in a syn (same side) relationship to one another because the chlorine isn't able to chelate like an oxygen atom does and impart the selectivity we saw previously.

Cram-Reetz Model

The Cram-Reetz model was developed as a method to predict which face of a molecule an incoming nucleophile (electron donor) will approach from and thus dictate which stereoisomer will predominate in a reaction. Cram's rule (as it's sometimes called) says that the nucleophile will approach from the less hindered side of the starting material or that's the most sterically accessible.

For instance, when a Grignard reaction occurs on an aldehyde containing a methyl group adjacent to it, the Grignard reagent (the phenyl group in this case) will want to approach from the bottom of the molecule since the methyl group is coming out at us. This would give a product in which the methyl group and the phenyl group are trans (opposite) to one another.

An example of a reaction that represent

Evans Model

Dave Evans, who is an organic chemist at Harvard University, developed a model for stereoselectivity that utilizes what's called a chiral auxiliary to 'direct' which face of a molecule a nucleophile will approach from. The presence of the chiral auxiliary is able to dictate which stereoisomer is formed in the reaction.

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