Abstract A technique for the Diels-Alder synthesis of endo-norbornene-cis-5,6dicarboxylic anhydride and its stereoisomer, exo-norbornene-cis-5,6-dicarboxylic anhydride, is explained. To prove that each stereoisomer was made in the experiment and to distinguish between the two molecules, the characteristic long range coupling affects seen in the H1-NMR spectra of bridged six member ring molecules are used. A method for the separation of the two molecules is also explained. This technique can be used as a tool to instruct organic chemistry students in the Diels-Alder reaction and how to use NMR techniques to prove the stereochemistry of the products involved in such reactions. Introduction The Diels-Alder reaction has long been an important tool for the organic chemist in that it not only creates new carbon-carbon bonds, but it also creates six-membered rings1. This attribute of the Diels-Alder reaction is especially valuable, because there are few synthetic methods of creating six-membered ring structures. The process of a DielsAlder reaction involves a substituted alkene reacting with a conjugated diene in the cis conformation to create two new carbon-carbon σ bonds which results in the formation of a six-membered ring, as shown below: Figure 1: Example Diels-Alder reaction
In the case of the reaction outlined in this report, the substituted alkene is maleic anhydride, and the conjugated diene is cyclopentadiene. The two undergo a Diels-Alder reaction to produce norbornene-cis-5,6-dicarboxylic anhydride, as shown:
Figure 2: Diels-Alder reaction between cyclopentadiene and maleic anhydride H O H H H H O
O O O O H
Depending upon the arrangement of the two molecules during the reaction, two separate stereoisomers can be formed. The endo isomer (the kinetic product because its transition state is abnormally stable due to overlap between maleic anhydride’s oxygen-carbon π bonds and the new π bonds being formed2) is formed under low temperature conditions which are easy to achieve in the laboratory setting. The chemistry department at the University of Nebraska at Kearney uses this reaction as a demonstration of the DielsAlder reaction. However, the exo isomer (the thermodynamic product because it is a more stable final product) is not formed at lower temperatures. It is the purpose of this experiment to determine a method for obtaining the exo isomer in a fashion that would allow an organic chemistry student to perform the same experiment, to separate the compounds, and to use NMR techniques to distinguish between the endo and exo isomers. All NMR spectra were obtained using a Bruker Avance 300 MHz NMR Spectrometer. The main attribute of the NMR spectra of the Diels-Alder products that allows differentiation between the endo and exo compounds is the characteristic and unusual “W-conformation” coupling seen in bridged six member rings3. Normally, protons that are separated by more than three bonds do not show coupling to one another. However, when the carbon atoms the protons are bonded to are strained as such that they form a “W,” an unusually large coupling between the protons can be observed in the H1 and H1-COSY NMR spectra. This unique coupling allows differentiation between the two stereoisomers obtained as the product in this experiment. After the synthesis reaction, the two molecules must be isolated from each other in order to run NMR analysis on each separate molecule. This was achieved through column chromatography using silica gel as the stationary phase and a mixture of hexane and ethyl acetate as the mobile phase. Methods and Materials...