Abstract
The thermal equilibration of himic anhydride [IUPAC (2-endo,3-endo)-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid anhydride] to (2-exo,3-exo)-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid anhydride and subsequent recrystallization of the exo-product can be performed as a standard undergraduate laboratory experiment requiring minimal equipment. The interpretation of the 1H NMR spectra for these norbornene carboxylic anhydride molecules promotes an appreciation of constrained ring systems and factors that affect chemical shifts and coupling constants.
Highlights
The Diels−Alder[1] reaction between cyclopentadiene[2] and maleic anhydride is an established undergraduate experiment. It works well in the time scale of a laboratory session, resulting in good yields of the major isomer which we will refer to as endo-himic anhydride but is known as himic anhydride or carbic anhydride.[3]. It is an example of the endo rule seen in practice and can be used to complement or underpin other undergraduate experiments.[4−7] This article takes the concept of the endo rule further and presents a reaction which requires a better understanding of the concept of kinetic versus thermodynamic control[8−10] and the theoretical reasons behind this observation
The reaction produces an important starting material for ring opening metathesis polymerization (ROMP) which has become an established approach to synthesize complex macromolecules.[12−19] in the context of modules that teach stereochemical concepts, both endo- and exo-himic anhydrides are each good examples of mesomeric compounds.[20]
Thermal Equilibration of Himic Anhydride batches can be determined by 1H NMR and/or by GC, and there are options available to allow the experiment to be tailored to different time scales and stages of an undergraduate program
Summary
This experiment has been trialed with groups of 10−12 Stage 3 UK undergraduate students repeated over a 3 year period. Further discussion on the interpretation of the spectra follows in the online Supporting Information This lab was developed to be the first day of a Stage 3 research project module which required an experiment that was not too challenging to perform and could be carried out without undue time pressure. The 1H NMR spectra do not present peaks explicable with a simple so-called “n + 1” rule ( 2nI + 1)[25] which was useful to explore constrained systems and the Karplus equation.[26] We found it more practically convenient to supply students with prerecorded examples of the more advanced NMR spectra, rather than run them for each student, to allow them to initially attempt to solve the structures on their own Following from this lab, a small group session the following week was found to be best to guide the students through the fuller spectroscopic interpretation. The literature searches and NMR interpretation were found to be overwhelmingly useful by students even if some struggled with some of the more advanced concepts as they built a skill set and awareness of resources which were used repeatedly throughout the subsequent research projects
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