In an experiment to determine the stereochemistry of the enzymatic hydration of disodium fumarate, students model both the anti and syn addition products (2S, 3S-malate and 2S, 3R-malate), using dihedral driving to ensure that the minimum energy conformer is located. The software then uses the Karplus relationship to estimate the vicinal H-H coupling, which is compared with the value measured by the students in their 1H nmr spectra of the product.
Modeling, including dihedral driving, is performed on 1-(o-chlorophenyl)- 2,2-dichloropropanol tosylate to identify and visualize the conformation that allows neighboring group participation by halogen in the rate-determining C-O bond cleavage. Students also follow the rate of reaction by nmr.
A Diels-Alder reaction between tetraphenylcyclopentadienone and dimethylacetylenedicarboxylate yields, after reverse cheleotropic loss of CO, dimethyl tetraphenyl phthalate. Modeling reveals the propeller shape of the product, which helps to explain its unusual 1H and 13C nmr spectra.
Here is a picture of the very crowded product, with the atoms drawn to full scale of their van der Waals radii. The four peripheral benzene rings are twisted at about 30 degrees relative to the central one, whereas the two ester functions are essentially orthogonal to the ring.
Modeling of the methyl m-nitrobenzoate product of nitration of methyl benzoate reveals the ring hydrogen between the two substituents to lie in the anisotropic deshielding zone of the nitro group, which is coplanar with the ring. The unusual downfield chemical shift of this hydrogen thus is explained. Note that the ester carbonyl is orthogonal to the ring, and thus too from from the ortho hydrogen to influence its shift. Here's a space-filling model of the structure:
You can just see the hydrogen, peeking out below the nitro group, on the top right.
Students synthesize the insect repellant, DEET, N, N-diethyl-m-toluamide. In the course of their nmr analysis, they discover the barrier to rotation of the amide nitrogen. Modeling, including p-VESCF calculations, demonstrates the non-coplanar minimum energy conformation and explains the source of the barrier.
The ball-and-stick model below shows the overall conformation nicely:
Students model the product of a combined aldol condensation/Michael addition, 2,6-di(p-chlorophenyl)-3,5-dimethyl-4-pyranone in two stereoisomeric forms, and compare computed vicinal H-H coupling constants with their experimentally measured ones to determine the stereoselectivity of this reaction.
The product of the Knoevenagel condensation between piperonal and nitromethane is modeled, with dihedral driving, to demonstrate the planarity of the fully conjugated system, and the associated delocalization energy.
