The Diels-Alder reaction is an attractive subject because of its highly ordered transition state and its interesting stereochemistry. It can be modelled efficiently without resorting to ab initio methods. Under kinetic control, most of Diels-Alder additions yield endo adducts. Exceptions to this rule reveal the different origins of this selectivity. In a first case, heteroatoms closing the diene to a five membered ring can favour both the exo transition states and the exo adducts. In another case, the use of a catalyst (a cyclic porphyrin trimer) that mimics biological enzymes shows the interest of semi-empirical models of large molecules.
Diels-Alder additions to benzene are very difficult because of the loss of aromaticity. Substituents on the benzene ring that force it to forgo its aromaticity can make these additions easier, provided that they do not induce steric hindrance. A simple thermodynamical cycle using the computed reaction enthalpies can provide numerical estimatates of the lost aromaticity.
The structural dependencies of hydrogen bonds are never easy to understand because of their relative weakness. The hydrogen bonds involving a proton bound to a carbon alpha to a carbonyl can be perturbed by the proximity of the oxygen. Searches for intramolecular hydrogen bonds in a database show that the relationship with the partial charge on the hydrogen is lost amid other structural factors such as conformational energies or rings.
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