Transition states and mechanisms of the hetero-Diels-Alder reactions of hyponitrous acid, nitrosoalkanes, nitrosoarenes, and nitrosocarbonyl compounds

The transition states and energetics of Diels-Alder reactions for a variety of nitroso compounds with dienes were explored with density functional the ory using the B3LYP functional and 6-31G* basis set. The reactions involve HNO, various nitrosoalkanes and arenes (RNO and ArNO), and nitrosoformaldeh yde (CHONO) as dienophiles with butadiene and a series of 1- and 2-substitu ted dienes. The mechanisms, activation energies, energies of reaction, ster eoselectivities, and regioselectivities are predicted for these reactions. These predictions are compared to available experimental data. The mechanis m is found to be concerted but involves highly asynchronous transition stat es. Although it is not evident in the products, we find that the endo path is very strongly favored over the exo alternative due to repulsion between the diene and nitrogen's lone pair. A range of experimental regioselectivit ies are reproduced by calculations and are found to hinge on a very sensiti ve balance between FMO interactions, electrostatics, and steric effects. A series of generalizations for predicting regioselectivity for untried diene -dienophile pairs are made.