Diels-Alder reactions of chiral isoimidium salts: a computational study

Recently, important efforts have been devoted to asymmetric Diels-Alder (DA) cycloadditions. Several chiral auxiliaries and catalysts were tested, originating a multitude of selectivities. Here, we study the ability of the isoimidium group as a dienophile activator in DA reactions, as well as its behavior in the induction of selectivity, when incorporated in chiral auxiliary units. We study dienophiles attached to isoimidium auxiliaries derived from (2R,5R)-2,5-diphenylpyrrolidine and from (R)-bis((R)-1-phenylethyl)amine, and show that they lead to low activation energies in DA additions to different dienes. Reported experimental regio- and endo/exo selectitivites are also fully rationalized. While diastereoselectivities originated by (2R,5R)-2,5-diphenylpyrrolidine based dienophiles can easily be rationalized by a C2 symmetric transition state, several transition states have to be simultaneously accounted for the rationalization of the selectivity obtained with dienophiles based on (R)-bis((R)-1-phenylethyl)amine. In this case, the C2 symmetric structure leads to opposite selectivities to those experimentally observed. Thus, while the structures of the dienophiles derived from these two amines seem similar, their behavior in the induction of stereoselectivity in DA reactions is quite different. Our models, which substantially differ from those previously proposed in the literature, can also be adapted to other reactions in which this type of chiral auxiliaries is used.