Ab initio studies of pericyclic reactions of aminoboranes.: [2+2] dimerization and [4+2] Diels-Alder reactions of H2BNH2, Me2BNMe2, and (F3C)2BNMe2

Examination of the dimerizations and Diels-Alder reactions of the aminoboranes H2BNH2, 1; Me2BNMe2, 2; and (F3C)(2)BNMe2, 3, employing the B3LYP/6-31G* model and,basis set, provides a reasonable picture of the experimental results. Dimerization of 1 requires less activation energy and is more exothermic than its Diels-Alder reaction. Both the Diels-Alder and dimerization reactions of 2 have high barriers and are endothermic. Thus one expects that I will dimerize rather than react with a diene, while 2 should prove unreactive toward dienes or itself. Both expectations match experiment. By contrast, the Diels-Alder reaction of 3 exhibits a lower activation barrier and is more exothermic than dimerization. The activation energy required for the former reaction is sufficiently small that the reaction should occur at room temperature, which it does. In broad terms, these observations are explained by a combination of steric and electronic considerations. Dimerization of 2 and 3 is inhibited by the bulky substituents, but dominates for unsubstituted 1. The Lewis acidity of the bis(trifluoromethyl)borane moiety in 3 enhances its reactivity toward a diene, while the lowered Lewis acidity of the dimethylborane moiety in 2 renders this compound inert.