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

Examination of the dimerizations and Diels-Alder reactions of the aminobora nes 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 re sults. Dimerization of 1 requires less activation energy and is more exothe rmic than its Diels-Alder reaction. Both the Diels-Alder and dimerization r eactions 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 unrea ctive toward dienes or itself. Both expectations match experiment. By contr ast, the Diels-Alder reaction of 3 exhibits a lower activation barrier and is more exothermic than dimerization. The activation energy required for th e former reaction is sufficiently small that the reaction should occur at r oom temperature, which it does. In broad terms, these observations are expl ained by a combination of steric and electronic considerations. Dimerizatio n of 2 and 3 is inhibited by the bulky substituents, but dominates for unsu bstituted 1. The Lewis acidity of the bis(trifluoromethyl)borane moiety in 3 enhances its reactivity toward a diene, while the lowered Lewis acidity o f the dimethylborane moiety in 2 renders this compound inert.