Computational studies of pericyclic reactions of aminoborane (F _3C)_2B-N(CH_3)_2: Ene reactions vs hydrogen transfers and regiochemical and conformational preferences

摘要

The aminoborane (F_3C)_2B-N(CH_3)_2 (1) undergoes olefin-like concerted pericyclic ene-type and hydrogen transfer reactions with alkenes. Representative examples of each were studied computationally using density functional, ab initio, and composite models. Both types of reactions proceed through six-membered cyclic chairlike transition states. Energy barriers for reactions observed experimentally are computed to be ≥48 kJ/mol (OG2R3 composite model); therefore, this value appears to be an upper limit for viable reactions under the experimental conditions. All reactions exhibit substantial exothermicities, suggesting that neither pathway should be reversible. However, the experimental reaction between 1 and H _2C-C(CH_3)(t-Bu) provides a mixture of hydrogen transfer and ene-type products, favoring the former. Computations indicate that the ene-type pathway requires a lower barrier than the hydrogen transfer pathway but is less exothermic; thus, the experimental result is rationalized on the basis of the ene-type pathway being reversible. Although 1 reacts with most alkenes of the formula H_2C-C(H)(CHR_1R_2) by one of the two pathways, it reportedly does not react with H_2C-C(H)(CHMeCl). Computations indicate that this results from a slight increase in the barriers for the pericyclic reactions, presumably stemming from the presence of the electron-withdrawing chloride. Warming the reactants should allow formation of the ene-type product.