Density Functional Investigation of Reaction of Borohydride Cation BH ₂⁺ with Propylene

The reactions of BH₂⁺ with propylene (CH ₂=CHCH₃) to form both the adducts BC₃H ₈^- and the H₂elimination products BC ₃H₆⁺ + H₂ have been investigated at the density functional B3LYP/6-311G(d,p) level of theory. It is shown that the electrophilic attacks of BH₂⁺ towards two olefinic carbons of H₂C=CHCH₃ and two subsequent 1,3-H-shifts may form four low-lying BC₃H₈⁺ isomers (with the relative energies in parentheses in kcal/mol): 1 BH₂⁺ · CH₂CHCH₃ (0.0), 1′ BH₂ ⁺ · CH₃CHCH₂ (6.3), 3 BHCH ₂CH₃⁺ (4.3), and 4 BHCH(CH₃) ₂⁺ (5.0), respectively. On the other hand, further H ₂-eliminations may also occur easily between B - C bonds of isomers 1 and 1′ and between C - C bonds of isomers 3 and 4 to form two dissociation products (P₁) HBCHCHCH₃⁺ + H ₂ and (P₂) HBC(CH₃)CH₂⁺ + H₂, with H₂-elimination from isomer 1 to be energetically most favorable. According to our calculated mechanism, the collisional stabilization processes of low-lying isomers 1, 1′, 3, and 4 may compete extensively with their H₂-eliminations processes for the title reaction, leading mainly to some linear carborane cations. This study may be helpful for understanding the stereochemical aspects of borohydride cations towards alkylenes.