DFT Studies on the Reactions of Boroles with Alkynes.

DFT calculations were performed to investigate the reactions of boroles with alkynes, in which boranorbornadiene, borepin and/or boracyclohexadiene can be formed depending on the substituents on the borole and alkyne. Our computational results indicated that formation of boranorbornadiene and borepin is generally kinetically favoured and that the products can interconvert. Formation of boracyclohexadiene is kinetically unfavourable in most reactions. However, certain substituents can promote the formation of boracyclohexadiene by stabilizing an energetically high-lying zwitterionic transition state that leads to boracyclohexadiene. Borafluorene reacted with most alkynes to give borepin exclusively due to its propensity to maintain its aromaticity. However, when it reacted with silyl-substituted alkyne(s), a novel product, different from boranorbornadiene, borepin and boracyclohexadiene was generated, which can be attributed to the well-known ability of silyl groups to migrate.