Mechanistic Investigation of Acene Reduction with Boron Triiodide

Boron triiodide (BI3) is a strong Lewis acid that has underexplored reactivity. Herein we detail a new reaction utilizing BI3 to reduce acenes to their monohydrogenated products while investigating the mechanistic features of these reductions. Both pure BI3 or BI3 generated in situ from KBH4 and I2 are competent in the reduction, which is performed at room temperature and results in isolated yields up to 99%. Acenes with substitution on terminal and central rings were explored, with heteroatoms on the central ring being cleaved in the reaction. Mechanistic experiments include deuterium labeling, reaction time course monitoring, radical trap experiments, and control experiments. We observed that water is a necessary additive for the reduction using pure BI3. Anhydrous HI was found to be insufficient to reduce acenes under these conditions, with or without BI3; however, HI formation was observed and subsequently consumed at rates that indicate its involvement in product formation. Ultimately, experimental evidence suggests that either a BI3•OH2 adduct protonates the acene to initiate the reaction, or a hydrolysis product of BI3 catalyzes HI addition to the acene. Subsequently, three possible pathways utilizing HI are outlined to generate the final dihydro products and iodine as the oxidized byproduct.