In Situ Formation of an Efficient Catalyst for the Semihydrogenation of Alkynes from Imidazolone and BH3

Abstract

We herein report the finding that a sterically encumbered imidazolone turns the simplest borane, i.e., BH3, into a potent catalyst for the semihydrogenation of alkynes. The efficiency of this catalyst system was demonstrated by the successful semihydrogenation of 15 different alkynes with excellent Z-selectivity. Kinetic studies show that the reaction rate depends on the alkyne concentration but is independent of the hydrogen pressure. Thus, the catalytic reaction can be operated at low hydrogen pressures (2 bar). Further mechanistic investigations and dispersion-corrected double-hybrid density functional theory (DFT) computations revealed that the imidazolone undergoes an initial protodeborylation with a tri-alkenylborane, formed in situ by hydroboration of the alkyne with BH3. The protodeborylation yields an imidazolonate borane that resembles a frustrated Lewis pair and is capable of activating dihydrogen.