Aluminum Hydrides Stabilized by N-Heterocyclic Imines as Catalysts for Hydroborations with Pinacolborane

The catalytic activity of the NHI-substituted aluminum hydrides {L^MesNAlH₂}₂(1a), {L^DipNAlH₂}₂(1b), {L^MesNAl(H)OTf}₂(2a), and {L^DipNAl(H)OTf}₂(2b) in the hydroboration of terminal alkynes (for 1), as well as carbonyl compounds (for 2) with pinacolborane (4,4,5,5-tetramethyl-1,3,2-dioxaborolane) is investigated (L^MesN = 1,3-dimesityl-imidazolin-2-imino, Mes = 2,4,6-trimethylphenyl, L^DipN = 1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-imino, Tf = triflyl). With the implementation of 1 as a catalyst, the hydroboration of selected terminal arylalkynes requires elevated temperature (80 °C) to proceed. The less sterically congested 1a produces faster conversions than the bulkier 1b. With the use of 2 as a catalyst, the hydroboration of carbonyl compounds occurs at room temperature. An increased steric hindrance of the catalyst (2a vs. 2b) does not mitigate the rate of conversion to a relevant degree.