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

The catalytic activity of the NHI‐substituted aluminum hydrides {L Mes NAlH 2 } 2 ( 1a ), {L Dip NAlH 2 } 2 ( 1b ), {L Mes NAl(H)OTf} 2 ( 2a ), and {L Dip NAl(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 Mes N = 1,3‐dimesityl‐imidazolin‐2‐imino, Mes = 2,4,6‐trimethylphenyl, L Dip N = 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.