Imidazolyl Alanes – Synthesis, Structures, and Reactivity Studies

Targeting the synthesis of Al/C based ambiphilic molecules, we investigated the dehydrohalogenation of a series of (benz)imidazole alane adducts. Depending on the steric bulk of the heterocycle, different dimeric products with various ring sizes were obtained. Dehydrohalogenation of the adduct of 1‐mesityl imidazole ( Mes Im) and 0.5 [ t Bu 2 AlBr] 2 furnished the dimer 2 , featuring a “classical” N‐heterocyclic carbene (NHC) and a mesoionic or “abnormal” NHC (aNHC) subunit within a single molecule. The dimer is bound loosely enough to allow thermally induced isomerization of 2 into the isomers 2 NHC (all NHC) and 2 aNHC (all aNHC). Dehydrohalogenation of the adduct of 1‐mesityl‐2‐methyl imidazole ( Mes Im Me ) and 0.5 [ t Bu 2 AlBr] 2 ( 4 ) yielded the dimeric compound 5 consisting of two N‐heterocyclic olefin (NHO) subunits. Although these six‐ and eight‐membered heterocycles show no FLP‐type reactivity towards small molecules like H 2 , CO or CO 2 , we observed an ambiphilic behavior of the imidazolyl alanes during our studies. Salt metathesis reactions using Mes Im resulted in the formation of 3 , which can be viewed as t Bu 2 AlBr adduct of an Al/N ambiphile. Utilizing heterocycles such as benzimidazole or spiroindole provided the entry point to C–H ( 7 , 9 ) and N–H ( 10 ) activation products, most likely resulting from a reactivity of intermediate species as Al/C ambiphiles.