Imidazolyl Alanes – Synthesis, Structures, and Reactivity Studies

Abstract

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 (MesIm) and 0.5 [tBu2AlBr]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 2NHC (all NHC) and 2aNHC (all aNHC). Dehydrohalogenation of the adduct of 1‐mesityl‐2‐methyl imidazole (MesImMe) and 0.5 [tBu2AlBr]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 H2, CO or CO2, we observed an ambiphilic behavior of the imidazolyl alanes during our studies. Salt metathesis reactions using MesIm resulted in the formation of 3, which can be viewed as tBu2AlBr 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.