Imidazolyl Alanes - Synthesis, Structures, and Reactivity Studies – 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 ($^{Mes}$Im) and 0.5 [tBu$_{2}$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 [tBu$_{2}$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 tBu$_{2}$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