Magnesiation and calciation of CH acidic N-alkyl imidazoles

Abstract

The reactivity of β-diketiminato alkylmagnesium, hydridomagnesium and hydridocalcium bases toward 1-methylimidazole, 1-tert-butylimidazole and 1-methylbenzimidazole enables the C-magnesiation and calciation of this family of N-heterocyclic molecules. Although 1-methylimidazole, 1-tert-butylimidazole, and 1-methylbenzimidazole are deprotonated at their C-2 positions by both the magnesium alkyl and hydride derivatives, these reagents can also leverage a level of regiodivergent kinetic discrimination and resultant C-4 metallation. In contrast to the straightforward C-2-deprotonation of 1-methylbenzimidazole provided by the magnesium hydride, a rapid cascade of apparent deprotonation, imidazole ring opening and twofold CC coupling was induced by the analogous calcium reagent. This process provides a tricalcium species comprising an unprecedented trianionic unit, which may be considered as a trimethylenemethane dianion isostere equipped with a pendent amide donor. While attempted boron functionalisation of the imidazolyl anions of the magnesium derivatives of 1-methylimidazole and 1-methylbenzimidazole with pinacolborane (HBpin) was unsuccessful, the 1-tert-butylimidazolyl analogue provided the C-2-borylated product irrespective of the position of initial C-2 or C-4 deprotonation. Although the calcium congeners resulting from 1-methylimidazole and 1-tert-butylimidazole were deduced to form similarly borylated products, study of their onward reactivity was prevented by their instability toward Schlenk-type redistribution in solution.