Suzuki cross coupling of 4-Bromobenzyl-(1H)-tetrazole for a rational ligand design in series of Fe(II) spin-crossover complexes

Abstract

In 1982 Haasnoot et al. introduced N-substituted tetrazoles as ligands for spin-crossover compounds (SCO). Since then they have been used amongst other systems as spin-crossover ligands. In contrast to other systems the synthesis of complicated tetrazoles encounters difficulties due to low yields or difficult synthetic pathways. Herein a new pathway for the synthesis of N-substituted tetrazoles via palladium-catalyzed cross coupling reactions is investigated. This establishes an additional method for the synthesis of formerly difficult accessible functionalized tetrazoles. Therefore, a systematic study of the SCO phenomenon using those compounds is drastically simplified. As a model system one of the most famous and best studied cross coupling reactions was chosen, the Suzuki-Miyaura reaction. The reaction was investigated with 4-bromobenzyl-1H -tetrazole as test substrate. Optimization of the reaction parameters gave conditions under which satisfying yields were achieved. The scope of this reaction was studied and the products were characterized and tested for their potential use as SCO-ligands in iron(II) compounds. Two further coupling reactions were investigated regarding their applicability for the introduction of the tetrazole moiety, the Heck and the Sonogashira reaction. For both reactions conditions were found with the tetrazole reacting with the substrate, although in far lower yields than in the Suzuki coupling. Therefore, this work introduces palladium-catalyzed cross coupling reactions as an alternative pathway for the synthesis of otherwise inaccessible N1-substituted tetrazoles.