New 1,2,4,5-tetrakis-(N-imidazoliniummethyl)benzene and 1,2,4,5-tetrakis-(N-benzimidazoliummethyl)benzene salts as N-heterocyclic tetracarbene precursors: synthesis and involvement in ruthenium-catalyzed allylation reactions

New tetraimidazolinium and tetrabenzimidazolium salts have been prepared. Upon reaction with tBuOK, they generate carbene ligands, which were associated in situ to [RuCp*(MeCN)3]PF6 to produce new ruthenium catalysts that are active for the substitution of allylic substrates by amines, phenols, and carbonucleophiles. The influence of the N-heterocyclic core as well as that of the N-substitutents at the periphery of the salts, on reactivity and regioselectivity have been examined.