Abstract
AbstractHerein we report a comparative study of nickel‐catalyzed syntheses of pyrimidines via dehydrogenative multi‐component coupling of alcohols and amidines using two different classes of nickel catalysts (1 a/1 b and 2 a/2 b) differing with respect to their mode of action during catalysis. The catalysts 1 a and 1 b are two tetracoordinate Ni(II)‐complexes containing two apparently redox‐inactive tetraaza macrocyclic ligands while the catalysts 2 a and 2 b are square planar Ni(II)‐complexes featuring redox‐active diiminosemiquinonato type scaffolds. Catalyst 1 a and 1 b dehydrogenate alcohols via a two‐electron hydride transfer pathway involving energetically demanding nickel‐centered redox events while in the presence of 2 a and 2 b dehydrogenation of alcohols proceeds via a one‐electron hydrogen atom transfer (HAT) pathway via synergistic participation of metal and ligand centered redox processes avoiding high energy nickel centered redox events. Detailed substrate screening and control experiments were performed to unveil the reaction sequence and understand the advantages/disadvantages of these two pathways.
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