C-H Activation at a Ruthenium(II) Complex - The Key Step for a Base-Free Catalytic Transfer Hydrogenation?

Ruthenium(II) complexes [((6)-cymene)RuCl(apypm)]BPh4 with bidentate 2-amino-4-(2-pyridinyl)pyrimidine (apypm) ligands catalyze the transfer hydrogenation of acetophenone. Their activities are strongly dependent on the substituent pattern of the pyrimidine ring. Complexes bearing a primary amino group in the 2-position of the pyrimidine ring do not perform the catalysis in terms of a bifunctional mechanism, although they possess protic hydrogen atoms at the amino moiety in close proximity to the metal site. Systems containing tertiary dialkylated amino substituents at the apypm ligand are the first examples of air-stable and phosphane-free transfer-hydrogenation catalysts that show high activities even in the absence of a base. A new mechanism for the catalyst activation in the absence of an external base is proposed on the basis of ESI-MS investigations and ab initio calculations combined with isotope labelling: C-H bond cleavage at the pyrimidine ring is the crucial step for the generation of the catalytically active species.