Transfer hydrogenation of acetophenone catalyzed by half-sandwich ruthenium(II) complexes containing amino amide ligands. Detection of the catalytic intermediates by electrospray ionization mass spectrometry

A series of natural amino acid (alanina, valina, phenylalanine and isoleucine) amides have been synthesized and fully characterized. They have been used as supporting ligands in the Ru(II) catalyzed asymmetric transfer hydrogenation (ath) of acetophenone in the presence of i-PrOH/KOH. Secondary amides impart high reactivity to the corresponding Ru(II) complexes, with TOFs up to 1680 h-1 and ees up to 47%. The amino acid and the substituents of the amide nitrogen govern the activity and the enantioselectivity of the catalytic processes. The pre-catalysts obtained by reacting (L)-phenylalanine p-anisidineamide and (L)-valine o-anisidineamide with [Ru(p-cymene)Cl2]2 have been isolated and characterized as the half-sandwich complexes [(η6-p-cymene)Ru(κ2-N,N′-aminoamidato)Cl] (10 and 11, respectively). An ESI-MS study conducted on the acetophenone reduction catalyzed by 11 has led to the detection of the organometallic intermediates involved in the catalytic cycle: the pre-catalyst 11, the 16e--complex [(η6-p-cymene)Ru(κ2-N,N′-diamide)] 12 and the hydride [(η6-p-cymene)Ru(κ2-N,N′-aminoamidato)H] 13.