Ruthenium complexes with novel tridentate N,P,N ligands containing a phosphonite bridge between two chiral oxazolines. Catalytic activity in cyclopropanation of olefins and transfer hydrogenation of acetophenone

We compare the coordination behavior of the new chiral ligand (S,S)-bis[1-(4-isopropyl-4,5-dihydrooxazol-2-yl)-1-methylethyl] phenylphosphonite (V), abreviated NOPONiPr and referred to as bis(oxazolinyl) phenylphosphonite, toward Ru(II) with that of NOPONMe2 (TV), Although the similarity between IV and V led to the expectation that they would coordinate in a similar manner, considerable differences were observed. Whereas reaction of [RuCl2(eta(6)-p-cymene)](2) with IV afforded the pentacoordinated complex [RuCl2(NOPONMe2)] 1, the chiral ligand V led instead to the dinuclear, chloride-bridged complex [Ru(mu-Cl)Cl(NOPONiPr)](2) (3). The crystal structures of 1 and 3 have been determined by X-ray diffraction, The mer arrangement of the NOPONMe2 ligand in 1 results from the presence of the four methyl groups, two of which would point toward each other and lead to a steric clash if a fac geometry were adopted. Preliminary theoretical calculations on 1 at the EHT level indicated that coordination of ethylene parallel to the RuCl2P plane would result in a stable situation and it is for steric reasons that ethylene does not coordinate to the metal, either parallel or perpendicular to the RuCl2P plane. Coordination of CO to 1 was found to be reversible, whereas bulkier ligands do not coordinate to Ru, Complex 3 is a rare example of a fully characterized complex in which a tridentate chiral bis(oxazoline)-type ligand coordinates in a non-mer fashion. It was evaluated in catalytic reactions such as asymmetric cyclopropanation of styrene with ethyl diazoacetate and transfer hydrogenation of acetophenone in propan-2-ol.