Ruthenium(II) Complexes Bearing a Ligand Derived from P,N- or P,N,O-Diphenylphosphinobenzoxazine: Synthesis, X-ray Characterization, and cis Diastereoselectivity in Styrene Cyclopropanation

A phosphino-oxazine based ligand (2-(2-(diphenylphosphino)phenyl)-2,4-dihydro-1Hbenzo[ d][1,3]oxazine) (L) showing a temperature-dependent equilibrium between a closed bidentate (LPN) and an opened tridentate (LPNO) form, has been synthesized and the coordination behavior toward ruthenium(II) centers studied. Employing different experimental conditions, two different species bearing the ligand either in its bidentate or tridentate coordination mode were isolated by reaction with Ru(PPh3)3Cl2. These species, respectively formulated as [Ru(PPh3)(LPNO)Cl2] (1) and [Ru(PPh3)(LPN)Cl2] (2), were fully characterized via NMR in solution and by X-ray structural determination. Notably, compound 2 reacts with an excess of ethyl diazoacetate (EDA) in CH2Cl2 to give a stable η3-diethylmaleate complex, [Ru(LPN)(cis-EtOOC-CH=CH-C(O)OEt)Cl2] (3). The crystal structure of 3 has been also determined. Substitution reactions with 4-picoline (4-Me-py) performed on 1 led to two new complexes, the neutral complex [Ru(4-Me-py)(LPNO)Cl2] (5) and the salt [Ru(4-Mepy) 2(LPNO)Cl](Cl) (6a). This latter catalyzed the intermolecular cyclopropanation of styrene with EDA in high yields and with elevated cis-diastereoselectivity (i.e., cis:trans = 80:20).