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 (L; 2-(2-(diphenylphosphino)­phenyl)-2,4-dihydro-1H-benzo­[d]­[1,3]­oxazine) showing a temperature-dependent equilibrium between a closed bidentate (LPN) and an opened tridentate (LPNO) form, has been synthesized and its coordination behavior toward ruthenium­(II) centers studied. Under different experimental conditions, two different species bearing the ligand in either 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 an X-ray structural determination. Notably, compound 2 reacts with an excess of ethyl diazoacetate (EDA) in CH2Cl2 to give a stable η3-diethyl maleate complex, [Ru­(LPN)­(cis-EtO­(O)­CCHCHC­(O)­OEt)­Cl2] (3). The crystal structure of 3 has also been 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-Me-py)2(LPNO)­Cl]­(Cl) (6a). The latter compound catalyzed the intermolecular cyclopropanation of styrene with EDA in high yields and with elevated cis diastereoselectivity (i.e., cis/trans = 80/20).