Chiral Phosphinooxazoline−Ruthenium(II) and −Osmium(II) Complexes as Catalysts in Diels−Alder Reactions

The synthesis and characterization of optically active phosphinooxazoline chloride complexes (S_M and R_M)-[(η⁶-p-MeC₆H₄iPr)MCl(PN)]A (M = Ru, Os; PN = phosphinooxazoline ligand; A = counteranion) and the derived aqua complexes (R_M and S_M)-[(η⁶-p-MeC₆H₄iPr)M(PN)(H₂O)](A)₂ are reported. The OPOF₂-containing compounds (R_M and S_M)-[(η⁶-p-MeC₆H₄iPr)M(OPOF₂)(PNiPr)][PF₆] (M = Ru, Os; PNiPr = (4S)-2-(2-diphenylphosphinophenyl)-4-isopropyl-1,3-oxazoline) have been also prepared and characterized. The molecular structures of (S_M)-[(η⁶-p-MeC₆H₄iPr)MCl(PNiPr)][SbF₆] (M = Ru, Os), (S_Ru)-[(η⁶-p-MeC₆H₄iPr)RuCl(PNInd)][SbF₆] (PNInd = (3aS,8aR)-2-(2-diphenylphosphinophenyl)-3a,8a-dihydroindane [1,2-d]oxazole), and (R_Ru)-[(η⁶-p-MeC₆H₄iPr)Ru(PNiPr)(H₂O)][SbF₆] and that of the OPOF₂-containing compounds (R_Ru and S_Ru)-[(η⁶-p-MeC₆H₄iPr)Ru(OPOF₂)(PNiPr)][PF₆] have been determined by X-ray diffractometric methods. Dichloromethane solutions of the aqua complexes [(η⁶-p-MeC₆H₄iPr)M(PN)(H₂O)][SbF₆]₂ are active catalysts for the Diels−Alder reaction between methacrolein and cyclopentadiene. The reaction occurs rapidly at room temperature with good exo:endo selectivity (from 85:15 to 96:4) and moderate enantioselectivity (up to 47%). The intermediate Lewis acid−dienophile compound (R_Ru and S_Ru)-[(η⁶-p-MeC₆H₄iPr)Ru(PNInd)(methacrolein)][SbF₆]₂ was isolated, and the molecular structure of the S epimer was determined by diffractometric means. The osmium complexes (S_Os and R_Os)- [(η⁶-p-MeC₆H₄iPr)Os(PN)(H₂O)][A]₂ (PN = PNiPr, A = SbF₆, BF₄; PN = PNInd, A = SbF₆) evolve to the phenyl-containing compounds (S_Os and R_Os)-[(η⁶-p-MeC₆H₄iPr)OsPh(PN‘)][SbF₆] (PN‘ = (4S)-2-(2-hydroxyphenylphosphinophenyl)-4-isopropyl-1,3-oxazoline (PNOHiPr), PN‘ = (3aS,8aR)-2-(2-hydroxyphenylphosphinophenyl)-3a,8a-dihydroindane[1,2d]oxazole] (PNOHInd)) and (S_Os and R_Os)-[(η⁶-p-MeC₆H₄iPr)OsPh(PNFiPr)][BF₄] (PNFiPr = (4S)-2-(2-fluorophenylphosphinophenyl)-4-isopropyl-1,3-oxazoline), respectively, in which the phosphinooxazoline ligand incorporates a hydroxy or fluoro functionality. On the basis of spectroscopic and crystallographic observations, a common pathway for these reactions is proposed.