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)-[(eta(7)-p-MeC(6)H(4)iPr)MCl(PN)]A (M = Ru, Os; PN = phosphinooxazoline ligand, A = counteranion) and the derived aqua complexes (R-M and S-M)-[(eta(6)-p-MeC(6)H(4)iPr)M(PN)(H2O)](A)(2) are reported. The OPOF2-containing compounds (R-M and S-M)-[(eta(6)-p-MeC(6)H(4)iPr)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)-[(eta(6)-p-MeC(6)H(4)iPr)MCI(PNiPr)][SbF6] (M = Ru, Os), (S-Ru)-[(eta(6)-p-MeC(6)H(4)iPr)RuCl(PNInd)][SbF6] (PNInd = (3aS,8aR)-2-(2-diphenylphosphinophenyl)-3a,8a-dihydroindane [1,2-d]oxazole), and (R-Ru)-[(eta(6)-p-MeC(6)H(4)iPr)Ru(PNiPr)(H2O)][SbF6] and that of the OPOF2-containing compounds (R-Ru and S-Ru)-[(eta(6)-p-MeC(6)H(4)iPr)Ru(OPOF2)(PNiPr)][PF6] have been determined by X-ray diffractometric methods. Dichloromethane solutions of the aqua complexes [(eta(6)-p-MeC(6)H(4)iPr)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)-[(eta(6)-p-MeC(6)H(4)iPr)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)-[(eta(6)-p-MeC(6)H(4)iPr)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)-[(eta(6)-p-MeC(6)H(4)iPr)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)-[eta(6)-p-MeC(6)H(4)iPr)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.