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

The synthesis and characterization of optically active phosphinooxazoline chloride complexes (SM and
RM)-[(η6-p-MeC6H4iPr)MCl(PN)]A (M = Ru, Os; PN = phosphinooxazoline ligand; A = counteranion)
and the derived aqua complexes (RM and SM)-[(η6-p-MeC6H4iPr)M(PN)(H2O)](A)2 are reported. The
OPOF2-containing compounds (RM and SM)-[(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 (SM)-[(η6-p-MeC6H4iPr)MCl(PNiPr)][SbF6] (M = Ru, Os),
(SRu)-[(η6-p-MeC6H4iPr)RuCl(PNInd)][SbF6] (PNInd = (3aS,8aR)-2-(2-diphenylphosphinophenyl)-3a,8a-dihydroindane [1,2-d]oxazole), and (RRu)-[(η6-p-MeC6H4iPr)Ru(PNiPr)(H2O)][SbF6] and that of the
OPOF2-containing compounds (RRu and SRu)-[(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 (RRu and SRu)-[(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
(SOs and ROs)- [(η6-p-MeC6H4iPr)Os(PN)(H2O)][A]2 (PN = PNiPr, A = SbF6, BF4; PN = PNInd, A =
SbF6) evolve to the phenyl-containing compounds (SOs and ROs)-[(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 (SOs and ROs)-[(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.