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

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posted on 27.03.2006, 00:00 by Daniel Carmona, Cristina Vega, Néstor García, Fernando J. Lahoz, Sergio Elipe, Luis A. Oro, M. Pilar Lamata, Fernando Viguri, Rosana Borao
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.