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

The synthesis and characterization of optically active phosphinooxazoline chloride complexes (<i>S</i><sub>M</sub> and <i>R</i><sub>M</sub>)-[(η<sup>6</sup>-<i>p</i>-MeC<sub>6</sub>H<sub>4</sub><i>i</i>Pr)MCl(PN)]A (M = Ru, Os; PN = phosphinooxazoline ligand; A = counteranion) and the derived aqua complexes (<i>R</i><sub>M</sub> and <i>S</i><sub>M</sub>)-[(η<sup>6</sup>-<i>p</i>-MeC<sub>6</sub>H<sub>4</sub><i>i</i>Pr)M(PN)(H<sub>2</sub>O)](A)<sub>2</sub> are reported. The OPOF<sub>2</sub>-containing compounds (<i>R</i><sub>M</sub> and <i>S</i><sub>M</sub>)-[(η<sup>6</sup>-<i>p</i>-MeC<sub>6</sub>H<sub>4</sub><i>i</i>Pr)M(OPOF<sub>2</sub>)(PN<i>i</i>Pr)][PF<sub>6</sub>] (M = Ru, Os; PN<i>i</i>Pr = (4<i>S</i>)-2-(2-diphenylphosphinophenyl)-4-isopropyl-1,3-oxazoline) have been also prepared and characterized. The molecular structures of (<i>S</i><sub>M</sub>)-[(η<sup>6</sup>-<i>p</i>-MeC<sub>6</sub>H<sub>4</sub><i>i</i>Pr)MCl(PN<i>i</i>Pr)][SbF<sub>6</sub>] (M = Ru, Os), (<i>S</i><sub>Ru</sub>)-[(η<sup>6</sup>-<i>p</i>-MeC<sub>6</sub>H<sub>4</sub><i>i</i>Pr)RuCl(PNInd)][SbF<sub>6</sub>] (PNInd = (3a<i>S,</i>8a<i>R</i>)-2-(2-diphenylphosphinophenyl)-3a,8a-dihydroindane [1,2-<i>d</i>]oxazole), and (<i>R</i><sub>Ru</sub>)-[(η<sup>6</sup>-<i>p</i>-MeC<sub>6</sub>H<sub>4</sub><i>i</i>Pr)Ru(PN<i>i</i>Pr)(H<sub>2</sub>O)][SbF<sub>6</sub>] and that of the OPOF<sub>2</sub>-containing compounds (<i>R</i><sub>Ru</sub> and <i>S</i><sub>Ru</sub>)-[(η<sup>6</sup>-<i>p</i>-MeC<sub>6</sub>H<sub>4</sub><i>i</i>Pr)Ru(OPOF<sub>2</sub>)(PN<i>i</i>Pr)][PF<sub>6</sub>] have been determined by X-ray diffractometric methods. Dichloromethane solutions of the aqua complexes [(η<sup>6</sup>-<i>p</i>-MeC<sub>6</sub>H<sub>4</sub><i>i</i>Pr)M(PN)(H<sub>2</sub>O)][SbF<sub>6</sub>]<sub>2</sub> are active catalysts for the Diels−Alder reaction between methacrolein and cyclopentadiene. The reaction occurs rapidly at room temperature with good <i>exo:endo</i> selectivity (from 85:15 to 96:4) and moderate enantioselectivity (up to 47%). The intermediate Lewis acid−dienophile compound (<i>R</i><sub>Ru</sub> and <i>S</i><sub>Ru</sub>)-[(η<sup>6</sup>-<i>p</i>-MeC<sub>6</sub>H<sub>4</sub><i>i</i>Pr)Ru(PNInd)(methacrolein)][SbF<sub>6</sub>]<sub>2</sub> was isolated, and the molecular structure of the <i>S</i> epimer was determined by diffractometric means. The osmium complexes (<i>S</i><sub>Os</sub> and <i>R</i><sub>Os</sub>)- [(η<sup>6</sup>-<i>p</i>-MeC<sub>6</sub>H<sub>4</sub><i>i</i>Pr)Os(PN)(H<sub>2</sub>O)][A]<sub>2</sub> (PN = PN<i>i</i>Pr, A = SbF<sub>6</sub>, BF<sub>4</sub>; PN = PNInd, A = SbF<sub>6</sub>) evolve to the phenyl-containing compounds (<i>S</i><sub>Os</sub> and <i>R</i><sub>Os</sub>)-[(η<sup>6</sup>-<i>p</i>-MeC<sub>6</sub>H<sub>4</sub><i>i</i>Pr)OsPh(PN‘)][SbF<sub>6</sub>] (PN‘ = (4<i>S</i>)-2-(2-hydroxyphenylphosphinophenyl)-4-isopropyl-1,3-oxazoline (PNOH<i>i</i>Pr), PN‘ = (3a<i>S,</i>8a<i>R</i>)-2-(2-hydroxyphenylphosphinophenyl)-3a,8a-dihydroindane[1,2<i>d</i>]oxazole] (PNOHInd)) and (<i>S</i><sub>Os</sub> and <i>R</i><sub>Os</sub>)-[(η<sup>6</sup>-<i>p</i>-MeC<sub>6</sub>H<sub>4</sub><i>i</i>Pr)OsPh(PNF<i>i</i>Pr)][BF<sub>4</sub>] (PNF<i>i</i>Pr = (4<i>S</i>)-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.