Highly enantioselective Diels–Alder catalysis with a chiral ruthenium bisoxazoline complex

When treated with AgSbF6, the cation (−)-[(η6-cymene)RuCl(L)]SbF6, where L=(+)-INDABOX=[3aR-[2(3′aR*,8′aS*),3′aβ,8′aβ]]-(+)-2,2′-methylenebis[3a,8a-dihydro-8H-indeno[1,2-d]-oxazol], yields a dication which is an efficient catalyst for the condensation of either methacrolein or ethylacrolein with cyclopentadiene in high ee. For example, the reaction of methacrolein with cyclopentadiene at −24 °C produced (S)-(+)-exo-2-methylbicyclo[2.2.1]hept-5-ene-2-carboxaldehyde in 95% conversion with de=96% and ee=91%. The monocationic catalyst precursor was characterized spectroscopically and by single crystal X-ray diffraction.