Chiral Octahedral Phosphano–Oxazoline Iridium(III) Complexes as Catalysts in Asymmetric Cycloaddition Reactions
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The synthesis and characterization of cationic iridium(III) aqua complexes of the formula [IrH(H2O)(PN*)(PP)][SbF6]2 (PN* = chiral phosphano–oxazoline ligand; PP = diphosphane) as well as that of the OPOF2-containing complex [IrH(OPOF2)(PNiPr)(dppp)][SbF6] (10) are reported. The X-ray molecular structures of [IrH(H2O)(PNInd)(dppe)][SbF6]2 (1), [IrH(H2O)(PNInd)(dppen)][SbF6]2 (2), and 10a have been determined. Dichloromethane solutions of these aqua complexes efficiently catalyze the enantioselective 1,3-dipolar cycloaddition of the nitrone N-benzylidenephenylamine N-oxide to methacrolein and Diels–Alder reactions between cyclopentadiene and trans-β-nitrostyrenes. In the first case, the catalytic reaction occurs with excellent endo selectivity and ee up to 85%; the Diels–Alder reaction occurs rapidly at room temperature with good endo:exo selectivity and ee up to 90%. The dipolar cycloaddition intermediates [IrH(methacrolein)(PNInd)(PP)][SbF6]2 (PP = (S,S)-chiraphos (11), (R)-prophos (12)) have been characterized, and the molecular structure of 11 has been determined by an X-ray structural analysis.
