Chiral Octahedral Phosphano–Oxazoline Iridium(III) Complexes as Catalysts in Asymmetric Cycloaddition Reactions

Abstract

The synthesis and characterization of cationic iridium(III) aqua complexes of the formula [IrH(H 2 O)(PN*)(PP)][SbF 6 ] 2 (PN* = chiral phosphano−oxazoline ligand; PP = diphosphane) as well as that of the OPOF 2 -containing complex [IrH(OPOF 2 )(PNiPr)(dppp)][SbF 6 ] (10) are reported. The X-ray molecular structures of [IrH(H 2 O)-(PNInd)(dppe)][SbF 6 ] 2 (1), [IrH(H 2 O)(PNInd)(dppen)][SbF 6 ] 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)][SbF 6 ] 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.

Recent Advances of Catalytic Asymmetric 1,3-Dipolar Cycloadditions

“…1,3-DC of C , N -diphenyl nitrone with methacrolein was conducted by Ni­(II) complex to give the 3,5-endocycloadduct, whereas the Zn­(II) complex was chosen for the reaction with bromoacrolein to give the 3,4-endocycloadduct. Carmona’s group demonstrated the utility of Rh and Ir/chiral bisphosphine complexes 34 (Scheme c) . The 3,4-endoisoxazolidine was obtained preferentially in the reaction of C , N -diphenyl nitrone and methacrolein.…”

Diastereoselective Synthesis of Chiral Octahedral Iridium(III) Phosphano–Oxazoline Complexes

“…The easy preparation and high modularity of the new chiral iridium(III) compounds make them good catalyst candidates for asymmetric transformations. Some results obtained in this area are reported in a companion paper …”

“…Their characterization includes the molecular structure determination by X-ray diffractometric means of some representative examples. In a companion paper we report the preparation and characterization of the derived aqua complexes [IrH(H 2 O)(PN*)(PP)][SbF 6 ] 2 and their application as catalyst precursors in asymmetric 1,3-dipolar and Diels–Alder cycloadditions.…”

Asymmetric Catalysis with Substitutionally Labile yet Stereochemically Stable Chiral-at-Metal Iridium(III) Complex

“…Recently, we introduced inert chiral-at-metal iridium­(III) complexes as asymmetric catalysts in which the catalysis is mediated through the ligand sphere. , We hypothesized that the high configurational inertness of iridium­(III) complexes might permit retaining metal-centered chirality even in the presence of exchange-labile ligands, and we selected Λ- 1 and Δ- 1 for investigating the scope of asymmetric catalysis with reactive but configurationally stable chiral-at-metal complexes (Figure ). The C 2 -symmetrical complex 1 contains two cyclometalating 5- tert -butyl-2-phenyl­benzoxa­zoles and two labile acetonitrile ligands.…”